ISSUE RS OF L REL ATIVIT Y

SPECIAL 100 YE A GENERA

SEPTEMBER 2015 ScientificAmerican.com

EINS
HOW RELATIVITY
CHANGED THE RULES
TEIN A BRIEF
HISTORY
OF OUR REALITY OF TIME
HIS LIFE, TRAVEL
CRISES &
BOLDEST THINGS
DREAMS HE GOT
WRONG
NEW QUEST
FOR A
THEORY OF
EVERYTHING
2015 Scientific American
 SEPTEMBER 2015

VO L U M E 3 1 3 , N U M B E R 3

I N T RO DUC TIO N
32 Einstein
THE
A hundred years ago Albert EINSTEIN
Einstein published his general
theory of relativity and rewrote
ISSUE
the rules of our physical world. 
By the Editors
ESS AY
34 Why He Matters
Einsteins mind had an astonishing
influence on our civilization.
By Brian Greene
H I STO RY
38 How Einstein
Reinvented Reality
Amid personal troubles, political
tension and fierce competition,
a grand theory was born.
By Walter Isaacson
Also: Relativity Primer
T H OUGH T EXP ER IMENTS
46 Head Trip
Einsteins thought experiments
forged new frontiers in physics. 
By Sabine Hossenfelder
COS M O LO GY
50 What Einstein Got Wrong

32
The visionary scientist repeatedly
misjudged the value of his ideas.
By Lawrence M. Krauss
A L EG ACY I N NU MBERS
56 Relativitys Reach
A map of Einsteins influence.
F U N DAM E N TAL P HYSICS A STUDY IN CONTR A STS
60 Cleaning Up after Einstein 80 Who Was Einstein, Really?
His quest for a theory of everything The great scientists life
failed, so todays physicists are and personality.
trying new strategies.
NEUROSC IENC E
By Corey S. Powell
82 Genius in a Jar
T H EO RY Decades of picking over Einsteins
68 A Brief History brain have brought us no closer to
of Time Travel understanding what made him so
COVER PHOTO REFERENCE: GETTY IMAGES

Moving forward is easy. Backward smart. By Brian D. Burrell ON THE C OVE R
Albert Einsteins enigmatic face has captivated
poses a challenge. By Tim Folger QUA NTUM PHYS ICS the world nearly as long as his theories have.
AST RO N O MY 88 Is the Cosmos Random? The scientists hallmark hair and soulful eyes have
74 The Black Hole Test Einsteins claim that God does not become an icon of genius. This special issue
investigates the great mans life, legacy and the
Does general relativity work in play dice with the universe has
future of his ideas on the 100th anniversary of
extreme gravity? By Dimitrios been seriously misinterpreted.  his greatest success, his general theory of relativity.
Psaltis and Sheperd S. Doeleman By George Musser Illustration by Daniel Adel.

2 Scientific American, September 2015 Illustration by Bernard Lee

2015 Scientific American

 6 From the Editor
8 Letters
12 Science Agenda
Budget meddling by Congress could cripple
crucial Earth science programs. B
 y the Editors

14 Forum
Brain researchers are overwhelmed with data.
Hackers can help. By Daniel Goodwin

16 Advances
Why Lyme disease may linger. Underwater greenhouses.
The SAT revamped. A toy car powered by evaporation.
14
29 The Science of Health
A Mediterranean-style diet in later stages of life
may slow memory loss. B y Dina Fine Maron

31 TechnoFiles
The new breed of virtual assistant is scary smart.
By David Pogue

94 Recommended
Recent takes on Einstein. B
 y Clara Moskowitz

95 Skeptic
How trustworthy are DNA and other
crime scene tests? By Michael Shermer

96 Anti Gravity
21
Birds show math skills; dogs showcase loyalty. 
By Steve Mirsky

98 50, 100 & 150 Years Ago

100 Graphic Science
Populous cities are supposed to emit less carbon
per capita, but they dont always. B
 y Amber Williams

ON THE WEB

General Relativity at 100

Take a quiz about Einstein, read about the real story
of E = mc2, explore an interactive relativity infographic,
watch a video about the theory, and more.
Go to www.ScientificAmerican.com/relativity
95

Scientific American (ISSN 0036-8733), Volume 313, Number 3, September 2015, published monthly by Scientific American, a division of Nature America, Inc., 1 New York Plaza, Suite 4500, New York, N.Y. 10004-1562. Periodicals
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4 Scientific American, September 2015

2015 Scientific American

From the Editor
Mariette DiChristinais editor
in chief of Scientific American. 
Follow her on Twitter @mdichristina

Editorial and Einstein

A long with his article submission, the scientist

included an apologetic note to the editors:
The article is somewhat long and not quite
easy to grasp. I should, therefore, not be aston
ished if you find it unsuited
for publication in your magazine.
Despite the authors concern, the pub
lisher, Gerard Piel, and the editor in chief,
Dennis Flanagan, ran the article in S  cientific
mentions Einstein, who boldly generalized the principle and
proclaimed the relativity of all intervals and epochs of time.
And in 1920long before the world ever heard of crowd
sourcing, the magazine sought to rectify the paucity
of lay explanations of the physics via a global
essay contest. The editors explained:
Mr. Eugene Higgins offered, through
the Scientific American, a prize of the
extraor
dinary amount of $5,000 for
American. They also framed the letter. the best popular essay on the Einstein
The author? Albert Einstein. His article, Theories of Relativity. The contest drew
On the Generalized Theory of Gravitation, exactly 300 essays from all parts of Eu
appeared in the April 1950 issue (right). rope and North America, from India and
I stumbled across this charming anecdote South Africa and South America. With the
while paging through one of our old scrap agreement of the judges, Mr. L. Bolton of
books, which included a clipping from the London got the award, with publication of
November 19, 1961, edition of the C  hicago Sun- his essay in a February 5, 1921, issue.
Times, along with others about our publication of the feature A century after Einsteins landmark De
submission from Einstein. cember 2, 1915, publication of Die Feldgleichungen der Gravi-
Of course, it wasnt our first encounter with Einstein. As you tation (The Field Equations of Gravitation), which we cele
would expect from a magazine that is now celebrating its 170th brate in this edition, we offer todays perspective on efforts to
year of reporting innovation, the editorial team and our scientist grasp the nature of spacetime. Issue editor Clara Moskowitz
authors covered his theories as he published them. For example, and the team have created a special report that is profound yet
Max Planck commented about the evolution of relativity theory playful and sparkles with the wonder of discoveryrather like
in a 1910 article, The Mechanical Theory of Nature: The prin the great man himself. We hope you enjoy reading it as much as
ciple of relativity, despite its youth, appears very promising. He we did putting it together.

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6 Scientific American, September 2015 Illustration by Nick Higgins

2015 Scientific American

Letters
editors@sciam.com

Californias tum foam of quark-antiquark and gluon

pairs that continually pop in and out of ex-
law mandating istence inside the proton. Thus, in a proton
vaccinations moving at 0.99999 times the speed of light,
the lifetime of this quantum foam is dilated
for school-age long enough that its feature of splitting
children is a gluons is captured by an observers quark-
gluon femtoscope. Conversely, the physicist
necessary measure. co-moving with the proton cannot observe
hannah peckler u niversity of california, these quantum fluctuations, because they
san francisco, school of nursing are short-lived, relative to him or her. This
person is therefore impervious to the
seething cauldron of quarks and gluons
that killed dinosaurs but spared mam- existing within the proton (and all of us).
mals, allowing our ancestors to take over. Because the fluctuations exist in both
But it wasnt so simple. Some dinosaurs frames, special relativity is upheld.
did survive: birds. Yet it is a mystery why
some (but not all) birds did so, but numer- PUBERTY ONSET
May 2015 ous very birdlike, feathered dinosaurs After reading Why Girls Are Starting Pu-
such as Velociraptor and its kin died. berty Early, by Dina Fine Maron [The Sci-
Many mammals did survive, particularly ence of Health], I was surprised that there
VACCINATION STRATEGY those that were smaller and had more gen- was not a mention of the impactof growth
In Wooing the Fence-sitters [Science eral diets. Yet a number of other mammals hormones in milk production. Can these
Agenda], the editors suggest that rather perished. The late Cretaceous was the hey- hormones bear some of the responsibility
than strong-arm tactics, we should use a day of metatherians (living marsupials for the early onset of puberty in girls?
more subtle social strategy of little nudg- and close relatives), but this entire group Vivian Fabbro Keenan
es to convince parents to vaccinate their almost went extinct when the asteroid that St. Petersburg, Fla.
children. But strong-arm tactics are noth- triggered the dinosaurs demise hit. In the
ing new to public health. How is a law that ensuing Paleogene, it was the placental Although it is clear that obesity is part of
mandates vaccinations for school-age chil- mam mals that took advantage of the the picturefat cells secrete estrogen,
dren, such as the one recently passed in metatherian demise and blossomed into which is the major hormone involved in
California, any different from, say, fluori- the many familiar groups we know today, puberty in girlsI was astounded that
dating drinking water? Although forming including our primate forebears. Maron ignored what is most certainly the
peer-advocacy groups and promoting pro- cause of both obesity and early-onset pu-
vaccine interactions with providers are RELATIVISTIC VOYAGE berty: a diet rich in high-fat animal prod-
part of the solution, I do not think these In The Glue That Binds Us, Rolf Ent, ucts, including dairy foods, which are
strategies alone are aggressive enough for Thomas Ullrich and Raju Venugopalan themselves rich in estrogen.
a time-sensitive and life-threatening pub- state that physicists think that when pro- Adam Dave
lic health issue. Californias law is a neces- tons and neutrons reach extreme speeds, via e-mail
sary measure to shift the immunization the gluons inside the protons split into
rate above the safety threshold. pairs of new gluons. MARON REPLIES: I t is not simple to iden-
Hannah Peckler But that would violate special relativi- tify any one factor responsible for earlier
University of California, San Francisco, tys tenet that the laws of physics are the puberty in girls. The bulk of evidence
School of Nursing same for all observers. Consider the per- points to those outlined in the article (such
spective of a tiny physicist riding a proton as obesity), yet other theories abound.
DINOSAUR EXTINCTION in a vacuum, surrounded by a tube that rac- Milk does contain some substances that
Stephen Brusatte refers to the extinction of es by at ever faster speeds.Our little physi- have weak estrogenic effects in humans,
the dinosaurs in Rise of the Tyranno- cist monitors his proton from time to time but they probably are not big drivers of
saurs. What was so vulnerable in the dino- and always finds it the same. Which physi- earlier puberty. Meanwhile although some
saurs that ensured their extinction while cists gluons are splittingthe tiny one or cows are also treated with a hormone relat-
allowing mammals to survive and thrive? one observing from outside the tube? ed to human growth hormone, there is little
Peter Stephen Charles M. Bagley, Jr. reason to think that it would affect hu-
via e-mail Seattle mans, and according to pediatric endocri-
nologist Paul Kaplowitz, naturally occur-
BRUSATTE REPLIES: T  he end-Cretaceous THE AUTHORS REPLY: E  insteins special ring and added hormones in these prod-
extinction is often viewed as a catastrophe theory of relativity holds true for the quan- ucts are quickly degraded in the stomach.

8 Scientific American, September 2015

2015 Scientific American

Letters

L E T T E R S TO T H E E D I TO R
Instead environmental chemicals that act
like hormones after being ingested appear Scientific American
to be of much greater concern. ESTABLISHED 1845 1 New York Plaza, Suite 4500
EDITOR IN CHIEF AND SENIOR VICE PRESIDENT New York, NY 10004-1562
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variety of health reasons. Yet it is hard to Fred Guterl Michael Mrak Letters may be edited for length and clarity.
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10 Scientific American, September 2015

2015 Scientific American

Science Agenda by the Editors
Opinion and analysis from Scientific Americans Board of Editors

Dont Blind
nasa to
Earths Climate
Budget meddling by Congress could
cripple Earth science programs

nasa was always supposed to look close to home as well as out

to the stars. In 1958 the U.S. Congress chartered the agency to
focus on phenomena in the atmosphere and space. Through
Earth-observing satellites, nasa has vastly improved weather
forecasting and natural disaster prediction and relief. It may
have even helped save the world when it spotted a dangerous,
growing hole in the planets protective ozone layer in the 1980s.
The data spurred the international community to ban ozone-
destroying chlorofluorocarbon chemicals.
This year congressional Republicans seem to have decided
they have had too much of a good thing and have moved to de- during the George W. Bush administration. Then, as now, ac-
crease nasas Earth science budget. They have been egged on tions were driven in large part by antiscientific opposition to evi-
by Senator Ted Cruz of Texas, who has big ambitionshe an- dence that global warming has a human trigger.
nounced his run for president this yearbut little respect for sci- Another agency, the National Oceanic and Atmospheric Ad-
ence. Cruz, chair of the Senate Subcommittee on Space, Science, ministration, also has eyes on Earth. But neither Republicans
and Competitiveness, said in a hearing that Earth science was nor Democrats in Congress have supported noaa with much
not part of the agencys core mission to space and, indeed, that fervor. For instance, in 2012 Democratic Senator Barbara Mikul-
it was not hard science at all. But this choice between our plan- ski of Maryland tried to move several noaa satellites to nasa.
et and others is a false one. And in the Obama administrations 2016 budget, noaa requested
In June the House of Representatives, led by Republicans, $30million for a study of ocean acidification, which is driven by
passed a budget of $18.5 billion (which is what the White House climate change. The House granted $8.4million, cut noaas total
had requested) but reshuffled where the money was to be spent. budget by about 5 percent and gave nasas Europa project that
It slashed Earth science funding by $260 million and added extra $110-million boost. Dont tell me that there isnt money avail-
money for planetary science that the agency did not ask for. For able, fumed Democratic Representative Sam Farr of California
example, nasa requested $30million for a robotic mission to Ju- during a House debate. It is just the priority where you give it.
piters icy moon, Europa, but the House gave it $140 million. The Are you going to save this planet or put all the money into the
Senate Appropriations Committee, also pushed by the Republi- moon of Jupiter?
can majority, approved a version of nasas budget that reduced nasa researchers have successfully placed rovers on Mars and
total funding by $239 million. At press time, Congress needed to tracked the depletion of groundwater that is exacerbating the
reconcile these competing bills. current drought in the American West. Of all federal agencies,
Several Republicans, such as Cruz and Representative John this one is best positioned to study the heavens and the major
Culberson of Texas, claim that funds used by the agency for gaz- environmental changes that affect our lives on Earth. Political
ing down at Earth would be better spent examining other worlds. extremists need to back off from their budgetary meddling and
The cuts, many say, actually pare back years of Earth science lar- let the agency do both its jobs. The clock is ticking: the new fiscal
gesse from the Obama administration that underfunded other year starts in October.
agency initiatives.
This argument is misleading. The current administration did SCIENTIFIC AMERICAN ONLINE
increase nasas Earth science budget but only to redress a nearly Comment on this article at S cientificAmerican.com/sep2015
40 percent cut such science suffered between 2001 and 2006,

12 Scientific American, September 2015 Illustration by Morgan Schweitzer

2015 Scientific American

Forum by Daniel Goodwin
Commentary on science in the news from the experts Daniel Goodwin is a doctoral researcher
in the Synthetic Neurobiology Group at the
Massachusetts Institute of Technology. This
piece was written while he was a research
scientist at the Simons Center for Data
Analysis in New York City.

in a field like neuroscience, helpers could be in

valuable in exploring the daunting wilderness of
newly revealed neural networks.
A few pioneers are leading the way. One is H.
Sebastian Seung, a professor at the Neuroscience
Institute and in the department of computer sci-
ence at Princeton University. A few years ago he
and his collaborators set out to map the retinas
neural connections. As they collected an over-
whelming mass of electron microscopy data, the
question was how they would ever manage to
interpret it all. Seungs familiarity with state-
of-the-art computing told him that no artificial-
intelligence algorithm in existence could possibly
handle the task alone.
The solutionthen almost unheard of in lab

Neuroscience
sciencewas to enlist thousands of human volun-
teers alongside a state-of-the art AI and harness their collective
brainpower. On December 10, 2012, Seung and his team launched

Needs Hackers
the online game EyeWire, in which players score points by help-
ing to improve a neural map. About a year and a half later the
games creators published their first discoveries in Nature, togeth-
er with a note sharing coveted co-author credit with the 2,183
Brain researchers are overwhelmed players who had reached the games top ranks and made the
paper possible. (Scientific American is part of Springer Nature.)
with data. Hackers can help Hackers are finding their own routes into neuroscience. In
late 2013 Brooklyn, N.Y.based designers Joel Murphy and Conor
There was a time w  hen neuroscientists could only dream of hav- Russomanno introduced OpenBCI, an open-source brain-com-
ing such a problem. Now the fantasy has come true, and they are puter interfacebasically a home-brewed electroencephalo-
struggling to solve it. Brilliant new exploratory devices are over- graphic device. Kits and plans are available from their Web site
whelming the field with an avalanche of raw data about the ner- for just a fraction of a standard EEGs cost, and by all accounts it
vous systems inner workings. The trouble is that even starting works just as well as the big-budget models. Their two-month-
to make sense of this bonanza of information has become a long Kickstarter campaign sold nearly 1,000 units and caught
superhuman challenge. the attention of academic research labs. Its just another example
Just about every branch of science is facing a similar disrup- of how traditional barriers are crumbling between institutional
tion. As laboratory-bench research migrates into the digital science and individuals with new ideas. In fact, some labs have
realm, programming is becoming an indispensable part of the begun posting research challenges with cash prizes on crowd-
process. At the same time, previously dependable sources of sourcing sites such as Kaggle and InnoCentive. These days if a
financial support are drying up. The result has been a painful research entity chooses not to explore such collaborative ap
scarcity of jobs and grantswhich, in turn, is impelling far too proaches, it is in danger of being left behind.
many gifted researchers to focus on their narrow areas of spe- The software-design community has demonstrated over the
cialization rather than investing time and energy into acquiring past 20 years that massive online collaborations can work won-
new, computer-age skills. In fields where data growth is especial- ders. Today the physical sciences are only beginning to discover
ly out of control, such as neuroscience, the demand for computer that potential. Established scientists would do well to recog-
expertise is growing as quickly as the information itself. nize that true hackers are motivated by challenge and honest
Science urgently needs hackershackers in the original, Tech pride in seeing what they can do.
Model Railroad Club of the Massachusetts Institute of Technolo-
gy sense of the word. Their engineering and design skills will be SCIENTIFIC AMERICAN ONLINE
useful, but what is most desirable is the true hackers resource- Comment on this article at ScientificAmerican.com/sep2015
fulness, curiosity and appetite for fresh challenges. Particularly

14 Scientific American, September 2015 Illustration by Wesley Bedrosian

2015 Scientific American

ADVANCES

A colorized micrograph of a black-legged tick,

which can carry up to five diseases.

F U R T H E R R E A D I N G S A N D C I TAT I O N S | SCIENTIFICAMERICAN.COM/SEP2015/ADVANCES

2015 Scientific American

D I S PATC H E S FR OM T H E FR O N TIE R S O F S C IE N C E , T E C H N O LO GY A N D M E D I C IN E IN S ID E

Airplanes face a new collision risk:

personal drones
A plot of basil flourishes
20 feet underwater
Is the sun a thief?
A toy car powered
by evaporation

HEA LTH

Lingering
Lyme
A new theory about long-lasting
Lyme disease symptoms
suggests treatment options

Lyme disease is a truly intractable puzzle.

Scientists used to consider the tick-borne
infection easy to conquer: patients, diag-
nosed by their bulls-eye rash, could be
cured with a weeks-long course of antibiot-
ics. But in recent decades the U.S. Centers
for Disease Control and Prevention has
realized that up to one in five Lyme patients
exhibits persistent debilitating symptoms
such as fatigue and pain, known as post-
treatment Lyme disease syndrome, and no
one understands why. The problem is
growing. The incidence of Lyme in the U.S.
has increased by about 70 percent over the
past decade. Today experts estimate that at
least 300,000 people in the U.S. are infect-
ed every year; in areas in the Northeast,
more than half of adult black-legged ticks
carry the Lyme bacterial spirochete, B  orrelia
burgdorferi. A lthough the issue is far from
settled, new research lends support to the
controversial notion that the disease lingers
because these bacteria evade antibiotics
and that timing drug treatments differently
could eliminate some persistent infections.
GETTY IMAGES

These ideas stem from the observation

of a few rogue bacterial cells. Kim Lewis,

September 2015, ScientificAmerican.com 17

2015 Scientific American

 ADVANCES

director of the antimicrobial discovery cen- of other bacterial species are removed from
ter at Northeastern University, and his col- a bath of antibiotics, they begin to grow
leagues grew B  .burgdorferi in the laboratory, again. This fact prompted Lewis and his col-
treated them with various antibiotics and leagues to try treating B  .burgdorferi w  ith
found that whereas most of the bacteria antibiotics in pulsed dosesadministering
died within the first day, a small percent- the drugs, stopping and then administering
agecalled persister cellsmanaged to them againto see if they could kill the
survive the drug onslaught. Scientists first persisters once they began to regrow. It
discovered persister cells in 1944 in S taphylo- worked, which suggests that if persisters are
coccus aureus, t he agent of staph infections, responsible for lasting infections in people,
and Lewis and others have observed them in treating patients on and off with antibiotics
other species of bacteria, toobut the could help. Lewis and his colleagues, as well
observations that B  .burgdorferi a lso form as the Johns Hopkins scientists, are also
persisters is new. exploring other treatment options, such as
These are some of the most robust per- different drugs and drug combinations.
sisters weve seen, says Lewis, whose results Not everyone agrees that persister cells
were published online in May in A  ntimicrobi- play a role in Lymes lingering symptoms.
al Agents and Chemotherapy.  Over days, in Theres been no evidence that this persister
the presence of antibiotic, their numbers phenomenon has any relevance for animals
dont decline. Researchers at Johns Hopkins or humans, says Gary Wormser, chief of the
University similarly identified B  .burgdorferi division of infectious diseases at New York
persister cells this past spring. Medical College. First, he says, lab studies of
Persisters are not antibiotic-resistant B.burgdorferi c annot account for the poten-
mutants; they are genetically identical to tial effects of the bodys immune system,
their vulnerable counterparts. Instead they which might be able to eliminate persisters
are bacteria that have gone into a dormant once the brunt of the infection has cleared.
state, ceasing the types of cellular activities Second, labs have yet to grow B  .burgdorferi
that antibiotics typically thwart. Previous isolated from people treated with antibiotics,
research has shown that when persisters and that raises questions about whether the
persisters are even viable
and capable of making
someone sick.
Identifying the causes
of and treatments for post-
treatment Lyme disease
syndrome is one of the
highest priority research
needs in the field, said
C. Ben Beard, chief of the
bacterial diseases branch
at the cdc s Division of
Vector-Borne Diseases, at
a cdc event in May 2014. So
although it is as yet unclear
whether B  .burgdorferi per-
LYME ON THE RISE sister cells drive some of
Lyme disease is expanding its geographical reach in almost every direction these enduring symptoms,
from its epicenters in the Northeast and upper Midwest, according to a study Lewis and his colleagues
published in August by CDC scientists. The reasons remain uncertain. will take their research to
Ongoing forest fragmentation could contribute to the problem: as people
the next levelthey will test
chop forests into smaller pieces, they unwittingly create landscapes well
whether pulse dosing helps
GERALD C. KELLEY Science Source

suited for the deer and small mammals that ticks tend to feed on.
Climate change may also foster new suitable habitats for the arachnids to clear B.burgdorferi infec-
and change the timing of tick feedings in ways that make young ticks tions in micein an attempt
and humansmore vulnerable to infection. Nearly every known tick-
to move one step toward a
borne disease in the U.S. has become more prevalent over the past decade.
Scientists have identified four new ones since 2013, bringing the total much needed answer.
up to an estimated 16. M.W.M.  Melinda Wenner Moyer

18 Scientific American, September 2015 COMMENT AT ScientificAmerican.com/sep2015

Math_Tutor_Conquer_Math_And_Science.indd 1 3/20/14 5:06 PM
2015 Scientific American
PSYC H OLO GY

Fibs with
Friends
Groups spot lies more often
than individuals do
A shifty gaze, fi dgety stance or sweaty palms sig-
nal a liar in classic film noirs. In real life, however, it
is surprisingly difficult to recognize when someone
is telling a tall tale. Even among trained profession-
als, the lie-detection accuracy rate is only slightly
better than pure chance. And courts tend to reject
polygraph evidence because the tests lack stan-
dardized questions for determining falsehoods. For
better odds, discussions of questionable claims
appear to be the way to go. Psychologists at the
University of Chicago have found that groups of
people are consistently more reliable at rooting out
fabrications than chance or individual judges.
For the study, participants were shown video-
taped statements, either by themselves or with
other people present, and then asked to guess
whether the speakers were telling the truth or
white lies. After 36rounds, the researchers found
that groups of evaluators scored just as well as
individuals in determining truths but were up to
8.5percent more accurate in exposing lies. Groups
of three or six were equally reliable at pinpointing
falsehoods. The slight edge arises as a result of
insights that emerge from conversations, says
Nadav Klein, one of the studys authors. By talking
out their observations with others, people gain
new perspectives, improving their understanding.
The results were published in June in the P  roceed-
ings of the National Academy of Sciences USA.
The scales of justice could be recalibrated
accordingly. For instance, judges could explicitly
instruct juries to evaluate witnesses for honesty, in
addition to asking them to consider the evidence
objectively, says R. Scott Tindale, a psychologist at
Loyola University Chicago. With that direction,
deliberations might be more likely to include con-
versations about credibility and thus to defeat
deception. No one advocates for mob mentality,
but when gauging mendacity, it is apparently wise
to compare notes.  Kat Long

Illustration by Thomas Fuchs September 2015, ScientificAmerican.com 19

2015 Scientific American

 ADVANCES

the relatively heavy traffic colli-

sion and avoidance system used
by larger aircraft to track the lo-
cations of nearby planes. As an
alternative, Berman, a former
chief investigator for the Nation-
al Transportation Safety Board,
thinks drone manufacturers
should program their craft to
avoid flying above certain alti-
tudes or into restricted airspace.
The faa has also mentioned
TECHNOLOGY
such geo-fencing software up-

Cloudy with
dates as a potential short-term fix but does
not require them for small drone makers.
One best-selling-drone manufacturer

a Chance has already taken such steps. DJI, a compa-

ny based in Shenzhen, China, makes the

of Drones worlds most popular small drones, with

its Phantom models costing about $1,000
apiece. Since 2014, DJI has pushed out
Civilian multicopters raise drone firmware updates to clearly show
collision risks for jet airliners operators the restricted airspaces around
AP THEORY airports, Washington, D.C., or national bor-
ASY 10/01/2015 2C 2.125W x 4.625H A near miss w  ith a personal drone forced ders. Operators who ignore the software
a Shuttle America flight to pull up while on warnings about restricted airspace and try
Advertisement final approach to land at LaGuardia Airport flying forward will find their drones simply

*REWARD*
Waterwise_Waterwisdom.indd 1 6/5/15 12:06 PM
in New York City earlier this year. It wasnt refusing to move. Its like flying into an in-
the first such incident. The U.S. Federal Avi- visible wall, DJIs Michael Perry says.
ation Administration currently receives Several other tactics could be hovering
about 60 reports from pilots every month just over the horizon. In February the faa

$10,000.00
The Theories of Relativity, Special
that represent potential drone sightings. No
one knows exactly the type or extent of
damage that a collision with a small drone
proposed rules for small drones that include
speed and altitude restrictions and access lim-
its to airspace where manned planes typically
could cause to a jet airliners engine or air- fly. Such rules could be finalized as early as
Relativity, String, Steady State and frame, but the agency plans to research 2016. On the technology side, nasa has been
Nebula Hypothesis (Accretion) have that possibility in the next fiscal year. Mean- working with industry partners to develop an
all been disproven. Youre invited while technologies and policies that could unmanned air traffic system that could track
to disprove The AP Theory. So deter such collisions remain up in the air. small drones at low altitudes. The space agen-
confident are we that The AP Theory The current prevention tactic is to stop cy also has tested a detect-and-avoid system
most logically describes the forma- repeat offenders. The faa works with local for larger unmanned aircraft such as its Ikha-
tion of water and our solar system law enforcement to contact drone operators na drone, a civilian version of the militarys
and how our atmosphere is being who carry out an unauthorized [unmanned Predator drone. Such technology could even-
held down without gravity in the aerial system] operation to educate them tually scale down for smaller drones.
most logical way that were offering about flight safety regulations. The agency These solutions have become more in-
a $10,000.00 REWARD to the first can also tack on civil penalties for careless cumbent as growing numbers of drones find
person to disprove it. Must include or reckless operation of drones. their way into the hands of ordinary consum-
But the faa needs to do more to avert ers. This year China exported 160,000 civilian
an example in nature (on Earth) and a
collisions than educate citizens, says Ben Ber- drones worth $120 million from January to
description of a successful experiment.
man, a Boeing 737 pilot for a major U.S. air- May, according to the Xinhua News Agency.
line. Most near-collision courses are going to Were in the process of going from these
RICHARD NEWSTEAD Getty Images

READ: www.aptheory.info
be misses, he explains. But if we roll the dice very niche hobby products to mass consum-
COMMENT: on near collisions with drones enough times er products, Perry says. Many consumers
aptheory@aptheory.info a year, eventually youll come up snake eyes. just entering this space dont know the rules
no purchase necessary
Small dronesclassified by the faa as and regulations in the way that model aircraft
weighing less than 55poundscannot carry hobbyists used to.  Jeremy Hsu

20 Scientific American, September 2015 COMMENT AT ScientificAmerican.com/sep2015

Untitled-1.indd 1 7/23/15 9:49 AM

2015 Scientific American

 laserglow.pdf 1 2015-07-17 6:17 PM

AG RICU LT U R E

Sunken Strawberries
The produce aisle goes undersea
in a new approach to farming
In transparentplastic bubbles 20 feet
beneath the surface of the Mediterranean
Sea, an experimental garden grows. The
strawberries, basil, beans and tomatoes
within these air-filled biospheres thrive in
their submerged homes. Surrounding water
provides the constant temperature and
humidity elusive at most terrestrial farms,
and freshwater trickles down the spheres
interiors after the seawater below evapo-
rates and then condenses.
These marine greenhouses, located off
the coast of Italy, represent a foray into under-
water farming by Ocean Reef Group, a div-
ing and scuba gear company. Company pres- C

ident Sergio Gamberini chose to grow his M

crops hydroponically after noticing, during

Y
an early trial, that soil brought along stow-
away insect pests. He hopes to introduce this CM

gardening approach to coastal developing MY

countries with arid lands. In fact, Gamberini

CY
has received requests for biospheres from
nations ranging from the Maldives to Saudi CMY

Arabia. His son, Luca Gamberini, admits The air temperature inside the underwater K

a long path lies ahead: Our dream is, on greenhouses stays close to a balmy 84 degrees
a large scale, utopic.  Sabrina Imbler Fahrenheit, with a humidity of 90.5 percent.
COURTESY OF SERGIO GAMBERINI (top); OLIVIER MORIN Getty Images (middle and bottom)

September 2015, ScientificAmerican.com 21

Laserglow_Your_Search_Your_Way.indd 1 7/23/15 1:42 PM

2015 Scientific American

ADVANCES

E DUC AT I O N

SAT Charts
New Territory
The revamped test puts a stronger
emphasis on graphic literacy

Fortune favors the prepared mind, a s

Louis Pasteur once said. So as school revs up
this month, so do SAT prep classes. Students
might be surprised, however, at the amount
of time dedicated to visual literacy skills.
The increased focus on graphics is designed
to prepare an estimated 1.6 million college- tables, charts and graphs and to correct a literate consumer of that information
bound pupils for the first redesign of the text so it accurately describes data found is valuable regardless of your career,
standardized college admissions test in more in accompanying figures. says Jim Patterson, an executive director
than a decade. Along with other updates, test Mounting evidence indicates that such at the College Board, the nonprofit
takers of the March 2016 exam will encoun- literacy is a key skill for success in college, corporation that owns and publishes
ter graphics not only in the math section as careers and daily life in general. In an increas- the SAT.
in past years but also in the reading and writ- ingly data-rich world, graphics now pop up Education experts agree that stu-
ing and language portions. Students will be routinely in formats ranging from political dents in many developed nations,
asked to interpret information presented in campaign literature to household bills. Being including the U.S., lack experience with
visual data. Apart from basic x - a nd
y-axis graphs, educators [around the
world] dont sufficiently teach students
how to represent information graphical-
ly, says Emmanuel Manalo, a professor
of education psychology at Kyoto Uni-
versity in Japan. The SATs new focus
most likely will nudge educators to shift
SOLO OR 2 DEVELOPS their lesson plans accordingly. Students,
PERSON PLAY! SPATIAL SKILLS! in other words, wont be the only ones
with bubble charts or scatter plots on
the mind this fallteachers will, too.
 Rachel Nuwer

ZOBRIST CUBE
TM
BIG DATA
DEMANDS
20,000 Puzzles in a Box! The intellectual work r equired to
interpret a graph taxes our brain
33 POLYCUBE PIECES & 52 PAGE CODE BOOK more than the effort involved in read-
ing the same information presented
as text, according to a new study
by Manalo and two researchers at
the University of Twente in the Neth-
$1000 PRIZE erlands. The team measured neuro-
BE FIRST TO SOLVE THE CODE abe+s from the 2d logical activity in students and found
edition of the codebook page 45 (first edition page that graphs elicited roughly 60per-
49), but hide the s piece. There may be multiple cent more electrical activity than
RYAN BALDERAS Getty Images

solutions dividing the prize. No more than one text or equations and 40to 50per-
winner per state or territory of the US, or non-US cent more than pictures and tables.
country of the world. See the web site for full rules. Manalo will next examine whether
practice diminishes the amount of
tapped brainpower.  R.N.
1 (855) 962-7478 www.ZobristCube.com Ages 6 - Adult

22 Scientific American, September 2015 COMMENT AT ScientificAmerican.com/sep2015

2015 Scientific American

 A ST RO N O MY

Solar
Swindle
The sun may have swiped
Sedna from a neighboring star

At the time of S  ednas discovery in

2003, it was the farthest body ever seen
in our planetary club. Its peculiar path
it never ventures near the giant plan- Sedna
etssuggested an equally peculiar his-
tory. How did it get there? The sun may
have snatched Sedna away from another Sedna (purple) and 2012 VP113 (blue) never come close to
the orbits of the four giant planets (red) or even to Plutos
star, new computer simulations show.
home in the Edgeworth-Kuiper belt (orange).
A clue to Sednas past came in 2012,
when observers spotted a second and
even smaller object with a similarly
elongated and remote orbit. Astrono- until more objects with odd orbits are found in these objects from Star Q, they should all
mers Lucie Jlkov and Simon Portegies the outer reaches of our solar system. When come closest to Earth on the same side of
Zwart of Leiden Observatory in the we have something like a dozen, I think well the sun. But if their orbits differ, the sun proba-
Netherlands and their colleagues decid- probably know, Brown says. If the sun stole bly is innocent of theft.  Ken Croswell
ed to investigate whether interstellar
robbery could produce the orbits of
both Sedna and its sidekick, 2012 VP113.
We show that its possible, Jlkov IN REASON WE TRUST
says. Moreover, the researchers recon-

Ifup,theacknowledge
structed the crime scene and even the
likely properties of the victim star, which topic comes
they dubbed Star Q. In work submit-
ted to M  onthly Notices of the Royal Astro-
nomical Society, t he astronomers say
Star Q was originally about 80 percent youre an atheist.
more massive than the sun. It passed
within 34 billion kilometers of usjust No big deal.
7.5 times greater than the distance from
the sun to Neptune. This proximity Now lets talk
means the star arose in the same stellar
group or cluster as the sun. Although about something

StarQ still exists, its fiercest light proba-
bly burned out long ago because of its interesting.
Photo: Brent Nicastro

greater mass. As a dim white dwarf, it

will be hard to find.
Join the nations largest association of
The new work makes a pretty con- freethinkers (atheists, agnostics) working
SOURCE: SCOTT S. SHEPPARD C arnegie Institution for Science

vincing case that Sedna could be cap- to keep religion out of government.
tured, says astronomer Scott Kenyon of For a free sample of FFRFs newspaper,
DANIEL C. DENNETT Freethought Today, phone
the Harvard-Smithsonian Center for Austin B. Fletcher Professor of Philosophy,
Astrophysics. But Sedna discoverer Mike
Brown of the California Institute of Tech-
Tufts University
Author, Breaking the Spell: Religion as a 1-800-335-4021
Natural Phenomenon n
nology contends that the object most FFRF Honorary Director FFRF is a 501(c)(3) educational charity

likely is native to our solar system and got

yanked outward by the gravitational tug Freedom From Religion Foundation
of the suns siblingsa simpler scenario.
The issue may remain unresolved
FFRF.ORG/reason-must-prevail
Graphic by Amanda Montaez September 2015, ScientificAmerican.com 23

2015 Scientific American

ADVANCES

AN I M A L B E H AV I O R
from what sounds like multiple callers might

From the make the warning seem more reliable. Birds

can adapt some very interesting and unique

Pages of strategies to protect their young, says Igic,

who is now at the University of Akron. The

Aesops Fables Pied currawongs

thornbills own hawk calls fooled the curra-
wongs for 8.3 seconds on average, but
are fearsome birds when Igic included the mimicked calls, too,
A seven-gram bird deceives but have enemies the currawongs were distracted twice as
a predator 40times its size of their own.
long. That padding may provide enough
by crying wolf time for the nestlings to escape the nest.
works, biologist Branislav Igic, then at the Visual mimicry, exemplified by harmless
In the classic story, a boy tries to repeat- Australian National University, and his col- king snakes that resemble venomous coral
edly fool his town into believing that there leagues positioned a taxidermied currawong snakes, is well known to animal behavior-
is a wolf on the prowl. This morality tale near thornbill nests while broadcasting nest- ists, but vocal mimicry has remained more
ends poorly for the boy, but a small Austra- ling distress calls. The thornbills sounded mysterious. This is further evidence of the
lian bird can do one better. When a pied their hawk alarm calls and even mimicked benefits some vocal mimics gain from their
currawong goes looking for brown thornbill the alarms of other species. Igic also tested unusual vocal behavior, says University of
nestlings to eat, the thornbill parents call 18 currawongs by broadcasting the sounds of Cape Town ornithologist Tom Flower, who
wolfor, actually, they call hawk. The false the thornbills mimetic and nonmimetic hawk was not involved in this study. He ultimate-

B. G. THOMSON Science Source

alarms fool the currawong into thinking calls. He found that the playback discouraged ly would like to see proof that deception
that its own predator, the brown goshawk, the currawongs from hunting. The results actually increases nestling survival. For
is nearby. The tiny thornbill thus effectively were published this past spring in the P  ro- now, unlike the fabled townspeople, the
outsmarts its large enemy. ceedings of the Royal Society B. currawongs have yet to catch on to the
To explore how this sophisticated ruse The researchers think that alarm calls thornbills lies.  Jason G. Goldman

YOUR ST RY
CAN CHANGE
SOMEONE ELSES.

ADDICTION IS HOPELESS WITHOUT YOU

Share your story of recovery or message of hope with someone who needs
to hear it. Visit drugfree.org and join the Stories of Hope community.

The Partnership for a Drug-Free America, Inc.

24 Scientific American, September 2015 COMMENT AT ScientificAmerican.com/sep2015

2015 Scientific American

 UNITED ARAB EMIRATES RUSSIA
I N TH E N EWS Dubai will house the first fully functional Scuba divers helped an international team

Quick
3-D-printed building, announced the citys of physicists install a spherical neutrino detector
Museum of the Future. A 20-foot-tall printer more than 4,000feet below the surface of

Hits will construct the 2,000-square-foot office. icy Lake Baikal in Siberia.

U.S.
Google will add every
railroad crossing in the
country to its Maps
INDONESIA
application. Visual and
A plane dumped several
audio alerts will signal
tons of cloud-seeding salt
these potential hazards to
over central Sumatra in
users. The Federal Railroad
a government-supported
Administration hasasked
effort to induce rain and
Apple, TomTom and other
relieve a seasonal heat
companies that provide
wave. The attempt failed
GPS services to do the same
MEXICO CUBA to produce any showers.
after a large year-over-year
increase in crossing acci A deep-sea submarine discovered a The World Health Organization
dents in 2014. 0.25-mile-long field of hydrothermal vents certified the island nation as
along a fault in the Gulf of Californias seafloor. the first country to eliminate the
For more details, visit www.Scientific
At 12,500 feet, they are the deepest high- transmission of HIV and syphilis
American.com/sep2015/advances temperature vents ever found in the Pacific. from mother to baby.

Adopt Me.
And change my world.

There are fewer

than 900 mountain
gorillas left on the planet.
Together we can save them.
Give A Gift to Yourself or to Someone You Love

gorillafund.org/adopt-a-gorilla | 1 (800) 851 0203

September 2015, ScientificAmerican.com 25

2015 Scientific American

ADVANCES

E N GI N E E R I N G

Water, Water Everywhere

A toy car runs on the energy of evaporation
The first vehicle p
 owered by evaporation as they move through environments of could enable it to power a wide range of
frequently zooms along a laboratory bench varying humidity. It can take up to 10 min- technologies, including robotics systems
these days at Columbia University. The utes to cross a table at the moment, but and generators. For now the focus is on
100-gram car relies on spore-coated tapes the biologists, chemists and engineers on the small: another prototype can power
that expand and contract like tiny muscles the project think upgrades to the engine two tiny LED lights.  Maria Temming

2 Bacterial spores on plastic

HOW IT tapes inside the chamber
WORKS absorb the moisture and
expand, causing the tapes
to lengthen. Small, acrylic
blocks at the ends of 3 The imbalance shifts
1 A person wets the paper the tapes now hang the center of mass of the
walls of a chamber that farther from the structure away from
encloses the front half of a axle than the the axis of rotation and
creates torque. The rotary Humid
rotary engine. Drops of blocks outside
water evaporate, creating the chamber. engine begins to turn. Dry
a humid environment Nanoscale cavities
within the chamber.
Spore- Humid
Wall lined with Acrylic block covered Wet
wet paper tape surface Displaced Dry
center of mass

Torque
Bacillus subtilis
spores

SOURCE: SCALING UP NANOSCALE WATER-DRIVEN ENERGY CONVERSION INTO EVAPORATION-DRIVEN ENGINES

AND GENERATORS, BY XI CHEN ET AL., IN NATURE COMMUNICATIONS, VOL. 6, ARTICLE NO. 7346; JUNE 16, 2015
Dry
tape
curls

Humid
tape
4 The engines axle is attens
connected to the cars Spore-covered tape
front wheels by a
rubber band so the 5 When the tapes inside the chamber
car moves forward. emerge into the dry air, their spores release
water and shrink. As a result, the tapes curl
up and pull their blocks closer to the axle
maintaining the off-kilter center of mass.
The wheels stop turning when all the water
has evaporated.
1 inch

26 Scientific American, September 2015 Illustration by 5W Infographics

2015 Scientific American

 BY THE NUMBER S

Chris:
Made possible by
OLD MOLD SOLD
A small sample o f Sir Alexander Flemings an organ donor
original penicillin culture (above) was put
up for sale by a London auction house in
July. In 1955 Fleming had given the fun-
gus to a neighbor to thank him for foiling
a robbery at the famed microbiologists
home. Fleming stumbled on the antibiotic
properties of P enicillium m
 old in 1928, and
over the course of the following decade
University of Oxford bacteriologists and
COURTESY OF CATHERINE SOUTHON AUCTIONEERS & VALUERS (top); SOURCES FOR STATISTICS: DISCOVERY AND DEVELOPMENT OF PENICILLIN, IN AMERICAN CHEMICAL SOCIETYS INTERNATIONAL HISTORIC

others expanded on his findings to devel-

CHEMICAL LANDMARKS www.acs.org/content/acs/en/education/whatischemistry/landmarks/flemingpenicillin.html (f irst two items); THE REAL STORY BEHIND PENICILLIN, BY HOWARD MARKEL, IN PBS NEWSHOURS
THE RUNDOWN. PUBLISHED ONLINE SEPTEMBER 27, 2013 www.pbs.org/newshour/rundown/the-real-story-behind-the-worlds-first-antibiotic ( t hird item); NEW BUSINESS MODELS FOR ANTIBIOTIC INNOVATION,

op the first antibiotic drug. Mass produc-

tion of penicillin during World WarII
cured millions of people of bacterial infec-
tions that would have been fatal other-
BY ANTHONY D. SO AND TEJEN A. SHAH, IN UPSALA JOURNAL OF MEDICAL SCIENCES,VOL. 119, NO. 2; MAY 2014 (fourth item); CATHERINE SOUTHON AUCTIONEERS & VALUERS (fifth item)

wise. Today antibiotics are considered one

of the greatest discoveries in medical his-
torythus the auction of a crusty petri
dish that would have been thrown out
long ago. In a letter accompanying the
sample, the microbiologists house-
keeper warned its recipient that the
contents were not to be confused with
Gorgonzola cheese!!!  Kat Long

$20
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COMMENT AT ScientificAmerican.com/sep2015 September 2015, ScientificAmerican.com 27

Untitled-3 1 2015 Scientific American 7/22/15 2:38 PM

 The Science of Health by Dina Fine Maron
Dina Fine Maron is an associate editor
at Scientific American. S he focuses on
medicine and health.

Brain Food
A Mediterranean-style diet may slow memory loss, even if adopted late in life
Whenever the fictional character P  opeye the Sailor Man man versity and his colleagues collected information about the di
aged to down a can of spinach, the results were almost instan etary habits and health status of about 2,000 Medicare-eligible
taneous: he gained superhuman strength. Devouring any solid New Yorkerstypically in their mid-70sover the course of four
object similarly did the trick for one of the X-Men. As we age and years on average. In 2006 the investigators reported that tighter
begin to struggle with memory problems, many of us would love adherence to a Mediterranean diet, which had previously been
to reach for an edible mental fix. Sadly, such supernatural effects linked to a lower risk of cardiovascular disease, was associated
remain fantastical. Yet making the right food choices may well with slower cognitive decline and a lower likelihood of acquir
yield more modest gains. ing Alzheimers disease. Because the researchers merely ob
A growing body of evidence suggests that adopting the Medi served dietary patterns and did not control themas would be
terranean diet, or one much like it, can help slow memory loss as the case in a clinical trialdoubts lingered, however. It was still
people age. The diets hallmarks include lots of fruits and vegeta possible that the apparent brain benefit was the result of chance
bles and whole grains (as opposed to ultrarefined ones) and a or some other trait common to folks who consistently follow a
moderate intake of fish, poultry and red wine. Dining mainly on Mediterranean diet in the U.S., such as educational achieve
single ingredients, such as pumpkin seeds or blueberries, howev ment or particular life choices.
er, will not do the trick. Seven years later researchers pinned down some answers. In
What is more, this diet approach appears to reap brain bene 2013 Martnez Gonzlez and his colleagues published findings on
fits even when adopted later in lifesometimes aiding cognition their massive PREDIMED study, an experiment that included al
in as little as two years. You will not be Superman or Superwom most 7,500 people in Spain. (PREDIMED stands for Prevention
an, says Miguel A. Martnez Gonzlez, chair of the department with Mediterranean Diet.) The investigators randomly assigned
of preventive medicine at the University of Navarra in Barcelona. study subjects to one of two experimental groups. In the first, par
You can keep your cognitive abilities or even improve them ticipants followed the Mediterranean diet with an additional
slightly, but diet is not magic. Those small gains, however, can helping of mixed nuts; in the second, they also adhered to the
be meaningful in day-to-day life. Mediterranean diet but were given additional extra virgin olive
oil. (Researchers felt that providing extra nuts and oils at no cost
F ROM FORK TO BRAIN to participants would guarantee that certain healthy fats were
Scientists long believed that altering diet could not improve eaten in quantities large enough to have measurable effects on
memory. But evidence to the contrary started to emerge about the studys outcomes.) The control group, against which the re
10 years ago. For example, Nikolaos Scarmeas of Columbia Uni sults of the experimental groups would be compared, was in

Illustration by Aleks Sennwald September 2015, ScientificAmerican.com29

2015 Scientific American

The Science of Health

structed generally on how to lose weight. Its members were given Adhering to the exact diet laid out in PREDIMED may not be
advice on eating vegetables, meat and high-fat dairy products the only way to gain cognitive benefits from food. In February,
that jibed with the Mediterranean diet, but they were discour Morris and her colleagues published online a study recommend
aged from using olive oil for cooking and from consuming nuts. ing a modified diet largely consistent with the Mediterranean diet
As expected, the results showed that either of the experimental but one cheaper to adopt in the U.S. Morriss so-called MIND diet
Mediterranean diet options led to significantly better cardiovascu emphasizes green, leafy plant and whole grain consumption. Its
lar outcomes. But when the scientists tested cognition in a subset staples include two veggie servings a day, two berry servings a
of study members, they also discovered that individuals in either week and, instead of the almost daily fish consumption required
of the Mediterranean diet groups performed better than the in the Mediterranean diet, fish only once a week.
weight-instruction group in a battery of widely accepted cognitive Morris found that even moderate adherence to the MIND diet
tests. This is surprising, of course, Martnez Gonzlez says. for an average of 4.5 years appeared to reduce Alzheimers risk
As intriguing as these findings are, they are still not conclu compared with the Mediterranean and another diet. She and her
sive; the researchers had not gathered any cognitive information colleagues judged that outcome by counting the number of cases
at the beginning of the study. Therefore, the possibility remains of clinically diagnosed Alzheimers among each group during the
that there was something different between the two experimental study period. (The comparison diets required stricter adherence
groups and the control groupbeyond their diet interventions to get the same cognitive benefit.) Better yet, the MIND diet may
that could account for the findings. be more achievable for the average persons wallet and for Ameri
Martnez Gonzlez sought to quiet such criticisms with a can culture. In the bigger picture, this finding suggests that peo
new study his team published in July in JAMA Internal Medi- ple improving their diet can make a difference for their memory,
cine. Drawing from a group of more than 300 participants who says Francine Grodstein, a professor focusing on healthy aging at
were also part of PREDIMED but at a specific site with more fi Brigham and Womens Hospital in Boston and Harvard Medical
nancial resources, the researchers conducted baseline cognitive School, who was not involved with the work.
measurements and compared them with that same groups results Why certain food choices might help the brain function bet
four years later. On average, people were 67 years old at the start of ter remains unclear. Perhaps these regimens known cardiovas
the study. The newest findings, Martnez Gonzlez says, are con cular benefits, which promote a good flow of blood and oxygen to
sistent with what he found in his earlier studies. These results are the brain, are key. But other factors may be at work. Of course,
also not definitive, however, because this substudy was relatively questions about when these dietary changes need to happen or
small. Yet, he notes, it is the first time scientists have seen im how diet stacks up against other factors, such as physical activi
provements in cognitive function from a randomized trial of the ty, sleep patterns and genetics also remain unanswered.
Mediterranean diet. Recently some researchers have begun broadening their focus
Can Americans, whose standard diet and way of life are often beyond food alone. In the European Union, a multicountry ran
substantially different from that of adults living in Spain, benefit domized trial beginning this year is designed to provide further
from the approach? That remains to be seen. The normal diet of insights into how diet, exercise and better control of blood pres
the people in the studys control group was still closer to a Medi sure could work together to promote brain health. (Hypertension
terranean diet than that of most Americans, so they already had is a leading cause of stroke, which can seriously harm mental pro
years of relatively healthy eating under their belts, which could cessing.) Although the study will not allow scientists to pinpoint
have helped their overall health. But Martnez Gonzlez believes which factor offers the greatest benefit, it should give them a bet
that the diet might provide even greater benefits for Americans ter understanding of how significant a role life changes can play.
because they have so much more room for improvement. Still, nu There is reason to be hopeful. A pilot study published in June
trition expert Martha Morris of Rush University says, only a ran in the Lancet found that making changes in diet and habits later
domized trial in the U.S. can truly answer the questionsome in life can slow the course of cognitive decline. Scandinavian re
thing she hopes to spearhead in the coming years. searchers divided a group of 1,260 people in Finland either to re
ceive standard nutrition and diet advice or to follow a specified
B EYOND DIET exercise plan and eat a modified Mediterranean dietall while
Proving that a particular cuisine a ffects cognitive health is one their blood pressure and other health indicators were monitored
thing. Getting a lot of Americans to eat more fruits, vegetables, and, if necessary, treated. Subjects in the experimental group
fish and olive oil is another matter altogether. Two major obstacles ended up doing significantly better on standard tests of cogni
are cost and ingrained habits. For PREDIMED, study participants tion. We could really see that [the intervention] can protect
were supplied with expensive extra virgin olive oil and told how to against or at least delay cognitive impairments, says lead study
prepare meals. To transfer this knowledge to the American popu author Miia Kivipelto, director of research and education at the
lation, you cant just show them food items, Martnez Gonzlez geriatric clinic at the Karolinska Institute in Stockholm. Unex
says. You have to show them how to shop for them, cook with pectedly, she says, those changes were visible within just two
them and prepare them to keep all the nutrients in line with the years. And best of all, superpowers are not required.
traditional Mediterranean diet. The first step in the right direc
tion, he says, would be for Americans to slash their consumption SCIENTIFIC AMERICAN ONLINE
of red meats and use poultry instead. But that still leaves a lot of Comment on this article at S cientificAmerican.com/sep2015
other steps to go before they are eating a Mediterranean diet.

30 Scientific American, September 2015

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 TechnoFiles by David Pogue
David Pogueis the anchor columnist for Yahoo Tech
and host of several NOVA miniseries on PBS.

Super Siri
A new breed of virtual assistant
is almost here. And it is scary smart

Virtual voice-controlled assistants such as Siri, Cortana and

Google Now are magical. You can say things such as Will I
need an umbrella in Dallas this weekend? or What flights are
overhead?or even jokey things like Is Santa Claus real?
Each time, you get an accurate (or witty) answer.
Behind the scenes, though, all their responses were scripted
in advance by writers and programmers. (In fact, Apple employs
a team of comedy writers exclusively for drafting Siris wise
cracks.) Their underlying software is still, in essence, a passel of
if/then statements.
Soon, though, your voice assistant will be much, much smarter.
After leaving Apple, three of Siris creatorsDag Kittlaus, Adam that Francophone optometristor further cement the dominance
Cheyer and Chris Brighamstarted a company called Viv Labs. of the worlds Amazons and Ubers? Will Vivs product selections
Whereas a Siri or a Cortana might know how to handle re be the final nail in the coffin of browsing and serendipity?
quests about weather, sports and about 20 other areas, Vivs And just how smart will Viv be? When you book a flight, will
knowledge and vocabulary will be extensible and unlimited. it take into account how bad traffic will be on your way to the air
They will tap into the databases of thousands of online services port that time of day? Will it know that a layover at Chicagos
stores, flight-booking sites, car-sharing services, flight trackers, OHare airport during the holidays is a fate worse than death?
restaurants, florists, dating sitesand understand how every And what about jobs? What happens to all the travel agents, flo
thing all fits together. rists and sommeliers that AI assistants such as Viv will displace?
You can ask Siri, Where does my sister live? and Whats Historically the economy adapts, Kittlaus replies. Most of
the weather in Boston? Cheyer explained to me, but you cant the U.S. population used to work on farmsnow its a tiny minor
say, Whats the weather where my sister lives? because that ity. Its only when peoples skills cant keep up with the rate of
integration hasnt been written by a human. But Viv will weave change that you run into trouble.
things together. The user interface isnt done yet, so the company hasnt re
Viv will also learn a huge portfolio about youyour prefer leased pictures or videos. But Viv works well, even now. Kittlaus
ences, credit-card numbers, addresses, and so on (with your per demoed Viv with a bunch of commands (for example: Send my
mission, of course). As a result, Viv can answer queries such as mom a dozen roses and Book me two first-class seats on the
Book me an appointment with a French-speaking optometrist first flight from SFO to JFK, returning Tuesday). Each time, Viv
whose office is on my way home from work, Find me a good presented a list of options, ranked by price or rating; Kittlaus
place to go take my kids to the Caribbean in the last week of Feb could execute the transaction with one more tap.
ruary, and I want to pick up a great bottle of wine on the way to Viv is a year away from public release. But Vivs competitors
my brothers housesomething that goes well with lasagna. arent sitting still; in the past few months Microsoft, Google and
In that last example, Viv consults one Web service that knows Apple all announced upgrades to the intelligence of their exist
the inventory of the wine in various stores, one that plots the ing voice assistants. And SoundHounds Hound is still in beta
route to your brothers home and one that knows the ingredients but already beating out the big dogs in complex voice searches
of lasagna. And in the case of the Caribbean trip, Viv can suggest and ultraspecific transaction requests. Although they dont quite
a resort package for you, which you can book on the spotno approach the sophistication and integration of Vivs smarts, the
searching required. direction these voice apps are taking is already clear: theyre
It would be convenient for the consumer. And a boon for Viv. growing smarter, faster and more disruptive every day.
Every time that you confirm one of Vivs proposed purchases,
the corresponding service (say, Uber, Hotels.com or Orbitz) will SCIENTIFIC AMERICAN ONLINE
pay Viv a cut. 10 ways to streamline Siri commands: ScientificAmerican.com/sep2015/pogue
Will this system level the playing field for smaller companies

Illustration by Peter and Maria Hoey September 2015, ScientificAmerican.com 31

2015 Scientific American

 CLEANING UP
AFTER EINSTEIN

EINS
HOW EINSTEIN
REINVENTED page 60
REALITY
page 38

WHAT EINSTEIN
GOT WRONG
page 50

HEAD TRIP
page 46

RELATIVITYS
WHY
REACH
HE MATTERS
page 56
page 34

100 YEARS OF
GENERAL RELATIVITY
Everyone knows what gravity is. A baby at three months will express sur-
prise if a box does not topple as expected; a one-year-old knows whether a precarious object will
fall or not depending on its shape. Scientists came to think of gravity as a pull to Earth and later, in
a more generalized way, as a force of attraction between any two masses.
Then came Albert Einstein. In 1915 he revealed in his general theory of relativity that gravity is
not a force so much as the by-product of a curving universe. In other words, what we think we know
about gravity from everyday experience is wrong.
The publication of  Die Feldgleichungen der Gravitation (The Field Equations of Gravita-
tion) on December 2, 1915, at first got little notice beyond academe. A few years later a solar eclipse
expedition led by Sir Arthur Eddington made an observation that vaulted the theory to fame over-
night. Starlight, as Einstein predicted, appeared to bend as it passed the sun, and Eddington con-

32 Scientific American, September 2015

2015 Scientific American

STEIN
GENIUS IN
A JAR
page 82
THE BLACK
HOLE TEST
page 74

WHO WAS
EINSTEIN, REALLY?
A BRIEF page 80
HISTORY OF
TIME TRAVEL
page 68

IS THE COSMOS
RANDOM?
page 88

firmed this bending firsthand. The New York Times f amously declared: Men of Science More or
Less Agog over Results of Eclipse Observations.
They were right to be agog. It would be hard to overstate how disruptive the idea of general
relativity was a century ago to prevailing notions of the universe and our physical world. All of a
sudden, space and time were no longer a mere backdrop to the real action of the cosmos. Space-
time, rather, had its own geometry, and its curvature dictated the movements of the heavenly
bodies and kept our feet planted firmly on the ground. Even light, the theory suggested, had to fol-
low its contours.
The relativity revolution went on to shape much of the 20th century. It influenced philosophy,
art, politics and pop culture. Its inventors name became synonymous with genius and inaugurated
Einstein as the worlds greatest scientific celebrity. He used his stature to play a major role in global
eventshe famously advocated for the development of the atomic bomb and then spent decades
bemoaning the mistake. He lobbied for the protection of the Jewish people and was an outspoken
critic of racism and an activist for civil rights. Even more, the fame surrounding Einstein and his
great idea marked a turning point in the public perception of science, establishing the 20th century
as the scientific age and ushering in a technological transformation that we are still living through.
The 100th anniversary of general relativity provides an opportunity to survey the incredible
pace of science and its effect on society. In the following pages we look back at what we learned from

September 2015, ScientificAmerican.com 33

2015 Scientific American

 WHY
Einsteins achievement and forward to ES S AY
the secrets it may yet reveal. An illumi-
nating graphic shows the multitude of
new fields of research the theory has
spawned (page 56). We examine the first
moment of inspiration that led the
genius on the path to relativity (page 38)
and celebrate his ability to expose such
truths through the power of pure thought
(page 46). Even Einsteins mistakes often
proved fruitful (page 50), and we see that
what is commonly thought to be one of
his greatest faultshis perceived intoler-

HE MATTERS
ance of quantum mechanicsis misun-
derstood (page 88). And we explore our
obsession with genius by investigating
the misguided attempts to locate the
source of Einsteins brilliance in his brain
anatomy (page 82).
The passage of 100 years of general The fruits of one mind shaped civilization
relativity is also important because of
what the idea still has not done: unite more than seems possible
with the other forces of nature to build a
unified theory of everything. Einstein By Brian Greene
spent his last years questing for a deeper
set of rules that would reign not just over
the realm of the cosmosgeneral relativ-
itys domainbut also the world inside
the atom, where quantum mechanics Albert Einstein once said that there are only two things
rules. He thought this dream was in that might be infinite: the universe and human stupidity. And, he confessed,
reach, but a century of toil by genera- he wasnt sure about the universe.
tions of physicists has not accomplished When we hear that, we chuckle. Or at least we smile. We do not take offense.
a single theory of nature. Relativity and The reason is that the name Einstein conjures an image of a warm-hearted,
quantum mechanics are just as incom- avuncular sage of an earlier era. We see the good-natured, wild-haired scientific
patible as they ever were. genius whose iconic portraitsriding a bike, sticking out his tongue, staring at us
Lately scientists have begun to take a with those penetrating eyesare emblazoned in our collective cultural memory.
new tack, probing some of the mysteries of Einstein has come to symbolize the purity and power of intellectual exploration.
the universe that have popped up since Einstein shot to fame within the scientific community in 1905, a year chris-
Einsteins age, such as dark matter and tened as his annus mirabilis. While working eight hours days, six days a week
dark energy, in hopes that these paths will at the Swiss patent office in Bern, he wrote four papers in his spare time that
eventually lead to the realization of Ein- changed the course of physics. In March of that year he argued that light, long
steins dream (page 60). Other researchers described as a wave, is actually composed of particles, called photons, an ob-
are attempting to poke holes in general servation that launched quantum mechanics. Two months later, in May, Ein-
relativity by testing it in the extreme realm steins calculations provided testable predictions of the atomic hypothesis, lat-
of black holes (page 74). And one of rela- er confirmed experimentally, cinching the case that matter is made of atoms.
tivitys weirdest consequencesthe possi- In June he completed the special theory of relativity, revealing that space and
bilities it introduces for time travelmay time behave in astonishing ways no one had ever anticipatedin short, that
also offer an avenue for uncovering deeper
secrets of nature (page 68).
Ultimately it is clear that no other sci- IN BRIEF
entific theory has been more important in
Einsteins first major achievements came in 1905, fined our notion of space and time. It launched
GETTY IMAGES (Einstein photograph)

shaping the course of 20th-century phys-

when he published four groundbreaking papers, new strands of research that scientists are still
ics, and no other scientists legacy looms
including his completion of special relativity. pursuing and made its creator a star.
larger over the 21st century than Einsteins. Ten years later h e expanded that theory to Over the past century E insteins ideas have in-
At this milestone anniversary, physics is include gravity, creating general relativity. The termingled with culture and art and shaped our
waiting for the next general relativity. We idea toppled Isaac Newtons physics and rede- world in infinite, indelible ways.
could use another Einstein. The Editors

34 Scientific American, September 2015 Illustration by Mario Wagner

2015 Scientific American

 September 2015, ScientificAmerican.com35

2015 Scientific American

distances, speeds and durations are all relative depending on Brian Greene is a professor of physics and mathematics
the observer. And to cap it off, in September 1905 Einstein de- at Columbia University who researches superstring theory.
rived a consequence of special relativity, an equation that would He is author of numerous books and co-founder and
chairman of the board of the World Science Festival.
become the worlds most famous: E =  mc2.
Science usually progresses incrementally. Few and far be
tween are contributions that sound the scientific alert that a rad-
ical upheaval is at hand. But here one man in one year rang the
bell four times, an astonishing outpouring of creative insight. Al-
most immediately, the scientific establishment could sense that
reverberations of Einsteins work were shifting the bedrock un-
derstanding of reality. For the wider public, however, Einstein
had not yet become Einstein.
That would change on November 6, 1919.
In special relativity, Einstein established that nothing can
travel faster than the speed of light. This set the stage for a con-
frontation with Newtons theory of gravity, in which gravity ex-
erts its influence across space instantaneously. Driven by this
looming contradiction, Einstein brazenly sought to rewrite the
centuries-old rules of Newtonian gravity, a daunting task that
even his ardent supporters considered quixotic. Max Planck, the
dean of German science, intoned, As an older friend, I must ad-
vise you against it.... You will not succeed, and even if you suc-
ceed, no one will believe you. Never one to yield to authority,
EINSTEINS FAMOUS EQUATION, E = mc2, c an be seen
Einstein pressed on. And on. For nearly a decade.
in his own handwriting from a later paper, published in 1946.
Finally, in 1915, Einstein announced his general theory of rel-
ativity, which offered a profound recasting of gravity in terms of
a startling new idea: warps and curves in space and time. In-
stead of Earth grabbing hold of a teacup that slips from your applaud me because everybody understands me, and they ap-
hand and pulling it to an untimely demise on the floor, general plaud you because no one understands you. It was a role Ein-
relativity says that the planet dents the surrounding environ- stein wore well. And the wider public, weary from World War I,
ment, causing the cup to slide along a spacetime chute that di- embraced him wholeheartedly.
rects it to the floor. Gravity, Einstein declared, is imprinted in As Einstein glided through society, his ideas about relativity,
the geometry of the universe. at least the version broadly reported, seemed to resonate with
During the 100 years since Einstein proposed the theory, other cultural upheavals. James Joyce and T. S. Eliot were splin-
physicists and historians have pieced together a coherent, if tering the sentence. Pablo Picasso and Marcel Duchamp were
complex, story of its genesis [see How Einstein Reinvented Re- cleaving the canvas. Arnold Schoenberg and Igor Stravinsky
ality, by Walter Isaacson, on page 38]. In some of my own gen- were shattering the scale. Einstein was unshackling space and
eral-level writings, Ive had the pleasure of retracing Einsteins time from outmoded models of reality.
climb, from elegant maneuvers to p  ieds en canard t o his final Some have gone further, portraying Einstein as the central in-
summit. Far from demystifying Einsteins creative leaps, how- spiration for the avant-garde movement of the 20th century, the
ever, perusing his process only adds luster to the astonishing scientific wellspring that necessitated a cultural rethink. Its ro-
novelty and overwhelming beauty of the proposal. mantic to believe that natures truths set off a tidal wave that
On November 6, 1919, four years after Einstein completed the swept away the dusty vestiges of an entrenched culture. But Ive
general theory of relativity, newspapers the world over trumpet- never seen convincing evidence pinning these upheavals to Ein-
ed just released astronomical measurements establishing that steins science. A widespread misinterpretation of relativity
the positions of stars in the heavens were slightly different than that it eliminated objective truthis responsible for many unjus-
what Newtons laws would have us expect, just as Einstein had tified invocations of Einsteins theories in the realm of culture.
predicted. The results triumphantly confirmed Einsteins theory Curiously, Einstein himself had conventional tastes: he preferred
ALBERT EINSTEIN ARCHIVES, HEBREW UNIVERSITY OF JERUSALEM

and rocketed him to icon status overnight. He became the man Bach and Mozart to modern composers and refused a gift of new
who had toppled Newton and who, in the process, had ushered Bauhaus furniture in favor of the well-worn traditional decor he
our species one giant step closer to natures eternal truths. already owned.
To top it off, Einstein made for great copy. While squinting in It is fair to say that many revolutionary ideas were wafting
the limelight and paying lip service to an ardent desire for soli- through the early 20th century, and they surely commingled.
tude, he knew how to entice the worlds interest in his mysteri- And just as surely, Einstein was a prime example of how break-
ous but momentous dominion. He would throw out clever quips ing from long-held assumptions could uncover breathtaking
(I am a militant pacifist) and gleefully play the public part of new landscapes.
the bemused genius of geniuses. At the premiere of C  ity Lights, A century later the landscapes Einstein revealed remain re-
while the cameras on the red carpet flashed, Charlie Chaplin markably vibrant and fertile. General relativity gave birth in the
whispered to Einstein something along the lines of, The people 1920s to modern cosmology, the study of the origin and evolution

36 Scientific American, September 2015

2015 Scientific American

of the entire universe. Russian mathematician Aleksandr Fried- How, then, did Einstein do it? How did he contribute so
mann and, independently, Belgian physicist and priest Georges much of such lasting importance? Whereas we can dismiss Ein-
Lematre used Einsteins equations to show that space should stein as the source of Cubism or atonal music, he is why we
be expanding. Einstein resisted this conclusion and even modi- imagine that someone can, in the privacy of his or her own
fied the equations by inserting the infamous cosmological con- mind, think hard and reveal cosmic truths. Einstein was social
stant to ensure a static universe. But subsequent observations as a scientist, but his big breakthroughs were solitary aha! mo-
by Edwin Hubble showing that distant galaxies are all rushing ments. Did those insights emerge because his brain had an un-
away convinced Einstein to return to his original equations and usual architecture? Because of a nonconformist perspective?
accept that space is stretching. An expanding universe today Because of a tenacious and uncompromising ability to focus?
means an ever smaller universe in the past, implying that the Maybe. Yes. Probably. The reality, of course, is that no one
cosmos emanated from the swelling of a primordial
speck, a primeval atom as Lematre called it. The

The centenary of general relativity

big bang theory was born.
In the decades since, the big bang theory has been
substantially developed (today the most widely held
version is inflationary theory) and, through various re- is a far cry from a backward
finements, has aced a spectrum of observational tests.
One such observation, which received the 2011 Nobel glance of historical interest.
Prize in Physics, revealed that for the past seven billion
years not only has space been expanding, but the rate
Einsteins general relativity is
of expansion has been speeding up. The best explana-
tion? The big bang theory augmented by a version of
tightly woven into the tapestry of
Einsteins long-ago-discarded cosmological constant.
The lesson? If you wait long enough, even some of Ein-
todays leading-edge research.
steins wrong ideas turn out to be right [see What Ein-
stein Got Wrong, by Lawrence M. Krauss, on page 50].
An even earlier insight from general relativity originated in knows. We can tell stories of why someone may have had this or
an analysis carried out by German astronomer Karl Schwarz that idea, but the bottom line is that thought and insight are
schild during his stint at the Russian front in the midst of shaped by influences too numerous to analyze.
World War I. Taking a break from calculating artillery trajecto- Eschewing hyperbole, the best we can say is that Einstein
ries, Schwarzschild derived the first exact solution of Einsteins had the right mind at the right moment to crack a collection of
equations, giving a precise description of the warped spacetime deep problems of physics. And what a moment it was. His nu-
produced by a spherical body like the sun. As a by-product, merous but comparatively modest contributions in the decades
Schwarzschilds result revealed something peculiar. Compress after the discovery of general relativity suggest that the timeli-
any object to a sufficiently small sizethe sun, say, to three ness of the particular intellectual nexus he brought to bear on
miles acrossand the resulting spacetime warp will be so se- physics had passed.
vere that anything approaching too closely, including light it- With all that he accomplished, and the continuing legacy he
self, will be trapped. In modern language, Schwarzschild had re- spawned, theres an urge to ask another speculative question:
vealed the possibility of black holes. Could there be another Einstein? If one means another ber ge-
At the time, black holes seemed far-fetched, a mathematical nius who will powerfully push science forward, then the answer
oddity that many expected to have no relevance to reality. But is surely yes. In the past half a century since Einsteins death,
observation, not expectation, dictates what is right, and astro- there have indeed been such scientists. But if one means an ber
nomical data have now established that black holes are real and genius to whom the world will look not because of accomplish-
plentiful. They are too far away for direct exploration at the mo- ments in sports or entertainment but as a thrilling example of
ment, but as theoretical laboratories, black holes are indispens- what the human mind can accomplish, well, that question speaks
able. Beginning with Stephen Hawkings influential calculations to usto what we as a civilization will deem precious.
in the 1970s, physicists have become increasingly convinced
that the extreme nature of black holes makes them an ideal
M O R E TO E X P L O R E
proving ground for attempts to push general relativity forward
and, most notably, to meld it with quantum mechanics [see E = mc 2 : A Biography of the Worlds Most Famous Equation. David Bodanis.
Penguin, 2000.
The Black Hole Test, by Dimitrios Psaltis and Sheperd S. Doel-
The Fabric of the Cosmos: Space, Time and the Texture of Reality. Brian Greene.
eman, on page 74]. Indeed, one of todays most hotly debated is- Knopf, 2004.
sues concerns how quantum processes may affect our under- The Collected Papers of Albert Einstein. P rinceton University Press. 
standing of the outer edge of a black holeits event horizonas http://einsteinpapers.press.princeton.edu
well as the nature of a black holes interior.
FROM OUR ARCHIVES
Which is all just to say that the centenary of general relativity
is a far cry from a backward glance of historical interest. Ein- The Future of String Theory: A Conversation with Brian Greene. B rian Greene;
November 2003.
steins general relativity is tightly woven into the tapestry of to-
days leading-edge research. s c i e n t i f i c a m e r i c a n . c o m /m a g a z i n e /s a

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HOW EINSTEIN
REINVENTED

REALITY
H I STO R Y

Albert Einstein created his most famous

theory amid personal strife, political tension
and a scientific rivalry that almost cost him
the glory of his discovery
By Walter Isaacson

The general theory of relativity began with a sudden

thought. It was late 1907, two years after the miracle year in which Albert
Einstein had produced his special theory of relativity and his theory of light
quanta, but he was still an examiner in the Swiss patent office. The physics
world had not yet caught up with his genius. While sitting in his office in Bern,
a thought startled him, he recalled: If a person falls freely, he will not feel
his own weight. He would later call it the happiest thought in my life.
The tale of the falling man has become an iconic one, and in some accounts
it actually involves a painter who fell from the roof of an apartment building
near the patent office. Like other great tales of gravitational discoveryGali-
leo dropping objects from the Leaning Tower of Pisa and the apple falling on
Isaac Newtons headit was embellished in popular lore. Despite Einsteins
propensity to focus on science rather than the merely personal, even he was
not likely to watch a real human plunging off a roof and think of gravitational
theory, much less call it the happiest thought in his life.
Einstein soon refined his thought experiment so that the falling man was
in an enclosed chamber, such as an elevator, in free fall. In the chamber, he
would feel weightless. Any objects he dropped would float alongside him.
There would be no way for him to tellno experiment he could do to deter-
GETTY IMAGES

mineif the chamber was falling at an accelerated rate or was floating in a

gravity-free region of outer space.

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 Then Einstein imagined that the man was in the same Walter Isaacson is CEO of the Aspen Institute.
chamber way out in space, where there was no perceptible He was chairman of CNN and managing editor
gravity, and a constant force was pulling the chamber up at an of Time m
 agazine. Isaacson is author of numerous
accelerated rate. He would feel his feet pressed to the floor. If books, including Steve Jobs (Simon & Schuster, 2011).
he dropped an object, it would fall to the floor at an accelerated
ratejust as if he stood on Earth. There was no way to make a
distinction between the effects of gravity and the effects of
being accelerated.
Einstein dubbed this the equivalence principle. The local
effects of gravity and of acceleration are equivalent. Therefore,
they must be manifestations of the same phenomenon, some chamber thought experiment indicated that gravity would
cosmic field that accounts for both acceleration and gravity. bend light. Imagine that the chamber is being accelerated
It would take another eight years for Einstein to turn his upward. A light beam comes in through a pinhole on one wall.
falling-man thought experiment into the most beautiful theory By the time it reaches the opposite wall, the light is a little clos-
in the history of physics. He would go from his sedate life as a er to the floor because the chamber has shot upward. And if
married father working at the Swiss patent office to living alone you could plot the beams trajectory across the chamber, it
as a professor in Berlin, estranged from his family and increas- would be curved because of the upward acceleration. The
ingly alienated from his Prussian Academy of Sciences col- equivalence principle says that this effect should be the same
leagues there by the rise of anti-Semitism. The decision last whether the chamber is accelerating upward or is resting still
year by the California Institute of Technology and Princeton in a gravitational field. In other words, light should bend when
University to put an archive of Einsteins papers online for free passing through a gravitational field.
permits a glimpse of him juggling the cosmic and the personal In 1912 Einstein asked an old classmate to help him with the
throughout this period. We can relish his
excitement in late 1907 as he scribbled down

Einstein faced two ticking clocks:

what he called a novel consideration, based
on the principle of relativity, on acceleration

he could sense that Hilbert was closing

and gravitation. Then we can sense his
grumpy boredom, a week later, as he reject-

in on the correct equations, and he

ed an electric companys patent application
for an alternating-current machine, calling
the claim incorrectly, imprecisely and
unclearly prepared. The coming years had agreed to give a series of four
would be full of human drama, as Einstein
raced against a rival to give mathematical formal Thursday lectures on his theory.
expression to relativity while struggling
with his estranged wife over money and his
right to visit his two young boys. But by 1915 his work climaxed complicated mathematics that might describe a curved and
in a completed theory that would change our understanding of warped four-dimensional spacetime. Until then, his success
the universe forever. had been based on his talent for sniffing out the underlying
physical principles of nature. He had left to others the task of
BENDING LIGHT finding the best mathematical expressions of those principles.
For almost four years a fter positing that gravity and accelera- But now Einstein realized that math could be a tool for discov-
tion were equivalent, Einstein did little with the idea. Instead eringand not merely describingnatures laws.
he focused on quantum theory. But in 1911, when he had finally Einsteins goal as he pursued his general theory of relativity
breached the walls of academia and become a professor at the was to find the mathematical equations describing two inter-
German Charles-Ferdinand University in Prague, he turned his woven processes: how a gravitational field acts on matter, tell-
attention back to coming up with a theory of gravity that would ing it how to move, and how matter generates gravitational
help him generalize special relativitythe relation between fields in spacetime, telling spacetime how to curve.
space and time that he defined in 1905. For three more years Einstein wrestled with drafts and out-
As Einstein developed his equivalence principle, he realized lines that turned out to have flaws. Then, beginning in the sum-
that it had some surprising ramifications. For example, his mer of 1915, the math and the physics began to come together.

IN BRIEF

Einsteins realization that gravity and He raced to discover t he correct math- gled on the home front, as he went Despite these challenges, E instein tri-
acceleration are equivalent put him on ematical formulas for his theory before a through a divorce from his first wife and a umphed and delivered one of the worlds
an eight-year path to generalize his spe- rival, mathematician David Hilbert, could separation from his sons while he court- supreme scientific works in his general
cial theory of relativity. do so first. Einstein simultaneously strug- ed a cousin whom he would later marry. theory of relativity.

40 Scientific American, September 2015

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 1907
Einstein realizes that a
person falling freely would
not feel his weightan
insight that set him on the
path to general relativity
1914
Einstein and his first wife, Mileva
Maric, separate. She moves from
Berlin, where they had been living,

Road to
to Zurich with their two sons

Relativity
Einstein faced
difficulties, both
scientific and personal, 1911 1912
while formulating Now a professor at the
German Charles-Ferdinand
The physicist begins an affair
with his cousin, Elsa Lwenthal,
general relativity University in Prague, Einstein whom he later marries
starts working to expand his
special theory of relativity to
include gravity

PERSONAL UNRAVELING A RIVALRY

By then, he had moved to Berlin to become a professor and The visit to Gttingen was a triumph. A  few weeks later Ein-
member of the Prussian Academy. But he found himself work- stein reported to a scientist friend that he was able to convince
ing pretty much without support. Anti-Semitism was rising, Hilbert of the general theory of relativity. In a letter to another
and he formed no coterie of colleagues around him. He split colleague, he was even more effusive: I am quite enchanted
with his wife, Mileva Maric, a fellow physicist who had been his with Hilbert!
sounding board in formulating special relativity in 1905, and Hilbert was likewise enchanted with Einstein and with his
she moved back to Zurich with their two sons, ages 10 and four. theory, so much so that he soon set out to see if he could do
He was having an affair with his cousin Elsa, whom he would what Einstein had so far not accomplished: produce the math-
later marry, but he lived by himself in a sparsely furnished ematical equations that would complete the formulation of
apartment in central Berlin, where he ate intermittently, slept general relativity.
randomly, played his violin and waged his solitary struggle. Einstein began hearing Hilberts footsteps in early October
Throughout 1915 his personal life began to unravel. Some 1915, just as he realized that his current version of the theory
friends were pressing him to get a divorce and marry Elsa; oth- which was based on an E  ntwurf, or outline, he had been refin-
ers were warning that he should not be seen with her or let her ing for two yearshad serious flaws. His equations did not
come near his two boys. Maric repeatedly sent letters request- account properly for rotating motion. In addition, he realized
ing money, and at one point Einstein replied with unbridled that his equations were not generally covariant, meaning that
bitterness. I find such a demand beyond discussion, he re they did not really make all forms of accelerated and nonuni-
sponded. I find your constant attempts to lay hold of every- form motion relative, nor did they fully explain an anomaly
thing that is in my possession absolutely disgraceful. He tried that astronomers had observed in the orbit of the planet Mer-
hard to maintain a correspondence with his sons, but they rare- cury. Mercurys perihelionits point of closest approach to the
ly wrote back, and he accused Maric of not delivering his letters sunhad been gradually shifting in a way not accounted for by
to them. Newtonian physics or by Einsteins then current version of his
Yet amid this personal turmoil, Einstein was able to devise, own theory.
by late June 1915, many elements of general relativity. He gave a Einstein faced two ticking clocks: he could sense that Hil-
weeklong series of lectures at the end of that month on his bert was closing in on the correct equations, and he had agreed
evolving ideas at the University of Gttingen in Germany, the to give a series of four formal Thursday lectures on his theory
worlds preeminent center for mathematics. Foremost among in November to the members of the Prussian Academy. The
the geniuses there was David Hilbert, and Einstein was partic- result was an exhausting monthlong whirlwind during which
GETTY IMAGES

ularly eagerperhaps too eager, it would turn outto explain Einstein wrestled with a succession of equations, corrections
all the intricacies of relativity to him. and updates that he rushed to complete.

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 JUNE 1915
Mathematician David Hilbert
attends a lecture where Einstein
describes his ideas about
general relativity. Hilbert begins
to race Einstein to devise the
mathematics of the theory NOVEMBER 1915
During his fourth lecture that
month at the Prussian State
Library, Einstein finally delivers
a paper reporting his field
equations for general relativity

SUMMER AND
FALL OF 1915
Einstein lives alone, eats
and sleeps intermittently,
and consoles himself with his
violin while he struggles to
produce equations to formalize
general relativity

Even as he arrived at the grand hall of the

Prussian State Library on November 4 to deliver
the first of his lectures, Einstein was still wres-
tling with his theory. For the last four years, he
began, I have tried to establish a general theory
of relativity. With great candor, he detailed the
problems he had encountered and admitted that
he still had not come up with equations that fully
worked. can learn a lot of fine and good things from me as well
Einstein was in the throes of the one of the most concentrat- that no one else can offer you so easily.... In the last few
ed frenzies of scientific creativity in history. At the same time, days I completed one of the finest papers of my life; when
he was dealing with personal crises within his family. Letters you are older, I will tell you about it.
continued to arrive from his estranged wife that pressed him
for money and discussed the guidelines for his contact with He ended with a small apology for seeming so distracted.
their two sons. Through a mutual friend, she demanded that he I am often so engrossed in my work that I forget to eat lunch,
not ask that his children come visit him in Berlin where they he wrote.
might discover his affair. Einstein assured the friend that in Einstein also engaged in an awkward interaction with Hil-
Berlin he was living alone and that his desolate apartment bert. He had been informed that the Gttingen mathematician
had an almost churchlike atmosphere. The friend replied, had spotted the flaws in the Entwurf equations. Worried about
referring to Einsteins work on general relativity, Justifiably so, being scooped, he wrote Hilbert a letter saying that he himself
for unusual divine powers are at work in there. had discovered the flaws, and he sent along a copy of his Novem-
On the very day that he presented his first paper, he wrote ber 4 lecture.
a painfully poignant letter to his elder son, Hans Albert, who In his second lecture, delivered on November 11, Einstein
was living in Switzerland: imposed new coordinate conditions that allowed his equations
to be generally covariant. As it turned out, the change did not
 esterday I received your dear little letter and was
Y greatly improve matters. He was close to the final answer but
delighted with it. I was already afraid you didnt want to making little headway. Once again, he sent his paper off to Hil-
write me at all anymore.... I shall press for our being bert and asked him how his own quest was going. My own
GETTY IMAGES

together for a month every year so that you see that you curiosity is interfering with my work! he wrote.
have a father who is attached to you and loves you. You Hilbert sent him a reply that must have unnerved Einstein.

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He said he had a solution to your great problem, and he invit- as rapidly as you, in my equations the electron would have to
ed Einstein to come to Gttingen on November 16 and have the capitulate, and the hydrogen atom would have to produce its
dubious pleasure of hearing it. Since you are so interested, I note of apology about why it does not radiate. The next day,
would like to lay out my theory in very complete detail this however, Hilbert sent a paper to a Gttingen science journal
coming Tuesday, Hilbert wrote. My wife and I would be very describing his own version of the equations for general relativi-
pleased if you stayed with us. Then, after signing his name, ty. The title he picked for his piece was not a modest one: The
Hilbert felt compelled to add a tantalizing and disconcerting Foundations of Physics, he called it.
postscript. As far as I understand your new paper, the solution It is not clear how carefully Einstein read Hilberts paper or
given by you is entirely different from mine. if it affected his thinking as he prepared his climactic fourth
lecture at the Prussian Academy. Regardless, he produced in
COMING TO A HEAD time for his final lecture on November 25entitled The Field
Einstein wrote four letters o  n November 15, a Monday, that Equations of Gravitationa set of covariant equations that
give a glimpse into his intertwined personal and professional described a general theory of relativity.
dramas. To Hans Albert, he suggested that he would like to It was not nearly as vivid to the layperson as, say, E = mc2.
travel to Switzerland at Christmas to visit him. Maybe it would Yet using the condensed notations of tensors, in which sprawl-
be better if we were alone somewhere, such as at a secluded ing mathematical complexities can be compressed into little
inn, he said to his son. What do you think? subscripts, the crux of the final Einstein field equation is com-
He then wrote his estranged wife a conciliatory letter that pact enough to be emblazoned on T-shirts worn by physics
thanked her for her willingness not to undermine my relations geeks. In one of its many variations, it can be written as:
with the boys. And he reported to a friend, I have modified
the theory of gravity, having realized that my earlier proofs had R - gR = -8 G T
a gap.... I shall be glad to come to Switzerland at the turn of
the year to see my dear boy. The left side of the equationwhich is now known as the
He also replied to Hilbert and declined his invitation to visit Einstein tensor and can be written simply as Gdescribes
Gttingen the next day. His letter did not hide his anxiety: The how the geometry of spacetime is warped and curved by mas-
hints you gave in your messages awaken the greatest of expec- sive objects. The right side describes the movement of matter
tations. Nevertheless, I must refrain from traveling to Gttin- in the gravitational field. The interplay between the two sides
gen.... I am tired out and plagued by stom-
ach pains. ... If possible, please send me a

Einstein was in the throes of the one

correction proof of your study to mitigate
my impatience.
As he hurriedly rushed to come up with
the precise formulation of his theory, Ein- of the most concentrated frenzies
stein made a breakthrough that turned his
anxiety into elation. He tested a set of of scientific creativity in history.
revised equations to see if they would yield
the correct results for the anomalous shift At the same time, he was dealing
in Mercurys orbit. The answer came out
right: his equations predicted the perihelion with personal crises within his family.
should drift by about 43 arc seconds per
century. He was so thrilled that he had heart
palpitations. I was beside myself with joy and excitement for shows how objects curve spacetime and how, in turn, this cur-
days, he told a colleague. To another physicist, he exulted, vature affects the motion of objects.
The results of Mercurys perihelion movement fill me with Both at the time and to this day, there has been a priority
great satisfaction. How helpful to us is astronomys pedantic dispute over which elements of the mathematical equations of
accuracy, which I used to secretly ridicule! general relativity were discovered first by Hilbert rather than
The morning of his third lecture, November 18, Einstein by Einstein. Whatever the case, it was Einsteins theory that
received Hilberts new paper and was dismayed by how similar was being formalized by these equations, one that he had
it was to his own work. His response to Hilbert was terse and explained to Hilbert during their time together in Gttingen
clearly designed to assert priority. The system you furnish that summer of 1915. Hilbert graciously noted this in the final
agreesas far as I can seeexactly with what I found in the last version of his paper: The differential equations of gravitation
few weeks and have presented to the Academy, he wrote. that result are, as it seems to me, in agreement with the mag-
Today I am presenting to the Academy a paper in which I nificent theory of general relativity established by Einstein.
derive quantitatively out of general relativity, without any guid- As he later summed it up, Einstein did the work and not the
ing hypothesis, the perihelion motion of Mercury. No gravita- mathematicians.
tional theory has achieved this until now. Within a few weeks Einstein and Hilbert were repairing their
Hilbert responded kindly and generously the following day, relationship. Hilbert proposed Einstein for membership in the
claiming no priority for himself. Cordial congratulations on Royal Society of Sciences in Gttingen, and Einstein wrote back
conquering perihelion motion, he wrote. If I could calculate with an amiable letter saying how two men who had glimpsed

September 2015, ScientificAmerican.com43

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transcendent theories should not be diminished by earthly emo- THE BASICS
tions. There has been a certain ill-feeling between us, the cause of
which I do not want to analyze, Einstein wrote. I have struggled
against the feeling of bitterness attached to it, and this with com-
Relativity Primer
plete success. I think of you again with unmixed geniality and General relativity redefined t he concept of gravityrather
ask you to try to do the same with me. Objectively it is a shame than a force pulling masses together, the theory exposed it as
when two real fellows who have extricated themselves from this a simple consequence of the geometry of space and time. The
shabby world do not afford each other mutual pleasure. notion grew out of a revelation from the more limited special
theory of relativity, which Albert Einstein conceived 10 years
THE BOLDEST DREAMS earlier. This theory established space and time as a single
Einsteins pride was understandable. At age 36, he had p roduced entity, spacetime (below). In his general theory of relativity,
a dramatic revision of our concept of the universe. His general Einstein described what happens when mass is present in
theory of relativity was not merely the interpretation of some spacetime (top right), causing it to curve and forcing objects
experimental data or the discovery of a more accurate set of traveling through it to follow a bent path. If enough mass is
laws. It was a whole new way of regarding reality. packed into a very small region, spacetime becomes infinitely
With his special theory of relativity, Einstein had shown curved, creating a black hole (bottom right).
that space and time did not have independent existences but
instead formed a fabric of spacetime. Now, with his general ver-
sion of the theory, this fabric of spacetime became not merely a Spacetime without Mass
container for objects and events. Instead it had its own dynam-
ics that were determined by, and in turn helped to determine, The special theory of relativity first established that
the motion of objects within itlike the way that the fabric of a the universe as we know it has four dimensions
three of space and one of time. In the absence of mass,
trampoline will curve as a bowling ball and some billiard balls
spacetime is essentially a grid, and the shortest path for
roll across it and in turn that the dynamic curving of the tram- an object to travel through it is a straight line. Because we
poline fabric will determine the path of the rolling balls and cannot portray four dimensions on this two-dimensional
cause the billiard balls to move toward the bowling ball. page, we show a simplified diagram of the three spatial
The curving and rippling fabric of spacetime explained dimensions with the position of an object at various
gravity, its equivalence to acceleration and the general relativi- times standing in for the missing fourth dimension.
ty of all forms of motion. In the opinion of Paul Dirac, the Nobel
laureate pioneer of quantum mechanics, it was probably the
greatest scientific discovery ever made. And Max Born, anoth-
er giant of 20th-century physics, called it the greatest feat of
human thinking about nature, the most amazing combination
of philosophical penetration, physical intuition and mathemat-
ical skill.
The entire process had exhausted Einstein. His marriage
had collapsed, and war was ravaging Europe. But he was as
happy as he would ever be. The boldest dreams have now been
fulfilled, he exulted to his best friend, engineer Michele Besso.
General c ovariance. Mercurys perihelion motion wonderfully
precise. He signed himself contented but quite worn-out.
Years later, when his younger son, Eduard, asked why he
was so famous, Einstein replied by using a simple image to
describe his fundamental insight that gravity was the curving
of the fabric of spacetime. When a blind beetle crawls over the
surface of a curved branch, it doesnt notice that the track it has
covered is indeed curved, he said. I was lucky enough to
notice what the beetle didnt notice.

M O R E TO E X P L O R E

The Field Equations of Gravitation. A  . Einstein in P reussische Akademie der

Wissenschaften, Sitzungsberichte, p ages 844847; December 2, 1915.
Einstein: His Life and Universe. W
 alter Isaacson. Simon & Schuster, 2007.

FROM OUR ARCHIVES

On the Generalized Theory of Gravitation. A  lbert Einstein; April 1950.

An Interview with Einstein. I . Bernard Cohen; July 1955.

s c i e n t i f i c a m e r i c a n . c o m /m a g a z i n e /s a

44 Scientific American, September 2015 Illustration by Nigel Holmes

2015 Scientific American

 Spacetime with Mass
When massbe it a star, a planet or a human beingis present,
spacetime bends around it so that an object traveling nearby must
follow a rounded trajectory that takes it closer to the mass. Just
as it is impossible to move in a straight line on the surface of a
sphere, it is likewise impossible to move in a straight line through
curved spacetime. This effect produces gravity, which we observe
as an attraction between two masses. On the left is a simplified
two-dimensional diagram of spacetime curvature, and below is an
approximation of the same situation in three dimensions.

Spacetime with Extreme Mass

One of the most startling consequences of general relativity is
the idea of black holes. These occur when mass is dense enough to
form a so-called singularitya point where spacetime is infinitely
curved. The black hole defines the region around the singularity where
gravity is so strong that nothing that enters can exit again. Physicists
now think that black holes are ubiquitous in the universe, often
resulting from the death of stars. The drawing at the right shows a
simplified picture of a black hole in a two-dimensional slice of space
with an objects trajectory as it falls in; an approximation of the same
situation in three dimensions of space appears below.

September 2015, ScientificAmerican.com45

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46 Scientific American, September 2015

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 HEAD
TRIP
THOUGHT
EXPERIMENTS

Einsteins thought experiments left a long and

somewhat mixed legacy of their own
By Sabine Hossenfelder

Gedankenexperiment, German for thought experiment, was Albert Einsteins

famous name for the imaginings that led to his greatest breakthroughs in physics. He
traced his realization of lights finite speedthe core idea of special relativityto his
teenage daydreams about riding beams of light. General relativity, his monumental
theory of gravitation, has its origins in his musings about riding up and down in an
elevator. In both cases, Einstein crafted new theories about the natural world by
using his minds eye to push beyond the limitations of laboratory measurements.

Einstein was neither the first nor the last theorist to ance on the power of thought was a spectacular success.
do this, but his remarkable achievements were pivotal On the other, many of his best known thought experi-
in establishing the gedankenexperiment as a corner- ments were based on data from real experimentation,
stone of modern theoretical physics. Today physicists such as the classic Michelson-Morley experiment that
regularly use thought experiments to craft new theo- first measured the constancy of the speed of light. More-
ries and to seek out inconsistencies or novel effects over, Einsteins fixation on that which can be measured
within existing ones. at times blinded him to deeper layers of realityal-
But the modern embrace of thought experiments though even his mistakes in thought experiments con-
raises some uncomfortable questions. In the search for tributed to later breakthroughs.
a grand unified theory that would wed the small-scale Here we will walk through some of Einsteins most
world of quantum mechanics with Einsteins relativistic iconic thought experiments, highlighting how they suc-
description of the universe at large, the most popular ceeded, where they failed and how they remain vital to
current ideas are bereft of observational support from questions now at the frontiers of theoretical physics.
actual experiments. Can thought alone sustain them?
How far can we trust logical deduction? Where is the THE WINDOWLESS ELEVATOR
line between scientific intuition and fantasy? Einsteins In his thought experiments, E
 insteins genius was in re-
legacy offers no certain answers: On one hand, his reli- alizing which aspects of experience were essential and

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2015 Scientific American

which could be discarded. Consider his most famous one: the ele- Sabine Hossenfelder is an assistant professor at Nordita,
vator thought experiment, which he began devising in 1907. Ein- the Nordic Institute for Theoretical Physics, in Stockholm.
She works on quantum gravity and physics beyond the
stein argued that inside a windowless elevator, a person cannot Standard Model. More of her writing can be found at her
tell whether the elevator is at rest in a gravitational field or is in- blog, Backreaction (http://backreaction.blogspot.com).
stead being hauled up with constant acceleration. He then conjec-
tured that the laws of physics themselves must be identical in both
situations. According to this principle of equivalence, lo-
cally (in the elevator), the effects of gravitation are
the same as that of acceleration in the absence of particles must add up to zero; one particle,
gravity. Converted into mathematical equa- then, could possess a spin value of up, and
tions, this principle became the basis for gen- the other could have a spin value of down.
eral relativity. In other words, the elevator The laws of quantum mechanics dictate
thought experiment motivated Einstein to that in the absence of measurement, nei-
make the daring intellectual leap that ul- ther of the particles possesses a definite
timately led to his greatest achievement, spin until one of the two speeding entan-
his geometric description of gravity. gled particles is measured. Once a mea-
surement of one particle is made, the
SPOOKY ACTION state of the other changes i nstantaneous-
Later in his career, E  instein fought hard ly, e ven if the particles are separated by
against the tenets of quantum mechanics, vast distances!
particularly the uncertainty principle, which Einstein believed this spooky action at a
dictates that the more you know about one aspect distance was nonsense. His own special theory of
of a fundamental particle, such as its position, the less relativity held that nothing could travel faster than
you can know about another related aspect of that particle, light, so there was no way for two particles to communicate with
such as its momentumand vice versa. Einstein thought that each other instantaneously from opposite sides of the universe.
the uncertainty principle was a sign that quantum theory was He suggested instead that the measurement outcomes must be
deeply flawed. determined prior to measurement by hidden variables that
During a years-long exchange with Danish quantum theorist quantum mechanics failed to account for. Decades of discussion
Niels Bohr, Einstein conceived of a series of thought experi- followed until 1964, when physicist John Stewart Bell developed
ments meant to demonstrate that it is possible to violate the un- a theorem quantifying exactly how the information shared be-
certainty principle, but Bohr dissected every one of them. This tween entangled particles differs from the information that Ein-
exchange bolstered Bohrs conviction that quantum uncertainty stein postulated would be shared through hidden variables.
was a fundamental aspect of nature. If not even the great Ein- Since the 1970s lab experiments with entangled quantum sys-
stein could devise a way to precisely measure both the position tems have repeatedly confirmed that Einstein was wrong, that
and the momentum of a particle, then certainly there must be quantum particles indeed share mutual information that can-
something to the uncertainty principle! not be accounted for by hidden variables. Spooky action at a dis-
In 1935, along with his colleagues Boris Podolsky and Nathan tance is real, but experiments have demonstrated that it cannot
Rosen, Einstein published what was meant to be his most po- be used to transmit information faster than light, making it per-
tent critique of the uncertainty principle. Perhaps because Po- fectly consistent with Einsteins special relativity. This counter-
dolsky, not Einstein, drafted the actual text of the paper, this intuitive truth remains one of the most mysterious conundrums
Einstein-Podolsky-Rosen (EPR) thought experiment was pre- in all of physics, and it was Einsteins stubborn, mistaken oppo-
sented not as an easy-to-imagine scenario of boxes, clocks and sition that proved crucial to confirming it.
light beams but as an abstract series of equations describing in-
teractions between two generalized quantum systems. ALICE AND BOB
The simplest version of the EPR experiment studies the par- Today some of the most significant thought experiments in phys-
adoxical behavior of entangled particlespairs of particles ics explore how to reconcile Einsteins clockwork, relativistic uni-
that share a common quantum state. It unfolds as follows: verse with the fuzzy uncertainties inherent to quantum particles.
imagine an unstable particle with a spin of zero decaying into Consider, for instance, the widely discussed black hole in-
two daughter particles, which speed off in opposite directions. formation paradox. If you combine general relativity and quan-
(Spin is a measure of a particles angular momentum, but coun- tum field theory, then you find that black holes evaporate, slow-
terintuitively, it has little to do with a particles rate of rotation.) ly radiating away their mass because of quantum effects. You
Conservation laws dictate that the spins of those two daughter also find that this process is not reversible: regardless of what

IN BRIEF

One of Einsteins enduring contribu- His intuition about falling elevators, Today some of the most important Yet there is a problem: these thought
tions to physics was his use of gedanken- for example, led to his greatest achieve- questions in theoretical physics involve experiments may be so far removed from
experiments, or thought experiments. ment, the general theory of relativity. thought experiments about black holes. empirical data as to be untestable.

48 Scientific American, September 2015 Illustrations by Nigel Holmes

2015 Scientific American

formed the black hole, the evaporating black hole always pro- however, Charlie would have to
duces the same featureless bath of radiation from which no in- compare his observations not only
formation about its contents can be retrieved. But such a pro- with Bobs measurement but
cess is prohibited in quantum theory, which states that any oc- also with Alicesinside the
currence can, in principle, be reversed in time. For instance, black hole. So he must hover at
according to the laws of quantum mechanics, the leftovers of a the horizon, measure the emit-
burned book still contain all the information necessary to reas- ted radiation, then jump in to
semble that book even though this information is not easily ac- tell Alice what he has found.
cessible. Not so for evaporating black holes. And so we arrive at Amazingly enough, Susskind
a paradox, a logical inconsistency. A union of quantum mechan- and Thorlacius showed that no
ics and general relativity tells us that black holes must evapo- matter how hard Charlie tries,
rate, but we conclude that the result is incompatible with quan- it is impossible for him to enter
tum mechanics. We must be making some mistakebut where? the black hole and compare his in-
The thought experiments created to explore formation with Alices before they are
this paradox typically ask us to imagine both torn apart by tidal forces. Their
a pair of observers, Bob and Alice, grisly fate suggests no violations of
who share a pair of entangled quantum mechanics can ever be measured by
particlesthose spooky entities anybody around a black hole, and so theorists can
from the EPR experiment. Al- commit this crime against nature with impunity.
ice jumps into the black hole, Suffice it to say, not all theorists are convinced
carrying her particle with her, that this argument is valid. One criticism of black
whereas Bob stays outside hole complementarity is that it might violate Ein-
and far away with his. With- steins equivalence principlethe one that grew
out Alice, Bobs particle is just out of his elevator thought experiment. Einsteins
typical, with a spin that might general relativity predicts that just as the eleva-
measure up or downthe in- tors passenger cannot distinguish between gravi-
formation that it once shared ty and acceleration, an observer crossing a black
with its entangled partner is lost, holes horizon should not notice anything unusual;
along with Alice. there is no way an observer can tell that he or she has
Bob and Alice play a central role in slipped past the point of no return.
one of the most popular proposed solutions Now let us return to the entanglement of Alice and Bob. If
to the paradox, called black hole complementarity. Proposed in the radiation that Bob sees from far outside the hole contains
1993 by Leonard Susskind, Lrus Thorlacius and John Uglum, all all the information that we thought vanished with Alice behind
then at Stanford University, black hole complementarity rests on the horizon, then this radiation must have been emitted with
Einsteins golden rule for a gedankenexperiment: a strict focus an extremely high energy; otherwise, it would not have es-
on that which can be measured. Susskind and his colleagues pos- caped the strong gravitational pull near the horizon. This ener-
tulated that the information falling in with Alice must come out gy is high enough to vaporize any infalling observer before he
later with the evaporating black holes radiation. This scenario or she slips past the black holes horizon. In other words, black
would usually create another inconsistency because quantum hole complementarity implies that black holes have a firewall
mechanics allows only pair-wise entanglement with one partner just outside the horizonand yet the firewall directly contra-
at a time, a property called monogamy of entanglement. That is, dicts the predictions of Einsteins equivalence principle.
if Bobs particle is entangled with Alices, it cannot be entangled At this point, we have ventured deep into the realm of theo-
with anything else. But black hole complementarity requires that ry. Indeed, we might never know the solutions to these puzzles.
Bobs particle be entangled with Alices and with the radiation But because those solutions could lead to an understanding of
the black hole later emits even though this violates monogamy. the quantum nature of space and time, these puzzles are, for
At first sight, then, black hole complementarity seems to ex- better or worse, some of the most vibrant areas of research in
change one inconsistency with another. theoretical physics. And it all goes back to Einsteins musings
But like a perfect crime, if no one actually witnesses this in- about falling elevators.
consistency, perhaps it can subvert natures otherwise strict
laws. Black hole complementarity relies on the argument that it
is physically impossible for any observer to see Alice and Bobs M O R E TO E X P L O R E

entangled particles breaking the rules. Einsteins Dice and Schrdingers Cat: How Two Great Minds Battled Quantum
To envision how this perfect quantum-mechanical crime Randomness to Create a Unified Theory of Physics. P aul Halpern. Basic Books, 2015.
could unfold, imagine a third observer, Charlie, hovering near
FROM OUR ARCHIVES
the black hole, keeping an eye on Alice and Bob. He watches as
Bob stays outside and as Alice falls in, measuring the black holes Black Holes and the Information Paradox. Leonard Susskind; April 1997.
A Quantum Threat to Special Relativity. David Z Albert and Rivka Galchen; March 2009.
emitted radiation all the while. In theory, information encoded
Burning Rings of Fire. J oseph Polchinski; April 2015.
in that radiation could tip off Charlie that Bob and Alice had vio-
lated the monogamy of their entanglement. To know for certain, s c i e n t i f i c a m e r i c a n . c o m /m a g a z i n e /s a

September 2015, ScientificAmerican.com49

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50 Scientific American, September 2015

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 WHAT
EINSTEIN
GOT COSMOLOGY

WRONG Everyone makes mistakes. But those

of the legendary physicist are
particularly illuminating
By Lawrence M. Krauss

Like all people, Albert Einstein made mistakes, and like many physicists he
sometimes published them. For most of us, the times when we go astray are happily
forgettable. In Einsteins case, even the mistakes are noteworthy. They offer insight
into the evolution of his thinking and with it the surrounding shifts in scientific con-
ceptions of the universe. Einsteins errors also lay bare the challenges of discovery at
the leading edge. When pushing the limits of understanding, it is difficult to know
whether ideas written down on paper correspond to real phenomena and whether a
radically new idea will lead to profound insights or will fizzle out.
Over the years Einsteinthe man who brazenly redefined the meaning of space and
timeunderestimated his discoveries and second-guessed himself surprisingly often.
Today three whole flourishing areas of cosmology are built on ideas he misjudged: grav-
itational lensing, gravitational waves and the accelerating expansion of our universe.

IN BRIEF

Despite his immense powers o f per- As a result, he dismissed the impor- Examining Einsteins errors offers in-
ception, Einstein repeatedly failed to tance of gravitational lensing, initially sight into his thought process, as well
grasp the meaning of some of his own doubted the reality of gravitational as a new perspective on the history
most significant ideas or else over- waves and failed to anticipate the dis- behind three of the most exciting ar-
looked their importance. covery of the expanding universe. eas of modern cosmology.

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 EINSTEINS DISTORTED LENS Lawrence Krauss is director of the Origins Project at Arizona
In the case of gravitational lensing, Einsteins crucial error was State University and also Foundation Professor in the School
to downplay one of his most famous results: his prediction that of Earth and Space Exploration and the physics department
there. He is author of nine books (including best sellers The
light bends in a gravitational field. In December 1936 he pub-
Physics of Star Trek and A Universe from Nothing) and a producer
lished a short paper in the journal S  cience, w
 ith the title Lens- of The Unbelievers, a documentary film on science and reason.
Like Action of a Star by the Deviation of Light in the Gravita-
tional Field. It began with a kind of innocence that would be
impossible to find in modern academic literature:  Some time
ago, R. W. Mandl [a Czech engineer] paid me a visit and asked
me to publish the results of a little calculation which I had made
at his request. This note complies with his wish.
The little calculation examined the possibility of extreme de
flections of light caused by gravity. It was a simple matter for Ein-
stein to show that given a massive enough intervening object and What Einstein missedas the irascible but brilliant California
a sufficiently close approach, light rays originating from well be- Institute of Technology astronomer Fritz Zwicky pointedly ar-
hind the object would be bent so strongly by gravity that they gued in a paper he submitted to the Physical Review within
could converge, producing a magnified image or multiple images months of Einsteins publicationwas that stars combine to form
of the distant sourceakin to the bending of light through a lens, galaxies. Individual stars might produce unobservably small lens-
hence the name gravitational lensing. Lensing has developed into ing effects, Zwicky noted, but lensing by massive galaxies, con-
one of the most important observational tools in modern cosmol- taining perhaps 100 billion stars, might be observable.
ogy because it offers a way to deduce the distribution of mass in Zwickys one-page paper, published in 1937, was remarkable.
the universe even in places where the matter is invisible. In it he proposed three uses for gravitational lensing that pres-
Einstein did not recognize either the magnitude or the impor- age almost all the applications that astronomers have managed
tance of the lensing effect, however. Rather he concluded in his to achieve in the intervening decades: testing general relativity,
1936 paper that the splitting of images caused by light passing a using lensing by galaxies to magnify more distant objects that
nearby star would be so small as to be essentially immeasurable, would otherwise be unobservable, and using lensing to measure
which undoubtedly explains the self-deprecating nature of the in- the masses of the largest structures in the universe. Zwicky
troduction to his paper. He was technically correct, but apparent- missed a fourth application that has turned out to be equally
ly it did not occur to him that stars are not the only objects that important, using the lensing by galaxies to probe the geometry
could produce such bending. and evolution of the universe on its largest scales.
Einsteins obliviousness is all the more surprising given the It is hard to imagine a larger underestimation of the signifi-
huge impact of gravitational lensing on his scientific reputation. cance of any calculation in physics.
Deflection of light by a massive object was a key observational
prediction of general relativity. In 1919 an expedition led by phys- STYMIED BY IMAGINARY SINGULARITIES
icist Arthur Eddington observed a solar eclipse and determined In the case of gravitational wavesripples in spacetimeEin-
that starlight passing by the sun bent just as Einstein expected. stein understood early on that they were implied by his theory
News of the confirmation appeared on the front pages of newspa- but for a time backtracked from his original, correct claims for
pers around the world, with the drama of a British expedition con- their existence. Today the detection of gravitational waves from
firming the work of a German scientist right at the end of World colliding black holes and exploding stars or from the inflation-
War I no doubt contributing to the publics fascination. Einstein ary era (an epoch of hyperfast expansion immediately after the
rapidly attained a level of scientific fame unequaled ever since. big bang) promises to open a vast new window on the universe.
There is a further twist to the story. Einstein had done the Einstein first predicted gravitational waves shortly after he
same light-bending calculation years earlier, in 1912. He had not finalized his general theory of relativity in 1916. Although the
recognized the cosmological importance of his result then, either. mathematics behind the waves is complex, the line of reasoning
Even worse, he had made a near-disastrous mathematical error: he employed is not. According to the laws of electromagnetism,
he performed his calculation using an early version of general rel- if we move an electrical charge back and forth, we generate an
ativity that predicted a light deflection by gravity half as big as oscillating disturbance that manifests itself as an electromag-
the true value. An expedition had been planned to search for the netic wave such as light. Likewise, if we move a pebble back and
bending of starlight by the sun during a 1914 solar eclipse, but it forth across the surface of a pond, we generate a pattern of wa-
was preempted by the outbreak of World War I. Einstein was ter waves. Einstein had demonstrated that matter curves space,
lucky that the observation never happened. If it had, the first pre- so matter in motion should produce an analogous, oscillating
diction of Einsteins emerging theory of gravity would have dis- disturbance of space. But then he started to doubt whether such
agreed with the data. How that would have affected his life, and disturbances were physically real.
the subsequent history of science, is anyones guess. Einstein announced this change of heart in a 1936 paper sub-
After the 1936 article was published, Einstein wrote to the ed- mitted to Physical Review (the same prestigious American jour-
itor with a charmingly incorrect assessment of his research: Let nal that published Zwickys lensing paper). The tale of how he
me also thank you for your cooperation with the little publica- made the error and later discovered his mistake is almost comi-
tion, which Mister Mandl squeezed out of me. It is of little value, cally twisted. He had moved to the U.S. from Germany three
but it makes the poor guy happy. years earlier, and clearly he was still not used to the way things

52 Scientific American, September 2015

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were done in the new world. Around the time he submitted his ties become infinitely large. That nonsensical result led them to
paper, entitled Do Gravitational Waves Exist? Einstein wrote a infer that such waves could not exist. In reality, Einstein misun-
letter to his colleague Max Born, stating,  Together with a young derstood the mathematics of his own theory. General relativity
collaborator, I arrived at the interesting result that gravitational tells us that nature is independent of the particular way that sci-
waves do not exist, though they had been assumed a certainty to entists choose to define coordinates in space; many seemingly bi-
the first approximation. This shows us that the non-linear gener- zarre results that come out of solving relativitys equations are
al relativistic field equations can tell us more or, rather, limit us now understood as mere artifacts of using the wrong coordinate
more than we have believed up to now. system. For example, around a black hole there is a radius, called
The paper that Einstein sent to the P  hysical Review n  o lon- the event horizon, inside of which one can never escape the pull
ger exists because it was never published there. Following nor- of the black hole. When writing down the geometry around a
mal procedure, the editor of the journal had sent his paper (co- black hole, many quantitiesincluding distance and timeseem
authored with Nathan Rosen, then Einsteins research assistant to blow up at the event horizon. These infinities are unphysical,
at the Institute for Advanced Study in Princeton, N.J.) out for however. In another set of coordinates, defined by the way that
peer review. A critical report came back from an anonymous ref- light moves through space, they disappear. The same is true for
eree and was forwarded to Einstein for a response. He was gravitational waves. There is no single coordinate system in
stunned to have had his work subject to review, given that this which planar gravitational waves can be described without ap-
policy was not the norm in the German publications he previ- parent singularities, but these are not real. By using two different,
ously had submitted to. overlapping coordinates, the singularities disappear.
In response, Einstein wrote a haughty letter to the editor: Still convinced of his argument, Einstein resubmitted his pa-
 We (Mr. Rosen and I) had sent you our manuscript for publica- per to the J ournal of the Franklin Institute, b
 ut before it could be
tion and had not authorized you to show it to specialists before it published, he, too, realized his mistake and informed the editors
is printed. I see no reason to address thein any case errone- he had discovered errors. The final published form, retitled On
ouscomments of your anonymous expert. On the basis of this Gravitational Waves, presents a solution to the general relativity
incident I prefer to publish the paper elsewhere. H e never again equations that use a different coordinate systemone appropriate
submitted a paper to the Physical Review. A  pparently he also for cylindrical rather than planar gravitational wavesin which
never read the referees report, written by the distinguished U.S. no singularities appear, just as Robertson had suggested.
cosmologist Howard Percy Robertson, which correctly ex- How did Einstein come to the correct conclusion in the end?
plained the crucial error in his thinking. According to his later assistant, Leopold Infeld, Robertson sought
Einstein and Rosen had tried to write a formula for gravita- out Infeld and kindly explained to him both the error in the origi-
tional plane waves (flat, evenly spaced waves, analogous to pond nal paper and the possible resolution, which Infeld related to Ein-
ripples from a rock that was dropped extremely far away), but in stein. Robertson apparently never revealed that he was the pa-
doing so they encountered a singularitya place where quanti- pers referee, nor did Einstein ever mention the original referees
report. The upshot is that Einstein never published
his erroneous claim disputing the existence of grav-
itational waves, but only thanks to the intervention
of a particularly diligent peer reviewer.
Einstein did not fare as well with regard to
black holes. He remained confused by the unphysi-
cal singularity at the event horizon and assumed
that nature must prohibit it somehow. He argued
that conservation of angular momentum would
cause particles in a collapsing object to stabilize in
orbits of finite radius, making it impossible for an
event horizon to form. He never accepted black
holes as physically real objects.

A BRILLIANT BLUNDER?
The most famous of Einstein s errors is his modifi-
cation of general relativity to allow a universe that
is not expanding. It became widely known because
he reportedly denounced it himself as a blunder.
When he completed general relativity in 1915, the
prevailing wisdom held that our galaxy, the Milky
Way, was surrounded by an infinite void that was
both static and eternal. But Einstein recognized
that the gravitational force caused by matter in gen-
eral relativity (as in Newtons theory) is universally
attractive, making a static solution impossible.
Gravity should cause the matter to collapse inward.

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 CASE STUDIE S

Einsteins Blunders
In three major cases, E
 instein shockingly underestimated the value of his findings or decided that a valid discovery was incorrect.
His discarded ideas have proved crucial to modern cosmology. Gravitational lensing is used to map galaxy clusters; gravitational
waves offer insights into the first moments of the big bang; the cosmological constant may regulate the evolution of the universe.

Gravitational Lensing
Observed distant galaxy position True distant galaxy position
When Einstein published his 1936 paper
on gravitational lensingbending of light
by gravityhe mistakenly concluded that
the phenomenon would be unobservable.
He thought only about lensing of stars by True distant
other stars, not the more pronounced Observed star position
lensing of galaxies by other galaxies, distant
which is why he did not publish his results star position
earlier. It is a good thing, too: The first
time Einstein calculated the lensing effect, Lensing angle from a
in 1912, he used an early form of his galaxy is far greater;
theory, and his estimate of the bending measuring how the
was too small. Had he published the Near star Near galaxy
light is bent can
erroneous prediction, it might have reveal the mass and
affected the ultimate acceptance of structure of the
general relativityand that would have intervening galaxy
been a big mistake. Lensing angle
from a single
star is very small
Earth

Gravitational Waves
Cylindrical wave,
whose plane General relativity implied the existence of
of oscillation gravitational waves, ripples in spacetime, but
rotates as the Einstein initially rejected his own prediction. He
wave moves was saved from publishing this faulty assertion
by another mistake: regarding peer review as
an insult. After withdrawing a paper in anger
over a reviewers critique, he realized his error:
he had tried to find a solution for waves oscil
lating in a constant direction as they move. He
subsequently derived the correct expression for
waves whose direction of oscillation rotates as
Planar wave, they move. Gravitational waves have since been
oscillating in well confirmed, albeit indirectly.
constant direction

Cosmological Constant
Einsteins Field Equation
In 1917 Einstein added a term, called the
cosmological constant, to the equations of
general relativity as a mathematical way
of keeping the universe static. When he
learned that the universe is expanding, he
discarded the constant. What he did not
realize is that such a term is a natural part
of the theory. Scientists now recognize R and g describe (lambda), G is the T represents
that the cosmological constant corre the structure the cosmological gravitational the energy
sponds to an energy within empty space; of spacetime constant, is a term constant density of
that energy may explain the accelerating that can describe matter and
expansion of the universe. a repulsive force c is the radiation
throughout space speed of light

54 Scientific American, September 2015 Illustrations by Jen Christiansen

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 In a 1917 paper, Cosmological Considerations in the General 1998 observed that the expansion of the universe is accelerat-
Theory of Relativity, Einstein therefore introduced an addi- ing, driven outward by something that seems to act just like a
tional, constant term in his equations for general relativity to cosmological constant. In this instance, one might say that Ein-
ensure a static universe. The cosmological term would provide a stein actually blundered twice: by introducing the cosmological
counteracting gravitational repulsion throughout all of space, constant for the wrong reason and again by throwing it out in-
holding back gravity as Einstein hoped. There was no physical stead of exploring its implications.
justification for this term, other than staving off collapse.
Within a decade after the introduction of the cosmological THE ERROR HE NEVER ADMITTED
constant, evidence began to mount that the universe wasnt Einstein s errors were intellectually fertile because they were
static after all. At first, Einstein was resistant. Belgian physicist all rooted in grand, provocative ideas about how physics works.
and Catholic priest Georges Lematre developed a model of an That is true even of what is generally regarded as his greatest er-
expanding universe, complete with a kind of big bang, in 1927, ror of all: his refusal to accept quantum mechanics as a funda-
which was two years before Edwin Hubble published his land- mental theory of nature.
mark paper documenting the recession of galaxies. Lematre Although Einstein had created the basis for quantum me-
later recalled being admonished by Einstein, Your calculations chanics with his theory of the photoelectric effect (for which he
are correct, but your physics is abominable! later won the Nobel Prize), he never completely shed the mind-
Eventually Einstein came around. He went to visit Hubble set of classical physics. The idea that the location of a particle is a
and looked through his telescope at Mount Wilson Observa matter of probability or that one particle can instantaneously in-
tory near Pasadena, Calif., and in 1933 Einstein reportedly fluence another one from a great distance struck him as absurd,
praised Lematres cosmological theory: This is the most beau- although his views on the quandaries of quantum theory were
tiful and satisfactory explanation of creation to which I have more nuanced than he is usually given credit for [see Is the Cos-
ever listened. mos Random? by George Musser, on page 88]. He spent most of
It was not lost on Einstein that in an expanding universe his later years attempting to merge the equations of gravity and
there was no longer any need for a cosmological constant to keep electromagnetism within a classical framework, into a so-called
things static. Even in 1919 he wrote that the constant was grave- unified field theory.
ly detrimental to the formal beauty of the theory. And in an oft- As part of that effort, Einstein became fascinated by a specula-
quoted reference in George Gamows book, M  y World Line: An tion introduced by German mathematician Theodor Kaluza in
Informal Autobiography, Gamow related the following anec- 1921 and later elaborated on by Swedish physicist Oskar Klein.
dote: Much later, when I was discussing cosmological problems They suggested that if the universe contains five dimensions
with Einstein, he remarked that the introduction of the cosmo- three of familiar space, one of time and a fifth dimension curled
logical term was the biggest blunder he ever made in his life. up so as to be invisibleit would be possible to create a single,
In retrospect, Einstein was completely mistaken in thinking combined description of electromagnetism and gravity. For Ein-
that the cosmological constant was worthless, but his introduc- stein, one of the attractive facets of the theory was that it was
tion of it was a blunder, for two reasons. Had he had the courage purely classical. Klein had shown that, in the model, the apparent
of his convictions, he would have recognized that general relativi- quantization of electrical charge could be a consequence of elec-
tys inconsistency with a static universe was a p  rediction. A
t a tromagnetism reflecting the geometry of the closed, circular
time when no one expected that the universe was dynamic on shape of the fifth dimension.
large scales, Einstein could have predicted cosmic expansion in- Einsteins effort to construct a unified field theory ultimately
stead of having to grudgingly accept it later. went nowhere, but his flawed ideas once again led to important
The introduction of the cosmological constant was also a breakthroughs. In calling attention to the extra dimensions of
blunder in a more fundamental way. Simply put, the constant Kaluza and Klein, Einstein may have helped inspire the higher-
could not work the way he intended: it would not allow the kind dimensional mathematics of modern string theory, a currently
of static universe that he was trying to match. That error arose in popular proposal for incorporating general relativity into quan-
part because once again Einstein used the wrong coordinate tum mechanics. Einstein probably would have been repelled by
frame for his calculations. But his conception was wrong from a the idea of having general relativity arise out of a quantum land-
physical perspective as well. Although it is possible to briefly bal- scape rather than the other way around. But as we have seen, he
ance the gravitational attraction of matter with the repulsion was anything but infallible.
from a cosmological constant, the smallest perturbation will pro-
duce runaway expansion or collapse. With or without the cosmo- M O R E TO E X P L O R E
logical constant, the universe must be dynamic.
The Origin of Gravitational Lensing: A Postscript to Einsteins 1936 Science
The cosmological constant ultimately proved far more dura-
Paper. J urgen Renn, Tilman Sauer and John Stachel in S cience, Vol. 275, pages 184
ble than the limited astronomical knowledge that inspired it. 186; January 10, 1997.
Although the constant was an ad hoc addition to his equations, Einstein versus the Physical Review. Daniel Kennefick in P hysics Today, Vol. 58, No. 9,
physicists now understand that when viewed through the lens pages 4348; September 2005.
of quantum theory, it corresponds to a possible energy residing FROM OUR ARCHIVES
in empty space. In fact, quantum physics requires the presence
A Cosmic Conundrum. L awrence M. Krauss and Michael S. Turner; September 2004.
of such a cosmological term. Moreover, the energy content of
The Right Way to Get It Wrong. D
 avid Kaiser and Angela N. H. Creager; June 2012.
empty space is not just a theoretical concept. In one of the most
astonishing measurements in recent history, two groups in s c i e n t i f i c a m e r i c a n . c o m /m a g a z i n e /s a

September 2015, ScientificAmerican.com55

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 A LEGACY IN
NUMBERS

R E L AT I V I T YS

REACH
A visualization of recent physics terms affirms the
enduring influence of Einsteins 100-year-old masterpiece

The outer limits of 21st-century phys- quantum cosmology. We then edited this
ics involve arcane pursuits with strange and list down to 61 keywords, each of which rep-
wonderful names like M-theory and de resents a research topic that has grown out
Sitter universes. Many of these endeavors of general relativity. The arXivs relativity
rely heavily on Albert Einsteins explanation section was scanned to discover which of
of how gravity emerges from the bending of the 61words were turning up most often in
space and time. the research reports.
With the assistance of the Office for Crea The data visualization here is the result.
tive Research (OCR), a New York City data Each incandescent colored dot stands for a
visualization firm, Scientific American de paper that touches on at least one element of
cided to look for some measure of how often general relativity or its spin-offs [see follow-
recent scientific papers in relevant areas ing three pages for details on how to inter-
of physics still lean on Einsteins 100-year- pret the visualization]. For an interactive
old achievement. version, go to www.ScientificAmerican.
OCR examined a years worth of the phys- com/sep2015/relativity-infographic.
ics literature for references to general rela- It is apparent at a glance that Einsteins
tivity or its conceptual offshoots. Specific ideas are still going strong. Thousands of
ally, OCR processed 2,435 abstracts of 2014 papers published every year make reference
physics papers from the arXiv.org repos to his progeny. General relativity seems cer-
itory through a powerful text-analysis pro- tain to continue to be a cornerstone of phys-
gram incorporated into IBMs Watson AI ics in decades to come. When we redo this
system. The software ex tracted keywords data visualization 100 years from now, we
that turned up repeatedly in abstracts from are betting that it will yield the same pointil-
a section of arXiv on general relativity and list explosion of color. The Editors

BOUQUETS OF LIGHT: In this view of the three-dimensional visualization, clusters of colored dots represent groupings
of research papers that cite particular keywords related to new areas of physics that have emerged from Einsteins theory.
To the right, for example, one cluster corresponds to the keywords black hole and event horizon.

56 Scientific American, September 2015 Graphics by the Office for Creative Research

2015 Scientific American

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Top View Each dot equals one Article dots are The size of the The color of the The dot outline
article from arXivs positioned near key dot is based on how dot indicates indicates publication
2014 general rela words (diamonds). often the 2014 number of authors. status. Many physi
tivity and quantum If the text has multiple article has been cists put their papers
cosmology database keywords, the dot is cited since publi- 1 author in arXiv before they
(2,435 total, some placed between terms, cation through the 2 authors are published.
are obscured). such as the article with middle of this year.
3 authors Currently
special relativity and
Noethers theorem. No citations 4 authors published
Some keywords are 5849 authors
linked with red lines. Awaiting peer
85 citations 850+ authors review/publication

Non-euclidean geometry
Non-Euclidean Kerr metric
Antimatter
Wormhole

Accelerating universe
Time travel
Gravitation Closed timelike curve
Gravitational
Exotic singularity
Frame dragging
Quantum Hall eect matter
Noethers theorem Schwarzschild metric

Special relativity

De Sitter space
Symmetry Time
Hubbles law Cosmic microwave Spacetime De Sitter
sitter universe
Big bang background radiation
Dark energy Minkowski space

Lambda-CDM model
Physical cosmology Gravitational wave Binary star
Pulsar
Dark matter Multiverse
Neutron star

Cosmological constant White dwarf

Standard model Supernova
Vacuum energy Schwarzschild
radius
Cosmic ination
Riemannian manifold Black holes
Supersymmetry Hawking radiation Event horizon
Black hole

Cosmology
String theory
Black hole
Loop quantum gravity information paradox

Euclidean space Quantum gravity Black hole

Accretion disk
disc thermodynamics

Hilbert space
Theory of everything
Gauge theories
M-theory

Gravitational constant
Knot theory
Equivalence principle

2015 Scientific American

 Gravitation String theory Spacetime
Physical cosmology

Black hole
When a lot of articles include
the same keyword, the word is
depicted as being higher.

Side View
Red lines descend and converge from
the brilliantly colored surface of the
visualization. Each line stands for an
abstract with at least one of the 61 most
frequently used keywords for general
relativity (not all are shown). The lines
terminate at a point indicating the
number of articles that mention the
listed keyword. Shorter lines correspond
to more frequent mentions.

Number of Articles
Containing a Keyword
500
Black hole

400
Physical cosmology

String theory

Gravitation

300

Dark matter
Spacetime

Special relativity
200 Time

Quantum gravity

Gravitational wave
Big bang
DATA SOURCE: ARXIV.ORG

Standard Model
Hawking radiation
Dark energy
100 Symmetry

September 2015, ScientificAmerican.com 59

2015 Scientific American

 FUNDAMENTAL

CLEANING
P H YS I C S

UPEINSTEIN
AFTER
A new generation of physicists hope
to succeed where Einstein failed
By Corey S. Powell

Leslie Rosenbergs attempt to understand the universe resem-

bles a makeshift home hot-water heater tank, capped with some wires and shoved into
a large, underground refrigerator. The experiment, housed in a laboratory adjacent to
his office at the University of Washington, is a supercooled, magnetized vacuum cham-
ber equipped with a sensitive detector that listens for the microwave ping of passing
particles called axions. These particles are invisible and, so far, entirely hypothetical.
Rosenberg has been on the trail of this particle ever since he was a postdoctoral
researcher at the University of Chicago in the early 1990s. In that time he has per-
formed experiment after experiment, achieving ever greater precision and yet al-
ways the same old empty results, hoping for the positive detection that could res-
cue Albert Einsteins biggestand most star-crossedidea.
Physicists call it the unified field theory, but it is more popularly and evocatively
known as the theory of everything. The idea has been to devise a single formulation
that sums up the behavior of all the known forces of physics. Einstein started this
quest nine decades ago. It bothered the great theorist that the two fundamental
forces guiding the behavior of the universegravity and electromagnetismap-
peared to play by different rules. He wanted to demonstrate that all types of matter
and energy are governed by the same logic.

IN BRIEF

At the end o f his life, Einstein tried He failed, in part because two of Physicists are making the attempt
to create a theory of everything, those forces, the weak and strong, again, starting with data on new
governing all forces in the cosmos. had yet to be discovered. types of particles and fields.

60 Scientific American, September 2015 Photograph by Timothy Archibald

2015 Scientific American

 PARTICLE SEEKERS
location: University
of Washington, Seattle
project: The ADMX detector,
inside a supercooled, magne
tized vacuum chamber, listens
for a microwave ping of
passing particles called axions.
The hypothetical particles
may account for the dark
matter that is supposed to
vastly outweigh visible matter.
who is doing it (back row, from
left): Ciera Cox, Nick Posey,
James Sloan, Clifford Plesha,
Richard Ottens, Josh Povick
and Kerkira Stockton;
(front row, from left): Hannah
LeTourneau, Leslie Rosenberg,
Xavier Frost, Ana Malagon,
Kiva Ramundo and Jacob Herr

2015 Scientific American

 Rolling up the universe into a single formula was a formida- Corey S. Powell is a science writer, blogger
ble ambition, even for Einstein. I want to know how God cre- and editor living in Brooklyn, N.Y. He is a visiting
ated this world, he wrote in an oft-cited 1920 letter to a Ger- scholar at New York Universitys Science, Health
man physics student. I am not interested in this or that phe- and Environmental Reporting Program. Follow
nomenon, in the spectrum of this or that element. I want to him on Twitter @coreyspowell
know his thoughts. The rest are details.
But as the Yiddish proverb goes, Man plans and God
laughs. Einstein pursued Gods thoughts for three decades to
no avail, running down one blind alley after another. When he
died, in 1955, he left behind a set of unsolved unified field equa-
tions scrawled on his blackboard. perhaps the universe contains an unknown class of particles that
The task of unification has fallen to subsequent generations are invisible to all our telescopes.
of physicists, who have broken the problem into myriad parts. Possibility number one is shunned by the vast majority of
What started as the grand vision of a singular genius has physicists because it is ad hoc, and it is also difficult to recon-
morphed into slow, grinding labor carried out by different cile with measurements of how galaxies move. The scientific
teams of physicists, each trying to solve a small piece of a vast mainstream has therefore lined up behind possibility number
cosmic puzzle. Rosenberg, for instance, does not obsess over an two, instigating dozens of crafty efforts to unmask the unseen
all-encompassing theory of everything. He is focused on his one dark particles. Which is where ADMX comes in.
vexing and specific problem: the axion. It has theoretical prop- Axions nicely match the inferred properties of dark matter, so
erties that could wipe away the need to modify Einsteins
equations of gravity. Well see what the data say, Rosen-
berg notes. I dont want to look into the mind of God.
Despite their narrow focus, Rosenberg and his compa-
triots have not taken their eyes off the prize. They are en-
gaged in a broader effort to hammer out flaws in the the-
oretical edifice that Einstein created and to build a more
complete model of particle physics from the ground up,
rather than from the top down. They seek to push the sci-
ence forward by finding out how nature really behaves,
not how scientists think it should (an approach that
Rosenberg dismisses as navel gazing). Other research-
ers are designing experiments to reveal a hidden aspect
of physics called dark energy or to detect two-dimension-
al quantum units that could be building blocks of our ap-
parently three-dimensional existence. Their hard data
may be just what todays physicists need to succeed where
Einstein failed.
We could actually test some of these crazy ideas
about the evolution of the universe, says physicist Josh-
ua Frieman of the University of Chicago. Almost certain-
ly, he believes, physicists will not get to a theory of every-
thing without them.

DARK SIDE OF THE UNIVERSE

A look at Rosenbergs Axion Dark Matter eXperiment
(ADMX) reveals the power of the small-is-beautiful ap-
proach. In its seemingly modest search for just one parti-
cle and one new set of physics rules, ADMX could also re-
fute concerns about general relativity and solve a major
cosmology puzzle in the process.
That puzzle dates back to the 1930s, when astronomers
began to realize that the universe appeared to be full of
some unseen component that makes its presence known
only by its gravitational pull on the visible stars. The dis-
covery turned even stranger in the 1980s, when new mod-
els of the big bang showed that the invisible (or dark)
stuffwhatever it iscould not consist of ordinary atoms.
That left two unsettling possibilities. Perhaps gravity does
not work the way Einstein thought it did at large scales, or

62 Scientific American, September 2015 Photograph by Sandy Nicholson

2015 Scientific American

if Rosenberg and his ADMX team detect them, they would pro- odd duck out, Rosenberg admits cheerily. Then the various
vide a more complete picture of how galaxies have formed and WIMP detectors kept getting better and better, without finding
evolved. They would also do away with the need to make ugly anything. The watershed moment came last year, when an ultra-
modifications to some of Einsteins gravity equations. Above all, sensitive WIMP finder called Large Underground Xenon (LUX),
axions would force a revision of the Stan- beneath the hills of South Dakota, switched
dard Model of particle physics. That model on. So far it, too, has come up empty.
is a comprehensive, yet clearly incomplete, Now is the make-or-break moment for
theory of fundamental particles and fields. HOLOGRAM HUNTERS Rosenberg to prove that axions are the an-
Finding the axion would validate a much de- location: Fermi National swer and to shore up general relativity
bated elaboration of the Standard Model, Accelerator Laboratory Einsteins idea that gravity comes from a
bringing physicists one step closer to a true (Fermilab), Batavia, Ill. curvature of spacetimein the process. The
theory of everything. project: The Holometer
concept behind ADMX is deliciously straight
Until recently, axions were considered a experiment will look for forward. If dark matter really consists of
long shot in the search for dark matter. Most tiny changes in a laser beam particles, there must be a continuous wind
of Rosenbergs colleagues were focusing sent down two perpendicular of them blowing through the earth and ev-
their attention on another class of particles pathways that suggest space erything on it (including you) all the time.
called WIMPs (weakly interacting massive and time are made of funda And if those particles are axions, theoreti-
particles), which were considered more the- mental quantum units cally they will very occasionally decay. The
a theory known as the
oretically attractive. I was always a little particles themselves are invisible, but in
holographic principle.
who is doing it (from left):
Sam Waldman, Ohkyung
Kwon, Robert Lanza,
Aaron Chou, Craig Hogan,
Ray Tomlin, Stephen Meyer,
Brittany Kamai, Lee McCuller,
Jonathan Richardson, Chris
Stoughton, Rainier Weiss
and Richard Gustafson

September 2015, ScientificAmerican.com63

2015 Scientific American

STRING THEORISTS
location: Stanford University,
Stanford, Calif.
project: String theorists attempt
to unify all of natures forces into
a single framework by thinking
of both particles and forces as
vibrations of loops of string.
Some versions of the theory
make predictions about the
beginning of the universe that
might manifest in imprints in the
radiation we can observe from that rare decay process, they should listen for; the frequency of the signal depends on the mass of the
the far reaches of the cosmos. turn into microwaves, which would axion, which is of course unknown.
produce a weak but detectable sig- The only way around this problem is to hop through the mi-
who is doing it (from left):
nal. Straightforward, yes, but diffi- crowave band frequency by frequency; the entire ADMX endeav-
Andrei Linde, Renata Kallosh,
Ahmed Almheiri, Leonard cult to execute in practice. or is essentially a process of flipping channels on a CB radio.
Susskind, Shamit Kachru, We have a cavity the size of an oil Rosenberg lights up when I offer that analogy: Ive always had
Patrick Hayden and drum, Rosenberg says, and its this interest in radio electronics. I played with the radio as a kid,
Lampros Lamprou cooled to 100 millikelvins, which is bouncing signals off the moon. Now were looking at signals us-
0.1 degree above absolute zero. The ing receivers so sensitive they could get four bars of cell-phone
extremely low temperature ensures reception on Mars! He is also proud that ADMX, unlike Ein-
that the detector itself produces al- steins endless explorations of the unified field theory, is guaran-
most no microwave noise. Next the cavity is magnetized to stim- teed to yield a concrete answer.
ulate the decay of axions. Then a small, pencil-shaped probe lis- By 2018 we will have completely covered the definitive
tens in for some microwaves that should not be there. Adding to search region for the axion, Rosenberg says. At that point, its
the challenge, nobody knows exactly what kind of microwaves to either there, or it isnt. In other words, we will have either a big

64 Scientific American, September 2015 Photograph by Timothy Archibald

2015 Scientific American

 tom-built camera strapped to the Blanco four-meter telescope
atop Cerro Tololo, a towering peak in Chile rising more than two
kilometers above sea level. The idea is to gather a vast number of
pictures of distant galaxies. Each image from the camera con-
tains 570 megapixels, a huge amount of data, and it will collect
about 400 images a night, 105 nights a year, over five years total.
The project is callednot surprisinglythe Dark Energy Survey,
and when it is complete in February 2018, the survey will have
examined 300 million galaxies and about 4,000 supernova ex-
plosions. (For comparison, a state-of-the-art automated superno-
va search conducted at the University of California, Berkeley,
from 1998 to 2000 turned up a grand total of 96.)
Like Rosenberg, Frieman used to work as a theorist but got
pulled over to the observational side by the idea of designing
actual tests. Now he has to confront the realities of the task.
Taking the data is hard, he says. Processing the data is hard.
Frieman and his team pick apart observations from the sur-
vey four different ways, each one designed to capture a specific
aspect of how dark energy behaves. One analysis zeroes in on a
class of exploding stars called type Ia supernovae, which act as
mile markers in space. Their brightness indicates their distance,
and their color indicates how quickly they are moving away from
us. Put together a bunch of those mile markers, and you get a
sense of how the expansion of the universe has been changing
over time. The other three kinds of analyses explore various pat-
terns of how galaxies cluster. Gravity tends to pull everything to-
gether, and dark energy tends to push everything apart. Map-

Weird as they are, dark matter

and dark energy can be thought
of as garnishes atop a reality that
Einstein would have recognized.
But what if that reality needs
adjustment to make progress?

new clue about how to build a theory of everything or one more ping how galaxy clusters change over cosmic time therefore re-
idea to scratch off the list. veals the intensity of the dark energy effect.
In the simplest models of dark energy, it is an unchanging
ENERGY OF EMPTY SPACE and ubiquitous feature of empty space. It turns out that the stan-
While Rosenberg whittles away a t the problem of dark matter, dard theories of particle physics can account for the existence of
other researchers are working toward a complete picture of such an energy; they just predict a value 10120 times too large. (It
physics by going after the other major unseen aspect of the uni- is sometimes called the worst prediction in all of physics.) Ac-
verse: dark energy. It is the opposite of dark matter in its effect, counting for the real, drastically smaller value of dark energy is
producing a repulsive force rather than a gravitational attrac- one of the most important tests for a prospective theory of every-
tion. Because dark energy counters the action of gravity, it has thing. Astronomers also do not know yet whether dark energy is
direct implications for how to interpret the equations of gener- truly constant. If Frieman finds that it changes over time, that is
al relativity. More profoundly, dark energy cannot be explained another thing that a theory of everything must explain.
within the current model of particle physics. It therefore pro- Before we reach that point, though, there is a more basic is-
vides a critical test for any would-be theory of everything. sue to settle. Our assumption is that dark energy is whats
One such test is being run by Chicagos Frieman. It uses a cus- driving the accelerated expansion, but we dont know that for

September 2015, ScientificAmerican.com65

2015 Scientific American

sure. It could be that on the largest scales, general relativity just the scale at which Hogan thinks the graini-
isnt the correct theory, Frieman says. A modified version of ness of space might show up. A full answer
relativity could potentially mimic the dark energy effect, some- could come within a year,, he predicts, and
thing that he will be investigating closely. One way or another, then something will happenhe is just not
there must be a theory that goes beyond Einsteins, and the sure what: If we dont see something, or we
Dark Energy Survey will help find it. do see something, either way its going to con-
strain peoples ideas. Nobody knows what the
IS LIFE A HOLOGRAM? hell to expect.
Weird as they are, d  ark matter and dark energy can still be
thought of as garnishes on the universe as we know it: an icing EINSTEINS DREAM, CONTINUED
of additional particles or fields atop the kind of reality that Ein- After Hogans comments, I was eager to speak
stein would have readily recognized. But what if that reality with Susskind to hear his take. Contrary to the
needs adjustment to make progress toward a more sweeping stereotype of the pensive, math-obsessed theo-
theory? What if spacetime itself has new, undetected properties rist, Susskind quickly launches into a discus-
that are not described by general relativity? sion about testable concepts. People
Craig Hogan, director of the Center for Particle Astrophysics bitch about theoretical physicists be-
at Fermi National Accelerator Laboratory, is exploring that ing frivolous with their ideas because
head-scratcher with an experiment he calls the Holometer. His they dont face the issues of falsifi- COSMIC CRUSADERS
quest is to find out whether space and time are constructed out ability. Thats nonsense. We all are location: Perimeter Institute
of fundamental units: a universe inherently built around ticks very concerned about falsifiability, for Theoretical Physics,
of time and marks on the ruler. In this alternative view, our he says. But if there is a laboratory Waterloo, Ontario
sense of living in a three-dimensional universe is an illusion. If test, he contends, the Holometer is
project: Theorists are explor
you could magnify space sufficientlydown to 10 trillion tril- not how to do it. ing other ways to understand
lion times as small as an atomyou would see two-dimensional A better bet, Susskind says, is to the whole universe within a
pixels that look three-dimensional only when viewed from a look to the edge of the observable single framework. One idea
large-scale perspective, like the dots on a television screen. universe for behavior that supports is that Einsteins relativity of
Each of those units would follow quantum rules, such as string theory. In this theory, all par- time can be replaced by a
having an amount of built-in uncertainty about its location. At ticles and forces are different modes relativity of size, leading to
a reformulation of general
large scales, space would appear continuous, as Einstein be- of vibrations in wiggling strings of
relativity in which time and
lieved, but it would have an underlying quantum structure. In energy, which makes it a unified ex
shape are meaningful but
this way, a pixelated universe would force quantum mechanics planation for all of them. (These size is not.
into relativity, removing a key obstacle to creating a unified strings are different from cosmic
who is doing it (back row,
theory of physics. strings, which may be defects in
from right): Daniel Carrasco
The idea of the apparent 3-D universe emerging from a 2-D spacetime.) It also makes predic-
Guariento, Gabriel Herczeg,
reality is known as the holographic principle, hence the name tions about physical conditions at Flavio Mercati, Sean Gryb
of Hogans experiment. Holometer is also something of a pun, the time of the big bang. More re- and Hamish Forbes;
riffing on the name of a 16th-century precision surveying de- markable, some versionsthe ones (front row, from right): Niall
vice. Hogans instrument, now collecting data at Fermilab, is Susskind works onmake predic- Murchadha, Henrique Gomes,
similarly designed to measure the lay of the land with unprece- tions about conditions at an even Andrea Napoletano, Julian
dented accuracy. It consists of a laser beam that is split in two, earlier stage, before our universe Barbour and Lee Smolin
sent down different tunnels, bounced off a mirror and then re- was born. Susskind believes astron-
combined. If space has a quantum structure, the uncertainty of omers might be able to identify evi-
the location associated with each pixel should create a jitter dence of that prior existence im-
within the device; that jitter would shift the two halves of the printed on radiation from the far reaches of our universe.
beam and knock them out of sync. In principle, the Holometer More likely, though, he believes the next strides toward the
can measure movements at the attometer scale: 1018 meter! unification of physics will come not from experiment or obser-
That may not be small enough, however. Any underlying quan- vation but from intense mathematical explorations of black
tum structure of space could be even more minuscule, far too sub- holes and space and time. Important things are going to hap-
tle to detect experimentally, some of Hogans colleagues have pen over the next five to 10 years, Susskind predicts. I dont
warned him. He took their skepticism as a dare. As we talk, he say were going to have a complete theory of everything; were
seems especially tickled by how acutely his experiment irks Leon- not even close. But there are going to be major insights into the
ard Susskind of Stanford University, one of the primary develop- connection between gravity and quantum mechanics.
ers of the holographic universe concept. Lenny has an idea of When that connection is revealed, Susskindlike most of to-
how the holographic principle works, and this isnt it. Hes pretty days theoristsexpects that quantum mechanics will come out
sure that were not going to see anything. We were at a confer- on top, with gravity and general relativity forced to live within its
ence last year, and he said that he would slit his throat if we saw framework. But because Einstein was the one who started us
this effect, Hogan recalls. down this path, it seems only fair to give the final word to one of
Their dispute should be settled soon. After collecting one todays leading Einsteinians, physicist Lee Smolin of the Perime-
hour of data, the Holometer is approaching Planck sensitivity, ter Institute for Theoretical Physics in Ontario.

66 Scientific American, September 2015

2015 Scientific American

 Smolin is convinced that many of his quantum-obsessed col- Smolin does not intend to sound mystical, but in some way he
leagues are literally thinking too small in their pursuit of an ulti- seems to be talking not about the physical universe but about the
mate theory. Quantum mechanics is only sensible as a theory of spirit of Einstein. One century ago a single man revealed a novel
a subsystem, he says, but general relativity is not a description way to think about the universe. Sixty years ago that life was
of subsystems. It is a description of the universe as a closed sys- snuffed out, as all human lives are sooner or later. But the mind
tem. If you want to understand the universe as a whole, then of Einstein still leaves a distinct imprint on todays researchers.
you have to think of it as Einstein did, in relativistic terms. They run new experiments in the service of an old ideal. The im-
That approach has led Smolin to the startling hypothesis that pulse seems unstoppable, as they keep recapitulating his search
the laws of physics may evolve over time and that the universe for a deeper truth, a higher enlightenment.
has a memory of its own historywhat he calls the principle of
precedence. In this way, he envisions moving beyond specific,
M O R E TO E X P L O R E
unexplained details of quantum mechanics (the strength of this
particular field or the mass of that particular particle) and re- Search for Hidden Sector Photons with ADMX Detector. A  . Wagner et al. in
Physical Review Letters, V
 ol. 105, No. 17, Article No. 171801; October 19, 2010.
garding them all as developmental aspects of the single, closed-
Time Reborn: From the Crisis in Physics to the Future of the Universe. L ee Smolin.
system universe. He even has a notion of how to test his idea. Houghton Mifflin Harcourt, 2013.
If we could evolve a system that is large and complex but still Mass and Galaxy Distributions of Four Massive Galaxy Clusters from Dark
described by a pure quantum state, we would force nature to in- Energy Survey Science Verification Data. P . Melchior et al. in M  onthly Notices
vent some novel systematics. We could imagine doing that with of the Royal Astronomical Society, V  ol. 449, No. 3, pages 22192238; May 21, 2015.
quantum devices, Smolin says. After creating the same system FROM OUR ARCHIVES
over and over in the lab, nature might start to develop a prefer-
On the Generalized Theory of Gravitation. A
 lbert Einstein; April 1950.
ence for a certain quantum state. It would be hard to distinguish
from the noises of experimental practice. But not impossible. s c i e n t i f i c a m e r i c a n . c o m /m a g a z i n e /s a

Photograph by Sandy Nicholson September 2015, ScientificAmerican.com67

2015 Scientific American

 TIME
THEORY

A BRIEF
HISTORY OF

TRAVEL We already have the means to skip ahead in time,

but going backward is a different wormhole
By Tim Folger

I found myself engulfed

in a turbulent wormhole.

68 Scientific American, September 2015

2015 Scientific American

 Luckily I was able to find a way out ...

... And to my relief came upon

the spacecraft I had left behind.

I drifted closer
to the window,
hoping to see
the face of
my commander.

?!

But instead found a younger version

of myself staring back at me!

Illustration by Asaf Hanuka September 2015, ScientificAmerican.com69

2015 Scientific American

 Tim Folger writes for National Geographic, Discover a nd
other national publications. He is also the series editor for
The Best American Science and Nature Writing, an annual
anthology published by Houghton Mifflin Harcourt.

H. G. Wells published h is first novel, T he Time

Machine, in 1895, just a few years before Queen Victo-
rias six-decade reign over the U.K. ended. An even
more durable dynasty was also drawing to a close: the
200-year-old Newtonian era of physics. In 1905 Albert
Einstein published his special theory of relativity,
which upset Isaac Newtons applecart and, to Wellss
presumed delight, allowed something that had been
impossible under Newtons laws: time travel into the
future. In Newtons universe, time was steady every- Betelgeusea star that is about 520 light-years
from Earthat 99.995 percent the speed of light,
where and everywhen; it never sped up or slowed
by the time he returned to Earth he would be
down. But for Einstein, time was relative. only 10 years older. Sadly, everyone he knew
would be long dead because 1,000 years would
have passed on Earth; it would be the year 3015.
Time travel is not only possible, it has already happened, Time travel to the future, we know we can do, Gott says. Its
though not exactly as Wells imagined. The biggest time traveler to just a matter of money and engineering!
date is Sergei K. Krikalev, according to J. Richard Gott, an astro- Jumping a few nanosecondsor centuriesinto the future
physicist at Princeton University. Over the course of his is relatively straightforward, despite practical chal-
long career, which began in 1985, the Russian cos- lenges. But going backward in time is harder.
monaut spent a little over 803 days in space. Einsteins special theory of relativity for-
As Einstein proved, time passes more slow- bade it. After another decade of work,
ly for objects in motion than for those Einstein unveiled his general theory of
at rest, so as Krikalev hurtled along at relativity, which finally lifted that re-
17,000 miles an hour onboard the striction. How someone would actu-
Mir s pace station, time did not flow ally travel back in time, however, is
at the same rate for him as it did a vexing problem because the
on Earth. While Krikalev was in equations of general relativity
orbit, he aged 148 of a second less have many solutions. Different so-
than his fellow earthlings. From lutions assign different qualities
another perspective, he traveled to the universeand only s ome o f
1 of a second into the future. the solutions create conditions that
48
The time-travel effect is much permit time travel into the past.
easier to see with longer distances Whether any of those solutions
and higher speeds. If Krikalev left describes our own universe is an open
Earth in 2015 and made a round-trip to question, which raises even more pro-

IN BRIEF

Traveling very fast a llows you to go for- A wormhole is one such curve. You Closed timelike curves are useful for Quantum mechanicsand indeed, the
ward in time. Traveling backward in time would enter it through a spherical open- testing theories about the cosmos. For nature of the universe itselfmight for-
is much harder, but mathematics says it ing. Once inside, everything you ob- example, if one were present at the start bid wormholes and therefore prevent
is possible through geometric structures served in space would be normal and so of our universe, it could have allowed the backward time travel. Physicists just do
called closed timelike curves. would the passage of time. universe to create itself. not know yet if this is the case.

70 Scientific American, September 2015 Illustrations by Nigel Holmes

2015 Scientific American

 found investigations: Just how much tweaking of fundamental ticians of the 20th centuryand one of the oddest. His many
physics would it take to allow backward time travel? Does the foibles included a diet of baby food and laxatives.
universe itself somehow prevent such journeys even if Ein- Gdel presented this model universe as a gift to Einstein on
steins equations do not rule them out? Physicists continue to his 70th birthday. The universe Gdel described to his skeptical
speculate, not because they imagine time travel will ever be friend had two unique properties: It rotated, which provided
practical but because thinking about the possibility has led to centrifugal force that prevented gravity from crunching togeth-
some surprising insights about the nature of the universe we in- er all the matter in the cosmos, creating the stability Einstein
habit, including, perhaps, how it came to be in the first place. demanded of any cosmic model. But it also allowed for time
travel into the past, which made Einstein deeply uneasy. In
A NEW WAY OF LOOKING AT TIME Gdels cosmos, space travelers could set out and eventually
With his special theory o f relativity, Einstein made time mallea- reach a point in their own past, as if the travelers had complet-
ble in a way that must have pleased Wells, who presciently be- ed a circuit around the surface of a giant cylinder. Physicists call
lieved that we inhabit a universe in which three-dimensional these trajectories in spacetime closed timelike curves.
space and time are knit together into a four-dimensional whole. A closed timelike curve is any path through spacetime that
Einstein arrived at his revolutionary results by exploring the im- loops back on itself. In Gdels rotating cosmos, such a curve
plications of two fundamental ideas. First, he argued that even would circle around the entire universe, like a latitude line on
though all motion is relative, the laws of physics must look the Earths surface. Physicists have concocted a number of different
same for everyone anywhere in the universe. Second, he types of closed timelike curves, all of which allow travel to
realized that the speed of light must be similarly the past, at least in theory. A journey along any of
unchanging from all perspectives: if everyone them would be disappointingly ordinary, how-
sees the same laws of physics operating, ever: Through the portholes of your space-
they must also arrive at the same result ship, you would see stars and planetsall
when measuring the speed of light. the usual sights of deep space. More im-
To make light a universal speed portant, timeas measured by your
limit, Einstein had to jettison two own clockswould tick forward in
commonsense notions: that all ob- the usual way; the hands of a clock
servers would agree on the mea- would not start spinning backward
surement of a given length and that even though you would be traveling
they would also agree on the dura- to a location in spacetime that existed
tion of times passage. He showed in your past.
that a clock in motion, whizzing past Einstein was already aware of the
someone at rest, would tick more slow- possibility of closed timelike curves
ly than a stationary clock at the persons back in 1914, says Julian Barbour, an in-
side. And the length of a ruler moving dependent theoretical physicist who lives
swiftly by would shorten. Yet for anyone who near Oxford, England. As Barbour recalls,
was traveling at the same speed as the clock and
ruler, the passage of time and the length of the rul-
er would appear normal.
At ordinary speeds, the time-and-space-dis-
torting effects of special relativity are negligible.
But for anything moving at a hefty fraction of the
speed of light, they are very real. For example,
many experiments have confirmed that the decay
rate of unstable particles called muons slows by
an order of magnitude when they are traveling at
close to the speed of light. The speeding muons,
in effect, are minuscule time travelerssubatom-
ic Krikalevshopping a few nanoseconds into
the future.

GDELS STRANGE UNIVERSE

Those speedy clocks a nd rulers and muons are all
racing forward in time. Can they be thrown into
reverse? The first person to use general relativity
to describe a universe that permits time travel
into the past was Kurt Gdel, the famed creator
of the incompleteness theorems, which set limits
GETTY IMAGES

on the scope of what mathematics can and can- TIME FOR LUNCH: After more than 803 days hurtling through space,
not prove. He was one of the towering mathema- Sergei K. Krikalev (left) had traveled 148 of a second into the future.

September 2015, ScientificAmerican.com71

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Einstein said, My intuition strives most vehemently against energy in a volume of space spontaneously fluctuates to less
this. The curves existence would create all kinds of problems than zero. Without negative energy, a wormholes spherical en-
with causalityhow can the past be changed if it has already trance and four-dimensional tunnel would instantaneously im-
happened? And there is the hoary grandfather paradox: What plode. But a wormhole held open by negative energy seems to
happens to a time traveler who kills his or her grandfather be- be hard, probably impossible, Carroll says. Negative energies
fore the grandfather meets the grandmother? Would the de- seem to be a bad thing in physics.
mented traveler ever be born? Even if negative energy kept a wormhole open, just when
Fortunately for fans of causality, astronomers have found no you would be on the verge of turning that into a time machine,
evidence that the universe is rotating. Gdel himself apparently particles would be moving through the wormhole, and every
pored over catalogs of galaxies, looking for clues that his theory particle would loop back around an infinite number of times,
might be true. Gdel might not have devised a realistic model of Carroll says. That leads to an infinite amount of energy. Be-
the universe, but he did prove that closed timelike curves are cause energy deforms spacetime, the entire thing would col-
completely consistent with the equations of general relativity. lapse into a black holean infinitely dense point in spacetime.
The laws of physics do not rule out traveling to the past. Were not 100 percent sure that that happens, Carroll says.
But it seems to be a reasonable possibility that the universe is
AN ANNOYING POSSIBILITY actually preventing you from making a time machine by mak-
Over the past few decades cosmologists have used Einsteins equa- ing a black hole instead.
tions to construct a variety of closed timelike curves. Gdel Unlike black holes, which are a natural consequence of
conjured an entire universe that allowed them, but general relativity, wormholes and closed timelike
more recent enthusiasts have warped space- curves in general are completely artificial con-
time only within parts of our universe. structsa way of testing the bounds of the
In general relativity, planets, stars, theory. Black holes are hard to avoid,
galaxies and other massive bodies warp Carroll says. Closed timelike curves are
spacetime. Warped spacetime, in turn, very hard to make.
guides the motions of those massive Even if wormholes are physically
bodies. As the late physicist John implausible, it is significant that they
Wheeler put it, Spacetime tells mat- fit in with the general theory of rela-
ter how to move; matter tells space- tivity. Its very curious that we can
time how to curve. In extreme cases, come so close to ruling out the possi-
spacetime might bend enough to bility of time travel, yet we just cant
create a path from the present back to do it. I also think that its annoying,
the past. Carroll says, exasperated that Einsteins
Physicists have proposed some exotic beautiful theory might allow for something
mechanisms to create such paths. In a 1991 so seemingly implausible. But by contemplat-
paper, Gott showed how cosmic stringsinfinite- ing that annoying possibility, physicists may gain
ly long structures thinner than an atom that may have a better understanding of the kind of universe we live
formed in the early universewould allow closed timelike in. And it may be that if the universe did not permit backward
curves where two strings intersected. In 1983 Kip S. Thorne, a time travel, it never would have come into existence.
physicist at the California Institute of Technology, began to ex-
plore the possibility that a type of closed timelike curve called a DID THE UNIVERSE CREATE ITSELF?
wormholea kind of tunnel joining two different locations in General relativity describes the universe on the largest scales.
spacetimemight allow for time travel into the past. In general But quantum mechanics provides the operating manual for the
relativity, if you connect two different regions of space, youre atomic scale, and it offers another possible venue for closed
also connecting two different regions of time, says Sean M. Car- timelike curvesone that gets at the origin of the universe.
roll, a colleague of Thornes at Caltech. On a very small scale1030 centimeteryou might expect
The entrance into a wormhole would be sphericala three- the topology of spacetime to fluctuate, and random fluctuations
dimensional entrance into a four-dimensional tunnel in space- might give you closed timelike curves if nothing fundamental
time. As is the case with all closed timelike curves, a trip through prevents them, says John Friedman, a physicist at the Universi-
a wormhole would be like any other journey, Carroll says. Its ty of WisconsinMilwaukee. Could those quantum fluctuations
not that you disappear and are reassembled at some other mo- somehow be magnified and harnessed as time machines?
ment of time. There is no respectable theory where that kind of Theres certainly no formal proof that you c ant h  ave macro-
science-fiction time travel is possible. For all travelers, he adds, scopic closed timelike curves, Friedman says. But the commu-
no matter what they do, time flows forward at one second per nity of people who have looked at these general questions would
second. Its just that your local version of forward might be bet pretty heavily against it.
globally out of sync with the rest of the universe. There is no doubt that the creation of a loop in spacetime on
Although physicists can write equations that describe worm- either a quantum scale or a cosmic one would require some very
holes and other closed timelike curves, all the models have seri- extreme physics. And the most likely place to expect extreme
ous problems. Just to get a wormhole in the first place, you physics, Gott says, is at the very beginning of the universe.
need negative energy, Carroll says. Negative energy is when the In 1998 Gott and Li-Xin Li, an astrophysicist now at Peking

72 Scientific American, September 2015

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University in China, published a paper in which they argued transformed into a queen.You would say thats against the
that closed timelike curves were not merely possible but essen- rules, Gott says. You cant change your pawn into a queen.
tial to explain the origin of the universe. We investigated the Well, yes, you can! You just never saw a game that extreme be-
possibility of whether the universe could be its own mother fore. Time-travel research is like that. Were testing the laws of
whether a time loop at the beginning of the universe would al- physics by looking at extreme conditions. Theres nothing logi-
low the universe to create itself, Gott says. cally impossible about time travel to the past; its just not the
Gott and Lis universe starts with a bout of inflationjust universe were used to. Turning a pawn into a queen could be
as in standard big bang cosmology, where an all-pervasive en- part of the rules of relativity.
ergy field drove the universes initial expansion. Many cosmol- Such wildly speculative ideas may be closer to philosophy
ogists now believe that inflation gave rise to countless other than to physics. But for now, quantum mechanics and general
universes besides our own. Inflation is very hard to stop once relativitypowerful, counterintuitive theoriesare all we have
it gets started, Gott says. It makes an infinitely branching to figure out the universe. As soon as people start trying to
tree. Were one of the branches. But you have to ask yourself, bring quantum theory and general relativity into this, the first
Where did the trunk come from? Li-Xin Li and I said it could thing to say is that they really have no idea what theyre doing,
be that one of the branches just loops around and grows up to says Tim Maudlin, a philosopher of science at New York Uni-
be the trunk. versity. Its not really rigorous mathematics. Its one piece of
A simple two-dimensional sketch of Gott and Lis self-start- mathematics that sort of looks like general relativity and an-
ing universe looks like the number 6, with the spacetime loop other little piece of mathematics that sort of looks like quan-
at the bottom and our present-era universe as the top tum theory, mixed together in some not entirely co-
stem. A burst of inflation, Gott and Li theorized, herent way. But this is what people have to do
allowed the universe to escape from the time because they honestly dont know how to go
loop and expand into the cosmos we in- forward in a way that makes sense.
habit today. Will some future theory eliminate
It is difficult to contemplate the the possibility of time travel into the
model, but its main appeal, Gott says, past? Or will the universe again turn
is that it eliminates the need for cre- out to be far stranger than we imag-
ating a universe out of nothing. Yet ine? Physics has advanced tremen-
Alexander Vilenkin of Tufts Univer- dously since Einstein redefined our
sity, Stephen Hawking of the Uni- understanding of time. Time travel,
versity of Cambridge and James which existed only in the realm
Hartle of the University of California, of fiction for Wells, is now a proved
Santa Barbara, have proposed models reality, at least in one direction. Is it
in which the universe does indeed arise too hard to believe that some kind of
out of nothing. According to the laws of symmetry exists in the universe, allow-
quantum mechanics, empty space is not re- ing us to travel backward in time? When I
ally empty but is filled with virtual particles put the question to Gott, he replies with
that spontaneously pop into and out of existence. an anecdote:
Hawking and his colleagues theorized that the universe Theres a story where Einstein was talking to a guy. The
burst into being from the same quantum-vacuum stew. But in guy pulled a notebook out and scribbled something down. Ein-
Gotts view, the universe is not made out of nothing; it is made stein says, Whats that? The guy says, A notebook. Whenever I
out of somethingitself. have a good idea, I write it down. Einstein says, Ive never had
any need for a notebook; Ive only had three good ideas.
A COSMIC CHESS GAME Gott concludes: I think were waiting for a new good idea.
For now, there is no way t o test whether any of those theories
might actually explain the origin of the universe. The famed M O R E TO E X P L O R E
physicist Richard Feynman compared the universe to a great
Rules for Time Travelers. S ean Carroll. Published online May 14, 2009. 
chess game being played by the gods. Scientists, he said, are www.preposterousuniverse.com/blog/2009/05/14/rules-for-time-travelers
trying to understand the game without knowing the rules. We Time Travel and Modern Physics. R evised. Frank Arntzenius and Tim Maudlin
watch as the gods move a pawn one space for- in Stanford Encyclopedia of Philosophy. P ublished online December 23, 2009. 
ward, and we learn a rule: pawns al- http://plato.stanford.edu/entries/time-travel-phys
ways move one space forward. But Make a Cosmic Ray Detector at Home and Test Relativity! E volutionEvidence.org
in F rom Quarks to Quasars. Published online April 20, 2014. w
 ww.fromquarks
what if we never saw the open- toquasars.com/make-a-cosmic-ray-detector-at-home-and-test-relativity
ing of a game, when a pawn When Einstein Met H. G. Wells: Encounters in the Fourth Dimension. E than
can move two spaces for- Siegel in M edium. Published online January 13, 2015. h ttps://medium.com/
ward? We might also as- starts-with-a-bang/when-einstein-met-h-g-wells-425372d21821
sume, mistakenly, that pawns FROM OUR ARCHIVES
always remain pawnsthat
How to Build a Time Machine. P aul Davies; September 2002.
they never change their
identityuntil we see a pawn s c i e n t i f i c a m e r i c a n . c o m /m a g a z i n e /s a

September 2015, ScientificAmerican.com73

2015 Scientific American

General relativity has never been tested
in places where the effects of gravity become
truly extremefor example, at the edge
of a black hole. That will soon change
By Dimitrios Psaltis and Sheperd S. Doeleman

THE
BLACK
A ST R O N O M Y

HOLETEST 2015 Scientific American

 MATTER FALLING i nto a black
hole, as shown in this simulation,
should create observable pheno
mena that can be used to test
Einsteins theory of gravity.

IN BRIEF

Einsteins general theory of relativity The Event Horizon Telescope (EHT), a These observations will explore wheth- If the EHT detects deviations from
has stood firm for a century, but it has global network of radio telescopes, er Sagittarius A* is a black hole or an Einsteins predictions, other instru-
never been tested in places where will perform such tests by resolving the exotic object such as a naked singulari- ments that come online in the next
gravity is extremely strong, such as event horizon of Sagittarius A*, the black ty. If it is a black hole, does it behave the several years will be able to inde
the edge of a black hole. hole at the center of the Milky Way. way general relativity says it should? pendently check those results.

September 2015, ScientificAmerican.com75

2015 Scientific American

Scientists have been trying u nsuc- Dimitrios Psaltis is a professor of astronomy and
cessfully to poke holes in Albert Einsteins physics at the University of Arizona. He has pioneered
the development of tests of Einsteins general theory
general theory of relativity for a full century. of relativity in strong gravitational fields using
So far, however, Einsteins theory has had observations of black holes and neutron stars in the
electromagnetic spectrum.
it easy. Every assessment to date has been
conducted in rather weak gravitational fields. Sheperd S. Doeleman is an astronomer at the
Massachusetts Institute of Technology and the Harvard-
To put general relativity to its greatest test, we Smithsonian Center for Astrophysics, where he leads teams
need to see whether it holds up where gravity that make ultrahigh-resolution observations of black holes.
He coordinates the Event Horizon Telescope project.
is extremely strong. And nowhere in the
universe today is gravity stronger than at the
edge of a black holeat the event horizon, the
boundary beyond which gravity is so over-
whelming that light and matter that pass tions possible from the surface of Earth, the EHT exploits a tech-
nique known as very long baseline interferometry (VLBI), in
through can never escape. which astronomers at radio dishes across the globe observe the
same target simultaneously, record the data they collect on hard
The interior of a black hole is unobservable, but the gravita- drives, and then later combine all those data using a supercom-
tional field surrounding these objects causes matter close to the puter to form a single image. By doing so, many telescopes locat-
horizon to produce huge amounts of electromagnetic radiation ed on different continents can form one virtual Earth-sized tele-
that telescopes can detect. Near the black hole, the crushing scope. The resolving power of a telescope is given by the ratio of
force of gravity compresses inflowing matter, known as the ac- the wavelength of light it observes to its size, and so VLBI rou-
cretion flow, into ever smaller volumes. This causes the infalling tinely makes images of the radio sky with detail that far surpass-
matter to reach temperatures of billions of degreeswhich, iron- es the magnifying power of any optical telescope.
ically, makes the vicinity immediately surrounding a black hole By advancing the technologies used in VLBI so that observa-
one of the brightest spots in the cosmos. tions can be made at the shortest radio wavelengths, the EHT
If we could observe a black hole with a telescope with enough will soon be able to meet all the challenges of black hole imaging.
magnifying power to resolve the event horizon, we could follow At these wavelengths (close to one millimeter in size), the Milky
matter as it spirals down toward the point of no return and see Way is largely transparent, enabling the EHT to observe Sagittar-
whether it behaves as general relativity says it should. There is, ius A* with a minimum of blurring from the intervening gas.
of course, a catch: developing a telescope that can resolve a These same wavelengths are also able to pierce the matter falling
black hole horizon poses several challenges. Notably we have to toward the black hole, allowing access to the innermost regions
contend with the black holes tiny size when viewed from Earth. surrounding Sagittarius A*s event horizon. And in a true Goldi-
Even the supermassive black holes now thought to inhabit the locks coincidence, the magnifying power of a globe-spanning
centers of most galaxies, which weigh in at millions or billions VLBI array at millimeter wavelengths is well suited to resolving
of our suns mass and in some cases have diameters larger than the event horizons of the nearest supermassive black holes.
our solar system, are so far away from Earth that they subtend In a parallel development, theoretical astrophysicists have
incredibly tiny angles on the sky. The nearest example is Sagit- developed mathematical models and computer simulations to
tarius A*, the four-million-solar-mass black hole at the center of explore a wide range of possible outcomes of these observations
the Milky Way; its event horizon would appear to be only 50 mi- and to develop tools to interpret them. Using novel supercom-
croarcseconds across, or roughly the size of a DVD seen on the puter algorithms, they have simulated the churn of matter just
moon. To resolve an object so small, a telescope must have an outside the black holes event horizon, and in all simulations
angular resolution more than 2,000 times finer than that they have found that the black hole casts a shadow on the
achieved by the Hubble Space Telescope. light coming off the accretion flow.
PRECEDING PAGES: COURTESY OF CHI-KWAN CHAN University of Arizona

What is more, such black holes are obscured from our view in University of Washington physicist James Bardeen predicted
two ways. First, they occur at the very centers of galaxies, deep the existence of a black hole shadow in 1973. By definition, any
within dense clouds of gas and dust that block most of the elec- light that crosses the event horizon can never return. Bardeen
tromagnetic spectrum. Second, even material that emits the identified the point outside the horizon where a photon will or-
light we want to detectthat glowing whirlpool of crushed mat- bit the black hole. If a light ray crosses this orbit heading in-
ter spiraling in toward the horizonis itself opaque to most ward, it is caught forever and spirals inward to the event hori-
wavelengths of light. Consequently, there are only a few wave- zon. Light rays originating between the event horizon and this
lengths of light that can escape from the black holes edge to be orbit can escape, but they have to be pointed almost radially
observed by us on Earth. outward, or they, too, risk being caught by the black holes gravi-
The Event Horizon Telescope (EHT) project is an internation- ty and having their trajectories bent backward toward the event
al effort to overcome these hurdles and perform detailed obser- horizon. We call this boundary the photon orbit.
vations of a black hole. To achieve the highest angular resolu- As far as light is concerned, the black hole acts like an opaque

76 Scientific American, September 2015

2015 Scientific American

object, with the photon orbit defining its BASICS
boundary. The contrast between the
bright ring of the photon orbit and the
dimmer interior is what is known as the
A Telescope the Size of Earth
shadow. The apparent size of this shadow At least nine radio telescopes a  nd arrays around the globe will together form
as seen by observers on Earth is actually the Event Horizon Telescope (EHT). Every telescope is located at high altitude
predicted to be quite a bit larger than the to minimize the absorption of the signals in Earths atmosphere. By spanning
photon orbit. This occurs because the in- the globe and operating at millimeter wavelengths, the array will achieve
tense gravitational field surrounding the an effective angular resolution that is comparable to a few millionths of an arc
black hole magnifies the shadow secondgood enough to spot a DVD on the moon.
through gravitational lensing. [For more
on gravitational lensing, see box in What
Einstein Got Wrong, by Lawrence M. SMT IRAM
Krauss, on page 54.] LMT 30-meter
telescope
The EHT is now poised to observe this
IRAM Plateau
shadow and other features of black holes. SMA, de Bure
JCMT interferometer
In 2007 and 2009 observations verified that
the technological approach was sound APEX,
ALMA
and that the ultimate science goal was APEX,
within reachby targeting Sagittarius A* ALMA
and another supermassive black hole at the
heart of the galaxy Virgo A (also known as
M87). These early observations linked to-
SPT SPT
gether sites in Hawaii, Arizona and Califor-
nia to successfully measure the extent of ra-
dio emission at a 1.3-millimeter wavelength
from both sources. In both cases, the mea-
surements matched the expected size of the black hole shadow. up to reveal the singularity within; others describe black holes
Observations planned with the full, planet-spanning web of that have no event horizon.
dishes will yield enough data to allow us to construct complete Naked singularities, unlike black holes, remain highly theo-
images of these black holes. An additional, equally important set retical: nobody has come up with a real-world recipe that would
of observations will use VLBI data to search for and trace the lead to their formation. Every astrophysically plausible comput-
trajectories of localized active regions (hotspots) as they circle er simulation of the gravitational collapse of a star leads to the
the black hole. Because general relativity predicts both what formation of a black hole with a horizon. Indeed, in 1969 Roger
these black holes should look like and how matter should orbit Penrose introduced the cosmic censorship hypothesis: the idea
them, these observations will allow us to perform a series of that physics somehow censors the nakedness of singularities by
tests of Einsteins theory of relativity in the place where its most always enshrouding them with a horizon.
extreme predictions become manifest. In September 1991 California Institute of Technology physi-
cists John Preskill and Kip Thorne made a bet with University of
CHECKING COSMIC CENSORSHIP Cambridge physicist Stephen Hawking that the cosmic censor-
The EHT will enable us t o answer a basic question: Is Sagittarius ship hypothesis is false and that naked singularities do exist. Two
A* a black hole? All available evidence suggests that the answer is and a half decades later the bet is still standing, begging for an
yes, but no one has ever directly observed a black hole, and other experiment that will settle it. Proving that Sagittarius A* has an
possibilities are consistent with general relativity. For example, event horizon would not conclusively disprove the existence of
Sagittarius A* could be something called a naked singularity. naked singularities elsewhere. Yet determining that the black
A singularity in physics is a place where the solution to an hole in the center of our Milky Way is a naked singularity would
equation is undefined and where the laws of nature as we un- allow us to directly observe phenomena at conditions where
derstand them no longer operate. General relativity predicts modern physics breaks down.
that the universe began in a singularityan initial moment
when all the contents of the cosmos were concentrated into a LOOKING FOR HAIR
single point of infinite density. The theory also tells us that a sin- Discrediting cosmic censorship w  ould not be a death blow to gen-
gularity, where gravity becomes infinite and matter is com- eral relativity; after all, its equations allow for naked singularities.
pressed to infinite density, lies at the center of every black hole. Yet we also expect the EHT to test a long-standing idea about
In a black hole, the event horizon hides the singularity from black holes called the no-hair theorem. And if the no-hair theorem
our universe. General relativity does not require all singularities is false, general relativity will, at minimum, have to be modified;
to be clothed by a horizon, however. There are an infinite the mathematical proof of this theorem leaves no wiggle room.
number of solutions to Einsteins equations in which the singu- The theorem says that any black hole that is surrounded by an
larities are naked. Some of these solutions describe normal event horizon can be completely described using just three prop-
black holes spinning so fast that their horizons have opened erties: mass, spin and electrical charge. In other words, any two

Maps by Terra Carta September 2015, ScientificAmerican.com77

2015 Scientific American

 METHODS

Testing Einstein The Shape of the Shadow

with Black Holes

Astrophysicists h  ave created sophisticat-
ed models based on Einsteins general
theory of relativity that predict how mat-
ter should behave in the vicinity of a a c
b
black hole. Soon, Event Horizon Tele-
scope observations of the black hole in A black hole casts a shadow on the emission from the hot matter surrounding it. The shape and size
of the shadow depend, in principle, on how fast the black hole is spinning, on the amount that light
the center of the Milky Way will tell us
rays are gravitationally bent in its vicinity, and on the orientation of the observer. Because of a lucky
whether reality matches those predic- coincidence, all three effects conspire to make the shadow nearly circular for all black holes and
tions. If it does not, Einsteins theory may observers a . This coincidence, however, occurs only if Einsteins theory is correct and the no-hair
need to be modified. theoremwhich states that a black hole can be completely described by its mass, spin and
chargeis satisfied. If observations reveal an elliptical shadow, as shown in images b and c ,
then Einsteins theory will not pass this test.

Tracking Closure Phase

Normal accretion flow Orbiting hotspot
Black holes sometimes flare up, and one
explanation is that the normal steady
accretion flow may be disrupted by hot
spots, regions of increased temperature
that orbit the black hole before dissipating.
The EHT will use trios of telescopes to mea
sure the difference in time of arrival of light
emitted by hotspots; with these data, it is
possible to triangulate the position of hot
spots. The simulation at the right shows Closure
SMT phase SMT
such a signal (called closure phase) based on
data from two different triangles. The orbit 100 LMT
Hawaii
of the hotspot creates a heartbeat pat
terna time signature in closure phase. 0
Measuring these signals will make it possi ALMA ALMA
ble to map the spacetime of the black hole
and test the predictions of Einsteins theory. 100
0 0.5 1 1.5 2
Time (hours elapsed)

Simulating a Complex Reality

COURTESY OF AVERY E. BRODERICK University of Waterloo and Perimeter Institute for Theoretical Physics ( a , b, c and top four images
in Tracking Closure Phase); COURTESY OF CHI-KWAN CHAN University of Arizona ( b ottom two images); TERRA CARTA (globes)
Scientists affiliated with the EHT are using
supercomputers to perform elaborate numer
ical simulations of accreting black holes that
exhibit the expected complexities of astro
physical objects. The right image depicts a
black hole in a fairly quiet state of emission; on
the left is a magnetically active region during
a flare. With these simulations, scientists have
developed algorithms that will allow them to
extract the properties of black hole shadows
from real-world observational data.

black holes with the same mass, spin and electrical charge are en- holes that produced them. But our simulations presented us
tirely identical, just as any two electrons are indistinguishable. with an unexpected and, ultimately, very pleasant surprise. No
Black holes, the theorem states, have no hairno geometric ir- matter how fast we let the black holes spin in our simulations,
regularities or distinguishing characteristics. and no matter where we placed our mock observers, the black
When we first started to think about imaging black holes us- hole shadows always appeared nearly circular with an apparent
ing VLBI, we thought we could use the shapes and sizes of black size equal to about five times the radius of the event horizon. Be-
hole shadows to learn the spins and orientations of the black cause of some lucky coincidenceand if there is a deep physical

78 Scientific American, September 2015

2015 Scientific American

reason for this, we still have not uncovered itno matter how cretion flow as it orbits the black hole, and that could provide
we alter the parameters in our models, the size and shape of the yet another way of checking to see whether the predictions of
black hole shadow remain practically unchanged. This coinci- general relativity hold up near the edge of a black hole.
dence is excellent news if our goal is to test Einsteins theory be-
cause it happens only if the general theory of relativity holds up EXTRAORDINARY EVIDENCE
[see box on opposite page]. If Sagittarius A* has an event hori- What happens if our observations appear to disagree with Ein-
zon, and if the size or the shape of its shadow deviates from our steins theory? To use an expression popularized by Carl Sagan,
predictions, that would constitute a violation of the no-hair the- extraordinary claims require extraordinary evidence. In the natu-
oremand, thus, of general relativity. ral sciences, extraordinary evidence often means one or more ver-
ifications of any claim by independent methods. In the coming
TRACING ORBITS AND MORE years, powerful optical and radio telescopes, as well as space-
EHT observations w  ill generate a great deal more data than are based gravitational-wave detectors, may provide such verification
used to make images. The antennas will record the full polariza- by monitoring the orbits of stars, neutron starstiny, incredibly
tion of the radiation emitted by the black hole, which will enable dense objects produced by the gravitational collapse of massive
us to create maps of the magnetic fields near the event horizon. starsand other objects around supermassive black holes.
Such maps could help us understand the physics behind the The optical interferometer GRAVITY, which is being built
powerful jets emanating from the centers of galaxies such as for use on the European Southern Observatorys Very Large
M87beams of extraordinarily energetic matter traveling near Telescope (VLT) in Chile, as well as next-generation 30-meter-
the speed of light for up to thousands of light-years. Astrophysi- class optical telescopes, will track the orbits of stars in our gal-
cists believe that magnetic fields near the event horizon of su- axy that lie fairly close to Sagittarius A*s event horizonat a
permassive black holes power these jets; mapping the magnetic distance only a few hundred times the radius of the black hole.
fields could help us test that hypothesis. Once completed, the Square Kilometer Array (SKA), a radio in-
We can learn other things by watching the motion of matter terferometer under construction in South Africa and in Austra-
around a black hole. The accretion flows around the black holes lia, will begin monitoring the orbits of rapidly spinning neu-
are expected to be highly turbulent and variable. Computer simu- tron stars, called pulsars, around the same black hole. And the
lations often show the presence of localized, short-lived, magneti- evolved Laser Interferometer Space Antenna (eLISA) will de-
cally active regions in themhotspots similar to magnetic erup- tect gravitational waves emitted as small compact objects orbit
tions on the surface of the sun. These hotspots, which may explain around supermassive black holes in nearby galaxies.
the brightness variations that are often seen in Sagittarius A*, Because of the very strong gravitational fields of the black
would circle the black hole at nearly the speed of light, along with holes, the elliptical orbits of these objects will shift (precess) rap-
the underlying accretion flow, completing full orbits in less than idly; this effect is so pronounced that the points of maximum dis-
half an hour. In some cases, they become gravitationally lensed as tance from the black holes should trace a complete circle in only
they move behind the black hole and generate nearly complete a few orbits. At the same time, the black holes will drag space-
Einstein ringsbright, gravitationally warped circles of light just time around with them, causing the orbital planes of objects
like those the Hubble Space Telescope has detected from distant within those spacetimes to precess as well. Measuring the rates
quasars. In other cases, they orbit around the black hole a few of orbital precession for objects at different distances from a
times before they lose their energy and dissipate. black hole will lead to a complete three-dimensional reconstruc-
Hotspots could complicate the process of making an image tion of spacetime around a black hole, providing many tests of
because the VLBI technique uses telescopes much like a time- general relativity in the presence of extremely strong gravity.
lapse camera, leaving the virtual shutter open for the full dura- Together all these instruments will help decide whether Ein-
tion of the observation and using the natural rotation of Earth steins general theory of relativityin particular, its predictions
to get as many different angles on the black hole as possible. If a about black holeswill survive intact for another century or be
bright spot in the accretion flow orbits the black hole, its ap- sacrificed on the altar of scientific progress.
pearance will be smeared, just as a photograph of a sprinter will
be blurry if the camera shutter is left open too long. M O R E TO E X P L O R E
Yet hotspots could also enable us to perform an entirely dif-
Detecting Flaring Structures in Sagittarius A* with High-Frequency VLBI.
ferent test of general relativity. The EHT can trace the orbits of Sheperd S. Doeleman et al. in Astrophysical Journal, V ol. 695, No. 1, pages 5974;
hotspots using a technique that goes by the fancy name of clo- April 10, 2009.
sure phase variability tracking. The method involves measuring Testing the No-Hair Theorem with Observations in the Electromagnetic Spec-
the delays between the time of arrival of light from the hotspot trum. II. Black Hole Images. Tim Johannsen and Dimitrios Psaltis in A strophysical
Journal, Vol. 718, No. 1, pages 446454; July 20, 2010.
at three telescopes and then using basic triangulation to infer
Jet-Launching Structure Resolved Near the Supermassive Black Hole in M87.
the position of the hotspot in the sky. Orbiting hotspots will pro- Sheperd S. Doeleman et al. in Science, Vol. 338, pages 355358; October 19, 2012.
duce distinctive signatures in the raw data collected by the tele- The Power of Imaging: Constraining the Plasma Properties of GRMHD Simulations
scopes. And in much the same way that Einsteins equations using EHT Observations of Sgr A*. Chi-Kwan Chan et al. in A strophysical Journal,
predict the size and shape of the black hole shadow, they also Vol. 799, No. 1, Article No. 1; January 20, 2015.
disclose everything we need to know about the orbits that FROM OUR ARCHIVES
hotspots should trace. This hotspot model is somewhat sche-
Portrait of a Black Hole. Avery E. Broderick and Abraham Loeb; December 2009.
matic, and reality may be more complex. Nevertheless, at full
sensitivity the EHT will be able to monitor structure in the ac- s c i e n t i f i c a m e r i c a n . c o m /m a g a z i n e /s a

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 A ST U DY I N
C O N T R A STS

WHO WAS
EINSTEIN,
Sure, he was pretty good at science. But science isnt everything
REALLY?

80 Scientific American, September 2015

WO R L D
CITIZE N
C UR M U D G E ON R O C K S TA R REBEL

Einstein was an outspoken He may have been the greatest A celebrity in his own lifetime, Einstein often rejected the publics
progressive whose views scientist of them all, but Einstein Einstein was arguably the first urge to place him on a pedestal.
sometimes provoked was also socially inept (by his theoretical physicist to have groupies. He bucked the status quo and was
a backlash. own admission) and, at times, On a visit to New York City in the viewed as an enemy by the Nazis,

2015 Scientific American

boorish, particularly to the 1920s, he drew large crowds. not least because he was a Jew.
women in his life.

ANTIFEMINIST HERO OF HEROES ICONOCLAST
In a written contract to his estranged The physicist, according to one survey, To punish me for my contempt
wife, he suggested that they could is considered the greatest hero of all of authority, fate has made me
continue to live together if she time, ahead of Mother Teresa, Gandhi an authority myself, in a message
met certain conditions, among them: and Martin Luther King. to a friend, 1930.

1
That my clothes and laundry
SCIENCE OF THE JEWS
are kept in good order. The Nazis championed the notion that
2
Einsteins relativity was a Jewish theory,
That I will receive my three mired in foggy thinking. Two German Nobel
meals regularly in my room. Prize winners promulgated the idea of
3
Aryan physics, which, unlike Einsteins
That my bedroom and study theory, did not attempt to destroy
are kept neat, and especially that

Illustrations by Scott Menchin

the world view of German man.
my desk is left for my use only.

ARDENT PACIFIST PUTTING THE MIND ON HOLD EINSTEIN THE BRAND IM NOT A CUBIST
Had he known that the Germans were not He disliked games and anything He didnt have to build his He rejected numerous attempts to
close to getting the A-bomb, he said, he recreational that required mental own personal brand. It equate his ideas about space, time and
would never have made his influential agility. When I get through with work, created itself. Rights to use gravity with the zeitgeist of the
personal appeal to F.D.R. to move ahead I dont want anything that requires Einsteins likeness in 20th centurythe upheavals taking place
with the Manhattan Project. the working of the mind. bobbleheads and other in art, music and philosophy. The theory of
knickknacks must be relativity had nothing to do, he said, with

procured from the same the growing subjectivityrelativism
RIGHT-WING TARGET company (Corbis Enter evident in the arts and philosophy.
A reactionary womens organization sent tainment) that manages
a letter to the State Department in 1932 rights for Steve McQueen,
recommending that Einstein be denied Muhammad Ali, Charlie
a visa because not even Stalin himself Chaplin, Thomas Edison,
is affiliated with so many anarcho- Johnny Cash, the Wright
Communist international groups. brothers and
Martin Luther King.

IN THE BEGINNING WAS
THE WORD
A Kickstarter campaign has raised
money to create a font based on
Einsteins handwriting.

CORBIS (Martin Luther King, Jr., a nd Marian Anderson); GETTY IMAGES (Paul Robeson, Einstein leaning on hand, Mother Teresa, Gandhi and
BAD DAD
His second son had schizophrenia.
Although he helped pay for his care, Einstein

Einstein sticking out tongue); BEN MEYER C altech Archives (E instein on bike); SCIENTIFIC AMERICAN (Einstein bobblehead); THE PICTURE DESK (Yoda)
RECKLESS IS HE EVERYMAN
never saw him after 1933. I have no special talent,
CHAMPION OF WORLD GOVERNMENT Artist Stuart Freeborn took
Einstein as inspiration for the Im only passionately curious,
At present we are far from possessing any
in a note to his biographer in 1952.

2015 Scientific American

supranational organization competent to character Yoda in S tar Wars.
render verdicts of incontestable authority SOCIAL INEPTITUDE
and enforce absolute submission to In rejecting the offer of the presidency
the execution of its verdicts. of Israel, he said that he lacked the natural
aptitude and the experience to deal
properly with people.
CIVIL-RIGHTS ADVOCATE
He was friends with Marian Anderson
and Paul Robeson and called racism
NOT A TREKKIE
a disease of white people.
Einstein derided science fiction.
DO YOU BELIEVE IN ALBERT?
A 2002 book by science writer and
editor Corey S. PowellGod in the
Equationsuggested that Einstein
might be a prophet of sorts.

September 2015, ScientificAmerican.com81

 EINSTEINS BRAINwas meticulously mapped by pathologist Thomas Harvey, who also supervised the dissection of the spec-
imen. In defiance of hospital protocol, Harvey took the tissues into his own possession and controlled access to them for decades.
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GENIUS
NEUROSCIENCE

IN A JAR The bizarre journey of Einsteins brain

illustrates the pitfalls in sciences search
for the origins of brilliance
By Brian D. Burrell

On April 18, 1955,

 Albert Einstein died at Princeton Hospital of a ruptured
aortic aneurysm. Within hours the pathologist on call, Thomas Harvey, acting on his
own initiative, removed the famed physicists brain without the familys permission. He
then preserved the organ, counter to Einsteins stated wish to be cremated. Harvey
managed to secure a retroactive blessing from Einsteins son Hans Albert, with the stip-
ulation that the brain would be used only for scientific purposes. But Harvey himself
lacked the expertise needed to analyze the organ, so he began to seek out specialists to
help him. It would take him 30years to find one. The quest changed the course of Har-
veys life and consigned his precious specimen to a fate that is at once strange, sad and
fraught with ethical complications.

IN BRIEF

Scientists have long s ought the ana as Harvey removed his brain without None of the studies of Einsteins brain Researchers have yet to demonstrate
tomical roots of genius in the brains of permission and took possession of it for or any of the other elite brains have that extraordinary intellectual achieve
renowned thinkers. decades while he sought out experts to been able to conclusively pinpoint the ment stems purely from nature rather
When Einstein died, p athologist Thom conduct all manner of analyses on it. source of mental greatness. than nurture.

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 Einstein was not the first renowned thinker to have his brain Brian D. Burrellis a lecturer in mathematics at the University
scrutinized in the name of science. The past is littered with similar of Massachusetts Amherst. He explored the neuroscience
tradition of studying brains of famous intellectuals in Postcards
examples. I found myself drawn into the curious history of these from the Brain Museum (Broadway, 2005). His latest book,
so-called elite brain studies around 15years ago, when I heard my with Harvard University neurologist Allan H. Ropper,
frustrated calculus students complaining that the Einsteins of the is Reaching Down the Rabbit Hole (St. Martins Press, 2014).
world have a neuroanatomical advantage over mere mortals such
as themselves. I found this idea dismayingmost peoples brains
are fully equipped to learn college-level calculusbut it prompted
me to investigate the scientific literature to see exactly what, if
anything, brain research has revealed about the source of mathe-
matical ability in particular and exceptional intellect in general. In
so doing, I found that, despite enthusiastic efforts
over the past two centuries to discern the anatomy
of talent or genius, scientists are not much closer to
finding it now than they were in the 1800s.
The case of Einsteins brain is perhaps the
most prominent example of how profound this
failure has been. As of this writing, half a dozen
reports on his brain, each highlighting a different
anatomical feature as the possible fount of his
brilliance, have come forthall to great media
fanfare. None has revealed a credible anatomical
basis for the mans aptitude. Instead they simply
add to the pile of flawed brain studies that have
collectively spawned what one critic has ruefully
termed a neuromythology of genius.

BRAIN EQUALS MIND?

The long and checkered tradition of studying the
brains of gifted people began 100 years before Ein-
steins death with the passing in 1855 of German
mathematician Carl Friedrich Gauss, the Einstein
of his day. Gausss University of Gttingen col-
leagues presided over his autopsy and removed his
brain. One of them, an anatomist named Rudolph
Wagner, then preserved the whole brain in an alco-
hol solution and later convinced Gausss son to al-
low him to keep it for research. Wagner obtained HARVEYS PHOTOGRAPHS s how the right lateral view (left)
the organ to bolster his firmly held belief in Ren and frontal view (right) of Einsteins brain prior to dissection.
Descartess philosophy of dualism: the idea of the
mind as something more than the sum of the
brains physical functions. Are human beings mere-
ly sophisticated machines, or are they endowed by God with a soul yond brain anatomy altogether. Perhaps it derived, as Wagner
substance? This question was the hot-button issue of the era. Ei- hoped, from the divine element, the ghost in the machine.
ther brain equals mind, or it doesnt. For Wagner, the very exis- Scientific materialists of that era, unhappy with Wagners
tence of God hung on the answer. findings, took the audacious step of founding brain-donation
Acquiring the brain of a celebrated genius opened the door to societies in hopes of identifying the physical underpinnings of
more acquisitions, and within seven years Wagner published exceptional talent. Membership hinged on the promise to be-
two detailed studies of the comparative anatomy of primate queath ones brain to ones fellows. By the end of the 19th cen-
SCIENTIFIC AMERICAN AND ST. MARTINS PRESS ARE AFFILIATES

brains. His data set included measurements of 964 brains from tury, as science began to usurp the role of religion, bestowing
people of all walks of lifeEnglish poet Lord Byron and French ones brain became a positively fashionable thing to do. Enthu-
naturalist Georges Cuvier, among them. Wagner found nothing siasm peaked at the founding of the brain societies, however,
to dispel his dualistic view of the mind. Neither brain weight nor and quickly waned in the absence of any substantiated find-
surface convolution patterns seemed to correspond with intel- ings. By the dawn of the 20th century the specimens had piled
lectual prowess. Cuviers brain was huge, but so was the brain of up, but most went unstudied or were lost to neglect.
a manual laborer. Gausss brain had an intricate pattern of Exactly what fueled Harveys obsession with Einsteins brain
grooves, or sulci, in the cerebral surface, but so did a washer- is unknown. He was aware of the historical precedents, of the
womans. Thus, the crucial difference between a genius and an many collections of celebrated brains. He may have simply been
average Joe, it seemed, had to lie below the surface or even be- overcome by curiosity. But the political atmosphere of the 1950s

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may have motivated him, too. Harvey knew that in the 1920s, the ropathologist at Bostons Brigham and Womens Hospital. But
search for the anatomy of genius had moved on to the cellular all tissues removed, as authorized to be retained by written per-
level. Soviet scientists, having amassed a pantheon of celebrated mission, are under the custody of the hospital and are never con-
brains, including those of Vladimir Lenin and Joseph Stalin, es- sidered the personal property of the attending pathologist. In
tablished a secretive research program to map the cortical layers defiance of protocol and his employers demands, Harvey refused
of the brains hemispheres based on neuronal patterns, a special- to relinquish his precious specimen and was eventually fired in
ty known as cytoarchitectonics. Outsiders were denied access to 1960. He packed his belongings and left for the Midwest, bring-
the specimens, and the Soviets always seemed poised to an- ing with him two jars. One contained the sugar-cube-size, celloi-
nounce a great discovery, although they never did. It was in this din-embedded chunks Keller had so carefully prepared; the oth-
atmosphere of cold war competitiveness and paranoia that Har- er held the undissected portions of Einsteins brain. He stowed
vey decided to appropriate Einsteins brain. the jars in a beer cooler along with the remaining sets of slides
and the calibrated photographs.
Harvey suffered several downward turns after
the hospital terminated him. His marriage fell
apart, and he lost his medical license. He then
took a job in a plastics-extrusion factory. He
moved frequently, at one point becoming a neigh-
bor and drinking companion of writer William S.
Burroughs. But he never lost interest in the brain,
and eventually, three decades after he removed it
from Einsteins corpse, Harvey found a neurosci-
entist to study it. Or rather she found him.
In 1985 Marian C. Diamond of the University
of California, Berkeley, requested four of Har-
veys tissue blocks. She was interested in study-
ing Einsteins glial cells. Glial cells act as a sup-
port system for neurons. In Diamonds previous
work with mice, she found that exposure to a
sensory-enriched environment produces a high-
er ratio of glial cells to neurons than does a non-
stimulating environment. She suspected that
Einstein might have possessed a high ratio of gli-
al cells to neurons in portions of his cortex asso-
ciated with higher neural functions such as men-
tal imagery, memory and attention.
When Diamond examined the material Har-
vey sent, she found what she was looking for in
one of the four tissue blocks and concluded that
the higher proportion of glial cells she observed
resulted from Einsteins enhanced use of this tis-
sue. In the ensuing media frenzy around her
SLICED AND DICED study, however, journalists gave the impression that this surfeit
By all accounts, Harvey was an eccentric man but scrupulous. was not the product of his deep thinking but the cause of it.
Once he acquired his hallowed relic, he approached it as methodi- It was not long before scientists themselves began to search
cally as any crime scene investigator. He photographed the corti- for anatomical explanations for Einsteins intellectual prowess.
cal surface from every angle, inserting a scale bar so that measure- Studies in the 1990s by Britt Anderson, then at the University
ments could be made from the images. He then took the specimen of Alabama at Birmingham, and psychologist Sandra Witelson
to the pathology laboratory at the University of Pennsylvania and of McMaster University in Ontario attributed it to other dis-
entrusted it to a gifted technician, Marta Keller. Under Harveys tinctive aspects of his brain. Anderson called attention to the
exacting instructions, while using the best practices at the time high density of cells in his prefrontal cortex. For her part, Wit
for neurological tissue preparation, Keller spent the next eight elson focused on the atypical absence of the so-called parietal
months dissecting portions of the cortex, embedding 240 num- operculum, part of a fissure that divides the parietal lobe. As a
bered chunks of it in blocks of a clear plastic material called celloi- result, she claimed, Einstein had an expanded cortical region
din and mounting 12sets of microscope slides with stained tissue associated with visuospatial and mathematical abilities.
slices. Harvey sent slide sets to several of his peers. None of them The ensuing decade saw the publication of many interesting
found anything unusual in the slides, but they did find something studies on anatomical anomalies in the brains of professional
strange about Harveys obsessive control over the brain. musicians and London taxi drivers but nothing on Einstein.
Pathologists generally have the latitude of removing, preserv- Then, in 2007, just around the time of Harveys death, neuro-
ing and studying organs, explains Umberto De Girolami, a neu- ophthalmologist Frederick E. Lepore of what is now the Rutgers

September 2015, ScientificAmerican.com85

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DESPITE HARVEYS CONSIDERABLE EFFORTS to prepare Einsteins brain for study, the source of the physicists genius
remains unknown. Today his brain is scattered in several locations. Harveys personal collection of drawings, photographs and tis-
sue slides (above) is housed at the National Museum of Health and Medicine in Silver Spring, Md.

86 Scientific American, September 2015

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Robert Wood Johnson Medical School discovered a previously guards the material closely. Harveys personal collection of
unreported cache of Harveys calibrated photographs of Ein- some 500 slides, as well as his calibrated photographs, went to
steins brain. He shared these with Dean Falk, a paleoanthro- the National Museum of Health and Medicine in Silver Spring,
pologist at Florida State University who works primarily on Md. Other slides and bits and pieces are distributed among a
brain evolution. Falk noticed some odd features in the topogra- dozen museums and university researchers. And speculation
phy of the brain, including a knob on the cortex known as the about the source of Einsteins brilliance continues.
Omega sign that had previously been linked to musical talent. Would science, and neuroscience in particular, be better off
It is interesting to contemplate, if Einsteins brain had been cre-
she wrote, that [Einsteins] ex- mated with his body? The point is
traordinary abilities may, to some now moot, but the question de-
degree, have been associated with
the unusual gross anatomy of his When you begin with the serves some consideration. In
1906, more than a century before
cerebral cortex.
In the most recent of the Ein-
assumption that geniuses Men and Falk did their research,
American anatomist Edward An-
stein brain studies, published on-
line in 2013, Falk and Weiwei Men
are different from thony Spitzka thought he had
found the key to mental acuity in
of East China Normal University
in Shanghai claimed to find an-
everyone else, the culprit the cross-sectional area of the cor-
pus callosum. In his report he
other anatomical explanation for would logically be any suggested that men of genius

anatomical anomaly you

the physicists prodigious powers were capable of their great ef-
of thought: in addition to his forts of the intellect . . . as it were,
unique cortical structure and cy-
toarchitectonics, Einstein had happen to come across. without taking pains. Authors
of subsequent elite brain studies,
enhanced communication routes including those focused on Ein-
between at least some parts of his stein, have echoed his suggestion
two cerebral hemispheres, they that mental greatness is purely a
claimed. They based their conjecture on measurements of the trick of nature. Yet none has shown it to be true.
cross-sectional area of the corpus callosum, the fiber bundle To my mind, this failure is ultimately a good thing because
connecting the left and right hemispheres, of Einsteins brain as the discovery of substrates of talentor lack thereofin the
compared with a control group. brain would have troubling practical and ethical implications.
As compelling as these proposed explanations for Einsteins If medical imaging could reveal anatomical correlates of talent,
achievements are at first glance, they all suffer from similar would parents then start screening children and directing
methodological defects. Terence Hines, a psychologist at Pace them to training regimens that accord with their neuroanato-
University, has been their most persistent critic. Hines observes my? Would they deny physics club or music lessons to a kid
that, among other scientific sins, the architects of these studies who lacks the Omega sign?
have tended to favor findings that support their preconcep- To a student who laments not being born with a math brain,
tions, downplaying aspects of Einsteins brain that are either I would point out that Einstein might not have been either. We
within normal limits or even deficient. Poorly chosen compara- dont know, and it doesnt matter. Behind the great achieve-
tive samples have further confused matters. Anderson, for in- ments of a Gauss or an Einstein is in all cases a life devoted to
stance, measured Einsteins brain against only five other brains contemplation, curiosity, collaboration and, perhaps most of
in her studyhardly enough to capture the range of human all, hard work.
variation and generate statistically significant conclusions.
Perhaps most troubling of all is the post hoc fallacy that
M O R E TO E X P L O R E
haunts almost every claim to have pinpointed the anatomical
substrate of genius: when you begin with the assumption that On the Brain of a Scientist: Albert Einstein. Marian C. Diamond et al. in
Experimental Neurology, Vol. 88, No. 1, pages 198204; April 1985.
geniuses are different from everyone else, the culprit would log-
Alterations in Cortical Thickness and Neuronal Density in the Frontal Cortex
ically be any anatomical anomaly that you happen to come of Albert Einstein. B ritt Anderson and Thomas Harvey in Neuroscience Letters,
across. And if you make enough measurements of anyones Vol. 210, No. 3, pages 161164; June 7, 1996.
brain, you will find something that sets it apart. The Exceptional Brain of Albert Einstein. S andra F. Witelson et al. in Lancet, Vol. 353,
No. 9170, pages 21492153; June 19, 1999.
NATURE VS. NURTURE The Corpus Callosum of Albert Einsteins Brain: Another Clue to His High Intelli-
gence? W  eiwei Men et al. in Brain. Published online September 24, 2013.
Today, some 60 years after H  arveys fateful decision, Einsteins The Cerebral Cortex of Albert Einstein: A Description and Preliminary Analysis
brain is scattered in several locations. Harvey returned the bulk of Unpublished Photographs. D  ean Falk, Frederick E. Lepore and Adrianne Noe
of it170 of the original 240 pieces embedded in celloidin, in Brain, Vol. 136, No. 4, pages 13041327; April 1, 2013.
along with the cerebellum and brain stemto Princeton Hospi- FROM OUR ARCHIVES
tal (since replaced with the University Medical Center of Prince
ton at Plainsboro) shortly before his death a decade ago. That High-Aptitude Minds. C hristian Hoppe and Jelena Stojanovic; Scientific American
Mind, August/September 2008.
material is now in the care of pathologist Elliot Krauss of the
Princeton medical center, who holds Harveys old job and who s c i e n t i f i c a m e r i c a n . c o m /m a g a z i n e /s a

September 2015, ScientificAmerican.com87

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 IS THE
RANDOM?COSMOS QUANTUM
P H YS I C S

Einsteins assertion that God

does not play dice with the universe
has been misinterpreted
By George Musser

Few of Albert Einsteins sayings h ave been as widely quot-

ed as his remark that God does not play dice with the universe. People
have naturally taken his quip as proof that he was dogmatically opposed
to quantum mechanics, which views randomness as a built-in feature of
the physical world. When a radioactive nucleus decays, it does so sponta-
neously; no rule will tell you when or why. When a particle of light strikes
a half-silvered mirror, it either reflects off it or passes through; the out-
come is open until the moment it occurs. You do not need to visit a labora-
tory to see these processes: lots of Web sites display streams of random
digits generated by Geiger counters or quantum optics. Being unpredict-
able even in principle, such numbers are ideal for cryptography, statistics
and online poker.
Einstein, so the standard tale goes, refused to accept that some things
are indeterministicthey just happen, and there is not a darned thing
anyone can do to figure out why. Almost alone among his peers, he clung
to the clockwork universe of classical physics, ticking mechanistically,
each moment dictating the next. The dice-playing line became emblemat-
ic of the B side of his life: the tragedy of a revolutionary turned reaction-
ary who upended physics with relativity theory but was, as Niels Bohr put
it, out to lunch on quantum theory.
Over the years, though, many historians, philosophers and physicists
have challenged this story line. Diving into what Einstein actually said,
they have found that his thinking about indeterminism was far more radi-
cal and nuanced than is commonly portrayed. It becomes a kind of a mis-
sion to get the story right, says Don A. Howard, a historian at the Univer-
sity of Notre Dame. Its amazing when you dig into the archives and see
the disparity from the common narrative. As he and others have shown,
Einstein accepted that quantum mechanics was indeterministicas well
he might, because he was the one who had d  iscovered its indeterminism.
What he did not accept was that this indeterminism was fundamental to
nature. It gave every indication of arising from a deeper level of reality
that the theory was failing to capture. His critique was not mystical but fo-
cused on specific scientific problems that remain unsolved to this day.
The question of whether the universe is a clockwork or a craps table
strikes at the heart of what we suppose physics to be: a search for simple
rules that underlie the wondrous diversity of nature. If some things hap-
pen for no reason, they mark the limits of rational inquiry. Fundamental
indeterminism would mean an end to science, worries Andrew S. Fried-
man, a cosmologist at the Massachusetts Institute of Technology. And yet
philosophers throughout history have supposed that indeterminism is
a prerequisite for human free will. Either we are all gears in the clock-

88 Scientific American, September 2015 Illustration by Shout

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 September 2015, ScientificAmerican.com89

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work, so that everything we do is preordained, or we are the George Musser is a contributing editor at Scientific
agents of our own destiny, in which case the universe must not American a nd author of the book S pooky Action at
a Distance, to be published by Scientific American/
be deterministic after all. This dichotomy has had very real Farrar, Straus and Giroux in November.
consequences for how society holds people responsible for
their actions. Assumptions about free will suffuse our legal sys-
tem; to be culpable, an offender must have acted with intent.
The courts continually wrestle with whether people are inno-
cent by reason of insanity, adolescent impulsiveness or rotten
social background.
Whenever people talk about a dichotomy, though, they usu-
ally aim to expose it as false. Indeed, many philosophers think it of motion: it does not lead to muddles such as singularities
is meaningless to say whether the universe is deterministic or (where quantities become infinite and thus indescribable) or
indeterministic. It can be either, depending on how big or com- chaos (where motion becomes unpredictable).
plex your object of study is: particles, atoms, molecules, cells, or- The tricky part is that determinism of the Schrdinger equa-
ganisms, minds, communities. The distinction between deter- tion is the determinism of the wave function, a nd the wave
minism and indeterminism is a level-specific distinction, says function is not directly observable, as the positions and veloci-
Christian List, a philosopher at the London School of Economics ties of particles are. Instead the wave function specifies the
and Political Science. If you have determinism at one particular quantities that can be observed and the likelihood of each even-
level, it is fully compatible with indeterminism, both at higher tuality. The theory leaves open what exactly the wave function
levels and at lower levels. The atoms in our brain can behave in is and whether it should be taken literally as a real wave out
a completely deterministic way while still giving us freedom of there in the world. Thus, it also leaves open whether observed
action because atoms and agency operate on different levels. randomness is instrinsic to nature or just a facade. People say
Likewise, Einstein sought a deterministic subquantum level that quantum mechanics is indeterministic, but thats too
without denying that the quantum level was probabilistic. quick, says philosopher Christian Wthrich of the University
of Geneva in Switzerland.
WHAT EINSTEIN OBJECTED TO Werner Heisenberg, another early pioneer of quantum theo-
How Einstein ever got tagged as antiquantum is almost as big a ry, envisioned the wave function as a haze of potential existence.
mystery as quantum mechanics itself. The very notion of quan- If it fails to pinpoint unequivocally where a particle is located,
taof discrete units of energywas his brainchild in 1905, and that is because the particle is not, in fact, located anywhere. Only
for a decade and a half he stood practically alone in its defense. when you observe the particle does it materialize somewhere.
Einstein came up with most of what physicists now recognize The wave function might have been spread out over a huge re-
as the essential features of quantum physics, such as lights pe- gion of space, but at the instant the observation is made, it
culiar ability to act as both particle and wave, and it was his abruptly collapses to a narrow spike at a single position, and the
thinking about wave physics that Erwin Schrdinger built on particle pops up there. When you so much as look at a particle
to develop the most widely used formulation of quantum theo- bam!it stops behaving deterministically and leaps to an end re-
ry in the 1920s. Nor was Einstein antirandomness. In 1916 he sult like a kid grabbing a seat in musical chairs. No law governs
showed that when atoms emit photons, the timing and direc- collapse. There is no equation for it. It just happens.
tion of emission are random. This goes against the popular Collapse became a core ingredient of the Copenhagen inter-
image of Einstein as an adversary to probability, says philoso- pretation, the view of quantum mechanics named for the city
pher Jan von Plato of the University of Helsinki. where Bohr had his institute and Heisenberg did much of his
But Einstein and his contemporaries faced a serious prob- early work. (Ironically, Bohr himself never accepted wave func-
lem. Quantum phenomena are random, but quantum theory tion collapse.) Copenhagen takes the observed randomness of
is not. The Schrdinger equation is 100 percent deterministic. quantum physics at face value, incapable of further explanation.
It describes a particle or system of particles using a so-called Most physicists accepted it, if only because of a psychological
wave function, which expresses particles wave nature and anchoring effect: it was a good enough story, and it was the first.
accounts for the undulating patterns that collections of parti- Although Einstein was not antiquantum, he was definitely
cles can form. The equation predicts what happens to the wave antiCopenhagen interpretation. He recoiled from the idea
SCIENTIFIC AMERICAN AND FARRAR, STRAUS AND GIROUX ARE AFFILIATES

function at every moment with complete certainty. In many that the act of measurement should cause a break in the con-
ways, the equation is more d  eterministic than Newtons laws tinuous evolution of a physical system, and that was the con-

IN BRIEF

I, at any rate, a m convinced that He is Closer examination, though, reveals Today many philosophers a rgue that free will. Even if everything that parti
not playing at dice, Albert Einstein wrote that Einstein did not reject quantum physics is b oth indeterministic and de cles do is preordained, the choices we
to a colleague in 1926. Repeated over the mechanics or its indeterminism, al terministic, depending on the level of make can be completely open because
years, his sound bite became the quin though he did thinkfor solid scientific reality being considered. the low-level laws governing particles
tessential put-down of quantum me reasonsthat the randomness could This view dissolves the much debated are not the same as the high-level laws
chanics and its embrace of randomness. not be a fundamental feature of nature. dilemma between determinism and governing human consciousness.

90 Scientific American, September 2015

2015 Scientific American

text in which he began to complain about divine dice rolling. you roll a six-sided die and it lands on, say, four, the range of
Its that, specifically, that Einstein is lamenting in 1926 and not one to six collapses to the actual outcome of four. A godlike
a blanket metaphysical assertion of determinism as an abso- demon, able to track all the atomic details affecting the die
lutely necessary condition, Howard says. Hes specifically in the exact way your hand sends the cube tumbling across the ta-
the thick of these arguments about whether or not wave func- blewould never speak of collapse.
tion collapse introduces discontinuities. Einsteins intuitions were backed up by his early work on
Collapse could not be a real process, Einstein reasoned. It the collective effects of molecular motionstudied by the
would require instantaneous action at a distancea mysterious branch of physics known as statistical mechanicsin which
mechanism ensuring that, say, the left side and right side of a he had demonstrated that physics could be probabilistic even
wave function both collapse to the same narrow spike even when
no force is coordinating them. Not just Einstein but every physi-

Einstein was trying

cist of his day thought such a process impossible; it would oper-
ate faster than light, in apparent violation of relativity theory. In
effect, quantum mechanics does not just give you dice to play
with. It gives you pairs of dice that always come up doubles, even to explain randomness,
if you roll one in Vegas and the other on Vega. For Einstein, it
seemed obvious that the dice must be loadedpossessing hid- not explain it away.
den attributes that fix their outcome in advance. But Copenha-
gen denied any such thing, implying the dice really do affect
each other instantly across the vastness of space. when the underlying reality was deterministic. In 1935 Einstein
Einstein was further troubled by the power that Copenhagen wrote to philosopher Karl Popper, I do not believe that you
accorded to measurement. What is a measurement, anyway? Is are right in your thesis that it is impossible to derive statisti-
it something that only conscious beings or tenured professors cal conclusions from a deterministic theory. Only think of clas-
can do? Heisenberg and other Copenhagenists failed to elabo- sical statistical mechanics (gas theory, or the theory of Brown-
rate. Some suggested that we create reality in the act of observ- ian movement).
ing itan idea that sounds poetic, perhaps a little too poetic. The probabilities in Einsteins way of thinking were just as
Einstein also thought it took a lot of chutzpah for Copenhagen objective as those in the Copenhagen interpretation. Although
ists to claim that quantum mechanics was complete, a final the- they did not appear in the fundamental laws of motion, they ex-
ory never to be superseded. He regarded all theories, including pressed other features of the world; they were not merely arti-
his own, as stepping-stones to something greater. facts of human ignorance. Einstein gave Popper the example
In fact, Howard argues that Einstein would have been happy of a particle that moves around a circle at steady speed; the
to entertain indeterminism as long as his concerns were ad- chance of finding the particle in a given arc of the circle reflects
dressedif, for example, someone could spell out what a mea- the symmetry of its path. Similarly, a die has a one-sixth chance
surement was and how particles could stay in sync without of landing on a given side because it has six equal sides. He did
acting at a distance. As a sign that Einstein considered indeter- understand better than most at that time that there was signifi-
minism a secondary concern, he made the same demands of de cant physical content in the details of statistical-mechanical
terministic alternatives to Copenhagen and rejected them, too. probabilities, Howard says.
Another historian, Arthur Fine of the University of Washington, Another lesson of statistical mechanics was that the quanti-
thinks Howard overstates Einsteins receptiveness to indeter- ties we observe do not necessarily exist on a deeper level. For
minism but agrees that the mans thinking was more solidly instance, a gas has a temperature, but a single gas molecule
grounded than the dice-playing sound bite has led generations does not. By analogy, Einstein came to believe that a subquan-
of physicists to assume. tum theory needed to mark a radical break from quantum me-
chanics. In 1936 he wrote, There is no doubt that quantum me-
RANDOM THOUGHTS chanics has seized hold of a beautiful element of truth. . . . How-
If you tug on Copenhagens loose ends, Einstein thought, you ever, I do not believe that quantum mechanics will be the
should find that quantum randomness is like every other type starting point i n the search for this basis, just as, vice versa, one
of randomness in physics: the product of deeper goings-on. The could not go from thermodynamics (resp. statistical mechan-
dancing of a dust mote in a shaft of sunlight betrays the com- ics) to the foundations of mechanics. To fill in that deeper lev-
plex motions of unseen air molecules, and the emission of a el, Einstein sought a unified field theory, in which particles de-
photon or radioactive decay of a nucleus is analogous, Einstein rive from structures that look nothing like particles. In short,
figured. In his estimation, quantum mechanics is a broad- conventional wisdom is wrong that Einstein repudiated the
brush theory that expresses the overall behavior of natures randomness of quantum physics. He was trying to explain the
building blocks but lacks the resolution to capture individual randomness, not to explain it away.
cases. A deeper, more complete theory would explain the mo-
tion in full without any mysterious jumps. DO YOUR LEVEL BEST
In this view, the wave function is a collective description, Although Einstein s overall project failed, h
 is basic intuition
like saying that a fair die, repeatedly tossed, will land roughly about randomness still holds: indeterminism can emerge from
the same number of times on each side. Wave function collapse determinism. The quantum and subquantum levelsor any
is not a physical process but the acquisition of knowledge. If other pair of levels in the hierarchy of natureconsist of dis-

September 2015, ScientificAmerican.com91

2015 Scientific American

 KEY CONCEPT istically. In some configurations, the die
ends up showing one dot; in others, two;

Realitys Many Realms and so on. Therefore, a single macro-

scopic condition (being rolled) can lead
Is the world deterministic or indeterministic? T  he answer depends not only on to multiple possible macroscopic out-
the basic laws of motion but also on the level at which a system is described. comes (showing one of six faces) [see box
Consider five atoms in a gas moving deterministically (top plot). They start at at left]. If we describe the die at a macro
nearly the same location and gradually spread out. On a macroscopic level (bottom level, we can think of it as a stochastic
plot), though, one would not see individual atoms but an amorphous puff of gas. system, which admits objective chance,
After a time, the gas might split at random into multiple puffs. This macrolevel says List, who has studied the meshing of
randomness is not an artifact of an observers ignorance about the microlevel; levels with Marcus Pivato, a mathemati-
it is an objective feature of nature, reflecting how atoms agglomerate. Analogously, cian at the University of Cergy-Pontoise
Einstein suspected that a deterministic subrealm of the universe leads to the in France.
randomness of the quantum realm. Although the higher level builds (in the
jargon, supervenes) on the lower one, it
is autonomous. To describe dice, you need
to work at a level where dice exist, and
Microlevel when you do so, you cannot help but ne-
glect atoms and their dynamics. If you
cross one level with another, you commit
the fallacy of a category mistake, which is
like asking about the political affiliations
of a tuna sandwich (to use an example
Trajectories of atoms
from philosopher David Z Albert of Co-
lumbia University). When we have phe-
nomena that can be described at multiple
levels, we have to be conceptually very
Position

careful in not mixing levels, List says.

For this reason, the die roll is not
Time merely apparently random, as people
sometimes say. It is truly random. A god-
Macrolevel like demon might brag that it knows ex-
actly what will happen, but it knows only
Trajectory of collection what will happen to the atoms. It does
of atoms
not even know what a die is because that
is higher-level information. The demon

 OL. 124, NO. 1; JANUARY 2015

never sees a forest, only trees. It is like
the protagonist of Argentine writer Jorge
Luis Borgess short story Funes, the
Memorious, a man who remembers ev-
erything and grasps nothing. To think is

SOURCE: EMERGENT CHANCE, BY CHRISTIAN LIST AND MARCUS PIVATO, IN THE PHILOSOPHICAL REVIEW, V
Position

to forget a difference, to generalize, to

abstract, Borges wrote. For the demon
Time to know which side the die lands on, you
In several places, the trajectory branches, an event have to tell it what to look for. The de-
that follows no clear law and occurs randomly.
mon would only be able to infer the high-
er-level history if the demon was given
our specification of how we partition the
physical level, List says. Indeed, the de-
tinct types of structures, so they abide by different types of mon might well come to envy our mortal perspective.
laws. The laws governing one level can leave a genuine element The level logic works the other way, too. Indeterministic mi-
of randomness even if the laws underneath it are completely crophysics can lead to deterministic macrophysics. A baseball
regimented. A deterministic microphysics does not induce a can be made of particles behaving randomly, yet its flight is en-
deterministic macrophysics, says philosopher Jeremy Butter- tirely predictable; the quantum randomness averages out. Like-
field of the University of Cambridge. wise gases consist of molecules executing enormously compli-
Think of a die at the atomic level. It can be constructed from catedand effectively indeterministicmovements, yet their
zillions of atomic configurations that look utterly indistinguish- temperature and other properties follow laws that are dead sim-
able to the eye. If you track any one of these configurations as ple. More speculatively, some physicists such as Robert Laugh-
the cube is rolled, it will lead to a specific outcomedetermin- lin of Stanford University suggest that the lower level is utterly

92 Scientific American, September 2015 Graphic by Jen Christiansen

2015 Scientific American

irrelevant. The building blocks could be anything and still pro- someones motivations rather than the physics backstory. Psy-
duce the same collective behavior. After all, systems as diverse chological explanations presume the kind of indeterminism
as water molecules, stars in a galaxy and cars on a highway that List is talking about. For example, game theorists model
obey the same laws of fluid flow. human decisions by laying out the range of options and show-
ing which one you will select if you are acting rationally. Your
FREE AT LAST freedom to choose a certain option steers your choice even if
When you think in terms of levels, t he worry that indetermin- you never plump for that option.
ism might mark the end of science evaporates. There is no big To be sure, Lists arguments do not explain free will fully.
wall around us, cordoning off a law-abiding chunk of the uni- The hierarchy of levels opens up space for free will by separat-
verse from the anarchic and inexplicable beyond. Instead the ing psychology from physics and giving us the opportunity to
world is a layer cake of determinism and indeterminism. The do the unexpected. But we have to seize the opportunity. If, for
earths climate, for example, supervenes on Newtons determin- example, we made every decision on a coin toss, that would still
istic laws of motion, but weather reports are probabilistic, count as macroindeterminism but would hardly qualify as free
whereas seasonal and longer-term climate trends are again will in any meaningful sense. Some peoples decision making
predictable. Biology, too, supervenes on deterministic physics, may be so debilitated that they cannot be said to act freely.
but organisms and ecosystems require different modes of de- This way of thinking about determinism also makes sense of
scription, such as Darwinian evolution. Determinism doesnt an interpretation of quantum theory that was developed in the
explain everything, says Tufts University philosopher Daniel years after Einsteins death in 1955: the many-worlds interpre-
C. Dennett. Why are there giraffes? Because it was deter- tation. Advocates argue that quantum mechanics describes a
mined that there would be? collection of parallel universesa multiversethat behaves de-
Human beings are embedded within this layer cake. We terministically in the large but looks indeterministic to us be-
have the powerful sense of free will. We often do the unpredict- cause we are able to see only a single universe. For instance, an
able, and in most of lifes decisions, we feel we were capable of atom might emit a photon to the left or to the right; quantum
doing otherwise (and often wish we had). For millennia, so- theory leaves the outcome open. According to the many-worlds
called philosophical libertariansnot to be confused with the interpretation, that is because the same situation arises in a zil-
political kindhave argued that human freedom requires par- lion parallel universes; in some, the photon goes deterministi-
ticle freedom. Something must break the deterministic flow of cally left, and in others, it goes right. Not being able to tell
events, such as quantum randomness or the swerves that which of those universes we reside in, we cannot predict what
some ancient philosophers thought atoms can undergo. will happen, so the situation from the inside looks inexplicable.
The trouble with this line of thought is that it would free the There is no true randomness in the cosmos, but things can ap-
particles but leave us enslaved. Whether your decision was pre- pear random in the eye of the beholder, says cosmologist Max
ordained at the big bang or made by a mutinous particle, it is Tegmark of the Massachusetts Institute of Technology, a promi-
not your decision. To be free, we need indeterminism not at the nent proponent of this view. The randomness reflects your in-
particle level but at the human level. And that is possible be- ability to self-locate.
cause the human and particle levels are autonomous. Even if That is very similar to saying that a die or brain could be
everything you do can be traced to earlier events, you can be constructed from any one of countless atomic configurations.
the author of your actions because neither you nor the actions The configurations might be individually deterministic, but be-
exist at the level of matter, only at the macrolevel of mind. This cause we cannot know which one corresponds to our die or our
macroindeterminism riding on microdeterminism may secure brain, we have to think of the outcome as indeterministic.
free will, Butterfield says. Macroindeterminism is not the Thus, parallel universes are not some exotic idea out there in
cause of your decision. It is y our decision. the cosmos. Our body and brain are little multiverses, and it is
Some might complain that you are still a puppet of the laws the multiplicity of possibility that endows us with freedom.
of nature, that your freedom is an illusion. But the word illu-
sion itself conjures up desert mirages and ladies sawed in M O R E TO E X P L O R E
half: things that are unreal. Macroindeterminism is not like
Review Essay: The Shaky Game (by Arthur Fine). D  on Howard in Synthese, V
 ol. 86,
that. It is quite real, just not fundamental. It is comparable to No. 1, pages 123141; January 1991.
life. Individual atoms are completely inanimate, yet enormous Freedom Evolves. D  aniel C. Dennett. Viking, 2003.
masses of them can live and breathe. Anything to do with Laws, Causation and Dynamics at Different Levels. J eremy Butterfield in Interface
agents, their intentional states, their decisions and choices: Focus, Vol. 2, No. 1, pages 101114; February 2012.
none of this features in the conceptual repertoire of fundamen- Free Will, Determinism, and the Possibility of Doing Otherwise. C hristian List
in N
 os, V
 ol. 48, No. 1, pages 156178; March 2014.
tal physics, but that doesnt mean those phenomena arent
Our Mathematical Universe: My Quest for the Ultimate Nature of Reality.
real, List observes. It just means that those are very much Max Tegmark. Knopf, 2014.
higher-level phenomena.
FROM OUR ARCHIVES
It would be a category mistake, not to mention completely
unenlightening, to describe human decisions as the mechanics 100 Years of Quantum Mysteries. M  ax Tegmark and John Archibald Wheeler;
of atoms in your brain. Instead you need to use all the concepts February 2001.
Was Einstein Right? G  eorge Musser; September 2004.
of psychology: desire, possibility, intention. Why did I choose
Finding Free Will. C hristof Koch; Scientific American Mind, M
 ay/June 2012.
water over wine? Because I wanted to. My desire explains my
action. Most of the times that we ask Why? we are seeking s c i e n t i f i c a m e r i c a n . c o m /m a g a z i n e /s a

September 2015, ScientificAmerican.com93

2015 Scientific American

Recommended by Clara Moskowitz For more recommendations,
MORE to go to ScientificAmerican.com/
EXPLORE sep2015/recommended

Einsteins Dice and

Schrdingers Cat:
How Two Great Minds Battled
Quantum Randomness to Create
a Unified Theory of Physics
by Paul Halpern. Basic Books, 2015 ($27.99)

Einstein notoriously 
disliked the inherent
randomness in quantum
theorywhich he de
scribed as God playing
dice with the universe
and he was not alone. His lifelong friend
and another giant of 20th-century phys-
ics, Erwin Schrdinger, was one of the
architects of quantum mechanics and yet
struggled with the mainstream interpre-
tation of the theory, which he parodied
in his famous Schrdingers cat thought
experiment. Physicist Halpern recounts
the two mens quest to find an overarch-
ing theory that would resolve these issues
and the occasions when vanity and ri
valry threatened, but never destroyed,
their friendship.

The Physicist and

New Takes on the Great Man the Philosopher:
Einstein, Bergson, and
Four recent tomes explore the loyalties, inspirations and trials of Albert Einstein
the Debate That Changed
Our Understanding of Time
The Road to Relativity: Einstein: His Space and Times by Jimena Canales.
The History and Meaning by Steven Gimbel. Princeton University Press, 2015 ($35)
of Einsteins The Foundation Yale University Press, 2015 ($25)
of General Relativity On April 6, 1922,
by Hanoch Gutfreund and Jrgen Renn. Einstein renounced Einstein clashed with the
Princeton University Press, 2015 ($35) religion at the age of 12, most famous philosopher
when he decided his of the day, Henri Bergson,
Any devotee o  f Einstein Jewish beliefs were about the nature of time.
will relish the chance incompatible with the Einstein espoused the picture he formulat-
to parse this annotated analytical mind-set of ed in general relativity of time as insepa-
facsimile of the physi- his truer devotion, science. Yet the world rable from space and lacking the absolute
cists original manu- never stopped seeing him as a Jew, and reality that humans tend to perceive in it.
script on general relativity. The authors over time he became a champion for his Bergson claimed that science alone cannot
provide a full English translation and oppressed people and a supporter of the describe time, which he said was closely
painstakingly explain, page by page, Ein- Zionist cause. Einstein had alienated intertwined with the vital impulse of life
steins text and equations, which lay out himself from the larger Jewish communi- and creative expression. Science historian
his theory and the path he took to derive ty, but the times forced him to realize that Canales describes how their debate initiat-
it. Their cogent descriptions and the his heritage was an inalienable part of ed a rift between physics and philosophy,
accompanying illustrations and docu- who he was, writes philosophy professor splitting the century into two cultures
ments open a fascinating window onto Gimbel in this look at Einsteins relation- and pitting scientists against humanists,
Einsteins otherwise inaccessible opus. ship to Judaism and his political activism. expert knowledge against lay wisdom.

94 Scientific American, September 2015 Illustration by Stephen Savage

2015 Scientific American

 Skeptic by Michael Shermer
Michael Shermer is publisher of Skeptic Viewing the world with a rational eye
magazine (www.skeptic.com). His new
book is The Moral Arc (Henry Holt, 2015).
Follow him on Twitter @michaelshermer

Forensic
Pseudoscience
How trustworthy are DNA
and other crime scene tests?

The criminal justice system has a problem, and its name is foren
sics. This was the message I heard at the Forensic Science Re
search Evaluation Workshop held May 2627 at the AAAS head
quarters in Washington, D.C. I spoke about pseudoscience but
then listened in dismay at how the many fields in the forensic sci
ences that I assumed were reliable (DNA, fingerprints, and so on)
in fact employ unreliable or untested techniques and show incon when firefighters spray water on a burning building with win
sistencies between evaluators of evidence. dows. He also noted that burn marks on the floor are not the re
The conference was organized in response to a 2009 publica sult of a liquid deliberately poured on it. When a fire consumes
tion by the National Research Council entitled S  trengthening Fo- an entire room, the extreme heat burns even the floor, along with
rensic Science in the United States: A Path Forward, w  hich the U.S. melting metal and leaving burn marks under a doorway thresh
Congress commissioned when it became clear that DNA was the old, which many investigators assume implies the use of an ac
only (barely) reliable forensic science. The report concluded that celerant. Most of the science of fire and explosive analysis has
the forensic science system, encompassing both research and been conducted by insurance companies looking to find evi
practice, has serious problems that can only be addressed by a na dence of arson so they dont have to pay off their policies, Lenti
tional commitment to overhaul the current structure that supports ni explained to me when I asked how his field became so fraught
the forensic science community in this country. Among the areas with pseudoscience.
determined to be flawed and in need of more research are: accura Itiel Dror of the JDI Center for the Forensic Sciences at Uni
cy and error rates of forensic analyses, sources of potential bias versity College London spoke about his research on cognitive fo
and human error in interpretation by forensic experts, fingerprints, rensicshow cognitive biases affect forensic scientists. For ex
firearms examination, tool marks, bite marks, impressions (tires, ample, the hindsight bias can lead one to work backward from a
footwear), bloodstain-pattern analysis, handwriting, hair, coatings suspect to the evidence, and then the confirmation bias can di
(for example, paint), chemicals (including drugs), materials (in rect one to find additional confirming evidence for that suspect
cluding fibers), fluids, serology, and fire and explosive analysis. even if none exists. Dror discussed studies that show that the
Take fire analysis. According to John J. Lentini, author of the de same expert examiner, evaluating the same prints but within dif
finitive book S cientific Protocols for Fire Investigation (CRC Press, ferent contexts, may reach different and contradictory decisions.
second edition, 2012), the field is filled with junk science. What Not just fingerprints. Even DNA analysis is subjective. When 17
does that pattern of burn marks over there mean? he recalled ask North American expert DNA examiners were asked for their in
ing a young investigator who joined him on one of his more than terpretation of data from an adjudicated criminal case in that ju
2,000 fire investigations. Absolutely nothing was the correct an risdiction, they produced inconsistent interpretations, Dror and
swer. Most of the time fire investigators find nonexistent patterns, his co-author wrote in a 2011 paper in S  cience and Justice.
Lentini elaborated, or they think a certain mark means the fire No one knows how many innocent people have been convicted
SCIENTIFIC AMERICAN AND HENRY HOLT ARE AFFILIATES

burned fast or slow, allegedly indicated by the alligatoring based on junk forensic science, but the National Research Council
of wood: small, flat blisters mean the fire burned slow; large, report recommends substantial funding increases to enable labs
shiny blisters mean it burned fast. Nonsense, he said. It may take to conduct experiments to improve the validity and reliability of
a while for a fire to get going, but once a couch or bed burns and the many forensic subfields. Along with a National Commission
reaches a certain temperature, you are not going to be able to dis on Forensic Science, which was established in 2013, its a start.
cern much about its cause.
Lentini debunked the myth of window crazing in which SCIENTIFIC AMERICAN ONLINE
cracks indicate rapid heating supposedly caused by an acceler Comment on this article at S cientificAmerican.com/sep2015
ant (arson). In fact, the cracks are caused by rapid cooling, as

Illustration by Izhar Cohen September 2015, ScientificAmerican.com 95

2015 Scientific American

Anti Gravity by Steve Mirsky
The ongoing search for fundamental farces Steve Mirskyhas been writing the Anti Gravity
column since a typical tectonic plate was about
34 inches from its current location. He also hosts
the S cientific American podcast Science Talk.

birdbrain should really be a compliment. Mexi

can jays in Arizona (which to Donald Trumps prob
able dismay cross the border with impunity) know
lots about legume load. That is, researchers report
in the Journal of Ornithology that they observed
the birds picking up peanuts in their beaks to gauge
the samples hefts. After literally weighing their op
tions, the birds would fly off with the densest nut
but enough about various congressional commit
tees that deal with science. And Donald Trump.
By the way, although the journal article refers
to the Mexican jays as A  phelocoma ultramarina, in
2011 the American Ornithologists Union decided
that A . ultramarina should probably be consid
ered a separate species called the transvolcanic jay.
And that the birds still to be called Mexican jays,
which include the subjects of the peanut study,

Numbers Games
should be referred to as A  phelocoma wollweberi.
Mexican jays used to be called gray-breasted jays, for anyone
keeping score at home. The thing to remember is that the birds
dont care, and if you worry about these designations too much,
Some animals can count; you could wind up aphelocomatose.
Dogs may not have the talents, let alone the talons, of some
others can be counted on birds, but their domesticated sophistication extends into realms
that look suspiciously ethical or moral. The journal Animal Be-
Its nice to know t hat the great man we celebrate in this special haviour r ecently featured a study in which 54 dogs watched an
issue had a warm sense of humor. For example, in 1943 Albert unknown person either help the dogs owner with a task or re
Einstein received a letter from a junior high school student who fuse to help. And the dogs later turned their noses up at a snack
mentioned that her math class was challenging. He wrote back, offer from that good-for-nothing creep who would not give a
Do not worry about your difficulties in mathematics; I can as hand to the most wonderful person in the world. Well, to be
sure you that mine are still greater. more accurate, the dogs were less likely to accept the snack from
Today we know that his sentiment could also have been di the rat punk than from somebody else, anyway. In a follow-up
rected at crows, which are better at math than those members interview, one weak-willed dog allegedly described its choice to
of various congressional committees that deal with science who take the tasty morsel as rough.
refuse to acknowledge that global temperatures keep getting Finally, in invertebrate news, heres a quote from a late June
higher. Studies show that crows can easily discriminate between story by the Australian Broadcasting Corporation about a find
a group of, say, three objects and another containing nine. They ing published in the P  roceedings of the Royal Society B: A tiny
have more trouble telling apart groups that are almost the same worm which procreates by jabbing a needle-like penis into its
size, but unlike the aforementioned committee members, at least own head has left biologists in Europe stunned. Thats some jab.
theyre trying. Of course, the flatworm is hermaphroditic; its actions would
A study in the P roceedings of the National Academy of Scienc- otherwise be mere self-abuse. But during times when it cant
es USA fi nds that the brain of a crow has nerve cells that special find a discrete mate, the male end bends around to the head
SCIENTIFIC AMERICAN IS PART OF SPRINGER NATURE

ize in determining numbersa method quite similar to what end to engage in whats technically called hypodermic insemi
goes on in our primate brain. Human and crow brains are sub nation. The sperm then travels to the midbody, where fertiliza
stantially different in size and organization, but convergent evo tion occurs. So it looks like it injects where it does because thats
lution seems to have decided that this kind of neuron-controlled the easiest place for the tail to wriggle around to reach. That is,
numeracy is a good system. (Crows are probably unaware of evo it did the math and used its head.
lution, which is excusable. Some members of various congressio
nal committees that deal with science pad their reactionary r SCIENTIFIC AMERICAN ONLINE
sums by not accepting evolution, which is astonishing.) Comment on this article at S cientificAmerican.com/sep2015
Crows are not the only avian types out there illustrating that

96 Scientific American, September 2015 Illustration by Matt Collins

2015 Scientific American

50, 100 & 150 Years Ago compiled by Daniel C. Schlenoff SCIENTIFIC F ind original articles and images in
AMERICAN the Scientific American a rchives at
Innovation and discovery as chronicled in Scientific American ONLINE  ScientificAmerican.com/magazine/sa

years, for then the birds would soon

September have pecked up every grain that fell September
1965 to the ground.
1865
Cities on Against the Sea Nitroglycerin
the Rise Twice during the thirteen years since for Blasting
Urbanized societies, it was built has the great concrete wall Glycerine, as we all
in which a majority along the waterfront of Galveston know, is the sweet
of the people live crowded together in withstood the furious onslaughts of a principle of oil, and
towns and cities, represent a new and raging sea lashed by a hurricane, and in is extensively used for purposes of the
fundamental step in mans social evolu- each case the seawall has stood perfectly. toilet, but it has now received an appli-
tion. Although cities themselves first In the latest storm the damage done cation of rather an unexpected nature.
appeared some 5,500 years ago, they to the city was chiefly in the business In 1847 Ascanio Sobrero discovered that
were small and surrounded by an over- section, north of Broadway, where the glycerine, when treated with nitric acid,
whelming majority of rural people; plan of grade raising has never been was converted into a highly explosive
moreover, they relapsed easily to village carried out. The writer at the request of substance, which he called nitro-glycer-
or small-town status. The urbanized the County Commissioners Court of the ine. It is oily, heavier than water, soluble
societies of today, in contrast, not County of Galveston inspected the work in alcohol and ether, and acts so power-
only have urban agglomerations of immediately after the two great storms fully on the nervous system that a single
a size never before attained but also of 1909 and 1915, and in neither case drop placed on the tip of the tongue will
have a high proportion of their popula- did he find the seawall damaged in the cause a violent headache that will last
tion concentrated in such agglomera- slightest degree, though heavy timbers for several hours. This liquid seems to
tions. Neither the recency nor the and logs were driven over it and badly have been almost forgotten by chemists,
speed of this evolutionary develop- damaged the boulevard.Brigadier- and it is only now that Mr. Nable [sic
ment is widely appreciated. Before General Henry M. Robert Alfred Nobel], a Swedish engineer, has
1850 no society could be described The author also wrote R  oberts Rules succeeded in applying it to a very impor-
as predominantly urbanized, and of Order, o
 riginally published in 1876. tant branch of his art, viz., blasting.
by 1900 only oneGreat Britain
could be so regarded. Today, only Brilliant(-ish)
65 years later, all industrial nations Invention
are highly urbanized. Messrs EditorsI venture to
submit for publication a plan,
to me apparently simple and
September feasible, but that I have never
1915 put to the test of experiment. It
is to do what man has already
War and done upon the earthmake use
Wildlife of the powers of the inferior
The war is having a animals given to him to be his
great influence on the servants to effect his purposes.
birds throughout Europe, especially on There are many birds noted for
the birds of passage. These birds were strength of wing and endur-
observed in places where they were ance in flight. The brown eagle
SCIENTIFIC AMERICAN, VOL. XIII, NO. 13 (NEW SERIES); SEPTEMBER 23, 1865

never seen before and were missed in and the American swan partic-
the localities where battles were raging. ularly suggest themselves. I
In Luxembourg, where otherwise mil propose to obtain a number of
lions of birds congregate in the leafy such birds and attach them by
forests, there are now scarcely any to jackets fitted around their bod-
be seen or heard. A nature lover there ies and cords to a frame work,
writes that whole oat fields have which shall sustain a basket
sprung up along the roads and in the large enough to hold a man.
market squares of the little towns and A slide show of other optimistic
villages where the horses have been inventions from 1865 is at
fed as the cavalry passed through. This DREAM OF FLIGHT: A creative plan to slip www.ScientificAmerican.com/
would never have been possible in other the surly bonds of Earth, 1865 sep2015/creative-inventions

98 Scientific American, September 2015

2015 Scientific American

 Graphic Science

A Bigger City Isnt

10

Salt Lake City

Atlanta Other cities (gray dots)

Always Better
Houston Density does not reliably predict
per capita carbon dioxide emissions
8 Cities will save us, e xperts argue. Higher densities of people mean
less energy consumption and lower carbon emissions per capitaa
Phoenix boon for the environment. People supposedly switch from driving
cars to using public transportation, for instance. But density in and
Salt Lake City, Atlanta, of itself isnt changing behavior, says Conor Gately, a graduate stu-
Houston, Phoenix
Per capita emissions are
dent in Boston Universitys department of Earth and environment.
determined by the amount Gately and his colleagues analyzed 33years worth of annual carbon
of carbon dioxide produced dioxide emissions by on-road vehicles across the U.S. Since 2000 the
by road transportation in 50 fastest-growing counties by population decreased their per capita
an area divided by the number emissions by only 12 percenta reduction that was not enough to
On-Road Carbon Dioxide Emissions (megagrams per capita)

of people living in that area. If

offset the total emissions growth in those same areas. The discrepan-
many suburbanites commute
6 cy probably comes down to the sprawl of suburbs, the researchers
into cities, the emissions from
their cars can counteract say. If public transportation between city and surrounding neighbor-
Denver
carbon savings achieved by hoods, for example, fails to keep pace with growth, more and more
having more people who dont people will drive into center cities for work and play and add pollu-
drive often living in city centers. tion to areas where they dont live. (Thats whats been happening in
Salt Lake City.)
Urban areas that are already dense and have the necessary green
Detroit
infrastructure, such as New York City, will see their per capita emis-
sions decline as they grow larger. In 2012 carbon emissions from vehi-
cles on the road accounted for 28 percent of the total fossil-fuel CO2
Los Angeles emissions for the entire countrya sizable chunk to target for reduc-
4 tions. The data suggest that solutions and regulations should be cus-

SOURCE: CITIES, TRAFFIC, AND CO 2: A MULTIDECADAL ASSESSMENT OF TRENDS, DRIVERS, AND SCALING RELATIONSHIPS, BY CONOR K. GATELY,
tomized for individual cities, not mandated as national catchalls. 
Amber Williams

LUCY R. HUTYRA AND IAN SUE WING, IN PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES USA, VOL. 112, NO. 16; APRIL 21, 2015
Boston
San Francisco
Already dense, the City by the Bay has continued to grow
over the past 20 years and its per capita emissions have
declined accordingly. Whether the trend will hold is
Washington, D.C. unknown: vehicle use dropped precipitously after the
2008 recession and may or may not be the new normal.
2 Baltimore
San Francisco
Seattle
New York City
Chicago
Dashed line = 1,650 persons
per square kilometer Chicago
Cities to the left and right of this The population of the
threshold follow different per capita Windy City declined from
emissions trends as they grow. 1990 to 2010 and because
its suburban population
rose sharply, the emissions 1980 1990 2000 2010
per resident increased.
0
0 1,650 2,000 4,000 6,000 8,000 10,000 12,000

Density (persons per square kilometer)

100 Scientific American, September 2015 Graphic by Tiffany Farrant-Gonzalez

2015 Scientific American