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Leonardo Santamaria
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Black Holes May Hide a Mind-Bending
Secret About Our Universe
Take gravity, add quantum mechanics, stir. What do you get?
Just maybe, a holographic cosmos.
Dennis Overbye
By Dennis Overbye
Published Oct. 10, 2022
Updated Oct. 11, 2022
For the last century the biggest bar fight in science has been between
Albert Einstein and himself.
On one side is the Einstein who in 1915 conceived general relativity,
which describes gravity as the warping of space-time by matter and
energy. That theory predicted that space-time could bend, expand, rip,
quiver like a bowl of Jell-O and disappear into those bottomless pits of
nothingness known as black holes.
On the other side is the Einstein who, starting in 1905, laid the
foundation for quantum mechanics, the nonintuitive rules that inject
randomness into the world — rules that Einstein never accepted.
According to quantum mechanics, a subatomic particle like an electron
can be anywhere and everywhere at once, and a cat can be both alive
and dead until it is observed. God doesn’t play dice, Einstein often
complained.
Gravity rules outer space, shaping galaxies and indeed the whole
universe, whereas quantum mechanics rules inner space, the arena of
atoms and elementary particles. The two realms long seemed to have
nothing to do with each other; this left scientists ill-equipped to
understand what happens in an extreme situation like a black hole or
the beginning of the universe.
But a blizzard of research in the last decade on the inner lives of black
holes has revealed unexpected connections between the two views of
the cosmos. The implications are mind-bending, including the
possibility that our three-dimensional universe — and we ourselves —
may be holograms, like the ghostly anti-counterfeiting images that
appear on some credit cards and drivers licenses. In this version of the
cosmos, there is no difference between here and there, cause and
effect, inside and outside or perhaps even then and now; household
cats can be conjured in empty space. We can all be Dr. Strange.
“It may be too strong to say that gravity and quantum mechanics are
exactly the same thing,” Leonard Susskind of Stanford University wrote
in a paper in 2017. “But those of us who are paying attention may
already sense that the two are inseparable, and that neither makes
sense without the other.”
That insight, Dr. Susskind and his colleagues hope, could lead to a
theory that combines gravity and quantum mechanics — quantum
gravity — and perhaps explains how the universe began.
Einstein vs. Einstein
The schism between the two Einsteins entered the spotlight in 1935,
when the physicist faced off against himself in a pair of scholarly papers.
In one paper, Einstein and Nathan Rosen showed that general relativity
predicted that black holes (which were not yet known by that name)
could form in pairs connected by shortcuts through space-time, called
Einstein-Rosen bridges — “wormholes.” In the imaginations of science
fiction writers, you could jump into one black hole and pop out of the
other.
In the other paper, Einstein, Rosen and another physicist, Boris
Podolsky, tried to pull the rug out from quantum mechanics by
exposing a seeming logical inconsistency. They pointed out that,
according to the uncertainty principle of quantum physics, a pair of
particles once associated would be eternally connected, even if they
were light-years apart. Measuring a property of one particle — its
direction of spin, say — would instantaneously affect the measurement
of its mate. If these photons were flipped coins and one came up heads,
the other invariably would be found out to be tails.
To Einstein this proposition was obviously ludicrous, and he dismissed it
as “spooky action at a distance.” But today physicists call it
“entanglement,” and lab experiments confirm its reality every day. Last
week the Nobel Prize in Physics was awarded to a trio of physicists
whose experiments over the years had demonstrated the reality of this
“spooky action.”
The physicist N. David Mermin of Cornell University once called such
quantum weirdness “the closest thing we have to magic.”
As Daniel Kabat, a physics professor at Lehman College in New York,
explained it, “We’re used to thinking that information about an object
— say, that a glass is half-full — is somehow contained within the
object. Entanglement means this isn’t correct. Entangled objects don’t
have an independent existence with definite properties of their own.
Instead they only exist in relation to other objects.”
Einstein probably never dreamed that the two 1935 papers had
anything in common, Dr. Susskind said recently. But Dr. Susskind and
other physicists now speculate that wormholes and spooky action are
two aspects of the same magic and, as such, are the key to resolving an
array of cosmic paradoxes.
Throwing Dice in the Dark
To astronomers, black holes are dark monsters with gravity so strong
that they can consume stars, wreck galaxies and imprison even light. At
the edge of a black hole, time seems to stop. At a black hole’s center,
matter shrinks to infinite density and the known laws of physics break
down. But to physicists bent on explicating those fundamental laws,
black holes are a Coney Island of mysteries and imagination.
In 1974 the cosmologist Stephen Hawking astonished the scientific
world with a heroic calculation showing that, to his own surprise, black
holes were neither truly black nor eternal, when quantum effects were
added to the picture. Over eons, a black hole would leak energy and
subatomic particles, shrink, grow increasingly hot and finally explode. In
the process, all the mass that had fallen into the black hole over the
ages would be returned to the outer universe as a random fizz of
particles and radiation.
This might sound like good news, a kind of cosmic resurrection. But it
was a potential catastrophe for physics. A core tenet of science holds
that information is never lost; billiard balls might scatter every which
way on a pool table, but in principle it is always possible to rewind the
tape to determine where they were in the past or predict their
positions in the future, even if they drop into a black hole.
But if Hawking were correct, the particles radiating from a black hole
were random, a meaningless thermal noise stripped of the details of
whatever has fallen in. If a cat fell in, most of its information — name,
color, temperament — would be unrecoverable, effectively lost from
history. It would be as if you opened your safe deposit box and found
that your birth certificate and your passport had disappeared. As
Hawking phrased it in 1976: “God not only plays dice, he sometimes
throws them where they can’t be seen.”
His declaration triggered a 40-year war of ideas. “This can’t be right,” Dr.
Susskind, who became Hawking’s biggest adversary in the subsequent
debate, thought to himself when first hearing about Hawking’s claim. “I
didn’t know what to make out of it.”
ImageA white, illustrated cat sits in the middle of the page, staring out, and dark
blue lines radiate from behind it like a scintillating star.
Credit...Leonardo Santamaria
Encoding Reality
A potential solution came to Dr. Susskind one day in 1993 as he was
walking through a physics building on campus. There in the hallway he
saw a display of a hologram of a young woman.
A hologram is basically a three-dimensional image — a teapot, a cat,
Princess Leia — made entirely of light. It is created by illuminating the
original (real) object with a laser and recording the patterns of reflected
light on a photographic plate. When the plate is later illuminated, a
three-dimensional image of the object springs into view at the center.
“‘Hey, here’s a situation where it looks as if information is kind of
reproduced in two different ways,’” Dr. Susskind recalled thinking. On
the one hand, there is a visible object that “looked real,” he said. “And
on the other hand, there’s the same information coded on the film
surrounding the hologram. Up close, it just looks like a little bunch of
scratches and a highly complex encoding.”
The right combinations of scratches on that film, Dr. Susskind realized,
could make anything emerge into three dimensions. Then he thought:
What if a black hole was actually a hologram, with the event horizon
serving as the “film,” encoding what was inside? It was “a nutty idea, a
cool idea,” he recalled.
Across the Atlantic, the same nutty idea had occurred to the Dutch
physicist, Gerardus ’t Hooft, a Nobel laureate at Utrecht University in
the Netherlands.
According to Einstein’s general relativity, the information content of a
black hole or any three-dimensional space — your living room, say, or
the whole universe — was limited to the number of bits that could be
encoded on an imaginary surface surrounding it. That space was
measured in pixels 10⁻³³ centimeters on a side — the smallest unit of
space, known as the Planck length.
With data pixels so small, this amounted to quadrillions of megabytes
per square centimeter — a stupendous amount of information, but not
an infinite amount. Trying to cram too much information into any
region would cause it to exceed a limit decreed by Jacob Bekenstein,
then a Princeton graduate student and Hawking’s rival, and cause it to
collapse into a black hole.
“This is what we found out about Nature’s bookkeeping system,” Dr. ’t
Hooft wrote in 1993. “The data can be written onto a surface, and the
pen with which the data are written has a finite size.”
The Soup-Can Universe
The cosmos-as-holograph idea found its fullest expression a few years
later, in 1997. Juan Maldacena, a theorist at the Institute for Advanced
Study in Princeton, N.J., used new ideas from string theory — the
speculative “theory of everything” that portrays subatomic particles as
vibrating strings — to create a mathematical model of the entire
universe as a hologram.
In his formulation, all the information about what happens inside some
volume of space is encoded as quantum fields on the surface of the
region’s boundary.
Dr. Maldacena’s universe is often portrayed as a can of soup: Galaxies,
black holes, gravity, stars and the rest, including us, are the soup inside,
and the information describing them resides on the outside, like a label.
Think of it as gravity in a can. The inside and outside of the can — the
“bulk” and the “boundary” — are complementary descriptions of the
same phenomena.
Since the fields on the surface of the soup can obey quantum rules
about preserving information, the gravitational fields inside the can
must also preserve information. In such a picture, “there is no room for
information loss,” Dr. Maldacena said at a conference in 2004.
Hawking conceded: Gravity was not the great eraser after all.
“In other words, the universe makes sense,” Dr. Susskind said in an
interview.
“It’s completely crazy,” he added, in reference to the holographic
universe. “You could imagine in a laboratory, in a sufficiently advanced
laboratory, a large sphere — let’s say, a hollow sphere of a specially
tailored material — to be made of silicon and other things, with some
kind of appropriate quantum fields inscribed on it.” Then you could
conduct experiments, he said: Tap on the sphere, interact with it, then
wait for answers from the entities inside.
“On the other hand, you could open up that shell and you would find
nothing in it,” he added. As for us entities inside: “We don’t read the
hologram, we are the hologram.”
Image
Credit...Leonardo Santamaria
Wormholes, wormholes everywhere
Our actual universe, unlike Dr. Maldacena’s mathematical model, has
no boundary, no outer limit. Nonetheless, for physicists, his universe
became a proof of principle that gravity and quantum mechanics were
compatible and offered a font of clues to how our actual universe
works.
But, Dr. Maldacena noted recently, his model did not explain how
information manages to escape a black hole intact or how Hawking’s
calculation in 1974 went wrong.
Don Page, a former student of Hawking now at the University of Alberta,
took a different approach in the 1990s. Suppose, he said, that
information is conserved when a black hole evaporates. If so, then a
black hole does not spit out particles as randomly as Hawking had
thought. The radiation would start out as random, but as time went on,
the particles being emitted would become more and more correlated
with those that had come out earlier, essentially filling the gaps in the
missing information. After billions and billions of years all the hidden
information would have emerged.
In quantum terms, this explanation required any particles now escaping
the black hole to be entangled with the particles that had leaked out
earlier. But this presented a problem. Those newly emitted particles
were already entangled with their mates that had already fallen into
the black hole, running afoul of quantum rules mandating that particles
be entangled only in pairs. Dr. Page’s information-transmission scheme
could only work if the particles inside the black hole were somehow the
same as the particles that were now outside.
How could that be? The inside and outside of the black hole were
connected by wormholes, the shortcuts through space and time
proposed by Einstein and Rosen in 1935.
In 2012 Drs. Maldacena and Susskind proposed a formal truce between
the two warring Einsteins. They proposed that spooky entanglement
and wormholes were two faces of the same phenomenon. As they put
it, employing the initials of the authors of those two 1935 papers,
Einstein and Rosen in one and Einstein, Podolsky and Rosen in the other:
“ER = EPR.”
The implication is that, in some strange sense, the outside of a black
hole was the same as the inside, like a Klein bottle that has only one
side.
How could information be in two places at once? Like much of quantum
physics, the question boggles the mind, like the notion that light can be
a wave or a particle depending on how the measurement is made.
What matters is that, if the interior and exterior of a black hole were
connected by wormholes, information could flow through them in
either direction, in or out, according to John Preskill, a Caltech physicist
and quantum computing expert.
“We ought to be able to influence the interior of one of these black
holes by ‘tickling’ its radiation, and thereby sending a message to the
inside of the black hole,” he said in a 2017 interview with Quanta. He
added, “It sounds crazy.”
Ahmed Almheiri, a physicist at N.Y.U. Abu Dhabi, noted recently that by
manipulating radiation that had escaped a black hole, he could create a
cat inside that black hole. “I can do something with the particles
radiating from the black hole, and suddenly a cat is going to appear in
the black hole,” he said.
He added, “We all have to get used to this.”
The metaphysical turmoil came to a head in 2019. That year two groups
of theorists made detailed calculations showing that information
leaking through wormholes would match the pattern predicted by Dr.
Page. One paper was by Geoff Penington, now at the University of
California, Berkeley. And the other was by Netta Engelhardt of M.I.T.;
Don Marolf of the University of California, Santa Barbara; Henry
Maxfield, now at Stanford University; and Dr. Almheiri. The two groups
published their papers on the same day.
“And so the final moral of the story is, if your theory of gravity includes
wormholes, then you get information coming out,” Dr. Penington said.
“If it doesn’t include wormholes, then presumably you don’t get
information coming out.”
He added, “Hawking didn’t include wormholes, and we are including
wormholes.”
Not everybody has signed on to this theory. And testing it is a challenge,
since particle accelerators will probably never be powerful enough to
produce black holes in the lab for study, although several groups of
experimenters hope to simulate black holes and wormholes in quantum
computers.
And even if this physics turns out to be accurate, Dr. Mermin’s magic
does have an important limit: Neither wormholes nor entanglement
can transmit a message, much less a human, faster than the speed of
light. So much for time travel. The weirdness only becomes apparent
after the fact, when two scientists compare their observations and
discover that they match — a process that involves classical physics,
which obeys the speed limit set by Einstein.
As Dr. Susskind likes to say, “You can’t make that cat hop out of a black
hole faster than the speed of light.”
Continue reading the main story
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