Lecture 42
Where is everybody? The Fermi Paradox
U Chicago
Life in the Universe - Prof. Richard Pogge The Ohio State University
This lecture is about the Fermi Paradox.
Apparent contradiction between estimates of the speed of colonization and the lack of evidence of extraterrestrials. Proposed solutions: We have come in contact with them (UFOs are real Hypothesis) Civilizations choose not to be noticed (the Zoo Hypothesis or an ethical decision like the Prime Directive) Civilizations by nature are self-destructive (Doomsday Hypothesis) Complex life never develops (the Rare Earth Hypothesis)
Interstellar Colonization is a rapid, exponential process.
The time to colonize the entire Galaxy is 10-100 Myr even with very modest assumptions Ships that travel at 0.01c and take 500 years between stars. 5,000 year wait time before dispatching the next ships. Inhabited region grows at 0.1% speed of light. Fills the Galaxy in ~100 Myr
Colonization times are small compared to the age of the Galaxy for reasonable assumptions.
The exponential population dynamics of colonization stands in stark contrast to the static assumptions of the Drake Equation.
Even a pessimistic computation with the Drake Equation could be a dramatic underestimate of the number of intelligent civilizations
So? Where is everybody?
NASA
Enrico Fermi (1901-1954)
Italian physicist best known for fundamental contributions to quantum theory, particle physics, and statistical mechanics. Won Nobel Physics Prize in 1938 Built the first nuclear reactor at Chicago in 1942. Worked on the Manhattan Project and at Los Alamos after WWII
The Fermi Paradox came out of a 1950 lunchtime conversation with colleagues at Los Alamos
If any civilizations have been around in our Galaxy for at least ~10100 Myr they should have colonized the entire Galaxy by now. We should have been visited many times over. But, we have no evidence of extraterrestrial visitation, nor have we found any alien artifacts on Earth.
1950 New Yorker Cartoon by Alan Dunn
The Fermi Paradox is a statement about the rapidity of colonization, not the frequency of civilizations
Only need one civilization around for at least 10 to 100 Myr to colonize an entire Galaxy. Galaxy is at least 10 Gyr old. Could colonize it 100 to 1000x over at that rate. But, it only has to happen once or twice.
STScI
If life and intelligence are very rare, but civilizations are long-lived, get at least a few
L N = N f p ne fl fi f c Age 5 Gyr = 100 Billion 0.2 110 10 1 10 Gyr =1
5 5
Number over the age of the Galaxy: 10 Gyr / 5 Gyr = 2 civilizations.
If life and intelligence are common, but civilizations are short-lived, get many over the age of the Galaxy
L N = N f p ne fl fi f c Age 100 yr = 100 Billion 0.5 11 0.11 10 Gyr = 50
Number over the age of the Galaxy: 10 Gyr/100 years 50 = 5 Billion civilizations!
A number of solutions have been proposed to explain the Fermi Paradox.
We are alone
and always have been (Rare Earth Hypothesis) because civilizations blow themselves up (Doomsday Hypothesis) because Nature eradicates us (Inhospitable Universe Hypothesis)
Civilizations have not colonized the Galaxy
because its hard because they dont want to
Civilizations have colonized the Galaxy but we dont know it
because they arent telling us (Zoo Hypothesis) because we cant tell
We do have evidence of their existence
but the government wont tell us (UFOs Are Real Hypothesis)
UFOs Are Real?
Extraterrestrial Visitations? No. Fuzzy photographs and anecdotal accounts of visits and abductions arent proof. Claims of secret government conspiracies? Most cant balance a budget let alone keep secrets for years
A.S. Graham, New Yorker, 1953
There are unexplained sightings, but failure to explain them does not justify wild leaps of the imagination.
Civilizations have colonized the Galaxy, but
They arent telling us Zoo Hypothesis Sentinel Hypothesis A Prime Directive (ethics)
We are too primitive to tell They are too advanced or too different from us.
Paramount Pictures
Many examples of fatal impacts between peoples in human history, even when well-intentioned.
Civilizations have colonized the galaxy but... we havent noticed (yet).
Perhaps aliens live among us, covertly monitoring our culture.
Perhaps aliens use a form of communication that we havent developed yet...
Makes our radio and microwave eavesdropping useless.
Civilizations have not colonized the Galaxy because it is too expensive
Use slow autonomous robotic probes instead of expensive starships Self-replicating Von Neumann machines Bracewell Messenger Probes
David Darling
because they dont want to
No precedent in human history
because civilizations self-destruct before they can colonize the Galaxy
Human populations tend to grow faster than sustainability Tendency toward aggression Poor long-range planning Irresponsibility with technology Doomsday Hypothesis Nuclear war Biological warfare Accidental contamination Nanotechnology catastrophe Environmental catastrophe Zombie apocalypse
NNSA/Nevada Site Office
because Nature can be cruel, which limits the lifetimes of even peaceful civilizations.
Possible exterminating events: Asteroid or Comet Impact Supernova or Gamma-Ray Burst sterilizes the planet Geological Catastrophe Massive volcanism (Deccan Flood) Deep ice age (Snowball Earth). Ample precedent for this in Earths geological history.
NASA
The Rare Earth Hypothesis posits that complex intelligent life and habitable Earths are very rare
We are a product of an extremely unlikely combination of geological & astronomical circumstances. Were in the right part of our Galaxy. Were around the right kind of star. Were on the right kind of planet. Earth has a large moon that stabilizes its rotation Complex life arose late, and intelligence even later Concludes that we are the only intelligent species.
NASA
