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Astronomy 141
Life in the Universe
Prof. Scott Gaudi

Lecture 7: The Fermi Paradox and the Rare Earth Hypothesis

Key Ideas

Reasonably advanced civilizations can colonize the Galaxy is 10-100 million years.
Fermi Paradox states: If complex, intelligent life exists in the Galaxy, why haven't we contacted them?
Some solutions to the Fermi Paradox
--We have come in contact with them (the UFOs Are Real Hypothesis)
--Civilizations choose not to be noticed (the Zoo Hypothesis)
--Civilizations by nature are self-destructive (Doomsday Hypothesis)
--Complex life never develops (the Rare Earth Hypothesis)
The Rare Earth Hypothesis
--The rise of complex, intelligent life on Earth required an extremely unlikely combination of events and circumstances.

Enrico Fermi (1901-1954)

Best known for contributions to:
quantum theory
particle physics
statistical mechanics
Won Nobel Prize 1938
Involved with Manhattan Project
Los Alamos Laboratory

The Fermi Paradox

What is the Fermi Paradox?

If any civilizations have been around in our Galaxy for at least ~10-100 million years,
they should have colonized the entire Galaxy by now, and so
we should have evidence of their existence.

Colonization Happens Rapidly

Travel at 0.01c
500 years to nearest star system
5,000 year wait time
Inhabited region grows at 0.1% speed of light
Colonization of entire galaxy in 100 million years

Colonization times are 10-100 million years for reasonable assumptions.


Not a statement of the frequency of civilizations
Statement of the rapidity of colonization
Only need one civilization around for at least 10-100 million years

Solutions to the Fermi Paradox

Possible Solutions

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 it's hard
--because they don't want to
Civilizations have colonized the Galaxy but we don't know it because..
--they aren't telling us (The Zoo Hypothesis)
--we can't tell
We do have evidence of their existence...
--But the governmenet isn't telling us (The UFOs Are Real Hypothesis)

UFOs Are Real?

Extraterrestrial Visitations? No.
"Extraordinary claims require extraordinary proof."
No extraordinary proof has been offered:
--Fuzzy photographs
--Anecdotal accounts of visits & abductions
--Claims of government conspiracies
The are unexplained sightings, but failure to explain them does not justify a leap to a truly wild explanations.

Civilizations have colonized the Galaxy

They aren't telling us
--"Zoo Hypothesis"
--Prime Directive
--"Sentinel Hypothesis"
We can't tell
--They are too advanced
--They are too different

Civilizations have not colonized the Galaxy

It's really hard.
- No obvious reason why it would be that hard.
They don't want to.
- No precedent in past human history.

We Are Alone

...because civilizations don't last long enough
Every indication that we grow faster than sustainability
Tendency toward aggression
Irresponsibility with technology
"Doomsday Hypothesis"
Many possible ends:
--nuclear war
--biological warfare
--accidental contamination,
--nanotechnological catastrophe
--environmental catastrophe

...because civilizations don't last long enough
Nature can be cruel?
Possible exterminating events:
--Comet/Asteroid collision
--Supernova/Gamma-Ray Bursts

The Rare Earth Hypothesis

The rise of complex, intelligent life on Earth required an extremely unlikely combination of events and circumstances.

Unlikely events and circumstances:
--Large moon
--Location in the galaxy
--Giant planet outside the Earth's orbit
--Lack of mass extinctions

Without the moon our spin-axis would wander

But! What would the Earth's spin rate be without a moon?
Recall, the moon is slowing down the Earth
Earth spun much faster in the past
Earth may have been spinning much more quickly if there wasn't a moon-forming impact!
If the Earth spun quicker, it would be stable (like a top).
Not clear a moon is necessary!

Implications of Various Solutions

Rare Earth
--We truly are unique
--No 'principle of mediocrity'
Doomsday Hypothesis
--This implies that all other civilizations have failed.
--If there were many civilizations, we are probably doomed
Zoo Hypothesis
--May one day encounter other civilizations

See A Note about Graphics to learn why the graphics shown in the lectures are generally not reproduced with these notes.

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