Extra-Terrestrial Intelligence:
How Common Is It?
George Lebo
29 November 2011
AST 2037
1
The Drake Equation
• First proposed by
Cornell astronomer
Frank Drake
• Actual mathematical
formula for estimating
number of intelligent
civilizations currently
in the Galaxy
2
What about number of planets
with life?
• Nlife = N* fP ne fl
• So …
• N* = 3 x 1011 stars
• fP = 1/200 fraction of “suitable stars”
(an educated guess)
• ne =1 suitable planets per suitable stars
• fl = fraction of these planets where life forms
3
What about number of planets
with life?
• Nlife = N* fP ne fl
• Nlife = 3 x 1011 stars * 1/200 * 1 * fl
• = 1.5 billion * fl
• If fl =1 , then we have 1.5 Billion life-bearing planets in
the Galaxy
• If fl =1 in a million, then we have 1500 life-bearing
planets in the Galaxy
• If fl =1 in a billion, then we are on the ONLY lifebearing planet in the Galaxy
4
Summary from Last Time
• Rarity/prevalence of life in the Galaxy depends on several
factors
• How many stars are there in the Galaxy?
• How many of them are suitable for Life?
• How many of those stars have suitable planets?
• How many of those suitable planets around suitable
stars produce Life?
• We can calculate an estimate of the number of civilizations
in the Galaxy using the Drake Equation
• A shorter version tells us how many planets have any life at
all
• Current estimates: as high as 1.5 billion, as low as 1 (us!)
5
The Drake Equation
• Nciv = N* fP ne fl fi ft Lciv / Lgal
• Nciv = number of current civilizations in the Galaxy
• N* = 3 x 1011 stars; fP = 1/200 fraction of “suitable
stars”; ne =1 suitable planets per suitable stars
• fl = fraction of these planets where life forms
• fi = fraction of life-bearing planets where intelligence
evolves
• ft = fraction of intelligence-bearing planets where
technology develops for communication
• Lciv = average lifetime of a civilization
• Lgal = lifetime of Galaxy
6
Intelligence Fraction
• What fraction of life-bearing planets produce intelligent
life?
• Remember definition of intelligence: capable of
developing technology sufficient for interstellar
communication (i.e. radio)
• Earth suggests this fraction is about 100%
• But … is this right?
7
Intelligence & Dinosaurs
• Dinosaurs were not
intelligent
• But they were still dominant
life on Earth … for about
200 Myr !!
• They were only wiped out by
a major extinction (K-T
asteroid impact)
• This allowed mammals and
(eventually) “intelligent” life
to dominate
• We would not survive that
impact either!!
• So, dinosaurs died from bad
luck, not stupidity (?)
8
Intelligence & Dinosaurs
• So, dinosaurs died from bad luck, not stupidity (?)
• No real evidence that intelligence is fundamentally “better”
than other traits
• If dinos around today, would they “win”?
9
Intelligence Fraction
• This is an endless debate
• But .. does the outcome REALLY matter?
• Even if intelligence is NOT inevitable, it happened once
out of two tries (us + dinosaurs)
• Would estimate fraction at ½ or 1/3 (Permian extinction
ended another “try”?)
• In short: 1 out of a few, so this is not a huge factor; it is
close to 1 (not 0.01, not 0.000001)
10
Technology Fraction
• Again, Earth suggests this fraction is 1
• But, what about intelligent life without technology?
• Idyllic scene
11
Human Population
• But … technology clearly provides an advantage in
numerical increase
• Human
population versus
time
•
Updated as of 11/30/12
– Total human
population is 7.1 billion
12
Technology & Malthus
• In early 1800s, Thomas Malthus predicted that the world
was rapidly approaching its maximum sustainable
population
• Why did it continue to grow? Technology – we now get
much more production per acre out of agriculture
(pesticides, irrigation, fertilizer)
• Corn yield versus
year
13
Technology & Health
• Technology (i.e. medicine) also increases population
via survival rates
14
Technology & Health
• Technology (i.e. medicine) also increases population
via survival rates – As of 11/30/12 – Life expectancy in
the US is 78.2 years.
15
Technology & Evolution
• So … evolutionary “success” is seen as improved
reproductive capability
• If have intelligence, sooner or later drive to survive results
in some technology development
• Even chimps do it!
16
Technology & Evolution
• “Technological” life reproduces/survives preferentially
• Eventually leads to dominant life being technological
• The more technology, the more likely it is to survive (even
if it does not directly kill off the others – contrary to all of
human history!)
• Theory of Neanderthal extinction: Peaceful nontechnological Neanderthals were victims of Cro-Magnon
genocide
• So … technology fraction is about 1
17
Technology & Evolution
• So … technology fraction is about 1
• But … does everyone with the technology to communicate
WANT to communicate?
18
Civilization Lifetime
• Why is this critical?
• Do the math so far …
• Nciv = N* fP ne fl fi ft Lciv / Lgal
•
= 3x1011 * 0.005 * 1 * 1 * 1 * 1 * Lciv / 1.3x1010 yrs
•
= 0.1 * Lciv
•
= Lciv / 10
• Carl Sagan estimated this as 10 Lciv (in 1974)
• Pessimistic case would be Lciv / 10 billion (a BIG
difference)
19
Civilization Lifetime
• So … Nciv depends an awful lot on Lciv
• Take L = 90 yrs (lifetime for which Earth has ad a
civilization capable of communicating via radio, so far
…)
• Then expect N = 9 civilizations in the Galaxy right now!
• On the other hand, could be as low as 90/1010  0
• Is L larger than this, though?
20
What Limits Civilization Survival?
• Malthus says we are limited
by food/resources (i.e.
starvation)
• Malthus thought we would
all die of starvation by 1900
• But … history shows that
technology advances can
dramatically improve our
ability to generate food
from limited natural
resources
• Mmmm ….
21
What Limits Civilization Survival?
• On the other hand, read Jared Diamond’s “Collapse”
• Many civilizations have in fact collapsed here on Earth
• This is often at the PEAK of their technological
advancement (i.e. the ancient Maya)
• Why?
22
Collapse of Civilization
• Why?
• More tech, leads to
more people
• More people leads to
more demand
• Exponential population
growth means
technology needs to
KEEP evolving (faster
and faster!)
• If it fails to keep up 
COLLAPSE!
• So, L could be long or
could be short
23
Natural Extinction?
• Asteroid impact,
implies L ~108 yrs
• Can technology avoid
even that?
• Gamma-ray bursts
can’t be avoided
• One heated atmosphere
already (!) March 3,
2003
• But … timescale is also
>108 yrs
• (If not, why not
previous extinctions …
or were there?)
24
Self-Limiting Lifetime?
• For the last 50 years,
humanity has had the
ability to destroy itself
completely
• MAD = Mutually
Assured Destruction
• For last 20 years,
fingers have been off
the triggers
• But … still a worry
• Could imply a short L
(~100 years??)
25
Number of Civilizations
• So … estimates range from L = 80 years to L = ??? (1000
years? 100,000 years? Longer? Mass extinctions seem
to happen every 50-100 million years)
• A “realistic” estimate is Nciv = Lciv / 10
• So … anywhere from 8 civilizations to 1000 to millions
(!)
• A pessimistic estimate is Lciv / 10 billion – which gives
ZERO even for long lifetimes (limited by mass extinction
timescale)
26
Distance between Civilizations
• Draw on Galaxy on board
• Mean distance estimate:
• Galaxy “Habitable Zone” is a circle-ish shape,
about 10 kpc in radius
• Area around each civilization averages R2/N
• So … radius around each civilization is about R/N
• This corresponds to anywhere from 3.3 kpc (for 9
civilizations) to 10 pc (for 1 million civilizations)
27
Distance & Communication
• We are not currently in regular radio contact with
extra-terrestrial civilizations
• Why not? Would we expect to be?
• Well … timescale for communication could be tough
• 1 pc = about 3 light years
• For “nominal” 9 civilizations, closest is about 3300 pc
away = 10000 light years
• If they sent us a message at the time of the pyramids …
it is only halfway here!!
28
Distance & Communication
• Even for an “optimistic” case,
average distance is 10 pc away
• This is 30 light years
• Play the “Contact” game:
• We start broadcasts around 1936
• Message arrives there in 1966
• They send a return message to us
within 1 year
• Would only have reached us in
1997
• So … just barely possible in the
most optimistic scenario
29
Summary
• Drake Equation guides estimate of number of civilizations
in the Galaxy right now
• Depends on many unknowns
• One of the most important is lifetime of civilizations
• We can estimate that this is at least 80 (100) years
• Could be as long as 100 million years (??), but depends on
ecological collapse, natural extinction, or violent selfdestruction
• “Realistic” estimates of number of civilizations ranges
from about 8 to as many as 1 million
• Pessimistic estimates would say “zero”
• Even for optimistic estimates, we would just BARELY be
at the threshold of being able to communicate with other
civilizations
31
FERMI’S CONJECTURE
IF WE ASSUME:
1.
LIFE WILL FORM WHERE IT CAN FORM.
2.
LIFE WILL NOT ARBITRARILLY EXTERMINATE
ITSELF.
3.
TECHNOLOGY WILL GROW AS IT HAS ON EARTH.
4.
WE WILL DEVELOP THE ABILITY TO SPACE TRAVEL.
THEN:
1.
INTELLIGENT LIFE WILL HAVE FORMED BILLIONS OF
YEARS AGO MANY PLACES IN OUR GALAXY.
2.
THEY WILL DETERMINE THAT THEIR CIVILIZATION
IS DOOMED AT SOME POINT.
3.
THEY WILL COLONIZE THE UNIVERSE.
4.
THEY WILL HAVE FOUND THE EARTH AS A
HABITABLE PLANET.
SO:
WHERE ARE THEY?