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 life-bearing 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

12

• Human population versus time

• Updated as of 11/30/12 – Total human population is 7.1 billion

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)

13

• Corn yield versus year

Technology & Health• Technology (i.e. medicine) also increases population

via survival rates

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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!

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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 non-technological Neanderthals were victims of Cro-Magnon genocide

• So … technology fraction is about 1

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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)

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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

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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 self-destruction

• “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?