1. Extra-Terrestrial Intelligence: How Common Is It?
Stephen Eikenberry
09 April 2019
AST 2037

2. The Drake Equation First proposed by Cornell astronomer Frank Drake
Actual mathematical formula for estimating number of intelligent civilizations currently in the Galaxy

3. What about number of planets with life?
Nlife = N* fP ne fl
So …
N* = 3 x 1011 stars
fP = 1/4 fraction of “suitable stars” (an educated guess)
ne =0.4 suitable planets per suitable stars
fl = fraction of these planets where life forms

4. What about number of planets with life?
Nlife = N* fP ne fl
Nlife = 3 x 1011 stars * 1/4* 1 * fl
= 30 billion * fl
If fl =1 , then we have 30 billion life-bearing planets in the Galaxy
If fl =1 in a million, then we have 30,000 life-bearing planets in the Galaxy
If fl =1 in 30 billion, then we are on the ONLY life-bearing planet in the Galaxy

5. 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 30 billion, as low as 1 (us!)

6. 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 = ¼ 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

7. 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?

8. 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 (?)

9. 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”?

10. 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 )

11. Technology Fraction Again, Earth suggests this fraction is 1
But, what about intelligent life without technology?
Idyllic scene

12. Human Population
But … technology clearly provides an advantage in numerical increase
Human population versus time

13. 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

14. Technology & Health
Technology (i.e. medicine) also increases population via survival rates

15. Technology & Health
Technology (i.e. medicine) also increases population via survival rates

16. 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!

17. 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

18. Technology & Evolution
So … technology fraction is about 1
But … does everyone with the technology to communicate WANT to communicate?

19. Civilization Lifetime
Why is this critical?
Do the math so far …
Nciv = N* fP ne fl fi ft Lciv / Lgal
= 3x1011 * 0.25* 0.4 * 1 * 1 * 1 * Lciv / 1.3x1010 yrs
= 2 * Lciv
Carl Sagan estimated this as 10 Lciv (in 1974)
Pessimistic case would be ~ Lciv / 10 billion (a BIG difference)

20. Civilization Lifetime
So … Nciv depends an awful lot on Lciv
Take L = 90 yrs (lifetime for which Earth has had a civilization capable of communicating via radio, so far …)
Then expect N = 180 civilizations in the Galaxy right now!
On the other hand, could be as low as 180/1010  0
Is L larger than this, though?

21. 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 ….

22. 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?

23. 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

24. 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?)

25. Self-Limiting Lifetime?
For the last 50 years, humanity has had the ability to destroy itself completely
MAD = Mutually Assured Destruction
For last 30 years, fingers have been off the triggers
But … still a worry
Could imply a short L (~100 years??)

26. Number of Civilizations
So … estimates range from L = 80 years to L = ??? (1000 years? 100,000 years? Longer? Mass extinctions seem to happen every million years)
A “realistic” estimate is Nciv = 2* Lciv
So … anywhere from 160 civilizations to 1000 to millions (!)
A pessimistic estimate is Lciv / 10 billion – which gives ZERO even for long lifetimes (limited by mass extinction timescale)

27. 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 1 kpc (for 100 civilizations) to 10 pc (for 1 million civilizations)

28. 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” 100 civilizations, closest is about 1000 pc away = 3000 light years
If they sent us a message at the time of the Julius Caesar … it is not yet here!!

29. 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

30. 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 180 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