Transcript ET's and Fermi's Conjecture, Lebo, 8-7-08
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?
•
N life = N * f P n e f l
•
So …
• • • •
N * = 3 x 10 11 stars f P = 1/200 fraction of “suitable stars” (an educated guess) n e =1 suitable planets per suitable stars f l = fraction of these planets where life forms
3
What about number of planets with life?
•
N life = N * f P n e f l
• • • • •
N life = 3 x 10 11 stars * 1/200 * 1 * f = 1.5 billion * f l If f l =1 , then we have the Galaxy 1.5 Billion l life-bearing planets in If f l =1 in a million, then we have planets in the Galaxy 1500 life-bearing If f l =1 in a billion, then we are on the bearing planet in the Galaxy ONLY life-
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
•
N civ = N * f P n e f l f i f t L civ / L gal
• • • • • • •
N civ = number of current civilizations in the Galaxy N * = 3 x 10 stars”; n e 11 stars; f P = 1/200 fraction of “suitable =1 suitable planets per suitable stars f l = fraction of these planets where life forms f i = fraction of life-bearing planets where intelligence evolves f t = fraction of intelligence-bearing planets where technology develops for communication L civ L gal = average lifetime of a civilization = 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 non technological 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 …
• • • •
N civ = N * = 3x10 11 f P n e f l f i f t L civ / L gal * 0.005 * 1 * 1 * 1 * 1 * L civ / 1.3x10
10 yrs = 0.1 * L civ = L civ / 10
• •
Carl Sagan estimated this as 10 L civ Pessimistic case would be L difference) civ (in 1974) / 10 billion (a BIG
19
• • •
Civilization Lifetime
So … N civ depends an awful lot on L civ 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 90/10 10
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 ~10 8 yrs Can technology avoid even that?
Gamma-ray bursts can’t be avoided One heated atmosphere already (!) March 3, 2003 But … timescale is also >10 8 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 N civ = L civ / 10
•
So … anywhere from 8 civilizations to 1000 to millions (!)
•
A pessimistic estimate is L civ / 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
R 2 /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 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.
3.
4.
4.
LIFE WILL NOT ARBITRARILLY EXTERMINATE ITSELF.
TECHNOLOGY WILL GROW AS IT HAS ON EARTH.
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.
3.
THEY WILL DETERMINE THAT THEIR CIVILIZATION IS DOOMED AT SOME POINT.
THEY WILL COLONIZE THE UNIVERSE.
THEY WILL HAVE FOUND THE EARTH AS A HABITABLE PLANET.
SO: WHERE ARE THEY?