• Probabilities
• Rates and totals
• The Drake equation
  • Computes the expected number of technical civilizations in the galaxy

• Do other civilizations exist in the galaxy or elsewhere?
• How might we estimate this statistically and what are the uncertainties?
• We would like to quantify whether life and other civilizations, in particular, might exist in the galaxy.

• But this isn't the whole story ....

f_h = Average number of life-suitable (habitable) planets within those systems having planets.

• The ecosphere size varies with stellar type, but we might expect the odds to be similar to our solar system, so we choose
  
  • f_h ~ 1/10
• Accept only F, G and K stars.

To be discussed in class -

There are specific regions in the galaxy which may be more suitable to the formation of terrestrial worlds and survivability of life.

f_s = Fraction of habitable planets on which at least simple life arises.

• How likely is it for life to form? Is life rare?
• It is certainly complex!

Laboratory experiments show that complex organic molecules can be formed in an atmosphere similar to that expected on the early earth.

• Harold Urey and Stanley Miller (1953)
• Made "primordial soup" mixture
  • water, methane, carbon dioxide, ammonia
• Passed simulated lightning through it.
• Produced "gunk" containing many of the amino acids found in life today.

• About a decade later constructed nucleotide bases in a similar manner.
• Both experiments did not closely resemble the early atmosphere.
  • But showed biological molecules can be synthesized by nonbiological methods.

• Using better knowledge of the primordial ocean and atmosphere.
• Various energy sources can produce amino acids and nucleotide bases.
• Possible energy sources: solar UV radiation, lightning, volcanic heat, natural radioactivity, and atmospheric shock waves produced by meteorites.

f_s = Fraction of habitable planets on which at least simple life arises.

• Making organics is easy as demonstrated by experiments, but creating life may not be. Some argue that life happens only under the right conditions.
• Most optimistic case:
  • f_s ~ 1

f_i = Fraction of life-bearing planets on which intelligence evolves.

• The appearance of a well-developed brain might not happen if left to random chance.
• But, natural selection tends to single out the more adaptable, more intelligent species.
• The optimistic view takes intelligence as inevitable:
  
  • f_i ~ 1

• Dinosaurs "ruled" the world for ~100 million years, but were pretty stupid.
• Was the mass extinction of the dinosaurs necessary for Homo Sapiens to evolve?

The Sky is Falling: Meteor Impacts

• A 1 km rock, traveling at 30 km/sec:
  • Equivalent to 170,000 Megatons TNT.
  • Not a global effect
  • A 10 km object releases 1000 times more energy
  • One hits every 10-50 million years.
  • 5 x 10⁹ people/ 10 x 10⁶ yrs = 500 people/yr
    (higher probability than a plane crash)

f_t = Fraction of those intelligent life planets that develop a technological society.

• It is hard to imagine an intelligent species avoiding technology.
• Technical civilizations arose independently in many areas of the world.
• Taking technological development to be inevitable,
  • f_t ~ 1

t_l = Average lifetime of a technological civilization. (years)

• How long does a technical civilization last?
• We've had one for ~100 years.
• There are many unknowns to our own future, let alone predicting how long another civilization might last.
• Suppose the average lifetime of a technical civilization is 1 million years
  • 1% of the reign of the dinosaurs
  • 100 times longer than human civilization has existed!
    
    1 million civilizations in our galaxy.

• Important - each term in the Drake equation (probably) gets more uncertain when proceeding from left to right.
• For lack of a better example we have adopted an Earth/human bias when estimating various terms.
• We do not know the uncertainties.

• For 1,000,000 civilizations in the galaxy
  • the average distance between them will be ~ 150 ly!!!
    
  • two-way communication will take at least 300 years!
• If the lifetime of a technical civilization is less than 3000 years
  • Average distance is so large that civilizations will die, on average, before two-way communications can be established!