Lec33(Intelligent Life in the Univ) - Cornell · PDF fileThe ecosphere size varies with...

Lec. 33: Intelligent Life in the Universe

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Intelligent Life in the Universe

Lecture 33 APoD: Easter Island Eclipse

Lec 33: Intelligent Life in the Universe 2

In-Class Question

1) Do you think life exists elsewhere in the Universe?

a) Yesb) Noc) Don’t knowd) Don’t care


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Lecture Topics

Probabilities

Rates and totals

The Drake equation Computes the expected number of

technical civilizations in the galaxy

Are we alone?

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, in particular, other civilizations might exist in the galaxy.


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Probabilities

How many dates might a guy get in this class?

N D

ND = number of dates

NW f ask f accept f show up _

NW = number of women in the classfask = fraction he asks out

faccept = fraction that acceptfshow_up = fraction that show up


Suppose Nw = 100 Shy guy: fask = 0.02 (2%) faccept = 0.50 (50%)

fshow_up= 1.00 (100%)

ND = 100 x 0.02 x 0.5 x 1.0 = 1 date

Outgoing guy: fask = 0.20 (20%) faccept = 0.10 (10%)

fshow_up= 0.50 (50%)

ND = 100 x 0.2 x 0.1 x 0.5 = 1 date


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Rates and Totals

Suppose

R* = Rate at which stars are born

tl = Average lifetime of a star

How many stars are alive at a given time?

The number of stars is:

N = R* x tl ( Rate times time )


Stars dead Stars not yet born

Total number of stars alive

Suppose: R* = 1 star/year (represented by spikes above).

Time

NowDeath line

10 yrs

10 stars would be alive at any given time.

And stars live only 10 years.


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Number of civilizations

Suppose that each star developed a civilization.

If the lifetime of the civilization is tl then the total number of civilizations alive is:

N R tT l But this isn’t the whole story ....


The Drake Equation

Attempts to quantify the number of civilizations that might exist in the galaxy.

Named after, Frank Drake pioneered this analysis

while at Cornell


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NT = Number of technological civilizations in the galaxy.

R* = Rate at which stars are born, averaged over the lifetime of the galaxy. (Stars/year)

fp = Fraction having planetary systems.

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

N R f f f f f tT p h s i t l


N R f f f f f tT p h s i t l

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

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

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

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


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N R f f f f f tT p h s i t l N f f f f f tTstarsyear p h s i t l 10

R* = Rate at which stars are born, averaged over the lifetime of the galaxy. (Stars/year)

There are ~100 billion stars in the galaxy today.

And the galaxy is about 10 billion years old. R* ~ 10 stars/year


N f f f f f tTstarsyear p h s i t l 10N f f f f tTstarsyear h s i t l 10 1

fp = Fraction having planetary systems.

If our understanding of star formation is correct, then planets are a natural consequence.

All stars could have planets, so we take

fp ~ 1

However, only ~5% of nearby sun-like stars have giant planets (depends highly on metallicity).


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N f f f f tTstarsyear h s i t l 10 1N f f f tTstarsyear s i t l 10 1 1

10

fh = 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

fh ~ 1/10

Accept only F, G and K stars.


Caveats: Galactic Habitable Zone Region in the Galaxy over which life and life

bearing worlds are likely to exist Requirements

Available material to build planets High enough metallicity to produce terrestrial planets Right mix of “heavy elements” to radioactively heat core

of planet (drives plate tectonics which regulate CO2 in the atmosphere)

Seclusion from cosmic threats Impacts by asteroids (depends on Jupiter) and comets

(affected by galactic tides, GMCs, and passing stars) Blasts of radiation (active galactic nucleus outbursts,

supernovae, and gamma ray bursts) Orbit near “co-rotation” circle – place where orbital period

of star equals rotation period of spiral arm pattern.


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MetallicityIn the outer parts of the galaxy, the metallicity will be too low for giant planet formation

Supernovae and stellar encounters are much more frequent in the interior of the galaxy

Galactic Hazards


Galactic Habitable Zone


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N f f f tTstarsyear s i t l 10 1 1

10


How likely is it life will 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.


The Urey-Miller Experiment

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.


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Cyril Ponnamperuma 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 means.

Astrobiology Studies the origin, evolution, and possible future of

life in the Universe

This is an area of active research

Primordial Soup


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Creating Organics is easy

Using better knowledge of the primordial ocean and atmosphere.

Various energy sources can produce amino acids and nucleotide bases.

Energy sources such as: solar UV radiation, lightning, volcanic heat, natural radioactivity, and atmospheric shock waves produced by meteorites.


N f f f tTstarsyear s i t l 10 1 1

10N f f tTstarsyear i t l 10 1 11

10


Making organics is easy, but creating life may not be. Some might argue that under the right conditions life has to happen.

Most optimistic case:fs ~ 1


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In-Class Question

1) What is the galactic habitable zone of the Milky Way?

a) Sufficient metals the build planetsb) Seclusion from cosmic threatsc) Inner regions of the galaxyd) a and be) b and c


N f f tTstarsyear i t l 10 1 11

10N f tTstarsyear t l 10 1 1 11

10

fi = 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:

fi ~ 1


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Dinosaurs and extinction Dinosaurs “ruled” the world for ~ 100 million

years, but were pretty stupid (technically). Was the mass extinction (due to an asteroid

impact) of the dinosaurs necessary for Homo Sapiens to evolve?


Other influences?

What role did Jupiter and Saturn have in allowing life to form on Earth. Cleared out cometary objects!

But also deflects them too

The Moon Stabilizes the orientation of the Earth’s spin axis

Otherwise we could have “days” that last a whole year!


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N f tTstarsyear t l 10 1 1 11

10N tTstarsyear l 10 1 1 1 11

10

ft = 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 as inevitable:

ft ~ 1



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.

N tTstarsyear l 10 1 1 1 11

10


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N tTstarsyear l 10 1 1 1 11

10NTstarsyear years 10 1 1 1 1 101

106


Suppose the average lifetime of a technical civilization is 1 millions years

1% of the reign of the dinosaurs

100 times longer than human civilization has existed!

1 million civilizations in our galaxy.


Uncertainties!

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.


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How far to our neighbors?

For 1,000,000 civilizations in the galaxythe average distance between them

will be ~ 150 ly!!! two-way communication will take at

least 300 years! But this is a large over prediction since

the Galactic Habitable Zone has much, much less than 1011 stars


How far? (cont’d)

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!

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