Longevity of Civilizations

ASTR 1420

Lecture 22

from a Nature paper

Drake Equation (Carl Sagan’s version)

N number of transmitting civilizations

N = N* × fplanet × fE × flife × fintell × fciv × fL

× × × ×

× × =

N

N* fplanet fEarth flife

fintell fciv flong

Delta t argument (Richard Gott 1993, Nature, 363, 315)

Tbegin TEnd

Past Future

If there is an End, where do we stand now in the time axis?

TNow

Lifetime of an event

Delta t argument (Richard Gott 1993, Nature, 363, 315)

Tbegin TEnd

Past Future

• t will range between 0 and 1.

• Calculates a probability of t being in the first or last 2.5%.

Probability of 0.025 ≤ t ≤ 0.975 ?

95%

• At the 95% confidence level, t will NOT be in the beginning 2.5% or in the ending 2.5% range.

€

t ≡Tpast

Tend −Tbegin

Delta t argument (Richard Gott 1993, Nature, 363, 315)

Tbegin TEnd

Past Future

€

t ≡Tpast

Tend −Tbegin

• 0.025 ≤ t ≤ 0.975

• Similarly, at the 99% confidence level

€

1

39TPast ≤ TFuture ≤ 39TPast

€

1

197TPast ≤ TFuture ≤197TPast

Delta t argument

€

1

39×Tpast < Tfuture < 39 ×Tpast

• the length of time something has been observable in the past is a rough measure of its future observability…

History of Human

• Homo Sapiens : ≈200,000 years.

o 200,000 / 39 < Future < 39*200,0005128 years < Future < 7.8 million years

• For our human civilization of 10,000 years

o 10,000 / 39 < Future < 39 * 10,000256 years < Future < 390,000 years

• Our industrial civilization of ≈200 yearso 200 / 39 < Future < 39 * 2005 years < Future < 7,800 years

If aliens are like human

6,400 ≤ N ≤ 9.8 million

R* 20 stars/yr

fplanet 1

nE 0.5

flife 0.5

fintell 0.5

fciv 0.5

L 5100 yrs

R* 20 stars/yr

fplanet 1

nE 0.5

flife 0.5

fintell 0.5

fciv 0.5

L 7.8 million yrs

SETI Debate

Ernst Mayr• Bioastronomy News (1995, V7, No. 3)

• German/American biologist• Harvard Biology Prof.• 7/5/1904 – 2/3/2005

Carl Sagan • Bioastronomy News (1995, V7, No. 4)

• We need “functional equivalent of human” not “prevalent humanoids”…

• Radio technology If Aztec civilization survived, would they develop radio technology in several millenia?

• We, humanoids, are very young, but we have ~5 billion years to spare in the future.

• American Astronomer• Cornell Professor• 11/9/1034 – 12/20/1996

SETI debate

Ben Zuckerman• UCLA Astronomy professor

Seth Shostak• SETI Astronomer

Zuckerman

3 simple postulates have major implications for SETI

1. after the development of technology, all civilizations will build similar TPF space telescopes

2. Intelligent life is curious about other life forms

3. Once they use space telescopes to discover nearby habitable or life-bearing planets, curiosity will lead them to visit these planet

4. Therefore, SETI program need to look more distant stars!

Shostak

• Zuckerman’s argument is only an extended version of Fermi Paradox

• Interstellar travel is not as easy as Zuckerman assumes

• Also, within 500 light years of us we can only find about a million star systems, so the probability of worlds with intelligent beings is smaller still.

In summary…

Important Concepts• Delta t argument

• Statistical approach to the longevity of civilizations

• Logics behind SETI proponent and opponent

Important Terms

UFOs : next class!