Contacting life "out there."

Key points: Drake equation; most uncertain terms in predicting existence of communicating life

"To consider the Earth as the only populated world in infinite space is as absurd as to assert that in an entire field sown with millet only one grain will grow.''-- Metrodorus of Chios (fourth century B.C.)

It seems likely there is life on the scale of stromatolites on many other planets - this type of life formed so quickly on Earth and is based on very robust bacteria. However, it wouldn't be much fun to try to carry on a conversation with alien bacteria. It is hard to tell on how many planets complex life (multi-cellular, specialized) has formed, and still more difficult to guess how many cases have led to intelligent beings. 

Our (maybe romantic) dream is to make contact with another civilization. To have one out there to talk to requires

"Sometimes I think we're alone. Sometimes I think we're not. In either case, the thought is staggering."

-- Buckminster Fuller

Although it looks easy in the movies and on TV just to hop on a rocket and find other civilizations, there are some major difficulties to be solved buttonex.jpg (1228 bytes)(See also http://www.lerc.nasa.gov/WWW/PAO/warp.htm) The best clue we can acquire with our existing technology is to "listen" for radio and television signals that might be coming (accidentally) from a civilization. Our civilization has been sending out such signals for about 70 years, so any civilization on the nearest stars that had been listening would now know that we exist. If they had figured out how to decode our transmissions, they might know a lot more!!

 

Quantifying the probability of success

How probabilities work:

If event A happens a fraction z of the time and event B happens a fraction y of the time, then both A and B will happen z times y of the time.

For example, if one coin comes up heads 0.5 of the time, then two coins will both come up heads

0.5 x 0.5 = 0.25 or 1/4 of the time

The Drake equation attempts to quantify the number of civilizations in the Milky Way capable of sending us signals (other life forms could well exist, but if we cannot learn of their existence, they might as well not exist for the purposes of science (and philosophy!))

The equation gives the number of intelligent and communicating civilizations:

N = N* x Nplanets x fplanets x plife x fintelligent x fradio x [ Lcivilization/L*]

 

We can now evaluate the terms in the Drake Equation. Each picture links back to the discussion of the topic.

1.) Number of stars in the galaxy: well determined, say 200 billion. mway1.jpg (157431 bytes)
2.) Number of planets per star: recent discoveries suggest that planets are common, so we can't be too far wrong if we say an average of 5. dopplerplanet1.jpg (31678 bytes)
3.) Fraction of planets suitable for life: the requirements are:
  • planet must be massive enough to retain an atmosphere
  • but it can't be so massive it also retains hydrogen and helium
  • it must be at the right distance from the star for a "comfortable" temperature
  • the star can't be too hot (too much ultraviolet will kill!)
  • the star can't be too cool (too little ultraviolet to make amino acids)

    This fraction is still pretty much speculation, but given all these conditions it  could be pretty small -- say 0.001 (?), 1 in a thousand.

habzone.gif (54161 bytes)
4.) Fraction of suitable planets where life forms: since life formed so quickly on Earth, we can assume the fraction is reasonably large, say 0.5. archean.gif (79135 bytes)
5.) Fraction of planets where life evolves to intelligent life: this appears to have been difficult on Earth (it took 4 billion years!). This single case suggests that a long-lived star like the sun is required. However, we have no independent checks so any answer will be very uncertain. We just guess at 0.1 (?). elevel.jpg (29712 bytes)
6.) Fraction of intelligent life forms that use radio: radio is a general and powerful  communication tool, but wouldn't be used by marine-dwelling intelligence for example. So we would be moderately safe with 0.3. greenbank_nrao.jpg (31894 bytes)
7.) Duration of radio-communicating period: highly uncertain. Could intelligent life overwhelm its planet and kill itself off through overcrowding and the resulting  famine, disease, and war? Then the duration would be only 100 - 1000 years. Or maybe the duration is set by the time between major asteroid-induced extinction events, 100 million years for Earth. Just to pick something in between, we can take 100,000 years, giving a fraction of the stellar life of  100,000/5 billion = 2 x 10-5 (???). impactpowerchart.gif (12004 bytes)

The numbers in blue would suggest about 300 radio communicating civilizations in the Milky Way. The most uncertain term, however, is the lifetime of a civilization, and the number of radio communicators could easily be 1 (us)! However, reviewing what we do and do not know, it appears likely that there are many planets with some form of primitive life, based on the rapidity of the appearance of primitive life on Earth. The biggest uncertainties in whether there are beings to communicate with are in the probability of intelligent life evolving and then in the behavior of that life!

A minor adjustment (within our uncertainties) in, say, the fraction of life that evolves to a high level of intelligence to 0.01, and to the lifetime of a radio emitting civilization to 10,000 years would leave us with 0.3 radio emitting civilizations in the galaxy - we are then a statistical anomaly, and should not expect that there is anyone to listen to.

However, Scientists in general have a prejudice against any class of event that happens only once and hence many scientists prefer the more optimistic estimates. And besides, their implications are much more interesting.

Finding that there is another civilization out there would be fascinating, particularly if we could understand its thought processes and activities by decoding the transmissions. It would be extremely exciting to scientists and many others -- who knows what form the transmissions would take, or in fact how we would reactlink to an extra topic

For more about SETI, try http://www.seti.org/

Test your understanding buttongrad.jpg (11232 bytes)

thinkera.jpg (8490 bytes)

 

 

 

Rodin's "The Thinker", from Cleveland Museum of Art, http://www.clevelandart.org/educef/rodin/html/index.html

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hypertext copyright.jpg (1684 bytes) G. H. Rieke