Saturday, July 22, 2006

Listening – Part 1

I am sorry to say that there is too much point to the wisecrack that life is extinct on other planets because their scientists were more advanced than ours. ~John F. Kennedy

Seth Shostak writes a regular SETI (Search for Extraterrestrial Intelligence) column for Space.com. The other day, he wrote a nice piece about misconceptions that the average person has about the SETI Project. Mr. Shostak obviously has a sense of humor as well as a ton of dedication. Good thing, since he obviously needs both in his line of work. SETI is the longest of long shots, particularly given its low funding levels and intermittent nature.

However, it was his mention of Frank Drake that got me to thinking about life “out there.” If you've ever watched or read Carl Sagan's Cosmos, you may recall the section where discusses an elaborate formula for calculating the number of technological civilizations that might be found in the galaxy. The equation was Frank Drake's creation, although probably not exactly in this form. In fact, Dr. Sagan presented a somewhat different version in a book he co-wrote fourteen years earlier, in 1966, with Russian I. S. Shklovskii, Intelligent Life In The Universe.

I need to digress a bit here. The Shklovskii book was an interesting collaborative effort. Shklovskii had written a book on extraterrestrial life published in the Soviet Union. Sagan, who had already corresponded with Shklovskii for several years, proposed that the two of them write such a book for the U. S. market. Shklovskii, in those Cold War days, couldn't leave Russia; Sagan couldn't go there. So Sagan took segments of Shklovskii's book and added his own sections (which he clearly marks as his own, so as not to co-opt Shklovskii's work). He then sent the chapters to the Soviet Union for Shklovskii's approval and/or revisions. What emerges is a well-written book that is not for the “UFOs kidnapping drunks offen the bayou” crowd. It is meticulous, filled with calculations, and a practical look at the possibilities of life elsewhere in the universe.

It's also got cartoons scattered through it. All work and no play ...

At any rate, Drake's equation, as expressed by Sagan in Cosmos computes a value for the number of technological civilizations (N) which is the product of the following factors (to save myself some HTML code, I'm using different literation):

S the number of stars in the galaxy, valued at about 4x1011:
P the fraction of stars with planets, 1/3;
E the number of planets in a system suitable for life, 2;
L1 the fraction of planets where life arises at least once, 1/3;
I the fraction of planets where intelligence appears, combined with T below for a value;
C the fraction of planets where intelligence leads to a technical civilization, with IxC=0.01;
L2 the fraction of a planet's lifetime is marked by a technical civilization, anywhere from 10-6 to 0.01.

The lower end of L2 yields a value of N=10, meaning 10 civilizations in the whole galaxy, making the job of the SETI project nigh on to impossible. If one takes the wildly optimistic value of .01, N equals about 10,000,000.

By way of comparison, here's how Sagan approached this formula in 1966. N is now the product of the following:

R the rate of star formation in the galaxy, valued at 10 stars/year;
P the fraction of stars with planets, valued at 1 (!);
E the number of planets in a system suitable for life, 1;
L1 the fraction of planets where life arises at least once, 1 (!);
I the fraction of planets where intelligence appears, 0.1;
C the fractons of planets where intelligence leads to a technical civilization, 0.1, with IxC=0.01;
L2 the lifetime of a technical civilization, anywhere from less than a hundred years to 100,000,000.

In the 1966 book, Sagan takes the optimistic view that L2 could be set at 10,000,000, leading to N=1,000,000. The different methods are interesting in themselves, but I was caught by how the estimate for P dropped from 1, in 1966, to 1/3. Current planet-hunters would tend toward his 1966 conclusion rather than the 1980. The overall approach seems to indicate that in 1966 the appearance of life was more of a sure thing, but there were fewer worlds suitable for life. In the 1966 book, this simple calculation is followed by summaries of much more complex approaches to the problem.

Basically, the results boil down to this: If technical civilizations last a long time, there are plenty of planets out there to talk to. If not, there's plenty of planets that had, have, or will have life, but none of us will be around long enough to carry on much of a conversation. Even if the civilizations are long-lived, calculations indicate that, on average, they would be anywhere from a few hundred to a few thousand light years apart. Therefore, conversations that do occur are going to have a lot of long pauses. And commuting from here to there is going to be murder.

I've got more to say, but I'll do that in Part 2.

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