Chapter 11: The Value of "L" and the Cosmic Bottleneck

The Drake Equation, a commonly used starting point for discussions about the likelihood of finding extraterrestrial intelligence, is now nearly a half-century old. It dates from 1961, a year after Frank Drake made his pioneering radio search for artificial signals from other worlds. That search, dubbed "Project Ozma," was a 200-hour scrutiny of two nearby, Sun-like stars for transmissions spectrally situated near the 1420 MHz line of neutral hydrogen, and was conducted with an 85-foot-antenna at the National Radio Astronomy Observatory in Green Bank, West Virginia (Kellermann and Seielstad 1985; Drake 1960). These efforts to find easy evidence of intelligence in other star systems provoked considerable public interest, including a major article in the Saturday Review (Lear 1960).

As a sequel, Drake organized a two-day conference a year later on searching for sentience in the galaxy - the so-called Green Bank Conference. The invitees comprised approximately 10 astronomers, biologists, and technical specialists. As a conference agenda, Drake composed a simple, linear equation (Drake 1965) for estimating the number of galactic civilizations that are sending signals we could detect. The last term in this famous formula is L, the lifetime of a signaling society. L is sui generis among the equation s factors for two reasons:

1. It is dependent on sociology, not astronomy or biology (the only other term that is similar in this regard is f(sub c), the fraction of intelligent species that develop a technical civilization).
2. It is arguably the term that we know, and perhaps can know, least about. At a conference in 1971, Carl Sagan noted that in trying to evaluate the terms of the Drake Equation "We are faced . . . with very difficult problems of extrapolating from, in some cases, only one example and in the case of L, from no examples at all. When we make estimates we cannot pretend that these values are reliable." (Sagan 1973).

This is a daunting caveat. It has not, however, squelched speculation on the value of L. The fact that these estimates are speculative can be gauged by the degree to which they differ. In a compilation by Steven Dick, published estimates for L range over five orders of magnitude (Dick 1996).

Clearly, the chances of finding a signal with SETI experiments depend strongly on the value of L. For example, if the invention of nuclear weapons is always nearly simultaneous with the development of radio and laser technology (as is the case for Homo sapiens), then it is seductive to argue that when a species is technically mature enough to make its presence known from afar, it is also ripe for effecting its own destruction. In that case, L might be only a few centuries or less, and the opportunity for intercepting a signal is very limited. Having some inkling of what L might be - even if that estimate has an uncertainty of a magnitude or two - is significant in motivating (or perhaps demoralizing) those seeking evidence of intelligence elsewhere.

The other reason for considering the value of L, quite independent of SETI, is that as a matter of self-interest, it s clearly of consequence to know if our species - or at least our culture - can reasonably hope for a long future.