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Probabilities: Longevity The search for extraterrestrial intelligence is principally a test to determine L. Our ability to answer this question is extremely limited. Soviet astronomer V.A. Ambartsumian summarized the problem in 1971: We do not have even a single example of the lifetime of a technical civilization.” As we know of only one technological species—our own—we are in effect trying to predict our own future. Just as there is as yet no predictive theory of biology, so there is no pre- dictive theory of history.’ We do not know how long our own civilization will survive. We have only opinions, making L the most politically loaded factor in the Drake equation. In the classic SETI paradigm, only a large value of L leads to significant numbers of civilizations coexisting at a given time. As Sagan saw it, L is strongly biased toward the small fraction of technological civilizations that achieve very long lifetimes (implying that most do not). That does not necessarily mean that they would be sending signals; the lifetime of transmitting projects may be far shorter than the lifetime of a technological society. In the original Drake equation, L meant the average lifetime of communicating civilizations, which might be a small subset of the total number of technological societies. The debate about the longevity of civilizations often blurs what we mean by their deaths. Many implicitly assume that extraterrestrial civilizations often end with the extinction of the intelligent beings who belong to them—a drastic outcome. Spoiling a civilization is one thing, and perhaps not too arduous, commented Shapley; complete annihilation is quite another, and vastly more difficult.° 94 —David Schwartzman and Lee J. Rickard, 1988! How Long Do Technological Civilizations Live?