Page Content (OCR)
56 Probabilities A further revision was presented in the SETI 2020 report, published in 2002. Here, n(e) is the number of planets per planetary system with an environment suitable for life; f(c) is the fraction of civilizations that develop a technology that releases detectable signs of their existence into space; L is the average length of time such civilizations release detect- able signals into space. Even as revised, the equation still focuses on civilizations that send out electromagnetic signals that we can detect. There could be many others that have not reached this stage or that left it behind long ago. Although a calculation of probabilities sounds scientific, the way that we have quantified the factors in the Drake equation has been heavily influenced by opinions and beliefs. As Goldsmith observed, everyone who deals with the probability of extraterrestrial life has some bias for or against finding another civilization.’ Different scientists, inserting different numbers for each factor in the equation, have come up with wildly varying conclusions. Those most optimistic about finding extraterrestrials once dominated discussions of this question. Drake, Sagan, and their colleagues proposed in 1961 that there are somewhere between 1000 and 100 million advanced civilizations in our galaxy. Nearly 30 years later, Sagan still estimated about 1 million civilizations more advanced than our own."° Asimov went farther, suggesting that technological civilizations may have developed on as many as 390 million planets in our galaxy and that nearly all of them are more technologically advanced than we are. MacGowan and Ordway estimated that 3 billion stars in the Milky Way have evolved intelligent communicating societies, although we need to know how many still exist." At the other extreme, deniers derive numbers as low as one communicat- ing civilization—our own. They back up their pessimism about SETI by inserting lower estimates of probability for one or more factors. When isolationists refuse to support SETI because they think we are alone, their hypothesis becomes a self-fulfilling prophecy.’ One can visualize the evolution of communicating civilizations as passing through a series of bottlenecks, suggested Rood and Trefil. All that is necessary to get a pessimistic result is to make one of them very narrow. For the optimistic result to hold, all of the bottlenecks must be wide.’ Individual preferences determine the width of these bottlenecks. Bota- nist William Burger, inserting his own arbitrary numbers for a long list of factors, produced an estimate of 3 to 30 Earth-like planets with technolo- gical civilizations in our galaxy.'* The derived conclusion is not absence, but scarcity. As Brin saw it, everyone involved in this debate has a favorite factor that they love to suppress. Uniqueness partisans squelch the number of stable