Still, statistically the probability that there are other thinking beings out there is good. Nobody knows how many stars there are in the Milky Way—estimates range from 100 billion or so to perhaps 400 billion—and the Milky Way is just one of 140 billion or so other galaxies, many of them even larger than ours. In the 1960s, a professor at Cornell named Frank Drake, excited by such whopping numbers, worked out a famous equation designed to calculate the chances of advanced life in the cosmos based on a series of diminishing probabilities.

不过，从统计角度来看，外层空间存在有思想的生物的可能性还是很大。谁也不清楚银河系里有多少颗恒星--估计有1000亿颗到4000亿颗--而银河系只是大约1400亿个星系之一，其他许多比我们的银河系还要大。20世纪60年代，康奈尔大学的一位名叫弗兰克?德雷克的教授为这么巨大的数字所振奋，根据一系列不断缩小的可能性，想出了一个著名的方程式，旨在计算宇宙中存在高级生命的可能性。

Under Drake's equation you divide the number of stars in a selected portion of the universe by the number of stars that are likely to have planetary systems; divide that by the number of planetary systems that could theoretically support life; divide that by the number on which life, having arisen, advances to a state of intelligence; and so on. At each such division, the number shrinks colossally—yet even with the most conservative inputs the number of advanced civilizations just in the Milky Way always works out to be somewhere in the millions.

按照德雷克的方程式，你把宇宙某个部分的恒星数除以恒星可能拥有行星系的数；再用那个商除以理论上能够存在生命的行星系数；再用那个商除以已经出现生命，而且生命提高到了有智力的状态的行星系数；如此等等。每这样除一次，那个数字就大大缩小--然而，即使以最保守的输入，仅在银河系里，得出的高等文明社会的数字也总是在几百万个。