Sphere

by Michael Crichton

“I don’t know what it is, but I know what it isn’t. It isn’t a spacecraft from another civilization.” Ted, standing nearby, turned away in annoyance. Harry and Ted had evidently had this same conversation already. “How do you know?” Norman asked. “A simple calculation,” Harry said, with a dismissing wave of his hand. “Trivial, really. You know the Drake equation?” Norman did. It was one of the famous proposals in the literature on extraterrestrial life. But he said, “Refresh me.” Harry sighed irritably, pulled out a sheet of paper. “It’s a probability equation.” He wrote: “What it means,” Harry Adams said, “is that the probability, p, that intelligent life will evolve in any star system is a function of the probability that the star will have planets, the number of habitable planets, the probability that simple life will evolve on a habitable planet, the probability that intelligent life will evolve from simple life, and the probability that intelligent life will attempt interstellar communication within five billion years. That’s all the equation says.” “Uh-huh,” Norman said. “But the point is that we have no facts,” Harry said. “We must guess at every single one of these probabilities. And it’s quite easy to guess one way, as Ted does, and conclude there are probably thousands of intelligent civilizations. It’s equally easy to guess, as I do, that there is probably only one civilization. Ours.”