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Probability 1: Why There Must Be Intelligent Life in the Universe
By Amir D. Aczel
Harcourt Brace
230 pp.; $22
Amir Aczel's Probability 1 argues that there must be intelligent life on other planets. Here is his argument: The probability of intelligent life arising on a habitable planet near a sun like ours cannot be all that small since here we are on planet Earth—evolved life forms with intelligence. Recent evidence indicates that planets and suns like ours are abundant throughout the universe. Provided there are sufficiently many opportunities for an event to occur, no matter how small the probability so long as it is positive, we can say with a probability arbitrarily close to 1 that the event will occur; that is, it is nearly certain. The probability of intelligent life arising on a planet like ours is not too small and the number of such planets gives all indications of being huge. QED.
Aczel's argument thus balances the probability of an event against the probabilistic resources for bringing about that event. The probabilistic resources for an event are the number of opportunities for its occurrence. Consider a coin tossed ten times. The probability of tossing ten heads in a row is approximately one in a thousand. Now, if all you have are ten coin tosses, then it's unlikely that you'll witness ten heads in a row. But if you have a million coin tosses, it's virtually certain (i.e., the probability is close to 1) that at some point you'll toss ten heads in a row.
The problem with Aczel's argument is that he never addresses the tension between the probability of an event and its probabilistic resources. He blithely assumes that intelligent life isn't all that improbable and that the probabilistic re sources (i.e., hospitable planets for intelligent life) are sufficiently abundant to render intelligent life a virtual certainty. But "the probability of life occurring on a single planet that is already within its star's habitable zone"—the probability so crucial to Aczel's argument—is not a well-defined quantity. To be sure, Aczel arbitrarily assigns it a number—"one in a trillion." Thus, by assuming a billion trillion habitable planets as probabilistic resources, Aczel renders life a virtual certainty (i.e., renders its probability effectively 1—the billion trillion probabilistic resources essentially wash out the one in a trillion improbability for intelligent life forming).
But what if the probability of intelligent life forming is not one in a trillion, but one in 10^40,000 (i.e., one followed by 40,000 zeros—a number incomparably bigger than a trillion)? This, for Fred Hoyle, is the probability of life forming in the entire universe throughout its entire duration. In that case, Aczel's argument falls apart. Indeed, in that case, despite a billion trillion habitable planets, intelligent life will still be vastly improbable, with probability nowhere near 1.
The subtitle of Aczel's book, "Why There Must Be Intelligent Life in the Universe," is therefore misleading. (The dust jacket even refers to the book as offering "proof" that we are not alone: "Aczel shows how the history of space discovery and probability theory come together to prove that we are not alone in the universe.") Aczel establishes nothing like that there must be intelligent life elsewhere in the universe. At best he establishes the conditional claim that if the probability of intelligent life arising on a habitable planet like ours is not too small and if the number of such planets is very large, then the probability of intelligent life elsewhere in the universe will be arbitrarily close to 1. This conditional claim is far less ambitious than his original one and is frankly not very interesting. Clearly, it makes for a much more exciting story to learn that we are not alone—that extraterrestrial intelligence is real—than to learn that the evidence is thoroughly inconclusive. Alas, the evidence for extraterrestrial intelligence is in fact inconclusive, and Aczel's book does nothing to change that.
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