November 15, 1998
The author is confident that alien life-forms are out there just waiting to be discovered.
By JOHN DURANT
Why There Must Be Intelligent Life in the Universe.
By Amir D. Aczel.
230 pp. New York:
Harcourt Brace & Company. $22.
very thoughtful person wonders about the possibility that life exists elsewhere in the universe. Hollywood has made a lot of money from movies based on the assumption that alien life-forms exist; and now Amir D. Aczel sets out to prove they do -- although there has been no well-authenticated ''contact,'' and despite the fact that life on earth is the only life we actually know.
Aczel is a mathematician, and his approach to the problem is to try to calculate the probability that life exists on at least one other planet. In gambling terms his title, ''Probability 1,'' means: take the bet; it's a certainty. Aczel assures us we can be confident that alien life-forms are out there just waiting to be discovered. Now, all my sympathies are with him. I hope very much there is life elsewhere in the universe -- if there isn't, what a terribly boring place the rest of the universe will turn out to be! But so far as proof is concerned, Aczel's argument leaves me entirely unconvinced.
I have no quarrel with the way Aczel tackles the problem. His starting point is a famous equation invented by the astronomer Frank Drake in 1961. Drake was a pioneer in the quest for extraterrestrial intelligence through the search for alien signals arriving from deep space. He wanted to know how many civilizations in space are currently capable of communicating with other civilizations. Drake's equation breaks the question down into a series of discrete steps: first, estimate how many potentially suitable extraterrestrial homes there are; then calculate how many of those are inhabited by extraterrestrial life-forms; and then establish how many of these life-forms have managed to invent satellite dishes or their equivalents.
In essence, ''Probability 1'' is a review of Drake's equation in the light of scientific evidence. Aczel argues convincingly that the number of planets suitable for life is extremely large. There are about 300 billion stars in our galaxy and possibly 100 billion galaxies in the universe -- which would yield 30,000,000,000,000,000,000,000 stars in the universe. Some may quarrel with Aczel's claim that as many as half of these stars can be presumed to have planets, and that on average one in nine of these planets is suitable for life; but few, I think, will disagree with his conclusion that there are a very large number of planets suitable for life.
The trouble starts when we come to estimate the probability that life will emerge on any of these planets. The problem is that we don't have a good theory of the origin of life on earth. Without one, it is anybody's guess how likely this event actually was. Out of thin air Aczel conjures the figure of 1 in a trillion for this likelihood and concludes that the probability of life existing on at least one other planet is virtually 1.
This is statistical sleight of hand. What if the real value for this crucial probability is far, far smaller? What if, in fact, the probability of life appearing in any one suitable home is tiny in relation to the number of suitable homes available? Now the answer to our question changes. Instead of the probability approaching 1, it can just as easily approach 0. By adjusting the value of the probability of life appearing, we can make the probability of life existing elsewhere in the universe pretty much anything we like.
Statistics are extremely powerful and important, and Aczel is a very clear and capable exponent of them. But statistics cannot substitute for empirical knowledge about the way the universe behaves. We now have no plausible way of arriving at robust estimates for the probability of life arising spontaneously when the conditions are right. So, until we either discover extraterrestrial life or understand far more about how at least one form of life -- terrestrial life -- first appeared, we can do little more than guess at the likelihood that life exists elsewhere in the universe. And as long as we're merely guessing, we should not dress up our interesting speculations as mathematical certainties.
John Durant is an assistant director of the Science Museum and a professor of public understanding of science at Imperial College, London.
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