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© Copyright 1998, Jim Loy
Pretend that the universe is infinitely large, and is full of stars. I will deal with the alternative assumption later.
Let's ask a question, "How many stars have a certain property?" That kind of depends on what the property is. Let's ask, "How many stars have a diameter of less than a certain amount?" The answer to a question like that is probably either "zero" or "infinity." If the question was, "How many stars are smaller than one inch?" then the answer is zero, such an object cannot be a star. If the question was, "How many stars are smaller than 500,000 miles, then the answer is infinity. In such an infinite universe, probability dictates just those two answers, zero or infinity.
How about some other feature, like color? How many stars have a certain color? We have to define a color as a range of wavelengths of light. Then we either get zero stars or infinitely many. It doesn't matter what feature we are asking about, number of planets, chemical composition...
Well, what if we have a particular star in mind? What if we ask, "How many stars have nine planets?" Assuming that the sun does have nine planets, we can not answer "zero." But, we can't answer "one" either. The answer must be "infinity." This brings us to the question, "How many planets have intelligent life?" Assuming that the earth does have intelligent life, we have to answer, "infinity."
What we see here is that, in an infinite universe, zero and infinity are the rule. One, and other numbers, are so unlikely as to be impossible.
There may be a way to get "one" as an answer, however. "How many stars are exactly like the sun in every way?" We are not talking ranges of numbers here, we are talking exact atoms in exact position at an exact time... It is then reasonable to say that there is only one sun. But is there only one sun? We apply probability. We classify every feature of the sun as a number, including time. And, we get infinity. There are infinitely many stars identical in every way to the sun, including having a planet with a town called Bozeman in which someone named Jim is writing this paragraph. Such stars are very rare.
Actually, the universe is thought to be of a finite size. Then the above arguments are more complex. The rule is no longer zero or infinity. The rule becomes zero, one, or many. And there is only one star which has a planet having a town called Bozeman in which someone named Jim was writing the previous paragraph. "One" becomes a very likely option, when we have a particular star in mind, and when our criteria become very narrow. Numbers besides zero, one, and many become possible, too. But, these are usually very unlikely.
So, is there intelligent life out there? That depends on the probability of life, and on the relative abundance of suitable planets. Many people would find "Yes" to be the most satisfying answer, logically and philosophically.
Pretend that intelligent life just happened once. Why here? Why now? Why us? Many people would say, "Not just once. There must be intelligent life out there."