( u n d e r c o n s t r u c t i o n . . . )
(1) Anthropic Principle
[¶1] It's often suggested that the existence of (intelligent) life on earth is evidence for abundant life in the universe. After all, since earth doesn't seem to be a particularly special planet, the appearance and evolution of life here suggests the same thing is likely elsewheres, among the myriad earth-like planets assumed to exist.
But that's a logical fallacy, by the following simple argument
(which is one among many variations of what's come to be called the
Anthropic PrincipleAnthropic Principle):
[¶3] A more statistical formulation of that same argument goes as follows. Consider a "sample space" consisting of, say, ten billion universes, each one identical to our own. Except, suppose intelligent life is so improbable that it only occurs once. Now, if we, on the "outside" of this experiment, randomly select one of these universes, the probability of finding that life is pretty much zero. And we correctly conclude that life is rare. On the other hand, from the "inside" of the experiment, that one civilization just finds itself in whatever universe it's in: no one-in-ten-billion probability is involved. So, when it begins asking about intelligent life elsewheres, it shouldn't reason just from its own existence that life is abundant. Regardless of life's improbability, any civilization that happens to arise automatically finds itself wherever it is.
[¶4] Thus, answers to questions about the statistical likelihood of life on other planets can only be legitimately answered by looking at other planets, not by looking at our own. We already know there's life on earth; but that's just logically given, a priori, by the Anthropic principle. What we'd really like to know is independent a posteriori, preferably direct empirical, evidence for other life. And so far we've seen none.
(2) Drake's Equation
[¶1] One way to construct a framework for indirectly estimating the probability of extraterrestrial life is Drake's equationDrake's Equation. We try to list the factors required for life to occur, estimate the probability for each factor, and then multiply them all together.
[¶2] For most such factors, probability estimates are ballpark at best. Given the extensive uncertainties surrounding such estimates, one typical simplifying assumption just takes each probability as either 0.1 or 1.0. For example, the probability...
[¶3] For the time being, let's just say that NDrake denotes the number of 0.1 factors required for (intelligent) life in Drake's equation. Then the probability that a randomly selected star hosts such life will just be 10 NDrake.
(3) Number of Stars
Sources vary, but current estimates suggest the observable universe
(4) Number of Civilizations
[¶1] Once our above two N's are both known (or assumed for the sake of argument), the expected number of civilizations in the observable universe will just be Nlife = Nstars 10 NDrake. This is obviously the roughest of rough numerical estimates, but its very form allows us to draw some interesting conclusions.
[¶2] In particular, if NDrake is much less than about 23, say if it's 15 or so, then we can expect the universe to be teeming with life. But if it's much greater than about 23, say if it's 30 or so, then life is so improbable that just our own existence is literally miraculous (requiring appeal to the anthropic principle to explain it), and it's beyond foolish to expect any other civilization. Either way, what's highly unlikely is that NDrake just happens to be exactly 23 or so, i.e., on the "boundary" where we'd expect to find one or just a few civilizations scattered across the universe. So, just the form of Drake's equation suggests that, in all likelihood, either the universe is teeming with life or we're absolutely alone.
[¶3] At present there's no observational evidence for other life. But if the universe were teeming with it, wouldn't we expect at least one such civilization to leave some trace footprint/signature/whatever detectable by our instruments? And, having thus far failed to find any such footprint, are we justified to tentatively suggest the universe probably isn't teeming with life? If that's the case, our preceding discussion concludes we're probably alone in the universe.