Do you ever lie awake at night wondering why you happen to be alive just now? Why it should be that your own particular bit of self-consciousness popped into existence in the twentieth century and not, say, during the reign of the Antonines or 10 million years hence? If you do, and your musings take a sufficiently rigorous form, you might arrive at a terrible realization: The human race is doomed to die out--and quickly.

So, at least, a handful of cosmologists and philosophers have concluded. Their reasoning is known as the Doomsday Argument. It goes like this: Suppose humanity were to have a happier fate, surviving thousands or millions of years into the future. And why not? The sun still has half its 10-billion-year life span to go. The earth's population might stabilize at 15 billion or so, and our successors could even colonize other parts of the galaxy, allowing a far greater increase in their numbers.

But think what that means: Nearly every human who will ever exist will live in the distant future. This would make us unusual in the extreme. Assume, quite conservatively, that a billion new people will be born every decade until the sun burns out. That makes a total of 500 quadrillion people. At most, 40 billion people have either lived in the past or are living now. Thus we would be among the first 0.00001 percent of all members of the human species to exist. Are we really so special?

But suppose, contrariwise, that humanity will be wiped out imminently, that some sort of apocalypse is around the corner. Then it is quite reasonable, statistically speaking, that our moment is the present. After all, more than 4 billion of the 40 billion humans who have ever lived are alive today, and with no future epochs to live in, this is far and away the most likely time to exist. Conclusion: Doom soon.

Even as transcendental a priori arguments go, this one is pretty breathtaking. For economy of premise and extravagance of conclusion, it rivals Saint Anselm's derivation of God's existence from the idea of perfection and Donald Davidson's proof that most of what we believe must be true or else our words would not refer to the right things.

As far as anyone knows, the Doomsday Argument was first publicly broached in 1983 at a meeting of the Royal Society in London. Its apparent author was Brandon Carter, a British astrophysicist (now living in France) famous for his work on black holes. A decade earlier, Carter had baptized the much-debated anthropic principle, which purports to explain why the laws of physics look the way they do: If they were any different, life could not have emerged, and, hence, we would not be here to observe them. We find ourselves living in this particular universe, in other words, because alternative universes are uncongenial to intelligent life. Carter was suggesting to the Royal Society that the same goes for time: We find ourselves living in this particular epoch because earlier and later ones are, for reasons we do not fully grasp, uncongenial to us. As interesting cosmological ideas often are, the Doomsday Argument was soon taken up by philosophers--notably John Leslie of the University of Guelph in Ontario.

Perhaps you are skeptical about, if not plain scornful of, the Doomsday Argument. It looks like logical trumpery. How could an abstract argument have such an experientially rich upshot? Yet it is difficult to find anything amiss in its logic. The sole assumption it requires--an eminently plausible one--is that if humanity endures, our cumulative numbers will increase. And the inference it makes is justified by the principle of probability known as Bayes' theorem, which dictates how a piece of evidence (we are living now) should affect the likelihoods we assign to competing hypotheses (doom sooner versus doom later).

Furthermore, the Doomsday Argument may not seem so unlikely once you consider all the forms doom could actually take. Don't think Ebola, greenhouse, nukes--think cosmos. An asteroid might bump into our planet (one wonders whether the Doomsday Argument occurred to the dinosaurs 65 million years ago). The Swift-Tuttle comet--dubbed the "Doomsday Rock" by the media--will be swinging awfully close on or about August 14, 2126. And that's the small stuff. The North Star could go supernova at any moment. In fact, the dread event might already have happened, in which case the news of it is traveling earthward in the form of lethal radiation that will obliterate us upon receipt.

The most delicious scenario of all is the one in which absolutely everything gets reduced to nothingness. Most cosmologists think the universe is no more than a tenth of the way through its historical progress from the Big Bang to the Big Crunch. But they cannot rule out the possibility that space is "metastable," meaning that it could spontaneously slip to a lower energy level at any moment. If this were to happen, a little bubble of "true vacuum" would appear without warning somewhere and begin to inflate at the speed of light. Its wall would contain tremendous energy, annihilating in a stroke everything before it: entire star systems, galaxies, galactic clusters, and eventually the cosmos itself.

Now that's a speculative bubble worth worrying about.

Jim Holt