Captains Kirk and Picard stumbled into them all the time. Alice found one beyond the looking glass. The children in C.S. Lewis' Chronicles of Narnia entered theirs by way of a musty old wardrobe. Considering their usefulness as a plot device, it's hardly surprising that science fiction and fantasy literature are filled with alternate universes of one kind or another. What is surprising, though, is that mainstream physicists have stumbled onto their own alternate universes, hidden amid the complexities of science's most arcane equations.

Nobody knows what form these parallel worlds might take, and it's far from clear that we could detect their existence, let alone step through a mirror or a space warp for a visit. But hints that ours is just one of many universes keep cropping up in all sorts of different theories—and in ways that can seem far stranger than fiction.

The first credible suggestion that alternate universes might exist came in the early 1950s when a young physics graduate student named Hugh Everett was toying with some of the more bizarre implications of quantum mechanics. That theory, accepted by all serious physicists, says that the motions of atoms and subatomic particles can never be predicted with certainty; you can tell only where, say, an electron will probably be a millisecond from now. It could quite possibly end up somewhere else.

Precisely what that fundamental uncertainty tells us about the basic nature of the subatomic world is a question theorists have been wrestling with for decades. The great Danish physicist Niels Bohr, for example, believed that before you pinned a particle down by measuring it, the particle was literally in several places at once. The act of measurement, he suggested, forced the particle to choose one location over all the others.

But Everett had another idea: when you locate the electron, he argued, the world splits into multiple universes. In each one, the electron has a different position—and all these many worlds, each equally real, go on to have their own futures. In this so-called many-worlds interpretation of quantum mechanics, the universe is incredibly prolific, since each particle in the cosmos produces a multitude of new universes in each instant—and in the next instant, every one of these new universes fragments again. Yet plenty of physicists consider this to be a perfectly valid idea. And if it's correct, the number of universes evolving in parallel is far greater than we could ever count.

It's all purely theoretical, though, since there's no conceivable way to make contact with even one of these alternate universes. So while each of us may spawn an uncountable number of parallel selves as the particles within us split and re-split, the chance of tapping into our other histories is precisely zero—and so, alas, is the chance of figuring out whether this interpretation of quantum mechanics is correct.

Things don't look much more certain in the second type of alternate universe, which comes not from quantum mechanics but from the other great physics revolution of the 20th century, Einstein's general theory of relativity. According to Einstein, objects with extremely large mass or high density stretch the fabric of space-time. Find something whose density approaches infinity—a black hole, for example—and that stretch can become a tear.

This rip in space-time, better known as a wormhole, could in theory serve as a shortcut to a distant part of the universe (characters on Star Trek: Deep Space Nine use wormholes the way New Yorkers use subways). But according to an idea proposed in the 1980s by Stephen Hawking, it could also lead out of our cosmos altogether, creating a "baby universe" that would then expand and grow, forming its own self-contained branch of space-time.

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