BRIAN GREENE There was a time when the word "universe" meant "all there is," everything. The notion of more than one universe, more than one "everything," seemed impossible. But perhaps, if we could go beyond our solar system, beyond the Milky Way, even beyond other distant galaxies, past the end of the observable universe, we'll find that there's more, a lot more, that our universe is not alone. There may be other universes. In fact, there might be new ones being born all the time.

So, what would it be like, if we could travel to some of these other universes? What would we see? Some would be vastly different from our own, with properties unlike anything we've ever seen. In fact, some universes in the multiverse might not have light or matter or anything recognizable at all. And there might be other universes with features not unlike the familiar ones we know, but where life takes a completely different form, perhaps communicating in ways we'd find utterly bizarre.

And the math shows that if we were able to visit enough of these universes, we might, eventually, find ones like ours, with a Milky Way galaxy, a solar system and an Earth, except with some slight differences.

In one, maybe the asteroid that killed off the dinosaurs 65 million years ago missed, and evolution charted a new course.

In another, there might be an Earth with people similar to us but better at multitasking.

But there's something even stranger. Somewhere out there, we should find exact copies of our universe, with duplicates of everything and everyone.

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