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Will We Discover Another Universe?

Physicists, exploring science's most arcane equations, have discovered what may be

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.

If that's true, then trillions of these baby universes exist, for that's how many black holes are believed to inhabit our cosmos. And those are just the naturally occurring ones; baby universes could in principle be manufactured as well. M.I.T. physicist Alan Guth realized in the late '80s that you might create a baby universe in the lab from just a few pounds' worth of matter by compressing the stuff to black-hole density.
We won't have the technology to do that in the next 100 years—or probably even in the next billion years. Nevertheless, a sufficiently advanced civilization might be able to master the intricacies of creating baby universes—maybe even selling kits to do it in science fairs. Unfortunately, the new space-time such a universe inhabits will be forever cut off from our cosmos by the black-hole bottleneck (which destroys everything that passes through it), and thus will be just as undetectable as those in quantum theory's many-worlds interpretation.

One more type of alternate universe remains, however, and in this case there's a chance of detection—albeit a highly circumstantial one.

Since 1965, astronomers have had powerful evidence that the cosmos began with a Big Bang and that everything has been expanding outward ever since. But in the 1970s and early '80s, U.S. and Russian physicists (including Guth) realized that powerful energy fields dominating the cosmos when it was a fraction of a second old could have turbocharged the expansion, forcing the universe to fly apart—or "inflate"—at a rate many times faster than the speed of light. (The light barrier can't be broken by things moving through space, but space itself is exempt from this universal speed limit.)

So far, we're still talking about one universe—though one vastly larger than the tiny patch, a mere 30 billion light-years across, that we can see. But then scientists, including the Russian emigre Andrei Linde, realized that this inflation was more flexible than anyone had thought. Energy fields of early-universe intensity could arise purely by chance in subatomic-size regions of even a normal cosmos.

Our universe could thus be the result of an inflationary bubble that formed in a pre-existing universe—an arena better described as a metauniverse, or metaverse. Other, parallel bubbles could have formed just as easily. (If two expanding bubbles somehow met, the result would be a wall of fiery energy spanning one side of the cosmos. No evidence of that to date.)

But if bubbles of inflation could percolate in a pre-existing metaverse, they might also spring forth from our cosmos. New universes could be sprouting from ours all the time, in fact spewing out into unimaginable dimensions to evolve along their own paths. These universes might have laws of physics dramatically different from our own—gravity so powerful that they collapse almost instantly, or so weak that stars can never form, to give just two examples. And they could give birth in turn to other universes, creating a metaverse in which universes bud off of universes in an endlessly self-reproducing fashion.

It gets even more bizarre. According to Princeton astrophysicist J. Richard Gott, the flow of time loses its meaning when you hop from one universe to another. In the timeless metaverse, in fact, a baby cosmos could beget a baby that would beget a baby that might ultimately give birth to the universe that started it all. "It's quite possible," says Gott, "that the universe could end up being its own great-grandmother."

In many ways, the physics of these budding cosmoses are like the baby universes Hawking and Guth hatch inside black holes. The difference is that unlike the details of what transpires in black holes, the evidence for or against inflation could be settled within just a few years. If the universe did inflate, that brief period of breakneck expansion should have left a telltale pattern imprinted on the radiation left over from the Big Bang, which still echoes around the universe in the form of electromagnetic microwaves. Two satellites set to be launched later this year are sensitive enough to detect such a pattern.

The detection would be a powerful boost to alternate-universe theories. "Confirming inflation," says Princeton physicist Paul Steinhardt, "would give us a lot more confidence about some of its implications." And that includes ideas that have lived until now only in the parallel universe of fantasy.