Are There Other Universes?

The irresistible, mind-boggling fantasy comes to just about everyone, sooner or later: What if everything we knew, our whole universe, was just a speck of dust on someone's shoulder?

Of course, that's not an idea astronomers take seriously. But many cosmologists are giving serious thought to a more scientific question: Do other universes exist?

At first glance, you can't help but wonder how anyone could have the chutzpah to ask a question like that. We can barely figure out this universe, and now we're wondering about others?

Believe it or not, theorists have an answer. And the answer appears to be, Yes.

To understand why, you have to go back to the Big Bang, that mysterious, mother-of-all-explosions that most astronomers believe spawned our universe. One second, according to theory, there was nothingness. The next, our cosmos sprang into existence. Nature seems to have pulled off the feat of getting something -- in fact, everything -- for nothing.

As unimaginable as that sounds, it comes straight out of the theory of quantum mechanics, a set of mathematical rules that describe how the universe works on the smallest scales, inside atoms. Quantum mechanics says that matter and energy can appear spontaneously out of the vacuum of space, thanks to something called a quantum fluctuation, a sort of hiccup in the energy field thought to pervade the cosmos.

Cosmologists say that a quantum fluctuation gave rise to the Big Bang. And the thing about quantum fluctuations is that they can happen anywhere, any time. And if our universe was born out of a quantum fluctuation, say theorists, then it's possible that other quantum fluctuations could have spawned other universes.

There's a reason some theorists want other universes to exist: They believe it's the only way to explain why our own universe, whose physical laws are just right to allow life, happens to exist. According to the so-called anthropic principle, there are perhaps an infinite number of universes, each with its own set of physical laws. And one of them happens to be ours. That's much easier to believe, say the anthropic advocates, than a single universe "fine-tuned" for our existence.

But there's a problem. If these other universes exist, there's no way for us to detect them.

Of course, as University of Arizona astronomer Chris Impey points out, there are parts of our own cosmos that we can't observe, because light from those extremely distant realms hasn't had time to reach us. "We know that our own physical universe is substantially, maybe enormously larger, than the visible universe," Impey says.

That doesn't mean, however, that Impey is prepared to accept the idea of other universes. For one thing, he says, cosmologists don't really understand the nature of quantum fluctuations, "because we don't have any quantum gravity theory yet." That means the idea of multiple fluctuations, and multiple universes, is "truly speculative."

Other astronomers are even more forceful in their resistance to the idea.

"It's not a testable idea," says Paul Steinhardt of Princeton University. Because the different universes would not be detectable by one another, he says, "You can't really prove it exists or doesn't exist." When you talk about multiple universes, Steinhardt says, you're not talking about science anymore. "In my view, you're into metaphysics."

Not everyone rejects the multiple-universe idea out of hand. At the University of California, Virginia Trimble is more accepting. "I find it neat. In much the same way that I think it would be neat if there were reincarnation."

Not exactly a resounding scientific endorsement.

But Andreas Albrecht, a cosmologist at the University of California at Davis, says the question isn't open for debate. Why? You can't argue with quantum mechanics. "As far as we can tell," Albrecht says, "that's the fundamental language that Nature speaks. Nature doesn't answer questions for certain; it answers questions by giving probabilities."

And in quantum mechanics, "There's a possibility that almost anything happens." Including other universes. And if cosmologists are queasy about that, they don't have a choice. "It comes out of the mathematics," Albrecht explains. "It's forced down our throats."

"Quantum mechanics will not give up these other alternatives on its own," says Albrecht. "And we really don't know what to make about that. On one hand it sounds totally metaphysical. On the other hand, it's all we have to work with at the moment."

So, how would Albrecht answer Impey, and Steinhardt, and others who challenge the idea of a multiple universe?

"I would say, 'Your instincts are great, but do something with them. Give me a theory that doesn't have multiple universes.' And they'd be stuck."

That's not to say Albrecht agrees with the anthropic advocates. All the "fine-tuning" which they believe is too good to be true, is only true for life as we know it. Of the advocates, Albrecht says, "They don't know a thing about life. They don't know what it takes to have life in the universe. There could be forms of life out there that we haven't even thought of! It's really stupid."

There is no shortage of published scientific papers, articles, and books on the anthropic theory, including some by experienced cosmologists. And there's some good science involved in these studies, Albrecht says, if you take out the stuff about the anthropic principle.

"People get into these studies because they are intrigued because there are real scientific issues to address. Then they say, 'Oh, this looks familiar, I'd better call it anthropic,' and they start using all these buzz words. But they [alienate] a large part of the scientific community when they do that. It's a kind of sloppiness."

"Science is about trying not to be sloppy, but it's hard because we're human. This is an example of our humanity creeping in and getting in the way of sheer rationality. I think with time it will get much better."

"In the end," Albrecht says, "we may understand that there are a bunch of other universes, but it won't be the way the anthropic guys want:. There's a lot of room between the total smorgasbord that the anthropic people want, and just having a few extra universes around."