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Last year a Stanford theorist named Shamit Kachru set out with some colleagues to calculate just how many different universes one particular version of string theory could produce. The number he came up with was a 1 followed by something like 100 zeros--roughly a hundred billion billion times the number of atoms in our universe. It was an answer that didn't please anyone. Says Max Tegmark, a theorist at the University of Pennsylvania: "People have tried very hard to get rid of these multiple universes and failed. They just don't like the concept; they think it's weird. And they're right. But don't we already have good evidence by now that the cosmos really is weird?" To Einstein's celebrated musing about whether God had a choice in creating the universe, the answer seems to be a resounding yes: all sorts of universes are possible.
Not everyone is convinced that the anthropic principle is sound evidence for a multiverse, though. "In my view," says cosmologist George Ellis, of the University of Cape Town in South Africa: "Belief in multiple universes is just as much a matter of faith as any other religious belief." Even scientists who are willing to entertain the anthropic position are wary, with good reason. "Astronomers have been burned over and over again," says SETI's Shostak, "on beliefs that seemed to imply we're special--that we're at the center of the solar system or the center of the galaxy, or that the Milky Way is the only galaxy in the universe. Every time, it turned out that we weren't special after all. We just didn't have enough knowledge."
Besides, it's easy to see the anthropic principle as an explanation of last resort. When he first began looking at it back in the late 1980s, particle theorist Steven Weinberg of the University of Texas hoped the anthropic principle might go away. But the opposite happened. "It's not something that we're particularly happy about," he says. Every physicist dreams of being able to calculate everything from a set of fundamental laws. But at the same time, Weinberg says, "it's important to be realistic. We may just have to get used to the fact that some of the things we call fundamental constants may be historical accidents."
For example, he observes, when it was first realized that planets go around the sun, astronomers hoped they might find an underlying principle that would explain why the planets orbit at the precise distances they do. But now we know the orbits are the result of pure chance. The elliptical shapes of planetary orbits, on the other hand, led to the truly profound discovery of Newton's laws of gravity. "My own feeling," says Brian Greene, a superstring theorist at Columbia University and author of the best-selling The Fabric of the Cosmos, "is that we can give a deeper explanation of why this universe, with its particular properties, came to be."
That may be the most important result of anthropic thinking: it pushes scientists to ask all sorts of new questions--questions that may ultimately provoke a new scientific revolution. For example, how improbable is our universe? If the answer is not very, there ought to be lots of universes like our own. Or if multiple universes come about through inflation, as M.I.T. cosmologist Alan Guth suspects, "does it produce all types of universes about equally, or does it produce just a few types? We don't know the answer--yet."