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BRIAN GREENE spent a good part of the last decade extolling the virtues of string theory. He dreamed that one day it would provide physicists with a theory of everything that would describe our universe – ours and ours alone. His bestselling book The Elegant Universe eloquently captured the quest for this ultimate theory.

“But the fly in the ointment was that string theory allowed for, in principle, many universes,” says Greene, who is a theoretical physicist at Columbia University in New York. In other words, string theory seems equally capable of describing universes very different from ours. Greene hoped that something in the theory would eventually rule out most of the possibilities and single out one of these universes as the real one: ours.

So far, it hasn’t – though not for any lack of trying. As a result, string theorists are beginning to accept that their ambitions for the theory may have been misguided. Perhaps our universe is not the only one after all. Maybe string theory has been right all along.

Greene, certainly, has had a change of heart. “You walk along a number of pathways in physics far enough and you bang into the possibility that we are one universe of many,” he says. “So what do you do? You smack yourself in the head and say, ‘Ah, maybe the universe is trying to tell me something.’ I have personally undergone a sort of transformation, where I am very warm to this possibility of there being many universes, and that we are in the one where we can survive.”

We keep banging into the possibility …