Physics World, May 3, 2011http://physicsworld.com/cws/article/indepth/45854 Brian Greene on whether our universe is just one among many – and whether theories of the multiverse will ever qualify as science As a popular-science author, Brian Greene is nothing if not ambitious. In his first book, The Elegant Universe, he set out to explain string theory – one of the most complex and mind-bending areas of physics – in a way that non-scientists could understand. A few years later, he tackled the nature of space and time in The Fabric of the Cosmos. But Greene's latest book The Hidden Reality goes further still, exploring one of the biggest questions of all: is our universe the only one that exists, or is it merely one among many? In this special audio interview (below),http://physicsworld.com/cws/article/indepth/45854 Physics World reviews editor Margaret Harris talks to Greene about some of the multiverse ideas described in the book, and whether parallel universes will ever become part of accepted scientific theory. The book "does not argue for [the multiverse] in a monolithic way", says Greene, a physicist at Columbia University in New York. Instead, he explains, it suggests that the possibility of parallel universes is something "worth thinking about, worth taking seriously" even though there are important questions about how to turn such speculations into science. One of the most compelling reasons for taking multiple universes seriously, Greene says, is the fact that many different areas of physics seem to hint at their existence. "People in physics who are trying to work out the deepest laws that might describe the small, the big and everything inbetween are repeatedly coming upon the idea that there may be other universes," he says. "It's not that some unidirectional fringe approach to physics has suggested this possibility. If you study cosmology, or relativity, or quantum mechanics, or string theory, and you follow these theories far enough, they each bump into some variation on the theme of parallel universes." Of course, this common multiverse theme could be a cosmic red herring. Without solid predictions or experiments to test such theories, we cannot be sure. Greene acknowledges that falsifiability – the ability to prove a theory wrong – is "the key question" for proponents of the multiverse. Indeed, he argues that some variants of multiverse theory "stand outside science" because they can never be falsified, even in principle. However, he also notes that mathematics has long provided a guide for exploring ideas that are beyond our current ability to test, and many of those ideas have later been confirmed through experiment or observation. In the near future, Greene hopes that data from collisions at CERN's Large Hadron Collider will turn up what he calls "circumstantial evidence" that favours his preferred option of a string-based multiverse. He is also looking forward to getting back to physics. "I find that these books are a nice change of pace," he says. "But I think it will be a long time before I embark on another one – if ever."
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