Is it preposterous to consider the existence of parallel universes? Or is it preposterous not to? Physicist Brian Greene would tend toward the latter view. The Columbia University theoretical physicist's latest book, The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos," follows up on his two earlier books for popular audiences, "The Elegant Universe" and "The Fabric of the Cosmos." Those works presented step-by-step guides to string theory and space-time, respectively, leavened with pop-culture references and analogies drawn from everyday life. Greene doesn't explain just one scenario in which unreachable universes co-exist alongside our own. He delves into nine possibilities, drawn from different corners of scientific speculation. As a string theorist, Greene is used to ... criticism. Like parallel universes, the idea that matter's fundamental building blocks are tiny vibrating strings or multidimensional membranes has often been knocked as unprovable, unverifiable, unfalsifiable speculation. "That's provocative nonsense," Greene [commented]. Theorists are not just pulling this stuff out of thin air. Rather, they're being led to seemingly wild conclusions while working within what he called "the tight straitjacket of mathematics." The math is clearly suggesting that there may be other universes out there.