Captains Kirk and Picard stumbled into them all the time. Alice found one beyond the looking glass. The children in C.S. Lewis' Chronicles of Narnia entered theirs by way of a musty old wardrobe. Considering their usefulness as a plot device, it's hardly surprising that science fiction and fantasy literature are filled with alternate universes of one kind or another. What is surprising, though, is that mainstream physicists have stumbled onto their own alternate universes, hidden amid the complexities of science's most arcane equations.
Nobody knows what form these parallel worlds might take, and it's far from clear that we could...