From the outside, an Einstein-Rosen bridge, as wormholes were originally known, looks a lot like its cousin, the black hole. Which you must keep in mind that "nothing can escape from a black hole — not even light."
Einstein and Rosen made a very bold supposition: What if a traveler fell into the mouth of something that looked like a black hole, but rather than being crushed by a singularity at the center of a black hole, instead emerged from another mouth, potentially many light years from where he or she started? This isn't as crazy as it sounds. Einstein's theory of general relativity — our current working model for how gravity and space work — has been confirmed with countless experiments. And, as ad hoc as it sounds, an Einstein-Rosen bridge is a perfectly valid solution to the equations of general relativity.
And it's not just a shortcut through space. In 1988, Caltech physicist Kip Thorne also showed something else: If you can build a wormhole, you can also turn it into a time machine. By dragging one of the mouths of the wormhole around space at nearly the speed of light, we can create a two-way tunnel connecting two points in time. Even better, you don't need to worry about mucking up history. A time machine built from the laws of general relativity is necessarily self-consistent, and thus your history will remain safely as you left it.
However, Einstein's original concept had a few flaws. For one thing, going through an Einstein-Rosen bridge, later theorists have concluded, would have to be a one-way trip, since one mouth always serves as the entrance and the other the exit. An even bigger problem with the wormhole Einstein envisioned was found in 1962, when John Archibald Wheeler demonstrated that an Einstein-Rosen bridge would collapse before anything, even a beam of light, could travel through.
Fortunately, wormhole design has improved considerably in the last 75 years. In 1988, Thorne and his students took up the problem of traversable wormholes, in large part because of a plea from his friend Carl Sagan, who was then working on the novel "Contact." Thorne found that it was theoretically possible to construct models of wormholes, but they would require the existence of as-yet-undiscovered "exotic matter" — strange stuff that has less than zero mass — to keep them open. Unlike Einstein-Rosen bridges, Thorne's model was bi-directional and, more important, stable.
This all might seem like good news, but the fine print on wormholes is pretty daunting when you get into it. For one thing, we've never discovered anything like the exotic matter needed to prop wormholes open, and for another, we're not sure how we — or even a super-civilization — could punch a hole through the universe to create one in the first place. Furthermore, the idea of time travel is so anathema to many respectable physicists that some, including Stephen Hawking, have proposed a "chronology protection conjecture," basically insisting that physics must somehow outlaw time machines in order to keep "the universe safe for historians."
-Los Angeles Times