Let's Do the Time Warp Again

It takes a lot of gravity to significantly warp time. A black hole has such enormous gravitational force that it creates a tear in space-time itself, but a black hole is no portal because it will suck a would-be time traveler into the cramped quarters of infinite density, forever. A properly engineered wormhole, however, theoretically creates a passage between two black holes that leads to another place in the universe through the space-time tear; a bit of galactic-scale fiddling with one end of the wormhole turns it into a time machine (more on wormhole engineering).




"What Kip Thorne and his colleagues noted," says Gott, "was that if you moved the wormhole mouth correctly, Jodie Foster (in the film version of Contact) could have come back before she left... . Jodie Foster would have been waiting at the same spot to shake hands with Jodie Foster when she arrived."
The notion of creating a hopeless causal loop in time is childishly easy to understand. In Back to the Future, Michael J. Fox finds himself fading from existence after journeying back in the souped-up DeLorean and attracting his mother's romantic interest at a time when she was supposed to be falling in love with Fox's future father. (According to one academic paper, this "is the first science fiction film to make explicit the incestuous possibilities that have always been at the heart of our fascination with time travel.")




Davies says the paradoxes of time travel have repelled some physicists, who were afraid of being ridiculed. Igor Novikov, a Russian astrophysicist who has written extensively on the subject, says that for decades, "very serious mathematicians, very serious physicists were not brave enough to declare that time travel is possible."



Many resolutions to the paradox have been proposed. One simply maintains that the universe won't let paradoxes be created: If you try to kill your grandfather, you won't be able to. You'll change your mind, or the gun won't go off, or you'll be a lousy shot. This notion, it will be noted, has serious implications for the existence of free will. Other approaches say there really are no paradoxes; every problem can be solved mathematically without producing a paradoxical inconsistency.