The Physics of Time Travel

Once the antigravity scaffolding is holding open the portal, the traveler passing through would emerge in a distant place. But time travelers, of course, want to journey not just geographically but temporally. So Thorne's next step was to desynchronize the two regions on either side of the wormhole.

To do this, he applied an old trick of Einstein's. A major consequence of Einstein's Special Theory of Relativity is that time slows for objects that move quickly. Thorne applied this principle to one of the two black holes that make up a wormhole. Imagine lassoing one of the black holes-perhaps by trapping it inside a cage of negative energy-and towing it around the universe at close to the speed of light. That black hole, and therefore that end of the wormhole, would age more slowly than the stationary end of the wormhole. Over time, the black holes would become desynchronized, two objects connected through the wormhole but existing in different eras. An explorer who entered the stationary end of the wormhole would exit the moving end, many years earlier than when he departed, making the wormhole a true time portal.




Or Try It on a Shoestring




The most recent development in the physics of time travel came in 1991, when Princeton astrophysicist J. Richard Gott III suggested that hypothetical objects called cosmic strings might enable an astronaut to travel backward in time. Cosmic strings are long, thin objects that some cosmologists believe coalesced out of the universe's very earliest days. They are infinitely long, no wider than a single atom, and so dense that a few miles of a single cosmic string would outweigh Earth itself.



Gott's proposal relies on idealized versions of cosmic strings. In order to be em-ployed in the service of a time traveler, two cosmic strings, perfectly parallel and traveling at nearly the speed of light, must whiz past one another like two cars traveling in opposite directions on a highway. As the strings pass each other, space-time would become profoundly distorted by the influence of these fast-moving filaments. A savvy time traveler, waiting in a nearby spaceship, could exploit those distortions by flying around the coupled strings. If he timed it just right, the twists in space-time would enable him to return to his starting point before he began-making the voyage a one-way trip back in time. Which means that, according to the laws of physics, journeys through time are conceivable, if rather difficult to arrange. It may be only a matter of time.