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The downside of traveling into the future this way is that you might be stuck there. Is there any way of going backward in time? Once again, Einstein may have provided the answer. His 1915 theory of general relativity showed that space and time are curved, and that the curvature can be large in the neighborhood of very massive objects. If an object is dense enough, the curvature can become nearly infinite, perhaps opening a tunnel that connects distant regions of space-time as though they were next door. Physicists call this tunnel a wormhole, in an analogy to the shortcut a worm eats from one side of a curved apple to the other.
In 1988, Kip Thorne, a physicist at Caltech, and several colleagues suggested that you could use such a wormhole to travel into the past. Here's how you do it: move one mouth of the wormhole through space at nearly the speed of light while leaving the other one fixed. Then jump in through the moving end. Like a moving astronaut, this end ages less, so it connects back to an earlier time on the fixed end. When you pop out through the fixed end an instant later, you'll find that you've emerged in your own past.
The problem with wormholes is that the openings are microscopic and tend to snap shut a fraction of a second after they're created. The only way to keep them open, as far as we know, is with matter that has negative density. In layman's terms, that's stuff that weighs less than nothing. This may sound impossible, but the Dutch physicist Hendrik Casimir theorized in 1948 that holding two plates of electrically conducting material very close together in a vacuum actually does create a region of negative density that exerts an inward pressure on the plates. The force predicted by Casimir has been verified in the laboratory.
Using this idea, Thorne and his colleagues proposed constructing a wormhole tunnel 600 million miles in circumference, with Casimir plates separated by only 400 proton diameters at the midpoint. Time travelers would have to somehow open doors in these plates to pass through the wormhole. The mass required for construction? Two hundred million times the mass of the sun. These are projects only a supercivilization could attempt--not something for 21st century engineers.
In 1991 I found another possible mechanism for time travel using cosmic strings, thin strands of energy millions of light-years long, predicted by some theories of particle physics (but not yet observed in the universe). You could try to construct a cosmic-string time machine by finding a large loop of cosmic string and somehow manipulating it so it would contract rapidly under its own tension, like a rubber band. The extraordinary energy density of the string curves space-time sharply, and by flying a spaceship around the two sides of the loop as they pass each other at nearly the speed of light, you'd travel into the past.
To go back in time by one year, unfortunately, you'd need a loop containing about half the mass-energy of an entire galaxy. Worse yet, the contracting cosmic-string loop would probably trigger the formation of a rotating black hole, trapping any time-travel regions inside. You would almost certainly be torn apart by near infinite space curvature before you could travel anywhere.