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Time travel paradoxes are very familiar to authors of science fiction. The question is, what are we physicists to make of them? Do they imply that time travel is simply not on, or that reality is subtler than we suppose? This is where opinions start to differ. Some physicists, most notably David Deutsch, think the way out of this is to assume that there are multiple realities, so that when you travel back into the past, the world you change is not the same one that you left, but a parallel imitation.

This topic is often cast in the parable of the grandmother paradox: you go back 50 years and kill your grandmother, ensuring that you were never born in the first place. One way around it is that if you go to a parallel world, and kill your parallel grandmother, you can return to your own time to find Granny still alive and well. That's a possible resolution. There isn't any consensus on it. Perhaps the existence of parallel realities is a worse prospect than that of causal loop paradoxes.

Some people feel that the problems of travel into the past are so great that there must be something in nature to prevent it actually happening. For a while Stephen Hawking flirted with this position, and formulated what he termed the chronology protection hypothesis. It implied that although the laws of physics would seem to allow travel backwards in time, in every practical case something would intervene to prevent it happening. Nature would always outmanoeuvre attempts to change the past. But we don't know, this is still an open question.

Today, most of the research in this field is being done finding more plausible ways to travel backwards in time. Gödel's idea of the rotating universe is just one scenario; there are others. The most popular is the wormhole in space, which is a little bit like a black hole but different. Wormholes were made famous by Jodie Foster, who fell into one in the film "Contact." This movie was based on Carl Sagan's book of the same name. In the movie what happens is that this wormhole is manufactured according to a prescription sent to earth by alien beings in a radio message. Jodie Foster gets dropped into what looks like a gigantic kitchen mixer, and 18 minutes later emerges at a different part of the galaxy. The wormhole in effect connects two distant points in space so as to form a shortcut. It's a little bit like drilling a hole from New York to Sydney. If you wanted to go see the Olympics the quick way would be to plunge through the hole, rather than fly the long way around the earth's surface. Einstein's theory of relativity tells us that space is curved by gravity, so imagine that it was warped in such a way that it connected earth with the center of the galaxy through a tube or a tunnel that might only be a few kilometers long — who knows?

The point is that if a wormhole is possible, it can be adapted for use as a time machine, as shown by Kip Thorne at Caltech, and his colleagues, and now the subject of an international cottage industry in research papers. To travel in time, what you do is this. You first plunge through the wormhole and exit at the remote end, then you zoom back home again through ordinary space at nearly the speed of light. If the circumstances are right, you can get back before you leave.

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