It is easy to dismiss time travel as nothing more than science fiction. After all, H. G. Wells wrote The Time Machine in the late 1800s, but still no one has built one that works. Don't give up yet, though: we are continuing to make discoveries that may show us the way forward - or back.
Time travel is inherent in the basics of general relativity. Einstein's theory predicts that time runs more slowly in strong gravity, so you grow old more slowly living in a bungalow than in a skyscraper: being closer to the ground, you are in marginally stronger gravity (see "Personal time warps"). So to make a time machine, you simply have to connect two regions where time flows at different rates.
Take, for instance, the Earth and the immediate vicinity of a black hole, where strong gravity makes time flow extremely slowly. Say you start two clocks ticking on Monday at the two locations. When Friday comes around on Earth, it will still be only Wednesday by the black hole. So if you could travel instantaneously from Earth to near the black hole, you could travel from Friday back to Wednesday. Hey presto: time travel.
The question is, can you? Yes - in principle. According to quantum theory, the fabric of space-time is a tangle of sub-microscopic shortcuts through space and time known as wormholes. A few steps along such a tunnel and you might emerge light years away on the other side of the galaxy, or years in the past or future. It is possible that ghostly particles called neutrinos might already be performing such a feat (New Scientist, 1 October, p 6).
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