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Chatting about time travel in a room overlooking a verdant quadrangle at the Massachusetts Institute of Technology seems strangely appropriate. The building dates from 1916 and looks its age: the high ceilings, echoing corridors and musty offices with heavy wooden doors have changed little in that time. If it weren't for a computer screen in the corner, the room's interior could almost date from that time.

The office belongs to Seth Lloyd, one of the world's leading theorists on quantum mechanics. We are talking about a paper he and his colleagues circulated a month or so earlier describing a subtle new twist on time travel. This kind of paper crops up every few years, and usually focuses on some kind of thought experiment that uses logic and reasoning, rather than equipment, to describe how time travel could actually occur for real.

Except, there is always a caveat; the unfortunate time traveller must journey to the edge of a black hole to perform the feat, for example. So there is never an experimental test of the ideas. It is all good fun and usually worth a few column inches, but there is rarely anything tangible to get your teeth into.

Lloyd has come up with a new way of looking at the problem using quantum mechanics, so I am keen to hear about his thought experiment and, of course, to learn about the inevitable caveat. He is telling me how photons could travel back in time when he says something extraordinary. "...and we've done the experiment", he says casually. "You've seen that paper too, haven't you?"

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Category: Science