It might be your greatest dream, I but for many physicists, time travel is their worst nightmare. "I think most of us would like to get rid of time machines if we possibly could," says Amanda Peet of the University of Toronto. "They offend our fundamental sensibilities." There's a very simple reason for this. Although the laws of nature seem to allow time machines to exist, they violate the principle of causality - the basic assumption that causes must precede their effects. The problem is, no one has come up with a definitive explanation for why time machines can't work. The best we have is Stephen Hawking's "chronology protection conjecture", which, in a nutshell, suggests that the universe has a built-in time cop. Whenever anyone is on the verge of constructing a working time machine the time cop intervenes, shutting the operation down before it has a chance to wreak havoc with the past. However, there are no time cops evident in the laws of physics, so at the moment the chronology protection conjecture is simply wishful thinking, a physicist crossing his fingers and hoping for the best. But that may be about to change.
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