
Time travel is a sexy science-fiction trope. It's right up there with faster-than-light travel (the two are, in fact, inextricably related, and chances are you if you invent one then you'll have invented both) as something that, as far as our current understanding of the universe works, is impossible. There are some fascinating loopholes involving wormholes and general relativity, but in order to get it working you need metric shit-joules of energy and something called …