To mark half a century of its “Doctor Who” science fiction drama series, the BBC is laying on a programme of special events that includes an hour-long talk by Prof Cox at the Royal Institution. So is the Tardis used by Doctor Who to flip across the centuries a viable prospect? Prof Cox says it is for travel into the future. You can do that by travelling in space at near light speeds and returning to find that more years have passed on Earth than you experienced in space. A simpler way might be to put yourself into suspended animation in a well-protected sealed chamber, only emerging when your desired number of years has passed. Either way you find yourself much younger than the world you re-enter.
But what about the return trip? This is where we hit problems. The universe’s speed limit says we cannot convey information through space faster than light can cross it. Science fiction writers, myself included, have sought to circumvent this limit by having vehicles leave conventional space-time and returning to it elsewhere. A wormhole is like a tunnel that takes you out of the universe at one point and puts you back into it at another. This could theoretically work for time as well as space. Two Russian mathematicians have suggested that the Large Hadron Collider at Cern could be used to create tiny wormholes. Of course it is possible that we might discover some effect in the universe that exerts its influence in less time than it takes light to travel. If we did, we might be able to modulate it to send signals.
Then the paradoxes come into play. If a man goes back and kills his father before he was conceived, does he disappear, and therefore not be able to kill his father? Isaac Asimov solved most of these in “The End of Eternity.” In that work time travellers do indeed alter the past, but are protected by an aura of ‘physiotime’ that shields them from any changes they have made that might affect their own person. And except at the end of the book, he postulates that time travellers cannot go farther back in time than the invention of the first time field. It’s well worth a read, and it’s a lot cleverer than “Doctor Who.”
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