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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Madsen Pirie 
Interesting! Hearing about time travel as a actual possibility seems a little daunting though!
Travelling at the speed of light is a fascinating subject. If we could travel at the speed of light and a forward facing laser was fired at a twin mission capsule say 10,000 km in front, would the laser beam be visible on the forward space capsule ? In theory, no because photons cannot travel faster than the speed of both capsules which are already at the speed of light and the photons would not leave the laser. Conversely, if we fired a rearward facing laser from the forward capsule towards the following one would the forward speed of the capsule cause the laser beam to cancel out the beams rearward intention, i.e. the photons are already travelling forward at the speed of light and would possibly stay put because the photons are locked in limbo. I have a feeling that Dr Brian Cox could possibly have an answer to this problem.
If you could communicate telepathically, wouldn’t that be quicker than the speed of light Madsen?
I have another suggestion. Take the human body travelling at the speed of light. The electrons which the brain uses to send electrical impulses to muscles and organs in order to give us movement etc. naturally travel at the speed of light within our bodies. If our astronauts were asleep with their feet towards the front of the space capsule, their ‘ brain wave electrons ‘ within their bodies would already be travelling at the speed of light before the brain triggered them and they might not move down into the legs or elsewhere. Would we see our astronauts in a time warp……frozen in time until we reduced our speed to the normal human experience on Earth ?
I just love topics about time travel and space. It was a bit of frustration for me not to make a living as a man of science. (well, girl)..