Golden Square, London, 21st October 2013
Having worked as a physicist, people often ask me if it is possible to travel in time. The answer is yes. Forwards, at least.
Einstein discovered that if you go on a round trip, your stay-at-home friends are all slightly older than you when you get back – you have gone forward in time. It really only starts to show when you travel close to light speed, when your friends - or their grey-haired grandchildren - would remark that you'd hardly aged at all. The effect is big enough to notice even on earth, sometimes. GPS navigation tools would drift off course if corrections were not made for the time travel effect.
This is a slight problem for accurate timekeeping. There's not too much point in trying to make atomic clocks too accurate, since if you move relative to the clock, your timeline will not be the same.
As far as we know, we can’t travel backwards in time. Except by going to Belgium.