Einstein’s theories of relativity suggest that time travel is possible. Einstein theorised that space and time are aspects of the same thing, space-time and that these are not constant. As you travel close to, or even exceed, the speed of light, time slows down until at the speed of light time stops. There have been experiments which substantiate this theory. Therefore to travel into Earth’s future all you need do is leave in a spaceship that travels close to the speed of light and return to Earth some time later. As time moves very much more slowly on our spaceship, time on Earth would be far in advance of time on the ship.
No practical time travel has yet been achieved but theories on how it might have been expounded. Using wormholes accelerated to the speed of light follows the same principle as the spaceship example. H G Wells famously created a time machine which worked due to some exotic matter in a rod conveniently dropped off into the hands of his hero. Black holes may hold the secret or it may result from developments in quantum mechanics.
I can almost hear you shout, what about time travel paradoxes and why have we not met any time travellers if this is possible. There are many possible answers to these questions. Perhaps Stephen Hawkings’ chronology protection conjecture is correct which suggests quantum level affects conspire to prevent time travel. It is possible that time travellers cannot interfere with past events as theorised by Novikov whose self consistency principle states that if an event exists that would give rise to a paradox, then the probability of that event happening is zero. There are others too including the existence of multiple universes where everything happens somewhere or simply that time travellers are somehow physically prevented from interacting with people out of there time zone.
Of course, science fiction loves it time travellers. Perhaps the most famous time traveller of our time has to be a Gallifreyan Time Lord simply known as the Doctor (for those that didn’t know, Gallifrey can be found at the centre of the milky way, co-ordinates 1001100 by 02). This BBC serial (Dr Who) has been delighting and scaring audiences since the early sixties and Russell T Davies’ resurrection of Doctor Who in 2005 has seen the show revitalised and better than ever. Christopher Eccleston did an excellent job as the ninth Doctor, but for only one series before passing the baton to David Tennant.