Travelling forwards in time is surprisingly easy. Einstein’s special theory of relativity, developed in 1905, shows that time passes at different rates for people who are moving relative to one another - although the effect only becomes large when you get close to the speed of light.

If one were to leave Earth in a spacecraft travelling at an appreciable fraction of lightspeed, turn around and come back, only a few years might have passed on board but many years could have gone by on Earth. This is known as the “twins paradox”, since a traveller undertaking such a journey would return to find herself much younger than her twin. This is just a hypothetical concept and not a realistic theory.