Doctor Who fans rejoice! Time travel is possible and has already happened. But it does not involve flying in a blue police box and will certainly not send you to another era. Albert Einstein was the first to suggest that time is relative – it speeds up and slows down according to how fast you move in relation to something else. Time also moves slower as gravity increases. So astronauts orbiting round the Earth at 7.66 km/s on the International Space Station experience time a tiny amount faster – just a fraction of a millisecond – than us down on the ground.
