Time travellers already walk among us. Thanks to Einstein’s theory of special relativity, astronauts orbiting on the International Space Station experience time ticking more slowly. At that speed the effect is minuscule, but ramp up the velocity and the effect means that one day humans might travel thousands of years into the future. Nature seems to be less fond of people going the other way and returning to the past, however some physicists have concocted an elaborate blueprint for a way to do it using wormholes and spaceships. It could even be used to hand yourself a present on Christmas Day, or answer some of the many questions that surround the universe’s great unknowns.