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For example, time passes more slowly if you travel at speed, though you need to start approaching the speed of light for the effect to be significant. This gives rise to the twin paradox, in which one of two identical twins becomes an astronaut and whizzes around in space at close to the speed of light, while the other stays on Earth. The astronaut will age more slowly than their Earthbound twin. "If you travel and come back, you are really younger than the twin brother," says Vlatko Vedral, a quantum physicist at the University of Oxford, in the UK. Twins Scott and Mark Kelly did this for real when Scott spent months in space, albeit not at speeds close to that of light.

Similarly, time passes more slowly for you if you are in an intense gravitational field, such as a black hole. "Your head is ageing quicker than your feet, because Earth's gravity is stronger at your feet," says Osborne.

Doctor Who used this as the setup for season 10 finale World Enough and Time, in which the Twelfth Doctor and his friends are trapped on a spaceship close to a black hole. At the front of the craft, closer to the black hole, time passes more slowly than at the rear. This means the small group of Cybermen at the rear of the craft are able to develop into a huge army in, from the Doctor's point of view, a matter of minutes. This effect of gravity on time also features in the plot of the film Interstellar.