Physics says time travel into the future is not a thought experiment — every astronaut who has orbited Earth has returned a fraction of a second younger than they would have been if they had stayed on the ground

Physics says time travel into the future is not a thought experiment — every astronaut who has orbited Earth has returned a fraction of a second younger than they would have been if they had stayed on the ground

Time travel into the future sounds like a fiction problem until it is written in the language of clocks. Every astronaut who has spent time in low Earth orbit has moved through time at a slightly different rate from people on the ground. The effect is tiny, far too small to feel, and it does not involve vanishing into another century. But it is real. When astronauts return from orbit, their own bodies and watches have experienced a little less t…