Will we ever be able to travel backwards in time?

Einstein’s theory of general relativity – our best theory of gravity, which has passed every experimental test so far – has some solutions that are sufficiently twisted to allow time travel to the past. These include wormholes and moving cosmic strings. Just as Magellan’s crew went west around the world and arrived back in Europe, a time traveller can go steadily toward the future and yet circle back through curved space-time to visit an event in their own past.

If you built a time machine in the year 3000 by twisting spacetime, you might be able to use it to go from 3002 to 3001, but you couldn’t use it to go back to 2016 because that was before the time machine was built. It would take a supercivilisation to even attempt such a project. To know whether these time machines could be created in practice, we may need to learn the laws of quantum gravity, or how gravity behaves on microscopic scales. It’s one of the reasons physicists find the possibilities of time travel so interesting.

J. Richard Gott, professor at Princeton University