Though we will never see this from the outside, our doomed traveller will eventually cross the black hole’s event horizon, the boundary beyond which no light – or anything else – can escape. This is the point of no return, and beyond it, the traveller would be forced toward the centre of the black hole. This means their experience of time could be fundamentally changed – and they might even be able to move back and forth in time.
Why so? In our ordinary lives, safely outside of a black hole, we can move how we like in the three dimensions of space, but must travel ceaselessly forward in the fourth dimension: time. But within the event horizon of a black hole, things are backwards. Inside, an astronaut would be forced to travel ceaselessly in space – toward the black hole’s centre – which means that some people think they may be able to move in time.
In this sense, a black hole can act like a time machine, allowing anyone brave enough to enter to travel back to times long before they crossed the event horizon, as far back as the creation of the black hole itself.
The only catch is that, as far as we can tell, there would be no way to exit the black hole, so no time traveller from the future can use this trick to come and visit us here on the surface of the Earth. But understanding what is possible – and thinking about how black holes manipulate the space and time around them – can provide physicists with the most precise tests of Einstein’s theories, and might lead to a deeper understanding of what exactly this thing we call time is. It beats flying round the world with an atomic clock strapped in the seat next to you.
*Chris Lintott is a professor of astrophysics at the University of Oxford and a lecturer at Gresham College, a co-presenter on the BBC’s The Sky at Night, and the author of the upcoming book Our Accidental Universe: Stories of Discovery from Asteroids to Aliens.
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