Why you age slower on a plane (and other incredibly strange effects of relativity)

Einstein correctly predicted that time slows when you're flying fast, but to experience "time dilation" most spectacularly, you'd have to travel into a black hole, says astrophysicist Chris Lintott.

One of my favourite scientific experiments involved flying four clocks twice around the world. In 1971, physicists Joseph Hafele and Richard Keating took atomic clocks – capable of losing no more than one second every 30 million years – on a commercial jet, flying first west and then east around the globe before returning to their laboratory in Washington DC. There, they compared the time on their well-travelled timepieces to a set of clocks that had remained static. Remarkably, the clocks disagreed: the act of travel had seemingly altered the passage of time.

The experiment was a test of a core principle of Einstein's theory of relativity, which is that time is not universal. The faster you travel, the slower time will pass for you. The effect is small – take a transatlantic flight from London to New York and your watch will be a ten-millionth of a second behind one left on the ground – but nonetheless you'll have aged a fraction more slowly than if you'd stayed at home. And Hafele and Keating's clocks could measure it. 

Another prediction of relativity says that gravity has an effect too. Get further from the Earth's gravitational pull, and time will speed up. This affects our own bodies: it means your head is ever so slightly older than your feet. Once again, the effect is incredibly small, but at greater distances from Earth, it becomes important. The GPS system that we all depend on to navigate, with its satellites 20,000km (12,400 miles) above the Earth, needs to take this into account in order to work properly.

Despite these phenomena, the Earth is, at the end of the day, a small planet in a big Universe. Around black holes, massive objects whose gravitational pull dwarves that of any planet, these relativistic effects become far more pronounced.

To understand why, imagine falling toward a black hole. (We'll assume you're in some magical spacecraft which keeps you safe from the distressing effect of "spaghettification", the terminal stretching that happens to anything that gets too close to a black hole.)

As you fall, you wouldn't notice any difference in time for you or your close surroundings. Looking at your watch, or feeling your pulse, you would perceive the same steady beat as second after second you approach near-certain doom. If your spacecraft's instruments let you look back to observe the Universe outside the black hole, though, you might notice something odd – events out there would appear to you to be speeding up. If you could watch Earth through a telescope, you would see the future of our planet and species play out for your enjoyment, running like a sped-up movie. If you could pick up a television signal, then you could watch the rest of humanity's broadcasts until the Sun's evolution into a red giant swallows the planet, albeit at speed.

You may also like:

Now switch perspective. Imagine you're on a space station orbiting at some safe distance from the black hole, watching your brave or unfortunate colleague fall in. The edge of the black hole, such as it is, is the event horizon, the point at which even things travelling at the speed of light cannot escape, and it seems reasonable to expect our receding friend to reach this point and then disappear. What you would actually see is stranger – if they are waving to us, we will see them wave slower and slower as they fall deeper into the black hole's gravitational well. A clock mounted on the outside of their spacecraft will seem to run slower compared to one safely installed on our station.

This effect is exploited in the film Interstellar, where astronauts who have explored a planet near a black hole emerge to find a changed Universe that has moved on without them. As the film makes clear, it makes no sense to ask whether the time passing near the black hole or far away is the "correct" time; relativity tells us that there's no such thing. 

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.

--

If you liked this story, sign up for The Essential List newsletter – a handpicked selection of features, videos and can't-miss news delivered to your inbox every Friday.

Join one million Future fans by liking us on Facebook, or follow us on Twitter or Instagram.