Who hasn’t daydreamed about skipping ahead a few centuries to see the future, or slipping back to fix a mistake?
So, is time travel possible? Surprisingly, partly yes. Travelling into the future isn’t just possible. It’s been measured, and you rely on it every time you open the map on your phone. Travelling into the past is a very different story.
Travelling into the future is real
The key idea comes from Albert Einstein. His theories of relativity, published in 1905 and 1915, showed that time doesn’t tick at the same rate for everyone.
Two things change how fast it passes. The first is speed: the faster you move, the slower time passes for you compared with someone standing still. The second is gravity: the closer you are to something massive like Earth, the slower your clock runs compared with someone further away.
In everyday life, the differences are far too small to notice. But they’re real, and they’ve been measured.
The clocks that flew around the world
In October 1971, physicist Joseph Hafele and astronomer Richard Keating put four extremely accurate atomic clocks on ordinary commercial flights. They flew them around the world twice, once heading east and once heading west. Then they compared them with clocks that had stayed behind at the US Naval Observatory.
The results matched Einstein. The clocks that flew east lost about 59 nanoseconds, or billionths of a second. The ones that flew west gained about 273 nanoseconds, according to Wikipedia’s account of the experiment.
Why the difference between directions? Earth itself spins eastward, so the eastbound plane was moving faster overall.
In a tiny way, those clocks had travelled in time.
Your phone relies on it
The best everyday example is GPS. The satellites that tell your phone where you are carry their own atomic clocks, and both of Einstein’s effects apply to them at once.
Their high speed makes their clocks run slightly slow. Being far from Earth, where gravity is weaker, makes them run slightly fast. The gravity effect wins, so overall the satellite clocks run a little faster than clocks on the ground, as NASA Space Place explains.
The difference is tiny. But GPS works out your position from incredibly precise timing, so engineers have to correct for it. Without those relativity corrections, your map would quickly drift off course.
Has anyone actually travelled in time?
In a small way, yes: astronauts.
The International Space Station orbits at about 28,000 kilometres an hour. Unlike the much higher GPS satellites, it’s close enough to Earth that the slowing effect of its speed outweighs the speeding-up effect of weaker gravity. So astronauts’ clocks run a fraction slower than ours.
Over a long career, that adds up. Russian cosmonaut Sergei Krikalev, who spent 803 days in orbit, is estimated to have travelled about 0.02 seconds into his own future.
That’s not much. But the principle is real. If a spaceship could travel at a large fraction of the speed of light, its crew could come home to find that years, even centuries, had passed on Earth, while only a short time had passed for them.
The catch? We have no way of going anywhere near that fast.
What about travelling into the past?
Going backwards is a very different problem.
Einstein’s equations do contain some strange solutions that seem to allow loops through time. The best-known idea is a wormhole: a shortcut through space-time connecting two distant points, which in theory could connect two different times as well. But nobody knows whether wormholes exist. And keeping one open would seem to need exotic kinds of matter or energy that have never been found.
Stephen Hawking suspected nature itself prevents it. He argued that warping space-time that much would likely trigger a burst of radiation powerful enough to destroy the spaceship, and perhaps space-time itself.
Then there are the paradoxes. The most famous is the grandfather paradox: if you travelled back and stopped your grandparents from meeting, you’d never be born, so who travelled back in time? Physicists have proposed ways around it, from the universe somehow stopping you to you ending up in a separate timeline. None of them can be tested.
For now, travel into the past stays firmly in science fiction.
The party nobody came to
Hawking ran a cheeky experiment of his own. On 28 June 2009, he held a party at Gonville and Caius College, Cambridge, with balloons, champagne and a banner welcoming time travellers.
There was a catch. He only sent the invitations out the day after the party, as Space.com reports, so the only people who could possibly turn up were time travellers from the future. He hoped copies of the invitation would survive for thousands of years, just in case.
Nobody came. Hawking later joked that the empty room was experimental evidence that time travel isn’t possible.
The story had a fitting epilogue. When he died in 2018, the ticket ballot for his Service of Thanksgiving at Westminster Abbey accepted dates of birth as late as 2038, as Texas Public Radio reported. Time travellers were, it seems, still welcome.
So, is time travel possible?
To the future, yes, and it’s happening all the time, in tiny amounts, to astronauts, aeroplane passengers and the satellites that guide your phone. A big jump forward would need speeds close to the speed of light, far beyond anything we can build.
To the past, almost certainly not, at least in any way physics currently understands. And nobody has turned up from the future to tell us otherwise.
As Hawking liked to point out, we’re all travelling forward in time anyway, at one second per second.
Hoping time crystals might be the missing piece? My post on what time crystals are explains why they sadly have nothing to do with time machines. And the barycentre is another example of gravity shaping the universe in ways we rarely notice.
Sources
- NASA Space Place: Is time travel possible?
- Wikipedia: Hafele-Keating experiment
- Wikipedia: Sergei Krikalev
- Space.com: Stephen Hawking’s memorial might have time travelers in attendance (2018)
- Texas Public Radio: Time travelers invited to Stephen Hawking service (2018)