Cinema-goers in 1985 could be forgiven for being disappointed with the world in 2015. In Marty McFly’s timeline, 2015 was a time of hoverboards, flying cars and miniature rehydratable pizzas. And though Pepsi may have obligingly created the bottle seen in Back to the Future II, the rest of these innovations seem a long way off.
But what of time travel itself? A mainstay of science fiction for over a century, time travel has appeared again and again in different guises over the years. From Marty McFly’s stylish DeLorean to the Victorian ingenuity of H G Wells’ Time Traveller, from the Doctor’s blue box to Hermione Granger’s Time Turner, time machines of one sort or another crop up everywhere in fiction. Setting aside the magic and the fantasy, what’s the state of the art real-world science of time travel and is it possible?
The answer is that it depends on what you mean by time travel. To understand that, let’s take a whistle-stop tour of our current best theory of space and time: Einstein’s General Theory of Relativity. The consequences of Einstein’s relativity can be crudely summed up in two statements:
1) The faster you move through space, the slower you move through time (as viewed by a stationary observer).
2) The effects of acceleration are indistinguishable from the effects of gravity.
(For a far more nuanced explanation, stay tuned for my general relativity article next month – it’s appearing in Popular Astronomy and I can’t publish online until after the print magazine is published.)
By moving faster in space, you can slow down the rate at which you move through time, letting the rest of the world age faster relative to you. If you could go fast enough, the world around you could appear to age by years while you only aged a few seconds. To take advantage of this, however, you’d have to first accelerate up to speed, and then decelerate back to a stop afterward. The processes of acceleration and deceleration also change the rate at which we pass through time. To see this in action, we need to think a bit about gravity.
Gravity and acceleration have the same effect, relativity tells us. By moving to a lower gravity environment (say, the weightlessness of space) we can move through time more quickly. The reverse of this was recently demonstrated to great effect in Interstellar where the high gravity around a black hole caused time to pass far more slowly for the intrepid explorers than for their loved ones back on Earth.
But don’t get carried away – even the astronauts in the International Space Station, travelling in low-gravity at over 17,000 mph barely see the effects of time dilation on their bodies. Right now, in fact, astronaut Scott Kelly is currently a little over halfway through spending a year in space while his twin brother Mark remains on Earth. When Scott returns, he’ll only be a few milliseconds younger than Mark. Without some extreme gravity object like a black hole, this effect won’t be catapulting you through time anytime soon, but this effect is important for our satellite navigation systems, however – failure to account for time passing at different rates at different points in a gravitational field would rapidly render our satnavs useless.
A limited form of forward time travel is technically possible, then, if we could somehow achieve high enough speeds or find objects with a strong enough gravitational pull. What of travel back in time?
There’s a precise mathematical relation describing how an object’s movement in time is linked to its movement in space. To slow time down to a standstill, an object would have to be accelerated right up to the speed of light, a feat which the equations of relativity tell us would require an infinite amount of energy. To go backwards in time in this formalism, the object would have to break the speed of light, and all current science indicates this is fundamentally impossible.
“But wait!,” the fans of Interstellar are surely saying, “Can’t the extreme gravity of black holes warp spacetime enough to allow travel back in time after all?”
Well, sort of. While no one really knows what happens in extreme gravity environments, there are proposals out there for objects tracing ‘closed timelike curves’, or closed loops in time. These are all sufficiently speculative that the only true statement is that no one really knows, but that travel back in time in all but a microscopic, unobservable level is highly unlikely.
All of this, of course, assumes that time is just a single linear dimension like the dimensions of space. But it doesn’t have to be, and if time arises from some other mechanism, then all bets are off. Hugh Everett famously postulated the Many Worlds Interpretation of quantum mechanics in which anything that can happen, does happen. Page and Wootters went further to suggest that the passage of time itself is an illusion borne of the putative colossal quantum entanglement of the universe at large (with recent experimental support for their principle, though I’m not convinced yet). More recently, Max Tegmark has somewhat echoed this sentiment, expressing his fascinating idea that time is a mathematical trick stemming from the structure of our paths through spacetime. (It’s hard to prove and I’m not sure I agree with it, but I love the idea.)
In the Back to the Future or Doctor Who sense, then, time travel is probably impossible. It’s hard to be sure though – before we can really answer the question we need a better idea of what time and space actually are. Perhaps, if Everett was right, there’s an alternate reality out there somewhere in which flying cars do exist in 2015, and Marty McFly can be found whizzing around on a hoverboard trying to escape from Griff. Sad as it is to not live in a reality with flying cars, let’s at least be thankful for one thing: at least the biggest recent movie in our reality isn’t Jaws 19.
[This article was originally written as a freelance piece but I never got around to pitching it to my usual outlets. The title is a reference to one of my favourite songs, The Future Soon by Jonathan Coulton.]