On the line – July 2009

Dan: Hello, I’m Dan Falk and I’m a science journalist and author. One of my favourite subjects is time, and I’ve been exploring its many intricacies while researching my new book called In Search of Time: Journeys along a Curious Dimension, published by the National Maritime Museum. For me, one of the most intriguing aspects of time is the problem of time travel.

Of all the plot devices used by science fiction writers, it’s hard to think of anything that captures the imagination quite as much as the idea of time travel.

And no wonder: imagine what it would be like if we had the same kind of freedom to move through time that we have to move through space. We could go back and see a loved one who’s no longer with us. We might be drawn to some of the pivotal moments in history. Or perhaps we could try to change history.

What interests me is the question of whether these kinds of stories are mere fantasy, or if time travel could happen in the real world… and, if so, how we can resolve the many paradoxes that it seems to entail.

Before we go any further, we have to make a distinction between travel into the future and travel into the past.

You might be surprised to learn that time travel into the future is not controversial. Einstein’s theory of relativity says that if you move quickly for a period of time, and then return to your starting point, you’ll have aged less than you would have if you had stayed put. In other words, you’ll have travelled into the future. But the effect only becomes noticeable if you approach the speed of light. The closer you get to light speed, the greater the time discrepancy.

Because no human beings have ever travelled that fast, the degree of time travel that we’ve engaged in so far has been quite modest. The Apollo astronauts, for example, returned to Earth having aged just a tiny amount less than their stay-at-home colleagues. The difference was very slight - just a few milliseconds - because their speed was still a snail’s pace compared with the speed of light. But the magnitude of that time difference is limited only by technology.

In principle, an astronaut could embark on a long voyage, and return to Earth to discover that many centuries had passed. For the astronaut, that would be the equivalent of travelling into the Earth’s future.

Now let’s turn to the problem of time travel into the past. As you can imagine, this kind of time travel presents more of a problem - the so-called 'grandfather paradox' - but we’ll get to that in just a moment. First of all, how would you actually do such a journey?

There’s nothing in the laws of physics that specifically prohibits time travel into the past. In fact, the second part of Einstein’s theory, known as general relativity, seems to suggest that it might actually be possible. In general relativity, matter warps both space and time. The larger the mass of a particular body, the greater the warping. And if the warping is extreme enough, one can imagine creating a 'loop' in time - what physicists call a 'closed timelike curve.'

So how do you achieve such a high degree of warping? There’s nothing in our solar system that even comes close to doing the trick. Instead, we would need some far more exotic astrophysical objects - perhaps a black hole, or better yet, a 'wormhole'… a kind of tunnel connecting one region of spacetime with another.

Now, this is quite speculative. We don’t know whether wormholes exist, or if traveling through them might be possible. But just knowing that they aren’t prohibited by the laws of physics is a stunning idea. If we ever discovered a wormhole that was not only stable but navigable, an astronaut could enter in one location and emerge not only in another place but at another time, either in the past or the future.

But if that kind of a journey really is possible, then we have a problem. Because travelling into the past gets us into difficulties involving cause and effect.

The best-known puzzle is the famous 'grandfather paradox.' Suppose you have a time machine at your disposal - either a wormhole, or some other exotic structure capable of bending spacetime in just the right way. You travel back to the time when your grandfather was a young man. Now, it would be rather evil to kill your grandfather just to test the laws of physics… so let’s say that you kill him by accident - perhaps you run over him in your car. Immediately, we have a problem:

Your grandfather won’t meet your grandmother, and so their child - your own mother or father - won’t be born. And therefore you won’t be born. But if you’re not born, how can you travel though time and kill your grandfather?

It is, obviously, a paradox… so we need some way to resolve it. Over the years, both physicists and philosophers have given a lot of thought to the grandfather paradox, and they’ve come up with a number of possible remedies.

One possibility is that the laws of nature somehow conspire to prevent you from killing your grandfather. Suppose you really are intent on killing him. You emerge from your trusty time machine, track down the house where grandpa lived, and soon enough you spot him.

Raising your pistol, you have him clearly within your sights. You can’t miss. You squeeze the trigger, and… well, presumably, you do miss. The question is, why? This is where it gets interesting. Nobody is suggesting that any sort of supernatural intervention takes place. But some physicists would argue that some mundane event - or series of mundane events - must prevent you from carrying out your task. Perhaps in spite of those hours of practice at the shooting range, you miss. Or, perhaps you have a change of heart at the last minute. Or maybe you slip on a banana peel on the way to grandpa’s house. One way or another, you’re prevented from achieving your goal.

But this does sound a bit strange, doesn’t it? It seems to be at odds with the notion of free will. There is some subtle physics here, and some rather subtle philosophy as well. It gets us into something called the 'autonomy principle', and it’s a bit more involved than what I can get into here, so I’ll leave you to catch the details in my book.

What these arguments boil down to, though, is the idea that you can’t change history. Roughly speaking, if you travel back in time, you can only do things that are consistent with the overall history of the universe. That is, if you travel into the past, you will only be able to do what you actually did. If grandpa remembers a crazed stranger trying to kill him when he was a young man, then the time traveller simply is that stranger. If a time traveller witnesses some historical event, so be it; but that means the time traveller was there all along.

But there is a third possibility. It involves what we might call 'parallel universes': In this scenario, you can travel into the past and kill your grandfather, but your actions don’t change history - that is, they won’t change the history of the world as you knew it, but merely the history of another, parallel world.

You can see how this idea of parallel universes successfully gets around the various paradoxes. But it does seem to carry a lot of metaphysical baggage. Is there any reason to suspect that these unseen universes actually exist?

Actually, modern physics does seem to hint at the existence of multiple universes. In quantum mechanics, for example, there’s something called the 'many worlds' interpretation of the theory, in which every time there’s an event with more than one possible outcome, all of the outcomes are said to occur - each in a different universe. This is a controversial idea, but a number of prominent scientists are taking it seriously. Our best theory of how the universe got started - the big bang model - also hints at extra, unseen regions of spacetime. Even string theory suggests that such regions might actually exist. So the idea of multiple universes might not be quite as crazy as it sounds.

But there’s still another possibility. It might be that the paradoxes are proof that backward time travel is simply impossible. The most famous endorsement of this position has come from physicist Stephen Hawking. He even gave this principle a name: he calls it the 'chronology protection conjecture'. Hawking admits that he doesn’t know what the mechanism actually is that would prevent backward time travel - but whatever it is, it 'makes the world safe for historians'. Even so, he does seem to have wavered on the possibility of time travel over the last few years.

For now, of course, the idea of time travel remains speculative. There are scientists who publish papers on the subject, and they’re usually greeted with a fair dose of skepticism. But even if time travel were shown to be impossible, that, too, would be a tremendously important discovery.

In the end, the quest to understand time travel is about more than an infatuation with tales from science fiction. Thinking about the problem of time travel is forcing scientists to confront some of the most profound issues in modern physics. It forces us to think about nature’s ultimate laws; to ask deep questions about cause and effect; to inquire about the fundamental nature of space and time. It’s only natural that we’d want to confront these questions, as we journey In Search of Time.