Time Travel and the Problem of Paradoxes

Graham Storrs joins us on the blog to discuss all things TIME TRAVEL.

“Paradox is the poisonous flower of quietism, the iridescent surface of the rotting mind, the greatest depravity of all.”  – Thomas Mann

What, you’re not a quietist? Never mind, we’ll come back to that.

As a writer of time travel novels, I spend a lot of time with paradox. It has become a friend. A shabby, disagreeable friend, I have to say, but one for whom I have an inordinate fondness. There are two ways of looking at paradox. Either it is a hideous monster of purest logic that prevents all possibility of time travel, or it is a sly creature of silken charm that whispers in the writer’s ear, urging creative trickery to make that story possible.

To be clear where I stand on the physics, let me just say that time doesn’t really work the way story-writers want it to. We don’t really travel in time. We travel in spacetime. Yes, you can describe space as a dimension something like the spatial dimensions to get a geometrical description of spacetime and, yes, it does seem as if you can move (in one direction) along that dimension at different rates. But consider this, if time is slowed in the vicinity of massive objects (which it is – ask Einstein), why does the Earth (a much smaller mass) not race ahead of the Sun in time, eventually leaving it far behind?

Time, as it affects us, is something like the ticking of a clock that can be different in each “inertial frame” (you’ll have to Google that one). If a spaceship moves past you at near the speed of light, the ticking of its clock seems slower than your own. If a suitably cooled and shielded man stands on the surface of the Sun, his clock also ticks relatively slower than yours. There’s a sense in which time is merely the rate at which events can unfold in your local spacetime. You can manipulate that rate by moving at different speeds or moving between different strength gravitational fields but, as far as we know, you can’t reverse it or even stop it, just slow it by applying unimaginable amounts of energy. You can maybe cut corners by moving through wormholes from one spacetime location to another by a route that is shorter than would be available in normal space, and that’s sort of like time travel, but not really.

There is a sense in which time travel, as we conceive it in science fiction, where we physically leave the present and reappear in the past or the future, would require jumping outside of time itself, jumping outside the Universe we know. If that is possible, no-one has ever worked out how. To a physicist, it probably just sounds silly. Yet that is the premise behind every single time travel story ever written (except for the Rip van Winkle and time dilation types).

As if this wasn’t argument enough against time travel, it’s very easy to conclude it is impossible just by looking at paradoxes. If I were able to travel back in time and shoot myself yesterday, I wouldn’t be alive today to travel back in time and shoot myself. Paradox! Most physicists conclude that this kind of paradox (known as the grandfather paradox) is sufficient reason to believe that we cannot travel back in time. The “many worlds” interpretation of quantum mechanics is a get-out-of-jail-free card for the grandfather paradox, since it allows a whole new Universe to be created whenever you go back and shoot yourself. So your own timeline is preserved and the one with you dead instead of alive is simply unfolding in parallel. The paradox is avoided but it’s an unsatisfactory kind of time travel if you ask me.

Paradoxes really are the spanner in the works for time travel. They don’t only happen for backwards travel, they also work if you travel into the future. Imagine you go forwards twenty years, meet your own sixteen-year-old child and immediately shoot yourself. You never went back and conceived the child. It’s a paradox!

You might have noticed that what all these paradoxes actually rule out is backwards time travel, not forwards time travel (in the previous example, the paradox arises because you can’t go back). And that makes sense, of course, because we are all, at this very moment, travelling forwards in time. If we went on a near-light-speed trip, or stood near the event horizon of a black hole, or had ourselves cooled to a fraction above absolute zero, we could slow down the rate at which time passes for us (our own local clock would run slower) and we would effectively zoom off into the future relative to everyone else. So going faster into the future is also possible. But note that you never leave the “present” for all the observers around you. They can still track your spaceship with radar, or monitor you on that event horizon, or watch you in your cryogenic chamber.

The time traveller who disappears from the present, goes to the future or the past, and then comes back, does seem to be a complete fiction. The physics of the real world, plus the logic of paradoxes seem to rule out such a thing. And yet time travel story writers from H G Wells onwards have imagined it just like that.

I’ve done it myself – knowing all that I know about the physics – because time travel stories are just so incredibly fascinating. However, in my own work, I have striven to remove the paradoxes and allowed myself the luxury of inventing new physics. The logic of paradoxes is a much more rigid barrier to creativity than the physics of reality. We can always imagine a future world in which what we think we now know about relativity or quantum theory is augmented or supplanted by a new understanding. But the logic of paradoxes does not depend on the science. It is eternal and unbending. You just cannot get around it.

So I used a version of time travel in one novel where you can jump around in time but there is only one, immutable timeline and your jumping around is already part of it. This is known as the Novikov self-consistency principle. Thus, if you go back and try to shoot yourself yesterday, you’ll miss, or the gun will backfire, or whatever. Something will always happen to prevent it. Why? Because that’s what must have happened in order for you to be alive the next day to go back in time!

In the Timesplash novels I used a different trick. My time travellers can go backwards in time and they can cause anomalies to their hearts’ content but the Universe can only be bent out of shape temporarily. It quickly springs back into it’s original form and spits out the time travellers like the irritants they are. So the past goes back to how it always was. No paradoxes, guaranteed. However – and this is what gives the books their dramatic interest – the disturbance caused, if it’s big enough, flows through to the present and causes massive acausal weirdness. It’s like a major acid trip but played out in the real world, not just someone’s mind, and it can be enough to topple buildings and kill people. But if you like chaos and the trippy effects of seeing causality completely messed up, jumping back in time is a great way to get people off and fuel the best underground parties ever.

Oh yes, and the quietists? They’re group of philosophers who think that by describing problems in a way that makes the misguided reasoning they’re based on apparent, they will put an end to all confusion, and set us on the path to intellectual quietude. And they really don’t like paradoxes. But then, who does? Maybe they should spend a bit more effort on clarifying the problem of time travel.

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