Today is Back to the Future Day, the day that Marty arrives in the future according to the 1989 movie. So it’s a fine day to discuss time travel.
Is time travel possible?
Well, to answer this, we first need to clarify what variety of time travel we’re considering.
What we usually have in mind is time travel to the past. But here there are distinct pictures that we have in mind.
The first is consistent history time-travel (which in my mind is the one that counts as genuine time travel). Here there is only a single history of the universe, every event happens at a time and a place, and at that time and place there’s only one thing that happens.
The time-travel part involves someone or something from the future traveling back to the past and interacting with it. Marty McFly travels to the past and hangs out with his mom and dad when they were teenagers. The Terminator travels back to try to kill Sarah Connor before she gives birth to the son who will destroy the machines. And so on. We all know the plot.
These scenarios raise an obvious problem, typically known as the grandfather paradox. Can I go to the past and kill my own grandfather before my mother is born? If I do, then my mother will never have been born, which will mean that I was never born, which will mean that I never traveled to the past, so my grandfather lived, so my mother was born, so I travel to the past and kill my grandfather, but then my mother was never born . . .
Considerations like these have led many physicists to make a ban on backwards time travel a fundamental principle of physics (they typically call it the “causality” condition which requires all causes to be earlier than their effects). But it’s not so clear that there’s actually any problem with this sort of time travel.
Many philosophers (notably including David Lewis) have argued that time travel to the past is perfectly consistent. Granted, there would be a problem if you killed your grandfather, but that’s OK — because you won’t kill your grandfather.
How do I know you won’t? Because you didn’t (obviously, since you’re here). Are you able to kill your grandfather? Well here it depends on what we mean by saying someone has an ability. You might have a loaded gun, be a great marksman, have the resolution to commit the murder, etc. But we know that for all that you won’t succeed. Because we know that your grandfather lives. Given the truth that your grandfather lived (which is shown by your existence), it cannot be true that your grandfather died.
Can I change the past? Again, we need to be clear about what we mean here. Obviously if something happened, I can’t make it be the case that it didn’t happen (at the same time and place in the single history of the world). That would be a contradiction.
However, if by “change” I mean causally contribute to some event happening, then if I can get to the past, there’s every reason to think that I’ll be part of the relevant goings-on at that time I’ve arrived. I just can’t make something happen if it didn’t happen.
But notice that we’re in exactly the same situation with respect to the future: If in the actual future history of the universe I have pizza for dinner, it would be inconsistent (and thus impossible) for me to have spaghetti instead. Given the actual future, nothing else can happen.
Of course, that doesn’t mean we don’t causally contribute to future events. (And it also doesn’t mean that we lack free will.) It just means that the actual course of events needs to be consistent.
There is one aspect that’s new in the case of time travel (or “backwards causation,” or “causal loops” — as the situations are also labeled). In this case there are global consistency constraints that don’t arise when all causation is in the forward direction. But it’s important to note that this has nothing in particular to do with questions of human agency, or what have you; it arises with simple inanimate objects too.
To see this, consider a tube (make it a worm hole, if you like) that can transport a billiard ball to a couple seconds in the past. We send the ball towards the tube, and we see it exit the tube before it enters. For a couple seconds there seem to be two balls on the table, then the first ball enters the tube and disappears. So far, no problem. It’s a simple consistent-history time travel story.
Now bend the tube so that the exiting ball would strike the entering ball, and so would knock it completely away from the tube. Then the ball never enters the tube, and so never exits, and so doesn’t interfere with the ball travelling along, and so it does enter, and so it prevents itself from entering . . . paradox.
What this means is that a causal loop like this is inconsistent, and so we know it will never happen. But this gives us no reason to suppose that time travel that is consistent would be impossible. At any given time everything that happens needs to be consistent with itself. Once we introduce backwards causation, we find that there are also consistency constraints on temporally extended processes.
So there are no metaphysical problems with time travel to the past.
Is time travel to the past physically possible? Well, we don’t know for sure because we don’t have the complete laws of physics, but given our current knowledge it seems that time travel to the past is indeed possible. Though it isn’t actual.
There are different ways of understanding the phrase “physically possible.” We might mean that something is possible given the laws of physics (and allowing the state to be different than the actual state). Or we might mean that something is possible given both the actual physical laws andand the actual physical state. If we have the latter meaning in mind, then (if the laws are deterministic) the only things that are physically possible are the things that actually happen.
But we usually have the former notion in mind. If I say it was physically possible for me to catch the train, what we mean is that I wouldn’t have violated any physical laws if I had been on time, but clearly the physical details would have been different.
Using this criterion, time travel to the past is possible according to General Relativity. There are solutions to the Einstein field equations that have closed timelike curves. This means that the laws of physics allow for time travel. (And clearly this would be consistent-history time travel because there’s only a single spacetime manifold.)
However, our spacetime is pretty clearly not the sort of spacetime that includes such curves (and it seems that there’s nothing we could do to change the future bits of spacetime in that respect). So there is no time travel to the past in our universe — even though such travel is physically possible.
What about time travel to the future? Well that’s a different story, one that (given how long this has gone) I should save for a future post.