It's a question asked by society for more than a hundred years. So it’s only fitting on Pretend To Be a Time Traveller Day that we enlist the help of Clas Weber, unit coordinator of PHIL3001 Metaphysics: a User's Guide to Time Travel, to explain theories around the possibility of time travel.
Think about the most embarrassing thing that’s ever happened to you. Wouldn’t it be great if you could go back in time and prevent it from happening? Ever since the publication of H. G. Wells' novel The Time Machine, people have been intrigued by the idea of time travel. Is it just fiction or is it real?
Time travel and physics
What do physicists say? One of the most successful theories for how the universe works, confirmed over and over again by countless experiments, is Albert Einstein’s General Theory of Relativity. Einstein’s teacher Hermann Minkowski gave a famous interpretation of this theory on which time is just another dimension among the three dimensions of space: things exist in four-dimensional space-time. This opens up the possibility that points in time are similar to points in space – destinations to which we can travel. And indeed, in the late 1940s, the brilliant mathematician Kurt Gödel proved that Einstein’s theory allows objects to travel along loop-like trajectories through space-time, such that they return to their point of origin in the past. According to Einstein’ theory, time travel is real. If you are interested in time travel from a scientific point of view, check out Sean Carroll’s great podcast on this issue.
Time travel and philosophy
Are these scenarios just oddities of Einstein’s formalism or do they correspond to genuine possibilities? Can we really travel in time? Physics may allow for time travel but perhaps logic does not? Philosophers who have thought about time travel are most worried by the idea that it would lead to paradoxes, i.e. logical contradictions. The most famous one is the so-called grandfather paradox.
Consider the following scenario: You possess a time machine that could take you back to the past. You also passionately hate your grandfather, a truly horrible man who committed many awful crimes. You pack your gun and travel back to when your grandfather was 15 years old. This is before he committed the crimes, but also before he had met your grandmother. You lurk in the bushes in front of his house. You see him leave the house. You aim. You are about to pull the trigger. But wait! Can you really kill him? Why not? Your gun is working, you practised this over and over again, you have a clear view. So, you can kill him. On the other hand, killing him means he will never meet your grandmother. Which means your mother will never be born, and you won’t be born either. But then you couldn’t take the trip with the time machine in the first place. So, you can’t kill him. You can kill him and you can’t kill him – paradox. Something has gone wrong…
The most influential philosophical analysis of the paradox is from David Lewis. According to Lewis, you won’t shoot your grandfather – otherwise, you wouldn’t be there to try. What prevents you from doing it? Some commonplace reason (not the gods of logic): your gun jams, something diverts the bullet, you trip on a banana peel. OK, so you won’t kill your grandfather, but does that mean you couldn’t kill him? Here, Lewis says: in one sense you could, in another sense you couldn’t. We need to assess what you can and can’t do relative to some set of facts. Relative to your training, your good eyesight, the conditions at your grandfather’s house, etc. you can shoot your grandfather. However, if you add to this the fact you travelled back in time, then you cannot kill him. There is no paradox – we have simply assessed what you can do relative to two different sets of facts. So, time travel is a real possibility both from the point of view of physics and that of philosophy.
Back to the horribly embarrassing thing that happened. Can you go back in time and prevent it? Well, you can try. But you will fail. Why? Because the terrible thing happened. That’s a fact about the past. Even if you now decided to travel back, you were already there when it happened and failed to prevent it back then. The past is the past and we cannot change it anymore.