The Logic of Time Travel in Fiction

A topic I'm really fascinated by: Time travel. It's fun to think about the logic of it, how it might work, in fiction or real life. 

I guess this interest has increased a bit since I started watching Doctor Who, but I think it first started after I read Harry Potter and the Prisoner of Azkaban, where they go back in time at the end to save Buckbeak and Sirius. I had a long-ish discussion with my sister to try to figure out how exactly things went as they did. More on that later.

Very common time travel logic question: "If you went back in time and killed your grandfather before he had children, what would happen?" 

Different people give different answers. But logically, if you killed your grandfather, you would never have been born to go back and kill him in the first place, thus eliminating the whole process. 

Then, what would actually physically happen if you tried to do this? Would there be a strange force stopping you from killing him? If you killed him, would you just vanish on the spot after he dies?

I think I've found an answer to this that I like. I think that if you go back in time and kill him, you'll create a new reality. So if you then go back to the future, to the point you left from, you don't exist anymore. You have changed the future. You still physically exist, but no one knows you. There are no records of you anywhere. Unless your sci-fi time travel device also allows you to travel between universes, you'll be stuck there.

And then there's time loops and paradoxes, which I find really bit annoying, at least as a plot device. Here's an example from Doctor Who:

The Doctor is trapped inside an inescapable prison. He has a tool, a sonic screwdriver, that can open most doors, but he doesn't have it with him inside the prison, so he can't use it to get out. His friend Rory is somewhere outside, and doesn't know that the Doctor is trapped. Suddenly, the Doctor appears in a flash of light and explains to Rory that he is from the future, and gives him the sonic screwdriver to go and let the "past Doctor" out of the prison. Rory does this, and then the Doctor is free, and able to go back in time to tell Rory how to let him out.

It's sort of an endless loop. Technically this should work I guess. Because nothing is stopping the freed Doctor to go back in time to let himself out when he's already been released. But — where did the actual "fix" come from? It feels like you have been cheated of the solution. It's like if you have a question you can't find the answer to. And suddenly your future self appears and gives you the answer. And then you know, so you can go back in time, and tell to it your past self. Again, there isn't really anything wrong with this scenario, but where did the answer actually come from? Where did you find it? There isn't any point in this loop where you actually figure out the answer for yourself. And this is why this annoys me so much when it's used in fiction. It's cheating. Because if this is possible, why doesn't the time traveling character use this technique to solve absolutely everything?

"The Laws of Time Travel" seems to be changing with the writer of the story. Everyone has their own view on what works and what happens. The following seven categories are taken from TV Tropes, but I'll try to give my own explanations and examples of how they work:

1:You can't fight fate!

Something has gone wrong in the past, maybe a character has died. The hero goes back in time to save them, but even if he saves them, some other kind of accident or similar kills them anyway, however hard they try to stop it. This kind of time travel story sort of says that you can't change anything. The future, the past and everything is set in stone. Some higher force controls everything.

2: Set Right What Once Went Wrong

The time traveler goes back to the past to change something. Maybe to stop an apocalypse, like in... Terminator? (I haven't really seen those films, I might be wrong) In the end, this works, and the future is changed to become a better place. The expression "Time can be rewritten" from Doctor Who sort of applies to this. It's possible to start in one bad future, go back to remove the source, and go back to live in a better future. It's sort of like having removed a piece of a chain, but everything still hangs together.

3: A Stable Time Loop

I think this is where the time traveling done in Prisoner of Azkaban fits. In this one, absolutely everything you do in the past already happened before you went back in time. All your actions led up to the moment you went back. This one is sort of a head scratcher. Because it's kind of implied that you can't actually change anything, but you still need to go back to make it happen. I'll try to make this more understandable by using the example from Harry Potter:

The reason Harry and Hermione goes back in time, is to save Buckbeak from getting killed, and save Sirius as well. But before they go back, they never actually saw Buckbeak getting his head chopped off, they just heard the sound of the axe, and Hagrid's howling. They go back in time, and lead Buckbeak away from where he's tied up. The executioner see's that he's missing, and chops a pumpkin in half in frustration, which makes a noise that sounds like a head getting cut off. Hagrid cries out with happiness that his beloved Buckbeak got away.

So this means that the killing never actually happened. Because they were already there to stop it, they just didn't know it. So, you can't really undo anything. If they had seen Buckbeak getting his head chopped off, if they had seen him die, it meant that their future selves never got the chance to go back and save him, ever.

I also remember a similar, simpler example from The Big Bang Theory:

Sheldon and Leonard agree that if any of them ever manage to create a time machine, they will go back to this exact moment to prove it to themselves. Of course, this doesn't happen, so they conclude that they never will build one.

I guess this is also where the previously mentioned example from Doctor Who fits. This, and category 7, sound to me like the most realistic version of time travel, and something that could maybe apply to real life if we ever become able to travel in time. But it doesn't appear in fiction that often, because it's so complicated to keep straight. Also, it's nearly impossible to execute properly, because then the writers need to have the entire story planned out from the start.

4: A Temporal Paradox

Ugh, paradoxes. This is where most of the solutions to the grandfather example fits. What happens if you travel back in time and undo something that was the very motivation or cause to make you go back in time at all? For example, after you have killed your grandfather: You might disappear at the spot, right after he dies. You might live, and go back to the future and find the world to be a completely different place. All of reality might collapse. Who knows.

You can also see it this way: What if you were a survivor of the holocaust, and wanted to go back and kill Hitler to stop all the suffering. If you do this, you will have eliminated your very motivation for going back in time in the first place! In theory, these would produce an infinite number of parallel universes.

A different kind of paradox can be this: Let's say you own an old thing, it can be whatever you like. You got this from a grandparent. You take this old item, go back in time, and sell it to someone. And without you knowing, this person is the one that sells/gives the item to your own grandparent when they were younger. Sort of a meaningless scenario, but still; where did that item actually come from? Everything just loops. And if the thing you took back was already old when you took it back, it would be even older when all the time has passed up to the point where your grandparent gives it to you, and then the loop repeats. So the thing should get older with each loop... which technically would turn the thing to dust at some point. Or it would just stop to exist. Now I'm giving myself a headache.

Or, to continue with the example from Harry Potter:

Harry is lying by the water, surrounded by Dementors that are trying to kill him and Sirius. His attempts to use the patronus charm to chase them away doesn't work. But suddenly he sees a figure on the other side of the lake that produces a stag-patronus that saves their life. He believes that this is somehow "the ghost" of his father. He then travels back in time, and finds himself at the spot where he saw the figure. And then he realizes that he didn't see his father, he saw himself. He then rushes forward and produces a perfect patronus and saves his own life. And later he explains that he only managed to do this because he knew that he had already done it.

So, at which point did he actually learn to use the spell? The skill appears out of nowhere because he already knows that it will work, because it already happened before... Of course, it can be argued that the happy feeling he gets when he realizes that he can save himself is what enables him to do the spell.

This video gives a fun explanation of both the paradox version and the "stable time loop", using cookies as an example.

5: Reset Button

If someone goes back and changes something, the future is "reset", and everyone wakes up having no memory of what happened. Or maybe they do remember, but the entire story is kind of deleted as if nothing happened at all.

Let's say you do something that makes your life miserable. You miss an important meeting, and then you lose your job, your girlfriend/boyfriend and your money. Your life is generally really bad right now. You go back in time and make sure you get to that meeting, and then you just vanish from existence, and the other you keeps on living your life the way you changed it to be. 

6: Trapped in the past

This is the one where someone ends up in the past without actually meaning to go there. Kind of like Samurai Jack who gets sent to the future by the demon Aku. In those cases the person usually has no means to go back home, and has to live out their life from the point they were sent to. To use another expression from Doctor Who, they have to "take the slow path".

7: Alternate Timeline

This is where my final solution for the grandfather paradox fits. If you change the past, you split off into a new timeline. There's the old one you came from where nothing was changed, and the new one you live in now, where you have been erased from existence in the future. Logically, you would be trapped in this timeline, with no means of going back to where you came from.

Also, in this one, none of your knowledge of the future will be of any use, because as soon as you went back, you became part of events and changed the future to something different. In a stable time loop you would have already been a part of shaping the future you left from. In this version, you actually change things to something different from what you remember.

In real life and other comments

Stephen Hawking once suggested that the absence of tourists from the future is an argument against the existence of time travel—a variant of the Fermi paradox. (wikipedia)

Interesting TV Tropes articles:

• Hitler's Time Travel Exemption Act: If you time-travel into the past and then try to kill Hitler, it won't work as intended.
• If you time travel, where do you actually land? Let's say you can only move in time, not space. If you disappear in 2012, and reappear in the same spot some time in the future or the past, you should technically end up in outer space, because the earth has moved!

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Last edited 15. may 2023