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A Theory of Time Travel for Stories
Jul. 31st, 2021 07:09 pmWas chatting about this with Rick and he asked me to write it up.
Most stories that involve time travel are, to quote my wife "bullshit". But they don't *have* to be. A little thought can establish models that allow for greater narrative cohesion while not getting in the way of storytelling very much.
The most straightforward model is the one where time travel is possible, but *changing* time isn't. The only timeline there is already includes everything you do with your time machine. This has been used in many great stories, notably Bill and Ted's Excellent Adventure (first movie only) and Ted Chiang's moving short story "The Merchant and the Alchemist's Gate".
But say that you don't just want a story about moving through time, but actually *changing* it. That gets more complicated. Notions of how time works are baked into our language and thought at a very low level. Think about the very phrase "time travel". That includes the word "travel" which is generally thought of as a change in position *over time*. So time travel is a change in time… over time? That makes little sense and we're not even past the name.
But there is a fairly straightforward way for changing the past to make sense (or enough sense for a story), and it's even one that many stories already seem to be using instinctively. I'm going to use the recent Loki TV series for examples, so there may be spoilers.
One way to make sense of "change in time over time" is to posit two different *kinds* of time. Or, if you like, two different *dimensions* of time. If ordinary time is a line, then time travel stories often function as if time were a *plane*. Our ordinary experience of time we can call time-X, since it moves along the x-axis. But time-travelers like the TVA live in time-Y, orthogonal to time-X. Time-Y acts like time-X in most ways. Cause precedes effect, time only moves in one direction, etc. The one big difference is that people in Time-Y can (technologically or otherwise) change their temporal X coordinate. But they *cannot* affect their Y-coordinate, which proceeds inexorably forward at 1 second per second.
In this way the TVA can travel to any arbitrary point in what ordinary people consider time. But they can't change their *own* past. In this way it makes sense to talk of a multiversal war in the "past" – that is, an earlier coordinate on the Y-axis, or to say that the utopia at the end of time (time-X) isn't finished "yet" (time-Y). On a smaller scale, this is why, when Sylvie ambushes various agents, or even when she escaped the very first time, the TVA doesn't just use time travel to fix the problem. Sylvie's life (after she was taken) is proceeding along the Y-axis just like the TVA, and the TVA's tech only affects Time-X, not time-Y.
In the more general case, this gives us a model where we can do all kinds of fun rule-breaking (in time-X) but still have a story that functions in a way that doesn't require diagrams and math to wrap your head around (along time-Y).
Now, there are some things in Loki that *look* like the TVA tech can affect time-Y, such as their "prisoner rewind" collars. But since those clearly don't reset the *consciousness* of the prisoners, I'm going to call that a complicated piece of themed teleportation, not actually time travel. (Admittedly, I'm hand-waving here, since I don't think the Loki writers were actually using this model.)
There's one place in Loki that definitely doesn't fit this model (as explained so far), but which I think actually does so in a way that supports the story beautifully. If one can't time travel along the Y-axis, how can He Who Remains know everything that Loki will do once they meet? The obvious (to me at least) answer is that HWR has gotten to time-*Z*, from which he can time travel along the Y-axis. Which also rather neatly explains how he won the (First?) Multiversal War. Sylvie seemed to end up in possession (and control) of HWR's time machine. Is she now living on time-Z, and capable of messing with time-Y. That would explain how the TVA seems to have forgotten Loki. Loki remembers all this because, once you pop "up" a level, as he and Sylvie did when meeting HWR, you never go "down" again.
Again, I don't think the Loki writers are actually using this model, but I think it fits season 1, and offers interesting possibilities for season 2. On the off-chance that anyone reading this feels like using this model, be my guest.
[From a nit-picker's POV, I realize that this model fits actual physics about as well as FTL does – which is to say "not at all". It breaks symmetry, it has shared reference frames, it ignores spacetime curvature, etc. But, like FTL, it allows for some fun stories that would not otherwise be possible.]
Most stories that involve time travel are, to quote my wife "bullshit". But they don't *have* to be. A little thought can establish models that allow for greater narrative cohesion while not getting in the way of storytelling very much.
The most straightforward model is the one where time travel is possible, but *changing* time isn't. The only timeline there is already includes everything you do with your time machine. This has been used in many great stories, notably Bill and Ted's Excellent Adventure (first movie only) and Ted Chiang's moving short story "The Merchant and the Alchemist's Gate".
But say that you don't just want a story about moving through time, but actually *changing* it. That gets more complicated. Notions of how time works are baked into our language and thought at a very low level. Think about the very phrase "time travel". That includes the word "travel" which is generally thought of as a change in position *over time*. So time travel is a change in time… over time? That makes little sense and we're not even past the name.
But there is a fairly straightforward way for changing the past to make sense (or enough sense for a story), and it's even one that many stories already seem to be using instinctively. I'm going to use the recent Loki TV series for examples, so there may be spoilers.
One way to make sense of "change in time over time" is to posit two different *kinds* of time. Or, if you like, two different *dimensions* of time. If ordinary time is a line, then time travel stories often function as if time were a *plane*. Our ordinary experience of time we can call time-X, since it moves along the x-axis. But time-travelers like the TVA live in time-Y, orthogonal to time-X. Time-Y acts like time-X in most ways. Cause precedes effect, time only moves in one direction, etc. The one big difference is that people in Time-Y can (technologically or otherwise) change their temporal X coordinate. But they *cannot* affect their Y-coordinate, which proceeds inexorably forward at 1 second per second.
In this way the TVA can travel to any arbitrary point in what ordinary people consider time. But they can't change their *own* past. In this way it makes sense to talk of a multiversal war in the "past" – that is, an earlier coordinate on the Y-axis, or to say that the utopia at the end of time (time-X) isn't finished "yet" (time-Y). On a smaller scale, this is why, when Sylvie ambushes various agents, or even when she escaped the very first time, the TVA doesn't just use time travel to fix the problem. Sylvie's life (after she was taken) is proceeding along the Y-axis just like the TVA, and the TVA's tech only affects Time-X, not time-Y.
In the more general case, this gives us a model where we can do all kinds of fun rule-breaking (in time-X) but still have a story that functions in a way that doesn't require diagrams and math to wrap your head around (along time-Y).
Now, there are some things in Loki that *look* like the TVA tech can affect time-Y, such as their "prisoner rewind" collars. But since those clearly don't reset the *consciousness* of the prisoners, I'm going to call that a complicated piece of themed teleportation, not actually time travel. (Admittedly, I'm hand-waving here, since I don't think the Loki writers were actually using this model.)
There's one place in Loki that definitely doesn't fit this model (as explained so far), but which I think actually does so in a way that supports the story beautifully. If one can't time travel along the Y-axis, how can He Who Remains know everything that Loki will do once they meet? The obvious (to me at least) answer is that HWR has gotten to time-*Z*, from which he can time travel along the Y-axis. Which also rather neatly explains how he won the (First?) Multiversal War. Sylvie seemed to end up in possession (and control) of HWR's time machine. Is she now living on time-Z, and capable of messing with time-Y. That would explain how the TVA seems to have forgotten Loki. Loki remembers all this because, once you pop "up" a level, as he and Sylvie did when meeting HWR, you never go "down" again.
Again, I don't think the Loki writers are actually using this model, but I think it fits season 1, and offers interesting possibilities for season 2. On the off-chance that anyone reading this feels like using this model, be my guest.
[From a nit-picker's POV, I realize that this model fits actual physics about as well as FTL does – which is to say "not at all". It breaks symmetry, it has shared reference frames, it ignores spacetime curvature, etc. But, like FTL, it allows for some fun stories that would not otherwise be possible.]