A few days ago a friend of mine introduced me to the album Revel in Time by Star One. It’s a progressive metal album where every song is based on a different time travel movie: there’s a song for terminator, bill and ted, back to the future, donnie darko, and my personal favorite song in the album - a song based on primer. The album kicks ass, even if the latter half is weaker than the first, and it’s gotten us on a time travel movie watch-spree so we can cover all the properties in this album.¹

Seeing those movies I got to thinking about how time travel is used in movies, less on how it’s used as a narrative tool and more how it’s used internally - does the time travel in this movie make sense? Or atleast, as much sense as it possibly could make, considering we’re talking about time travel here? Most movies I’ve seen treat it as a kind of “don’t think about it too hard!” element and try to hand-brush away how it actually works, which is fine, but I am an overanalyzing bitch² and I want my silly stories where we go meet aristotle make sense goddamnit.

So I wanna talk about time travel - how does it work, and what “system” of time travel makes most internal sense?

To be clear I’m not gonna discuss the mechanism of time travel itself, and we’re just going to assume going forward that the hypothetical time traveler can do so with zero problems and warp instantly to a given point in the past (we can already time travel to the future by patiently waiting, so that’s not particularly interesting to discuss). What I want to discuss is the timestream itself and what happens to it, or in other words - If I go back in time and kill my grandpa, what happens?

Ruleset 1: Big Ball of Wibbily Wobilly… Timey Wimey… Stuff.

As seen in: Dr. Who, Bill and Ted

“Just don’t fuckin’ think about it too hard!” Under this ruleset, if it could even be called that, time travel works however it needs to work for this particular situation. Paradoxes be damned! Or maybe not be damned. Or half-damned? Depends on the episode and how the writer was feeling that morning, killing your grandpa may be totally safe, annihilate you from the timestream, or turn you into a goose. This is honestly the safest path for an author because it is basically admitting to your audience you don’t expect them to keep a consistent idea of how the time travel works in your movie and just accept it as existing.

Final Verdict: eh sure, who cares.

Ruleset 2: Singular Timeline

As seen in: Back to the Future, Erased, That Time You Killed Me

Under these rules, only one set of events can exist at one time; only one timeline. This means that if an event is modified in the past, the future changes accordingly, cascading the changes. This ruleset creates the Grandfather Paradox - If you go back in time and kill your own grandfather when he was an infant, what happens? If your grandfather doesn’t reach maturity, your dad isn’t born, meaning you aren’t born, meaning you don’t kill your grandfather. So your grandfather does survive. But then that means you are born, so you go kill your grandfather. So he’s dead. But if he’s dead you’re dead, so he’s alive. So you’re alive, so he’s dead. So you’re dead so he’s alive. So he’s dead. He’s alive. Dead. Alive. Dead. Alive.

Your grandfather is simultaneously alive and not alive. Not alternating between the two, but simultaneously two opposing states: an impossibility; a paradox.

How this is typically resolved in media of this sort is with there being a… “universal” timestream, let’s call it, that acts independently of any given timeline. For example - if you go back in time and kill your grandpa, even though it’s several years before in your timestream, it happens later on the “universal” timestream, because you did it after being born in the logical sequence of events. This allows time travelers (and authors) some flexibility, as seen in Back to the Future where Marty slowly disappears from the timestream under a countdown that only affects him. This countdown until when Marty is erased from history operates under the “universal” timestream rather than his position in his own timeline.

This method can work well for telling stories, but I find it a little unsatisfying as a ruleset because it says that the solution for time travel is creating a different, true timestream, so we still end with some form of uninterrupted timeline. Additionally, it’s up to the individual author to determine how they resolve paradoxes because this ruleset doesn’t actually resolve them.

Final Verdict: Decent, but you could do better.

Ruleset 2.5: Singular, Preset Timeline

As seen in: Terminator 1, Attack on Titan, Dirk Gently BBCA

This is a sub-type of the singular timeline where the time travel does not actually modify the timeline, because the timeline always incorporated the changes created by the time traveler.

This is a much trickier form of time travel to pull off both for the time traveler and for the author because the time traveler is now disallowed from doing anything that may possibly impact them prior to the point of time traveler: they cannot give themselves any new information, they cannot actually fix the issue they’re out to resolve (otherwise they wouldn’t have originally time traveled), and god forbid meeting themselves.

However, this form of time travel is logically sound. It allows for the Bootstrap Paradox (which isn’t actually a paradox, I’ll get to it in a second) but it doesn’t violate any internal rules. There’s a single timeline, where the events are preset, and there is no possibility for two contradicting truths to simultaneously occur. It is interesting.

One interesting facet of these rules is the aforementioned Bootstrap Paradox: A situation where a time traveler’s actions in the past unwittingly make them commit certain actions in the future, which then lead them to do these actions in the past. For example - let’s say you time travel and give your past self the instructions on how to make a time machine, but only on the condition your past self does the same as their first time travel. Your past self accepts, builds a time machine, and then goes into the past to give their past self the instructions on how to make a time machine. The question arises - who built the first time machine? Who made these instructions in the first place? Under the bootstrap paradox: No one did. This “time loop” is a universal constant that was simply going to occur, and the time machine’s plans is part of it. It’s not really a paradox, as no contradiction occurs at any point, it’s just a somewhat nonsensical event.

But it’s not nonsense or self-contradictory, it simply is hard for us to comprehend; it doesn’t make sense to us, but it doesn’t mean it doesn’t make sense. There are many things we just accept to exist without questioning them too hard (ourselves, for example, or if you believe in god - why god exists), so it’s not impossible that the time loop simply exists too.

Final Verdict: Cool but rigid.

Ruleset 3: Multiple Timelines

As seen in: Terminator 2, Primer, Outer Wilds

This ruleset is the most flexible of the bunch: under this ruleset there can exist multiple, incompatible timelines at any given point. When a time traveler goes to the past, they don’t go to their own past, they go to a different timeline’s past and any changes they do will not ripple back towards them. In this ruleset you can kill your grandpa as many times as you please without worrying that you might poof out of existence. You can kill yourself before you go in the time machine and all will be fine in the world³.

I like this interpretation of time travel very much because despite disallowing some specific, cool ideas in time travel (like the bootstrap paradox) it can be as complex or as simple as you want: In Terminator 2 the time travel and its consequences is as simple as can be, meanwhile Primer’s time travel is such a goddamn mess even XKCD can’t tell what the fuck is going on.

It also leaves many questions open for the author to answer regarding the time travel: Is it possible to return to your original timeline at all? What occurs to the timelines left by the time traveler, do they persist or whiff out of existence? Can multiple time travelers from separate timelines clash into the same target timeline? It can be a lot of fun, but also very, very messy.

Final Verdict: Love it.