An Incomplete Index of Time Travel Philosophies

I have few true loves in this life.  Good movies is one; maple candy is another.  High on the list, on some sort of cloud-like throne, sits theoretical discussion of time travel.  More specifically, I like the different ways different movies handle the mechanics of treating space and time like some filthy whore, poking through various wholes willy-nilly.  Here then is an index of some of the classic approaches.
But first, some definitions.

Timeline:  one particular set of events / reality.  That is, say we all live in timeline A.  The Big Bang happens, a bajillion years later you're born, you die, the world moves on and eventually ends.  That batch of events in timeline A.  If, however, some dude figures out how to travel through time, goes back and introduces bananas to North America long before they were supposed to be, and thereby changes history, everyone is now living in a new set of events.  This new set of events is a new timeline, timeline B.

The Grandmother Paradox:  this is a pretty standard time travel paradox.  If you go back in time and kill your grandma before she gives birth, what happens to you?  Do you cease to exist because your grandmother died before she had kids or what?  And if you do suddenly disappear, does your grammy come back to life because you no longer exist?  And if so, then you'll snap back into existence and make everyone's head hurt in the process of trying to figure out what the crap is going on.

The Butterfly Effect:  no, not the movie.  This is the idea that if you send a butterfly back in time to the land of the dinosaurs, a single flap of its wings will change the winds a little bit, and over time that slight change in wind will lead to different storm patterns down the line, different people getting rained on, and different lives for everyone in the world.  Simply put, a little change goes a long way when you've got years to wait.

With that out of the way, here are some types of time travel:

One Timeline Time Travel

I'll let my favorite trailer ever explain this one:



This is the kind of time travel where there is only one timeline, and everyone just jumps around in that.  Like, imagine a movie of all of time.  You can fast forward, you can rewind, and you can jump around to whenever you like, but at the end of the day you can't change it (we'll pretend you're too cheap for video editing software).  You can move all around in this single time line, you can select the scene, but you can't change anything.  It's all preset.  If you go back in time, you have always been a part of history.  You've always been back there doing whatever it is that you do.  If you go back and try and kill your own grandma, it'll turn out that you fail at the last second or she isn't actually your grandma.  Usually, it'll turn out that you're your own grandma because that's a fun plot twist.  This kind of time travel often ends up with a protagonist going back in time to solve some problem only to discover that by going back in time they've caused the problem they were trying to solve because everyone loves irony.

Examples:  12 Monkeys, Bill & Ted's Excellent Adventure, Star Trek IV:  The Voyage Home


Branching Timeline Time Travel

This is the other obvious approach to time travel.  Under this philosophy, if you go back and change something, it changes the future.  If you go back to the 80s and make someone's parents never meet, when you go to the future that person won't exist.  Kill Hitler in the 20s or 30s and he'll never rise to power.  And so forth.  Depending on the mood of the story, changes due to your actions can either be subtle or enormous.  For example, in Back to the Future, our hero goes back and somehow gets his dad to beat up a bully.  This brings him to the future where he still exists (the exact right sperm and egg still got together to make him), but his family is happier and lives in a nicer house where they live happily ever after until Doc shows up just in time to have a cliffhanger ending.  Contrast this with The Butterfly Effect, where changing one childhood event can make our hero end up in places he's never been when he gets back to the present.

Examples:  Back to the Future, The Butterfly Effect, Primer

Time Is Like A River

This school of thought says that you can go try and change stuff in the past, but history is inevitable and has a way of correcting itself.  So if, say, your sweetheart dies in a horrible accident and you invent time travel so that you can go back in time and save her, you might be able to save her from getting run over by a stagecoach, but then she'll just drown or something.  If she didn't die horribly, then you'd never have the motivation to invent time travel, and that'd just end up weird.  Also, some things are just inevitable.  In these stories fate is fate, and you can't work around it.  You can go back in time and set a big-ass rock in the middle of the river of time, but time will just flow on around your rock.  You'll have changed little bits of flow right near your rock, but the water is still all going to the same place, and most other parts of the river won't be affected.

Sometimes, though, a little rock and change the course of a whole river...  Some stories have moments where time is particularly mutable, moments where a little change could divert the course of history.  This variation is called The Trousers of Time by Terry Pratchett, and I see no need to come up with a new name.  The metaphor there is that for the most part time just rumbles along, but occasionally will reach a point not unlike the divide between legs in a pair of trousers.  At those points, it's up to the individuals at hand and their actions to determine which pant leg time will tumble on down.

Examples:  The Time Machine

One Present Time Travel

This kind of time travel is similar to branching timeline time travel in that if you go back in time and do something, it changes the future.  The difference here is that if you go back in time and change something, it doesn't tend to have lasting effects; there is no butterfly effect.  The clearest example of this I can think of is in Day of the Tentacle, a 3rd person puzzle video game featuring a time travel mechanic.  Early in the game one of your characters is stuck in a tree, and to get them out you have to go back in time and cut down the tree.  Having done so, the tree just disappears in the future and the girl previously stuck in the tree falls suddenly to the ground.  No lives are changed; everyone who was previously born is still born.  Simply removing the tree doesn't do anything beyond removing the tree.

This kind of time travel can be tricky; sometimes characters remember things that end up changed and sometimes they don't.  That is, the girl who just fell out of the tree definitely knows there used to be a tree there, but the guy who comes and talks to her right after she falls out of it doesn't seem to care that a tree just disappeared.  Some people know that something just changed, and some people don't.  This can lead to plot holes.

The way I like to visualize this is with a set of moving bars.  Bear with me.  Imagine your timeline, which looks like this:

Imagine that the timeline is always growing; each moment gets tacked on to the end as it happens.  Now some asshole invents time travel and heads back a few decades to try and fix their broken life.  They find themselves at some point in the past, which I'll represent with a red bar:


Both the blue bar and the red bar are always moving forward at the same rate.  Each moment that passes by the blue bar gives exactly enough time for a moment's worth of time to pass by the red bar, so they're always the same distance apart.  I like to think of the red bar as being a part of the present.  You might imagine a universal clock that's completely independent from the timeline always ticking away.  Each moment that it ticks moves both the blue bar and the red bar one moment further through time.  You might also think of the present as a movie playing, and when someone time travels away from it someone starts up another copy of the movie running on a screen right next to the original present.  The only moments that the universe is keeping track of at any given point are the ones where someone from the original present happens to be.  I'm not sure which of these ideas are most applicable.

So there's someone from the blue present at the red present, and the moment that someone does something at the red present, it echoes out through the future and changes everything beyond it.  Except that the universe is only keeping track of the two times where people from the original present happen to be, so if you do something at the red bar, the reality at the blue bar just sort of suddenly adjusts.  If you cut down the tree at the red bar, it disappears at the blue bar.  People will probably remember the tree has always having been there, or they might just feel like "there should be a tree here".  Or another example, if you appear and tell some dude in a Hazmat suit to go find your mum in 10 years time at the red bar, he'll suddenly remember this happening at the blue bar.

Examples:  Day of the Tentacle (a video game), Lost (probably), Star Trek:  First Contact


I'm sure this "index" is incomplete.  I haven't seen every time travel movie ever, and I'm missing some important ones like Terminator 2.  The first Terminator looks like it should be one timeline time travel, but I always hear that the sequel is branching timeline time travel.  I should do some research.

Before I sign off, there's one more thing I want to mention.  The movie Primer has a really cool time travel related idea that more stories should pick up.  In that movie, time is like space in that you have to pass through each point between you and your destination in order to get there.  So in space, if you want to get from your computer to the fridge, you have to walk through each point in space that exists between you and your fridge.  Then after that if you want to get to yesterday, you have to pass through each moment between now and yesterday to get there.  This means that you have to sit in a special box for 24 hours if you want to get to 24 hours before now.  This leads to lots of... stuff.  Weird stuff.

Alright readers, I know you're out there because you keep either commenting on my posts or talking to me about them at dinner.  Dad.  Tell me what philosophies of time travel I've missed, tell me what movies fall into what categories, and tell me good time travel movies I should check out.  Just don't get into Donnie Darko stuff; we can leave that for another time.  I plan on writing a Donnie Darko walkthrough at some point.  Or maybe I'll just link to one.