For ages I've been wanting to make a video analyzing time travel in fiction – not the

magical or physical mechanisms by which the time travel is supposedly achieved , but rather,

the different ways time travel can influence causality (and thus the plot) within the universe

of each story.

Needless to say, there are spoilers ahead!

Let's start with Ender's Game by Orson Scott Card – time travel in this book is actually

100% realistic: the characters experience slower passage of time when they travel close

to light speed, allowing just a few days or months to pass for those traveling while years

pass on earth or other planets.

It's traveling forward through time like we normally do, but at different rates.

This kind of time travel doesn't "change the past" or allow characters to make different

decisions than the ones they already did – it's all one consistent historical trajectory.

The original Planet of the Apes film is similar, where astronauts experience extreme time dilation

and then crash land on a strange ape-ruled planet that (major spoiler) turns out to just

be earth in the distant future.

But what about actual time-travel time travel?

Well, I would say there are two big distinguishing features between different types of time travel

in fiction.

The first is whether or not the time traveler is there when history happens the "first time

around" – that is, is there a kind of "self-consistency" where, since time travel takes you to the

past, when the past happened the first time, the time-traveling version of you was always

there to begin with?

Or does the very act of time traveling to the past change what happened and force the

universe onto a different trajectory of history from the one you experienced prior to traveling?

And the second distinguishing feature is: who has free will when somebody is time traveling.

Like, whose actions are allowed to move history onto a different trajectory, and whose aren't?

One of the simplest time travels is "do-over" time travel, where you essentially get to

re-play history starting exactly as it was at a certain point, with the only caveat being

you remember your experiences from already having tried various possible future timelines

(while no one else does).

It's essentially like playing a video game where you can start a level over with the

foresight of what you did wrong the first time.

For example, in Groundhog Day Bill Murray's character relives the same day over and over

again, and though he can make different choices each time, he always starts back at the same

point (except with new memories of his previous choices).

That is, until he figures out the one exact set of choices that frees him from the loop.

I consider "A Christmas Carol" to be in this vein, too, even though it may not seem like

time travel.

But because Scrooge gets to visit the future of his current timeline, even though he has

no ability to affect the timeline directly while "visiting", he can still change his

actions in the present based on what he learns, essentially getting a “do-over.”

The video game Braid is built on the idea of “do-overs”, where you get to rewind

a few seconds and try something different (though there are some things that are immune

to going back in time and don't "rewind", which is what makes the game interesting).

Braid also has another kind of time travel, where you go back to your past as a separate

individual, and the past version of you is there with no free will, just doing exactly

what you did the first time around, while "time-traveling you" can change the course

of history.

This is also how the video "Clock Blockers" by the Corridor Digital youtube channel works.

And then there's time travel where the very act of going to the past or future creates

a fully new trajectory of history because time-traveling you weren't there the first

time around, and now you are.

This includes the typical "anything goes" time travel movies like Bill & Ted's Excellent

Adventure, Back to the Future, Star Trek First Contact, and so on, where you can kind of

instantly jump back and forth to any point in time you want, potentially resulting in

multiple versions of yourself.

From a causality perspective, anything you do in the past (and even just the act of going

back in time) redirects the course of history onto a new timeline – in Back to the Future,

Marty's interference with his parents falling in love results in the timeline of history

being redirected towards a version of the future where he doesn't exist and so he starts

to disappear from photos and real life.

And even after correcting that major deviation, his interactions with his parents while he's

in the past result in them being very different people when he returns to his present time;

he accidentally caused history to progress in a slightly different direction.

The movie ”Looper" is similar, but there's a little more circularity because when you

jump to the past, you cause history to branch onto on a trajectory where, in the future,

the younger you also goes back in time the same way you just did.

Both you and your past self still have enough free will to change that forward course of

history, though, which results in weirdness like you getting new memories when your past

self does things you yourself didn't do, or if they lose a body part, suddenly you'll

lose it too, replaced by an old scar on your own body.

So, changes to the present affect not just future timelines, but also future timelines

that wrap back around to the present!

The indie film Primer is in the same vein, except that it takes the plot device of time

travel to the extreme, with time travel within time travel within time travel, time-traveling

characters interacting with other time-traveling versions of themselves, bringing time machines

with them to the past inside other time machines, and so on.

But beyond the complexity, there are two things that make Primer stand out: first, time travel

to the past isn't an instantaneous jump, but actually takes time: to go back 6 hours, you

sit in the time machine for what feels like 6 hours.

And time travel can't take you back to a time before a given time machine was initially

activated, since of course, the machine can only be taking you back in time inside it

if it's turned on, so the first time it was turned on is the farthest back in time you

can go.

There's a nice logic to it.

Which brings us to perhaps my all time favorite of all fictional time travel: Harry Potter

and the Prisoner of Azkaban.

It's an "instantly jump back in time" kind of time travel that doesn't actually generate

any new timelines.

It manages that because in this universe, while you were experiencing your initial,

pre-time-travel passage through a particular point in history, your "time-traveling clone"

was also already there, doing everything you'll eventually do when you time travel yourself.

For example, Harry and friends are saved from dying by their time-traveling selves, the

first time through that timeline.

It makes so much sense – if you go back in time, you really and truly were present

at that point in time all along!

This also means that during the period of overlap, the time-traveling you has no actual

free will, since everything you do has in some sense already been done, which Harry

comprehends when he realizes he has to save his past self because he was already saved

by his future self when he was in the past.

I think I love this kind of time travel because it manages to be logically consistent: it's

time travel to the past where you can't change the past, because the past already happened.

And there's only one timeline – the one in which time travelers arrive from the future,

do stuff, and at some later date, leave to go to the past.

Logical consistency is a primary thing that, you may have noticed, I think lays the foundation

for good time travel stories – not because logical consistency is important in an of

itself, but because, most of the time, in order to care about the characters in a story,

we have to believe that actions have consequences.

If everything is just a meaningless series of events, then we almost don't have a story.

So it's really helpful if there are rules by which the universe of the story functions,

whatever those rules may be.

Speaking of actions with consequences, I finally got the kick in the pants I needed to make

this video from my friends at the Corridor Digital YouTube channel.

They've asked me to help promote their new YouTube Red Original Series, "Lifeline”,

which, minor spoilers ahead... is a sci fi action thriller with time travel in it.

What kind of time travel, you ask?

Essentially, if somebody dies in the future, that sends a message back to the present,

which allows people to jump forward to just before the time the person dies and change

the trajectory of history from that point onwards, averting their death.

But as you might imagine, things eventually go awry.

Anyway, you can check out the first episode of Lifeline for free on the Corridor Digital

channel or by following the links onscreen or in the description . And fun facts: I actually

know the Corridor guys from back before MinutePhysics, when I was doing special effects for the "freddiew"

channel.

We also all grew up in neighboring towns in Minnesota and even competed against each other

in high school sports , though we didn't know each other at the time.

But enough trivia – go check out their show!