This video is sponsored by the YouTube Red Sci-Fi Series "Lifeline".

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 planet ruled by apes

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,

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 Charles Dickens<i> </i>"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 pretty much any time

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 while the past version of you is there with has no free will,

it just does exactly what you did the first time around,

while the "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 &amp; 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

(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.

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,

and find it immediately replaced by an old scar.

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,

with 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 in a given machine.

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 a kind of time travel where you "instantly jump back in time"

and can interact with yourself

but it doesn't generate new timelines.

It manages that because in the Harry Potter universe,

while you were experiencing your initial,

pre-time-travel passage through a particular point in history,

your "time-traveling clone" is also already there,

doing everything you’ll eventually do when you time travel yourself.

For example, Harry and his friends are saved from dying

by their time-traveling selves, first time history gets to that point.

It just makes so much sense

- if you go back in time,

and of course you really and truly were at that location in time at that time

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.

I highly recommend you check out the first the first episode of Lifeline which you can do for free on the corridor digital channel

or by following the links on screen 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 happen to grow 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 lifeline!