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  • 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 fictionnot 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 Cardtime 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 ofdo-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 movieLooper" 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!

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

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