Temporal Mutability

Temporal Mutability — AKA "The Sliding Scale Of How Easy It Is For Time Travelers To Change The Past, And Why".

Time Travel is one of the richest concepts in Speculative Fiction; altering the past is easily one of the richest Time Travel plots.

Apparently, people (or at least SF writers) in general have a deep-seated dissatisfaction with the past—either their own recent past, or with the whole history of the world—because every time the subject of time travel comes up, characters inevitably start wondering whether they can use their Time Machine to change the past. Even if the characters have no intention of changing the past—even if the characters don't actually travel to the past at any point—some smartass will ask about the Grandfather Paradox, which will in turn lead to a discussion on the possibility (and morality) of altering the past:

Could you go back and save your brother from that fatal car crash? Could you punch your boss in the face, then go back and stop yourself? Could you prevent World War II by going back to 1930 and killing Adolf Hitler?

And if you could, should you?

Seeing as time travel is currently just a pipe dream, there's really no saying what would be possible when traveling to the past in Real Life. Writers are thus free to invent and follow whatever chronophysics they like, and as long as it's consistent the fans will usually accept it.

These settings tend to fall into one of the following categories (arranged here from least changeable to most changeable):


 * 1) You Already Changed the Past: AKA Block time or Eternalism. Past, present, and future are an immutable whole. Consequently all time travel to the past results in the creation of a Stable Time Loop, by virtue of the fact that the past--including the interference of all those time travelers—already happened. Changing the past is out of the question—but there is the possibility that the history books don't tell the whole story. Even so, your attempt to travel back to 1930 and assassinate Hitler is almost certainly doomed.
 * 2) Enforced Immutability: In theory, the past could be changed, but some force stymies anyone who tries. Maybe Time Police or Clock Roaches menace anyone who violates the Temporal Prime Directive, or maybe the past can only be visited via Intangible Time Travel.
 * 3) Rubber Band History: Time is mostly immutable, like a wide river following a well-worn path. Travelers can make changes to the past, but these changes inevitably get smoothed over by the passing years. For example, it would be possible to travel back to 1930 and assassinate Hitler, but World War Two (or some equally bad conflict) would still happen anyway. Setting Right What Once Went Wrong works, but only in the short term; Making A Better World, unfortunately, doesn't work. Unless you were to apply a sufficiently large change, one that would stretch the rubber band until it snaps, freeing history to run in a different direction.
 * 4) Temporal Balancing Act: There's no rubber band, so there's nothing to prevent you from making major, permanent changes to the past if you want to. But at the same time, it's possible for a conscientious time traveler like yourself to leave the past exactly as you found it. Or to change the past, then change your mind and go back again and un-change the past. Or to intentionally arrange a Stable Time Loop.
 * 5) Temporal Chaos Theory: The Butterfly Effect is in full force. Simply by being in the past in the first place, you alter the past, both overtly and in ways too subtle to notice. And these changes inevitably snowball, eventually rendering the Present or Future (almost) completely unrecognizable. And sometimes, the universe hates you, so every change to the past only makes the present worse.
 * It bears mentioning that over short enough time periods, settings that fall under Temporal Chaos Theory may not be distinct from those that fall under Temporal Balancing Act.

And in any setting where changing the past is possible, the alteration generally happens in one of two ways:


 * Overwriting the timeline: The old timeline ceases to exist, and is replaced by the new series of events resulting from your time travel (implying that Time itself exists in a sort of Meta-Time). The change to the timeline may be instantaneous, or it may cause a Delayed Ripple Effect, allowing you to race against San Dimas Time to Set Right What Once Went Wrong before you find yourself Ret-Gone. You may find yourself the only person who realizes the past has changed. Overwriting the timeline is also prone to causing Temporal Paradoxes. Progress can be tracked with a Ripple Effect Indicator if one is available.


 * Branching timelines: Your time-traveling causes a new timeline to split off the original, and both timelines exist (can be temporarily or permanently if it happens to be Another Dimension identical to your own but shifted in time) as Alternate Universes of each other. Depending on the setting, you may or may not be able to return to your native timeline after you've caused it to split. Thus, there's no danger of accidentally erasing yourself from existence—at worst, you'll prevent one alt-timeline's equivalent of you from existing. On the other hand you can't truly Set Right What Once Went Wrong, either: for every timeline that you fix, there's another that you don't.

And, fitting none of the above categories:


 * Timey-Wimey Ball: The series says outright that time travel follows no rhyme or reason. Or, it starts off following the rules of one of the above categories, only to later contradict these rules (sometimes Justified by stating that the original time travel expert was wrong, or that this new case is some kind of special exception to the general rules of time travel).

As an aside, it's interesting that no one ever seems to be nearly as concerned about time travelers altering the present or the future. No one says "But what if saving that guy somehow causes World War Three — because it didn't 'really' happen or it's not the 'correct' outcome, and humans are not supposed to change history?"

See also this page, for a more in-depth discussion.

Film

 * The first Terminator movie fell squarely into this category. Later movies retconned it into... something a lot less straightforward.

Literature

 * Animorphs is complicated, mostly due to including multiple methods of time travel. When using the "accidentally due to huge explosion" method, the result is a Stable Time Loop, but when the Sufficiently Advanced Aliens get involved, the timeline is mutable.
 * It's even more complicated than that—the first of the two times the former method (a Sario Rip) appears, . And apparently there was a risk of both versions of themselves being annihilated when they caught back up to the point when the explosion happened, even though after that, the first iteration of them would be gone, back into the past. As for the Time Matrix, it turned out to be even more powerful than it seemed.
 * In Dragonriders of Pern, turns out their crisis (Thread's back after 400 years, and they only have one Weyr, with nowhere near enough dragons to protect the whole continent) was caused by Lessa's trip back in time to bring the other five Weyrs forward to solve that crisis.
 * This is how Time Travel works in Harry Potter -- the trick is to make sure not to let the not-yet-time-traveled-you see the time-traveled-you so that you don't know that you were in the past until the moment you make the decision to time travel. It's said that terrible things happen to those who try to mess with time (changing the past), but this is never shown in the books.
 * Harry Harrison's The Technicolor Time Machine has that, although this is not shown until the very end of the book, when the protagonist (a film director who wanted to make a cheap movie about Vikings using a time machine) realizes that the only reason the Vikings settled Vinland (and were the first Europeans to reach the New World) is because of the movie. Their Viking friend Ottar reveals that he is, in fact, Thorfinn Karlsefni, whom history recorded as the leader of the expedition. Furthermore, the director's name is Barney Hendrickson, which is eerily similar to the Real Life historical figure Bjarni Herjólfsson, who was key in ensuring Karlsefni's success.

Live-Action TV

 * Andromeda features this twice, only to turn into a Timey-Wimey Ball whenever a tesseract is nearby.
 * The first time happens when the ship accidentally ends up in the past, about a year after Hunt got trapped in the black hole. Hunt even asks Rommie if she believes in fate. Rommie does, but Hunt doesn't, claiming "a man makes his own fate". They arrive just before the final battle of the Nietzschean Rebellion, where the Nietzschean fleet obliterates the remains of the High Guard (although that turns into a Pyrrhic Victory for them). Knowing one ship won't make a difference in the battle, Hunt prepares to travel back to the future. Then 3 times as many Nietzschean ships appear as recorded by history. Realizing the Nietzscheans still need to have a Pyrrhic Victory for the gallaxies to plunge into chaos, Hunt uses a device that wipes out the "extra" 2/3 of the fleet and leaves. Tyr later reveals that he knew about this all along from old stories but didn't feel like sharing.
 * Later on, Hunt's fiancée tries to pull the Andromeda Ascendant from the black hole in the past and, actually, looks like she has a chance of doing that. Then a Nietzschean ship shows up and forces her to abandon the efforts. Rommie later tells Hunt that she did manage to nudge the ship a little, which put it in a position to be able to be rescued 300 years later by the Eureka Maru.

Video Games

 * Ellone in Final Fantasy VIII claims that changing the past is impossible; this appears to be why, with a side of Stable Time Loop. The influences that Squall and Ultimecia have on their respective pasts are already in effect in their presents - in particular, Squall, and Ultimecia's efforts to change the past are at least heavily implied to directly bring about the events she is trying to prevent.

Western Animation

 * Beast Wars uses this form of time travel, with the eponymous conflict resulting in drastic changes to Planet Earth, allowing the events of the original Transformers to occur.
 * It starts out that way, since the "alien planet" turned into Earth All Along thanks to time travel. But then it turns into Type 3 when Megatron nearly kills Optimus Prime, with that action "breaking" the rubber band.
 * He also has Rampage blow up the top of a mountain and watches as it changes in a future recording.

Literature

 * In his preface for The Great Divorce, ~C. S. Lewis~ cited as an inspiration a short story (whose author he could no longer remember) from an American SF magazine about a man who traveled into the past "and there, very properly, found raindrops that would pierce him like bullets and sandwiches that no strength could bite--because, of course, nothing in the past can be altered." (I love this concept, but practically speaking it involves forgetting that air is a material. If you can't move the raindrops due to causality, then you can't walk through the air either.)
 * Time Scout presents Enforced Immutability that approaches Rubber Band. You can change anything so long as it doesn't matter in any way. If it does matter, you can't change it. Something will happen. Usually to you. Things that can be changed are enforced by uptime laws. And taxed accordingly.

Live-Action TV

 * Star Trek had a Temporal Prime Directive. However, this is poorly enforced; the rules of the universe amount to Type 4.
 * Lost seems to be following this rule, in fact continually referring to "the rules" and expounding that history cannot be changed. Desmond's stories drift slightly into Rubber Band History, in that he can prevent individual instances of Charlie's death, but it will still happen as soon as he fails to intervene.
 * Doctor Who has aspects of almost all of them, ocassionally it depending on who's at the helm. Previously Enforced Immutability was implied via the Timelords who're all dead now. These days the term Fixed Point is used to describe events which are basically historical lynch pins: you can technically change them, but you shouldn't seeing as the universe will fall apart. They also had the Reapers (beyond creepy monsters from outside time who came to "sterilise the wounds" if things got too messed up, but you had to really screw history up to get them. Mostly it's Rubber Band History in effect, with the occasional burst of You Already Changed the Past for variety's sakes (It's even possible to avoid Paradoxes if you're clever enough to create a Paradox Machine which holds the current time in place regardless of what you do to the past (hense why the third season finale had a bunch of transformed humans killing off their ancestors and suffering no ill effects despite essentially erasing their history.) It helps that the main character can see all of time and space in his head at his whim and pick and choose whether he wants to/can afford to be a conscientious Temporal Balancing Actor or just screw with everything.

Tabletop Games

 * Continuum is like this. Theoretically, you can change the past, but when the lives of the countless sentients of the post-Aquarian future depend on the past not being changed, it's going to stay unchanged.

Web Comics

 * In Homestuck, any deviation from the original timeline creates a Doomed Timeline, which is as bad as it sounds. On the other hand, timelines have a limited ability to interact with one another, and the creation of a Doomed Timeline may have consequences reflected in the Alpha Timeline (Davesprite being the most visible example of this phenomenon). While most of these cross-timeline shenanigans have been necessary to the proper continuation of the Alpha Timeline, it's been hinted that the right combination of Doomed Timelines could throw the Alpha Timeline permanently Off the Rails, or at least off of Lord English's rails.
 * That being said, Stable Time Loops happen all the time. They can even extend to Alternate Universes.

Tabletop Games

 * In Feng Shui, trying to change history without capturing any Feng Shui sites will inevitably result in this. Big world events happen with the perpetrators having different names and everything eventually comes around to something resembling the present day. But once you start capturing Feng Shui sites, you can start making changes stick, and can even bring about a Critical Shift if enough sites are in your power.

Film

 * Back to The Future used the Overwriting the Timeline version of this, with Delayed Ripple Effects.
 * In Primer, Abe and Aaron are able to "preserve causality" by insulating themselves from the outside world prior to their trip back in time, but when they want to, they're able to manipulate past events to their own advantage. It's never specified whether they're Rewriting the Past or causing Branching Timelines.
 * Presumably, they're rewriting the past. Remember the watch experiment? They were able to tell time was looping inside the box because when the box was started, and then later a watch was placed inside and the box closed and reopened immediately, the time elapsed on the watch was an even multiple of the external time past. The watch was circulating through a time loop a huge number of times, each internal loop presenting a finite probability that it would be the one that was reopened, and by building a bigger box and putting people in it (who can decide to exit at the external beginning of the loop), time travel happens.
 * Primer may actually fall under Temporal Chaos Theory, as the characters never travel back further than a week—not enough time for wild divergences to manifest.

Literature

 * The Clown Service by Guy Adams uses a version of this. Changing the past is possible but highly perilous. The characters only dare to do it to

Live-Action TV

 * Star Trek: The Original Series: Time works this way when the Enterprise encounters the Guardian Of Forever.
 * The End Of Eternity:

Tabletop Games

 * Feng Shui revolves around the battle for Places of Power that generate powerful chi. Whoever controls enough Feng Shui sites can change the course of history. The only major catch is that there are only four major "junctures" of time available in the Netherworld, and whatever happens in one juncture, you cannot go back and try to stop what already happened from occurring because time flows normally in each of the junctures.

Video Games

 * Chrono Trigger allows major and deliberate changes to the timeline, and one rather spectacular instance of Tricked-Out Time. Sometimes, your changes will come back to bite you in the ass, but for the most part that's fixable too—however, the sequel Chrono Cross gives this a Cerebus Retcon and takes it to level five.
 * Achron is a fantastic multiplayer example of the 'Overwriting the timeline' alteration style. Which is to say... the old timeline ceases to exist, and is replaced by the new series of events resulting from the time travel (time itself existing in a sort of Meta-Time). The change to the timeline causes a Delayed Ripple Effect, allowing you to race against San Dimas Time to Set Right What Once Went Wrong (or Make Wrong What Once Went Right) before you find your forces Ret-Gone. You are achronal. Overwriting the timeline can cause Temporal Paradoxes, but they don't tend to happen accidentally.
 * In the Sam & Max episode, "Chariots of the Dogs", Bosco accidentally goes back in time and changes his history. When Sam and Max go back in time to stabilize the past they in addition cause Bosco to never been born. They fix it all by the end.

Comic Books

 * The original Marvel UK comics run of Transformers Generation 1 had this approach, with time-hopping characters from the future causing (both directly and indirectly) horrific effects to the timestream that result in the catastrophic "Time Wars" and numerous other paradoxes. The situation is eventually contained - barely - but the 'future' is no longer set, and indeed takes a wildly different path.

Literature

 * Orson Scott Card's Pastwatch: The Redemption of Christopher Columbus uses the Overwriting the Timeline version of this in order to sabotage Columbus's discovery of the New World and prevent the massacre of Native Americans and the Atlantic slave trade. Temporal paradoxes don't result, because causality and chronology are regarded as completely separate things.
 * The author includes an explanation of the mathematicians and physicists to the historians about why it's possible for a time traveler to exist when his own timeline has been erased. Essentially, causality is claimed not to be real. Changes in the past cause the points in time from that point on to be overwritten in the same way as a VHS tape is overwritten by a new recording. There can be no alternate timeline because there is no "space" for it to be written. The time traveler's memories are not altered, as the physical state of the brain remains unaffected by the overwriting.

Anime and Manga

 * This is how time travel works in the Dragon Ball Z universe, and the reason killing larval Cell in the main timeline didn't do anything to stop the fully-grown one from another timeline. Trunks isn't able to prevent his Bad Future from happening, but he can use the power he gained in the past to stop it from getting any worse.

Comic Books

 * This is how it used to work in Marvel Comics, particularly around the Stan Lee editorial era, flowing into the Wolfman era. It kind of stopped somewhere in the 90's.

Film

 * The Abramsverse and the Mirrorverse in Star Trek explicitly do this, presumably to preserve the main continuity.
 * The non-canonic Countdown comic shows events still occuring after Spock and Nero travel to the past, meaning the old timeline still exists.

Video Games

 * The Legend of Zelda falls under this category. In The Legend of Zelda: Ocarina of Time, time-travel shennanigans (or, rather, mental time travel shennanigans) cause the timeline to split into three branches, with one point of divergence known, and the other only theorized.

Web Comics

 * The time travel of Narbonic was revealed to be this variant in the "Director's Cut" version currently running. Dave Davenport becomes unstuck in time, changes his own past, and as a result... becomes someone who never smoked and never had a nicotine addiction.

Comic Books

 * Rip Hunter in Booster Gold states that the future is in flux but major events in the past (called "solidified time") are immutable. This strange time scheme hinging on the present is particularly odd when you consider Booster is from the future himself making the present his past, and Rip spent most of his time in the future.
 * Since the comic takes place in a Shared Universe, the rule is, in practice, "Booster can only change stuff that wouldn't screw up the storylines of other DC Comics." So, he can go back in time and save some random little girl's puppy from being killed, since the girl and the dog are just background characters, but he can't save Bruce Wayne's parents, since that would kind of negate the entire Batman series.
 * This is driven home via a recurring subplot where Booster keeps trying to go back and save Barbara Gordon from being crippled. Every time he tries, no matter how Crazy Prepared he is, he fails, because the event is solidified time.

Literature

 * Perry Rhodan falls into this category mostly due to being a Long Runner with changing authors. It's generally a mix of #1 and #3, but the past has on occasion been changed (including one old issue featuring a trip into the past that ended up affecting the outcome of a battle in the present, including dead soldiers spooking their superiors by suddenly being alive again). The series also once featured a "time police" for one arc, though its role was not so much to actually 'police' time as to simply mercilessly attack any civilizations discovered to be experimenting with time travel in the here-and-now.

Live-Action TV

 * Doctor Who, being the Trope Namer, flirts with both this and Enforced Immutability (see above).
 * Andromeda turns into this from You Have Already Changed The Past after the introduction of tesseracts. Trance somehow manages to swap places with her future self. Then they add the Route of Ages and multiple realities.

Take only Ripple Effect Proof Memories, leave only Gray's Sports Almanac.