Temporal Mutability

Temporal Mutability — AKA The Sliding Scale of How Easy It Is for Time Travellers 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? Nope.

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. You Can't Fight Fate. You can't change history. Not one line. 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. Any attempt to change the past is not merely futile but vulnerable to a variation of the Self-Fulfilling Prophecy: an effort to Set Right What Once Went Wrong may be tragically revealed as the reason it went wrong in the first place. Alternately, an accidental involvement in something that Once Went Right might be the reason it went right (that was probably left out of the history books).

2. Enforced Immutability: In theory, the past could be changed, but some force stymies anyone who tries. Maybe the 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 II (or some equally bad conflict) would happen anyway because the social factors propelling him would still be in play. Setting Right What Once Went Wrong works, but only in the short term. You could prevent your brother from taking that fatal journey, but due to his poor driving skills, you merely delayed his death by a few months. Either that, or he gets killed by some other accident. 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, no time police, 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: 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. The alteration could occur from either the Point of Divergence onward or it may even ripple into the past creating an entirely new chain of events from start to finish. Overwriting the timeline is also prone to causing Temporal Paradoxes. Progress can be tracked with a Ripple Effect Indicator if one is available. Not to mention the realization that since you have erased a whole timeline, anywhere from a few dozen to a few billion people will never have existed in the first place, so you effectively killed them all and erased their existence. Nice Job Breaking It, Hero.

• 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. With this new, diverging timeline being entirely separate to the original, any threat of a temporal paradox is completely eliminated. 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 III — 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.

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Examples:

Type 1: You Already Changed the Past

Fan Works

• Split Second (My Little Pony) has this, although the gods are the exception. For them, any changes overwrite their own pasts.
  • In An Eternity Divided, the sequel, Death uses his time magic to overwrite the past. Death becomes a mare with no memory of ever having been a stallion. And, to top it off, the author went back and edited every chapter that even a reference to Death appeared in to switch Death's gender.

Film — Live-Action

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

Literature

• Animorphs:
  • 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, it's short-circuited when Jake dies, his consciousness merges with the other version of himself, and he proceeds to prevent the whole thing from happening — also, he was apparently the only one really there because of that. 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.
• Artemis Fowl: In "The Time Paradox", Artemis himself actually spells this trope out right at the start:
  Artemis: Ah yes, the trusty time paradox. If I go back in time and kill my grandfather, will I cease to exist? I believe that any repercussions [of our impending trip] are already being felt. If I have been to the past, then I have already been back.
• 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 galaxies 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 gives Edea the idea for the founding of SeeD thirteen years in his own past, and Ultimecia's efforts to change the past are at least heavily implied to directly bring about the events she is trying to prevent.
• The method of heart time travel introduced in Kingdom Hearts 3D [Dream Drop Distance] is (confusingly) explained to follow specific unique rules, one of which is this. The past cannot be changed, anything you do while time traveling is the cause of an event as it originally happened, and returning to your proper time period will “etch” the memories of your time travel adventures onto your heart to subconsciously guide your actions into a Stable Time Loop but you otherwise cannot consciously remember what you did while time traveling. It has ramifications for later games.
  • In Kingdom Hearts χ, it’s revealed that Maleficent learned about heart time travel and attempted it, but the Master of Masters arranged a Gambit Pileup to minimize the impact of her actions in the past because he knew she wasn’t aware of the fact that You Can't Fight Fate and would do something reckless in the past when trying to. As such, she ends up in a simulation of world whose past she wants to change, and is only informed after arriving.
• In Pokémon this is generally how Time Travel works as seen in Pokémon Legends: Arceus, with how Arceus sending the Protagonist back in time means they're indirectly responsible for Hisui being named Sinnoh, the destruction of The Temple of Arceus (and the creation of the Spear Pillar), and Giratina’s Heel–Face Turn

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.
  • And then back to the Stable Time Loop in the finale, when an Autobot shuttle wasn't on the records of the Ark because the protagonists used it to save the day and get home.
• In Gargoyles, the Phoenix Gate allows the user to travel through time, but every attempt to change things runs into You Already Changed the Past, resulting in the world as we always knew it and letting us know that this is why it was always that way. There at one point was a degree of "Enforced Immutability:" When Goliath saves a character who vanished and was thought dead and tries to allow him to return to his old life, increasingly improbable Diabolus ex Machina events happen to said character to make his death a certainty. Eventually, Goliath realizes that the universe just won't let him send that character home, and brings that character to the present. And now we know why he disappeared in the first place: he was brought to the present by Goliath!
  • Xanatos learns how to properly take advantage of this by using a coin meant to be sent to his past self a thousand years later. The coin wasn't worth much for it's time, but would now go for the 20 grand he would ultimately use to kickstart his fortune. He also made sure to write a letter that would be sent to himself a week before these events occurred, so he would know the instructions for how to pull it off.
• In spite of Rick's hatred of Time Travel, the Rick and Morty episode "Rattlestar Ricklactica" features a Stable Time Loop in which their interference in the past is part of the original timeline.

Type 2: Enforced Immutability

Fan Works

• In The Parselmouth of Gryffindor, while theoretically time-travel ought to allow you to change the past, in practice, magic does its best to manipulate probablity so that the result is a Stable Time Loop. The farther back in time you go and the more efforts you make to change the past, the more unstable space-time becomes until you're either obliterated or kicked back to your home time period. Try too hard and you might cause a Time Crash, and you can bet you won't be there anymore when the universe reboots.

Film — Animation

• Spider-Man: Across the Spider-Verse introduces the concept of "canon events": events in every corner of the multiverse that define the Spider-Man of that dimension. In every case shown to Miles, this event is a personal tragedy for that Spider-Man, including (but not limited to) Uncle Ben's death. Aside from preventing denizens of the multiverse from ending up in the wrong dimension, the duty of Miguel O'Hara's Spider-Society is to ensure these canon events are not altered, lest such an alteration threatens to completely destroy that dimension. Miguel is adamant that You Cannot Fight Fate Because Destiny Says So, but when confronted with the knowledge that a canon event may claim his father's life, Miles decides to Screw Destiny.

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."
• 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. Reality seems to agree with humans on "what matters" to a degree that is almost comedic: if you tried to kill someone important to history, your gun would jam. If it's someone who never made the history books, go crazy! You would think that to the universe, to physics, it's all just atoms, but it seems reality cares a great deal about the "important" people and events in history enough to step in.
• In Stephen King's 11/22/63, the larger the change to the past, the more the past resists changing in the form of Contrived Coincidence which will stop or kill the agent of change. Large changes can, with great difficulty, be effected, but inevitably end up being for the worse.
• The Golden Hamster Saga: In Freddy's Final Quest, Signor Goldoni invents a time machine, but determines that because going back and altering history would cause a paradox, and because a human would hardly be able to do anything without altering history, it must be impossible for humans to time travel. He calls it the Goldonian Law of Time Travel. Instead, he sends the hamster Freddy, the cat Sir William, the guinea pigs Enrico and Caruso, and the robotic hamster Tjark back in time because animals have no history or cultural influence.

Live-Action TV

• Doctor Who:
  • The franchise has aspects of almost all of them, depending largely on Rule of Drama and who's at the helm.
  • Before the revival and the Time War, Enforced Immutability was implied via the Time Lords. 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 (hence 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 time and space in their head at whim and pick and choose whether they want to/can afford to be a conscientious Temporal Balancing Actor or just screw with everything.
• 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.
• Star Trek had a Temporal Prime Directive. However, this is poorly enforced; the rules of the universe amount to Type 4.
• Kamen Rider Gaim: Late in the series when Mai becomes the Woman of the Beginning, she travels back in time to try and prevent the disasters that have happened over the course of the story. However, when she tries to warn her friends, her very explicit statements like "Do not put on that belt!" somehow come out of her mouth as the same vague warnings she gave the first time around. DJ Sagara explains that time is completely immutable even to godlike beings like themselves; once something happens, it's set in stone and there's nothing anyone can do about it.

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.

Video Games

• Kingdom Hearts III applies Wrong Context Magic to Magic A Is Magic A in regards to the time travel rules from Kingdom Hearts 3D [Dream Drop Distance] mentioned above under Type 1, first as a Hand Wave to some Early-Installment Weirdness and next as a plot point, which becomes especially relevant in the Re:Mind DLC. First we have Dark Riku in San Fransokyo offhandedly mentioning that he had to play by the rules to time travel, unlike “a certain wizard” the heroes know (presumably in reference to the Timeless River world from Kingdom Hearts II). The more plot-relevant example comes in the final battle when the heroes die and Sora has to journey through the Final World to retrieve their hearts from the Lich. After breaking one set of universal rules (regarding the power of waking), Sora then proceeds to break the rules of heart time travel by sending his friends’ hearts to the past to a point before they all died and replacing the original history with a new one where they survive. The Re:Mind scenario has Chirithy explain to Sora that doing this caused a Time Crash, and that since he shouldn’t try to change the past anymore than he already has, he’ll have to use that space-time rift he created to visit both of the contradictory events now occupying the same timeline for this adventure. Chirithy’s warning edges closer to “Enforced Immutability” territory than before, as All There in the Manual indicates that Sora breaking the rules of time travel to accomplish all this resulted in him vanishing from existence during the ending.

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.
    • In fact, Stable Time Loops are Dave's preferred method of time travel, because the Alpha Timeline's method of enforcing immutability is to kill off any time travelers who aren't supposed to be there (or to put it another way, doomed timeline = doomed time traveler).

Western Animation

• In the What If...? episode "What If… Doctor Strange Lost His Heart Instead of His Hands?", an alternate Strange uses the Time Stone over and over to try undoing his beloved Christine's death, only to find out she will always die no matter what. It's particularly forceful in that, while usually she dies from a car crash that Strange can't seem to avoid no matter what, she also dies from such things as a heart attack, an armed robber, and the building she's in catching fire.

Type 3: Rubber-Band History

Film — Live-Action

• Looper seems to adhere to this, mostly, with some bits from type 4 put in.

Literature

• Time travel in Cirque du Freak seems to work this way. Anything that happens will still happen, even if the "actors" for any given role are different people. For example, the books explain that if someone went back in time to kill Adolf Hitler, you could hypothetically succeed. However, someone else would rise to lead Nazi Germany, and World War II would still happen. It's just that the man known as Furher would not be Adolf Hitler. The basic principle for this is the key to Darren's salvation.
• Changing the past is explained to work this way in Asimov's The End of Eternity, down to the elastic band metaphor. Permanent changes can be made, but it's very difficult.

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.

Visual Novels

• In Steins;Gate, altering the timeline is quite easy once you get around to inventing time travel, to the point that you can create a divergent world line with a single 32-kanji text message. However, all world lines within a 1% divergence of the original still fall within its Attractor Field and always lead to the same end result over time - a phenomenon facilitated by the fact that people will always and only die at the time they were predicted to, with the exception of certain individuals involved in critical (and permanent) divergence points. Mayrui is unfortunately not such a person. Kurisu, on the other hand, is.

Type 4: Temporal Balancing Act

Fan Works

• Ask The New Hope's Peak seems to be mostly this and involves branching paths, but with some elements of Type 5 as well. When the future children of some of the characters from 2034 accidentally end up in the present, they themselves try not to actively cause any serious changes at first. However, the quantum systems that they brought back with them by their act of time travel end up setting things off in different directions. This leads to, among many other things, the deaths of some people who they knew to be alive in their version of the future. They still remember what the future was like and decide to try and use that to help push things in the right direction or stop some events from playing out, but things also seem to be moving of their own accord as well.
• Child of the Storm mostly has this kind of time travel. You don't necessarily have to change things, but the Butterfly Effect can still ensue. Moreover, the series' primary Magnificent Bastard, Doctor Strange a.k.a. Taliesin, student of Merlin and Court Physician/Bard of Camelot uses his powerful abilities as a Seer and skills at Time Magic to outright exploit the Butterfly Effect with (almost) perfect precision, and steadily manipulate the timeline to where he wants it to be. Why? He's preparing the Earth to face off against Thanos.
• A Student Out of Time is mainly this, but with elements of both 2 and 3 as well. It's possible for history to be changed, but a series of inconvenient coincidental phenomenon- dubbed the "ankle effect" after its first appearance lead to Hajime tripping and twisting his ankle- will happen and try to keep history on its original path, with its severity seemingly proportional to the importance of the event. However, if the event is changed, then history will carry on in a new direction. These new changes and their butterflies also don't seem to produce ankle effects of their own.

Film — Live-Action

• 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.

Literature

• As detailed in Night Watch, this seems to be the case on Discworld. When Sam Vimes and a convict he's chasing both get sent back in time, the convict kills Vimes' mentor, and Vimes is thereafter given the chance to fulfill that mentor's role in history (although he does need to have this all explained to him by the History Monks). In the end, both versions of history converge on the same present day.
• The Star Trek: Department of Temporal Investigations series spends a good amount of time explaining the temporal science behind Star Trek since it is so central to its theme; not an easy task with how fast and loose other works play with the rules. The physics of time travel fall closest to this category, with a lean towards Type 3, since while changes to the timeline will not snowball indefinitely into the future, it can take a very long time for the rubber band to correct large changes — potentially centuries or millennia, which might as well be forever on a human timescale. Smaller changes can still be dangerous, but unlike Type 5, time travelers aren't likely to destroy an entire civilization just by sneezing.
• Time Salvager has elements of Type 3 in that the timeline can self-correct small disturbances, but large ones (like openly using future technology or saving the life of someone who should have died) will permanently alter it. In addition, the rubber band is weaker in some places than in others; World War II, for example, is off-limits to all but a select few because of how easy it would be to make irrevocable changes there. To ensure the timeline is preserved, all time travelers are highly trained and adhere to a strict set of rules to avoid changing history. Break those rules, and an auditor will hunt you down.

Live-Action TV

• Star Trek: The Original Series: Time works this way when the Enterprise encounters the Guardian Of Forever.
  • It tends to work this way generally. Technology used by those who frequently use time travel can insulate you (or you can get lucky and be outside the normal reality when the thing that made the change happens) and that leaves you with Ripple-Effect-Proof Memory and "not disappearing when something screws with your past" ability, the better to Set Right What Once Went Wrong. In the end, this culminated in the reboot movies: such a massive change to the history of Star Trek: The Original Series happened and stuck that everything that happens after is up for grabs. The Leonard Nimoy Spock, who made the trip with the villain who caused the change, is the only one who remembers the old reality. We are at the highest level of easy "go back, do something to change history, get back and have a new timeline but you're still you in every way" ability if you have the means to make the trip. What keeps us out of type five territory is that the changes won't necessarily be large and sweeping - it's just fine to leave the Nexus a little earlier than you entered and save the crew or snag two whales from the past to talk to that alien probe thingy.
• The End Of Eternity: Eternity is initially set up as Rubber-Band History, but is revealed to be a Temporal Balancing Act at the end.
• The final season of Agents of S.H.I.E.L.D. has the team travel back in time in a chase after the Chronicoms, who are trying to dismantle S.H.I.E.L.D. before it has a chance to become strong enough to stop them. Theoretically they could have succeeded in stopping the Chronicoms and preserving their original timeline, however the team's butting heads over whether they should try to improve their own futures as well causes enough ripples to cause the Malick family to all survive into the 70s and Nathaniel to copy Daisy's powers, get Mack and his brother orphaned as children, get Daisy's mom killed but sparing her sister, and destroy a majority of S.H.I.E.L.D. anyway.

Music

• According to Insane Clown Posse's "Dark Carnival" mythos, there is a place where one can play with a jack-in-the-box and see what the afterlife holds for them. However, this is not set in stone; the fate revealed by the Riddle Box can be changed via doing enough good deeds. Other aspects of the Dark Carnival would probably fit into other categories on this page, though, which is understandable as it is a Metaphysical Place to begin with.

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.

Western Animation

• Milo Murphy's Law has this. There's a time travel organization that sends its agents into the past to deliberately alter certain events, with the two agents seen most often (Cavendish and Dakota) tasked with making sure the pistachio nut doesn't go extinct. The episode "Missing Milo" also has the duo unintentionally set up a Stable Time Loop involving a peach, while the main character plans to do the same when his friends discover a note from the 1960s that is addressed by him but that he never wrote.

Type 5: Temporal Chaos Theory

Comic Books

• The Transformers (Marvel): The UK series 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.

Fan Works

• MS Paint Masterpieces: Time travel itself is an act which changes the future. If you go forwards in time, you vanish from the present while travelling and so arrive in a future shaped by your absence. If you go backwards, your appearance in the past changes causality and erases the future you returned from. Two possible futures are shown in the comic: the first is one in which Dr Wily attacks time and time again until his execution; this timeline is erased when he travels forward in time, arriving in a new timeline in which he vanished for 37 years before suddenly reappearing. Wily then returns to the present, erasing this second possible future which relied upon his absence, and the events of the comic continue down a new third timeline.

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.
  • The author also invokes The Time Traveller's Dilemma, given this version of the trope, as altering the timeline means, effectively, killing billions of humans, reasonably pointing out that the only way this can be acceptable is if the Godzilla Threshold has been reached.
• Just about all of these are discussed in Ray Bradbury's short story "A Sound of Thunder," involving time travel used to let rich folks hunt dinosaurs for amusement. They aren't sure exactly what rules time travel operates under, but discuss the prevailing theories: you can't change the past because you've already changed it, you can only change the past in minor ways, you can only change the past in direct and predictable ways, or the simple act of stepping on a butterfly in the past leads to chain reaction of events that completely alters the present in ways no one can foresee. Turns out, in the end, Temporal Chaos Theory is correct.
• In Alfred Bester's "The Probable Man" This prevents the protagonist from returning to his own time, he can go back to the same point in the past repeatedly but will always end up in a new timeline when he travels back to his own time and quickly realizes that this will happen no matter how many times that he tries. In the end he decides to return to the past just before he had left to return to the future (foreshadowed at the start of the story when an unseen shooter in the distance had distracted his pursuers) and live out the rest of his life there, implying an end to the cycle.

Live-Action TV

• This appears to be the case in The Flash (2014). It's eventually revealed that the show's timeline is not the original one, as Eobard Thawne (AKA the Reverse-Flash) has traveled back in time to try to kill Barry Allen (AKA the Flash) when Barry was little. He fails, as young Barry is saved by his future self from the same timeline as the Reverse-Flash. Barry's mother dies instead, his father goes to prison, and Barry is raised by Detective Joe West instead of the Allens. Despite this, he still becomes a forensic scientist but with an interest in anything "weird", given his experiences on the night of his mother's death. In the original timeline, Barry's transformation into the Flash may have indeed been a random accident. In the new version of events, Dr. Harrison Wells, AKA Eobard Thawne deliberately engineers Barry's transformation several years ahead of "schedule", which also results in dozens of other "meta-humans" now roaming the city. When Barry finds out that there is evidence of his future self traveling back in time to that night, he is both excited and dejected, as the fact appears to indicate that he's destined to fail. However, other episodes appear to indicate that time is indeed infinitely mutable, especially when Wells warns Barry to let the events play out exactly as they had after Barry accidentally travels back in time by one day, claiming that averting one disaster will likely cause an even greater one to strike.
  • A later episode reveals the existence of Time Wraiths, who hunt down and suck out the life force of speedsters who meddle with time. At the end of season 2, Barry goes back to that night and saves his mother, watching the Barry from the end of season 1 vanish. Season 3 is focused on the new reality he has created, which Thawne dubs Flashpoint. Barry eventually cancels Flashpoint by allowing Eobard to kill his mother, but this creates another timeline, similar to the old one but different in a few respects. One of them is the presence of Savitar, who is eventually revealed to be a time remnant of Barry, who was created in order to stop Savitar himself from murdering Iris, which Barry only saw because he accidentally time-traveled to the future while trying to stop Savitar, who murders Iris West, which Barry sees when he accidentally visits the future. Barry and his team then work on changing small details in the hope that it will change the future, but the show seems to go into You Can't Fight Fate. Eventually, HR uses an illusion to pretend to be Iris, keeping the Stable Time Loop in place.
• The Legends of Tomorrow spin-off appears to go back to the Rubber-Band History option with the oft-repeated phrase "time wants to happen". Apparently, one has to try really hard to make a major change to the timeline, since it will always try to "snap back" to the proper course. It also doesn't help that the Time Masters are there to ensure no undue changes to the timeline (Fridge Logic ensues when you start wondering why they allow speedsters to do it at will). Near the end of season 1, the device that allows the Time Masters to monitor the timeline is destroyed, rendering them impotent. Savage uses the opportunity to try to destroy the timeline with three Doomsday Devices at different points in time, which, as he claims, will roll the timeline back to Ancient Egypt, allowing him to start over. Also, Rip claims he tried to save his family multiple times, only for something to always prevent him from doing that. He later finds out it was the Time Masters' doing.

Web Original

• SCP-2003: a time machine created by the Foundation in order to study the future and promote the development of a positive future for humanity. Unfortunately, one thing they've found is there are individuals whose presence and actions affect causality disproportionately, even to completely unconnected events happening thousands of kilometers away. For example, Scenario XE has the birth of a boy in Turkmenistan and the election of the Prime Minister of New Zealand on the same day in 2049 cause a chain of events leading to a society-destroying nuclear war between Israel and Greater Indonesia in 2058. Other futures have included things like the sun spontaneously becoming a black hole or a gamma ray burst destroying all life. These futures are unstable and can be changed with the most mundane of actions, and there's no telling what the results may be.

Branching Timelines

Anime and Manga

• Dragon Ball
  • This is how it works in the Dragon Ball 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.
  • Becomes a plot point in Dragon Ball Xenoverse, which kicks off with major events in the main Dragon Ball timeline suddenly changing for the worse. The Time Patrol realizes that someone with incredible time-manipulating powers must be behind it, because they know that's not how time travel works in their universe.
  • In Dragon Ball Super, Beerus claims that when a god alters the timeline, no split occurs and the future is actively rewritten. Whether this is actually true or not is dubious, as his destruction of Zamasu did not affect Future Trunks's timeline at all. Nor did it affect Goku Black, who has been implied to be the same Zamasu Beerus just destroyed, who hopped between timelines. Black himself claims his Time Ring removes him from causality.
  • "Here's the thing: multiverse theory's a bitch."

Comic Books

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

Film — Live-Action

• Star Trek (2009):
  • This is explicitly the case for the Kelvin Timeline (or "Abramsverse"), presumably to preserve the main continuity. An accident sends both Nero and the original Spock over a hundred years into the past, radically altering certain events. While Spock Prime doesn't have to worry about paradoxes, he's also unable to return home. Star Trek: Picard shows the original timeline is still intact despite Spock's and Nero's disappearances.
• In Avengers: Endgame, Dr. Banner explains that you can't change the past, and that going back in time to do so would only result in the creation of a new timeline. For that reason, going back in time to stop Thanos wouldn't change anything in their own history but they can take the Infinity Stones from before they were destroyed and use them to undo his acts in the present.

Literature

• Star Trek: Department of Temporal Investigations: Since the series wants to create a semi-consistent set of rules for a franchise that has spent decades holding the Timey-Wimey Ball, it's a necessity to have both branching and overwriting timelines. The physics of time travel are left deliberately vague (as they are difficult to understand even within the setting), but in general, traveling back in time to making changes will spawn an alternate timeline in superposition with the original. Depending on how things shake out, one may collapse and be overwritten by the other, the practical consequence of which is the inability to Set Right What Once Went Wrong for any Fish out of Temporal Water from that timeline.

Live-Action TV

• By the end of the final season of Agents of S.H.I.E.L.D., the team's attempts to stop the Chronicoms from meddling too much in the past still create a timeline firmly divergent from their own as early as the 80s. Because the form of time travel they've been using only lets them leap forward in predetermined jumps, they're resigned to staying in the new timeline...until Jemma is able to rebuild a device, the parts of which had been planted with various S.H.I.E.L.D. agents, that opens a Quantum Tunnel to the original timeline and brings Fitz with it. With the link established but temporary, they create a much larger one that sends both the team and the Chronicoms back to the original timeline moments after they left, while Deke stays in the new timeline to give a nearly-destroyed S.H.I.E.L.D. a chance at survival.

Tabletop Games

• Timelines are continuously splintering off from one another in Sentinels of the Multiverse, with occasional crossover events (for example, the villain Iron Legacy is from a timeline where his daughter was murdered by Baron Blade and he decided the only way to ensure a peaceful world was by force). There are a few moments that are fixed points - for example, Vanessa Long (who in one timeline grew up to be Visionary) will always develop superpowers, but how she uses those superpowers changes depending on which timeline. The two most important timelines, storywise, are the "RPG timeline" (the timeline that leads into the Sentinel Comics RPG) and the "Tactics timeline" (the Darker and Edgier timeline that leads into the Sentinel Tactics game), which branch depending on what happens during the fight with OblivAeon.

Video Games

• In The Legend of Zelda: Ocarina of Time, time-travel shenanigans (or, rather, mental time travel shenanigans) cause the timeline to split into three branches, with one point of divergence known, ^note The ending, where Zelda sends Link's mind back in time to prevent his removing the Master Sword from its pedestal and the other previously only theorized.^note The final boss fight, depending on if Link wins or loses
• According to its creators, this is what Einstein's use of the Hitler's Time Travel Exemption Act in the Command & Conquer: Red Alert Series boiled down to. When he Ret Goned Hitler from history to prevent the original World War 2, all he did was create an alternate timeline in which several Weird Historical Wars replaced it. The original Einstein from "our" history (kinda) returned to his laboratory in Trinity. Which makes it even more sad in a way. He didn't really undo the genocide of the Jews, and he unknowingly condemned an alternate universe to even more mayhem, in which his Alternate Self ends up getting Ret Goned too.
  Einstein: Time will tell. Sooner or later, time will tell.

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.
• Like Dragon Ball above, time travel in The Adventures of Dr. McNinja works like this with the point of divergence being when the time traveler arrives to prevent whatever problem they're trying to solve. A recurring time traveler has had to help his future selves save the world several times over, as he keeps going back to prevent a new catastrophe.

Western Animation

• In the Rick and Morty episode "The Vat of Acid Episode", Morty asks Rick to make him a Save Scumming remote, and Rick who despises Time Travel (and wanted to screw with Morty at the time) obliges. The remote worked by transporting Morty to a parallel universe at the exact point in time he pushed the button or died, leaving a melted or mangled corpse in his place. Morty then begs Rick to undo it, so he merges the branched timelines so that all the dead Mortys never existed in the first place and every action he reset actually happened.

Timey-Wimey Ball

Comic Books

• Booster Gold:
  • Rip Hunter 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 and is also Booster's future son.
  • 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 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.
• In Disney Ducks Comic Universe, the fact that there are various authors with different opinions and nobody to control, if you search long enough, you'll most certainly find examples of all the sub-tropes, and a few original ideas.

Live-Action TV

• Andromeda turns into this from You 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.
• Doctor Who, being the Trope Namer, flirts with both this and Enforced Immutability (see above). Basically, the rules of time travel are fixed and immutable... within a single story. The writers don't even pretend it's consistent beyond that. This gets a lampshade in "The Day of the Doctor" when the Tenth Doctor is terrified at the idea of changing history, only for the Eleventh Doctor to blow him off since they do it all the time.

Magazines

• 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.

Video Games

• Ikemen Sengoku usually subscribes to the Temporal Chaos Theory with the main character unknowingly preventing Nobunaga Oda's historical death on her very first day in the Sengoku period, but each route differs on how exactly the temporal stream responds to her and Sasuke making major changes to the past — in some routes, no negative repercussions result from their actions, but in others the wormhole that sent them to the past actively tries to send them back to their original time with Sasuke theorizing in Nobunaga's Dramatic route that a temporal force is trying to undo the changes they made to the timeline by sending them back. Ultimately, given that Ikemen Sengoku is much more focused on the "romance" part of its Time-Travel Romance premise, its inconsistent time travel rules can be chalked up to its creators not wanting every route to feel the same.

Web Comics

• Times Like This: A comic with time travel as its focus, but the environment is mostly satirical humor with a little drama thrown in. Changing the past can have reverberations on the present, but there's no paradoxes that come with, say, giving yourself a massage.

Other

Anime and Manga

• One Piece avoids all of this by having its time travel only go forward, not backward. In fact, the phrase "mankind cannot return to the past" shows up several times, and characters have to come to terms with not being able to change the past despite time travel existing.

Fan Works

• With This Ring incorporates several different temporal mechanics on different occasions.
  • Rubber Band History is apparently the general rule, because maintaining larger changes requires more energy input, and maintaining a paradox would require an impractically large amount. Ex-Time-Trapper Paul speculates that it would be possible to work around it by the affected parties leaving one universe and travelling to another, but the universe left behind would implode.
  • The rubber band can be worked around by using Temporal Balancing Act rules; if you manage to create a Stable Time Loop that supports your changes, then the universe won't need to self-correct. This is the solution offered to Doctor Strange, when trying to go back in time and save a woman from a car crash; he needs to persuade his younger self that she died, so that he will still prepare to go back and save her.
  • At the start of the story, changes will overwrite the timeline. Then, Time Trapper Paul alters the structure of the multiverse, to switch it over to branching timelines, so that he can go home to his original timeline without it clashing with — or destroying — every other possibility.
  • The Time Trapper, who has become cut off from his past, exists in a meta-version of You Already Changed the Past; if anything happens that would change the identity of the person who became the Time Trapper, it will then turn out that that new person was the Time Trapper all along. Meta-originally, it was Controller Jevek Jos Jar, but then Vandal Savage had been the Time Trapper all along, then Mandated!Paul.

Video Games

• In the fourth Deponia game, Deponia Doomsday what kind of time travel rules your trip follows depends on what model of time machine you are using, one guy at a bar for time travelers is mocked for owning a deterministic model because it doesn't allow any changes to the timeline, at one point you have to use it to go back to a location you visited some time ago and set things up the way they were when you first visited to solve a puzzle.
• Dungeon Fighter Online's flavor of time travel somehow manages to throw several of these rules into a blender and hit "purée". For starters, going into the past at all creates a pocket dimension of time with no immediate effect on the current timeline, meaning no inherent risk of causing a Stable Time Loop or whatnot. If the end result of events that occur in that "time pocket" sufficiently align with the current timeline, the time pocket will cease to exist completely and any minor alterations made within that time pocket will be deleted and/or fixed by the Clock Roaches. However, if events are so drastically changed that events such that the final consequences in that time pocket are different from the current timeline, the time pocket will overwrite the current timeline and become the new timeline, destroying the old one in the process.
• Legacy of Kain takes an axe to all of this and more. Blood Omen has Kain altering the past, only to create a future where vampires are all hunted down and killed by Moebius the Timestreamer's mercenary army. However, Soul Reaver has Kain insisting that all events are preordained, and that everyone plays out the part written for them by fate. But immediately afterwards, in Soul Reaver 2 and Defiance, we find out that Raziel is the only being capable of changing fate, which Kain structures his entire Xanatos Gambit Roulette of defying fate around and disrupting the Gambit Pileup created by his, Moebius', the Elder God's, and Hash'ak'gik's manipulations throughout Nosgoth's history. And that's not even getting into the many, time-derailing paradoxes present throughout the series, most of which revolve around the incredibly complicated existence of the Soul Reaver. In short, the rules of time travel in Legacy of Kain depend on and act to the advantage of whoever is on top of the giant Gambit Pileup, which results in Xanatos Speed Chess from many different sides, and where the effects of defying fate can either result in events already predestined to happen or create completely alternate futures that deviate entirely from the timeline, such as seen in Blood Omen 2.
  Kain: "History abhors a paradox, Raziel. Even now, the time-stream strains to divert itself, finding its old course blocked by your refusal to destroy me. The future is reshuffling itself to accommodate your monumental decision."

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

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