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Ontological Inertia

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Everything that happens stays happened. You can't steer a train.

Ontological Inertia acts as a buffer against changes to the cosmic status quo: You cannot (well, not completely) undo something that already exists.

On the other hand, what can happen instead is if you do change something in history that is significant, the timeline "fractures", a whole new universe is created at that point, and you and the new event are in a completely different reality with the change you caused. Thus, either you go back to your universe where the change never happened, or you end up going forward to the equivalent time in the new universe with the change that you made propagating from that point. If you don't like the result, you can try to go back and change time again, in which case, guess what, time "fractures" again to compensate for that new incident, and the cycle starts all over again.

As discussed in Analysis.Ontological Inertia, it's a particular case of You Can't Fight Fate. See also In Spite of a Nail. Contrast with (but not the exact opposite ofnote ) No Ontological Inertia. May explain Hitler's Time Travel Exemption Act.


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    Anime & Manga 
  • Dragon Ball Z goes with this during the Android Saga, when Future Trunks comes back to avert the Bad Future. He notes that anything he does in the past won't affect his future, just create an Alternate Timeline that has no bearing on his time. Among other things, despite Present Goku surviving the heart virus and Krillin and Trunks destroying Present Cell before he can be completed, Future Goku stays dead and the main Cell continues to exist.

    Comic Books 
  • Sonic the Hedgehog (Archie Comics): After defeating Dr. Eggman in issue #200, the citizens of New Mobotropolis take to celebrating as soon as they hear the news, believing the Eggman Empire is finally finished. They discover the hard way that just because Eggman's gone down doesn't mean his assets are, and his resources and technology are immediately seized by the Iron Queen, who wastes no time at all in coming to the city and taking it over. Monkey Khan chews Sonic and co. for thinking that it'd be that easy.

    Fan Works 
  • Brother on Brother, Daughter on Mother posits what the author describes as a "Grand Unified Theory of Star Trek Time Travel" that works like this. Reshek Taryn describes the multiverse using the analogy of a rope of infinite length, consisting of many strands composed of probabilistic outcomes that are all being pulled in the same direction. The pulling force means that smaller changes made by time travelers have a tendency to true up into Stable Time Loops over time and result in more-or-less the same outcome in the long run. Larger incursions, however, cause the "rope" to fray off vastly divergent timelines (among them the Mirror Universe), which weakens the overall timestream: Taryn briefly mentions a time war (implied to be the canon version of Star Trek: Enterprise) causing an entire strand to be erased. This in turn is the reason for her own job as part of Starfleet's "temporal SWAT team".
  • Paradoxus: Any given timeline is defined by certain key events that cannot be changed without branching the time-space continuity off. Id est, in timeline A, Bloom and Stella are assassinated, while in timeline B, they live for some reason (Eudora spares them, the Burning Legion invasion never happens, etc.). Thing is, B is an alternate version of A. If you somehow manage to prevent A's defining events (an impossible feat in itself), you are going to jump to/create B instead of staying in a modified version of A. When you travel back from when you came, you return to the same future you were trying to change (A's future). To get to B's future, you'll have to go the long way. Unfortunately, chances are you will first fade out from existence and the events there are totally random, meaning you lose any foreknowledge you got from time-traveling. All in all, A will keep existing as a separate timeline because its key events have ontological inertia — they cannot be undone. In this fic, there are two defining events for the timeline: the aforementioned deaths of Bloom and Stella and the Burning Legion invasion of Magix. Besides those two, things can be changed to varying extents, although it's not an easy task. For example, Daphne has to send back her memories so her past self can start planning how to change non-defining events that cana potentially save Magix from extinction.
  • With This Ring: History can be altered, but it requires additional energy to come from somewhere. The bigger the changes, the more energy is required, and a paradox basically requires infinite energy, which is why the universe will always find some other way to resolve it. So, if someone you love dies in a car crash, inspiring you to go back in time and save them, you won't be able to, because if they were saved, then why would you go back? Inevitably, either your efforts will fail to stop the crash, or the person will die in some other way. (On the other hand, if you manage to set things up so your younger self still believes that they died in a car crash, and thus goes on to build a time machine, that may work.)

    Films — Animation 
  • The Futurama movie Bender's Big Score relies heavily on this: the time travel in the film is "paradox-free", meaning that any issues created through time travel self-correct and don't significantly change the present. The most commonly given example is the idea of two versions of a person existing due to time travel with no clear Stable Time Loop, which would cause one of the versions to simply die abruptly, "resolving" the paradox. Of course, the whole process has limits; if something massively, utterly irreconcilable were to occur, then it would cause the universe to simply give up and break open.

    Films — Live-Action 
  • Discussed in Avengers: Endgame when the heroes consider Time Travel to undo the Decimation from 5 years prior. One suggests going back in time and kill Thanos as a baby before he could start his mad conquest, only to be corrected that changing the past won't affect the present, it would only create an alternate reality that has absolutely no bearing to the current state of affairs. This bears out in the climax when Past Nebula is killed by Present Nebula, who doesn't then cease to exist, and Past Thanos and his army are all killed, which doesn't prevent the Snap from having happened.
  • In the first three Terminator movies, good Terminator androids, bad Terminator androids, and one human are sent back in time to either prevent the upcoming apocalypse or kill off the future leader of the human rebellion. As each successive movie shows, attempts to change the future by either side will inevitably fail as long as there exists a demand for more Terminator movies.
  • Explored in The Time Machine (2002), with the time traveler's fiancée Emma acting as fate bait. It's a singularly interesting example: if she doesn't die, he doesn't invest the time and effort into creating the time machine. Her death cannot be changed, or he cannot go back to change. Down to the particular time limit (read: that very night, no matter what he tries). It's not like going back in time and stepping on a mosquito, the flow of time cannot continue if she does not die. Paradox, anyone? Of course, all that's really important is that he believes she died. He could go back in time, fake her death and bring her into the present with him.

  • Writer Fritz Leiber agreed with this trope in his Change War series of stories involving time travel, and devised the "Law of Reality Conservation" as a way to show how things couldn't just un-happen. In that context, it states that you can change the past (in fact he named one of the stories in the series, "Try and Change the Past"), but Fate will force a coincidental event to ensure that history proceeds down its intended path without paradox; every time you try to prevent one historical trend or event, a similar one will take its place in history.
  • Discworld:
    • The plot of Thief of Time deals with this kind of idea. When the Procrastinators (which are sort of spindles that regulate the flow of time ) slip out of control and time starts moving in weird ways, the History Monks attempt to re-regulate everything by dumping the excess time into various places (like the ocean, for example, which is "always big and wet" no matter what the time, and nobody cares if fishermen are suddenly drawing up a catch of weird fish that they've only ever seen as fossils). In fact, the entire history of the Discworld is supposedly pieced together from all the scrapped bits of the actual time-line that they could find after a temporal blow-out which shattered all of history. They didn't just change the past, they literally pieced it all together from scraps and hoped nobody noticed the inconsistencies. That's why some things in Discworld just don't make sense historically. Even the History Monks aren't sure why the Disc's history seems to keep the same general shape in spite of various things messing with time. Some ideas include the Theory of Narrative Causality (the Trope Namer) and the Historical Imperative, which appears to be equal parts this trope and a Pun on Kant.
    • The books also bring up the already-discussed concept of "steam engine time", which shows that human society tends to avert No Plans, No Prototype, No Backup (i.e., if Thomas Savery hadn't invented the steam pump, one of his contemporaries who were working on the same subject would have patented roughly the same thing, and history might have been back on track just in time for James Watt to make the engine efficient enough to use portably, and no boiling-over tea kettle need be involved).
    • Played and justified in exactly so many words in Mort. As Death's assistant, Mort attempts to save a princess from assassination, changing the predestined course of history. Historical inertia pushes back, creating a shrinking bubble reality the heroes must escape from.
    • Discussed by the History Monk Lu-Tze in Night Watch. Samuel Vimes finds both himself and a serial killer thrown 30 years into the past, where the killer murders John Keel, the man who mentored Vimes, thus altering history. By an astounding coincidence, Vimes winds up with a wound on his eye matching one Keel had, making it clear that he's going to take his place, mentor his younger self, and play Keel's role in an imminent revolution. Lu-Tze makes it explicit that it wasn't always this way; things really did happen the way Vimes remembers them in his own past. But the fact that such an improbable coincidence has occurred suggests that history is trying to correct itself into something at least resembling events as they were supposed to happen.
  • Dragonlance: The world as created by the gods does not allow past events to be changed. Unfortunately, several races have come into existence that were not intended at the time of the world's birth. They are more or less fine in the present, but all the "you can't change anything" rules of time travel don't apply to kender, Dwarves, and Draconians. Cue Tasslehoff Burrfoot, the kender Hero of the Lance, who tags along with Caramon on his trip to Istar in Dragonlance Legends.
  • Johnny Maxwell Trilogy: In Johnny and the Bomb, a Five-Man Band of schoolkids travel back to World War II and accidentally create an alternate timeline by stealing a bike, which ultimately results in a bomb which fell on the neighbourhood killing dozens of people who would have otherwise lived. When they travel to the alternate present, the world in general is pretty much identical to the original timeline, since the deaths of a few more civilians in the War didnít have much of an impact on history. However, there are minor changes, most notably the eponymous Johnny was never born (one of the people killed was his paternal grandmother), but he continues to exist despite his friends suggesting he should just vanish. Similarly, one of them gets left behind and survives to the present. When they meet him, he has become hugely wealthy, first by "inventing" fast food burgers and then investing in industries he knows will do well. He did consider trying to change history for the better, but found that one man, even a very rich one, couldn't affect world events by much and everything happened pretty much the way it was supposed to.
  • In Locksmith's Closet, Lock and Gary bury a geode in the present, travel to the future, dig it up again and bring it back to the present with them. Then, in the present, Lock digs it up again. Meanwhile, Gary is videotaping the one they brought back in the future to see if it disappears. It doesn't. They later use this as a literal money-making scheme.
    Gary: I think the rule must be... you can't change your own personal past.
  • A defining trope of Shakugan no Shana. When a person's existence is eaten by a Rinne, it damages the fabric of really, so, the Powers That Be replace them with a "Torch" to act as a shock absorber. They look the same, and even have the original's memories. As their flame burns out over time, their presence and impact on the world lessens — they become apathetic and do little, people overlook them — until they disappear completely. When this happens, no-one remembers them, and it is as though they never had existed, ever. This happens all the time.
  • In Time Patrol, there is a principle of "temporal inertia" which acts like this. It is very difficult to make substantial changes to the timeline, since most likely subsequent events will coalesce in a way that maintains the overall historical status quo. However, the flip side of the principle is that once changes are made to the timeline, it is similarly very difficult to undo those changes and return the timeline to its original status.
  • Time Wars has a Law of Historical Inertia; any change you make will be like a stone dropped in the river of time: History will simply flow around it and, for the most part, end up exactly where it was before (so if you wanted to actually change it, you'd essentially need a really big "stone" to divert the river, the consequences of which could be disastrous).
  • In To Say Nothing of the Dog, it is discovered that any change that someone tries to make in the past will be automatically rectified by the time-stream. For instance, if a bucket is removed from a historically significant bucket brigade, it will be replaced with a convenient barrel. By the end of the book, it turns out that all of the hero's misadventures through time were a direct result of a change somebody from the distant future will have made in the past. More than that, sometimes the time stream simply won't allow changes to be made. People who tried to go back and take out Hitler were deposited far enough away (time-wise) that they couldn't do it.

    Live-Action TV 
  • A fundamental part of Being Erica has the title character being able to go back in time to try to fix her regrets about her life. However, inevitably she will discover 1) she is unable to prevent the regrettable action from occurring 2) she is able to prevent the event from occurring, but something else happens that causes the same effect on her in the future or 3) she figures out that her regret had another fundamental cause. Instead, the point of the exercise is to gain perspective on what's going on in her current-timeline life. The few times she does manage to change a major event, the effect is usually erased by the end of the episode.
  • Doctor Who speaks of "fixed points in time" (also called "temporal nexuses" in the radio dramas), moments and individuals in the timeline which cannot be changed without causing catastrophic ripple effects. Even the Daleks don't mess with them.
  • FlashForward (2009) has elements of this. It is possible to change your future but very, very hard. In one case a character kills himself to prevent a future where he causes the death of a woman. In the altered timeline the person who replaces him on the team ends up causing the death instead.
  • Hercules: The Legendary Journeys has an episode in which the characters are worried about how their time travel might affect the present, but Hercules assures them that time will correct itself, so nothing will change.
  • In the final episode of Kamen Rider Decade, when Big Bad Apollo Geist is defeated, his forcible merger and destruction of the multiverse continues unabated. In fact, if anything it actually speeds up. This leads to Decade receiving a What the Hell, Hero? speech from his predecessors.
  • Lost plays with this in Season 5. For example, when handling a nuke, Daniel assures them that it can't explode because the island still exists in the future they came from.
  • The Orville: In "Lasting Impressions", Gordon falls in love with a 21st-century girl after using her extensive cell phone data to create a holo-simulation of her rich life. When her ex-boyfriend returns and she breaks off with Gordon to reunite, he erases the boyfriend from the simulation so he can have her for himself. Unfortunately, he was such a motivating factor for so many of her life decisions, that without her she's a completely different person. Gordon realizes that he fell in love with her because she felt genuine and real, and if he starts rewriting her to fit his desires, she'll lose that quality.
  • Outlander:
    • The entire plot of Season 2 revolves around Jamie and Claire first trying to prevent The Rising from taking place, and then attempting to succeed against the British. No matter what they do, things still go the way Claire remembers them from history, with the Scottish being decimated at the Battle of Culloden.
    • Mary invokes this in season 4 as, now living in America, she knows the Revolution is coming and tells Jamie nothing they do can prevent it from happening.
  • The Twilight Zone (1959):
    • In "Back There", during a discussion about traveling back to time to the day before the Wall Street Crash, Peter Corrigan argues that history cannot be changed as the events of October 24, 1929 are a part of established history. When he is sent back in time himself, he learns that some things can be changed. Peter was unable to prevent Lincoln's assassination but inadvertently changes history in a more minor way. The police officer who believed his story made a name for himself for seemingly predicting the assassination. As a result, he became Chief of Police, a councilman and a millionaire after investing in real estate. In the original history, his great-grandson William was an attendant at the Potomac Club but a member of the club in the altered history.
    • In "Of Late I Think of Cliffordville", when the protagonist realizes that he is still 70 years old (but looks like 30) and no one believes his ideas of "inventions" that he knows will be coming, he asks Miss Devlin to return him to where he started. She says she can but that will be a time set in motion by events as they are now and there is a fee. He sells the only thing he has of worth to the guy who was sweeping the streets and returns to his "present." Only thing is, now that guy, who has been the janitor in his building now owns the company, and he is the janitor!
  • The Twilight Zone (2019): In "The Comedian", Samir gains the ability to remove people from existence by making fun of them, but discovers that they are retroactively erased from the past, and everything they affected reverts to how it would have been if they hadn't been there. After wiping out his girlfriend's mentor in a bout of misplaced jealousy, her prestigious lawyer career is undone and she becomes a minimum wage waitress, because it was the mentor who motivated her to become something more. In the end, Samir makes jokes about himself to the audience and vanishes. Everyone he erased returns retroactively, because he was never around to erase them.

    Tabletop Games 
  • Enforced in Continuum. Spanners have a saying: "the universe is". Time is stable, unchanging, and any attempts to change the known result in reality-rending paradoxes that must be repaired. This is not a natural law of the universe, but rather an artificial one that is strenuously enforced by the Inheritors, humanity's post-Singularity descendants that occupy all time from 2400 AD forward and do not take kindly to any change that might affect their own existence.

    Video Games 
  • In Command & Conquer: Red Alert, Einstein tries to erase Hitler from history to prevent World War II. He succeeds and an even worse war between Russia and the Allies takes the place of World War II. It's implied that the Einstein that did the erasing will not see any changes. All he has done is create a divergent timeline which exists simultaneously with ours. Otherwise, he wouldn't be able to go back to his lab in 1946. This is then thrown out the window in Red Alert 3, where Soviet time travelers kill Einstein in the same manner before he invents his time machine. When they go back, they immediately see changes. One change? No nuclear weapons. Another change? Japan is now a superpower to rival the Soviets and the Allies. Interestingly, many of Einstein's inventions are still made, but by other people, including the Chronosphere.
  • Dragon Age: Origins:
    • The Dalish Keeper Zathrian summoned a spirit and bound it to a wolf in order to inflict a werewolf curse on the human barbarians who murdered his son and raped his daughter. Centuries later, when the curse has been passed down to the descendants of those who savaged Zathrian's children, the Warden is tasked with killing the wolf — Witherfang — who has spread the curse to Zathrian's clan. Killing Witherfang will not end the curse; it will only enable Zathrian to cure the Dalish elves who have been inflicted. Likewise, killing Zathrian will not end the curse, and those inflicted with it will remain as werewolves. Only persuading Zathrian to lift the curse himself will cure all those who have been turned into werewolves.
    • The Grey Wardens, once they commit to the Joining ritual, are forever tainted by the corruption of the Darkspawn Blight. Even if the Archdemon leading a Blight is eliminated, Grey Wardens will remain tainted until their Calling, a point at which the corruption threatens to consume their mind. Once a Warden experiences their Calling they will enter the Deep Roads to seek death fighting Darkspawn underground before the corruption completely overtakes them.
  • Fate/Grand Order: During the Seventh Singularity, Gilgamesh reveals that while Chaldea's efforts will undo the Singularities that alter human history, they won't prevent any humans killed in a Singularity from dying in the proper timeline. The recollection of how they die gets changed to something more mundane, such as a person killed by a wyvern in the Orlean Singularity having history changed so they died by either natural causes or an accident.
  • The conclusion of Final Fantasy is a version of this: the Light Warriors shatter a Stable Time Loop by killing Chaos. In the process, they themselves are shunted into the newly created timeline wherein Garland never abducted the princess and the Four Fiends never existed... and their memories are lost in the process.
  • Ontological Inertia is such a strong force in the Legacy of Kain series that the titular vampire spends most of his immortal life looking for a way to thwart it at a key point in his past. It turns out the trick is to have time displaced versions of something interact, for instance the first example in the series involves him fighting an enemy when they're both using the same sword (and another character is completely immune to it because he carries a future version of his own soul around with him). This is the origin of Kain's memorable "edge of the coin" speech:
    "But supposing you toss a coin enough times. Supposing one day, it lands on its edge..."
  • In The Messenger (2018), the game world is under an enduring curse laid on the land by the Demon King that will create an endless cycle of demon invasions and Messenger quests. However, killing him doesn't do anything about this curse, because he made sure to put the source onto the founder of the island's society Phantom, who is sealed in a magical music box. Thus, in spite of the Demon King being the Big Bad, he's not the final boss, and the endgame is focused on reaching and freeing Phantom.
  • Mentioned in Shadow of Destiny. It's ultimately revealed to be the driving force behind the entire story.
  • Trinity: After successfully stopping the Trinity test of the first A-bomb (which would apparently destroy most of New Mexico), a mysterious voice explains that since the history that produced your character depends on atomic weapons, reality will arrange for smaller explosions to occur every time a nuclear weapon is supposed to detonate from then on. Smaller meaning nukes as we know them.

    Western Animation 
  • Code Lyoko:
    • Strangely used: although there's a Reset Button that the heroes can use to travel into the immediate past and undo most of the damage the Big Bad causes, if anyone dies before they use it, they'll stay dead, even after the past is changed; their death still possesses ontological inertia in the new timeline. Presumably they'd just drop dead from no apparent cause, but since the heroes never allowed anyone to die in the course of their adventures, the viewer never really saw how it'd work.
    • Additionally, the heroes, (and only the heroes, even if someone else were brought into the fold of this whole "XANA" shenanigan for this particular problem of the week) would remember the events of the erased day, presumably so someone would know the world had been saved to begin with. One of the flashback episodes demonstrates that you retain your memory if you've been scanned into the supercomputer.
    • The Reset Button even becomes a plot point because they learn XANA grows more powerful each time they reset, showing just who can ignore it.

    Real Life 
  • The first law of motion (an object tends to stay in motion unless acted on by another force) could be considered an example of this. In theory, an object moving in a frictionless environment such as outer space will keep moving at the same speed forever, unless it runs into something that influences it; for example, the Voyager space probes will continue on their current paths through space for thousands of years until they approach some of the nearest stars to the Sun, which may change the route they take from then on. On Earth, however, there is obviously friction, which tends to slow stuff down (you can still observe this law in effect however, for example, does your car stop moving the instant you take your foot off the gas pedal? of course not, hence the need for brakes).
  • The game of peekaboo helps infants learn the concept of object permanence by playing with it, making a person seem to disappear, then reappear. The reason is that very young children process the world through themselves and, thus, if something is out of the reach of their senses then it ceases to exist until further notice. In other words, for them, objects and people have no Ontological Inertia.
  • This is the basis for the "many worlds" interpretation of time travel, and also as a means of explaining why there are no evidence of time paradoxes from people travelling back in time if time travel is possible in the future; each time travel event splits the timeline for the two choices; one timeline exists where the traveller doesn't go back in time and stays where they are (Timeline 1), and another for when they do go back (Timeline 2), effectively meaning that nothing is changed. The number of timelines then begins to grow exponentially when you consider all the other different variables that would create different outcomes.
  • Larry Niven wrote an article explaining why time travel doesn't happen — you invent a time machine, go back and change the past, which runs into the future when somebody invents a time machine and goes back and changes the past... until eventually you arrive at a timeline where time travel is never invented because somebody killed the inventor before he invented it just to stop all the Timey-Wimey Ball nonsense.