->''Anything in existence will continue to exist until a sufficient force acts against it.''

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

Writer Creator/FritzLeiber 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 [[RetGone 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.

On the other hand, what can happen instead is if you do change something in history that is significant, the time line "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. So 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.

Simon Hawke's ''Literature/TimeWars'' has a similar Law of Historical Inertia, and any change you make will be like a stone dropped in the river of time: History will simply flow round 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 disaster).

A particular case of YouCantFightFate. See also InSpiteOfANail. Contrast with (but not the exact opposite of) NoOntologicalInertia. May explain HitlersTimeTravelExemptionAct.

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!!Examples

[[folder:Anime & Manga]]
* Used in ''HigurashiNoNakuKoroNi''.
** And in the ''Kai'' finale, [[spoiler:averted entirely]].
* A defining trope of ''ShakuganNoShana''. When a person's existence is eaten by a Rinne, they are gone but replaced by a "Torch", who acts 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. [[FridgeHorror This happens all the time.]]
* A key point in ''VampirePrincessMiyu'' is that Miyu fights against and defeats monsters but - as a curse she obtained by refusing to become a monster herself - is always unable to reverse any of the evil they have done. For example, when she defeats a ghost who has been luring women travelling on underground trains [[spoiler: to an abandoned station where they are transformed into statues, she can seal the station, but [[DownerEnding can do nothing about the statues already created]] [[NightmareFuel who remain]] [[FateWorseThanDeath imprisoned, petrified and weeping forever]]. ]]
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[[folder:Films]]
* Explored in the 2002 version of ''[[Film/TheTimeMachine2002 The Time Machine]]'', 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. [[TakeAThirdOption He could go back in time, fake her death and bring her into the present with him]].
* In the first three ''{{Terminator}}'' movies, good terminator androids, bad terminator androids, and one human are sent back in time to either prevent the upcomming 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.
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[[folder:Literature]]
* The plot of Terry Pratchett's ''Discworld/ThiefOfTime'' deals with this kind of idea. When the Procrastinators (which are sort of spindles that regulate the flow of time in Discworld) 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 [[spoiler: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.]]
** To clarify, 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 TheoryOfNarrativeCausality (the TropeNamer) and the Historical Imperative, which appears to be equal parts this trope and [[IncrediblyLamePun a pun on Kant]].
*** The Discworld books also bring up the alrady-discussed concept of "steam engine time", which shows that human society tends to avert NoPlansNoPrototypeNoBackup (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 [[InSpiteOfANail history might have been back on track]] just in time for James Watt to make the engine efficient enough to use portably, and no [[ForWantOfANail boiling-over tea kettle]] need be involved).
** Played and justified in exactly so many words in Terry Pratchett's ''Mort''. As Death's assistant, Mort attempts to [[spoiler:save a princess from assassination]], changing the predestined course of history. Historical inertia pushes back, [[spoiler:creating a shrinking bubble reality the heroes must escape from]].
* In ''To Say Nothing of the Dog'' by Connie Willis, 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 [[spoiler: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.
* In PoulAnderson's ''Literature/TimePatrol'' series of short stories/novellas, there is a principle of "temporal inertia" which acts like this. It is very difficult to make substantial changes to the time-line, 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 time-line, it is similarly very difficult to undo those changes and return the time-line to its original status.
* In the ''{{Dragonlance}}'' Legends trilogy, the flow of time can't be altered.[[spoiler:..unless one of the Graygem races, like a kender, were to travel back in time. Cue Tasslehoff Burrfoot, the kender Hero of the Lance, who tags along with Caramon on his trip to Istar.]]
* In ''[[Literature/LocksmithsCloset 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.
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[[folder:Live Action TV]]
* {{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.
* TheLegendaryAdventuresOfHercules had an episode where they were worried about how their time travel might affect the present, but Hercules assured them that Time would correct itself, so nothing would change.
* In the final episode of ''Series/KamenRiderDecade'', when BigBad [[Series/KamenRiderX 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 WhatTheHellHero speech from his predecessors.
* FlashForward2009 had elements of this. It was 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.
* In the Series/TwilightZone episode "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 know will be coming, he asks [[TheDevil Miss Devlin]] to return him to where he started. She says she can but that is 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 a 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!
** Also, in "Back There", Russell Johnson finds himself in Washington DC before Lincoln's assassination. He tells a cop, but can't stop the act when he is drugged by John Wilkes Booth. When Johnson's character returns to his present, the man who was butler at his club is now one of the wealthiest members. His great grandfather was the copy and afterwards became famous and rich because he tried to stop the assassination as well.
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[[folder:Tabletop Games]]
* The ''{{Dragonlance}}'' Saga even uses the stone in a time-stream example. 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 more or less are fine in the present, but all the "you can't change anything" rules of time travel don't apply to [[SpannerInTheWorks kender, Dwarves, and Draconians]].
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[[folder:Video Games]]
* Spoilers for the {{Infocom}} InteractiveFiction game ''Trinity'': [[spoiler: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 [[{{AFGNCAAP}} 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.]]
* Mentioned in ShadowOfDestiny (aka ''Shadow of Memories''). [[spoiler:It's ultimately revealed to be the driving force behind the entire story.]]
* The conclusion of ''VideoGame/FinalFantasyI'' is a version of this: [[spoiler:the Light Warriors shatter a StableTimeLoop by killing Chaos. In the process, they themselves are shunted into the newly-created time-line wherein Garland never abducted the princess and the Four Fiends never existed... and their memories are lost in the process.]]
* In CommandAndConquer: 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 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.
* OntologicalInertia is such a strong force in the ''[[LegacyOfKain Legacy of Kain]]'' series that the titular vampire spends most of his immortal life looking for a way to [[SetRightWhatOnceWentWrong thwart it]] at a [[TakeAThirdOption key point]] in his past. This is the origin of Kain's memorable "[[HeadsTailsEdge edge of the coin]]" speech:
-->''But supposing you toss a coin enough times. Supposing one day, it lands on its edge...''
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[[folder:Western Animation]]
* Strangely used in ''CodeLyoko''. Although there's a ResetButton that the heroes can use to travel into the immediate past and undo most of the damage the BigBad 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 time-line. 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 ResetButton even becomes a plot point because they learn [[spoiler:Xana grows more powerful each time they reset]] showing just who can ignore it.
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