A type of TemporalParadox. The name comes from the most famous variation - namely "what would happen if you travelled back in time and killed your own grandfather?", but also applies to anything that happens while time travelling that should logically make your original time travel trip impossible or unnecessary.

For example, if you killed your grandfather in the past, you should [[RetGone never have been born]], and therefore you couldn't have travelled to the past kill your grandfather. Destroyed the time machine? Okay, but how did you use said machine to travel into the past in the first place? Kill the evil overlord while he's a child? Then [[HitlersTimeTravelExemptionAct you shouldn't have any reason to travel into the past to kill him]], because he never grew up to [[DoomedHometown destroy your village]] and all.

So, then, killing your grandfather causes you to not exist, and since you don't exist, you never killed him. Which means he survives, so you exist, so you do go back to kill him. Which means he doesn't, so you don't; therefore he does, so you do, etc ... [[LogicBomb are you confused yet?]]

Alternately, this whole snafu can be ignored outright if you're using AlternateUniverse-style time travel, where the time period ''you'' came from is separate from (and unaffected by) any meddling you do in the past. (The downside is that returning "home" might be a tricky matter....)

If the universe runs on StableTimeLoop, this type of paradox is simply impossible, as all changes that [[TimeTravelTenseTrouble will have been going to happen]] have [[YouAlreadyChangedThePast "already" happened anyway]], so you can't cause a change that will negate itself -- something will have to intervene, no matter what or how.

May lead to MyOwnGrampa, though in this case the person killed (probably) wasn't originally your grandfather anyway.

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!!Examples:

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[[folder:Film]]

* The plot of the first ''Film/BackToTheFuture'' may be the most well known example, even though it's A. not Marty's ''grand''father it involves and B. he doesn't kill him, but rather accidentally [[spoiler:takes his place as his mother's object of affection]]. The rest of the movie has Marty trying to correct things before he's erased from existence.
** Though within the semi-canon/non-canon of the Telltale ''VideoGame/BackToTheFuture'' game, Marty does encounter his paternal grandfather and affects his own existence in time.
* Inverted in ''Film/StargateContinuum'', where Cam Mitchell winds up going back in time, and eventually (ten years down the road) keeping his Grandfather alive as a way of [[SetRightWhatOnceWentWrong setting right what Ba'al had messed up]].
** Said paradox was directly referenced before, when Mitchel found out that he doesn't exist in new timeline because Ba'al killed his grandfather.
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[[folder:Literature]]
* Creator/RobertAHeinlein:
** In ''Literature/TimeEnoughForLove'', Lazarus Long just glosses over the possibility of creating a paradox while time-traveling by saying that it's impossible to create one. So he has sex with his mother, meets his younger self, enlists in the Army and fights in WorldWarI. ItMakesSenseInContext.
** In ''Literature/TheDoorIntoSummer'', this is boiled down to the time-traveler protagonist waiting just outside of a room where he also is prior to his time-traveling activities, and briefly wondering what would happen if he ran in and slashed his counterpart's throat. He doesn't do it, of course, because that would be stupid and accomplish nothing, but he notes in present tense that he ''still'' hasn't figured it out.
* The Creator/CharlesStross novella ''Palimpsest'' has a twist: killing your own grandfather is the initiation rite for the TimePolice.
** The graduation ceremony is stepping back in time a few minutes and killing ''yourself''.
* Paradoxically inverted in "Grandpa", a short story by Edward M. Lerner. In it the protagonist, Professor Fitch, survives two assassination attempts by his grandson, and preempts a third by deciding not to have children.
* Averted in ''Discworld/NightWatch'', where Vimes' mentor is murdered while he's in the past; [[YouWillBeBeethoven he ends up taking over his identity,]] [[StableTimeLoop teaching his younger self everything that his mentor taught him.]]
** Also a DiscussedTrope in ''Discworld/TheLastContinent'' when Ponder Stibbons tries to explain the idea to Archchancellor Ridcully, but runs up against the latter's LiteralMinded {{Metaphorgotten}} tendencies. "Why would I want to kill my grandfather? I rather liked the old boy".
* In ''Literature/JohnnyAndTheBomb'', Bigmac suggests going back in time to [[HitlersTimeTravelExemptionAct kill Hitler]]. Johnny warns him of the dangers should he accidentally kill his grandfather, but Bigmac says it's safe since his grandfather doesn't look anything like Hitler. (Fortunately, by the time they obtain actual time travel, he's forgotten the plan.) Then they fall victim to an actual grandfather paradox: [[spoiler:their time travel results in Wobbler's grandfather being killed in a World War II bombing]].
* In MichaelCrichton's ''Literature/{{Timeline}}'', one of the protagonists raises this paradox to the CorruptCorporateExecutive who's trying to send them back in time. The latter changes the subject to a long discussion about how it would be nearly impossible for one person to make the Mets beat the Yankees (ie, the forces of history are too large for one person to decisively change). When the protagonist presses the point, the Exec {{Hand Wave}}s it and moves on.
* The end of ''TheSagaOfDarrenShan'' explains how the story is an endless paradox because [[spoiler:the rule of Destiny is that if you kill someone, somebody else will take their place and do exactly as they would do (as an example, Evanna says that if you were to kill Hitler, somebody else would've taken his place and done exactly as he did.) and seeing as Darren goes through everything just to go back in time to stop the whole thing from happening, someone else will see the Cirque, join Mr.Crepsley and go on all the exact adventures Darren did, eventually having to stop themselves from seeing their best friend talking to Mr.Crepsley and then someone ELSE taking THEIR place and so on. Darren says that afterward you could read the books again and change all the names and it'd still be technically correct.]]
* Inverted in two separate ways in {{Rant}}: [[spoiler: Traveling back in time to kill your parents will cause you to be outside of time, and therefore immortal (in-universe this is known as "severing ties"). Going back and impregnating your mother, or a direct matriarch of your family (grandmother, great-grandmother) will result in gaining heightened faculties (this is known in-universe as "stoking". Combo points for [[{{Squick}} impregnating each one down the line until you are born]].)]]
* Creator/GregoryBenford's ''Timescape'' describes a unique, quantum-mechanical approach to Grandfather Paradoxes. If a time-travelling signal were to prevent its own transmission, the signal and everything involved in triggering it would be in an ''indeterminate'' state where it neither does, nor doesn't, occur -- like Schrödinger's Cat before the box is opened.
* In one classic sci-fi story, the protagonist decides to try to commit suicide in a grand way by going back in time to shoot his grandfather. He does so. Nothing happens, so he turns the gun on himself. The narrative continues on to note that the sound of the gunshot does nothing to disturb his grandmother and his grandfather's best friend as she tells him to make sure he pulls out in time...
* [[DiscussedTrope Discussed]] in Creator/StephenKing's ''[[ElevenTwentyTwoSixtyThree 11/22/63]]''.
-->"Yeah, but what if you went back and killed your own grandfather?"
-->He stared at me, baffled. "Why the fuck would you do that?"
-->That was a good question, so I just told him to go on.
** Though he initially hand-waves, King does answer the question implicitly: Nothing would happen to the traveler. People who go through the portal are 'out' of the timestream and thus not eliminated as a result of their actions.
* Discussed and inverted in ''[[Literature/LocksmithsCloset Locksmith's Closet]]''. When Lock and Gary travel into the future and find it uninhabited, Gary remarks that "at least we don't have to worry about our grandkids coming along and shooting us just to see what'll happen."
* Happens literally in René Barjavel novel ''Le voyageur imprudent'' written in 1943 (hence the first novel to enunciate the grandfather paradox) where the time-traveler (Pierre Saint-Menoux) tries to assassinate Napoleon Bonaparte before his rise to power: [[spoiler: at the last moment, a soldier jumps to take the bullet and save Bonaparte. This soldier is of course the time-traveler ancestor. The time-traveler is then wiped out from existence.]]
* In PoulAnderson's ''Literature/TimePatrol'' stories, you can have effects without causes, so that killing your grandfather merely means you exist without a parent. Fortunately this means you can yank history back on course. Or unfortunately, since in the process you eliminate the future created by the change, and everyone in it.
* Discussed in ''Literature/TheDresdenFiles'' novel ''Literature/ColdDays'', when Harry asks Vadderung what would happen if he were to travel back in time and attempt to kill his grandfather. The answer is that [[BadassGrandpa Harry's grandfather would beat him senseless]], because [[spoiler: Harry's grandfather is Ebenezer [=McCoy=], Harry's mentor and the White Council's Blackstaff.]]
** Eventually he gives a more serious answer, it would destroy the current timeline and create two new ones, one where he failed and one where he succeeded.
* Parodied in one of Brian Aldiss's "Three Enigma" stories. A time traveller goes back in time and falls in love with his grandmother, causing his grandfather to commit suicide. "I can see this is going to be awkward," says the time traveller as he fades out of existence.
* A different take, not involving relatives, is in ''A Dry, Quiet War'' by Tony Daniel. The protagonist returns to his home planet after fighting a war twelve billion years in the future at the end of time, apparently to hold back the spread of entropy so the universe has a chance to exist in the first place. He has a run-in with several deserters from that war who are terrorizing the place. If he does anything to harm them however, it changes the future and everything he's fought for has been for nothing.
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[[folder: Live Action TV ]]

* For the [[Series/DoctorWho Time Lords]], Grandfather Paradox is an actual ''person'' who went back and, yes, killed his grandfather, which doomed him to a sort of undead temporal limbo. He's the Time Lord equivalent of the Bogeyman, and the splinter group/terrorist cult FactionParadox considers him their spiritual leader, partly cause [[ForTheEvulz it pisses off the Time Lords.]]
** We actually meet him. He's quite literally the AnthropomorphicPersonification of FutureMeScaresMe - he's ''everybody's'' evil future self.
** And FactionParadox has the entire trope as a sport for initiates. Want to get in? Kill momma. ''[[MindScrew Before you were conceived]]''.
** At one point, his appearance is described as eerily similar to the Ninth Doctor, which at that point would have been the Doctor's future self. In fact, the first time the Doctor encounters a FactionParadox agent, the agent calls him "Gramps".
** Martha brings this up at the start of her first trip in the TARDIS.
--> "What if I kill my grandfather?"
--> "Are you planning to?"
--> "No."
--> "Well, then."
** In "Last of the Time Lords", The Master constructs a Paradox Machine specifically to evade the consequences of this paradox, [[spoiler:as his army of psychopathic laser balls happen to be the descendants of humans, from the end of the universe]].
* Parodied in ''Series/TheXFiles'' in "Clyde Bruckman's Final Repose": the title character cites this as a reason why he shouldn't help Mulder and Scully catch a serial killer -- because one of the people he might save could be the grandmother of the person who'd invent a time machine that would mean Clyde's father never met his mother and therefore he'd never have been born. As his ability to see the future had made his life a misery, Clyde suddenly realizes that wouldn't be a bad thing after all.
* Referenced in ''{{Series/Continuum}}'', when terrorists Liber8 attempt to eliminate their adversary Kiera Cameron- both displaced to 2012 from 2077- by killing her grandmother before she can give birth; Kiera attempts to counter this by threatening the pregnant mother of Liber8's founder. [[spoiler:The paradox is apparently proven irrelevant when the grandmother of Liber8 renegade Matthew Kellog is killed before she has children and nothing happens to him, but Kiera's ally Alec Sadler notes that lack of evidence is not proof that nothing will happen.]]
* Inverted in ''Series/{{Haven}}'', where after traveling back in time Duke accidentally ''saves'' his grandfather (who had already had a son). When Duke learns who he is, the issue becomes whether he needs to make sure he dies or let him continue living. [[spoiler: Turns out it's neither. Duke is a part of a StableTimeLoop in which Sarah Vernon ends up killing Roy Crocker, because he found out from Duke that she was going to kill him, so he went after her first.]]
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[[folder: Tabletop Games ]]

* ''TabletopGame/GeniusTheTransgression''[='=]s stance on the subject: "And yes, if you kill your own grandmother before your father is born, you will cease to exist. The universe, it turns out, doesn't care that much if your grandmother gets shot in the head and there's no shooter. You still go poof."
* The old ''Series/DoctorWho'' RPG encourages [=GMs=] to be cruel to players who try this. One popular result is that, if you go back in time and point a gun at your grandfather, then the young version of your grandfather will leap out of the way, pull his own gun and shoot ''you'' dead. Paradox? What paradox?
* ''[[http://dig1000holes.wordpress.com/time-temp/ Time and Temp]]'' uses office temps (hence the name of the game) as field agents because their [[{{Mooks}} unimportance]] minimizes the risk of accidental GrandfatherParadox. At least until their vital mission (explained using the same bland corporate-speak as any other boring day job) gives way to their [[ItsAllAboutMe selfish]] [[NiceJobBreakingItHero foibles]]; the worst-case scenario is to RetGone [[ApocalypseHow all of existence]], but usually they just get slapped with an [[HilarityEnsues Incident Report]].

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[[folder: Video Games ]]

* In ''VideoGame/MetalGearSolid3'', if you kill one of the bosses instead of just knocking him out, you get the "TIME PARADOX" NonstandardGameOver--said boss is integral to the plot of the previous games in the series, which take place chronologically after [=MGS3=]. Hilariously, the HD rereleases of the game have an achievement for doing this entitled, "[[WhyDontYouJustShootHim Problem Solved, Series Over]]."
* Narrowly averted in ''[[VideoGame/TheLegendOfZeldaOracleGames The Legend of Zelda: Oracle of Ages]]''; Ralph attempts to destroy his own ancestor Queen Ambi (who is possessed), ''knowing'' that it will remove him from existence and [[HeroicSacrifice willing to take that consequence to protect the people of Labrynna]]. Subverted when [[CurbStompBattle she proves more than capable of defending herself]].
* In [[http://achrongame.com/paradox.html one of the demo videos]] for the [[RealTimeStrategy RTS]] game, ''{{Achron}}'', they build a mech, then send it back in time to destroy the factory that built it.
** The way the game handles this is pretty interesting since it involves "time waves" and a point in the past where time manipulation is impossible. Time waves sweep through the timeline from past to future and, to quote the [[http://www.achrongame.com/wiki/index.php/Main_Page wiki]], "a time wave is what makes the past affect the future". So, when a time wave hits the time the factory was destroyed, the factory simply gets destroyed and the mech survives. When the next time wave hits, the mech will be destroyed because the factory doesn't exist in the future and the factory survives and so on. The "final" version of the event is the one when the event hits the point where time is immutable. [[MindScrew You know what, just watch the video.]]
* In ''VideoGame/ChronoTrigger'', Marle disappears from existence due to her being mistaken for her ancestor Queen Leene who was kidnapped at the time she landed in the past, and since they stopped looking for Leene thinking they had found her, she was killed, thus causing Marle to not exist. Fortunately Leene hasn't been killed ''yet'', so our heroes are able to go rescue her, which causes Marle to start existing again. While this is clearly a grandmother paradox, everyone seems to remember that she existed at least long enough for her to cause herself to have never existed.
** Time travel in the Chrono series ''[[TimeyWimeyBall usually does not work that way]]''. Throughout the rest of the game, the worst you get is a bad ending if you try something similar. The person who wrote this part of the game quit after this segment, which is why it's so out of place.
* In ''Videogame/PokemonMysteryDungeonExplorers Of Time/Darkness/Sky'', [[spoiler: the player character traveled back in time from a BadFuture to change the past. When they succeed, they have just enough time to say one last goodbye to their partner before they fade away.]] Ultimately averted, as the lack of RetGone makes the point moot.
* One of the death scenes in ''VideoGame/TimeGal'' has [[PlayerCharacter Reika]] fire her gun into a bunch of cavemen — and promptly dematerialize because she just shot one of her ancestors.
* In the true ending of [[TheKingOfFighters The King of Fighters XIII]], [[spoiler: Ash]] stops his time traveller ancestor [[spoiler: Saiki]] from returning to the past after losing the fight to the player. Soon after, [[spoiler: Ash]] fades away taking [[spoiler: Saiki]] with him.
* In ''VideoGame/SpaceQuest 5'', anything that causes Beatrice's death will cause Roger to cease to exist. She is the eventual mother of his son, who saved Roger's life in the previous game. No Bea means no son, no son means Roger was never saved. The death screen will explain this each time.
* Invoked in ''VideoGame/BioShockInfinite'' [[spoiler:ending: Elizabeth travels back in time and kills Booker, her own father, before her conception in every possible timeline, thus forcing the universe to block off all branching timelines where Booker becomes Comstock, kidnaps Booker-from-another-universe's daughter Anna, who grows up to become Elizabeth and develops reality-warping and time-traveling powers as a result of her dimension shift. That way, the only timelines that are left are the ones where Anna/Elizabeth grows up peacefully with a never-baptized Booker]].
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[[folder: Web Comics ]]

* BobAndGeorge [[http://www.bobandgeorge.com/archives/010325c One more reason to hate time travel!]] (On top of SchrodingersButterfly questions of whether their acts can affect the author.)
* [[Series/{{Mythbusters}} Adam and Jamie]] decide to put this trope and the MyOwnGrandpa trope to the test when they went back in time in ''IrregularWebcomic''. They botch it up when they accidentally swapped grandmothers, making ''each other'' their grandfathers (Adam is Jamie's grandfather, Jamie is Adam's). Incidentally though, this does make them their own great-great-grandfathers, proving that this trope and the MyOwnGrandpa trope is possible. Myth confirmed!
** Incidentally, the people that ''should'' have been Adam and Jamie's grandfathers themselves went back in time to the Jurassic period, where they were eaten by an Allosaurus that used the time machine to go to the future and become president.
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[[folder: Web Original ]]

* A {{Newgrounds}} cartoon "Grandbunny Paradox" made fun of this. It featured a bunny and a stick figure. The bunny went back in time to kill his grandmother and finds himself turned into a sheep, because his grandfather married a sheep instead of a bunny. The stick figure decides to do the same and kills his grandmother only to find himself turned into a tomato. He doesn't like being a tomato so he goes back and shoots the guy who sold him the gun to kill his grandmother...only to find himself now holding grenades.
* In the [[http://www.abyssandapex.com/200710-wikihistory.html International Association of Time Travelers]] skit, which is mostly dedicated to [[HitlersTimeTravelExemptionAct going back in time and killing Hitler]], this ends up being the fate of [[spoiler:[=AsianAvenger=]]].

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[[folder: Western Animation ]]

* In an episode of ''{{Futurama}}'', Fry goes back to Roswell in 1947 and accidentally gets his grandfather killed in an atomic blast [[DefiedTrope while trying to avert this trope.]] He doesn't stop existing because he ''also'' ends up doing his grandmother, becoming [[MyOwnGrampa his own grandfather]]. Or, as he put it, "I did the nasty in the pasty." [[spoiler:This becomes a key plot point in later episodes, as this event caused him to have brain damage that made him immune to a number of things.]]
* In ''TheAdventuresOfSonicTheHedgehog'', one episode in the four-episode arc about the Chaos Emeralds starts with Sonic foiling a plot by Robotnik to prevent his ancestors from marrying and thus eliminate Sonic from the timeline. Sonic succeeds in sending Robotnik packing, but then '''causes the paradox himself''' by ordering a chili dog from his maternal ancestor, causing his paternal ancestor to become impatient waiting to be served and leave. After Sonic disappears, Tails solves the paradox in about a minute by forcing the meeting to happen.
* ''Wonder Warthog'' employs this with one case of glaring inconsistency. He is hanging out in a bar with Stoneage Warthog and The Hog from the Future (I may have the names wrong), and the latter decides to explore the nature of a paradox by shooting the former with a zapgun. Since Stoneage Warthog was the direct ancestor of the others, they cease to exist, while the city is retroactively turned into a crime-ridden cesspool because WW wasn't around to do anything. The Hogs are then immediately returned (since Future Hog couldn't have killed Stoneage Hog if he didn't exist) and everything is fine... except the city is ''still'' a hellhole, necessitating the heroes to fix it the traditional way.
* Lampshaded in an episode of ''InvaderZIM'', where Zim tries to kill Dib using a time machine. The resulting LogicBomb causes GIR's head to explode.

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