History Main / GrandfatherParadox

* A DiscussedTrope in ''Literature/AQuantumMurder'' by Creator/PeterFHamilton. The conclusion is you can due to the existence of [[TheMultiverse parallel realities]]. In short, you're conceived in one universe, then go back in time and kill your grandfather in another universe.

* Mentioned in ''VideoGame/Injustice2'': Reverse-Flash ends up trapped in the present, [[YouCanNeverGoHomeAgain unable to return to his home in the future]], because one of his ancestors died at the hands of the Regime (which the Flash was originally a part of, and whom Reverse-Flash personally blames).

* ''VisualNovel/SteinsGate'' explains that [[spoiler: this is why Okabe been failing to change the timeline in his favor, specifically when he needs to save Kurisu from being stabbed. The whole plot kicked off when Okabe accidentally sent the first D-Mail after seeing Kurisu lying in a pool of blood, [[ForWantOfANail drastically changing events]] so that, among other things, the above event was prevented from happening, allowing Okabe and Kurisu to fall in love with each other. Without that, he wouldn't even be trying to save Kurisu in the first place. However, nothing says that he can't change events so that [[TrickedOutTime they're similar enough that his past self effectively does the same thing as before]].]]

For example, if you killed your grandfather in the past, you should [[RetGone never have been born]], and therefore you couldn't have traveled to the past to 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 EvilIverlord 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.

For example, if you killed your grandfather in the past, you should [[RetGone never have been born]], and therefore you couldn't have traveled to the past to 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.

* ''TabletopGame/FengShui'' goes with a belt-and-suspenders approach. Actually changing history is hard and requires taking possession of feng shui sites; doing something minor like shooting your own grandfather changes nothing. Johnny Wong will return to the present to discover that his grandmother met someone functionally identical to his old grandfather and the only difference is that his name is now Johnny Fong. If you ''do'' make the effort to make your changes stick, you can shift history so that you never existed... but accessing the Netherworld (the method of time travel) immediately locks in your personal timeline at that point, and you will never be affected by future shifts. So Johnny Wong returns to a present where he doesn't exist and never did, but he still exists because he's locked to a timeline where he existed. (The Netherworld is full of people who had one too many shifts happen to them and retreated from a world that they no longer know and that no longer knows them.)

* 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?

-->-- ''WesternAnimation/MyLittlePonyFriendshipIsMagic'' fanfic ''Fanfic/TemporalIrreguRarity''

* ''Franchise/DoctorWhoExpandedUniverse'': For the 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 Literature/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 Faction Paradox 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 Faction Paradox agent, the agent calls him "Gramps".

* ''WesternAnimation/WonderWarthog'' 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 Stone-age 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.

* 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.

*
The ''WesternAnimation/TimeWarpTrio'' episode, “Dude, Where’s My Karma?”, has Joe’s uncle, Mad Jack, go back in time to prevent their ancestor, Prince Karna getting engaged with Princess Lakshmi and ends up with Joe and Jody being faded away.

* In ComicBook/{{JSA}} ComicBook/MisterTerrific mentions having read "By His Bootstraps" and really liking it when he first meets the time-traveling Hawkgirl and Mr. Terrific II, and later in the story sends the 74th Century conqueror Black Barax packing when he claims that he used the JSA's Time Spectacles to look into the future and see that he was Barax's direct ancestor, threatening to shoot himself if Barax doesn't cease his attack and go home. After some heroics by his future counterpart briefly delay him, Terry gets the gun again and starts pulling the trigger—-Barax, unwilling to take the risk, goes home. By the way, the JSA doesn't ''have'' Time Spectacles.

* In ComicBook/{{JSA) ComicBook/MisterTerrific mentions having read "By His Bootstraps" and really liking it when he first meets the time-traveling Hawkgirl and Mr. Terrific II, and later in the story sends the 74th Century conqueror Black Barax packing when he claims that he used the JSA's Time Spectacles to look into the future and see that he was Barax's direct ancestor, threatening to shoot himself if Barax doesn't cease his attack and go home. After some heroics by his future counterpart briefly delay him, Terry gets the gun again and starts pulling the trigger—-Barax, unwilling to take the risk, goes home. By the way, the JSA doesn't ''have'' Time Spectacles.

[[folder:Anime and Manga]]

* Parodied in one of Brian Aldiss's "Three Enigma" stories. A time traveler 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 traveler as he fades out of existence.