Grandfather Paradox

"Nibblonian 1: "It's a genetic abnormality which resulted when you went back in time and performed certain actions which made you your own grandfather." Fry: "I did do the nasty in the pasty!""

- Futurama.

A type of Temporal Paradox. 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 the original time travel impossible or unnecessary.

If you kill your grandfather or destroy the time machine in the past, you should never have been born or never been able to travel into the past in the first place. If you kill the evil overlord while he's a child, you shouldn't have any reason to travel into the past to kill him, since in the changed present he was killed before he destroyed your village. So killing your grandfather causes you to not exist to kill him. Which means he survives. Which means you do exist and proceed to kill him. And so on.

This type of paradox can be ignored if the type of time travel being used involves Alternate Universes instead.

If the universe runs on Stable Time Loop, this type of paradox is impossible. Since all changes that will have been going to happen "already" happened, you can't cause a change that will negate itself.

May lead to My Own Grampa, though in this case the person killed wasn't originally your grandfather anyway.

Film

 * The plot of the first Back to The Future may be the most well known example, even though it's A. not Marty's grandfather it involves and B. he doesn't kill him, but rather accidentally . 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 Back to the Future game, Marty does encounter his paternal grandfather and affects his own existence in time.
 * Inverted in Stargate: Continuum, where Cam Mitchell winds up going back in time, and eventually (ten years down the road) keeping his Grandfather alive as a way of 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.

Literature
""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."
 * In Robert Heinlein's Time Enough for Love, 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 World War I.
 * In Door into Summer, 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 Charles Stross novella Palimpsest has a twist: killing your own grandfather is the initiation rite for the Time Police.
 * 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 Night Watch, where Vimes' mentor is murdered while he's in the past; he ends up taking over his identity, teaching his younger self everything that his mentor taught him.
 * In Johnny and The Bomb, Bigmac suggests going back in time to 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:.
 * In Michael Crichton's Timeline, one of the protagonists raises this paradox to the Corrupt Corporate Executive 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 Waves it and moves on.
 * The end of The Saga of Darren Shan explains how the story is an endless paradox because
 * Inverted in two separate ways in Rant:
 * Gregory Benford'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...
 * Discussed in Stephen King's 11/22/63.


 * 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.
 * 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:

Live Action TV
""What if I kill my grandfather?" "Are you planning to?" "No." "Well, then.""
 * 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 Faction Paradox considers him their spiritual leader, partly cause it pisses off the Time Lords.
 * We actually meet him. He's quite literally the Anthropomorphic Personification of Future Me Scares Me - he's everybody's evil future self.
 * And Faction Paradox has the entire trope as a sport for initiates. Want to get in? Kill daddy. Before you were conceived.
 * Not necessarily: conception lags sex by several hours or even days. It is completely possible for a man to have sex and then die before the resulting child is 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".
 * Martha brings this up at the start of her first trip in the TARDIS.


 * In "Last of the Time Lords", The Master constructs a Paradox Machine specifically to evade the consequences of this paradox,.
 * Oh so inverted by the Lost episode "The Variable".
 * Parodied in The X-Files 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.

Tabletop Games

 * Genius: The Transgression'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 Doctor Who 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?
 * Time and Temp uses office temps (hence the name of the game) as field agents because their unimportance minimizes the risk of accidental Grandfather Paradox. At least until their vital mission (explained using the same bland corporate-speak as any other boring day job) gives way to their selfish foibles; the worst-case scenario is to Ret-Gone all of existence, but usually they just get slapped with an Incident Report.

Video Games

 * In Metal Gear Solid 3, if you kill one of the bosses instead of just knocking him out, you get the "TIME PARADOX" Nonstandard Game Over. The reason is quite meta: you killed a boss which starred in a previous game, because 3 is a prequel to 1 and 2, which means you killed a boss in 3 which exists in 1 and 2, so with that boss dead he shouldn't exist in 1 and 2 anymore, so...
 * Narrowly averted in 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. Subverted when she ends up kicking his ass instead.
 * In one of the demo videos for the 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 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. You know what, just watch the video.
 * In Chrono Trigger, 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.
 * In of Pokémon Mystery Dungeon Explorers Of Time/Darkness

Web Comics

 * Bob and George One more reason to hate time travel! (On top of Schrodinger's Butterfly questions of whether their acts can affect the author.)

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 International Association of Time Travelers skit, which is mostly dedicated to going back in time and killing Hitler, this ends up being the fate of.

Western Animation

 * In an episode of Futurama, Fry goes back to Roswell in 1947 and accidentally gets his grandfather killed in an atomic blast while trying to avert this trope. He doesn't stop existing because he also ends up doing his grandmother, becoming his own grandfather. Or, as he put it, "I did the nasty in the pasty."
 * In The Adventures of Sonic the Hedgehog, 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.