Temporal Paradox - Television Tropes & Idioms

Toriyama follows the multiverse approach in Dragon Ball Z: Future Trunks travels back in time to change the past, although he knows that this will not affect his own past or future (since each timeline exists as a separate dimension, changes made in one time line will not affect the others). Because Cell was also traveling around, but then Trunks prevents Cell's trip, fans have theorized that there are at least five time lines: the time line chronicled by the anime and manga (Line 1), the time line Trunks-who-participates-in-the-Cell-Games (this is Future Trunks, mentioned above) originates from (Line 2), and the time line Cell-who-killed-Trunks-to-steal-the-time-machine-and-terrorized-the-main-series-time-line originates from (Line 3), plus two theoretical/implied time lines. The first implied time line is the one visited by the Trunks native to Line 3, the Trunks killed by Cell so that he could go back to Line 1. This time line would proceed identically as Line 1 up to the point of Cell's discovery; after that, all fans have been able to guess is that the androids were somehow defeated. The commonly-accepted (suggested by the Daizenshuu guidebooks) theory is that Bulma built the deactivation switch (just like she did in Line 1) but Kuririn didn't destroy it, and Trunks took it back to Line 3 to use on the androids there. (Of course, it was the very discovery of Cell that lead Trunks and Kuririn back to Dr. Gero's lab in order to get the blueprints Bulma used to build the switch... so maybe Piccolo defeated them...?) The other theoretical time line is one Cell would have visited, if the Trunks native to Line 2 didn't kill him, using the strength gained from his training in Line 1. This line would proceed identically to Line 1 up until Cell's discovery, as well, so the Trunks visiting this time line probably won't be strong enough to defeat Cell, if he comes from a time line where Cell is waiting to kill him if he kills the androids and...
The actual objective of Servant Archer/Heroic Spirit Emiya in Fate Stay Night. He tries to kill his younger self (the protagonist, Emiya Shirou) to force a contradiction within Gaia, which he hopes will cause his whole existence to be erased to keep reality from breaking from the impossibility of the event. He himself admits that this would have a very low chance of happening, considering that by meeting Archer, Shirou is already set on not becoming Heroic Spirit Emiya, so the death of a "different" Emiya Shirou shouldn't affect Heroic Spirit Emiya in the slightest.
- Besides, It's said that the Heroic Spirits are removed from the time axis and await their summonings in the Seat of Heroic Spirits. So, even in the case that Shirou actually did want to become a Heroic Spirit, as Archer is no longer bound by the rules of time, Shirou's death would not form a paradox and free him from his destiny. Archer's whole objective in UBW was both attempt this plan anyway in the off-chance that it actually succeeded, and to make sure his past self didn't have to see his ideals betray him like he did.

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The Marvel Universe has a simple solution for this in the novel trilogy Time's Arrow. There are a large—but not infinite—number of alternate universes, that deal with what ifs. If someone in those timelines goes back in time to change something, it will create a new timeline that's an offshoot of one's own from that point. No going back and killing Hitler, Cyclops notes when told this—the idea being that if you do so, your own timeline will be unaffected. Oddly, this doesn't seem to be the case in the comic universe.
- Except when it is that way. You don't think that any two comic writers actually agree on how this stuff works, do you? That said, the Earth X series (including Universe X and Paradise X) suggests a couple of different versions of this. In the end, it is fundamentally, philosophically important that the idea that alternate universes branch off only as a result of time travel is true.

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In the Back To The Future movies, Doc Brown is very concerned with paradoxes. Paradoxen. Paradoci?
- However, the effects of time travel are different in the various movies. For instance: in the first movie, Marty's accidental stopping his parents getting together was starting to delete him from existence; in the second movie, Biff interferes with his past, and the changed present has Biff saying he sent Marty to a school in Switzerland, yet this never affects the time-travelling Marty.
- Fanon has justified this in various ways; for example, saying the time traveller is only affected by his own changes to the timeline, or by saying that he will be unaffected as long as there's somewhere in the timeline for him to "slot in" - changes to his situation in the new timeline are shrugged off, as long as he exists somewhere.
- The documentaries on the DVD set mention how the justification was that there's some entity that regulates time itself. The partial deletion over time of Marty, why both Jennifers fainted when they met each other, and why even with relatively major changes to the timeline, Marty's family, home, and association with Doc Brown and Jennifer remain largely the same. They wanted to explore this aspect, but couldn't find a way to incorporate it into the films without it being obtrusive. The Other Wiki has more information here.
- While Present!Marty and Present!Doc vanished from the Present in order to travel to the Future, Future!Marty and Future!Doc still managed to keep living in the Future like they didn't time travel to the future at all. At the same time.
The Lake House is a story about a mailbox that delivers letters from Kate to Alex two years ago and vice versa. Alex dies in a car accident on Valentines Day. Two years later, when Kate realizes that, she sends a message to Alex two years ago telling him not to be there, and he survives. It should be noted that the Korean movie this movie is based on dealt with the paradox differently: The female character sends the warning back in time, but the male character remains dead. Meanwhile, the insertion of the warning splits off an alternate universe where the male character survives, and the movie ends with the male character meeting the female character, just as the female character is moving into the house, before she's even gotten the first letter. It's okay, though. The guy has quite a story to tell her. Since the movie ends there, by the way, it's unknown whether the female character would have ever started the letter-exchanging if the guy hadn't...ugh, it's all sort of vague, really.
Frequency depicts basically the same situation as The Lake House—due to abnormal sunspot activity, a police officer and his long-deceased father are able to communicate across a 30-year gulf of time over the same CB radio set. The son first saves his father from dying in a firefighting mishap, only to discover that he died of lung cancer a few years later anyway. But he then manages to convince him to quit smoking.
Donnie Darko: After sleepwalking away from the place where he was supposed to die, the titular character is caught in an unstable time loop that he must close. When he moves himself and the jet engine that should have killed him back into the past, he closes the loop by dying in the way that he should have from the beginning, negating everything in the time loop. This causes everything that was changed by his time travel to exist outside of the normal timeline without affecting it. Maybe.
In the new Star Trek film Vulcan is destroyed by a Romulan from the future with the help of Red Matter, which was developed by the Vulcan Academy. To this Spock remarks that there is only one timeline, when you travel back in time, you reshape it from this point on.
- Actually, the movie has a deliberate subversion. Spock Prime tells Jim that he can't tell the Young Spock any of what Spock Prime just told him. (Implying, but not stating, that a paradox would result). Later in the movie, Spock Prime meets Young Spock who calls his elder counterpart on it. Word Of God also explicitly affirms that the old universe still exists, we just won't be seeing anymore stories about it. This is an Alternate Universe.
The whole plot of the movie Butterfly Effect revolves around the main character's ability to travel back in time and change parts of his life. Every change causes his brain to physically rewire itself with the new memories, though, and this causes intense pain for him.

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David Weber's The Apocalypse Troll has the characters discussing the theories about time travel — one (it's not possible) has been disproved by the fact that one character just did, to arrive in the time of the discussion; the other two, that the future will be altered by what she did or that her presence has caused an alternate world to split off, can't be proved or disproved by anything they can do now. They end up assuming the alternate world and thereafter ignore the question.
The Time Scout novels avoid Temporal Paradox by the timeline including built-in safeguards; safeguards which are dangerous to time travelers. The most prominent are first, that you can't change anything that's important to the timeline—some improbable accident will occur to prevent it, no matter what you try—which is dangerous, as although some people are obviously important to the timeline, there are even more who aren't obviously important, but just as crucial; and second, that if a time traveler ever arrives at a time where they already exist, the most recent version dies instantly to prevent them from doing anything to their past selves that would undermine their current presence.
In The Drawing Of The Three, Roland kills the man who murdered Jake, who Roland met in The Gunslinger. He spends the first part of The Waste Lands fighting off insanity because of the paradox this creates.
The Care Taker Trilogy has an interesting take on this: there are no alternate universes, and while changing the future/past is possible, doing anything that would create a paradox is impossible simply because it would create a paradox. It's said that there is some natural "force" that prevents paradoxes from occuring. Exactly how that works is not explained, because the protagonist apparently doesn't have the necessary education to understand the specifics.
Time travel is forbidden in The Dresden Files because it might end up destroying the fabric of reality. Characters capable of seeing the future can't be specific about their visions for the same reason.

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In Quantum Leap, it appears that Sam is affected by the changes he makes to history only after he leaps, and this has some bearing on his occasional manifestation of previously unmentioned skills (and previously unmentioned/nonexistent family members). Al, on the other hand, seems to be affected instantly, but only when probability of a new event becomes sufficiently high. (In one episode, Sam assures Al's untimely death. When the probability reaches 100%, Al is replaced by another character, but he reappears when Sam reduces the probability.)
Doctor Who once (in "The Time Meddler") had characters speculate that if history was changed, their memories would be updated with the new version instantly — though later events imply this is not actually the case. In "Father's Day", we see that creating a true paradox (which seems to require not only a change to history which undermines the traveller's presence, but that the traveller witnesses himself doing this by being present in the same time zone twice) has the effect of releasing killer flying time monkeys, which eat everything on your planet. No, really.
- The series does tend to imply that the "Laws of Time" are more of a legal code than physical law: in "Smith and Jones", the Doctor notes that crossing one's own timeline is dangerous and forbidden, "except for cheap tricks."
  - Although there's the recurring concept of fixed events as opposed to unfixed ones - events that must happen in a specific way, as opposed to ones that could happen any way. The Doctor, of course, has the inherent ability to tell them apart. And, of course, no one else does. Usually.
- Another interesting use of the temporal paradox concept comes in "Last of the Time Lords", in which the Master brings humans back in time from the end of the universe to kill humanity... which would normally make no sense, which is why he turned the TARDIS into a "Paradox Machine" to keep the paradox stable. Destroying this acts as a Reset Button which sets everything on the surface back to the way it was at the end of the last episode.
- Also, "Blink", the episode that gave us the Timey Wimey Ball, has a paradox at its heart. The Doctor is only able to tell Sally Sparrow what's going on via DVD Easter Eggs because Sally wrote it all down at the time and gives it to him at the end of the episode.
- Overall, however, one has to give credit to Doctor Who, in that a show with a time traveler as a central character delves into temporal paradoxes relatively infrequently; in most cases, the time travelling is just a way to set stories in different periods, the temporal version of Adventure Towns.
  - It gets much weirder in the Expanded Universe, which features Faction Paradox, a group whose hat is Temporal Paradox. In fact, part of their initiation ritual involves traveling back in time and killing off your own ancestors. Yes, really.
  - At one point, they infected the Third/Fourth Doctor with Faction biodata during a regeneration that wasn't supposed to happen (when he was shot on Dust, instead of the canon radiation poisoning on Metebelis Three), causing the Eighth Doctor to disrupt his own timeline so that the Third Doctor was shot on Dust, permitting the Faction to infect him with the biodata, which caused him to tinker with the past so he could be infected with the biodata...BOOM!
In The Big Bang Theory, in one episode, Sheldon, Leonard, Raj, and Walowitz buy the original time machine prop. Events in the episode lead to Sheldon and Leonard trying to decide if Leonard could have gone back in time to stop himself from buying the time machine, leading Sheldon to say, basically, "No."
Lost: Subverted when Sayid attempted to kill Ben, which simply caused him to grow up into the man he already was.

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In the RPG Feng Shui there are no temporal paradoxes, because history rewrites itself to accommodate changes in the timeline. For instance, if Donald Wong goes back in time and kills his great-great-grandfather, when he returns to the present, he'll find that everyone now knows him as Donald Fong, a person with a very similar life to Donald Wong. He'll remember his old life as Donald Wong, but everyone else will always have known him as Donald Fong. In extreme cases - such as when someone controls enough feng shui sites to cause a critical shift (i.e. they change reality) - people can get written out of the timeline entirely; they still exist, but they have no past in the current timeline, because their version of history simply doesn't exist anymore.
The german RPG The Dark Eye takes a similar approach in declaring the time a dynamic, "healing" weave. An example to solve the grandfather paradox is to have the person get stranded in time, get a life, meet a woman, marry and have kids and thus becoming his own grandfather.

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In Achron paradoxes are a deterministic and fully logical gameplay element. So pretty much a Aversion on the illogical/indefinite portion of the trope.
The Metal Gear Solid games are well known for situations occuring in which the player can create a paradox of sorts, by killing someone in a prequel who is known to be alive in chronologically later games. Of particular note is Revolver Ocelot, in the third game, whose death during certain scenes results in an instant Nonstandard Game Over. It's especially surreal when your CO from the future starts chewing you out for causing a Time Paradox. The fact that EVA (who is never mentioned in any other game in the series AFAIK) is protected by a similar Nonstandard Game Over adds an additional layer of surreality.
- She's since appeared in 2 more. They were just giving her a paradox vaccine.
- The "CO from the future" part is actually a joke that is, for lack of a better way of saying it, lost in translation. In the original Japanese language track, the voice actor who plays this character is also the voice actor who does the Japanese dub of Doc Brown in Back To The Future.
In the Space Quest series of games, Roger Wilco is saved from certain death in Space Quest IV by a mysterious man who is later revealed to be his grown-up son from the future. Roger meets the future mother of Roger Jr. (though she doesn't know it yet) in Space Quest V and if she's killed during the course of the story, Roger Jr. and therefore Roger Sr. as well cease to exist, and it's Game Over.
- This troper is pretty sure that Roger had enough time between the end of Space Quest V and the beginning of Space Quest VI to get Beatrice pregnant. Of course, Beatrice couldn't have been considered his wife...
- That would be a Have A Nice Death, not a Non Standard Game Over.
  - However, in one way to kill off Beatrice picking her up while she's frozen and having her break into bite-sized pieces, you get a slightly different Have A Nice Death message.
One route in the game Ever17 features a time-travel attempt to set right what once went wrong with an obvious fix. Unfortunately, this also undoes the events that triggered the attempt, resulting in a very unpleasant paradox◊.
Time-travelling in miniature into your own brain can have equally unpleasant results in The Hitchhiker's Guide to the Galaxy Infocom text adventure. When the minaturisation wears off, you cause the head you're in to explode. You're fine... but you won't be when the minature you, now inside your own head, expands to full-size...
In Tales Of Phantasia, Time Travel divides the timeline around halfway through the game in such a way that the object of your characters' revenge and the final boss of the game are two different people.
- Not only that, but at the end of the game, a sword you acquired in the future is sent back to the past. The character who takes it promises to "seal it away", the concept of "ontological paradox" is apparently entirely foreign to the protagonists.
In Persona 3, the Temporal Paradox does have quite a devastating effect - the main female love interest wants to go back in time and save/see the main character before he sacrificed himself, which might bring about The End Of The World As We Know It.
The main villains of Tales of the World: Narikiri Dungeon 3 are the main characters from the future, and they are messing up the timeline to unseal the "Demon King Jababa" and defeat him before he can destroy their village. The thing is, the game implies that he got out and destroyed the village because they released him.
World Of Warcraft has a quest that plays with this. While doing a survey for the Bronze Dragonflight (keepers of time), the player is assisted by his future self. Later on, he needs to do the same thing again to protect his past self.
- A really mind bending example occurs with the key to Karazahn. The arch-mage Khadgar sends the player to retrieve the fragments to the key; however, he can't repair the key. Instead, he sends the players back in time some thirty years to his master, Medeivh, so he can repair the key for them. Medievh comments that it will take time to repair the key, so he gives them a spare key that he has on him, adding that he'll repair their key later and give it to Khadgar. So the key that he gives to Khadgar is the one that Khadgar sent the players back in time to repair and... ow.
Timeshift stays light on the plot side, but that doesn't stop the nastier issues from occurring. By the end of the game, the protagonist has been sent back in time and had his time machine damaged by a bomb that later never went off, sent back in time and had his or her time machine damaged by a giant mechanical spider that later was distracted somewhere else at the time and then ceased to exist, can accidentally lead to the death of an individual who earlier would go on to assist the protagonist, was assisted by and provided assistance to capture La Resistance who later never were captured and then were La Resistance to an entirely different matter. All to prevent a time traveling Mad Scientist from replacing 1940s Europe with his own special dictatorship which also prevented the time traveling Mad Scientist and protagonist's time machines from ever being developed. That's before we get to the Wild Mass Guessing that the protagonist survived the bombing once sans timesuit and then went back in time to try to stop the Mad Scientist from ever going back in the first place, or that the protagonist is a time-clone of one of the other characters as implied by the paradox warning triggered by attempting to remove his or her time suit. OWWW!
- It does strongly note that the Beta Suit cannot ever benefit from the Reverse Time effects, nor can you benefit other people. For example, you cannot use Reverse Time to repeatedly kill/revive someone, your ammo/grenades will never return to you, your health won't restore from bullets never hitting you, and in one case, you cannot prevent someone from being pulverized by stepping into the path of a wind tunnel. You can, however, do some suitably amazing and crazy shit, such as reversing time and shooting from two seperate locations simultaneously; once in real time, then reverse time, move to the new location, and open fire once real time kicks in again. The Beta Suit's "mission", however, is to recover the Origin Drive from Krone's Alpha Suit.
The Infocom Interactive Fiction game Trinity combines this with a Stable Time Loop: Moments before London is destroyed by a nuclear attack, you go back in time to sabotage the original Trinity test and prevent atomic weapons from ever being used.... Unfortunately, as your Spirit Advisor explains, that would change history so that you were never born, thus creating a paradox. The universe, fortunately, resolves the paradox by making a small explosion every time one of the atomic weapons that should not exist is detonated (i.e., destroying a city instead of destroying most of New Mexico as it should) and sends you back to London before the bomb is dropped to do it all over again....
Notably averted in inFAMOUS with a character who still exists despite drastically changing his own past. The idea of alternate universes is never so much as mentioned; rather, the character in question travels through time via his own superpower, with immunity from changes in the timeline granted by Required Secondary Powers.
The Ubisoft Prince Of Persia trilogy is entirely based around this concept.
- In the first game the Prince's father find the Hourglass of Time whilst invading another land's fortress, and the Prince is tricked into releasing the sand within by the evil vizier. This turns pretty everyone except the Prince, the vizier and the princess from this other land, Farah into sand monsters, leading the Prince on a quest to undo it all. He teams up with Farah, but she dies during the adventure until he manages to get to the hourglass and insert the dagger, reversing everything up to the point where they originally invaded the fortress. Waking up in camp with the dagger he visits Farah and tells he everything, though she doesn't remember now naturally, since it never happened. The vizier enters, the Prince kills him, then gives the dagger to Farah and leaves, asking her to call him "Kakalukia", a word with significance to her that she told him during the adventure together.
- Cue the second game, 7 years later. The Prince is informed by some old dude that he should have died during the first game, but cocked it up through messing round with the time continuum and is now hunted by the Dahaka, a guardian of the sands, which seeks to kill him and restore the balance. To stop this, the Prince decides to travel to the Island of Time to stop the sands ever being created by travelling back through time to kill the Empress of Time. He rescues a woman, Kaileena, and sets about trying to get to the Empress whilst dodging the Dahaka and a strange wraith-like figure seemingly out to get him. Whilst travelling to the Throne room he is confronted by both, but escapes when the Dahaka kills the wraith and buggers off. Kaileena reveals herself to be the Empress, the Prince kills her and travels back to the present, only to be confronted by the Dahaka again, since the sands turned out to have been created by the act of killing the Empress. He almost gives up hope when he discovers the mask of the Sand Wraith, which allows him to co-exist with himself in the same time-line. He then goes back in time, revealing himself to be the strange wraith-like figure, who wasn't trying to kill the prince but in fact save him. When confronting his past self with the Dahaka outside the throne room he dodges the Dahaka, allowing it to kill his old self, which reverts him back to the Prince. Confused yet? He then proceeds to confront the Empress again, but this time throw her through a portal into the present, planning to kill her here, thus still creating the sands, but not in a time frame that would allow them to be found by his father. Then the Dahaka shows up again, now trying to kill the Empress, but together they manage to defeat it and set sail for Babylon, the Prince's home.
- Which takes us to the third game. Arriving at Babylon they find the place ransacked because by retconning his past the vizier of course never died, got hold of the dagger, and proceeded to attack it, looking for the Sands of Time. Kaileena is captured, but when the Prince tries to rescue her the vizier stabs himself with the dagger, turning into a sand god or something, killing Kaileena and infecting the Prince with the sands. Princey manages to swipe the dagger though, escape, and sets about to kill the vizier again. Along the way he bumps into Farah, who had been captured way back when the vizier got the dagger, and discovers that the sands have manifested within him as the Dark prince; a seperate personality that tries to convince him to look out only for himself. He catches up with the vizier, is soundly beaten and thrown into a well, finds his father, who is dead again, and has a crisis moment where the Dark Prince tries to take over. He resists, fights the vizier again, kills him with the dagger, Kaileena appears and cleanses him of the sands, and all seems well. Then the Dark prince pulls him into his own mind, tries to screw it all up but he resists, gets rid of him too and gets the girl. Alls well that ends well. Aside from the dead father and ruined city.
The flash game chronotron revolves around the players ability to travel back to the begining of the stage (so that multiple version of the player exists at the same time). It is quite possible to either kill a past self, or bar their passage to the time machine - resulting in a time paradox "death", complete with a penrose triangle warning sign.

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Paradox is notably averted in Schlock Mercenary, where it's put forth that causality is never sacrificed. If a time traveler were to go back and kill his/her grandfather, even though another one of them wouldn't be born as a result, they've already caused the event, so they'll still exist. Changes due to time travel are likened to "overwriting" the original timeline (though it's also worth mentioning that, within the Schlockiverse, time travel is normally impossible).
- This troper wonders if the author didn't get that idea from Orson Scott Card, as this is the same idea of time travel that was set forth in Pastwatch: The Redemption of Columbus.
  - The comic itself likens it to the most common time travel power in video games: "Load Game".
Evil Katarakis of Starslip Crisis hasn't thought his brilliant plan all the way through:
Imagine... enjoying your favourite sandwich... then going back and stealing the sandwich from yourself before you eat it! So you get two sandwiches.
- Deep Time isn't exactly the best off, either: one member spent spare weekends going back in time to kill Hitler, then back again to stop himself from killing Hitler. Another spends forty years on dead-end research, then tells a third agent to stop him from wasting his life on the matter. In yet another case, accidentally blowing up a planet causes an entire Deep Time spaceship to spontaneously fall apart.
  - At one point, people in the present protect themselves against attackers from the future by identifying the future guys' ancestors, and making sure they're on all the ships that the future guys are attacking.
Red Mage accuses Chaos of having an unworkable plan because of a variant on this. Chaos intends to destroy all of space and time which would destroy all the events leading up to his being summoned to destroy all of space and time.
- Reaction: I'd better not create a temporal paradox! I'd hate it if everything were destroyed the way I wanted it to be destroyed!

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The Danny Phantom movie The Ultimate Enemy is one big temporal paradox. In the original timeline, Danny's family and friends are killed, he goes mad with grief and kills himself (people with a Split Personality can do that and survive), and his evil self terrorizes the world for ten years. Thanks to Danny and some timely interference by the Dungeon Master, this timeline was erased, but his evil self was in the past when it was erased, so he still exists even with the events that caused his existence never happening. His evil self even pointed out the paradox. "Don't you get it? I still exist. That means you still turn into me." The Observants mention something about him still being there because "he exists outside of time."
Naturally enough, the animated series of Bill And Teds Excellent Adventure often courted this trope. One obvious example is the episode in which Bill and Ted neglect to buy Bill's father an antique railroad watch as a birthday present, to replace the one he lost as a child. Ted's initial plan is actually perfectly sound: take the original watch from Bill's father when he 'loses' it in the past, then give it to him in the present. This plan fails however, so they travel even further back in time to obtain the watch before Bill's father inherits it. Of course, this should mean that Bill's father wouldn't miss the watch in the first place, but the episode simply ignores this.

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Transformers Armada, in the "Drift" episode. Starscream is blasted with the Requiem Blaster, then Highwire somehow apparently warps the kids back in time, but in an Alternate Universe, where both the Autobots and Decepticons are imprisoned and slowly being digested within Unicron. Before he expires, Hot Shot reveals that the Minicons are actually Unicron's cells, and the Transformers were being used by them. Then the kids travel further back in time to when the Minicons were created. Here they tell them to escape from Cybertron, eventually resulting in them coming to Earth and all subsequent events in the story. Then, back in the present, Perceptor stops Thrust from blasting Starscream. Therefore, the kids had to go back in time to trigger the sequence of events that led them to Cybertron and ultimately the time travel event itself.

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Link saving himself in The Legend Of Zelda: The Return.

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Another Sandra Bullock film, Premonition, mixes this trope with You Cant Fight Fate: Linda's attempts to prevent her husband's death cause it, but she does get pregnant before he dies, and prevents herself from going crazy and getting committed, which she could not have done had she not had the premonitions of the future.
Referenced in the Movie Deja vu by agent Carlin right before they send a note back in time:
Technician: It would have gone faster if you had written it [the note] yourself
Carlin: Yeah, then I recognize my own hand writing and the universe explodes.
Fathering the guy who will send you back in time counts, too.

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As Lost season 5 deals with a Stable Time Loop, this type of paradox is emerging. Kate, Sawyer, and Juliet save Ben's life, allowing Ben to grow up and turn the wheel, which causes the time travel in the first place. There may be physical examples as well: in the future, Richard gives Locke a compass. Then Locke travels to 1954 and gives it back to Richard. While it's possible Richard now has two compasses (and must later give Locke the "newer" one,) or the compass was never created.
- The other major season 5 storyline has a similar problem. Jack's goal is to set off a bomb that will prevent their plane from crashing, meaning they'll never come to the island; completely erasing everything that's happened on the show. This means Jack will never have been there to detonate it. Interestingly, it is suggested that the explosion may end up doing the opposite of what Jack wants and leads to the plane crashing.
- Happens in "Catch-22" when Desmond set out into the jungle after a parachutist on the island and made sure all of the details of his quest exactly matched his vision of the event. The events of the vision only happened because he saw the vision. Also, Daniel Faraday is sent by his mother, Eloise Hawking, to the island, even though she knew full well that she would be the one to kill him when he arrives.
In an episode of Wizards Of Waverly Place, Harper travels back in time from the future. At the end of the episode, present Harper sees a hat that future Harper is wearing. She asks where she got it, and future Harper gives present Harper the hat. But then you begin to wonder where the hat came from in the first place.

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The ultimate time paradox story is Heinlein's "'—All You Zombies—'", in which the protagonist turns out to be hisheritthey's own mother, father, mentor, the man who stole her baby, the man who dropped him off at the orphanage, and the bartender he tells her story to.
- Another Heinlein story, By His Bootstraps, takes things nearly as far. Among other hijinks, the main character gets a book from the future, which he copies into another one (the same one, when it's new?) when it's become too old and falling apart. A good way to avoid ontological paradoxes.
  - Avoid nothing, that just gets around the paradox of an aging object. The ontological paradox is still in effect as: Where did the information come from?
Averted — by the characters, no less — in Isaac Asimov's short story The Red Queen's Race. They wind up creating a Stable Time Loop instead. A scientist conducts an experiment to send modern scientific texts back in time, translated into ancient Greek. His translator, fearing a Temporal Paradox, only translates the parts that would account for the oddly anachronistic scientific advances already in our ancient history, like Hero's steam engine or the infamous Baghdad Battery.
- Of course, this is an example of Technology Levels; there's no reason a scientific genius of the time couldn't have invented those on their own with the tools of the time, and it only seems anachronistic because we expect steam power to Officially show up in the 1800s, and electric power Officially not long after that.
  - It's a little more complicated than that; many 1800s-vintage technologies like efficient, powerful steam engines really did depend on other technologies not developed until a few centuries before that time. Like the use of machine tools to work metal. Put James Watt back in 300 BC, and he'd have a hard time duplicating his engine without first duplicating the metalworking technology of his era.
In Artemis Fowl and the Time Paradox, Opal Koboi from the past travels to the present, and possesses Artemis' mother, making her appear ill. This forces present day Artemis to travel back in time to get the cure from the past Artemis. Opal then uses Artemis returning to the present to return to a few days before the present to make Artemis' mother ill in the first place. Ironically, this is all so she can aquire the secret of time travel.
Played with in the latest Thursday Next book, where they find that despite the existance of the Chronoguard, no one has actually invented time travel yet, so they assume that the technology much have been sent from the future and eventually they'll find the spot on the timeline where someone invented it to close the gap. As one character describes it, it's like they're running the technology "off of borrowed credit." This causes trouble however, when the Chronoguard begins to realize that no one in the timeline ever invented time travel. The resulting paradox causes the system to unravel and gets rid of any further possibility of Time Travel in the series (although it seems everyone in the populace has a Ripple Effect Proof Memory).
In Harry Potter and the Prisoner of Azkaban, Harry and Hermione travel back in for a number of reasons. During this time travel, Harry manages to save himself from dementors using an Expecto Patronum charm. The event is noted to have happened earlier in the book with Harry only glimpsing his mysterious saviour and thinking it looked a lot like his dad.

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In The Legend of Zelda: Ocarina of Time Link learns the Song of Storms from a man who claims that he learned it when a kid played that song seven years ago and messed up his windmill. Link then travels back in time seven years and plays the song, messing up the man's windmill.
- Now the real question is, where did the Song of Storms originate?
  - Termina's version of Flat (of the Composer brothers) composed it, as revealed in Majora's Mask, as a way to calm the spirit of his brother Sharp, who had cursed the Ikana Canyon so that his ancient masters would roam the land again (and for his own emo reasons about letting him know how wronged he felt). How did it end up on the other side of the portal to Hyrule is the actual mystery.
- To even further compound the Temporal Paradox, at least from the viewer's perspective, the background music within the windmill is the Song of Storms... even before Link learns the song.
- The Legend Of Zelda: Oracle of Ages features a few instances, notably during one of Link's interactions with the gorons:
  - In an item trading puzzle, you trade a rock briquette to a goron in exchange for a family heirloom and then trade the same heirloom to an ancestor of the previously mentioned goron so he can hand it down across generations to his descendant who trades it to you so you can trade it to his ancestor who hands it down to him who trades it to you...
The issue of paradox is averted in Second Sight. Throughout the game, it appears that John Vattic is coming across information pertaining to peoples deaths, and then projecting himself back through time to avert them. The finale reveals that the parts which Vattic thought were the "present" were actually potential futures he was seeing through precognition. So he was predicting deaths which hadn't actually happened, rather than averting deaths through time travel (which would create the paradox of why he would need to travel back in the first place.

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In the first Futurama movie, the "paradox-free time travel" isn't quite paradox-free: there remains an ontological paradox surrounding the origin of the name "Lars," as future-Fry chose that name when he realized that the injuries he sustained when Bender attempted to kill him made him Lars. From whence did the name come? No one knows.
- I do. Fry was trying to say "Ow, my larynx", but the damage to his larynx made it sound like "Lars". The original Lars must have chosen the name at that moment. Paradox, schmarado- Oh, wait. The original Lars wouldn't have existed since without a Lars to send Fry into depression he wouldn't have gone back in ti- Oh, no. I've gone cross-eyed.
In Invader Zim, an entire episode (Bad Bad Rubber Piggy) has one scene that demonstrates this perfectly: After GIR finds out that Zim intends to send a robot back to the past to destroy Dib, it leads to this classic line of dialogue:
GIR: Wait... if you destroy Dib in the past, then he won't ever be your enemy, so you won't have to send a robot back, so then he will be your enemy, so then you WILL have to send a robot BACK... (head explodes)

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Improperly invoked in Light And Dark The Adventures Of Dark Yagami, , after Blud learns that Matt survived a car crash with "Yotsuba", he decides to write Matt's name in his Death Note in thefuture to kill him in the past. This results in the past changing, with Matt dying and Yotsuba surviving. Dark claims the reason why Blud is telling him and Light this now, rather than at the point in the future when he writes the name is “Its one of those time parradoks that they have in back to the future”.

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Millennium concludes with a massive paradox barrelling its destructive way into the future whose time travel efforts caused it.
Both averted and played with in Primer. As one of the characters says, "The last revision is apparently the one that counts." We find characters gradually losing their worries about causality; they wind up going back in time to relive the events of that same week in their original place — apparently intending to do everything right this time. It appears that causing a paradox causes some kind of mild brain trauma to the time traveler involved. But then there's that other version of yourself that you drugged up and locked in the basement so you could replace him...
Déjà Vu contradicted itself on terms of Temporal Paradox. First, it is implied that anything changed in past changes the present, as Doug causes the death of his partner, that was thought caused by the ferry explosion. Later, it is implied that the past has already been changed, as the message "U CAN SAVE HER" in Claire's house was written by him, but in the end, it is contradicted, because if he prevented the explosion, he could never have been assigned to the case, and thus could never do the time travelling, and so on...
- unless like it was stated we are dealing with an alternative timeline way of time travel. the movie took place mostly in timeline B, timeline A involved him traveling back in time, and failing to stop the ferry explosion, but leaving himself clues / inspiration motivators, to let him know that he can change things. doug then travels to timeline C where he saves the day, but dies. back in timeline B nothing has changed and doug from timeline A's body is found, mabye, eventually, but they can see timeline C in their viewing machine. of course, how many times did it take doug to get the time travel right? i smell outtake reels.
French-Canadian movie based from a cult tv show Dans une galaxie près de chez vous 2 featured a spatio-dimensional rip (shaped like a zipper) who goes to present Earth. The Capitain was able to chuck down a DVD with their plea (NOT to destroy the ozone layer) recorded on it. It backfired when the video got featured on You Tube and ridiculed as "Star Wars Twit" (Being bad at pronounciation dosen't help). Nevertheless, it might have pushed a younger vesion of the Capitain to go into space, directly and indirectly setting the events of the show into place.
- The previous troper seeming slightly unclear in his dialogue, This Troper would like to straighten the whole thing up: "Dans Une Galaxie Près de Chez Vous" (Litterally, In a Galaxy Near You), is a parody of the Sci-Fi genre. Set in 2034, it follows the quest of the space ship "Romano Fafard", on a mission to find a new planet, to move the inhabitants of Earth, which has been ravaged by years of pollution. In the second movie based on the series, it becomes evident that there is no good planet. However, the ship discovers a rip through the fabric of time. (comically closed by a giant zipper) Unable to pass through it, they send to earth a video explaining the tragic fate of earth, and encourage them to avoid such a fate. However, the seeming absurdity of the claims, as well as the captain's slight speech impediment, makes the video pass for a joke, and it ends up ridiculed on Youtube. However, the epilogue to the movie shows past versions of Bob and Flavien, respectively the pilot and radar operator of the ship, who appear to take it relatively seriously.
The recent Star Trek movie casually plays with both types.

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In Strange Attractors by William Sleator, almost any time travel to the past causes instability in the universe. As those instabilities add up, the entire universe can "go chaotic", essentially becoming a huge mass of paradoxes. The only noticeable effect of this is that electrical lighting flickers.
- That's nothing. The timeline in that series is so fragile you can cause paradoxes by going so much as five minutes into the *future*.
In Ted Chiang's short story The Merchant and the Alchemist's Gate, the titular gate can transport anyone exactly twenty years into the future, or twenty years back. This leads to increasingly more improbable shenanigans, starting with a Stable Time Loop involving a treasure map, and reaching its arguable peak when a character's wife meets her husband's younger self in the past, takes him to the bedroom, and upon descovering his lack of the, er, skills that the husband has in the present, teaches him how to please a woman, over the course of weeks. It's also implied that the husband married her in the present because, when he saw her, she reminded him of the middle-aged woman who took his virginity. This troper learned that in certain circumstances, adultery is quite justifiable.
In L. Sprague de Camp's short story A Gun for Dinosaur, four characters (two hunters and two guides) travel to the Cretaceous period for a dinosaur-hunting safari. One of the hunters, Holtzinger, is killed by a tyrannosaur, and the other, James, is blamed for his death because he recklessly fired the shots that woke the dinosaur up. Later, James, swearing revenge, tries to go back to just before the expedition arrived so he can kill the guides once they emerge from the time machine. Instead, the space-time continuum snaps him back to the present to prevent a paradox, killing him messily.

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In the Star Trek universe, time travelers (and the writers) are generally immune to the effects of changes they make to the timeline, and can therefore find themselves in an Alternate Universe where they should not exist (as in "The City on the Edge of Forever" (TOS), "Yesterday's Enterprise" (TNG), or "Cold Front" (Enterprise)).
Star Trek Deep Space Nine has Chief Miles O'Brien going forward in time a few hours and then, when he feels he's about to die, sends his future self in the past to take his place and prevent the disaster.
Chief Miles O'Brien and Chief Miles O'Brien: I hate temporal mechanics.
- One way this could work is that the original Present Miles was from and the Past he goes to have become Alternate Universes. The "first" universe, what was called the Present, Miles would never return to. However, the show just continues in the changed Past universe.
- And in ""Trials and Tribbleations":
  Lucsly: So you're not contending it was a pre-destination paradox?
  Dulmur: A time loop? That you were meant to go into the past?
  Sisko: Um... no.
  Dulmur: Good.
  Lucsly: We hate those.
When, in the Red Dwarf episode "Tikka To Ride", Lister attempts to explain the temporal paradox of the last season finale, the camera explodes. Repeatedly.

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In the Time Travel RPG Continuum, if a time traveler creates a paradox, they accumulate "frag," and if they accumulate too much, it eventually causes them to unravel. What's more, unchecked temporal paradoxes will eventually lead to the unraveling of reality itself. Much of the game centers around the players, who are part of "The Continuum" trying to fix paradoxes deliberately created by time travelers (known as "narcissists") who don't believe the official line on paradoxes, and who want to mess with the timeline for their own personal gain.
Time Travel is rare in Warhammer 40000, but the Warp does strange things sometimes. Take, for example, the waaagh of one Ork Warboss traveling back through time via warp-storm, meeting up with his army's past self, then attacking it so he could have two copies of his favorite gun. The remaining Orks were so confused by the result that the waaagh was called off.)
Averted in World Of Darkness fanline Genius The Transgression. As the game puts it, it turns out the universe doesn't particularly care if your grandmother gets shot and there's no shooter — barring external intervention, you pop out of existence if you pull the trigger and the bullet hits home. This can have some interesting consequences, as the angry young lad seeking to avert a massacre in his country's history did not discover…

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Chrono Trigger, just Chrono Trigger.
- It is implied ingame, by the main characters no less, that there was an 'entity' or outside force making them travel through time. This means it's less of a headache, if someone with UBER POWER is driving the time bus.
- The DS remake manages to make it even worse. There's now a sidequest that involves combining an object with itself. Causality is Crono's bitch.
- Chrono Cross. Like Chrono Trigger, Only More So.
Talesof Phantasia and it's sequel Narakiri Dungeon 1 are even more confusing.
Time Hollow for the DS subverts these, for the most part, by having a few people remember all alterations - as such, you can't hit the "prevent myself from adjusting time" snag by fixing the thing you wanted to change. There is, however, of all things, a cat that ends up in a near-ontological paradox avoided only by the fact that it's locked in time and cannot age.
Super Robot Wars Reversal's plot. In the future, the world is turned into a Crapsack World. Chance encounter with Duminuss causes Raul/Fiona to be thrown back to the past, before the world goes gloomy. At that point, he/she decides to screw the bad future and make paradoxes here and there, incidentally making the future brighter when they come back in their own time.
Ecco the Dolphin's story consisted of Ecco seeing the Asterite, who tells him to go back in time and recover his missing globe from the Asterite of the past. That means that in the past, a Dolphin (Ecco) stole the Asterite's globe, and only because of a request from the future.
As part of its recurring themes of You Cant Fight Fate and Screw Destiny, the Legacy Of Kain series establishes that time travel always creates a Stable Time Loop, where if an event in history is changed, time itself immediately rediscovers the path of least resistance and continues along it, like a river flowing around a stone. Truly dramatic time paradoxes can only occur when the past and present forms of the Soul Reaver blade appear together, such as when Kain travels back in time, Soul Reaver in hand, to murder King William the Just, who would later become the nefarious Nemesis, also armed with the Soul Reaver.
- It gets very interesting when you note that there is only ONE Soul Reaver that exists, and yet at one point in the series, no less than ~~THREE~~ FOUR Soul Reavers were within ten feet of one another. Space was literally WARPING around them, pulsating, throbbing, and could possibly strain the player's eyes if stared at for long enough.
  - Speaking of four Soul Reavers, by the end of Defiance, one SWORD Soul Reaver, and TWO SPECTRAL Soul Reavers are all joined together thus marking the first occourence of WEAPONIZED Paradox. This Troper wants one too.

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Super Stupor's Clockstopper can change history with his "Time Punch". (And he'd rather be surfing TVTropes than fighting crime.)
Well, I'm confused. How is a flashback to the childhoods of the Cheer! girls even possible? Weren't they, you know, boys? Just how much of the past did Miranda rewrite to cover up Anne's mistakes? Is it like what happens when a misfile occurs? Argh...maybe it's best to pretend this isn't canon, especially seeing as there are lots of people who still remember.
In the Surreptitious Machinations arc of General Protection Fault, Empress Trudy travels back in time to give her younger self the necessary information on what she must do to take over the world. Near the end of the arc, Nick and Ki's son Todd reveals that the entire Bad Future he and Empress Trudy came from was the product of a temporal paradox, since it could not have happened without Empress Trudy advising her younger self, which would not be possible if it did not previously exist. It is heavily implied that Pandemonium was responsible for the existence of the alternate future in the first place. As a result of the future being changed, Todd, the Empress and all other objects from the alternate future fade from existence, but the Empress teleports to a different time just before she fades, and the Gamester finds and recruits Todd.

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Using the Time Monkey Idol in the Kim Possible episode A Sitch in Time The Supreme One managed to cause the very events that led her to take over the Time Monkey Idol in the first place, or something like that.
- Shego told herself to grab the Time Monkey. The original way Shego came to own the Time Monkey is not shown. It's quite possible that Shego grabbed the Time Monkey in the first place without being asked to by future Shego, but that future Shego simply wanted to make sure present Shego did. Future Shego also may have passed on some tips, which may have meant the Future Shego wanted Present Shego to do a better job than what she did first time around. or something like that.
  - Or not. It's Future!Shego (and not the future Present!Shego) that is The Supreme One in the future, as revealed by her Evil Gloating. In fact, Present!Shego dropped from the timeline. Future!Shego wouldn't even need to advice Present!Shego to take the Time Monkey in the first, making the movie suffer from MST 3 K Mantra.
  - Simple, really (well, as much as it gets with time travel): The Supreme One tells Shego to take the Time Monkey while everyone's watching Kim fight the stone monkey. The other villains are arrested and Shego slips away, then travels back and forth to enact her evil plan until she is The Supreme One. Then she goes back to herself what to do.
  - You're missing the point. Future Shego sent Ron's mom to Norway, getting Ron and Kim seperated and "out of sync", which is the only reason supervillain treo (and Drakken) were able to grab the time monkey. Shego even says that's her reason for doing that, and it worked. If not for Future Shego's meddling, Kim Possible would have stopped present Shego from getting the time monkey and thus couldn't send Ron across the globe, and thus wouldn't have defeated KP to get the monkey, and thus couldn't send Ron across the globe—(Stack Overflow Exception) Thus, a stable time loop, predestination paradox.
The Fairly Oddparents special, "The Secret Origin of Denzel Crocker". Timmy goes back in time to find out why Crocker was so miserable and to try to fix it. He finds out that as a child, Crocker himself had fairy godparents—and that they were Cosmo and Wanda, something that they don't remember—and figures out that he must've done something to lose his fairies. He tries to warn the young Crocker, but inadvertently ends up being the one who reveals the secret (with some help from both '70s Cosmo and modern Cosmo's stupidity). Furthermore, as Jorgen shows up to erase everyone's memories of there being fairies, young Crocker manages to get his hands on the DNA tracker that AJ had built so that they'd know when Crocker was around, and managed to get Cosmo's DNA to use in it, and managed to covertly write a memo on the back of it that fairy godparents exist without Jorgen noticing, allowing him to keep that knowledge after his memory of fairies was erased...which means that if Timmy had never interfered, Crocker would be neither miserable nor fairy-obsessed. However, whereas when Timmy left for the past, Crocker was using a very primitive and likely useless "fairy finder", the Crocker in the present that Timmy returned to was using the tracker that AJ had built, implying that he had created an alternate timeline, and leaving one to wonder what happened in the original timeline. Of course, considering it's explcicitly stated in The Movie that few kids keep their fairies past their first year, much less until adulthood when they would leave anyway, we can guess...
In Justice League, Lord Chronos was at first a meek scientist who invented time travel. He used it to steal things from history that would not affect the timeline. Then his wife nagged him about his lack of imagination, and one trip to the Wild West later he's decided that stealing the most famous items from history and setting himself up as master of space and time is the better way to go. Reality itself falls apart, so he decides to go to the beginning of time and do it all over again. Batman and Green Lantern manage to reset history.

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