You Already Changed The Past aka: You Cannot Change The Future
"Things have their shape in time, not space alone. Some marble blocks have statues within them, embedded in their future... Any moment now, Janey's watchband will break. Somewhere, the fat man is already lumbering toward the shooting gallery, steps heavy with unwitting destiny."
You go back in time to Set Right What Once Went Wrong, only to discover that the "changes" you're making to the past were what "already" happened anyway. In other words, there was no "first time around" - the past only happened once, there were no different "versions" of it, and the changes you made to the past ultimately created the very past you read about in the history books before leaving on the trip.
It's like being Time's own personal Unwitting Pawn.
This does not necessarily mean that You Can't Fight Fate. For example, if Bob wanted to go back in time to stop Alice's death, he could simply convince his past self that Alice still died in the future. Following this logic, Alice never dies at all — and Bob suddenly remembers how several months ago, some "other" Bob came up to him insisting that Alice was going to die of something and the two of them had to go save her, which they did, so she's still very much alive and well all along. (Do you have a headache yet?) Or to avoid the headache and ensuing paradox, Future Bob could go back and save Alice in such a way that Past Bob still thinks that she died.
Needless to add, grammar can sometimes become thoroughly useless at trying to put the point across, as all sense of tense gets thrown of the window. This trope is easier to observe rather than analyze.
This trope arguably makes the most sense when considering time travel from a scientific point of view, see the Novikov self-consistency principle.note Novikov's self-consistency principle was named "the Law of Conservation of History" by Larry Niven in his short piece "The Theory and Practice of Time Travel," published at least ten years prior to Novikov's work. Of course, Larry Niven is a Science Fiction writer, which may explain why nobody cares. Alternatively, this is an illustration of something called Stigler's Law of Eponymy: nothing ever gets named after the first person to discover it. However, the number of time-travel plots that it allows for are extremely limited and the logic gets complicated very quickly. This, however, also has the side-effect of creating a 'self-correcting universe' usually by a slew of Contrived Coincidences (ie. if you try to shoot your grandfather the gun will jam; if you try poisoning him he will recover; if you try strangling him you will be overcome; if you wear Power Armor from the future you will have second thoughts; if you try sending a bomb back through time and detonating it directly inside his chest the time machine will break down). This can also lead to a scenario where the only reason why the past is not changed is because someone else says 'you cannot' and you take his advice. Meaning the advice itself is a part of the universe's self-correcting nature.
Thus, most time travel stories that involve altering the past will provide some of the characters with Ripple-Effect-Proof Memory. This makes less sense, but it makes for a narrative convenience. If a You Already Changed The Past plot is used, the time travel will probably be a one-off thing, since repeating it would most likely get tedious.
The Ancient Greeks and Vikingsloved the notion that You Cannot Change The Future, and their works heavily imply that they believed in this specific notion of time (which even the Gods were trapped in). Although they used predictions rather than time travel, the effect is the same. Many first-time readers of the classics who don't buy into this notion of time, or don't realize this is whyYou Can't Fight Fate in the classics, have a hard time accepting The Fatalist behavior of classical Greek and European heroes.
See also Self-Fulfilling Prophecy. Compare Retroactive Preparation, where having changed the past already works to your favor. Related to Stable Time Loop where you go back in time, because you already changed the past.
In one, Lum goes into the future where she brings back Ataru's diary. He reads it and believes things will go right for him, but attempting to cause them makes everything go horribly wrong. It's later found that when writing the entry about everything that went wrong, his tears blur the ink, causing it to look like he wrote about things going well.
In the other, the cast goes back in time to prevent Mendo from getting claustrophobia and nyctophobia. As a result, young Mendo pisses off the modern Mendo, causing him to attack his younger self. While hiding from his older self, young Mendo was trapped in a dark jar, causing him to grow deathly afraid of dark and tight spaces.
Tsubasa Reservoir Chronicle. Details would be massive spoilers, but suffice to say that time travels differently in different universes, and something the heroes do midway through in a world that later turns out to be their own past sets up the very premise of the story, as revealed in the finale.
Also, one character who pulls a Face-Heel Turn halfway through is fated to pull a Heel-Face Turn back, given that his future self, who actually reincarnates in the past is the protagonist's father. Yeah...see, all grammar is useless. In fact, depending on which angle you see it from, the whole story wouldn't have happened if the past had not already included the influence of the future.
However, the first instance of tinkering with time that we knew of was not an example of this - the group visits the world of Shara twice, before and after visiting Shura which turns out to be the past of Shara; and the effects of their actions are quite visible. CLAMP seems to have lost track of their time-travelling system as the Mind Screw got more and more complicated...
In Rave Master, after much time is spent freaking out over what horrible ways they've twisted the past, Sieg, Elie, and Haru (but mostly Sieg) discover that all their actions caused the future they were trying to protect by not taking those actions. Haru made it very clear to the knight that the criminals he brought had invaded the castle ten days earlier, and that the knight was to take credit for catching them, which we see him talking about at the time Haru gave 50 years later. Getting Resha kidnapped enforced the king's decesion to have her fake her own death, leading her into the future where she get's amnesia and meets Haru, and ditching Sieg in the past leads to him being there to set the whole time loop up and make sure they mess with the past like they're supposed to.
Steins;Gate: Understanding this concept is what allows Okabe (with some help from his "future" self) to turn the constant stream of Downer Endings into something much more pleasant. His early episodes boast about being "able to cheat the universe itself" doesn't look so silly anymore by the end.
That said, the series doesn't follow this trope entirely with regards to time travel; it's more like a combination of For Want of a Nail and Rubber-Band History most of the time. Sending messages to the past can have fairly far-reaching implications (such as changing the actual biological sex of a person), and the butterfly effect is explicitly called out by name. However, some events have greater inevitability than others (such as Mayuri's death in the beta world line by a series of increasingly Contrived Coincidences).
A Blade series had Doctor Doom lure the Daywalker to his castle, where Doom then proposed Blade with going back in time and saving his mother from a vampire attack. Blade asked him why he should do it, and Doom replies with "Because I've already seen you do it in the past." Doom is nice enough to give him a serum which would suppress his bloodthirst though.
An issue in The Mighty Thor series had an storyline where Loki sends himself back through time with the aid of Hela to accomplish certain tasks that had already been mentioned in a previous issue, but with certain details left unclear. Turns out that Loki was responsible for many of the major events in Asgardian history, but it's left unclear whether they still would have happened had he never gone back in time. Even he isn't completely sure.
He lampshaded this trope, saying that he cannot change a past and make future comes different way, but he can make sure it will go a way it did.
The Terminator gives a rare example of the good guys directly benefiting from the immutability of time. The machines sent back a Terminator to kill Sarah Connor before her son John Connor was born, in response, the rebels send back... the guy who becomes John's father.
Also, in a deleted scene, it turns out that Cyberdyne, the company that built Sky Net and the original Terminators, acquired the remains of the Terminator. The sequel shows that they'd begun reverse engineering the Terminator, which would presumably have led to the creation of the Terminators had the events of the sequel not occurred, so it happened on both sides.
In the reboot movie Star Trek, 200-something years in the future, Spock fails to save Romulus because he doesn't get there in time, and he and Nero and his crew are sent back to the present. Nero, in revenge, destroys planet Vulcan, which Spock Prime tells Kirk was because of his failure to save Romulus. So, Spock (presumably) now knows his failure to save Romulus in the future was what caused the destruction of Vulcan in the present, so, when the time comes, he can simply leave for Romulus earlier, to make sure he gets there in time, thus averting the destruction of both planets. But, if he averts these events, there's no way for him to know he should leave for Romulus earlier, and-oh no, I've gone cross-eyed.
This movie runs on the parallel-universe model of time travel. Romulus is destroyed in the original Star Trek universe where all the previous series and movies took place. The time travel causes a new universe to branch off, in which Vulcan is destroyed but Romulus presumably won't be. If someone from the reboot universe goes back in time to before Vulcan's destruction to tell Spock to leave earlier to save Romulus, then both planets will be saved in the resulting universe.
But the problem with that theory is that Nero arrived in the TOS time line twenty-something years before the destruction of Vulcan. He destroyed Kirk's father's ship, remember? The ball was in motion before Kirk was even born, and Spock would presumably have been at least in his youth by then.
Nero came from the Prime universe, where Romulus had been destroyed. In the rebooted universe, if Romulus is in fact saved, the reboot universe version of Nero will have had no reason to go back in time. If someone then goes back in time from beyond that point, while making sure that Romulus is still saved, then they'd find that Nero never came from the future because that future was averted in the timeline they came from. Or else the re-rebooted universe becomes separated from the rebooted universe with the arrival of the person that tries to save Vulcan, just like Nero came back from the Prime universe to the rebooted universe in time to face the USS Kelvin. So, there would be three time branches: the Prime Universe (Original Star Trek), the Rebooted Universe (Star Trek 2009, diverging from the encounter of the Narada with USS Kelvin, where Vulcan is destroyed but Romulus is saved) and the Re-rebooted Universe (diverging from the unknown person arrives from the future to some point between the USS Kelvin's destruction and the destruction of Vulcan, and that person manages to save Vulcan and tell Spock to arrive to Romulus earlier).
This gets covered regarding the movie Happy Accidents very well here.
This is a central theme of brilliant Spainish Mystery thriller Time Crimes.
In Déjà Vu, the first few attempts at actually changing the past just end up causing things the characters and audience have already seen happen. Eventually, for the sake of having a happy ending, they do manage to make a change that works.
This could be a case of subversion, as it was mentioned in passing during the course of the movie that a big enough change could change the future (i.e., not having the ferry blow up). As the Other Wiki has a diagram showing at least four runs◊ of the timeline are needed to explain how the events of the movie are possible, perhaps several trips of smaller changes adds up to one big enough change.
An interesting case is the movie Paycheck. What happens to the protagonist (he is administered a procedure which would erase all of his memories from the coming two years; when he is finished, he's told these two years already happened) would be a perfect example of this trope. Only there's no time travel (though the plot revolves around a future-seeing machine).
In the 2007 film Premonition, Sandra Bullock lives the week of her husband's death out of order. She's unsuccessful in her attempts to save him, as on the last day she accidentally causes his death by preventing another one.
Played with in Back to the Future, where Marty goes back in time and introduces 1985 concepts to 1955, but the movie implies that he only changes the source of the original idea without actually altering their progression into the modern day. He didn't invent skateboards but he introduced skateboarding to Hill Valley earlier than would have caught on naturally, and he didn't write Johnny B. Goode, but hearing his guitar solo inspired Chuck Berry over the phone. Since these things don't actually change the future, it looks like they were always that way.
His comment also inspires the busboy Goldie Wilson to go into politics, although Wilson is still mayor in the original timeline, meaning he would have found inspiration elsewhere.
One of the signs that Bill and Ted are clever if not book smart is their recognition of this trope; they realize that to solve a problem in the present, they can use their time machine to plant helpful items in the past, and then they'll be there for their present selves to discover - and they keep reminding each other "Once this is over, we have to go back and place all that stuff!"
In fact, the entire climax of the second movie is B&T and the Big Bad performing dueling versions of this. Except that, as Bill points out, only the winner can change history, so all the things the villain thought he planted were just decoys B&T placed to lull him into a false sense of security. In fact, as dumb as Bill and Ted usually are, they tend to be very smart when it comes to this trick.
There Will Be Time by Poul Anderson. A substantial number of humans have had the innate ability to Time Travel since before recorded history (possibly because it was inserted into the genome by future travelers). So little of human history is known exactly, and the book's scope is so great (from Jesus' crucifixion to a far-future postapocalyptic revival of civilization—at least) that the inability to change the past comes up only rarely—but the protagonist is nearly broken when his Byzantine wife dies of an illness because other travellers have abducted him to the future.
It is established fairly early in the story that it is impossible to change anything that the hero knows about what will happen. Every attempt he makes to save his father (who died in WW-II) is prevented in some way.
Eoin Colfer's Artemis Fowl Time Paradox The matter is discussed before they actually Time Travel and Artemis presumes that whatever happened in the past cannot be changed. It turns out he's right. It also lets a huge variety of crazy actions take place.
Strangely, the previous book in the series, The Lost Colony, completely averts this trope in favor of shooting hoops with the Timey-Wimey Ball. Artemis manages to bring multiple people Back from the Dead by firing a stun-gun into the recent past.
Diana Gabaldon's Outlander series runs into this a lot.
His short —All You Zombies— involves a time agent making sure he completes the correct steps to finish the changes he remembers happening earlier in his life.
And in his "By His Bootstraps", the protagonist is introduced to time travel by a man form the future, and shortly finds himself meeting himself twice, and each self gets trapped into saying and doing the same things he saw and heard said before. Eventually, he gets the drop on the man who introduced him to time travel by traveling into that man's past 10 years, only to find out he's waiting for himself.
Technically they were only trying to figure out where he'd been buried. The protagonist's mother happened to be a bit Genre Blind though, and tried to save him.
The Dandelion Girl, a short story by Robert F. Young; the trope specifically applies to Julie. In the end, it is revealed that Anne and Julie are actually one and the same person; Anne/Julie's real name is probably Julianne. And so Mark has always been married to the same girl-from-the-future all along, as "Julie" had traveled further back in time to meet him in his 20's.
In Dragonflight, F'nor returns from the past to warn his friends that an expedition they're planning is going to fail. Unfortunately they now know that if they don't go it'll create a paradox because the guy who warned them won't come back in time to warn them... so his warning has exactly the opposite effect. Knowing they're going to fail they have to set out anyway.
And of course we have the situation where there are too few riders in the present due to many of them having suddenly disappeared in the past. So someone travels into the past to gather some more, thus causing the shortage. (Although it's not as futile as it sounds because their numbers would have declined anyway due to a coming period of long inactivity. This way their disappearance is useful.)
This is without even mentioning events from a more recent novel in which a Gold somehow randomly jumps to a few centuries in the past after being given a BAD mixture of gene-altering medicines in an effort to cure a plague running through the dragons themselves, which results in said Gold crash-landing in a time when one of the last trained geneticists is still alive, thereby resulting in the creation of aforementioned medicines when they would not have otherwise been made, but had already been made anyway because in the past the sick dragon had already crashed....good god my head hurts.
And even THAT ties back to the original trilogy of books by establishing, at long last, just what the 'Ancient-timers' room was made for, and what the colorful diagrams REALLY were.
Also noteworthy is the climax of All the Weyrs of Pern, where the AIVAS reveals to Jaxom that two of the three antimatter charges used to divert the Red Star from its orbit have to be placed in the past in order to have the proper effect, and that those past explosions are what caused the so-called Long Intervals in which Thread did not fall. Of course, if those hadn't occurred, none of the events of those books would have occurred either, including the discovery of the AIVAS itself.
He encounters the brand-new original manuscript of a poem he'd studied in his own century, and wonders how it would pick up the stains he'd seen on it in his own time. A poet he recently met then walks in carrying some food, puts it down, and picks up the manuscript with his greasy hands to look it over.
He encounters a 17th century book with an inscription in it that shakes him up. He later travels accidentally to that century, and on encountering the then-new book, writes the pig-Latin inscription addressed to himself that he would read in the future.
The climax of Harry Potter and the Prisoner of Azkaban. They go to the past to save Buckbeak and Sirius, and Harry wants to see a mysterious figure that he believes to be his father. Buckbeak never died; the thumping sound was the executioner taking his frustration out on a fence (pumpkin in the movie). The mysterious figure was Harry from the future saving himself, his dad really is dead. Then they save Sirius, who rides off with Buckbeak. Plus they hear a couple strange noises, which turns out to be their time traveling selves.
To Say Nothing of the Dog involves time traveling historians (which first appeared in her Doomsday Book) who spend a lot of effort to repair the "incongruity" caused when one of them inadvertently brings a cat forward from Victorian England (they're extinct in 2057). This involves trying to make sure that the cat's owner winds up with the "Mr. C" that her diary specifies after they've accidentally introduced her to a different man. It turns out that all perceived incongruities are the continuum's self-correcting system.
Blackout and All Clear have a similar example. Some historians go back to WWII era, then find that they can't get home. They agonize over every little thing they do, worried that the slightest change might cause the Germans to win the war. It turns out that the things they did, the people they saved, and so on, were exactly the tipping points to let ENGLAND win the war. Their future, in which the third reich fell, predicates on them getting stuck in the past and doing the things they're convinced will ruin everything.
Used extensively in Suzumiya Haruhi this seems to be the whole purpose of future(er) Asahina. Who is suspected to be the superior of Present(or rather not-so-future) Asahina, and puts her younger self trough all the missions and trouble she already went trough herself. So she already changed the past because she will order herself to go to the past and change it so she can get to the future and order herself to change the past.
Minor example in So You Want To Be A Wizard by Diane Duane: Nita and Kit are stopped for a moment on their way to a world gate by a loud bang on the other side of a door they are about to open. It turns out at the end of the book that it was Nita herself, coming back from the future a little earlier than planned and trying to avoid meeting their younger selves.
This was true in the novel The Time Traveler's Wife by Audrey Niffenegger. The main character was constantly going into both the past and future, but everything was pre-set. Everything he did when he went into the past, he had "already done", and once something happened, he could never change it; in situations where he already knew what was going to happen, he had to act in the way he had already acted, he didn't have any choice.
Then again, Ford and Arthur's presence in the past is almost completely negligible. For example, they simply show up in the alien ship. It was already supposed to crash on Earth and they had nothing to do with that.
Isaac Asimov's short story "The Red Queen's Race" has a character who tries to make this trope happen. He was asked to translate several modern books on physics into ancient Greek, with the work being beamed back into humanity's past. History fails to change because the translator was very careful to leave out most of the advanced material.
Specifically, the translator only includes information which would account for discoveries and advances already present in our own time line.
Inverted in The End Of Eternity. Despite changing history themselves all the time, the Eternals are certain that the existence of their organization is guaranteed by this trope—how can they eliminate their own existence? The exception, an Eternal who is certain that time loops are intrinsically unstable, turns out to be correct, and Eternity eliminates itself.
Unborn Tomorrow, a short story by Dallas McCord Reynolds. A wealthy man wants a private eye to locate a time traveler from the future and get the secret of eternal life. He believes such time travellers would go to the Oktoberfest, where everyone would be too drunk to notice anything strange about them. The secretary is surprised when her boss curtly turns down this chance to get drunk on someone else's money. The private eye explains that he's already taken the assignment three times, and each time the time travelers sent him back to this point in the time line, with a massive hangover from drinking too much German beer. There's no way he's getting another hangover piled on top of the previous three, not for any amount of money!
The Skull by Philip K. Dick. An assassin is sent back in time to kill the founder of a subversive religion before he gives a famous speech, only to realize that the Founder is himself — the 'miracle' that inspired the religion's creation was him appearing after he'd been killed (he'd arrived at the wrong point in time) thus 'coming back from the dead'. The Rousing Speech supposedly given by the Founder never actually happened, but was a result of history being embellished after his death.
An interesting example in Pastwatch: The Redemption of Christopher Columbus by Orson Scott Card. The book takes place in an technologically-advanced, but dying, Earth and the protagonists are trying to find at which point in history they need to change events. While researching Christopher Columbus, they find that a vision he wrote about in his diaries was actually a hologrammatic projection sent from a parallel future to their own.
Happens quite a lot in Count and Countess, in which the two eponymous characters exchange letters with each other despite living more than a hundred years apart. Notably, Elizabeth, living in the 1500s, knows that her ancestor Matyas Hunyadi (in the 1400s) held the throne of Hungary for a very long time. In an attempt to save Vlad Dracula's life, she warns him not to try to make a grab for the throne, or he will probably be killed. As a result, Vlad stays as far away from Hunyadi as possible. Which gives Hunyadi plenty of time to rouse the Black Forces against Vlad and stop him in his tracks.
Stanislaw Lem pumped this to the max by time travellers creating the whole world from the physical constants up. What started as an attempt to make things better for everyone ended with our reality because of bureaucracy, competition, attempts of personal gain, human mistakes etc.
Though it seems SG-1 held to the "Alternate timelines/universes" first. The 20th episode of Season 1 had the "Quantum Mirror" which put Daniel Jackson in an alternate timeline/universe. "1969" was the 21st episode of Season 2.
Stargate Continuum shows the present universe being erased by Baal's actions in the past. As a part of the SG-1 team consciously try to outrun the phenomenon, the stargate wormhole somehow shields them from it. So, while there are alternate realities in the Stargate Verse, those may be unrelated to time travel. Either that, or the writers just can't decide.
In the Murder Most Horrid episode "A Determined Woman", a female scientist working on a time machine becomes so frustrated with her idiot husband's antics that she kills him. Several years later she is released from prison, finishes her time machine and goes back to try and save her husband, only to find that his confusion between the two versions of her is what caused his erratic behavior in the first place.
After Hobgoblins, a film, was shown on Mystery Science Theater 3000, Tom Servo tried to go back in time to stop the movie from being made by hunting down the director and... kicking him in the shin. Upon Tom's return to his present, Crow pulls up an article where the director claimed that his inspiration for Hobgoblins was that time when a squat red robot ran up to him out of the blue and kicked him in the shin...
In the Hercules: The Legendary Journeys two part adventure "Armageddon Now", Callisto goes back in time to prevent who she thinks was Xena (because her army was in the village) from killing her parents. While trying to protect her family from Xena's army, the adult Callisto accidentally kills her own father & mother.
Correction: she only accidentally kills her father. Apparently, she likes to throw knives at anyone behind her without checking first. Only after a What Have I Done moment (and, yes, she forgets that she's a goddess), she realizes she can't change anything and incinerates her mother.
In the series 4 episode "The Fires of Pompeii", The Doctor doesn't want to avert the destruction of Pompeii, is convinced to avert it anyway, and then is forced to cause the disaster in order to avert a larger catastrophe.
Blink: "You're reading aloud from a transcript of a conversation you're still having?"
The rift in space time that already exists in Cardiff is created by the Doctor's actions in "The Unquiet Dead".
The TARDIS explosion at the end of Series 5 was successfully prevented (in a fairly static timeline, probably) with the help of River Song, who wouldn't exist until a series of events that could only have happened after (?) it. Adding to the temporal weirdness which comes with this is the fact that she was around to see a crack in "Flesh and Stone", even though, from her perspective, the cracks never existed (or maybe they did, but she had just played a direct role in making sure they were completely erased from the universe). Every encounter with River can basically be summed up as "an inexplicably Stable Time Loop".
Inverted in Farscape. Sent back in time and desperate to keep things the way they were, the crew screws up, with each attempt to force what they know to be history resulting in the present time line getting worse. In the end, rather than the noncombatant survivors of the battle being spared in a hasty but well-regarded treaty, they are butchered by enraged enemy soldiers. The memorial to peace becomes a lament of the senseless slaughter.
The only time-travel arc on Babylon 5 involves this trope, and it is absolutely central to both the Myth Arc and the background mythology of the show. Babylon 4 appears in Babylon 5 space four years after it disappears (the episode "Babylon Squared." The events leading up to that appearance are explained in the two-parter "War Without End," in which we find out that Babylon 4 was taken to the year 1260 AD (or so) to help the Minbari and their allies gather to fight the Shadows. To prevent this from happening, the Shadows sent a big bomb to Babylon 4 just as it was about to come on line in 2254. However, the White Star also goes back in time (because Delenn, Sinclair, Sheridan, and Ivanova see it in a recording), destroys the bomb, and (as it turns out) takes it back in time as well. However, this is not before the time device (sent by Draal and transported by Zathras) malfunctions, dropping Babylon 4 into 2258, leading to the events of "Babylon Squared." Sinclair then realizes that he must take Babylon 4 back in time himself, and then uses the triluminary device to turn himself into a Minbari—specifically, Valen, who led them in the First Shadow War, organized their society, and effectively became the main prophet of their religion. The Stable Time Loop is fully completed, so to speak, by the fact that when Valen dies, he eventually gets reincarnated as Sinclair.
Valen/Sinclair doesn't need to be reincarnated. From his point of view, he is born in the 23rd century as a human, goes throgh the War and subsequent events of the series up to "War Without End" and then goes back in time to the 13th century as a Minbari and lives out his life as Valen. The Minbari think he is Valen reincarnated when they enounter him at the Battle of the Line because he has Valen's soul; not knowing about the time travel, they don't see that Sinclair will become Valen in the future before travelling back to the past.
This concept became a major plot point in the fifth season of LOST (which Hurley couldn't quite grasp) though it was put to the test in the cliffhanger finale...
Particularly annoying with Sayid shooting young Ben, which was not only implied to have already happened, Kate and Sawyer's interference in order to put things right seems to actually have caused Ben to become evil, as Richard says that because the island healed him he would always be "one of them" and that he would "lose his innocence". So by trying to kill him, they effectively caused what they were trying to prevent. Nice going, guys!
Quantum Leap was somewhat inconsistent on this trope. It's the trope namer for Set Right What Once Went Wrong, but Sam and Al always remember how things used-to-be even as Sam changes the past (Al keeps track of the current timeline's history using his handheld computer-link to Mission Control). In episodes that directly impacted Al or Sam, they would have the entire memory of both things happening. For example:
Sam successfully tries to save his brother's life in Vietnam, which alters history and results in Al becoming a prisoner of war.
Actually Al was already going to be a prisoner of war. Sam could have changed that, or save his brother. Al allowed him to save his brother by not telling him he was one of the prisoners.
The series finale had a somewhat omniscient bartender asking Sam if there was anything that he wanted to do differently. Sam remembers when Al was invisibly dancing with his first wife, Beth, who became heartbroken when she thought Al was killed in Vietnam and married someone else by the time Al was released. Sam then travels to that moment and tells Beth that Al is alive and coming home - the next leap only shows a black screen, with epilogue text stating that Al and Beth had celebrated their 39th anniversary and Sam was never seen again.
In one episode, Congress is reviewing Project Quantum Leap's funding and leans on Al (acting as the project's representative) to have Sam alter history in ways beneficial to the US. Al tries to get Sam to prevent the U-2 spy plane incident, but Sam is in the past protecting a young attorney. At the end of the episode, the Congressman in charge of the committee is about to cut the project's funding when, in the past, Sam unintentionally corrects the attorney on a key piece of Constitutional law which she had wrong and she says could have made her fail the bar. Cut to the present, where the obstructionist Congressman is replaced by an older version of the attorney, who approves the project's funding for another year. It's never made explicit, but Al's surprise at the sudden change suggests that he's aware of the change.
However, in the episode about the Kennedy assassination, while Sam can't prevent himself from killing JFK, it then appears the reason he was sent back there was to prevent Jackie Kennedy from being killed, which most viewers would have assumed had already happened, whether Sam had anything to do with it or not.
In the Outer Limits episode "Tribunal", history professor and Holocaust scholar Aaron Zgierski is taken back to Auschwitz by time-traveler Nicholas Prentice (who turns out to be Zgierski's own great-grandson). While there, they rescue Aaron's "older" sister (who is only eight at the time) by bringing her into the future to live out her life free of Nazi oppression. History recorded Aaron's sister as dying at Auschwitz after being "dragged away" by a couple of guards, who were actually Zgierski and Prentice in disguise.
Another episode from the Prentice arc subverts this when a new recruit into the temporal agency goes back in time and prevents her father's death due to eco-terrorists' bomb going off prematurely. This results in another member of the agency suddenly vanishing. His colleagues figured out that, without her father to tamper with the bomb, it went off as planned and killed a lot of innocent people, including an ancestor of the temporal agent who disappeared. Reluctantly, the girl has to let her father sacrifice himself. However, she does alter her mother's fate somewhat by giving her a coping mechanism (in her timeline, her mother's a wreck; in the altered one, she is an accomplished artist).
In yet another Prentice episode, Prentice wants to change the past by convincing a Civil War buff (who has pro-Confederate views) of the wrongness of his convictions by taking him and his friend to just before the Battle of Gettysburg. Originally, the buff was going to assassinate a black President in his own future. Instead, the buff takes this opportunity to try to alter the course of the battle in the Confederate favor. He accidentally uses Prentice's time machine (shaped as an old-fashioned camera) to transport a Confederate general through time. His attempts at preventing the (from his viewpoint) catastrophe result in him getting shot for cowardice. Prentice takes the friend back to his time, and the latter finds an old newspaper with the picture of his dead friend. Meanwhile, in the Future, the transporter Confederate general appears at the moment of the original assassination, and he ends up being the presidential assassin (he was actually aiming for a man dressed as Abraham Lincoln, who was standing next to the president).
In The Sarah Connor Chronicles episode "Allison from Palmdale", Cameron malfunctions and believes herself to be Allison Young, the resistance fighter on whom her appearance was based. She calls "her" mother, who says this must be some mistake since she doesn't have a daughter "yet"...
Claire Young: [rubbing her pregnant belly] That's a beautiful name though. "Allison"...
Given how time travel appears to work in this universenote every instance of time travel seems to set up divergent timelines, however, it's probably that Allison's mother would have chosen that name anyway.
In the Haven episode "Sarah", Duke is sent back in time to the point when his grandfather Roy was killed, and decides to try and avert this. Instead he accidentally tips the guy off that he's about to be killed and by whom, sending Roy racing off to start the shootout that ends in his death.
As mentioned, Greek and Germanic mythology tended to hammer on the idea (relying on prophecies instead of time travel) that You Cannot Change The Future. Even the Gods can't change the outcome of the story. (How many steps is Thor destined to take in the final battle of Ragnarok?) Not only that, but historians actually posit that Viking culture went into a prolonged funk over it, presaging the rise of Ingmar Bergman and Werner Herzog by centuries. (The lack of sunlight in wintertime didn't help.) Vikings in particular lamented the decline in pagan beliefs for exposing them to the horrors of existentialism, making them less resigned to the inevitability of death in battle. Meanwhile, the Greeks preferred to set up stories where characters would have hubris enough to believe this trope did not apply to them, and then brutally swat them down in order to provide an entertaining Aesop. For examples, see You Can't Fight Fate and Self-Fulfilling Prophecy.
The concept of Predestination. This concept is prevalent in all Calvinist churches (Reformed, Presbyterian, Baptist, congregational, Pentecostal) and in nutshell means that the life and final depository of a human being is pre-ordained and pre-determined by God and he or she can do nothing to avoid it. In other words, people are selected either to Heaven or Hell before they even were born.
This same concept is prevalent in Islam. The only way to avert the predestination is to get killed in Holy War, which earns you an automatic admission to Paradise.
In the time-traveller role-playing game Continuum, it's an ironclad article of Spanner faith that there is only one universe — including one past and one future. A player will meet fellow spanners who've been affected by changes that are in the player's Yet, and you'd better do them or risk Frag.
Featured in one of the videos leading into The SimpsonsRide at Universal. Professor Frink arrives looking for Doc Brown's Institute of Future Technology, only to find it replaced by a clownish theme park (Krustyland). He decides to stop it by going "back to the future, I mean past." He gets into a Delorean and accelerates into a time jump. Two years ago, a broker is telling Doc Brown that he'll be able to keep the Institute open for years to come. At that point, the Delorean materializes and runs over the broker. Frink jumps out and Brown yells at him "You ruined everything! Now I'll have to sell the Institute of Future Technology to that mercenary clown!" Krusty promptly pastes a Krustyland logo over the IFT logo on the front sign.
Brown then shows Krusty to his limo, and Krusty tells him to tear tickets at the front gate after he gets a haircut.
Legacy of Kain uses this as an important plot point; more than one character has goals achievable only by finding ways to subvert this. Interestingly, while two entire games in the series take place in the past, while this trope is in effect, it still manages to have one of the most complicated sets of Gambit Roulette ever. One of the protagonists is himself a walking subversion, and thus finds himself endlessly manipulated by pretty much everyone, because he's the only one who can alter time in their favor.
Kain: "Suppose you flip a coin enough times. Suppose one day... it lands on its edge."
Interestingly the first game in the series (Blood Omen) used a different kind of time travel - the kind where you go back, do something, and return to the present to find the effects of your actions having taken place. This was played straight with no mention of the paradox that would ensue from this kind of time travel. Two games later when time travel was reintroduced as a much more significant plot element, the rules were established around this instance of time travel, and it turns out Kain's original trip back to the past, as depicted in the first game without RetCons, already met the criteria for how someone can achieve a true alteration to the time line.
Final Fantasy VIII ending has this trope, with Ultimecia giving her sorceress powers to Edea in the past and Squall suggesting the SeeD idea at the same moment, setting up the organization that Squall is raised by.
TimeSplitters: Future Perfect is an example of this situation. Sergeant Cortez does, at least once per time period, meet a future version of himself that saves him, then when he reaches the period the future version is from, he then goes back to save his past self, fulfilling the time loop. EX: in the stage "What Lies Below", Cortez is saved by himself from the future, then has to defend his future self from zombies while the future self kills ghosts. Shortly afterward he wants to follow his future self, but his future self tells him "but where would you be if I hadn't shown up?". Future and Past Cortez part, then Past Cortez finds the ghost gun that Future Cortez found, steps into a wormhole, and ends back up where he was before, becoming Future Cortez. Now Future Cortez (who is controlled by the player) must kill ghosts while past Cortez shoots the zombies. When past Cortez asks to stay in a team, future Cortez tells him exactly what he heard from himself at this point. "Where would you be if I hadn't shown up?". The two part, and Cortez continues.
This does create an odd situation however in the "Scotland the Brave" stage, where Cortez finds a locked door, and his future self gives him a key, telling him to pass it on when he's done. When he reaches this point, he finds his past self, and he gives him the key, and tells him to pass it on when he's done, then leaves. The question remains however: where did the key come from in the first place if Cortez was passing it between time periods but never stopped to find it?
That's an easy one! The key has always existed. No matter how far back in time you go the key will always be passed down from future Cortez to past Cortez basically making the key The Constant or maybe a Time Abyss. Though, wait a second, wouldn't the key break down over time and deteriorate until it eventually snaps in the lock as Cortez uses it? Oh Crap...
The key had to come from somewhere initially, however it could just be a Deus ex Machina. As for degradation, it's either only been used a few times in its existence, made of something magical, or because the character using isn't Link.
After playing this trope straight for the entire game the final boss fight subverts it. Halfway through the fight Cortez realizes that he's outgunned and decides to go back in time and aid his past self to defeat the boss. This would be fine except that future Cortez never showed up to help you defeat the boss in the first place. The game never explains this Time Paradox and thus completely subverts the trope.
The game actually features two completely different forms of time travel, and it is implied that use of the first tore the universe a new one enabling the second.
In Prince of Persia: Warrior Within, the Prince travels to the Island of Time in hopes of preventing the Sands of Time, the source of all his misfortunes, from ever being created. He defeats the Empress of Time, only to discover that she is the sands in corporeal form, and that the events that led him here were of his own making. The second half of the game is about the Prince deciding to Screw Destiny and subvert this.
In Ōkami, the evil Orochi was defeated by a legendary hero named Nagi and a miraculous white wolf, who died in the attempt. The wolf was actually Amaterasu in physical guise, and was resurrected a century later as you, the player character. But then you travel back in time and discover — you were the one who defeated Orochi then too, and the wolf who died was a different version of you.
Braid features this when you try to rescue the princess - and then see that she was running away from you instead...
* A Double Subversion occurs at the very end of Futurama: The Game. Farnsworth learns from his own crew-from-the-future that Mom's imminent purchase of Planet Express will lead to her taking over the Universe, and says the deal's off. Mom tells him that it is destined to occur anyway, because "No one can alter the continuum of time!" Farnsworth replies "Oh yeah? Watch me!" and tears up the contract. Then Mom sweetens the deal by offering him a Nice Hat… the very same hat he had entered the room wearing at the beginning of the game.
A three-part episode of Red vs. Blue had Church travel back in time and try to change history to prevent both his 'death' and that of Tex, as well as attempting to stop the other difficulties that the Blue Team had to encounter at the time (such as the problem with Lopez's switch, and Tucker getting blasted by an RPG). He ends up accidentally causing, or failing to prevent, every major event of the series up to that point including his own 'demise'.
He then wants to try again only for another version of him to tell him that it's all been tried a dozen times already by other versions of Church who all failed. Eventually, Church gives up.
At least, that's what the character believes. The character with enough nigh-omnipotent abilities to force things on the track he remembers.
Sarda did this to himself. As a young wizard, he time-traveled back to the beginning of the universe, only to find that a White Mage had gotten there first. After living through all of creation being formed around him, Sarda planned to put that White Mage into a pocket dimension before she could go back in time to the universe' start...only for that pocket dimension to be the beginning of the universe.
Sarda: So now I know how she got there and what it feels like when I utterly screw with someone's lifelong ambitions.
In Sluggy Freelance Bun-Bun's whole adventure in Timeless Space was based off this trope. As Uncle Time put it, "Life's so much funner with the paradox rules turned off."
Notably the alternate timelines created by Time players are "doomed timelines", they and anything and everything from them is marked for destruction, this can still be very useful though as they can create armies of doomed copies, they will all die off in a relatively short time, but if you just need a lot of bodies to fight something that would get a lot of people killed anyway it doesn't actually matter. Of course, it turns out that the creation of the doomed timelines and their involvement in the alpha timeline was also predestined, so either way, You Can't Fight Fate.
As explained by one character in a particularly nasty future, the branching timelines are the reason you can't technically change the past. See, since a future-you didn't come back in time to the present when you were taking The Slow Path to the current future, you didn't do that, so if you go back in time, you create an alternate past where you did, and the versions of your friends and family in your current future-present will never see you again, and also their lives still suck.
At the end of A Very Potter Sequel Hermione asks future!Draco what was really supposed to happen during their first year and Draco says that this was how it all played out originally, it just makes sense now that he lived through it.
Played straight on Futurama, where Fry ends up in 1947 and spends half the episode just trying to make sure his grandfather doesn't die. After the Professor warns him not to change the past unless he was already destined to change the past, Fry's extreme caution and stupidity result in his grandfather being vaporized by an atomic bomb. However, subsequent events make it clear that the man Fry killed was not actually Fry's grandfather, and that his real grandfather is Fry himself. Of course, this is immediately followed by the Professor launching a full-blown assault on Area 51 with his intergalactic starship to retrieve the parts necessary to get home...
Professor: Choke on that, causality!
Also, Zoidberg is the Roswell alien.
The movie "Bender's Big Score" tinkers with this a lot, for example Bender was responsible for the destruction of New York seen in the first episode. This gets especially weird when Bender travels back in time and turns out to be responsible for giving the secret of time travel to himself.
Goliath tried to convince Demona in the past not to turn evil, and she seems to take it all to heart. Unfortunately, one guy, even the love of your life, telling you to "stay good" is trumped by centuries of of being brutalized by humans. It's a true Tear Jerker to realize that Demona and Goliath were once really and truly Happily Married.
During the same incident, Demona also tried to warn her younger self of the destruction of her clan in order to turn her against the humans. Demona is in massive denial, however; she herself was one of the major causes of that very destruction, in one of her anti-human plots.
Xanatos uses this to his advantage. He gives two period coins to the Illuminati, along with a letter. The coins are like pennies in the past, but by the present they're very valuable and are the coins that started his fortune. The letter of course, is to tell him to do just that.
Later, Goliath attempts to use the time-traveling Phoenix Gate to save Griff from being killed during the Blitz in WWII London, after being accused of abandoning or murdering Griff by his companions. With incident after increasingly improbable incident occurring that indicates the universe has decided Griff is its new Chew Toy, Goliath ultimately concludes that fate will not allow Griff to get home and uses the Phoenix Gate to bring Griff back with him to the present, thus causing his original disappearance.
Goliath winds up in a Bad Future (really All Just a Dream), and Elisa keeps egging Goliath on to use the Phoenix Gate to fix things. This is a hint that Elisa isn't what she seems, as by now, she should know how the Gate works.
Used on an episode of Justice League Unlimited. Brainiac 5 imports heroes from the past because history mentioned an incident where heroes traveled to the future. He tries to avoid mentioning how it turned out, of course, just to be sure things go the way they're supposed to, with only two of the three returning.
Nobody dies. Supergirl just decided to stay in the future.
In the Darkwing Duck episode "Paraducks", Gosalyn warns Darkwing not to interfere into the past when they went back in time to his childhood. At first he doesn't and returns to the present, only to find that S.H.U.S.H. doesn't exist, the King, a two bit thug from Darkwing's childhood has taken over St. Canard and he serves as the King's cowardly lackey, never became Darkwing Duck. They go back and time and shut down the King for good and give little Drakey Mallard (Darkwing) the courage he needed.
The Fairly Oddparents special "The Secret Origin of Denzel Crocker": Timmy and 21st-century Cosmo were the ones responsible for making Crocker lose his fairy godparents and giving him the opportunity to partially get around the ensuing mass mindwipe thus causing his obsession with fairies, which also indirectly led to his own birth due to the disappointed scientists at Crocker's presentation in the '80s investing in Dinkleburg's parachute pants and causing him to break up with Timmy's mom, thereby getting his parents together.
Of course, in the original timeline, 1970s-era Cosmo would have exposed himself and Wanda as fairies due to his own stupidity.
Somewhat subverted in the Invader Zim episode "Bad, Bad Rubber Piggy" had Zim send a rubber piggy into Dib's past at crucial points to kill him, only he survives by an inch each time (though everytime he comes close to death, he's given robotic body parts from his father due to losing his own) and after his tampering with the past results in an angry Dib in a Humongous Mecha bent on killing him, he sends his last piggy to the past to warn him not to send any piggies to the past in the first place. Unfortunately the premise of the piggies was they replaced something in the timeline they're sent to and it replaces Zim's brain at the end.
In Powerpuff Girls Mojo Jojo goes to the past to kill the adolescent Professor Utonium before he can create the Girls. The Girls pursue him. It turns out that in the past Professor was a lazy ass and a bully with no interest in becoming a scientist and creating the Girls, if it weren't for Mojo's interference and the consequent encounter with and rescue by the Girls that gave him inspiration.
Mojo should be glad for this, considering that, if not for the Chemical X accident when Professor Utonium created the Girls, Mojo would be just another monkey.
In the season 2 episode "It's About Time" of My Little Pony: Friendship Is Magic, Twilight Sparkle is visited by her future self (from a week later, looking entirely worn) and told "whatever you do, don't..." with the sentence being cut off. Past Twilight then spends the whole week worrying about and trying to prevent whatever happens during the next week, with each incident causing her to gain the looks of Future Twilight, indicating she hasn't changed the future at all. She only then learns later that nothing actually happens. So she goes back into the past to tell her past self "Whatever you do, don't... worry about the future" only to end up being pulled back into the future right where it cut off for Past Twilight, setting the events into motion for the whole episode.
Played with in Beast Wars. While it is entirely possible to change the past and thus the future, thus finally answering Dinobot's soul searching about the nature of time travel and what that means for free will (if the past is immutable, than our ability to choose anything is a cosmic illusion). By changing the past, Dinobot learns that it is possible for an individual's choice to matter to the universe. Ironically however, with this new knowledge, it means that Megatron can change the past for the worst, and that the only choice Dinobot has is to invoke this trope.
The question that once haunted my being has been answered: the future is not fixed. My choices are my own. And yet... how ironic, for I now find that I have no choice at all. I am a warrior! Let the battle be joined.
Played with again in the finale. Dinobot II, being influenced by the original Dinobot's spirit sends the Maximals information concerning an Autobot shuttle docked in the Ark. Black Arachnia points out that the historical records never mentioned that the Autobots had a shuttle. Rhinox then exclaims that history is still being made, and uses the shuttle to help save the day.
¡Mucha Lucha!: In the episode "Woulda Coulda Hasbeena", Mr. Hasbeena told his students about how a mysterious flash caused him to lose a fight and it destroyed his career. Then, a time portal leading back to that day opened and he decided to change the past. The three Mascaritas went after him to stop him. Hasbeena then tried to get rid of them by using his signature move, which caused the flash he tried to prevent in the first place.
Duck Dodgers: The Martian Commander believed that Dodgers wouldn't be so heroic if not for a football game he played in college before being frozen. The Queen wasn't so sure of this given his previous attempts to get rid of Dodgers by messing with his past. The Martian Commander was so obsessed about proving he was right about it he forgot his mission was to stop Dodgers. He instead helped Dodgers to win the game.
Warp explains this to Starfire in an episode of Teen Titans, saying that he went into the past to steal a certain clock because...that's what happened (he stole the clock in the past). She can't change it because that was what occurred and the timeline has the event hard coded. Except she does.
This has been used as an argument against time travel (or at least time travel into the past) being physically possible no matter what advanced scientific discoveries become possible. If it will ever be discovered, then anyone traveling into the past would arrive in the timeline of that past. Since up to this point nobody is known to have been credibly been visited by someone from the future, the argument goes, nobody in any future can ever do it.