If heroes from our world visit the Alternate Universe, they won't care what happens there. Typically, neither do the audience.
This goes further than individual death. Absolutely anything horrible can happen in an Alternate Universe — Zombie Apocalypse, life-threatening plague — and our Earth will still be safe, if we can close the portal. Many heroes will not leave the Planetville of the week until its problems are solved, but for the Alternate Universe, all they care about is getting out alive.
The reason for this is that an Alternate Universe often feels like a cheap copy of our own. It's just an extra us, so its people aren't unique characters. Under the Second Law of Metafictional Thermodynamics, that makes it expendable. It is extremely rare for so much as a single refugee to escape a doomed Alternate Universe, because that refugee will ruin Cast Speciation for the Earth-1 version unless the refugee will become a Suspiciously Similar Substitute. In-universe, this is often simply pragmatism from the characters: Works that feature alternate universes heavily often use the "infinite variations" model of the multiverse. The leads simply can't afford to worry about saving every single possible universe.
Things are even worse if there's only one world, but Time Travel can change it back and forth. People in the future can die a hundred times each, but since the Time Travel will bring them back, who cares?
A related issue is that too many alternates can cheapen established stories. If the heroes have saved Earth from the Planet Looters, the Zombie Apocalypse, The Virus, the Nietzsche Wannabe and the Circus of Fear, that's impressive. If there's an Alternate Universe for each one of those villains winning, it means our heroes aren't impressive — they lost as often as they won, and we're from the universe where they just happened to win five times in a row. This gets even more egregious when there are infinite alternate universes; for every event in "our" universe, there are infinite others in which it didn't happen...
On the other hand, "You can have an infinity of apples but still not a single orange", as a scientist explained this theory: So it's still perfectly possible that the heroes win everywhere, or at least more often than they lose.
This callousness doesn't apply to worlds that are less obviously mirrors of our own. Wonderland, Narnia, and dimensions with only nonhuman life are all technically alternate universes, but they're so different that losing them actually hurts.
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Anime and Manga
Either averted or played straight (depending on how you look at it) in the anime/manga series Bokurano. with every Humongous Mecha battle, an alternate reality that is strikingly similar to that of the universe of the main characters is simply wiped out of existence. Averted in that the main characters go through a series of angst regarding the fact that to keep their own universe protected they have to wipe out others. Played straight in that, well, they end up doing it anyway. For the most part.
Noein plays with this. All the Dragon Knights believe that universes outside their own are somehow virtual and unreal, Haruka is arguing with them all the way, and Karasu is the only one who's convinced/realizes that the "virtual universe" thingie connected to a real universe rather than creating an artificial existence. In the end, it's up to Tobi to drop the anvil - that peoples' existences are confirmed by interactions and specifically bonds, not by observing a collapsing waveform.
Super Robot Wars Original Generation had an inversion and subversion to this. In it, a group of heroes fleeing their own universe after a coup against the corrupt Federation fails, and they try to rebuild their powerbase in our universe. They get several things right, but are surprised when several major things that happened to our universe just didn't happen in theirs. The subversion lies in their belief of recruiting the heroes of OUR universe to help them at any cost, and the main characters treat them as important as any other person.
However it's hard to say if this really counts, because aside from one main female characters' alternate counterpart who is so radically different they don't even know it's her until the end of the game, the rest of the alternate universe characters are all people that don't exist in the main universe, or at least have never appeared in story there. It's hard for the main characters to write Axel off as "oh it's just an alternate Axel Almer" when they don't know of any other Axel Almer.
The Inspector, the anime adaptation of Original Generation 2, plays the trope a bit straighter; Beowulf, already established in the games as the parallel version of Kyosuke Nambu, goes from merely being Axel's unseen arch-nemesis to being the Big Bad of the series who mercilessly slaughters his world's version of the SRX Team in the Cold Opening of the first episode.
The anime Dual Parallel Trouble Adventure, which features two parallel universes, take this to the ultimate extreme by having the characters seemingly view themselves as more important than everybody in BOTH worlds. At the end of the series Kazuki and Mitsuki Sanada merge the two universes together into one new universe, with only the main characters remembering the events of the show..
In Dragon Ball Z, Cell comes from a timeline different than that of Trunks or the regular timeline. While the regular timeline and the one Trunks is from are eventually saved, no one seems to ever care about this third timeline and after awhile it's simply forgotten.
Somewhat justified. The implication of Cell arriving in the main timeline is that people on Earth in that timeline are already dead (because Cell killed them before traveling) or alive and well (because the Androids were killed by Trunks, forcing Cell to kill him and came back in time to find them instead).
All the Z Fighters are dead in that "forgotten" timeline as Cell killed the Trunks from it. So it is justified as the main protagonists of the story (with maybe the exception of Bulma and side characters like Chi-chi) are gone. Additionally, due to how time travel works in DBZ, the only person who could travel to the third timeline is Cell himself.
The central conflict in Higurashi no Naku Koro ni Rei is about this. At first, Rika is willing to do ANYTHING to get back to her 'own' universe without even contemplating the morality of it, up to and including matricide. Then as she relaxes a little bit and lets herself become a part of the universe she contemplates the good parts of it and why it could be wrong to regard her original universe as a universe A and the one she's currently inhabiting as a universe B. In a Bittersweet Ending typical of the series, she ends up committing an unspeakable sin to get back home anyway, only after acknowledging the meaning of it enough to be truly scarred by it. Sure, another characters tells her 'it was all a dream,' but it was rather probably an attempt at comforting through deceit.
Asura Cryin' spends a good deal of time getting us acquainted with the main universe of the show, though it's hinted early on there's another one. When several characters from the main universe (which is actually World-2) are killed off, the show changes gears and goes to World-1. While Tomoharu knows he doesn't belong here, an effort is being made to Save Both Worlds; they're both considered equally real, though there's still only one Tomoharu between them.
Inverted in 11eyes. 3 characters who were all technically strangers in the beginning, seeing how all of them come from a different parallel world get brutally murdered. In the end, not only does the person who did it get off clean because she was (technically) the good guy, but everything ends all and well, because in the world where they end up in, has all three of them alive and well, even though they are not the same characters we have come to know and love.
The Code Geass manga Nightmare of Nunnally has the heroine use her Geass power to see an alternate universe - namely, the "mainline" universe of the Code Geass anime. No interference here, though Nunnally says she's happier with the manga reality, mainly since in the "other" timeline, her beloved brother Lelouch and half-sister Euphemia are dead.
Mirai Nikki: Discussed by Minene and Yuki when they both travel to the Alternate Timeline. Minene insists Yuki must invoke this trope if he wish to succed. Yuki defies it and tries to save everyone's versions. He makes the correct choice.
Comic books have lots of alternate universes, from the DC comics Elseworlds and "imaginary stories" to Marvel creations like "What if?" and Marvel Zombies. Almost every one of these ends with a bucketload of corpses, or maybe the entire universe getting destroyed. It's not our Wolverine, we don't care if this one dies.
Except it was revealed that time travel in the MU just splits off alternate histories, so the original Crapsack World timeline still happened, just not in the mainstream books; the attempt didn't save the DOFP X-Men.
Heavily subverted in Paradise X, in which a number of heroes go to immense lengths to save alternate universes (though some prove unsalvageable). The only thing they won't do is travel into the past to reset a universe, since this will just create another alternate (stranding the people they were trying to save). It pays off—even archvillain Annihilus from the Negative Zone is willing to help out, out of gratitude for the assistance.
Marvel at least makes a small concession to this trope, in that the 'main' universe is Earth-616, implying that it is just another one among many. DC, on the other hand, traditionally had its main reality as Earth-1, implying it to be the real world that all the others are merely copies of. Since their latest multiverse shakeup, however, the main world is now New Earth, with 52 numbered alternates in existence.
Several series of Marvel's Exiles title deal with characters in some way exiled from their various realities, usually previously unseen, working to steer the course of events of other realities. A 2009 series seemed to this reader to be running on the principle that if a reality stopped being interesting to read then it'd cease to exist, or is that too literal? (The series itself ended at issue six.)
Settings, alternate or not, that may not be attracting enough audience to continue publication, often have an apocalypse to stimulate attention, and may then be cut off anyway. Marvel's "2099", "Heroes Reborn", Batman's "No Man's Land", and Wildstorm's "World's End" are examples of apocalypses where the other factors may apply.
In Grant Morrison's Earth 2 graphic novel, the Justice League discovers the existence of an alternate Earth run by villains, and Batman specifically says it's not their problem. (He later changes his mind.)
Similarly, the heroes of the Marvel universe are entirely aware that there's an alternate Earth out there where the Nazis won WW2 and are turning the whole planet into a concentration camp. (First revealed in Excalibur back in the 1980s.) No one seems to care enough to do anything about it, not even Kitty Pryde, who is a first-hand witness, and Jewish.
The Multiverse was such a constant until Crisis that people did, in fact, care about the people in most of the more prominent universes. In Crisis itself, the death of the original Crime Syndicate is just heartbreaking; they're supervillains, but they're still part of the world, and they fight more bravely for it than anybody else does for theirs. A few of them run headlong into a wave of antimatter by the end...
This is rather subverted in Infinite Crisis, as those who saved the single remaining universe decide that they saved the wrong Earth. Infinite Crisis is all about trying to stop these former heroes from re-writing the world in their mold. They nearly succeed.
The Multiverse was brought back at the end of Infinite Crisis, because of the temporary return of the Infinite Earths caused there to be too much aspects to return which made it impossible for New Earth to return to its original Post-Crisis state. Because of the events of 52, Infinite Crisis' immediate sequel, fifty one of the fifty-two were now radically different from each other. People were happy about this... until Countdown to Final Crisis came along and destroyed one earth (but not its universe), and destroyed a universe so utterly that it had to be rebuilt from scratch. Then a deadly mutative virus permanently altered all life in that same rebuilt universe. Grant Morrison has officially stated that Countdown never happened (minus Earth 51 being destroyed which is what caused Final Crisis).
In the Pre-Crisis era, DC Comics struck a balance between readers not caring about alternate continuity characters and the need to maintain the status quo of its mainstream continuity characters with its Earth-Two continuity. Under DC's multiverse system, Earth-Two was DC's original continuity from before the Continuity Reboot of the Silver Age. Since the Earth-Two characters were the original pre-reboot versions of the characters, it was expected that readers would still care about them since they weren't from some throwaway continuity. However, since the Earth-Two characters had been displaced by the reboot versions of the characters, there was less need for DC to maintain the status quo for the Earth-Two characters since they had the reboot versions to maintain the status quo. In the Earth-Two continuity, Superman, Batman, Wonder Woman, and the Flash all got married and Batman was later killed off.
In the comic version of Wanted the Villain Protagonists jumped to other dimensions with the specific purpose of stealing things (such as an irradiated condom) and killing superheroes. While they were there they felt perfectly free to trash other realities to make their own go the way they wanted.
Usually subverted in The Authority comics in that the title group (who have a Cool Ship capable of traveling between realities) are more than willing to move into other realities and tell the locals how things should be done - Or Else!
They do show respect for other realities at times, usually as long as they're run right - or if the Authority wants their help. In one instance they had to temporarily evacuate the world's entire population to other realities, and are seen negotiating with The Meritocracy, their gender-flipped counterparts in a gender-flipped reality. (They in turn put the decision to a worldwide vote rather than decide for themselves, resulting with a majority agreement from that world's people.)
On the other hand, in one arc they destroy an alternate Italy and in another they cause the death of everyone on an alternate earth trying to power their ship.
Age of Apocalypse killed off almost the entire alternate cast, with only a handful escaping back to 616, but the series still had a cult following and Marvel eventually acceded to demands that Blink be brought back, meaning that someone must have cared.
On a Post-Zero Hour encounter between every possible version of the Legion of Super-Heroes and the Time Trapper, the heroes criticize the trapper for toying with their histories, claiming they're not variants but people.
Subverted in the first issue of Planetary. Doc SavageExpy Doc Brass and his Super Team created a machine that uses a short-lived pocket multiverse as a supercomputer. Unfortunately for them, the pocket universes experience billions of years in the real-world seconds before they're destroyed, and a familiar-looking super team from one of the universes figures out what was going to happen to them — and how to get to the universe where Doc Brass and friends were.
Taken to an extreme in IDW's Transformers comics with regards to the Dead Universe. That Shockwave rendered an entire universe uninhabitable from the moment of its origin is mentioned only in passing, and when the Dead Universe collapses, the only important thing about it is that Optimus Prime and his team escaped in time.
This is a very common theme in Fan Fiction as a whole. The easiest way to do a Crossover or Self Insert story is to have the existence of parallel universes which somehow intersect.
Chrono Trigger: Crimson Echoes has three timelines, the one that was created after the Big Bad from the original game died, one where the myths of a certain legendary sword never existed, and one that was mentioned in the official sequel Chrono Cross, where the aforementioned Big Bad was not around to influence evolution and a long-dead race was revived to wage war on the now-ordinary humans. This fan-game's Watcher kinda doesn't like that these alternate timelines exist, as his workers, if you did something to muck up the timeline to make it different from normal, will not allow you to return to base until you make the necessary changes that keep history on track.
The Ranma ˝ fanfic The Converging Series (sequel to Descents and Inversions): the non-canon main characters are accidental time travellers from different alternate futures who are descended from various pairings of the canon characters. While the Only Sane Man recognizes the existence of coexisting alternate realities (having had to deal with his reality-hopping amazon "half-sister" in the prequel), all the others decide to force the current reality into their own, to ensure their own continuous existence.
In Applejack's reharmonizing chapter in the Pony POV Series has her seeing a large number of these staring into the Truth, a pool that shows the viewer all kinds of uncomfortable truths. She sees the Bad Future presented in "Epilogue" along with others. The most heartbreaking is the "Orangejack" timeline, where she discovers that she could've lived perfectly happily without returning home and met the love of her life, even having children. She's heartbroken at the realization her children won't exist because of her choice, until Celestia reveals that universe still exists and they'll live on in that path.
''The Infinite Loops has Loopers refer to this attitude as Sakura Syndrome, and generally frown upon it. Yes, reality is stuck in a time loop but not only will your fellow loopers remember your actions, if you break the world too hard you might wind up in the Eiken Loop or accidentally destroy your universe.
The 2001 film The One is about an interdimensional criminal who's been going through every universe and killing his counterparts to steal their lifeforce, and is down to the last one (ours). Grave consequences are implied if there's only one of one person in the multiverse. Notably, this is a rather constrained multiverse: it is explained that every time a sun turns into a black hole (or something like that) a new universe is created and so far this has happened 125 times. At the very end of the movie, the villain is sent to the "Hell Universe" which serves as a jail for all the others... there's an entire universe dedicated to being a prison system. No mention of the original inhabitants or what became of them.
The third Men In Black movie has Griffin, an alien capable of seeing all timelines at once though he's not certain which one he's in. This means that he's often fretting whether or not this is the timeline that something disastrous happens based on minor actions that seem insignificant to others.
In the Magic: The Gathering novel Test of Metal, bolas at one point uses clockworking to attack Tezzeret with a massive swarm of undead copies of himself from timelines where he lost the fight. Later Tezzeret says that he can forgive Bolas despite an oath to kill him because he did kill him, over and over again.
The former Trope Namer, Larry Niven's "All the Myriad Ways", actually deals with an inversion - because billions of new alternate worlds are created every second (every time anyone makes a choice, even such a minor choice as what color socks to put on, or even to put on socks at all, a new universe is born; multiply that by the billions of people on the earth, and...), people no longer value their own lives, because they know alternate versions of themselves will do better if they die — and why not commit murder, rape, robbery, or suicide, if you were always destined to do so in at least one timeline? The story ends by showing nine very different outcomes to the same story with only the last line changed on a whim of the protagonist.
In the story collection of the same name, this is followed by an essay where Niven explains his dislike for the concept of "infinite divergent worlds", reasoning that it essentially nullifies free will, as no one can really choose any action if, in the bigger picture, they choose every action.
L. Neil Smith has named this "Niven's Fallacy", pointing out that you are the only one you have. Only your choices count, since you can only live one life, in one universe at a time. Your perspective is of a single existence at a time, and your actions shape that existence from your perspective. You are you, your double is someone else.
...unfortunately that breaks down as soon as you have to make your next decision. It doesn't matter what you do - all of the resulting Yous will be your current You, and not the Yous of a (current) double.
Avoided in Diana Wynne Jones's Chrestomanci sequence, where events in alternate Earths do affect events in the main character's world (which isn't ours, and isn't World One, either. Nor is our Earth - we're World 12B.) However, saving a life from another world can throw the whole system into disarray if you're not careful... so any traffic between the worlds has to be carefully regulated.
While several of Philip K. Dick's novels and stories happen in alternate or subjective realities, the best is arguably The Man in the High Castle, where a native of a universe where the Axis won WWII discovers an alternate USA (and also reads of a third) where they did not. None of these is our universe. Another is Eye In The Sky, where a group of people each gain control of reality as they individually wake from a coma. Reality in this case is according to prejudice and their wishes, rather than an alternate, but the idea of decisions causing the change holds true.
In the Sterkarm novels by Susan Price, amoral tycoons are quite happy to strip-mine the past for natural resources via time-travel. They don't care that by so doing they'll screw up the future, because it's only possible to get to the past of an alternate "dimension", and therefore it's not their future.
Played with in the Discworld novel Night Watch, in which Lu Tze tells Sam Vimes that despite there being multiple alternate timelines, and a theory that states everything which could physically happen must happen in one of them, there are nevertheless events which haven't, such as Sam killing his wife, showing that individual choices do matter.
This was "Sam Vimes as he is now", implying that in an alternate universe where Sam Vimes grew into a completely different person, he could have done such thing, but that would not be the Sam Vimes who can be recognised as the person he is.
In Jingo, Sam's PDA somehow gets switched with one from a different timeline. He's horrified as he realizes that, if he had made the wrong choice at the wrong time, a lot more people would have died. Including Vimes himself: "Things to do today... Die..."
And then there was Men at Arms, where Pratchett all but says at the start that there were a lot of coincidences and lucky breaks that made the novel's happy ending possible—and then notes that in most universes, it didn't happen that way. Cuddy and Detritus didn't fall through the weakened street. Or Edward d'Eath didn't do anything with his rage, and just nursed his grudge alone. "In a million universes, this was a very short book."
It's a plot point in Lords and Ladies, where the weakening boundaries between the Discworld and Fairyland are also causing the boundaries between alternate realities to weaken, enough for the protagonist Granny Weatherwax to remember the lives of her alternate selves. Realizing it helps her figure out how to Borrow a swarm of bees at the climax.
Just to explain how badass Granny is: Her main problem with Borrowing bees was that there was more than one body, all under the same hive mind. By becoming aware of all the other alternate Granny Weatherwaxes out there, and their thoughts, she realizes that she is one woman living different lives and thinking different thoughts at the same time - and that compared to that, a swarm of bees is easy.
She also realises that, unlike the Esme Weatherwax who married Mustrus Ridcully, she has the Virgin Power to bridle the unicorn.
In H. Beam Piper's Paratime stories, the home timeline carefully guards the secret of inter-timeline travel and takes advantage of resources from less developed (or completely uninhabited) timelines. The Paratime Police suppress gross exploitation such as inter-timeline slave-trading, but the bottom line is that Homeline's interests come first.
After Piper and before Turtledove, there's Keith Laumer's Imperium stories, where the "Maxoni-Cocini drive" allows access to parallel timelines - but at the risk of destroying one's home time-line in an unspecified chrono-nuclear disaster. In fact, our Earth is in the middle of a Blight made up of timelines where the M-C drive went horribly wrong.
Laumer's Dinosaur Beach explores parallel time and the Timesweepers who have to clean up the messes left by previous time-travelers while fighting off others who don't want the extant lines cleaned up.
Harry Turtledove has made a living off of this trope. All of his books, aside from fantasy novels, deal with this trope in some way (and most of them are pretty good) but the most blatant is the Crosstime Traffic series, in which eponymous company has solved our earth's Malthusian troubles by developing "Chronophysics" and the technology to go to parallel worlds. Although this mostly gives him license to drop modern teenagers into period pieces, as the books are obviously written for teenagers, and they are pared down from his normal book length, forcing him to sacrifice the plot and world development which is omnipresent in his best works.
By the way, the Crosstime Traffic series is a Shout-Out to Piper, as the names of the people who developed the technique in Turtledove's stories are clearly based on those who developed the Paratime technique.
Played with in The Talisman, by Stephen King and Peter Straub. When flipping between Alternate Universes of our Earth and The Territories and others at the end, someone will switch minds with and take over the body of their double from that universe. ...unless they don't have one (as is the case with Jack, the protagonist), where they disappear from their home world and appear in the other.
Comes up in the World Gates trilogy by Holly Lisle, especially when one character talks to the fellow that was her husband, only he's a still-alive version in another universe.
The Chronicles of Amber series has the Amberites treat the "shadows" as less valuable than the "real" world containing Amber. How much less valuable? Caine murdered one of his alternate selves and dumped the corpse in Amber as part of faking his own death. He was one of the "good" guys. To be fair, the "nothing but us is real" mentality starts to unravel a bit once the main characters figure out that they're not entirely real either - and attitudes towards Shadow-dwellers seem to be at their most sociopathic early in the series, with most of the major characters softening a bit as time goes on.
In his novel A Dark Travelling, alternate worlds are referred to as "bands", and three of them have become dystopian "Darkbands". The protagonists find themselves caught up in an attempt to liberate a Darkband, with the end result being a splitting of the band into two new ones, one free, and one where the liberators failed and were killed, as the battle went both ways. The main character expresses a desire to visit his own grave when that band is liberated.
This is the premise of the Star Trek: Myriad Universes series of novels from Pocket Books, exploring various "what-if" scenarios in the Star Trek universe. This is also the premise of the first novel in the Crucible trilogy, Provenance Of Shadows, which explores both the aftermath of "City on the Edge of Forever" and an alternate universe where McCoy did save Edith Keeler and Spock and Kirk never came back for him, leaving him stuck in the alternate past forever.
His short story "The Infinite Assassin" deals with this from the inside: the protagonist is remarkably uniform between worlds, so he can leave one world, complete his cross-universe mission, and return to a completely different set of bosses who'll nevertheless recognize him. His sense of identity is correspondingly diffuse: "I am the ones who succeed."
And I wonder: in how many infinite sets of worlds will I take one more step? And how many countless versions of me will turn around instead, and walk out of this room? Who exactly am I saving from shame, when I'll live and die in every possible way?
If that's not mind-boggling enough, check this out: the protagonist's ultimate defeat involves being blasted into "Cantor Dust". If your victory condition has been mathematically restricted to a nowhere dense subset of the space of all universes, then you can win in one, you can win in many, you can even win in an "uncountably infinite" number of universes. But that's still an infinitesimal subset, meaning you've been defeated in "100% of all universes".
"Singleton" is about an AI who's specifically designed not to have alternate versions of herself.
Robert Reed likes this trope. In a more straight example in his novel, Down The Bright Way, there are thousands of alternate Earths, each of which started diverging around the time apes started becoming more intelligent. In Mere, a race of aliens has quantum sized structures in their brains that cause them to see a faint "aura" around some objects, which they interpret as being glimpses into alternate universes. Hyperfiber in his Great Ship universe is an extremely durable material that gains its strengths from spreading any damage and energy over hundreds of alternate dimensions, making it nearly impossible to destroy.
The Doctor Who New Adventures novel Blood Heat subverts this; after bringing peace to the Alternate Universe where the action of the novel takes place, the Doctor reveals that because it's an artificial timeline generated from the 'real' universe, it's siphoning energy from the 'real' universe that will cause the original one to end billions of years prematurely unless he destroys it. This does not stop him from feeling great guilt at the very real lives he is being forced to end in the Alternate Universe, nor his companions from angrily lashing out at him for this when he tries to justify it with this trope for their sake. Later novels in the series reveal that an old enemy of his created the artificial universe precisely to put the Doctor in this position.
Played with in Simon Hawke's Time Wars series book The Pimpernel Plot: After the time commandos effective wipe out an alternate universe by retroactively preventing the event that accidentally created it, Colonel Forester deliberately invokes this trope by asserting the people in the alternate universe were just potential people, little more than ghosts who never properly existed in the first place, because otherwise they'll have to accept being the worst mass murderers in history.
In Stephen Baxter 's The Time Ships, the Time Traveler is horrified at discovering that he's changed history by leaving the account of his first travels (published by H. G. Wells as The Time Machine, of course) even though the world he's averted is the horrible Crapsack World of Morlocks and Eloi. He's so distraught he tries to prevent himself from inventing the machine, which of course goes horribly wrong. Fortunately it turns out that he's creating alternate universes, not destroying history.
Live Action TV
Played reasonably straight in the Buffy the Vampire Slayer episode "The Wish". By the end of the episode, Giles and Oz are the only main characters left alive; then Giles manages to hit the Reset Button and restore the original universe.
"The Wish" is downright gleeful in its desire to have the main characters kill each other: besides vamp!Willow and vamp!Xander killing Cordelia mid-episode, the climax has Oz kill vamp!Willow, vamp!Xander kill Angel, Buffy kill vamp!Xander, and the Master kill Buffy.
"The Wish" is an unusual example of this trope in that some characters come to realize that they're living in an alternate reality. Cordelia was brought into that world from the main reality, and Giles and the "white hats" investigated her story. In the end Giles works out how to fix things. When asked why he's sure he even should - why he thinks the original universe would be better than the one he lives in - he answers simply "Because it has to be."
In the follow-up episode "Doppelgangland", the heroes have no problem with returning Vampire Willow to her own universe rather than staking her, even though she's killed people there in the past and fully intends to continue killing people once she gets back, apparently it's okay because she's not hurting anyone from our Buffyverse and Willow thinks she's kind of cool. She ended up being staked by Wishverse Oz almost immediately after returning to her universe, but the heroes had no way of knowing that would happen.
One of their better half-aversions, where the death of an alternate is treated as acceptable but still tragic, is the Star Trek: The Next Generation episode "Parallels." A Negative Space Wedgie rips a hole in time, and five billion plus Starships Enterprise need to get back to their home dimensions. This takes place in a second (for lack of a better term) universe where Riker is captain, and he leads the effort to fix the hole. Then a third Riker tries to stop him — the third Riker is crazed, from a horrible universe where the Borg have conquered the galaxy, and doesn't want to go back. The second Riker has his Enterprise fire on the third, intending to dissuade it, but the travails of the third ship had already done such a number that even a light shot blew it apart. Riker wasn't happy. The Techno Babble that ended the episode was effectively a Reset Button, and while Worf retained his memory of it, it effectively didn't happen.
As a more straight example of the trope, the same episode has the second universe's Geordi dying — and nobody in his own universe cares, as if they knew their Geordi wasn't "real."
On the other hand, that episode also featured some emotion regarding the fate of a parallel character when Troi informs Worf (who'd been bouncing around between realities) that in that particular universe they are married and have children - and when Worf returns to his original universe it's implied that the Worf of that universe won't come back to replace him.
For the 2009 reboot, Word of God used this episode to explain how the new timeline started in this film can coexist with the Prime timeline from past Trek stories.
Seen in Star Trek: Voyager. A time-space hiccup causes there to be two Voyagers in the same place at the same time drawing off the same power source. One of the Harry Kims is killed (and one of the Naomi Wildmans dies not long after being born), so the other Voyager sends their Kim to the functioning ship before self-destructing to take out an invading alien force. Naomi and Harry's status as alternates is never mentioned again. This is somewhat an inversion of the trope, since it is implied that the destroyed Voyager is the "real" Voyager of the series! (though this, too, is never again mentioned)
In Star Trek Deep Space Nine, there is an episode where Time Travel Disease keeps sending O'Brien forward in time to see himself die. (See: O'Brien Must Suffer). He goes to great lengths to save himself each time until the disease kills him, at which point Alternate Timeline O'Brien goes back in his place to save the station. In a subversion of the main trope, Alternate O'Brien feels very bad about Regular O'Brien's death
Similarly, the Heroes episode "Five Years Gone" is set in a dark future with a high body count, none of which will matter if our heroes prevent the dark future from happening. And in season two, Peter visits a future where 93% of the population is dead. That is, the human population. Of Earth. The planet.
The season two example is made worse by the fact that Peter's girlfriend is abandoned there when his powers deposit him back in the 'present'. So he attempts to save her by stopping that future happening... but doesn't that mean that it would be impossible to rescue her?
It's just as likely she'd reappear at the proper time in this timeline. It could be like throwing a dart at a dartboard only to have someone move the board before the dart hits; the dart wouldn't cease to exist, it would just hit the bare spot on the wall where the board used to be.
Yes, but... if you throw the dart and someone takes away the board after it hits, the dart will go away with it. So...
One of two things can happen: Either he changes the future, and past him now teleports to the new future and leaves her there, or past him still teleports to the Bad Future, in which case he could still save her by traveling back to his past self and coming to that future with him, or just keeping his past self from taking his girlfriend. Either way he'll have to do some time traveling if he plans on bringing her back to the present.
In Season 3 Peter's apparently forgotten that she ever existed...
Season 3 also introduced yet another possible ugly future which contains, among other things Hiro's (apparent) death at the hands of his best friend Ando, Claire killing Peter, and a redeemed Sylar cratering Costa Verde (and killing Matt's future wife in the process) when his own son is killed and he loses control of Ted's powers. Also the mass proliferation of super-people has the Earth primed to EXPLODE, But none of it has happened yet.
Neither will it happen, since that future is dead as well. We seem to be locked on target for Five Years Gone, however...
In the alternate universe seen in "There But For the Grace of God", the Goa'uld actually succeed in conquering Earth and killing the counterparts of SG-1 (except for Teal'c, who never defected from Apophis in this universe). (Teal'c got to die offscreen when the base was programmed to self destruct.)
As an exception, "Point of View" had "our" team travel to an Alternate Universe to help stop the Goa'uld invasion of Earth (although they still did it only after they found out that the alternate Samantha Carter couldn't stay in their universe). Although, oddly, this episode, while not following the trope, does explicitly state it. Teal'c (rather nonchalantly) kills his alternate, and when he's questioned about it by his (incredibly freaked out) teammates, he doesn't hesitate to matter-of-factly state "ours is the only reality of consequence". While this seems unusually callous of Teal'c, Fridge Brilliance may be relevant: he's The Atoner, so he would be particularly willing to kill a version of himself who was still guilty of what the "real" Teal'c was trying to atone for.
In the episode Ripple Effect, Alternate Dr. Frasier (who somehow only popped up in one of the 20+ SG-1 teams along with Carter's snake-brained love-interest Martouf; both are dead in this world) comes from a version of Earth where the Ori plague was still ravaging the world and a cure was still unfeasible. Alternate Frasier outright demands that her reality be taken seriously by Stargate Command, and she receives help (their cure) from them.
Yes, she receives the cure, but only after being told multiple times by General Landry that he can't place her universe's needs above his own. For a while, it seems like this trope will be played straight and it is only because SG1 manages to save the day AND figure out a way to get the alternates back home that Alternate Frasier's Earth is saved.
The incident with Dr. Frasier's still-alive version turning up also nicely averts the 'Bob-2 doesn't matter' segment of this trope, in that everyone who had gone through her death was treating meeting this version as a rather special and warm opportunity.
The episode does also follow...or perhaps invert the trope: one of the other SG1 teams is planning to sacrifice "Earth-1" to save their own Earth. Technically, the other SG-1 wasn't planning on sacrificing Earth-1. They just wanted to save their Earth by getting the prime universe's ZPM, figuring that a three-week ride on the Daedalus instead of an Earth-to-Atlantis gate wasn't too bad.
Subverted in the audiobook "Gift of the Gods", which revealed that Daniel Jackson from "our" universe was Killed Off for Real before the episode "Fair Game" and replaced by an alternate universe counterpart.
The main role of O'Neill's friend Major Kowalski is to die in every single timeline, whether they find it or create it with time travel.
In the season 10 episode "The Road Not Taken", Carter and her counterpart in another universe are experimenting with an ancient device simultaneously. Something goes wrong and our Sam is transported to the other side. The other Sam wasn't so lucky. No one from the other side seems too upset about this, whereas our SG-1 is extremely worried for the duration of our Sam's absence.
Played with in the movie Stargate Continuum. The team gets sent to an alternate timeline where the stargate was lost at sea and the SGC was never founded. When they suggest that they use the stargate to travel back in time and set things back the way they were, the alternate universe Landry chews them out for thinking they had the right to alter the lives of every human on the planet. They wind up having to do this to save the earth by the end of the movie anyways. It just took them a few years for a threat to rise up.
Subverted in "McKay and Mrs Miller"; the techies have no qualms doing great damage to an alternate reality until they find out that life also exists in it (a chance that was considered astronomically small).
The penultimate episode: The episode is set in a separate alternate universe where Sheppard is a CSI-style detective in Vegas hunting down a rogue Wraith that somehow got to Earth. Alternate!Sheppard is chasing down the Wraith and ultimately ends up dying to take it out. His death is treated tragically, however before the Wraith dies, it transmits a signal throughout the Multiverse shouting Earth's location. The Alternate!Woolsey's response to this is that it's pointless to worry about saving every possible universe and is sufficiently pleased to have prevented the invasion in his own. Unfortunately for the primary versions of the cast, the message makes it to their universe, setting up the finale.
In fact, the only reason the Wraith in the primary universe even get the message is because they just happened to have upgraded their Hive ship with a ZPM at exactly the right time to have more sensitive sensors.
Sliders both avoids and endorses this trope through its first two seasons. Despite an agreement not to interfere with the worlds they arrive in, the Sliders tend to get involved in local politics and generally try to make things better. Unless they are on any kind of doomed world, in which case they typically just try to survive until the jump, unless the apocalypse will come before the wormhole, in which case they're destined to stop it.
They also show a great deal more concern when one of their doubles dies, and have at several points considered staying behind to "fill the gap", before being talked out of it by the others (usually Arturo).
As the series went along, things became more polarized overall on this subject. Season 4 has Quinn refer to his home Earth (the one the show started at, not the one he was born on) as 'Earth Prime' constantly, and many episodes focus on how "wrong" a world is when X happened instead of Y (like episode 2 of that season, where they encounter a world focused on religion instead of technology. Because all of the science didn't somehow predate their modern technology, it was somehow backwards... all somehow gathered from a glance at a newspaper). This is the same season that introduced a massive cross-reality war between mankind and their Cro-Magnon ancestors.
Strangely, only two worlds seem to have produced Kromaggs. Both times alongside humans. In one case, the humans ended up kicking the Kromaggs out and, when the Kromaggs obtained sliding tech from an alternate Quinn (the one from the pilot), they start on their interdimensional conquest. The other world has Kromaggs treated like second-class citizens with an underground movement to help them.
Similarly, in Doctor Who, Mickey decides to stay in the parallel universe, replacing his counterpart Ricky, instead of going home, because he feels he can help make this world better. (And because his alt!grandmother is still alive.)
And Rose is considered to have as much of a "happy ending" as she can without the Doctor — her mother and father are reunited. Only it's the parallel counterpart of her Dad — the home version is still dead, and the local version of her mother is not around for various reasons.
The season 4 finale confuses things once more by having Rose make a herculean effort to contact the Doctor to warn him of a crisis that threatens every universe. The Doctor's world isn't so far into crisis as the alternate one, where "the stars are going out". By the end, a reshuffle has taken place: Mickey's granny is revealed to have died, and he and Rose have concluded their unfinished business; so he returns home. The Doctor's almost-clone goes with Rose and Jackie to the alternate universe. Meaning that there's a character in the alternate universe who is - sort of - the counterpart of a character in the Doctor's universe, even though he originates from the Doctor's universe himself. Confused?
The original series also subverts this by having the Doctor traumatized by seeing an alternate Earth being destroyed in the story "Inferno".
Deconstructed in Farscape's final season episode "Prayer". Long story short, John needs some information, and to get it he has to kill someone in an alternate universe where everyone on Moya was combined for some reason. And that someone is the combined Chiana-Aeryn, Aeryn being his love and Chiana being his little sister-figure. He points his gun, she starts begging for her life in a way that makes it clear she doesn't take it seriously because she can't believe John would do this...a tear rolls down her eye...John puts down the gun, says he can't do it. Which is probably why he brought his arch-nemesis/shadow, who predictably grabs John's hand and the gun and executes Chiaeryn. He also had reason to believe that they were all going to die within the arn if he hadn't become involved. This is hopefully why he shrugged off the deaths of two other crewmembers fairly easily.
In Lexx, they simply abandon their own universe and jump to another after dooming the first one to get consumed by an armed menace...
In 7Days, it is not unusual for most of the main cast to have killed each other before a Backstep.
There is also an episode where the Sphere glitches, and Frank ends up in a Mirror Universe, where the US is a dictatorship, everything is written backwards, and all characters are their polar opposites. Naturally, some characters get killed, and Frank gets back to his own reality.
In Kamen Rider Decade, something is causing alternate realities to meld together, so the Kamen Riders entrust Decade with the task of destroying dimensions in order to stop the chaos. While traveling the dimensions (almost all alternate versions of the past Kamen Rider shows), Decade instead befriends the other Riders and helps them solve potentially world-shaking crises before moving on. In the final arc, the original Riders call Decade to task for not doing his job, and turn on him. However, it ends up a subversion, as it turns out that destruction was the correct course of action. Decade's goal was to bridge the worlds, then destroy them to end the merging, at which point those connections would bring everything back as it was and restore balance to the multiverse.
Played with, and ultimately subverted, in Fringe. At first, it seems that Walter feels this way about his counterpart when he steals his counterpart's son after our Peter dies of a rare disease, but we later find out he intended to sent Peter back after curing him. Because of our Walter's action, which has also caused fissures in reality and mass casualties in the parallel universe, Walternate felt this way about OUR side, and used his position as Secretary of Defense to prepare for a war with it. Most of the third season is spent with episodes switching between universes, enabling the audience to gain sympathy for the parallel universe while believing that only one universe can survive, until the season's final episode shows that the opposite is true - the survival of each universe is dependent on that of the other, and if one is destroyed the other will ultimately fall apart as well, so they have to work together and learn to trust each other. In the fourth season, Walter's dealing with a lot of guilt over the damage he did to the parallel universe. So, thoroughly subverted in the end.
Interestingly, Walter originally created the interdimensional "window" to study a universe with more advanced technology. One of his demonstrations is a RAZR-like cell phone (in the 80s), which he has copied from the other side and shows to the generals.
In Supernatural, this occurs in the alternate universe that is our (or something like it) reality, and in others. It is also inverted in universes in which people live. Averting the Titanic disaster, for example.
Community episode Remedial Chaos Theory explores this. Jeff rolls a die to decide who gets pizza and creates six different timelines, each one of them starting events that develop depending on who leaves the group. In the main timeline, Abed catches the die, but in the timeline where Troy leaves, things go very bad, very quickly. Pierce gets shot in the leg and dies, Annie gets locked away in a mental institution, Jeff loses an arm, Troy destroys his larynx, Shirley falls Off The Wagon and Britta...dyes her hair blue. Abed and Troy decide that since this is obviously the darkest timeline, they should commit to being evil and find way to the main timeline, kill their alternate selves and reclaim their lives. This becomes a plot point later in the season, as Abed starts seeing Evil!Abed in times of great insecurity, culminating in Evil!Abed taking over Abed's body in the season finale. Of course, since Community is not a sci-fi show, any of this may just be Abed's imagination.
In an episode of Earth: Final Conflict, Liam and Augur have a shuttle accident and find themselves in an alternate universe where the Taelons are invading Earth. Human civilization has also taken a different track, eschewing large settlements and preferring to live in harmony with nature with trade centers being the only permanent places with structures. Nevertheless, they are better prepared to fight the Taelons due to millennia of conflict amongst themselves (they have energy weapons, for example). A number of La Résistance members in this 'verse are killed (including Sandoval's double Jason, who is the leader), but Liam and Augur (along with Jason's girlfriend) manage to escape. Only a few episodes mention the other universe, but it is quickly forgotten.
The GURPS setting Infinite Worlds notes that neither Infinity nor Centrum quite view people from other worldlines as real people. Among other things, it notes that some filmmakers have taken to filming real battles on other worldlines with thousands of people dying to use as footage in their historical epics.
Also in GURPS, the Inertial Brake device in Warehouse 23 is described as working by dumping the ship's inertia into an alternate timeline, where it will likely cause some localized disaster.
BIONICLE does this when Takanuva ends up in an Alternate Universe rules by a corrupt Empress. That serial featured more on-screen deaths than the rest of the canon combined at the time of writing. It's a partial subversion, since Takanuva is genuinely shocked by some of the things he sees, such as his younger self getting impaled by iron spikes, and he tries to help the people in that universe as well as reaching his own goals of getting home, but after he gets out of there , (And accidentally cuts the Empress in half by way of a closing interdimensional portal), he takes only a few moments to wonder about the fate of that universe before continuing on with his quest.
Played straight when a bunch of Takanuvas are taken from their original universes, turned evil, and then get smashed to bits by one of the good guys with a Warhammer. Yes, it's a Crowning Moment Of Awesome, but when that Fridge Logic hits you, it hits hard.
Fire Emblem Awakening averts this in a big way. Since a large amount of characters are from alternate futures where the heroes have had a lot less luck, in a Fire Emblem game especially, seeing any character as expendable can doom you down the road. And then there's how the characters, ingame, feel about the future characters. All of the future characters are children of the 'present' heroes.
LEGO Star Wars. "In an infinite universe, all things are possible..." Though it's really just the one parallel universe where you must blow everything up to collect a million Lego studs - despite the fact that you're using Star Wars Lego figures on a generic suburban Lego Town...
In Star Ocean The Last Hope, the party accidentally travel to a parallel universe where Edge indirectly destroys an alternate 1960s Earth and everyone on it. The rest of the party plays this trope completely straight, writing off Edge's accidental genocide as unimportant since it was just a parallel world that took the damage. However, Edge himself averts this trope by realising just what he has done, and in response has a Heroic BSOD that lasts for the next ten or so hours of the game.
The game itself plays this straight, as no matter what the characters' opinions are, the event has no importance to the plot besides making Edge angst and adding Meracle to the party.
Lampshaded in Ben There, Dan That!, when Ben remarks that he doesn't need to worry about disposing of anyone's corpse, since he's in a parallel dimension, and thus none of his actions have any meaning beyond their contribution to the accomplishment of his goals.
Averted in Muv-Luv where Takeru cares about his friends no matter what universe they're in.
Inverted by the protagonists in BioShock Infinite, who on several occasions solve their problems by hopping to an alternate universe where things are better for them with no intention to make it back to their original universe. The gunsmith they needed is dead? Go to a dimension where he's alive. His tools confiscated by the police? Hop to a dimension where the police didn't take them.
Averted in the Burial At Sea DLC, which is centered around Elizabeth going to other universes and trying to right the wrongs by the Comstocks of every reality and reuniting the Bookers with their daughters.
Played straight by the Big Bad of the story, who firmly believed that only his offspring could rule Columbia after his death; unfortunately, constant exposure to dimensional travel had sterilized him, so he decided to get around this by opening a portal to another world and stealing the daughter of his alternate self.
Interestingly, the Many Worlds interpretation of quantum physics can be interpreted in a way that this is how Star Trek-style transporters work. They kill you here and build a clone out of the atoms that are somewhere else. Normally, quantum physics is interpreted as only having one of each particle, but it could be interpreted as that there's multiple particles, and the alternate universes where two are swapped is just as likely. As such, there's a universe, just as likely as this one, where the atoms you're being built of now made you up before. You're effectively being transported to an alternate universe where your atoms are already there. Alternate you gets transported to this universe by the same method.
Also by Kris Straub: In Checkerboard Nightmare, some kind of time-travel mishap flings past-Chex into the present. Upon realizing that the many-worlds hypothesis must be in effect, present-Chex fixes this problem by killing his past self.
In Narbonic, a Time Travel device is powered by harnessing all the energy in, and in the process destroying, alternate universes "where they probably don't want to exist as much."
Inverted in Goats, when one character discovers that the "simulations" he's been running and deleting are actually real universes (at least as real as the one he lives in, which is also being run on a computer somewhere) being destroyed, and immediately has a Heroic BSOD, moaning that he is the worst mass-murderer in history.
Kevin & Kell also played with this trope - with Kevin's sister, Danielle, dying in a Heroic Sacrifice, only to be replaced with her double from the Human world (long story). Most of the main cast knows the truth, but have accepted her as part of the family; even if a few would rather believe that she returned from the dead than she's from another dimension.
Averted in the To Save Her spinoff of Pastel Defender Heliotrope; even though there are many, many parallel universes, Altenates of the same character are deliberately treated as separate people, even when the alternate of a dead character steps in as a Suspiciously Similar Substitute of the original.
Averted and played straight in Sluggy Freelance. The main characters are surprisingly apathetic when they set up their TV to view alternate universes, and discover most of the Myriad Ways end with one of them causing Armageddon. (It being a filler-guest story made in cut-and-paste style may have had something to do with this.) This is averted, however, during the "Aylee" and "That Which Redeems" arcs, where Torg tries his hardest to save alternate versions of the Sluggyverse; the death of the Zoe from the Dimension of Lame is actually one of the saddest moments in the series, and profoundly affects Torg from there on out. Then again, there was the Punyverse... it blew up, all of it, and it was a very remote reality with no obvious alternative versions of anything in the usual dimension, but they got away and just didn't care, presumably because it was such a stupid place and impossible to take seriously. To be fair, The punyverse was being destroyed by an atomic chain reaction. There was nothing they could do but escape, and they had only seconds to escape in.
Subverted in General Protection Fault: the rest of the cast thinks Trudy is actually her counterpart from the "Emperor Nick" universe. That Trudy switched with "the real" Trudy, to give the latter a chance at true redemption. (So what they think is a new Trudy is actually still the old Trudy)
When Karkat realizes that the demon who showed up from another universe and destroyed his game session was actually an alternate-universe version of Jack Noir, the Derse agent who helped him overthrow the Black Queen, Karkat says he feels betrayed even though he knows they're technically separate people.
When Dave travels back in time to prevent some bad decisions that doomed his timeline, he gets annoyed at being called "the other Dave" or any implication that he's not "really" Dave. Meanwhile, in the doomed alternate future, the readers actually see his doomed timeline blink out of existence. Rose, at least, survives this event in a fashion by merging with her past self.
Later, Aradia pulls the same trick as Dave about a couple hundred times, and ends up with an army of alternates of herself from doomed timelines. The alternates know beforehand that they're doomed to premature death—it's one of the laws governing time travel in paradox space—and they're okay with this because Aradia's completely emotionless. When all of the clones do get slaughtered, her friends don't seem terribly concerned about this, mostly because they have bigger problems to deal with than the philosophical implications of time travel clones.
Getting back to Dave: Due to weird time shit, he occasionally encounters corpses of himself from alternate timelines, and he's more creeped out by this than his stoic facade will let him admit. When Terezi tells Dave that he can achieve god-tier power by killing one of his doomed-timeline clones—who, as above, is doomed to die somehow, anyway—Dave refuses to do it. Then Terezi breaks down crying after seeing the doomed Dave die, even though alpha timeline Dave is still alive.
When we get a look at the Homestuck afterlife, we see that doomed-timeline alternates—Dave, Aradia, and a doomed John—are also there, and their souls are separate from their alpha timeline selves. This has created some weird moments, such as when John learns that his doomed timeline self dated Vriska and she treats him with flippant familiarity, even though this John never got to know her too well. Then, later, a dreambubble is destroyed, killing all of the dead alternates in it, which the fandom did actually respond to with genuine sadness.
Terezi actually has the power to split timelines, by flipping a coin and placing separate courses of action on either outcome. She was responsible for setting up Dave's scenario of killing himself, as each represented whether she'd assist Dave in reaching the god-tier or not. She later repeats the trick by flipping a coin to decide whether to kill Vriska or not. The timeline where she lets Vriska go results in Jack finding and killing them all.
Although the actual coin flips she does are entirely pointless, as she points out: for the first one, she didn't even look at the result; rather it was Dave's decision of which result counted as "show me now" or "show me later" which split the timeline. In the second case, the coin flip was constant across all timelines, because Vriska manipul8ed the coin to get her preferred outcome. The two timelines were entirely a result of her decision of whether or not to go through with killing Vriska.
The fandom was actually pretty sad to hear that the alternate Rose and Dave from the Alpha universe were dead, even though the main timeline Rose and Dave are still fine.
Final Blasphemy has Jeremy being shown several possible futures the author had planned, some of which had alternate timelines.
In Wapsi Square, the plan to save Jin resulted in the deaths of many versions of Shelly from previous versions of the "Groundhog Day" Loop. Granted, these Shellys only had around a year left before the timeline reset, but they still died.
In Goblins we have the Maze of the Many, a multiversal dungeon crawl where alternate versions of the player characters combat each other and the dungeon to get to the treasure room first. If a party gets wiped out or does not reach the treasure in time, they pop back into existence when they entered, their memories of the dungeon lost. "Our" versions of Minmax, Farogath and Kin have failed over two million times. The author throws in a filler detailing the background of some of the alternates every now and then.
In the Darwin's Soldiers tie-in story Card Of Ten, almost a team of anti-matter duplicates of the main characters is killed, but it has no effect on the main storyline.
In Ink City, Caroline justifies her decision to help Starscream find a way to destroy Optimus Prime this way. Yes, if they're successful, it puts a universe filled with innocents at risk... but it's not her home world that's at stake.
An Alternate Universe counterpart of SCP-507 (justifiably and correctly) worries about this happening, and tries to hide his identity by cooperating with the 'real one' to refuse to disclose which of them is 'original' to that universe. Neither is, and their fears prove perfectly founded.
Transformers generally avoids this by being a multiverse rather than a universe, and thus all timelines are valid, and occasionally even have an effect on each other. Time Travel stories generally involve a large tear in the fabric of time and space, and any threat coming through from another universe where things happened differently tends to mean something is terribly, horribly wrong, often to the point of Unicron being involved.
In fact, later Transformers materials have indicated Unicron is a multiversal constant—one of the few things that is unchanged in the entire multiverse. If he is destroyed in one universe, that's fine—he still exists in the others. This doesn't prevent them from messing with each other, though—in Marvel's G1 comic, the Unicron of the Marvel continuity plucked Galvatron from both the future and an alternate universe to serve as his herald—presumably since Megatron, in that reality, was dead, and that Galvatron (and Unicron) had succeeded.
And the British comics go further then this by also having another, much stronger, Galvatron from a different future who came back in time to kill Unicron. He abandons this plan when he accidentally changes history and realizes that, since he continued to exist, he was just in an alternate timeline. This is a slight justification to Galvatron not caring about the fate of a universe that wasn't his own though; he's a bit of a jerk and more than a bit mad.
Futurama plays with this trope in one episode. When the Planet Express crew arrives in an alternate, palette swapped universe, they argue with their counterparts over whose universe is Universe A. ("Why do we have to be 'B'?" "This place kinda feels like a B...") Eventually they decide on the names Universe A and Universe 1. Also both Benders genuinely care about their perpendicular universe counterparts, because if there's one thing Bender loves, it's Bender.
Narrator: Meanwhile, in Universe A, Hermes A heads towards the sun. ...A.
In Bender's Big Score, Professor Farnsworth and the Harlem Globetrotters discover a law of physics detailing how the universe protects itself from time paradoxes: It arbitrarily kills any clones created via Time Travel. This happens several times over the course of the film; some clones are mourned more than others.
In "The Late Phillip J. Fry" a time machine that can only go forward in time takes Fry, Bender, and the Professor further and further into the future, until the universe they knew dies out and a new universe, completely identical to the old one, comes into being. They consider this a completely acceptable substitute for the real thing. This is actually an inversion, since it portrays the original universe as expendable and the original Leela was shown missing Fry and being miserable for the rest of her life, but Fry isn't particularly concerned because he just continues off with the new identical Leela in the new universe. It's also partially played straight in the Fry, Bender, and the Professor kill their counterparts upon arriving in the new universe and aren't particularly concerned.
There is also an episode where the crew takes Fry on a tour of the Universe and its tourist attractions. One is a scenic outlook to a parallel universe. The difference? Everyone in the parallel universe is wearing cowboy hats. When Fry asks about there being an infinite numbers of these parallels, he is told that there is in fact only one parallel to their own. His response is that's probably enough. It's also why the alternate universes mentioned above are referred to as "perpendicular" rather than parallel.
Justice League had an inverted version of this: in an alternate universe where Lex Luthor kills The Flash, Superman returns the favor, and the Justice League becomes the "Justice Lords," a totalitarian force that ensure peace and justice by suppressing dissent and lobotomizing former villains. When they find out about the series' main Earth, they decide to head over, incapacitate the Justice League, and "show them the light". The Justice League's response is a pure embodiment of this trope: they depower the Justice Lords and apparently stick them in prisons on the Justice League's Earth. So much forthatworld. (They still have their Batman, though. They'll be fine.)
J'onn J'onzz: You understand that if we do change the past, you — this version of you — will never have existed? Alternate Batman:Nothingwould make me happier.
The crowning irony of this statement is that alternate Batman was 'created' by Vandal Savage murdering his parents instead of Joe Chill. How he would feel if he'd known they'd be murdered either way is never revealed, because none of the league have the heart to tell him. On the other hand, Alterbat seems to be saying, "I risk my life every day fighting Alternazis. Death was always a possibility. Go get the bastards!"
There's also an episode where Superman is seemingly killed (in fact, thrown into the far future), and without him the Justice League fails to stop another plot by Vandal Savage. Which doesn't turn out the way he plans and instead wipes out nearly all life and hideously mutates the rest. Except for Savage himself, who being immortal is unaffected. After centuries in the literal hell on Earth he created, Savage comes to regret his villainy, and teams up with Superman to prevent himself form defeating the Justice League by sending Superman back in time. Once they succeed, the heroic Savage fades into nonexistence, assumably leaving only the usual supervillain version. The heroic version of Savage considers that an acceptable sacrifice. Superman doesn't seem to give it much thought at all.
Off course he didn't, the entire planet was wiped already wiped out save for Vandal Savage.
In the not-officially-related Justice League: Crisis on Two Earths when Batman's Evil Counterpart Owlman learns that every decision creates a new universe he decides that not only do other universes not matter, but nor does his own, because any choice he makes is meaningless. After all, in another universe he automatically made a different one. He resolves to destroy Earth-Prime to destroy the multiverse in response.
Ignoring the fact that once he shows up there and reintroduces human existence to the apparently-dead world, he vastly increases instability and uncertainty, reintroducing decision and instantly splitting off infinite universes (which he and his bomb then inhabit) in which he succeeds or fails in infinite ways. Batman, too, ignores the fact that any triumph over Owlman is just as meaningless because it happened differently on another world that split off.
Fridge Brilliance sets in when Owlman's Last Words ("It doesn't matter") and expression indicate that he knows this, meaning he's succeeded somewhere else.
By definition, Owlman couldn't succeed: Every iteration of Earth Prime that Owlman ends up on has already split off due to the choice made to move there from an alternate reality. The odds he'd end up on the 'correct' Earth Prime is one in infinity. Even in the iterations where he succeeded, the 'true' Earth Prime remains intact and thus The Multiverse.
Alternatively, you could assume Owlman accounted for these objections and that there was something different about Earth-Prime that prevented the normal splintering caused by decisions. In any case, the story overall could be seen as a subversion of the trope because the Justice League (sans Batman) seem to take genuine interest in fixing the alternate world's problems irrespective of it's impact on their own world.
In fact this is explicitly stated as being the case, all other worlds split out from Earth Prime. Earth Prime is a "Fixed Point" and obliterating that reality will negate every variant spreading out from it.
In the underrated and at-times quite creepy Fox's Peter Pan and the Pirates, one episode has Peter and the gang entering an alternate universe where copies of Peter Pan, Wendy and the Lost Boys are slaves, toiling endlessly to prevent the Corc (their version of the crocodile that Captain Hook and everyone else are afraid of) from awakening. Peter Pan et al have an adventure in this alternate universe, but are unable to save these copies of themselves from their lifetime of misery and slavery. The copies help the originals escape, and that's it.
Subverted in Batman: The Brave and the Bold: When a Depopulation Bomb threatens to kill all life on Earth, Batman and Red Hood teleport it to another dimension. It turns out (as in, we find out they already knew) that dimension's earth was made up entirely of zombies, so the bomb's explosion was completely harmless.
Averting this trope seems to be the logic Samurai Jack lives by in his Bad Future. He repeatedly turns down the chance to return to the past and kill Aku before he could ever take over the world and gives it up to help his new friends. While this may seem stupid in that he'd negate the Bad Future so he'd never need to save his friends, it may be that he's just too kind to risk that this trope is true. Thus the best option would be to destroy Aku in the future and then return to the past and destroy him there.
One episode where he finds a time portal seems to indicate that he eventually will do exactly that; the portal's guardian indicates that only The Chosen One may use the portal and trounces Jack in combat. After Jack has been knocked out, the portal itself tells the guardian not to kill him; he may not be the chosen one, but he will be many years later, when he returns with a beard, a crown, grey hair, and an army of liberated peoples from all over the planet.
An interesting idea from the TV special Rudolph's Shiny New Year. Father Time explains that, every New Year's Eve, a new island appears in the Archipeligo of Last Year. On these islands, "time stands still", and the world remains as it did during that year forevermore. Since this is a children's show, we get to see The Theme Park Version of a couple of years (One Million BC is depicted as a paradise of jolly cavemen and singing dinosaurs, 1776 is defined by Independence Day and Ben Franklin, and 1023 is apparantly the year in which all the world's fairy tales took place), but Fridge Logic sets in after that. Think about it; in 1964 The Beatles will never break up, but at the same time the Civil Rights movement will barely get off the ground. In 1944, World War II will wage forever, Pearl Harbor is still in flames, and Hitler will never die. In 2001... need we go on? And there is the implication that all the people on these islands are the same people who actually existed in these time periods, meaning that somewhere out there are multiple versions of you, one for each year of your life, trapped forever in a nebulous 12-month span, never to grow, or learn, or live on. And if you indeed weren't from the real world and merely were a reflection living upon one of the islands, how would you ever know?
Inverted in Rick And Morty: in "Rick Potion No. 9", Rick and Morty inadvertently cause the end of the world and the former simply takes them to an alternate universe where they both died and take their counterparts' place, making this an example of and Expendable Main Universe. Morty is clearly traumatized by the notion that his friends and family are all doomed and he will spend the rest of his life with identical strangers.
Given how nonchalant Rick is about this, it's quite clear that he considers every universe to be this.
alternative title(s): What Measure Is An Alternate Reality; Who Cares About Alternate Dimensions; Who Cares About Alternate Timelines; Who Cares About Alternate Universes; Anyone Can Die In Other Universes; All The Myriad Ways