History Main / EquivalentExchange

!!!TropeNamer:

* As seen in the description, the First Law of Thermodynamics makes this trope not just OlderThanDirt, but Older Than Everything But The Universe Itself. ''Manga/FullmetalAlchemist'' may be the TropeNamer, but it was UsefulNotes/IsaacNewton (who had a passion for alchemy himself) who was the historical TropeCodifier ("For every action there is an equal and opposite reaction"), forming the fundamental basis of physics, and indeed, all of science itself. The ''second'' law of thermodynamics states that you can't really get even equivalent exchange -- you'll always "lose"[[note]]In scare quotes because the energy still ''exists'', it's just not doing anything useful.[[/note]] some of the input energy to waste heat.

* ''[[Fanfic/MiraculousLadybugSaltShots Miraculous Ladybug Salt-Shots: A Price to Pay]]''; Thanks to Adrien pulling a FaceHeelTurn, Hawkmoth/Gabriel gets his hands on the Miraculouses he needs to MakeAWish to bring his wife Emilie back. Despite being warned that this will result in [[BalancingDeathsBooks somebody else dying]], neither he or his son care: since reality will be rewritten in the process, both assume that nobody will be able to connect the deaths to ''them''. But the Wish proves them wrong: the sacrifice turns out to be Marinette's dad, who gets killed in a car accident... caused by Gabriel. While he [[ScrewTheRulesIHaveMoney bribes the judge]] to escape legal consequences, Emilie is so enraged by her husband's blithe attitude over taking a life that she divorces him and [[IHaveNoSon disowns Adrien]] for defending his father. Thus, while the ExactWords of their Wish was granted, Gabriel is still seen as a murderer, and Adrien is judged harshly as well.

* ''Fanfic/MiraculousLadybugSaltShots'': Seen in ''[[https://archiveofourown.org/works/44607043/chapters/121613383 A Price to Pay]]''; Thanks to Adrien pulling a FaceHeelTurn, Hawkmoth/Gabriel gets his hands on the Miraculouses he needs to MakeAWish to bring his wife Emilie back. Despite being warned that this will result in [[BalancingDeathsBooks somebody else dying]], neither he or his son care: since reality will be rewritten in the process, both assume that nobody will be able to connect the deaths to ''them''. But the Wish proves them wrong: the sacrifice turns out to be Marinette's dad, who gets killed in a car accident... caused by Gabriel. While he [[ScrewTheRulesIHaveMoney bribes the judge]] to escape legal consequences, Emilie is so enraged by her husband's blithe attitude over taking a life that she divorces him and [[IHaveNoSon disowns Adrien]] for defending his father. Thus, while the ExactWords of their Wish was granted, Gabriel is still seen as a murderer, and Adrien is judged harshly as well.

* ''[[https://archiveofourown.org/works/44607043/chapters/121613383 A Price to Pay]]'': Thanks to Adrien pulling a FaceHeelTurn, Hawkmoth/Gabriel gets his hands on the Miraculouses he needs to MakeAWish to bring his wife Emilie back. Despite being warned that this will result in [[BalancingDeathsBooks somebody else dying]], neither he or his son care: since reality will be rewritten in the process, both assume that nobody will be able to connect the deaths to ''them''. But the Wish proves them wrong: the sacrifice turns out to be Marinette's dad, who gets killed in a car accident... caused by Gabriel. While he [[ScrewTheRulesIHaveMoney bribes the judge]] to escape legal consequences, Emilie is so enraged by her husband's blithe attitude over taking a life that she divorces him and [[IHaveNoSon disowns Adrien]] for defending his father. Thus, while the ExactWords of their Wish was granted, Gabriel is still seen as a murderer, and Adrien is judged harshly as well.

* The fountain in ''Film/PiratesOfTheCaribbeanOnStrangerTides'' takes life from one to extend another's. However, the rate of exchange isn't exactly equal. The one on the receiving end gets the person's entire natural lifespan, regardless of their age or actual life expectancy, meaning that the recipient is going to get around 100 years from the deal no matter the sacrifice.

** In the final chapter, it's revealed that time travel also works this way [[spoiler:through Felix's actions in the original timeline]]. When Master Fu regains his memories of all the past timelines and hells Marinette and Adrien get theirs as well, he explains to them [[spoiler:that after Felix defeated his father, who was the Butterfly villain Papillon, but lost his LoveInterest and partner Bridgette/Ladybug in the process, he steals the Rabbit Miraculous and goes back in time [[SetRightWhatOnceWentWrong to prevent the deaths of both Bridgette and his mother Amelie, the latter of which was what set his father to become Papillion]]. While it did save both of them, as the universe demanded a balance it led his aunt Emilie to fall into a coma, while her husband Gabriel would kill Felix's father for the Butterfly Miraculous. His own selfish act also caused Felix's soul to be tainted in the later timelines, [[TookALevelInJerkass with him becoming more selfish and cruel]] because he can't physically remember his past sins while his soul does, and the changes in his personality [[AllForNothing means that even with Bridgette alive, the two can no longer get together]]]].

** Once one becomes an [[PsychicPowers esper]], they can never use magic, and vice-versa, without suffering extreme damage to their bodies.

* This was part of the side effect in ''Fanfic/AlyaAndTheHaremReality'' where Alya desperately tries to stop Monarch from using the Ladybug and Cat Miraculous by wishing that people love Marinette in an attempt to waste the wish with something seemingly harmless. The result is that in the new timeline Marinette [[{{Polyamory}} is a triage with Alya and Chloe]] and Kagami also shows romantic attraction to the former. In exchange, Adrien was never allowed to leave his home and develop romantic ties with either Marinette or Kagami, while Nino was never locked in a cage with Alya or fell in love with her. In addition, some relationships are more distant or toxic, such as Chloe with her father or Kagami with her mother.

** In the final scene of "[[Recap/TheTwilightZoneS3E98TheDummy The Dummy]]", Jerry Etherson and Willie have switched places so that Jerry has become the dummy and Willie the ventriloquist. In his closing narration, Rod Serling says that he has gone from "boss to blockhead."
** In "[[Recap/TheTwilightZoneS4E113TheParallel The Parallel]]", Major Robert Gaines loses contact with Earth and is accidentally transported to a parallel universe in which he is a colonel. At the end of the episode, shortly after he returns home, the space program receives a transmission from Colonel Robert Gaines.

** In "The World Next Door", Barney Schlessinger and his counterpart from the RetroUniverse switch places as each wants what the other has in life. The Barney of our universe has longed to be a famous and successful inventor for years. He is delighted when he meets the alternate version of his wife Katie, who admires "his" inventions greatly as opposed to his own Katie who belittles him for them. The alternate Barney has grown tired of the pressure that comes with fame and people's expectations for the next great invention so he settles in our universe, where he can live in blissful anonymity and doesn't have to invent anything else if he doesn't want to.
** In "The Convict's Piano", Ricky Frost is able to escape his [[MiscarriageOfJustice unjust prison sentence]] in 1986 when he plays "Someone to Watch Over Me" on the old piano and is transported back in time to 1928. He realizes that he has picked the right song and the right time because he remains in the past after he stops playing the piano, in contrast to his previous trips to 1899 and 1917. The gangster Mickey Shaughnessy is transported forward in time to 1986 when he plays "S' Wonderful". [[PointOfNoReturn He is trapped 58 years in his future]] as the piano is subsequently destroyed.
** In "Something in the Walls", the creatures that live in walls are able to enter Sharon Miles' room at the Crest Ridge Sanitarium by coming through a crack formed by a leak. They absorb Sharon into the wall and one of them assumes her form and takes over her life.
* This appears to be how the Phoenix Talisman works in ''Series/{{Warehouse 13}}'', though its behavior is not consistent. It will resurrect its user after their death, and then the trauma of that death will be transmitted to a random person in proximity. However, it's not always a one-to-one substitution; during Mac Pherson's demonstration of the Talisman, two random mooks die in exchange for the single demonstrator. It may be proportional to the amount of trauma the user suffers.

* If a wizard [[RitualMagic ritually]] sacrifices a cat to get some magical mojo, what determines how much power the wizard receives: the value of the cat's life to the wizard, the value of the cat's life to the cat, or the value of the cat's life to a deity (the latter is often why a VirginSacrifice is considered better)?

[[quoteright:350:[[Manga/FullmetalAlchemist https://static.tvtropes.org/pmwiki/pub/images/fma_ee.png]]]]
[[caption-width-right:350:You can't make a metal weapon without any metal first.]]

* In ''Fanfic/MementoVivere'', a ''VideoGame/FinalFantasyX'' fanfiction, Rikku’s dilemma is deciding whether to pursue a romance with Auron at the risk of [[spoiler:bringing Sin back]].

* In ''Franchise/PowerRangers'' there are some examples of the SixthRanger being limited due to their awesome power. The most notable were the Green Ranger and the Titanium Ranger. Rita created a magic candle linked to the Green Ranger’s powers, such that every time he used them, it would burn, and if the candle burned out he would die. The Titanium Ranger was branded with a cursed tattoo of a cobra on his back. Every time he morphed, the cobra would move up a little. If it got to his neck, he would die.

* The ''VideoGame/MonkeyIsland'' games have this as an element of Voodoo magic. In ''VideoGame/TheCurseOfMonkeyIsland'', Guybrush needs to find a diamond as big as or bigger than the one that turned his fiancé to stone. In ''VideoGame/EscapeFromMonkeyIsland'', he needs to make an Ultimate Insult talisman as large as or bigger than the original in order to counter its effects. (No explanation is given on why his enemies can counter Guybrush’s insults even when he has the larger talisman, although it ''is'' only the ''second'' biggest monkey head Guybrush has ever seen...)

** Although, as the laws of thermodynamics are also technically just statistical laws, there is nothing in physics preventing it from happening, especially at the quantum level, but any large scale reduction of entropy is so vanishingly unlikely that it’s not worth discussing. Unless the universe was perfectly uniform, even more incredibly cold, and with no usable energy, aka maximum entropy, [[NaturalEndOfTime heat death]]. This could allow the entire universe to act as one quantum object and after effectively forever, the entire universe could quantum tunnel [[DefiedTrope back to minimum entropy]], meaning a new [[CausedTheBigBang Big Bang]]. If this could happen once, it could happen and could have happened [[EternalRecurrence many, many times]], to say the least. This universe also includes the more likely but still unlikely scenario of entropy reducing just enough to create a [[https://en.m.wikipedia.org/wiki/Boltzmann_brain Boltzmann Brain]] of [[ParanoiaFuel a human brain filled with]] [[CosmicHorrorStory memories of an ordinary early 21st century life]].