A causal loop is a paradox of time travel that occurs when a future event is the cause of a past event, which in turn is the cause of the future event. Both events then exist in spacetime, but their origin cannot be determined. A causal loop is also called bootstrap paradox, predestination paradox or ontological paradox in fiction.
The term "bootstrap paradox" refers to the expression "pulling yourself up by your bootstraps"; the use of the term for the time-travel paradox was popularized by Robert A. Heinlein's story By His Bootstraps. It is a paradox in the sense that an independent origin of the events that caused each other cannot be determined, they simply exist by themselves. Some works of fiction, for example Somewhere in Time, have a version of the paradox where an object from the future is brought to the past, where it ages until it is brought back to the past again, apparently unchanged from its previous journey. An object making such a circular passage through time must be identical whenever it is brought back to the past, otherwise it would create an inconsistency.
A self-fulfilling prophecy may be a form of causality loop, only when the prophecy can be said to be truly known to occur, since only then events in the future will be causing effects in the past. Otherwise, it would be a simple case of events in the past causing events in the future. Predestination does not necessarily involve a supernatural power, and could be the result of other "infallible foreknowledge" mechanisms. A notable fictional example of a self-fulfilling prophecy occurs in classical play Oedipus Rex, in which Oedipus becomes the king of Thebes, whilst in the process unwittingly fulfills a prophecy that he would kill his father and marry his mother. The prophecy itself serves as the impetus for his actions, and thus it is self-fulfilling.[page needed]
Bootstrap paradox in fiction
Day of the Tentacle
The cult video game Day of the Tentacle is a point-and-click adventure that includes time travel as a device to solve puzzles, as well as a plot device. Purple Tentacle drinks toxic waste and turns into a megalomaniac. Towards the end of the adventure it is revealed that it was a time traveling megalomaniac Purple Tentacle who polluted the water in the first place. The paradox is openly acknowledged in the dialogue.
The bootstrap paradox is used several times throughout the science-fiction series Doctor Who, most recently in the episode "Before the Flood", which begins with the Doctor explaining to the audience how the paradox works. At the end of the episode, The Doctor explains how his plan to save everyone came from viewing future events, which leads him and Clara to wonder how the plan originated in the first place, questioning the paradox.
The paradox was also prominent in the episode "Blink", in which Sally Sparrow, at the end of the episode, gives the Doctor a transcript of his half of a conversation during the episode, but before the events happened from his perspective. Then, when the events happen from his perspective, the Doctor reads from the transcript, for Sparrow to write down.
In the charity shorts "Space" and "Time", characters Amy, Rory and the Doctor are met with versions of themselves from seconds in the future who give them specific instructions based on what they'd seen seconds earlier.
In the charity short "Time Crash", the Doctor saves the Tardis in front of a younger version of himself from his past, but the method was incredibly complex and the younger Doctor was unaware of it, learning about it from his future self. His future self only knows how to do it because his younger self just witnessed it.
The causal loop was used in the series when the Flash's suit from the future was seen and then the modern Flash's suit was altered to match it.
The Flipside of Dominick Hide
In the 1980 BBC Television play, The Flipside of Dominick Hide, the protagonist travels back in time from the year to 2130 in search of his great-grandfather. During his trip, Dominick Hide develops a sexual relationship with a woman in 1980 who becomes pregnant. It is revealed that, as a result of this affair, Dominick becomes his own great-grandfather.
In the book and film of Harry Potter and the Prisoner of Azkaban, time travel plays a heavy role. When characters Harry and Hermione use a time turner, they go to the night before when Harry and Sirius Black were attacked by dementors, only to be saved by a mysterious figure whom Harry believed was somehow his deceased father. When they are watching and find that no one comes to save them, Harry saves himself, realizing that he was the one to do this all along. The paradox is further elaborated in the film.
In the version of the paradox in the film Predestination, the main character goes back to the past and impregnates their past self, becoming their own parents by being born from themself. The plot is based on the short story " '—All You Zombies—' " by Robert A. Heinlein, this story further develops themes explored by the author in a previous work, "By His Bootstraps".
The short story "The Skull" by Philip K. Dick is an example of a bootstrap paradox. The protagonist travels back in time in order to seek out and kill a man whose skull has since become a religious relic... only to discover that the skull is, in fact, his own.
The movie Time Lapse (film) is built entirely around the concept of bootstrap paradox. Don D'Amassa states that "The greatest difficulty in creating a story of this type is not so much the plotting of the various times loops, but to render them in such a way that the reader can follow the logic."
The Time Traveler's Wife
The book The Time Traveler's Wife by Audrey Niffenegger (which was made into a movie) feature the bootstrap paradox. Clare approaches Henry because he has appeared to her as a time traveler many times. But he has only appeared to her as a time traveler because he has fallen in love with her.
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