A causal loop is a paradox of time travel that occurs when a later (future) event is the cause of an earlier (past) event, through some sort of time travel. The past event is then partly or entirely the cause of the future event, which is the past event's cause. Since a causal loop has no independent origin, it is also called a bootstrap paradox or an ontological paradox.
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, thus they may be said to have been predestined to occur. Predestination does not necessarily involve a metaphysical power, and could be the result of other "infallible foreknowledge" mechanisms. The predestination paradox allows time travel to be self-consistent, similar to the Novikov self-consistency principle.
Bootstrap paradox in fiction
The bootstrap paradox has been used in fictional stories and films. In the 1980 romance film Somewhere in Time, based on Richard Matheson's 1975 novel Bid Time Return, an elderly woman gives a young man a pocket watch in 1972. He travels back in time to 1912 and gives the pocket watch to her, which she carries with her until 1972 when she meets the young man and gives the watch to him. The concept is named from the Robert Heinlein story "By His Bootstraps", which is considered the "ultimate time travel paradox tale" of its time. 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."
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. 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.
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