Predestination paradox

A predestination paradox, also called a causal loop or causality loop, is a paradox of time travel that is often used as a convention in science fiction. It exists when a time traveller is caught in a loop of events that "predestines" him to travel back in time. This paradox is in some ways the opposite of the grandfather paradox, the famous example of the traveller killing his own grandfather before his parent is born, thereby precluding his own travel to the past by cancelling his own existence.

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Because of the possibility of influencing the past while time travelling, one way of explaining why history does not change is by saying that whatever has happened was meant to happen. A time traveller attempting to alter the past in this model, intentionally or not, would only be fulfilling his role in creating history, not changing it. The Novikov self-consistency principle proposes that contradictory causal loops cannot form, but that consistent ones can.

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Paradox: A paradox is an apparently true statement or group of statements that seems to lead to a contradiction or to a situation that defies intuition. Typically, either the statements in question do not really imply the contradiction, the puzzling result is not really a contradiction, or the premises them...

Time travel: Time travel is the concept of moving forward and backward to different points in time, much as we do through space. It also includes traveling sideways in time between parallel realities or universes....

Science fiction: Science fiction is a form of speculative fiction principally dealing with the impact of imagined science and technology upon society and persons as individuals. In common with most fiction, science fiction is written mainly to entertain people. The borders of this genre are not well defined, and the...