Grandfather paradox

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The grandfather paradox is a proposed paradox of time travel first described (in this exact form) by the science fiction writer René Barjavel in his 1943 book Le Voyageur Imprudent (The Imprudent Traveller).[1] The paradox is this: suppose a man travelled back in time and killed his biological grandfather before the latter met the traveller's grandmother. As a result, one of the traveller's parents (and by extension the traveller himself) would never have been conceived. This would imply that he could not have travelled back in time after all, which means the grandfather would still be alive, and the traveller would have been conceived allowing him to travel back in time and kill his grandfather. Thus each possibility seems to imply its own negation, a type of logical paradox.

Despite the name, the grandfather paradox does not exclusively regard the impossibility of one's own birth. Rather, it regards any action that makes impossible the ability to travel back in time in the first place. The paradox's namesake example is merely the most commonly thought of when one considers the whole range of possible actions. Another example would be using scientific knowledge to invent a time machine, then going back in time and (whether through murder or otherwise) impeding a scientist's work that would eventually lead to the very information that you used to invent the time machine.

An equivalent paradox is known (in philosophy) as autoinfanticide, going back in time and killing oneself as a baby.[2]

The grandfather paradox has been used to argue that backwards time travel must be impossible. However, a number of possible ways of avoiding the paradox have been proposed, such as the idea that the timeline is fixed and unchangeable, the idea that the time traveller will end up in a parallel timeline, while the timeline in which the traveller was born remains independent or the possibility of the time traveller saving his grandfather's life instead of killing him so that he could later be born and travel back in time so that he could save his grandfather's life, exactly the opposite of the original paradox.

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Scientific theories

Novikov self-consistency principle

The Novikov self-consistency principle and Kip S. Thorne expresses one view on how backwards time travel could be possible without a danger of paradoxes. According to this hypothesis, the only possible timelines are those which are entirely self-consistent, so that anything a time traveler does in the past must have been part of history all along, and the time traveler can never do anything to prevent the trip back in time from being made since this would represent an inconsistency. In layman's terms, this is often called determinism. It conflicts with the notion of free-will. Succinctly, this explanation states that if time travel is possible, then actions are determined by history.

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