A predestination paradox (also called causal loop, causality loop, and, less frequently, closed loop or closed time loop) is a paradox of time travel that is often used as a convention in science fiction. It exists when a time traveler is caught in a loop of events that "predestines" or "predates" them to travel back in time. Because of the possibility of influencing the past while time traveling, one way of explaining why history does not change is by saying that whatever has happened must happen. A time traveler attempting to alter the past in this model, intentionally or not, would only be fulfilling their role in creating history as we know it, not changing it. Or that the time-traveler's personal knowledge of history already includes their future travels to their own experience of the past (for the Novikov self-consistency principle).
In layman's terms, it means this: the time traveller is in the past, which means they were in the past before. Therefore, their presence is vital to the future, and they do something that causes the future to occur in the same way that their knowledge of the future has already happened. It is very closely related to the ontological paradox and usually occurs at the same time.
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Famous quotes containing the word paradox:
“A good aphorism is too hard for the teeth of time and is not eaten up by all the centuries, even though it serves as food for every age: hence it is the greatest paradox in literature, the imperishable in the midst of change, the nourishment whichlike saltis always prized, but which never loses its savor as salt does.”
—Friedrich Nietzsche (18441900)