A temporal paradox, time paradox, or time travel paradox is a paradox, an apparent contradiction, or a logical contradiction that is associated with the idea of time and time travel. In physics, temporal paradoxes fall into two broad groups: consistency paradoxes exemplified by the grandfather paradox; and causal loops. More broadly, a variation of the Fermi paradox also applies to time travel.
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 may involve an event, a person or object, or information. The terms boot-strap paradox, predestination paradox or ontological paradox are sometimes used in fiction to refer to a causal loop.
The consistency paradox or grandfather paradox occurs when the past is changed in any way, thus creating a contradiction. A time traveler can do anything that did happen, but can't do anything that didn't happen. Doing something that didn't happen results in a contradiction. Consistency paradoxes occur whenever changing the past is possible.
The Fermi paradox can be adapted for time travel, and phrased "if time travel were possible, where are all the visitors from the future?" Answers vary, from time travel not being possible, to the possibility that visitors from the future can not reach any arbitrary point in the past, or that they disguise themselves to avoid detection.
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