A temporal paradox, time paradox, or time travel paradox is a paradox, an apparent contradiction, or logical contradiction 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. Other paradoxes associated with time travel are a variation of the Fermi paradox and paradoxes of free will that stem from causal loops such as Newcomb's paradox.
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. If one were ever to go back in time and kill one's grandfather in his childhood, it would result in one of the time traveler's parents, and ergo the time traveler, not being born. If the time traveler weren't born, then he never went back in time to kill his grandfather in the first place. Therefore, he lives to offspring the time traveler's parents, and therefore the time traveler. There is, thus no predicted outcome to this. Consistency paradoxes occur whenever changing the past is possible.
A possible resolution is that 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.
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 cannot reach any arbitrary point in the past, or that they disguise themselves to avoid detection.
Newcomb's paradox is a thought experiment showing an apparent contradiction between the expected utility principle and the strategic dominance principle. The thought experiment is often extended to explore causality and free will by allowing for "perfect predictors": if perfect predictors of the future exist, for example if time travel exists as a mechanism for making perfect predictions, then perfect predictions appear to contradict free will because decisions apparently made with free will are already known to the perfect predictor.
- Francisco Lobo (2003). "Time, Closed Timelike Curves and Causality". Nato Sci.ser.ii. 95: 289–296. arXiv:gr-qc/0206078. Bibcode:2003ntgp.conf..289L.
- Jan Faye (November 18, 2015), "Backward Causation", Stanford Encyclopedia of Philosophy, retrieved May 25, 2019
- Nicholas J.J. Smith (2013). "Time Travel". Stanford Encyclopedia of Philosophy. Retrieved November 2, 2015.
- Leora Morgenstern (2010), Foundations of a Formal Theory of Time Travel (PDF), p. 6, retrieved November 2, 2015
- Klosterman, Chuck (2009). Eating the Dinosaur (1st Scribner hardcover ed.). New York: Scribner. p. 60. ISBN 9781439168486. Retrieved 2 February 2013.
- "Carl Sagan Ponders Time Travel". NOVA. PBS. December 10, 1999. Retrieved April 26, 2017.
- Wolpert, D. H.; Benford, G. (June 2013). "The lesson of Newcomb's paradox". Synthese. 190 (9): 1637–1646. doi:10.1007/s11229-011-9899-3. JSTOR 41931515.
- Craig (1987). "Divine Foreknowledge and Newcomb's Paradox". Philosophia. 17 (3): 331–350. doi:10.1007/BF02455055.
- Craig, William Lane (1988). "Tachyons, Time Travel, and Divine Omniscience". The Journal of Philosophy. 85 (3): 135–150. doi:10.2307/2027068. JSTOR 2027068.