Editing Temporal paradox (section)

==Types==
{{anchor|Causal loop}}
{{redirect|Causal loop|the cause and effect diagram|causal loop diagram|the plot device|time loop}}
Temporal paradoxes fall into three broad groups: bootstrap paradoxes, consistency paradoxes, and Newcomb's paradox.<ref name="Stanford backward">{{citation|url=https://plato.stanford.edu/entries/causation-backwards/|title=Backward Causation|author=Jan Faye|date=November 18, 2015|access-date=May 25, 2019|encyclopedia=Stanford Encyclopedia of Philosophy}}</ref> Bootstrap paradoxes violate causality by allowing future events to influence the past and cause themselves, or "[[bootstrapping]]", which derives from the idiom "{{linktext|pull oneself up by one's bootstraps}}."<ref name="Klosterman">{{cite book |last1=Klosterman |first1=Chuck |title=Eating the Dinosaur |date=2009 |publisher=Scribner |location=New York |isbn=9781439168486|edition=1st Scribner hardcover|pages=[https://books.google.com/books?id=lZurDFJtAWwC&lpg=PA60&pg=PA60 60–62]}}</ref><ref>{{cite web|last=Ross |first=Kelley L. |title=Time Travel Paradoxes |date=1997 |url=http://www.friesian.com/paradox.htm |url-status=dead |archive-url=https://web.archive.org/web/19980118212457/http://www.friesian.com/paradox.htm |archive-date=January 18, 1998 }}</ref> Consistency paradoxes, on the other hand, are those where future events influence the past to cause an apparent contradiction, exemplified by the [[grandfather paradox]], where a person travels to the past to prevent the conception of one of their ancestors, thus eliminating all the ancestor's descendants.<ref name="Nato" /> [[Newcomb's paradox]] stems from the apparent contradictions that stem from the assumptions of both [[free will]] and foreknowledge of future events. All of these are sometimes referred to individually as "causal loops." The term "[[time loop]]" is sometimes referred to as a causal loop,<ref name="Klosterman" /> but although they appear similar, causal loops are unchanging and self-originating, whereas time loops are constantly resetting.<ref>{{cite book|title=Time Travel in Popular Media|last1=Jones|first1=Matthew|last2=Ormrod|first2=Joan|year=2015|publisher=[[McFarland & Company]]|isbn=9780786478071|page=207}}</ref>

===Bootstrap paradox===
A bootstrap paradox, also known as an '''information loop''', an ''information paradox'',<ref name="Everett" /> an ''ontological paradox'',<ref name="smeenk">{{Citation |last1=Smeenk|first1=Chris|last2=Wüthrich|first2=Christian|editor-last=Callender|editor-first=Craig|contribution=Time Travel and Time Machines|title=The Oxford Handbook of Philosophy of Time|year=2011|publisher=Oxford University Press|isbn = 978-0-19-929820-4|page=[https://books.google.com/books?id=PrapBAAAQBAJ&lpg=PP1&pg=PT720 581]}}</ref> or a "predestination paradox" is a paradox of time travel that occurs when any event, such as an action, information, an object, or a person, ultimately causes itself, as a consequence of either [[retrocausality]] or [[time travel]].<ref name="Smith">{{cite web |url=http://plato.stanford.edu/entries/time-travel/index.html#CauLoo |last=Smith|first=Nicholas J.J.|date=2013 |title=Time Travel |website=Stanford Encyclopedia of Philosophy |access-date=June 13, 2015}}</ref><ref name="Lobo" /><ref>{{cite book|last=Rea|first=Michael|title=Metaphysics: The Basics|date=2014|publisher=Routledge|location=New York|isbn=978-0-415-57441-9|edition=1. publ.|page=[https://books.google.com/books?id=v1IsAwAAQBAJ&lpg=PP1&pg=PA78 78]}}</ref><ref>{{cite book|last1=Rea|first1=Michael C.|title=Arguing about Metaphysics|date=2009|publisher=Routledge|location=New York [u.a.]|isbn=978-0-415-95826-4|page=204}}</ref>

Backward time travel would allow information, people, or objects whose histories seem to "come from nowhere".<ref name="Smith" /> Such causally looped events then exist in [[spacetime]], but their origin cannot be determined.<ref name="Smith" /><ref name="Lobo" /> The notion of objects or information that are "self-existing" in this way is often viewed as paradoxical.<ref name="Lobo" /><ref name="Everett">{{cite book|last1=Everett|first1=Allen|last2=Roman|first2=Thomas|title=Time Travel and Warp Drives|date=2012|publisher=University of Chicago Press|location=Chicago|isbn=978-0-226-22498-5|pages=[https://books.google.com/books?id=Dm5xt_XbFyoC&lpg=PP1&pg=PA136 136–139]}}</ref><ref>{{cite book|last=Visser|first=Matt|title=Lorentzian Wormholes: From Einstein to Hawking|date=1996|publisher=Springer-Verlag|location=New York|isbn=1-56396-653-0|page=213}}</ref> A notable example occurs in the 1958 [[science fiction]] [[short story]] "[[—All You Zombies—]]", by [[Robert A. Heinlein]], wherein the main character, an [[intersex]] individual, becomes both their own mother and father; the 2014 film ''[[Predestination (film)|Predestination]]'' is based on the story. Allen Everett gives the movie ''[[Somewhere in Time (film)|Somewhere in Time]]'' as an example involving an object with no origin: an old woman gives a watch to a playwright who later travels back in time and meets the same woman when she was young, and shows her the watch that she will later give to him.<ref name="Everett" /> An example of information which "came from nowhere" is in the movie ''[[Star Trek IV: The Voyage Home]]'', in which a 23rd-century engineer travels back in time, and gives the formula for [[list of Star Trek materials#Transparent aluminum|transparent aluminum]] to the 20th-century engineer who supposedly invented it.

===Predestination paradox===
Smeenk uses the term "predestination paradox" to refer specifically to situations in which a time traveler goes back in time to try to prevent some event in the past.<ref name="smeenk" />

The "predestination paradox" is a concept in time travel and temporal mechanics, often explored in science fiction. It occurs when a future event is the cause of a past event, which in turn becomes the cause of the future event, forming a self-sustaining loop in time. This paradox challenges conventional understandings of cause and effect, as the events involved are both the origin and the result of each other. A notable example is found in the TV series ''[[Doctor Who]]'', where a [[Father's Day (Doctor Who)|character saves her father in the past]], fulfilling a memory he had shared with her as a child about a strange woman having saved his life. The predestination paradox raises philosophical questions about free will, determinism, and the nature of time itself. It is commonly used as a narrative device in fiction to highlight the interconnectedness of events and the inevitability of certain outcomes.

===Consistency paradox===
The consistency paradox or grandfather paradox occurs when the past is changed in any way that directly negates the conditions required for the time travel to occur in the first place, thus creating a contradiction. A common example given is traveling to the past and preventing the conception of one's ancestors (such as causing the death of the ancestor's parent beforehand), thus preventing the conception of oneself. If the traveler were not born, then it would not be possible to undertake such an act in the first place; therefore, the ancestor proceeds to beget the traveler's next-generation ancestor and secure the line to the traveler. There is no predicted outcome to this scenario.<ref name="Smith" /> Consistency paradoxes occur whenever changing the past is possible.<ref name="Lobo" /> A possible resolution is that a time traveller ''can'' do anything that ''did'' happen, but ''cannot'' do anything that ''did not'' happen. Doing something that did not happen results in a contradiction.<ref name="Smith" /> This is referred to as the [[Novikov self-consistency principle]].

==== Variants ====
{{anchor|Retro-suicide paradox}}{{Anchor|Hitler's murder paradox}}
The grandfather paradox encompasses any change to the past,<ref name="NicholasSmith2">{{cite encyclopedia |title=Time Travel |encyclopedia=Stanford Encyclopedia of Philosophy |url=http://plato.stanford.edu/entries/time-travel/index.html#CauLoo |access-date=November 2, 2015 |date=2013 |author=Nicholas J.J. Smith}}</ref> and it is presented in many variations, including killing one's past self.<ref name="horwich">{{cite book |last1=Horwich |first1=Paul |title=Asymmetries in Time: Problems in the Philosophy of Science |date=1987 |publisher=MIT Press |isbn=0262580888 |edition=2nd |location=Cambridge, Massachusetts |page=116}}</ref><ref name="stanford backward2">{{citation |author=Jan Faye |title=Backward Causation |date=November 18, 2015 |url=https://plato.stanford.edu/entries/causation-backwards/ |encyclopedia=Stanford Encyclopedia of Philosophy |access-date=May 25, 2019}}</ref> Both the "retro-suicide paradox" and the "grandfather paradox" appeared in letters written into ''[[Amazing Stories]]'' in the 1920s.<ref name="Nahin 1999">{{cite book |last=Nahin |first=Paul J. |url=https://archive.org/details/timemachinestime0000nahi_m8y6 |title=Time Machines: Time Travel in Physics, Metaphysics, and Science Fiction |date=1999 |publisher=Springer-Verlag |isbn=0-387-98571-9 |edition=2nd |location=New York |access-date=2022-02-19 |url-access=registration}}</ref> Another variant of the grandfather paradox is the "Hitler paradox" or "Hitler's murder paradox", in which the protagonist travels back in time to murder [[Adolf Hitler]] before he can rise to power in Germany, thus preventing [[World War II]] and the [[Holocaust]]. Rather than necessarily physically preventing time travel, the action removes any ''reason'' for the travel, along with any knowledge that the reason ever existed.<ref>{{cite book |last1=Brennan |first1=J.H. |url=https://books.google.com/books?id=UBf_HjZHUxkC |title=Time Travel: A New Perspective |date=1997 |publisher=Llewellyn Publications |isbn=9781567180855 |edition=1st |location=Minnesota |page=23}}</ref>

Physicist John Garrison et al. give a variation of the paradox of an electronic circuit that sends a signal through a time machine to shut itself off, and receives the signal before it sends it.<ref>{{cite journal |last1=Garrison |first1=J.C. |last2=Mitchell |first2=M.W. |last3=Chiao |first3=R.Y. |last4=Bolda |first4=E.L. |date=August 1998 |title=Superluminal Signals: Causal Loop Paradoxes Revisited |journal=Physics Letters A |volume=245 |issue=1–2 |pages=19–25 |arxiv=quant-ph/9810031 |bibcode=1998PhLA..245...19G |doi=10.1016/S0375-9601(98)00381-8 |s2cid=51796022}}</ref><ref>{{cite book |last1=Nahin |first1=Paul J. |title=Time Machine Tales |date=2016 |publisher=Springer International Publishing |isbn=9783319488622 |pages=335–336}}</ref>

===Newcomb's paradox===
{{main|Newcomb's paradox}}
Newcomb's paradox is a [[thought experiment]] showing an apparent contradiction between the [[expected utility hypothesis|expected utility]] principle and the [[strategic dominance]] principle.<ref name="Wolpert">{{cite journal |first1=D. H. |last1=Wolpert |first2=G. |last2=Benford |title=The lesson of Newcomb's paradox |journal=[[Synthese]]|date=June 2013 |volume=190 |issue=9 |pages=1637–1646 |doi=10.1007/s11229-011-9899-3 |jstor=41931515|s2cid=113227 }}</ref> 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{{how?|date=September 2024}}, then perfect predictions appear to contradict free will because decisions apparently made with free will are already known to the perfect predictor{{what?|date=September 2024}}.<ref>{{cite journal |author=Craig |year=1987 |url=http://www.leaderu.com/offices/billcraig/docs/newcomb.html |title=Divine Foreknowledge and Newcomb's Paradox |journal=Philosophia |volume=17 |issue=3 |pages=331–350 |doi=10.1007/BF02455055|s2cid=143485859 |url-access=subscription }}</ref><ref>{{cite journal |last=Craig |first=William Lane |author-link=William Lane Craig |year=1988 |title=Tachyons, Time Travel, and Divine Omniscience |journal=[[The Journal of Philosophy]] |volume=85 |issue=3 |pages=135–150 |jstor=2027068|doi=10.2307/2027068 }}</ref> [[Predestination]] does not necessarily involve a [[supernatural]] power, and could be the result of other "infallible foreknowledge" mechanisms.<ref>{{cite journal |last=Craig|first=William Lane|year=1987 |url=http://www.leaderu.com/offices/billcraig/docs/newcomb.html |title=Divine Foreknowledge and Newcomb's Paradox |journal=Philosophia |volume=17 |issue=3 |pages=331–350 |doi=10.1007/BF02455055|s2cid=143485859|url-access=subscription }}</ref> Problems arising from infallibility and influencing the future are explored in Newcomb's paradox.<ref>{{cite book|last=Dummett|first=Michael|title=The Seas of Language|isbn=9780198240112|year=1996|publisher=Oxford University Press|pages=356, 370–375}}</ref>