Causal loop: Difference between revisions

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[[File:Causal loop billiard ball.svg|thumb|right|Top: original billiard ball [[trajectory]].{{clear}}Middle: the ball emerges from the future at a different trajectory from the original, and collides with its past self, changing its trajectory.{{clear}}Bottom: the changed trajectory causes the ball to enter and exit the time machine in exactly the same way that changed its trajectory. The changed trajectory is its own cause, without an origin.]]

A '''causal loop''', also known as a '''bootstrap paradox''', '''information loop''', '''ontological paradox''', and a '''predestination paradox''', is a theoretical proposition, wherein by means of either [[retrocausality]] or [[time travel]], an event (an action, information, object, or person)<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">{{cite book |arxiv=gr-qc/0206078 |bibcode=2003ntgp.conf..289L |chapter= Time, Closed Timelike Curves and Causality |title=The Nature of Time: Geometry, Physics and Perception |volume=95 |pages=289–296 |series=NATO Science Series II |last=Lobo|first=Francisco|year=2003 |isbn=1-4020-1200-4 }}</ref> is among the causes of another event, which is in turn among the causes of the first-mentioned event.<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> Such causally looped events then exist in [[spacetime]], but their origin cannot be determined.<ref name="Smith" /><ref name="Lobo" /> A hypothetical example of a causality loop is given of a [[billiard ball]] striking its past self: the billiard ball moves in a path towards a time machine, and the future self of the billiard ball emerges from the time machine ''before'' its past self enters it, giving its past self a glancing blow, altering the past ball's path and causing it to enter the time machine at an angle that would cause its future self to strike its past self the very glancing blow that altered its path. In this sequence of events, the change in the ball's path is its own cause, which might appear paradoxical.<ref name="Thorne">{{cite book | last = Thorne | first = Kip S. | author-link = Kip Thorne | title = Black Holes and Time Warps | publisher = W. W. Norton | year= 1994 | isbn = 0-393-31276-3|pages=509–513| title-link = Black Holes and Time Warps }}</ref>

==Terminology in physics, philosophy, and fiction==