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A causal loop, in the context of time travel or retrocausality, is a sequence of events (actions, information, objects, people)〔Smith 2013.〕〔Lobo 2002, p. 3.〕 in which an event is among the causes of another event, which in turn is among the causes of the first-mentioned event.〔Rea 2015, p. 78〕〔Rea 2009, p. 204.〕 Such causally-looped events then exist in spacetime, but their origin cannot be determined.〔〔 A theorized 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.〔Thorne 1994, pp. 509–513.〕 ==Terminology in physics, philosophy, and fiction== Backwards time travel would allow for causal loops involving events, information, or objects whose histories form a closed loop, and thus seem to "come from nowhere."〔 The notion of objects or information which are "self-existing" in this way is often viewed as paradoxical,〔 with several authors referring to a causal loop involving information or objects without origin as a ''bootstrap paradox'',〔Everett 2012, pp. 136-139〕〔Visser 1996, p. 213. "A second class of logical paradoxes associated with time travel are the bootstrap paradoxes related to information (or objects, or even people?) being created from nothing."〕〔Klosterman 2009, pp. 60-62〕〔Toomey 2012, p. 343.〕 an ''information paradox'',〔 or an ''ontological paradox''.〔Callender 2011, p. 720〕 The use of "bootstrap" in this context refers to the expression "pulling yourself up by your bootstraps" and to Robert A. Heinlein's time travel story ''By His Bootstraps''.〔〔Ross 1997.〕 An example of this type of paradox involving information is given by Everett: suppose a time traveler copies a mathematical proof from a textbook, then travels back in time to meet the mathematician who first published the proof, at a date prior to publication, and allows the mathematician to simply copy the proof. In this case, the information in the proof has no origin.〔 Everett gives the movies ''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 gives her the same watch that she will later give to him.〔 Krasnikov writes that these bootstrap paradoxes - information or an object looping through time - are the same; the primary apparent paradox is a physical system evolving into a state in a way that is not governed by its laws.〔Krasnikov 2001, p. 4〕 He does not find this paradoxical, and attributes problems regarding the validity of time travel to other factors in the interpretation of general relativity.〔Krasnikov 2001, pp. 14-16〕 A 1992 paper by physicists Andrei Lossev and Igor Novikov labeled such items without origin as ''Jinn'', with the singular term ''Jinnee''.〔Lossev 1992, p. 2311-2312.〕 This terminology was inspired by the Jinn of the Quran, which are described as leaving no trace when they disappear.〔Toomey 2012, pp. 200-203〕 Lossev and Novikov allowed the term "Jinn" to cover both objects and information with reflexive origin; they called the former "Jinn of the first kind", and the latter "Jinn of the second kind".〔Lossev 1992, p. 2315-2317.〕〔〔Toomey 2012, p. 208〕 They point out that an object making circular passage through time must be identical whenever it is brought back to the past, otherwise it would create an inconsistency; the second law of thermodynamics seems to require that the object become more disordered over the course of its history, and such objects that are identical in repeating points in their history seem to contradict this, but Lossev and Novikov argued that since the second law only requires disorder to increase in ''closed'' systems, a Jinnee could interact with its environment in such a way as to regain lost order.〔〔 They emphasize that there is no "strict difference" between Jinn of the first and second kind.〔Lossev 1992, p. 2320〕 Krasnikov equivocates between "Jinn", "self-sufficient loops", and "self-existing objects", calling them "lions" or "looping or intruding objects", and asserts that they are no less physical than conventional objects, "which, after all, also could appear only from either infinity, or a singularity."〔Krasnikov 2001, p. 8-9〕 The term ''predestination paradox'' is used in the ''Star Trek'' franchise to mean "a time loop in which a time traveler who has gone into the past causes an event that ultimately causes the original future version of the person to go back into the past."〔Okuda 1999, p. 384.〕 This use of the phrase was created for a sequence in a 1996 episode of ''Star Trek: Deep Space Nine'' titled "Trials and Tribble-ations",〔Erdmann 2001, p. 31〕 although the phrase had been used previously to refer to belief systems such as Calvinism and some forms of Marxism which encouraged followers to strive to produce certain outcomes while at the same time teaching that the outcomes were predetermined.〔Robert 1960, p. 17. "Marxism, though represented as a 'monistic' philosophy, is permeated with dualism, of two sorts. One of these, underlying all of the foregoing discussion, has been widely recognized—the contrast between the scientific (or pseudo-scientific) analysis of the laws of social development, and the energetic devotion to revolutionary action to make the inevitable come true. This might be called the 'predestination paradox,' which Marxism shares with Augustinian and Calvinist theology."〕 Smeenk and Morgenstern use 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, but ends up helping to cause that same event.〔Smeenk 2011, p. 4〕〔Callender 2011, p. 581〕〔Morgenstern 2010, p. 6.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Causal loop」の詳細全文を読む スポンサード リンク
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