Causal loop: Difference between revisions

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Backwards time travel would allow for causal loops involving events, information, people or objects whose histories form a closed loop, and thus seem to "come from nowhere."<ref name="Smith" /> The notion of objects or information that are "self-existing" in this way is often viewed as paradoxical,<ref name="Lobo" /> with several authors referring to a causal loop involving information or objects without origin as a ''bootstrap paradox'',<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}} "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."</ref><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 name="Toomey2012">{{cite book|last=Toomey|first=David|title=The New Time Travelers|date=2012|publisher=W. W. Norton & Company|___location=New York, New York|isbn=978-0-393-06013-3|url=https://archive.org/details/newtimetravelers00toom}}</ref>{{Rp|343}} an ''information paradox'',<ref name="Everett" /> or 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> The use of "bootstrap" in this context refers to the expression "[[bootstrapping|pulling yourself up by your bootstraps]]" and to [[Robert A. Heinlein]]'s time travel story "[[By His Bootstraps]]".<ref name="Klosterman"/><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> 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>

An example of a causal loop paradox involving information is given by Allan 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.<ref name="Everett"/> A similar example is given in the television series ''[[Doctor Who]]'' of a hypothetical time-traveler who copies Beethoven's music from the future and publishes it in Beethoven's time in Beethoven's name.<ref>{{cite web|last=Holmes|first=Jonathan|work=Radio Times|url=http://www.radiotimes.com/news/2015-10-10/doctor-who-what-is-the-bootstrap-paradox|title=Doctor Who: what is the Bootstrap Paradox?|date=October 10, 2015}}</ref> A solution to the causal loop paradox involving information has been published.<ref>Riggs, P.J. (2018), "The Temporal Epistemic Anomaly", ''Manuscrito – Rev. Int. Fil. Campinas'' '''41''', 3: 1–28.</ref> 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 gives her the same watch that she will later give to him.<ref name="Everett" />

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.<ref name="Krasnikov2001">{{citation|last=Krasnikov|first=S.|year=2001|title=The time travel paradox|journal=Phys. Rev. D|volume=65|issue=6|page=06401 |arxiv=gr-qc/0109029|bibcode = 2002PhRvD..65f4013K |doi = 10.1103/PhysRevD.65.064013 |s2cid=18460829}}</ref>{{Rp|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.<ref name="Krasnikov2001" />{{Rp|14–16}}