Editing Time travel (section)

==== Self-consistency principle ====
The [[Novikov self-consistency principle]], named after [[Igor Dmitrievich Novikov]], states that any actions taken by a time traveler or by an object that travels back in time were part of history all along, and therefore it is impossible for the time traveler to "change" history in any way. The time traveler's actions may be the ''cause'' of events in their own past though, which leads to the potential for [[causal loop|circular causation]], sometimes called a predestination paradox,<ref>{{cite book|last1=Erdmann|first1=Terry J.|last2=Hutzel|first2=Gary|title=Star Trek: The Magic of Tribbles|date=2001|publisher=Pocket Books|isbn=978-0-7434-4623-5|page=31}}</ref> 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=581}}</ref> or bootstrap paradox.<ref name="smeenk" /><ref>{{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> The term bootstrap paradox was popularized by [[Robert A. Heinlein]]'s story "[[By His 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&pg=PA60 60–62]}}</ref> The Novikov self-consistency principle proposes that the local laws of physics in a region of spacetime containing time travelers cannot be any different from the local laws of physics in any other region of spacetime.<ref>{{cite journal|first=John|last=Friedman|author2=Michael Morris|author3=Igor Novikov|author4=Fernando Echeverria|author5=Gunnar Klinkhammer|author6=Kip Thorne|author7=Ulvi Yurtsever|url=http://authors.library.caltech.edu/3737/|title=Cauchy problem in spacetimes with closed timelike curves|journal=Physical Review D|volume=42|year=1990|issue=6|pages=1915–1930|doi=10.1103/PhysRevD.42.1915|pmid=10013039|bibcode=1990PhRvD..42.1915F|access-date=2009-01-10|archive-date=2011-09-28|archive-url=https://web.archive.org/web/20110928215902/http://authors.library.caltech.edu/3737/|url-status=live}}</ref>

The philosopher Kelley L. Ross argues in "Time Travel Paradoxes"<ref>{{citation|first1=Kelley L.|last1=Ross|url=http://www.friesian.com/paradox.htm|title=Time Travel Paradoxes|year=2016|access-date=April 26, 2017|archive-date=January 18, 1998|archive-url=https://web.archive.org/web/19980118212457/http://www.friesian.com/paradox.htm|url-status=live}}</ref> that in a scenario involving a physical object whose world-line or history forms a closed loop in time there can be a violation of the [[second law of thermodynamics]]. Ross uses the film ''[[Somewhere in Time (film)|Somewhere in Time]]'' as an example of such an ontological paradox, where a watch is given to a person, and 60 years later the same watch is brought back in time and given to the same character. Ross states that [[entropy]] of the watch will increase, and the watch carried back in time will be more worn with each repetition of its history. The second law of thermodynamics is understood by modern physicists to be a [[Statistical mechanics|statistical]] law, so [[Fluctuation theorem|decreasing entropy and non-increasing entropy]] are not impossible, just improbable. Additionally, entropy statistically increases in systems which are isolated, so non-isolated systems, such as an object, that interact with the outside world, can become less worn and decrease in entropy, and it's possible for an object whose world-line forms a closed loop to be always in the same condition in the same point of its history.<ref name="Gott" />{{rp|23}}

In 2005, Daniel Greenberger and [[Karl Svozil]] proposed that [[Quantum mechanics|quantum theory]] gives a model for time travel where the past must be self-consistent.<ref name="greenberger">{{cite book|doi=10.1007/3-540-26669-0_4|title=Quo Vadis Quantum Mechanics?|year=2005|arxiv=quant-ph/0506027|bibcode=2005qvqm.book...63G|chapter=Quantum Theory Looks at Time Travel|series=The Frontiers Collection|last1=Greenberger|first1=Daniel M.|last2=Svozil|first2=Karl|isbn=978-3-540-22188-3|page=63|s2cid=119468684}}</ref><ref>{{cite news|last=Kettlewell|first=Julianna|url=http://news.bbc.co.uk/2/hi/4097258.stm|title=New model 'permits time travel'|work=BBC News|date=June 17, 2005|access-date=April 26, 2017|archive-date=April 14, 2017|archive-url=https://web.archive.org/web/20170414111240/http://news.bbc.co.uk/2/hi/4097258.stm|url-status=live}}</ref>