Editing Time travel (section)

== Philosophy ==
{{main|Philosophy of space and time}}
Philosophers have discussed the [[philosophy of space and time]] since at least the time of [[ancient Greece]]; for example, [[Parmenides]] presented the view that time is an illusion. Centuries later, [[Isaac Newton]] supported the idea of [[absolute time and space|absolute time]], while his contemporary [[Gottfried Wilhelm Leibniz]] maintained that time is only a relation between events and it cannot be expressed independently. The latter approach eventually gave rise to the [[spacetime]] of [[Theory of relativity|relativity]].<ref>{{citation|title=The Dictionary of Philosophy|editor=Dagobert D. Runes|page=318|section=Time|year=1942|publisher=Philosophical Library}}</ref>

=== Presentism vs. eternalism ===
Many philosophers have argued that relativity implies [[Eternalism (philosophy of time)|eternalism]], the idea that the past and future exist in a real sense, not only as changes that occurred or will occur to the present.<ref name="Crisp">{{citation|chapter=Presentism, Eternalism, and Relativity Physics|title=Einstein, Relativity and Absolute Simultaneity|author=Thomas M. Crisp|editor1=William Lane Craig|editor2=Quentin Smith|page=footnote 1|chapter-url=https://thomasmcrisp.files.wordpress.com/2017/07/presentism-eternalism-and-relativity-physics.pdf|year=2007|access-date=2018-02-01|archive-date=2018-02-02|archive-url=https://web.archive.org/web/20180202012816/https://thomasmcrisp.files.wordpress.com/2017/07/presentism-eternalism-and-relativity-physics.pdf|url-status=live}}</ref> Philosopher of science [[Dean Rickles]] disagrees with some qualifications, but notes that "the consensus among philosophers seems to be that special and general relativity are incompatible with presentism".<ref>{{citation|author=Dean Rickles|year=2007|title=Symmetry, Structure, and Spacetime|url=https://books.google.com/books?id=gDwJYtfoCh8C&pg=PA158|page=158|publisher=Elsevier |access-date=July 9, 2016|isbn=9780444531162|archive-date=March 24, 2023|archive-url=https://web.archive.org/web/20230324181809/https://books.google.com/books?id=gDwJYtfoCh8C&pg=PA158|url-status=live}}</ref> Some philosophers view time as a dimension equal to spatial dimensions, that future events are "already there" in the same sense different places exist, and that there is no objective flow of time; however, this view is disputed.<ref>{{citation|author=Tim Maudlin|title=The Metaphysics Within Physics|isbn=9780199575374|year=2010|chapter=On the Passing of Time|publisher=Oxford University Press |chapter-url=https://philocosmology.rutgers.edu/images/uploads/TimDavidClass/05-maudlin-chap04.pdf|author-link=Tim Maudlin|access-date=2018-02-01|archive-date=2021-03-08|archive-url=https://web.archive.org/web/20210308030324/https://philocosmology.rutgers.edu/images/uploads/TimDavidClass/05-maudlin-chap04.pdf|url-status=live}}</ref>

[[Presentism (philosophy of time)|Presentism]] is a school of philosophy that holds that the future and the past exist only as changes that occurred or will occur to the present, and they have no real existence of their own. In this view, time travel is impossible because there is no future or past to travel to.<ref name="Crisp" /> Keller and Nelson have argued that even if past and future objects do not exist, there can still be definite truths about past and future events, and thus it is possible that a future truth about a time traveler deciding to travel back to the present date could explain the time traveler's actual appearance in the present;<ref name="Presentism">{{cite journal|first=Simon|last=Keller|author2=Michael Nelson|title=Presentists should believe in time-travel|url=http://people.bu.edu/stk/Papers/Timetravel.pdf|journal= Australasian Journal of Philosophy|volume=79|issue=3|pages=333–345|date=September 2001|doi=10.1080/713931204|s2cid=170920718|url-status=dead|archive-url=https://web.archive.org/web/20081028211537/http://people.bu.edu/stk/Papers/Timetravel.pdf|archive-date=October 28, 2008}}</ref> these views are contested by some authors.<ref name="Bourne">{{cite book|author=Craig Bourne|title=A Future for Presentism|url=https://books.google.com/books?id=DJMSDAAAQBAJ|date=7 December 2006|publisher=Clarendon Press|isbn=978-0-19-921280-4}}</ref>

=== The grandfather paradox ===
{{main|Grandfather paradox}}
A common objection to the idea of traveling back in time is put forth in the grandfather paradox or the argument of auto-infanticide.<ref name="horwich">{{cite book|last1=Horwich|first1=Paul|title=Asymmetries in Time: Problems in the Philosophy of Science|date=1987|publisher=MIT Press|location=Cambridge, Massachusetts|isbn=978-0262580885|page=116|edition=2nd}}</ref> If one were able to go back in time, inconsistencies and contradictions would ensue if the time traveler were to change anything; there is a contradiction if the past becomes different from the way it ''is''.<ref name="NicholasSmith">{{cite encyclopedia|url=http://plato.stanford.edu/entries/time-travel/index.html#CauLoo|author=Nicholas J.J. Smith|date=2013|title=Time Travel|encyclopedia=Stanford Encyclopedia of Philosophy|access-date=November 2, 2015|archive-date=August 18, 2018|archive-url=https://web.archive.org/web/20180818152007/https://plato.stanford.edu/entries/time-travel/index.html#CauLoo|url-status=live}}</ref><ref name="lobo">{{cite journal|title=Time, Closed Timelike Curves and Causality|journal=The Nature of Time: Geometry|volume=95|pages=289–296|author=Francisco Lobo|year=2003|arxiv=gr-qc/0206078v2|bibcode=2003ntgp.conf..289L}}</ref> The paradox is commonly described with a person who travels to the past and kills their own grandfather, prevents the existence of their father or mother, and therefore their own existence.<ref name="sagan-nova" /> Philosophers question whether these paradoxes prove time travel impossible. Some philosophers answer these paradoxes by arguing that it might be the case that backward time travel could be possible but that it would be impossible to actually ''change'' the past in any way,<ref name="unchangeable">{{cite web|author=Norman Swartz|title=Time Travel: Visiting the Past|url=https://www.sfu.ca/~swartz/time_travel1.htm|date=1993|access-date=February 20, 2016|archive-date=August 18, 2018|archive-url=https://web.archive.org/web/20180818151754/https://www.sfu.ca/~swartz/time_travel1.htm|url-status=live}}</ref> an idea similar to the proposed [[Novikov self-consistency principle]] in physics.

=== Ontological paradox ===
==== Compossibility ====
According to the philosophical theory of [[compossibility]], what ''can'' happen, for example in the context of time travel, must be weighed against the context of everything relating to the situation. If the past ''is'' a certain way, it's not possible for it to be any other way. What ''can'' happen when a time traveler visits the past is limited to what ''did'' happen, in order to prevent logical contradictions.<ref>{{cite journal|last=Lewis|first=David|title=The paradoxes of time travel|journal=[[American Philosophical Quarterly]]|volume=13|pages=145–52|year=1976|url=http://www.csus.edu/indiv/m/merlinos/Paradoxes%20of%20Time%20Travel.pdf|bibcode=1996gr.qc.....3042K|arxiv=gr-qc/9603042|access-date=2010-09-06|archive-date=2017-08-28|archive-url=https://web.archive.org/web/20170828174937/http://www.csus.edu/indiv/m/merlinos/paradoxes%20of%20time%20travel.pdf|url-status=live}}</ref>

==== 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>