Editing Time travel (section)

=== Quantum physics ===
{{main|Quantum mechanics of time travel}}
==== No-communication theorem ====
When a signal is sent from one location and received at another location, then as long as the signal is moving at the speed of light or slower, the mathematics of [[Relativity of simultaneity|simultaneity]] in the theory of relativity show that all reference frames agree that the transmission-event happened before the reception-event. When the signal travels faster than light, it is received ''before'' it is sent, in all reference frames.<ref name="Jarrell">{{cite web|url=http://www.physics.uc.edu/~jarrell/COURSES/ELECTRODYNAMICS/Chap11/chap11.pdf|title=The Special Theory of Relativity|access-date=October 27, 2006|last1=Jarrell|first1=Mark|pages=7–11|archive-url=https://web.archive.org/web/20060913173236/http://www.physics.uc.edu/~jarrell/COURSES/ELECTRODYNAMICS/Chap11/chap11.pdf <!-- Bot retrieved archive -->|archive-date=September 13, 2006}}</ref> The signal could be said to have moved backward in time. This hypothetical scenario is sometimes referred to as a [[tachyonic antitelephone]].<ref>{{cite journal|last=Kowalczyński|first=Jerzy|date=January 1984|title=Critical comments on the discussion about tachyonic causal paradoxes and on the concept of superluminal reference frame|journal=[[International Journal of Theoretical Physics]]|publisher=[[Springer Science+Business Media]]|volume=23|issue=1|pages=27–60|doi=10.1007/BF02080670|bibcode=1984IJTP...23...27K|s2cid=121316135}}</ref>

Quantum-mechanical phenomena such as [[quantum teleportation]], the [[EPR paradox]], or [[quantum entanglement]] might appear to create a mechanism that allows for faster-than-light (FTL) communication or time travel, and in fact some interpretations of quantum mechanics such as the [[Bohm interpretation]] presume that some information is being exchanged between particles instantaneously in order to maintain correlations between particles.<ref name="Bohm">{{cite web|url=http://plato.stanford.edu/entries/qm-bohm/|title=Bohmian Mechanics|date=March 27, 2017|access-date=April 26, 2017|last1=Goldstein|first1=Sheldon|archive-date=January 12, 2012|archive-url=https://web.archive.org/web/20120112030926/http://plato.stanford.edu/entries/qm-bohm/|url-status=live}}</ref> This effect was referred to as "[[action at a distance (physics)#spooky action at a distance|spooky action at a distance]]" by Einstein.

Nevertheless, the fact that causality is preserved in quantum mechanics is a rigorous result in modern [[Quantum field theory|quantum field theories]], and therefore modern theories do not allow for time travel or [[Superluminal communication|FTL communication]]. In any specific instance where FTL has been claimed, more detailed analysis has proven that to get a signal, some form of classical communication must also be used.<ref name="Nielsen and Chuang">{{cite book|last1=Nielsen|last2=Chuang|first1=Michael|first2=Isaac|title=Quantum Computation and Quantum Information|url=https://archive.org/details/quantumcomputati00niel_056|url-access=limited|publisher=Cambridge|year=2000|page=[https://archive.org/details/quantumcomputati00niel_056/page/n55 28]|isbn=978-0-521-63235-5}}</ref> The [[no-communication theorem]] also gives a general proof that quantum entanglement cannot be used to transmit information faster than classical signals.

==== Interacting many-worlds interpretation ====
A variation of [[Hugh Everett]]'s [[many-worlds interpretation]] (MWI) of quantum mechanics provides a resolution to the grandfather paradox that involves the time traveler arriving in a different universe than the one they came from; it's been argued that since the traveler arrives in a different universe's history and not their own history, this is not "genuine" time travel.<ref>{{citation|title=Time Travel and Modern Physics|date=December 23, 2009|author1=Frank Arntzenius|author2=Tim Maudlin|url=http://plato.stanford.edu/entries/time-travel-phys/|encyclopedia=Stanford Encyclopedia of Philosophy|access-date=August 9, 2005|archive-date=May 25, 2011|archive-url=https://web.archive.org/web/20110525025650/http://plato.stanford.edu/entries/time-travel-phys/|url-status=live}}</ref> The accepted many-worlds interpretation suggests that all possible quantum events can occur in mutually exclusive histories.<ref name="many-worlds">{{cite web|url=http://plato.stanford.edu/entries/qm-manyworlds/|title=Many-Worlds Interpretation of Quantum Mechanics|date=January 17, 2014|access-date=April 26, 2017|last1=Vaidman|first1=Lev|archive-date=December 9, 2019|archive-url=https://web.archive.org/web/20191209220612/http://plato.stanford.edu/entries/qm-manyworlds/|url-status=live}}</ref> However, some variations allow different universes to interact. This concept is most often used in science-fiction, but some physicists such as [[David Deutsch]] have suggested that a time traveler should end up in a different history than the one he started from.<ref name="deutsch">{{cite journal|last=Deutsch|first=David|author-link=David Deutsch|year=1991|title=Quantum mechanics near closed timelike lines|journal=Physical Review D|volume=44|issue=10|pages=3197–3217|doi=10.1103/PhysRevD.44.3197|pmid=10013776|bibcode=1991PhRvD..44.3197D|s2cid=38691795|url=http://pdfs.semanticscholar.org/8e99/3e3e9b0952198a51ed99c9c0af3a31f433df.pdf|archive-url=https://web.archive.org/web/20190228075930/http://pdfs.semanticscholar.org/8e99/3e3e9b0952198a51ed99c9c0af3a31f433df.pdf|url-status=dead|archive-date=2019-02-28}}</ref><ref>{{citation|author=Pieter Kok|title=Time Travel Explained: Quantum Mechanics to the Rescue?|url=https://www.youtube.com/watch?v=uz9eLjO2BrA |archive-url=https://ghostarchive.org/varchive/youtube/20211211/uz9eLjO2BrA| archive-date=2021-12-11 |url-status=live|date=February 3, 2013}}{{cbignore}}</ref> On the other hand, Stephen Hawking has argued that even if the MWI is correct, we should expect each time traveler to experience a single self-consistent history, so that time travelers remain within their own world rather than traveling to a different one.<ref name="Hawking warp"/> The physicist Allen Everett argued that Deutsch's approach "involves modifying fundamental principles of quantum mechanics; it certainly goes beyond simply adopting the MWI". Everett also argues that even if Deutsch's approach is correct, it would imply that any macroscopic object composed of multiple particles would be split apart when traveling back in time through a wormhole, with different particles emerging in different worlds.<ref name="Everett MWI">{{cite journal|last=Everett|first=Allen|title=Time travel paradoxes, path integrals, and the many worlds interpretation of quantum mechanics|journal=Physical Review D|volume=69|issue=124023|pages=124023|year=2004|doi=10.1103/PhysRevD.69.124023|arxiv=gr-qc/0410035|bibcode=2004PhRvD..69l4023E|s2cid=18597824}}</ref>