Editing Time (section)

== Travel ==
{{Main|Time travel}}
{{See also|Time travel in fiction|Wormhole|Twin paradox}}

Time travel is the concept of moving backwards or forwards to different points in time, in a manner analogous to moving through space, and different from the normal "flow" of time to an earthbound observer. In this view, all points in time (including future times) "persist" in some way. Time travel has been a [[plot device]] in fiction since the 19th century. Travelling backwards or forwards in time has never been verified as a process, and doing so presents many theoretical problems and contradictory logic which to date have not been overcome. Any technological device, whether fictional or hypothetical, that is used to achieve time travel is known as a [[Time travel|time machine]].

A central problem with time travel to the past is the violation of [[causality]]; should an effect precede its cause, it would give rise to the possibility of a [[temporal paradox]]. Some interpretations of time travel resolve this by accepting the possibility of travel between [[Many-worlds interpretation|branch points]], [[Multiverse|parallel realities]], or [[universe]]s. The many-worlds interpretation has been used as a way to solve causality paradoxes arising from time travel. Any quantum event creates another branching timeline, and all possible outcomes coexist without any [[wave function collapse]].<ref>{{Cite journal |last=Ghosh |first=Arnub |date=11 May 2024 |title=Revolutionizing Quantum Mechanics: The Birth and Evolution of the Many-Worlds Interpretation |url=https://arxiv.org/html/2405.06924v1 |journal=Bangladesh University of Engineering and Technology |arxiv=2405.06924v1 |via=arXiv |archive-date=31 December 2024 |access-date=28 February 2025 |archive-url=https://web.archive.org/web/20241231201147/https://arxiv.org/html/2405.06924v1 |url-status=live }}</ref> This interpretation was an alternative but is opposite from the [[Copenhagen interpretation]], which suggests that wave functions do collapse.<ref>{{Cite journal |last=Crease |first=Robert P. |date=2019-09-02 |title=The bizarre logic of the many-worlds theory |url=https://www.nature.com/articles/d41586-019-02602-8 |journal=Nature |language=en |volume=573 |issue=7772 |pages=30–32 |doi=10.1038/d41586-019-02602-8 |bibcode=2019Natur.573...30C |archive-date=3 February 2025 |access-date=28 February 2025 |archive-url=https://web.archive.org/web/20250203133712/https://www.nature.com/articles/d41586-019-02602-8 |url-status=live }}</ref> In science, hypothetical faster-than-light particles are known as [[Tachyon|tachyons]]; the mathematics of Einstein's relativity suggests that they would have an ''imaginary [[Invariant mass|rest mass]]''. Some interpretations suggest that it might move backward in time. General relativity permits the existence of [[Closed timelike curve|closed timelike curves]], which could allow an observer to travel back in time to the same space.<ref>{{Cite journal |last1=Tobar |first1=Germain |last2=Costa |first2=Fabio |date=2020-10-22 |title=Reversible dynamics with closed time-like curves and freedom of choice |url=https://iopscience.iop.org/article/10.1088/1361-6382/aba4bc |journal=Classical and Quantum Gravity |volume=37 |issue=20 |pages=205011 |doi=10.1088/1361-6382/aba4bc |issn=0264-9381|arxiv=2001.02511 |bibcode=2020CQGra..37t5011T }}</ref> Though for the [[Gödel metric]], such an occurrence requires a globally rotating universe, which has been contradicted by observations of the [[Redshift|redshifts]] of distant galaxies and the [[cosmic background radiation]].<ref>{{Cite journal |last=Luminet |first=Jean-Pierre |date=2018-05-31 |title=Closed Timelike Curves and Singularities |url=https://doi.org/10.37282/991819.18.32 |journal=Inference: International Review of Science |volume=4 |issue=1 |doi=10.37282/991819.18.32 |issn=2576-4403}}</ref> However, it has been suggested that a slowly rotating universe model may solve the [[Hubble tension]], so it can not yet be ruled out.<ref>{{cite journal
 | title=Can rotation solve the Hubble Puzzle?
 | first1=Balázs Endre | last1=Szigeti | first2=István | last2=Szapudi
 | first3=Imre Ferenc | last3=Barna | first4=Gergely Gábor | last4=Barnaföldi
 | journal=Monthly Notices of the Royal Astronomical Society
 | volume=538 | issue=4 | date=April 2025 | pages=3038–3041
 | doi=10.1093/mnras/staf446 | doi-access=free | arxiv=2503.13525 }}</ref>

Another solution to the problem of causality-based temporal paradoxes is that such paradoxes cannot arise simply because they have not arisen. As illustrated in numerous works of fiction, [[free will]] either ceases to exist in the past or the outcomes of such decisions are predetermined. A famous example is the [[grandfather paradox]], in which a person is supposed to travel back in time to kill their own grandfather. This would not be possible to enact because it is a historical fact that one's grandfather was not killed before his child (one's parent) was conceived. This view does not simply hold that history is an unchangeable constant, but that any change made by a hypothetical future time traveller would already have happened in their past, resulting in the reality that the traveller moves from. The [[Novikov self-consistency principle]] asserts that due to causality constraints, time travel to the past is impossible.