Editing Temporal paradox (section)

== Proposed resolutions ==
=== Logical impossibility ===
Even without knowing whether time travel to the past is physically possible, it is possible to show using [[modal logic]] that changing the past results in a logical contradiction. If it is necessarily true that the past happened in a certain way, then it is false and impossible for the past to have occurred in any other way. A time traveler would not be able to change the past from the way it ''is,'' but would only act in a way that is already consistent with what ''necessarily'' happened.<ref name="Norman">{{citation |author=Norman Swartz |title=Beyond Experience: Metaphysical Theories and Philosophical Constraints |date=2001 |url=https://www.sfu.ca/~swartz/beyond_experience/chap08.htm |pages=226–227 |publisher=University of Toronto Press}}</ref><ref>{{cite book |last1=Dummett |first1=Michael |title=The Seas of Language |date=1996 |publisher=Oxford University Press |isbn=0198236212 |edition=New |location=Oxford |pages=368–369}}</ref>

Consideration of the grandfather paradox has led some to the idea that time travel is by its very nature paradoxical and therefore logically impossible. For example, the philosopher [[Bradley Dowden]] made this sort of argument in the textbook ''Logical Reasoning'', arguing that the possibility of creating a contradiction rules out time travel to the past entirely. However, some philosophers and scientists believe that time travel into the past need not be logically impossible provided that there is no possibility of changing the past,<ref name="NicholasSmith2"/> as suggested, for example, by the [[Novikov self-consistency principle]]. Dowden revised his view after being convinced of this in an exchange with the philosopher [[Norman Swartz]].<ref name="Swartz">{{cite web |author=Norman Swartz |year=1993 |title=Time Travel - Visiting the Past |url=https://www.sfu.ca/~swartz/time_travel1.htm |access-date=2016-04-21 |publisher=SFU.ca}}</ref>

=== Emergent time and entropic reconfiguration ===
A recent proposed resolution argues that if time is not an inherent property of the universe but is instead [[emergent phenomena|emergent]] from the laws of [[entropy]], as some modern theories suggest,<ref>{{cite journal |last=Rovelli |first=Carlo |date=2009 |title=Forget time |journal=Foundations of Physics |volume=41 |issue=9 |pages=1475–1490 |arxiv=0903.3832 |bibcode=2011FoPh...41.1475R |doi=10.1007/s10701-011-9561-4 |s2cid=9891985}}</ref><ref>{{cite journal |last=Verlinde |first=Erik |date=2011 |title=On the Origin of Gravity and the Laws of Newton |journal=Journal of High Energy Physics |volume=2011 |issue=4 |page=29 |arxiv=1001.0785 |bibcode=2011JHEP...04..029V |doi=10.1007/JHEP04(2011)029 |s2cid=118622227}}</ref> then it presents a natural solution to the Grandfather Paradox.<ref>{{cite web |title=Resolving the Grandfather Paradox through Emergent Time |url=https://filmboards.com/t/Science/Resolving-the-Grandfather-Paradox-3561233/ |website=Filmboards |access-date=14 March 2025}}</ref> In this framework, "time travel" is reinterpreted not as movement along a linear continuum but as a reconfiguration of the present state of the universe to match a prior entropic configuration. Because the original chronological sequence—including events like the time traveler’s birth—remains preserved in the universe’s irreversible entropic progression, actions within the reconfigured state cannot alter the causal history that produced the traveler. This avoids paradoxes by treating time as a thermodynamic artifact rather than a mutable dimension.

=== Illusory time ===
Consideration of the possibility of backward time travel in a hypothetical universe described by a [[Gödel metric]] led famed logician [[Kurt Gödel]] to assert that time might itself be a sort of illusion.<ref name="Yourgrau">{{cite book |last1=Yourgrau |first1=Palle |url=https://books.google.com/books?id=GtY907BlhxYC&q=2005+A+World+Without+Time:+The+Forgotten+Legacy+of+G%C3%B6del+and+Einstein&pg=PA134 |title=A World Without Time: The Forgotten Legacy of Godel and Einstein |date=4 March 2009 |publisher=Basic Books |isbn=9780786737000 |location=New York |page=134 |access-date=December 18, 2017}}</ref><ref name="holt">{{cite magazine |last=Holt |first=Jim |date=2005-02-21 |title=Time Bandits |url=https://www.newyorker.com/magazine/2005/02/28/time-bandits-2 |magazine=The New Yorker |access-date=2017-12-13}}</ref> He suggests something along the lines of the [[Eternalism (philosophy of time)|block time]] view, in which time is just another dimension like space, with all events at all times being fixed within this four-dimensional "block".{{Citation needed|date=September 2016}}

=== Physical impossibility ===
[[Sergey 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 these 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}}

==== Self-sufficient loops ====
A 1992 paper by physicists Andrei Lossev and [[Igor Dmitriyevich Novikov|Igor Novikov]] labeled such items without origin as ''Jinn'', with the singular term ''Jinnee''.<ref name="Lossev1992">{{cite journal|last1=Lossev|first1=Andrei|last2=Novikov|first2=Igor|date=15 May 1992|title=The Jinn of the time machine: non-trivial self-consistent solutions|journal=Class. Quantum Gravity|volume=9|issue=10|pages=2309–2321|url=http://thelifeofpsi.com/wp-content/uploads/2015/01/Lossev-Novikov-1992.pdf|doi=10.1088/0264-9381/9/10/014|bibcode=1992CQGra...9.2309L|s2cid=250912686 |access-date=16 November 2015|archive-url=https://web.archive.org/web/20151117014658/http://thelifeofpsi.com/wp-content/uploads/2015/01/Lossev-Novikov-1992.pdf|archive-date=17 November 2015|url-status=dead}}</ref>{{Rp|2311–2312}} This terminology was inspired by the [[Jinn]] of the [[Quran]], which are described as leaving no trace when they disappear.<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|200–203}} Lossev and Novikov allowed the term "Jinn" to cover both objects and information with the reflexive origin; they called the former "Jinn of the first kind", and the latter "Jinn of the second kind".<ref name="Everett" /><ref name="Lossev1992" />{{Rp|2315–2317}}<ref name="Toomey2012" />{{Rp|208}} They point out that an object making circular passage through time must be identical whenever it is brought back to the past, otherwise it would create an inconsistency; the [[second law of thermodynamics]] seems to require that the object tends to a lower energy state throughout its history, and such objects that are identical in repeating points in their history seem to contradict this, but Lossev and Novikov argued that since the second law only requires entropy to increase in ''closed'' systems, a Jinnee could interact with its environment in such a way as to regain "lost" entropy.<ref name="Everett" /><ref name="Toomey2012" />{{Rp|200–203}} They emphasize that there is no "strict difference" between Jinn of the first and second kind.<ref name="Lossev1992" />{{Rp|2320}} Krasnikov equivocates between "Jinn", "self-sufficient loops", and "self-existing objects", calling them "lions" or "looping or intruding objects", and asserts that they are no less physical than conventional objects, "which, after all, also could appear only from either infinity or a singularity."<ref name="Krasnikov2001" />{{Rp|8–9}}

==== Novikov self-consistency principle ====
{{main|Novikov self-consistency principle}}
The self-consistency principle developed by [[Igor Dmitriyevich Novikov]]<ref name="Friedman1990">{{cite journal |ref={{harvid|Friedman|1990}}|last1=Friedman|first1=John |last2=Morris|first2=Michael S.|last3=Novikov|first3=Igor D.|last4=Echeverria|first4=Fernando |last5=Klinkhammer|first5=Gunnar |last6=Thorne|first6=Kip S.|last7=Yurtsever|first7=Ulvi| 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 | doi=10.1103/PhysRevD.42.1915 | pages=1915–1930 | bibcode=1990PhRvD..42.1915F | pmid=10013039|url-access=subscription}}</ref>{{Rp|at=p. 42 note 10}} expresses one view as to how backward [[time travel]] would be possible without the generation of paradoxes. According to this hypothesis, even though [[general relativity]] permits some [[exact solutions in general relativity|exact solutions]] that allow for [[time travel]]<ref>{{citation|last=Krasnikov|first=S.|year=2002|title=No time machines in classical general relativity|journal=Classical and Quantum Gravity|volume=19|issue=15|page=4109|arxiv = gr-qc/0111054 |bibcode = 2002CQGra..19.4109K |doi = 10.1088/0264-9381/19/15/316 |s2cid=16517920}}</ref> that contain [[closed timelike curves]] that lead back to the same point in spacetime,<ref>{{cite journal |last=Gödel|first=Kurt| title=An Example of a New Type of Cosmological Solution of Einstein's Field Equations of Gravitation | journal=Rev. Mod. Phys. | year=1949 | volume=21 | pages=447–450 | doi=10.1103/RevModPhys.21.447 | issue=3|bibcode = 1949RvMP...21..447G | doi-access=free }}</ref> physics in or near [[closed timelike curves]] (time machines) can only be consistent with the universal laws of physics, and thus only self-consistent events can occur. Anything a time traveler does in the past must have been part of history all along, and the time traveler can never do anything to prevent the trip back in time from happening, since this would represent an inconsistency. The authors concluded that time travel need not lead to unresolvable paradoxes, regardless of what type of object was sent to the past.<ref name="Thorne" />

[[File:Grandfather_paradox_billiard_ball.svg|right|thumb|Top: original billiard ball trajectory. Middle: the billiard ball emerges from the future, and delivers its past self a strike that averts the past ball from entering the time machine. Bottom: The billiard ball never enters the time machine, giving rise to the paradox, putting into question how its older self could ever emerge from the time machine and divert its course.]]
Physicist [[Joseph Polchinski]] considered a potentially paradoxical situation involving a [[billiard ball]] that is fired into a [[wormhole]] at just the right angle such that it will be sent back in time and collides with its earlier self, knocking it off course, which would stop it from entering the wormhole in the first place. [[Kip Thorne]] referred to this problem as "Polchinski's paradox".<ref name="Thorne">{{cite book | last = Thorne | first = Kip S. | author-link = Kip Thorne | title = Black Holes and Time Warps | publisher = W. W. Norton | year= 1994 | isbn = 0-393-31276-3|pages=509–513| title-link = Black Holes and Time Warps }}</ref> Thorne and two of his students at Caltech, Fernando Echeverria and Gunnar Klinkhammer, went on to find a solution that avoided any inconsistencies, and found that there was more than one self-consistent solution, with slightly different angles for the glancing blow in each case.<ref>{{cite journal |ref={{harvid|Echeverria|1991}}| first=Fernando | last=Echeverria |author2=Gunnar Klinkhammer |author3=Kip Thorne | url=http://authors.library.caltech.edu/6469/ | title=Billiard balls in wormhole spacetimes with closed timelike curves: Classical theory | journal = Physical Review D | volume = 44 | year=1991 | issue=4 | doi=10.1103/PhysRevD.44.1077 | pages=1077–1099| pmid=10013968 |bibcode = 1991PhRvD..44.1077E | url-access=subscription }}</ref> Later analysis by Thorne and [[Robert Forward]] showed that for certain initial trajectories of the billiard ball, there could be an infinite number of self-consistent solutions.<ref name="Thorne" /> It is plausible that there exist self-consistent extensions for every possible initial trajectory, although this has not been proven.<ref name="Earman1995187188">{{cite book | last = Earman | first = John | title = Bangs, Crunches, Whimpers, and Shrieks: Singularities and Acausalities in Relativistic Spacetimes | publisher = Oxford University Press |year= 1995 | isbn = 0-19-509591-X}}</ref>{{Rp|184}} The lack of constraints on initial conditions only applies to spacetime outside of the [[Chronology protection conjecture|chronology-violating region of spacetime]]; the constraints on the chronology-violating region might prove to be paradoxical, but this is not yet known.<ref name="Earman1995187188" />{{Rp|187–188}}

Novikov's views are not widely accepted. Visser views causal loops and Novikov's self-consistency principle as an ''ad hoc'' solution, and supposes that there are far more damaging implications of time travel.<ref>{{cite book | last = Nahin | first =Paul J. | title = Time Machines: Time Travel in Physics, Metaphysics, and Science Fiction | publisher =American Institute of Physics |year= 1999 | isbn = 0-387-98571-9|pages=345–352}}</ref> Krasnikov similarly finds no inherent fault in causal loops but finds other problems with time travel in general relativity.<ref name="Krasnikov2001" />{{Rp|14–16}} Another conjecture, the [[cosmic censorship hypothesis]], suggests that every closed timelike curve passes through an [[event horizon]], which prevents such causal loops from being observed.<ref>{{cite journal |last1=Visser |first1=Matt |date=15 April 1997 |title=Traversable wormholes: The Roman ring |journal=Physical Review D |volume=55 |issue=8 |pages=5212–5214 |arxiv=gr-qc/9702043 |bibcode=1997PhRvD..55.5212V |doi=10.1103/PhysRevD.55.5212 |s2cid=2869291}}</ref>

==== Parallel universes ====
The interacting-multiple-universes approach is a variation of the [[many-worlds interpretation]] of quantum mechanics that involves time travelers arriving in a different universe than the one from which they came; it has been argued that, since travelers arrive in a different universe's history and not their history, this is not "genuine" time travel.<ref name="Stanford time travel">{{citation |author1=Frank Arntzenius |title=Time Travel and Modern Physics |date=December 23, 2009 |url=http://plato.stanford.edu/entries/time-travel-phys/ |encyclopedia=Stanford Encyclopedia of Philosophy |access-date=May 25, 2019 |author2=Tim Maudlin}}</ref> Stephen Hawking has argued for the [[chronology protection conjecture]], 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 world rather than traveling to a different one.<ref name="Hawking warp">{{cite web |last1=Hawking |first1=Stephen |year=1999 |title=Space and Time Warps |url=http://www.hawking.org.uk/space-and-time-warps.html |access-date=February 25, 2012 |archive-date=February 10, 2012 |archive-url=https://web.archive.org/web/20120210233225/http://www.hawking.org.uk/space-and-time-warps.html |url-status=dead }}</ref>

[[David Deutsch]] has proposed that [[quantum computation]] with a negative delay—backward time travel—produces only self-consistent solutions, and the chronology-violating region imposes constraints that are not apparent through classical reasoning.<ref name="Deutsch">{{cite journal |last1=Deutsch |first1=David |author-link=David Deutsch |date=15 November 1991 |title=Quantum mechanics near closed timelike lines |journal=Physical Review D |volume=44 |issue=10 |pages=3197–3217 |bibcode=1991PhRvD..44.3197D |doi=10.1103/PhysRevD.44.3197 |pmid=10013776}}</ref> However Deutsch's self-consistency condition has been demonstrated as capable of being fulfilled to arbitrary precision by any system subject to the laws of classical [[statistical mechanics]], even if it is not built up by quantum systems.<ref>{{cite journal |last1=Tolksdorf |first1=Juergen |last2=Verch |first2=Rainer |date=2021 |title=The D-CTC condition is generically fulfilled in classical (non-quantum) statistical systems |journal=Foundations of Physics |series= |volume=51 |issue=93 |page=93 |arxiv=1912.02301 |bibcode=2021FoPh...51...93T |doi=10.1007/s10701-021-00496-z |authorlink2= |authorlink1= |s2cid=208637445}}</ref> Allen Everett has also argued 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, with different particles emerging in different worlds.<ref name="Everett MWI">{{cite journal |last=Everett |first=Allen |year=2004 |title=Time travel paradoxes, path integrals, and the many worlds interpretation of quantum mechanics |journal=Physical Review D |volume=69 |issue=124023 |pages=124023 |arxiv=gr-qc/0410035 |bibcode=2004PhRvD..69l4023E |doi=10.1103/PhysRevD.69.124023 |s2cid=18597824}}</ref>