Novikov self-consistency principle: Difference between revisions

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{{Short description|Assertion that timeTime-travel paradoxes are impossible, as paradox-causing events cannot occur}}

The '''Novikov self-consistency principle''', also known as the '''Novikov self-consistency conjecture''' and [[Larry Niven]]'s '''law of conservation of history''', is a [[principle]] developed by Russian physicist [[Igor Dmitriyevich Novikov]] in the mid-1980s. Novikov intended it to solve the problem of [[Temporal paradox|paradoxes in time travel]], which is theoretically permitted in certain solutions of [[general relativity]] that contain what are known as [[closed timelike curve]]s. The principle asserts that if an event exists that would cause a paradox or any "change" to the past whatsoever, then the [[probability]] of that event is zero. It would thus be impossible to create [[time paradox]]es.

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Among the co-authors of this 1990 paper were [[Kip Thorne]], [[Mike Morris (physicist)|Mike Morris]], and Ulvi Yurtsever, who in 1988 had stirred up renewed interest in the subject of time travel in general relativity with their paper "Wormholes, Time Machines, and the Weak Energy Condition",<ref>{{cite journal | first=Kip | last=Thorne |author2=Michael Morris |author3=Ulvi Yurtsever | journal=[[Physical Review Letters]] | volume = 61 | issue=13| pages=1446–1449 | doi= 10.1103/PhysRevLett.61.1446 | title= Wormholes, Time Machines, and the Weak Energy Condition | year=1988 | url=http://authors.library.caltech.edu/9262/1/MORprl88.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://authors.library.caltech.edu/9262/1/MORprl88.pdf |archive-date=2022-10-09 |url-status=live | bibcode=1988PhRvL..61.1446M | pmid=10038800}}</ref> which showed that a new general relativity solution known as a [[Wormhole#Traversable wormholes|traversable wormhole]] could lead to closed timelike curves, and unlike previous CTC-containing solutions, it did not require unrealistic conditions for the universe as a whole. After discussions with the lead author of the 1990 paper, John Friedman, they convinced themselves that time travel need not lead to unresolvable paradoxes, regardless of the object sent [[Through the Wormhole|through the wormhole]].<ref name = "time warps">{{cite book| first= Kip S. | last= Thorne|title=Black Holes and Time Warps: Einstein's Outrageous Legacy|url=https://archive.org/details/blackholestimewa0000thor| url-access= registration | quote= Polchinski's paradox. |year=1994|publisher=W. W. Norton|isbn=978-0-393-31276-8|pages=[https://archive.org/details/blackholestimewa0000thor/page/510 510]–}}</ref>{{rp|509}}

[[File:Grandfather paradox billiard ball.svg|thumb|right|upright=0.7|"Polchinski's paradox"]] [[File:Causal loop billiard ball.svg|thumb|right|upright=0.7|Echeverria and Klinkhammer's resolution]]

By way of response, physicist [[Joseph Polchinski]] wrote them a letter arguing that one could avoid the issue of free will by employing a potentially paradoxical thought experiment involving a [[billiard ball]] sent back in time through a wormhole. In Polchinski's scenario, [[The Billiard Ball|the billiard ball]] is fired into the [[wormhole]] at an angle such that, if it continues along its path, it will exit in the past at just the right angle to collide with its earlier self, knocking it off track and preventing it from entering the wormhole in the first place. Thorne would refer to this scenario as "[[Polchinski's paradox]]" in 1994.<ref name = "timewarps">{{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}}</ref>{{rp|510–511}}

Upon considering the scenario, Fernando Echeverria and Gunnar Klinkhammer, two students at [[California Institute of Technology|Caltech]] (where Thorne taught), arrived at a solution to the problem, that lays out the same elements as the solution Feynman and Wheeler<ref>{{cite journal | first1=John | last1=Wheeler | first2=Richard | last2=Feynman | title=Classical Electrodynamics in Terms of Direct Interparticle Action | journal = Reviews of Modern Physics | volume = 21 | year=1949 | issue=3 | pages=425–433| doi=10.1103/RevModPhys.21.425 | bibcode=1949RvMP...21..425W | doi-access=free }}</ref> termed the "glancing blow" solution, to evade inconsistencies arising from causality loops. In the revised scenario, the ball from the future emerges at a different angle than the one that generates the paradox, and delivers its younger self a glancing blow instead of knocking it completely away from the wormhole. This blow alters its trajectory by just the right degree, meaning it will travel back in time with the angle required to deliver its younger self the necessary glancing blow. Echeverria and Klinkhammer actually found that there was more than one self-consistent solution, with slightly different angles for the glancing blow in each situation. Later analysis by Thorne and [[Robert Forward]] illustrated that for certain initial trajectories of the billiard ball, there could actually be an infinite number of self-consistent solutions.<ref name = "timewarps" />{{rp|511–513}}

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the same authors show that Deutsch's CTC fixed point condition can also be fulfilled in any system

subject to the laws of classical [[statistical mechanics]], even if it is not built up by quantum systems. The authors conclude that hence,

Deutsch's condition is not specific to quantum physics, nor does it depend on the quantum nature of a [[physical system]] so that it can be fulfilled.

In consequence, Tolksdorf and Verch argue that Deutsch's condition is not sufficiently specific to allow statements about time travel scenarios or their hypothetical realization by quantum physics.

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* The story ''[[The Merchant and the Alchemist's Gate]]'' (2007) by [[Ted Chiang]] explores the interplay between free will and self-consistent time-travel.

* ''[[Steins;Gate]]'' (2009): Cited by Makise Kurisu during her presentation on time travel.

* ''[[Harry Potter and the Methods of Rationality]]'': In [[Eliezer Yudkowsky]]'s exposition on rationality, framed as a piece of Harry Potter fanfiction, Harry attempts to use his Time Turner to influence the past and comes to the conclusion that the Novikov self-consistency principle applies.

* ''[[Orthogonal (series)|Orthogonal]]'': A science-fiction novel series by [[Greg Egan]] that applies the principle.

* The [[Netflix]] series ''[[Dark (TV series)|Dark]]'' is largely based on the notion that the possibility of time travel tempts the characters to try to change the past, which only leads them to cause the events they were trying to prevent in the first place.

* ''[[Quantum Break]]'' (2016): A video game by [[Remedy Entertainment]], centers heavily on the question whether the past can be changed or not. Some of the characters in the plot are driven to change it, whereas others, who have already tried doing so in vain, have resigned themselves to come to the conclusion that the Novikov self-consistency principle seemingly applies.

* ''[[Outer Wilds]]'' (2019): A video game involving time travel which does not follow the principle, causing a game over if the player experiments to test it.