Editing Many-worlds interpretation (section)

{{Short description|Interpretation of quantum mechanics}}
[[File:Schroedingers cat film.svg|thumb|upright=1.4|The quantum-mechanical "[[Schrödinger's cat]]" paradox according to the many-worlds interpretation. In this interpretation, every quantum event is a branch point; the cat is both alive and dead, even after the box is opened, but the "alive" and "dead" cats are in different branches of the multiverse, both of which are equally real, but which do not interact with each other.]]
{{Quantum mechanics|cTopic=[[Interpretation of quantum mechanics|Interpretations]]}}

The '''many-worlds interpretation''' ('''MWI''') is an [[interpretation of quantum mechanics]] that asserts that the [[universal wavefunction]] is [[Philosophical realism|objectively real]], and that there is no [[wave function collapse]].<ref name="dewitt73">{{cite book |last1=Everett |first1=Hugh |title=The Many-Worlds Interpretation of Quantum Mechanics |last2=Wheeler |first2=J. A. |last3=DeWitt |first3=B. S. |last4=Cooper |first4=L. N. |last5=Van Vechten |first5=D. |last6=Graham |first6=N. |publisher=[[Princeton University Press]] |year=1973 |isbn=0-691-08131-X |editor-last1=DeWitt |editor-first1=Bryce |editor-link1=Bryce DeWitt |series=Princeton Series in Physics |location=Princeton, New Jersey |page=v |language=en-us |author-link1=Hugh Everett III |author-link2=John Archibald Wheeler |author-link3=Bryce DeWitt |author-link4=Leon Cooper |editor-last2=Graham |editor-first2=R. Neill}}</ref> This implies that all [[Possible world|possible]] outcomes of quantum measurements are physically realized in different "worlds".<ref name=tegmark97>{{Cite journal|arxiv=quant-ph/9709032|last1=Tegmark|first1=Max|title=The Interpretation of Quantum Mechanics: Many Worlds or Many Words?|journal=Fortschritte der Physik|volume=46|issue=6–8|pages=855–862|year=1998|doi=10.1002/(SICI)1521-3978(199811)46:6/8<855::AID-PROP855>3.0.CO;2-Q|bibcode=1998ForPh..46..855T|s2cid=212466 }}</ref> The evolution of reality as a whole in MWI is rigidly [[Determinism|deterministic]]<ref name=dewitt73/>{{rp|9}} and [[principle of locality|local]].<ref name="DeterminismLocal">{{Cite book |author1=Brown |first=Harvey R. |author-link1=Harvey Brown (philosopher) |title=Quantum Nonlocality and Reality: 50 years of Bell's theorem |author2=Christopher G. Timpson |publisher=Cambridge University Press |year=2016 |isbn=9781316219393 |editor=Mary Bell |pages=91–123 |chapter=Bell on Bell's Theorem: The Changing Face of Nonlocality |doi=10.1017/CBO9781316219393.008 |quote=On locality:"Amongst those who have taken Everett’s approach to quantum theory at all seriously as an option, it is a commonplace that—given an Everettian interpretation—quantum theory is (dynamically) local-there is no action-at-a-distance" on determinism:"But zooming-out (in a God’s-eye view) from a particular branch will be seen all the other branches, each with a different result of measurement being recorded and observed, all coexisting equally; and all underpinned by ([[supervenient]] on) the deterministically, unitarily, evolving universal wavefunction" |editor2=Shan Gao |arxiv=1501.03521 |s2cid=118686956}}</ref> Many-worlds is also called the '''relative state formulation''' or the '''Everett interpretation''', after physicist [[Hugh Everett III|Hugh Everett]], who first proposed it in 1957.<ref name="everett56">[[Hugh Everett]] [https://www.pbs.org/wgbh/nova/manyworlds/pdf/dissertation.pdf Theory of the Universal Wavefunction], Thesis, Princeton University, (1956, 1973), pp. 1–140.</ref><ref name=everett57>{{cite journal | last1 = Everett | first1 = Hugh | author-link = Hugh Everett | year = 1957 | title = Relative State Formulation of Quantum Mechanics | url = http://www.univer.omsk.su/omsk/Sci/Everett/paper1957.html | journal = Reviews of Modern Physics | volume = 29 | issue = 3 | pages = 454–462 | bibcode = 1957RvMP...29..454E | doi = 10.1103/RevModPhys.29.454 | access-date = 2011-10-24 | archive-url = https://web.archive.org/web/20111027191052/http://www.univer.omsk.su/omsk/Sci/Everett/paper1957.html | archive-date = 2011-10-27 | url-status = dead | url-access = subscription }}</ref> [[Bryce DeWitt]] popularized the formulation and named it ''many-worlds'' in the 1970s.<ref name="dewitt71">{{cite journal |author1=DeWitt |first=Bryce S. |date=1970 |title=Quantum mechanics and reality |journal=Physics Today |volume=23 |issue=9 |pages=30–35 |bibcode=1970PhT....23i..30D |doi=10.1063/1.3022331 |doi-access=free}} See also {{cite journal |last1=Ballentine |first1=Leslie E. |last2=Pearle |first2=Philip |last3=Walker |first3=Evan Harris |last4=Sachs |first4=Mendel |last5=Koga |first5=Toyoki |last6=Gerver |first6=Joseph |last7=DeWitt |first7=Bryce |date=1971 |title=Quantum-mechanics debate |journal=Physics Today |volume=24 |issue=4 |pages=36–44 |bibcode=1971PhT....24d..36. |doi=10.1063/1.3022676}}</ref><ref name=dewitt73/><ref name="dewitt67">[[Cécile DeWitt-Morette|Cecile M. DeWitt]], [[John A. Wheeler]] (eds,) The Everett–Wheeler Interpretation of Quantum Mechanics, ''Battelle Rencontres: 1967 Lectures in Mathematics and Physics'' (1968).</ref><ref name="dewitt72">[[Bryce Seligman DeWitt]], The Many-Universes Interpretation of Quantum Mechanics, ''Proceedings of the International School of Physics "Enrico Fermi" Course IL: Foundations of Quantum Mechanics'', [[Academic Press]] (1972).</ref>

In modern versions of many-worlds, the subjective appearance of wave function collapse is explained by the mechanism of [[quantum decoherence]].<ref name=tegmark97/> Decoherence approaches to interpreting quantum theory have been widely explored and developed since the 1970s.<ref name=zeh>[[H. Dieter Zeh]], On the Interpretation of Measurement in Quantum Theory, ''Foundations of Physics'', vol. 1, pp. 69–76, (1970).</ref><ref name=zurek91>[[Wojciech Hubert Zurek]], Decoherence and the transition from quantum to classical, ''Physics Today'', vol. 44, issue 10, pp. 36–44, (1991).</ref><ref name="zurek03">[[Wojciech Hubert Zurek]], Decoherence, einselection, and the quantum origins of the classical, ''Reviews of Modern Physics'', 75, pp. 715–775, (2003).</ref> MWI is considered a mainstream [[interpretation of quantum mechanics]], along with the other decoherence interpretations, the [[Copenhagen interpretation]], and [[hidden variable theories]] such as [[De Broglie–Bohm theory|Bohmian mechanics]].<ref>{{Cite journal |last1=Schlosshauer |first1=Maximilian |title=A snapshot of foundational attitudes toward quantum mechanics |last2=Kofler |first2=Johannes |last3=Zeilinger |first3=Anton |journal=Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics |date=August 2013 |volume=44 |issue=3 |pages=222–230 |language=en |doi=10.1016/j.shpsb.2013.04.004|doi-access=free |arxiv=1301.1069 |bibcode=2013SHPMP..44..222S }}</ref><ref name=tegmark97/>

The many-worlds interpretation implies that there are many parallel, non-interacting worlds. It is one of a number of [[multiverse]] hypotheses in [[physics]] and [[philosophy]]. MWI views time as a many-branched tree, wherein every possible quantum outcome is realized. This is intended to resolve the [[measurement problem]] and thus some [[quantum paradox|paradoxes]] of [[Quantum mechanics|quantum theory]], such as [[Wigner's friend]],<ref name=everett56/>{{rp|4–6}} the [[EPR paradox]]<ref name=everett57/>{{rp|462}}<ref name=dewitt73/>{{rp|118}} and [[Schrödinger's cat]],<ref name=dewitt71/> since every possible outcome of a quantum event exists in its own world.