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{{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=978-1-316-21939-3 |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-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.

== Overview of the interpretation ==
The many-worlds interpretation's key idea is that the [[linear combination|linear]] and [[unitary (physics)|unitary]] dynamics of quantum mechanics applies everywhere and at all times and so describes the whole universe. In particular, it models a measurement as a unitary transformation, a correlation-inducing interaction, between observer and object, without using a [[Wave function collapse|collapse postulate]], and models observers as ordinary quantum-mechanical systems.<ref name=wallace2012/>{{rp|35–38}} This stands in contrast to the [[Copenhagen interpretation]], in which a measurement is a "primitive" concept, not describable by unitary quantum mechanics; using the Copenhagen interpretation the universe is divided into a quantum and a classical domain, and the collapse postulate is central.<ref name=wallace2012/>{{rp|29–30}} In MWI, there is no division between classical and quantum: everything is quantum and there is no collapse. MWI's main conclusion is that the universe (or [[multiverse]] in this context) is composed of a [[quantum superposition]] of an uncountable<ref name="Heresy"/> or undefinable<ref name=wallace2010/>{{rp|14–17}} amount or number of increasingly divergent, non-communicating parallel universes or quantum worlds.<ref name=dewitt73/> Sometimes dubbed Everett worlds,<ref name=dewitt73/>{{rp|234}} each is an internally consistent and actualized [[alternative history]] or timeline.

The many-worlds interpretation uses [[decoherence]] to explain the measurement process and the emergence of a quasi-classical world.<ref name=wallace2010/><ref name=saunders2010/> [[Wojciech H. Zurek]], one of decoherence [[theory]]'s pioneers, said: "Under scrutiny of the environment, only [[pointer state]]s remain unchanged. Other states decohere into mixtures of stable pointer states that can persist, and, in this sense, exist: They are einselected."<ref name="ZurekNature2009">{{cite journal | last = Zurek | first = Wojciech | author-link = Wojciech Hubert Zurek | title = Quantum Darwinism | date = March 2009 | arxiv = 0903.5082 |bibcode = 2009NatPh...5..181Z |doi = 10.1038/nphys1202 | volume=5 | issue=3 | journal=Nature Physics | pages=181–188| s2cid = 119205282 }}</ref> Zurek emphasizes that his work does not depend on a particular interpretation.{{efn|"Relative states of Everett come to mind. One could speculate about reality of branches with other outcomes. We abstain from this; our discussion is interpretation-free, and this is a virtue."<ref name=ZurekNature2009/>}}

The many-worlds interpretation shares many similarities with the [[decoherent histories]] interpretation, which also uses decoherence to explain the process of measurement or wave function collapse.<ref name=saunders2010/>{{rp|9–11}} MWI treats the other histories or worlds as real, since it regards the universal wave function as the "basic physical entity"<ref name=everett57/>{{rp|455}} or "the fundamental entity, obeying at all times a deterministic wave equation".<ref name="everett56"/>{{rp|115}} The decoherent histories interpretation, on the other hand, needs only one of the histories (or worlds) to be real.<ref name=saunders2010/>{{rp|10}}

Several authors, including Everett, [[John Archibald Wheeler]] and [[David Deutsch]], call many-worlds a theory or [[metatheory]], rather than just an interpretation.<ref name="Heresy"/><ref name=skyrms1976/>{{rp|328}} Everett argued that it was the "only completely coherent approach to explaining both the contents of quantum mechanics and the appearance of the world."<ref>{{Citation |last=Everett |first=Hugh |title=Hugh Everett letter to David Raub, 7-April-1980 |date=1980-04-07 |url=https://calisphere.org/item/ark:/81235/d83t9dk9g/ |access-date=2023-08-26}}</ref> Deutsch dismissed the idea that many-worlds is an "interpretation", saying that to call it an interpretation "is like talking about dinosaurs as an 'interpretation' of fossil records".<ref name="MAD"/>{{rp|382}}

=== Formulation ===
In his 1957 doctoral dissertation, Everett proposed that, rather than relying on external observation for analysis of isolated quantum systems, one could mathematically model an object, as well as its observers, as purely physical systems within the mathematical framework developed by [[Paul Dirac]], [[John von Neumann]], and others, discarding altogether the ''ad hoc'' mechanism of [[wave function collapse]].<ref name=everett56/><ref name=dewitt73/>

=== Relative state ===
Everett's original work introduced the concept of a ''relative state''. Two (or more) subsystems, after a general interaction, become ''correlated'', or as is now said, [[quantum entanglement|entangled]]. Everett noted that such entangled systems can be expressed as the sum of products of states, where the two or more subsystems are each in a state relative to each other. After a measurement or observation one of the pair (or triple, etc.) is the measured, object or observed system, and one other member is the measuring apparatus (which may include an observer) having recorded the state of the measured system. Each product of subsystem states in the overall superposition evolves over time independently of other products. Once the subsystems interact, their states have become correlated or entangled and can no longer be considered independent. In Everett's terminology, each subsystem state was now ''correlated'' with its ''relative state'', since each subsystem must now be considered relative to the other subsystems with which it has interacted.

In the example of [[Schrödinger's cat]], after the box is opened, the entangled system is the cat, the poison vial and the observer. ''One'' relative triple of states would be the alive cat, the unbroken vial and the observer seeing an alive cat. ''Another'' relative triple of states would be the dead cat, the broken vial and the observer seeing a dead cat.

In the example of a measurement of a continuous variable (e.g., position ''q''), the object-observer system decomposes into a continuum of pairs of relative states: the object system's relative state becomes a [[Dirac delta function]] each centered on a particular value of ''q'' and the corresponding observer relative state representing an observer having recorded the value of ''q''.<ref name="everett56"/>{{rp|57–64}} The states of the pairs of relative states are, post measurement, ''correlated'' with each other.

In Everett's scheme, there is no collapse; instead, the [[Schrödinger equation]], or its [[quantum field theory]], relativistic analog, holds all the time, everywhere. An observation or measurement is modeled by applying the wave equation to the entire system, comprising the object being observed ''and'' the observer. One consequence is that every observation causes the combined observer–object's wavefunction to change into a quantum superposition of two or more non-interacting branches.

Thus the process of measurement or observation, or any correlation-inducing interaction, splits the system into sets of relative states, where each set of relative states, forming a branch of the universal wave function, is consistent within itself, and all future measurements (including by multiple observers) will confirm this consistency.

=== Renamed many-worlds ===
Everett had referred to the combined observer–object system as split by an observation, each split corresponding to the different or multiple possible outcomes of an observation. These splits generate a branching tree, where each branch is a set of all the states relative to each other. [[Bryce DeWitt]] popularized Everett's work with a series of publications calling it the Many Worlds Interpretation. Focusing on the splitting process, DeWitt introduced the term "world" to describe a single branch of that tree, which is a consistent history. All observations or measurements within any branch are consistent within themselves.<ref name=everett56/><ref name=dewitt73/>

Since many observation-like events have happened and are constantly happening, Everett's model implies that there are an enormous and growing number of simultaneously existing states or "worlds".{{efn|"every quantum transition taking place on every star, in every galaxy, in every remote corner of the universe is splitting our local world on earth into myriads of copies of itself."<ref name=dewitt71/> DeWitt later softened this extreme view, viewing splitting as decoherence driven and local, in line with other modern commentators.<ref>{{Cite book |url=https://www.degruyter.com/document/doi/10.1515/9781400842742/html |title=The Everett Interpretation of Quantum Mechanics: Collected Works 1955-1980 with Commentary |date=2012-05-20 |publisher=Princeton University Press |isbn=978-1-4008-4274-2 |editor-last=Barrett |editor-first=Jeffrey A. |doi=10.1515/9781400842742 |quote=DeWitt eventually softened his view on the necessity of a splitting universe occurring with every atomic interaction when he broadly embraced the decoherence approaches proposed by Dieter Zeh, Wojciech Zurek, James B. Hartle, Murray Gell-Mann, and others beginning around 1970. |editor-last2=Byrne |editor-first2=Peter}}  </ref>}}

=== Properties ===

MWI removes the observer-dependent role in the [[quantum measurement]] process by replacing [[wave function collapse]] with the established mechanism of [[quantum decoherence]].<ref>{{cite journal|doi=10.1016/j.physrep.2019.10.001 |first=Max |last=Schlosshauer |title=Quantum decoherence |journal=Physics Reports |volume=831 |date=2019-10-25 |pages=1–57 |arxiv=1911.06282|bibcode=2019PhR...831....1S |s2cid=208006050 }}</ref> As the observer's role lies at the heart of all "quantum paradoxes" such as the [[EPR paradox]] and von Neumann's "boundary problem", this provides a clearer and easier approach to their resolution.<ref name=everett57/>

Since the Copenhagen interpretation requires the existence of a classical domain beyond the one described by quantum mechanics, it has been criticized as inadequate for the study of cosmology.<ref name="gell-man1990"/> While there is no evidence that Everett was inspired by issues of cosmology,<ref name="Heresy"/>{{rp|7}} he developed his theory with the explicit goal of allowing quantum mechanics to be applied to the universe as a whole, hoping to stimulate the discovery of new phenomena.<ref name=everett57/> This hope has been realized in the later development of [[quantum cosmology]].<ref>{{Cite book |last1=Gell-Mann |first1=Murray |last2=Hartle |first2=James B. |date=January 1997 |title=Quantum Mechanics in the Light of Quantum Cosmology |url=http://www.worldscientific.com/doi/abs/10.1142/9789812819895_0036 |language=en |publisher=World Scientific |volume=4 |pages=347–369 |doi=10.1142/9789812819895_0036 |arxiv=1803.04605 |isbn=978-981-02-2844-6}}</ref>

MWI is a [[philosophical realism|realist]], [[deterministic]] and [[principle of locality|local]] theory. It achieves this by removing wave function collapse, which is indeterministic and nonlocal, from the deterministic and local equations of quantum theory.<ref name="DeterminismLocal"/>

MWI (like other, broader multiverse theories) provides a context for the [[anthropic principle]], which may provide an explanation for the [[fine-tuned universe]].<ref>[[Paul C.W. Davies|Paul C. W. Davies]], ''Other Worlds'', chapters 8 & 9 ''The Anthropic Principle'' & ''Is the Universe an accident?'', (1980) {{ISBN|0-460-04400-1}}.</ref><ref>[[Paul C.W. Davies|Paul C. W. Davies]], ''The Accidental Universe'', (1982) {{ISBN|0-521-28692-1}}.</ref>

MWI depends crucially on the [[linear combination|linearity]] of quantum mechanics, which underpins the [[quantum superposition|superposition principle]]. If the final [[theory of everything]] is non-linear with respect to wavefunctions, then many-worlds is invalid.<ref name="dewitt71"/><ref name="dewitt73"/><ref name="everett57"/><ref name="dewitt67"/><ref name="dewitt72"/> All [[quantum field theory|quantum field theories]] are linear and compatible with the MWI, a point Everett emphasized as a motivation for the MWI.<ref name="everett57"/> While quantum gravity or [[string theory]] may be non-linear in this respect,<ref name="penrose"/> there is as yet no evidence of this.<ref name="weinberglinearitydoft">[[Steven Weinberg]], ''Dreams of a Final Theory: The Search for the Fundamental Laws of Nature'' (1993), {{ISBN|0-09-922391-0}}, pp. 68–69.</ref><ref name="weinberglinearityannalsop">[[Steven Weinberg]]. ''Testing Quantum Mechanics'', Annals of Physics Vol. 194, #2 (1989), pp. 336–386.</ref>

Weingarten<ref>{{cite journal|first1=Don|last1=Weingarten|title=Macroscopic Reality from Quantum Complexity|url=https://link.springer.com/article/10.1007/s10701-022-00554-0|journal=Foundations of Physics|date=5 April 2022|issn=1572-9516|article-number=45|volume=52|issue=2|doi=10.1007/s10701-022-00554-0|arxiv=2105.04545|bibcode=2022FoPh...52...45W }}</ref> and Taylor & McCulloch<ref>{{cite journal|first1=Jordan|last1=Taylor|first2=Ian|last2=McCulloch|title=Wavefunction branching: when you can't tell pure states from mixed states|url=https://quantum-journal.org/papers/q-2025-03-25-1670/|journal=Quantum|date=25 March 2025|issn=2521-327X|page=1670|volume=9|doi=10.22331/q-2025-03-25-1670|arxiv=2308.04494|bibcode=2025Quant...9.1670T }}</ref> have made separate proposals for how to define wavefunction branches in terms of quantum circuit [[Quantum_complexity_theory|complexity]].

=== Alternative to wavefunction collapse ===
As with the other interpretations of quantum mechanics, the many-worlds interpretation is motivated by behavior that can be illustrated by the [[double-slit experiment]]. When [[photon|particles of light]] (or anything else) pass through the double slit, a calculation assuming wavelike behavior of light can be used to identify where the particles are likely to be observed. Yet when the particles are observed in this experiment, they appear as particles (i.e., at definite places) and not as non-localized waves.

Some versions of the Copenhagen interpretation of quantum mechanics proposed a process of "collapse" in which an indeterminate quantum system would probabilistically collapse onto, or select, just one determinate outcome to "explain" this phenomenon of observation. Wave function collapse was widely regarded as artificial and ''[[ad hoc]]'',<ref>{{Cite book |last=Wimmel |first=Hermann |url=https://books.google.com/books?id=I63sCgAAQBAJ |title=Quantum Physics And Observed Reality: A Critical Interpretation Of Quantum Mechanics |date=1992-05-26 |publisher=World Scientific |isbn=978-981-4505-46-8 |page=45 |language=en}}</ref> so an alternative interpretation in which the behavior of measurement could be understood from more fundamental physical principles was considered desirable.

Everett's PhD work provided such an interpretation. He argued that for a composite system—such as a subject (the "observer" or measuring apparatus) observing an object (the "observed" system, such as a particle)—the claim that either the observer or the observed has a well-defined state is meaningless; in modern parlance, the observer and the observed have become entangled: we can only specify the state of one ''relative'' to the other, i.e., the state of the observer and the observed are correlated ''after'' the observation is made. This led Everett to derive from the unitary, deterministic dynamics alone (i.e., without assuming wave function collapse) the notion of a ''relativity of states''.

Everett noticed that the unitary, deterministic dynamics alone entailed that after an observation is made each element of the [[quantum superposition]] of the combined subject–object wave function contains two "relative states": a "collapsed" object state and an associated observer who has observed the same collapsed outcome; what the observer sees and the state of the object have become correlated by the act of measurement or observation. The subsequent evolution of each pair of relative subject–object states proceeds with complete indifference as to the presence or absence of the other elements, ''as if'' wave function collapse has occurred,<ref name=dewitt73/>{{rp|67,78}} which has the consequence that later observations are always consistent with the earlier observations. Thus the ''appearance'' of the object's wave function's collapse has emerged from the unitary, deterministic theory itself. (This answered Einstein's early criticism of quantum theory: that the theory should define what is observed, not for the observables to define the theory.){{efn|"Whether you can observe a thing or not depends on the theory which you use. It is the theory which decides what can be observed."—[[Albert Einstein]] to [[Werner Heisenberg]], objecting to placing observables at the heart of the new quantum mechanics, during Heisenberg's 1926 lecture at Berlin; related by Heisenberg in 1968.<ref name="einstein26">[[Abdus Salam]], ''Unification of Fundamental Forces'', Cambridge University Press (1990) {{ISBN|0-521-37140-6}}, pp 98–101</ref>}} Since the wave function ''appears'' to have collapsed then, Everett reasoned, there was no need to actually assume that it ''had'' collapsed. And so, invoking [[Occam's razor]], he removed the postulate of wave function collapse from the theory.<ref name=dewitt73/>{{rp|8}}

=== Testability ===

In 1985, [[David Deutsch]] proposed a variant of the [[Wigner's friend]] thought experiment as a test of many-worlds versus the Copenhagen interpretation.<ref name="deutsch1985">{{cite journal|last=Deutsch |first=D. |author-link=David Deutsch |year=1985 |title=Quantum theory as a universal physical theory |journal=[[International Journal of Theoretical Physics]] |volume=24 |number=1 |pages=1–41 |doi=10.1007/BF00670071 |bibcode=1985IJTP...24....1D|s2cid=17530632 }}</ref> It consists of an experimenter (Wigner's friend) making a measurement on a quantum system in an isolated laboratory, and another experimenter (Wigner) who would make a measurement on the first one. According to the many-worlds theory, the first experimenter would end up in a macroscopic superposition of seeing one result of the measurement in one branch, and another result in another branch. The second experimenter could then interfere these two branches in order to test whether it is in fact in a macroscopic superposition or has collapsed into a single branch, as predicted by the Copenhagen interpretation. Since then Lockwood, Vaidman, and others have made similar proposals,<ref name="vaidman_stanfordencyclopedia">{{cite book |last=Vaidman |first=Lev |title=Many-Worlds Interpretation of Quantum Mechanics |url=http://plato.stanford.edu/entries/qm-manyworlds/ |publisher=The Stanford Encyclopedia of Philosophy|year=2018 }}</ref> which require placing macroscopic objects in a coherent superposition and interfering them, a task currently beyond experimental capability.

== Probability and the Born rule ==

Since the many-worlds interpretation's inception, physicists have been puzzled about the role of probability in it. As put by Wallace, there are two facets to the question:<ref name="wallace2003"/> the ''incoherence problem'', which asks why we should assign probabilities at all to outcomes that are certain to occur in some worlds, and the ''quantitative problem'', which asks why the probabilities should be given by the [[Born rule]].

Everett tried to answer these questions in the paper that introduced many-worlds. To address the incoherence problem, he argued that an observer who makes a sequence of measurements on a quantum system will in general have an apparently random sequence of results in their memory, which justifies the use of probabilities to describe the measurement process.<ref name=everett56/>{{rp|69–70}} To address the quantitative problem, Everett proposed a derivation of the Born rule based on the properties that a measure on the branches of the wave function should have.<ref name=everett56/>{{rp|70–72}} His derivation has been criticized as relying on unmotivated assumptions.<ref name="ballentine1973">{{cite journal |author1=Ballentine |first=L. E. |date=1973 |title=Can the statistical postulate of quantum theory be derived?—A critique of the many-universes interpretation |journal=Foundations of Physics |volume=3 |issue=2 |pages=229–240 |bibcode=1973FoPh....3..229B |doi=10.1007/BF00708440 |s2cid=121747282}}</ref> Since then several other derivations of the Born rule in the many-worlds framework have been proposed. There is no consensus on whether this has been successful.<ref>{{cite book |first=N. P. |last=Landsman |chapter-url=http://www.math.ru.nl/~landsman/Born.pdf |quote=The conclusion seems to be that no generally accepted derivation of the Born rule has been given to date, but this does not imply that such a derivation is impossible in principle. |chapter=The Born rule and its interpretation |title=Compendium of Quantum Physics |editor-first=F. |editor-last=Weinert |editor2-first=K. |editor2-last=Hentschel |editor3-first=D. |editor3-last=Greenberger |editor4-first=B. |editor4-last=Falkenburg |publisher=Springer |year=2008 |isbn=978-3-540-70622-9 }}</ref><ref name="kent2009">{{Cite book |last1=Kent |first1=Adrian |title=Many Worlds? Everett, Quantum Theory and Reality |publisher=Oxford University Press |year=2010 |editor=Saunders |editor-first=S. |chapter=One world versus many: The inadequacy of Everettian accounts of evolution, probability, and scientific confirmation |bibcode=2009arXiv0905.0624K |editor2=Barrett |editor-first2=J. |editor3=Kent |editor-first3=A. |editor4=Wallace |editor-first4=D. |arxiv=0905.0624}}</ref><ref>{{cite journal |last1=Kent |first1=Adrian |year=1990 |title=Against Many-Worlds Interpretations |journal=International Journal of Modern Physics A |volume=5 |issue=9 |pages=1745–1762 |arxiv=gr-qc/9703089 |bibcode=1990IJMPA...5.1745K |doi=10.1142/S0217751X90000805 |s2cid=14523184}}</ref>

=== Frequentism ===

[[Bryce DeWitt|DeWitt]] and Graham<ref name="dewitt73"/> and Farhi et al.,<ref>{{cite journal |last1=Farhi |first1=Edward |last2=Goldstone |first2=Jeffrey |last3=Gutmann |first3=Sam |date=1989 |title=How probability arises in quantum mechanics |journal=Annals of Physics |volume=192 |issue=2 |pages=368–382 |bibcode=1989AnPhy.192..368F |doi=10.1016/0003-4916(89)90141-3}}</ref> among others, have proposed derivations of the Born rule based on a [[Frequentist probability|frequentist]] interpretation of probability. They try to show that in the limit of [[Uncountable set|uncountably many]] measurements, no worlds would have relative frequencies that didn't match the probabilities given by the Born rule, but these derivations have been shown to be mathematically incorrect.<ref>{{Cite journal|last=Benioff|first=Paul|author-link=Paul Benioff|date=October 1978|title=A note on the Everett interpretation of quantum mechanics|journal=[[Foundations of Physics]]|language=en|volume=8|issue=9–10|pages=709–720|doi=10.1007/BF00717501|bibcode=1978FoPh....8..709B|s2cid=123279967|issn=0015-9018}}</ref><ref>{{cite journal |last1=Caves |first1=Carlton M. |author-link1=Carlton M. Caves |last2=Schack |first2=Rüdiger |date=2005 |title=Properties of the frequency operator do not imply the quantum probability postulate |journal=[[Annals of Physics]] |volume=315 |issue=1 |pages=123–146 |arxiv=quant-ph/0409144 |bibcode=2005AnPhy.315..123C |doi=10.1016/j.aop.2004.09.009 |s2cid=33263618}}</ref>

=== Decision theory ===
A [[decision theory|decision-theoretic]] derivation of the Born rule was produced by [[David Deutsch]] (1999)<ref>{{Cite journal|arxiv=quant-ph/9906015|last1=Deutsch|first1=David|title=Quantum Theory of Probability and Decisions|journal=Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences | volume=455 | issue=1988 | pages=3129–3137 | year=1999 | doi=10.1098/rspa.1999.0443 | bibcode=1999RSPSA.455.3129D |s2cid=5217034}}</ref> and refined by Wallace<ref name="wallace2003">{{cite journal | last1 = Wallace | first1 = David | year = 2003 | title = Everettian Rationality: defending Deutsch's approach to probability in the Everett interpretation | journal = Stud. Hist. Phil. Mod. Phys. | volume = 34 | issue = 3| pages = 415–438 | bibcode = 2003SHPMP..34..415W | doi = 10.1016/S1355-2198(03)00036-4 | arxiv = quant-ph/0303050 | s2cid = 1921913 }}</ref><ref>{{cite arXiv |eprint=quant-ph/0211104|last1=Wallace|first1=David|title=Quantum Probability and Decision Theory, Revisited|year=2002 }}</ref><ref>{{cite arXiv |eprint=quant-ph/0312157 | last1=Wallace | first1=David | title=Quantum Probability from Subjective Likelihood: Improving on Deutsch's proof of the probability rule|year=2003 }}</ref><ref>{{cite arXiv |eprint=0906.2718|last1=Wallace|first1=David|title=A formal proof of the Born rule from decision-theoretic assumptions|class=quant-ph|year=2009}}</ref> and Saunders.<ref>{{cite journal | last1 = Saunders | first1 = Simon | year = 2004 | title = Derivation of the Born rule from operational assumptions | journal = Proc. R. Soc. Lond. A | volume = 460 | issue = 2046| pages = 1771–1788 | bibcode = 2004RSPSA.460.1771S | doi = 10.1098/rspa.2003.1230 | arxiv = quant-ph/0211138| s2cid = 1459183 }}</ref><ref>{{cite book|arxiv=quant-ph/0412194|last1=Saunders|first1=Simon|title=Quo Vadis Quantum Mechanics?|pages=209–238|year=2004|doi=10.1007/3-540-26669-0_12|chapter=What is Probability?|series=The Frontiers Collection|isbn=978-3-540-22188-3|s2cid=117218061}}</ref> They consider an agent who takes part in a quantum gamble: the agent makes a measurement on a quantum system, branches as a consequence, and each of the agent's future selves receives a reward that depends on the measurement result. The agent uses decision theory to evaluate the price they would pay to take part in such a gamble, and concludes that the price is given by the utility of the rewards weighted according to the Born rule. Some reviews have been positive, although these arguments remain highly controversial; some theoretical physicists have taken them as supporting the case for parallel universes.<ref name="newsci">{{Cite news | last = Merali | first = Zeeya | title = Parallel universes make quantum sense | magazine = New Scientist | issue = 2622 |date=2007-09-21 | url = https://www.newscientist.com/article/mg19526223.700-parallel-universes-make-quantum-sense.html | access-date = 2013-11-22 }} (Summary only).</ref> For example, a ''[[New Scientist]]'' story on a 2007 conference about Everettian interpretations<ref>{{cite web| url = http://www.perimeterinstitute.ca/conferences/many-worlds-50| title = Perimeter Institute, Many worlds at 50 conference, September 21–24, 2007}}{{cite web| url = http://www.perimeterinstitute.ca/video-library/collection/many-worlds-50-2007| title = Videos| access-date = 2020-02-24| archive-date = 2020-02-24| archive-url = https://web.archive.org/web/20200224164736/http://www.perimeterinstitute.ca/video-library/collection/many-worlds-50-2007}}</ref> quoted physicist Andy Albrecht as saying, "This work will go down as one of the most important developments in the history of science."<ref name="newsci"/> In contrast, the philosopher [[Huw Price]], also attending the conference, found the Deutsch–Wallace–Saunders approach fundamentally flawed.<ref>{{Cite book |last1=Price |first1=Huw |title=Many Worlds? Everett, Quantum Theory and Reality |publisher=Oxford University Press |year=2010 |editor=Saunders |editor-first=S. |chapter=Decisions, Decisions, Decisions: Can Savage Salvage Everettian Probability? |editor2=Barrett |editor-first2=J. |editor3=Kent |editor-first3=A. |editor4=Wallace |editor-first4=D. |arxiv=0802.1390}}</ref>

=== Symmetries and invariance ===
In 2005, Zurek<ref name="zurek2005">{{cite journal |last1=Zurek |first1=Wojciech H. |author-link=Wojciech H. Zurek |year=2005 |title=Probabilities from entanglement, Born's rule from envariance |journal=Physical Review A |volume=71 |issue=5 |article-number=052105 |arxiv=quant-ph/0405161 |bibcode=2005PhRvA..71e2105Z |doi=10.1103/physreva.71.052105 |s2cid=18210481}}</ref> produced a derivation of the Born rule based on the symmetries of entangled states; Schlosshauer and Fine argue that Zurek's derivation is not rigorous, as it does not define what probability is and has several unstated assumptions about how it should behave.<ref>{{cite journal |last1=Schlosshauer |first1=M. |last2=Fine |first2=A. |year=2005 |title=On Zurek's derivation of the Born rule |journal=Foundations of Physics |volume=35 |issue=2 |pages=197–213 |arxiv=quant-ph/0312058 |bibcode=2005FoPh...35..197S |doi=10.1007/s10701-004-1941-6 |s2cid=119100306}}</ref>

In 2016, Charles Sebens and [[Sean M. Carroll]], building on work by [[Lev Vaidman]],<ref>Vaidman, L. "Probability in the Many-Worlds Interpretation of Quantum Mechanics". In: Ben-Menahem, Y., & Hemmo, M. (eds), The Probable and the Improbable: Understanding Probability in Physics, Essays in Memory of Itamar Pitowsky. Springer.</ref> proposed a similar approach based on self-locating uncertainty.<ref>{{cite journal |last1=Sebens |first1=Charles T. |last2=Carroll |first2=Sean M. |year=2016 |title=Self-Locating Uncertainty and the Origin of Probability in Everettian Quantum Mechanics |journal=The British Journal for the Philosophy of Science |volume=69 |issue=1 |pages=25–74 |arxiv=1405.7577 |doi=10.1093/bjps/axw004 |s2cid=53648469}}</ref> In this approach, decoherence creates multiple identical copies of observers, who can assign credences to being on different branches using the Born rule. The Sebens–Carroll approach has been criticized by [[Adrian Kent]],<ref>{{Cite journal|last=Kent|first=Adrian|author-link=Adrian Kent|date=February 2015|title=Does it Make Sense to Speak of Self-Locating Uncertainty in the Universal Wave Function? Remarks on Sebens and Carroll|journal=Foundations of Physics|language=en|volume=45|issue=2|pages=211–217 |arxiv=1408.1944|doi=10.1007/s10701-014-9862-5|issn=0015-9018|bibcode=2015FoPh...45..211K|s2cid=118471198}}</ref> and Vaidman does not find it satisfactory.<ref>{{Cite book|last1=Vaidman|first1=Lev|chapter=Derivations of the Born Rule|title=Quantum, Probability, Logic: Itamar Pitowsky's Work and Influence |editor=Meir Hemmo |editor2=Orly Shenker |publisher=Springer Nature Switzerland |year=2020|id=PhilSci: [http://philsci-archive.pitt.edu/15943 15943]}}</ref>

=== Branch counting ===
In 2021, [[Simon Saunders]] produced a branch counting derivation of the Born rule. The crucial feature of this approach is to define the branches so that they all have the same magnitude or [[norm (mathematics)|2-norm]]. The ratios of the numbers of branches thus defined give the probabilities of the various outcomes of a measurement, in accordance with the Born rule.<ref name=BranchCounting>{{Cite journal|last=Saunders|first=Simon|author-link=Simon Saunders|date=24 November 2021|title=Branch-counting in the Everett interpretation of quantum mechanics.|journal=Proceedings of the Royal Society A|language=en|volume=477|issue=2255|pages=1–22 |article-number=20210600 |doi=10.1098/rspa.2021.0600|issn=|arxiv=2201.06087|bibcode=2021RSPSA.47710600S |s2cid=244491576 }}</ref>

== Preferred basis problem ==

As originally formulated by Everett and DeWitt, the many-worlds interpretation had a privileged role for measurements: they determined which [[Basis (linear algebra)|basis]] of a quantum system would give rise to the eponymous worlds. Without this the theory was ambiguous, as a quantum state can equally well be described (e.g.) as having a well-defined position or as being a superposition of two delocalized states. The assumption is that the preferred basis to use is the one which assigns a unique measurement outcome to each world. This special role for measurements is problematic for the theory, as it contradicts Everett and DeWitt's goal of having a reductionist theory and undermines their criticism of the ill-defined measurement postulate of the Copenhagen interpretation.<ref name="skyrms1976">{{cite journal |author1=Skyrms |first=Brian |author-link1=Brian Skyrms |date=1976 |title=Possible Worlds, Physics and Metaphysics |journal=Philosophical Studies |volume=30 |issue=5 |pages=323–332 |doi=10.1007/BF00357930 |s2cid=170852547}}</ref><ref name=ballentine1973/> This is known today as the ''preferred basis problem''.

The preferred basis problem has been solved, according to Saunders and Wallace, among others,<ref name="saunders2010">{{Cite book |last1=Saunders |first1=Simon |url=http://users.ox.ac.uk/~lina0174/introduction.pdf |title=Many Worlds? Everett, Quantum Theory and Reality |publisher=Oxford University Press |year=2010 |editor=Saunders |editor-first=S. |chapter=Many Worlds? An Introduction |author-link=Simon Saunders |editor2=Barrett |editor-first2=J. |editor3=Kent |editor-first3=A. |editor4=Wallace |editor-first4=D.}}</ref> by incorporating decoherence into the many-worlds theory.<ref name="gell-man1990">{{Cite book |last1=Gell-Mann |first1=M. |title=Complexity, Entropy, and the Physics of Information |last2=Hartle |first2=J. B. |publisher=Addison-Wesley |year=1990 |editor=Zurek |editor-first=W. H. |chapter=Quantum mechanics in the light of quantum cosmology |arxiv=1803.04605}}</ref><ref>{{cite journal |author1=Saunders |first=Simon |date=1993 |title=Decoherence, relative states, and evolutionary adaptation |journal=Foundations of Physics |volume=23 |issue=12 |pages=1553–1585 |bibcode=1993FoPh...23.1553S |doi=10.1007/BF00732365 |s2cid=119754481}}</ref><ref>{{cite journal |author1=Saunders |first=Simon |date=1995 |title=Time, quantum mechanics, and decoherence |url=http://philsci-archive.pitt.edu/463/1/part1.pdf |journal=Synthese |volume=102 |issue=2 |pages=235–266 |doi=10.1007/BF01089802 |s2cid=14550985}}</ref><ref>{{cite journal |author1=Hartle |first=James B. |date=2011 |title=The quasiclassical realms of this quantum universe |journal=Foundations of Physics |volume=41 |issue=6 |pages=982–1006 |arxiv=0806.3776 |bibcode=2011FoPh...41..982H |doi=10.1007/s10701-010-9460-0 |s2cid=118469123}}</ref> In this approach, the preferred basis does not have to be postulated, but rather is identified as the basis stable under environmental decoherence. In this way measurements no longer play a special role; rather, any interaction that causes decoherence causes the world to split. Since decoherence is never complete, there will always remain some infinitesimal overlap between two worlds, making it arbitrary whether a pair of worlds has split or not.<ref name = stappbasis>{{cite journal | last1 = Stapp | first1 = Henry | year = 2002 | title = The basis problem in many-world theories | url = http://www-physics.lbl.gov/~stapp/bp.PDF | journal = Canadian Journal of Physics | volume = 80 | issue = 9 | pages = 1043–1052 | doi=10.1139/p02-068|arxiv = quant-ph/0110148 |bibcode = 2002CaJPh..80.1043S | s2cid = 18634782 }}</ref> Wallace argues that this is not problematic: it only shows that worlds are not a part of the fundamental ontology, but rather of the ''emergent'' ontology, where these approximate, effective descriptions are routine in the physical sciences.<ref>{{cite journal |author1=Wallace |first=David |date=2003 |title=Everett and structure |journal=Studies in History and Philosophy of Science |volume=34 |issue=1 |pages=87–105 |arxiv=quant-ph/0107144 |bibcode=2003SHPMP..34...87W |doi=10.1016/S1355-2198(02)00085-0 |s2cid=15222560}}</ref><ref name="wallace2010">{{Cite book |author1=Wallace |first=David |title=Many Worlds? Everett, Quantum Theory and Reality |publisher=Oxford University Press |year=2010 |editor=Saunders |editor-first=S. |chapter=Decoherence and Ontology, or: How I Learned To Stop Worrying And Love FAPP |editor2=Barrett |editor-first2=J. |editor3=Kent |editor-first3=A. |editor4=Wallace |editor-first4=D. |arxiv=1111.2189}}</ref> Since in this approach the worlds are derived, it follows that they must be present in any other interpretation of quantum mechanics that does not have a collapse mechanism, such as Bohmian mechanics.<ref name=BrownWallace>{{cite journal | last1 = Brown | first1 = Harvey R | author-link = Harvey Brown (philosopher) | last2 = Wallace | first2 = David | year = 2005 | title = Solving the measurement problem: de Broglie–Bohm loses out to Everett | url = http://philsci-archive.pitt.edu/archive/00001659/01/Cushing.pdf | journal = Foundations of Physics | volume = 35 | issue = 4 | pages = 517–540 | doi=10.1007/s10701-004-2009-3|arxiv = quant-ph/0403094 |bibcode = 2005FoPh...35..517B | s2cid = 412240 }}</ref>

This approach to deriving the preferred basis has been criticized as creating circularity with derivations of probability in the many-worlds interpretation, as decoherence theory depends on probability and probability depends on the ontology derived from decoherence.<ref name=kent2009/><ref name=zurek2005/><ref name="baker">{{cite journal |author=Baker |first=David J. |year=2007 |title=Measurement outcomes and probability in Everettian quantum mechanics |url=http://philsci-archive.pitt.edu/2717/1/OutcomeProbEverett.pdf |journal=Studies in History and Philosophy of Science |volume=38 |issue=1 |pages=153–169 |bibcode=2007SHPMP..38..153B |doi=10.1016/j.shpsb.2006.05.003}}</ref> Wallace contends that decoherence theory depends not on probability but only on the notion that one is allowed to do approximations in physics.<ref name=wallace2012/>{{rp|253–254}}

== History ==
MWI originated in Everett's [[Princeton University]] [[PhD]] thesis "The Theory of the [[Universal wavefunction|Universal Wave Function]]",<ref name=dewitt73/> developed under his thesis advisor [[John Archibald Wheeler]], a shorter summary of which was published in 1957 under the title "Relative State Formulation of Quantum Mechanics" (Wheeler contributed the title "relative state";<ref name=geons-wheeler>{{cite book|first=John Archibald |last=Wheeler |author-link=John Archibald Wheeler |title=Geons, Black Holes and Quantum Foam |isbn=0-393-31991-1 |year=2000 |publisher=W. W. Norton & Company |pages=268–270 }}</ref> Everett originally called his approach the "Correlation Interpretation", where "correlation" refers to quantum entanglement). The phrase "many-worlds" is due to Bryce DeWitt,<ref name=dewitt73/> who was responsible for the wider popularization of Everett's theory, which had been largely ignored for a decade after publication in 1957.<ref name=Heresy/>

Everett's proposal was not without precedent. In 1952, [[Erwin Schrödinger]] gave a lecture in [[Dublin]] in which at one point he jocularly warned his audience that what he was about to say might "seem lunatic". He went on to assert that while the [[Schrödinger equation]] seemed to be describing several different histories, they were "not alternatives but all really happen simultaneously". According to David Deutsch, this is the earliest known reference to many-worlds; [[Jeffrey A. Barrett]] describes it as indicating the similarity of "general views" between Everett and Schrödinger.<ref>{{cite book | last1=Deutsch |first1=David | author-link=David Deutsch |chapter=Apart from Universes|title=Many Worlds? Everett, Quantum Theory and Reality |editor=S. Saunders |editor-link1=Simon Saunders |editor2=J. Barrett |editor-link2=Jeffrey A. Barrett |editor3=A. Kent |editor-link3=Adrian Kent |editor4=D. Wallace |publisher=Oxford University Press|year=2010}}</ref><ref>{{cite book |last1=Schrödinger |first1=Erwin |editor1-last=Bitbol |editor1-first=Michel |editor-link=Michel Bitbol |title=The Interpretation of Quantum Mechanics: Dublin Seminars (1949–1955) and other unpublished essays |date=1996 |publisher=OxBow Press}}</ref><ref>{{cite book |last=Barrett |first=Jeffrey A. |author-link=Jeffrey A. Barrett |title=The Quantum Mechanics of Minds and Worlds |publisher=Oxford University Press |year=1999 |isbn=978-0-19-158325-4 |pages=62–63 }}</ref> Schrödinger's writings from the period also contain elements resembling the [[modal interpretation]] originated by [[Bas van Fraassen]]. Because Schrödinger subscribed to a kind of post-[[Ernst Mach|Machian]] [[neutral monism]], in which "matter" and "mind" are only different aspects or arrangements of the same common elements, treating the wave function as physical and treating it as information became interchangeable.<ref>{{cite book |last=Bitbol |first=Michel |title=Schrödinger's Philosophy of Quantum Mechanics |date=1996 |publisher=Springer Netherlands |isbn=978-94-009-1772-9 |location=Dordrecht |oclc=851376153 }}</ref>

[[Leon Cooper]] and Deborah Van Vechten developed a very similar approach before reading Everett's work.<ref>{{Cite journal |last1=Cooper |first1=Leon N. |last2=Van Vechten |first2=Deborah |date=1969-12-01 |title=On the Interpretation of Measurement within the Quantum Theory |url=https://pubs.aip.org/ajp/article/37/12/1212/1048185/On-the-Interpretation-of-Measurement-within-the |journal=American Journal of Physics |language=en |volume=37 |issue=12 |pages=1212–1220 |doi=10.1119/1.1975279 |bibcode=1969AmJPh..37.1212C |issn=0002-9505|url-access=subscription }}</ref> Zeh also came to the same conclusions as Everett before reading his work, then built a new theory of [[quantum decoherence]] based on these ideas.<ref>{{Cite journal |last=Camilleri |first=Kristian |date=2009 |title=A history of entanglement: Decoherence and the interpretation problem |url=https://linkinghub.elsevier.com/retrieve/pii/S1355219809000562 |journal=Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics |language=en |volume=40 |issue=4 |pages=290–302 |doi=10.1016/j.shpsb.2009.09.003 |bibcode=2009SHPMP..40..290C |url-access=subscription }}</ref>

According to people who knew him, Everett believed in the literal reality of the other quantum worlds.<ref name="MAD">{{cite book |author1=Byrne |first=Peter |title=The Many Worlds of Hugh Everett III: Multiple Universes, Mutual Assured Destruction, and the Meltdown of a Nuclear Family |date=2010 |publisher=Oxford University Press |isbn=978-0-19-955227-6}}</ref> [[Mark Oliver Everett|His son]] and wife reported that he "never wavered in his belief over his many-worlds theory".<ref name="newsciMark">{{Cite news | last = Aldhous | first = Peter | title = Parallel lives can never touch | magazine = New Scientist | issue = 2631 |date=2007-11-24 | url = https://www.newscientist.com/article/mg19626311.800-interview-parallel-lives-can-never-touch.html | access-date = 2007-11-21 }}</ref> In their detailed review of Everett's work, Osnaghi, Freitas, and Freire Jr. note that Everett consistently used quotes around "real" to indicate a meaning within scientific practice.<ref name="Heresy">{{cite journal | last1 = Osnaghi | first1 = Stefano | last2 = Freitas | first2 = Fabio | last3 = Olival Freire | first3 = Jr | author-link3 = Olival Freire, Jr. | year = 2009 | title = The Origin of the Everettian Heresy | journal = Studies in History and Philosophy of Modern Physics | volume = 40 | issue = 2| pages = 97–123 | doi=10.1016/j.shpsb.2008.10.002| bibcode = 2009SHPMP..40...97O | citeseerx = 10.1.1.397.3933 }}</ref>{{rp|107}}

== Reception ==

MWI's initial reception was overwhelmingly negative, in the sense that it was ignored, with the notable exception of DeWitt. Wheeler made considerable efforts to formulate the theory in a way that would be palatable to Bohr, visited Copenhagen in 1956 to discuss it with him, and convinced Everett to visit as well, which happened in 1959. Nevertheless, Bohr and his collaborators completely rejected the theory.{{efn|Everett recounted his meeting with Bohr as "that was a hell&nbsp;... doomed from the beginning". [[Léon Rosenfeld]], a close collaborator of Bohr, said "With regard to Everett neither I nor even Niels Bohr could have any patience with him, when he visited us in Copenhagen more than 12 years ago in order to sell the hopelessly wrong ideas he had been encouraged, most unwisely, by Wheeler to develop. He was undescribably[sic] stupid and could not understand the simplest things in quantum mechanics."<ref name=Heresy/>{{rp|113}}}} Everett had already left academia in 1957, never to return, and in 1980, Wheeler disavowed the theory.<ref name="SchrodingersCatWheeler">[[John Gribbin]], ''[[In Search of Schrödinger's Cat]]'', {{ISBN|978-0552125550}}, p. 246.</ref>

=== Support ===

One of the strongest longtime advocates of MWI is David Deutsch.<ref name="deutsch98">[[David Deutsch]], ''The Fabric of Reality: The Science of Parallel Universes And Its Implications'', Penguin Books (1998), {{ISBN|0-14-027541-X}}.</ref> According to him, the single photon interference pattern observed in the [[double slit experiment]] can be explained by interference of photons in multiple universes. Viewed this way, the single photon interference experiment is indistinguishable from the multiple photon interference experiment. In a more practical vein, in one of the earliest papers on quantum computing,<ref name="deutsch85">{{cite journal | last1 = Deutsch | first1 = David | author-link = David Deutsch | year = 1985 | title = Quantum theory, the Church–Turing principle and the universal quantum computer | journal = Proceedings of the Royal Society of London A | volume = 400 | issue = 1818| pages = 97–117 | doi=10.1098/rspa.1985.0070|bibcode = 1985RSPSA.400...97D | citeseerx = 10.1.1.144.7936 | s2cid = 1438116 }}</ref> Deutsch suggested that parallelism that results from MWI could lead to "a method by which certain probabilistic tasks can be performed faster by a universal quantum computer than by any classical restriction of it". He also proposed that MWI will be testable (at least against "naive" Copenhagenism) when [[Reversible computing|reversible computers]] become conscious via the reversible observation of spin.<ref name="davis86">[[Paul C.W. Davies]], J. R. Brown, ''The Ghost in the Atom'' (1986) {{ISBN|0-521-31316-3}}, pp. 34–38: "The Many-Universes Interpretation", pp. 83–105 for [[David Deutsch]]'s test of MWI and reversible quantum memories.</ref>

=== Equivocal ===
Philosophers of science James Ladyman and Don Ross say that MWI could be true, but do not embrace it. They note that no quantum theory is yet empirically adequate for describing all of reality, given its lack of unification with [[general relativity]], and so do not see a reason to regard any interpretation of quantum mechanics as the final word in [[metaphysics]]. They also suggest that the multiple branches may be an artifact of incomplete descriptions and of using quantum mechanics to represent the states of macroscopic objects. They argue that macroscopic objects are significantly different from microscopic objects in not being isolated from the environment, and that using quantum formalism to describe them lacks explanatory and descriptive power and accuracy.<ref name="ladymanross">{{cite book|last1=Ladyman|first1=James|url=https://books.google.com/books?id=YC91DgAAQBAJ|title=Every Thing Must Go: Metaphysics Naturalized|last2=Ross|first2=Don|date=2007|publisher=Clarendon Press|isbn=978-0-19-927619-6|pages=179–183|language=en}}</ref>

=== Rejection ===

Some scientists consider some aspects of MWI to be [[unfalsifiable]] and hence unscientific because the multiple parallel universes are non-communicating, in the sense that no information can be passed between them.<ref name=Bunge>{{cite book |author1=Bunge, M. |year=2012 |title=Evaluating Philosophies |chapter=Parallel Universes? Digital Physics? |series=Boston Studies in the Philosophy and History of Science |volume=295 |pages=152–153 |publisher=Springer |location=New York |doi=10.1007/978-94-007-4408-0|isbn=978-94-007-4407-3 }}</ref><ref name=Ellis>{{cite journal |author1=Ellis, G. |author2=Silk, J. |year=2014 |title=Scientific method: Defend the integrity of physics |journal=Nature |volume=516 |issue=7531 |pages= 321–323 |doi=10.1038/516321a |doi-access=free |bibcode=2014Natur.516..321E |pmid=25519115 }}</ref>

[[Victor J. Stenger]] remarked that [[Murray Gell-Mann]]'s published work explicitly rejects the existence of simultaneous parallel universes.<ref name="stenger1995">{{cite book |last=Stenger |first=V. J. |title=The Unconscious Quantum: Metaphysics in Modern Physics and Cosmology |publisher=Prometheus Books |year=1995 |isbn=978-1-57392-022-3 |lccn=lc95032599}}</ref> Collaborating with [[James Hartle]], Gell-Mann worked toward the development of a more "palatable" ''post-Everett quantum mechanics''. Stenger thought it fair to say that most physicists find MWI too extreme, though it "has merit in finding a place for the observer inside the system being analyzed and doing away with the troublesome notion of wave function collapse".{{efn|"Gell-Mann and Hartle, along with a score of others, have been working to develop a more palatable interpretation of quantum mechanics that is free of the problems that plague all the interpretations we have considered so far. This new interpretation is called, in its various incarnations, '''post-Everett quantum mechanics''', alternate histories, consistent histories, or decoherent histories. I will not be overly concerned with the detailed differences between these characterizations and will use the terms more or less interchangeably."<ref name="stenger1995"/>{{rp|176}}}}

[[Roger Penrose]] argues that the idea is flawed because it is based on an oversimplified version of quantum mechanics that does not account for gravity. In his view, applying conventional quantum mechanics to the universe implies the MWI, but the lack of a successful theory of [[quantum gravity]] negates the claimed universality of conventional quantum mechanics.<ref name="penrose">{{cite web|last=Penrose |first=Roger |author-link=Roger Penrose |title=Roger Penrose Looks Beyond the Classic-Quantum Dichotomy |publisher=Sciencewatch |date=August 1991 |url=http://www.sciencewatch.com/interviews/roger_penrose2.htm |access-date=2007-10-21 |archive-url=https://web.archive.org/web/20071023063403/http://www.sciencewatch.com/interviews/roger_penrose2.htm |archive-date=2007-10-23 }}</ref> According to Penrose, "the rules must change when gravity is involved". He further asserts that gravity helps anchor reality and "blurry" events have only one allowable outcome: "electrons, atoms, molecules, etc., are so minute that they require almost no amount of energy to maintain their gravity, and therefore their overlapping states. They can stay in that state forever, as described in standard quantum theory". On the other hand, "in the case of large objects, the duplicate states disappear in an instant due to the fact that these objects create a large gravitational field".<ref name="ball">{{Cite web|first=Philip |last=Ball |author-link=Philip Ball |date=2015-02-17 |access-date=2021-09-23 |title=Too many worlds|url=https://aeon.co/essays/is-the-many-worlds-hypothesis-just-a-fantasy|website=[[Aeon.co]]}}</ref><ref>{{Cite web|title=If an Electron Can Be in Two Places at Once, Why Can't You?|url=https://www.discovermagazine.com/the-sciences/if-an-electron-can-be-in-two-places-at-once-why-cant-you|website=[[Discover Magazine]]}}</ref>

Philosopher of science [[Robert P. Crease]] says that MWI is "one of the most implausible and unrealistic ideas in the history of science" because it means that everything conceivable happens.<ref name="ball"/> Science writer [[Philip Ball]] calls MWI's implications fantasies, since "beneath their apparel of scientific equations or symbolic logic, they are acts of imagination, of 'just supposing{{'"}}.<ref name="ball"/>

Theoretical physicist [[Gerard 't Hooft]] also dismisses the idea: "I do not believe that we have to live with the many-worlds interpretation. Indeed, it would be a stupendous number of parallel worlds, which are only there because physicists couldn't decide which of them is real."<ref>{{Cite news|last=Melinda|first=Baldwin|date=2017-07-11|title=Q&A: Gerard 't Hooft on the future of quantum mechanics|journal=Physics Today|issue=7 |article-number=5262 |url=https://physicstoday.scitation.org/do/10.1063/PT.6.4.20170711a/abs/|language=EN|doi=10.1063/PT.6.4.20170711a}}</ref>

[[Asher Peres]] was an outspoken critic of MWI. A section of his 1993 textbook had the title ''Everett's interpretation and other bizarre theories''. Peres argued that the various many-worlds interpretations merely shift the arbitrariness or vagueness of the collapse postulate to the question of when "worlds" can be regarded as separate, and that no objective criterion for that separation can actually be formulated.<ref>{{Cite book|last=Peres|first=Asher|title=[[Quantum Theory: Concepts and Methods]]|publisher=Kluwer Academic Publishers|year=1995|isbn=0-7923-2549-4|page=374|author-link=Asher Peres}}</ref>

=== Polls ===
A poll of 72 "leading quantum [[cosmologist]]s and other quantum field theorists" conducted before 1991 by L. David Raub showed 58% agreement with "Yes, I think MWI is true".<ref name=":1">{{cite book |pages=170–171 |title=The Physics of Immortality:Modern Cosmology, God and the Resurrection of the Dead |first= Frank |last= Tipler| author-link=Frank J. Tipler |date=1994|quote=In the "yes" column were Stephen Hawking, Richard Feynman, and Murray Gell-Mann}}</ref>

[[Max Tegmark]] reports the result of a "highly unscientific" poll taken at a 1997 quantum mechanics workshop. According to Tegmark, "The many worlds interpretation (MWI) scored second, comfortably ahead of the [[consistent histories]] and [[Bohm interpretation]]s."<ref>{{cite web | url = http://space.mit.edu/home/tegmark/quantum.html | title = Max Tegmark on many-worlds (contains MWI poll) }}</ref>

In response to [[Sean M. Carroll]]'s statement "As crazy as it sounds, most working physicists buy into the many-worlds theory",<ref name=carrollcrazy>{{cite web |url=http://preposterousuniverse.blogspot.com/2004_04_01_preposterousuniverse_archive.html#108087902367974365 |archive-url=https://web.archive.org/web/20040908014703/http://preposterousuniverse.blogspot.com/2004_04_01_preposterousuniverse_archive.html#108087902367974365 |first=Sean |last=Caroll |title=Preposterous Universe |date=1 April 2004 |archive-date=8 September 2004 }}</ref> [[Michael Nielsen]] counters: "at a quantum computing conference at Cambridge in 1998, a many-worlder surveyed the audience of approximately 200 people&nbsp;... Many-worlds did just fine, garnering support on a level comparable to, but somewhat below, Copenhagen and decoherence." But Nielsen notes that it seemed most attendees found it to be a waste of time: Peres "got a huge and sustained round of applause…when he got up at the end of the polling and asked 'And who here believes the laws of physics are decided by a democratic vote?{{'"}}<ref>{{cite web |url=http://www.qinfo.org/people/nielsen/blog/archive/000060.html |archive-url=https://web.archive.org/web/20040520222807/http://www.qinfo.org/people/nielsen/blog/archive/000060.html |first=Michael |last=Nielsen |title=Michael Nielsen: The Interpretation of Quantum Mechanics | archive-date=20 May 2004 |date=3 April 2004 }}</ref>

A 2005 poll of fewer than 40 students and researchers taken after a course on the Interpretation of Quantum Mechanics at the Institute for Quantum Computing University of Waterloo found "Many Worlds (and decoherence)" to be the least favored.<ref>[http://www.iqc.ca/~qipcourse/interpret/survey.html Survey Results] {{webarchive |url=https://web.archive.org/web/20101104152717/http://www.iqc.ca/~qipcourse/interpret/survey.html |date=2010-11-04 }}</ref>

A 2011 poll of 33 participants at an Austrian conference on [[quantum foundations]] found 6 endorsed MWI, 8 "Information-based/information-theoretical", and 14 Copenhagen;<ref name=poll2011>{{cite journal |arxiv=1301.1069 |last1=Schlosshauer |first1=Maximilian |title=A Snapshot of Foundational Attitudes Toward Quantum Mechanics |journal=Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics |volume=44 |issue=3 |pages=222–230 |last2=Kofler |first2=Johannes |last3=Zeilinger |first3=Anton |year=2013 |doi=10.1016/j.shpsb.2013.04.004 |bibcode=2013SHPMP..44..222S |s2cid=55537196 }}</ref> the authors remark that MWI received a similar percentage of votes as in Tegmark's 1997 poll.<ref name=poll2011/>

== Speculative implications ==
{{see also|Parallel universes in fiction}}

DeWitt has said that Everett, Wheeler, and Graham "do not in the end exclude any element of the superposition. All the worlds are there, even those in which everything goes wrong and all the statistical laws break down."<ref name="dewitt71" /> Tegmark affirmed that absurd or highly unlikely events are rare but inevitable under MWI: "Things inconsistent with the laws of physics will never happen—everything else will&nbsp;... it's important to keep track of the statistics, since even if everything conceivable happens somewhere, really freak events happen only exponentially rarely."<ref name="TegmarkEverything">{{Cite web |last=Tegmark |first=Max |title=The Universes of Max Tegmark |url=https://space.mit.edu/home/tegmark/crazy.html |access-date=2023-08-26 |website=space.mit.edu}}</ref> [[David Deutsch]] speculates in his book ''[[The Beginning of Infinity]]'' that some fiction, such as [[alternate history]], could occur somewhere in the [[multiverse]], as long as it is consistent with the laws of physics.<ref name="beginning">David Deutsch. ''Beginning of Infinity'', Penguin Books (2011), {{ISBN|978-0-7139-9274-8}}, p. 294.</ref><ref name="GribbinSixImpossibleThings">[[John Gribbin]], ''Six Impossible Things'', Icon Books Limited (2021), {{ISBN|978-1-7857-8734-8}}.</ref>

According to Ladyman and Ross, many seemingly physically plausible but unrealized possibilities, such as those discussed in other scientific fields, generally have no counterparts in other branches, because they are in fact incompatible with the universal wave function.<ref name=ladymanross/> According to Carroll, human decision-making, contrary to common misconceptions, is best thought of as a classical process, not a quantum one, because it works on the level of neurochemistry rather than fundamental particles. Human decisions do not cause the world to branch into equally realized outcomes; even for subjectively difficult decisions, the "weight" of realized outcomes is almost entirely concentrated in a single branch.<ref name=deeplyhidden/>{{rp|214-216}}

''[[Quantum suicide]]'' is a [[thought experiment]] in [[quantum mechanics]] and the [[philosophy of physics]] that can purportedly distinguish between the [[Copenhagen interpretation]] of quantum mechanics and the many-worlds interpretation by a variation of the [[Schrödinger's cat]] [[thought experiment]], from the cat's point of view. ''Quantum immortality'' refers to the subjective experience of surviving quantum suicide.<ref name="mit_edu">{{cite web |url=http://space.mit.edu/home/tegmark/quantum.html#immortality |title=Quantum immortality |last=Tegmark |first=Max |date=November 1998 |access-date=25 October 2010 }}</ref> Most experts believe the experiment would not work in the real world, because the world with the surviving experimenter has a lower "measure" than the world before the experiment, making it less likely that the experimenter will experience their survival.<ref name=wallace2012>{{cite book |last1=Wallace |first1=David |title=The Emergent Multiverse: Quantum Theory According to the Everett Interpretation |date=2012 |publisher=Oxford University Press |isbn=978-0-19-954696-1 }}</ref>{{rp|371}}<ref name=vaidman_stanfordencyclopedia/><ref name=deeplyhidden>{{cite book |last=Carroll |first=Sean |url=https://books.google.com/books?id=f16IDwAAQBAJ |title=Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime |date=2019-09-10 |publisher=Penguin |isbn=978-1-5247-4302-4 |language=en }}</ref><ref>{{cite book |last=Deutsch |first=David |year=2011 |chapter=The Beginning |title=The Beginning of Infinity |publisher=Penguin Group }}</ref>

== See also ==
{{cols|colwidth=35em}}
* [[Alternate history]]
* [[Consistent histories]]
* [[Many-minds interpretation]]
* "[[The Garden of Forking Paths]]"
* [[Parallel universes in fiction]]
* ''[[The Beginning of Infinity]]''
* [[Mathematical universe hypothesis]]
* [[Multiverse]]
{{colend}}

== Notes ==
{{notelist|30em}}

== References ==
{{reflist}}

== Further reading ==
* Peter Byrne, ''The Many Worlds of Hugh Everett III: Multiple Universes, Mutual Assured Destruction, and the Meltdown of a Nuclear Family'', Oxford University Press, 2010.
* Jeffrey A. Barrett and Peter Byrne, eds., "The Everett Interpretation of Quantum Mechanics: Collected Works 1955–1980 with Commentary", Princeton University Press, 2012.
* Julian Brown, ''Minds, Machines, and the Multiverse'', Simon & Schuster, 2000, {{ISBN|0-684-81481-1}}
* [[Sean M. Carroll]], ''Something deeply hidden'', [[Penguin Random House]], (2019)
* [[Paul C.W. Davies]], ''Other Worlds'', (1980) {{ISBN|0-460-04400-1}}
* {{cite journal | last1 = Osnaghi | first1 = Stefano | last2 = Freitas | first2 = Fabio | last3 = Olival Freire | first3 = Jr | year = 2009 | title = The Origin of the Everettian Heresy | url = http://stefano.osnaghi.free.fr/Everett.pdf | journal = Studies in History and Philosophy of Modern Physics | volume = 40 | issue = 2 | pages = 97–123 | doi = 10.1016/j.shpsb.2008.10.002 | bibcode = 2009SHPMP..40...97O | citeseerx = 10.1.1.397.3933 | access-date = 2009-08-07 | archive-url = https://web.archive.org/web/20160528110359/http://stefano.osnaghi.free.fr/Everett.pdf | archive-date = 2016-05-28 }} A study of the painful three-way relationship between [[Hugh Everett]], [[John A Wheeler]] and [[Niels Bohr]] and how this affected the early development of the many-worlds theory.
* {{cite book |editor-last=Vaidman |editor-first=Lev |editor-link=Lev Vaidman |title=The Many-Worlds Interpretation of Quantum Mechanics |year=2024 |url=https://www.mdpi.com/books/reprint/9238-the-many-worlds-interpretation-of-quantum-mechanics |publisher=[[MDPI]] |isbn=978-3-7258-1070-3}}
* David Wallace, Worlds in the Everett Interpretation, ''Studies in History and Philosophy of Modern Physics'', 33, (2002), pp.&nbsp;637–661, {{arxiv|quant-ph/0103092}}
* [[John A. Wheeler]] and [[Wojciech Hubert Zurek]] (eds), ''Quantum Theory and Measurement'', [[Princeton University Press]], (1983), {{ISBN|0-691-08316-9}}

== External links ==
{{Sister project links|auto=1|wikt=y|b=y|v=y}}
* {{IEP|url-id=everett|title=Everettian Interpretations of Quantum Mechanics}}
* [https://plato.stanford.edu/entries/qm-everett/ Everett's Relative-State Formulation of Quantum Mechanics] – [[Jeffrey A. Barrett]]'s article on Everett's formulation of quantum mechanics in the [[Stanford Encyclopedia of Philosophy]].
* [https://plato.stanford.edu/entries/qm-manyworlds/ Many-Worlds Interpretation of Quantum Mechanics] – [[Lev Vaidman]]'s article on the many-worlds interpretation of quantum mechanics in the [[Stanford Encyclopedia of Philosophy]].
* [http://ucispace.lib.uci.edu/handle/10575/1060 Hugh Everett III Manuscript Archive (UC Irvine)] – [[Jeffrey A. Barrett]], Peter Byrne, and James O. Weatherall (eds.).
* [http://www-physics.lbl.gov/~stapp/bp.PDF Henry Stapp's critique of MWI, focusing on the basis problem] Canadian Journal of Physics 80, 1043–1052 (2002).
* [https://www.scientificamerican.com/article/hugh-everett-biography/ Scientific American report on Many Worlds and Hugh Everett.]

{{Quantum mechanics topics|state=expanded}}
{{Time travel}}

{{DEFAULTSORT:Many-Worlds Interpretation}}
[[Category:Interpretations of quantum mechanics]]
[[Category:Quantum measurement]]
[[Category:Multiverse]]
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