View source for Interpretations of quantum mechanics

{{short description|Area of physical and philosophical debate}}
{{Quantum mechanics|cTopic=Interpretations}}
An '''interpretation of quantum mechanics''' is an attempt to explain how the mathematical theory of [[quantum mechanics]] might correspond to experienced [[reality]]. Quantum mechanics has held up to rigorous and extremely precise tests in an extraordinarily broad range of experiments. However, there exist a number of contending schools of thought over their interpretation. These views on interpretation differ on such fundamental questions as whether quantum mechanics is [[determinism|deterministic]] or [[stochastic]], [[Principle of locality|local]] or [[Quantum nonlocality|non-local]], which elements of quantum mechanics can be considered real, and what the nature of [[measurement in quantum mechanics|measurement]] is, among other matters.

While some variation of the [[Copenhagen interpretation]] is commonly presented in textbooks, many other interpretations have been developed.
Despite a century of debate and experiment, no consensus has been reached among physicists and [[Philosophy of physics|philosophers of physics]] concerning which interpretation best "represents" reality.<ref>
[[Murray Gell-Mann]] – Quantum Mechanics Interpretations – Feynman Sum over Histories – EPR Bertlemann's https://www.youtube.com/watch?v=f-OFP5tNtMY Richard P Feynman: Quantum Mechanical View of Reality 1 (Part 1) https://www.youtube.com/watch?v=72us6pnbEvE
</ref><ref>{{Cite journal|last1=Schlosshauer|first1=Maximilian|last2=Kofler|first2=Johannes|last3=Zeilinger|first3=Anton|author-link3=Anton Zeilinger|date=2013-08-01|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|doi=10.1016/j.shpsb.2013.04.004|issn=1355-2198|arxiv=1301.1069|bibcode=2013SHPMP..44..222S|s2cid=55537196}}</ref><ref>{{Cite journal |last=Gibney |first=Elizabeth |date=2025-07-30 |title=Physicists disagree wildly on what quantum mechanics says about reality, Nature survey shows |url=https://www.nature.com/articles/d41586-025-02342-y |journal=Nature |language=en |volume=643 |issue=8074 |pages=1175–1179 |doi=10.1038/d41586-025-02342-y |pmid=40739026 |bibcode=2025Natur.643.1175G |issn=1476-4687|url-access=subscription }}</ref>

== History ==
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The definition of quantum theorists' terms, such as ''[[wave function]]'' and ''[[matrix mechanics]]'', progressed through many stages. For instance, [[Erwin Schrödinger]] originally viewed the electron's wave function as its charge density smeared across space, but [[Max Born]] reinterpreted the absolute square value of the wave function as the electron's [[probability density function|probability density]] distributed across space;<ref name="jammer1974" />{{Rp|24–33}} the [[Born rule]], as it is now called, matched experiment, whereas Schrödinger's charge density view did not.

The views of several early pioneers of quantum mechanics, such as [[Niels Bohr]] and [[Werner Heisenberg]], are often grouped together as the "[[Copenhagen interpretation]]", though physicists and historians of physics have argued that this terminology obscures differences between the views so designated.<ref name="jammer1974">{{cite book|last=Jammer |first=Max |author-link=Max Jammer |title=Philosophy of Quantum Mechanics: The interpretations of quantum mechanics in historical perspective |url=https://archive.org/details/philosophyofquan0000jamm |url-access=registration |year=1974 |publisher=Wiley-Interscience|isbn=9780471439585 }}</ref><ref>{{Cite journal|last=Camilleri|first=Kristian|date=2009-02-01|title=Constructing the Myth of the Copenhagen Interpretation|url=https://muse.jhu.edu/article/259149|journal=Perspectives on Science|volume=17|issue=1|pages=26–57|issn=1530-9274|doi=10.1162/posc.2009.17.1.26|s2cid=57559199}}</ref> Copenhagen-type ideas were never universally embraced, and challenges to a perceived Copenhagen orthodoxy gained increasing attention in the 1950s with the [[De Broglie–Bohm theory|pilot-wave interpretation]] of [[David Bohm]] and the [[many-worlds interpretation]] of [[Hugh Everett III]].<ref name="jammer1974"/><ref>{{Citation |last=Vaidman |first=Lev |author-link=Lev Vaidman |title=Many-Worlds Interpretation of Quantum Mechanics |date=2021 |url=https://plato.stanford.edu/archives/fall2021/entries/qm-manyworlds/ |encyclopedia=The Stanford Encyclopedia of Philosophy |editor-last=Zalta |editor-first=Edward N. |editor-link=Edward N. Zalta |access-date=2023-08-25 |edition=Fall 2021 |publisher=Metaphysics Research Lab, Stanford University}}</ref><ref name="Tipler1994">{{cite book|author=[[Frank J. Tipler]]|title=The Physics of Immortality: Modern Cosmology, God, and the Resurrection of the Dead|url=https://books.google.com/books?id=PqhI7eo92tYC&q=leading+cosmologists+and+other+quantum+field+theorists|year=1994|publisher=Anchor Books|isbn=978-0-385-46799-5}}</ref>

The physicist [[N. David Mermin]] once quipped, "New interpretations appear every year. None ever disappear."<ref name=":22">{{Cite journal|last=Mermin|first=N. David|author-link=N. David Mermin|date=2012-07-01|title=Commentary: Quantum mechanics: Fixing the shifty split|journal=[[Physics Today]]|volume=65|issue=7|pages=8–10|doi=10.1063/PT.3.1618|issn=0031-9228|bibcode=2012PhT....65g...8M|doi-access=free}}</ref> (Mermin also coined the saying "Shut up and calculate" to describe many physicists' attitude to quantum theory, a remark which is often misattributed to [[Richard Feynman]].<ref>{{cite journal|doi=10.1063/1.1768652|title=Could Feynman have said this?|journal=[[Physics Today]]|volume=57|issue=5|pages=10–11|year=2004|last1=Mermin|first1=N. David|bibcode=2004PhT....57e..10M|doi-access=}}</ref>) As a rough guide to development of the mainstream view during the 1990s and 2000s, a "snapshot" of opinions was collected in a poll by Schlosshauer et al. at the "Quantum Physics and the Nature of Reality" conference of July 2011.<ref name="ReferenceA">
{{cite journal| arxiv=1301.1069
 | 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
 | date=2013-01-06
 | last1=Schlosshauer
 | first1=Maximilian
 | last2=Kofler
 | first2=Johannes
 | last3=Zeilinger
 | first3=Anton
 | author-link3=Anton Zeilinger
 | doi=10.1016/j.shpsb.2013.04.004
 | bibcode=2013SHPMP..44..222S
 | s2cid=55537196
}}</ref> The authors reference a similarly informal poll carried out by [[Max Tegmark]] at the "Fundamental Problems in Quantum Theory" conference in August 1997. The main conclusion of the authors is that "the [[Copenhagen interpretation]] still reigns supreme", receiving the most votes in their poll (42%), besides the rise to mainstream notability of the many-worlds interpretations: "The Copenhagen interpretation still reigns supreme here, especially if we lump it together with intellectual offsprings such as [[quantum information|information-based interpretations]] and the [[quantum Bayesian]] interpretation. In Tegmark's poll, the Everett interpretation received 17% of the vote, which is similar to the number of votes (18%) in our poll."

Some concepts originating from studies of interpretations have found more practical application in [[quantum information science]].<ref>{{Cite journal|last1=Barnum|first1=Howard|last2=Wehner|first2=Stephanie|author-link2=Stephanie Wehner|last3=Wilce|first3=Alexander|date=August 2018|title=Introduction: Quantum Information Theory and Quantum Foundations|journal=[[Foundations of Physics]]|language=en|volume=48|issue=8|pages=853–856|doi=10.1007/s10701-018-0188-6|bibcode=2018FoPh...48..853B|s2cid=126293060|issn=0015-9018|doi-access=free}}</ref><ref>{{Cite journal|last1=DiVincenzo|first1=David P.|author-link=David DiVincenzo|last2=Fuchs|first2=Christopher A.|date=2019-02-01|title=Quantum foundations|journal=[[Physics Today]]|volume=72|issue=2|pages=50–51|doi=10.1063/PT.3.4141|bibcode=2019PhT....72b..50D|s2cid=241052502|issn=0031-9228|doi-access=free}}</ref>

== Interpretive challenges ==
{{More citations needed section|date=August 2024}}
# Abstract, mathematical nature of [[quantum field theories]]: the [[mathematical formulation of quantum mechanics|mathematical structure of quantum mechanics]] is abstract and does not result in a single, clear interpretation of its quantities.
# Apparent [[quantum indeterminacy|indeterministic]] and irreversible processes: in [[classical field theory]], a physical property at a given location in the field is readily derived. In most mathematical formulations of quantum mechanics, measurement (understood as an interaction with a given state) has a special role in the theory, as it is the sole process that can cause a nonunitary, irreversible evolution of the state.
# Role of the [[observer (quantum physics)|observer]] in determining outcomes. Copenhagen-type interpretations imply that the wavefunction is a calculational tool, and represents reality only immediately after a measurement performed by an observer. [[Many-worlds interpretation|Everettian interpretations]] grant that all possible outcomes are real, and that measurement-type interactions cause a branching process in which each possibility is realised.<ref>{{Citation |last=Bacciagaluppi |first=Guido |title=The Role of Decoherence in Quantum Mechanics |date=2012 |url=https://plato.stanford.edu/archives/win2012/entries/qm-decoherence/ |encyclopedia=The Stanford Encyclopedia of Philosophy |editor-last=Zalta |editor-first=Edward N. |editor-link=Edward N. Zalta |access-date=2023-08-25 |edition=Winter 2012 |publisher=Metaphysics Research Lab, Stanford University}}</ref>
# [[Quantum entanglement|Classically unexpected correlations]] between remote objects: [[Quantum entanglement|entangled quantum systems]], as illustrated in the [[EPR paradox]], obey statistics that seem to violate [[Principle of locality|principles of local causality]] by [[action at a distance]].<ref>''La nouvelle cuisine'', by John S. Bell, last article of Speakable and Unspeakable in Quantum Mechanics, second edition.</ref>
# Complementarity of proffered descriptions: [[complementarity (physics)|complementarity]] holds that no set of classical physical concepts can simultaneously refer to all properties of a quantum system. For instance, wave description ''A'' and particulate description ''B'' can each describe quantum system ''S'', but not simultaneously. This implies the composition of physical properties of ''S'' does not obey the rules of classical [[propositional calculus|propositional logic]] when using [[Logical connective|propositional connectives]] (see "[[Quantum logic]]"). Like contextuality, the "origin of complementarity lies in the [[non-commutativity]] of operators" that describe quantum objects.<ref>{{Cite book |last=Omnès |first=Roland |title=Understanding quantum mechanics |date=1999 |publisher=Princeton University Press |isbn=978-0-691-00435-8 |location=Princeton, N.J}}</ref>
# [[Quantum contextuality|Contextual]] behaviour of systems locally: [[Quantum contextuality]] demonstrates that classical intuitions, in which properties of a system hold definite values independent of the manner of their measurement, fail even for local systems. Also, physical principles such as [[Identity of indiscernibles|Leibniz's Principle of the identity of indiscernibles]] no longer apply in the quantum domain, signaling that most classical intuitions may be incorrect about the quantum world.

== Influential interpretations ==
=== Copenhagen interpretation ===
{{Main|Copenhagen interpretation}}

The [[Copenhagen interpretation]] is a collection of views about the meaning of [[quantum mechanics]] principally attributed to [[Niels Bohr]] and [[Werner Heisenberg]]. It is one of the oldest attitudes towards quantum mechanics, as features of it date to the development of quantum mechanics during 1925–1927, and it remains one of the most commonly taught.<ref name="Siddiqui2017">{{cite journal |title=How diverse are physics instructors' attitudes and approaches to teaching undergraduate level quantum mechanics?|year=2017|last1=Siddiqui|first1= Shabnam|last2=Singh|first2=Chandralekha|journal=European Journal of Physics|volume=38 |issue=3 |pages=035703|doi=10.1088/1361-6404/aa6131|bibcode=2017EJPh...38c5703S|doi-access=free}}</ref><ref>Bell, John S. (1987), Speakable and Unspeakable in quantum Mechanics (Cambridge: Cambridge University Press)</ref> There is no definitive historical statement of what is ''the'' Copenhagen interpretation, and there were in particular fundamental disagreements between the views of Bohr and Heisenberg.<ref name="Faye-Stanford">{{Cite book|last=Faye|first=Jan|title=[[Stanford Encyclopedia of Philosophy]]|publisher=Metaphysics Research Lab, Stanford University|year=2019|editor-last=Zalta|editor-first=Edward N.|editor-link=Edward N. Zalta|chapter=Copenhagen Interpretation of Quantum Mechanics|author-link=Jan Faye|chapter-url=https://plato.stanford.edu/entries/qm-copenhagen/}}</ref><ref name="camilleri2015">{{cite journal|first1=K. |last1=Camilleri |first2=M. |last2=Schlosshauer |title=Niels Bohr as Philosopher of Experiment: Does Decoherence Theory Challenge Bohr's Doctrine of Classical Concepts? |arxiv=1502.06547 |journal=Studies in History and Philosophy of Modern Physics |volume=49 |pages=73–83 |year=2015 |doi=10.1016/j.shpsb.2015.01.005|bibcode=2015SHPMP..49...73C |s2cid=27697360 }}</ref> For example, Heisenberg emphasized a sharp "cut" between the observer (or the instrument) and the system being observed,<ref name="PauliEinstein">{{cite book|first=Wolfgang |last=Pauli |author-link=Wolfgang Pauli |chapter=Albert Einstein and the development of physics |title=Writings on Physics and Philosophy |editor-first1=C. P. |editor-last1=Enz |editor-link1=Charles Enz |editor-first2=K. |editor-last2=von Meyenn |publisher=Springer-Verlag |location=Berlin |orig-year=1958 |year=1994 |bibcode=1994wpp..book.....P }}</ref>{{Rp|133}} while Bohr offered an interpretation that is independent of a subjective observer or measurement or collapse, which relies on an "irreversible" or effectively irreversible process that imparts the classical behavior of "observation" or "measurement".<ref>{{citation |title=Against 'measurement' |author=[[John Stewart Bell|John Bell]] |journal=Physics World |volume=3 |number=8 |year=1990 |pages=33–41 |doi=10.1088/2058-7058/3/8/26}}</ref><ref>{{citation |author=[[Niels Bohr]] |title=Niels Bohr: Collected Works |date=1985 |url=https://www.nbarchive.dk/publications/bcw/ |volume=6: Foundations of Quantum Physics I (1926–1932) |pages=451–454 |editor=Jørgen Kalckar |orig-date=May 16, 1947}}</ref><ref>{{citation |author=Stenholm |first=Stig |title=Quantum Optics, Experimental Gravitation, and Measurement Theory |pages=121 |year=1983 |editor=Meystre |editor-first=Pierre |chapter=To fathom space and time |publisher=Plenum Press |quote=The role of irreversibility in the theory of measurement has been emphasized by many. Only this way can a permanent record be obtained. The fact that separate pointer positions must be of the asymptotic nature usually associated with irreversibility has been utilized in the measurement theory of Daneri, Loinger and Prosperi (1962). It has been accepted as a formal representation of Bohr's ideas by Rosenfeld (1966). |editor1-link=Pierre Meystre}}</ref><ref>{{citation |author=Haake |first=Fritz |title=Classical motion of meter variables in the quantum theory of measurement |date=April 1, 1993 |journal=[[Physical Review A]] |volume=47 |issue=4 |pages=2506–2517 |bibcode=1993PhRvA..47.2506H |doi=10.1103/PhysRevA.47.2506 |pmid=9909217}}</ref>

Features common to Copenhagen-type interpretations include the idea that quantum mechanics is intrinsically indeterministic, with probabilities calculated using the [[Born rule]], and the principle of [[Complementarity (physics)|complementarity]], which states certain pairs of complementary properties cannot all be observed or measured simultaneously. Moreover, properties only result from the act of "observing" or "measuring"; the theory avoids assuming [[counterfactual definiteness|definite values from unperformed experiments]]. Copenhagen-type interpretations hold that quantum descriptions are objective, in that they are independent of physicists' mental arbitrariness.<ref name="omnes">{{cite book |last=Omnès |first=R. |title=The Interpretation of Quantum Mechanics |publisher=Princeton University Press |year=1994 |isbn=978-0-691-03669-4 |location=Princeton, New Jersey |language=en-us |oclc=439453957 |author-link=Roland Omnès}}</ref>{{Rp|85–90}} The statistical interpretation of wavefunctions due to Max Born differs sharply from Schrödinger's original intent, which was to have a theory with continuous time evolution and in which wavefunctions directly described physical reality.<ref name="jammer1974"/>{{rp|24–33}}<ref>{{cite journal|title=Matrix Theory before Schrödinger: Philosophy, Problems, Consequences |first=Mara |last=Beller |journal=Isis |date=December 1983 |volume=74 |number=4  |pages=469–491 |doi=10.1086/353357 |jstor=232208|s2cid=121780437 }}</ref>

=== Many worlds ===
{{Main|Many-worlds interpretation}}

The [[many-worlds interpretation]] is an interpretation of quantum mechanics in which a [[universal wavefunction]] obeys the same deterministic, [[CPT symmetry|reversible]] laws at all times; in particular there is no (indeterministic and [[irreversibility|irreversible]]) [[wavefunction collapse]] associated with measurement. The phenomena associated with measurement are claimed to be explained by [[Quantum decoherence|decoherence]], which occurs when states interact with the environment. More precisely, the parts of the wavefunction describing observers become increasingly [[quantum entanglement|entangled]] with the parts of the wavefunction describing their experiments. Although all possible outcomes of experiments continue to lie in the wavefunction's support, the times at which they become correlated with observers effectively "split" the universe into mutually unobservable [[alternate histories]].

=== Quantum information theories ===
[[Quantum information]]al approaches<ref>
{{cite news
 | url=https://www.newscientist.com/article/mg16922784-300-in-the-beginning-was-the-bit/
 | title=In the beginning was the bit
 | work=New Scientist
 | date=2001-02-17
 | access-date=2022-01-18 }}</ref><ref>{{cite book |last1=Janas |first1=M. |last2=Cuffaro |first2=M. E. |last3=Janssen |first3=M. |date=2022 |title=Understanding Quantum Raffles |url=https://www.springer.com/gp/book/9783030859381  |language=en-us}}</ref> have attracted growing support.<ref>{{cite news
 | url=http://www.dailycamera.com/science-columnists/ci_22444536/kate-becker-quantum-physics-has-been-rankling-scientists
 | title=Quantum physics has been rankling scientists for decades
 | work=Boulder Daily Camera
 | author=Kate Becker
 | date=2013-01-25
 | access-date=2013-01-25
}}</ref><ref name="ReferenceA"/> They subdivide into two kinds.<ref name=Timpson>"Let us call the thought that information might be the basic category from which all else flows informational immaterialism."  [http://users.ox.ac.uk/~bras2317/iii_2.pdf Information, Immaterialism, Instrumentalism: Old and New in Quantum Information.] Christopher G. Timpson</ref>
* Information ontologies, such as J. A. Wheeler's "[[it from bit]]". These approaches have been described as a revival of [[idealism|immaterialism]].<ref name=Timpson/>
* Interpretations where quantum mechanics is said to describe an observer's knowledge of the world, rather than the world itself. This approach has some similarity with Bohr's thinking.<ref>"Physics concerns what we can say about nature". (Niels Bohr, quoted in Petersen, A. (1963). The philosophy of Niels Bohr. ''Bulletin of the Atomic Scientists'', 19(7):8–14.)</ref> Collapse (also known as reduction) is often interpreted as an observer acquiring information from a measurement, rather than as an objective event. [[James Hartle]] writes,
<blockquote>
The state is not an objective property of an individual system but is that information, obtained from a knowledge of how a system was prepared, which can be used for making predictions about future measurements. ... A quantum mechanical state being a summary of the observer's information about an individual physical system changes both by dynamical laws, and whenever the observer acquires new information about the system through the process of measurement. The existence of two laws for the evolution of the state vector&nbsp;... becomes problematical only if it is believed that the state vector is an objective property of the system&nbsp;... The "reduction of the wavepacket" does take place in the consciousness of the observer, not because of any unique physical process which takes place there, but only because the state is a construct of the observer and not an objective property of the physical system.<ref>{{cite journal | last1 = Hartle | first1 = J. B. | year = 1968 | title = Quantum mechanics of individual systems | journal = Am. J. Phys. | volume = 36 | issue = 8| pages = 704–712 | doi=10.1119/1.1975096| bibcode = 1968AmJPh..36..704H| arxiv = 1907.02953 | s2cid = 123454773 }}</ref>
</blockquote>

=== Relational quantum mechanics ===
{{Main|Relational quantum mechanics}}

The essential idea behind [[relational quantum mechanics]], following the precedent of [[special relativity]], is that different observers may give different accounts of the same series of events: for example, to one observer at a given point in time, a system may be in a single, "collapsed" [[eigenstate]], while to another observer at the same time, it may be in a superposition of two or more states. Consequently, if quantum mechanics is to be a complete theory, relational quantum mechanics argues that the notion of "state" describes not the observed system itself, but the relationship, or correlation, between the system and its observer(s). The [[Quantum state vector|state vector]] of conventional quantum mechanics becomes a description of the correlation of some ''degrees of freedom'' in the observer, with respect to the observed system. However, it is held by relational quantum mechanics that this applies to all physical objects, whether or not they are conscious or macroscopic. Any "measurement event" is seen simply as an ordinary physical interaction, an establishment of the sort of correlation discussed above. Thus the physical content of the theory has to do not with objects themselves, but the relations between them.<ref>
{{cite web
 |url=http://plato.stanford.edu/entries/qm-relational/
 |title=Relational Quantum Mechanics (Stanford Encyclopedia of Philosophy)
 |publisher=Plato.stanford.edu
 |access-date=2011-01-24}}</ref><ref>For more information, see {{cite journal |doi=10.1007/BF02302261 |author=Carlo Rovelli |year=1996 |title=Relational Quantum Mechanics |journal=[[International Journal of Theoretical Physics]] |volume=35 |issue=8 |pages=1637–1678 |arxiv=quant-ph/9609002|bibcode = 1996IJTP...35.1637R |s2cid=16325959 |author-link= Carlo Rovelli
}}</ref>

=== QBism ===
{{Main|Quantum Bayesianism}}

[[QBism]], which originally stood for "quantum Bayesianism", is an interpretation of quantum mechanics that takes an agent's actions and experiences as the central concerns of the theory. This interpretation is distinguished by its use of a [[Bayesian probability#Dutch book approach|subjective Bayesian]] account of probabilities to understand the quantum mechanical [[Born rule]] as a [[normative]] addition to good decision-making. QBism draws from the fields of [[quantum information]] and [[Bayesian probability]] and aims to eliminate the interpretational conundrums that have beset quantum theory.

QBism deals with common questions in the interpretation of quantum theory about the nature of [[wavefunction]] [[Quantum superposition|superposition]], [[Measurement problem|quantum measurement]], and [[Quantum entanglement|entanglement]].<ref name=":21">{{cite journal|last=Timpson|first=Christopher Gordon|year=2008|title=Quantum Bayesianism: A study|url=http://users.ox.ac.uk/~bras2317/qb_s.ps|format=postscript|journal=Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics|volume=39|issue=3|pages=579–609|arxiv=0804.2047|doi=10.1016/j.shpsb.2008.03.006|bibcode=2008SHPMP..39..579T|s2cid=16775153}}</ref><ref>{{Cite journal|last=Mermin|first=N. David|author-link=N. David Mermin|date=2012-07-01|title=Commentary: Quantum mechanics: Fixing the shifty split|journal=Physics Today|volume=65|issue=7|pages=8–10|doi=10.1063/PT.3.1618|issn=0031-9228|bibcode=2012PhT....65g...8M|doi-access=free}}</ref> According to QBism, many, but not all, aspects of the quantum formalism are subjective in nature. For example, in this interpretation, a quantum state is not an element of reality—instead it represents the [[Degree of belief|degrees of belief]] an agent has about the possible outcomes of measurements. For this reason, some [[Philosophy of science|philosophers of science]] have deemed QBism a form of [[anti-realism]].<ref>{{Cite book|title=Bananaworld: Quantum Mechanics for Primates|last=Bub|first=Jeffrey|publisher=Oxford University Press|year=2016|isbn=978-0198718536|location=Oxford|pages=232}}</ref><ref>{{Cite book|title=Every Thing Must Go: Metaphysics Naturalized|url=https://archive.org/details/everythingmustgo00lady|url-access=limited|last1=Ladyman|first1=James|last2=Ross|first2=Don|last3=Spurrett|first3=David|last4=Collier|first4=John|publisher=Oxford University Press|year=2007|isbn=9780199573097|location=Oxford|pages=[https://archive.org/details/everythingmustgo00lady/page/n196 184]}}</ref> The originators of the interpretation disagree with this characterization, proposing instead that the theory more properly aligns with a kind of realism they call "participatory realism", wherein reality consists of ''more'' than can be captured by any putative third-person account of it.<ref>For "participatory realism," see, e.g.,<br />{{Cite book|title=Information and Interaction: Eddington, Wheeler, and the Limits of Knowledge|last=Fuchs|first=Christopher A.|year=2017|isbn=9783319437606|editor-last=Durham|editor-first=Ian T.|chapter=On Participatory Realism|arxiv=1601.04360|oclc=967844832|editor2-last=Rickles|editor2-first=Dean|bibcode=2016arXiv160104360F}}<br />{{Cite web|url=http://fqxi.org/grants/large/awardees/view/__details/2016/fuchs|title=Does Participatory Realism Make Sense? The Role of Observership in Quantum Theory|last1=Fuchs|first1=Christopher A.|last2=Timpson|first2=Christopher G.|website=FQXi: Foundational Questions Institute|access-date=2017-04-18}}</ref><ref>{{Cite book|title=What is Quantum Information?|last=Cabello|first=Adán|publisher=Cambridge University Press|year=2017|isbn=9781107142114|editor-last=Lombardi|editor-first=Olimpia|editor-link=Olimpia Lombardi|pages=138–143|chapter=Interpretations of quantum theory: A map of madness|arxiv=1509.04711|editor2-last=Fortin|editor2-first=Sebastian|editor3-last=Holik|editor3-first=Federico|editor4-last=López|editor4-first=Cristian|bibcode=2015arXiv150904711C|doi=10.1017/9781316494233.009|s2cid=118419619}}</ref>

=== Consistent histories ===
{{Main|Consistent histories}}

The [[consistent histories]] interpretation generalizes the conventional Copenhagen interpretation and attempts to provide a natural interpretation of [[quantum cosmology]]. The theory is based on a consistency criterion that allows the history of a system to be described so that the probabilities for each history obey the additive rules of classical probability. It is claimed to be [[consistent]] with the [[Schrödinger equation]].

According to this interpretation, the purpose of a quantum-mechanical theory is to predict the relative probabilities of various alternative histories (for example, of a particle).

=== Ensemble interpretation ===
{{Main|Ensemble interpretation}}

The [[ensemble interpretation]], also called the statistical interpretation, can be viewed as a minimalist interpretation. That is, it claims to make the fewest assumptions associated with the standard mathematics. It takes the statistical interpretation of Born to the fullest extent. The interpretation states that the wave function does not apply to an individual system{{spaced ndash}}for example, a single particle{{spaced ndash}}but is an abstract statistical quantity that only applies to an ensemble (a vast multitude) of similarly prepared systems or particles. In the words of Einstein:

{{Blockquote|The attempt to conceive the quantum-theoretical description as the complete description of the individual systems leads to unnatural theoretical interpretations, which become immediately unnecessary if one accepts the interpretation that the description refers to ensembles of systems and not to individual systems.|Einstein in ''Albert Einstein: Philosopher-Scientist'', ed. P.A. Schilpp (Harper & Row, New York)}}

The most prominent current advocate of the ensemble interpretation is Leslie E. Ballentine, professor at [[Simon Fraser University]], author of the text book ''Quantum Mechanics, A Modern Development''.

=== De Broglie–Bohm theory ===
{{Main|De Broglie–Bohm theory}}

The [[de Broglie–Bohm theory]] of quantum mechanics (also known as the pilot wave theory) is a theory by [[Louis de Broglie]] and extended later by [[David Bohm]] to include measurements. Particles, which always have positions, are guided by the wavefunction. The wavefunction evolves according to the [[Schrödinger wave equation]], and the wavefunction never collapses. The theory takes place in a single spacetime, is [[Principle of locality|non-local]], and is deterministic. The simultaneous determination of a particle's position and velocity is subject to the usual [[uncertainty principle]] constraint. The theory is considered to be a [[hidden-variable theory]], and by embracing non-locality it satisfies [[Bell's inequality]]. The [[measurement problem]] is resolved, since the particles have definite positions at all times.<ref>{{cite journal | last1 = Maudlin | first1 = T. | year = 1995 | title = Why Bohm's Theory Solves the Measurement Problem | journal = Philosophy of Science | volume = 62 | issue = 3| pages = 479–483 | doi=10.1086/289879| s2cid = 122114295 }}</ref> Collapse is explained as [[Phenomenology (physics)|phenomenological]].<ref name="CushingFine2013">{{cite arXiv |eprint=quant-ph/9511016 |first1=D. |last1=Durr |first2=N. |last2=Zanghi |title=Bohmian Mechanics as the Foundation of Quantum Mechanics |date=Nov 14, 1995 |last3=Goldstein |first3=S. }} Also published in {{cite book |last1=Cushing |first1=J. T. |url=https://books.google.com/books?id=By34CAAAQBAJ&q=%22Bohmian+Mechanics+as+the+Foundation+of+Quantum+Mechanics%22&pg=PA21 |title=Bohmian Mechanics and Quantum Theory: An Appraisal |last2=Fine |first2=Arthur |last3=Goldstein |first3=S. |date=17 April 2013 |publisher=Springer Science & Business Media |isbn=978-94-015-8715-0 |pages=21–43}}</ref>

=== Transactional interpretation ===
{{Main|Transactional interpretation}}
The [[transactional interpretation]] of quantum mechanics (TIQM) by [[John G. Cramer]] is an interpretation of quantum mechanics inspired by the [[Wheeler–Feynman absorber theory]].<ref>
{{cite web
 |url         = http://www.npl.washington.edu/npl/int_rep/qm_nl.html
 |title       = Quantum Nocality – Cramer
 |publisher   = Npl.washington.edu
 |access-date  = 2011-01-24
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20101229074514/http://www.npl.washington.edu/npl/int_rep/qm_nl.html
 |archive-date = 2010-12-29
}}</ref> It describes the collapse of the wave function as resulting from a time-symmetric transaction between a possibility wave from the source to the receiver (the wave function) and a possibility wave from the receiver to source (the complex conjugate of the wave function). This interpretation of quantum mechanics is unique in that it not only views the wave function as a real entity, but the complex conjugate of the wave function, which appears in the Born rule for calculating the expected value for an observable, as also real.

=== Consciousness causes collapse ===
{{Main|Consciousness causes collapse}}
[[Eugene Wigner]] argued that human experimenter consciousness (or maybe even animal consciousness) was critical for the collapse of the wavefunction, but he later abandoned this interpretation after learning about [[quantum decoherence]].<ref>{{cite journal|first=Michael |last=Esfeld |year=1999 |title=Essay Review: Wigner's View of Physical Reality |journal=[[Studies in History and Philosophy of Modern Physics]] |volume=30B |pages=145–154}}</ref><ref name="Schreiber">{{cite arXiv |eprint=quant-ph/9501014 |first=Zvi |last=Schreiber |title=The Nine Lives of Schrödinger's Cat |year=1995 }}</ref>
Some specific proposals for consciousness caused wave-function collapse have been shown to be [[unfalsifiable]] and more broadly reasonable assumption about consciousness lead to the same conclusion.<ref>{{Cite journal |last1=de Barros |first1=J. Acacio |last2=Oas |first2=Gary |date=October 2017 |title=Can We Falsify the Consciousness-Causes-Collapse Hypothesis in Quantum Mechanics? |url=http://link.springer.com/10.1007/s10701-017-0110-7 |journal=Foundations of Physics |language=en |volume=47 |issue=10 |pages=1294–1308 |doi=10.1007/s10701-017-0110-7 |issn=0015-9018|arxiv=1609.00614 |bibcode=2017FoPh...47.1294D }}</ref>

=== Quantum logic ===
{{Main|Quantum logic}}

[[Quantum logic]] can be regarded as a kind of propositional logic suitable for understanding the apparent anomalies regarding quantum measurement, most notably those concerning composition of measurement operations of complementary variables. This research area and its name originated in the 1936 paper by [[Garrett Birkhoff]] and [[John von Neumann]], who attempted to reconcile some of the apparent inconsistencies of classical Boolean logic with the facts related to measurement and observation in quantum mechanics.

=== Modal interpretations of quantum theory ===
Modal interpretations of quantum mechanics were first conceived of in 1972 by [[Bas van Fraassen]], in his paper "A formal approach to the philosophy of science". Van Fraassen introduced a distinction between a ''dynamical'' state, which describes what might be true about a system and which always evolves according to the Schrödinger equation, and a ''value'' state, which indicates what is actually true about a system at a given time. The term "modal interpretation" now is used to describe a larger set of models that grew out of this approach. The ''[[Stanford Encyclopedia of Philosophy]]'' describes several versions, including proposals by [[Simon B. Kochen|Kochen]], [[Dennis Dieks|Dieks]], Clifton, Dickson, and [[Jeffrey Bub|Bub]].<ref>{{cite encyclopedia
|url=http://www.science.uva.nl/~seop/entries/qm-modal/
 |title=Modal Interpretations of Quantum Mechanics
 |encyclopedia=Stanford Encyclopedia of Philosophy
 |publisher=Science.uva.nl
 |date=2002-11-12
 |access-date=2011-01-24|last1=Lombardi
 |first1=Olimpia|author1-link=Olimpia Lombardi
 |last2=Dieks
 |first2=Dennis
}}</ref> According to [[Michel Bitbol]], Schrödinger's views on how to interpret quantum mechanics progressed through as many as four stages, ending with a non-collapse view that in respects resembles the interpretations of Everett and 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 wavefunction as ontic and treating it as epistemic 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>

=== Time-symmetric theories ===
Time-symmetric interpretations of quantum mechanics were first suggested by [[Walter Schottky]] in 1921.<ref>{{cite journal| author=Schottky, Walter | authorlink=Walter Schottky |title= Das Kausalproblem der Quantentheorie als eine Grundfrage der modernen Naturforschung überhaupt |journal= Naturwissenschaften | volume= 9 | issue =25 | pages= 492–496 |date=1921|bibcode=1921NW......9..492S |doi=10.1007/BF01494985 | s2cid=22228793 | url=https://zenodo.org/record/1428318 }}</ref><ref>{{cite journal| author=Schottky, Walter |title= Das Kausalproblem der Quantentheorie als eine Grundfrage der modernen Naturforschung überhaupt |journal= Naturwissenschaften | volume= 9 | issue =26 | pages= 506–511 |date=1921|bibcode=1921NW......9..506S |doi=10.1007/BF01496025 |s2cid= 26246226 |url= https://zenodo.org/record/1428318 }}</ref> Several theories have been proposed that modify the equations of quantum mechanics to be symmetric with respect to time reversal.<ref>{{cite journal | last1 = Watanabe | first1 = Satosi | year = 1955 | title = Symmetry of physical laws. Part III. Prediction and retrodiction | journal = Reviews of Modern Physics | volume = 27 | issue = 2| pages = 179–186 | doi=10.1103/revmodphys.27.179| bibcode = 1955RvMP...27..179W| hdl = 10945/47584 | s2cid = 122168419 | hdl-access = free }}</ref><ref>{{cite journal |last1=Aharonov |first1=Y. |author-link1=Yakir Aharonov |display-authors=etal |year=1964 |title=Time Symmetry in the Quantum Process of Measurement |journal=Physical Review |volume=134 |issue=6B |pages=B1410–1416 |bibcode=1964PhRv..134.1410A |doi=10.1103/physrev.134.b1410}}</ref><ref>[[Yakir Aharonov|Aharonov, Y.]] and [[Lev Vaidman|Vaidman, L.]] "On the Two-State Vector Reformulation of Quantum Mechanics". ''Physica Scripta'', Volume T76, pp. 85–92 (1998).</ref><ref>{{cite journal | last1 = Wharton | first1 = K. B. | year = 2007 | title = Time-Symmetric Quantum Mechanics | journal = Foundations of Physics | volume = 37 | issue = 1| pages = 159–168 | doi=10.1007/s10701-006-9089-1| bibcode = 2007FoPh...37..159W| s2cid = 123086913 }}</ref><ref>{{cite journal | last1 = Wharton | first1 = K. B. | year = 2010 | title = A Novel Interpretation of the Klein–Gordon Equation | journal = Foundations of Physics | volume = 40 | issue = 3| pages = 313–332 | doi=10.1007/s10701-009-9398-2| bibcode = 2010FoPh...40..313W| arxiv = 0706.4075| s2cid = 121170138 }}</ref><ref>{{cite journal | last1 = Heaney | first1 = M. B. | year = 2013 | title = A Symmetrical Interpretation of the Klein–Gordon Equation | journal = Foundations of Physics | volume =  43| issue = 6| pages =  733–746| doi=10.1007/s10701-013-9713-9| arxiv =1211.4645| bibcode =2013FoPh...43..733H| s2cid = 118770571 }}</ref> (See [[Wheeler–Feynman absorber theory|Wheeler–Feynman time-symmetric theory]].) This creates [[retrocausality]]: events in the future can affect ones in the past, exactly as events in the past can affect ones in the future. In these theories, a single measurement cannot fully determine the state of a system (making them a type of [[hidden-variables theory]]), but given two measurements performed at different times, it is possible to calculate the exact state of the system at all intermediate times. The collapse of the wavefunction is therefore not a physical change to the system, just a change in our knowledge of it due to the second measurement. Similarly, they explain entanglement as not being a true physical state but just an illusion created by ignoring retrocausality. The point where two particles appear to "become entangled" is simply a point where each particle is being influenced by events that occur to the other particle in the future.

Not all advocates of time-symmetric causality favour modifying the unitary dynamics of standard quantum mechanics. Thus a leading exponent of the two-state vector formalism, [[Lev Vaidman]], states that the two-state vector formalism dovetails well with [[Hugh Everett]]'s [[many-worlds interpretation]].<ref>[[Yakir Aharonov]], Lev Vaidman: ''The Two-State Vector Formalism of Quantum Mechanics: an Updated Review''. In: Juan Gonzalo Muga, Rafael Sala Mayato, Íñigo Egusquiza (eds.): ''Time in Quantum Mechanics'', Volume 1, Lecture Notes in Physics 734, pp.&nbsp;399–447, 2nd ed., Springer, 2008, {{ISBN|978-3-540-73472-7}}, {{doi|10.1007/978-3-540-73473-4_13}}, {{arXiv|quant-ph/0105101}}, [https://books.google.com/books?id=InKru6zHQWgC&pg=PA443 p. 443]</ref>

=== Other interpretations ===
{{Main|Minority interpretations of quantum mechanics}}
As well as the mainstream interpretations discussed above, a number of other interpretations have been proposed that have not made a significant scientific impact for whatever reason. These range from proposals by mainstream physicists to the more [[occult]] ideas of [[quantum mysticism]].

== Related concepts ==
Some ideas are discussed in the context of interpreting quantum mechanics but are not necessarily regarded as interpretations themselves.

=== Quantum Darwinism ===
{{Main|Quantum Darwinism}}
Quantum Darwinism is a theory meant to explain the emergence of the [[Classical physics|classical world]] from the [[Quantum mechanics|quantum world]] as due to a process of [[Charles Darwin|Darwinian]] [[natural selection]] induced by the environment interacting with the quantum system; where the many possible [[quantum states]] are selected against in favor of a stable [[pointer state]]. It was proposed in 2003 by [[Wojciech Zurek]] and a group of collaborators including Ollivier, Poulin, Paz and Blume-Kohout. The development of the theory is due to the integration of a number of Zurek's research topics pursued over the course of twenty-five years including [[pointer state]]s, [[einselection]] and [[decoherence]].

=== Objective-collapse theories ===
{{Main|Objective-collapse theory}}
Objective-collapse theories differ from the [[Copenhagen interpretation]] by regarding both the wave function and the process of collapse as ontologically objective (meaning these exist and occur independent of the observer). In objective theories, collapse occurs either randomly ("spontaneous localization") or when some physical threshold is reached, with observers having no special role. Thus, objective-collapse theories are realistic, indeterministic, no-hidden-variables theories. Standard quantum mechanics does not specify any mechanism of collapse; quantum mechanics would need to be extended if objective collapse is correct. The requirement for an extension means that objective-collapse theories are alternatives to quantum mechanics rather than interpretations of it. Examples include 
* the [[Ghirardi–Rimini–Weber theory]]<ref>
{{cite book
 |chapter-url=http://www.romanfrigg.org/writings/GRW%20Theory.pdf
 |first=Roman |last=Frigg
 |title=Compendium of Quantum Physics
 |chapter=GRW Theory (Ghirardi, Rimini, Weber Model of Quantum Mechanics)
 |date=2009 |pages=266–270
 |publisher=Springer
 |access-date=2011-01-24
 |archive-date=2016-06-24
 |archive-url=https://web.archive.org/web/20160624052814/http://www.romanfrigg.org/writings/GRW%20Theory.pdf
 |url-status=dead
 |doi=10.1007/978-3-540-70626-7_81
 |isbn=978-3-540-70622-9 |editor-first1=Daniel |editor-last1=Greenberger
 |editor-first2=Klaus |editor-last2=Hentschel 
 |editor-first3=Friedel |editor-last3=Weinert
}}</ref>
* the [[continuous spontaneous localization model]]
* the [[Penrose interpretation]]

== Comparisons ==
The most common interpretations are summarized in the table below. The values shown in the cells of the table are not without controversy, for the precise meanings of some of the concepts involved are unclear and, in fact, are themselves at the center of the controversy surrounding the given interpretation. For another table comparing interpretations of quantum theory, see reference.<ref>{{Cite book|title=What is quantum information?|last1=Olimpia|first1=Lombardi|author1-link=Olimpia Lombardi|author2=Fortin, Sebastian|last3=Federico|first3=Holik|last4=Cristian|first4=López|year=2017|isbn=9781107142114|pages=138–144|oclc=965759965|arxiv = 1509.04711|doi = 10.1017/9781316494233.009|chapter = Interpretations of Quantum Theory: A Map of Madness|s2cid=118419619}}</ref>

No experimental evidence exists that distinguishes among these interpretations. To that extent, the physical theory stands, and is consistent with itself and with reality. Nevertheless, designing experiments that would test the various interpretations is the subject of active research.

Most of these interpretations have variants. For example, it is difficult to get a precise definition of the Copenhagen interpretation as it was developed and argued by many people.

{| class="wikitable sortable" style="text-align:center;"
|-
! Interpre&shy;tation
! Year pub&shy;lished
! Author(s)
! [[Determinism|Determ&shy;inistic]]?
! [[Wave function#Ontology|Ontic wave&shy;function]]?
! Unique <br />history?
! [[Hidden-variable theory|Hidden <br />variables]]?
! [[Wavefunction collapse|Collapsing <br />wave&shy;functions]]?
! Observer <br />role?
! [[Principle of locality|Local <br />dyna&shy;mics]]?
! [[Counterfactual definiteness|Counter&shy;factually <br />definite]]?
! Extant <br />[[universal wavefunction|universal <br />wave&shy;function]]?
|-
| [[Ensemble interpretation]]
|1926
|[[Max Born]]
| {{okay|Agnostic}}
| {{no}}
| {{yes}}
| {{okay|Agnostic}}
| {{no}}
| {{no}}
| {{no}}
| {{no}}
| {{no}}
|-
| [[Copenhagen interpretation]]
|1927
|[[Niels Bohr]], [[Werner Heisenberg]]
| {{no}}
| {{okay|Some}}<ref>{{citation |chapter=The Earliest Missionaries of the Copenhagen Spirit |author=John L. Heilbron |year=1988 |title=Science in Reflection |editor=E. Ullmann-Margalit |pages=201–233 |quote=This resolution of EPR, which Rosen later characterized as a stipulation that '[physical] reality is whatever quantum mechanics is capable of describing', was applauded for its clarity by Bohr's close associates. Heisenberg, Klein, and Kramers particularly liked the reduction of the EPR thought experiment to the familiar problem of the diaphragm with holes. Perhaps the most interesting responses came from Bohr's old friend, the physicist C. W. Oseen, and from his new ally, the physicist-philosopher Philipp Frank. Oseen had understood at last what he now recognized that Bohr had been saying all along: before a measurement an atom's state with respect to the quantity measured is undefined. Frank saw that Bohr had indeed transfixed EPR on an essential ambiguity. What Frank liked most was the implication that physicists should avoid the term and concept of 'physical reality'. He understood Bohr to mean that complementarity characterized measuring procedures, not the things measured. Bohr acknowledged that that was indeed what he had had in mind.}}</ref>
| {{yes}}
| {{no}}
| {{okay|Some}}<ref>{{citation |title=Niels Bohr on the wave function and the classical/quantum divide |author=Henrik Zinkernagel |year=2016 |doi=10.1016/j.shpsb.2015.11.001 |journal=Studies in History and Philosophy of Modern Physics |volume=53 |pages=9–19 |arxiv=1603.00353 |bibcode=2016SHPMP..53....9Z |s2cid=18890207 |quote=For a start, discussions of the Copenhagen interpretation in the literature are ambiguous between two different views of the wave function, both of which of course accept the Born interpretation. Sometimes the Copenhagen (and Bohr’s) interpretation is associated with the epistemic view of the quantum state, according to which the quantum state is but a representation of our knowledge of the physical system, and thus not a real existing entity in itself. On this view the 'collapse' of the wave function is not a physical process, and it just reflects an update of our information about the system; see e.g. Zeilinger (1999). By contrast, the Copenhagen interpretation has also been associated with an ontological view of the quantum state, in which the wave function somehow describes a real wave, and the collapse is a real physical process – presumably induced by the observer. This ontological view is usually attributed to von Neumann in his 1932 textbook exposition of quantum mechanics; see e.g. Henderson (2010). [...] Thus, for Bohr, the wave function is a representation of a quantum system in a particular, classically described, experimental context. Three important points need to be made regarding this contextuality: 1) When a measurement is performed (that is, when an irreversible recording has been made; see below), then the context changes, and hence the wave function changes. This can formally be seen as a 'collapse' of the wave function, with the square quotes indicating that we are not talking about a physical process in which a real wave collapses.}}</ref>
| {{no}}<ref name="Heisenberg-observer-reality">{{citation |author=W. Heisenberg |chapter=The Development of the Interpretation of the Quantum Theory |title=Essays dedicated to Niels Bohr on the occasion of his seventieth birthday |editor=W. Pauli |year=1955 |publisher=Pergamon Press |quote=Of course it is entirely justified to imagine this transition, from the possible to the actual, moved to an earlier point of time, for the observer himself does not produce the transition; but it cannot be moved back to a time when the compound system was still separate from the external world, because such an assumption would not be compatible with the validity of quantum mechanics for the closed system. We see from this that a system cut off from the external world is potential but not actual in character, or, as BOHR has often expressed it, that the system cannot be described in terms of the classical concepts. We may say that the state of the closed system represented by a Hilbert vector is indeed objective, but not real, and that the classical idea of 'objectively real things' must here, to this extent, be abandoned.}}</ref><ref>{{citation |title=Essays 1958–1962 on Atomic Physics and Human Knowledge| author=Niels Bohr |page=3 |year=1958 |chapter=Quantum Physics and Philosophy—Causality and Complementarity |quote=The description of atomic phenomena has in these respects a perfectly objective character, in the sense that no explicit reference is made to any individual observer and that therefore, with proper regard to relativistic exigencies, no ambiguity is involved in the communication of information.}}</ref>
| {{yes}}
| {{no}}
| {{no}}
|-
| [[De Broglie–Bohm theory]]
|1927–<br />1952
|[[Louis de Broglie]], [[David Bohm]]
| {{yes}}
| {{yes}}{{efn|Both particle <small>AND</small> guiding wavefunction are real.}}
| {{yes}}{{efn|Unique particle history, but multiple wave histories.}}
| {{yes}}
| {{okay|Phenomen&shy;ological}}
| {{no}}
| {{no}}
| {{yes}}
| {{yes}}
|-
| [[Quantum logic]]
|1936
|[[Garrett Birkhoff]]
| {{okay|Agnostic}}
| {{okay|Agnostic}}
| {{yes}}{{efn|But quantum logic is more limited in applicability than Coherent Histories.}}
| {{no}}
| {{no}}
| {{okay|Interpre&shy;tational{{efn|Quantum mechanics is regarded as a way of predicting observations, or a theory of measurement.}}}}
| {{okay|Agnostic}}
| {{no}}
| {{no}}
|-
| Time-<br />symmetric theories
|1955
|[[Satosi Watanabe]]
| {{yes}}
| {{no}}
| {{yes}}
| {{yes}}
| {{no}}
| {{no}}
| {{no}}<ref>{{Cite journal |doi = 10.1038/s41598-018-26018-y|pmid = 29769645|pmc = 5955892|title = Nonlocal Position Changes of a Photon Revealed by Quantum Routers|journal = Scientific Reports|volume = 8|issue = 1|pages = 7730|year = 2018|last1 = Elitzur|first1 = Avshalom C.|last2 = Cohen|first2 = Eliahu|last3 = Okamoto|first3 = Ryo|last4 = Takeuchi|first4 = Shigeki|bibcode = 2018NatSR...8.7730E|arxiv = 1707.09483}}</ref>
| {{no}}
| {{yes}}
|-
| [[Many-worlds interpretation]]
|1957
|[[Hugh Everett]]
| {{yes}}
| {{yes}}
| {{no}}
| {{no}}
| {{no}}
| {{no}}
| {{yes}}
| {{okay|Ill-posed}}
| {{yes}}
|-
| [[Consciousness causes collapse]]
|1961–<br />1993
|[[Eugene Wigner]], [[Henry Stapp]]
| {{no}}
| {{yes}}
| {{yes}}
| {{no}}
| {{yes}}
| {{yes|Causal}}
| {{no}}
| {{no}}
| {{yes}}
|-
| [[Many-minds interpretation]]
|1970
|[[H. Dieter Zeh]]
| {{yes}}
| {{yes}}
| {{no}}
| {{no}}
| {{no}}
| {{okay|Interpre&shy;tational{{efn|Observers separate the universal wavefunction into orthogonal sets of experiences.}}}}
| {{yes}}
| {{okay|Ill-posed}}
| {{yes}}
|-
| [[Consistent histories]]
|1984
|[[Robert B. Griffiths]]
| {{no}}
| {{no}}
| {{no}}
| {{no}}
| {{no}}{{efn|In the consistent histories interpretation the collapse is a legitimate calculational procedure when describing the preparation of a quantum system, but it amounts to nothing more than a convenient way of calculating conditional probabilities.}}
| {{no}}{{efn|In the consistent histories interpretation, observers are necessary to select a specific family of consistent histories (i.e., a framework), thus enabling the calculation of probabilities of physical events. Observers, however, play a purely passive role, similar to a photographer choosing a particular framing when taking a picture.}}
| {{yes}}
| {{no}}
| {{yes}}
|-
| [[Transactional interpretation]]
|1986
|[[John G. Cramer]]
| {{no}}
| {{yes}}
| {{yes}}
| {{no}}
| {{yes}}{{efn|In the TI the collapse of the state vector is interpreted as the completion of the transaction between emitter and absorber.}}
| {{no}}
| {{no}}{{efn|The transactional interpretation is explicitly non-local.}}
| {{yes}}
| {{no}}
|-
| [[Objective-collapse theory|Objective-collapse theories]]
|1986–<br />1989
|[[Giancarlo Ghirardi]], Alberto Rimini, Tullio Weber, [[Roger Penrose]]
| {{no}}
| {{yes}}
| {{yes}}
| {{no}}
| {{yes}}
| {{no}}
| {{no}}
| {{no}}
| {{no}}
|-
| [[Relational quantum mechanics|Relational interpretation]]
|1994
|[[Carlo Rovelli]]
| {{no}}<ref>{{Cite journal | doi=10.1007/s10701-019-00234-6| title=The Notion of Locality in Relational Quantum Mechanics| journal=Foundations of Physics| volume=49| issue=2| pages=96–106| year=2019| last1=Martin-Dussaud| first1=P.| last2=Rovelli| first2=C.| last3=Zalamea| first3=F.| bibcode=2019FoPh...49...96M| arxiv=1806.08150| s2cid=50796079}}</ref>
| {{no}}
| {{okay|Agnostic{{efn|Comparing histories between systems in this interpretation has no well-defined meaning.}}}}
| {{no}}
| {{yes}}{{efn|Any physical interaction is treated as a collapse event relative to the systems involved, not just macroscopic or conscious observers.}}
| {{yes|Intrinsic{{efn|The state of the system is observer-dependent, i.e., the state is specific to the reference frame of the observer.}}}}
| {{okay|Possibly{{efn|The interpretation was originally presented as local,<ref>{{Cite journal|last1=Smerlak|first1=Matteo|last2=Rovelli|first2=Carlo|date=2007-03-01|title=Relational EPR|journal=Foundations of Physics|language=en|volume=37|issue=3|pages=427–445|doi=10.1007/s10701-007-9105-0|issn=0015-9018|arxiv=quant-ph/0604064|bibcode=2007FoPh...37..427S|s2cid=11816650}}</ref> but whether locality is well-posed in RQM has been disputed.<ref>{{cite journal|last=Pienaar |first=Jacques |title=Comment on 'The Notion of Locality in Relational Quantum Mechanics' |journal=Foundations of Physics |volume=49 |pages=1404–1414 |year=2019 |issue=12 |doi=10.1007/s10701-019-00303-w |arxiv=1807.06457 |bibcode=2019FoPh...49.1404P|s2cid=119473777 }}</ref>}}}}
| {{no}}
| {{no}}
|-
| [[Quantum Bayesianism|QBism]]
|2010
| Christopher Fuchs, Rüdiger Schack
| {{no}}
| {{no}}{{efn|A wavefunction merely encodes an agent’s expectations for future experiences. It is no more real than a probability distribution is in [[Bayesian probability#Dutch book approach|subjective Bayesianism]].}}
| {{okay|Agnostic{{efn|Quantum theory is a tool any agent may use to help manage their expectations. The past comes into play only insofar as an agent’s individual experiences and temperament influence their priors.}}}}
| {{no}}
| {{yes}}{{efn|Although QBism would eschew this terminology. A change in the wavefunction that an agent ascribes to a system as a result of having an experience represents a change in his or her beliefs about further experiences they may have. See [[Doxastic logic]].}}
| {{yes|Intrinsic{{efn|Observers, or more properly, participants, are as essential to the formalism as the systems they interact with.}}}}
| {{yes}}
| {{no}}
| {{no}}
|}
{{notelist}}

== The silent approach ==

Although interpretational opinions are openly and widely discussed today, that was not always the case. A notable exponent of a tendency of silence was [[Paul Dirac]] who once wrote: "The interpretation of quantum mechanics has been dealt with by many authors, and I do not want to discuss it here. I want to deal with more fundamental things."<ref>P. A. M. Dirac, The inadequacies of quantum field theory, in Paul Adrien Maurice Dirac, B. N. Kursunoglu and E. P. Wigner, Eds. (Cambridge University, Cambridge, 1987) p. 194</ref> This position is not uncommon among practitioners of quantum mechanics.<ref name="QOE">{{cite book |author=Duarte |first=F. J. |title=Quantum Optics for Engineers |publisher=CRC |year=2014 |isbn=978-1439888537 |location=New York}}</ref> 
Similarly [[Richard Feynman]] wrote many popularizations of quantum mechanics without ever publishing about interpretation issues like quantum measurement.<ref>{{Cite journal |last=Zeh |first=H. D. |date=July 2011 |title=Feynman's interpretation of quantum theory |url=http://link.springer.com/10.1140/epjh/e2011-10035-2 |journal=The European Physical Journal H |language=en |volume=36 |issue=1 |pages=63–74 |doi=10.1140/epjh/e2011-10035-2 |issn=2102-6459|arxiv=0804.3348 }}</ref> 
Others, like [[Nico van Kampen]] and [[Willis Lamb]], have openly criticized non-orthodox interpretations of quantum mechanics.<ref>van Kampen, N. G. (2008). "The scandal of quantum mechanics". American Journal of Physics 76: 989.</ref><ref>Lamb, W. E. (2001). "Super classical quantum mechanics: the best interpretation of nonrelativistic quantum mechanics". American Journal of Physics. 69: 413–421.</ref>

== See also ==
{{cols|colwidth=26em}}
* [[Bohr–Einstein debates]]
* [[Einstein's thought experiments]]
* [[Glossary of quantum philosophy]]
* [[Local hidden-variable theory]]
* [[List of unsolved problems in physics#Foundations of physics]]
* [[Philosophical interpretation of classical physics]]
* [[Popper's experiment]]
* [[Superdeterminism]]
* [[Quantum foundations]]
{{colend}}

== References ==
{{reflist}}

== Sources ==
* {{cite journal | last1 = Bub | first1 = J. | last2 = Clifton | first2 = R. | year = 1996 | title = A uniqueness theorem for interpretations of quantum mechanics | journal = Studies in History and Philosophy of Modern Physics | volume = 27B | pages = 181–219 | doi = 10.1016/1355-2198(95)00019-4 }}
* [[Rudolf Carnap]], 1939, "The interpretation of physics", in ''Foundations of Logic and Mathematics'' of the ''[[International Encyclopedia of Unified Science]]''. Chicago, Illinois: University of Chicago Press.
* Dickson, M., 1994, "Wavefunction tails in the modal interpretation" in Hull, D., Forbes, M., and Burian, R., eds., ''Proceedings of the PSA'' 1" 366–376. East Lansing, Michigan: Philosophy of Science Association.
* --------, and Clifton, R., 1998, "Lorentz-invariance in modal interpretations" in Dieks, D. and Vermaas, P., eds., ''The Modal Interpretation of Quantum Mechanics''. Dordrecht: Kluwer Academic Publishers: 9–48.
* Fuchs, Christopher, 2002, "Quantum Mechanics as Quantum Information (and only a little more)". {{arxiv|quant-ph/0205039}}
* --------, and A. Peres, 2000, "Quantum theory needs no 'interpretation{{'"}}, ''Physics Today''.
* Herbert, N., 1985. ''Quantum Reality: Beyond the New Physics''. New York: Doubleday. {{ISBN|0-385-23569-0}}.
* Hey, Anthony, and Walters, P., 2003. ''The New Quantum Universe'', 2nd ed. Cambridge University Press. {{ISBN|0-521-56457-3}}.
* {{cite journal | last1 = Jackiw | first1 = Roman | author-link = Roman Jackiw | last2 = Kleppner | first2 = D. | year = 2000 | title = One Hundred Years of Quantum Physics | doi = 10.1126/science.289.5481.893 | pmid = 17839156 | journal = [[Science (journal)|Science]] | volume = 289 | issue = 5481| pages = 893–898 | arxiv = quant-ph/0008092 | bibcode = 2000quant.ph..8092K | s2cid = 6604344 }}
* [[Max Jammer]], 1966. ''The Conceptual Development of Quantum Mechanics''. McGraw-Hill.
* --------, 1974. ''The Philosophy of Quantum Mechanics''. Wiley & Sons.
* Al-Khalili, 2003. ''Quantum: A Guide for the Perplexed''. London: Weidenfeld & Nicolson.
* de Muynck, W. M., 2002. ''Foundations of quantum mechanics, an empiricist approach''. Dordrecht: Kluwer Academic Publishers. {{ISBN|1-4020-0932-1}}.
* [[Roland Omnès]], 1999. ''Understanding Quantum Mechanics''. Princeton, New Jersey: Princeton University Press.
* [[Karl Popper]], 1963. ''Conjectures and Refutations''. London: Routledge and Kegan Paul. The chapter "Three views Concerning Human Knowledge" addresses, among other things, instrumentalism in the physical sciences.
* [[Hans Reichenbach]], 1944. ''Philosophic Foundations of Quantum Mechanics''. University of California Press.
* {{cite journal | last1 = Tegmark | first1 = Max | author-link = Max Tegmark | last2 = Wheeler | first2 = J. A. | year = 2001 | title = 100 Years of Quantum Mysteries | journal = [[Scientific American]] | volume = 284 | issue = 2| pages = 68–75 | doi=10.1038/scientificamerican0201-68| bibcode = 2001SciAm.284b..68T | s2cid = 119375538 }}
* [[Bas van Fraassen]], 1972, "A formal approach to the philosophy of science", in R. Colodny, ed., ''Paradigms and Paradoxes: The Philosophical Challenge of the Quantum Domain''. Univ. of Pittsburgh Press: 303–366.
* [[John A. Wheeler]] and [[Wojciech Hubert Zurek]] (eds), ''Quantum Theory and Measurement'', Princeton, New Jersey: Princeton University Press, {{ISBN|0-691-08316-9}}, LoC QC174.125.Q38 1983.

== Further reading ==
Almost all authors below are professional physicists.
* [[David Z Albert]], 1992. ''Quantum Mechanics and Experience''. Cambridge, Massachusetts: Harvard University Press. {{ISBN|0-674-74112-9}}.
* [[John S. Bell]], 1987. ''Speakable and Unspeakable in Quantum Mechanics''. Cambridge University Press, {{ISBN|0-521-36869-3}}. The 2004 edition ({{ISBN|0-521-52338-9}}) includes two additional papers and an introduction by [[Alain Aspect]].
* [[Dmitry Blokhintsev|Dmitrii Ivanovich Blokhintsev]], 1968. ''The Philosophy of Quantum Mechanics''. D. Reidel Publishing Company. {{ISBN|90-277-0105-9}}.
* [[David Bohm]], 1980. ''Wholeness and the Implicate Order''. London: Routledge. {{ISBN|0-7100-0971-2}}.
* {{cite arXiv |author=Adan Cabello |date=15 November 2004 |title=Bibliographic guide to the foundations of quantum mechanics and quantum information |eprint=quant-ph/0012089}}
* [[David Deutsch]], 1997. ''[[The Fabric of Reality]]''. London: Allen Lane. {{ISBN|0-14-027541-X}}; {{ISBN|0-7139-9061-9}}. Argues forcefully ''against'' instrumentalism. For general readers.
* {{cite book |title=  Quantum Optics for Engineers | publisher= CRC |location=New York |year=2014 |isbn=978-1439888537 | author = F. J. Duarte |author-link= F. J. Duarte }} Provides a ''pragmatic'' perspective on interpretations. For general readers.
* [[Bernard d'Espagnat]], 1976. ''Conceptual Foundation of Quantum Mechanics'', 2nd ed. Addison Wesley. {{ISBN|0-8133-4087-X}}.
* [[Bernard d'Espagnat]], 1983. ''In Search of Reality''. Springer. {{ISBN|0-387-11399-1}}.
* [[Bernard d'Espagnat]], 2003. ''Veiled Reality: An Analysis of Quantum Mechanical Concepts''. Westview Press.
* [[Bernard d'Espagnat]], 2006. ''On Physics and Philosophy''. Princetone, New Jersey: Princeton University Press.
* [[Arthur Fine]], 1986. ''The Shaky Game: Einstein Realism and the Quantum Theory. Science and its Conceptual Foundations''. Chicago, Illinois: University of Chicago Press. {{ISBN|0-226-24948-4}}.
* Ghirardi, Giancarlo, 2004. ''Sneaking a Look at God's Cards''. Princeton, New Jersey: Princeton University Press.
* [[Gregg Jaeger]] (2009) [https://www.springer.com/physics/quantum+physics/book/978-3-540-92127-1 ''Entanglement, Information, and the Interpretation of Quantum Mechanics''.] Springer. {{ISBN|978-3-540-92127-1}}.
* [[N. David Mermin]] (1990) ''[http://www.cambridge.org/catalogue/catalogue.asp?isbn=0521388805 Boojums all the way through.]'' Cambridge University Press. {{ISBN|0-521-38880-5}}.
* [[Roland Omnès]], 1994. ''The Interpretation of Quantum Mechanics''. Princeton, New Jersey: Princeton University Press. {{ISBN|0-691-03669-1}}.
* [[Roland Omnès]], 1999. ''Understanding Quantum Mechanics''. Princeton, New Jersey: Princeton University Press.
* [[Roland Omnès]], 1999. ''[[Quantum Philosophy]]: Understanding and Interpreting Contemporary Science''. Princeton, New Jersey: Princeton University Press.
* [[James L. Park]], 1967-1992.<ref>{{Cite web |title=James L. Park Main Scientific Publications |url=https://quantum-thermodynamics.unibs.it/james_l_park_publications.htm }}</ref>
* [[Roger Penrose]], 1989. ''[[The Emperor's New Mind]]''. Oxford University Press. {{ISBN|0-19-851973-7}}. Especially chapter 6.
* [[Roger Penrose]], 1994. ''[[Shadows of the Mind]]''. Oxford University Press. {{ISBN|0-19-853978-9}}.
* [[Roger Penrose]], 2004. ''[[The Road to Reality]]''. New York: Alfred A. Knopf. Argues that quantum theory is incomplete.
* Lee Phillips, 2017. ''[https://arstechnica.com/science/2017/07/a-brief-history-of-quantum-alternatives/3/ A brief history of quantum alternatives]''. Ars Technica. 
* {{cite journal
 |last1=Styer |first1=Daniel F. |author-link=Daniel F. Styer
 |date=March 2002
 |title=Nine formulations of quantum mechanics
 |journal=[[American Journal of Physics]]
 |volume=70 |issue= 3|pages=288–297
 |bibcode= 2002AmJPh..70..288S
 |doi=10.1119/1.1445404
 |last2=Balkin |first2=Miranda S.
 |last3=Becker |first3=Kathryn M.
 |last4=Burns |first4=Matthew R.
 |last5=Dudley |first5=Christopher E.
 |last6=Forth |first6=Scott T.
 |last7=Gaumer |first7=Jeremy S.
 |last8=Kramer |first8=Mark A.
 |last9=Oertel |first9=David C.
 |last10=Park |first10=Leonard H.
 |last11=Rinkoski |first11=Marie T.
 |last12=Smith |first12=Clait T.
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* {{cite journal |last1=Baggott |first1=Jim |title='Shut up and calculate': how Einstein lost the battle to explain quantum reality |journal=Nature |date=25 April 2024 |volume=629 |issue=8010 |pages=29–32 |doi=10.1038/d41586-024-01216-z |language=en|doi-access=free |pmid=38664517 |bibcode=2024Natur.629...29B }}

== External links ==
{{Wikiquote}}
{{Wikiversity|Making sense of quantum mechanics}}
* ''[[Stanford Encyclopedia of Philosophy]]'':
** "[http://plato.stanford.edu/entries/qm-bohm/ Bohmian mechanics]" by Sheldon Goldstein.
** "[http://plato.stanford.edu/entries/qm-collapse/ Collapse Theories.]" by Giancarlo Ghirardi.
** "[http://plato.stanford.edu/entries/qm-copenhagen/ Copenhagen Interpretation of Quantum Mechanics]" by [[Jan Faye]].
** "[http://plato.stanford.edu/entries/qm-everett/ Everett's Relative State Formulation of Quantum Mechanics]" by Jeffrey Barrett.
** "[http://plato.stanford.edu/entries/qm-manyworlds/ Many-Worlds Interpretation of Quantum Mechanics]" by [[Lev Vaidman]].
** "[http://plato.stanford.edu/entries/qm-modal/ Modal Interpretation of Quantum Mechanics]" by Michael Dickson and [[Dennis Dieks]].
** "[https://plato.stanford.edu/entries/qt-issues/ Philosophical Issues in Quantum Theory]" by Wayne Myrvold.
** "[https://plato.stanford.edu/entries/quantum-bayesian/ Quantum-Bayesian and Pragmatist Views of Quantum Theory]" by Richard Healey.
** "[http://plato.stanford.edu/entries/qt-entangle/ Quantum Entanglement and Information]" by [[Jeffrey Bub]].
** "[http://plato.stanford.edu/entries/qm/ Quantum mechanics]" by Jenann Ismael.
** "[https://plato.stanford.edu/entries/qt-quantlog/ Quantum Logic and Probability Theory]" by Alexander Wilce.
** "[http://plato.stanford.edu/entries/qm-relational/ Relational Quantum Mechanics]" by Federico Laudisa and [[Carlo Rovelli]].
** "[http://plato.stanford.edu/entries/qm-decoherence/ The Role of Decoherence in Quantum Mechanics]" by Guido Bacciagaluppi.
* ''[[Internet Encyclopedia of Philosophy]]'':
** "[http://www.iep.utm.edu/int-qm/ Interpretations of Quantum Mechanics]" by Peter J. Lewis.
** "[http://www.iep.utm.edu/everett/ Everettian Interpretations of Quantum Mechanics]" by Christina Conroy.

{{Quantum mechanics topics}}

[[Category:Interpretations of quantum mechanics| ]]
[[Category:Quantum measurement| ]]
[[Category:Epistemology of science]]
[[Category:Philosophy of physics]]
[[Category:Philosophical debates]]
[[Category:Reality]]