Editing Quantum mechanics (section)

== Philosophical implications ==
{{Main|Interpretations of quantum mechanics}}

{{unsolved|physics|Is there a preferred interpretation of quantum mechanics? How does the quantum description of reality, which includes elements such as the "[[superposition principle|superposition]] of states" and "[[wave function collapse]]", give rise to the reality we perceive?}}
Since its inception, the many counter-intuitive aspects and results of quantum mechanics have provoked strong [[philosophical]] debates and many [[interpretations of quantum mechanics|interpretations]]. The arguments centre on the probabilistic nature of quantum mechanics, the difficulties with [[wavefunction collapse]] and the related [[measurement problem]], and [[quantum nonlocality]]. Perhaps the only consensus that exists about these issues is that there is no consensus. [[Richard Feynman]] once said, "I think I can safely say that nobody understands quantum mechanics."<ref>{{Cite book |last=Feynman |first=Richard |title=The Character of Physical Law |title-link=The Character of Physical Law |publisher=MIT Press |year=1967 |isbn=0-262-56003-8 |pages=129 |author-link=Richard Feynman}}</ref> According to [[Steven Weinberg]], "There is now in my opinion no entirely satisfactory interpretation of quantum mechanics."<ref>{{Cite journal |arxiv=1109.6462 |doi=10.1103/PhysRevA.85.062116 |title=Collapse of the state vector |journal=Physical Review A |volume=85 |issue=6 |pages=062116 |year=2012 |last1=Weinberg |first1=Steven |bibcode=2012PhRvA..85f2116W |s2cid=119273840}}</ref>

The views of [[Niels Bohr]], Werner Heisenberg and other physicists are often grouped together as the "[[Copenhagen interpretation]]".<ref>{{Cite journal |last=Howard |first=Don |date=December 2004 |title=Who Invented the 'Copenhagen Interpretation'? A Study in Mythology |url=https://www.journals.uchicago.edu/doi/10.1086/425941 |journal=Philosophy of Science |volume=71 |issue=5 |pages=669–682 |doi=10.1086/425941 |s2cid=9454552 |issn=0031-8248}}</ref><ref>{{Cite journal |last=Camilleri |first=Kristian |date=May 2009 |title=Constructing the Myth of the Copenhagen Interpretation |url=http://www.mitpressjournals.org/doi/10.1162/posc.2009.17.1.26 |journal=Perspectives on Science |volume=17 |issue=1 |pages=26–57 |doi=10.1162/posc.2009.17.1.26 |s2cid=57559199 |issn=1063-6145}}</ref> According to these views, the probabilistic nature of quantum mechanics is not a <em>temporary</em> feature which will eventually be replaced by a deterministic theory, but is instead a <em>final</em> renunciation of the classical idea of "causality". Bohr in particular emphasized that any well-defined application of the quantum mechanical formalism must always make reference to the experimental arrangement, due to the [[complementarity (physics)|complementary]] nature of evidence obtained under different experimental situations. Copenhagen-type interpretations were adopted by Nobel laureates in quantum physics, including Bohr,<ref name="BohrComo">{{Cite journal |last1=Bohr |first1=Neils |author-link=Niels Bohr |year=1928 |title=The Quantum Postulate and the Recent Development of Atomic Theory |journal=Nature |volume=121 |issue=3050 |pages=580–590 |bibcode=1928Natur.121..580B |doi=10.1038/121580a0 |doi-access=free}}</ref> Heisenberg,<ref>{{Cite book |last=Heisenberg |first=Werner |author-link=Werner Heisenberg |title=Physics and philosophy: the revolution in modern science |date=1971 |publisher=Allen & Unwin |isbn=978-0-04-530016-7 |edition=3 |series=World perspectives |location=London |oclc=743037461}}</ref> Schrödinger,<ref>{{Cite journal |last=Schrödinger |first=Erwin |year=1980 |orig-date=1935 |editor-last=Trimmer |editor-first=John |title=Die gegenwärtige Situation in der Quantenmechanik |trans-title=The Present Situation in Quantum Mechanics |journal=Naturwissenschaften |volume=23 |issue=50 |pages=844–849 |doi=10.1007/BF01491987 |jstor=986572 |s2cid=22433857 |language=de}}</ref> Feynman,<ref name="Feynman" /> and [[Anton Zeilinger|Zeilinger]]<ref name=MaKoflerZeilinger>{{Cite journal |last1=Ma |first1=Xiao-song |last2=Kofler |first2=Johannes |last3=Zeilinger |first3=Anton |date=2016-03-03 |title=Delayed-choice gedanken experiments and their realizations |journal=Reviews of Modern Physics |volume=88 |issue=1 |page=015005 |doi=10.1103/RevModPhys.88.015005 |issn=0034-6861 |arxiv=1407.2930 |bibcode=2016RvMP...88a5005M |s2cid=34901303}}</ref> as well as 21st-century researchers in quantum foundations.<ref name=":25">{{Cite journal |last1=Schlosshauer |first1=Maximilian |last2=Kofler |first2=Johannes |last3=Zeilinger |first3=Anton |date=1 August 2013 |title=A snapshot of foundational attitudes toward quantum mechanics |journal=Studies in History and Philosophy of Science Part B |volume=44 |issue=3 |pages=222–230 |arxiv=1301.1069 |bibcode=2013SHPMP..44..222S |doi=10.1016/j.shpsb.2013.04.004 |s2cid=55537196}}</ref>

[[Albert Einstein]], himself one of the founders of [[Old quantum theory|quantum theory]], was troubled by its apparent failure to respect some cherished metaphysical principles, such as [[determinism]] and [[principle of locality|locality]]. Einstein's long-running exchanges with Bohr about the meaning and status of quantum mechanics are now known as the [[Bohr–Einstein debates]]. Einstein believed that underlying quantum mechanics must be a theory that explicitly forbids [[action at a distance]]. He argued that quantum mechanics was incomplete, a theory that was valid but not fundamental, analogous to how [[thermodynamics]] is valid, but the fundamental theory behind it is [[statistical mechanics]]. In 1935, Einstein and his collaborators [[Boris Podolsky]] and [[Nathan Rosen]] published an argument that the principle of locality implies the incompleteness of quantum mechanics, a [[thought experiment]] later termed the [[Einstein–Podolsky–Rosen paradox]].{{refn|group=note|The published form of the EPR argument was due to Podolsky, and Einstein himself was not satisfied with it. In his own publications and correspondence, Einstein used a different argument to insist that quantum mechanics is an incomplete theory.<ref name="spekkens">{{cite journal |author2-link=Robert Spekkens |first1=Nicholas |last1=Harrigan |first2=Robert W. |last2=Spekkens |title=Einstein, incompleteness, and the epistemic view of quantum states |journal=[[Foundations of Physics]] |volume=40 |issue=2 |pages=125 |year=2010 |doi=10.1007/s10701-009-9347-0 |arxiv=0706.2661 |bibcode=2010FoPh...40..125H |s2cid=32755624}}</ref><ref name="howard">{{cite journal |last1=Howard |first1=D. |title=Einstein on locality and separability |journal=Studies in History and Philosophy of Science Part A |date=1985 |volume=16 |issue=3 |pages=171–201 |doi=10.1016/0039-3681(85)90001-9 |bibcode=1985SHPSA..16..171H}}</ref><ref>{{Cite journal |last=Sauer |first=Tilman |date=1 December 2007 |title=An Einstein manuscript on the EPR paradox for spin observables |url=http://philsci-archive.pitt.edu/3222/ |journal=Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics |volume=38 |issue=4 |pages=879–887 |doi=10.1016/j.shpsb.2007.03.002 |issn=1355-2198 |bibcode=2007SHPMP..38..879S |citeseerx=10.1.1.571.6089}}</ref><ref>{{cite encyclopedia |last=Einstein |first=Albert |title=Autobiographical Notes |encyclopedia=Albert Einstein: Philosopher-Scientist |year=1949 |publisher=Open Court Publishing Company |editor-last=Schilpp |editor-first=Paul Arthur}}</ref>}} In 1964, [[John Stewart Bell|John Bell]] showed that EPR's principle of locality, together with determinism, was actually incompatible with quantum mechanics: they implied constraints on the correlations produced by distance systems, now known as [[Bell inequalities]], that can be violated by entangled particles.<ref>{{Cite journal |last=Bell |first=John Stewart |author-link=John Stewart Bell |date=1 November 1964 |title=On the Einstein Podolsky Rosen paradox |journal=[[Physics Physique Fizika]] |volume=1 |issue=3 |pages=195–200 |doi=10.1103/PhysicsPhysiqueFizika.1.195 |doi-access=free}}</ref> Since then [[Bell test|several experiments]] have been performed to obtain these correlations, with the result that they do in fact violate Bell inequalities, and thus falsify the conjunction of locality with determinism.<ref name="wiseman15" /><ref name="wolchover17" />

[[Bohmian mechanics]] shows that it is possible to reformulate quantum mechanics to make it deterministic, at the price of making it explicitly nonlocal. It attributes not only a wave function to a physical system, but in addition a real position, that evolves deterministically under a nonlocal guiding equation. The evolution of a physical system is given at all times by the Schrödinger equation together with the guiding equation; there is never a collapse of the wave function. This solves the measurement problem.<ref>{{cite encyclopedia |url=https://plato.stanford.edu/entries/qm-bohm/ |last=Goldstein |first=Sheldon |title=Bohmian Mechanics |encyclopedia=Stanford Encyclopedia of Philosophy |year=2017 |publisher=Metaphysics Research Lab, Stanford University}}</ref>
[[File:Schroedingers cat film.svg|thumb|upright=1|The [[Schrödinger's cat]] thought experiment can be used to visualize the many-worlds interpretation of quantum mechanics, where a branching of the universe occurs through a superposition of two quantum mechanical states.]]
Everett's [[many-worlds interpretation]], formulated in 1956, holds that <em>all</em> the possibilities described by quantum theory <em>simultaneously</em> occur in a multiverse composed of mostly independent parallel universes.<ref>{{Cite encyclopedia |first=Jeffrey |last=Barrett |encyclopedia=Stanford Encyclopedia of Philosophy |publisher=Metaphysics Research Lab, Stanford University |year=2018 |editor-last=Zalta |editor-first=Edward N. |title=Everett's Relative-State Formulation of Quantum Mechanics |url=https://plato.stanford.edu/entries/qm-everett/}}</ref> This is a consequence of removing the axiom of the collapse of the wave packet. All possible states of the measured system and the measuring apparatus, together with the observer, are present in a real physical quantum superposition. While the multiverse is deterministic, we perceive non-deterministic behavior governed by probabilities, because we do not observe the multiverse as a whole, but only one parallel universe at a time. Exactly how this is supposed to work has been the subject of much debate. Several attempts have been made to make sense of this and derive the Born rule,<ref name=dewitt73>{{cite book |editor-last1=DeWitt |editor-first1=Bryce |editor-link1=Bryce DeWitt |editor-last2=Graham |editor-first2=R. Neill |last1=Everett |first1=Hugh |author-link1=Hugh Everett III |last2=Wheeler |first2=J. A. |author-link2=John Archibald Wheeler |last3=DeWitt |first3=B. S. |author-link3=Bryce DeWitt |last4=Cooper |first4=L. N. |author-link4=Leon Cooper |last5=Van Vechten |first5=D. |last6=Graham |first6=N. |title=The Many-Worlds Interpretation of Quantum Mechanics |series=Princeton Series in Physics |publisher=[[Princeton University Press]] |location=Princeton, NJ |year=1973 |isbn=0-691-08131-X |page=v}}</ref><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 |arxiv=quant-ph/0303050 |bibcode=2003SHPMP..34..415W |doi=10.1016/S1355-2198(03)00036-4 |s2cid=1921913}}</ref> with no consensus on whether they have been successful.<ref name="ballentine1973">{{cite journal |first1=L. E. |last1=Ballentine |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 |doi=10.1007/BF00708440 |bibcode=1973FoPh....3..229B |s2cid=121747282}}</ref><ref>{{cite book |first=N. P. |last=Landsman |chapter=The Born rule and its interpretation |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. |title=Compendium of Quantum Physics |editor-first1=F. |editor-last1=Weinert |editor-first2=K. |editor-last2=Hentschel |editor-first3=D. |editor-last3=Greenberger |editor-first4=B. |editor-last4=Falkenburg |publisher=Springer |year=2008 |isbn=978-3-540-70622-9}}</ref><ref name="kent2009">{{Cite book |last1=Kent |first1=Adrian |author-link=Adrian Kent |title=Many Worlds? Everett, Quantum Theory and Reality |publisher=Oxford University Press |year=2010 |editor=S. Saunders |chapter=One world versus many: The inadequacy of Everettian accounts of evolution, probability, and scientific confirmation |arxiv=0905.0624 |bibcode=2009arXiv0905.0624K |editor2=J. Barrett |editor3=A. Kent |editor4=D. Wallace}}</ref>

[[Relational quantum mechanics]] appeared in the late 1990s as a modern derivative of Copenhagen-type ideas,<ref>{{Cite journal |last=Van Fraassen |first=Bas C. |author-link=Bas van Fraassen |date=April 2010 |title=Rovelli's World |url=http://link.springer.com/10.1007/s10701-009-9326-5 |journal=[[Foundations of Physics]] |volume=40 |issue=4 |pages=390–417 |doi=10.1007/s10701-009-9326-5 |bibcode=2010FoPh...40..390V |s2cid=17217776 |issn=0015-9018}}</ref><ref>{{cite journal|first1=Claudio |last1=Calosi |first2=Timotheus |last2=Riedel |title=Relational Quantum Mechanics at the Crossroads |journal=Foundations of Physics |volume=54 |page=74 |year=2024 |issue=6 |doi=10.1007/s10701-024-00810-5|doi-access=free |bibcode=2024FoPh...54...74C }}</ref> and [[QBism]] was developed some years later.<ref name=":23">{{Cite encyclopedia |last=Healey |first=Richard |encyclopedia=[[Stanford Encyclopedia of Philosophy]] |publisher=Metaphysics Research Lab, Stanford University |year=2016 |editor-last=Zalta |editor-first=Edward N. |title=Quantum-Bayesian and Pragmatist Views of Quantum Theory |url=https://plato.stanford.edu/entries/quantum-bayesian/}}</ref><ref>{{cite book|first=Blake C. |last=Stacey |chapter=Toward a World Game-Flavored as a Hawk's Wing |title=Phenomenology and QBism: New Approaches to Quantum Mechanics |pages=49–77 |editor-first1=Philipp |editor-last1=Berghofer |editor-first2=Harald A. |editor-last2=Wiltsche |year=2023 |publisher=Routledge |isbn=9781032191812 |doi=10.4324/9781003259008-3}}</ref>