Editing Philosophy of physics (section)

==Philosophy of quantum mechanics==
{{Main|Quantum foundations}}
[[Quantum mechanics]] is a large focus of contemporary philosophy of physics, specifically concerning the correct interpretation of quantum mechanics. Very broadly, much of the philosophical work that is done in quantum theory is trying to make sense of superposition states:<ref>{{cite web|url=https://www.youtube.com/watch?v=J8k_2oD66mI&t=128| archive-url=https://ghostarchive.org/varchive/youtube/20211211/J8k_2oD66mI| archive-date=2021-12-11 | url-status=live|title=Eleanor Knox (KCL) – The Curious Case of the Vanishing Spacetime|last=BristolPhilosophy|date=19 February 2013|access-date=7 April 2018|via=YouTube}}{{cbignore}}</ref> the property that particles seem to not just be in one determinate position at one time, but are somewhere 'here', and also 'there' at the same time. Such a radical view turns many common sense metaphysical ideas on their head. Much of contemporary philosophy of quantum mechanics aims to make sense of what the very empirically successful formalism of quantum mechanics tells us about the physical world.

===Uncertainty principle===
{{Main|Uncertainty principle}}
The [[uncertainty principle]] is a mathematical relation asserting an upper limit to the accuracy of the simultaneous measurement of any pair of [[conjugate variables]], e.g. position and momentum. In the formalism of [[Operator (physics)#Operators in quantum mechanics|operator notation]], this limit is the evaluation of the [[Canonical commutation relation|commutator]] of the variables' corresponding operators.

The uncertainty principle arose as an answer to the question: How does one measure the location of an electron around a nucleus if an electron is a wave?  When quantum mechanics was developed, it was seen to be a relation between the classical and quantum descriptions of a system using wave mechanics.

==="Locality" and hidden variables===
{{main|EPR paradox|Bell's theorem}}
[[Bell's theorem]] is a term encompassing a number of closely related results in physics, all of which determine that [[quantum mechanics]] is incompatible with [[Local hidden-variable theory|local hidden-variable theories]] given some basic assumptions about the nature of measurement. "Local" here refers to the [[principle of locality]], the idea that a particle can only be influenced by its immediate surroundings, and that interactions mediated by [[Field (physics)|physical fields]] cannot propagate faster than the [[speed of light]]. "[[Hidden-variable theory|Hidden variables]]" are putative properties of quantum particles that are not included in the theory but nevertheless affect the outcome of experiments. In the words of physicist [[John Stewart Bell]], for whom this family of results is named, "If [a hidden-variable theory] is local it will not agree with quantum mechanics, and if it agrees with quantum mechanics it will not be local."<ref>{{cite book  | first = John S. | last = Bell | author-link = John Stewart Bell | title = Speakable and Unspeakable in Quantum Mechanics | publisher = Cambridge University Press | date = 1987  | page = 65 | isbn = 9780521368698 | oclc = 15053677}}</ref>

The term is broadly applied to a number of different derivations, the first of which was introduced by Bell in a 1964 paper titled "On the [[EPR paradox|Einstein Podolsky Rosen Paradox]]". Bell's paper was a response to a 1935 [[thought experiment]] that [[Albert Einstein]], [[Boris Podolsky]] and [[Nathan Rosen]] proposed, arguing that quantum physics is an "incomplete" theory.<ref name="EPR">{{cite journal | title = Can Quantum-Mechanical Description of Physical Reality be Considered Complete? | date = 1935-05-15 | first1 = A. | last1 = Einstein |first2=B. |last2 = Podolsky |first3=N. |last3 = Rosen | author-link1 = Albert Einstein | author-link2 = Boris Podolsky | author-link3 = Nathan Rosen | journal = [[Physical Review]] | volume = 47 | issue = 10 | pages = 777–780 | bibcode = 1935PhRv...47..777E |doi = 10.1103/PhysRev.47.777 | doi-access = free }}</ref><ref name=Bell1964>{{cite journal | last1 = Bell | first1 = J. S. | author-link = John Stewart Bell | year = 1964 | title = On the Einstein Podolsky Rosen Paradox | url = https://cds.cern.ch/record/111654/files/vol1p195-200_001.pdf | journal = [[Physics Physique Физика]] | volume = 1 | issue = 3| pages = 195–200 | doi = 10.1103/PhysicsPhysiqueFizika.1.195 }}</ref> By 1935, it was already recognized that the predictions of quantum physics are [[probability|probabilistic]]. Einstein, Podolsky and Rosen presented a scenario that involves preparing a pair of particles such that the quantum state of the pair is [[quantum entanglement|entangled]], and then separating the particles to an arbitrarily large distance. The experimenter has a choice of possible measurements that can be performed on one of the particles. When they choose a measurement and obtain a result, the quantum state of the other particle apparently [[Wave function collapse|collapses]] instantaneously into a new state depending upon that result, no matter how far away the other particle is.  This suggests that either the measurement of the first particle somehow also influenced the second particle faster than the speed of light, ''or'' that the entangled particles had some unmeasured property which pre-determined their final quantum states before they were separated.  Therefore, assuming locality, quantum mechanics must be incomplete, as it cannot give a complete description of the particle's true physical characteristics. In other words, quantum particles, like [[electron]]s and [[photon]]s, must carry some property or attributes not included in quantum theory, and the uncertainties in quantum theory's predictions would then be due to ignorance or unknowability of these properties, later termed "hidden variables".

Bell carried the analysis of quantum entanglement much further. He deduced that if measurements are performed independently on the two separated particles of an entangled pair, then the assumption that the outcomes depend upon hidden variables within each half implies a mathematical constraint on how the outcomes on the two measurements are correlated. This constraint would later be named the ''Bell inequality''. Bell then showed that quantum physics predicts correlations that violate this inequality. Consequently, the only way that hidden variables could explain the predictions of quantum physics is if they are "nonlocal", which is to say that somehow the two particles can carry non-classical correlations no matter how widely they ever become separated.<ref name="C.B. Parker 1994 542">{{cite book | first = Sybil B. | last = Parker | title = McGraw-Hill Encyclopaedia of Physics | edition = 2nd | page = [https://archive.org/details/mcgrawhillencycl1993park/page/542 542] | date = 1994 | publisher = McGraw-Hill | isbn = 978-0-07-051400-3 | url = https://archive.org/details/mcgrawhillencycl1993park| url-access = registration }}</ref><ref name = "ND Mermin 1993-07">{{cite journal | last = Mermin |first = N. David |author-link=N. David Mermin |title = Hidden Variables and the Two Theorems of John Bell | journal = [[Reviews of Modern Physics]] | volume = 65 |pages = 803–15 | number = 3| date = July 1993  | url = http://cqi.inf.usi.ch/qic/Mermin1993.pdf |arxiv=1802.10119|doi = 10.1103/RevModPhys.65.803 |bibcode = 1993RvMP...65..803M |s2cid = 119546199 }}</ref>

Multiple variations on Bell's theorem were put forward in the following years, introducing other closely related conditions generally known as Bell (or "Bell-type") inequalities. The first rudimentary experiment designed to test Bell's theorem was performed in 1972 by [[John Clauser]] and [[Stuart Freedman]].<ref>{{cite press release |url=https://www.nobelprize.org/prizes/physics/2022/press-release/ |title=The Nobel Prize in Physics 2022 |date=October 4, 2022 |work=[[Nobel Prize]] |publisher=[[The Royal Swedish Academy of Sciences]] |access-date=6 October 2022}}</ref> More advanced experiments, known collectively as [[Bell test]]s, have been performed many times since. To date, Bell tests have consistently found that physical systems obey quantum mechanics and violate Bell inequalities; which is to say that the results of these experiments are incompatible with any local hidden variable theory.<ref name="NAT-20180509">{{cite journal |author=The BIG Bell Test Collaboration |title=Challenging local realism with human choices |date=9 May 2018 |journal=[[Nature (journal)|Nature]] |volume=557 |issue=7704 |pages=212–216 |doi=10.1038/s41586-018-0085-3 |pmid=29743691 |bibcode=2018Natur.557..212B |arxiv=1805.04431 |s2cid=13665914 }}</ref><ref>{{Cite web|url=https://www.quantamagazine.org/physicists-are-closing-the-bell-test-loophole-20170207/|title=Experiment Reaffirms Quantum Weirdness|last=Wolchover|first=Natalie|author-link=Natalie Wolchover|date=2017-02-07|work=[[Quanta Magazine]]|language=en-US|access-date=2020-02-08}}</ref>

The exact nature of the assumptions required to prove a Bell-type constraint on correlations has been debated by physicists and by philosophers. While the significance of Bell's theorem is not in doubt, its full implications for the [[interpretation of quantum mechanics]] remain unresolved.

===Interpretations of quantum mechanics===
{{Main|Interpretation of quantum mechanics}}
In March 1927, working in [[Niels Bohr]]'s institute, [[Werner Heisenberg]] formulated the principle of uncertainty thereby laying the foundation of what became known as the [[Copenhagen interpretation]] of quantum mechanics. Heisenberg had been studying the papers of [[Paul Dirac]] and [[Pascual Jordan]].  He discovered a problem with measurement of basic variables in the equations. His analysis showed that uncertainties, or imprecisions, always turned up if one tried to measure the position and the momentum of a particle at the same time. Heisenberg concluded that these uncertainties or imprecisions in the measurements were not the fault of the experimenter, but fundamental in nature and are inherent mathematical properties of operators in quantum mechanics arising from definitions of these operators.<ref>Niels Bohr, ''Atomic Physics and Human Knowledge'', p. 38</ref>

The Copenhagen interpretation is somewhat loosely defined, as many physicists and philosophers of physics have advanced similar but not identical views of quantum mechanics. It is principally associated with Heisenberg and Bohr, despite their philosophical differences.<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.|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> 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 that objects have certain pairs of complementary properties that cannot all be observed or measured simultaneously.<ref>{{cite book|last=Omnès |first=Roland |author-link=Roland Omnès |chapter=The Copenhagen Interpretation |title=Understanding Quantum Mechanics |publisher=Princeton University Press |year=1999 |pages=41–54 |doi=10.2307/j.ctv173f2pm.9 |s2cid=203390914 |quote=Bohr, Heisenberg, and Pauli recognized its main difficulties and proposed a first essential answer. They often met in Copenhagen ... 'Copenhagen interpretation has not always meant the same thing to different authors. I will reserve it for the doctrine held with minor differences by Bohr, Heisenberg, and Pauli.}}</ref> Moreover, the act of "observing" or "measuring" an object is irreversible, and no truth can be attributed to an object, [[counterfactual definiteness|except according to the results of its measurement]]. Copenhagen-type interpretations hold that quantum descriptions are objective, in that they are independent of any arbitrary factors in the physicist's mind.<ref name="omnes">{{cite book|first=R. |last=Omnès |author-link=Roland Omnès |title=The Interpretation of Quantum Mechanics |publisher=Princeton University Press |year=1994 |isbn=978-0-691-03669-4 |oclc=439453957 }}</ref>{{Rp|85–90}}

The [[many-worlds interpretation of quantum mechanics]] by [[Hugh Everett III]] claims that the wave-function of a quantum system is telling us claims about the reality of that physical system. It denies wavefunction collapse, and claims that [[superposition principle|superposition]] states should be interpreted literally as describing the reality of many-worlds where objects are located, and not simply indicating the indeterminacy of those variables. This is sometimes argued as a corollary of [[scientific realism]],<ref>David Wallace, 'The Emergent Multiverse', pp. 1–10</ref> which states that scientific theories aim to give us literally true descriptions of the world.

One issue for the Everett interpretation is the role that probability plays on this account. The Everettian account is completely deterministic, whereas probability seems to play an ineliminable role in quantum mechanics.<ref>David Wallace, 'The Emergent Multiverse', pp. 113–117</ref> Contemporary Everettians have argued that one can get an account of probability that follows the Born rule through certain decision-theoretic proofs,<ref>David Wallace, 'The Emergent Multiverse', pg. 157–189</ref> but there is as yet no consensus about whether any of these proofs are successful.<ref name=kent2009>{{Cite book|arxiv=0905.0624|last1=Kent|first1=Adrian|chapter=One world versus many: The inadequacy of Everettian accounts of evolution, probability, and scientific confirmation|title=Many Worlds? Everett, Quantum Theory and Reality |editor=S. Saunders |editor2=J. Barrett |editor3=A. Kent |editor4=D. Wallace |publisher=Oxford University Press|year=2010|bibcode=2009arXiv0905.0624K}}</ref><ref>{{cite journal | last1 = Kent | first1 = Adrian | year = 1990 | title = Against Many-Worlds Interpretations | arxiv = gr-qc/9703089 | journal = International Journal of Modern Physics A | volume = 5 | issue = 9| pages = 1745–1762 |bibcode = 1990IJMPA...5.1745K |doi = 10.1142/S0217751X90000805 | s2cid = 14523184 }}</ref><ref>{{Cite book| last1=Price |first1=Huw | chapter=Decisions, Decisions, Decisions: Can Savage Salvage Everettian Probability?|title=Many Worlds? Everett, Quantum Theory and Reality |editor=S. Saunders |editor2=J. Barrett |editor3=A. Kent |editor4=D. Wallace |publisher=Oxford University Press|year=2010|arxiv = 0802.1390}}</ref>

Physicist [[Roland Omnès]] noted that it is impossible to experimentally differentiate between Everett's view, which says that as the wave-function decoheres into distinct worlds, each of which exists equally, and the more traditional view that says that a decoherent wave-function leaves only one unique real result. Hence, the dispute between the two views represents a great "chasm". "Every characteristic of reality has reappeared in its reconstruction by our theoretical model; every feature except one: the uniqueness of facts."<ref>{{cite book|last1=Omnès|first1=Roland|title=Quantum philosophy : understanding and interpreting contemporary science|date=2002|publisher=Princeton University Press|location=Princeton|isbn=978-1400822867|page=213|edition=1st paperback |others=Arturo Spangalli (transl.)|language=fr|chapter=11}}</ref>