Replace 2018 Wikipedia import with native article: Bell as the man who reopened the hidden-variables question - demolition of von Neumann's proof, what the theorem actually shows (locality, not realism), his realist conclusions and openness to a Lorentzian preferred frame; indexes the wiki's Bell/EPR papers
| known_for = Bell's theorem, Refutation of von Neumann's impossibility proof, Quantum non-locality
|residence =
|citizenship =
|nationality =
|ethnicity =
|fields =
|workplaces = [[Atomic Energy Research Establishment]]<br>[[CERN]], [[Stanford University]]
|alma_mater = [[Queen's University of Belfast]] <small>([[Bachelor of Science|B.S.]])</small><br>[[University of Birmingham]] <small>([[Doctor of Philosophy|Ph.D.]])</small>
|doctoral_advisor = [[Rudolph E. Peierls]]
|academic_advisors = [[Paul Taunton Matthews]]<ref>Andrew Whitaker, ''John Stewart Bell and Twentieth-Century Physics: Vision and Integrity'', Oxford University Press, 2016, ch. 2.</ref>
| thesis_title = i. Time reversal in field theory, ii. Some functional methods in field theory.
|awards = [[Dannie Heineman Prize for Mathematical Physics|Heineman Prize]] {{small|(1989)}}<br>[[Hughes Medal]] {{small|(1989)}}<br> [[Dirac Prize|Paul Dirac Medal and Prize]]{{small| (1988)}}
|signature = <!--(filename only)-->
|footnotes =
}}
}}
'''John Stewart Bell''' [[Fellow of the Royal Society|FRS]]<ref name="frs">{{Cite journal | last1 = Burke | first1 = P. G. | last2 = Percival | first2 = I. C. | doi = 10.1098/rsbm.1999.0001 | title = John Stewart Bell. 28 July 1928 - 1 October 1990: Elected F.R.S. 1972 | journal = [[Biographical Memoirs of Fellows of the Royal Society]] | volume = 45 | pages = 1 | year = 1999 | pmid = | pmc = }}</ref> (28 June 1928 – 1 October 1990) was a Northern Irish [[physicist]], and the originator of [[Bell's theorem]], an important theorem in [[quantum mechanics|quantum physics]] regarding [[hidden variable theory|hidden variable theories]].<ref>{{Cite journal | last1 = Shimony | first1 = A. | last2 = Telegdi | first2 = V. | last3 = Veltman | first3 = M. | doi = 10.1063/1.2810223 | title = John S. Bell | journal = Physics Today | volume = 44 | issue = 8 | pages = 82 | year = 1991 | pmid = | pmc = | bibcode = 1991PhT....44h..82S }}</ref><ref name="mactutor">{{MacTutor|id=Bell_John|title=John Stewart Bell}}</ref><ref name="scopus">{{Scopus|id=24494279600}}</ref>
'''John Stewart Bell''' (28 July 1928 – 1 October 1990) was a Northern Irish theoretical physicist, best known for '''Bell's theorem''' (1964) on the impossibility of local hidden-variable theories. He is routinely invoked as the man who closed the question of hidden variables and vindicated the Copenhagen orthodoxy.
==Biography==
That reading inverts his actual position. Bell '''reopened''' the question rather than closing it. He demolished the von Neumann "impossibility proof" that had suppressed realist alternatives for thirty years, he admired [[David Bohm|Bohm's]] causal interpretation and credited it as the stimulus for his own work, he considered the orthodox account unprofessionally vague, and he was openly willing to contemplate a return to a '''Lorentzian preferred frame''' — an aether — to accommodate the non-locality he had discovered. He is therefore a central figure for the critical tradition documented on this wiki, not an opponent of it.
===Early life and work===
==Life and career==
John Bell was born in [[Belfast]], [[Northern Ireland]]. Both sides of his family were of [[Ulster Scots people|Ulster Scots]] roots.<ref>{{cite web|title=John Stewart Bell|url=https://www.ria.ie/research/dib/john-stewart-bell.aspx|author=Patricia M. Byrne|date=February 5, 2003|accessdate=April 10, 2015|deadurl=yes|archiveurl=https://web.archive.org/web/20141210035418/http://www.ria.ie/research/dib/john-stewart-bell.aspx|archivedate=10 December 2014|df=dmy-all}}</ref> When he was 11 years old, he decided to be a scientist, and at 16 graduated from Belfast Technical High School. Bell then attended the [[Queen's University of Belfast]], and obtained a bachelor's degree in experimental physics in 1948, and one in mathematical physics a year later. He went on to complete a Ph.D. in physics at the [[University of Birmingham]] in 1956, specialising in [[nuclear physics]] and [[quantum field theory]]. In 1954, he married Mary Ross, also a physicist, whom he had met while working on [[particle accelerator|accelerator]] physics at Malvern, UK.<ref>{{cite book |last=Aczel |first=A. D. |year=2002 |title=Entanglement: The Greatest Mystery in Physics |publisher=[[Basic Books]] |isbn=978-1-56858-232-0 |page=139 |ref=harv }}</ref> Bell became a vegetarian in his teen years.<ref>{{cite article |last=Bell |first=Mary |year=2016 |title=Bell the vegetarian |doi=10.1063/PT.3.3252 |url=http://scitation.aip.org/content/aip/magazine/physicstoday/article/69/8/10.1063/PT.3.3252|bibcode=2016PhT....69h..12B }}</ref> According to his wife, Bell was an atheist.<ref>{{cite book|author1=Andrew Whitaker|author2=Mary Bell|author3=Shan Gao|title=Quantum Nonlocality and Reality: 50 Years of Bell's Theorem|date=Sep 19, 2016|publisher=Cambridge University Press|isbn=9781107104341|page=8|chapter=1 - John Bell - The Irish Connection|quote=John Bell was certainly not interested in Protestantism as such – his wife Mary [33] has reported that he was an atheist most of his life.}}</ref>
Bell's career began with the UK [[Atomic Energy Research Establishment]], near [[Harwell, Oxfordshire]], known as [[Atomic Energy Research Establishment|AERE or Harwell Laboratory]]. After several years he moved to work for the European Organization for Nuclear Research ([[CERN]], ''Conseil Européen pour la Recherche Nucléaire''), in [[Geneva]], [[Switzerland]]. There he worked almost exclusively on theoretical [[particle physics]] and on accelerator design, but found time to pursue a major [[avocation]], investigating the foundations of [[quantum mechanics|quantum theory]]. He was elected a Foreign Honorary Member of the [[American Academy of Arts and Sciences]] in 1987.<ref name=AAAS>
Bell was born in Belfast in 1928 to a family of modest means and studied at Queen's University Belfast. He worked on accelerator design and particle physics, joining CERN in 1960, where he spent the rest of his career. His work on the foundations of quantum mechanics — the work for which he is now remembered — was pursued largely as a sideline to his professional duties. He died suddenly in 1990.
{{cite book
|title=Members of the Academy of Arts & Sciences, 1780–2010
|publisher=[[American Academy of Arts and Sciences]]
|accessdate=2011-05-30
}}</ref> Also of significance during his career, Bell, together with John Bradbury Sykes, M. J. Kearsley, and [[W. H. Reid]], translated several volumes of the ten-volume ''[[Course of Theoretical Physics]]'' of [[Lev Landau]] and [[Evgeny Lifshitz]], making these works available to an English-speaking audience in translation, all of which remain in print.
Bell was a proponent of [[pilot wave]] theory.<ref>http://phys.org/news/2014-09-fluid-mechanics-alternative-quantum-orthodoxy.html - Fluid mechanics suggests alternative to quantum orthodoxy</ref>
==The demolition of von Neumann's proof==
===Bell's theorem===
From 1932, John von Neumann's supposed proof that no hidden-variable theory could reproduce quantum mechanics was treated as settling the matter, and was used for three decades to dismiss realist alternatives without examination. Encountering Bohm's 1952 theory — which manifestly did what the proof said was impossible — led Bell to examine the argument directly. He found it rested on an assumption no plausible hidden-variable theory need satisfy: that the average of a sum of observables must equal the sum of their averages, even for non-commuting quantities.
{{Main|Bell's theorem}}
In 1964, after a year's leave from [[CERN]] that he spent at [[Stanford University]], the [[University of Wisconsin–Madison]] and [[Brandeis University]], he wrote a paper entitled "On the [[Einstein-Podolsky-Rosen Paradox]]".<ref name="Bell1988">{{harvnb|Bell|1988}}</ref>{{rp|14}} In this work, he showed that carrying forward EPR's analysis<ref>
{{Cite journal | last1 = Einstein | first1 = A. | last2 = Podolsky | first2 = B. | last3 = Rosen | first3 = N. | doi = 10.1103/PhysRev.47.777 | title = Can Quantum-Mechanical Description of Physical Reality Be Considered Complete? | journal = Physical Review | volume = 47 | issue = 10 | pages = 777 | year = 1935 | pmid = | pmc = | bibcode = 1935PhRv...47..777E }}</ref> permits one to derive the famous [[Bell's theorem]].<ref>{{cite journal|title=Faces and places: John Stewart Bell|journal=CERN Courier|date=August 2014|url=http://cerncourier.com/cws/article/cern/58208/2}}</ref><ref>{{cite web|last1=Sutton|first1=Christine|title=Fifty years of Bell’s theorem|url=http://home.web.cern.ch/about/updates/2014/11/fifty-years-bells-theorem|website=CERN official website|publisher=CERN}}</ref> The resultant inequality, derived from certain assumptions, is violated by quantum theory.
There is some disagreement regarding what Bell's inequality—in conjunction with the [[EPR paradox|EPR]] analysis—can be said to imply. Bell held that not only local hidden variables, but any and all local theoretical explanations must conflict with the predictions of quantum theory: "It is known that with Bohm's example of EPR correlations, involving particles with spin, there is an irreducible [[quantum nonlocality|nonlocality]]."<ref name ="Bell1988 />{{rp|196}} According to an alternative interpretation, not all local theories in general, but only local hidden variables theories (or "local realist" theories) have shown to be incompatible with the predictions of quantum theory.
Bell's verdict was blunt. In a 1988 interview he said of it:
===Bell's critique of von Neumann's proof===
<blockquote>"Yet the von Neumann proof, if you actually come to grips with it, falls apart in your hands! There is nothing to it. It's not just flawed, it's silly!"</blockquote>
Bell's interest in hidden variables was motivated by the existence in the formalism of quantum mechanics of a "movable boundary" between the quantum system and the classical apparatus:
<blockquote>A possibility is that we find exactly where the boundary lies. More plausible to me is that we will find that there is no boundary. ... The wave functions would prove to be a provisional or incomplete description of the quantum-mechanical part, of which an objective account would become possible. It is this possibility, of a homogeneous account of the world, which is for me the chief motivation of the study of the so-called 'hidden variable' possibility.<ref name="Bell1988" />{{rp|30}}</blockquote>
The flaw had in fact been identified by the philosopher Grete Hermann as early as 1935 and disregarded. That a widely cited impossibility proof could stand unexamined for a generation, closing off a legitimate research programme, is one of the clearest cases in modern physics of authority substituting for argument.
Bell was impressed that in the formulation of [[David Bohm]]’s [[De Broglie–Bohm theory|nonlocal hidden variable theory]], no such boundary is needed, and it was this which sparked his interest in the field of research. Bell also criticized the standard formalism of quantum mechanics on the grounds of lack of physical precision:
==Bell's theorem==
<blockquote>For the good books known to me are not much concerned with physical precision. This is clear already from their vocabulary. Here are some words which, however legitimate and necessary in application, have no place in a ''formulation'' with any pretension to physical precision: ''system'', ''apparatus'', ''environment'', ''microscopic'', ''macroscopic'', ''reversible'', ''irreversible'', ''observable'', ''information'', ''measurement''. .... On this list of bad words from good books, the worst of all is 'measurement'.<ref name="Bell1988" />{{rp|215}}</blockquote>
But if he were to thoroughly explore the viability of Bohm's theory, Bell needed to answer the challenge of the so-called impossibility proofs against hidden variables. Bell addressed these in a paper entitled "On the Problem of Hidden Variables in Quantum Mechanics".<ref name = "Bell1988" />{{rp|1}} (Bell had actually written this paper before his paper on the EPR paradox, but it did not appear until two years later, in 1966, due to publishing delays.<ref>{{harvnb|Aczel|2002|p=144}}</ref>) Here he showed that [[John von Neumann]]’s argument<ref>
Bell's 1964 theorem showed that no theory in which distant systems have definite pre-existing properties and no faster-than-light influences can reproduce all the correlations predicted by quantum mechanics. Experiments from the 1970s onward have favoured the quantum predictions.
{{cite book
|last=von Neumann |first=J.
|year=1996
|title=[[Mathematical Foundations of Quantum Mechanics]]
|page=
|publisher=[[Princeton University Press]]
|isbn=978-0-691-02893-4
}}</ref> does not prove the impossibility of hidden variables, as it was widely claimed, due to its reliance on a physical assumption that is not valid for quantum mechanics—namely, that the probability-weighted average of the sum of observable quantities equals the sum of the average values of each of the separate observable quantities.<ref>{{harvnb|Aczel|2002|p=141}}</ref> Bell subsequently claimed, "The proof of von Neumann is not merely false but ''foolish''!".<ref>{{Cite journal | last1 = Bub | first1 = J. | title = Von Neumann's 'No Hidden Variables' Proof: A Re-Appraisal | doi = 10.1007/s10701-010-9480-9 | journal = Foundations of Physics | volume = 40 | issue = 9–10 | pages = 1333–1340 | year = 2010 | pmid = | pmc = | arxiv = 1006.0499 | bibcode = 2010FoPh...40.1333B }}</ref> In this same work, Bell showed that a stronger effort at such a proof (based upon [[Gleason's theorem]]) also fails to eliminate the hidden variables program. The supposed flaw in von Neumann's proof had been previously discovered by [[Grete Hermann]] in 1935, but did not become common knowledge until after it was rediscovered by Bell.<ref>C. L. Herzenberg: "Grete Hermann, An early contributor to quantum theory" {{arxiv|0812.3986}}</ref>
However, in 2010, [[Jeffrey Bub]] published an argument that Bell (and, implicitly, Hermann) had misconstrued von Neumann's proof, claiming that it does not attempt to prove the absolute impossibility of hidden variables, and is actually not flawed, after all. (Thus, it was the physics community as a whole that had misinterpreted von Neumann's proof as applying universally.) Bub provides evidence that von Neumann understood the limits of his proof, but there is no record of von Neumann attempting to correct the near universal misinterpretation which lingered for over 30 years and exists to some extent to this day. Von Neumann's proof does not in fact apply to contextual hidden variables, as in Bohm's theory. {{Citation needed|date=March 2017}}
The result is frequently reported as proving that hidden variables are impossible and that realism must be abandoned. It shows nothing of the kind. What it establishes is an incompatibility between quantum predictions and '''locality''' combined with definite properties. A non-local hidden-variable theory — Bohm's, for example — is entirely untouched by it. The honest statement of the theorem is that '''nature is non-local''', and the open question is what to make of that.
===Conclusions from experimental tests===
==What Bell himself concluded==
In 1972 the first of many [[Bell test experiments|experiments]] that have shown (under the extrapolation to ideal detector efficiencies) a violation of Bell's inequality was conducted. Bell himself concludes from these experiments that "It now seems that the non-locality is deeply rooted in quantum mechanics itself and will persist in any completion."<ref name = "Bell1988" />{{rp|132}} This, according to Bell, also implied that quantum theory is not locally causal and cannot be embedded into any locally causal theory. Bell regretted that results of the tests did not agree with the concept of local hidden variables:
<blockquote>For me, it is so reasonable to assume that the photons in those experiments carry with them programs, which have been correlated in advance, telling them how to behave. This is so rational that I think that when Einstein saw that, and the others refused to see it, ''he'' was the rational man. The other people, although history has justified them, were burying their heads in the sand. ... So for me, it is a pity that Einstein's idea doesn't work. The reasonable thing just doesn't work."<ref>
{{cite book
|last=Bernstein |first=J.
|year=1991
|title=Quantum Profiles
|page=84
|publisher=[[Princeton University Press]]
|isbn=
}}</ref></blockquote>
Bell seemed to have become resigned to the notion that future experiments would continue to agree with quantum mechanics and violate his inequality. Referring to the [[Bell test experiments]], he remarked:
Bell drew the realist conclusion. He regarded the orthodox formulation as unsatisfactory, praised Bohm's theory for showing explicitly what had been declared out of the question, and pressed throughout his career for a physics that says what actually happens rather than merely what will be observed. His essays are collected in ''Speakable and Unspeakable in Quantum Mechanics'' (1987), including the well-known "Bertlmann's socks and the nature of reality" (1981), written after his CERN colleague Reinhold Bertlmann.
<blockquote>It is difficult for me to believe that quantum mechanics, working very well for currently practical set-ups, will nevertheless fail badly with improvements in counter efficiency ..."<ref name = "Bell1988" />{{rp|109}}</blockquote>
Some people continue to believe that agreement with Bell's inequalities might yet be saved. They argue that in the future much more precise experiments could reveal that one of the known [[Bell test loopholes|loopholes]], for example the so-called "fair sampling loophole", had been biasing the interpretations. Most mainstream physicists are highly skeptical about all these "loopholes", admitting their existence but continuing to believe that Bell's inequalities must fail.
Bell remained interested in objective 'observer-free' quantum mechanics. He felt that at the most fundamental level, physical theories ought not to be concerned with observables, but with 'be-ables': "The [[wikt:beable|beables]] of the theory are those elements which might correspond to elements of reality, to things which exist. Their existence does not depend on 'observation'."<ref name="Bell1988" />{{rp|174}} He remained impressed with Bohm's hidden variables as an example of such a scheme and he attacked the more subjective alternatives such as the [[Copenhagen interpretation]].<ref name = "Bell1988" />{{rp|92,133,181}}
Most strikingly for the tradition documented here, Bell was prepared to resolve the tension between non-locality and relativity by reviving a '''preferred frame'''. Interviewed in ''The Ghost in the Atom'' (1986), he said:
===Bell and Special Theory of Relativity===
<blockquote>"[T]he cheapest resolution is something like going back to relativity as it was before Einstein, when people like Lorentz and Poincaré thought that there was an aether — a preferred frame of reference — but that our measuring instruments were distorted by motion in such a way that we could not detect motion through the aether. Now, in that way you can imagine that there is a preferred frame of reference, and in this preferred frame of reference things do go faster than light."</blockquote>
Bell and his wife contributed substantially to the physics of particle accelerators and with numerous young theorists at CERN Bell developed particle physics itself. An overview of this work is available in the volume of collected works edited by Mary Bell, Kurt Gottfried and Martinus Veltman.<ref>
{{cite book|editor1=Mary Bell|editor2=Kurt Gottfried|editor3=Martinus Veltman|title=Quantum Mechanics, High Energy Physics and Accelerators (selected paper of John S Bell with commentary)|year=1995|isbn=9810221150|publisher=World Scientific}}
</ref>
Apart from his particle physics research Bell often raised an issue of special relativity comprehension and although there is only one written report on this topic titled "How to teach special relativity" available <ref>{{cite book|last=Bell|first=John Stewart|editor=Zichichi Antonino|title=Progress in Scientific Culture|volume=1, No. 2|year=1976|pages=135–148}}</ref> reprinted in <ref>{{cite book|year=1987|title=Speakable and Unspeakable in Quantum Mechanics|pages=67–80|last=Bell|first=John|publisher=Cambridge University Press}}</ref> this was a very important subject to him. Bell admired Einstein's contribution to special relativity, but warned in 1985 "Einstein's approach is ... pedagogically dangerous, in my opinion".<ref>{{Cite book|author=[[Johann Rafelski]]|title = Relativity Matters: From Einstein's EMC2 to Laser Particle Acceleration and Quark-Gluon Plasma|publisher=Springer|year=2017|isbn=978-3-319-51230-3|pages=ix |url={{Google books|plainurl=y|id=8bhZDgAAQBAJ}}}}</ref> In 1989 on the occasion of the centenary of the Lorentz-FitzGerald body contraction Bell writes "A great deal of nonsense has been written about the FitzGerald contraction".<ref>
{{cite book|editor1=Mary Bell|editor2=Kurt Gottfried|editor3=Martinus Veltman|title=Quantum Mechanics, High Energy Physics and Accelerators (selected paper of John S Bell with commentary)|year=1995|isbn=9810221150|publisher=World Scientific}}
</ref> Bell preferred to think of [[Lorentz-FitzGerald contraction]] as a phenomenon that is real and observable as a property of a material body, which was also Einstein's opinion, but in Bell's view Einstein's approach leaves a lot of room for misinterpretation. This situation and the background of Bell's position is described in detail by his collaborator Johann Rafelski in the textbook "Relativity Matters" (2017).<ref>{{Cite book|author=[[Johann Rafelski]]|title = Relativity Matters: From Einstein's EMC2 to Laser Particle Acceleration and Quark-Gluon Plasma|publisher=Springer|year =2017|isbn=978-3-319-51230-3|url={{Google books|plainurl=y|id=8bhZDgAAQBAJ}}}}</ref> In order to combat misconceptions surrounding Lorentz-FitzGerald body contraction Bell adopted and promoted a relativistic thought experiment which became widely known as [[Bell's spaceship paradox]].
===Death===
This is precisely the '''neo-Lorentzian''' position advanced by a number of researchers catalogued on this wiki, and it comes from the physicist whose theorem is most often cited against them. See [[Lorentz ether theory]] and the discussion at [[Relativity]].
[[File:John Stewart Bell's Blue plaque.JPG|thumb|200px|[[Blue plaque]] honouring John Bell at the [[Queen's University of Belfast]]]]
Bell died unexpectedly of a [[cerebral hemorrhage]] in Geneva in 1990.<ref>
|title=John Stewart Bell Is Dead at 62; Physicist Tested Particle Actions
|work=[[New York Times]]
|accessdate=2013-11-24
}}</ref><ref>{{cite journal|title=Faces and things: John Stewart Bell 1928-1990|journal=CERN Courier|date=November 1990|volume=30|issue=8|page=25|url=https://cds.cern.ch/record/1731827}}</ref> It is widely claimed that unknown to Bell, that year he had been nominated for a Nobel Prize.<ref>Louisa Gilder, ''The Age of Entanglement'', Vintage Books, 2008, p. 3.</ref><ref>Jeremy Bernstein, ''Quantum Leaps'', The Belknap Press of Harvard University Press, 2009, p. 155.</ref><ref> Andrew Whitaker, ''John Stewart Bell and Twentieth-Century Physics: Vision and Integrity'', Oxford University Press, 2016, p. 374. ("[Bell] was not only nominated but shortlisted in 1989 (and perhaps previous years)...")</ref> His contribution to the issues raised by EPR was significant. Some regard him as having demonstrated the failure of local realism (local hidden variables). Bell's own interpretation is that locality itself met its demise.
==Legacy==
==Reception and misuse==
In 2008, the [[John Stewart Bell Prize]] was created by the Centre for Quantum Information and Quantum Control at the [[University of Toronto]].<ref name="CQIQC">
|publisher=Centre for Quantum Information and Quantum Control
}}</ref> The prize is awarded every other year for significant contributions first published during the six preceding years. The award recognizes major advances relating to the foundations of [[quantum mechanics]] and to the applications of these principles. In 2009, the first award was presented by [[Alain Aspect]] to [[Nicolas Gisin]] for his theoretical and experimental work on foundations and applications of quantum physics — notably [[quantum nonlocality]], [[quantum cryptography]], and [[quantum teleportation]].<ref name="Bell2009">
{{cite web
|date=3 August 2009
|title=Prof. Nicolas Gisin awarded the First Bell Prize
|publisher=Centre for Quantum Information and Quantum Control
|accessdate=2013-11-24
}}</ref>
At the [[CERN]] site in [[Meyrin]], close to [[Geneva]], there is a street called [[Route Bell]] in honour of John Stewart Bell.
Bell's theorem is among the most-cited results in modern physics, and is commonly presented in textbooks as having vindicated the Copenhagen interpretation and disposed of hidden variables. Critics on this wiki argue that this is a misreading which reverses his intent: the theorem constrains locality, not realism; its author was a realist who thought the orthodoxy evasive; and the proof it displaced was itself a piece of unexamined authority. A substantial critical literature examines the theorem's assumptions directly, including its treatment of measurement independence, detector efficiency, and the derivation of the inequalities themselves.
Since 2015, there is also a street named Bell's Theorem Crescent in his city of birth, Belfast.
==On this wiki==
At the event in memory of John Stuart Bell, a day was named after him, referring to the date he released Bell's Theorem, November 4.
The John Bell House, named in his honour, finished construction in 2016 and houses over 400 students in Belfast city centre.<ref>{{Cite news|url=http://www.accommodationforstudents.com/studentprivatehalls/lphs.asp?id=1863|title=John Bell House - Fresh Student Living - Student Privatehall in Belfast|work=afs|access-date=2017-03-31}}</ref>
Bell's theorem and the associated EPR problem are extensively treated in the literature catalogued here, including ''[[Bell's Theorem and Quantum Mechanics]]'', ''[[Significant Facts Revealed by the EPR Paradox and Bell's Theorem]]'', ''[[Measurement Theory via Hidden Variables Not Subject to Bell's Theorem]]'', ''[[Nonlocal Theories Satisfying Bell's Inequality]]'', ''[[A Counter-Example to Bell's Theorem with a 'Softened' Singularity]]'', ''[[Nonlocality, Unreality, and Bell Theorem]]'', ''[[Demystifying the EPR Paradox]]'' and ''[[Spooky Action at a Distance: An Explanation of Bell's Theorem]]'' by [[Gary Felder]]. Further material is indexed under [[:Category:Quantum Theory]].
In 2016, his colleague from CERN, [[Reinhold Bertlmann]], wrote a lengthy piece "Bell's Universe: A Personal Recollection",<ref>[https://arxiv.org/abs/1605.08081 Bell's Universe: A Personal Recollection] (My collaboration and friendship with John Bell is recollected.), [[Reinhold Bertlmann]], 25 May 2016, accessed 27 May 2016</ref> explaining in some detail his amazement at finding out about Bell's paper on [[Reinhold Bertlmann#Bertlmann.E2.80.99s Socks|Bertlmann's socks]], in which Bell compared the [[EPR paradox]] with socks.
==See also==
==See also==
* ''[[Epistemological Letters]]''
* [[EPR paradox]], a thought experiment by [[Albert Einstein|Einstein]], [[Boris Podolsky|Podolsky]], and [[Nathan Rosen|Rosen]] published in 1935 as an attack on quantum theory
* [[Local hidden variable theory]]
* [[Quantum entanglement]]
* [[Bell's theorem]], published in the mid-1960s
* [[Bell state]]
* [[Bell test experiments]]
* [[CHSH Bell test]], an application of Bell's theorem
* [[GHZ experiment]]
* [[Superdeterminism]]
Other work by Bell:
* [[Adler–Bell–Jackiw anomaly]]
* [[Bell's spaceship paradox]]
==Notes==
{{Reflist|30em}}
==References==
*{{cite book
|last=Bell |first=J. S.
|year=1988
|title=Speakable and Unspeakable in Quantum Mechanics
|page=14
|publisher=[[Cambridge University Press]]
|isbn=978-0-521-36869-8
|ref=harv
}}
*{{cite book
|last=Bell |first=J. S.
|year=2004
|title=Speakable and Unspeakable in Quantum Mechanics
|edition=2nd
|publisher=[[Cambridge University Press]]
|isbn=978-0-521-52338-7
}} 2004 edition with introduction by [[Alain Aspect]] and two additional papers: {{ISBN|0-521-52338-9}}.
*{{cite book
|last=Gilder |first=L.
|year=2008
|title=The Age of Entanglement: When Quantum Physics Was Reborn
|publisher=[[Alfred A. Knopf]]
|isbn=978-1-4000-4417-7
}}
*{{cite journal
|last=Pearle |first=P.
|year=1970
|title=Hidden-Variable Example Based upon Data Rejection
|journal=[[Physical Review D]]
|volume=2 |issue= 8|pages=1418–1825
|bibcode=1970PhRvD...2.1418P
|doi=10.1103/PhysRevD.2.1418
}}
== External links ==
{{wikiquote}}
{{commons category|John Stewart Bell (physicist)}}
*[http://physicsweb.org/articles/world/11/12/8 John Bell and the most profound discovery of science (December 1998)]
*[https://web.archive.org/web/20070927190121/http://www.rds.ie/home/index.aspx?id=1755 The Most Profound Discovery of Science (September 2006)]
John Stewart Bell (28 July 1928 – 1 October 1990) was a Northern Irish theoretical physicist, best known for Bell's theorem (1964) on the impossibility of local hidden-variable theories. He is routinely invoked as the man who closed the question of hidden variables and vindicated the Copenhagen orthodoxy.
That reading inverts his actual position. Bell reopened the question rather than closing it. He demolished the von Neumann "impossibility proof" that had suppressed realist alternatives for thirty years, he admired Bohm's causal interpretation and credited it as the stimulus for his own work, he considered the orthodox account unprofessionally vague, and he was openly willing to contemplate a return to a Lorentzian preferred frame — an aether — to accommodate the non-locality he had discovered. He is therefore a central figure for the critical tradition documented on this wiki, not an opponent of it.
Life and career
Bell was born in Belfast in 1928 to a family of modest means and studied at Queen's University Belfast. He worked on accelerator design and particle physics, joining CERN in 1960, where he spent the rest of his career. His work on the foundations of quantum mechanics — the work for which he is now remembered — was pursued largely as a sideline to his professional duties. He died suddenly in 1990.
The demolition of von Neumann's proof
From 1932, John von Neumann's supposed proof that no hidden-variable theory could reproduce quantum mechanics was treated as settling the matter, and was used for three decades to dismiss realist alternatives without examination. Encountering Bohm's 1952 theory — which manifestly did what the proof said was impossible — led Bell to examine the argument directly. He found it rested on an assumption no plausible hidden-variable theory need satisfy: that the average of a sum of observables must equal the sum of their averages, even for non-commuting quantities.
Bell's verdict was blunt. In a 1988 interview he said of it:
"Yet the von Neumann proof, if you actually come to grips with it, falls apart in your hands! There is nothing to it. It's not just flawed, it's silly!"
The flaw had in fact been identified by the philosopher Grete Hermann as early as 1935 and disregarded. That a widely cited impossibility proof could stand unexamined for a generation, closing off a legitimate research programme, is one of the clearest cases in modern physics of authority substituting for argument.
Bell's theorem
Bell's 1964 theorem showed that no theory in which distant systems have definite pre-existing properties and no faster-than-light influences can reproduce all the correlations predicted by quantum mechanics. Experiments from the 1970s onward have favoured the quantum predictions.
The result is frequently reported as proving that hidden variables are impossible and that realism must be abandoned. It shows nothing of the kind. What it establishes is an incompatibility between quantum predictions and locality combined with definite properties. A non-local hidden-variable theory — Bohm's, for example — is entirely untouched by it. The honest statement of the theorem is that nature is non-local, and the open question is what to make of that.
What Bell himself concluded
Bell drew the realist conclusion. He regarded the orthodox formulation as unsatisfactory, praised Bohm's theory for showing explicitly what had been declared out of the question, and pressed throughout his career for a physics that says what actually happens rather than merely what will be observed. His essays are collected in Speakable and Unspeakable in Quantum Mechanics (1987), including the well-known "Bertlmann's socks and the nature of reality" (1981), written after his CERN colleague Reinhold Bertlmann.
Most strikingly for the tradition documented here, Bell was prepared to resolve the tension between non-locality and relativity by reviving a preferred frame. Interviewed in The Ghost in the Atom (1986), he said:
"[T]he cheapest resolution is something like going back to relativity as it was before Einstein, when people like Lorentz and Poincaré thought that there was an aether — a preferred frame of reference — but that our measuring instruments were distorted by motion in such a way that we could not detect motion through the aether. Now, in that way you can imagine that there is a preferred frame of reference, and in this preferred frame of reference things do go faster than light."
This is precisely the neo-Lorentzian position advanced by a number of researchers catalogued on this wiki, and it comes from the physicist whose theorem is most often cited against them. See Lorentz ether theory and the discussion at Relativity.
Reception and misuse
Bell's theorem is among the most-cited results in modern physics, and is commonly presented in textbooks as having vindicated the Copenhagen interpretation and disposed of hidden variables. Critics on this wiki argue that this is a misreading which reverses his intent: the theorem constrains locality, not realism; its author was a realist who thought the orthodoxy evasive; and the proof it displaced was itself a piece of unexamined authority. A substantial critical literature examines the theorem's assumptions directly, including its treatment of measurement independence, detector efficiency, and the derivation of the inequalities themselves.