Bell’s theorem is one of the most significant results in quantum mechanics. So why, wonders Iulia Georgescu, did John Bell publish it in a little-known, short-lived journal called Physics?
Prologue A memorial conference for John Bell, open to the public, was held at Rutgers University around 20 years ago. I gave a short talk there on Bell's views about David Bohm's “hidden variables” formulation of quantum mechanics, a version of quantum mechanics often called the de Broglie–Bohm theory or Bohmian mechanics. This theory was in fact discovered by Louis de Broglie in 1927. In 1952, it was rediscovered and developed by Bohm, who was the first to appreciate its connection to the predictions of standard quantum mechanics. I did not publish the talk. I have decided that it would be appropriate to publish a very lightly edited version of it here, in this volume devoted to the work of Bell on the foundation of quantum mechanics. One reason for doing so is that the connection between Bell and Bohm continues to be somewhat underplayed, with the strength of his advocacy of Bohmian mechanics not properly appreciated. For example, about half of the papers in Bell's collected works on the foundations of quantum mechanics deal with Bohmian mechanics. But in his fine introduction to the revised edition of this great book [1], Alain Aspect mentions this theory in only a single sentence, and parenthetically at that. For several decades after Bell proved his nonlocality theorem, based in part on Bell's inequality, it was widely claimed that Bell had shown that hidden variables – and Bohmian mechanics in particular – were impossible, that they were incompatible with the predictions of quantum mechanics. For example, the great physicist Eugene Wigner, who, unlike most of his contemporaries, was profoundly concerned with the conceptual foundations of quantum mechanics and usually wrote on the subject with great clarity and insight, has written that This [hidden variables] is an interesting idea and even though few of us were ready to accept it, it must be admitted that the truly telling argument against it was produced as late as 1965, by J. S. Bell … This appears to give a convincing argument against the hidden variables theory.
Although skeptical of the prohibitive power of no-hidden-variables theorems, John Bell was himself responsible for the two most important ones. I describe some recent versions of the lesser known of the two (familiar to experts as the "Kochen-Specker theorem") which have transparently simple proofs. One of the new versions can be converted without additional analysis into a powerful form of the very much better known "Bell's Theorem," thereby clarifying the conceptual link between these two results of Bell.
Combining twenty-six original essays written by an impressive line-up of distinguished physicists and philosophers of physics, this anthology reflects some of the latest thoughts by leading experts on the influence of Bell's theorem on quantum physics. Essays progress from John Bell's character and background, through studies of his main work, and on to more speculative ideas, addressing the controversies surrounding the theorem, and investigating the theorem's meaning and its deep implications for the nature of physical reality. Combined, they present a powerful comment on the undeniable significance of Bell's theorem for the development of ideas in quantum physics over the past 50 years. Questions surrounding the assumptions and significance of Bell's work still inspire discussion in the field of quantum physics. Adding to this with a theoretical and philosophical perspective, this balanced anthology is an indispensable volume for students and researchers interested in the philosophy of physics and the foundations of quantum mechanics.
John Stewart Bell and the debate on the significance of his contributions to the foundations of quantum mechanics, M.Jammer John Bell as a deep critical testimony of modern physics, V. de Alfaro magic moments with John Bell, R. Bertlmann what if Bell had come before Copenhagen?, J. Cushing quantum mechanics vs. realism in the context of two-state oscillations, D. Home one or two bell theorems?, B. Despagnat self interference of two photons, J. Rarity polarization correlation analysis of the simultaneous two-photon emission of metastable hydrogen, H. Kleinpoppen unneeded use of Bell inequalities conceals the physics in the EPR subject, O. Piccioni Bell's other theorem and its connection with nonlocality - Part 1 H. Brown Bell's other theorem and its connection with nonlocality - Part 2 A. Elby historical reflections on the notion of an objective reality, P .Feyerabend interference, indistinguishability and nonlocality, L. Mandel quantum physics from the viewpoint of realism, F. Selleri (Part contents).
John Stewart Bell (1928-1990), a truly deep and serious thinker, was one of the leading physi-cists of the 20th century. He became famous for his discovery that quantum mechanics impliesthat nature is nonlocal, i.e., that there are physical inuences between events that propagatefaster than light.From 1960 until his death Bell worked at CERN in Geneva on the physics of particle accel-erators, making a number of important contributions to high-energy physics and quantum eldtheory. Noteworthy was his discovery in 1969, together with Roman Jackiw, of the so-called\Bell-Jackiw-Adler anomaly (discovered independently by Stephen Adler), a mechanism ex-plaining physical e ects such as neutral pion decay (which are unexplainable on the basis of thesymmetries of the classical eld Lagrangian), in terms of an \anomalous term arising from therenormalization of quantum eld theory. Since then this mechanism has become an importantcornerstone of quantum eld theory. Another important contribution was the argument he gavein 1967 for why weak interactions should be described using a gauge theory.John Bell was one of the leading experts|perhaps the leading expert|on the foundations ofquantum mechanics. The book collecting his articles on this subject, Speakable and unspeakablein quantum mechanics, is unsurpassed for clarity and depth and it is still the best reference forwhoever wishes to learn about the eld [4].Bell strongly opposed the \Copenhagen interpretation of quantum physics, according towhich macroscopic objects, such as chairs and planets, do exist out there, but electrons andother microscopic particles do not. According to the Copenhagen view, the world is divided intotwo realms, macro and micro, \classical and \quantum, logical and contradictory|or, as Bellput it in one of his essays, into \speakable and \unspeakable. Along with Albert Einstein,Erwin Schrodinger, Louis de Broglie and !David Bohm, Bell was one of the few physicistscompelled by his conscience to reject the Copenhagen interpretation.Bell emphasized that the empirical facts of quantum physics do not at all force us to renouncerealism. There is, in fact, a realist theory (!Bohmian mechanics, also known as the de Broglie-Bohm theory) that accounts|insofar as the nonrelativistic theory is concerned|for all of thesefacts in a most elegant way. This theory describes a world in which electrons, quarks andthe like are point particles, always having positions that move in a manner dictated by thewave function. It should be taught to students, Bell insisted, as a legitimate alternative tothe prevailing orthodoxy. After GianCarlo Ghirardi, Alberto Rimini, and Tullio Weber in 1986succeeded in formulating a second kind of realist theory, Bell encouraged the further developmentof this theory as well [4, p. 201]. He thought that such a theory contained the seeds of a1