John Stewart Bell (28 July 1928 – 1 October 1990) was a physicist from Northern Ireland and the originator of Bell's theorem, an important theorem in quantum physics regarding hidden-variable theories. In 2022, the Nobel Prize in Physics was awarded to Alain Aspect, John Clauser, and Anton Zeilinger for work on Bell inequalities and the experimental validation of Bell's theorem.
Biography
Early life and work Bell was born in Belfast, Northern Ireland to a working class family. Due to financial hardship, neither parent and none of his three older siblings completed high school, typically dropping out of school by age 14 to work. When he was 11 years old, he decided to be a scientist, and encouraged by his mother, at 16 he graduated from Belfast Technical High School. Then in an exceptionally rare occurrence for someone of his background, Bell attended the Queen's University of Belfast, where, in 1948, he obtained a bachelor's degree in experimental physics and, a year later, a bachelor's degree in mathematical physics. He went on to complete a PhD 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 accelerator physics at Malvern, UK. Bell became a vegetarian in his teen years. According to his wife, Bell was an atheist. Bell's career began with the UK Atomic Energy Research Establishment, near Harwell, Oxfordshire, known as AERE or Harwell Laboratory. In 1960, 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 theory. He was elected a Foreign Honorary Member of the American Academy of Arts and Sciences in 1987. 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. In 1987, inspired by Ghirardi–Rimini–Weber theory, he also advocated collapse theories. He said about the interpretation of quantum mechanics: "Well, you see, I don't really know. For me it's not something where I have a solution to sell!"
Critique of von Neumann's proof Bell was impressed that the formulation of David Bohm's nonlocal hidden-variable theory did not require a "movable boundary" between the quantum system and the classical apparatus:
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. Bell also criticized the standard formalism of quantum mechanics on the grounds of lack of physical precision:
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". 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". (Due to publishing delays, this paper did not appear until 1966, two years after his more famous work on the Einstein–Podolsky–Rosen (EPR) paradox.) He showed that John von Neumann's no hidden variables proof does not prove the impossibility of hidden variables, as 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. This 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. Bell reportedly said, "The proof of von Neumann is not merely false but foolish!" 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. However, in 2010, Jeffrey Bub published an argument that Bell (and, implicitly, Hermann) had misconstrued von Neumann's proof, saying 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. Bub's conclusion has, in turn, been questioned.
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, Bell wrote a paper entitled "On the Einstein–Podolsky–Rosen paradox". In this work, he showed that carrying forward EPR's analysis permits one to derive the famous Bell's theorem. The resultant inequality, derived from basic assumptions that apply to all classical situations, is violated by quantum theory.
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