John Bell was one of the most outstanding theoretical physicists during the second half of the 20th century, and Andrew Whitaker's biography gives an excellent account of his numerous achievements during his long tenure at CERN, the European center for research in particle physics. Bell made important contributions in particle physics, but he became widely known in the broader physics community for his contributions to elucidate the foundations of quantum mechanics. When he was a graduate student of Rudolf Peierls, he showed his talent early on by giving an independent proof of the CPT theorem: the invariance in a local quantum field theory of the combined transformation of charge conjugation, parity, and time reversal. He joined the theory Division at CERN where he was highly appreciated for his work in theoretical and experimental particle physics. One of his early contributions are two papers with Martinus Veltman on neutrino beams to produce the intermediate bosons later found experimentally at CERN, and considered to be the first major triumph at this research center. Bell also became well known for this paper with Roman Jackiw on the ABJ anomaly in quantum field theory, and for his review of neutral K mesons with Jack Steinberger.

At CERN, Bell initially did not discuss his lifelong interest and deep concerns about the foundations of quantum mechanics. He considered this topic inappropriate to the mission of this research center dedicated primarily to particle physics. Instead, in 1964 he wrote his celebrated papers on this topic while he was on leave at the Brandeis University Physics Department and at the Stanford Linear Accelerator Center (SLAC). I had the good fortune to overlap with him at SLAC, where we had numerous arguments about the *measurement problem* in quantum mechanics. We contributed a tongue-in-cheek paper on this problem to a *festschrift* in honor of Vicky Weiskopf, remarking that “Current interest in such questions is small. The typical physicist feels that they have long been answered, and that he will fully understand just how, if ever he can spend *twenty minutes* thinking about it.” Now interest in this problem has exploded, with an endless series of papers on the subject, and Bell spent the rest of his life promoting and defending his views about it.

Bell's best known contribution was a theoretical *inequality* leading to a direct experimental test that distinguished a general class of deterministic *local* hidden-variable theories from conventional quantum mechanics.^{1} He was stimulated in this endeavor after reading David Bohm's 1951 description of quantum theory as a statistical theory with a non-local quantum potential giving rise to instantaneous interaction among particles separated by an arbitrary spatial distance. Bohm's theory is a non-relativistic theory, and such interactions would violate the principles of special relativity, but surprisingly this problem did not seem to bother Bell. His inequality led to experimental tests, first carried out by John Clauser, and extended by Alain Aspect and many others, to close some loopholes that ruled out attempts to explain the origin of the lack of determinism in quantum mechanics by *local* hidden variables. In his last paper^{2} on this subject, entitled “Against Measurement,” Bell concluded, like an ancient prophet:

“Suppose for example that quantum mechanics was found to resist precise formulation. Suppose that when formulation beyond FAPP (for all practical purposes) is attempted, we find an immovable finger pointing outside the subject, to the mind of the observer, to the Hindu scripture, to God, or even only to Gravitation? Would not that be very, very interesting?”

During his life Bell received many honors. In 1987, he became an Honorary Foreign member of the American Academy of Arts and Sciences, and in 1995 he was awarded the Dannie Heiman Prize for Mathematical Physics of the American Physical Society, and the Hughes Medal of the Royal Society. His work on the foundation of quantum mechanics also contributed to the development of quantum information, which has become a very active theoretical and experimental field. Whitaker devotes several pages to argue that, were it not for his premature death, Bell would have received the Nobel Prize in Physics. His book also presents some of the history of particle physics in the second half of the twentieth century, but it is centered primarily on the contributions made at CERN. At the end there is a short section on the birth of quantum information and Bell's contribution to it.

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*Michael Nauenberg is an emeritus professor of Physics at the University of California in Santa Cruz. He has done research on the foundations of quantum mechanics and on a broad range of subjects in physics and its history.*