John Bell and Bell’s Theorem

John Stewart Bell (1928-1990), a truly deep and serious thinker, was one of the leading physicists of the 20th century. He became famous for his discovery that quantum mechanics implies that nature is nonlocal, i.e., that there are physical influences between events that propagate faster than light. From 1960 until his death Bell worked at CERN in Geneva on the physics of particle accelerators, making a number of important contributions to high-energy physics and quantum field theory. 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 explaining physical effects such as neutral pion decay (which are unexplainable on the basis of the symmetries of the classical field Lagrangian), in terms of an “anomalous” term arising from the renormalization of quantum field theory. Since then this mechanism has become an important cornerstone of quantum field theory. Another important contribution was the argument he gave in 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 of quantum mechanics. The book collecting his articles on this subject, Speakable and unspeakable in quantum mechanics, is unsurpassed for clarity and depth and it is still the best reference for whoever wishes to learn about the field [4]. Bell strongly opposed the “Copenhagen interpretation” of quantum physics, according to which macroscopic objects, such as chairs and planets, do exist out there, but electrons and other microscopic particles do not. According to the Copenhagen view, the world is divided into two realms, macro and micro, “classical” and “quantum,” logical and contradictory—or, as Bell put it in one of his essays, into “speakable” and “unspeakable.” Along with Albert Einstein, Erwin Schrödinger, Louis de Broglie and →David Bohm, Bell was one of the few physicists compelled by his conscience to reject the Copenhagen interpretation. Bell emphasized that the empirical facts of quantum physics do not at all force us to renounce realism. There is, in fact, a realist theory (→Bohmian mechanics, also known as the de BroglieBohm theory) that accounts—insofar as the nonrelativistic theory is concerned—for all of these facts in a most elegant way. This theory describes a world in which electrons, quarks and the like are point particles, always having positions that move in a manner dictated by the wave function. It should be taught to students, Bell insisted, as a legitimate alternative to the prevailing orthodoxy. After GianCarlo Ghirardi, Alberto Rimini, and Tullio Weber in 1986 succeeded in formulating a second kind of realist theory, Bell encouraged the further development of this theory as well [4, p. 201]. He thought that such a theory contained the seeds of a