Abstract Abner Shimony read a pre-print of Bell’s paper, and realized its brilliance. The old experiments discussed by Bohm and Aharonov were indecisive. New experiments were urgently needed. John Clauser, who as a student had become greatly frustrated by Bohr’s quantum obscurity, reached the same conclusion. Clauser became obsessed, encouraged by support from Bell, Bohm, and de Broglie. Shimony and Clauser agreed to collaborate. They alighted on an experiment in which pairs of entangled photons are produced by atomic cascade emission, and developed a form of Bell’s inequality more suited to laboratory conditions. By 1970, the growing interest in foundations gave birth to a new journal and an international conference at which familiar arguments were aired by the old faithful, and at which newcomers gave fresh perspectives. Clauser published his results in 1972. Quantum mechanics prevails: local hidden variables do not exist. Other results soon followed, one contradictory, most confirmatory.
Abstract Abner Shimony read a pre-print of Bell’s paper, and realized its brilliance. The old experiments discussed by Bohm and Aharonov were indecisive. New experiments were urgently needed. John Clauser, who as a student had become greatly frustrated by Bohr’s quantum obscurity, reached the same conclusion. Clauser became obsessed, encouraged by support from Bell, Bohm, and de Broglie. Shimony and Clauser agreed to collaborate. They alighted on an experiment in which pairs of entangled photons are produced by atomic cascade emission, and developed a form of Bell’s inequality more suited to laboratory conditions. By 1970, the growing interest in foundations gave birth to a new journal and an international conference at which familiar arguments were aired by the old faithful, and at which newcomers gave fresh perspectives. Clauser published his results in 1972. Quantum mechanics prevails: local hidden variables do not exist. Other results soon followed, one contradictory, most confirmatory.
During the 1960s, Bell produced his seminal work on the foundations of quantum theory, first showing that, contrary to von Neumann’s argument, hidden variable theories were allowed but then that they had to be non-local. Thus, he showed that quantum theory did not respect local causality, and he outlined experiments that could be used to test whether the assumption of local realism was true in quantum theory: Bell’s theorem, or Bell’s inequality, or Bell’s inequalities. His work was followed up by John Clauser, Abner Shimony, Michael Horne, and Richard Holt, who produced the CHSH inequality. Bell had moved to CERN, where he worked on the theory of neutrino experiments, and on nuclear and elementary particle physics, making crucial suggestions concerning the use of gauge theory for each type of physical force and also producing the Adler–Bell–Jackiw anomaly, or the ABJ anomaly.
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.
Abstract In 1957, Bohm and Yakir Aharonov alighted on experiments on correlated photons performed some years earlier. Although the experimenters had not looked for EPR-type correlations, Bohm and Aharonov found no contradictions with quantum theory, concluding that entanglement is a real property of matter. Inspired by Bohm, in 1964 John Bell first dispatched von Neumann’s impossibility proof, readmitting the possibility of local hidden variables, and showed how experiment could decide if they existed. Many physicists later misunderstood this as a challenge to Copenhagen. But, as Bohm and Aharonov explained: ‘ . . . there is no inconsistency in the quantum-mechanical conclusion that such correlations exist, but there is, in [Bohr’s] point of view, no way even to raise the question of what is their origin’. If the particles cannot be considered as separable, Einstein’s gaping explanatory hole returns. Interested experimenters had first to disinter the question, which Bell had buried in an obscure journal.
It’s well known the role of John Bell’s article On the Einstein- Podolsky-Rosen paradox, published in 1964, for having aroused a renewed interest toward the research about Quantum Mechanics foundations, though only after some years it was observed the consequences in stimulating and orienting the reflection and the research work of physicists concerned with the topic. In fact, the interest and the participation in the debate expanded following the article by Clauser, Horne, Holt and Shimony in 1969 on the proposal of experimental tests on Bell’s inequality verify, as well as after contributions and discussion at Varenna Conference in 1970 and the out- comes publication of first generation experimental tests conducted between 1972 and 1976. From John Bell’s scientific correspondence 10 unpublished letters are selected and, from November 1970 to January 1980, these docu- ments testify and confirm an increased involvement in the research on Quantum Mechanics foundations and involve, as interlocutors of the corre- spondence, authors such as Max Jammer, Daniel Evrard, Francois Bonsack Chabat, Louis de Broglie, David Bohm and Basil Hiley.
Abstract This chapter presents a detailed discussion of Bell inequalities and their experimental tests. Starting from the Aharonov-Bohm version of the Einstein-Podolsky-Rosen (EPR) paradox, we review EPR’s criteria for realism and completeness and Bell’s locality condition, before deriving the Clauser-Horne-Shimony-Holt (CHSH) inequality. This is followed by an exposition on Bell’s original inequality and other Bell inequalities by Wigner, as well as Clauser and Horne. We then discuss the milestones in the experimental violation of Bell inequalities, from the experiment of Freedman and Clauser in the 1970s, to the efforts of Aspect et al. in the 1980s, the experiments by the teams of Zeilinger and Gisin in the 1990s, and culminating the loophole-free tests in 2015. Finally we turn to a brief discussion of the interpretations of quantum mechanics, focusing on contrasting the realist views held by, e.g., Einstein and Bell, and the informational views with proponents such as Brukner and Zeilinger