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.
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.
Hidden variable theories, as encouraged by the EPR paper, that proliferated in the 1950s, were taken quite seriously in some academic quarters, despite the existence of the 1932 von Neumann ‘proof’ that supposedly showed that these hidden theories were incompatible with the stochasticity of quantum mechanics. In 1964, John Bell, a particle theorist, reexamined the von Neumann proof, found it ‘wanting’, and offered his own proof that reaffirmed that hidden variable theories were incompatible with quantum mechanics. Bell added a twist: his proof was in the form a neat inequality theorem that was easy to comprehend and became an instant success. Bell's theorem led to a new generation of experimentalists to rediscover quantum entanglement, and to conduct these experiments in the visible spectrum, to test for evidence of hidden variable theories. The results were negative thus reaffirming the validity of quantum mechanics and its indeterminacy. In this chapter the premises of Bell's theorem are explained as well as its link to the probabilities of quantum entanglement.
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.
By 1935, the Copenhagen interpretation had become the orthodoxy. Einstein needed to find a situation in which it is possible in principle to acquire knowledge of the state of a quantum system without disturbing it in any way. Working with two young theorists, Boris Podolsky and Nathan Rosen, Einstein devised an extraordinarily cunning challenge based on entangled particles. We can discover the state of one particle with certainty by making measurements on its entangled partner. All we have to assume is that the particles are local: any measurement we make on one in no way affects or disturbs the other. Through the work of David Bohm and John Bell, the challenge posed by EPR became accessible to experiment, and Bell devised a simple test for all locally realistic theories. All the experiments performed to date suggest that the standard quantum formalism is correct: in any realistic interpretation, quantum particles are non-local.
Quantum mechanics makes surprising predictions that defy our classical understanding of the world.Experiments done on entangled particles-particles with linked properties-prove that actions on one particle can affect the other one no matter the distance.In response, physicists came up with local hidden variable theories in an attempt to understand this quantum phenomena through a classical lens.John Bell formulated a way to test whether nature is dictated by local hidden variable theories or quantum mechanics, and since then it has been the goal of numerous physicists to experimentally test the theorem and improve upon the results.This paper describes the history of these experiments, their pros and cons, their advancements, and the potential applications that they will enable.
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.
The first round of the Einstein–Bohr debates took place when Einstein challenged Bohr’s principle of complementarity at the Solvay conference in 1927 and Bohr successfully defended it. The most serious challenge, however, came in 1935 when a paper by Einstein, Podolsky, and Rosen (EPR) argued that quantum mechanics was incomplete through a gedanken experiment motivating an approach based on hidden variables. In this chapter, EPR’s arguments about the incompleteness of quantum mechanics and Bohr’s reply to them are presented. The ultimate answer came almost 30 years later, almost ten years after Einstein’s death, and was nothing that Einstein would have expected. Bell’s inequality and the subsequent Bell-CHSH inequality, that are satisfied by all theories based on the “self-evident truths” of reality and locality are discussed. The startling results that quantum mechanics violates Bell’s inequality and the experimental results are in agreement with the prediction of quantum mechanics are presented.