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arXiv:1410.4935 · European Journal of Physics

Mathematical and physical meaning of the Bell inequalities

Emilio Santos

Published Jun 29, 2016• 8 citations
arXivHTMLPDF

Abstract

Abstract It is shown that the Bell inequalities are closely related to the triangle inequalities involving distance functions amongst pairs of random variables with values { 0 , 1 } . A hidden variables model may be defined as a mapping between a set of quantum projection operators and a set of random variables. The model is noncontextual if there is a joint probability distribution. The Bell inequalities are necessary conditions for its existence. The inequalities are most relevant when measurements are performed at space-like separation, thus showing a conflict between quantum mechanics and local realism (Bell's theorem). The relations of the Bell inequalities with contextuality, the Kochen–Specker theorem, and quantum entanglement are briefly discussed.

DOI
10.1088/0143-0807/37/5/055402

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Abstract It is shown that the Bell inequalities are closely related to the triangle inequalities involving distance functions amongst pairs of random variables with values { 0 , 1 } . A hidden variables model may be defined as a mapping between a set of quantum projection operators and a set of random variables. The model is noncontextual if there is a joint probability distribution. The Bell inequalities are necessary conditions for its existence. The inequalities are most relevant when measurements are performed at space-like separation, thus showing a conflict between quantum mechanics and local realism (Bell's theorem). The relations of the Bell inequalities with contextuality, the Kochen–Specker theorem, and quantum entanglement are briefly discussed.
Abstract Bell inequalities are consequences of local realism while violated by quantum mechanics. In particle physics, entangled high energy particles can be produced from a common source, and the decay of each particle plays the role of measurement. However, in a hidden variable theory, the decay could be determined by hidden variables. This loophole killed such approaches to Bell test in particle physics. It is a special form of measurement-setting or free-will loophole, which also exists in other systems. Using entangled baryons, we present new inequalities of local realism with the explicit assumption of the dependence of the decays on hidden variables, as well as the consideration of the statistical mixture of polarizations and the separation of local hidden variables for objects with spacelike distances. These violations closes the measurement-setting loophole once and for all. We propose to use the processes ηc→ΛΛˉ\eta _c\rightarrow \varLambda {\bar{\varLambda }}ηc​→ΛΛˉ ηc→ΛΛ¯ and χc0→ΛΛˉ\chi _{c0} \rightarrow \varLambda {\bar{\varLambda }}χc0​→ΛΛˉ χc0→ΛΛ¯ to test our inequalities, and show that their violations are likely to be observed with the data already collected in BESIII.
The conceptually puzzling features of quantum mechanics as a statistical theory all have their source in the impossibility of relating the probability assignments defined by the quantum state to distributions over determinate values of the physical magnitudes. While some interpretations of quantum mechanics, as well as “hidden variable” modifications of the theory, have proposed several constructions for assigning values to magnitudes (beyond the value assignments defined by the quantum state alone), a variety of theorems exist which impose severe restrictions on such theoretical constructions. This note will deal with Bell's proof (in [2]), that any hidden variable theory satisfying a physically reasonable locality condition is characterized by an inequality which is inconsistent with the quantum statistics. Bell's result has initiated a series of experiments designed to refute the entire class of local hidden variable theories. We show that Bell's inequality actually characterizes a feature of hidden variable theories which is much weaker than locality in the sense considered physically motivated.
A number of additional assumptions is discussed playing a role in derivations of the Bell inequalities along with the locality condition. An analogy between quantum mechanics and statistical thermodynamics is exploited to demonstrate that the ex istence of conditional probabilities p(ai|,Lambda)generally assumed in stochastic hidden variables theories, may be questionable within the domain of application of quan tum mechanics. It is demonstrated how this could block derivations of the Bell inequalities and of the Kochen-Specker theorem. The analogy also provides a pos sible understanding of the nonlocality of quantum mechanics as evidenced by the Borchers-Schlieder theorem.
We provide necessary and sufficient conditions for several observables to have a joint distribution. When applied to the bivalent observables of a quantum correlation experiment, we show that these conditions are equivalent to the Bell inequalities, and also to the existence of deterministic hidden variables. We connect the no-hidden-variables theorem of Kochen and Specker to these conditions for joint distributions. We conclude with a new theorem linking joint distributions and commuting observables, and show how violations of the Bell inequalities correspond to violations of commutativity, as in the theorem.
Abstract The relation between the Bell inequalities, locality and the existence of joint probability distributions is discussed in different realist interpretations of quantum mechanics. We distinguish four realist interpretations, viz., the objectivistic one, the contextualistic one, the strictly nonobjectivistic and the quasi‐objectivistic interpretation. Conclusions are differing largely in different interpretations. We also distinguish between two kinds of locality, viz, macrolocality and Einstein/Bell locality. From a classical model of stochastic measuring processes a definition of Einstein/Bell locality is derived that differs from the Bell/Clauser/Horne/Shimony factorizability condition. It is demonstrated that only in the quasi‐objectivistic interpretation the Einstein/Bell locality condition plays a role in the derivation of a Bell inequality for quantities that are experimentally relevant. It is argued that even in this interpretation it is not possible to arrive at the conclusions that the Bell inequalities stem from the locality condition because of the tacit assumption of an additional property, namely the existence of probability distributions conditionalized on the dispersionfree states of the hidden variables. Consideration of a phase space representation of the Schrödinger equation demonstrates that this latter assumption is at odds with the statistics of quantum systems.
Noncontextual hidden variables theories, assigning simultaneous values to all quantum mechanical observables, are inconsistent by theorems of Gleason and others. These theorems do not exclude contextual hidden variables theories, in which a complete state assigns values to physical quantities only relative to contexts. However, any contextual theory obeying a certain factorisability condition implies one of Bell's Inequalities, thereby precluding complete agreement with quantum mechanical predictions. The present paper distinguishes two kinds of contextual theories, ‘algebraic’ and ‘environmental’, and investigates when factorisability is reasonable. Some statements by Fine about the philosophical significance of Bell's Inequalities are then assessed. INTRODUCTION In most of the discussions of Bell's Inequalities little notice is taken of the historical circumstances of their discovery. J. S. Bell was told by J. Jauch of the work of A. Gleason [1957] which showed that simultaneous values cannot be assigned to all observables of a quantum mechanical system in a way that respects their algebraic structure (except in the simple case in which the relevant Hilbert space has dimension less than three). Gleason's theorem thus precludes a type of hidden variables theory that has come to be called ‘noncontextual’. The question of agreement between the statistical predictions of quantum mechanics and those of a noncontextual hidden variables theory does not even arise, since such a theory cannot be consistently formulated.