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arXiv:quant-ph/0510101

Bell's Theorem Reexamined

Luiz Carlos Ryff

Published Oct 13, 2005• 0 citations
arXivHTMLPDF

Abstract

An Einstein-Podolsky-Rosen (EPR)-like argument using events separated by a time-like interval strongly suggestes that measuring the polarization state of a photon of an entangled pair changes the polarization state of the other distant photon. Trough a very simple demonstration, the Wigner-D'Espagnat inequality is used to show that in order to prove Bell's theorem neither the assumption that there is a well-defined space of complete states $λ$ of the particle pair and well-defined probability distribution $ρ(λ)$ over this space nor the use of counterfactuals is necessary. These results reinforce the viewpoint that quantum mechanics implicitly presupposes some sort of nonlocal connection between the particles of an entangled pair. As will become evident from our discussion, relinquishing realism and/or free will cannot solve this apparent puzzle.

DOI
10.48550/arxiv.quant-ph/0510101

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An Einstein-Podolsky-Rosen (EPR)-like argument using events separated by a time-like interval strongly suggestes that measuring the polarization state of a photon of an entangled pair changes the polarization state of the other distant photon. Trough a very simple demonstration, the Wigner-D'Espagnat inequality is used to show that in order to prove Bell's theorem neither the assumption that there is a well-defined space of complete states λλλ of the particle pair and well-defined probability distribution ρ(λ)ρ(λ)ρ(λ) over this space nor the use of counterfactuals is necessary. These results reinforce the viewpoint that quantum mechanics implicitly presupposes some sort of nonlocal connection between the particles of an entangled pair. As will become evident from our discussion, relinquishing realism and/or free will cannot solve this apparent puzzle.
Most physicists agree that the Einstein–Podolsky–Rosen–Bell paradox exemplifies much of the strange behavior of quantum mechanics, but argument persists about what assumptions underlie the paradox. To clarify what the debate is about, we employ a simple and well-known thought experiment involving two correlated photons to help us focus on the logical assumptions needed to construct the EPR and Bell arguments. The view presented in this paper is that the minimal assumptions behind Bell’s inequality are locality and counterfactual definiteness but not scientific realism, determinism, or hidden variables as are often suggested. We further examine the resulting constraints on physical theory with an illustration from the many-worlds interpretation of quantum mechanics—an interpretation that we argue is deterministic, local, and realist but that nonetheless violates the Bell inequality.
A critical reconsideration of the EPR (Einstein-Podolsky-Rosen) paper shows that the EPR argument can be developed without using the concept of `element of physical reality', thus eliminating any philosophical element in the logical chains of the paper. Deprived of its philosophical ornament, the EPR argument plainly reduces to require what quantum mechanics can not do: to assign definite values to two incompatible physical quantities. Hidden variables theories built up according to Bell - type theorems are formulated on the basis of the assumption that the locality condition implies the statistical independence between two measurements space - like separated. This assumption is valid only with the additional one that statistical dependence between two measurements requires a causal connection between them. This additional assumption rules out the possibility that statistical dependence may due to an intrinsic property of the physical system under study. Therefore, hidden variables theories are built up with a restriction which leads them to be disproved by experiment. Quantum mechanical non - locality, invoked for describing EPR - type experiments, is strictly connected to the hypothesis (NDV hypothesis) according to which the twin photons of entangled pairs do not have a definite polarization before measurements. Both hypotheses are used only for describing EPR experiments and not for making predictions. Therefore, they can be dropped without reducing the predictive power of quantum mechanics concerning entangled photons pairs. Furthermore, both hypotheses can be experimentally tested by a modification of a standard experimental apparatus designed for studying entangled photons pairs.
In their original paper, Einstein, Podolsky, and Rosen (EPR) questioned the probabilistic nature of the quantum theory using an argument based on the quantum-mechanical correlations between two distant particles which had once interacted. Bell later proved that the quantum theory and any local theory differ significantly in their predictions for these experimentally measurable correlations. It is shown that the arguments of EPR can be extended to apply equally well to certain properties of a single particle or system and that, in analogy with Bell's theorem, the predictions of the quantum theory for certain single-particle experiments are in disagreement with the predictions of any local theory. For example, based on either the quantum theory or existing experimental observations, it is possible to arrange for a photon to have a negligible probability of not being absorbed by some system of finite dimensions, such as one of a large number of atoms, within a finite time interval. It is shown that any local theory which satisfies this requirement must also require that negligible interference effects occur in an optical interferometer at sufficiently low intensities from an incoherent source in combination with sufficiently large separation LLL between the two optical paths. Time-reversal invariance is used to estimate the requirement $L\ensuremath{\gg}\frac{3{\ensuremath{\lambda}}^{2}}{4{\ensuremath{\pi}}^{2}{r}_{c}}$, where $\ensuremath{\lambda}$ is the wavelength of the light and rc{r}_{c}rc​ is the classical radius of the electron; at optical wavelengths this distance is on the order of 10 m. The same separation should also be required between the photon detectors in any optical experiment based on Bell's theorem. A review of the literature indicates that no existing experimental data, either from optical interferometers or experiments based on Bell's theorem, can be used to distinguish between the local and quantum theories. An investigation of low-intensity optical interference at large path separations is proposed for this purpose.
Entangled quantum systems can connect to the environment via a Bell state measurement. This applies, for example, to teleportation and entanglement swapping. Although the results are well understood, it is not entirely clear whether they involve nonlocal action or whether they are predetermined. This can best be decided from a model, provided it predicts the key measurement results. Models based on the fact that the partners of an entangled pair have the same value of a statistical parameter cannot be applied here. This is because the partner particles of the resulting entangled states after a teleportation or an entanglement swapping never had contact before. The question then is, what connects entangled photons? Therefore, this paper presents a local realistic model that reproduces the quantum mechanical predictions for expectation values with polarization measurements, but is not based on shared statistical parameters. Instead, the coupling of the entangled particles is based on initial conditions and conservation of spin angular momentum. The model refutes Bell's theorem and also explains teleportation and entanglement swapping in a local way. The manuscript is thus a step forward toward a complete theory describing quantum physical reality as thought possible by Einstein, Podolsky, and Rosen.
We use the Greenberger-Horne-Zeilinger [in Bell's Theorem, Quantum Theory,and Conceptions of the Universe, edited by M. Kafatos (Kluwer Academic, Dordrecht, 1989)] approach to present three demonstrations of the failure of Einstein-Podolsky-Rosen (EPR) [Phys. Rev. 47, 777 (1935)] local realism for the case of two spinless particles in a two-particle interferometer. The original EPR assumptions of locality and reality do not suffice for this. First, we use the EPR assumptions of locality and reality to establish that in a two-particle interferometer, the path taken by each particle is an element of reality. Second, we supplement the EPR premises by the postulate that when the path taken by a particle is an element of reality, all paths not taken are empty. We emphasize that our approach is not applicable to a single-particle interferometer because there the path taken by the particle cannot be established as an element of reality. We point out that there are real conceptual differences between single-particle, two-particle, and multiparticle interferometry.
We argue that the conclusion of Bell theorem, namely, that there must be spatial non-local correlations in certain experimental situations, does not apply to typical individual measurements performed on entangled EPR pairs. Our claim is based on three points, (i) on the notion of quantum {\it complete measurements}; (ii) on Bell results on local yet distant measurements; and (iii) on the fact that perfect simultaneity is banned by the quantum mechanics. We show that quantum mechanics indicates that, while the measurements of the pair members are indeed space-like separated, the pair measurement is actually a sequence of two complete measurements, the first one terminating the entanglement and, therefore, the second one becoming unrelated to the initial preparation of the entangled pair. The outstanding feature of these measurements is that neither of them violates the principle of locality. We discuss that the present measurement viewpoint appears to run contrary to the usual interpretation of "superposition" of states with its concomitant "collapse" on measurement.