It is argued in this work that in order to explain the conflict between Bell's inequality (BI) and quantum mechanics (QM), it is neither necessary to give up the notion of Einstein locality (LOC), nor that of reality, as is done in some recent work. The definite existence of EPR's “elements of reality” is shown by using a result in a former thought experiment by M. Renninger. Einstein locality is saved by sticking to the failure of counterfactual definiteness (CFD) at the level of individual quantum processes.
It is argued in this work that in order to explain the conflict between Bell's inequality (BI) and quantum mechanics (QM), it is neither necessary to give up the notion of Einstein locality (LOC), nor that of reality, as is done in some recent work. The definite existence of EPR's “elements of reality” is shown by using a result in a former thought experiment by M. Renninger. Einstein locality is saved by sticking to the failure of counterfactual definiteness (CFD) at the level of individual quantum processes.
The notion of the Einstein-Podolsky-Rosen (EPR) "element of reality" is much discussed in the literature on the foundations of quantum mechanics. Recently, it has become particularly relevant due to a proposed criterion of the physical reality of a given quantum mechanical observable [A. L. O. Bilobran and R. M. Angelo, Europhys. Lett. 112, 40005 (2015)]. We examine this proposal and its consequently related measure of non-locality [V. S. Gomez and R. M. Angelo, Phys. Rev. A 97, 012123, (2018)] and argue that the criterion is ill-described as quantifying physical reality without introducing serious inconsistency with the basic notions of realism that under-gird enquiry. We agree that this reality criterion demonstrates, along with the famous GHZ results, that general quantum observable values make for poor elements of reality. However, we also argue that this does not mean no such elements of reality are to be found in quantum theory. By arguing for, and adopting, probability distributions as these elements of reality instead, we demonstrate that the criterion of physical reality is actually one of observable predictability. We then examine the relationship of realism-based non-locality to the Bell form and find that, despite the flawed premise, this measure does indeed codify non-locality that is not captured by Bell inequalities.
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.
The concept of elements of physical reality (e.p.r.) in quantum mechanics as defined by Einstein, Podolsky and Rosen (EPR) is discussed in the context of the EPR–Bohm and the EPR–Bell experiments on a pair of spin 1/2 particles in the singlet state. It is argued that EPR's definition of e.p.r. is appropriate to the EPR–Bell experiment rather than to the EPR–Bohm experiment, and that Bohr's interpretation of e.p.r. is also consistent with such a viewpoint. It is shown that the observed correlation between the spins of the two particles in the EPR–Bell experiment is just a manifestation of the correlation that exists between the wave functions of the particles in the singlet state and a consequence of the fact that a Stern–Gerlach magnet does not change the state of a particle but only transforms its wave function into a representation defined by the axis of the magnet. As such, the correlation is suggested to be an affirmation of Einstein's concept of locality, and not an evidence for nonlocality.
The Einstein-Podolsky-Rosen argument for the incompleteness of quantum mechanics involves two assumptions: one about locality and the other about when it is legitimate to infer the existence of an element-of-reality. Using one simple thought experiment, we argue that quantum predictions and the relativity of simultaneity require that both these assumptions fail, whether or not quantum mechanics is complete. EPR’s (1935) argument for the incompleteness of QM turns upon the following sufficient condition: “If, without in any way disturbing a system, we can predict with certainty (i.e., with probability equal to unity) the value of a physical quantity, then there exists an element of physical reality corresponding to this physical quantity.“ EPR apply this condition in a case where knowledge gained from measuring one quantum system allows one to predict with certainty a value for a physical quantity on another.
Maudlin has claimed that no local theory can reproduce the predictions of standard quantum mechanics that violate Bell’s inequality for Bohm’s version (two spin-half particles in a singlet state) of the Einstein-Podolsky-Rosen problem. It is argued that, on the contrary, standard quantum mechanics itself is a counterexample to Maudlin’s claim, because it is local in the appropriate sense (measurements at one place do not influence what occurs elsewhere there) when formulated using consistent principles in place of the inconsistent appeals to “measurement” found in current textbooks. This argument sheds light on the claim of Blaylock that counterfactual definiteness is an essential ingredient in derivations of Bell’s inequality.
Abstract The 1935 Einstein–Podolsky–Rosen (EPR) paper, “Can Quantum–Mechanical Description of Physical Reality Be Considered Complete?”, stands as a landmark in the foundational debates over quantum mechanics. While the paper has since become one of Einstein’s most cited works, it initially received little attention, and the feature of non-locality, which was central to the argument in the paper, was largely neglected for decades. In this paper, I offer two reasons for this early neglect of non-locality in the immediate aftermath of EPR. First, early responses to the EPR paper defended the completeness of quantum mechanics by reformulating the thought experiment the authors had presented, thereby obscuring the feature of quantum mechanics the authors had wished to highlight. Bohr’s influential reply, for instance, substituted the specific thought experiment presented by EPR with the two-slit experiment, thus suppressing the entanglement between spatially separated systems. A second reason for the marginalization of non-locality was the tendency to see Einstein’s critique of quantum mechanics as proceeding from a commitment to determinism, though this in fact played no role in the EPR paper or in his later writings and correspondence. This paper argues that the failure to recognize non-locality was partly a result of the rhetorical manoeuvring by EPR’s critics, but also to a broader philosophical climate dominated by concerns over determinism, which effectively eclipsed all other conceptual issues, Only with Bell’s theorem in the 1960s did non-locality return as a serious focus of foundational inquiry.