With the Nobel Prize attributed to Aspect, Clauser, and Zeilinger, the international scientific community acknowledged the fundamental importance of the experimental violation of Bell's inequalities. It is however still debated what fails in Bell's hypotheses, leading to these inequalities, and usually summarized as "local realism", or maybe more appropriately "classical local realism". The most common explanation is "quantum non-locality", that remains however fully compatible with relativistic causality; this makes wondering whether any non-local phenomenon is really involved in these experiments. Here we want to recapitulate another option, sometimes called "predictive incompleteness", closely related to the idea that the usual state vector $ψ$ is incomplete indeed, as it was claimed by Einstein, Podolsky and Rosen. However, the right way to complete $ψ$ has nothing to do with hidden variables, but requires to specify the measurement context, as it was claimed by Bohr. Here we will consider the simple case of two spin 1/2, or two qubits, in order to keep the argument simple, but it does apply generally in quantum mechanics.
With the Nobel Prize attributed to Aspect, Clauser, and Zeilinger, the international scientific community acknowledged the fundamental importance of the experimental violation of Bell's inequalities. It is however still debated what fails in Bell's hypotheses, leading to these inequalities, and usually summarized as "local realism", or maybe more appropriately "classical local realism". The most common explanation is "quantum non-locality", that remains however fully compatible with relativistic causality; this makes wondering whether any non-local phenomenon is really involved in these experiments. Here we want to recapitulate another option, sometimes called "predictive incompleteness", closely related to the idea that the usual state vector ψ is incomplete indeed, as it was claimed by Einstein, Podolsky and Rosen. However, the right way to complete ψ has nothing to do with hidden variables, but requires to specify the measurement context, as it was claimed by Bohr. Here we will consider the simple case of two spin 1/2, or two qubits, in order to keep the argument simple, but it does apply generally in quantum mechanics.
It is known that "quantum non locality", leading to the violation of Bell's inequality and more generally of classical local realism, can be attributed to the conjunction of two properties, which we call here elementary locality and predictive completeness. Taking this point of view, we show again that quantum mechanics violates predictive completeness, allowing the making of contextual inferences, which can, in turn, explain why quantum non locality does not contradict relativistic causality. An important question remains: if the usual quantum state ψ is predictively incomplete, how do we complete it? We give here a set of new arguments to show that ψ should be completed indeed, not by looking for any "hidden variables", but rather by specifying the measurement context, which is required to define actual probabilities over a set of mutually exclusive physical events.
In this article we argue that what is usually called non locality, leading to the violation of Bell's inequality and more generally of classical local realism, can be explained by considering contextual inferences, that are both possible and necessary in quantum physics. They do not show up in classical physics, where contexts may be discarded because physical properties are non-contextual. This analysis allows us to explain why non locality does not contradict relativistic causality. Our argument is general from a probabilistic point of view, and it strongly suggests that the usual quantum formalism must be completed, by specifying the measurement context.
In order to understand nonlocal phenomena, the corresponding processes must be treated as scattering experiments and investigated in the frame of quantum eld theory. Quantum mechanics is not su cient for this task, because it is only concerned with the in uence of a potential on a physical object. But in processes with nonlocal e ects the role of interaction between physical objects must be su ciently respected. All this is shown rst of all for the special case of the spin-1/2-experiment, and then generalized to arbitrary scattering processes. 1. What is the problem? Nonlocal phenomena are discussed and analyzed, since Einstein, Podolsky and Rosen in their famous paper [1] have drawn attention to a di culty of quantum mechanics. By means of their also famous gedankenexperiment the three authors have demonstrated their thesis that quantum mechanics is incomplete. So called local theories were developed in order to repair this pretended fault. The work of Bell [2] consisted in describing mathematically a general kind of such theories, and inferring the inequality bearing his name from characteristic features of such theories by mere logical deduction. In quantum mechanics a corresponding condition can be given, too. These two inequalities are not compatible with one another. Hence it seems to be quite natural to decide between them by experiment. Since all results of experimental investigations from Aspect et al. [3, 4] up to Erven et al. [5] have shown that Bell's inequality is not compatible with the empirical results, local theories nowadays are quite generally refuted. On the other hand all these experiments are supporting the results of quantum mechanics. Hence one could infer that quantum mechanics is correct. But that would be an illegal conclusion, because the agreement with experiment is a necessary, but no su cient condition for the validity of a theory. Hence one would convert a double negation into a positive assertion. In this paper the thesis is claimed and proved that quantum mechanics, indeed, is incomplete, not on the philosophical level, where Bohr and Einstein were quarrelling, but in an immediately physical sense.
This article presents a general discussion of several aspects of our present understanding of quantum mechanics. The emphasis is put on the very special correlations that this theory makes possible: They are forbidden by very general arguments based on realism and local causality. In fact, these correlations are completely impossible in any circumstance, except for very special situations designed by physicists especially to observe these purely quantum effects. Another general point that is emphasized is the necessity for the theory to predict the emergence of a single result in a single realization of an experiment. For this purpose, orthodox quantum mechanics introduces a special postulate: the reduction of the state vector, which comes in addition to the Schrödinger evolution postulate. Nevertheless, the presence in parallel of two evolution processes of the same object (the state vector) may be a potential source for conflicts; various attitudes that are possible to avoid this problem are discussed in this text. After a brief historical introduction, recalling how the very special status of the state vector has emerged in quantum mechanics, various conceptual difficulties are introduced and discussed. The Einstein–Podolsky–Rosen (EPR) theorem is presented with the help of a botanical parable, in a way that emphasizes how deeply the EPR reasoning is rooted into what is often called “scientific method.” In another section the Greenberger–Horne–Zeilinger argument, the Hardy impossibilities, as well as the Bell–Kochen–Specker theorem are introduced in simple terms. The final two sections attempt to give a summary of the present situation: One section discusses nonlocality and entanglement as we see it presently, with brief mention of recent experiments; the last section contains a (nonexhaustive) list of various attitudes that are found among physicists, and that are helpful to alleviate the conceptual difficulties of quantum mechanics.
In a recent note, Hance and Hossenfelder (arXiv:2211.01331) recall that "locally causal completions of quantum mechanics are possible, if they violate the assumption [called statistical independence or measurement independence] that the hidden variables do not in any way depend on measurement settings" and that, consequently, the experimental violations of Bell inequalities "show that maintaining local causality requires violating statistical independence". However, Hance and Hossenfelder also argue that "we should (...) look for independent experimental evidence that can distinguish the two different options: non-locality and statistical independence, or locality and violations of statistical independence" and that "the unwillingness to consider theories without statistical independence may be the reason we do not yet have a locally causal theory for the foundations of physics that is consistent with general relativity". Here, we recall that there is a third option, namely, rejecting that measurement outcomes are governed in any way by hidden variables. Moreover, we argue that some recent results in the search for principles singling out the sets of quantum correlations for Bell and Kochen-Specker contextuality scenarios point out that this third option is scientifically more plausible and answers the question of why and how nature produces quantum nonlocality.
The Nobel Prize in Physics 2022 was awarded to three experiment physicists: Alain Aspect, John F. Clauser and Anton Zeilinger, for their contributions to “experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science”. In this paper, the history, basics, and applications related with the Bell inequalities are reviewed. The discussion of fundamental issues on quantum mechanics dated back to the year 1927 when the fifth Solvay conference was held, highlighting intense arguments between Albert Einstein and Niels Bohr. At the time, Einstein expressed, publicly for the first time, his critical attitude towards quantum mechanics. While the arguments did not reach a decisive conclusion, Bohr’s probabilistic interpretation of quantum mechanics, the Copenhagen interpretation, had since been well circulated among the community. Einstein’s arguments with Bohr lasted many years after the conference. The arguably most famous one was the Einstein-Podolsky-Rosen (EPR) paradox, proposed in a 1935 paper written by the three authors. They stated, by flawless reasoning, that quantum mechanics is ‘incomplete’. Bohr replied immediately. So did Erwin Schrödinger, who, in particular, coined the term ‘entangled’ for the first time to describe the bizarre quantum mechanical property. Nevertheless, the EPR paper received few citations at its publication, until 1990’s when quantum information science started blossoming. From today’s perspective, the paper resembles the ‘sleeping beauty’ who slumbered for decades before fully awakening. It has been widely accepted that it was John S. Bell who eventually resolved the EPR paradox. But many important figures before him should not go unnoticed, including John A. Wheeler, Chien-Shiung Wu, David Bohm, to name a few. In 1964, Bell wrote down a mathematical inequality which satisfies local hidden variable theory but which can be violated in quantum mechanics. The significance is that Bell’s inequality brings the rather philosophic debates on the EPR paradox down to be experimentally testable. Hence, an experiment is here appealing, as is the case in all fields of physics. To the end, John F. Clauser, alongside Stuart J. Freedman, conducted an experiment in 1972 to test a Bell inequality, the Bell-Clauser-Horne-Shimony-Holt (CHSH) inequality, proposed by him and his collaborators in 1969. The results, however, leaves loopholes open. One of the loopholes was closed in a later experiment led by Alain Aspect in 1982. The Bell-CHSH inequality was later tested under a much stricter condition by Anton Zeilinger’s team. Two most important loopholes were both closed by three teams in 2015 independently. Zeilinger’s team was among them. Almost all experiments to date have confirmed quantum mechanical predictions. Zeilinger’s team also performed the first teleportation of unknown quantum states, the first entanglement swapping, along with other contributions pioneering the quantum information science. In retrospect, the foundational study of quantum theory has been developed over a century. It once drew much of quantum physics founders’ attention, went virtually unnoticed while other physical fields advanced, had for quite a while been considered to have very weak relationship with major physics, and finally revived in recent decades leading to rapid progress in quantum information science and quantum technology. We believe that the Nobel Prize in Physics this year is not only an award and recognition to past achievements, but also heralds a new era of science in future.