I s it possible to bring experiments to bear on metaphysical theses such as realism? In recent developments in quantum mechanics particularly associated with the late John Bell it has been claimed that a certain type of realism, what is known as local realism, has indeed been refuted by a series of experiments, culminating in those carried out by Alain Aspect and his collaborators in Paris in 1982. Crudely local realism means in this context that atomic and subatomic entities, the subject matter of quantum mechanics, possess definite sharp values for all their attributes at all times, and, in addition, these attributes cannot be affected instantaneously by operations such as measurements performed on other microentities at different spatial locations from the one whose attributes are in question. In other words, realism is supplemented with a denial of the possibility of instantaneous action-at-a-distance. Now relativity theory, the other great pillar of modern theoretical physics, is standardly understood as committed to such a denial. So an experimental refutation of local realism would force us to give up either locality (the no-instantaneous-action-at-a-distance principle) or the realist thesis itself. The first horn of this dilemma is closed off by appeal to relativity theory, and so the conclusion of the argument would appear to be the refutation of realism.
I s it possible to bring experiments to bear on metaphysical theses such as realism? In recent developments in quantum mechanics particularly associated with the late John Bell it has been claimed that a certain type of realism, what is known as local realism, has indeed been refuted by a series of experiments, culminating in those carried out by Alain Aspect and his collaborators in Paris in 1982. Crudely local realism means in this context that atomic and subatomic entities, the subject matter of quantum mechanics, possess definite sharp values for all their attributes at all times, and, in addition, these attributes cannot be affected instantaneously by operations such as measurements performed on other microentities at different spatial locations from the one whose attributes are in question. In other words, realism is supplemented with a denial of the possibility of instantaneous action-at-a-distance. Now relativity theory, the other great pillar of modern theoretical physics, is standardly understood as committed to such a denial. So an experimental refutation of local realism would force us to give up either locality (the no-instantaneous-action-at-a-distance principle) or the realist thesis itself. The first horn of this dilemma is closed off by appeal to relativity theory, and so the conclusion of the argument would appear to be the refutation of realism.
I am very grateful to the Master and Fellows of Trinity College for inviting me to give the Tarner Lectures. What I want to do in this book, stated boldly and baldly, is to consider the relevance of developments in modern theoretical physics to metaphysical questions about the ultimate nature of reality. By metaphysical questions I mean the very general sorts of questions that arise out of a critical examination of the principles, concepts and fundamental presuppositions that lie behind modern physics. How should we interpret the claims that physicists seem to be making about the world? Are they intended to be understood as literally true, for example, and exactly what are those claims, given that the pages of theoretical physics monographs are largely filled with abstract mathematical formalism? In the first chapter, I shall be looking at a range of philosophical points of view in the framework of which such questions can be addressed, and attempt the difficult task of adjudicating between the different approaches. At the end of the day I shall be arguing for a pretty straightforward metaphysical realism, that the external world exists independently of our knowledge of it. In the second chapter I shall look at arguments from various branches of modern physics that are sometimes claimed to bring in an essential role for subjectivity in physics, bringing the observer into a central role, creating his own reality.
According to the orthodox interpretation of quantum mechanics (Bohr being its leading voice), the attributes of a physical object (position, momentum, spin, etc.) can be assigned only when they have been measured. Einstein advocated the local realist viewpoint that an object had definite attributes whether they had been measured or not. The Copenhagen school view that measurement actually produces an object’s property would imply that the measurement of one part of an entangled quantum state can instantaneously fix the value of another part, no matter how far the two parts have been separated. Einstein, Podolsky, and Rosen devised a thought experiment to shine a light on this “spooky action-at-a-distance” feature; its discussion and debate have illuminated the meaning of quantum mechanics (QM). It led later to Bell’s theorem showing that these seemingly philosophical questions can lead to observable results. The experimental vindication of the orthodox theory has sharpened our appreciation of the nonlocal features of quantum mechanics. Nevertheless, the counter-intuitive QM picture of objective reality still troubles many, leaving one to wonder whether quantum mechanics is ultimately a complete theory.
This open letter challenges Annals of Physics' Editors and Bell's supporters on this front: in the context of Bell's theorem -- after AoP (2016:67) -- ‘it's a proven scientific fact that a violation of local realism has been demonstrated theoretically and experimentally.' We show that such claims under the Bellian canon are curtailed by its foundation on a naive realism that is known to be false; ie, under Bohr's old insight (in our terms), a test may disturb the tested system. Further: (i) We define a general all-embracing local realism -- CLR, commonsense local realism -- the union of local-causality (no causal influence propagates superluminally) and physical-realism (some physical properties change interactively). (ii) Under CLR, with EPR-based variables (and without QM), a thought-experiment delivers a local-realistic account of EPRB and GHZ in 3-space. (iii) Under EPR, mixing common-sense with undergrad math/physics in the classical way so favored by Einstein, we interpret QM locally and realistically. (iv) We find the flaw in Bell's theorem: Bell's 1964:(14a) ≠ Bell's 1964:(14b) under EPRB. (v) EPR (1935) famously argue that additional variables will bring locality and causality to QM's completion; we show that they are right. (vi) Even more famously, Bell (1964) cried ‘impossible' against such variables; we give the shortest possible refutation of his claim. (vii) Using Bell's (1988:88) moot gloss on a fragment of von Neumann's work, we conclude: ‘There's nothing to Bell's theorem -- nor Bellian variants like CHSH (1969), Mermin (1990), Peres (1995); nor Bellian endorsements like those by Bricmont, du Sautoy, Goldstein et al., Maudlin, Norsen, Shimony -- it's not just flawed, it's silly; its assumptions nonsense; it's not merely false but foolish' and misleading. (viii) Our results accord with common-sense, QM, Einstein's principles, EPR's belief and Bell's hopes and expectations.
I consider the extent to which the applicability of the concept of classical realism is constrained, irrespective of the validity or not of the quantum formalism, by existing experiments both in the EPR–Bell setup, including recent experiments testing ‘nonlocal realistic’ theories, and in the area of ‘macroscopic quantum coherence’. Unless we are willing to sacrifice one or more other intuitively plausible notions such as that of the conventional ‘arrow of time’, it appears impossible, in either context, to maintain the classical notion of realism.
Recent experiments of Gröblacher et al proved the violation of a Leggett-type inequality that was claimed to be valid for a broad class of non-local hidden-variable theories. The impossibility of constructing a non-local and realistic theory, unless it entails highly counterintuitive features, seems thus to have been experimentally proved. This would bring us close to a definite refutation of realism. Indeed, realism was proved to be also incompatible with locality, according to a series of experiments testing Bell inequalities. The present paper addresses the said experiments of Gröblacher et al and presents an explicit, contextual and realistic, model that reproduces the predictions of quantum mechanics. It thus violates the Leggett-type inequality that was established with the aim of ruling out a supposedly broad class of non-local models. We can thus conclude that plausible contextual, realistic, models are still tenable. This restates the possibility of a future completion of quantum mechanics by a realistic and contextual theory which is not in a class containing only highly counterintuitive models. The class that was ruled out by the experiments of Gröblacher et al is thus proved to be a limited one, arbitrarily separating models that physically belong in the same class.
Spooky Action at a Distance In the context of correlation experiments involving pairs of experiments performed at very nearly the same time in very far-apart experimental regions, Einstein famously said [1], But on one supposition we should in my opinion hold absolutely fast: “The real factual situation of the system S2 is independent of what is done with system S1 which is spatially separated from the former.” This demand is incompatible with the basic ideas of standard (Copenhagen/orthodox) quantum mechanics, which makes two relevant claims: (1) Experimenters in the two labs make “local free choices” that determine which experiments will be performed in their respective labs. These choices are free in the sense of not being predetermined by the prior history of the physically described aspects of the universe, and they are localized in the sense that the physical effects of these free choices are inserted into the physically described aspects of the universe only within the laboratory in which the associated experiment is being performed. (2) These choices of what is done with the system being measured in one lab can (because of a measurement-induced global collapse of the quantum state) influence the outcome of the experiment performed at very close to the same time in the very faraway lab. This influence of what is done with the system being measured in one region upon the outcome appearing at very nearly the same time in a very faraway lab was called “spooky action at a distance” by Einstein, and was rejected by him as a possible feature of “reality.” John Bell's Quasi-classical Statistical Theory Responding to the seeming existence in the quantum world of “spooky actions,” John Bell [2] proposed a possible alternative to the standard approach that might conceivably be able to reconcile quantum spookiness with “reality.” This alternative approach rests on the fact that quantum mechanics is a statistical theory.We already have in physics a statistical theory called “classical statistical mechanics.”