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CRC Press eBooks

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Bernard d’Espagnat

Published May 4, 2018• 0 citations
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Abstract

A view most physicists adhere to is that scientific research should keep aloof from implicit metaphysical hypotheses. This prompts most of them to make it a principle that the only "existing" physical entities are those that could somehow be observed, directly or indirectly. In quantum mechanics the assumption is made, that, with the exception of nondynamical quantities such as masses and electric charges as well as of time. All these observable quantities are described by Hermitean operators, with the help of which the quantum rules tell us all that can, or could, be observed, in any circumstances whatsoever. One basic feature of the pilot-wave theory is that the quantum mechanical predictions are exactly reproduced for any ensemble E in which the hidden variables are distributed at random. This suffices to show how difficult it is to build up tests to discriminate the two theories on the basis of their observational predictions.

DOI
10.1201/9780429503283-4

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A view most physicists adhere to is that scientific research should keep aloof from implicit metaphysical hypotheses. This prompts most of them to make it a principle that the only "existing" physical entities are those that could somehow be observed, directly or indirectly. In quantum mechanics the assumption is made, that, with the exception of nondynamical quantities such as masses and electric charges as well as of time. All these observable quantities are described by Hermitean operators, with the help of which the quantum rules tell us all that can, or could, be observed, in any circumstances whatsoever. One basic feature of the pilot-wave theory is that the quantum mechanical predictions are exactly reproduced for any ensemble E in which the hidden variables are distributed at random. This suffices to show how difficult it is to build up tests to discriminate the two theories on the basis of their observational predictions.
Any theory whatsoever which, in all circumstances, attributes definite properties to non-interacting physical objects necessarily violates some of the predictions of quantum mechanics. The very existence of the hidden variables theories should, however, prompt to have a second look at the problem. The theory necessarily implies the existence of a violent nonlocality effect which, as a matter of fact, may reasonably be considered as similar to—and even more surprising than—the nonseparability of the orthodox theory. The simultaneous existence of non-separability effects and of the principle that the velocity of propagation of interactions is finite raises nontrivial conceptual problems. One possible way to study these problems is to make use of the fact that hidden variables theories exist which reproduce correctly the verifiable predictions of quantum mechanics. For the ensembles the expectation values coincide, of course, with the observed value.
Introduction When I was a student I had much difficulty with quantum mechanics. It was comforting to find that even Einstein had had such difficulties for a long time. Indeed they had led him to the heretical conclusion that something was missing in the theory: ‘I am, in fact, rather firmly convinced that the essentially statistical character of contemporary quantum theory is solely to be ascribed to the fact that this (theory) operates with an incomplete description of physical systems.’ More explicitly, in ‘a complete physical description, the statistical quantum theory would … take an approximately analogous position to the statistical mechanics within the framework of classical mechanics …’. Einstein did not seem to know that this possibility, of peaceful coexistence between quantum statistical predictions and a more complete theoretical description, had been disposed of with great rigour by J. von Neumann. I myself did not know von Neumann's demonstration at first hand, for at that time it was available only in German, which I could not read. However I knew of it from the beautiful book by Born, Natural Philosophy of Cause and Chance , which was in fact one of the highlights of my physics education. Discussing how physics might develop Born wrote: ‘I expect … that we shall have to sacrifice some current ideas and to use still more abstract methods. However these are only opinions.
Bell’s inequality is an empirical constraint on theories with hidden variables, which Einstein, Podolsky and Rosen argued are needed to explain observed perfect correlations if keeping locality. One way to deal with the empirical violation of Bell’s inequality is by openly embracing nonlocality, in a theory like the pilot-wave theory. Nonetheless, recent proposals have revived the possibility that one can avoid nonlocality by resorting to superdeterministic theories. These are local hidden variables theories which violate statistical independence which is one assumption of Bell’s inequality. In this paper I compare and contrast these two hidden variable strategies: the pilot-wave theory and superdeterminism. I show that even if the former is nonlocal and the other is not, both are contextual. Nonetheless, in contrast with the pilot-wave theory, superdeterminist contextuality makes it impossible to test the theory (which therefore becomes unfalsifiable and unconfirmable) and renders the theory uninformative (measurement results tell us nothing about the system). It is questionable therefore whether a theory with these features is worth its costs.
Scientific realism is the view that our best scientific theories can be regarded as (approximately) true. This is connected with the view that science, physics in particular, and metaphysics could (and should) inform one another: on the one hand, science tells us what the world is like, and on the other hand, metaphysical principles allow us to select between the various possible theories which are underdetermined by the data. Nonetheless, quantum mechanics has always been regarded as, at best, puzzling, if not contradictory. As such, it has been considered for a long time at odds with scientific realism, and thus a naturalized quantum metaphysics was deemed impossible. Luckily, now we have many quantum theories compatible with a realist interpretation. However, scientific realists assumed that the wave-function, regarded as the principal ingredient of quantum theories, had to represent a physical entity, and because of this they struggled with quantum superpositions. In this paper I discuss a particular approach which makes quantum mechanics compatible with scientific realism without doing that. In this approach, the wave-function does not represent matter which is instead represented by some spatio-temporal entity dubbed the primitive ontology: point-particles, continuous matter fields, space-time events. I argue how within this framework one develops a distinctive theory-construction schema, which allows to perform a more informed theory evaluation by analyzing the various ingredients of the approach and their inter-relations.