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Fortschritte der Physik

On Some Consequences of the Breakdown of Counterfactual Definiteness in the Quantum World

Willy De Baere

Published Nov 1, 1998• 1 citation
Publisher

Abstract

The crucial role of counterfactual reasoning in generating inconsistencies between local subquantum theories and quantum mechanics is stressed. It is argued that the physical justification of such reasoning requires the fulfilment of a criterion of actualizability of hypothetical situations. It is demonstrated that in a subquantum framework this criterion is equivalent to the assumption of actual reproducibility of individual subquantum states. Therefore, inconsistencies between local subquantum theories and quantum mechanics disappear if a nonreproducibility property of nature, in full agreement with the empirical fact that past, present and future are basically distinct, is invoked. The proposal also leaves the universal validity of the locality principle untouched, in agreement with all existing empirical evidence.

DOI
10.1002/(sici)1521-3978(199811)46:6/8<843::aid-prop843>3.0.co;2-1

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Comment on “Nonlocal character of quantum theory,” by Henry P. Stapp [Am. J. Phys. 65 (4), 300–304 (1997)]

Abner Shimony, Howard Stein

A 1997 paper by Henry Stapp attempts to achieve a definitive argument for his long-sustained thesis that quantum mechanics implies a nonlocality inconsistent with the locality of relativity theory. His argument assumes both the validity of counterfactual reasoning under certain circumstances and the free choice of performance of experiments, and it specifically refrains from any assumptions about “elements of physical reality” as understood by Einstein, Podolsky, and Rosen. In Sec. I we drastically condense his argument but retain his central principle of inference, called “LOC2,” which he claims is a consequence of relativistic locality. By careful attention to counterfactual reasoning we then throw doubt upon this claim and hence upon his argument as a whole. In Sec. II we answer Stapp’s reply to our critique of the 1997 paper. Section III is a postscript, commenting briefly on Stapp’s answer to Sec. II.

American Journal of Physics•Aug 1, 2001•18 citations
The crucial role of counterfactual reasoning in generating inconsistencies between local subquantum theories and quantum mechanics is stressed. It is argued that the physical justification of such reasoning requires the fulfilment of a criterion of actualizability of hypothetical situations. It is demonstrated that in a subquantum framework this criterion is equivalent to the assumption of actual reproducibility of individual subquantum states. Therefore, inconsistencies between local subquantum theories and quantum mechanics disappear if a nonreproducibility property of nature, in full agreement with the empirical fact that past, present and future are basically distinct, is invoked. The proposal also leaves the universal validity of the locality principle untouched, in agreement with all existing empirical evidence.
The empirical validity of the locality (LOC) principle of relativity is used to argue in favour of a local hidden variable theory (HVT) for individual quantum processes. It is shown that such a HVT may reproduce the statistical predictions of quantum mechanics (QM), provided the reproducibility of initial hidden variable states is limited. This means that in a HVT limits should be set to the validity of the notion of counterfactual definiteness (CFD). This is supported by the empirical evidence that past, present, and future are basically distinct. Our argumentation is contrasted with a recent one by Stapp resulting in the opposite conclusion, i.e. nonlocality or the existence of faster-than-light influences. We argue that Stapp's argumentation still depends in an implicit, but crucial, way on both the notions of hidden variables and of CFD. In addition, some implications of our results for the debate between Bohr and Einstein, Podolsky and Rosen are discussed.
Recent discussions by Mermin [1] and Stapp [2] in this journal on non-locality and counterfactuality are shown to contain linguistic problems that require verification. As such they can at most provide us with two subjective choices for the meaning of 'counterfactual statements' in quantum mechanics. We shall show that the word 'counterfactual' is in fact inappropriate here and should be replaced by the word 'hypothetical'. Mermin's choice imposes a strictly contextual meaning based upon an interpretation of counterfactuality which he used to refute, without proof as we shall see, Stapp's logical proof [3] of non-locality in quantum theory. In linguistic theory both authors' choices of meaning: counterfactual versus hypothetical are equally acceptable and therefore some of the issues they discussed lie outside the domain of physics. The issues they discussed are further confused by the fact that in his reply Stapp [2] seems to have adopted Mermin's counterfactual interpretation against his own original [3] hypothetical interpretation. In the rest of this paper we shall adopt the hypothetical sense of Stapp's original statements but we modify his crucial statement LOC2 appropriately, then following his argumentations, we shall show that there is no conflict between relativity and quantum mechanics. We suggest that this should be the natural (pragmatic) choice of meaning in defining the predictions of events in the Hardy experiment.
In the paper the proof of the non-locality of quantum mechanics, given recently by H. Stapp and D. Bedford, and appealing to the GHZ example, is analyzed. The proof does not contain any explicit assumption of realism, but instead it uses formal methods and techniques of the Lewis calculus of counterfactuals. To ascertain the validity of the proof, a formal semantic model for counterfactuals is constructed. With the help of this model it can be shown that the proof is faulty, because it appeals to the unwarranted principle of Elimination of Eliminated Conditions (EEC). As an additional way of showing unreasonableness of the assumption (EEC), it is argued that yet another alleged and highly controversial proof of non-locality of QM, using the Hardy example, can be made almost trivial with the help of (EEC). Next the question is considered whether the validity of the proof in the GHZ case can be restored by adopting the assumption of partial realism. It is argued that although this assumption makes the crucial step of the original reasoning valid, it nevertheless renders another important transition unjustified, therefore the entire reasoning collapses.
A 1997 paper by Henry Stapp attempts to achieve a definitive argument for his long-sustained thesis that quantum mechanics implies a nonlocality inconsistent with the locality of relativity theory. His argument assumes both the validity of counterfactual reasoning under certain circumstances and the free choice of performance of experiments, and it specifically refrains from any assumptions about “elements of physical reality” as understood by Einstein, Podolsky, and Rosen. In Sec. I we drastically condense his argument but retain his central principle of inference, called “LOC2,” which he claims is a consequence of relativistic locality. By careful attention to counterfactual reasoning we then throw doubt upon this claim and hence upon his argument as a whole. In Sec. II we answer Stapp’s reply to our critique of the 1997 paper. Section III is a postscript, commenting briefly on Stapp’s answer to Sec. II.