Bernard d'Espagnat

Towards a separable «empirical reality»?

Abstract“To be” or “to be found”? Some contributions relative to this modern variant of Hamlets question are presented here. They aim at better apprehending the differences between the points of view of the physicists who consider that present-day quantum measurement theories do reach their objective and those who deny they do. It is pointed out that these two groups have different interpretations of the verbs “to be” and “to have” and of the criterion for truth. These differences are made explicit. A notion of “empirical reality” is constructed within the representation of which the physicists of the first named group can consistently uphold their claim. A detailed way of sharpening this definition so as to make empirical reality free of nonlocal actions at a distance is also described.

A note on measurement

Abstract Grounded on the quantum measurement riddle, a general argument against the universal validity of the superposition principle was recently put forward by Bassi and Ghirardi (Phys. Lett. A 275 (2000) 373). It is pointed out that this argument is valid only within the realm of the philosophy of “objectivistic realism” which is not a necessary part of the foundations of physics, and that recent developments including decoherence theory do account for the appearance of macroscopic objects without resorting to a break of the principle.

Consistent histories and the measurement problem

Abstract The Physical grounds and significance of Griffiths “consistent histories” theory are investigated. A simple derivation of the consistency conditions is given. The question is studied whether or not this theory makes it possible to generally consider a quantum measurement as informing us of the value the measured quantity had , before the measurement was done. It is shown that the answer is negative, unless the logical rule known as modus ponens is given up, which does not seem advisable.

Empirical reality, empirical causality, and the measurement problem

Does physics describe anything that can meaningfully be called “independent reality,” or is it merely operational? Most physicists implicitly favor an intermediate standpoint, which takes quantum physics into account, but which nevertheless strongly holds fast to quite strictly realistic ideas about apparently “obvious facts” concerning the macro-objects. Part 1 of this article, which is a survey of recent measurement theories, shows that, when made explicit, the standpoint in question cannot be upheld. Part 2 brings forward a proposal for making minimal changes to this standpoint in such a way as to remove such objections. The “empirical reality” thus constructed is a notion that, to some extent,does ultimately refer to the human means of apprehension and of data processing. It nevertheless cannot be said that it reduces to a mere name just labelling a “set of recipes that never fail.” It is shown that our usual notion of macroscopic causality must be endowed with similar features.

Are the quantum rules exact? The case of the imperfect measurements

Should we doubt the exactness of the predictive quantum rules of calculation? Although this question is sometimes raised in connection with the one on how to physically understand quantum mechanics, these two questions should not be mixed up. It is recalled here that even the first one is stil an object of controversy, and it is shown (a) that in one specific case the arguments put forward in support of such doubts are hardly cogent but (b) that, nevertheless, at least in one specific other context, the question is worth attention. This is the problem of repeated imperfect measurements. Relative to it, a theoretical possibility is shown of discriminating between the thesis that the quantum rules are exact and a powerful theory of which it is proved that it cannot be reconciled with the assumption.

Influences, histories, and reality

It is stressed that any theory of which it is claimed that it is compatible both with standard realism and with the experimental data is subject to severe constraints. One is that it must either incorporate superluminal influences or negate the free will of the experimentalist. The other one is that, in it. it is only at the price of accepting “backward causality” that a measurement can he interpreted as revealing the value the measured quantity had, just before, rather than just after, the measurement look place.

GianCarlo Ghirardi and the interpretation of quantum physics

This paper comprises a few notes illustrating the impact of GianCarlo Ghirardis achievements, even on the thinking of a non-realist.

Appearance of a local world

Abstract Why is it that nonlocality effects are not normally observed for pairs of systems one of which is (or both are) macroscopic? This question is investigated in detail for the paradigm case of a measurement process, the instrument being viewed as a quantum system. It is shown that whenever only classical dynamical quantities can be measured on the instrument the correlations between the instrument and the measured system cannot exhibit any violation of the local causality assumption. Implications are drawn concerning the empirical reality notion.