It is demonstrated that neither the arguments leading to inconsistencies in the description of quantum-mechanical measurement nor those “explaining” the process of measurement by means of thermodynamical statistics are valid. Instead, it is argued that the probability interpretation is compatible with an objective interpretation of the wave function.

ZusammenfassungFasst man die Ψ‐Funktion in der Quantenmechanik als eine vollständige Beschreibung eines realen Sachverhaltes auf, so ist die Hypothese einer schwer annehm‐baren Fernwirkung impliziert. Fasst man die Ψ‐Funktion aber als eine unvollständige Beschreibung eines realen Sachverhaltes auf, so ist es schwer zu glauben, dass für eine unvollständige Beschreibung strenge Gesetze für die zeitliche Abhängigkeit gelten.‐ A. E.

The program of a physical concept of information is outlined in the framework of quantum theory. A proposal is made for how to avoid the intuitive introduction of observables. The conventional and the Everett interpretations in principle may lead to different dynamical consequences. An ensemble description occurs without the introduction of an abstract concept of information.

The physical limitations, due to quantum mechanics, on the functioning of computers are analyzed.

Summarya) Bell tries to formulate more explicitly a notion of “local causality”: correlations between physical events in different space‐time regions should be explicable in terms of physical events in the overlap of the backward light cones. It is shown that ordinary relativistic quantum field theory is not locally causal in this sense, and cannot be embedded in a locally causal theory.b) Clauser, Home and Shimony criticize several steps in Bell's argument that any theory of local “beables” is incompatible with quantum (...) mechanics. It is contended that the Clauser‐Horne derivation of a Bell‐type inequality circumvents his weak steps. The Clauser‐Horne derivation must assume that there are no undetected correlations between choices of controllable variables in two space‐like separated regions. Methodological considerations support this assumption.c) In response to criticism by Shimony, Home, and Clauser, Bell tries to clarify the argument of “The theory of local beables”, and to defend as permissible the hypothesis of free variables.d) Bell's reply to an earlier criticism by Shimony, Clauser, and Home is answered. The convergence of Bell's position towards theirs is noted. (shrink)

“To be” or “to be found”? Some contributions relative to this modern variant of Hamlet's 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. (shrink)

The projection postulate, which prescribes “collapse of the state vector” upon measurement, is not an essential part of quantum mechanics. Rather it is only an optional discarding of certain branches of the state vector that are expected to be irrelevant for the purpose at hand. However, its use is hazardous, and there are examples of repeated measurements for which the conventional application of the projection postulate leads to incorrect results.

Sometimes it is possible in quantum theory for a system to interact with another system in such a way that the information contained in the wave function becomes very scrambled and apparently incoherent. We produce an example which is exactly calculable, in which a macroscopic change is induced in the environment, and all phase information for the system is apparently lost, so that a measurement has seemingly been made. But actually, although the wave function has been badly scrambled, all the (...) original information is still present. We call this situation one of “latent order.”Subsequently, the system interacts again with the environment, wiping out the macroscopic change, and the wave function once again becomes manifestly coherent. Thus the apparent measurement has been undone, and leaves no aftereffect. Thus, our “measurement” has disappeared without a trace. We call such a measurement a “haunted measurement,” and we believe that until the measurement process is rigorously understood, the concept of measurement is ambiguous. It is just not good enough to say that an amplification stage occurs “somewhere” in the process.We also point out the connection between the haunted measurement and delayed-choice experiments and discuss a haunted version of the “Schrödinger's Cat” experiment and of the Einstein-Podolsky-Rosen experiment. (shrink)

A brief review of the conceptual difficulties met by the quantum formalism is presented. The main attempts to overcome these difficulties are considered and their limitations are pointed out. A recent proposal based on the assumption of the occurrence of a specific type of wave function collapse is discussed and its consequences for the above-mentioned problems are analyzed.

The relation between quantum measurement and thermodynamically irreversible processes is investigated. The reduction of the state vector is fundamentally asymmetric in time and shows an observer-relatedness which may explain the double interpretation of the state vector as a representation of physical states as well as ofinformation about physical states. The concept of relevance being used in all statistical theories of irreversible thermodynamics is demonstrated to be based on the same observer-relatedness. Quantum theories of irreversible processes implicitly use an objectivized process (...) of state vector reduction. The conditions for the reduction are discussed, and it is concluded that the final (subjective) observer system may be carried by a space point. (shrink)

Let A 1 , A 2 , A 3 A 4 be four observables, the compatible observables among them being (A 1 , A 3 ), (A 1 , A 4 ), (A 2 , A 3 ), (A 2 , A 4 ). In order that the empirical data be reproducible by a quantum or a classical theory, the two-point correlation functions $$\{ C_{ij} = \left\langle {A_i A_j } \right\rangle :i,j a compatible pair\} $$ must necessarily satisfy $$|X_{13} X_{14} (...) - X_{23} X_{24} | \leqslant \left( {1 - X_{13} ^2 } \right)^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} \left( {1 - X_{14} ^2 } \right)^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} + \left( {1 - X_{23} ^2 } \right)^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} \left( {1 - X_{24} ^2 } \right)^{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}} (*)$$ where Xij=CijC ii −1/2 C jj −1/2 . In the case ofGaussian data, this inequality is alsosufficient; If (*) holds, there is a Gaussian joint distribution for A 1 , A 2 , A 3 , A 4 which reproduces the Gaussian data for compatible pairs. It follows that Bell's inequality is satisfied by all true-false propositions about the Gaussian data. A further consequence of the analysis is thatquantum Gaussian fields satisfy Bell's inequality for all true-false propositions aboutfield measurements.The maximum violation of (*) corresponds to Rastall's example in the case of two-valued observables. (shrink)

Classical relativistic physics assumes that spatially separated events cannot influence one another (“locality”) and that values may be assigned to quantities independently of whether or not they are actually measured (“realism”). These assumptions have consequences—the Bell inequalities—that are sometimes in disagreement with experiment and with the predictions of quantum mechanics. It has been argued that, even if realism is not assumed, the violation of the Bell inequalities implies nonlocality—and hence that radical changes are necessary in the foundations of physics. We (...) show that this conclusion does not follow unless the locality hypothesis is strengthened in an implausible manner. (shrink)

The proof of Bell's inequality is based on the assumption that distant observers can freely and independently choose their experiments. As Bell's inequality isexperimentally violated, it appears that distant physical systems may behave as a single, nonlocal, indivisible entity. This apparent contradiction is resolved. It is shown that the “free will” assumption is, under usual circumstances, an excellent approximation.I have set before you life and death, blessing and cursing: therefore choose life.... —Deuteronomy XXX, 19.

Instead of the usual asymptotic passage from quantum mechanics to classical mechanics when a parameter tended to infinity, a sharp boundary is obtained for the domain of existence of classical reality. The last is treated as separable empirical reality following d'Espagnat, described by a mathematical superstructure over quantum dynamics for the universal wave function. Being empirical, this reality is constructed in terms of both fundamental notions and characteristics of observers. It is presupposed that considered observers perceive the world as a (...) system of collective degrees of freedom that are inherently dissipative because of interaction with thermal degrees of freedom. Relevant problems of foundation of statistical physics are considered. A feasible example is given of a macroscopic system not admitting such classical reality.The article contains a concise survey of some relevant domains: quantum and classical Bell-type inequalities; universal wave function; approaches to quantum description of macroscopic world, with emphasis on dissipation; spontaneous reduction models; experimental tests of the universal validity of the quantum theory. (shrink)

We discuss the concept of spontaneous breaking of gauge symmetry in super-conductors and superfluids and, in particular, the circumstances under which the absolute phase of a superfluid can be physically meaningful and experimentally relevant. We argue that the study of this question pushes us toward the frontiers of what we understand about the quantum measurement process, and underline the need for a new theoretical framework that keeps pace with modern technological capabilities.

The interpretations of measurements in Bohm's and Everett's quantum theories are compared. Since both theories are based on the assumption of a universally valid Schrödinger equation, they face the common problem of how to explain that arrow of time, which in conventional quantum theory is represented by the collapse of the wave function. Its solution requires, in a statistical sense, a very improbable initial condition for thetotal wave function of the universe. The historical importance of Bohm's quantum theory is pointed (...) out. (shrink)

We show that a method exists for deducing from Einstein locality all the possible inequalities for linear combinations of correlation functions. This allows us to show also that there is a complete observational equivalence of deterministic local theories and probabilistic local theories.

The formulation of a relativistic theory of state-vector reduction is proposed and analyzed, and its conceptual consequences are elucidated. In particular, a detailed discussion of stochastic invariance and of local and nonlocal aspects at the level of individual systems is presented.

A general algorithm is given for determining whether or not a given set of pair distributions allows for the construction of all the members of a specified set of higher-order distributions which return the given pair distributions as marginals. This mathematical question underlies studies of quantum correlation experiments such as those of Bell or of Clauser and Horne, or their higher-spin generalizations. The algorithm permits the analysis of rather intricate versions of such problems, in a form readily adaptable to the (...) computer. The general procedure is illustrated by simple derivations of the results of Mermin and Schwarz for the symmetric spin-1 and spin-3/2 Einstein-Podolsky-Rosen problems. It is also used to extend those results to the spin-2 and spin-5/2 cases, providing further evidence that the range of strange quantum theoretic correlations does not diminish with increasing s. The algorithm is also illustrated by giving an alternative derivation of some recent results on the necessity and sufficiency of the Clauser-Horne conditions. The mathematical formulation of the algorithm is given in general terms without specific reference to the quantum theoretic applications. (shrink)