Foundations of Physics, Vol. 21, No. 11, 1991

Book Review Quantum Paradoxes and Physical Reality. By Franco Selleri, edited by Alwyn van der Merwe. Kluwer Academic Publishers, Dordrecht, 1990, x + 373 pp. $124.00 (hardcover). During the past twenty-five years or so, foundational research in quantum mechanics gained greater attention among physicists and philosophers than ever before. True, the problem of how to interpret quantum mechanics is as old as quantum mechanics itself, due to the fact that its mathematical formalism, resulting from a complicated process of trial and error, preceded its own interpretation, a unique development in the history of physical science. The debate on this matter, on which even the very originators of the theory like Planck, Einstein, Bohr, or de Broglie disagreed with each other, has never reached a conclusion but was generally thought to belong to philosophy rather than to physics and therefore to be of little interest for the experimental physicist in his laboratory. Only when it became clear that the mysterious correlations of the Einstein-Podolsky-Rosen paradox or the Bell inequalities have empirical implications and that philosophical issues, such as the problem of local realism, were thereby brought down from the realm of abstract speculation into the reach of empirical investigation, did the situation change: experimentalists discovered a new field of interesting activities, the theoretical elaboration of which gave rise to a voluminous literature and even to the appearance of a periodical ("Epistemological Letters") dealing exclusively with these problems. Well-attended international conferences convened on this subject which became even part of the syllabus of modern textbooks on quantum mechanics. It is therefore no exaggeration to say that the study of what has been called "quantum paradoxes" (de Broglie's box, Schr6dinger's cat, Wigner's friend, the EPR paradox, etc.) became a specialized, almost autonomous, branch of scientific investigation with its own research program and terminology. For historical reasons it is perhaps not inappropriate to dub it "quantum paradoxology." 1335 0015-9018/91/1100-1335506,5o/0 ~ 1991 Plenum Publishing Corporation

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Franco Selleri's Quantum Paradoxes and Physical Reality, edited by Alwyn van der Merwe, is an excellent introduction to this subject and an authoritative guide through its precarious intricacies. It is a greatly expanded elaboration of an earlier successful nontechnical paperback Die Debatte um die Quantentheorie which has been published also in French, Italian, and Japanese translations. It has therefore the didactic advantage of leading the reader, step by step, to a deeper understanding of the problems discussed in order to reach soon the level of sophisticated critical scholarship. The first chapter on "Quantum Theorists and the Physical World" offers in historical perspective a panorama of what the founders of quantum mechanics thought about questions concerning the objective reality of microphysical entities in the sense of whether they have an observerindependent existence, concerning the possibility of a realistic spacetime description of them, as well as concerning the reign of causality or determinism in nature. The next chapter, entitled "Is quantum mechanics a complete theory?", offers at first, for the case of the two-dimensional Hilbert space of spin-l/2 state functions, a simple proof of von Neumann's theorem concerning the impossibility of complementing the theory by hidden variables and explains in terms of a clever counterexample why von Neumann's presuppositions prevent his conclusion to be universally valid. Only thereafter is a general proof of the theorem presented, first in its original language of functional analysis and subsequently in the formalism of the propositional calculus as given by Jauch and Piron. Regrettably the interesting Kochen and Specker version is mentioned only en passant. The following two chapters on "The wave-particle duality" and the "Properties of quantum waves" contain a detailed account of the Copenhagen-Goettingen complementarity interpretation according to which the undulatory and corpuscular manifestations of quantum-mechanical objects are mutually exclusive; also the "consciousness approach" endorsed by von Neumann, London and Bauer, Wigner, and others as well as Wheeler's delayed choice experiments are discussed in great detail. An equally meticulous account is given of the opposing point of view associated with the names of Einstein and de Brogtie, according to whom those manifestations are conjunctive in the sense that every microphysical entity consists both of a corpuscle and a wave. It is at this point that Setleri, after having presented a well-documented unbiased account of these various options, finds it appropriate to propound his own personal preference: a modification of the Einsteinde Broglie doctrine. According to Selleri every microphysical entity consists of a corpuscle, in the sense that the very small region of space occupied by

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it is the seat of all the energy and momentum (as well as charge, etc.), associated with the entity; but it also consists of a spatially extended wave, traveling in close association with the corpuscle and being devoid of energy and momentum. Selleri, siding with Einstein, argues for a realistic and local interpretation. In his view every direct observation in microphysics is ultimately the detection of a particle; the dynamics of this process, e.g., the triggering of a detector or the activization of a grain in a photographic emulsion, is determined by the corpuscle, whereas the geometry of the process (i.e., which counter or which grain is affected) is determined by the wave. For example, the famous double-slit experiment, of which Feynman said that it "is impossible, absolutely impossible, to explain it in any classical way, and which is at the heart of quantum mechanics," can according to Selleri be expounded in terms of causality, space and time, somehow as follows: the corpuscular component of each individual particle passes in any case through only one slit, whereas its associated undulatory component always passes through all those slits which are open the fact that the geometry depends on whether only one slit or whether both slits are open, explains the production of the well-known (nonadditive) interference patterns. Furthermore, Selleri endorses determinism and explains the apparent indeterminism of fundamental processes,, as, e.g., disintegrations, as grounded in the insufficiency of our knowledge of the initial condition. This explanation does not, of course, imply any intrusion of human consciousness into the physical process; the role of the observer is here the same as in the case of statistical mechanics. As suggested already by the double-slit experiment but elicited more explicitly by all those experiments which involve a so-called splitting of the wave function, Selteri's interpretation has to postulate the existence of "empty" waves, which, lacking energy and momentum, cannot be observed directly. Their physical reality, Selleri points out, may nevertheless be empirically testable due to the fact that they are capable of giving rise to changes in the transition probabilities of systems with which they happen to interact. This idea, conceived by Selleri already in 1969, is greatly substantiated in the book by the analysis of a number of remarkable experiments such as those on induced laser emission, Tarozzi's experiment on optical photons, or Croca's experiment using neutron interferometry. In Chap. 5, "The Einstein-Podolsky-Rosen paradox," and Chap. 6, "The EPR Paradox in the real world," which also contains new derivations of Bell's inequality, Selleri gives a lucid account of the theoretical aspects of this topic and a detailed analysis of how far experimental evidence confirms the relevant theoretical conclusions. Selleri's critical description of the important experiments on the EPR correlations and Bell's inequalitics, which begins with the 1972 Freedman-

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Clauser experiment but pays special attention to the more recent experiments of the eighties, is in itself a valuable source of information and may be regarded as an updated supplement of the celebrated Clauser and Shimony review, published 1978 in Reports on Progress in Physics. But it was not for this purpose that Selleri compiled it. As is well known, most of these experiments are generally interpreted as yielding results that contradict the assumption of locality or separability. Selleri, an ardent advocate of local realism, meets this challenge by saving no effort to show that there are enough auxiliary assumptions and technical deficiencies to question the unambiguity of the results. To give just one example; because of the imperfect efficiency of the available detectors it is possible, as he and Zeilinger have shown in 1988, to construct a local model, having additional variables which determine whether a specific photon in the atomic-cascade photon experiments will trigger the detector or not, so as to reach full agreement with the factually observed results. Selleri even does not hesitate to point out "that the mathematical structure of quantum mechanics is very useful and accurate in making empirical predictions, but that it does not need to be true in nature." It is therefore no surprise that the last chapter, "Perspectives of physical realism," which presents Selleri's philosophical credo, concludes with a statement that Einstein once wrote to Max Born: "... I cannot seriously believe in it [-quantum mechanics] because the theory is incompatible with the principle that physics is to represent reality in time and space, without spookish long-distance effects." Of course, it would be asking too much from a book of such a wide scope to give exhaustive coverage both in its historical account and its thematic treatment of all the topics dealth with. Thus, for example, one may miss any reference to J. L. Destouches' interesting study of "Using de Broglie's physical waves to solve the EPR paradox" (Nuovo Cimento B 58, 1980), which shows a striking similarity to Selleri's approach; or one may point out that Jon Jarrett's penetrating analysis (1984) of the conditions of Bell's inequality and its implications for a "peaceful coexistence" of quantum mechanics with Einstein's relativity are not mentioned at all, even though Selleri's basic position would not be affected thereby. Selleri has written with rigor and clarity a superb book which is understandable and informative for every one interested in quantum mechanics and which is stimulating and thought provoking also for those who do not share the author's philosophical conviction. Max Jammer Department of Physics Bar-Ilan University Ramat-Gan, Israel