Roland Omnès's * Understanding Quantum Mechanics * is yet another indication of the renewed interest in the foundations of quantum mechanics among physicists. Several of the founders of quantum mechanics (such as Bohr, Heisenberg, Einstein and Schrodinger) were much concerned with the general epistemological and ontological implications of the new discipline. In more recent times, though, most of the discussion of foundational issues was confined to philosophers of science and largely ignored by physicists.

The basic attitude among physicists seemed to be that the mathematical formalism of the theory worked marvellously well for making predictions (which is surely the case), and that was enough. However, difficulties remained, as demonstrated by such long-standing questions as the measurement problem (roughly, how the formalism of quantum mechanics could account for, or even be consistent with, the obvious fact that measurements produce definite, "classical" outputs) and the classical limit (how one might hope to recover, even in reasonable approximation, the predictions of classical mechanics in the macroscopic domain).

One can identify two central factors that contributed to the remarkable change in attitude on the part of some physicists. First was the work of John Bell (mid-1960s), which proved that one of our most cherished expectations about the nature of the physical world - namely, an objective, observer-independent physical reality together with the absence of instantaneous influences over large distances - is incorrect.

Second, experimentalists have become so accomplished that what were once only "thought experiments" can now actually be performed to test the foundations of quantum theory. This brought "foundations" into the real world and made it relevant and respectable to working scientists. Omnès is one of the many internationally known theoretical physicists to enter the arena in recent years.

The essential message of this book is that one really does not need any new quantum principles to fashion an interpretation of the mathematical formalism to yield a complete account of our physical world. The central difficulty in obtaining the everyday features of our observed macroscopic world from quantum mechanics has always been the superposition principle for wave functions. It is superposition that underpins the interference effects that are the hallmark of the quantum domain. Such interference effects are almost always absent in the classical regime of our experience.

The basic conceptual conundrum is to account for an approximate theory (Newtonian or classical mechanical) in terms of a putatively more fundamental theory (quantum mechanics), when the latter formalism has features (such as superposition and interference phenomena) that are alien to the former. This is what the problems of measurement and of the classical limit are about.

Omnès and other researchers claim a resolution of these foundational issues because of the effect of decoherence, which means the destruction of quantum interference. While the concept of decoherence is not new, the efficiency of apparently innocuous influences, such as the interaction of a large system with just a few photons, in producing decoherence is remarkable. Formally, what decoherence accomplishes is to ensure that the probabilistic predictions of quantum mechanics behave as do those of classical mechanics. The rub is that even this richer representation of quantum mechanics does not account for how one specific, actually observed result comes about, rather than another of the myriad possibilities.

For Omnès this is a "false" problem, but for others it may well remain a question of central importance. These and many other fascinating technical and philosophical matters are lucidly discussed in this book, and anyone truly interested in attempts to understand our most empirically successful theory to date will surely want to read it.

Just two caveats, though: alternative points of view are at times given unfairly biased negative presentations (such as hidden variables). And, while the book is advertised as suitable for beginning (graduate) students, too often the discussions Omnès includes are so elliptical that such a reader will be reduced simply to taking the alleged result solely on the word of the author.

James T. Cushing is professor of physics, University of Notre Dame, Indiana, United States.

## Understanding Quantum Mechanics

Author - Roland Omnès

ISBN - 0 691 00435 8

Publisher - Princeton University Press

Price - £21.95

Pages - 307

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