COMMENSAL ISSUE 101

Quantum mechanics - and in this book and in this review the non-relativistic theory (NRQM) only is considered - has a peculiarity not shared with any other branch of physics : seventy-odd years after its birth the interpretation of NRQM is still being argued over, not just by philosophers and hedge-philosophers, but by many in the trade. A long time ago when I was at Niels Bohr's institute in Copenhagen I shared an office with an amiable Frenchman called Bernard d'Espagnat. In those days and ever since he has spent much of his time thinking and writing about what was the real meaning of the quantum mechanics which many of us accepted as being difficult to understand but which gave amazingly accurate predictions of phenomena in the physical world. Despite great advances in quantum electrodynamics, elementary particle theory and high energy physics in general, certain aspects of NRQM have continued to present problems of interpretation.

However in recent years the activities of a number of people, not only d'Espagnat but John Bell, Roland Omnès and others have largely solved many of these difficulties. In addition to these theoretical studies, numbers of very ingenious experiments using modern techniques have been carried out, mainly in France, which have made concrete the basis of the newer interpretation. This new book by Roland Omnès (who is Professor of Physics at the University of Paris XI) gives a comprehensive account of the present state of thought in this field which has long attracted the attention of not only scientists but many with a more philosophical interest.

Do not believe the blurb on the back of the book, which claims that it is aimed at "beginning students and experienced physicists, mathematicians and philosophers". It is a difficult book, even for practitioners of the art, including myself. The author begins with a historical survey ranging from the early days of NRQM to the present, and continues with numerous 'philosophical' discussions of the significance of the theoretical and experimental developments which he describes.

One of the most fundamental problems with QM is that of describing an experiment. The actors in the experiment - atoms, electrons, photons...... behave in a 'quantum' way while they are observed by macroscopic apparatus which is supposed to behave in a 'classical' way. This dichotomy is the basis of the Gedankenexperiment involving the well-known cat of Schrödinger. Omnès shows how the relatively new concept of 'decoherence' enables us to treat such a dualistic situation in a unified and systematic way, so that the cat may be retired to the philosophical menagerie along with Buridan's ass and other beasts. (An explanation of decoherence is inappropriate in a review such as this; the reader is referred to the book where it is explained in detail. Omnès also describes an experiment in which decoherence is actually measured. It is not another woolly concept like, say, deconstruction, which I gather nobody understands.)

Another difficulty in NRQM which Omnès discusses at length is to do with what is 'reality' in the context of such apparent contradictions as wave/particle duality. He also deals with the Einstein-Podolsky-Rosen (EPR) phenomenon . This has been much argued about since the original EPR paper in 1935. He describes how the late John Bell (who worked at CERN, Geneva) has given a definitive explanation of EPR. Bell's approach requires accepting that in QM we can no longer assume separability, which is the idea that distant objects may be looked upon as necessarily unconnected. Under certain circumstances two particles (eg those emitted in an EPR experiment) are in a single 'entangled' quantum state which is a superposition, no matter how far they recede from each other. This possibility was considered long ago by Bohr; more recently Bell described experiments which could prove this doctrine. Such experiments have now been carried out by Aspect and others. The suggestion which is often put around that EPR makes instantaneous communication possible, contrary to the basic principles of relativity, is thus shown to be unfounded. All these matters are explained clearly by Omnès.

The book concludes with descriptions of several ingenious experiments which exemplify the apparently implausible assertions of NRQM. For example, Omnès tells how a single atom can be trapped and observed while it undergoes 'quantum jumps'.

Although the non-mathematical reader will find parts of the book decidedly recondite, judicious skipping will enable him/her to get an accurate idea of the present state of the 'interpretation' of NRQM which, due to the efforts of a few able people, has now become much clearer.

Alan Edmonds