By Jean Bricmont
Springer
In this book, Jean Bricmont aims to challenge Richard Feynman’s famous statement that “nobody understands quantum mechanics” and discusses some of the issues that have surrounded this field of theoretical physics since its inception.
Bricmont starts by strongly criticising the “establishment” view of quantum mechanics (QM), known as the Copenhagen interpretation, which attributes a key role to the observer in a quantum measurement. The quantum-mechanical wavefunction, indeed, predicts the possible outcomes of a quantum measurement, but not which one of these actually occurs. The author opposes the idea that a conscious human mind is an essential part of the process of determining what outcome is obtained. This interpretation was proposed by some of the early thinkers on the subject, although I believe Bricmont is wrong to associate it with Niels Bohr, who relates the measurement with irreversible changes in the measuring apparatus, rather than in the mind of the human observer.
The second chapter deals with the nature of the quantum state, illustrated with discussions of the Stern–Gerlach experiment to measure spin and the Mach–Zender interferometer to emphasise the importance of interference. During the last 20 years or so, much work has been done on “decoherence”. This has shown that the interaction of the quantum system with its environment, which may include the measuring apparatus, prevents any detectable interference between the states associated with different possible measurement outcomes. Bricmont correctly emphasises that this still does not result in a particular outcome being realised.
The author’s central argument is presented in chapter five, where he discusses the de Broglie–Bohm hidden-variable theory. At its simplest, it proposes that there are two components to the quantum-mechanical state: the wavefunction and an actual point particle that always has a definite position, although this is hidden from observation until its position is measured. This model claims to resolve many of the conceptual problems thrown up by orthodox QM: in particular, the outcome of a measurement is determined by the position of the particle being measured, while the other possibilities implied by the wavefunction can be ignored because they are associated with “empty waves”. Bricmont shows how all the results of standard QM – particularly the statistical probabilities of different measurement outcomes – are faithfully reproduced by the de Broglie–Bohm theory.
This is probably the clearest account of this theorem that I have come across. So why is the de Broglie–Bohm theory not generally accepted as the correct way to understand quantum physics? One reason follows from the work of John Bell, who showed that no hidden-variable theory can reproduce the quantum predictions (now thoroughly verified by experiment) for systems consisting of two or more particles in an entangled state unless the theory includes non-locality – i.e. a faster-than-light communication between the component particles and/or their associated wavefunctions. As this is clearly inconsistent with special relativity, many thinkers (including Bell himself) have looked elsewhere for a realistic interpretation of quantum phenomena. Not so Jean Bricmont: along with other contemporary supporters of the de Broglie–Bohm theory, he embraces non-locality and looks to use the idea to enhance our understanding of the reality he believes underlies quantum physics. In fact he devotes a whole chapter to this topic and claims that non-locality is an essential feature of quantum physics and not just of models based on hidden variables.
Other problems with the de Broglie–Bohm theory are discussed and resolved – to the author’s satisfaction at least. These include how the de Broglie–Bohm model can be consistent with the Heisenberg uncertainty principle when it appears to assume that the particle always has a definite position and momentum; he points out that the statistical results of a large number of measurements always agree with conventional predictions, and these include the uncertainty principle.
Alternative ways to interpret QM are presented, but the author does not find in them the same advantages as in the de Broglie–Bohm theory. In particular, he discusses the many-worlds interpretation, which assumes that the only reality is the wavefunction and that, rather than collapsing at a measurement, this produces branches that correspond to all measurement outcomes. One of the consequences of decoherence is that there can be no interaction between the possible measurements, and this means that no branch can be aware of any other. It follows that, even if a human observer is involved, each branch can contain a copy of him or her who is unaware of the others’ presence. From this point of view, all the possible measurement outcomes co-exist – hence the term “many worlds”. Apart from its ontological extravagance, the main difficulty with many-worlds theory is that it is very hard to see how the separate outcomes can have different probabilities when they all occur simultaneously. Many-worlds supporters have proposed solutions to this problem, which do not satisfy Bricmont (and, indeed, myself), who emphasises that this is not a problem for the de Broglie–Bohm theory.
A chapter is also dedicated to a brief discussion of philosophy, concentrating on the concept of realism and how it contrasts with idealism. Unsurprisingly, it concludes that realists want a theory describing what happens at the micro scale that accounts for predictions made at the macro scale – and that de Broglie–Bohm provide just such a theory.
The book concludes with an interesting account of the history of QM, including the famous Bohr–Einstein debate, the struggle of de Broglie and Bohm for recognition, and the influence of the politics of the time.
This is a clearly written and interesting book. It has been very well researched, containing more than 500 references, and I would thoroughly recommend it to anyone who has an undergraduate knowledge of physics and mathematics and an interest in foundational questions. Whether it actually lives up to its title is for each reader to judge.
