BOOK REVIEW Looking for a quantum ontology

Every student or teacher or philosopher of science must at some point have wondered: is there really a problem with quantum mechanics, or is it just that the theory is counterintuitive, the mathematics complicated, the world indeterministic or the textbooks badly written? The unambiguous answer given in this extremely important and well-written book is that there is indeed a genuine problem with ordinary quantum mechanics, but also that there is a solution. The problem is usually called the measurement problem, but it should rather be called the problem of the meaning of the wave-function. In quantum mechanics, the ‘complete description’ of any system is supposed to be given by its wave-function. But a wave-function is just a vector in an abstract space and it is not at all clear what that sentence means. To explain the problem, consider first classical mechanics. In this theory, the notion of ‘force’ (acting instantaneously throughout the universe) is also obscure. Nevertheless, there are particles in the universe, on which the forces act—which determines their motion. Similarly, in classical electromagnetism, the notion of waves propagating in vacuum is obscure, but again, the waves act on particles and guide their motion. Similar remarks hold for the curved space–time of General Relativity. In all those theories, there is an ontology, to use the expression of Dürr and Teufel, namely something that exists independently of any human observation or even independently of the existence of mankind itself and whose evolution is described by the laws of physics. There is nothing of the sort in ordinary quantum mechanics. Indeed, in the latter, the abstract vector called the wave-function has no meaning whatsoever, except that it enters into an algorithm that predicts (very accurately) ‘results of measurements’. There is no ontology in ordinary quantum mechanics—there is nothing ‘out there’ that the theory speaks about. Note that this problem has nothing to do with the issue