Interpreting the Quantum World

The object of this book is the physical interpretation of the abstract formalism of quantum theory. This issue has been controversial from the early days of quantum mechanics, more than 70 years ago. Many of the best minds struggled with this problem, only to reach conflicting conclusions. Obviously, there is no similar interpretation problem for classical mechanics, because the mathematical symbols that appear in the latter simply coincide with experimentally observable quantities. On the other hand, the quantum formalism is based on a complex vector space in which the dynamical evolution is generated by unitary operators. Everyone agrees on how to manipulate the mathematical symbols; the thorny problem is to relate them to the observable physical reality. The traditional answer is to introduce ‘observers’ who sense the quantum world by interacting with it. While they are engaged in that interaction, the observers must obey quantum dynamics—this is needed for consistency of the formalism. Yet, after completion of the measuring process, the same observers must be given a mundane, classical, objective description, so that the ‘quantum measurement’ ends with a definite result, as we experience in everyday’s life. Quantum mechanics itself does not predict, in general, that result. It predicts only probabilities for the various possible outcomes of a measurement, once we specify the procedure used for the preparation of the physical system. This ad hoc approach is sufficient for the purposes of experimental physics, and it can even be rationalized by some theoretical physicists (including the author of this review). However, it is considered as unacceptable by philosophers of science. Bub’s book gave me an opportunity to understand why. Bub’s goal is to liberate the quantum world from its dependence on observers. Various possibilities are carefully examined. The book contains an amazing wealth of information, including numerous excerpts of correspondence between Einstein, Schrödinger, Pauli, Born, and others. I have particularly been impressed by the two long chapters (75 pages) which analyze in exhaustive detail the celebrated ‘no go’ theorems of Bell and of Kochen and Specker, namely the contradictions that would arise in any attempt to supplement the