Quantum World . By Jeffrey Bub

Jeffrey Bub himself once mentioned that whenever someone arrives at a position in the philosophy of quantum mechanics s/he seems to lose momentum. Here we have a clear counterexample: since his first book on the subject in 1974, which pioneered many of the subsequent development, Bub has gained a great deal of momentum. His position today has the additional virtue of providing sufficient conditions for the tenability of a whole range of alternative interpretations, thus locating quite clearly many of the diverse ideas found in recent literature. Moreover, he can demonstrate optimal desirable properties for his own favorite interpretation within that range. For physics students and for much of the general public, interpretation of quantum theory begins with certain mystifying pronouncements by the great scientists of Copenhagen. Familiarity and authority have dulled the sense of mystery, and left us with remnants delicately balanced between vacuity and inconsistency. For the specialist, however, the story of interpretation begins a few years after the Copenhagen breakthrough with von Neumann’s unification of matrix and wave mechanics in the Hilbert space formalism. Not that this had its meaning written on its face. Von Neumann strove mightily to add some, through his discussion of measurement ( `̀ collapse of the wave packet,’’ `̀ projection postulate’’) and of propositions representable by projection operators. Fundamental to his effort was what is now commonly called the `̀ eigenstate± eigenvalue link.’’ That is the semantic rule which says that an observable pertaining to a given system has a value if and only if the system is in the corresponding eigenstate of that observable. If that is so, then not only do incompatible observables never have simultaneous values ± ± but in fact most of time most observables have no definite value at all. Accordingly, to assure that at least the outcome of a measurement ( and the measured observable at that time) have a definite value, that `̀ collapse’’ postulate had to be added.