  In his book `Mathematical Foundations of Quantum Mechanics', von Neumann
asserted the following: the Compton-Simon experiment showed that the state
vector must collapse upon measurement of any self-adjoint operator. Comparing
von Neumann's account with the Compton-Simon paper, we find that von Neumann
had misinterpreted the experiment as consisting of two successive measurements
(which gave identical results), whereas the experiment only measured two angles
on the same photographic plate. Note, however, that the state vector must
collapse upon measurement of an additively-conserved quantity; otherwise the
conservation law could be violated. Next, it turns out that the mathematical
problem of explaining collapse is not fully defined until one specifies the
nature of the apparatus. If the apparatus does not have a `classical
description', the problem is insoluble, even if the measurement is only
approximate (Fine, Simony, Brown, Simony and Busch); but if it does, the
problem is soluble within Schroedinger dynamics (with a time-dependent
hamiltonian) for additively-conserved observables. The solution, a modification
of Sewell's, shows that the state vector has collapsed, but it does not reveal
the eigenvalue of the collapsed state. The collapse is irreversible, and
results from the interplay of additive conservation laws with the quantum
measurement postulate. Indeed, the quantum measurement problem - as expounded
by Wigner - may be better understood as the problem of establishing that the
two are compatible with each other; it has little relevance to actual
measurements.
