ABSTRACT
This chapter provides the formalism of quantum mechanics to the description of measurements. It describes how an atomic system behaves in a slowly varying applied field. In the Stern-Gerlach (S-G) experiment an atomic or molecular beam is passed through an inhomogeneous magnetic field. The field causes a spatial separation of systems having different values of the magnetic moment and allows one to determine the moment by measuring the deflection of the emergent beams. The S-G magnets and the recombining fields merely force the wave function to undergo a sequence of contortions, which are then carefully undone; the experiment is vacuous, and is not a measurement. The mathematical formalism of quantum mechanics also allows to compute the beam trajectory. If one accepts J. von Neumann's formulation of the quantum mechanics, one is led to far reaching and Dot entirely palatable conclusions.
