Looking for the Source of Change

Abstract

In most theories of the quantum measurement process changes in an observer’s perception of a state can take place without forces, as for example if a state is prepared in an eigenstate of Jx (x component of angular momentum) but Jz is measured. In the “special state” theory (explained in the previous article) any change in wave function requires forces. This allows experimental tests to distinguish these ideas and in the present article two examples of such tests are considered. The first is a kind of double Stern–Gerlach experiment, the second a check for angular momentum changes in a polarizer.