Wavefunction Tails in the Modal Interpretation

Abstract

Consider the general form of the state of a quantum system, (eq. 1), where the are (non-degenerate) eigenvectors of an operator, A, corresponding to an observable, A. According to the standard interpretation of quantum mechanics (the precise details of which do not concern me here), A does not have a definite value on the system in the state . But now suppose we measure A by coupling the system to a measuring apparatus. The composite system is then (eq. 2) where the are the indicator-states of the apparatus. Again, the standard interpretation tells us that the apparatus is not in a definite indicator-state. But how can this be? Measurements do have definite results. The apparatus is always in a definite state. I have just reviewed one version of the measurement problem. The modal interpretation of quantum mechanics proposes a solution.