Conceptual Problems Related to Time and Mass in Quantum Theory

The concept of proper time cannot be just taken over from classical theory and applied to quantum theory. There are a number of serious ambiguities related to it. Similarly, the concept of mass has some inconsistencies attached to it. We argue that both of these concepts only make sense if they are included in the theory as dynamical variables, and subsequently as conjugate operators. We discuss the necessary uncertainty principle between them.