The Modal Interpretation of Algebraic Quantum Field Theory

In a recent article, Dieks has proposed a way to implement the modal interpretation of (nonrelativistic) quantum theory in relativistic quantum field theory. We show that his proposal fails to yield a well-defined prescription for which observables in a local spacetime region possess definite values. On the other hand, we demonstrate that there is a well-defined and unique way of extending the modal interpretation to the local algebras of relativistic quantum field theory. This extension, however, faces a potentially serious difficulty in connection with ergodic states of a field.