Causality in the Schroedinger picture: Diagrammatical approach

The Schroedinger picture of Einstein causality is discussed in the framework of relativistic local field theory. The essential tool is the diagrammatic expansion of the time-evolution kernel which elucidates the propagation of the disturbance at each step of the interaction. At the level of the wave function, the causal character of the theory is rather complicated. However if the spacelike density matrix ρs is defined by integrating over the timelike field variables, it receives no influence of the signal. Any local disturbance is shown to propagate inside the forward light cone. It is also shown that causality is most clearly seen in the Wigner representation of the density matrix. The result automatically includes a proof of nonrelativistic causality