Evanescent modes are not necessarily Einstein causal

Previously, experiments with microwave signals have shown that evanescent modes can travel faster than light. Several theoretical investigations have proven that in the case of signals with unlimitedly high frequency components, such superluminal velocities do not violate Einstein causality, thus group, signal, and energy velocities are c where c is the vacuum velocity of light. In this letter I shall show that frequency band limitation is a fundamental property of signals and that such signals containing only evanescent modes can violate Einstein causality. Received: 12 December 1998 / Accepted: 14 December 1998