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Superluminal microwave propagation and special relativity

In the first part of this paper we verify, on the basis of the numerical solution of Maxwell equations, that waves propagating down a microwave guide can travel with superluminal group velocity, thus confirming experimental results previously reported in the literature. Then, we have to face the question of superluminal motions within the theory of Special Relativity. It is not widely recognized that all such theoretical and experimental results do not place relativistic causality in jeopardy. For instance, it seems possible to solve also the known causal paradoxes devised for “faster than light” motion. Here we show, in detail and rigorously, how to solve the oldest causal paradox, originally proposed by Tolman, which is the kernel of many further tachyon paradoxes. The key to the solution is a careful application of tachyon mechanics, as it unambiguously follows from Special Relativity.