In this paper we explain why and how tachyons do not imply any causality violation within relativistic theories. Particular emphasis is given to the role of the Stückelberg-Feynman reinterpretation principle, both in ordinary physics and in tachyon physics.

Although the existence of tachyons is not ruled out by special relativity, it appears that causal paradoxes will arise if there are tachyons. The usual solutions to these paradoxes employ some form of the reinterpretation principle. In this paper it is argued first that the principle is incoherent, second that even if it is not, some causal paradoxes remain, and third, the most plausible “solution,” which appeals to boundary conditions of the universe, will conflict with special relativity.

It is suggested that the existence of a preferred cosmological reference frame has a number of generally unrecognized implications which are by no means inconsistent with the assumptions and consequences of special relativity. Indeed, the assumption that there exists a cosmological fundamental reference frame for light propagation leads to an intelligible interpretation of relativistic effects and of the null observations of Michelson-Morley-type experiments. Hence all such experiments are impotent for the detection of any such fundamental frame, and it is shown (...) that this applies also to the ingenious coupled-mirrors experiment proposed by S. Marinov. Only astronomical observation allied to cosmological theory can reveal the existence of this frame. (shrink)

We comment on the previous paper by L. Basano. In particular, we show that its Section 2 is kinematically incorrect (the dynamics of a two-body interaction through tachyon exchange, incidentally, has already been thoroughly expounded in one paper of ours). Its Section 1 is simply a rather subjective introduction. As to its Section 3, containing indeed interesting problems, we again briefly refer to our earlier work. Our conclusions are still in favor of “au revoir to tachyons!,” even if it is (...) known that inside our cosmos tachyons are more likely to have a role in physics (e.g., in elementary particle and black-hole physics) as “exchanged objects” rather than as “asymptotically free” objects. (shrink)

It is shown that in addition to the usual difficulties related to causality, the theory of superluminal particles also exhibits paradoxical symmetry violations. In the second part of the paper a conventional paradox is revisited: causality violations at the macroscopic level follow from simple statistical arguments.