Faster-than-c signals, special relativity, and causality

TL;DR

This paper examines whether faster-than-light signals, like the Scharnhorst effect, can coexist with special relativity and causality, concluding that such phenomena are compatible and do not necessarily violate fundamental principles.

Contribution

It clarifies the compatibility of superluminal propagation with relativity and causality, focusing on the Scharnhorst effect as a physically plausible example.

Findings

Faster-than-c signals do not inherently violate special relativity.

The Scharnhorst effect's superluminal propagation is constrained to avoid causality violations.

Superluminal phenomena can be consistent with fundamental physical principles.

Abstract

Motivated by the recent attention on superluminal phenomena, we investigate the compatibility between faster-than-c propagation and the fundamental principles of relativity and causality. We first argue that special relativity can easily accommodate -- indeed, does not exclude -- faster-than-c signalling at the kinematical level. As far as causality is concerned, it is impossible to make statements of general validity, without specifying at least some features of the tachyonic propagation. We thus focus on the Scharnhorst effect (faster-than-c photon propagation in the Casimir vacuum), which is perhaps the most plausible candidate for a physically sound realization of these phenomena. We demonstrate that in this case the faster-than-c aspects are ``benign'' and constrained in such a manner as to not automatically lead to causality violations.