Abstract
We show that linear superpositions of plane waves involving a single-valued, covariantly stable dispersion relation always propagate outside the light cone unless . This implies that there is no notion of causality for individual dispersion relations since no mathematical condition on the function (such as the front velocity or the asymptotic group velocity conditions) can serve as a sufficient condition for subluminal propagation in dispersive media. Instead, causality can only emerge from a careful cancellation that occurs when one superimposes all the excitation branches of a physical model. This happens automatically in local theories of matter that are covariantly stable. Hence, we find that the need for nonhydrodynamic modes in relativistic fluid mechanics is analogous to the need for antiparticles in relativistic quantum mechanics.
- Received 2 August 2023
- Accepted 29 March 2024
DOI:https://doi.org/10.1103/PhysRevLett.132.162301
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.
Published by the American Physical Society