Does stability of relativistic dissipative fluid dynamics imply causality?

Abstract

We investigate the causality and stability of relativistic dissipative fluid dynamics in the absence of conserved charges. We perform a linear stability analysis in the rest frame of the fluid and find that the equations of relativistic dissipative fluid dynamics are always stable. We then perform a linear stability analysis in a Lorentz-boosted frame. Provided that the ratio of the relaxation time for the shear stress tensor ${\ensuremath{\tau}}_{\ensuremath{\pi}}$ to the sound attenuation length ${\ensuremath{\Gamma}}_{s}=4\ensuremath{\eta}/3(\ensuremath{\epsilon}+P)$ fulfills a certain asymptotic causality condition, the equations of motion give rise to stable solutions. Although the group velocity associated with perturbations may exceed the velocity of light in a certain finite range of wave numbers, we demonstrate that this does not violate causality, as long as the asymptotic causality condition is fulfilled. Finally, we compute the characteristic velocities and show that they remain below the velocity of light if the ratio ${\ensuremath{\tau}}_{\ensuremath{\pi}}/{\ensuremath{\Gamma}}_{s}$ fulfills the asymptotic causality condition.