Hydrodynamics as sound-speed approaches light-speed

Abstract

I present the simplest 3+1 dimensional quantum field theory for which the speed of sound can be arbitrarily close to the speed of light. Examining the hydrodynamics, I find cases where the shear viscosity is finite, but the "shear relaxation coefficient" appears always to be divergently large.

Comments: 10 pages including figures. Added acknowledgments, corrected minor typos