Resurrecting the Strong KSS Conjecture

Abstract

Many counterexamples to the proposed KSS bound $\frac\eta s \ge \frac 1 {4\pi}$ depend on constructing systems with large numbers of species. As a result, the entropy density grows large and $\frac \eta s$ can be made arbitrarily small. However, these constructions do not affect the dimensionless shear viscosity $\frac {\eta T}{\epsilon + P}$, which agrees with the traditional $\frac \eta s$ only for vanishing chemical potential. This raises the possibility that a KSS-like bound holds for all systems, not just UV-complete quantum field theories, contrary to the previous understanding.