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 $\frac{\eta T}{ϵ+P}$, 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.

Comments: 3 pages; comments welcome