Resurrecting the Strong KSS Conjecture

TL;DR

This paper investigates the validity of the KSS bound on shear viscosity, proposing that a similar bound might hold universally, beyond just UV-complete quantum field theories, despite known counterexamples affecting the entropy-based ratio.

Contribution

The paper introduces a new perspective suggesting a KSS-like bound could be universally valid, independent of the entropy density considerations affected by large species numbers.

Findings

Counterexamples affect the entropy-based ratio but not the dimensionless shear viscosity

The traditional $rac{ ext{eta}}{s}$ bound may not be universally applicable

A KSS-like bound might hold for all systems regardless of UV-completeness

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.