Viscosity Bound and Causality Violation

In recent work we showed that, for a class of conformal field theories (CFT) with Gauss-Bonnet gravity dual, the shear viscosity to entropy density ratio, $eta/s$, could violate the conjectured Kovtun-Starinets-Son viscosity bound, $eta/sgeq1/4pi$. In this paper we argue, in the context of the same model, that tuning $eta/s$ below $(16/25)(1/4pi)$ induces microcausality violation in the CFT, rendering the theory inconsistent. This is a concrete example in which inconsistency of a theory and a lower bound on viscosity are correlated, supporting the idea of a possible universal lower bound on $eta/s$ for all consistent theories.