The viscosity bound in string theory

Abstract

The ratio of shear viscosity to entropy density $\eta/s$ of any material in nature has been conjectured to have a lower bound of $1/4\pi$, the famous KSS bound. We examine string theory models for evidence in favour of and against this conjecture. We show that in a broad class of models quantum corrections yield values of $\eta/s$ just above the KSS bound. However, incorporating matter fields in the fundamental representation typically leads to violations of this bound. We also outline a program to extend AdS/CFT methods to RHIC phenomenology.