The viscosity bound in string theory

Understand

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.