From thermodynamics to the bound on viscosity

We show that the generalized second law of thermodynamics may shed much light on the mysterious Kovtun–Son–Starinets (KSS) bound on the ratio of viscosity to entropy density. In particular, we obtain the lower bound η/s+O(η3/s3)⩾1/4πη/s+O(η3/s3)⩾1/4π. Furthermore, for conformal field theories we obtain a new fundamental bound on the value of the relaxation coefficient τπτπ of causal hydrodynamics, which has been the focus of much recent attention.