Viscosity of Strongly Coupled Gauge Theories

We extend the correspondence between black hole physics and thermodynamics to non-equilibrium processes. The shear viscosity is shown to correspond to the absorption cross section of low-energy graviton by the black hole. We show that the ratio of shear viscosity to volume density of entropy in theories with gravity duals is equal to a universal value of ħ/(4π). We conjecture that this value is a lower limit on the ratio of shear viscosity to entropy density for a very wide class of systems. Possible applications of this viscosity bound conjecture are mentioned.