SHEAR VISCOSITY IN QCD AND WHY IT IS HARD TO CALCULATE

Shear viscosity is a dynamical property of fluid systems close to equilibrium, describing resistance to shear flow. After reviewing the physics of viscosity and the reason why it is usually difficult to compute, I discuss its importance within the theory of QCD and the obstacles to carrying out such a computation. A diagrammatic analysis requires extensive resummations and even then convergence is poor at physically relevant couplings. Lattice approaches require a poorly controlled analytical continuation of data from the Euclidean to the Minkowski domain. At present, our best results for QCD shear viscosity come from the hydrodynamical interpretations of experiments, with first-principles calculations trailing behind.