Colloquia

A dimensionless measure of fluidity is the ratio of shear viscosity to entropy density. In this talk we will argue that fluidity is a sensitive probe of the strength of correlations in a fluid. We will also discuss evidence that the two most perfect fluids ever observed are also the coldest and the hottest fluid ever created in the laboratory. The two fluids are cold atomic gases (~10^(-6) K) that can be probed in optical traps, and the quark gluon plasma (~10^{12} K) created in heavy ion collisions at RHIC (Relativistic Heavy Ion Collider at Brookhaven National Laboratory). Remarkably, both fluids come close to a bound on the shear viscosity that was first proposed based on calculations in string theory, involving non-equilibrium evolution of back holes in 5 (and more) dimensions.