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In search of the perfect fluid

Shear viscosity measures the amount of internal friction in a simple fluid. In kinetic theory shear viscosity is related to momentum transport by quasi-particles, and the uncertainty relation implies that the ratio of shear viscosity eta to entropy density s is bounded by a constant multiplied by hbar/k_B, where hbar is Planck's constant and k_B is Boltzmann's constant. A specific bound has been proposed on the basis of string theory. In a large class of theories that can be studied using string theory methods the constant is 1/(4π). Experiments at RHIC indicate that eta/s of the quark gluon plasma is close to this prediction. We will refer to a fluid that saturates the string theory bound as a perfect fluid. In this contribution we summarizes the theoretical and experimental information on the fluidity of the main classes of strongly interacting quantum fluids.