  Substantial collective flow is observed in collisions between large nuclei at
high energy, as evidenced by single-particle transverse momentum distributions
and by azimuthal correlations among the produced particles. The data are
well-reproduced by perfect fluid dynamics. In a separate development,
calculation of the dimensionless ratio of shear viscosity eta to entropy
density s by Kovtun, Son and Starinets within AdS/CFT yields eta/s = 1/4pi, and
they conjectured that this is a lower bound for any physical system. It is
shown that the transition from hadrons to quarks and gluons has behavior
similar to helium, nitrogen, and water at and near their phase transitions in
the ratio eta/s. Therefore it is possible that experimental measurements can
pinpoint the location of this transition or rapid crossover in QCD via the
viscosity to entropy ratio in addition to and independently of the equation of
state.
