  Substantial collective flow is observed in collisions between large nuclei at
RHIC (Relativistic Heavy Ion Collider) 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. A
calculation of the dimensionless ratio of shear viscosity $\eta$ to entropy
density $s$ by Kovtun, Son and Starinets within anti-de Sitter space/conformal
field theory yields $\eta/s = \hbar/4\pi k_B$ which has been conjectured to be
a lower bound for any physical system. Motivated by these results, we show 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
$\eta/s$. We suggest that experimental measurements can pinpoint the location
of this transition or rapid crossover in QCD.
