Global collective flow in heavy ion reactions from the beginnings to the future

The ratio of shear viscosity to volume density of entropy can be used to characterize how close a given fluid is to being perfect. Using string theory methods, we show that this ratio is equal to a universal value of \(\hbar/4\pi k_B\) for a large class of strongly interacting quantum field theories whose dual description involves black holes in anti--de Sitter space. We provide evidence that this value may serve as a lower bound for a wide class of systems, thus suggesting that black hole horizons are dual to the most ideal fluids.

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.