In search of the perfect fluid

TL;DR

This paper reviews theoretical and experimental insights into the shear viscosity to entropy density ratio in quantum fluids, highlighting the concept of a perfect fluid that saturates a string theory bound, with implications for quark-gluon plasma.

Contribution

It synthesizes current knowledge on the eta/s ratio in strongly interacting quantum fluids, emphasizing the significance of the string theory bound and experimental findings.

Findings

Quark-gluon plasma has eta/s close to the theoretical bound.

The eta/s ratio is a key indicator of fluid perfection.

String theory predicts a universal lower bound for eta/s.

Abstract

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/(4\pi). 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.