Nearly Perfect Fluidity: From Cold Atomic Gases to Hot Quark Gluon Plasmas

Understand

Shear viscosity is a measure of the amount of dissipation in a simple fluid.

• In kinetic theory shear viscosity is related to the rate of momentum transport by quasi-particles, and the uncertainty relation suggests that the ratio of shear viscosity eta to entropy density s in units of hbar/k_B is bounded by a constant.
• Here, hbar is Planck's constant and k_B is Boltzmann's constant.
• A specific bound has been proposed on the basis of string theory where, for a large class of theories, one can show that eta/s is greater or equal to hbar/(4 pi k_B).