Based on a calculation of the shear viscosity η and the entropy density s of certain strongly coupled field theories via the AdS/CFT-correspondence, Kovtun, Son and Starinets conjectured that their ratio is bounded below by the universal number ħ/4πkB, a bound that appears to hold for all existing fluids. The substances that come closest to being perfect in the sense of a minimum value of η/s are the quark gluon plasma and ultracold atoms at infinite scattering length, the so called unitary Fermi gas. The talk provides an introduction to the origin and interpretation of this bound from a simple physics perspective. Moreover, quantitative results for η/s are presented in the normal phase of the unitary Fermi gas. They are consistent with the Kovtun, Son and Starinets bound and are also in good agreement with experiments.
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