The anti-de Sitter/conformal field theory ( AdS/CFT ) correspondence implies that small perturbations of a black hole correspond to small deviations from thermodynamic equilibrium in a dual field theory .
For gauge theories with an Einstein gravity dual , the AdS/CFT correspondence predicts a universal value for the ratio of the shear viscosity to the entropy density , \eta / s = 1 / 4 \pi .
It was conjectured recently that all fluids conform to the lower bound \eta / s \geq 1 / 4 \pi .
This conjectured bound has been the focus of much recent attention .
However , despite the flurry of research in this field we still lack a proof for the general validity of the bound .
In this essay we show that this mysterious bound is actually a direct outcome of the interplay between gravity , quantum theory , and thermodynamics . Email : shaharhod @ gmail.com