A sticky business: the status of the conjectured viscosity/entropy density bound

Abstract

There have been a number of forms of a conjecture that there is a universal lower bound on the ratio, eta/s, of the shear viscosity, eta, to entropy density, s, with several different domains of validity. We examine the various forms of the conjecture. We argue that a number of variants of the conjecture are not viable due to the existence of theoretically consistent counterexamples. We also note that much of the evidence in favor of a bound does not apply to the variants which have not yet been ruled out.

23 pages, 4 figures, added references, corrected typos, added subsection in response to Son's comments in arXiv:0709.4651

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• Shear Viscosity from Gauss-Bonnet Gravity with a Dilaton Coupling
• A bound on the speed of sound from holography
• Lovelock theories, holography and the fate of the viscosity bound
• Shear Viscosity from AdS Born-Infeld Black Holes
• Fluid Dynamics and Viscosity in Strongly Correlated Fluids
• Hydrodynamics of spacetime and vacuum viscosity
• Hydrodynamics with conserved current via AdS/CFT correspondence in the Maxwell-Gauss-Bonnet gravity
• The first order hydrodynamics via AdS/CFT correspondence in the Gauss-Bonnet gravity
• Universal relations of transport coefficients from holography
• Minimum Shear Viscosity over Entropy Density at Phase Transition? --- A Counterexample
• Breaking rotations without violating the KSS viscosity bound
• From Thermodynamics to the Bound on Viscosity
• Shear Viscosity in Weakly Coupled N-Component Scalar Field Theories
• Binary superfluids: Low-energy properties and dissipative processes from spontaneous emission of massive phonons
• Shear viscosity and imperfect fluidity in bosonic and fermionic superfluids
• Viscosity to entropy density ratio: violation of the lower bound at small temperatures
• Transport and spectral functions in high-temperature QCD
• Resurrecting the Strong KSS Conjecture