Paul M. Hohler

University of Maryland, College Park, MD

In order to form a more perfect fluid: Does a fundamental bound on η/s exist?


There have been a number of forms of a conjecture that there is a universal lower bound on the ratio, η/s, of the shear viscosity, η, to the entropy density, s, with several different domains of validity. Various forms of the conjecture will be examined. It will be argued that a number of variants of the conjecture are not viable due to the existence of theoretically consistent counterexamples. Furthermore, much of the evidence in favor of a bound does not apply to the variants which have not yet been ruled out.


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