Experimental Tests of the Holographic Entropy Bound

TL;DR

The paper proposes that the Kovtun–Son–Starinets viscosity bound on the ratio $\eta/s$ arises from the generalized covariant entropy bound, linking holographic ideas to measurable fluid properties. By analyzing entropy flow through lightsheets in a flowing fluid and incorporating viscous backreaction, the authors derive the bound $\frac{\eta}{s} \ge \frac{1}{4\pi}$ (in natural units). The approach connects macroscopic transport coefficients to quantum-gravitational entropy bounds, offering a path to experimental tests of holography and suggesting extensions to other transport phenomena. Overall, the work reframes a fundamental bound on fluid transport as a tangible probe of holographic entropy bounds in laboratory systems.

Abstract

Kovtun, Son and Starinets proposed a bound on the viscosity of any fluid in terms of its entropy density. The bound is saturated by maximally supersymmetric theories at strong coupling, but can also easily be challenged experimentally to within a factor of 10 already today. We argue that this bound follows directly from the generalized covariant entropy bound, bringing holography within the reach of experimental investigation.

Figures (2)

• Figure 1: i) illustrates the CEB: all matter contained within the area $A$ has to pass the lightsheet, and this is still true even if there are velocities and velocity gradients as in ii). However in the case of the GCEB, where we are only interested in entropy passing through the lightsheet from $A$ to $A'$, matter inside both $A$ and $A'$, that would not have been counted at rest can move into the lightsheet as depicted in iii). It is only in this case that velocity gradients give new challenges to the bound.
• Figure 2: A large droplet of fluid, out of which a small volume element gets sampled by a lightsheet starting at $B_+$ and terminated at $B_-$.