Viscosity Bound and Causality in Superfluid Plasma

TL;DR

This paper investigates the relationship between viscosity bounds and causality in strongly coupled plasmas, showing that the link does not hold in theories with low temperature phase transitions through an explicit AdS/CFT model.

Contribution

The authors provide an explicit AdS/CFT model demonstrating that viscosity bounds do not imply causality violations in theories with phase transitions.

Findings

Viscosity bounds are not universally linked to causality violations.

Low temperature phase transitions break the viscosity-causality connection.

Explicit model confirms the independence of viscosity bounds and causality in certain regimes.

Abstract

It was argued by Brigante et.al that the lower bound on the ratio of the shear viscosity to the entropy density in strongly coupled plasma is translated into microcausality violation in the dual gravitational description. Since transport properties of the system characterize its infrared dynamics, while the causality of the theory is determined by its ultraviolet behavior, the viscosity bound/microcausality link should not be applicable to theories that undergo low temperature phase transitions. We present an explicit model of AdS/CFT correspondence that confirms this fact.