The status of the KSS bound and its possible violations (How perfect can a fluid be?)

TL;DR

This paper reviews the KSS bound on fluid viscosity to entropy ratio in quantum field theories with gravity duals, examining its validity, potential violations, and implications for physical phenomena like RHIC and phase transitions.

Contribution

It provides a comprehensive review of the KSS bound, analyzing conditions under which it may be violated and its significance in theoretical and experimental contexts.

Findings

The KSS bound is generally valid but can be violated under certain conditions.

Violations of the bound are related to phase transitions and specific theoretical models.

The bound's relevance to experimental data from RHIC is discussed.

Abstract

In this work we briefly review the Kovtun-Son-Starinet (KSS) computation of the ratio eta/s for quantum field theories with gravitational dual and the related conjecture that it is bound from below by 1/(4 pi). We discuss the validity of the bound and the nature of its possible violations, its relevance for RHIC, its connection with phase transitions and other related issues.