Shear viscosity to entropy density ratio: A powerful tool for gravity theories and strongly coupled fluids

In this perspective review, we present a concise yet multifaceted overview of the pivotal role played by the shear viscosity to entropy density ratio across various physical contexts. After summarizing some of the main aspects of the bound obtained by Kovtun, Son and Starinets, we examine potential sources of its violation, exploring the insights these may offer and their connections to fundamental causality conditions. We also review a range of experimental tests conducted in diverse, yet complementary, physical systems, discussing the prospects opened by upcoming measurements.