| Issue |
EPL
Volume 151, Number 6, September 2025
|
|
|---|---|---|
| Article Number | 69001 | |
| Number of page(s) | 7 | |
| Section | Gravitation, cosmology and astrophysics | |
| DOI | https://doi.org/10.1209/0295-5075/ae0365 | |
| Published online | 18 September 2025 | |
Shear viscosity to entropy density ratio: A powerful tool for gravity theories and strongly coupled fluids
1 Dipartimento di Fisica, Università di Pisa, Polo Fibonacci - Largo B. Pontecorvo 3, I-56127 Pisa, Italy
2 INFN, Sezione di Pisa, Polo Fibonacci - Largo B. Pontecorvo 3, I-56127 Pisa, Italy
3 Scuola Internazionale Superiore Studi Avanzati (SISSA) - Via Bonomea 265, I-34136 Trieste, Italy
4 Institute for Fundamental Physics of the Universe (IFPU) - Via Beirut 2, I-34014 Trieste, Italy
5 INFN, Sezione di Trieste - Via Valerio 2, I-34127 Trieste, Italy
6 INFN, Laboratori Nazionali del Gran Sasso - Via G. Acitelli, 22, I-67100 Assergi (AQ), Italy
Received: 17 July 2025
Accepted: 4 September 2025
Abstract
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.
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