The First Big Success: \(\eta /s\) and Hydrodynamics
What is Hydrodynamics? It is just the effective field theory description of a system close to equilibrium, valid at sufficiently long times and sufficiently large distances . Notice that, despite the misleading name, it can be applied to very generic systems, and not necessarily only to fluids ! Hydrodynamics is not the theory of fluids but it is much more general and it is simply the low energy effective description of a specific system.