I will discuss novel effective field theories for hydrodynamics. After identifying the relevant low-energy degrees of freedom, I will show how the symmetries of the problem can be efficiently implemented using supersymmetry. Interestingly, a conserved entropy current naturally arises as a Noether current in superspace. If time permits, I will also show how to derive such effective actions from holography.

It is widely believed that hydrodynamics is the low energy effective theory of any quantum field theory. In this talk I will derive the local low energy effective action for uncharged dissipationless conformal fluids through holography. This can be achieved by solving a double-Dirichlet problem for linearised gravitational perturbations between the conformal boundary of a (4+1)-dimensional AdS-black brane background and a stretched horizon. Interestingly dissipationless fluid dynamics is captured by Goldstone bosons of the broken symmetries by the classical solution ending on the second boundary. I will discuss how the effective action approach might be inconsistent unless there is a coupling to a dissipative dynamical IR sector which for simplicity can be replaced by a membrane paradigm-like boundary condition on a stretched horizon and I will also comment on the limits of validity of this simplification.