January 23: “Bound on thermalization from hydrodynamic fluctuations” by Luca Delacretaz

Abstract: Interacting systems thermalize. They can do so arbitrarily slowly, but not arbitrarily fast: the time scale necessary for a quantum many-body system to reach local thermal equilibrium has been conjectured to be bounded below by the Planckian time, \hbar/T. I will show that consistency of hydrodynamics (or diffusion), which generically emerges at late times, implies that this local equilibration time indeed has a lower bound. The key tool is the derivation of universal corrections to diffusion at intermediate times using EFT techniques: when these corrections are large the system cannot have thermalized. For the special case of CFTs, combining this argument with scale invariance allows one to prove the conjectured Planckian bound. I will also discuss spin chains, where the knowledge of these universal corrections to diffusion can allow for precision tests of thermalization, and a more accurate identification of a thermalizing system’s dissipative universality class with limited numerical resources.