Thermalization of 2d Quantum Field Theories

<!--HTML-->I will show that (1+1)d QFTs typically thermalize more slowly than the expected "Planckian" equilibration time 1/T. This arises due to causality, which imposes strong constraints on the emergence of hydrodynamic sound in low dimensions. Subluminality of sound further allows one to define a thermodynamic analogue of Zamolodchikov's C-function, and constrain the sign of the TT-bar term in the low energy limit of any 2d QFT.