Relativistic hydrodynamics from projection operator method

We study relativistic hydrodynamics in the linear regime, based on Mori's projection operator method. In relativistic hydrodynamics, it is considered that ambiguity about the fluid velocity occurs from a choice of a local rest frame: the Landau and Eckart frames. We find that the difference of the frames is not the choice of the local rest frame, but rather that of dynamic variables in the linear regime. We derive hydrodynamic equations in the both frames by the projection operator method. We show that natural derivation gives the linearized Landau equation. Also, we find that, even for the Eckart frame, the slow dynamics is actually described by the dynamic variables for the Landau frame.