Semi-Classical Limit of the Klein-Gordon Equation to Relativistic Euler Equations via an Adapted Modulated Energy Method

We show the convergence of the solutions to the massive nonlinear Klein-Gordon equation toward solutions to a relativistic Euler with a potential-type system in the semi-classical limit. In particular, the momentum and the density of Klein-Gordon converge to the momentum and the density of the relativistic Euler system in Lebesgue norms. The relativistic Euler with potential system is equivalent to the standard relativistic barotropic Euler system, differing only by a change in the unknowns. The proof relies on the modulated energy method adapted to the wave equation and the relativistic setting: a modulated stress-energy method.