Wave Speeds in the Macroscopic Extended Model for Ultrarelativistic Gases

Equations determining wave speeds for a model of ultrarelativistic gases are investigated. This model is already present in literature; it deals with an arbitrary number of moments and it was proposed in the context of exact macroscopic approaches in Extended Thermodynamics. We find interesting results: the whole system for the determination of the wave speeds can be divided into independent subsystems and some wave speeds, but not all of them, are expressed by square roots of rational numbers. Moreover, the remarkable property that these wave speeds for the macroscopic model are the same of those in the kinetic model is proved.