Dr Harri Niemi (Frankfurt University)

Transient relativistic fluid dynamics from the Boltzmann equation

A general derivation of relativistic fluid dynamics from the Boltzmann equation using the method of moments will be presented. The main difference between the presented approach and the traditional 14-moment approximation by Israel and Stewart is that we will not close the fluid-dynamical equations of motion by truncating the expansion of the distribution function. Instead, we keep all the terms in the moment expansion and truncate the exact equations of motion for the moments according to a systematic power counting scheme in Knudsen and Reynolds number. We show that the Boltzmann equation contains an infinite number of microscopic time scales and demonstrate that, in order to derive the fluid-dynamical equations of motion, it is essential to consider only the slowest of these time scales. We further test the validity of different approximations by comparing to the direct numerical solutions of the Boltzmann equation.