image The subtle differences between the non-relativistic moments (26Popup Equation) and the relativistic moments (67Popup Equation) may be studied in papers on the relativistic kinetic theory, e.g. Lichnerowicz & Marrot [29], Chernikov [12, 10, 11] and Marle [32] also in the book by de Groot, van Leeuven & van Weert [13].