Normal modes of relativistic systems in post Newtonian approximation
  We use the post Newtonian (pn) order of Liouville's equation (pnl) to study the normal modes of oscillation of a relativistic system. In addition to classical modes, we are able to isolate a new class of oscillations that arise from perturbations of the space-time metric. In the first pn order; a) their frequency is an order q smaller than the classical frequencies, where q is a pn expansion parameter; b) they are not damped, for there is no gravitational wave radiation in this order; c) they are not coupled with the classical modes in q order; d) in a spherically symmetric system, they are designated by a pair of angular momentum eigennumbers, (j,m), of a pair of phase space angular momentum operators (J^2,J_z). Hydrodynamical behavior of these new modes is also investigated; a) they do not disturb the equilibrium of the classical fluid; b) they generate macroscopic toroidal motions that in classical case would be neutral; c) they give rise to an oscillatory g_{0i} component of the metric tensor that otherwise is zero for the unperturbed system. The conventional classical model are, of course, perturbed to order q. These, however, have not been discussed in this paper. 
Y. Sobouti and V. Rezania