Light Scattering from Nonequilibrium Stationary States: The Implication of Broken Time-Reversal Symmetry

We report a theroetical prediction of a new feature in the spectrum of light scattered from a fluid in a nonequilibrium stationary state with a temperature gradient. The spectrum is not symmetric in the frequency shift and the Brillouin components have different intensities. The phenomenon is linked to the breaking of time-reversal symmetry and to the appearance of a static correlation function between momentum and number densities which is zero in equilibrium, but has a $\frac{1}{k^2}$ dependence in the stationary state. We suggest a light scattering experiment by means of which these predictions can be verified.