Thermal Fluctuations in a Layer of Liquid CS_{2} Subjected to Temperature Gradients with and without the Influence of Gravity

We report data for nonequilibrium density fluctuations in a layer of liquid CS2 subjected to temperature gradients on Earth and in a satellite. The structure factor S(q) was measured using a calibrated shadowgraph. Upon removing gravity, S(q) increased dramatically at small wave vector, until the fluctuations generated by thermal noise were limited only by the 3 mm sample thickness. The results agree with theory to within a few percent on Earth and are ∼14% below theory in microgravity, demonstrating that the use of equilibrium Langevin forces is appropriate in this nonequilibrium situation.