begin{center}ATTENTION : salle inhabituelle / unusual roomend{center} It is an experimental fact that fluids in a nonequilibrium steady state generically have long range space correlations. Some non reversible linear stochastic partial differential equations provide phenomenological models of this effect. We analyse this phenomenon in more detail starting from microscopic models admitting a hydrodynamic scaling limit. Boundary driven lattice gases provide important examples but long range space correlations can appear also in their equilibrium states if the microscopic dynamics violates time reversal in a “sufficiently strong” way. begin{center}(Séminaire de la Fédération de Physique Statistique Paris-Sud)end{center}

Universita La Sapienza, Roma