Hi Say I have an oven, from which atoms are effusing through a very small aperture. By "small" I mean such that it does not disturb the thermal equilibrium of the atoms. Now say I choose to e.g. shine laser light onto the atoms transversely to the direction from which the atoms emerge out of the oven. Under perfect condition, i.e. when treating the motion of the atoms classically, the atomic beam is collimated. Is it correct to say that the laser beams are the reason why Liouville's theorem is invalid in this case? I.e., that the beams act dissipatively, thus the volume in phase space is not conserved? Best, Niles.