Hi All, First off, everyone on this forum is amazing. Period. Second, I had a thought experiment the other day that was interesting and was hoping you could comment on it: The System: Imagine an infinitely long hollow cylinder in one direction, made of a metal (perhaps aluminum). At the beginning of the cylinder is a stationary black-body radiator that has a high emissivity (say, carbon black). Somewhere in the middle of the cylinder is a movable mirror that may move up or down the cylinder, acting as a piston. Suppose the entire system is at thermal equilibrium. The mirror-piston and metallic cylindrical container have a low emissivity, so that the carbon black emitter is emitting more thermal radiation that either the mirror or the cylinder. In addition, at any time a mirror may be slid in front of the carbon black emitter, effectively reflecting radiation that would otherwise have traveled down the cylinder back onto the emitter. Finally, whenever the emitter is uncovered, thermal radiation travels down the cylinder from the emitter to the mirror-piston, producing a small radiation pressure on the mirror-piston. My Question: Imagine this engine cycle: The mirror-piston travels down the cylinder, due to the radiation pressure The emitter cover is slid over the emitter The mirror-piston is compressed up the cylinder The emitter cover is retracted, uncovering the emitter Since compressing the piston requires less work than expanding it (less radiation pressure), the temperature of the system would spontaneously decrease. My only explanation would be that the work required to slide the emitter cover into place over the emitter might be the culprit - However, since it is a finite quantity of work, it could always be compensated for by allowing the mirror-piston to travel further down the cylinder. Thoughts?