The photon discussion a few weeks back got me reading about cavity radiation. I'm puzzled. Perhaps somebody can point me in the right direction. For those who don't already know, cavity radiation is a surprising phenomenon which required quantum theory to model. Metals radiate energy in the form of light. Each metal has a characteristic "radiance" for each temperature, the amount of energy it radiates. If a block of a metal is hollowed out and a small port is drilled to see in, the radiance of the cavity is substantially higher than that of the surface of the metal. As if that weren't shocking enough, it turns out that the radiance of cavities is the same no matter what kind of metal is used. (This is so counterintuitive that I almost don't believe it!) My book suggests that this is intuitively sound because if cavity radiation had different values, a violation of the second law of thermodynamics would occur. Set up two cavity radiators of a metal X and a different metal Y like so: XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXX XXX YYY YYYYYYYYYYYYYY XXXXXXXXXXXX XX YY YYYYYYYYYYYY XXXXXXXXXXX YYYYYYYYYYY XXXXXXXXXXXX XX YY YYYYYYYYYYYY XXXXXXXXXXXXXX XXX YYY YYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY If X and Y have the same temperature and Y has a higher level of cavity radiation, then X will get hotter and Y will get cooler. In other words, you would have a perpetual motion machine. So the radiance of each cavity must be the same. So far so good, but this argument should also apply to surface radiance, which we know to be different: XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY If Y has a higher level of surface radiance than X, then one would expect X to grow hotter, also making possible a perpetual motion machine. Clearly, something went wrong somewhere. Can anybody clue me in?