On Sep 5, 12:14=A0pm, Jarek Duda gmail.com> wrote:

I forgot in previous posts to analyze one little component in your
example: the gas that is contained in a transparent container. I have
been analyzing the expansion of the photon gas. The gas, presumably
molecular, is staying in place and doesn't expand as the mirrors move
apart. I think that what happens to the molecular gas may shed some
light on thermodynamics.
I assume the gas is not an ideal gas, so that at very low
temperatures it condenses and sublimates. As the IR radiation expands,
the temperature of the gas goes down. Eventually, the gas
crystallizes.
Most gases at higher concentrations do the following. At some
point in the expansion, the gas may turn to a liquid. At a still lower
temperature, the liquid turns solid.
At low concentrations, some gases will turn directly to a
crystalline solid. This is the opposite of sublimation. One unusual
gas, helium IV, turns to a superfluid instead of a crystal.
In any case, the entropy contained by the molecules decreases.
At absolute zero in temperature, the entropy of the molecules turns to
zero. Yet many other people here and I claim that the total entropy
has increased.
This is not a contradiction. What happened is that the entropy
between the two mirrors has increased as the volume between the two
mirrors increased. As the mirror moves apart, the disorder in the
electromagnetic waves increases. However, the entropy density
decreases. The entropy density, which is the ratio of entropy to
volume between the mirrors, decreases because the entropy spreads out.
The molecular gas, constrained in volume by your transparent
container, only contains a small fraction of the entropy.
This may be one of the things that confuse you. As the
electromagnetic waves spread out, the entropy spreads with it.
Therefore, the entropy density really does approach zero. However, the
total entropy between the two mirrors increases because the volume of
radiation increases.
The final crystal, at zero degrees Kelvin, has no entropy just as
described in the Third Law of thermodynamics. Therefore, the system
you described has created a localized area where entropy has
decreased. Thus, what you are describing is a sort of refrigeration.
However, the second law wasn't violated.
The entropy has been sucked out of the molecular gas. However, the
process is irreversible because the mirrors are moving apart. In fact,
the inertia of the mirrors would keep them moving apart even when the
radiation is neglibible. So this is an irreversible reaction.