Re: Entropy question

In article <E29Ne.3016$zb.2582@trndny04>,
Craig Franck <craig.franck@xxxxxxxxxxx> wrote:
>I remember reading an account of the fact that when a gas expands
>in a container entropy goes up, but when gas and dust clouds in
>space do the opposite and condense, entropy increases as well. (I
>think it was in a Roger Penrose book.)
>
>Why exactly is this? From a statistical POV, I don't see how N
>number of particles know which kind of system they are participating
>in or if they are behaving in the most likely way.

A flip way of defining entropy is that things do what they do, and they
won't undo it. If you drop a rock, it falls to the ground, and won't
levitate back up. If you drop a helium-filled balloon it will rise, and
won't come down until the helium leaks out of it. If you drop a
helium-filled balloon on the Moon it will hit the ground because there's
no atmosphere to bouy it up.

There's enough gas over large regions in interstellar space that it forms
a considerable gravitating mass, and it "falls" together. Unless the
gravity goes away, the gas won't disperse again without some new
influence. The amounts of gas we deal with in the laboratory are much
too small for self-gravitation to have any effect. But we can compress
ten moles of gas to 2000 psi, and gravity can't.

--
"The polhode rolls without slipping on the herpolhode lying in the
invariable plane." -- Goldstein, Classical Mechanics 2nd. ed., p207.
.

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