wandering sets, part 3 : Maxwell's demon

[spoonless]

I gave 3 examples of things that dissipate in part 2: friction, electrical resistance, and hurricanes. I feel like I understand fairly well why we call these dissipative, although I've always felt or hoped that there is some unifying principle that sheds more light on the subject and explains why them and not other things. But there's a forth

Comments

[sapience]

Wouldn't gas particles be bouncing back the other way, too? And the more of them that get trapped, the more of them will be bouncing around and be likely to cross back.

[spoonless]

Under normal conditions (without a demon) yes... every now and then, a particle from one side will cross over to the other, and this happens in both directions and is balanced. So in the long run, there remain roughly equal number of particles on either side of the box

[spoonless]

Visual aid courtesy of wikipedia...

http://en.wikipedia.org/wiki/File:Maxwell%27s_demon.svg

[spoonless]

Actually the Wikipedia image may do more to confuse than help, as it depicts a slightly different version of Maxwell's demon than the simplified one I'm presenting. In their version, there are two different kinds of molecules, represented by blue and red, and the demon is sorting them. But this is unnecessary, the same paradox presents itself if you just have one kind and you want to move them all to one side.