Is "Maxwell's Demon" Possible?

Question

"Maxwell's Demon" is a "demon", metaphorically, that guards a "gate" between two gas-filled containers. When the demon sees a faster-than-average molecule flying towards the gate in one container, he opens the gate, letting the fast particle through. This process of selectively letting molecules through could raise the temperature of one container and lower the other's, violating the 2nd Law of Thermodynamics. Is such a "demon" possible? Isn't evaporation a natural example of a "demon?"

Answer 1

The question is non-trivial.
(about my English, I am Russian).

I don't think evaporation is the case, because total entropy anyway increases due to mixing liquid molecules with gas molecules. In the case of enclosed space, equilibrium is reached soon, and temperatures of liquid and gas phases are no longer different.
Open space means that the system is so large, that all liquid evaporates without reaching equilibrium. It is possible to make work from the temperature difference between gas and liquid phases but for the price of increasing entropy, as always.
So, what does the demon do in this case? He sorts the molecules by their energy, lets faster ones to leave the liquid phase, but does it for the price of mixing them with molecules of another type, thereby decreasing order.

Answer 2

The laws of thermodynamics don't have quite the status of some other laws, because they are essentially statistical. The second law says that if you leave things to themselves they will get into a random state in which energy is distributed evenly. That's why anything that can undo the randomness, like Maxwell's demon, can stop the law working.

I suppose evaporation is an example here, but I'm not enough of a physicist to say whether this is a watertight example (excuse the pun). A better example is probably any living organism, which actively pumps entropy out of its body to maintain its internal order for as long as it lives.

Answer 3

Well, the demon is possible but it requires energy to operate. Here's a nice article and it looks like thermodynamic laws still hold.

http://en.wikipedia.org/wiki/Maxwell's_demon