don@allegra.UUCP (D. Mitchell) (03/22/84)
Yes, all the air molecules in a room could move to one side, but that is not inconsistent with statistical thermodynamics or the definition of entropy. First, let me define two terms: MICROSTATE: If you have a box of gas, the actual positions and velocities of all the molecules is the [highly detailed] microstate of the system. MACROSTATE: Properties of a more average nature like temperature and pressure, are descriptions of the macrostate. ENTROPY depends on the probability of macrostates! For example, in a box of gas, any configuration of the molecules is EQUALLY LIKELY. Those are the microstates. However, there are many more configur- ations that correspond to "evenly distributed gas" than there are that correspond to "all the gas on one side of the room". Those are macrostates, and the second one is very improbably. Finally, thermodynamics says "entropy tends to increase". This does not mean (as many people say) that the disorder of a system increases. It means that a system tends toward the most probably macrostate. In some cases, that actually means the system becomes MORE ORDERLY. For example, a growing crystal, or a gravitating dust cloud condensing into planets, or even the chemical processes that formed life all represent increases in entropy.