Figure 4.

Diagram of Maxwell’s Gedanken experiment. (a) Gas molecules are in the left chamber. For every molecule, you have one bit of information about which chamber it is in. (b) The door between the chambers is opened and the gas equalizes pressure: the second law of thermodynamics says entropy increases. For each molecule you now have zero bits of information about which chamber it is in. (c) Gas molecules start in both chambers with zero bits of information about which chamber they are in. (d) Maxwell proposed a “demon” who sits at the door between chambers and only briefly opens the door any time a molecule is about to go through the door from the left chamber to the right chamber. After some time, you again have one bit of information for each molecule. But entropy has decreased in violation of the second law of thermodynamics!