An Etymological Dictionary of Astronomy and Astrophysics

A → thought experiment meant to raise questions
about the possibility of violating the → second law of thermodynamics. A wall separates two compartments filled with gas. A little “demon” sits by a tiny trap door in the wall. He is able to sort hot (faster) molecules from cold molecules without expending energy, thus bringing about a general decrease in → entropy and violating the second law of thermodynamics. The → paradox is explained by the fact that such a demon would still need to use energy to observe and sort the molecules.
Thus the total entropy of the system still increases.

See also: Named after James Clerk Maxwell (→ maxwell), who first thought of this experiment; → demon.