Maxwell’s Demon

In his 1867 thought experiment, “Maxwell’s Demon,” James Clerk Maxwell attempted to show that thermodynamics is not strictly reducible to mechanics. Maxwellian Demons are mechanical devices that carry out measurements on a thermodynamic system, manipulate the system so as to extract work from it, and erase all records of the measurement outcomes. If successful, they decrease the total entropy of the universe, thereby violating the Second Law of Thermodynamics. According to the prevalent contemporary approach, the Demon fails, since measurement or erasure necessarily result in entropy increase. This article provides a brief overview of statistical mechanics focusing on the notions of macrovariables and macrostates, and describes in mechanical terms information processes such as measurement and erasure. It is shown that Maxwellian Demons are compatible with the principles of mechanics. After a detour through reduction, probability, information, and computation in statistical mechanics, it is seen that Maxwell was right after all.