Demons in Physics

Maxwell’s ”finite being” was born in 1867, as one ”who knows the paths and velocities of all the molecules” ([7], p.213), and who, unlike us, is ”clever enough” ([7], p.213), and can violate the second law of thermodynamics. It was baptized as a demon by Lord Kelvin ([7], p. 214), and ever since has fueled the discussions in the foundations of statistical mechanics. Since its inception, and contrary to Maxwell’s own intention, the demon has suffered numerous exorcism attempts, physical and information– theoretic,1 all aiming to demonstrate that it cannot achieve its goal; all culminating in (or, more precisely, starting from) the common belief that the second law of thermodynamics is an a priori truth. In their monograph The Road to Maxwell’s Demon, Hemmo & Shenker put an end to this long tradition of exorcism, and argue convincingly that under a very mild assumption, common to classical and quantum mechanics, namely, the deterministic character of the dynamics,2 the demon is consistent with both mechanics and statistical mechanics. They do so by painstakingly leading their readers through simple constructions that allow them to depict statistical mechanics as a theory based on three ingredients: (a) deterministic micro–dynamics on phase space, (b) a partition