Demons, Engines and the Second Law

O ne manifestation of the sec­ ond law of thermodynamics is that such devices as refrigera­ tors, which create inequalities of tem­ perature, require energy in order to operate. Conversely, an existing in­ equality of temperature can be ex­ ploited to do useful work-for exam­ ple by a steam engine, which exploits the temperature difference between its hot boiler and its cold condenser. Yet in 1871 the Scottish physicist James Clerk Maxwell suggested, in his Theory of Heat, that a creature small enough to see and handle indi­ vidual molecules might be exempt from this law. It might be able to cre­ ate and sustain differences in temper­ ature without doing any work: " . . .if we conceive a being whose faculties are so sharpened that he can follow every molecule in its course, such a being, whose attri­ butes are still as essentially finite as our own, would be able to do what is at present impossible to us. For we have seen that the molecules in a vessel full of air at uniform tempera­ ture are moving with velocities by no means uniform . . . . Now let us sup­ pose that such a vessel is divided into two portions, A and B, by a divi­ sion in which there is a small hole, and that a being, who can see the in­ dividual molecules, opens and closes this hole, so as to allow only the swifter molecules to pass from A to B, and only the slower ones to pass from B to A. He will thus, without expenditure of work, raise the tem­ perature of B and lower that of A, in contradiction to the second law of thermodynamics." The "being" soon came to be called Maxwell's demon, because of its far­ reaching subversive effects on the natural order of things. Chief among