Armed with this demonstration, Benn

Armed with this demonstration, Bennett returned to Szilard’s paper to confirm that the Demon’s operation really consisted of two steps: a measurement step and a memory step (because the Demon has to remember where he put the slow and fast molecules). Brillouin had suggested that the entropie increase was a result of the measurement step. Bennett (1987) refuted this idea by devising a reversible measuring device that allowed measurements to be made without an increase in the entropy. In effect, he showed how the Demon could find out where the molecules were without the need for a headlamp. He then proposed that the true source of the increasing entropy was in the memory step. Since the demon will eventually run out of memory space if he does not clear outdated information from his memory, at some point he must destroy information; and according to Landauer’s demonstration, the destruction of information has to be paid for by an increase in entropy. If one supposes a demon (or a computer) with a very large memory, he could of course simply remember all the measurements. The trouble with this scenario, Bennett explains, is that “the cycle would not then be a true cycle: every time around, the engine’s memory, initially blank, would acquire another random bit [of information]. The correct thermodynamic interpretation of this situation would be to say that the engine increases the entropy of its memory in order to decrease the entropy of its environment” (p. 116).