From Paul G. Ellis
Stephen Battersby uses Maxwell's demon to discuss the physical nature of information (14 May, p 28). But how can the demon “see the motions of air molecules” or operate “a frictionless door” or “slide a partition” without some physical interaction?
Surely, any useful modern analysis has to include the demon's use of photons or the like (which are subject to Heisenberg's Uncertainty Principle) to measure molecules' momentum vectors in order to determine whether they should pass into a hotter location. Similarly, some account is needed of the physical interaction involved in operating the door or inserting a partition.
Battersby mentions Leo Szilard's approach to the thought experiment. Though Szilard estimates the molecule's work of pushing a partition, he appears to me to ignore the physics both of the initial insertion of the partition and of the demon measuring which end the molecule is in.
Chichester, West Sussex, UK
