Maxwell, James demon

The earliest hint that physics and information might be more than just casually related actually dates back at least as far as 1871 and the publication of James Clerk Maxwell s Theory of Heat, in which Maxwell introduced what has become known as the paradox of Maxwell s Demon. Maxwell postulated the existence of a hypothetical demon that positions himself by a hole separating two vessels, say A and B. While the vessels start out being at the same temperature, the demon selectively opens the hole only to either pass faster molecules from A to B or to pass slower molecules from B to A. Since this results in a systematic increase in B s temperature and a lowering of A s, it appears as though Maxwell s demon s actions violate the second law of thermodynamics the total entropy of any physical system can only increase, or, for totally reversible processes, remain the same it can never decrease. Maxwell was thus the first to recognize a connection between the thermodynamical properties of a gas (temperature, entropy, etc.) and the statistical properties of its constituent molecules. 

One spectacular use of a rotaxane was to illustrate James Clerk Maxwell s 19th century thought experiment known as Maxwell s demon as shown in Fig. 1.23. Maxwell proposed several experiments that would violate the Second Law of Thermodynamics to show how the entropy of an isolated system could be reduced without expending energy. To do so he invoked the idea of a demon who effortlessly operates a frictionless door between two compartments which contain particles at different temperatures. Whenever a particle approaches the door the demon decides whether to open it and allow the particle through. In this way the particles can be sorted so that one compartment contains only hot particles and the other only cold. A similar example can be envisaged in which particles at an equilibrium pressure are moved from one compartment to the other to increase its pressure without any work apparently being done. 

M. Goldman, The Demon in the Aether. The Life of James Clerk Maxwell, Paul Harris, Edinburgh, 1983. 

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