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An imaginary being whose action appears to contradict the second law of thermodynamics, which identifies the natural direction of change with the direction of increasing entropy. There has always been a certain degree of discomfort associated with the acceptance of the law, particularly in relation to the time reversibility of physical laws and the role of molecular fluctuations. In 1867, J. C. Maxwell considered, in this connection, the action of “a finite being who knows the paths and velocities of all the molecules by inspection.” This being was later referred to as a demon by Lord Kelvin, and the usage has been generally adopted. See Entropy, Thermodynamic principles, Time, arrow of
The activity of Maxwell's demon can be modeled by a trapdoor in a partition between two regions full of gas at the same pressure and temperature. The trapdoor needs to be restrained by a light spring to ensure that it is closed unless it is struck by molecules traveling from the left (see illustration). Its hinging is such that molecules traveling from the right cannot open it. The essential point of Maxwell's vision was that molecules striking the trapdoor from the left would be able to penetrate into the right-hand region but those present on the right would not be able to escape back into the left-hand region. Therefore, the initial equilibrium state of the two regions, that of equal pressures, would be slowly replaced by a state in which the two regions acquired different pressures as molecules accumulated in the right-hand region at the expense of the left-hand region. Only a slightly more elaborate mechanical arrangement is needed to change the apparatus to one in which the temperatures of the two regions move apart. In each case, the demonic trapdoor appears to be contriving a change that is contrary to the second law, for an implication of that law is that systems in either mechanical equilibrium (at the same pressure) or thermal equilibrium (at the same temperature) cannot spontaneously diverge from equilibrium.
As frequently occurs in science, the resolution of a paradox or the elimination of an apparent conflict with a firmly based law depends on a detailed analysis of the proposed arrangement. Numerous analyses of this kind have shown that the activities of Maxwell's demon do not in fact result in the overthrow of the second law.