Demons

There are several theories in physics that apply probability theory to various problems. Examples of this can be statistical mechanics and statistical thermodynamics. Physicist James Clerk Maxwell came up with a thought experiment in the late nineteenth century that has puzzled people for over a century. The thought experiment deals with how someone might be able to violate the second law of thermodynamics by creating a situation that might decrease entropy in the system. Entropy can be thought of as the tendency for any system to loose energy. Entropy is a function of state, like the internal energy. It measures the relative degree of order (as opposed to disorder) of the system when in this state. An understanding of the meaning of entropy thus requires some appreciation of the way systems can be described microscopically (Sethna).

Maxwell had the insight to think of two systems placed side by side....
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