Abstract
A basic assumption implicit in the application of thermodynamics to the electromagnetic field is that the laws of thermodynamics are locally valid for radiative emission and absorption processes. This means that a certain minimum amount of entropy must be created by the radiative process itself. It is shown, by considering the extreme case in which the spontaneous emission of a natural spectral line is the only process taking place, that this assumption is correct, and that its validity is essentially a consequence of the uncertainty principle as expressed by the reciprocal relationship between natural line breadth and lifetime.

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