The existence of fluctuations and their agreement with theory is confirmed by studying systems at temperatures above 0K. Random thermal noise (Nyquist frequency) in transmission lines as a result of the thermal motion of electrons, limits the signal which such lines can carry [4].
The fluctuation-dissipation theory relates the fluctuations of a physical quantity of a system in equilibrium to a dissipation process which can be realized when the system is subject to an external force driving it away from equilibrium. It has been recognized that this theory makes it possible to find the nonequilibrium properties of a system from the analysis of thermal fluctuations of the system in equilibrium states. Therefore, the statistical mechanics of irreversible processes may, in a sense, be reduced to that of equilibrium states, which however has to deal with time-dependent fluctuations [4].
Moreover, any distribution function in statistical mechanics contains the probabilities of all possible fluctuations. Fluctuations are part of any macroscopic condition, equilibrium included [6].