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Fluctuation–dissipation theorem
(section)
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===Brownian motion=== For example, [[Albert Einstein]] noted in his 1905 paper on [[Brownian motion]] that the same random forces that cause the erratic motion of a particle in Brownian motion would also cause drag if the particle were pulled through the fluid. In other words, the fluctuation of the particle at rest has the same origin as the dissipative frictional force one must do work against, if one tries to perturb the system in a particular direction. From this observation Einstein was able to use [[statistical mechanics]] to derive the [[Einstein–Smoluchowski relation]] <math display="block"> D = \mu \, k_\text{B} T, </math> which connects the [[Fick's law of diffusion|diffusion constant]] ''D'' and the particle mobility {{mvar|μ}}, the ratio of the particle's [[Terminal velocity|terminal]] [[drift velocity]] to an applied force; {{math|''k''<sub>B</sub>}} is the [[Boltzmann constant]], and {{mvar|T}} is the [[absolute temperature]].
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