## The Fluctuation-Dissipation Theorem

University of Michigan

The equilibrium state of a system is in general a state of rest. However,
if one looks under great magnification one sees that there are always spontaneous
microscopic fluctuations about this state. These fluctuations are what are also sometimes
called noise. The fluctuation-dissipation theorem tells us how to calculate the spectrum
of these fluctuation. Most physicists have at least heard of the theorem, but few
seem to know exactly what it is or how it is used. My aim will be to do just that;
describe what the theorem is and to illustrate how it is to be used.