Fluctuation-dissipation theorem

The fluctuation-dissipation theorem (FDT) is a powerful tool in statistical physics for predicting the behavior of systems that obey detailed balance. Given that a system obeys detailed balance, the theorem is a general proof that thermal fluctuations in a physical variable predict the response quantified by the admittance or impedance of the same physical variable, and vice versa. The fluctuation-dissipation theorem applies both to classical and quantum mechanical systems.