Although Johnson–Nyquist noise shares many similarities with phonon noise (e.g. the noise spectral density depends on the temperature and is white at low frequencies), these two noise sources are distinct. Johnson–Nyquist noise arises from the random thermal motion of electrons, whereas phonon noise arises from the random exchange of phonons. Johnson–Nyquist noise is easily modeled at thermal equilibrium, where all components of the circuit are held at the same temperature. A general equilibrium model for phonon noise is usually impossible because different components of the thermal circuit are nonuniform in temperature and also often not time invariant, as in the occasional energy deposition from particles incident on a detector. The transition edge sensor typically maintains the temperature through negative electrothermal feedback associated with changes in internal electrical power.