Cavity-induced changes in atomic spontaneous emission rates are often interpreted in terms of quantum electrodynamical zero-point field fluctuations. A completely classical method of computing this effect in terms of the unquantized normal mode structure of the cavity is presented here. Upon applying the result to a classical dipole radiating between parallel mirrors, we obtain the same cavity correction as that for atomic spontaneous emission in such a cavity. The theory is then compared with a recent experiment in the radio-frequency domain.

I provide here a classical calculation of the Lamb shift that is of the same order of magnitude as the quantum Bethe result. This contradicts Jackson's claim that a classical calculation can not get the Lamb shift right—even to within an order of magnitude.