  By enhancing electron position fluctuations, equilibrium electromagnetic
radiation modifies the potential for an electron in a Hydrogen atom. This can
have significant effects for weakly bound states and especially at finite
temperature. This implies a 2% correction to Bethe's value for 2S-2P Lamb shift
for weak fluctuations, but the effect is an order of magnitude larger for
strong fluctuations where it provides direct measure of the proton diameter.
