Giant retardation effect in electron-electron interaction

In electron-electron interactions in electromagnetic systems, retardation in the exchange of a virtual photon is essentially important as the first-order quantum electrodynamics correction. However, the retardation effect is generally so small that it is buried in unretarded electric and magnetic interactions and thus has yet to be directly probed. Here, we present a giant contribution of the retardation effect in an electron-electron interaction via observing strong electric-dipole-allowed radiative transition rates. The relative transition rates are obtained for two dominant radiative transitions from the 1s2s(2)2p(1/2)2p(3/2) inner-shell excited state of boronlike tungsten and bismuth ions to 1s(2)2s(2)2p(1/2) and 1s(2)2s(2)2p(3/2), and it was found that the transition rate ratio between the two transitions is affected by the retardation effect up to more than 100%.