A new study throws QED theory into question. Pohl, R. et al. The Size of the Proton. Nature 466, 213–216 (2010).
The root-mean-square charge radius, rp, has been determined with an accuracy of 2 per cent (at best) by electron–proton scattering experiments. The present most accurate value of rp (with an uncertainty of 1 per cent) is given by the CODATA compilation of physical constants. This value is based mainly on precision spectroscopy of atomic hydrogen and calculations of bound-state quantum electrodynamics (QED). However, using an exotic version of hydrogen (muonic hydrogen in which a proton is orbited by a negative muon) and pulsed laser spectroscopy to measure the muonic Lamb shift, the value of rp is 5 standard deviations off of previous value. This implies either that the Rydberg constant has to be shifted by −110 kHz/c (4.9 standard deviations), or the calculations of the QED effects in atomic hydrogen or muonic hydrogen atoms are insufficient.