Proton radius puzzle
Precision tests of QED are limited by our knowledge of the hadronic effects. In particular, predictions for the Lamb shift are suffered from a bad knowledge of the nuclear structure. One can reverse the problem and to determine nuclear parameters from spectroscopy. Studying the Lamb shift in ordinary and muonic hydrogen, values of the proton charge radius have been derived. The ordinary-H value and the muonic-H value do not agree. The e-p scattering value supports the one from ordinary hydrogen.
I will review the situation with determination of the proton charge radius, which is likely the biggest (in terms of sigmas) contradiction in the contemporary QED-related studies.