Crisis and catharsis in atomic physics

Precise measurement of an atomic hydrogen transition resolves the proton size puzzle The spectrum of the simplest atomic species, the hydrogen (H) atom with only a single electron, was initially described with Bohr's theory of 1913 and was refined with newer theories, from Schrödinger's quantum mechanics to Dirac's relativistic formalism and ultimately Feynman-Schwinger-Tomonaga's quantum electrodynamics (QED). The latter includes the effects of virtual particles that emerge from the vacuum and deals with the problem that the energy of a charged point particle is infinite. The comparison between theory and precise measurements of the H atom ran into a crisis in 2010, when measurements on muonic hydrogen (where muons replace electrons) ( 1 ) led to two different values of the size of the proton, r p (see the figure). A decade-long period of the “proton size puzzle” spurred renewed experimental activity and many far-reaching hypothetical theories. On page 1061, Grinin et al. ( 2 ) report the precision measurement of the 1S-3S transition to help finally resolve this crisis.