PerspectivePhysics

Crisis and catharsis in atomic physics

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Science  27 Nov 2020:
Vol. 370, Issue 6520, pp. 1033
DOI: 10.1126/science.abf0589

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Summary

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, rp (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.

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