Crisis and catharsis in atomic physics

• Published in: Science (American Association for the Advancement of Science) Vol. 370; no. 6520; p. 1033
• Main Author: Ubachs, Wim
• Format: Journal Article
• Language: English
• Published: United States The American Association for the Advancement of Science 27.11.2020
• Subjects: Atomic physics; Catharsis; Crises; Humans; Hydrogen; Muons; Physics; Protons; Quantum electrodynamics; Quantum mechanics; Single electrons; Spectrum Analysis
• ISSN: 0036-8075; 1095-9203
• DOI: 10.1126/science.abf0589

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.