Muon-orbited protons are 0.84 femtometers in radius — 4% smaller than those in regular hydrogen. If the discrepancy was real, meaning protons really shrink in the presence of muons, this would imply unknown physical interactions between protons and muons — a fundamental discovery. 100s of papers speculating about the possibility have been written. After Pohl’s muonic hydrogen result, a team of physicists set out to remeasure the proton in regular, “electronic” hydrogen. Finally, the results are in: The proton’s radius is 0.833 femtometers, give or take 0.01, a measurement exactly consistent with Pohl’s value. Both measurements are more precise than earlier attempts, and they suggest that the proton does not change size depending on context; rather, the old measurements using electronic hydrogen were wrong.

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