A scandal at the heart of the atom remains. The most precise reading yet of the proton's radius confirms that it can seem smaller than quantum theory demands. The radius cannot be measured directly but can be inferred. In 2010, a group led by Randolf Pohl at the Max Planck Institute of Quantum Optics in Garching, Germany, used an exotic atom -a hydrogen with its electron replaced by the heavier muon -to put the proton's radius at 0.8418 femtometres.
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