In 2010, Randolf Pohl of the Max Planck Institute of Quantum Optics (Quantamagazine) announced his team precisely measured the size of the proton by substituting the electron of a hydrogen atom by a muon. The size of the proton in presence of the muon was smaller than with an electron which would imply unknown physical interactions between protons and muons.
A team led by Eric Hessels of York University in Toronto has remeasured the proton in regular hydrogen. Finally, the proton’s radius is around 0.833 femtometer, a measurement exactly consistent with Pohl’s value. The proton does not change size depending on context! (Quantamagazine)
