The proton might truly be smaller than was thought. Experiments on an exotic form of hydrogen first found1 a puzzling discrepancy with the accepted size in 2010. Now, evidence from a German and Russian team points to a smaller value for the size of the proton with ordinary hydrogen, too.
The results, which appeared on 5 October in Science2, could be the first step towards resolving a puzzle that has made physicists doubt their most precise measurements, and even their most cherished theories.
Still, “before any resolution, this new value has to be confirmed”, says Jan Bernauer, a physicist at the Massachusetts Institute of Technology in Cambridge. If other labs confirm it, he adds, “then we can find why the old experiments were wrong, hopefully”.
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