There鈥檚 a problem with protons, but we鈥檙e one step closer to solving it. We aren鈥檛 quite sure what the radius of a proton is and a new way of measuring has helped to fix the puzzle.
Until 2010, we were fairly sure about the size of the proton. We had measured it using hydrogen atoms, which are made of one proton and one electron. But in 2010, a measurement using muonic hydrogen 鈥� where the electron is replaced by another particle called a muon 鈥� found a value 4 per cent lower.
鈥淭hat led to some speculation as to whether muons interact differently with protons than electrons do, and if that were the case it would break the laws of physics,鈥� says Eric Hessels at York University in Canada. The standard model of particle physics would not accommodate a difference in those interactions, so we would need some sort of strange new force.
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Hessels and his colleagues developed a new way to measure the proton鈥檚 radius by sending hydrogen atoms through two sets of radio waves with slightly different frequencies. This excited some of the atoms into a higher energy state.
By measuring how many of the atoms remained in the lower energy state, the researchers could determine the difference in energy between the original lower energy state and the excited state. They used that energy difference to calculate the proton鈥檚 radius.
The final result matched the muonic hydrogen measurement, supporting the idea that the proton is smaller than we had thought at around 0.83 femtometres, or less than one million billionth of a metre.
鈥淭here are more and more measurements that seem to pile up at the smaller radius now, so maybe this puzzle will diffuse away,鈥� says Thomas Udem at the Max Planck Institute of Quantum Optics in Germany.
Other experiments are under way that will provide further evidence of the proton鈥檚 radius, he and Hessels say.
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