Physicists think they’ve resolved the proton size puzzle
A confusing gap
A vacuum chamber used to measure electronic transitions in atomic hydrogen
Credit: Axel Beyer/MPQ
A vacuum chamber used to measure electronic transitions in atomic hydrogen
Credit: Axel Beyer/MPQ
Subsequent measurements by various groups were inconclusive. For example, in 2013 the same international team performed muon-based experiments that confirmed their 2010 value, producing a measurement of 0.84 femtometers for the proton radius, with a deviation of 7 sigma. Another experimental variation in 2016 involved replacing the electron with a muon in an atom of deuterium, a heavier isotope of hydrogen, with a neutron as well as a proton and an electron. The idea was that the presence of a neutron would change the way electrons and muons perceive the proton’s charge. This also corresponds to the result of 2010.
However, two experiments using ordinary hydrogen to measure the proton radius produced mixed results: a 2017 study also confirmed the 2010 result, while a 2018 measurement was consistent with the higher value before the 2010 experiment. In 2019, scientists at York University chose to perform an electronic measurement of the proton radius, hoping to reconcile the various conflicting results of a consensus. The result: Their measurement of 0.833 femtometers agreed with the smaller value from the 2010 study.
This brings us to the last two papers, both of which involved experiments with hydrogen atoms in a vacuum chamber. They used lasers to control electrons and measured transitions between energies; this allowed them to deduce the exact dimensions of the proton’s charge radius. Based on the combined results, the proton has a radius of about 0.84 femtometers, or less than a million billionth of a meter, which is once again consistent with the 2010 measurement that started the debate.
“The proton radius should be a universal property; it should give the same result no matter which way you look at it,” Juan Rojo, a physicist at Vrije University of Amsterdam in the Netherlands, who was not involved in either experiment, told New Scientist. “That’s why these two articles are quite interesting, because they offer different perspectives to the same number.”
