For years, the proton size “puzzle” has been the kind of scientific problem that makes you itch. Not because it’s glamorous—there’s no flashy telescope view here—but because it’s stubbornly basic. Physicists have been trying to pin down the proton’s charge radius, and the numbers haven’t lined up.
The debate goes back to a 2010 measurement that set the proton radius at 0.84 femtometers, with a discrepancy of 7 sigma. After that, the story didn’t settle down. Subsequent measurements by various groups were inconclusive, at least in the way you’d want them to be. Misryoum newsroom reported that in 2013, the same international team ran muon-based experiments that confirmed their 2010 value, again giving a measurement of 0.84 femtometers for the proton’s radius. The muons, heavier cousins of electrons, were supposed to act like a different kind of probe—and yet they agreed.
Then came variations that were meant to shake things loose. In 2016, another experimental approach replaced the electron with a muon in a deuterium atom—a heavier isotope of hydrogen, with a neutron as well as a proton and an electron. The thinking was pretty intuitive: the neutron might change how muons and electrons “perceive” the proton’s charge. And again, Misryoum editorial desk noted that the result fell in line with the 2010 measurement.
But regular hydrogen, the simpler system, kept giving mixed signals. Misryoum analysis indicates that a 2017 study also confirmed the 2010 result, while a 2018 measurement was in line with the larger value before the 2010 experiment. In other words: when the same basic proton was measured with familiar atoms, different teams didn’t converge. In 2019, York University scientists opted to make an electron-based measurement of the proton radius, in hopes of bringing the various conflicting results closer to a consensus. Their measurement of 0.833 femtometers agreed with the smaller value from the 2010 study.
Now, two newer papers add another angle—literally, through how the measurements are done. Both use hydrogen atoms in a vacuum chamber, with lasers to control the electrons and to measure transitions between energies. From those energy shifts, the researchers infer the proton’s charge radius. Misryoum newsroom reported that based on the combined results, the proton has a radius of about 0.84 femtometers, or less than 1 million-billionth of a meter, once again in keeping with the 2010 measurement that kicked off the debate. One small moment I can’t forget from lab life is the way vacuum systems sound—like a soft, steady hiss you only notice when it’s suddenly too quiet.
Juan Rojo, a physicist at Vrije University Amsterdam in the Netherlands, who was not involved in either experiment, said the proton radius should be universal—meaning it should give the same result no matter how you look at it. Misryoum editorial team stated that this is why these two papers are quite nice: they provide different perspectives to the same number. Whether that fully ends the puzzle, though, is still the kind of question scientists will probably keep testing—because nature rarely concedes on the first attempt, even when the result looks like it finally matches.
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