How big is the proton anyway?

For almost a decade, there has been a serious disagreement in the physics community about one simple quantity: how large is a proton? Different ways of measuring the size of a proton — one using electrons, another using muons — gave results that disagreed by about 5 %. This 5 % disagreement was a big deal since each measurement claimed to be accurate to better than 1 %. It was unclear if this discrepancy was because of some genuinely weird physics, or because one of these measurements was incorrect.

Now, a careful high-precision measurement using hydrogen atoms that Branco Weiss Fellow Amar Vutha had initiated as a postdoc (and worked on for eight years since) has shown that there isn’t really any disagreement at all: The proton’s size is 0.84 femtometers, whether measured using electrons or using muons. Case closed.

Read the news on Quanta Magazine

Read the news on the New Scientist website

Read the paper in Science Magazine