Few things in physics have been more thoroughly studied than the proton, a fundamental building block of atoms. So it was a shock in July when Paul Knowles of the University of Fribourg in Switzerland claimed the proton is 4 percent smaller than everyone has thought for more than 50 years.
In the past, physicists have used electrons to measure the proton’s size indirectly. When a laser zaps an electron orbiting a proton, the electron undergoes what is called the Lamb shift, absorbing energy and jumping to a higher energy level. According to quantum electrodynamics, the Lamb shift is partly a function of the proton’s size; this allows physicists to infer its measurements. But instead of lasing electrons, Knowles examined protons with particles called muons, which he calls “the electron’s fat cousin.” Muons, he says, are more sensitive to the proton’s size, and so their Lamb shift gives a much more reliable estimate.
In quantum physics, a 4 percent mistake is a mighty error. “Either there’s a problem with quantum electrodynamics,” Knowles says, “or there’s some funny physics going on that no one understands yet.”