Protons Four Percent Smaller than Previously Thought: Study

According to a study published in the July 8 issue of Nature, we’ve been wrong about protons for years. While the canonical assumption has been that the radius of a proton is 0.8768 femtometers (1 femtometer = 1 x 10⁻¹⁵ meters), it turns out that it’s actually 0.8418 femtometers.

That might not sound like much of a difference, but it could change the face of quantum electrodynamics.

The team explains how they measured the radius, a process which involved using very finely tuned laser beams to “very accurately” infer the size of the proton by measuring the energy difference between two subatomic particles:

In order to determine the proton radius, we have replaced the single electron of hydrogen atoms with a negatively-charged muon, thus producing an exotic atom called muonic hydrogen. Muons are much like electrons, but they are 200 times heavier. According to the laws of quantum physics, the muon must, therefore, travel 200 times closer to the proton than the electron does in an ordinary hydrogen atom. In turn, this means that the muon energy levels are significantly influenced by the proton size.

Using a specially designed laser, we have measured the energy difference between two muon orbits and from that we have inferred the size of the proton very accurately.

Assuming the team’s measurements are accurate, the  most straightforward explanation for the discrepancy is that the Rydberg Constant, which is used to calculate the size of the proton, is incorrect: If this is the case, some other key formulae and fundamental constants may need to be revised. If not, though, it could cause a shakeup in all of quantum electrodynamics. Physicist Jeff Flowers, who was not involved in the measurement but wrote an essay accompanying it in Nature, writes that “If experimental discrepancies are confirmed rather than errors being found, high-accuracy work such as that by Pohl and colleagues, not the high-energy collisions of giant accelerators, may have seen beyond the standard model of particle physics.”

(via National Geographic, Live Science. title image via Matthew Wallace)