We may have been overestimating the proton for the last 60 years, if a new experiment has anything to say about it. A group of researchers have tried a new method of measuring the proton's radius that involved getting a muon to orbit it instead of an electron. The new approach is ten times more accurate than the way it has been done since the invention of quantum mechanics, and it has produced a value for the proton's radius that is four percent smaller than the currently accepted one. If the new measurement is incorrect and the proton is not actually smaller, the theory of quantum electrodynamics itself may need an adjustment. The currently accepted value of the proton's radius is .876 femtometers. This value isn't consistently measured by any one experiment, but is instead a "world average" of all the attempted measurements done by spectroscopy on a hydrogen atom, and the errors were large enough to provide room for a new, more exact measurement. Unfortunately, the new measurement provides a value that's completely outside these error bars.

The easiest way of studying protons is to use hydrogen, which is nothing more than a simple interaction between an electron and a proton. By watching what energy the electron needs to transition between the orbitals surrounding the proton, researchers can get an idea of how big the proton is.

To get a better measurement, the team of researchers wanted to "work in a system which is very sensitive to the proton radius," said Aldo Antognini, one of the co-authors of the paper. What they needed was a very small energy transition to observe, and a large platform in which to observe it.

For the transition, they needed look no further than a Lamb shift. A Lamb shift occurs when an electron moves between the 2s and 2p energy levels in an atom. The difference in binding energy between the two is very small, and leaves little room for external effects to muck up the measurements.

To get a highly accurate picture of the Lamb shift, the scientists generated protons orbited by muons, also known as muonic hydrogen. Muons are unstable elementary particles, with the same charge and spin as an electron—but they're 200 times heavier. Its size would allow the researchers to make more precise binding energy measurements.

Muonic hydrogen is not easy to make; in fact, the researchers were using the only laboratory in the world that can produce muons en masse. On top of that, only one percent of the muons generated stayed in their 2s excited state long enough to be experimented with—the rest immediately decayed. In all, Dr. Antognini noted that the experiment took more than eight years to complete.

The researchers found that the muonic hydrogen needed to be shot with a laser with a frequency of 50 terahertz in order to transition up to the 2p state. When they plugged this measurement into a quantum electrodynamics equation that relates proton radius to binding energies, they found the needed energy indicated a proton radius of 0.841 femtometers—four percent smaller and five standard deviations off the currently accepted radius of 0.876 femtometers.

The scientists are not yet sure whether their findings will upset the theory of quantum electrodynamics, or our understanding of the proton itself. "You can assume the theory is correct, or you can assume the radius is correct and the theory is wrong," Dr. Antognini told Ars.

If it's the theory and predictions that are off, this doesn't spell the end of quantum electrodynamics. More likely, the related equations need adjusting, not unlike how the calculation of Lamb shifts provided an adjustment for the energy theories Dirac first laid down.

On the other hand, if the theory holds and we've actually been overestimating the proton, there will be a sizable shakeup in the fundamentals of particle physics. The authors of the paper hope to continue probing the proton with similar tests on muonic helium atoms, as well as analyzing extra data they've collected on muonic hydrogen and deuterium.

Nature, 2010. DOI: 10.1038/nature09250 (About DOIs).