How Big Is a Proton

One of three particles that make up all atoms, the tiny positively charged proton has recently been given a precise measurement by researchers at York University’s Faculty of Science.

Their breakthrough study, published in Science, finds the proton to have a radius of only 0.833 femtometres, a measurement smaller than a trillionth of a millimetre and about five per cent smaller than the official value given more than a decade ago based on older measurements. 

The radius appeared to have a different size when measured with electrons, as opposed to muons, the electron’s heavier cousin. This unexpected difference, first noted by physicists a decade ago, has been referred to as the “proton radius puzzle.” Following a long process of experimentation, the York researchers have now been able to resolve it. 

“After eight years of working on this experiment, we are pleased to record such a high-precision measurement, which solves the elusive proton radius puzzle,” says Distinguished Research Professor Eric Hessels, Department of Physics & Astronomy, the internationally recognized physicist and expert in atomic physics who led the York University study. “The level of precision required to determine the proton size made this the most difficult measurement our laboratory has ever attempted.”

The result has far-reaching consequences for our understanding of the laws of physics, such as the theory of quantum electrodynamics, which describes how light and matter interact.  ■