The proton seems to be 0.00000000000003 millimeters (4 percent) smaller than researchers previously thought, according to work published in today’s issue of Nature. The new measurements could mean that there is a gap in existing theories of quantum mechanics.

Something is wrong. The most intriguing possibility is that previously undetected particles are changing the interaction of the muon (used to measure the proton’s size) and the proton. Such particles could be the “superpartners” of existing particles, as predicted by a theory known as supersymmetry, which seeks to unite all of the fundamental forces of physics, except gravity.

Pohl, R. et al. Nature 466, 213-217 (2010).