On measuring the size of the proton

The proton shrinks in size

Tiny change in radius has huge implications.

By Geoff Brumfiel

The proton seems to be 0.00000000000003 millimeters smaller than researchers previously thought, according to work published in the July 8 issue of Nature.

The difference is so infinitesimal that it might defy belief that anyone, even physicists, would care. But the new measurements could mean that there is a gap in existing theories of quantum mechanics. "It's a very serious discrepancy," says Ingo Sick, a physicist at the University of Basel in Switzerland, who has tried to reconcile the finding with four decades of previous measurements. "There is really something seriously wrong someplace. ..."

The radius of the proton probably derives from the position uncertainty of the three quarks that move about inside it and is in a reciprocal relation of some sort to the momentum of the electronic or muonic atom. Then the smaller proton radius observed with the heavier muon results from the greater momentum of the muonic atom.