Physicists at the SLAC accelerator have measured, with much greater precision than ever before, the variation in the weak nuclear force, one of the four known physical forces, over an enormous size scale (a distance of more than ten proton diameters) for so feeble a force. Although the results were not surprising (the weak force diminished with distance as expected) this new quantitative study of the weak force helps to cement physicists' view of the sub-nuclear world.

The SLAC work is, in effect, a 21st century analog of the landmark 18th experiments in which the intrinsic strength of the electromagnetic and gravitational forces were measured (by Charles Coulomb and Henry Cavendish, respectively) through careful observation of test objects causing a torsion balance to swing around. The weak force, in the modern way of thinking, is a cousin of the electromagnetic (EM) force; both of them are considered as different aspects of a single "electroweak" force.

The EM force is much better known to physicists and to non-experts: it's responsible for all electric, magnetic, and optical phenomena, and keeps atoms intact and holds atoms together in all the molecular and crystalline forms which make up our world. Over sizes larger than the atom, the strength of the EM force is prescribed by Coulomb's law, which states that the force between two charged objects (say, two electrons) is proportional to the charges of the electrons and inversely to the square of the distance between them.

For sub-atomic distances the Coulomb way of describing electron scattering gets complicated because of vacuum polarization, a process which takes into account the fact that at short distances an electron can longer be portrayed as a lone pointlike particle; instead we must view it as accompanied by a cloud of virtual particles sprouting out of the vacuum. These extra short-lived particles serve to redefine, or "renormalize," the effective electron charge and along with it the very nature of the EM force mediating the interaction with the other electron.