© David Kaplan.
The electromagnetic force, in its full quantum description, has the property that the coupling, or strength of charge, is different when it is measured at different energies. Probing an electron at high energy is equivalent to probing it at short range. The electromagnetic field around the electron has quantum fluctuations that include the appearance of virtual pairs of electrons and positrons as shown on the left. Those virtual effects change the perceived coupling as one gets closer or farther from the electron. The plot on the right gives the measured strength of the coupling,, at different scattering energies (given in MeV). (Unit: 2)