Relativistic Quantum Mechanics predicts that the ratio of the magnetic moment of a lepton to its spin is exactly 2. The effect of Quantum Foam slightly alters this value, and experiments are designed to measure the difference in the case of electrons and muons. Both the experimental measurements and the theoretical calculations are difficult. The Feynman diagram above shows a contribution to the difference for an electron (e), which is interacting with a static electromagnetic field represented by the X in the figure. The Quantum Foam intervenes in the form of light by light scattering via a virtual chargeless pion (π0). The physical π0 decays electromagnetically to two photons (π0→γγ); therefore, the virtual π0 can couple the fixed electromagnetic field X and the propagating electron e. In the past a discrepancy between experiment and theory for the muon suggested the presence of new physics, but the discrepancy was eventually traced to an error in the calculation of the hadronic light by light scattering contribution.
Reading from left to right, the i and the -i in the figure represent the forward flow of time. In Special Relativity the time axis can be considered imaginary (i=√-1) with respect to space. The colored lines are meant to suggest the color and anticolor of the π0's quark-antiquark valence pair.