The Feynman Diagram of the Magnetic Field

According to the standard model of particle physics, all the electromagnetic force is an exchange of photons or particles of light between different charged particles. The way a particle behaves in the magnetic field has everything to do with how it interacts with photons. We can even write down the equations describing those interactions just by drawing this little diagram. So if you calculate, use that diagram to calculate the muon's magnetic moment, you'll get what we call the classical contribution.