The field approach restarted around the middle of the 19th century where and when Faraday demonstrated his lines-of-forces using iron fillings sprinkled over a sheet of paper set atop a permanent magnet. Minute inspections reveal these tiny iron fillings seem to congregate along or around invisible magnetic lines, closely packed near the two opposite poles and farther apart in-between. The varying density is an indication of the strength of the magnetic field intensity at various points not its varying speed and direction of motion.

This lack of understanding of magnetic field speed and direction of motion was never resolve by the introduction of a quantum field theory. However, Dirac thought the mystery lies with the missing source of magnetic field in matter indicated by one of Maxwell’s classical field equations that the divergence of the magnetic field inside matter is always zero: divB=0 but asserting the existence of the magnetic dipole moment. Furthermore, Dirac theory predicted that the electron should have a magnetic dipole moment twice the classical Bohr magneton. The agreement between theory and experiment boost confidence in Dirac theory of the electron but fail to resolve the problem with negative energy states. Resolving the speed and direction of motion of the magnetic field is equivalent to resolving the problem of negative energy states using a theory of space-time quantization as square of energy.