The electromagnetic field, like gravity, is a force field; it is the agent that transmits force from one electric charge to another. It consists of two component fields, electric and magnetic. Unlike gravity, which is always attractive, the EM force can be attractive, repulsive, or even sideways. Its field nature was suggested by Michael Faraday in 1845 and field equations were developed by James Maxwell in 1864. These equations predicted EM waves (oscillations in field intensity) that travel at a speed of 300,000 km/second, which turned out to be the same as the speed of light. Not only did Maxwell’s equations thus explain the entire field of optics, they also predicted a vast spectrum of EM radiation that includes radio and TV waves, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays.
The quantum nature of the EM field was discovered by Max Planck while studying radiation from a hot object. He found that the data made sense if the radiation is not infinitely divisible, but consists of discrete amounts of energy that he called quanta. These quanta are now called photons. Planck also found that the energy of a photon is proportional to its frequency of oscillation.
Later Einstein showed that absorption of radiation occurs in the same discrete amounts of energy, suggesting that the quanta are actually particles. This was the first round in the fields-vs.-particles battle which goes on even today. In QFT a photon is a field and only a field. It spreads out in space and exhibits interference effects. When a photon is absorbed, no matter how spread out it may be, its energy is deposited into the absorbing atom. This is called quantum collapse.
The internal complexity of a quantum field is described by a number called spin or helicity. The gravitational field is the most complex of all fields, with a spin of 2 Planck units. The EM field, with its component electric and magnetic fields, has spin 1. The two fields introduced so far are summarized in the following table:
|EM||1||Photon (γ)||charged fields||green|