A relativistic quantum field theory that explains the behavior of electromagnetic fields. In terms of agreement with experiment it has proved to be one of the most successful theories modern science has come up with. Initially developed by Paul Dirac and further refined by Freeman Dyson, Richard P. Feynman, Julian Schwinger, and Sin-Itiro Tomonaga, the last three received a Nobel Prize in 1965 for their work developing this theory. Sometimes abbreviated as QED.
In the theory, the electromagnetic field is described as being carried by a massless spin-1 particle called the photon. Electromagnetic forces occur when electrically charged particles emit and absorb photons; the transfer of momentum that arises when a photon is emitted or absorbed is where the forces come from. This foundation provides with extraordinary precision explanations and predictions for various electromagnetic phenomena such as the photoelectric effect, Compton scattering, pair production and pair annihilation, bremsstrahlung, and radiative transitions of atoms.
One of the biggest problems with the theory as it was being developed in the late 1940's was that there were infinite quantities in its results. Richard Feynman solved these problems using a process called renormalization that involved subtracting other infinite quantities from the results. This technique does not have any firm mathematical foundation and it leaves many of the more mathematically-inclined physicists uneasy, but nevertheless the resulting theory has proven to be extraordinarily accurate in its predictions. An unrepentant Feynman later remarked: "It is not philosophy we are after, but the behavior of real things."
Efforts have been made that have proceeded in a similar fashion to explain the other fundamental forces of the universe, which are collectively known as quantum field theories. Somewhat satisfactory results have been achieved for the weak nuclear force and the strong nuclear force (quantum chromodynamics), but a quantum theory of gravity has thus far remained elusive.