ABSTRACT
Notwithstanding that it is possible to consider all electromagnetic fields as intermediaries, transmitting interactions between charged particles, it is fruitful to study the concepts connected to free fields, i.e., fields detached from the charges producing or absorbing them. To free electromagnetic fields, also referred to as radiation fields, one may associate many of the properties we are so familiar with for matter, e.g., energy, momentum and angular momentum. Even in the framework of classical physics, radiation fields take up a position almost on an equal footing with matter. In quantum physics, the autonomy of the radiation field is fully developed through the emergence of the photon concept. The photon, the elementary excitation of the electromagnetic quantum field, appears as a particle just as “fundamental” as the massive elementary particles attached to other quantum fields.
