On the number and spin of photons in classical electromagnetic fields

Abstract

A relativistically invariant expression for the number of photons in a free classical electromagnetic field through the currents, that created the field, is derived based on the formula for the total energy–momentum of the field. It is demonstrated that it corresponds to the classical limit of the photon number operator known from the quantum electrodynamics. An expression for the total spin moment of the classical electromagnetic field is derived and it is shown that it can be interpreted in terms of spin moments of elementary photons. Similar to the total number of photons the classical spin moment is the limit of the quantum operator of the total spin. It is revealed that the total number of photons as well as the total spin moment are related to each other through a second order tensor, which can be expressed through currents that created the field.