Gn. Afanasiev et Yp. Stepanovsky, THE HELICITY OF THE FREE ELECTROMAGNETIC-FIELD AND ITS PHYSICAL MEANING, Nuovo cimento della Società Italiana di Fisica. A. Nuclei, particles and fields, 109(3), 1996, pp. 271-279
The notion of helicity for the free electromagnetic field is analysed.
The generalized helicity is introduced which is a conserved quantity
coinciding with the difference of the right and left circularly polari
zed photons composing the electromagnetic field. It seems that it comp
letes the list of the zilch-type invariants found by Lipkin and Ragusa
. The gauge-invariant expression for the energy of the free gravitatio
nal field is obtained which strongly resembles the well-known bilinear
expression for the total number of photons composing the electromagne
tic field.