Abstract
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 polarized photons composing the electromagnetic field. It seems that it completes the list of the zilch-type invariants found by Lipkin and Ragusa. The gauge-invariant expression for the energy of the free gravitational field is obtained which strongly resembles the well-known bilinear expression for the total number of photons composing the electromagnetic field.
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