“…It follows immediately that each of the angular momenta described above has an infinite number of higher and lower extensions [45,47], as do other quantities, including energy and linear momentum. If we consider the helicity, for example, and replace, superficially, the fields with their time derivatives, we obtain
This is the 00-zilch [48,60,80], which was discovered before the helicity, in fact, and takes on a value equivalent to
per photon, with the ω 2 here deriving from the product of two time derivatives. That the 00-zilch is conserved follows from the Maxwell-like equations (2.22) in the same way that the conservation of helicity follows from Maxwell’s equations themselves (2.20) [45,47].…”