Hello,
I have seen in various places in the literature the expression for the spectral energy of the vacuum, but I don't seem to find a book/article where the derivation of this expression is shown.
Also sometimes people state at its dependence on the third power of frequency is the only form...
I know that in the vector potential formulation one can use a scalar Green function (to find the said potential and from then on the electric and magnetic fields), and that this works because the components of the potential are in the same direction as those of the source - i.e. a current in the...