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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 compatible with Lorentz invariance.

Do you know where I could find out how this is shown, too?

Thank you!

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# Spectral energy of the vacuum

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