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In this work we extend the Zeldovich formula, which was originally derivedfor the free electromagnetic field and was interpreted as the number ofphotons. We show that our extended formula gives a universal dimensionlessmeasure of the overall strength of electromagnetic fields: free fields andfields produced by various sources, in classical and in quantum theory. Inparticular, we find that this number (the Zeldovich number) for macroscopicsystems is huge, of the order of $10^{20}$. For the hydrogen atom in the groundstate it is equal to 0.025 and for the xenon atom it is around 50.

The Zeldovich number: A universal dimensionless measure for the electromagnetic field