We wish to "quantize" the electromagnetic energy of free space for a multimode field. The field intensity of free space should be independent of position such that | A 0 (r) | should be independent of r for each mode of the field. The mode function satisfying these conditions is: A 0 ( r ) = e k e i k ⋅ ⋅ r where k · e k = 0 in order to have the transversality condition ∇ · A (r, t) satisfied for the Coulomb gauge in which we are working.