This equation shows the remarkable result, called the "Kaluza miracle", that the precise form for the electromagnetic stress–energy tensor emerges from the 5D vacuum equations as a source in the 4D equations: field from the vacuum. This relation allows the definitive identification of A μ μ with the electromagnetic vector potential. Therefore, the field needs to be rescaled with a conversion constant k such that A μ μ → → k A μ μ .