Abstract

It is shown that Einstein?s proof for E = mc² is actually incomplete and therefore is not yet valid. A crucial step is his implicit assumption of treating the light as a bundle of massless particles. However, the energy-stress tensor of massless particles is incompatible with an electromagnetic energy-stress tensor. Thus, it is necessary to show that the total energy of a light ray includes also non-electromagnetic energy. It turns out, the existence of intrinsic difference between the photonic and the electromagnetic energy tensors is independent of the coupling of gravity. Nevertheless, their difference is the energy-stress tensor of the gravitational wave component that is accompanying the electromagnetic wave component. Concurrently, it is concluded that Einstein?s formula E = mc² necessarily implies that the photons include nonelectromagnetic energy and that the Einstein equation of 1915 must be rectified.