' which is in a state of uniform motion of translation relative to
, or, briefly, relative to every "Galileian" system of co-ordinates. In contrast to classical mechanics, the Lorentz transformation is the deciding factor in the transition from one such system to another.
By means of comparatively simple considerations we are led to draw the following conclusion from these premises, in conjunction with the fundamental equations of the electrodynamics of Maxwell: A body moving with the velocity
, which absorbs[13] an amount of energy
in the form of radiation without suffering an alteration in velocity in the process, has, as a consequence, its energy increased by an amount
In consideration of the expression given above for the kinetic energy of the body, the required energy of the body comes out to be