ABSTRACT
An electromagnetic pulse is a mechanical system that travels at the speed of light, carrying a mass proportional to the total energy content. In contrast, the masses of the charged particles are fixed quantities that have no reference to the particle’s state of motion and its associated energy. Another way of expressing this lack of mechanical unity between fields and particles comes from the physically evident expression for the total momentum density. The topic is the coordinate translation that grows linearly in time, or equivalently, the introduction of a new coordinate system with a constant relative velocity. The space-time transformations of the new kinematics are called Lorentz transformations, although it was Albert Einstein who, in 1905, first understood their significance as describing the full physical equivalence of reference frames in uniform relative motion.
