represents a dynamic 3d hypersheet (composed of fields & particles. Therefore I’ll refer to it as Field-Particle Hypersheet, or 3d FPHS) moving with a velocity c in the 4th dimension in an embedding 4d hyperspace. This is very easy to see. Let us take any observer, located anywhere in 3d space (3d FPHS), and moving with any possible velocity. Relative to itself, the observer does not move through space (dr=0). Hence, the above equation becomes (putting dr=0):