A neo explanation of the “mass-velocity relation”

A reinterpretation of the well-known formula of the “mass-velocity relation” is exactly derived from a new viewpoint with new concepts, such as the finiteness of the transmitting velocity of force (TVF), effective action, and the coupled effect of the TVF for two EM fields, etc. Then, a true meaning hidden in the Lorentz factor is exploited: i.e., when a charged particle is moving at a speed v under an EM field, the effective action exerted on it by the field varies inversely with the speed ratio β= v / U, where U is the TVF, which probably is equal to the propagation velocity of EM field. The actual reduction of the effective action gives a false impression of mass gain. Accordingly, it is a major mistake in orientation to ascribe the (genuine) electrodynamics of moving bodies to any observation, or to any motion of an observer, while disregarding the facts of mutual action.