Apply JD's analogy to the momentum-energy four vector of a particle. In its
proper frame its 3-momentum and 3-velocity are zero. In another inertial
frame, it has non-zero 3-momentum/velocity. This spatial component of the
momentum-energy four vector is merely a projection (a shadow) of the
magnitude invariant 4-vector upon the space axes of the new frame. It must
follow, by the JD analogy, that the particle is not really moving.