Abstract

The central concept of the theory of relativity is relative velocity. The velocity of a material body is not an intrinsic property of this body; it depends on the free choice of reference system. Relative velocity is thus reference-dependent; it is not an absolute concept. We stress that even zero-velocity must be relative. Every reference system possesses its own zero-velocity relative to exactly that one system. The theory of relativity formulated in terms of relative velocities, with many zero-velocities, does not imply the Lorentz isometry group. Moreover, we discuss a conceptual dichotomy: two different rival concepts of reference system: the Minkowski space-time observer-monad as time-like vector field, versus the Einstein space-time coordinate tetrad.