Abstract

The central concept of the relativity theory is a relative velocity. Relativity of the velocity means that the velocity is not absolute concept. The velocity of positive-mass body is not an intrinsic property of this body; it depends on the free choice of the reference system, it is reference-dependent. We stress that the zero velocity must be relative. Every reference system possess the own zero-velocity relative to exactly this system. Many zeroth-velocities contradict to group structure, and therefore the relativity theory in terms of relative velocities must be formulated within the groupoid structure, which is not a group. There is a dichotomy: two di erent concepts of a positive mass body, a tetrad versus a monad.