String theory
By law of inertia, when no force is applied to an object, the object holds its velocity, which is speed and direction.  An object either in uniform motion, which is constant speed in an unchanging direction, or staying at rest, which is zero velocity, experiences inertia.  This exhibits Galilean invariance—its mechanical interactions proceeding without variation—also called Galilean relativity since one cannot perceive whether one is at rest or in uniform motion.