Relativity is a theory of space and time that provides the foundation for much of physics. Relativity is an outgrowth of a single idea, the principle of relativity, and principle of relativity that physical laws be independent of the reference frame used to represent them. Relativity is a law about laws. The program of relativity program of relativity is to express equations of physics in such a way that, if true in one system of space-time coordinates, are true in any coordinate system, and thereby transcend coordinates. The primary state of motion, that exhibited by free particles, is inertial—in a straight line at constant speed. Free particles of and by themselves cannot change their states of motion. Transformation is central to relativity. Relativity is concerned with space and time and how the two are related through motion.