Motion and inertia

Abstract This chapter explores the question of what it means for something to move, and why physics cannot be done without an answer to that question. It does so mostly in the context of Newtonian physics, leaving considerations of the theory of relativity to the next chapter. We cannot simply define motion of one body as relative to another body if we want to do physics—we have to introduce the idea of a ‘rest frame’ that defines which bodies are at rest (Newton called this rest frame ‘absolute space’). But physics also satisfies the relativity principle—it is impossible to distinguish the rest frame from another frame moving at constant speed in that frame. So what physics really requires is not a preferred rest frame, but a family of inertial frames, all moving at uniform speeds relative to one another. The notion of ‘spacetime’ has been introduced as a way of understanding this family of inertial frames, but philosophers of physics disagree as to whether spacetime explains the nature of motion in physics, or merely codifies it. The chapter concludes by explaining how gravity can be thought of as a change in the structure of the inertial frames: though it was Einstein who first saw this clearly, it has nothing to do with relativity and makes sense even in Newtonian physics.