Absolute space and Newton's theory of relativity

Newton's metaphysical picture of space and time provides the conceptual background for his theory of motion. Philosophical discussions of absolute space and time, however, underemphasize Newton's concern with the relativity of motion. From a modern perspective, this is usually seen as a concern that Newton himself did not take seriously enough, especially in comparison with contemporaries such as Huygens and Leibniz. In one sense, however, Newton pursued the problem of the relativity of motion further than his contemporary critics. In fact, while they defended the relativity of motion as a general principle, only Newton tried to develop what may legitimately be called a theory of relativity: a systematic theoretical account of what is objective in the description of physical interactions, and a principled distinction between the objective properties and those that depend on the choice of a frame of reference. On this basis Newton articulated, more clearly than any of his contemporaries, the conceptual revisions imposed by the relativity of motion on prevailing notions of force, inertia, and causality. We can see this from the history of his use of the Galilean relativity principle, which became Corollary V to the Laws of Motion. Moreover, while his critics demanded a mechanistic alternative to his theory of gravitation, Newton not only saw the empirical power of his theory, and its exemplary power for the theory and practice of physics in general; he also saw that the peculiar nature of gravity placed the problem of the relativity of motion in a dramatically new light. This is seen in his development and use of Corollary VI.

By studying the progress of Newton's thought about these relativity principles, and the profound changes in his views between early manuscripts such as De Gravitatione and the first drafts of the Principia, we can see why Newton did not regard them as undermining his aim to determine “the true motions” in the solar system. On the contrary, he saw it as enabling him to separate the local problem of “true motion” for a given system of bodies, from the global problem of how that system might be moving with respect to absolute space. In other words, Newton, having acknowledged that absolute space is unobservable, and motion with respect to it therefore unknowable, nonetheless could solve the problem of “the system of the world.” Indeed, the history of his thinking shows that Newton introduced the theory of absolute space precisely in order to articulate his theory of relativity. Newton's use of relativity principles sheds light on the relation between mathematical principles and causal explanation in physics.