 The reader should note at the outset that this article presupposes familiarity with some of the basic concepts of relativity theory; in addition, section 3 presupposes familiarity with some relativity standard machinery from theoretical physics (e.g., Lagrangian mechanics). It would not be appropriate, in this philosophical article, to explain all of the background details here from the ground up. In lieu of doing so, we have (a) provided extensive references to literature in which the relevant concepts are explained further, (b) highlighted more technical subsections of this article which can be skipped on first reading, and (c) provided throughout non-technical summaries of the relevant conceptual points. 