The Law of Inertia plays a key role in the scheme of constructive axioms for the General Theory of Relativity. A new formulation of this law which avoids the circularity problems inherent in previous formulations is presented. The empirical status of this law and the manner in which it provides a non-conventional foundation for the Law of Motion and the definition of physical forces is established. First, quite general path structures are discussed which are not defined at the outset in (...) terms of geodesic paths and which require for their description only the local differential topological structure. Secondly, a number of theorems are presented which serve as purely local differential topological criteria for singling out from among the general path structures the geodesic ones that represent the inertial structure of spacetime. (shrink)

The claim that distant simultaneity with respect to an inertial observer is conventional arose in the context of a space-and-time rather than a spacetime ontology. Reformulating this problem in terms of a spacetime ontology merely trivializes it. In the context of flat space, flat time, and a linear inertial structure (a purely space-and-time formalism), we prove that the hyperplanes of space for a given inertial observer are determined by a purely spatial criterion that depends for its validity only on the (...) two-way light principle, which is universally regarded as empirically verified. All (empirically determined) “spacetime” entities, such as the conformal structure or light surface equation, are used in a purely mathematical manner that is independent of and hence isneutral with respect to the ontological status that is ascribed to them. In this regard, our criterion is significantly stronger than thespacetime criterion recently advanced by D. Malament, which appeals explicitly to the conformal orthogonality of spacetime vectors and to the invariance of the conformal-orthogonal structure of spacetime under the causal automorphisms of spacetime. Once the hyperplanes of space for a given inertial observer have been determined by our empirical and purely spatial criterion, the following holds: there exists one and only one $\vec \varepsilon $ -synchronization procedure, namely the standard procedure proposed by Einstein, such that the planes of common time are thesame as the nonconventional hyperplanes of space for the inertial observer. It follows that our criterion provides an empirical even if indirect method for determining that the one-way speed of light is the same as the average two-way speed of light. In addition, two inertial observers that are not at rest with respect to each other necessarily havedifferent hyperplanes of space, and consequently their respective spatial views cannot be encompassed in a single three-dimensional space. Hence, our purely spatial criterion provides an empirical motivation for adopting the more comprehensive spacetime ontology. (shrink)

The central aim of this dissertation is to clarify, defend and develop a realist field ontology of the causal-inertial structure of spacetime forcefully advanced by Hermann Weyl. Weyl's field ontology of spacetime structure may roughly be described as follows. The Special and General as well as the non-relativistic spacetime theories are principle theories of spacetime structure. They all postulate various structural constraints, and events within spacetime are held to satisfy these constraints. When interpreted physically, these mathematical structures correspond to physical (...) structural fields on spacetime. ;The thesis begins with a discussion of the significance of Riemann's Dynamical Hypothesis within the context of the debate between geometrical realism and conventionalism. In particular, Grunbaum's unyielding insistence that Riemann's work must be seen as the immediate progenitor of his brand of geometrical conventionalism is challenged and shown to be unsupported by a textual analysis of the relevant works by Riemann. The first two chapters provide a historical/exegetical analysis of Riemann's Habilitationsvortrag. Chapter 3 presents a general account of geometrical conventionalism apart from issues of its alleged historical ancestry as seen by Grunbaum. This is followed by a critical examination of Grunbaum's interpretation of Riemann. ;Grunbaum has repeatedly argued that Weyl's interpretation of Riemann accords with his. It is shown, however, that Weyl's understanding of Riemann is the direct opposite of that of Grunbaum. Chapter 4 provides an examination of Weyl's philosophy of geometry and his interpretation of Riemann. It is shown that Weyl considers Riemann's work to be essentially progenitorial of his version of geometrical realism. . . . UMI. (shrink)