User:Marc Goossens/Schröter-Schelb Spacetime

Schröter-Schelb spacetime or SS-ST in short, is an axiomatic development of the spacetime model underlying General Relativity, the Lorentzian manifold. It is one of several proposals to bring clearer physical motivation to and understanding of this mathematical structure in an axiomatic way. SS-ST was developed during the 1980's and 1990's by the German physicists Joachim Schröter and Udo Schelb, at Paderborn University  and published in full by Schelb  in 1997. In contrast to the older top-down Ehlers-Pirani-Schild proposal, Schröter-Schelb axiomatics works bottom-up, starting from primitive physical concepts.

Motivation
Mathematically, the canvas underlying the general theory of relativity takes the form of a Lorentzian manifold. This structure from differential geometry is a continuum of spacetime points (events), which can locally be consistently described by a set of 4 coordinates. At each event a mathematical device, the Lorentzian metric (tensor) or 'metric field' distinguishes light signals and particles, singling out their allowable worldline trajectories. In doing so, this 'metric field' also encodes the influence of gravitation on both particles and light.

This 4-dimensional manifold and its metric field are specialized mathematical concepts. As such they have no immediate physical interpretation or motivation. The particle and light worldlines to which they give rise do possess a clear physical meaning, but mahematically these are derived concepts. Therefore, one should like obtain a clearer physical insight of the meaning of the prior mathematical entity, the metric, itself, and ascertain to what extent we may take the model of a 4-dimensional continuum for granted.

Schröter and Schelb propose to tackle this by reconstructing the Lorentzian manifold, starting from a handful of primitive concepts, reality points, elementary events and signals. In the course of the construction, fastest signals, clocks and directions complete this set of primitives. For each, the physical meaning is described explicitly.

Some characteristics of the construction
In the same publication, Schelb points out some further aspects of the SS-ST approach to the foundations of spacetime geometry:
 * In the stepwise development summarized above, the originators develop and explore each subsequent refinement of the theory in its own right. This clarifies the precise role and extent of each of the assumptions (axioms).
 * Indeed, the 1997 Schelb memoir painstakingly erects each of these stages of completion as a separate physical theory, in the strict meta-theoretical sense of the structuralist methodology of Günther Ludwig.
 * SS-ST sets out to introduce all necessary concepts needed with a direct, operational physical interpretation.
 * The construction makes no use of (even rudiments of) other physical theories. In particular, no notion of quantum mechanics or atomic clocks is employed in order to define 'good' (i.e. sufficiently regular etc.) clocks.
 * Starting from scratch, SS-ST also makes no prior mathematical assumptions beyond set theory and the real numbers. Mathematical choices as to whether the metric or the connection is to be taken as the primary notion, or whether a geometry with torsion should be allowed, are avoided: the resulting Lorentzian manifold arises directly from physically motivated axioms.
 * The SS-ST construction uses or recovers several concepts familiar from spacetime research such as signals, causal structure, the Alexandrov topology etc. These established concepts had mostly been introduced starting from the full structure of a Lorentzian manifold. In contrast, in SS-ST they are derived early on in the process of constructing mathematical spacetime from "first concepts". Causal structures, for example, are usually defined by classifying tangent vectors to curves using light cones, all of which are concepts derived from the manifold and metric structures. SS-ST gets by using properties of signals only. The derived causal structure is subsequently used to build the Alexandrov topology on the set of events.).

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