Dedicated to my father, George Szekeres, on his eightieth birthday An axiomatic approach to relativity theory is proposed which is based entirely on a single reflexive relation called signalling. This generalises and simplifies earlier schemes of Kronheimer and Penrose and of Carter which are based on causality relations. Many of the features of space-times, such as photon paths and topology, have their counterparts in general signal spaces.
CASUAL SPACESThere is a school of thought [7] which regards Euclidean geometry as an excellent piece of mathematical physics, that is, an axiomatic system which applies to the real world. It was in something of this spirit in 1968 that George Szekeres, following a suggestion of M.L. Urquhart, attempted to give an axiomatic foundation of the kinematic part of Einstein's special theory of relativity [9]. This work was subsequently enlarged on by John Schutz [8].In a related vein there is Kronheimer and Penrose's axiomatic approach to space and time based entirely on causal relations [6]. This work is motivated more by general relativity than special, and attempts to encompass some of the unusual causal properties which can arise in that theory. However it does allow for structures more general than manifolds, and may eventually pave the way to a theory of discrete space-times which could be reconciled with the paradoxes of quantum theory.One thing which is not permitted in the Kronheimer-Penrose theory (hereinafter referred to as KP) is closed timelike lines such as occur in the Godel universe [3]. Carter [1] has extended the idea of a causal space to include the possibility of such causal loops. The resulting structure is one he calls an etiological space. In this paper I shall adopt a less arcane terminology.By a pre-causal space we shall mean a triple (X, -<,