Some Splitting Theorems for Stably Causal Spacetimes

We prove a splitting theorem for stably causal spacetimes and another splitting theorem for finitely compact spacetimes admitting a proper time synchronizable reference frame.     

The Causal Boundary of Spacetimes Revisited

We present a new development of the causal boundary of spacetimes, originally introduced by Geroch, Kronheimer and Penrose. Given a strongly causal spacetime (or, more generally, a chronological set), we reconsider the GKP ideas to construct a family of completions with a chronology and topology extending the original ones. Many of these completions present undesirable features, like those which appeared in previous approaches by other authors. However, we show that all these deficiencies are due to the attachment of an “excessively big” boundary. In fact, a notion of “completion with minimal boundary” is then introduced in our family such that, when we restrict to these minimal completions, which always exist, all previous objections disappear. The optimal character of our construction is illustrated by a number of satisfactory properties and examples.     

Conformal Proper Times According to the Woodhouse Causal Axiomatics of Relativistic Spacetimes

On the basis of the Woodhouse causal axiomatics, we show that conformal proper times and an extra variable in addition to those of space and time, together give a physical justification for the ‘chronometric hypothesis’ of general relativity. Indeed, we show that, with a lack of these latter two ingredients and of this hypothesis, clock paradoxes exist for which the unparadoxical asymmetry cannot be recovered when using the ‘clock and message functions’ only. These proper times originate from a given conformal structure of the spacetime when ascribing different compatible projective structures to each Woodhouse particle, and then, each defines a specific Weylian ‘sheaf structure’. In addition, the proper time parameterizations are defined via path-dependent conformal scale factors, which act like sockets for any kind of physical interaction and also represent the values of the variable associated with the extra dimension.     

A Variational Theory for Light Rays in Stably Causal Lorentzian Manifolds: Regularity and Multiplicity Results

This paper is dedicated to the study of light rays joining an event p with a timelike curve γ in a light–convex subset &\Lambda; of a stably causal Lorentzian manifold . We set up a functional framework, defined intrinsically, consisting of a family of manifolds and a positive functional Q defined on them. The critical points of Q on approach, as , the lightlike, future pointing geodesics joining p and γ. We prove some regularity results, including the C ¹–regularity of , the C ²–regularity of Q on and the C ²–regularity of its critical points. Using them, we develop a Ljusternik–Schnirelman theory for light rays, obtaining some multiplicity results, depending on the topology of the space of all lightlike curves joining p and γ. Received: 9 April 1996 / Accepted: 27 December 1996     

Dispersing billiards without focal points on surfaces are ergodic

Billiards are considered on two-dimensional, smooth, compact Riemannian manifolds with dispersing scatterers. We prove that these billiards are ergodic if only Vetier's conditions for the absence of focal points hold.Partially supported by the Hungarian National Foundation for Scientific Research, grant No. 819/1 and by Central Research Fund, grant No. 501/5/4/1984