A conformal Hopf-Rinow theorem for semi-Riemannian spacetimes

The famous Hopf-Rinow Theorem states, amongst others, that a Riemannian manifold is metrically complete if and only if it is geodesically complete. The Clifton-Pohl torus fails to be geodesically complete proving that this theorem cannot be generalized to compact Lorentzian manifolds. On the other hand, Hopf and Rinow characterized metric completeness also by properness. García-Heveling and the author recently obtained a Lorentzian completeness-compactness result for open manifolds with a similar flavor. In this manuscript, we extend the null distance used in this approach and our theorem to proper cone structures and to a new class of semi-Riemannian manifolds, dubbed $(n-\nu ,\nu )$-spacetimes. Moreover, we demonstrate that our result implies, and hence generalizes, the metric part of the Hopf-Rinow Theorem.