  It is shown that the space of null geodesics of a causally simple Lorentzian
manifold is Hausdorff if it admits an open conformal embedding into a globally
hyperbolic spacetime. This provides an obstruction to conformal embeddings of
causally simple spacetimes into globally hyperbolic ones irrespective of
curvature conditions. Examples of causally simple spacetimes are given not
conformally embeddable into globally hyperbolic ones.
