  The set N of all null geodesics of a globally hyperbolic (d+1)-dimensional
spacetime (M,g) is naturally a smooth (2d-1)-dimensional contact manifold. The
sky of an event is the subset of N defined by all null geodesics through that
event, and is an embedded Legendrian submanifold of N diffeomorphic to a
(d-1)-dimensional sphere. It was conjectured by Low that for d=2 two events are
causally related iff their skies are linked (in an appropriate sense). We use
the contact structure and knot polynomial calculations to prove this conjecture
in certain particular cases, and suggest that for d=3 smooth linking should be
replaced with Legendrian linking.
