TWISTOR LINKING AND CAUSAL RELATIONS IN EXTERIOR SCHWARZSCHILD SPACE

We investigate the causal structure of M, the spacetime consisting of that portion of Schwarzschild space outside the event horizon. Two poi nts, p and q in M which do not lie on a common null geodesic are chron ologically related if and only if their celestial spheres, P and Q hav e non-zero linking number in N, the space of null geodesics of M.