The Escape of Photons from Gravitationally Intense Stars

Abstract

A star of mass m and Schwarzschild radius r₀ is called gravitationally intense if the ratio 2 m / r₀ is only slightly less than unity. It is shown that, if photons are emitted in all directions from a point on the surface of a gravitationally intense star, only those emitted in directions lying witnin a slender critical cone escape to infinity; photons emitted in directions outside that cone are recaptured by the star. In the limit as 2 m / r₀ tends to unity, the critical cone degenerates into a line normal to the surface of the star. Since light rays are reversible, it follows that in this limit not only do all escaping photons issue in directions perpendicular to the surface, but also all photons reaching the star from infinity fall perpendicularly on its surface.