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tiny-tim
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Mar24-10, 03:25 AM
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Gravity can effectively delay light but it certainly can't stop it (the local speed of light, ie in any local frame, is always c). The closest to stopping light is that gravity near r = 3M round a black hole can make light orbit a large number of times before carrying on see the PF Library on photon sphere
Accordingly, a photon with L/E = 3√3M can orbit on the photon sphere ([itex]r\ =\ 3M[/itex]) with period [itex]6\pi\sqrt{3}M[/itex], or can approach the photon sphere, circling ever closer either just outside or just inside it with approximately the same period, but never quite reaching it.

Lens and mirror effects:

Similarly, a photon with L/E slightly greater than 3√3M may circle the photon sphere a number of times before returning to distant space.

So a black hole can act as a lens giving rise to n ring-shaped images of a background star, each ring corresponding to light which has circled 1,2,3,..n times around, for some positive integer n (which depends on the distance beyond the black hole).

And it can act as a mirror giving rise to n ring-shaped images of a foreground star, in the same way.

These effects are too faint to be observed, but the "zeroth ring", in which light is focussed without circling the black hole at all, has been observed, and is known as gravitational lensing