Can light orbit a black hole just outside its event horizon?

Yes they can, but not for long. The event horizon of a non-rotating black hole is located at a distance, R = 2 G M/c^2. But just outside this horizon at a distance of R = 3 G M/c^2, an incident photon is deflected into a circular 'photon orbit'. These orbits, however, are unstable and after a few orbits, the photon will either spiral inward into the event horizon, or manage to escape to 'infinity' with a very large redshift. Matter can orbit a black hole at distances much greater than the photon orbit radius depending on their velocity, however, for rotating black holes, the so-called Lenz-Tirring 'frame dragging' effect prevents any long-term, stable orbits for matter within about R = 10 G M/c^2.


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