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I understand that light can be in an unstable orbit at Schwarzschild radial coordinate r=3M (geometric units) around a Schwarzschild black hole. I also understand that the local* circular orbital speed of a massive particle around the hole is
[tex] v_o = r\frac{d\phi}{dt} = \sqrt{\frac{M}{r(1-2M/r)}}[/tex]
which is unstable for r <= 6M. The equation suggests that the local orbital speed equals the speed of light at r = 3M, which (sort of) confirms the orbit of light there.
If this is correct, my question is: if a particle in roughly circular orbit slowly spirals in from 3M < r < 6M, will it only approach the speed of light as it passes r = 3M or can it actually reach it?
*With local I mean as measured by an observer static at radial coordinate r.
[tex] v_o = r\frac{d\phi}{dt} = \sqrt{\frac{M}{r(1-2M/r)}}[/tex]
which is unstable for r <= 6M. The equation suggests that the local orbital speed equals the speed of light at r = 3M, which (sort of) confirms the orbit of light there.
If this is correct, my question is: if a particle in roughly circular orbit slowly spirals in from 3M < r < 6M, will it only approach the speed of light as it passes r = 3M or can it actually reach it?
*With local I mean as measured by an observer static at radial coordinate r.