Re:The Nobel Prize in Physics 2020
by a1369209993 on Tuesday October 06, 2020 @06:12PM
(#24702946)
> what constrains it from orbiting
Not quite what you seem to be asking, but the event horizon is the point where[0] the escape velocity is equal to the speed of light; the orbital velocity at that distance is greater (by a factor of ln 2 IIRC, so v_orbital ≈ 1.44c). There's a more distant distance, called the innermost stable orbit or the photon sphere, where the orbital velocity is equal to c (so photons will orbit if you emit them tangentially at this height), but the escape velocity is only ~0.69c.
0: It's not quite right to say that that's because the escape velocity is equal to c, though.
Not quite what you seem to be asking, but the event horizon is the point where[0] the escape velocity is equal to the speed of light; the orbital velocity at that distance is greater (by a factor of ln 2 IIRC, so v_orbital ≈ 1.44c). There's a more distant distance, called the innermost stable orbit or the photon sphere, where the orbital velocity is equal to c (so photons will orbit if you emit them tangentially at this height), but the escape velocity is only ~0.69c.
0: It's not quite right to say that that's because the escape velocity is equal to c, though.