I realize that there is a distance from a black hole where photons can orbit. I understand that this orbit is very unstable, but I am concerned with photons just outside this range. Could the black hole cause them to 'u-turn' so that you could see some light from your star when looking at the black hole?


1 Answer 1


The answer is yes.

Roughly speaking, for a spinless Schwarzschild black hole, light that gets to a closest approach to the black hole of just a little greater than $1.5 r_s$, where $r_s$ is the Schwarzschild radius, can be bent through any angle. The closer the closest approach is to $1.5 r_s$ the bigger the angle becomes, and the light can even loop around the black hole more than once. A 180 degree deflection is caused when inwardly directed light from far away passes a black hole at a closest approach of $1.76r_s$ - see https://physics.stackexchange.com/questions/766132/would-someone-falling-into-or-orbiting-a-black-hole-see-themselves .

Viewing this spectacle from a distance much larger than $r_s$, an observer would see the black hole surrounded by a ring of light that is heading towards them after being bent around the black hole. The apparent radius of this ring would be just a little larger than $3\sqrt{3}r_s/2$.

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    $\begingroup$ also related in Physics SE: Could a trajectory around a large mass ever deflect by more than 180 degrees due to general relativistic effects? (thought it asks about a particle's trajectory rather than light) $\endgroup$
    – uhoh
    Aug 13, 2023 at 11:19
  • $\begingroup$ The movie Interstellar famously paid for actual astrophysicists to do a real physics simulation of light paths to generate the famous black hole image that looks like it's wearing a hat, which shows how you can see every part of the accretion disc due to the bizarre light bending that's going on. It's not exactly a companion star's light but I think it does a good job of showing that no matter which direction it comes from, the light can reach you where you are. Of course the amount of light that reaches any given observer would be tiny compared to the incident light. $\endgroup$ Aug 14, 2023 at 15:34
  • $\begingroup$ OK, now I want someone to create a video of life on a planet orbiting at 1 AU around a G2 star orbiting a super massive black hole at 1.76 r_s, where r_s is about 4 AU (or 1 AU is about 1/4 r_s). (The black hole would need to have a mass of a little over 200,000,000 solar masses.) Bah, just realized that the hypothetical planet would be well outside the star's Hill sphere. I'll have to dream up another example... $\endgroup$ Aug 14, 2023 at 17:27

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