If black holes change the path of incoming light in a gradual way (from very slight changes to 180 degrees), only depending on how close the light is passing by, the black hole external layer (where a perpendicular light beams are curved but are not doomed to fall into the black hole) would in fact be acting as a light reflecting body since the incoming light from any nearby star (with light beams arriving at all possible distances to the center of black hole) would be reflected in all possible output directions.

In that case, why arent black holes bright due to the contribution of the reflected light from all nearby stars?


As you can see in the raytracing animation, the observer at the center bottom of the scene does not receive any rays that come from the direction of the black hole:

black hole ray tracing

Therefore a black hole has a shadow which has approximately 2.5 times the radius of its event horizon.

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    $\begingroup$ WOW!!!! this is sooo cool!!! thanks!!!. I think however this kind of proves my point. I was not saying that the rays would come directly from the black hole but from the surrounding region (the observer at the bottom center does see light coming from the star). I think what your animation proves is that the density of rays that have a really big deflection is really really low and therefore probably barely visible. Thanks again $\endgroup$ – SandiaDeDia Aug 6 '18 at 0:43
  • $\begingroup$ @SandiaDeDia The answer does not prove your point. Note that the animation does not show any light that comes to the observer from the direction of the black hole. See the "shadow" link in the answer for the visual representation of what the observer actually sees. The black hole remains black despite its gravitational lending. $\endgroup$ – safesphere Aug 6 '18 at 3:00
  • $\begingroup$ @safesphere I always meant from the "corona" and not the black hole itself. I may have expressed myself wrong. My question is then that since the light from any star reaching the black hole creates a shining corona for all observers (as can be seen in the animation) how come the sum of all the light that reaches the black hole from neighboring stars is not enough to create a clear corona around it. $\endgroup$ – SandiaDeDia Aug 6 '18 at 5:49
  • $\begingroup$ @SandiaDeDia It could be enough if an orphan black hole crossed our view of some bright galaxy, like Andromeda. However, black holes typically are surrounded by large clouds of dust whether in the galaxy center of in the center of a supernova. They can be seen only by radio telescopes. Check this out: eventhorizontelescope.org $\endgroup$ – safesphere Aug 6 '18 at 5:52
  • $\begingroup$ That corona is called the primary Einstein ring $\endgroup$ – Yukterez Aug 7 '18 at 2:54

Black holes make gravitational 'lenses'. Lenses don't just bend light toward an observer, they also bend light away. There's situations where a background star can be seen in duplicate, but also where a background of black void can be seen in duplicate. There is no net light-enhancement.

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  • $\begingroup$ I think my idea is that the black hole will always reflect some of the incoming light to any observer around it. For the observer this light comes from a small region on the outter part of the black hole, so summing up the light form all the stars reaching the black hole is weird that looking at a black hole there is not a clear brightness around the corona. The ray animation suggests that only a tiny portion of light is able to significantly change its path, so maybe this is why we don't see it shine so clearly (even though it does shine a little bit). $\endgroup$ – SandiaDeDia Aug 6 '18 at 3:21
  • $\begingroup$ @SandiaDeDia No light from any source bent by a black hole comes from the direction of the black hole. A black hole bends light around itself, so you always see the black hole itself as black circle, although it can be surrounded by a "corona" of the bent light. The only light you may see coming from the direction of the black hole is the light emitted or reflected by the objects (stars, gas, etc.) located between you and the black hole. This light is emitted toward you, not toward the black hole. Just like with a light bulb on the background of a night sky, the sky remains dark regardless. $\endgroup$ – safesphere Aug 6 '18 at 4:32

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