How do you see the back of your head with a black hole?

So, a black hole is a region in spacetime whose gravitational attraction is so great that even objects moving at the speed of light, including quanta of light itself, i.e. photons, cannot leave it. Usually such a strong gravitational pull is created by supermassive neutron stars.


Precisely because these objects attract and "don't let go" even of light, they are called black holes. Light does not leave them at all, is not reflected from them and is not emitted by them in any way, light is only attracted and "sucked" by them. This means that such objects cannot be seen. They can only be distinguished against the background of something luminous, and in this case they will look exactly like a black hole.


It is also obvious that objects relatively far away from the black hole are not attracted to it so strongly to be absorbed. Including light, flying at some respectable distance from the black hole, only changes its direction, but still is not "sucked in" by it.


Thus, there are two regions of space relative to each black hole:


a region in which light can fly past a black hole (that's almost the whole cosmos);

and a small, by cosmic standards, region next to the black hole, inside which light will be absorbed by the black hole and will never be able to fly out.

If there are two such mutually exclusive regions, then there is a boundary between them (it is called the event horizon). The light which finds itself on this boundary and moving in the right direction can enter the orbit around the black hole. That is, it will not be "sucked in" by the black hole, and yet it will no longer be able to move away from it, but instead will forever orbit around the black hole. This will be the boundary state between "passing by" and "being absorbed".


One can think of a very interesting "application" to this effect, already announced in the title of the article. The fact is that if one were at this boundary, one would be able to see the back of one's head without using any other means or tools than one's own eyes. After all, the light reflected from the back of his head, having orbited around the black hole in a full circle, would again return exactly to the man's eye.


However, this is not the best way to see the back of one's head. After all, according to modern concepts, it is impossible to accelerate a person to the speed of light, i.e. to go into orbit around a black hole in this region of space, so he will have only to fall there. And the object, getting inside the event horizon, will probably get into singularity in the end, and before that it will be torn off due to high gradient of the black hole's gravitational force (tidal forces).


So still in order to find out how your nape looks it is better to use proven methods: two mirrors or a camera.


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