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Why a black hole is actually black? Why light when enters into it does not bright inside?
Question Date: 2015-04-02
Answer 1:

Good question! The answer doesn't have to do with whether or not light is "bright" inside a black hole; instead, it has to do with the way light travels near a black hole.

As you may know, a black hole is an object which has a huge mass packed into a small volume, so its gravity is very strong. But according to Einstein's theory of general relativity, this strong gravity means that the spacetime around the black hole is very strongly curved (imagine putting a heavy object on a trampoline - the mass of the object bends the surface of the trampoline). Now, what role does light play? Well, because the spacetime is curved, beams of light travel in curves rather than straight lines. It turns out that the spacetime around a black hole is so strongly bent that a light beam that's shot out from the surface of the black hole gets pulled back into the black hole, so it can't escape. That means that if someone was on the surface (or even inside) the black hole and pointed a flashlight out towards you, none of the light from the flashlight would be able to escape! That makes the black hole look black to you - no light comes out of it.

Your friend inside the black hole, however, will still see light coming out of the flashlight - the light doesn't get any dimmer or black. It's just that the light (and your friend!) can never escape from the black hole to reach you.

I hope this helps!

Answer 2:

Black holes have such powerful gravity that the escape velocity - that is, the speed needed to get out - is faster than light. Because not even light travels fast enough to get out, it's not possible to see anything outside of a black hole from outside. Thus, they look black.

There is no reason why you could not see things inside of a black hole if you were yourself already inside, however. That said, you wouldn't be seeing anything for long, because black holes have other properties that would kill you rather quickly (you would be pulled apart).

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