# Seeing The Light in a Black Hole

I assume if light can't escape a black hole, then light is in a black hole. Does the light 'shine' inside a black hole

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Could you please elaborate on your question? It's rather skimpy right now... – Manishearth Dec 16 '12 at 17:49
Could I simply say that your assumption is wrong..! – Waffle's Crazy Peanut Dec 16 '12 at 18:01
big ole meanies – pairunoyd Dec 16 '12 at 18:05

## 4 Answers

This is really just an expansion on dmckee's answer, and if he wants to expand his answer I'll delete this one.

Anyhow, a light ray follows a path called a null geodesic. In flat spacetime this is just a straight line (or what we normally think of as a straight line) and the light can shine in any direction. So if you shine a torch at something the light can travel from the torch to the object then back to your eye, and you can see the object.

In curved spacetime light no longer travels in straight lines, which is why you get gravitational lensing. At the event horizon of a black hole the curvature is so intense that light can no longer move outwards. No matter which way you shine your torch the light always follows a path that moves inwards and eventually hits the singularity at the centre of the black hole.

So inside the event horizon a torch won't illuminate things. The light from the torch can only reach the object you're trying to see if the object is closer to the centre than the torch is. But then the light can't get back from the object to you. So you can't see things in the sense that you can use a torch to see things in a dark room.

However as twistor59 says in his comment to dmckee's answer, you can still see light from outside. If you have fallen past the event horizon you can still see the light shining from the outside world because you are nearer the centre of the black hole than the outside world is. So in this sense the answer is that yes, light shines into a black hole just as it does outside it.

If you want to pursue this further this blog posting gives a nice introduction and has useful links to more detailed descriptions.

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We can't say for certain what happens to light in a black hole. However, energy conservation doesn't dictate that light continues to shine, only that the energy doesn't disappear. Consider light shining on a dark surface; the photons are absorbed, but their energy is converted to heat in the surface.

When light is absorbed into a black hole, the energy of the photon is added to the energy of the black hole, which increases the effective mass of the black hole.

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Short-short answer: No.

From the horizon inward all geodesics point toward the center, so you can never see anything coming back toward you.

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It depends what the OP meant by "shine inside the BH". Light is "shining" in from the outside. (There is also some HR from the past horizon, but I doubt that's what the OP was talking about). – twistor59 Dec 16 '12 at 17:48
@twistor59 Yes. I see what you mean. – dmckee Dec 16 '12 at 18:11

Yes, light is shining there. If you would get into black hole and survive you would be able to see the light. However, there can be more space-time inside black hole than can be imagined. For example it can be the whole another Universe there. So, shining would not cycle there but would shine as usual.

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