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Old Jan 23rd 2015, 03:42 PM   #1
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Photons and Black Holes

Sorry if this seems like a dumb question and many of my assumptions based on my understanding here may be wrong. I always hear about how light cannot escape a black hole. Can light even enter a black hole in the first place? I've seen examples where massive objects can bend light and I'm assuming alter its course giving us the illusion that some distant object were are seeing is actually in a different spot then it really is.

So my question from the way I understand it is this. If a black hole warps space so much, would light be bent so much that it would actually be deflected by the black hole as it reached the event horizon or does it enter the black hole. Again this question is based on my limited knowledge so I'm happy with whatever answer I get just so that I can try to understand how a black hole effects light next to the event horizon.

Last edited by plard56; Jan 23rd 2015 at 03:54 PM.
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Old Jan 23rd 2015, 04:18 PM   #2
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Light is deflected by any gravitational body. It can be deflected by a black hole - the closer it gets, the more it will be bent.

Light that gets too close will be pulled to the event horizon. What happens after that is a matter of your reference frame. Observers outside the black hole never see anything cross the event horizon. But if you drop through the event horizon yourself, you will not notice anything different as you cross.
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Old Jan 25th 2015, 06:52 AM   #3
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Originally Posted by plard56 View Post
Sorry if this seems like a dumb question and many of my assumptions based on my understanding here may be wrong. I always hear about how light cannot escape a black hole.
And when you ask why, you will hear a succession of fairy tales, such as all geodesics lead to the central singularity or space is falling inward like a waterfall. The real reason is signalled by this. Light doesn't get out because the "coordinate" speed of light at the event horizon is zero. If you were to contact Tom Moore on Don Koks on this they'll confirm it.

Originally Posted by plard56 View Post
Can light even enter a black hole in the first place?
Yes. Have a look at the formation and growth of black holes. Kevin Brown refers to two interpretations, one of which leads to the "frozen star" black hole as originally proposed by Oppenheimer. Kevin Brown doesn't like it, but I'm confident that it's right. IMHO the black hole grows like a hailstone. Imagine you're a water molecule. You alight upon the surface, and you get buried by other water molecules. You don't pass through the surface, the surface passes through you. And you end up inside the hailstone. You "entered" it. IMHO it's similar for light.

Originally Posted by plard56 View Post
I've seen examples where massive objects can bend light and I'm assuming alter its course giving us the illusion that some distant object were are seeing is actually in a different spot then it really is.
No problem re gravitational lensing.

Originally Posted by plard56 View Post
So my question from the way I understand it is this. If a black hole warps space so much
It doesn't. A concentration of energy "conditions" the surrounding space altering its metrical qualities, which we model as curved spacetime. See Baez: "Similarly, in general relativity gravity is not really a 'force', but just a manifestation of the curvature of spacetime. Note: not the curvature of space, but of spacetime. The distinction is crucial".

Originally Posted by plard56 View Post
would light be bent so much that it would actually be deflected by the black hole as it reached the event horizon or does it enter the black hole. Again this question is based on my limited knowledge so I'm happy with whatever answer I get just so that I can try to understand how a black hole effects light next to the event horizon.
I'm not clear what you're asking here. This is the question you should ask yourself:

You're standing on a gedanken planet holding a laser pointer straight up. The light doesn't curve round, or slow down as it ascends, or fall down. It goes straight up. Now I wave my magic wand and make the planet denser and more massive. The light still doesn't curve round, or slow down as it ascends, or fall down. I make the planet even denser and more massive. The light still doesn't curve round, or slow down as it ascends, or fall down. I make the planet even denser and more massive, and take it to the limit such that it's a black hole. At no point did the light ever curve round, or slow down as it ascends, or fall down. So why doesn't the light get out?

Last edited by Farsight; Jan 25th 2015 at 06:57 AM.
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Old Jan 25th 2015, 12:37 PM   #4
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Originally Posted by Farsight View Post
And when you ask why, you will hear a succession of fairy tales, such as all geodesics lead to the central singularity or space is falling inward like a waterfall. The real reason is signalled by this. Light doesn't get out because the "coordinate" speed of light at the event horizon is zero. If you were to contact Tom Moore on Don Koks on this they'll confirm it.
I'm not going to start (another) question/answer thread with you because that isn't what this thread is about. A discussion on geodesics and the idea of a photon going less than c is far beyond what this post needs.

Let's just leave it at that and discuss the problem in another thread, shall we?

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Old Jan 25th 2015, 02:40 PM   #5
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Sorry Dan but I think it's important to get this right. Plard56 has to understand why light cannot escape a black hole to understand that light going vertically into a black hole isn't deflected at all. Professor Tom Moore's exact words were:

"As the planet's mass approaches the black hole limit, the signal emitted from the surface will seem to move more and more slowly away from the surface (and will also be seen to be increasingly red-shifted as observed from infinity). When the surface of the planet coincides with the black hole's event horizon, the signal will stop moving outward from the surface (and the redshift observed at infinity will go to infinity). So light no longer escapes."

Moreover follow the link above and see the second paragraph:



Most people probably including yourself have been taught in line with this:

"Finally, we come to the conclusion that the speed of light is not only observed to be constant; in the light of well tested theories of physics, it does not even make any sense to say that it varies."

I'm afraid it's wrong. Don Koks rewrote the article, see this:

"Einstein talked about the speed of light changing in his new theory. In the English translation of his 1920 book "Relativity: the special and general theory" he wrote: "according to the general theory of relativity, the law of the constancy of the velocity [Einstein clearly means speed here, since velocity (a vector) is not in keeping with the rest of his sentence] of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity [...] cannot claim any unlimited validity. A curvature of rays of light can only take place when the velocity [speed] of propagation of light varies with position." This difference in speeds is precisely that referred to above by ceiling and floor observers."
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Last edited by Farsight; Jan 25th 2015 at 03:01 PM.
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Old Jan 25th 2015, 03:44 PM   #6
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Originally Posted by Farsight View Post
Sorry Dan but I think it's important to get this right. has to understand why light cannot escape a black hole to understand that light going vertically into a black hole isn't deflected at all.
The point of my argument is that this is a question from a HS level class. He is not going to be able to replicate the arguments you are giving because he can't understand them.

Correct or not your comments are not standard teaching at this level.

Thread closed.

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