1. ## Re: Is a black hole made of matter ?

not_Fritz, I'm not a troll. I don't know if the sentence I used is ad hominem, however your observation that trolls use the same type of sentence seems aimed at implying that I'm a troll, and inferring that I'm a troll just because I used a type of sentence that trolls use is probably not something good either.

But yes, I could have avoided that comment about NASA; although I don't think it's wrong I also realize that it could be unpleasant. It was mostly out of disappointment, I guess I posted too quick.

kencrowder, no, I didn't get offended, it's just that I had ended up on that NASA page because my daughter asked me for reliable websites for her high school science essay about BHs and I thought well, what more reliable than NASA. So I wanted to highlight that at least in my intentions I did try to choose my sources (poorly, now I can say ).

In my experience people like Thorne and Davies need at least an hour to establish context for statements like the ones you quote.
OK, it's convincing. It's of course annoying to accept that even straight and apparently self-contained sentences like the one I quoted carry the risk of meaning something very different from what the scientist meant, and this makes scientific information at a general public level much less meaningful, but it's probably just the way it is.

I also would have never guessed that NASA shouldn't be considered a reliable source for this type of info. But as of now that's convincing as well.

Thanks everybody for all the answers and for a very interesting discussion.

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3. ## Re: Is a black hole made of matter ?

Originally Posted by Glitch
Is 2GM / c2 the radius of the Event Horizon or the radius of the singularity?

I ran the equation using Sol as an example, and came up with 2,954 meters

G = 6.6738 × 10-11
M = 1.9891 × 1030 kg
c = 299,792,458 m/s
It's the radius of the Schwarzschild event horizon. In classical GR the Schwarzschild metric has a singularity at the center of the hole with zero radius. Quantum corrections are left out in that remark.

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5. ## Re: Is a black hole made of matter ?

Originally Posted by SantiBailors
not_Fritz, I'm not a troll. I don't know if the sentence I used is ad hominem, however your observation that trolls use the same type of sentence seems aimed at implying that I'm a troll, and inferring that I'm a troll just because I used a type of sentence that trolls use is probably not something good either.
In saying I am "implying" you are a troll, you are making an unsuccessful attempt at reading my mind. I was simply calling attention to a defect in your argument and posing a question that only you can answer about your intentions.

But yes, I could have avoided that comment about NASA; although I don't think it's wrong I also realize that it could be unpleasant. It was mostly out of disappointment, I guess I posted too quick.
I think it is thoroughly wrong to argue in that manner. It is an example of ad hominem fallacy. People frequently resort to that fallacy when they don't want to think through an issue. It's common. It's also utterly fallacious and unscientific. Even scientists fall prey to it.

OK, it's convincing. It's of course annoying to accept that even straight and apparently self-contained sentences like the one I quoted carry the risk of meaning something very different from what the scientist meant, and this makes scientific information at a general public level much less meaningful, but it's probably just the way it is.

I also would have never guessed that NASA shouldn't be considered a reliable source for this type of info. But as of now that's convincing as well.
Popular sources can inspire, but they will never, I fear, avoid misleading folks and appearing contradictory.

Thanks everybody for all the answers and for a very interesting discussion.
Thanks for continuing to explore.

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7. ## Re: Is a black hole made of matter ?

Originally Posted by not_Fritz_Argelander
It's the radius of the Schwarzschild event horizon. In classical GR the Schwarzschild metric has a singularity at the center of the hole with zero radius. Quantum corrections are left out in that remark.
That is what I thought. Thanks.

8. ## Re: Is a black hole made of matter ?

I finally had time to watch the video. The documentary is badly edited and misleading at several points.

Kip Thorne, does seem to say in brief and continuous snippet, that is not explainable by bad editing, that the matter has gone into a black hole no longer exists.

The context of classical GR without Quantum corrections is the only context where that statement could be made and be true. In the context of GR with quantum corrections it is false.

Details on sorting out the context differences between classical GR and GR with quantum corrections are here:
No-hair theorem - Wikipedia, the free encyclopedia

So the seeming contradiction is traceable to that "context" information which is not carefully laid out for the documentary's viewer.

Thorne's remarks are only sensible in the pure classical GR context. In a real world with quantum corrections, they are false.

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10. ## Re: Is a black hole made of matter ?

Is it true that if you fell into a black hole and could survive long enough to turn around and look out, you'd see the universe speed up, (like a video tape on fast forward?) I have a feeling this might be one of those urban legends I heard as a kid, but from what I understand about GR, weird things happen with time and gravity, right?

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12. ## Re: Is a black hole made of matter ?

Originally Posted by Celestica
Is it true that if you fell into a black hole and could survive long enough to turn around and look out, you'd see the universe speed up, (like a video tape on fast forward?) I have a feeling this might be one of those urban legends I heard as a kid, but from what I understand about GR, weird things happen with time and gravity, right?
It's not an urban legend. It's true. It doesn't even need black holes. Gravitational time dilation / gravitational redshift happens everywhere.

It was verified on Earth in 1959: Pound

GPS won't work as well without taking it into consideration: Global Positioning System - Wikipedia, the free encyclopedia

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14. ## Re: Is a black hole made of matter ?

Fritz comes to my rescue once again, thanks!

This is going to be a long shot. Do you have any idea about the difference in elapsed time for someone inside a black hole and someone outside of it? I imagine myself falling into a black hole, looking out and seeing the evolution of the universe over a course of millions of years, while only a few minutes pass for me inside the black hole. The reason(s) being, I've read that gravity inside those things is ridiculously strong, to the extent that even light can't escape, and time itself might stop entirely (or come extremely close). So I imagine the time dilation would be enormous, hope that makes sense.

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16. ## Re: Is a black hole made of matter ?

Originally Posted by Celestica
Fritz comes to my rescue once again, thanks!

This is going to be a long shot. Do you have any idea about the difference in elapsed time for someone inside a black hole and someone outside of it? I imagine myself falling into a black hole, looking out and seeing the evolution of the universe over a course of millions of years, while only a few minutes pass for me inside the black hole. The reason(s) being, I've read that gravity inside those things is ridiculously strong, to the extent that even light can't escape, and time itself might stop entirely (or come extremely close). So I imagine the time dilation would be enormous, hope that makes sense.

I think that you may intend the numbers in your question as figurative. The falling observer sees the distant observer age more rapidly. How fast depends on details such as the mass of the black hole. the falling observer see himself as colliding with the central singularity in a finite amount of time. Whether he sees "millions" of years aging in a few minutes of falling depends on details.

Something like this problem was given as homework at Cornell last year, BTW. So I'm going to be a bit reticent. You want to consider the problem in Gullstrand-Painleve' coordinates. The "rain" version of the coordinates tells the tale.

Gullstrand

The history of this coordinate system is an interesting one. Einstein rejected them as erroneous, but in this case Einstein was quite mistaken.

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18. ## Re: Is a black hole made of matter ?

There is other legend, which said A crossing horizon will be able see "video" from this moment up to the end of the Universe. This legend is not true. Indeed at the moment when A passed event horizon, time outside horizon will be infinity, so it looks like A can see history of all Universe up to the end. But problem that A can detect only limited number of photons, so "video" will be very fragmented and not so interesting because of A will be able see only very small area near black hole event horizon. After crossing the event horizon, A not able to recognized in what order photons coming in from above, so video for A inside of black hole will be really very strange.

Of cause this is in fame of classical general relativity answer. In case of combination of general relativity and quantum mechanics picture can be different.

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