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Old Jul 17th 2017, 11:33 AM   #11
Pmb
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I'd like to point out that in geometry there are two kinds of curvature: intrinsic and extrinsic. The former is the kind used in GR.

Examples of the later are the surface of a cone and the surface of a cylinder. This kind of curvature relies on the presence of a higher dimension into which a surface is being curved into. The surfaces of these two objects have no intrinsic curvature. That's why objects of this shape are easy to wrap as presents, i.e. because a sheet of wrapping paper doesn't have to be deformed (intrinsic curvature) to cover them. Notice how hard it is to wrap a basketball with a sheet of wrapping paper.
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Old Jul 17th 2017, 11:47 AM   #12
wad
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Except that Hawkins theorizes radiation from black holes which "seems to imply that gravitational collapse is not as final and irreversible as we once thought."
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Old Jul 17th 2017, 02:02 PM   #13
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Originally Posted by wad View Post
Except that Hawkins theorizes radiation from black holes which "seems to imply that gravitational collapse is not as final and irreversible as we once thought."
I'm confused. Can you clarify something for me? What is the exception that you're referring to? I.e. when you say "Except that ..." what comment are you responding to? Thanks.
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Old Jul 17th 2017, 04:01 PM   #14
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"forever"

Hawkins theorizes (not that his theory is reality) in Chapter 7 of "A Brief History of Time" that "eventually" the energy equivalent of mass in a BH will be returned to the universe in the form of radiation and that if the theory is correct the BH will "just disappear" taking with it any singularity that might be inside.
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Old Jul 17th 2017, 04:16 PM   #15
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Originally Posted by wad View Post
"forever"

Hawkins theorizes (not that his theory is reality) in Chapter 7 of "A Brief History of Time" that "eventually" the energy equivalent of mass in a BH will be returned to the universe in the form of radiation and that if the theory is correct the BH will "just disappear" taking with it any singularity that might be inside.
That was an incomplete answer. I now see that you're really talking about this statement
Originally Posted by Woody View Post
However it is probably moot, since the actual black-hole will be forever hidden from the rest of the universe by the event-horizon.
Hawking made that statement with the assumption that nothing falls into the black hole while its emitting Hawking radiation. That is not a true assumption in general. First of all there is the 3LK CMB radiation which enters the black hole. Then there is always interstellar material like the various gases that are in space which will also get pulled in. Then there other things like matter from an accretion disk, stellar collisions, etc. So one cannot say that a black hole will eventually evaporate as Hawking so blindly states.

And you ignored the part where he said if the theory is correct. That theory came from quantum gravity which is not a well verified theory. In fact Hawking's theory has never been tested. However GR is well tested so asserting that Woody's statement is false is either out of context or just plain wrong.
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Old Jul 17th 2017, 04:52 PM   #16
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I understand that the origin of the Hawking theory is of pure mathematical and untested derivation. Don't know that Hawking or any of us know for sure anything that is true or false in this reality. "except" was my poor choice of words - my purpose was to introduce the Hawking theory as another perspective on the permanence of black holes.
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Old Jul 17th 2017, 05:02 PM   #17
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Originally Posted by wad View Post
...my purpose was to introduce the Hawking theory as another perspective on the permanence of black holes.
I see. Most of the members here who answer questions know about that.
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Old Jul 18th 2017, 07:43 AM   #18
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Does the Forum have rules of conduct that explain who is permitted to say what. Are only the 'members who answer questions' supposed to respond or contribute to questions / issues? If so who are those members?
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Old Jul 18th 2017, 07:54 AM   #19
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Originally Posted by wad View Post
Does the Forum have rules of conduct that explain who is permitted to say what. Are only the 'members who answer questions' supposed to respond or contribute to questions / issues? If so who are those members?
Of course not. Why would you ask such an obviously wrong question? Perhaps you assumed that since I mentioned that most members here know that it meant that I was saying that you shouldn't mention it? If so then you're incorrect. I was merely making a point. And when I speak of "most members" I'm referring only to those members who are here to help people understand physics.

In he future I recommend that you don't read more into a post than is actually stated in that post. In this case there is nothing in the comment Most of the members here who answer questions know about that. which suggests or even hints to the notion that you are not permitted to say something, to answer something or to contribute.

Do you have a reason for me not to mention what I believe most people here know?
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Old Jul 18th 2017, 08:11 AM   #20
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Yes. Wanted to know if there are any Q & A protocols of which new members (versus people here) should be aware.
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