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Black hole question 
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#1
Oct2512, 02:57 PM

P: 325

So are all black holes infinitely small, which is what they call the singularity point? I'm talking about the actual distance the matter of a black hole takes up. Does a super massive black hole have the same physical size as any other black hole(which i guess would be infinitely small) and just a must larger radius of effective gravity?



#2
Oct2512, 03:11 PM

P: 299

The event horizon, the only part of the black hole that you can "see", is proportional to the mass that generated the black hole. The best theory of gravity we have, General Relativity, predicts that there is a singularity at the centre. General Relativity is thought to break down in this regime and quantum effects take over.



#3
Oct2512, 03:13 PM

P: 5,632

The singularity at the very center of a BH is a point of virtually zero size....a singularity.
About all we know is that it appears space and time get mixed up there in quantum fluctuations...a quantum foam. No matter is believed to exist inside a BH....it has been compressed out of existence. 'Big' and 'small' BH refer to the size of the external event horizon....It's usually quoted with a radius r dependent of the mass M which has been swallowed: r = 2M called the Schwarszchild radius. This turns out to be a solution of formulas from Einstein's general relativity....Once something goes inside the event horizon, nothing gets out. Check out the first section here for more: http://en.wikipedia.org/wiki/Black_hole 


#4
Oct2512, 03:16 PM

P: 325

Black hole question
So the event horizon is proportional to the mass that generated the black hole(so maybe a collapsing star...). but is all of that mass that used to be the star condensed into an infinitely small point, or is it just packed more tightly together than it was in the star, making it have a measurable radius of actual matter that makes it up.(say a couple kilometers or something...)



#5
Oct2512, 03:38 PM

PF Gold
P: 6,338

As was pointed out, the math says it is a dimensionless point with infinite density, but that just means it's where the math model that we currently use breaks down and again, "we really don't know WHAT is going on in there". The hope is that some day a theory of quantum gravity will explain what's REALLY happening. 


#6
Oct2512, 05:21 PM

P: 325

I still havent really got my answer. (Im probably not being clear enough). Are ALL black holes an infitely small point and their area of gravitational influence is what makes them big or small? Or are there black holes the size of, say, a marble, or the earth, but much much denser? (But theres actual matter making it up and to get to the singularity of the BH you would have to "dig" through physical matter).



#7
Oct2512, 05:32 PM

PF Gold
P: 6,338

EDIT: and the "size" is the diameter of the event horizon. 


#8
Oct2512, 06:46 PM

P: 490

We therefore won't know what the speed of mass at the singularity might be moving at, and therefore the size of it's event horizon might not give an indication of the amount of mass used to make it. 


#9
Oct2512, 06:55 PM

P: 251




#10
Oct2612, 02:41 AM

P: 490

That's a fudge. 


#11
Oct2612, 03:51 AM

P: 665

Black holes aren't just some sort of black 'star' hanging in the sky that swallow anyone who gets too close. They are places where time and space literally end. We can't even guess what that might mean for the existence of matter. Long before you get to the singularity itself, ordinary matter ceases to exist as first the orbiting electrons are forced into the nucleus then the nucleus itself is crushed into ??? and then snuffed out as space itself ceases to exist and time ends. 


#12
Oct2612, 09:28 AM

P: 610




#13
Oct2612, 10:36 AM

C. Spirit
Sci Advisor
Thanks
P: 5,633

Just to clarify...not all black holes have a point  singularity. Rotating (kerr) black holes have ring singularities.



#14
Oct2612, 10:39 AM

P: 5,632

That is NOT what has been stated. GR works inside the event horizon down to the singularity...at the singularity GR and QM break down...it is only at the singularity that we have no theory...this is pretty clearly described in the above posts.... 


#15
Nov3012, 09:17 AM

P: 538

How can something have infinite density? That would mean the volume is zero, right? If the volume of something is zero, then how can it even exist? Zero volume would be literally nothing.



#16
Nov3012, 09:23 AM

PF Gold
P: 6,338




#17
Nov3012, 07:30 PM

P: 490

If the math on on the inside is not right how can the math be right on the outside unless one allows for wishfull thinking. 


#18
Nov3012, 07:43 PM

P: 490

How can GR work if it does not explain the singularity.It's nonesense, you would be better off explaining how an egg boils. Sorry if this sounds abrupt but to state with conviction that we know how GR works inside the event horizon down to the singularity is just plain wrong. 


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