
#1
Jun1511, 11:22 AM

P: 2,453

Why cant we know the mass distribution inside a black hole. If we are observing from outside the event horizon I couldn't tell how the mass was moving around inside the black hole, I could just figure out how much was in there by measuring the G field.




#2
Jun1511, 12:58 PM

Sci Advisor
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P: 3,853

Because it's spherically symmetric. All you can tell is the total mass, not how it's distributed as a function of r. The same holds for Newtonian gravity: you can't tell from measuring the Earth's gravitational field how much mass is located in the mantle and how much in the core. From the outside it all looks the same.




#3
Jun1611, 02:18 AM

P: 2,453

suppose it wasn't uniformly distributed . I know what your saying its like a gauss's law argument




#4
Jun1611, 07:18 AM

Mentor
P: 16,476

Mass distribution of a black hole?
It doesn't matter if it is uniform or not, as long as it is spherically symmetric all you can determine is the total mass.




#5
Jun1611, 05:32 PM

P: 2,453

Ok suppose it wasn't spherically symmetric. Suppose I had a curtain around a circle and inside this circle I had a big heavy rock on one side and then 2 other lighter rocks randomly placed. classically I could tell the mass distribution by walking around the curtain and measuring the gravitational field . But in the case of a black hole where I have some big rocks entering a BH at random places, once they cross the event horizon I will not be able to map out the mass distribution by going around and measuring the G field.




#6
Jun1611, 08:01 PM

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P: 16,476





#7
Jun1711, 12:06 PM

P: 125

In that case, mass distribution would have circular symmetry, but not spherical. Would one be able to determine the radius of the ring, perhaps? 



#8
Jun1711, 12:30 PM

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P: 8,005

It's usually said that black holes settle down quickly to a state where only three parameters observable from outside: mass, charge and angular momentum. The lingo seems to be that black holes have "no hair".
http://archive.ncsa.illinois.edu/Cyb...Evolution.html http://jila.colorado.edu/~ajsh/insidebh/schw.html Some proposed tests (which probably didn't get funded, but judging by the authors, the ideas are probably good): http://arxiv.org/abs/0903.0100 



#9
Jun1711, 12:31 PM

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P: 16,476





#10
Jun1711, 12:52 PM

P: 2,453





#11
Jun1711, 01:09 PM

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P: 16,476

Hmm, interesting idea. I have no clear objection to it, but it does make me doubt the correctness of my previous statement.




#12
Jun1711, 01:40 PM

P: 125

Is it possible that the time dilation would screw it up? Again, I worry that I'm out of date (I read a teenagerlevel book on black holes a while back, which is where I remember most of my "knowledge" about BHs), but I was under the impression that time dilation inside a black hole was, for all intents and purposes, infinite?




#13
Jun1711, 01:54 PM

P: 1,115





#14
Jun1711, 04:41 PM

P: 2,453





#15
Jun1711, 09:37 PM

Emeritus
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P: 7,436

Try http://www.desy.de/user/projects/Phy...k_gravity.html
"How does the gravity get out of the black hole?" 



#16
Jun1811, 02:04 AM

P: 2,453

If I read that article correctly. "How does the gravity get out of the black hole?" Then it seems that the G field of the BH is created from the matter right before it goes into the BH . Like it freezes its field in place. Or do i have it wrong?




#17
Jun1811, 03:09 AM

P: 1,115

"Nevertheless, the question in this form is still worth asking, because black holes can have static electric fields, and we know that these may be described in terms of virtual photons. So how do the virtual photons get out of the event horizon? Well, for one thing, they can come from the charged matter prior to collapse, just like classical effects. In addition, however, virtual particles aren't confined to the interiors of light cones: they can go faster than light! Consequently the event horizon, which is really just a surface that moves at the speed of light, presents no barrier." Can't say that handwavy argument particularly satisfies me. Relative to the outside, the charge is a totally frozen entity. Whence therefore is there any possibility of 'virtual particle exchange' when the trapped charge's internal machinery has ground to a complete stop? Maybe an expert on GR here would care to answer that. 



#18
Jun1811, 03:41 PM

Emeritus
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P: 7,436

The sense in which "nothing can get out of a black hole" is the sense in which if you make a disturbance in the field, the disturbance won't propagate beyond the event horizon. The field itself exists everywhere, it doesn't need to "get out". So if you look at the information point of view, the information about what went in is stored in the field, and it doesn't need to get out. Information about what happens inside the black hole sitll won't get out. 


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