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Offline Saganist

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« on: 24/01/2008 02:28:50 »
Hello,

This is an awesome forum! Thanks Naked Scientists!

My girlfriend Judy and I have been reading books by Stephen Hawking (Introducing Stephen Hawking and the Universe in a Nutshell) and Lisa Randall's Warped Passages.

We came up with questions about black hole singularities. At a site called Cambridge Relativity we could send questions directly to Professor Hawking, but I felt a little shy about asking such a busy man before asking on public forums. Here goes.

On evaporating black holes:

Once a black hole has taken in all the matter within its gravitational field, what happens to the singularity?

Will it lose its visible event horizon if no gas or dust or matter surrounds it?

The singularity's density is said to continue increasing to infinity, but is there in fact a density limit, such as somewhere beyond Bose Einstein condensate and strange matter states?

During the final contraction of the singularity, (when there is no more matter falling in) as its gravity from the center pulls on the outer area of the singularity; Is any radiation emitted by such a process? I know we're talking about a singularity which may not have a distinguishable center and outer area, but geez, there are huge black holes out there with singularities containing huge amounts of .... stuff.

Thanks so much.

Saganist





 

Offline neilep

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« Reply #1 on: 24/01/2008 02:54:55 »
Awesome First post !!

I'm not going to ruin this thread by attempting to answer your fantastic questions (cos I don't know the answers but I know a man who does !).... I just wanted to say a big welcome to you and Judy !!

Rock On !!
 

Offline Saganist

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« Reply #2 on: 24/01/2008 03:00:14 »
Hi Neilep,

Thanks for the welcome! This place R O C K S !

Cheers.

Saganist
 

Offline DoctorBeaver

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« Reply #3 on: 24/01/2008 08:08:04 »
Welcome Saganist & Judy. I'm glad you like TNS.

Warped Passages is 1 of my favourite text books. I'm just tackling it for the 3rd time. In fact, I've asked questions here relating to some of the issues raised in that book.

As to your questions... erm... dunno.  ???

I would imagine that once a singularity has taken in all available matter it will remain in a more-or-less stable state; although Hawking radiation will gradually erode it.

The event horizon is merely an area of space and is not, in itself, visible. What we see is the emission of radiation from particles as they cross the horizon. From that, we can deduce that if nothing more is crossing the horizon, there will be nothing to see. Literally, nothing to see. It would just be a nasty big black blob like a giant blackhead on the nose of spacetime.

There can be no further contraction of a singularity. A singularity is a mathematical point - therefore it has zero size. Something of zero size cannot contract. According to Hawking, though, it will continue to evaporate until it goes phut.

Personally, I take the view you stated; that there comes a point when the density reaches a maximum and will increase no further. I find the idea of zero size & infinite density absurd and worthy only of inclusion in a Spike Milligan book or a poem by Edward Lear.
 

Offline Soul Surfer

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« Reply #4 on: 24/01/2008 23:36:53 »
The concept of a singularity makes no physical sense. It is only a getout by mathematicians and modellers who can't immediately think what might stop things contracting to a singularity. 

Unfortunately we can't see what is going on because it lies behind the event horizon which does have a finite size.

The only black holes that we are aware of are stellar mass black holes formed in large supernovae and and the supermassive ones that appear to live in the middle of large galaxies.  Black holes are pretty small about a mile across for every solar mass that they have in them so a billion solar mass black hole is only about the size of our solar system.

As for the hawking radiation black holes get colder as they get bigger and even stellar mass black holes are only a tiny fraction of  degree above absolute zero and so much colder than the microwave background and therefore not evaporating at all.

As to what goes on inside.  I have never seen any good detailed suggestions of what goes on. The ordinary laws of physics show what is likely to happen so this is my own ideas.   

Just think about a few of the possibilities about what goes on inside a black hole.  For simplicity let us think of it as a single event and not worry too much about stuff falling in afterwards.

One very simple approach is that it all collapses down towards the middle going faster and faster somehow misses passes through the middle and bounces back again so we have a highly dense and heated material expanding.

Another possibility is that as the material collapses and gets denser so the gravitational field increases further so a second event horizon forms, the first event horizon occurs when the gravity stops light escaping into the universe but the material inside the hole has a higher field than that and the light cant even get to that first event horizon as the material collapses this second event horizon will collapse too and get smaller and more intense it will also emit hawking radiation eventually it will collapse to a size where the energy loss from the hawking radiation balances out the energy in the hole so it might achieve some sort of stable state.

Now neither of these take angular momentum into account and this is probably the most important feature.  It is highly unlikely that any black hole will form without it having some rotation so as it collapses it spins faster and faster until the energy in the angular momentum balances out the gravitational energy in the hole. 

but in all three cases we have things happening that will turn the collapse into an expansion or a radiating object.
 
 

Offline Saganist

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« Reply #5 on: 24/01/2008 23:45:27 »
Hi Doc B.

Thanks for these answers. These concepts seem to pose further questions as in the 'then what?' category. There has been a posting while I type this one, but I see that this post still has discussion matter, so here goes.

That Warped Passages book is indeed requiring repeated readings. There are so many terms to remember. I cringe at the next steps: Learning the Greek alphabet and learning the math! of all these theories. But I fear I would require more minutes than there are molecules in the multiverse, to do this.

If only spacetime, free of any matter, remains around the event horizon, would this then be considered a naked singularity?

I too dislike infinity outside of pure mathematics. 'Phut' is as good a term as I can imagine for the singularity's eventual demise. The interesting concept for me when trying to visualise such a process is that time has stopped from the event horizon down to the singularity.

My understanding thus far about black holes is that time ....ends or is at zero, within the Swarzchild radius?

If this is so, whatever the singularity is up to, that process is almost infinitely slow to observers outside of the radius. It could well be the biggest boom ever, but at such a slow rate of time, perhaps a 'phut', even, might be wishful thinking?

Whatever the final light show, some piece of spacetime and energy which had been excluded from the 4 dimensional universe while it remained inside the radius, will now try to reintegrate with the rest of spacetime?

The other aspect I like to ponder is that final process for the singularity. Its naked and its now beginning to decay.

It may get to ... infinity somewhere in the process of becoming infinitely dense, but it is theorised as then beginning to decay, eventually? How can something come back from infinity?

On its way to infinite density, matter and energy will get squeezed. That suggests increase in temperature during some intital phase in the making of the singularity? There are more and more atomic collisions, which would generate heat from friction?

At some moment, extremely low or zero atomic movement will then rapidly cool the singularity to smaller volumes? This is where some form of heat radiation would have to be emitted?

Add pressure and the atoms are bound into one big system. Add more pressure and the nuclei pop into quark soup. Add more pressure ..... then what?

My gut feeling is that something goes ballistic somewhere. But being that time is at zero, from our viewpoint, what could we observe?

Getting back to 'infinite' density. If such an infinity were to 'blow', would this not be a new 'Big Bang'? It would be a comparable bang but occurring within an existing spacetime and multiverse, which are not necessarily the medium which hosted our 'Big Bang'? It would blow when the singularity squeezed smaller than the 'string' scale of spacetime itself? The bang would nevertheless only be proportional to the size of the singularity's accumulated mass? But any mass would do strange things at ....infinite density? The thought of a huge black hole with trillions of solar masses in digestibles going to infinite density .... Ouch!

Cheers.

Saganist
 

Offline Saganist

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« Reply #6 on: 24/01/2008 23:48:04 »
Hi SoulSurfer,

Thanks for this very interesting reply. I was posting my reply to Doc when your reply appeared. Checking it out.

Cheers.

Saganist
 

Offline Dick1038

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« Reply #7 on: 25/01/2008 19:12:31 »
Perhaps matter is compressed down to its constituent "strings" at the center of the BH, perhaps on the order of the Planck length, 10‾43.
« Last Edit: 25/01/2008 19:15:14 by Dick1038 »
 

Offline syhprum

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« Reply #8 on: 25/01/2008 21:04:11 »
Electrons are regarded as point masses hence of infinite density.
 

Offline Saganist

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« Reply #9 on: 26/01/2008 02:11:52 »
Hi guys,

Very interesting ideas here.

A second event horizon is an interesting idea. It makes me think of some sort of quanta of gravity? As in: After a certain amount of gravity density, a new threshold is reached?

I read somewhere that if a collapsing star is rotating too quickly a black hole will not form?

Cheers.

PH

 

Offline Dick1038

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« Reply #10 on: 26/01/2008 18:18:48 »
I would venture to guess that analogously to how Newton's laws break down at relativistic speeds, General Relativity or even Quantum Theory may break down at extreme gravity.
 

Offline Soul Surfer

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« Reply #11 on: 26/01/2008 18:45:37 »
Theorists assume that this is what will happen but precisely how they will breakdown is not yet clear.

Please note when I referred to a second event horizon I was referring to what I think is likely to happen and not what appears in any textbook.  I was thinking about the process of this rotating material collapsing.  Let me say it again in another way as it collapses it rotates faster heats up, emits radiation and particles and its gravitational field increases now initially these photons and particles can just reach to the edge of the first event horizon before they fall back into the black hole.  However as the gravitational field increases they will not be able to get that far and will fall back earlier.  so from the inside the black hole will become "smaller".  I have put this in brackets because space time has become very funny and that may not be what it looks like from the inside.

Now this process of radiating energy and the particles falling back takes a significant length of time to happen ie the time that it takes the particles to leave the core perform their orbits and fall back to the surface. 

Now as the collapse continues the radiation will become more intense eventually sapping the energy in the core of the black hole enough to reach some sort if stable state as the energy in transit becomes a significant part of the energy in the object.  This will happen long before any sort of mathematical singularity is reached.  It should also be possible to model it using perfectly conventional relatavistic fluid dynamics modelling.  The sort of stuff people are using to model neutron stars and their collapse into black holes.

Is there anyone out there working on this?

Lets get the thinking this far before we go to the next phase of creating a new universe.
 

Offline DoctorBeaver

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« Reply #12 on: 26/01/2008 19:55:13 »

If only spacetime, free of any matter, remains around the event horizon, would this then be considered a naked singularity?

No. A naked singularity is a singularity without an event horizon. In the instance of a black hole having no more matter to consume, the event horizon would still be there.

Quote
I too dislike infinity outside of pure mathematics. 'Phut' is as good a term as I can imagine for the singularity's eventual demise. The interesting concept for me when trying to visualise such a process is that time has stopped from the event horizon down to the singularity.

My understanding thus far about black holes is that time ....ends or is at zero, within the Swarzchild radius?


Not quite. Time and space, in effect, swap within the event horizon - sort of, (ish). What was ahead of you in space (the singularity) becomes ahead of you in time. Your descent into it becomes inevitable. Interestingly, if you try to accelerate away from it, you hasten your descent into it.

Quote
If this is so, whatever the singularity is up to, that process is almost infinitely slow to observers outside of the radius. It could well be the biggest boom ever, but at such a slow rate of time, perhaps a 'phut', even, might be wishful thinking?

There can be no outside observers. No light or information can escape so there would be nothing to observe.

Quote
Whatever the final light show, some piece of spacetime and energy which had been excluded from the 4 dimensional universe while it remained inside the radius, will now try to reintegrate with the rest of spacetime?

If Hawking is correct, and black holes do indeed leak, then yes, particles from inside will re-integrate with "normal" spacetime.

Quote
The other aspect I like to ponder is that final process for the singularity. Its naked and its now beginning to decay.

It may get to ... infinity somewhere in the process of becoming infinitely dense, but it is theorised as then beginning to decay, eventually? How can something come back from infinity?

There's that word again - infinity. As I stated previously, I don't believe infinite density is ever reached. Ian (Soul Surfer) expressed it nicely - "It is only a getout by mathematicians and modellers who can't immediately think what might stop things contracting to a singularity."

Quote
On its way to infinite density, matter and energy will get squeezed. That suggests increase in temperature during some intital phase in the making of the singularity? There are more and more atomic collisions, which would generate heat from friction?

At some moment, extremely low or zero atomic movement will then rapidly cool the singularity to smaller volumes? This is where some form of heat radiation would have to be emitted?

That may well be the case; but the heat could not escape and so would not radiate away.

Quote
Add pressure and the atoms are bound into one big system. Add more pressure and the nuclei pop into quark soup. Add more pressure ..... then what?

If quarks are, as seems to be the case, fundamental, then they cannot be squeezed. Maybe applying enough pressure to quarks produces energy and nothing else.

Quote
My gut feeling is that something goes ballistic somewhere. But being that time is at zero, from our viewpoint, what could we observe?

Getting back to 'infinite' density. If such an infinity were to 'blow', would this not be a new 'Big Bang'? It would be a comparable bang but occurring within an existing spacetime and multiverse, which are not necessarily the medium which hosted our 'Big Bang'? It would blow when the singularity squeezed smaller than the 'string' scale of spacetime itself? The bang would nevertheless only be proportional to the size of the singularity's accumulated mass? But any mass would do strange things at ....infinite density? The thought of a huge black hole with trillions of solar masses in digestibles going to infinite density .... Ouch!

It has been theorised that the Big Bang may have been an exploding black hole. I have no opinion on that. Maybe our illustrious colleagues here can shed further light on it.
 

Offline Kuba

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« Reply #13 on: 26/01/2008 22:36:39 »
Hi to everyone,

Very interesting ideas one can find here. I have just recently became interested in BH so that I do not have much of the "insider" knowledge about them but hopefully imagination enough to learn and understand more. There is nothing else so inspiring as an exchange of ideas even if they sounds quite crazy. So, there is a question which is bothering me to which perhaps someone could cast some light. Generally what is exactly happening to the matter inside the BH? Is there a balance between compressing it and dissipating in a form of energy? I like the idea of matter density reaching at least in some BH a critical point (a certain mass/density "quantum" level) and exploding but not into our own universe but into another one maybe new, maybe adjacent one. Maybe the matter inside BH do not behave chaotically at all but becomes rather highly orgenised into some sort of "streams"/"waves". Furthermore perhaps we cannot exclude quantum effects on this basis as the highly orgenized matter under enormous pressure could   "regain"/"express" quantum characteristics. Finally deterministic behavior   is a result of collapse of the quantum characteristics as a consequence of the Interaction. The matter inside BH could perhaps under certain conditions  loose its primary deterministic nature. Ok, looking forward to your comments.

Best wishes,

Kuba
 

Offline DoctorBeaver

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« Reply #14 on: 26/01/2008 22:50:23 »
Welcome to TNS, Kuba.

No-one is entirely sure what happens inside a black hole so any answers to your questions must be conjecture or untried theories. I'm not sure what theories are currently de rigeur, but I dare say there are those among us here who can elucidate.
 

Offline Saganist

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« Reply #15 on: 27/01/2008 05:22:06 »
Hi all,

Great posts all!

If a black hole has an invisible event horizon its not naked?

If it somehow has no Swarzchild radius in 4D spacetime, then its naked?

Nature apparently abhors them.


I am just getting to string and brane theories in Dr Randall's book. Effects in more complex spacetimes and 'bulk' as covered in the Hawking book: The Universe in a Nutshell, will be discussed.

I've just learned about the Higgs mechanism and now I begin looking at theories with and without tachyons.

The collider I am keeping an eye on this weekend is the Rolex 24hr race from Daytona. The event website has included 3 track cams this year.

Cheers.

PH 
« Last Edit: 27/01/2008 05:25:07 by Saganist »
 

Offline JimBob

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« Reply #16 on: 27/01/2008 07:57:36 »
The event horizon is merely an area of space and is not, in itself, visible. What we see is the emission of radiation from particles as they cross the horizon.

If there is radiation escaping from a black hole then how can it be a black hole?
 

Offline DoctorBeaver

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« Reply #17 on: 27/01/2008 08:50:56 »
Quote
The collider I am keeping an eye on this weekend is the Rolex 24hr race from Daytona. The event website has included 3 track cams this year.

 ;D
 

Offline DoctorBeaver

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« Reply #18 on: 27/01/2008 08:53:14 »
The event horizon is merely an area of space and is not, in itself, visible. What we see is the emission of radiation from particles as they cross the horizon.

If there is radiation escaping from a black hole then how can it be a black hole?


Ask him --->  [O8)]
 

Offline Soul Surfer

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« Reply #19 on: 27/01/2008 10:08:56 »
Ever since Stephen Hawking pointed out that black holes can emit radiation most people have thought that they die away in a long but comparable time with the current age of the universe  this is most definitely not the case  for example  about the smallest possible black hole a supernova could create is about the mass of the sun.  This has a radius of about 1 mile and a temperature lower than a millionth of a degree above absolute zero. Far cooler than the microwave background so it's gaining energy from the microwave background and will not start to evaporate until the universe gets below its own temperature which is millions of times the current age of the universe. Even then it would take more than two x 10^58 years to evaporate an almost unimaginably long time.

you can see all this from

http://xaonon.dyndns.org/hawking/

« Last Edit: 27/01/2008 10:11:21 by Soul Surfer »
 

Offline syhprum

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« Reply #20 on: 27/01/2008 10:26:43 »
Although 'recently' generated black holes due to stellar collapse will have a very long life it has been suggested that a source of anti matter at the galactic centre may be due to 10^13 Kg black holes created at the time of the big bang decaying.
Also it has been suggested that tiny black holes may be created by the LHC with a life of 10^-24 seconds
 

Offline Soul Surfer

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« Reply #21 on: 27/01/2008 18:54:24 »
I agree that tiny black holes generated during the early stages of the big bang could be "exploding" now the explosions are rather small on an interstellar scale and would be remarkably difficult to detect.
 

Offline DoctorBeaver

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« Reply #22 on: 27/01/2008 22:39:25 »
Would they really explode? Surely, if the mass is gradually leaking away they would just fizzle out.

Or could it be that the leakage of particles would eventually cause the black hole to go sub-critical (i.e. not enough matter left inside for it to be a black hole any longer) and all the stuff remaining inside it suddenly reappear?
 

Offline Kuba

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« Reply #23 on: 27/01/2008 23:42:37 »
Hi again

thanks for the welcome. Yes I agree that the questions about what could really be happening inside the BH is much of a theory only (if not a bit of fantasy even) however it seems very attractive to me to follow this path as it may result in some new ideas. Imagination is powerful enough to - how knows - maybe even create things. The point is:
1. BH absorb matter in a certain way
2. They release some energy and I understand that the energy given away ~ ("returned") to the normal (our) space is a sort of equal to the matter taken up otherwise the BH would appear unstable and either collapse or explode.
We dont see, know of BH exploding but do we know of BH disappearing (in other words collapsing) and furthermore what would appear collapsing in our universe could demonstrate itself as explosion in parallel one (for example).
We can imagine inside BH the matter gets squeezed so much that it must (almost by intuition) loose its "normal space" properties (on the basis there is no output of either matter or equivalent energy into another, undetectable in our universe space). If so then what other properties this not-energy and not-matter state would posses. would it present  features ascribed to both energy and matter in the same time. Is it thinkable that it could be neither matter not energy but a cold, motionless state of suspension. Cold because no energy would be transfered or exchanged (beyond the one given up to our space  as a result of some sort of excess) and motionless because the movement would loose its sense as the result could appear in the same time or even before the reason. And time would simply ceased to exist. In other words all things/actions (if possible at all) would happen at once, even before they were invented or initialized. Sorry I have to break in this moment as something unexpected came across. Will try to continue later. Please feel free to give any coments as it is very interesting and important to discuss and think of all possibilities.

Best

Kuba     




     
 

Offline Saganist

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« Reply #24 on: 29/01/2008 05:52:50 »
Hi guys,

I am getting very nice info on black holes from the Cambridge Relativity site newbielink:http://www.damtp.cam.ac.uk/user/gr/public/ [nonactive]

I am seeing a lot of familiar terms: Lorenzians, Hamiltonians, Euclidians, AdS, and MTheory.

I also see cosmic strings and textures discussed.

There's a little character named an instanton that is most ... naked!

Pretty cool stuff.

I get interesting google search results for black hole websites.

The Daytona 24hr race was most pleasant to keep up with, via the wensite's cams and live timing. newbielink:http://www.grandamerican.com/ [nonactive]

Cheers.

Saganist
 

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