# Thread: Gravity of a Black Hole + other short stories

1. ## Gravity of a Black Hole + other short stories

OK today I was driving into work and I started thinking about the speed of gravity. I think at one point I was sucked into the thinking that Gravity is not limited by the speed of light.
The first argument was: That the gravitational waves from the sun are not perpendicular to the light waves of the sun. I think I got past this argument by realizing that the gravitational wave merely sets the landscape and that the earth follows that landscape. This would seem to be proof against a gravitron that travels at the speed of light. In any case I digress ...

The next argument against gravity traveling at the speed of light is the black hole. How could a black hole have a gravitational field if gravity was limited by the speed of light. OK again learning from my previous thoughts. I thought of the landscape that was created by a massive star and realized that the total mass stays roughly the same just becomes much more ( infinitely more ??? ) dense. So the landscape already exists even before the black hole becomes a black hole. But I am confused here.

Firstly, as matter/energy/mass is sucked into the black hole then the propagation of gravity of that extra matter should not be able to escape once it has passed the event horizon.

Secondly, at the exact moment that a star becomes a black hole ( the point where escaping light can no longer escape ) what happens to the landscape around the star? At first thought I would think that almost nothing happens as the mass still exists and the calculations could be based from the center of the gravitational mass. And that really there is little difference in all of that as the size of the star from the last point where light can escape to the size of the star at the first point where light cant escape is not that much different. But IF gravity is limited to the speed of light AND the mass/energy that is causing the gravitational waves is located at a point in space-time that is curved inwards ... wouldnt at the point that a star becomes a black hole wouldnt it loose its gravitational field?

Could all of a BHs mass live on the event horizon and not in its singularity?

2. Originally Posted by tommac
Could all of a BHs mass live on the event horizon and not in its singularity?
As I understand it, we do not yet have a test that can distinguish between the two situations with current data about known black holes.

3. Originally Posted by antoniseb
As I understand it, we do not yet have a test that can distinguish between the two situations with current data about known black holes.
OK then ... what if we did ...

What would be the conclusion in both cases?

4. Are you, basically, asking if the gravity effect (or that which causes it) can escape itself at its own escape velocity? There seems to be some circularity in there, to my (on these topics) simple mind anyway.

5. Established Member
Join Date
Nov 2005
Posts
2,961
The following is my non-expert opinion. For a supermassive blackhole the matter in the accreation disk orbits at perhaps 1/2 c, just outside the event horizon. The gravity is strong, but the tidel forces are not distructive to humans or a sturdy space craft in free fall. I don't think anything happens when you cross into the event horizon, except escape from the black is not possible. You can continue to orbit the singularity long term, unless you collide with something, or lots of tiny somethings. Even if you are falling toward the black hole, you will (all but surely) miss the singularity (which has approximately zero diameter. If you miss, your spagettified remains do a sling shot manuver which carries you back almost to the event horizon. Other matter and plazma should do likewise, so I would expect several percent of the total mass to be distributed inside the event horizon volume, perhaps mostly just inside the event horizon.
The average density just outside the singularity may be extremely high, denying access to the singularity to newly arriving matter/plasma.
Mainstream opinion is gravity travels at the speed of light. There can be no outside observer. Inside the event horizon observers will typicaly observe events at less than relativistic speeds except close to the singularity. Neil
Last edited by neilzero; 2008-Jun-24 at 07:23 PM.

6. Originally Posted by tommac
OK then ... what if we did ...

What would be the conclusion in both cases?
What if the mass gets spread out on whatever event horizon it is formed on, and as the black hole acquires mass, and the event horizon expands, new shells form?

Who knows? If you can devise, and pay for a test, you might be able to make a big name for yourself. This is unknown territory, and we are interested in studying black holes because of this.

7. Established Member
Join Date
Nov 2004
Posts
887
If we allow matter to stick to the EH, what is its thickness? Does our model allow for infinite mass on the EH? How does the model explain this?

8. Originally Posted by neilzero
You can continue to orbit the singularity long term, unless you collide with something, or lots of tiny somethings.
I donīt think you could orbit the singularity once you cross the EH. It will be in your 'future', necessarily.

9. Established Member
Join Date
Jun 2006
Posts
2,440

## Oh, How I Wish I Was in a Wikiworld . . .

Some problems with the original question:

1. Gravitons are theoretical particles. They may not exist. They have not been detected directly and probably never will be. Among other things, a detector with enough shielding to screen out neutrinos from gravitons would be so massive it would itself collapse into a black hole. Gravitational waves imply a large number of coherent gravitons, but they haven't been detected, either. Gravitons are postulated in an attempt to apply highly successful quantum principles to gravity. So far it hasn't worked very well, unresolvable infinities intrude in the calculations, although the string people have made some progress.

2. The BH solutions are GR. Ignore the gravitons, GR/SR describe a smooth universe without quantum particles. Einstein's argument against BH's was that particles at the event horizon would have to be accelerated to c. Schwarzchild's et al. response was that the particles fall through the event horizon. Since we observe objects (SMBH) that cannot reasonably be anything else, BH's probably exist as postulated.

Try the WIKI articles on Gravitons and Speed of Gravity, with the usual caveats about WIKI.

10. Established Member
Join Date
Apr 2008
Posts
356

## Suggest a good review paper

Originally Posted by antoniseb
As I understand it, we do not yet have a test that can distinguish between the two situations with current data about known black holes.
Originally Posted by alainprice
If we allow matter to stick to the EH, what is its thickness? Does our model allow for infinite mass on the EH? How does the model explain this?
Ditto John Mendenhall, plus this:

It seems worthwhile to point out that the suggestion (by someone in my Ignore list) that all the mass of a black hole could live on the horizon (according to line antoniseb quoted) is utterly incompatible with gtr, our current gold standard theory of gravitation.

Here is a review of evidence bearing on the question of whether or not gtr is terribly wrong about event horizons:

Ramesh Narayan
Evidence for the Black Hole Event Horizon
http://arxiv.org/abs/astro-ph/0310692

neilzero, would you like me to try to comment on how what you wrote compares to current mainstream opinion in the research literature? (Not sure how seriously you intended us to take your post.) Don't worry, it looks mostly OK, although there are a few things you may have gotten badly wrong.

11. ignore list ... geeze ... how fifth grade.

Anyway ... That is not what I was saying. In fact that is what I was was questioning. I dont see how to get around the fact that gravity needs to travel faster than light to escape a the EH ( from the inside ).

Either one of two things need to be true:
1) gravity can travel faster than the speed of light ( light doesnt travel fast enough to escape )
2) the mass either resides or is somehow represented by the either the EH or external to the EH.

Now does time stop at the EH? Sorry ... but if TS is curved to the point where TS is so curved that light cant escape ... what would be the reference of time relative to us at the EH?

Originally Posted by Chris Hillman

Ditto John Mendenhall, plus this:

It seems worthwhile to point out that the suggestion (by someone in my Ignore list) that all the mass of a black hole could live on the horizon (according to line antoniseb quoted) is utterly incompatible with gtr, our current gold standard theory of gravitation.

Here is a review of evidence bearing on the question of whether or not gtr is terribly wrong about event horizons:

Ramesh Narayan
Evidence for the Black Hole Event Horizon
http://arxiv.org/abs/astro-ph/0310692

neilzero, would you like me to try to comment on how what you wrote compares to current mainstream opinion in the research literature? (Not sure how seriously you intended us to take your post.) Don't worry, it looks mostly OK, although there are a few things you may have gotten badly wrong.

12. Established Member
Join Date
Jun 2006
Posts
2,440

## Stuck

Originally Posted by tommac
ignore list ... geeze ... how fifth grade.

Anyway ... That is not what I was saying. In fact that is what I was was questioning. I dont see how to get around the fact that gravity needs to travel faster than light to escape a the EH ( from the inside ).

Either one of two things need to be true:
1) gravity can travel faster than the speed of light ( light doesnt travel fast enough to escape )
2) the mass either resides or is somehow represented by the either the EH or external to the EH.

Now does time stop at the EH? Sorry ... but if TS is curved to the point where TS is so curved that light cant escape ... what would be the reference of time relative to us at the EH?
Gravity (in this case, gravitation, as a picky poster on another thread pointed out), doesn't travel. The warps in spacetime don't go away, they are simply there. For the external observer, the BH has three characterisitics: its mass, its angular momentum, and its charge. For all practical purposes, the charge is zero, since any charge will quickly attract enough particles of the opposite sign to go neutral. According to Hawking, a BH also has some non-zero temperature.

As long as the BH and the surrounding objects are in free fall, or uniformally accelerated (in orbit), the influence of gravitation between them will appear to be instantaneous. It's not, really, but that's the way it appears under GR, and a good thing, too, else we would slowly spiral into the sun.

What happens to the junk falling in at the event horizon? Who knows, we haven't seen one up close. Historically, there has been a lot of discussion about 'frozen' surfaces. Speculating, I bet it falls through in real time, since we don't see a bright visible light glow from the SMBH at the center of the galaxy. But it's just speculating.

13. Can I speculate? ;-)

Could all mass get pancaked into the event horizon? If time in the relative to us stops at the EH of a black hole then stuff can never really enter past that point as there is no time. Space also contracts ...

Take this along with the idea that things that fall behind the event horizon can not send out a gravitational wave. Then it only makes sense for all of the new mass to get stuck on the EH in a very compacted way. In time relative to itself ... it falls into the hole ... but what it perceives as miliseconds is really an eternity relative to us.

Hmmm ... wait ... I still have a problem with the initial mass of the black hole ... and the initial gravitational waves that is pulling in this matter ... please help.

14. Established Member
Join Date
Nov 2005
Posts
2,961
You guys are hard enough to follow without looking at research papers. My thoughts come from thinking. Has anyone, besides me analyzed sling shot manuvers around the singularity nor how a quark can get inside a singularity that is smaller than a quark? How about a trillion quarks per second trying to enter a quark size singularity = major traffic jam? Most black hole hypothesis is relative to a very distant observer. Has anyone other than fiction writers and I analyzed local observers? My guess is frozen surfaces don't happen for local observers. My guess is time stops for very distant observers, but not for local observers. I can handle your debunking of my ideas. I even suspect information can be sent outside the event horizon if you relay frequently = lots of repeaters or many stage rockets. Clearly present chemical rockets won't get far outside the event horizon even with 1000 stages.
Suggesting that all... raises a flag for me. Most of the mass perhaps? Neil

15. OK let me take a shot.

Locally ... time flows like normal relative to you. The singularity seems far in the distance. No matter which way you steer your spacecraft you point towards the singularity ... no slingshot manuvering right towards it. You see everything pulling away from you in all directions you see expansion. The only thing that stays a constant distance away from you is the singularity.

Originally Posted by neilzero
You guys are hard enough to follow without looking at research papers. My thoughts come from thinking. Has anyone, besides me analyzed sling shot manuvers around the singularity nor how a quark can get inside a singularity that is smaller than a quark? How about a trillion quarks per second trying to enter a quark size singularity = major traffic jam? Most black hole hypothesis is relative to a very distant observer. Has anyone other than fiction writers and I analyzed local observers? My guess is frozen surfaces don't happen for local observers. My guess is time stops for very distant observers, but not for local observers. I can handle your debunking of my ideas. I even suspect information can be sent outside the event horizon if you relay frequently = lots of repeaters or many stage rockets. Clearly present chemical rockets won't get far outside the event horizon even with 1000 stages.
Suggesting that all... raises a flag for me. Most of the mass perhaps? Neil

16. Established Member
Join Date
Nov 2004
Posts
887
Originally Posted by neilzero
You guys are hard enough to follow without looking at research papers. My thoughts come from thinking. Has anyone, besides me analyzed sling shot manuvers around the singularity nor how a quark can get inside a singularity that is smaller than a quark? How about a trillion quarks per second trying to enter a quark size singularity = major traffic jam? Most black hole hypothesis is relative to a very distant observer. Has anyone other than fiction writers and I analyzed local observers? My guess is frozen surfaces don't happen for local observers. My guess is time stops for very distant observers, but not for local observers. I can handle your debunking of my ideas. I even suspect information can be sent outside the event horizon if you relay frequently = lots of repeaters or many stage rockets. Clearly present chemical rockets won't get far outside the event horizon even with 1000 stages.
Suggesting that all... raises a flag for me. Most of the mass perhaps? Neil
I agree, except for the repeaters.

17. Was my question in #4 of the category "not even wrong"? I guess I'm asking why gravity would need to move faster than light. The only reason that light can't escape is gravity, right? It is not c that causes light being unable to escape, but the shear amount of gravity, and fotons just being unable to move fast enough to overcome it. I don't understand how that would apply to gravity itself. What am I missing?

18. Established Member
Join Date
Apr 2008
Posts
356

## Some corrections and suggetions

Neil, you still haven't clarified how seriously you intend us to take your questions. The default assumption (agreed?) is that those who post questions in Q&A at BAUT seek serious answers from those who have considerable knowledge about current mainstream physics/astro/cosmology.

Originally Posted by neilzero
I can handle your debunking of my ideas.
Good, because some of what you said is either incorrect or DMS (doesn't make sense).

Originally Posted by neilzero
You guys are hard enough to follow without looking at research papers. My thoughts come from thinking. Has anyone, besides me analyzed sling shot manuvers around the singularity

When you mention "singularity" or "curved spacetime", by default we naturally assume the theory you have in mind is gtr. But gtr is a fairly subtle theory which requires considerable mathematical background to master. So my question is: how can you claim to "analyze sling shot manuevers" without having first mastered gtr? If you are not using gtr, then what theory are you using and why?

I have the sense that you actually resent being pointed at survey papers on the arXiv! I find that very hard to understand. Please note that survey papers are not research papers; they are guides for serious students to the most important recent research papers, written by an expert in the field.

Would help if I explicitly stated something which upon reflection I can see might not have been obvious to you? Namely: my intent is that if anyone doesn't understand anything he/she reads in a review paper I cite, he/she will ask a followup question, which I would try to answer.

FYI, at this point I am still assuming that you are indeed asking about the current State of the Art concerning infall into a black hole, in which case you presumably want feedback from those who know about that. Please let me know ASAP if that is not the case. TIA!

With that said:

Originally Posted by neilzero
nor how a quark can get inside a singularity that is smaller than a quark?
This doesn't make sense in gtr for at least two reasons:
• gtr is a relativistic classical field theory, but quark is a concept from relativistic quantum field theory.
• in gtr, curvature singularities are not something you can "get inside".

Most likely you don't yet realize that in gtr the event horizon of a black hole is not a curvature singularity. When you represent the exterior region of your hole in the standard Schwarzschild coordinate chart, the event horizon is a coordinate singularity, analogous to the North Pole in standard polar spherical chart on the ordinary sphere. In various other charts, the event horizon does not appear as a coordinate singularity (but it still has a global significance).

BTW, the curvature singularity lurking in the future interior region of the Schwarzschild vacuum turns out to be quite different from that lurking inside the Kerr vacuum (with nonzero angular momentum), but in no case is it appropriate to think of it as "pointlike". Like everything else in gtr, this gets tricky, because small perturbations of the Kerr vacuum suggest that the picture we infer from the Schwarzschild vacuum is actually closer to the generic situation. So as someone already mentioned, you should think of it as located at a finite "distance" in the future of an infalling observer, not as being a pointlike object (or a ring).

Going back to infall, FWIW (which should be something), I have analyzed infall into a black hole (Schwarzschild and Kerr vacuums, Vaidya null dust, Reissner-Nordstrom and Kerr-Newman electrovacuum) as treated in gtr, although, for reasons hinted at above, "slingshot wrt the singularity" doesn't really make sense in gtr.

Originally Posted by neilzero
My guess is frozen surfaces don't happen for local observers. My guess is time stops for very distant observers, but not for local observers.
Right, although the notionf of a "frozen surface" (of a star undergoing gravitational collapse) is misleading since the amplitude decays exponentially (so the star, as seen by distant static observers, appears to simply "redden and wink out").

Originally Posted by neilzero
I even suspect information can be sent outside the event horizon if you relay frequently = lots of repeaters or many stage rockets. Clearly present chemical rockets won't get far outside the event horizon even with 1000 stages.
That's not true, according to gtr.

As it happens, I have analyzed influx and outflux in various black hole models commonly used in gtr in great detail, most recently in posts to Physics Forum. To follow the discussion, you need to know about coordinate charts, vector fields (treated as first order linear differential operators on our manifold), covector fields, and frame fields. See
http://en.wikipedia.org/wiki/User:Hillman/Archive
for links to some Wikipedia articles (in the last versions I helped edit and am willing to vouch for).

Eric Poisson,
A Relativist's Toolkit
University of Cambridge Press.

Originally Posted by neilzero
Most black hole hypothesis is relative to a very distant observer. Has anyone other than fiction writers and I analyzed local observers?
"Hypothesis"? Didn't you mean "analysis of signals from an infalling observer in a black hole model"?

Painleve 1921 and Eddington 1922 found charts suitable for this purpose but may not have appreciated their discovery. Historians agree that the first researchers to understand the global geometry of the Schwarzschild vacuum solution wer probably (independently) Synge, Kruskal, and Szekeres (about 1960). I can tell you a lot more about all this, but for now let me suggest another link which I think you will find very helpful:
(from Andrew Hamilton, Astrophysical and Planetary Sciences, University of Colorado).

Originally Posted by slang
I guess I'm asking why gravity would need to move faster than light.
Did someone tell you that? They were wrong. See
http://www.math.ucr.edu/home/baez/ph...k_gravity.html
The only reason that light can't escape is gravity, right? It is not c that causes light being unable to escape, but the shear amount of gravity, and fotons just being unable to move fast enough to overcome it. I don't understand how that would apply to gravity itself. What am I missing?[/QUOTE]

19. Originally Posted by Chris Hillman
Did someone tell you that? They were wrong.
No, it seemed to me that what was what the OP was asking, or rather, that his question was so tough to answer because it was the wrong question to ask. But since I cannot even wrap my mind around relativity I'll assume that it is me misunderstanding things, not OP. Thanks for the link!

20. More thoughts from the drive into work this AM.

1) At the moment that the black hole becomes a black hole rather than a dying star ... whatever the state of the gravitational landscape that it has created is what it will be for the life of the black hole ( until it starts consuming which I will discuss later ) Once the star collapses to the point that it becomes a black hole ( I think once its size is smaller than event horizon that it leaves behind ) no gravitational waves can leave the localized space-time inside the event horizon.

2) For matter entering into the black hole the same is true. Whatever its effect on the landscape is at the point it passes the event horizon is how it will permanently effect the gravitational landscape.

3) A BHs mass is equal to the sum of all of the mass that passed through the event horizon at any point in history.

21. More thoughts on this.

There is definitely no slingshot. Once you are past the event horizon your only future is the singularity.

The singularity may not be a point as you state.

Someone smarter than me can probably do the math to show the distance from a black hole that you would need to be to have the same gravity as a jump on the surface of the earth.

So think about being on the surface of the earth you jump ... would you ever slingshot around the other side of the earth. No ... you will hit the earth.

Lets take the sun ... ( assume that your space ship can take the heat of the sun ) If your spaceship was near the surface of the sun it would take quite a lot of thrust to leave the surface to get back to some sort of orbit.

There is no slingshot ... just a certain future that ends exactly at the singularity.

Now if you have a ring singularity ... you may be able slip though it ... I think that is a wormhole.

Originally Posted by tommac
OK let me take a shot.

Locally ... time flows like normal relative to you. The singularity seems far in the distance. No matter which way you steer your spacecraft you point towards the singularity ... no slingshot manuvering right towards it. You see everything pulling away from you in all directions you see expansion. The only thing that stays a constant distance away from you is the singularity.

22. Established Member
Join Date
Jun 2006
Posts
2,440

## This is a Point

Originally Posted by Chris Hillman

This doesn't make sense in gtr for at least two reasons:
• gtr is a relativistic classical field theory, but quark is a concept from relativistic quantum field theory.

And an important one. Many Q&A threads result from questions attempting to combine classical and quantum principles. It doesn't work. Getting the two to work together is one of the big challenges of the 21st century. If we're lucky, we'll live long enough to see it.

23. Established Member
Join Date
Nov 2005
Posts
2,961
Hi Chris: Your thoughtful explanations are appreciated. I'm sure I learned a little and I shall watch for survey papers which I may understand better. My interests are broad, but do not include learning math much beyond calculus.
I did not realize the term singularity was exclusive to gtr. I should perhaps type "the center region of the event horizon sphere".
Does "Schwartzchild vacuum" mean we assume negligible matter in the accreation disk and just inside the event horizon. This thread seems to imply that vacuum may not be a reasonable assumption? I shall consult Kerr vacuum in wikipedia and some other words you mentioned.
Shouldn't we expect lots of angular momentum inside the event horizon, from an engineering rather than theoretical viewpoint? Sorry I tend to think like an engineer rather than a scientist. Neil

24. Established Member
Join Date
Nov 2004
Posts
887
Singularity is not exclusive to any theory. A singularity is any situation where the solution cannot be determined. This is usually caused by infinites showing up preventing a meaningful answer.

25. Established Member
Join Date
Jun 2006
Posts
2,440

## Quantum Black Holes (Not).

Originally Posted by tommac

Once the star collapses to the point that it becomes a black hole ( I think once its size is smaller than event horizon that it leaves behind ) no gravitational waves can leave the localized space-time inside the event horizon.
Too many ifs, plus once again the presently impossible to reconcile nature of (undeteced) quantum gravitons with GR field theory. If there are gravitons, and if gravity waves are coherent graviton behavior, then there might be a problem, since gravity waves escaping from inside the event horizon sounds like a pretty good trick. But there are better GR and QM people than us on this forum. Anyone?

26. Originally Posted by John Mendenhall
Too many ifs, plus once again the presently impossible to reconcile nature of (undeteced) quantum gravitons with GR field theory. If there are gravitons, and if gravity waves are coherent graviton behavior, then there might be a problem, since gravity waves escaping from inside the event horizon sounds like a pretty good trick. But there are better GR and QM people than us on this forum. Anyone?
Even in GR ... gravity needs to be emmited at the speed of light. If space-time itself is so warped that light cant escape then IF gravity travels at the speed of light ... it too would not be able to escape from inside the event horizon. So this is not purely a gravitron question ...

But for gravitrons I think you have a larger issue explaining the non-parrallel-ness of gravity and light emmitted from the sun on the earth.

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•