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creativedreams
2008-Dec-05, 12:07 PM
Thinking maybe dark matter is..."the ashes of burned atoms"...so to speak?
Possibly caused by the crushing of molecules and atoms entering black holes and being shot out; maybe the ashes from the burning stars too?
Which brings to mind maybe black holes are just a type of nuclear fire (reaction) of which its energy is creating gravity?
Which brings to mind maybe it's energy (both kinetic and potential) that creates gravity and not mass?
Which brings to mind maybe gravity can be classified to several forms like electro-magnetic, magnetic, centripical, etc.?
Which brings to mind maybe there is both positive and negative gravity in the form of elecro-magnetics etc.?

Hornblower
2008-Dec-05, 01:02 PM
Thinking maybe dark matter is..."the ashes of burned atoms"...so to speak?
Possibly caused by the crushing of molecules and atoms entering black holes and being shot out; maybe the ashes from the burning stars too?
Which brings to mind maybe black holes are just a type of nuclear fire (reaction) of which its energy is creating gravity?
Which brings to mind maybe it's energy (both kinetic and potential) that creates gravity and not mass?
Which brings to mind maybe gravity can be classified to several forms like electro-magnetic, magnetic, centripical, etc.?
Which brings to mind maybe there is both positive and negative gravity in the form of elecro-magnetics etc.?
Which brings to mind that you appear to be displaying a vivid imagination, but not doing any rigorous testing of your ideas.

Neverfly
2008-Dec-05, 02:53 PM
Weren't you already answered in another thread that stars don't burn material and that Dark Matter is not ashes, already?

thorkil2
2008-Dec-05, 05:14 PM
You have made black holes out to be of substance. They are not. They consist mostly of empty space (highly distorted space-time, really), with a singularity in the middle of it all. The boundaries of the black hole are simply intervals from the singularity. The Event Horizon is the virtual surface that marks the distance at which escape velocity falls below c, so nothing gets "shot out." As for the rest, you'll need to go back to your Physics I text for some definitions, as well as a look at asymmetry and General Relativity to see why gravity can't be classified into the types you suggest and perhaps a hint as to why it isn't +/-. Speculation is great, but it needs to be founded on a sound understanding of physical principles. Einstein speculated and changed the world, but he started those speculations as a trained Physicist.

Cougar
2008-Dec-05, 06:17 PM
You have made black holes out to be of substance. They are not. They consist mostly of empty space...

I doubt this. Could you reference any publication that describes or represents black holes in such a manner?

grant hutchison
2008-Dec-05, 07:12 PM
You have made black holes out to be of substance. They are not. They consist mostly of empty space ...I doubt this. Could you reference any publication that describes or represents black holes in such a manner?it seems pretty standard. Here's Kip Thorne:
If the hole were a solid body squeezed into such a small circumference, its average density would be 200 million tons per cubic centimeter ... But the hole is not a solid body. General Relativity insists that the 10 solar masses of stellar matter, which created the hole by imploding long ago, are now concentrated at the hole's very center--concentrated into a minuscule region of space called a singularity. That singularity, roughly 10-33 centimeter in size (a hundred billion billion times smaller than an atomic nucleus), should be surrounded by pure emptiness, aside from the tenuous interstellar gas that is falling inward now and the radiation the gas emits. There should be near emptiness from the singularity out to the horizon ...

(Black Holes and Time Warps: Einstein's Outrageous Legacy)Grant Hutchison

pzkpfw
2008-Dec-05, 08:12 PM
Isn't this (side question) about whether the Black Hole is it's event horizon or is the compressed stuff right at the centre?


(I've always pictured the whole from event horizon on down as "the" black hole (like Kip Thorne?) but I've gathered recently that not everyone sees it that way.)

astromark
2008-Dec-05, 08:32 PM
I see this explanation of a black hole taking over this odd thread... The OP 'creativedreams' would seem to be inviting us to educate him when he could do it himself... But this is a questions and answers page and it is one., and now a discussion regarding the state of mater in a black hole. All I have to add here is this. The density of mater in the BH is such that the state changes. Density is extreme. It could not be correctly described as empty. The very existence of a singularity has not actually been proven at all. It would seem to be the obvious conclusion however. I do not see empty space as any part of a BH description.

grant hutchison
2008-Dec-05, 08:48 PM
Isn't this (side question) about whether the Black Hole is it's event horizon or is the compressed stuff right at the centre?


(I've always pictured the whole from event horizon on down as "the" black hole (like Kip Thorne?) but I've gathered recently that not everyone sees it that way.)The "region of space from which light cannot escape" definition seems to be pretty standard. I guess the last word should go to John Wheeler, who coined the name "black hole" and should therefore know what he was talking about when he used the expression. His textbook (with Edwin Taylor) Exploring Black Holes: Introduction to General Relativity has a chapter entitled "Inside Black Holes", in which they discuss trajectories thorugh the (empty) space below the event horizon.

Grant Hutchison

kleindoofy
2008-Dec-05, 08:51 PM
Thinking ...
Really?


... maybe dark matter is ... ashes ...
Well, for one I can see, feel, measure, and interact with ashes ..

Ken G
2008-Dec-05, 09:08 PM
it seems pretty standard. Here's Kip Thorne:
Obviously Kip Thorne knows more formal general relativity in his pinky than I do in my brain, but his word choice exposes some weaknesses in regard to both the philosophy and history of science when he states that "General Relativity insists [my bold] that the 10 solar masses of stellar matter, which created the hole by imploding long ago, are now concentrated at the hole's very center...". I would point out that it is very backwards logic to claim that physical theories "insist" things in regard to fundamentally untested claims, it's just plain philosophically and historically unsound (examples abound, of course). In fact what general relativity is, like any scientific theory, is a mathematical construct that connects observations in a simplified and effective way. As such, it is no more than a proxy for the observations that gave it its justification.

That fact raises a rather sticky issue when one addresses the issue of what is fundamentally unobservable. We got knocked on our keesters in quantum mechanics when we made claims about things we thought we could know even though they were unobservable, and I'm not sure that Kip Thorne is not simply making the same mistake again when he asserts that theories "insist" things about unobservable regions. The theory simply has never had to confront any data from that region, and probably never will unless we have a volunteer to fly into a black hole (and at least know the truth themself). What can we say about a theory in regard to a class of data it has never confronted? That's one of the most difficult issues we face in trying to understand what science really is-- historical examples where extrapolations worked great, or failed utterly, abound.

As such, I would have felt more comfortable with the scientific validity of Thorne's remark had he simply said "general relativity, a vastly successful theory, predicts such and such, so in the absence of anything better to go on, this is the picture that will do." Why do we forever seem to need to pretend we know more about the universe than we really do? All the so-called "scientific revolutions" that stem from that simple mistake get kind of wearying.

Klaus
2008-Dec-05, 10:45 PM
Roger Penrose's Singularity Theorem proves that a singularity exists in a black hole.

EDIT: I think the problem stems from more or less the language used. Of course, black holes are still theoretical but I'm sure most physicists and astrophysicists are 100% certain they exist. And same with Dr. Penrose's revelation, which was proved mathematically using topology.

grant hutchison
2008-Dec-05, 10:58 PM
As such, I would have felt more comfortable with the scientific validity of Thorne's remark had he simply said "general relativity, a vastly successful theory, predicts such and such, so in the absence of anything better to go on, this is the picture that will do." Why do we forever seem to need to pretend we know more about the universe than we really do? All the so-called "scientific revolutions" that stem from that simple mistake get kind of wearying.I feel you're misjudging him. Thorne's usually pretty careful, and I think he was pretty careful in my quotation.
I'm comfortable with the idea that arithmetic insists that 2+2=4. That's it's job. General relativity's insistence on a singularity is much the same thing: that's what falls out of the sums. Notice that Thorne then uses the word "should" twice, in a conditional sense, in what follows: the singularity "should" be surrounded by emptiness.

I actually admired the balanced juxtaposition as I copied out the words: general relativity "insists" on a singularity; so there "should" be emptiness within the event horizon. His message seems as clear as day to me: we have the maths, we don't have the observations.

Grant Hutchison

creativedreams
2008-Dec-05, 11:00 PM
sorry but i'm going to have a hard time ever believing all that matter is just compressing and staying in a singularity. My intuition tells me it is being burned (so to speak for basic understanding) in a type of reaction we have no comprehension of yet into a type of ashes (so to speak for basic understanding) so small and undetectible that we have no comprehension yet. Just my theory.

seanhogge
2008-Dec-05, 11:06 PM
Bertrand Russell has a few good theories, too.

Neverfly
2008-Dec-06, 12:29 AM
sorry but i'm going to have a hard time ever believing all that matter is just compressing and staying in a singularity. My intuition tells me it is being burned (so to speak for basic understanding) in a type of reaction we have no comprehension of yet into a type of ashes (so to speak for basic understanding) so small and undetectible that we have no comprehension yet. Just my theory.

Ok so.. You ignored the provided answer in the other thread...
Started a new thread...

Then claimed your "intuition" is superior?

"Gentleman, don't confuse me with facts. My mind's made up."

Ok so - The best course for posters here is just to not bother answering your questions then right?

Your Hypothesis ("Theory") could find a home in ATM forum...

creativedreams
2008-Dec-06, 12:52 AM
sorry I changed the question to this thread because i felt it was the place it should have been. You guys have some good facts I wasn't aware of and apoligize for seeming ignorant and answering myself with unsupported speculation.I really like reading all your guys posts as I am fascinated by all the unknowns of the universe. I really like your input and it is helping me see things from inside the box.

Tim Thompson
2008-Dec-06, 01:14 AM
sorry but i'm going to have a hard time ever believing all that matter is just compressing and staying in a singularity.
You and everyone else. But not to worry. Since "compressing and staying in a singularity" in fact has nothing at all to do with the standard theory of black holes, there is nothing to have a "hard time" with yet.


My intuition tells me it is being burned (so to speak for basic understanding) in a type of reaction we have no comprehension of yet into a type of ashes (so to speak for basic understanding) so small and undetectible that we have no comprehension yet. Just my theory.
Perhaps not out of the realm of possible, assuming we wish to avoid the arrogance of assuming we know it all. But neither theory nor observation, conducted by people of outstanding ability, is consistent with your intuition. Personally, I choose their discoveries over your intuition every time, and likewise over my own intuition most of the time.

The standard theory of black holes is simple. There is an event horizon, which we can measure & detect (i.e., Remillard, et al., 2006 (http://adsabs.harvard.edu/abs/2006ApJ...646..407R)). That's it for observation. We cannot, even in principle, observe, detect or measure any other of the various ingredients expected by theory for black holes. But we can in fact detect it, so we are encouraged to believe that general relativity is not complete hogwash after all (for that and a few zillion other reasons, i.e. Will, 2006 (http://relativity.livingreviews.org/Articles/lrr-2006-3/index.html)).

The singularity is not a point, nor is it a miniscule volume of space, nor in fact does it "exist" at all outside of our fertile imaginations (which is also a "not to worry" thing, since it isn't supposed to exist physically anyway). The singularity exists mathematically , and only mathematically. It is a point (or collection of points, like the ring singularity of a rotating black hole) where general relativity fails completely, where the mathematics fails completely, and so it is a place where nobody can say what is happening at all. In the absence of a quantum theory of gravity, or some theory to replace general relativity, nobody knows what the singularity is (if it is anything), where it goes (if it goes anywhere) or what it does (if it does anything). But my intuition is that the singularity is simply a symptom of the classical field nature of general relativity. Choose a quantum theory of gravity (string theory, for instance, and the singularity vanished, to be replaced by a complete mathematical description of what is physically happening. Not that it will do a lot to help us out, since we can't look inside the event horizon anyway.

Neverfly
2008-Dec-06, 01:57 AM
sorry I changed the question to this thread because i felt it was the place it should have been. You guys have some good facts I wasn't aware of and apoligize for seeming ignorant and answering myself with unsupported speculation.I really like reading all your guys posts as I am fascinated by all the unknowns of the universe. I really like your input and it is helping me see things from inside the box.

Above you will see a post from Tim Thompson.

Pay attention to that man.

Also, Grant Hutchison, KenG and others.

They are knowledgeable and can increase your understanding ten fold.

If you want a hundred fold, you'll need to attend college.

But in the meantime, I addressed your points directly and hopefully you took it in stride.

Intuition means very little in cosmology. We are evolved to the conditions we know here on Earth.

I may not be posting or able to post much longer, but I'm not exactly a contributing brain here, but I will encourage you to pay attention to answers, not dismiss them in favor of intuition.

creativedreams
2008-Dec-06, 02:00 AM
Thanks Tim, what does math have to say about a possible wormhole in the singularity?

creativedreams
2008-Dec-06, 02:09 AM
It's cool neverfly...was taken in stride. I don't get offended very easily as I just use the criticism to understand the perspective of others. And hopefully learn a little.

thorkil2
2008-Dec-06, 03:17 AM
it seems pretty standard. Here's Kip Thorne:Grant Hutchison

Thank you. I'm just now reading the question, but Kip Thorne is the source I would have looked to first as well.

Jeff Root
2008-Dec-06, 04:41 AM
My perspective on what black holes ARE:

Black holes are theorized places where gravity is very, very, very intense.

I could go on to talk about their properties or how they might form or be
observed, but that one line says everything about what they ARE.

-- Jeff, in Minneapolis

DrRocket
2008-Dec-06, 05:26 AM
I feel you're misjudging him. Thorne's usually pretty careful, and I think he was pretty careful in my quotation.
I'm comfortable with the idea that arithmetic insists that 2+2=4. That's it's job. General relativity's insistence on a singularity is much the same thing: that's what falls out of the sums. Notice that Thorne then uses the word "should" twice, in a conditional sense, in what follows: the singularity "should" be surrounded by emptiness.

I actually admired the balanced juxtaposition as I copied out the words: general relativity "insists" on a singularity; so there "should" be emptiness within the event horizon. His message seems as clear as day to me: we have the maths, we don't have the observations.

Grant Hutchison

Piling on a bit ---

I think you have stated Thornes position rather accurately. General relativity does indeed, insofar as one can humanize a theory, "insist" one a singularity. One simply takes the hypothesized initial conditions, turns the crank on the machine, and out pops the prediction of a singularity.

I think Ken's point is that there is no way to verify that the singularity on which general relativity "insists" is physically real. It probably is not real. A singularity is a set of points at which the mathematics of the machine breaks down and fails to provide a rigorous answer. It is a region in which the curvature tensor fails to exist in a mathematical sense (becomes infinite to some physicists). So Ken is probably correct in his thinking that if we insist on believing that the singularity is a physical thing or that the theory is accurate even very close to the predicted singularity then we are deluding ourselves.

We know a little more in fact. The theory predicts that mass in the general region of the purported singularity is pretty dense. We know that where particles are very very close to one another that quantum theory is important. We know that general relativity and quantum theory are incompatible. So we actually know something very important -- we know that we don't know how to predict the behavior of nature in detail in such circumstances.

But the theory does, with any reasonable interpretation of the word, "insist' on a singularity. The theory could be, and probably is, wrong. Quite often people who are insistent are also wrong.

WayneFrancis
2008-Dec-06, 05:44 AM
It's cool neverfly...was taken in stride. I don't get offended very easily as I just use the criticism to understand the perspective of others. And hopefully learn a little.

I've heard it said that black holes might be an entrance to a wormhole to another part of the universe (white hole) or other universes completely.

We've never seen anything like a white hole so that leaves other universes to be the better of the 2 ideas. But then if you believe in muliple universes each with its own set of physical parameters you probably wouldn't like where you popped out if you did survive the journey.

Jumping into a black hole to get to another universe is as smart as jumping into a blender to get to another universe. The black hole will just make you into a much finer paste.

Neverfly
2008-Dec-06, 06:59 AM
I've heard it said that black holes might be an entrance to a wormhole to another part of the universe (white hole) or other universes completely.

Black Hole theory arose from the mathematics.
So saying that they might be this or that actually makes little sense.

That might make sense if we had first seen a black hole and then tried to theorize about it.

Ken G
2008-Dec-06, 03:11 PM
I'm comfortable with the idea that arithmetic insists that 2+2=4.But that's not the same thing. Arithmetic has authority over calculations like 2+2, that's its proper realm, but only observations have authority over reality-- that's their proper realm in an empirical science. Theories never "insist" anything except on their own predictions. One can certainly say that GR "insists" that GR makes a certain prediction, but in my eye, Thorne's comment is either intended as a statement about reality, or is easily mistaken by the casual reader as making a statement about reality. Theory has no such jurisdiction, and virtually always runs afoul of reality when we forget that. (Symmetry principles, and their related conservation laws, come the closest to transcending their origin in observations, but even symmetries were made to be broken.)


General relativity's insistence on a singularity is much the same thing: that's what falls out of the sums. Again, what falls out of sums is a prediction-- nothing more. If Thorne is intended to be heard as describing a prediction of a theory, there is no issue. To me, his words are quite likely to be taken as something more, as would tend to occur on this very thread.

On the other hand, the idea that nothing special happens when you cross the event horizon, that it "should" be similar to the space on the outside of it, locally, is more of an observationally based idea. None of that comes from general relativity, it is simply not contradicted by GR. If no observations require it be different in there, we start with the presupposition that it is not, until we have reason for something else. But that is not because of general relativity. All we get from GR is that a global coordinatization that includes us gets a bit weird as we cross the event horizon, to reflect the physical prediction that light cannot escape the horizon. Then we do observations that suggest light does not in fact cross the horizon, and we base our understanding of that region on those observations and the theory that unifies them. The only number that comes from GR in that explanation is the 10-33 number, which is a scale we have never penetrated anywhere close to with observation and in my view is meaningful only as a lower bound on where we know our theories must fail.

Notice that Thorne then uses the word "should" twice, in a conditional sense, in what follows: the singularity "should" be surrounded by emptiness.
The word "should" isn't much better, actually. It's not an issue of degree of certainty, it's an issue of the purpose of a physical theory. It's true that if we drop a ball 99 times, it "should" also fall on the 100th time, but that's not because of the theory of gravity, it's because of the 99 previous drops that are unified by that theory.


I actually admired the balanced juxtaposition as I copied out the words: general relativity "insists" on a singularity; so there "should" be emptiness within the event horizon. His message seems as clear as day to me: we have the maths, we don't have the observations.
But the issue is, do maths "insist" on things, or do they simply make predictions? What if a scientist in Newton's day said, "the maths of classical mechanics, which are marvelously effective, insist that particles have an exact position and a momentum at the same time, so they should have an exact position and momentum at the same time, even though we cannot observe it." To my ear, that is precisely the same statement Thorne is making-- it is just that GR is to our generation what Newtonian mechanics was to theirs, and we have neither learned nothing from history, nor even understand what our own science is, if we similarly use it to "insist" on what "should" be the case in realms we have no observations about.

ETA: I want to be clear, because I don't mean to criticize Thorne for what I am hearing in his remarks that may not be there (indeed I don't really mean to be critical at all, the language has to be so exquisitely careful that some misunderstanding is almost impossible to avoid). We can agree that GR predicts that the region just inside the event horizon is locally no different than just outside, the event horizon makes a global statement about how light can connect two distant regions-- and the GR prediction is that it is a one-way connection. We have observations that validate GR in other ways, and we have observations that suggest that the one-way connection idea might be working out. But if we want to know the conditions inside an event horizon, even if we for some reason did not accept GR, we would still have no reason to think they were any different just inside than just outside. It's not from the theory we think that, it's from not having any good reason to think otherwise. If Thorne had just said that, I'd have no problem. The problem I do have is with the use of words like "insist" and "should" being applied to a region that reality, if one does accept GR, has cordoned off from our observation. Philosophically, we have every reason to expect that if reality has cordoned off that region, it may be for a reason we do not yet understand, and does not show up in the theory of GR beyond the cordoning itself. The other problem I had was with statements about the nature of the singularity, even its size, when our physics has never approached anywhere close to those kinds of scales. To me, both these are classic examples of statements that are very likely going to be viewed as extremely naive in a future as distant from us as we are from the Newtonian era-- as in the comparison I made about how a similar statement made by Newtonian experts of that day would sound to us today.

Buttercup
2008-Dec-06, 03:58 PM
Perhaps black holes are the result of God making donuts... :lol:

grant hutchison
2008-Dec-06, 04:32 PM
But the issue is, do maths "insist" on things, or do they simply make predictions? What if a scientist in Newton's day said, "the maths of classical mechanics, which are marvelously effective, insist that particles have an exact position and a momentum at the same time, so they should have an exact position and momentum at the same time, even though we cannot observe it."I would find that entirely unexceptionable, for the reasons I've given. This is a matter of English interpretation, not science.
I believe that you and I agree, and we agree with Thorne. There's just some sort of meta-disagreement going on about whether we agree or not. We may need to agree to disagree about that. :lol:

Frankly, I think you have a very reasonable hammer there, but it sometimes makes you see things that look like nails when no hammering is actually required. :) And I don't think Thorne's little bit of text is a viable nail for your "true-purpose-of-science" hammer.

Grant Hutchison

Ken G
2008-Dec-06, 04:57 PM
I would find that entirely unexceptionable, for the reasons I've given.I think it all comes down to one's opinion about "scientific revolutions". If one is comfortable with the idea that scientists just go with their best current understanding as a kind of pretense that it is a complete understanding, not because one really believes that but simply because it simplifies the mental effort required to entertain other possibilities, then one will simply expect there to be a "revolution in thought" the next time a new breakthrough is made. Personally, that approach I find "wearying", because it seems preferable to simply recognize what we have observed, and what we have not observed, and stop being surprised every time we break into a new regime we have not seen before-- and find something fundamentally new there. There's just no need to think of any of that as a "revolution" where some old view needs to be "overthrown".


I believe that you and I agree, and we agree with Thorne. There's just some sort of meta-disagreement going on about whether we agree or not. We may need to agree to disagree about that. All right, we will agree, and agree that we agree, but disagree on what we are agreeing on. ;)


Frankly, I think you have a very reasonable hammer there, but it sometimes makes you see things that look like nails when no hammering is actually required.I understand your point here, so I'll just let the thread go, and if at some point the nature of that "nail" emerges more clearly, I'll jump in and say-- "see, it was a nail after all." But I will just point out that this "hammer" is not only important to use on hopelessly confused versions of science that come mostly from quacks and flakes, because their ideas of are no significance anyway, in general. The use on mainstream science from top practitioners is much more important, because even though the "transgressions" are far slighter, their impact on a general misperception about what science is can be so much more far-reaching, and just set up for the next "shocking revolution".

grant hutchison
2008-Dec-06, 05:21 PM
Personally, that approach I find "wearying", because it seems preferable to simply recognize what we have observed, and what we have not observed, and stop being surprised every time we break into a new regime we have not seen before-- and find something fundamentally new there. There's just no need to think of any of that as a "revolution" where some old view needs to be "overthrown".I think this illustrates my point about how we're just differing on a matter of English interpretation.
I find the word "insist" to be a very neat choice of expression for what mathematical theories do when they get beyond the edge of their observational support, because it carries exactly the right connotation of pushing things just a little farther than is strictly decorous or appropriate. As rational folk, we are not convinced by mere insistence. We ask for evidence. So for me the word conveys exactly what I think you want conveyed.

Grant Hutchison

Ken G
2008-Dec-06, 05:35 PM
I think this illustrates my point about how we're just differing on a matter of English interpretation.
I find the word "insist" to be a very neat choice of expression for what mathematical theories do when they get beyond the edge of their observational support, because it carries exactly the right connotation of pushing things just a little farther than is strictly decorous or appropriate.Yes, you have put your finger on a key difference in English interpretation-- I find the word "insist" to mean "must be true, and anyone who claims otherwise is being foolish", like an appeal to authority, or a logical imperative. It hangs the authority of science on the claim, and lessens that authority every time it is later found wanting. I don't see it as used to mean the "insisting" character of a precocious child, but if one gives the word that interpretation, then I can agree with its use completely.

Spaceman Spiff
2008-Dec-06, 05:54 PM
I think this illustrates my point about how we're just differing on a matter of English interpretation.
I find the word "insist" to be a very neat choice of expression for what mathematical theories do when they get beyond the edge of their observational support, because it carries exactly the right connotation of pushing things just a little farther than is strictly decorous or appropriate. As rational folk, we are not convinced by mere insistence. We ask for evidence. So for me the word conveys exactly what I think you want conveyed.

Grant Hutchison

(my emphasis) In fact one might say that as scientists, we insist on evidence. But I worry that this is not how most people would interpret such a statement on a casual reading, and I think this is one of KenG's concerns (although I won't insist on it :)).

Is it possible that Thorne might have been cloyly pointing out a sudden lack of attire of "the emperor" as he entered a regime of nature that he had no business entering...er, umm, so to speak?:whistle:

Cougar
2008-Dec-06, 06:01 PM
You have made black holes out to be of substance. They are not. They consist mostly of empty space ...
I doubt this. Could you reference any publication that describes or represents black holes in such a manner?

it seems pretty standard. Here's Kip Thorne:



If the hole were a solid body squeezed into such a small circumference, its average density would be 200 million tons per cubic centimeter ... But the hole is not a solid body. General Relativity insists that the 10 solar masses of stellar matter, which created the hole by imploding long ago, are now concentrated at the hole's very center--concentrated into a minuscule region of space called a singularity. That singularity, roughly 10-33 centimeter in size (a hundred billion billion times smaller than an atomic nucleus), should be surrounded by pure emptiness, aside from the tenuous interstellar gas that is falling inward now and the radiation the gas emits. There should be near emptiness from the singularity out to the horizon ... (Black Holes and Time Warps: Einstein's Outrageous Legacy)

I'm glad I doubted, since this has obviously spawned a very interesting discussion. Of course I greatly respect Kip Thorn, as I do Penrose and Hawking. But the Penrose-Hawking singularity theorems (http://en.wikipedia.org/wiki/Penrose-Hawking_singularity_theorems) speak to results within the system of General Relativity, and essentially state that General Relativity alone is unable to provide meaningful solutions at this particular extreme. Thorne is of course correct in his description of what GR provides... His book is a tremendous glimpse into GR... and black holes.

But my intuition suggests coming at it from the observations as opposed to from the theory. So we have white dwarfs. Very dense bodies. Not black holes, but pretty extreme. Then neutron stars. Come on! These are very compact bodies. There is a minimum of "space" between particles. The physical "laws" stopping these phenomena from gravitationally contracting further are fairly well known. I think it's a bit... arrogant to think that there is utterly no physical law maintaining the "solidity" of a black hole just because humans haven't yet discovered one.


http://www.xmission.com/~dcc/coug-cautious.jpg

Ken G
2008-Dec-06, 06:05 PM
General relativity does indeed, insofar as one can humanize a theory, "insist" one a singularity. One simply takes the hypothesized initial conditions, turns the crank on the machine, and out pops the prediction of a singularity.But as you know, I'm claiming an important difference between the word "insist" and the word "predict". Yes, any theory can be said to insist on its predictions, but the important word there is not insist, it is predictions, so to use the former without the latter is especially problematic.


I think Ken's point is that there is no way to verify that the singularity on which general relativity "insists" is physically real.Yes, that is indeed part of what I'm saying, but that's not the controversial part-- most, including Thorne, might well agree with that part. I'm further asking, what is the actual purpose of a scientific theory? I claim it is solely to organize and unify previous observations, to motivate new ones, and to make predictions about those new ones, all with the intent to establish a kind of conceptual usefulness of the theory that is never known in regard to some new class of observables until the observations are done. But at what point does it "graduate" to being a description of reality that gains precedence over the observations used to test it? Never, with one exception-- we need to have a concept of a "similar observation" to one that has already been done, so that the theory can serve as a proxy for all those similar observations.

This is a very nebulous and tricky area of science-- there is no "theory of similar observations", and that absence is the source of the very debate we are having about how to best word the predictions of a well-supported and excellent theory like GR.


It probably is not real. A singularity is a set of points at which the mathematics of the machine breaks down and fails to provide a rigorous answer.Yes, this is my objection to the second part of Thorne's comments, about the characteristics of the singularity. He recognizes that our physics breaks down before we get to the mathematical singularity, so he means the term as "a place where our physics breaks down", moreso than "a mathematical point". But I would argue that simply knowing our physics breaks down at some point does not in any way logically imply it does not break down much sooner than that! We are just now bringing online an instrument that can probe size scales of about 10-17 cm. That is the physically relevant length scale we can now talk about-- not 10-33 cm, I view the latter as nothing but a lower limit on where we already know our physics fails. Should we expect to drive down 16 orders of magnitude in scale and not discover anything fundamentally new? It seems highly disingenuous to propose that the extra factor of 10 we get from the LHC will likely find something new (as opposed to simply verifying the Higgs), but 16 more powers of 10 will not.


So we actually know something very important -- we know that we don't know how to predict the behavior of nature in detail in such circumstances.That is true, and that is what Thorne is pointing to. But even that is part of what I'm objecting to-- the implication is, we should expect our physics to work until we have good reason to think it will not, rather than the more reasonable statement that we should expect our physics to work only for observations that we have reason to believe are similar to ones we've already done. Symmetry principles seem to be the most powerful for extrapolating that idea-- perhaps because symmetries have a wider domain in the meaning of "similar observations" than do scale changes, the latter being notorious for discovering new surprises (though I admit that many orders of scale may be needed, as when they span from quantum mechanics, to classical physics, to dark energy effects). But perhaps I overstate the reliability of symmetries, as they too know their domains of applicability.


But the theory does, with any reasonable interpretation of the word, "insist' on a singularity. The theory could be, and probably is, wrong. Quite often people who are insistent are also wrong.That is taking Grant's meaning of "insist", like a kind of stubbornness that we do well to take with some skepticism. If that is the interpretation given to the word, I'd agree it is fully appropriate, but I think that is only the interpretation of those who are well enough acquainted with science to already know Thorne's answer, so they are not the relevant audience for the remarks.

Ken G
2008-Dec-06, 06:15 PM
But my intuition suggests coming at it from the observations as opposed to from the theory.Actually, I'd say that's more than intuition, that's the definition of empirical science.

I think it's a bit... arrogant to think that there is utterly no physical law maintaining the "solidity" of a black hole just because humans haven't yet discovered one.
This relates to the issue of whether what is meant by "black hole" is everything inside the event horizon, or just whatever exists at the center. I think most take the former meaning, whereas you take the latter, so there probably is no actual disagreement there-- except as to the meaning of "singularity" as in DrRocket's post.

Cougar
2008-Dec-06, 07:12 PM
...This relates to the issue of whether what is meant by "black hole" is everything inside the event horizon, or just whatever exists at the center....

Oh, right. I wasn't much thinking of the event horizon, which is of course a special... region with a particular quality, but it may not define the radius where the black hole becomes the solid body I hypothesize...

So is there "empty space" between the event horizon and this hypothetical body? Well, first of all, space is everywhere, even at the center. It's a question of how congested is that space?

:think:

Jeff Root
2008-Dec-06, 07:29 PM
Very interesting sidetrack, Grant and Ken!

On the question of exactly what is comprised by the term "black hole":

When we talk about the Earth, we may intend to include the planet's
atmosphere and magnetic fields; when we talk about the Sun, we may
intend to include the corona, or even the solar wind and the heliosphere.

Black holes are named for how they would look if we could approach
them closely enough. What we would see would be a black region
bounded primarily by the photon sphere, not by the event horizon.
The event horizon is the closest to the center of a black hole from
which light can be emitted and reach the outside, but if there is no
matter falling into the black hole, no light will be emitted from that
region. Instead, we would see starlight framing the black hole in a
neat circle located just above the photon sphere, with complete
darkness from the photon sphere on down.

So I would insist that the photon sphere is the absolute minimum
that must be included in the term "black hole".

-- Jeff, in Minneapolis

Jeff Root
2008-Dec-06, 07:47 PM
Cougar,

I agree that some force might exist that could stop matter in a black
hole from being crushed without limit by its own gravity. But since there
is no indication that any such force exists, the "default" is obviously to
expect that the matter will be crushed without limit.

Since a force strong enough to resist the further crushing of matter by
its own gravity would have to be very, very much stronger than any of
the known forces, I personally doubt that any such force exits. Also, if
the force only has effects on matter inside the event horizons of black
holes, then it is forever unknowable. Hence I consider the possibility of
such a force as something that should be considered but also something
which could be safely ignored.

-- Jeff, in Minneapolis

kleindoofy
2008-Dec-06, 08:04 PM
I've heard it said that black holes might be an entrance to a wormhole ...
And I've heard it said that black holes are a good place to cool a six pack.

However, I prefer to place my trust in people who have not only just thought about things but who have done extensive, critical research, exploring all reasonable theories, who understand the math, understand the astronomical background and the consequences of theories, take corresponding data and related phenomena into account, and who formulate models based on their findings - not on their wishes -, models which they themselves would gladly abandon when somebody else proves them false.

These theories are usually a pretty boring read, seldom make the six o'clock news, and don't get used for the scripts of SF movies.

Random information which one hears tends not to be all too reliable.

Spaceman Spiff
2008-Dec-06, 09:10 PM
Cougar,

I agree that some force might exist that could stop matter in a black
hole from being crushed without limit by its own gravity. But since
there is no indication that any such force exists, the "default" is obviously
to expect that the matter will be crushed without limit.

Since a force strong enough to resist the further crushing of matter by
its own gravity would have to be very, very much stronger than any of
the known forces, I personally doubt that any such force exits. Also, if
the force only has effects on matter inside the event horizons of black
holes, then it is forever unknowable. Hence I consider the possibility of
such a force as something that should be considered but also something
which could be safely ignored.

-- Jeff, in Minneapolis

(my emphasis). "....matter crushed without limit"
So what does that mean? I read that and I see only words that provide no physical understanding whatsoever. (Jeff Root: I am not putting you on the spot; you just happened to be the one who uttered the phrase, and in doing so inspired this post...:)).

The iron core of a massive star, initially supported by electron degeneracy pressure, undergoes collapse due primarily to a critical change in the equation of state (http://en.wikipedia.org/wiki/Equation_of_state) -- the electrons undergo inverse beta decay (http://en.wikipedia.org/wiki/Inverse_beta_decay) with the nucleons and so are no longer around in sufficient number to support the structure. We are pretty darned sure that within neutron stars there are several major transitions in the equation of state. In particular, the deep interior almost certainly does not contain neutrons, but some other baryon that obeys Fermi-Dirac statistics (http://en.wikipedia.org/wiki/Fermi_Dirac_statistics) (such as hyperons (http://en.wikipedia.org/wiki/Hyperon)). The question in my mind is -- what phenomenon leads to the complete failure of these statistics and what is the result? One idea that has been kicked around is that some sort of "phase change" leads to all of the matter falling into the ground state in a Bose-Einstein condensate (http://en.wikipedia.org/wiki/Bose_Einstein_condensate), which obey a different set of statistics. This model (http://arxiv.org/abs/gr-qc/0407075) might be completely wrong, but at least they attempted to address the problem.

The arguments of macroscopic dynamical instabilities, e.g., speed of sound exceeding c, the increase in 3*P (pressure) relative to rho*c^2 (matter energy density) in the stress energy tensor, the equation of state becoming too soft, just do not jive (in my mind at least) with what we know about the quantum world. So some structure becomes dynamically unstable -- "thus it becomes a black hole with a singularity" does not address the question of what happens to the equation of state of the matter within that structure. Something is missing. And that is why I think "black holes" are so important to our understanding of the world -- they force a confrontation between our two most successful standard models, between which at present lies a deep chasm (or dare I say, a black hole?) in our understanding.

grant hutchison
2008-Dec-06, 09:11 PM
Is it possible that Thorne might have been cloyly pointing out a sudden lack of attire of "the emperor" as he entered a regime of nature that he had no business entering...er, umm, so to speak?:whistle:I'm sure he was, as you say, flagging the transition to an unobserved regimen.
In my quotation, he chooses "should be" over "is" twice twice, when writing about the black hole interior. To me, that's a pointed disclaimer:
"Where's the professor?"
"She is in her office." (Based on my personal observation.)
"She should be in her office." (Based on the timetable, but I haven't looked.)

Grant Hutchison

cjameshuff
2008-Dec-06, 09:49 PM
(my emphasis). "....matter crushed without limit"
So what does that mean? I read that and I see only words that provide no physical intuition whatsoever. (Jeff Root: I am not putting you on the spot; you just happened to be the one who uttered the phrase, and in doing so inspired this post...:)).

Well, at the event horizon, photons can not escape. This means they can not move outward at all, they can only move inward. The forces holding matter together (and more importantly, holding it apart) are similarly limited...matter within the event horizon can not exert an outward force on matter further out...so down it all goes, with nothing to stop it, no matter how hot or dense it gets.

It's more complex than this, there's subtleties due to the changing reference frame of an object falling into the black hole that I don't fully understand, but this seems to suffice as a general description of what goes on. Inside a black hole as described by relativity, matter only moves toward the singularity, it can not move away, and can not hold position.

Ken G
2008-Dec-06, 10:18 PM
So is there "empty space" between the event horizon and this hypothetical body? Kip Thorne was pointing out that the theory of GR gives us no reason to expect a difference in the conditions just inside the event horizon and those just outside. The event horizon is nothing but a "coordinate singularity" in that theory, so it has no local physical manifestations, but does have the global property that things can only pass across it in one direction. This is sort of like the way we only perceive time as proceeding in one direction, even though the individual laws of physics allow time to go either way. The one-way character of time is some kind of aggregrate or global property of how the laws act, moreso than a property of the fundamental laws themselves. Somehow, the global topology of an event horizon results in one-way connectivity, but we have no reason to suspect the local conditions change in any fundamental way across an event horizon. So Thorne concludes that our expectation should be that space inside is just as empty as space outside.

I agree that this is a reasonable expectation, but it's not because of anything the theory of general relativity says-- it is simply a default expectation that the theory does nothing to change. If there is a change, it will only be described by some other theory that stems from the strange unobservability of the insides of the event horizon-- a theory that likely makes most or all the same predictions as GR in the observable realm, but has advantages in some larger regime, like string theory or some such thing.

Spaceman Spiff
2008-Dec-06, 10:27 PM
Well, at the event horizon, photons can not escape. This means they can not move outward at all, they can only move inward....

It's more complex than this, there's subtleties due to the changing reference frame of an object falling into the black hole that I don't fully understand, but this seems to suffice as a general description of what goes on. Inside a black hole as described by relativity, matter only moves toward the singularity, it can not move away, and can not hold position.

Thanks; I think I understood all of that. But it does not answer my question, and it does not answer the question about how black holes form -- if they "actually" do form, as they're usually described (complete with a "singularity"). And the part in bold, above. What is this singularity, which GR tells us is the fate of all matter that crosses the event horizon? What is the equation of state of the matter and energy that fall in? Saying that the matter ends up in the singularity tells me nothing about the physical fate of that matter/energy or about how such a thing formed in the first place. In fact, it was my impression (from previous experience and Ken G's comments above) that the singularity is a place-holder for "our model does not adequately address these conditions." To the extent that is true, it seems useless to point at that and say that's (whatever "that" is) the fate of matter and energy which cross the event horizon.

Ken G
2008-Dec-06, 10:31 PM
In my quotation, he chooses "should be" over "is" twice twice, when writing about the black hole interior. To me, that's a pointed disclaimer:
"Where's the professor?"
"She is in her office." (Based on my personal observation.)
"She should be in her office." (Based on the timetable, but I haven't looked.)
Just to clarify what I'm saying in regard to the choice of the word "should", I do see the above exchange as a perfectly valid way to use that word-- but I do not see it necessarily as being like the way Thorne used it. In the above exchange, it seems to me that a perfectly equivalent translation, perhaps into a language that lacked the word "should", would be that the professor is usually in her office at this time, so unless today is any different somehow, one would expect her to be there now. This implies a comparison to all those other days when she was in her office at this time-- what we might call "similar observations". But what are the "similar observations" to what is inside an event horizon? One might claim that what is just outside is similar to what is just inside, but then one is merely assuming what Thorne is claiming is an expectation from GR.

Alternatively, one might say that the professor "should" be there because she is known to be punctual and she is scheduled to be in her office, even though today is not a typical day for her. Then what counts as a "similar observation" is that she normally follows a schedule, even though the schedule for today is unusual and untested. But we still have the question, can we use what we know about the professor in past situations to judge how she'll relate to a schedule like the one she has today? This is just a very tough issue, there's no known systematic way to deal with this issue in science-- and the guide provided by history of science, and professors, is: beware of such a use of "should".

Leaving analogy space, the question at hand is, should we attribute any physical local significance to the global unobservability of the inside of an event horizon? It seems innocuous enough to attribute no such significance, but what seems innocuous, and what is real science, are not always the same. It seems very possible to me that if nature sets up a principle of veiling its singularities, then you may well be in the situation like in image charges in conductors. I've made that analogy before-- the field outside a conductor can be predicted, and tested, by imagining there is an image charge inside the conductor, but the fields inside the conductor in no way match that solution. You would never know that if you could not test those interior fields.

Jeff Root
2008-Dec-06, 10:34 PM
What cjameshuff said. Three things I might add:

1) The ever-deepening of the gravity well expands spacetime without
limit in the radial direction even as the collapsing matter is crushed by
its own gravity without limit in the circumferential direction, so that
relative to the rest of the Universe, the matter never actually reaches
infinite density -- My interpretation is that it gets stretched more and
more in the time dimension as it gets deeper in the gravity well;

2) Uncertainty appears to prevent us from saying meaningfully that
the collapsing matter will ever be crushed more than a finite amount.
Beyond the limits of uncertainty, what happens can't be defined.

3) From an outsider's point of view, the limits of uncertainty are
superceeded by the limits of the event horizon, anyway.

-- Jeff, in Minneapolis

Cougar
2008-Dec-06, 10:44 PM
...matter within the event horizon can not exert an outward force on matter further out...

...unless it is done in some vague way similar to neutron stars. Or perhaps Uncertainty insists a certain lowest level equation of state.... :)

cjameshuff
2008-Dec-06, 10:45 PM
Well, at the event horizon, photons can not escape. This means they can not move outward at all, they can only move inward.

I should have stated this more clearly...since photons only move at the speed of light, if they were able to rise any distance above the event horizon, they would be able to escape.

Spaceman Spiff
2008-Dec-06, 10:57 PM
Yes, this is my objection to the second part of Thorne's comments, about the characteristics of the singularity. He recognizes that our physics breaks down before we get to the mathematical singularity, so he means the term as "a place where our physics breaks down", more so than "a mathematical point". But I would argue that simply knowing our physics breaks down at some point does not in any way logically imply it does not break down much sooner than that!

This seems to be a highly relevant statement regarding my questions (http://www.bautforum.com/questions-answers/82035-what-black-holes-really-2.html#post1382194), above.

Jeff Root
2008-Dec-06, 11:09 PM
Cougar,

The thing that cjameshuff pointed out which I think is key to the idea
that anything falling into a black hole is required and destined to fall in
forever, with no possibility of being stopped, is the fact that whatever
force would try to stop the falling matter would have to propagate
outward faster than the speed of light. Everything nearing the center
of a black hole gets pulled apart by the gravitational tide. Everything
gets spaghettified, because the forces holding the matter together are
transmitted through the matter at speeds limited to c. Stopping the
gravitational collapse inside the event horizon requires a force that is
transmitted upward through the matter at a speed greater than c.

-- Jeff, in Minneapolis

Ken G
2008-Dec-06, 11:57 PM
The thing that cjameshuff pointed out which I think is key to the idea
that anything falling into a black hole is required and destined to fall in
forever, with no possibility of being stopped, is the fact that whatever
force would try to stop the falling matter would have to propagate
outward faster than the speed of light.Actually, this was the incorrect aspect of his post. If this were true, black holes could not exert gravity on anything, even on our side of the event horizon. But forces are carried by virtual particles, not real ones, and are not subject to the usual limitations on speed. Indeed, I would tend to see this as evidence against the mathematical singularity idea-- I might imagine that even virtual particles might have a tough time escaping a true singularity.

Jeff Root
2008-Dec-07, 12:03 AM
So there is no limit on the speed of virtual particles? So the electric force,
for example, is transmitted faster than light? No? So what did you mean?

-- Jeff, in Minneapolis

DrRocket
2008-Dec-07, 12:29 AM
Actually, this was the incorrect aspect of his post. If this were true, black holes could not exert gravity on anything, even on our side of the event horizon. But forces are carried by virtual particles, not real ones, and are not subject to the usual limitations on speed. Indeed, I would tend to see this as evidence against the mathematical singularity idea-- I might imagine that even virtual particles might have a tough time escaping a true singularity.

I am a bit puzzled by this.

Black holes are explained, insofar as they are explained, by general relativity. In that context, black holes don't exert gravity they are simply a manifestation of extreme curvature of space-time, and that curvature is present well outside of the event horizon, the curvature being what we experience as gravity.

It is further my understanding that particles, real or virtual that crross from our side to the other side of an event horizon, don't come back. It is also my understanding that Hawking radiation (assuming that it exists) is the result of one of a pair of virtual particles crossing the event horizon while the other remains outside, hence they don't re-combine and annihilate one another.

So I don't understand your statement, but it sounds to me as though the idea behind it is that if nothing whatever escapes an event horizon, then neither could gravitons, in which case black holes would not exert a gravitational influence beyond the event horizon. But that would require some sort of a theory of quantum gravity, which we don't have. I don't think general relativity, which we do have, permits things to go from inside the event horizon to outside the event horizon, and notions that permit such things require an application of quantum theory, which is a bit speculative.

I think it is also necessary to distinguish between the event horizon of a black hole, which is not a singularity with the singular surface on the other side of the event horizon which is the result of a general relativistic calculation ( I won't call it a prediction because it is simply a manifold of points at which the curvature tensor ceases to make sense, and I also won't say that general relativity "insists" on it either :-)). I don't think anybody anywhere has any idea what happens to a particle, virtual or otherwise, that get to the location of the singularity, or even gets extremely close to it.

Ken G
2008-Dec-07, 12:52 AM
Black holes are explained, insofar as they are explained, by general relativity. In that context, black holes don't exert gravity they are simply a manifestation of extreme curvature of space-time, and that curvature is present well outside of the event horizon, the curvature being what we experience as gravity.That's quite true, but if one thinks of gravity as being mediated by the graviton, a virtual particle, then one thinks of general relativity as a kind of effective theory, awaiting quantization in a proper field theory. Don't get me wrong, I am no fan of deciding in advance what a "proper" theory really is, and I have no idea if gravitons will ever be considered a proper theory or not. Also, I am no expert in the use of virtual particles in making actual calculations or predictions. I'm just saying that virtual particles have ways of avoiding the normal rules, and this empowers them to do what they do, in ways that real particles could never do.

It is true that real forces cannot send signals at faster than the speed of light, and this relates to the fact that virtual particles have to end up having effects that obey the laws of physics, like energy conservation and causality limitations. But they only have to obey those laws in terms of their effects on real things-- one cannot imagine that the virtual particles themselves are ruled by those laws. As such, there is no reason virtual particles cannot escape black holes, and cannot "swim upstream" inside the event horizon, and cannot go back in time and so forth. Their net result cannot be to send a signal that something has changed in the center of the black hole (the mass itself is not allowed to vary in the standard solution we talk about-- time dependent solutions are much more difficult), but they can say that there is something there-- such as a charge. Indeed, net charge is one of the allowed attributes of a black hole.


It is further my understanding that particles, real or virtual that crross from our side to the other side of an event horizon, don't come back.It sounds like you are talking about Hawking radiation here, which is not about virtual particles per se, but is more about how virtual particles interface with real particles-- in that virtual particles can, under some circumstances, turn into real ones. The "circumstances" are that in some situations, virtual particles find they can obey the normal laws (especially conservation of energy), at which point they can become real. Emission of (real) photons is, I believe, an example of this. But if you are just talking about the electric force, rather than photon emission, you can deal with virtual photons that themselves need not obey any of those laws-- but their effects must, because the laws are based on those effects.

For example, we need some way to know what the charge of a black hole is, if you take the picture that the electric forces are mediated by virtual particles coming from that charge. This all gets into very murky country, far from any personal expertise of mine, because virtual particles are a fundamentally quantum mechanical concept, and general relativity is not unified with quantum mechanics. So it's an incomplete language that lets us use both notions at once, and I may be overlooking important details.

For one thing, if a virtual particle has a plane-wave wave function, then it has no known location-- it is everywhere. If it has no location, it has no direct connection to the charge that emitted it-- and so, it can be propagating toward that charge as easily as away from it. If we extend that concept to charges in a black hole, we might say that some of the virtual particles are coming from outside the event horizon and propagating in toward that charge that is responsible for them. In that picture, it may not be necessary to have any cross the event horizon, as you say. But they can still produce forces inside the event horizon, in contradiction to the claims I was objecting to.


But that would require some sort of a theory of quantum gravity, which we don't have. I don't think general relativity, which we do have, permits things to go from inside the event horizon to outside the event horizon, and notions that permit such things require an application of quantum theory, which is a bit speculative.Many theorists don't think this is a fundamental problem, and they expect gravitons to work. I really don't know how they reach that conclusion.

creativedreams
2008-Dec-07, 01:12 AM
do particles enter the black hole the same speed and maintain its direction from any direction? Is the event horizon a perfect sphere? If not what determines its axis and how does math work from any point?

grant hutchison
2008-Dec-07, 01:19 AM
do particles enter the black hole the same speed and maintain its direction from any direction? Is the event horizon a perfect sphere? If not what determines its axis and how does math work from any point?Particles cross the event horizon at the speed of light, irrespective of where they fall from, and what their original speed was.
The event horizon of a non-rotating black hole is spherical. The shape of the event horizon of a rotating black hole seems to depend on the coordinates you choose: in one set of coordinates it's spherical; in another, it has an equatorial bulge.

Grant Hutchison

creativedreams
2008-Dec-07, 01:26 AM
Thanks. Maybe black holes are the "bigbang" for a new smaller universe? Just a thought.

cjameshuff
2008-Dec-07, 02:27 AM
Actually, this was the incorrect aspect of his post. If this were true, black holes could not exert gravity on anything, even on our side of the event horizon. But forces are carried by virtual particles, not real ones, and are not subject to the usual limitations on speed. Indeed, I would tend to see this as evidence against the mathematical singularity idea-- I might imagine that even virtual particles might have a tough time escaping a true singularity.

My understanding is that this is just one of the instances where GR and QM conflict. In a black hole as described by relativity, gravity is not a force, let alone a force carried by gravitons, and this issue does not exist.

Ken G
2008-Dec-07, 02:42 AM
My understanding is that this is just one of the instances where GR and QM conflict. In a black hole as described by relativity, gravity is not a force, let alone a force carried by gravitons, and this issue does not exist.But it has to come up-- we need to have a charged black hole interact with its surroundings via quantized fields, often treated in terms of virtual particles that do get out somehow (or are already out, but still have to be able to affect everywhere, even inside the event horizon). Just because GR does not include virtual particles explicitly does not mean it can make statements about forces that are untrue. I agree that we don't know a lot here, because we have no unified description, but it only adds to that problem if we claim that forces cannot get from the central singularity to other places inside the event horizon, on the grounds that relativity would need to be violated to do so.

Jeff Root
2008-Dec-07, 02:55 AM
Particles cross the event horizon at the speed of light, irrespective
of where they fall from, and what their original speed was.
That can't be right. Anything falling from an infinite distance will
be moving at the speed of light as it crosses the event horizon,
relative to an observer magically stationary on the event horizon.
But my SuperSpacecraft could hover just above the event horizon
until its fuel is gone. It would then begin to fall, accelerating as it
would in lesser gravity, and would not reach the speed of light at
the event horizon, relative to the magically-stationary observer.

-- Jeff, in Minneapolis

grant hutchison
2008-Dec-07, 03:25 AM
That can't be right. Anything falling from an infinite distance will
be moving at the speed of light as it crosses the event horizon,
relative to an observer magically stationary on the event horizon.
But my SuperSpacecraft could hover just above the event horizon
until its fuel is gone. It would then begin to fall, accelerating as it
would in lesser gravity, and would not reach the speed of light at
the event horizon, relative to the magically-stationary observer.It doesn't matter how close you are to the horizon when you turn off your engines: you'll still hit it at lightspeed relative to an observer stationary at the event horizon.
The relevant equation derived from the Schwarzschild metric is:

vr/c = sqrt[(rs/r - rs/r0)/(1 - rs/r0)]

where vr is the velocity measured by a stationary observer at radius r, rs is the Schwarzschild radius, and r0 is the starting radius. All radii measured in Schwarschild coordinates. Set r = rs, to place your observer at the event horizon, and my result follows.
I won't torment you with the corresponding calculation for the velocity in Schwarzschild coordinates, which goes to zero at the event horizon. :)

See Taylor and Wheeler's Exploring Black Holes: Introduction to General Relativity for more detail.

Grant Hutchison

publius
2008-Dec-07, 03:50 AM
I won't torment you with the corresponding calculation for the velocity in Schwarzschild coordinates, which goes to zero at the event horizon. :)

Grant Hutchison

You don't really have to torment, saying "reaches the speed of light" is enough, if you understand to speed of light to mean "null path" and not the constant 'c'. A null path is always locally 'c' to any observer, of course. However, it is zero at the horizon to all stationary observers, save for the limiting case of the observer right at the horizon, who doesn't really exist, at which case the limit is 'c' as expected. Well, it's 0/0 *c. :lol:


-Richard

grant hutchison
2008-Dec-07, 03:05 PM
You don't really have to torment, saying "reaches the speed of light" is enough, if you understand to speed of light to mean "null path" and not the constant 'c'.The "torment" was really a reference to our previous tripartite discussion of the decay of the infaller's velocity towards zero at the horizon, for all distant stationary observers. It went on for rather a long time, as I recall. :)


I realised last night that the formulation

vr/c = sqrt[(rs/r - rs/r0)/(1 - rs/r0)]

collapses very readily when rs << r, r0. So we can readily reconstruct the standard Newton version:

vr2 = 2GM(1/r - 1/r0)

That's always nice. :)

Grant Hutchison