Talk:Black hole/Archive 1

message: I strongly disagree that objects fall to the center of a black hole.  At least one recent referreed paper havs shown that no object can cross an event horizon (and thus fall to the center).  I will go get my reprint later, and include a reference.  Beyond that, most everything else is correct.
One item that I would really like to mention is that a gravitational field is equivalent to a region of space with excess volume (e.g. more than 4pi/3 r^3 inside a spherical surface of 4pi r^2.)  This is one way of explaining how a wavefront of light is bent by the field.  When falling into a black hole, space is observed to expand to an extreme, sometimes called "hyperexpansion".  This is the same hyperexpansion which smoothed out many inhomogenieties after the "big bang".  In fact, a black hole has a radius where the circumference is a minimum!
Joseph D. Rudmin
:That would be surprising. Any general relativity textbook contains a discussion on how a falling object crosses the horizon and hits the singularity in a finite amount of proper time. I don't know what paper you're referring to, unless it's the so-called "gravastar" theory of black holes, which is highly speculative. -- CYD
::Here is my reference to the article I mentioned:
"Does the Principle of Equivalence Prohibit Trapped Surfaces from Forming in the General Relativistic Collapse Process?", Darryl Leiter and Stanley Robertson, _Foundation of Physics Letters_, Volume 16 (2003), Number 2, pp 143-161.
::I don't think that this correction to black hole theory will invalidate most other work, because most matter falling into a black hole never returns anyway.
::I will now create the page for the Schwarzchild metric, and give further details there on how errors in that metric resulted in the myth that an object will fall to the center of a black hole.
::Joseph D. Rudmin
I have edited this article because it was to much like prose writing, didn't realize the contraversy -Whoops! This is what was not retooled because Iw asn't sure how to handle it:
Once the gradient becomes large enough, close to the singularity, to tear atoms apart. The point at which the tidal forces become fatal depends on the size of the black hole. For a very large black hole such as those found at the center of galaxies, this point will lie well inside the event horizon (see also ), so the astronaut may cross the event horizon painlessly and live . Conversely, for a small black hole, those tidal effects may become fatal long before the astronaut reaches the event horizon.
This is my first attempt at a major edit, please advise.--Tznkai 01:10, 25 May 2005 (UTC)
replies: text: The article is now protected, per request of Jwrosenzweig. --Modemac 20:55, 13 Feb 2004 (UTC)
thread_title: Things falling in

message: User:Aranoff just made a few comments about objects falling into black holes that I find difficult to believe or otherwise suspicious, but since I'm no expert I thought I'd explain my disagreement here. They are:
:It is not possible to observe an object falling through the surface of a black hole, as it takes forever to get to the surface as viewed by an external observer. Since it is not possible to observe this effect, the effect is not part of physics.
This looks like a controversial statement at best, a gross oversimplification at worst. The laws of physics are being used to make these predictions in the first place, how can it not be a part of physics? Besides, the effect that Aranoff is claiming is unobservable seems quite observable to me; just drop a probe in and have it report on how things look from its frame of reference as it falls. 
:It cannot report on how things look! My statement is correct!
::No, it can report on how things look, right up until the edge of the event horizon. As I mentioned below. And as far as the probe is concerned (according to theory), nothing will be slowing its fall. Bryan 15:23, 29 Apr 2004 (UTC)
thread_title: More things falling in

message: :There is another point. The word singularity means a mathematical error, which means that the theory is not mathematically sound here.
Firstly, that's not at all what the word singularity means. 
:Yes, singularity means division by zero!
::That is one way to get a singularity, yes (there are others, see mathematical singularity for more details). But that is not a mathematical error, and in any event the singularity only occurs right at the very center of the black hole; there's nothing implausable about the predictions general relativity makes about approaching or passing through the event horizon. Bryan 15:23, 29 Apr 2004 (UTC)
replies: text: Once again I direct you to read mathematical singularity; "division by zero" is only one particular way to get a singularity, there are many others, and in any event a singularity is not an indication of a "mathematical error." Furthermore, although 1/x is not infinite when x=0, it does approach infinity as x approaches 0. This is sufficient to produce the predictions that you've been objecting to. Bryan 21:25, 12 Jun 2004 (UTC)

text: The above statement is false. The center of the event horizon does not exist. 1/x is not defined for x=0.
Variations of which has been addressed here several times already, so I don't have anything new to add in response. Bryan 02:00, 16 Jun 2004 (UTC)
thread_title: Singularity

message: I deleted the assertion that "nothing can interact with its interior".  AFAIK, matter that falls into a black hole will cross the event horizon in a finite time (in its own frame) and once inside it can interact with other matter that is in there. Is this correct?10:57, 31 May 2004 (UTC)
replies: text: In addition, particles cannot exceed the speed of light so nothing else can interact with the object, hence the term hole. 
This is wrong physiscs (things can interact gravitationally with the black hole) and wrong etymology (it is called a "hole" simply because things fall into it — or, in Nerdish, because its gravitational potential is lower than that of surrounding space 8-)Jorge Stolfi 14:42, 3 Jun 2004 (UTC)
thread_title:  Things cannot interact with the interior?

message: A misconception some people have is that a black hole is a two-dimensional object on a flat surface, as a hole in the ground is a two-dimensional opening in the plane of the earth's surface. So, I added the text about ball-shaped.
: Correction: a rotating black hole is probably an oblate (flattened) speroid rather than a prolate (enlogated) one, at least for reasonable rotation rates.  In any case the American football is not an ellipsoid, it is technically a piece of a torus  -- a surface swept by a circle rotating about an axis on its plane. (Only that in this case the torus is degenerate because the axis cuts the circle.)Jorge Stolfi 10:57, 31 May 2004 (UTC)
thread_title:  Shape of black hole 

message: I thought that the correct term was hypermass? Stargoat 22:55, 14 Jul 2004 (UTC)
thread_title:  Hypermass 

message: Quoted from the article: 
::According to classical general relativity, black holes can be entirely characterized according to three parameters: mass, angular momentum and electric charge, a principle summarized by the saying "black holes have no hair".
As someone with very little familiarity with physics, I have absolutely no idea how or why that principle is summarized in that saying.  Perhaps someone could explain? (And maybe add some clarification to the article itself?) :) --Slartibartfast 09:01, 23 Sep 2004 (UTC)
thread_title:  "Black holes have no hair"? 

message: This excellent article merits a better opening.
A black hole is a theoretical concentration of mass with a gravitational field so strong that its escape velocity exceeds the speed of light.
*Is there any need for the word "theoretical" in the 1st phrase ?
**Absolutely, see above. 
*What does "its" refer to ? The mass ? The gravitational field ? The black hole ?
**Good point.  I tried to improve it, what do you think? - Taxman 12:26, Sep 23, 2004 (UTC)
-- Frau Holle 10:37, 23 Sep 2004 (UTC)
thread_title:  Intro needs improvement 

message: In 2004 a cluster of black holes was detected, - this is useless 
without a reference - there's a huge difference if they are stellar black holes are supermassive black holes. Maybe it's a reference to this expectation of bh's in the GC? 
Please look through http://arxiv.org to find the article if this is not the right one.
Boud 11:10, 23 Sep 2004 (UTC)
thread_title:  reference missing for black hole cluster 

message: ...you throw antimatter into a black hole? Since antimatter makes matter explode on contact, would the black hole explode, or does being a black hole somehow make things different?
:No. When anti matter and matter anihilate they produce a  lot of energy. This energy typically goes into producing gamma rays. A gamma ray is a very high energy form of light. But the reason black holes are black is because light cannot escape from them. So assuming the antimatter gets as far as the event horizon (i'e doesn't meet any matter before getting there)Then we would see it go on perfectly normally. Once inside, it could meet some matter and they may well be a gigantic explosion, but we would never know because non of the energy would escape. Theresa Knott  (taketh no rest) 18:07, 23 Sep 2004 (UTC)
thread_title:  What happens if.... 

message: I am thinking about adding a section to the black hole article, but I would like to get people's opinions on whether this will produce any greater clarity or simply put people off. An almost universal (no pun intended) "problem" when people talk about black holes, this article included, is the rather casual way in which they talk about its "radius". For example, in the Schwarzschild metric that appears in the article, it is quite clear (apart from the coordinate singularity there) that the event horizon occurs where r = 2M. What is not usually mentioned is that this r coordinate is almost wholly *arbitrary*. The only special things about the coordinates for the metric in the form given are that i) the metric is independent of t (as well as being spherically symmetric), and ii) as space becomes asymptotically flat as r\to\infty, the length scale of the r coordinate agrees with flat space distance.
As the article on the Kerr metric points out, different r coordinates even though they agree at infinity, need not agree anywhere else. So this is where we have our problem - what can people actually mean to say when they say, "the radius of this black hole is 2M"? If we were to use the distance measured by a hypothetical traveller as he moves towards the event horizon, say from a distance of 100M where space is fairly flat, this is clearly false. The "proper distance" he measures on his way towards the event horizon is not 98M, it is infinite. This excellent page http://casa.colorado.edu/~ajsh/schwp.html illustrates the problem.
To cut a long story not terribly short, what we are doing descriptively when we make statements about black hole size is talking about places in flat space where there *would* be things other than a black hole if the space was flat there. So by saying the radius is 2M, what we actually mean is, there is definitely *no* black hole at places we can visit where we would have considered our distance from the putative black hole centre was *greater* than 2M had there been no black hole there. The black hole is actually a logical hole as well - we can only define its size in terms of where we know it isn't!
In fact, defining where the event horizon is in any case is actually problematic too. This other excellent article http://www.mathpages.com/home/kmath339.htm talks about a closely related issue, that a black hole cannot even be defined in a Universe that does not last forever. Loosely speaking, since anyone falling into what *would* be a black hole (i.e. no escape possible) sees infinite time passing in the exterior world as he crosses the horizon, he also sees the surrounding Universe end, if it is going to. Whatever else this means, it also means that he cannot cross any kind of horizon in this case.
All we can take away from this is that the "size" of a black hole, in terms of the region of what would be flat space if it weren't there that it affects, can only be said to be *proportional* to M, and not in fact precisely 2M or any other figure.
Anyway, I welcome (polite!) comments on what do with this info/discussion. Should I try to boil it down into a section on the black hole page, or another, or might it merit a separate page by itself, "The problem of the radius", or "Problems in talking about Black Holes" or so...? TIA. Bosmon 22:32, 23 Sep 2004 (UTC)
replies: text: What you're saying is all true, but I'm not convinced it should go in the article. Essentially, there's a problem with the whole English language when we deal with relativity. The only solution is to use the language of mathematics, but this article needs to be intelligible to people who don't know differential geometry. It's also not just a problem with the radius. Well, just my 2 cents. BTW, the idea about seeing the end of the exterior universe is very interesting! Too bad we don't seem to live in a closed universe :-) --Bcrowell 03:03, 25 Sep 2004 (UTC)
thread_title:  The problem of the radius 

message: Some time ago I read an article by Marek Abramowicz in Scientific American about how the concept of centrifugal force can change radically when you approach a black hole.
It has some rather interesting effects and consequences and I believe it should be added to the main article. Perhaps not as explained below, but hey I'm trying my best...
We all know that light bends around a black hole, in fact there is a certain radius where the light would bend just the right amount so that it formed a perpetual 'ring'. It would neither fall into the black hole, or escape! (In reality it would be a sphere - but let's keep it simple.)
It gets very interesting if you were to build a donut shaped space-station at this radius with the black hole in the centre (or hole of the donut!). First, this radius is OUTSIDE the event horizon by some distance, so you're safe. Second, if you were an occupant of this station and there was a central corridor that ran the entire circumference of this station. You could look down it and see your own back!
The light would be bent in such a way that the circular corridor would appear to be straight, this has a direct influence on the so-called centrifugal force experienced by any object travelling around the black hole. You can check the corridor is still circular and which side is the inside or outside using a straight ruler - but that's another interesting aside.
Now imagine a space-ship doing laps around a black hole outside that radius. The centrifugal force the ship would experience would be similar to the everyday kind. However at the described radius it wouldn't experience any of that force at all - it could do laps without having to counter any sideways forces. For all purposes it would be going in a straight line.
Going closer to the black hole (again we're still outside the event horizon) the space-ship doing the circular laps would have to counter a force opposite to normal. To all purposes the space-ship would be doing laps of the universe, not the black hole!
I thought about this a bit more... this also creates interesting effects when you observe any light falling into the black hole - above and below this magical radius. Basically as you get closer to the black hole (and looking away from it) you'll see more and more of the universe, eventually the stuff behind you because it's being bent by, surprise, the black hole.
At the magical radius, if the black hole was truely black, it would look like a perfectly flat, black surface that extended forever, the universe above it. If you imagine this black surface as a 'lake', the universe would be the 'sky'.
Eventually you'll get to the point where the universe would appear to be a white hole as the light will get bent further and further, much like the view from a fish-eye lens.
I hope that isn't too confused and perhaps someone would do some research and add a reference.
:I hope this doesn't burst your bubble, but as far as I knew Centrifugal Force does not exist in General Relativity, or many other recent competing theories. But regardless of the terminology used, you raise some interesting points. You may like to read this article for a similar take on what you've explained in your post.
::I vaguely recall reading exactly this description of centrifugal forces in a popularization a while back. Hm...  gives a nearly useless one-liner that indicates the magic radius where the force inverts is 1.5 times that of the event horizon... ah, here we are;  goes into detail about how the extreme gravity of black holes affects centrifugal force. Looks like the article I remember is the July 1995 Discover magazine, but it's only available online if you're a registered subscriber:  Bryan 05:33, 24 Sep 2004 (UTC)
thread_title:  Centrifugal Force and Black Holes 

message: "The closer he gets to the event horizon, the longer the photons he emits take to escape from the black hole's gravitational field. A distant observer will see the astronaut's descent slowing as he approaches the event horizon, which he never appears to reach." (emphasis mine)
This is wrong too.  Light always travels at the same speed from the viewpoint of every observer.  No time to change this right now, maybe someone else can do this.
:I'm not so sure I agree with this one. From an observers point of view, granted, light appears to always be moving at a constant speed, due to time dilation. But surely the photon is still affected and its traversal slowed by gravity regardless of the presense of an observer?
:: The sentence in the article is essentially correct, perhaps a bit misleading. The distant observer will see the astronaut's descent slowing. The *traversal* of the photon is not slowed, photons always appear to move at the speed of light in every frame they pass through. However, you must bear in mind that the astronaut is passing through a lot more space (proper distance) than you think he is! I.e. imagine that by some miracle the astronaut could "pause" at each spot on his way towards the horizon, and get out a metre rule and lay it end to end, spot to spot. He would find much more "space" on his way than an exterior observer would measure if he didn't know the black hole to be there. By this interpretation, the photons do indeed "take longer to escape". However, since the astronaut and the exterior world have no common idea of simultaneity, the idea of "take longer" is meaningless - take longer than what? Now will someone please comment on my "problem of the radius" proposal! Bosmon 00:29, 25 Sep 2004 (UTC)
replies: text: The statement in the article is correct. From the point of view of a distan observer, the photons do take longer and longer to arrive as the infalling object nears the horizon, assuming they are emitted at a constant (proper) rate. -- CYD
How can half of the mass-energy be converted into energy without violating baryon and lepton number? lysdexia 05:38, 16 Oct 2004 (UTC)
thread_title:  this is also wrong 

message: i do not mean to challenge established ideas of black holes, but it does not make sense that objects approaching the event horizon never seem to pass through the horizon from the view of a distant observer. if that is true, then nothing would ever appear to fall in, and all things near a black hole would appear to remain near it, which doesnt make sense. i understand the immense gravitational forces distort the rate of time, slowing it, however my understanding of GR is very superficial right now, and so i am not real confident that i understand what is going on exactly. perhaps this is a well countered argument and i just have not seen the answer yet. 
Quietly 04:27, 2005 Jan 3 (UTC)
:That is exactly how it is. An old name for "black hole" was "frozen star", because a distant observer will continuously receive light from an instant before the star collapsed past the horizon. However, objects "stuck" at the horizon will become dimmer and dimmer as time progresses, because of the distortion of spacetime. (The long story: suppose a distant observer (Alice) is looking at the light emitted by an infalling body (Bob). Bob emits a certain amount of light over a period of one second (on his clock.) However, Alice receives this light over a period of, say, five seconds, because of the distortion of spacetime near the black hole. Later on, Bob has gotten even closer to the black hole. He sends out the same amount of light over one of his seconds. Alice now receives the light over a period of five million years. So it looks to Alice as though Bob is getting fainter and fainter. In fact, for all intents and purposes, Bob will very quickly become completely "black" from Alice's point of view.) -- CYD
::Furthermore, the light being emitted by Bob will become increasingly red-shifted. Visible light has a wavelength of roughly 500nm (very roughly), but when Bob is at the point where one of his seconds is seen as taking 5 million years from Alice's perspective any visible light he emits will be seen as radio waves with a wavelength of approximately 80,000 kilometers (hope I got that calculation right). This is effectively undetectable. Bryan 08:42, 3 Jan 2005 (UTC)
thread_title:  appearance of objects falling towards the event horizon 

message: Question
Details of a black hole's structure are calculated from Albert Einstein's general theory of relativity: a “singularity” of zero volume and infinite density pulls in all matter and energy that comes within an event horizon, defined by the Schwarzschild radius, around it. In 1965, R. Penrose proved the singularity theorem, which says that a singularity must reside inside every imploding star, and therefore every black hole. 
In general, a singularity is a point at which an equation, surface, etc., blows up or becomes degenerate. 
At the singularity, though, the laws of physics, including General Relativity, break down. Enter the strange world of quantum gravity. In this bizzare realm in which space and time are broken apart, cause and effect cannot be unraveled. Even today, there is no satisfactory theory for what happens at and beyond the singularity. Singularity might also mean that our current physical knowledge is either beautifuly wrong (somewhere) or just incomplete. 
This brings me to the questions: How could black holes form if there is singularity? What is exactly meant by infinite density? Is negative energy only possible when the mass is positive (meaning above zero)? What if the mass is also imaginary (or complex-valued)? Orionix 13:59, 8 Feb 2005 (UTC)
replies: text: So, um, that's a nice summary of the history of black holes, but what are you asking, exactly?  Is it possible to have a black hole with imaginary mass?  Short answer: no, not in classical GR.  The mass of the black hole is a parameter in the metric, which is strictly real-valued.  Lethe | Talk 08:37, Feb 10, 2005 (UTC)
Sorry, i didn't formulate my question correctly. 
The problem is the singularity. Classical GR incompletely describes gravity. Inflationary models are becomming the leading candidates now. Inflation also predicts that a characteristic pattern of long-wavelength gravitational waves would have been created in the early universe. These waves are literally gravitons - the hypothetical particles that carry the gravitational force - that have been stretched to macroscopic lengths by the cosmic expansion. The detection of these waves would provide a unique signature of inflation. -- Orionix 00:02, 9 Mar 2005 (UTC)
thread_title:  Negative energy with complex-valued mass: Is it possible? 

message: Is there any proof, besides X-rays, for the existence of black holes? What does Einstein's theory of Relativity have to do with black holes?
And how would gravity affect light? Graham P.(user)
::Black holes are predicted by general relativity (though there is doubt whether GR correctly describes the geometry of space and time). Basically, it's impossible to understand the structure of black holes without understanding GR theory. 
::Black holes will remain theoretical (at least for now) but there are good reasons to think they exist. The observation and analysis of supernovas and binary stars using radio telescopes is a good indication. Black holes could be the strong sources of gravitational waves (also predicted by general relativity), if these exist. -- Orionix 00:02, 9 Mar 2005 (UTC)
thread_title:  Proof 

message: http://news.google.ca/news?gl=ca&ned=ca&hl=en&ie=UTF-8&q=Black+holes+don%27t+exist&btnG=Search+News
:Don't believe everything you read.--Joke137 14:26, 4 Apr 2005 (UTC)
replies: text: I read one of the original papers. It seemed that the author was arguing from metaphor: He made an approachable metaphor for black holes, and then seemed to argue that whatever held true for the metaphor held true for black holes as well. I forwarded it to a physicist friend of mine, who it turns out had received another paper on the same topic from the same conference. He had this to say: "Both papers are mainly techno-gibberish with a lot of buzz words strung together without substance." --kwami 23:46, 7 Apr 2005 (UTC)
thread_title: Black holes don't exist

message: Aren't arguments for unitarity a bit older than the Hawking announcement? Not wanting downplay Hawking's achievements, but sometimes a "Hawking said this", "Hawking said that", sort of style is overused. --Pjacobi 18:34, July 19, 2005 (UTC)
:I absolutely agree. I just moved the paragraph about Hawking down when I added the section about unitarity. I'm not sure who first pointed out the problem, or any of the history. In any case, the paper just came out today, a year after the conference for which his acheivements were plastered over every newspaper in creation. –Joke137 18:41, 19 July 2005 (UTC)
thread_title:  Unitarity 

message: I boldly switched the two biggest sections and reorganized a lot of the smaller sections. I was thinking of the Big Bang page as I did this, which is, I think, a very mature and well organized page. It seems to make sense to have a discussion of the evidence for black holes before talking about more technical things such as the event horizon and singularity. Everything you need to read the evidence section is, I think, contained in the introduction and history sections. –Joke137 21:23, 19 July 2005 (UTC)
thread_title:  reorganization