Talk:Black hole

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Black Hole Emitting Light?

I recently read an article in Scientific America stating that "as matter whips around a black hole, it radiates light perpendicular to its orbital radius." This doesn't seem plausible, as it has been stated that black holes pull in light (in the wikipedia article as well). - Akios.Metta 08:00, 11 November 2006 (UTC)

You are probably talking about photons coming out of the accretion disc. As long as the photons didn't pass the event horizon they can escape the pull of the black hole.Melamed katz 20:48, 11 November 2006 (UTC)

Irony in the article

How is it that "observable" emissions from black holes (ultraviolet rays, etc) are even observable in the first place? All frequencies of radiation on the electromagnetic spectrum travel at the same speed: the speed of light. If light (I'm assuming visible light) cannot escape, how can X-rays? Ninetigerr 06:36, 16 November 2006 (UTC)ninetigerr

Black hole#Observation covers this already. The radiation is emitted by material orbiting outside the event horizon where light can still escape (or in the case of Hawking radiation it's emitted from the empty space just outside the event horizon). Bryan 06:48, 16 November 2006 (UTC)

Colonize Black Holes

This sounds stupid, but would it be possible to colonize a black hole? Please respond to this question. Mrld 02:33, 27 November 2006 (UTC)

The short answer is "no", the long answer is "Nnnoooooo!" But see Frederik Pohl's Heechee series for an interesting sci-fi treatment of the subject. Doc Tropics 02:48, 27 November 2006 (UTC)

It's not stupid. Certainly, we couldn't colonize a black hole in the sense of "landing on it". However, it would be possible (theoretically) to put a space station in orbit around a black hole, or even build some large structure (a ring or a sphere, for example) around (but outside of) the black hole. The reason for doing this is that spinning black holes could actually serve as energy sources. There is something called an ergosphere around a spinning black hole, outside of its event horizon. It is theoretically possible to throw something into the ergosphere and get back something with higher energy. The practical details of how to do all this are -- of course -- well beyond our current abilities. --MOBle 19:33, 2 December 2006 (UTC)

So will a distant observer (ie us here on earth) ever see a black hole form

Sorry to go back to this old chestnut, as it seems to have been siezed upon by an editor with lots of "interesting" ideas. But i have come across this problem before, and never had it satisfactorily debunked. I'm going to phrase it carefully: Given an arbitrarily long but finite amount of time (distant observer's local time) will a distant observer ever see a star become a black hole. Time dilation during gravitational collapse suggests not.--Mongreilf 14:00, 30 November 2006 (UTC)

Define "see".

The problem with question is that -- strictly speaking -- we cannot know whether or not something is a black hole until the infinitely far in the future. (See Hawking & Ellis' The large-scale structure of spacetime, or Thorne's Black Holes and Time Warps.) A black hole is defined as a region from which nothing can ever escape. We can't know if anything ever escapes until we know everything that ever happens. In this sense: no, we can never "see" a black hole form.

Now, Physics is more or less the science of predicting what will happen, given some knowledge of the present state of the Universe. In this case, we measure a certain amount of mass (for instance, by watching stars in orbit around the mass), and the size of the space it fills up. If it is smaller than a certain size, General Relativity predicts that nothing will ever escape. In this sense, we can deduce that a black hole exists. Any time astronomers talk about "seeing" a black hole, this is basically what they mean. (Of course, General Relativity might actually be wrong...)

Personally, I think this is sufficiently well explained in the Evidence section. On the other hand, I'm not the target audience. If you have any ideas on how to make it clearer, please share. --MOBle 19:33, 2 December 2006 (UTC)

see as in a distant observer looking at a warm collapsing remnant star, emmiting radiation (though horribly red shifted), then later no longer emmitting anything due to the escape velocity exceeding light. and i mean in a GR model of this situation, rather than reality. all from the point of view of the distant observer —The preceding unsigned comment was added by Mongreilf (talk • contribs) 13:06, 3 December 2006 (UTC).

(Well, that's not really the definition of a black hole, but...) Okay, so now we can ask if it is ever possible to see something go so dim that it stops emitting anything -- like a photon. Presumably, if it ever does grow so dim, there is some last photon emitted. How can we possibly decide if we've seen the last photon? I maintain that we can't, unless we know everything that will ever happen, or use Physics to predict everything that will ever happen. This is the same answer as above.

I write this only in the hopes of clarifying my previous comment with a helpful analogy. As noted, the definition of a black hole does not involve dimness or escape velocity. --MOBle 23:05, 3 December 2006 (UTC)

yes, fair enough about the last photon, but presumably we are able to model and extrapolate a decline in photons and use a lack of photons received as statistical evidence that such a model is good. very little science is based on "total information". also i must be very old school, i thought escape velocity > light speed was one of the definitions of a black hole.--Mongreilf 12:21, 4 December 2006 (UTC)

Regarding the "model", this is just what I mean by "use Physics to predict everything that will ever happen". I agree that very little science is based on "total information", but this is precisely the unusual feature of the definition of a black hole; it is both a strength (in the sense that it allows us to prove interesting mathematical theorems and a weakness (in the sense that it requires a knowledge of the future of the Universe). If I recall correctly, Hawking's decision to use this "teleological" definition is insightfully narrated in Thorne's book.

The currently accepted definition is this one that basically involves an (absolute) event horizon. The original notion did use the somewhat weaker apparent horizon, which sort of boils down to "escape velocity" > speed of light. This is also the heuristic definition, but it is not exactly correct. --MOBle 22:56, 4 December 2006 (UTC)

Mass of black holes

On the main page about black holes at the beginning where it mentions the masses expected for black holes, it has been suggested in J-P Luminent's "Black Holes" (Cambridge press, (c)1992)(its an older book but its points are still valid), that it is possible to have a black hole of any mass if its contents are compressed sufficiently. In the book he mentions that a black hole could form when a mass the size of a mountain, is compressed down to a small Tv, which is significantly less massive than the mentioned sizes predicted on the main page. Additionally, the next generation of particle accelerators are predicted that they may even be able to create micro miniature black holes with lifetimes in tiny fractions of seconds, as was suggested by a fairly recent poplar science article. Chris2746 07:55, 3 December 2006 (UTC)

It is true that a black hole could have (basically) any mass. Rock weighs about 2700 kilograms per cubic meter. I guesstimate that a mountain might weigh 10¹³ kg. In geometrized units, this is 10^-14 meters. (Google calculation) Multiply by two to get the size of the black hole, and you get something the size of a proton. That's a pretty small TV. Conclusion: Luminent was either wrong or talking about something else. One way to find out is to look for another source. The particle accelerator issue was dealt with above. --MOBle 23:05, 3 December 2006 (UTC)

"Simple Overview"

I'm not a big fan of this addition. I think the existing article is very clear, and certainly understandable by anyone with a middle-school education. The new section is redundant, and it itself needs work on tone and clarity. Thoughts? —The preceding unsigned comment was added by 169.229.95.134 (talk) 03:05, 10 December 2006 (UTC).

The concept is not necessarily clear to such people; this addition was prompted by queries which made that clear. Reading the article, it explains in scientific terms that a bblack hole is "an object with a gravitational field so strong that nothing can escape it", and covers the history and formation and evidence for/of them. But it doesn't describe, in laymans intuitive terms, what a black hole actually is. That's a common problem with some types of articles. It's easy to assume knowledge and background that many readers don't have. A summary overview that explains a scientific concept in lay-terms is often very beneficial as a sort of orienting overview of counter-intuitive concepts such as a mass collapsing into a zero (as opposed to small) space (singularity), and placing that in the context of "normal" matter. A student or lay-person with little physics knowledge can read the two paragraph overview and gain a fairly accurate understanding of the field, including enough understanding to make sense of (or discuss) other aspects of the topic. Some touching up might not hurt but the basic idea is valuable. FT2 ^{(Talk | email)} 03:37, 10 December 2006 (UTC)

Black Hole: Destructive Force or Life Builder

Black Hole Theories:

For all my adult life I have wondered if our understanding of Black Holes have been flawed. It is taught in grade school and even our colleges around the world that Black Holes destroy everything in there paths, this belief is from only looking at similar occorances here on earth, such as, Hurricanes & Tornados. While it is a common understanding that these earthly forces do have a path of destruction, does it always mean that all aspects of occurances throughout the universe having the same appearance as those on earth, make them a destructive force?

In science we have been taught to look at all the aspects of what is not understood so that we can conclude a new understanding. So, how about this aspect of Black Holes, Life Builder.

As we look into the the heavens and admire the shear beauty of the universe we find one specific denominator, what is it? Well, it's not the darkness. It's not the great number of lights. It's not the shear number of galaxies or planets that inhabit the galaxies. Actually it is one thing that 99.99% of all human see everytime that we see the moon in the night sky or pictures of our beautiful planet that we call home. The one common denominator is in fact that not only as with the earth and the moon, every planet that we see in pictures are "ROUND" not square or mashed together but "ROUND".

We have been taught that matter and light do not escape the gravitational forces of a Black Hole and that once caught in the clutches of the event horizon, all life as we know it is doomed for destruction and out of existance. It is my belief that this teaching and understanding is flawed.

As with the mining of minerals from beneath the surface of our planet earth and mingling it with fire and other minerals transforms Iron Ore into steel, it would not be far fecthed to believe that Black Holes do the same thing and with the same type of results with the creation of life in the form of "ROUND" Planets.

If we look at the event horizon of a Black Hole we would notice not only light but also debris in the form of astroids and whatever else was caught up in the outer bands of this massive looking storm. With all these particles being directed to the center of the Black Hole, this debris would be compressed into a molten ball eventually being redeposited into the vastness of space. Towards the end of the life of the Black Hole and one the deposits of debris begin to run out, this may very well be where each Solar System is born into existance with a new sun. As with our own Sun, we do know that it has a strong gravitation pull on all the planets that surround it along with debris fields known as the Milkey Way, revolve around that center focus.

I pose this one Question: Could it be that our Sun as we know it, is the actual reminants of a Black Hole, burning off the last remaining particles of light and debris resulting in the heat and light that nurtures the life on the surface of our planet that we call home? Jdsparks 20:04, 10 December 2006 (UTC)

To answer your questions briefly (a more detailed answer might be obtained by asking on the Wikipedia science reference desk:

1. The sense in which black holes "destroy" everything is, that an object which gets too close to one, will be unable to escape, and will ultimately suffer destruction molecule by molecule through spaghettification, due to the extreme forces of gravity operating within a black hole. That should count as "destruction" in any book, at least as far as those living where it came from are concerned. See the Simple Overview section of this article for more.
2. The description is nothing to do with earthly weather, and everything to do with what happens when you put a finite mass in a relatively tiny amount of space until it collapses under the force of gravity, and then travel too close to it.
3. Whilst it is possible that in some ways black holes build and create "life", in whatever form that might mean, we have no evidence of it now. Wikipedia is not a crystal ball, and we do not put ideas in here that aren't verifiable (or at least whose credible and noteworthy proposal is verifiable) in the real world somehow. This is (at the moment at least) just speculation, and we don't put our own ideas in Wikipedia, however attracted we might feel towards them, but only things which seem to have some kind of notable support or the like.
4. The reason things are (roughly) round is similar to the reason that the oceans and your glass of water are (roughly) flat. There's no great mystery to it. Gravity attracts matter equally in all directions, and the shape which a centrally directed and rotationally symmetrical force tends to create is a sphere (in 3 dimensions) or a circle (in two).
5. Your belief may well be interesting for you. But it wouldn't get into encyclopedia brittanica or any other encyclopedia, because it isn't shared by those who have experience of such things. You might find this page WP:OR on personal views and beliefs, useful.
6. A black hole is not going to create a ball of some kind at the centre, molten or otherwise. The centre of a black hole is a point, known as a singularity, and any object sufficiently close to it will be shredded into pieces, then the pieces into atoms, then the atoms into subatomic particles, and into the zero size point that is the singularity. There's no solid object in the middle for planetary formation, the gravitational field would be too strong for that to exist.
7. The Milky Way doesn't lie around the sun and isn't a debris field of any kind. In fact the sun is close to the edge, in one of the spiral arms of the Milky Way galaxy, nothing to do with the centre.
8. Last question, no it couldnt be. A black hole is simply a black hole, they don't have "remnants" that "burn off last particles". You might be thinking of other forms of star, but a black hole is not that. It isn't "burning" anything. It's collapsing into zero space, due to the incredibly high gravitational field it creates in its immediate vicinity.

Hope this helps. FT2 ^{(Talk | email)} 21:40, 10 December 2006 (UTC)

Table?

This useful looking table was on the french wikipedia. Any use?

Types théoriques de trous noirs en fonction du moment cinétique (J) et de la charge électrique (Q). La masse (M) est toujours strictement positive.

	M > 0
	J = 0	J ≠ 0
Q = 0	Schwarzschild	Kerr
Q ≠ 0	Reissner-Nordström	Kerr-Newman

FT2 ^{(Talk | email)} 02:13, 11 December 2006 (UTC)

nearest candidates

Hello, I've added a table with the nearest candidates. Please double-check. Have fun. Dream about visit. -- Rwst 17:10, 15 December 2006 (UTC)

Black holes in fiction?

There should be an article for Black holes in fiction, similar things exist for a lot of stars and there is even one for White holes in fiction. Or does it exist somewhere under another name? There is a bit in Black hole (disambiguation), but very little.

Creatures of Light and Darkness contains the concept, though not the name, something called Skagganauk Abyss where there is no space or time. I'm fairly sure I also recall an SF story that features something we would now call a Black Hole, probably taken from the author's reading of science.

--GwydionM 20:10, 21 December 2006 (UTC)

I say go ahead and start such an article if you are inclined. I'm not sure it would be appropriate to have a section in this article on that topic. Such a section is liable to get too large and distract from the scientific focus of this article. A link from here to the BH in fiction article would be appropriate, however. -Joshua Davis 22:16, 21 December 2006 (UTC)

Now added. --GwydionM 17:56, 23 December 2006 (UTC)

GA failed

This failed GAC for these reasons: the biggest problem is that it's undercited. One section has no cites. Multiple paragraphs, several in a row have no cites. Refs come after punctuation, not in the middle of a sentence. The X-1 photo has a tag that it's about to be deleted.Rlevse 03:28, 7 January 2007 (UTC)

Bad section

The section Event horizon is incorrect. It states that nothing can escape a black hole because the escape velocity is greater than the speed of light. This is an old, incorrect, non-relativistic explanation of a black hole. The real reason that nothing can escape a black hole is that space-time is curved due to gravity and only an inward direction remains. Berrick (talk) 11:31, 10 January 2007 (UTC)

• Can you provide a reference? Given the kind of metrics used inside black holes, I'm not sure what your saying is a good presentation without backup explainations for laymen. WilyD 14:15, 10 January 2007 (UTC)

Until this is fixed, I've patched the intro to remove the technical reason (whether escape velocity or curvature) and in doing this, tried to make it more helpful, for example by explaining the observation is impossible for the same reason. FT2 ^{(Talk | email)} 17:49, 10 January 2007 (UTC)

I've looked into it, and find it very hard to find a short, satisfying explanation why nothing can escape from inside the event horizon. It is because light cones are rotated which, ultimately, is caused by the gravitational field. You can find this in any textbook on GR that treats black holes. The problem is that there's a whole theory of general relativity between gravitation and the rotation of light cones, which is, off course, too much for an encyclopedia. So I suppose it is a bit to straightforward to say that nothing can escape from within the event horizon because space-time is curved.

To state that something isn't true, as in this case the section, I shouldn't need to provide a reference. It is up to the writer/defender of the original piece to provide references of his/her writing. For arguments see the discussion on the physics project page

To keep things simple and correct I propose to replace

• Inside the event horizon, the escape velocity is more than the speed of light. This is why anything inside the event horizon, including a photon, is prevented from escaping across the event horizon by the extremely strong gravitational field.

by

• Due to the extremely strong gravitational field, anything inside the event horizon, including a photon, is prevented from escaping across the event horizon.

If nobody objects I will replace this in a few days Berrick 22:11, 12 January 2007 (UTC)

If you see something like this which you feel is incorrect or uncertain and you want to demand that others justify it, then you can put a {{cn}} tag right after it and it produces an effect like this. JRSpriggs 05:00, 13 January 2007 (UTC)

Alternative models: plasma cosmology

The alternative models section mentions plasma cosmology in a rather etherial style. Since plasma cosmology is not accepted as mainstream science, perhaps this bit should be removed. Man with two legs 17:34, 11 January 2007 (UTC)

It's worth summarizing some of the alternative and opposing views; if nothing else it is relevant to balance and also provides relevant ideas of other people's proposals related to black hole observations and theory. But it shouldn't be promoted as such, just stated as factual, and the space or approach should also reflect its support base - scientists or not, tiny or notable, and so on. But just because it's not mainstream scientifically accepted is not, by itself, the criterion for appropriate removal. FT2 ^{(Talk | email)} 20:53, 11 January 2007 (UTC)

Charged black holes & magnetic fields

I came here to check for information on charged black holes, their magnetic fields and the interaction with an accretion disk. But there doesn't appear to be much coverage of this topic, whether here or on the charged black hole page. I'd think this an important, if not vital sub-topic as it relates to relativistic jet formation, for example, which regulates galaxy mass accumulation, &c.[1][2] So I'd like to include this on the to-do list for completeness. Thanks. — RJH (talk) 23:32, 11 January 2007 (UTC)

Any black hole of stellar dimensions or larger would not have a significant charge. If there were one, charged ions of the opposite charge would quickly be drawn into it and neutralize its charge. JRSpriggs 08:03, 12 January 2007 (UTC)

True, a black hole is likely to have a very small charge, however, it certainly can have a tiny bit of charge. I can try to take some time on ADS to see if I can dig up a paper on the subject - you're right,the charge and B-field of a black hole is likely to be insignificant. But perhaps not uninteresting - I've certainly discussed the subject in a professional setting as an astronomer, if only of idle interest. WilyD 14:42, 12 January 2007 (UTC)

Thanks. If not from charge then perhaps something on the generation of a relativistic jet from the rotational energy of the black hole, in combination with the magnetism of the accretion disk?[3] I.e. how is the energy transferred from the black hole to the jet? The concept is unclear enough in my mind that an explanation on this page would be helpful. — RJH (talk) 22:35, 12 January 2007 (UTC)

I think that the energy driving the jets is coming from the accretion disc rather than the black hole itself. I am just guessing, but I think that the gravitational energy of infalling material is converted to kinetic energy in the disc and then a small part of it goes to heat due to friction and then it shoots out sideways in the jets because it cannot go back out via the equator which is full of accreting stuff. The plasma in the disc captures and amplifies ambient magnetic fields as it spins and they force the escaping particles into a narrow beam. JRSpriggs 04:51, 13 January 2007 (UTC)

error in the simple overview section

There is an error in the simple overview section. Gravity is an extreamly weak force and is not responsible for keeping subatomic or atomic particles together. This section should briefly mention the strong and weak nuclear forces. —The preceding unsigned comment was added by 63.78.207.104 (talk) 18:10, 20 January 2007 (UTC).

I rewrote the section to correct that error. The force responsible for keeping matter from collapsing is the Pauli force which is a consequence of the Pauli exclusion principle, the Exchange interaction, and the Kinetic energy resulting from the Uncertainty principle. Gravity, Electromagnetism, and the Strong force all tend to pull matter together. Gravity is dominant only for very large bodies. I do not think that the Weak force is a significant player in this, so I did not mention it. JRSpriggs 11:47, 21 January 2007 (UTC)

If the sun were a black hole

I heard that if the sun were to be a black hole of the same mass, nothing would happen to the planets, they won't "fall" in the black hole. However, the gravity of the black hole is so powerful that things within the black hole's horizon have no hope of coming back. So, if the sun were to be a black hole, where its horizon would be located and why planets would not "fall" in the black hole despite its overwhelming gravitational force? —The preceding unsigned comment was added by 24.201.98.14 (talk) 17:27, 22 January 2007 (UTC).

The strength of the gravitational field of an object is determined by its mass. If the Sun were replaced by a black hole of the same mass, its gravitational field (outside of the Sun's radius) would be the same as it is now. As Newton discovered (and is still true in Einstein's theory), the gravitational force outside a spherical shell is the same as if its mass were all concentrated at the center, whereas inside the spherical shell the gravitational force is zero. Therefor, at any place inside the Sun, the Sun's gravitational force is the same as if it were being generated by the ball of solar material inside that radius, crushed to a point at the center. So (making the very crude approximation that the Sun's density is constant), the Sun's gravitational force is proportional to the distance from the center (Mass/radius² is proportional to radius). For a black hole the entire mass is crushed to the center, so its gravitational force keeps increasing as you get closer. The black hole's Schwarzschild radius (where the event horizon is) would be about 3km (2 miles) - much less than the Sun's radius. JRSpriggs 07:18, 23 January 2007 (UTC)

One thing, thinking about general astro physics, not only black holes by themselves, is that so much energy is released if the sun collapsed to a black hole, it would be a supernova. The outer parts would be ejected in high speed, have large effect on the planets. -- BIL 08:43, 29 March 2007 (UTC)

Inside the spherical shell the gravity is NOT zero. Between the original radius and the event horizon gravity increases exponentially as one moves towards the center, producing enormous tidal forces and the famous spaghettification effect.--kwg06516 22:46, 19 April 2007 (UTC)

You seem to be misreading the example given. The relevant part was "under Newtonian gravity, both a spherically symmetric star and a same-mass black hole are equivalent to point masses", meaning their effects on objects distant from them are identical. The shell discussion was part of the proof for this equivalence. --Christopher Thomas 23:05, 19 April 2007 (UTC)

Kruskal diagram

Can someone explain what that is? Maybe an article about it would help--there isn't one now. 67.117.130.181 22:23, 15 February 2007 (UTC)

I think you are looking for Kruskal-Szekeres coordinates. It does need more work and a diagram or two. JRSpriggs 07:47, 16 February 2007 (UTC)

By the way, where in this article is "Kruskal diagram" mentioned (the subsection and paragraph number)? JRSpriggs 08:07, 18 February 2007 (UTC)

The Kruskal diagram is a spacetime diagram that lets you figure out the order of events near the boundary of a black hole. It's not mentioned in the article but there are a fair number of Google hits for it. I heard about it in astronomy class and had hoped to find out more about it here. It's somewhat hard to understand from the references I've looked at, given my very limited prior knowledge. Thanks. 67.117.130.181 09:09, 19 February 2007 (UTC)

See section 31.5 of Gravitation (book). JRSpriggs 09:49, 19 February 2007 (UTC)

Possible restructure?

Theoretical physics is a really tough subject for an encyclopedia, since readers can range from 12-year-olds (or younger) to physics undergraduates (not to mention science fiction enthusiasts or would-be writers looking for background). I don't see how a single presentation of a subject like black holes can meet the needs of audiences at both ends of the range.

So I suggest the article should be restructured into 2 major sections, or possibly separate but linked articles, one "popular" and one "technical".

The popular section / article would contain:

• Non-technical definition. It would still have to mention general relativity in order to explain why a spaceship can't escape, because a Newtonian explanation only explains why unpowered objects and particles can't escape.
• Anatomy of a black hole: singularity, event horizon, ergosphere (if the hole spins fast), etc.
• Formation of black holes.
• Evaporation of black holes via Hawking radiation.
• Black holes at the centers of galaxies.
• Black holes and earth (as at present).
• Link to wormhole with brief note.
• Black holes in fiction. BTW at the time of writing I can't find in this article a link to a separate article "Black holes in fiction", as discussed above.
• Link to "Alternative theories" section of the "technical" page / section, plus very brief description (more of a note).

"Technical" page / section: all the other content.

I would prefer separate pages, since that both articles would still be pretty long. I think the "popular" page should be the main entry, since more readers of Wikipedia are likely to be non-technical, but the "popular" page should have a link to the "technical" page at the top and in "see also".

The "popular" page would have to be checked by a competent physicist (I'm not even an incompetent physicist) to ensure that it contains no more distortions than those that are inevitable when describing a highly mathematical subject in plain English. Then it might be a good idea to lock it, to prevent editing without competent supervision. I think the "technical" page should remain open, sop that new theories and discoveris can be incorporated.

Question: May, Moore & Lintott's book "Bang!" says (p 77 in UK edition) that the orbit of a star labeled S21 proves there's a large black hole at the centre of the Milky Way - the orbit's minimum radius is 17 light-hours so that limits the radius of the central body, the orbital period is 15.2 years and that determines the mass of the central body, and the central body's mass and maximum radius mean it has to be a black hole (discounting the exotic alternatives to black holes). Can anyone find an academic reference which describe this analysis? I've googled and found nothing.

Note: http://cosmology.berkeley.edu/Education/BHfaq.html has a good FAQ, which should be in the "links" section of the "popular" page.Philcha 14:04, 17 February 2007 (UTC)

Generally, we like to have articles begin with a non-technical explanation and work up to more technical stuff. Feel free to re-arrange the sections accordingly. As far as protection is concerned, that is determined by whether the article is a target for attacks by vandals, not by its level of sophistication. JRSpriggs 08:12, 18 February 2007 (UTC)

Just random talk, and I guess a question

I've always found black holes rather novel. Since I'm not (yet) educated in astronomy or even really basic physics, my speculations are anecdotal and not based on any real science. Still, I always thought it would be fun if the "pre-big bang" pinhead-sized universe was the other end of the event horizon of a black hole in some other universe in the big multiverse.

It would be novel and cool, and if I've considered it, then it must have been considered by a million other people, and has been disproved (or at least discussed). Does anyone have any information about that?

Maybe I should write a science-fiction book (if one hasn't been written about this already). CaTigeReptile 18:28, 23 February 2007 (UTC)

• FWIW, the Universe probably does not exist before the Big Bang (of course, our knowledge of the first $10^{-43}$ seconds is non-existent. and probably shaky until $10^{-10}$ seconds or something - I'd have to ask a cosmologist.

Anyways, I've seen it suggested you can get Cosmic inflation from Brane theory in a way that's conceptually similar to what you're talking about. There are probably some papers on it - I really don't want to try to read them though. WilyD 18:40, 23 February 2007 (UTC)

Yes, many people have considered that idea. However, I am not aware that anyone has figured out a plausible way to change the collapse into an expansion. JRSpriggs 09:01, 24 February 2007 (UTC)

In string theory there is a theoretical proof that if the universe (or just one dimension of the universe) were crushed rather than expanded in the Big Bang, the particle-particle interactions would be the same as if it had actually expanded. By this theory the universe could be an ever shrinking point, rather than an expanding volume, but we simply can't tell the difference. Or the universe could be a line or a disk. Purely theoretical though, and to my knowledge no one has shown that this theory can be applied to the singularity inside a black hole, although several theorists have postulated on the possible consequences of such. Brian Greene discusses the "sub-plank" universe theory in his book "The Elegant Universe," though he doesn't mention the suggestion of a universe inside a black hole. Someguy1221 14:59, 11 March 2007 (UTC)

Thanks for your answers! CaTigeReptile 12:23, 19 March 2007 (UTC)

Time Dilation

Does time stop at the event horizon relative to an outside observer, or is it just a false image portrayed by the delayed photons? If time stopped outside the event horizon, then wouldn't a black hole be frozen in time and what way would time be dilated within the event horizon? —The preceding unsigned comment was added by Emorej717 (talk • contribs) 20:08, 23 February 2007 (UTC).

Time dilation does not work that way - but yes, it goes to infinity at the event horizon for an outside observer. So does the redshift, however, so the object emits no radiation, as it stops moving. WilyD 20:29, 23 February 2007 (UTC)

The event horizon is a light-like hyper-surface so events at the same place (but different coordinate-times) on that hyper-surface, although distinct, have no (spatial or temporal) separation. However, objects falling across the horizon continue to experience time while crossing and afterwards, until they are annihilated at the central singularity. JRSpriggs 08:56, 24 February 2007 (UTC)

Alternative explanation paragraph excluded

Removed from the article:

Finally, plasma cosmologists suggest that Birkeland currents provide an alternative explanation for the observed phenomenon, whereby plasmas transfer energy over great distances to smaller regions where it may be periodically or catastrophically released. Anthony Peratt explains the flickering of electromagnetic radiation: "The flickering of a light in Los Angeles does not mean that the supply source, a waterfall or hydroelectric dam in the Pacific Northwest, has abruptly changed dimensions or any other physical property. The flickering comes from electrical changes at the observed load or radiative source, such as the formation of instabilities or virtual anodes or cathodes in charged particle beams that are orders of magnitude smaller than the supply. Bizarre and interesting non-physical interpretations are obtained if the flickering light is interpreted by a distant observer to be both the source and supply." Plasma cosmology in this manner is a minority view not within mainstream science

This basically functions as an Unduly weighted advertisement for plasma cosmology. According to science notability and fringe reporting standards, this coverage is not warranted here since plasma cosmology is ignored in both the mainstream and in the popular press. No one takes Peratt, for example, to be an expert in black hole alternative models. --ScienceApologist 19:45, 1 March 2007 (UTC)

Yes in the Wikipedia of 1543 ‘On the Revolutions of the Celestial Spheres’ by Copernicus was deleted from the article on ‘Astronomy’ because it was an ‘unduly weighted’ advertisement for ‘heliocentricity’ and because it was not in the mainstream or the popular press. This was an appropriate edit by Ptolemaicists. Nothing should ever be allowed to interfere with dogma. DasV 16:38, 17 April 2007 (UTC)

work

this article is great but, it needs a lot of work.ψ — Preceding unsigned comment added by Drock211 (talk • contribs) 00:31, 6 March 2007 (UTC)

WP:SOFIXIT Could you be more specific? JRSpriggs 06:51, 6 March 2007 (UTC)

It could use more in-line citations, especially when it comes to the parts about describing what the actual black hole really is. Diez2 16:43, 7 March 2007 (UTC)

The biggest problem is that one can't say much about what a black hole is - our understanding of space-time breaks down inside of the event horizon (which is why the radii of black holes are given in terms of the event horizon and not in terms of what is inside). In fact, due to the cosmic censorship principle, it will be impossible (as far as we know now) to know what's going on inside the black hole. The only practical thing to do is talk about the black hole's effect on surrounding space. Saj210 01:39, 23 March 2007 (UTC)

Accretion disks

Neither the Black hole article nor the Accretion disk article explains why the infalling matter forms a disk. It's easy to present an arm-waving explanation for why infallng particles follow a spiral path - most gas clouds rotate; most black holes rotate, the gravitomagnetic effects of this rotation drag the matter nearest the event horizon into a spiral path, friction drags the next layer into a spiral path, etc. But why is the matter concentrated into a disk, when the heat generated by the friction ought to dissipate the disk into a more amorphous cloud? And can anyone point to a more scientific explanation of the spiral paths? (note to self: [4])

It would be also be helpful if someone could point to estimates of the density of accretion disks near the event horizon, preferably with comparisons to e.g. the outer layers of the sun.Philcha 14:42, 17 March 2007 (UTC)

My understanding is that much of this is just the result of angular momentum and friction. In an amorphous cloud, you may have net angular momentum (rotation of the cloud), but particles follow all manner of orbits, resulting in a cloud that's a spheroid. Near a black hole, the cloud is dense enough for particles to interact with each other, and the net result of such interactions is for non-average momentum components to be cancelled out (particles moving relative to the average flow feel drag). End result: In the inner disc, all particles have approximately the average angular momentum, and follow approximately circular orbits in the plane of the disc.

Within the disc, you have the inner parts orbiting more quickly than the outer parts, due to being deeper in the black hole's potential well. This causes shearing between adjacent streams of orbiting matter. Drag between such streams tends to rob energy from more closely-orbiting (faster-moving) streams and give it to the farther-oribiting streams, causing the innermost layers to spiral further inwards. As there is friction involved, some of the energy winds up as heat, causing net movement of the disc inwards as well.

I don't have numbers for the density of accretion discs around black holes, but there are many papers about this (search for rapidly-variable x-ray sources, as these are thought to be black hole accretion discs that switch between two different almost-stable modes of operation).

If any lurking astrophysics-type sees a mistake in my post, by all means correct it. I am not an astrophysicist. --Christopher Thomas 17:45, 17 March 2007 (UTC)

Rotating black holes: frame-dragging at / within event horizon?

The article Rotating black hole says that space-time is dragged at the speed of light at the outer boundary of the ergosphere and faster than light within the ergosphere. It also says, "Particles falling within the ergosphere are forced to rotate faster and thereby gain energy. Because they are still outside the event horizon, they may escape the black hole," which implies that the ergosphere is entirely outside the event horizon. So:

• Is there any frame-dragging within the event horizon of a rotating black hole?
• How does the frame-dragging rate vary with increasing "depth" (a) between the outer boundary of the ergosphere and the event horizon? (b) inside the event horizon?
• What happens just outside the outer boundary of the ergosphere? If space-time is dragged at less than the speed of light, is it possible for an object just outside the ergosphere to drain rotational energy from the black hole? If there is no dragging, why is there a discontinuity?Philcha 21:56, 17 March 2007 (UTC)

Also, does the frame-dragging rate vary with "latitude", i.e. is it different at the equator and near the poles? If so, is space-time dragged at light speed at all points on the ergosurface?Philcha 23:59, 18 March 2007 (UTC)

Rotating black hole: 2 event horizons

The article Rotating black hole says a rotating black hole has 2 event horizons, inner and outer. What are the consequences of this? For example does it mean that a completely external observer can see nothing within either horizon, an observer within the outer horizon can see things within the outer horizon and outside it but not within the inner one (as well as outside the outer horizon), and an observer within the inner horizon can see things which are within the inner horizon and outside it?

Rotating black hole also says, "If the spin is great enough, the two will eventually merge and shrink towards the singularity." Is this an actual process over time or a limiting case? If it's a limiting case: what is the limit, e.g. as the rotation speed tends to infinity? what happens to the ergosphere at the limit? Philcha 22:42, 17 March 2007 (UTC)

Rotating black hole: 2 photon spheres

Rotating black hole says, "Due to the two event horizons, a rotating black hole is also required to have two photon spheres, an inner and an outer one. The greater the spin of the black hole is, the farther from each other the photon spheres move. A beam of light travelling in a direction opposite to the spin of the black hole will circularly orbit the hole at the outer photon sphere. A beam of light travelling in the same direction as the black hole's spin will circularly orbit at the inner photon sphere." This raises a lot of questions for non-physicists like me:

• Are a a rotating black hole's photon spheres true spheres, or are they oblate like the ergosphere?
• What are the limiting conditions for possible photon orbits in the 2 photon spheres (for non-rotating BHs, the semi-major axis can't be smaller than the Schwarzchild radius)?
• What happens to the outer sphere at extremely high spin speeds, when the radius of the outer event horizon is close to zero? And what happens to the outer sphere at the limiting spin speed, when the radius of the outer event horizon is actually zero? How far can it extend?
• If a photon grazes the edge of a photon sphere but at an angle to the spin plane, what happens to the component of the photon's motion normal to the spin plane?
• Does the outer photon sphere have any effect on a photon travelling tangentially to the inner photon sphere and in the same direction as the black hole's spin?Philcha 23:27, 17 March 2007 (UTC)

I didn't really get this bit at all. It starts talking about the 2 event horizons without explaning why there are 2. In fact, the first thing it does it compare the inner and outer event horizons of two different black holes. Confusing! User:Mijin|Mijin]] 13:55, 03 April 2007 (UTC)

Please review the recent edits

I've completed pass 1 of the edit discussed in Talk item "Possible restructure?" (17 Feb 2007). Please check it for errors, inconsistencies and serious omissions. For comparison, the previous version of the article is at [[5]]

The objectives of the edit were:

• Make as much as possible of the content intelligible to someone who is neither a physicist nor (like me) a long-time fan of "hard" science fiction. I thought the previous version's "Simple overview" was just a summary in almost equally technical language, took most of the concepts for granted and therefore was very unlikely to help a non-specialist reader.
• Simplify the language, and reduce the technical terms used to a small and consistent subset, to minimise the reader's learning curve (the old article used a few sets of synonymous or near-synonymous terms).
• Improve the structure: reduce the extent to which the same topic was covered in different sections (often quite widely separated); reduce duplication; order the sections in a way that would be most helpful to the non-specialist. In the process I also moved a lot of the images so that they are now aligned with the sections to which they are most relevant.
• Keep the parts that are likely to be most interesting for physicists (and extremely hard to explain in fairly simple language, at least for me!).

At present I think it may be unnecessary to create separate technical and non-technical articles.

I've left all of the original article in but hidden by HTML comments, in case anyone thinks it necessary to copy and paste parts of it. I know it makes the download too large, but hopefully that can be resolved within a month.

If within the next 2 weeks the edit is not reverted and there are no comments which force me to re-think the whole approach, I'll start researching the questions I've raised in recent Talk posts - but please try to answer them!

The article (old and new versions) needs a lot more citations / references. I'll be searching for these too. Please add any that you know of!

Does anyone know of any better diagrams for the anatomy of black holes, especially rotating? There are so many features that the existing diagrams don't illustrate, e.g. photon sphere, the 2 photon spheres and 2 event horizons of rotating black holes.Philcha 21:20, 20 March 2007 (UTC)

You should probably ping Wikipedia talk:WikiProject Physics about this, to get a faster response. --Christopher Thomas 20:40, 21 March 2007 (UTC)

Thanks, I've put a request for review on Wikipedia talk:WikiProject Physics. Thanks also for asking them to look at the questions I've posted, e.g. re rotating black holes.Philcha 10:19, 23 March 2007 (UTC)

Here are some general comments on the article:

The "Supermassive black holes at the centers of galaxies" section has some severe problems. First of all, the section is incredibly timid. As it currently stands, the section suggests that Sag A* is the only well-studied supermassive black hole and that black hole candidates have only been found in a few other galaxies. This simply gives the wrong impression of current astronomical research. Many active galaxies (including all Seyfert galaxies, all radio galaxies, all quasars, many LINERs, and many ULIRGs) potentially contain black holes. Although no one has conclusively demonstrated that the active galactic nuclei contain black holes, that is the general premise adopted by extragalactic astronomers. Moreover, the page never even uses the term "active galactic nucleus", which is embarassing. Furthermore, the section needs to communicate that the masses of many (potential) black holes have been measured in many nearby galaxies, including many otherwise-normal galaxies (see NGC 4594, for example). This section needs significant revision.

Thanks for the comments, Dr. Submillimeter.
I agree this section should link to active galactic nucleus, and will fix this a.s.a.p.
I'm less sure about adding more info, as I think the "Supermassive black holes at the centers of galaxies" section has enough to tell the non-specialist reader that the evidence is strong to overwhelming and, as your next comment says, the length of Black hole is a concern.Philcha 22:38, 23 March 2007 (UTC)

Having said that, I also think that the entire article is too long at the moment. Much of the material should be summarized on this page and discussed more in depth on other pages. For example, the discussion on "Entropy and Hawking radiation" probably belongs on either the Hawking radiation page or its own page (Entropy and Hawking radiation).

Dr. Submillimeter, I sympathise, but have mixed feelings. Articles which are major entry points to WP tend to be long (e.g. Dinosaur, Global warming) because they carry the overhead of summarising and linking to related topics. At the same time I understand that no reader wants to plough on forever. This dilemma is one I've seen on WP before: for each major entry point WP needs a plan which defines how much detail goes where. Would you like to refer this question to Wikipedia:WikiProject Astronomy, Wikipedia:WikiProject Astronomical objects and Wikipedia:WikiProject Physics and get the 3 projects moving towards agreement - I've seen from the Talk pages that you're a regular contributor in these discussions.
Before I started the recent edit, I suggested that the article needs to be split into 2, "popular" and "technical". What do you think of that idea?
Re "Entropy and Hawking radiation" specifically, I think Black hole needs to explain Hawking radiation in enough detail for a non-specialist to understand the basic idea and that there are still uncertainties about how or even whether it works, and I have an open (?vacant) mind on how best to achieve that.Philcha 22:38, 23 March 2007 (UTC)

Also I've just seen another discussion where an article was turned down for FA status and one of the grounds was excessive dependency on links to other articles - apparently there's an FA guideline that says "the perfect Wikipedia article should be as self-contained as possible ....". That might be a reason for keeping it all together.Philcha 13:00, 6 April 2007 (UTC)

The "Black holes and Earth" section is very flaky. I suggest editing it accordingly.

Dr. Submillimeter, please explain in more detail what you mean by "very flaky". If you think it's misleading or seriously incomplete, please suggest how to fix it.
On the other hand if you mean that it's a bit "pop science", I favour including "pop science" because I think WP's objective is / should be to inform and motivate non-specialists (specialists have plenty of channels already), so it needs "Hot topics" (or "cool topics", in other dialects) to appeal to non-specialists and to get search engine hits.Philcha 22:38, 23 March 2007 (UTC)

If I see other things, I will let you know about it. I also suggest notifying people at Wikipedia:WikiProject Astronomy and Wikipedia:WikiProject Astronomical objects. Dr. Submillimeter 11:48, 23 March 2007 (UTC)

Thanks, I'll do that now. I've already notified Wikipedia:WikiProject Physics, and will ensure that all projects are aware of notifications to other projects.Philcha 22:38, 23 March 2007 (UTC)

I apologize for not answering earlier; I did not see some of the questions buried in the text.

In terms of splitting the article, I really think that the first step is to move some of the lengthier sections to separate pages rather than to split the page into basic and advanced pages. This article should attempt to give a higher-level summary of the topic; the subpages can be used to discuss the extensive details. Anyhow, that is my opinion.

I referred to the "Black holes and Earth" section as flaky because it is based on vague speculation and very improbable events. The chances of any stellar-mass object crossing through the Solar System as described in that section are very close to nil; this is a common galactic dynamics problem that I studied as a graduate student. The particle accelerator section seems to discuss something at great length that the section itself says is improbable. I would guess that the article would be improved by deleting the entire "Black holes and Earth" section altogether. Dr. Submillimeter 22:11, 28 March 2007 (UTC)

No need to apologise for not answering earlier - 5 days is pretty good, and I know you have a day job.

Re splitting the article, I admit my own opinions on this keep changing - I originally proposed such a split, then edited the article and thought no split was needed, and now the length is making me favour splitting again. I suppose the criterion should be whether a section can be summarised in a way that is informative enough but still significantly shorter - if "yes", then split it out. My own inclination is to keep the article in one piece until it looks good enough to be stable, then review for splitting - otherwise there's a danger that e.g. a summary will be edited but not the linked detail article.

Re flakiness of the "Black holes and Earth" section, I still favour keeping it for the reasons I mentioned earlier, and because dispelling misconceptions is an achievement (John Locke described himself as an "under-labourer" preparing the way for the "master-builders" of science [6]). Sounds like you're a genuine expert on why it's unlikely that a black hole would travel through the solar system - please edit this bit and add the references (your own work if appropriate!)Philcha 11:29, 4 April 2007 (UTC)

Conflict between "no hair" theorem and Hawking radiation?

As far as I understand it (?????), the "no hair" theorem implies that black holes have no temperature but the Hawking radiation theory implies that they do. Is there a conflict?Philcha 10:34, 21 March 2007 (UTC)

The "no hair" theorem (NHT) states that a black hole has no other caracteristics beyond its mass, eletric charge and angular momentum. In other words, two black holes that have exactly the same value for these three quantities are completely identical in all respacts, since there are no other caracteristics by which they could be distinguished from each other. The NHT does not prevent these black holes from having a Hawking temperature because their temperature is not an independent quantity. (given the other threee, the temperature can be calculated). Dauto 20:03, 22 March 2007 (UTC)

Thanks! I've edited the "no hair" section to emphasise "independent".Philcha 10:01, 23 March 2007 (UTC)

Need for a "Theoretical dilemmas" section?

Should the article have a "Theoretical dilemmas" section which summarises, in the simplest possible language, some of the difficulties in current black hole theory? It could also refer to e.g. the quantum mechanics objections to the zero-volume singularities predicted by general relativity. If such a section is added, I suggest it should be placed at the end of the plain-language theoretical material and just before the sections about finding black holes.Philcha 10:34, 21 March 2007 (UTC)

Inconsistency re what is seen as star collapses below event horizon?

The "History" section says that Oppenheimer & Snyder called black holes "frozen stars" because they thought the event horizon would show a red-shifted image of the star as its surface met the horizon. But the discussion of time dilation in "Falling into a black hole" says time dilation will red-shift the light to zero energy, i.e. there's nothing to see at the horizon. Any explanations?Philcha 12:40, 21 March 2007 (UTC)

From what I understand, regardless of whether the matter was visible at the horizon, they considered it to be there, justifying the "frozen star" impression. This is mainly an artifact of the coordinate system used (described under Schwarzschild metric), as opposed to representing what actually happens to the matter. So, I'm not sure why the two statements are inconsistent. --Christopher Thomas 20:44, 21 March 2007 (UTC)

Black_hole#History_of_the_black_hole_concept says, "The mathematics showed that an outside observer would see the surface of the star frozen in time at the instant where it crosses that radius." The description of time dilation and gravitational redshift in Black_hole#Before_the_falling_object_crosses_the_event_horizon implies that the last light-rays from the collapsing star's surface are redshifted to zero or infinitesimal energy (zero if emitted at the horizon, infinitesmal if emitted just before) and are therefore not observable in practice and possibly not in theory. To me, "frozen in time" suggests the image lasts as long as the black hole. But that would require infinite time dilation, which would imply infinite redshift and therefore the light ray would have zero energy. It sounds like either the article or my mind needs clarification (probably both!).Philcha 10:39, 23 March 2007 (UTC)

In this context, "see" means "detect through some unspecified mechanism". It is not meant to suggest that the infalling matter is still emitting detectable amounts of light. Does this clear up the perceived inconsistency? --Christopher Thomas 16:46, 23 March 2007 (UTC)

Hi, Christopher Thomas. I think the fundamental problem is that the the "History" section never did have any references / citations which would have allowed either of us to check the actual words (or equations - you appear to be more of a physicist than I am), except for Michell's Newtonian theory.

I'm not persuaded by "'see' means 'detect through some unspecified mechanism'" because AFAIK the whole of relativity, starting with special, is based on the idea that the only way of detecting events is via electromagnetic radiation which has a fixed speed. ("Discovering Relativity for Yourself" by Sam Lilley; Cambridge University Press, 1981; ISBN-10: 052129780X).
Can anyone supply references / citations which describes precisely but not too mathematically the reasoning behind the name "frozen star"?.Philcha 22:03, 23 March 2007 (UTC)

The full description, with references, should be at Schwarzschild metric. In Schwarzschild coordinates, infalling objects appear to approach but never reach the horizon. That is what is meant by "would see (i.e., assess) an object as frozen at (i.e., arbitrarily close to) the horizon". This view of black holes was realized to be incorrect when alternative coordinate systems were proposed that did not have a discontinuity at the event horizon. More detailed discussion should be in the talk archives for this page, as for a while we had people showing up every few months saying "black holes can't exist because of (broken argument involving the event horizon)", which resulted in detailed descriptions of all of these effects being posted. --Christopher Thomas 22:53, 23 March 2007 (UTC)

Thanks! While composing my reply I've found that it has become rather long, so I'll start by proposing how the "History" section of Black hole should handle the issue and inviting comments:

.... In 1930, the astrophysicist Subrahmanyan Chandrasekhar argued that, according to special relativity, a non-radiating body above 1.44 solar masses (the Chandrasekhar limit), would collapse since there nothing known at that time could stop it from doing so. His arguments were opposed by Arthur Eddington, who believed that something would inevitably stop the collapse. Eddington was partly right: a white dwarf slightly more massive than the Chandrasekhar limit will collapse into a neutron star. But in 1939, Robert Oppenheimer published papers (with various co-authors) which predicted that stars above about three solar masses (the Tolman-Oppenheimer-Volkoff limit) would collapse into black holes for the reasons presented by Chandrasekhar. Oppenheimer and his co-authors used Schwarzschild's system of coordinates (the only coordinates available in 1939), which produced mathematical singularities at the Schwarzschild radius, in other words the equations broke down at the Schwarzschild radius because some of the terms were infinite. This was interpreted as indicating that the Schwarzschild radius was the boundary of a "bubble" in which time "stopped". For a few years the collapsed stars were known as "frozen stars" because the calculations indicated that an outside observer would see the surface of the star frozen in time at the instant where its collapse takes it inside the Schwarzschild radius. But many physicists could not accept the idea of time standing still inside the Schwarzschild radius, and there was little interest in the subject for over 20 years. In 1958 David Finkelstein broke the deadlock over "stopped time" and introduced the concept of the event horizon by presenting the Eddington-Finkelstein coordinates, which enabled him to show that "The Schwarzchild surface r = 2m is not a singularity but acts as a perfect unidirectional membrane: causal influences can cross it but only in one direction". Note that at this stage all theories, including Finkelstein's, covered only non-rotating, uncharged black holes. In 1963 Roy Kerr extended Finkelstein's analysis by presenting the Kerr metric (coordinates) and showing how this made it possible to predict the properties of rotating black holes. In addition to its theoretical interest, Kerr's work made black holes more believable for astronomers, since black holes are formed from stars and all known stars rotate. ....

-- End of proposed text, start of ramblings ---

The "History" section is getting too long, but I think the Oppenheimer-Finkelstein-Kerr story is important in order to explain why interest in black holes was near zero until it soared in the early 1960s. Perhaps we need to put most of it in a separate article and just leave a one-sentence-per-item summary in the main Black hole article.

Christopher Thomas, your comment (22:53, 23 March 2007)appears to confirm that there is a conflict in the Black hole article (and in the physics 1939-1958) and to points to the resolution: Oppenheimer & co-authors' use of the Schwarzschild coordinates created a deadlock because it implied a "bubble" of "stopped time", and Finkelstein broke the deadlock. I've found some Web pages that support this interpretation: [7] says astrophysicists disagreed over the implications of "frozen star" until "Finkelstein discovers a new reference frame for the Schwarzschild geometry and resolves the Oppenheimer-Snyder paradox";[8] provides the Finkelstein quote I've used (unfortunately there are signs that [9] may not suitable for the "links" section of Black hole, see [10]).

Can anyone provide evidence of when the Kruskal coordinates were first published? I've googled and got nothing. The articles on Martin Kruskal and George Szekeres are stubs and the only clues they give are birthdates - Kruskal was 14 and Szekeres 28 when Oppenheimer & co. published their paper on gravitational collapse to "frozen star".

Also can anyone provide a reference and summary for the Oppenheimer-Snyder paper. I've picked up the ref for Oppenheimer-Volkov, and that appears only to deal with the gravitational collapse, not with the "stopped time" / "frozen star" issue. I've googled for Oppenheimer-Snyder and got nothing.

Your comment also put me on the trail of a few other things that might be worth using in various articles, e.g.: I took my first look at the articles on Schwarzschild metric, Eddington-Finkelstein coordinates and Kruskal coordinates, which I'd avoided because I'm no mathematician; [11] explains in relatively simple language the differences between the relevant co-ordinate systems and might help in adding less technical explanations for the co-ordinate systems; and I'll check the archived Talk pages for issues which have caused difficulties and might need clearer explanation.Philcha 11:29, 24 March 2007 (UTC)

How to explain space & time swapping roles at event horizons?

I've seen various plain-language explanations of why nothing can escape from within the (outer) event horizon, e.g. Black_hole#What_makes_it_impossible_to_escape_from_black_holes? currently says both "bend space-time so much that, at all points within a certain distance of the center (within the event horizon, see below) all directions lead to the center" and "bends space-time so tightly inside the event horizon that: space and time swap roles." From other reading, e.g. [12] I've concluded that "space and time swap roles" is the best I've seen, since it prepares the way for what the inner horizon does in rotating black holes - any event horizon makes space and time swap roles, the outer one makes the location of the singularity a falling object's future, the inner one reverts space & time to their normal roles (I'm still struggling to grasp the implications, but that's another matter). But how to explain this swapping in simple terms?

Intro

When and why did this article article get its new intro (section 0)? 500 versions back it was IMHO much preferable. The concept of Newtonion black holes should be discussed in the history section only. --Pjacobi 19:51, 25 March 2007 (UTC)

I added the sub-section about Newtonian black holes because I thought it important to make the point that only general relativity gives a satisfactory explanation - a lot of popular science stuff falls back on Newtonian concepts, especially escape velocity, either deliberately or as an accidental result of trying too hard to provide a simple, non-mathematical explanation.

I added the sub-section about the need for "abnormal compression" to deal with the fairly obvious point that enormous mass alone is not enough. But I think it's too long, and will probably remove the item which explains the shell theorem. I still think some reference to the inverse square law aspect of gravity is necessary.Philcha 15:34, 26 March 2007 (UTC)

That a star collapsing into a black hole suddenly starts sucking in everything in the vicinity is indeed a common misconception. That may deserve a sentence in the intro. The Newtionion stuff I still prefer to ban to the History. --Pjacobi 16:28, 26 March 2007 (UTC)

Where are the FA discussions?

Out of the curiousity I wanted to habe a look at the FA nomination and removal discussions, but wasn't able to locate them. Does anybody know the links? --Pjacobi 16:36, 26 March 2007 (UTC)

The first box at the top of this page (beginning "Black hole is a former featured article") has a "show" link at the right edge, alongside "Article milestones". Clicking this will show links to these discussions.Philcha 08:34, 27 March 2007 (UTC)

Heavens, JavaScript wizards at work! Didn't notice this. --Pjacobi 12:09, 27 March 2007 (UTC)

Photon sphere

The photon sphere section states a radius of 1.5 times the Schwarzschild radius. I could have sworn this was 4/3 the Schwarzschild radius. Anyone who's recently looked it up care to comment? --Christopher Thomas 17:46, 26 March 2007 (UTC)

I did: photon sphere; [13]; [14]; [15]; and lots of others. All say 1.5, for a non-rotating, uncharged black hole.

Rotating black holes have 2 photon spheres, but I haven't found anything which states the relation between the gap and the rotation rate (there must be a relation, because current theory expects rotating black holes to lose rotational energy and, when rotation stops, there is only 1 photon sphere. Can anyone help?Philcha 08:20, 27 March 2007 (UTC)

Light in the photon spheres of a rotating black hole

A recent edit of Black_hole#Two_photon_spheres has added "This beam will then split itself in two. Both pieces will move into the Hole." I think some clarifications are needed: (a) does it apply to only to the inner photon sphere (as the current wording implies) or to both? (b) why does the beam split and what causes the split? (c) why should both parts move inwards rather than e.g. one inwards and one outwards?Philcha 10:23, 28 March 2007 (UTC)

Gravity

How can we be sure black holes actually have the high gravity that they are assumed to have? If they merely had an immensely high gavitational field wouldn't it be theoretically possible to "fill one up"? How can we be sure black holes don't simply exist at absolute zero, therefore with zero internal pressure, making a super-vaccuum of sorts that "sucks" in everything that gets close. This would also explain why nothing can escape a black hole, since at absolute zero there is no energy, which means no mass, which means the object doesn't exist. So the big question here is are we sure black holes are actual existing objects or could they possibly be an area of "non-exisitance", so to speak. —The preceding unsigned comment was added by 69.141.105.22 (talk • contribs) on 12:55, 6 April 2007.

Well, for one thing, a vacuum wouldn't bend light the way black holes do. Only a gravitational field can do that. Izbitzer 20:25, 21 April 2007 (UTC)

Temporary "unreferenced" sections

I have temporarily marked a couple of paragraphs as "unreferenced" under "Objects which are thought to be black holes" and "Black holes and Earth". I will add references on 9 April 2007 when I return to work. (Expect something by John Kormendy.) Dr. Submillimeter 22:13, 6 April 2007 (UTC)

Rewrite of several sections

I've taken a good look at this article, and several sections seem to have changed into very strange states. I've made an attempt to bring them back into shape, without making any other major changes. This should wind down in another hour or so.

However, there are at least two other descriptions that could be added to the "escape is impossible" section that I don't feel up to writing right now. If WP:Physics lurkers or other denizens want to add them, great. One description is to define coordinates such that space near the hole appears to be flowing into it. In this description, light emitted outwards for the horizon propagates at C but doesn't get anywhere (in the more usual description, these are rays from the part of the edge of the light cone that's parallel to the black hole's worldline). A second description, from the old "fishing for the singularity" thread, defines coordinates such that the distance to the event horizon is actually infinite. This is good for giving a qualitative illustration of forces on any object attempting to fish things out being infinite. If neither of these descriptions is widely used, then by all means leave them out; the section just looks a bit strange saying "many descriptions exist, depending on how you set up coordinates" and then talking about only one of them.

I've also placed {{fact}} tags at a couple of places in the escape section. It should be trivial to give suitable textbook references for them; I just don't have appropriate books handy. --Christopher Thomas 23:15, 9 April 2007 (UTC)

I agree with your last comment ("many descriptions exist..." then talking about only one ...). In fact I don't find the description of tilting light cones helpful: it would require a (non-technical) definition of "light cone"; and a light cone's radius normally expands with time, so it would then be necessary to explain why light cones in a BH are no longer cones. Philcha 13:38, 17 April 2007 (UTC)

Locally, they're definitely still cones (as locally, space can be treated as Euclidean). They just have the future light-cone pointing inward. See Image:Bh-light-cones-1.png for a (rather ugly) diagram illustrating what I'm talking about. --Christopher Thomas 17:55, 17 April 2007 (UTC)

Black Hole hits Black Hole

I'm just wondering if anyone know this, what would happen if two black holes collided?--Andy mci 09:14, 12 April 2007 (UTC)

The two black holes would merge together to form a bigger one. (At one point, it was theorized that some gamma ray bursts were produced when two black holes merged, but I am not certain if the theory is accepted any more.) Dr. Submillimeter 09:23, 12 April 2007 (UTC)

"Artist's Impression" near the top

Wouldn't light that passes near the event horizing be extremely red-shifted? —The preceding unsigned comment was added by Dragonfall (talk • contribs) on 01:52, 19 April 2007.

Light emitted from near a black hole is red-shifted, but light from distant sources that passes near a black hole isn't. More accurately, it's blue-shifted on the way down and red-shifted by an equal amount on the way back up, with no net shift (unless the hole is rotating, but that's another story). --Christopher Thomas 02:03, 19 April 2007 (UTC)

Limitations of "no hair" theorem??

In March 2007 I included in the "no hair" section a para on the limitations of the "no hair" theorem - in particular that it doesn't work if the cosmological constant is non-zero, and a non-zero cosmological constant is one of the more widely supported explanations for the apparent acceleration in the universe's expansion (summarised from the no hair theorem article). I notice this para has been removed. Why? Should it be reinstated? Philcha 23:41, 19 April 2007 (UTC)

Photon sphere of non-rotating, uncharged black hole

This section says, "Second, the orbit is dynamically unstable; small deviations from a perfectly circular path will grow into larger deviations very quickly, causing the photon to either escape or fall into the hole." But the photon sphere only affects photons whose path is tangential. So why should such photons not follow a perfectly circular path? Philcha 23:51, 19 April 2007 (UTC)

This was a rewrite of previous text which had implied that photons coming from outside the photon sphere could be captured into circular orbits in the photon sphere. I felt that this replacement version was more correct. Regarding the current version, aligning a photon along a perfectly tangential orbit requires infinite precision and zero divergence, both of which are unphysical (even if it's initially set propagating on a tangential path with infinite precision, its wavefunction will spread out above and below the photon sphere, resulting in continuing attenuation of the component that stays on the sphere). --Christopher Thomas 00:04, 20 April 2007 (UTC)