Talk:Black hole/Archive 10

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Sandbox article

I want to try and improve this article without messing up work already done or stepping on other people's toes. Therefore, I have created my sandbox version of this article. Please feel free to suggest improvements at the talk page there. Thanks. MP ^{(talk•contribs)} 21:57, 24 December 2008 (UTC)

Observation and Facticity

I came here looking for an evidence (as opposed to surmise or inference) section. Shouldn't there be one? Lycurgus (talk) 08:47, 28 December 2008 (UTC)

The section Techniques for finding black holes probably comes closest to 'evidences'. In my sandbox version, I may incorporate 'evidence' as one of the sections. MP ^{(talk•contribs)} 11:58, 28 December 2008 (UTC)

Do singularities never form?

I know this is not "question and answer, and so I apologize, but here goes: From the article I understand that the falling of matter into the center of the black hole, even when the initial gravitational collapse occurs, take infinite time from the outside universe point of view. Does that means that singularities never actually form the normal point of view - that they would only form after an infinitely long time? [Ilan] 79.183.108.7 (talk) 19:19, 20 January 2009 (UTC)

No, it doesn't mean that. "Take infinite time from the outside universe point of view" is misleading. It's better to say you'll never see anything get to the point of crossing the event horizon if you remain outside, but that's simply because you see by means of light reaching your eyes, and no light from the event horizon or inside it can escape to reach your eyes. It may well be that there's no singularity in real black holes, but not for that reason. -- BenRG (talk) 15:06, 23 January 2009 (UTC)

Actually Ilan is completely correct, BenRG. You don't see them form not just because the light is blocked, but also because they don't form, ever. Since before ever happens they would have evaporated in a time considerably less than eternity.--Michael C. Price ^talk 13:40, 2 March 2009 (UTC)

A similar question is "do black holes ever grow?". This might be a simpler question as it prevents people from bringing up evaporation. Those two quotes from the article seem to contradict each other: "Any black hole will continually absorb interstellar dust from its direct surroundings" and "the distant observer will see an object falling into a black hole slow down as it approaches the event horizon, taking an infinite time to reach it." My guess is that the apparent contradiction is due to the distinction between "Observer Reference Frame" and "Observer Receive Time". A brick thrown into a black hole will eventually enter it after a finite time in the Observer's reference frame, but the receive time is infinite. I wish this were clarified, with maybe a formula giving the finite time it takes in the Observer's reference frame for concreteness. 24.6.174.244 (talk) 18:47, 4 April 2009 (UTC)

The statement "the distant observer will see an object falling into a black hole slow down as it approaches the event horizon, taking an infinite time to reach it." is incorrect because, as an object approaches the event horizon, the horizon expands due to the approaching mass becoming incorporated into the black hole itself. (We might expect this process to start after it crosses the event horizon, but in fact it starts beforehand.) --Michael C. Price ^talk 19:58, 9 April 2009 (UTC)

The statement actually is correct. What actually happens is that the external observer sees the object slow as it approaches the "new" event horizon. Now, even that is a peculiar statement due to the no local way in which the event horizon is defined, it actually already includes the object falling in in the future. (TimothyRias (talk) 20:10, 9 April 2009 (UTC))

You're probably right (although it seems to imply that two black holes would never merge). If you have a ref' for this I'll add it to the article. --Michael C. Price ^talk 21:05, 9 April 2009 (UTC)

Upon reflection I am less convinced that objects take infinite external time to reach the horizon. Consider two objects in sequence: the first object is swallowed as the 2nd object approaches the now-expanding-horizon/radius. Sort of implies that any object of finite extent is at least mostly swallowed in finite external time as well.--Michael C. Price ^talk 22:12, 10 April 2009 (UTC)

The problem is that the event horizon doesn't expand, it is already there, and the distant observer while "see" the first object eternally red-shift as it approaches the event horizon of the two objects combined. This all sounds weird, and it all has to do with the global way in which the event horizon is defined (namely as the outer hull of the causal past of future timelike infinity) this definition already knows about the fate of the second particle. By the very way this event horizon is defined an outside observer can never see anything cross the horizon. In a way it is somewhat tautological. (TimothyRias (talk) 22:33, 10 April 2009 (UTC))

Tautological and teleological, yes. But the absolute horizon does expand (e.g. from a point to a volume at the centre of a collapsing star, until the entire star is inside). Ah, I see, the "frozen star" business. So the distant observer never "sees" the absolute horizon, by definition..... Is that what you mean? --Michael C. Price ^talk 23:30, 11 April 2009 (UTC)

It's truly meaningless to ask when a merger/swallowing happens with respect to events outside. Since nothing inside an event horizon is in the causal past of anything outside, you can always pick a foliation of spacetime where nothing inside the event horizon has "happened yet", at least until after the black hole evaporates. Given any two black holes, as long as one doesn't evaporate in the causal past of the other's formation, you can always pick a foliation of spacetime in which they form at the same instant, even if they would appear to be timelike separated from a cosmological perspective (for example, they form a billion years apart in the same galaxy). Given two black holes that eventually merge you can always pick a spacetime foliation in which the merger precedes the formation of either hole—in other words, the event horizon first forms at the location of the merger, then expands in both directions to the initial locations of the collapsing stars. Intuitive ideas about simultaneity go out the window when you talk about black holes. You can talk about what you see, but not about what you "observe" in the special relativistic sense. -- BenRG (talk) 11:24, 12 April 2009 (UTC)

That may be true, but we are not talking about the events that occur inside the horizon, but those that occur on or just outside the horizon.--Michael C. Price ^talk 16:42, 12 April 2009 (UTC)

What about this argument? Count Iblis (talk) 12:12, 12 April 2009 (UTC)

That's a very interesting reference, which implies (correctly, IMO) that black holes never form in the first place, due to quantum effects.--Michael C. Price ^talk 16:42, 12 April 2009 (UTC)

Escape

Apparently some people feel that the explanation of event horizons according to escape velocity is insufficient to explain why light and matter cannot escape from within a black hole's event horizon. The following is an example of this:

"However, the argument can only be seen as an incomplete analogy. It explains neither why light should be affected by gravity in the first place, why it cannot travel beyond the horizon, nor why a rocket-powered spaceship would not be able to break free."

Regarding light, it is a well established fact that gravity acts on photons. This is something that has been and can be easily verified. The space ship is a bit more of a complicated scenario. Yes, it is possible to escape a gravitational force without reaching the escape velocity. A space ship can manage to escape to any given distance away from Earth without ever reaching the necessary escape velocity. Based on this notion, one would think that with the appropriate force one could push an object just past the event horizon of a black hole without ever having to reach the speed of light. This, however, is untrue. While the "escape velocity" explanation is not a direct explanation of why this is impossible, it does infer sufficient information to conclude said impossibility. To make this more understandable, let's think of this in terms of energy. To reach the escape velocity at or above the speed of light, a projectile would be required to gather infinite kinetic energy, which is clearly impossible. For a rocket with a constant force, the same velocity would not be required, but the same energy would be necessary as a sum of kinetic and potential energy. Because the energy would still have to be infinity, and it is impossible for an object to gather infinite energy, it is impossible for any object to escape from within a black hole's event horizon, regardless of speed. Because of the fact that the escape velocity explanation supplies the required escape velocity, included within that explanation is the inferred fact that the energy would have to be infinite. Likewise, this escape velocity explanation is sufficient to explain the impossibility of an escape from within a black hole's event horizon. Andrew Nutter  Talk | Contribs  13:44, 10 February 2009 (UTC)

No, the escape velocity argument fails in both cases. In the case of light does not only give Newtonian theory no mechanism for gravity to act on photons, it would also predict that light in fact does initially escape from a black hole, but would later fall back onto the black hole.

Your second argument fails even more massively. It is quite easy to compute that you need only a finite amount of energy to climb out of any newtonian gravity well (and in particular from the schwarzschild radius of a point mass). This is made quite explicit that the Newtonian potential is non singular (outside the origin). Now, of course you can try to make a similar argument in the context of GR, but then actual the energy need to escape from a black hole would be undefined, simply because it is impossible. (You can however show that the energy per kilogram needed to break free from a BH approaches infinity as you approach the event horizon. But such an approach is plagued with ambiguities. (TimothyRias (talk) 14:12, 10 February 2009 (UTC))

I see that TimothyRias has already addressed two of my main concerns. Within GR, the simple escape velocity argument makes no sense (the current version of the article correctly states what is really going on); within Newtonian physics, it doesn't give the right results. And in particular, the Newtonian escape velocity argument is certainly not "sufficient to explain the impossibility of an escape from within a black hole's event horizon" - that a particle does not reach escape velocity does not mean that it is confined within the Schwarzschild radius, on the contrary: as particles come arbitrarily close to escape velocity, they will reach an arbitrarily large distance from the mass. But they will always come back in finite time. (Unless they have a rocket motor on board - even a very weak one suffices!).

As for "it is a well established fact that gravity acts on photons" - within Newtonian gravity, it's not all that clear. You must add it as an additional assumption (which, of course, you can do). Markus Poessel (talk) 08:49, 11 February 2009 (UTC)

I should add that there is a nice simplified model for why it's impossible to escape from black holes, which does give the correct result for the properties of the event horizon: The river model of black holes. Markus Poessel (talk) 08:56, 11 February 2009 (UTC)

I don't know if you were interested in adding the river model to the article, but for what it's worth I'm opposed to this. There's nothing mathematically wrong with writing the metric in that form, but practically every conclusion someone might reasonably draw from the river picture ends up being wrong. It's suggestive that the river speed equals the Newtonian escape velocity, but when you're going outward at the river speed you're hovering at a constant distance from the hole, which is not at all how Newtonian escape velocity works, as you pointed out above. In the time-reversal the river flows out instead of in, so you'd expect white holes to be gravitationally repulsive, but they aren't. You don't feel any resistance moving with respect to the "flow", even at the speed of light. It isn't clear how orbits would work—it seems like everything should spiral in, but in reality you're far more likely to slingshot around a hole than fall in. This analogy seems to reinforce the common vacuum-cleaner misconception of black holes and obscure the fact that black holes gravitate just like any other object of the same mass (as long as you don't get too close). You'd need an expert to tell you which of the plausible conclusions you draw from this analogy are correct, and if you have an expert handy you may as well appeal directly to the expert instead of the analogy. -- BenRG (talk) 20:23, 21 February 2009 (UTC)

I was thinking about whether or not the river model would make a suitable addition, yes. While there may well be reasonable objections, I think your statement that "practically every conclusion someone might reasonably draw from the river picture ends up being wrong" is not a fair assessment.

• At one particular point, going outward with the river speed, you're hovering at a constant distance from the hole. You complain that this is "not at all how Newtonian escape velocity works". From my point of view, that's a big plus. At that particular hovering-point, we're at the black hole's horizon, and the hovering you describe is exactly what happens to light at the horizon, and which no Newtonian approximation can describe. Score one for the river model.
• Time-reversal: I'm not overly worried about not being able to describe White Holes, seeing how far those are removed from reality.
• Missing drag/resistance/friction: That is a good point. It should be mentioned when explaining the analogy, but I don't think it creates too much confusion.
• Orbits: What's the problem there? It's just the same as explaining Newtonian gravity without the formula: at each time-step, you have a shift towards the center, and a shift in the body's current direction of motion. Whether the result is a spiral or some other orbit cannot be deduced from the simplified version in either case. But neither is there any reason to believe that everything "should spiral in", as you claim (and again, in either case). Or that we've suddenly entered vacuum-cleaner territory (except from the horizon inwards).

Also, there are two sides here. The strength of the river analogy are the issues where people are likely to get the right idea: that the black hole's action is not that of a force, but that of the reference frame subtly shifting (the fish being carried away by the river). That the equivalence principle holds. That no-one falling into a black hole will feel a dramatic change as they cross the horizon, but that a global perspective reveals this to be the point of no return. That the horizon is localized, and that the inability to escape is much more absolute than in the Newtonian escape-velocity picture. For me, all these are key insights when it comes to understanding how black holes work, and the river model makes them accessible without the mathematics.

As for "appealing directly to the expert" - there are two aspects here. One is that, yes, from the river analogy, people could work out for themselves some of the key properties of black holes. The other, and in my view far more important, is that the river analogy can be used by an expert to describe many of the key features of a black hole without the mathematics, successfully connecting his or her explanations to what the audience already knows (fish in a river, how much simpler can you get?). Markus Poessel (talk) 10:35, 23 February 2009 (UTC)

Black hole twins

This following link is new and interesting, it discusses the properties of the union, frequency and discovery of black hole twins: Dancing black hole twins spotted

“ Reseachers have seen the best evidence yet for a pair of black holes orbiting each other within the same galaxy - BBC 4 March 2009 ”

Candidate for adding on the Black_hole#Growth section?

NevilleDNZ (talk) 00:30, 5 March 2009 (UTC)

Can a particle travel 10 times faster than light's speed in black hole?

Can a particle travel 10 times faster than light' speed in black hole? can it be experimented?? If so, what would happen?

Athos, Porthos, and Aramis (talk) 00:23, 11 March 2009 (UTC)

The speed of a particle in a black hole could be measured with a laboratory which falls inside. Any outside observer will never get the information on the results of the measurement: in the frame of coordinates of this observer, the falling laboratory never reaches the horizont. So the brave curious musketeers have to stay at the falling lab.
I expect, the lab (and musketeers) will be destroyed by the tide forces before to detect any significant violation of causality or the Newtonian mechanics in the frame of the falling lab. dima (talk) 06:34, 11 March 2009 (UTC)

Sandbox article

I've spent a couple days trying to improve this article without getting my work deleted minutes after I post it so I created a sandbox version. I spent a lot of time incorporating information from the above sandbox version by MP, the current article, and the french version which is a featured article. Please feel free to make any improvements to the article. I think the layout makes more sense and I tried to get rid of a lot of repeated info. Thanks.--Ben Harkness (talk) 03:57, 17 March 2009 (UTC)

it's nice that you and mpatel have your own versions of the article, but the real article is still being revised too. how are you going to manage putting your ideas back into the main article?  —Chris Capoccia ^T⁄_C 05:59, 17 March 2009 (UTC)

Well it's not really my version, I just changed the layout and created a sort of "hybrid" article. The only reason I created it in my sandbox is because that way it won't be rolled back right after I make changes. I tried changing just the intro a couple weeks ago but it was reverted by the next time I checked and I wasn't given a reason as to why. I just thought that by making so many edits, anti-vandals wouldn't even take the time to read what I've done and would revert it right away. So, I hoped by making this version that people could openly make improvement's to it and, when it's ready, replace the current article with it. I would like to use it as a development article. Thanks.--Ben Harkness (talk) 14:29, 17 March 2009 (UTC)

I removed the "what makes it impossible..." and "effects of falling into a black hole" sections and did my best to include their information in other suitable sections. I also left out alternative models because I'm not yet sure where to include the info and and unsure if it deserves it's own section. I am moving the article without the above mentioned changes to my sandbox so the information won't be lost until we can find a suitable place.--Ben Harkness (talk) 00:39, 19 March 2009 (UTC)

Lead and "Introduction and terminology"

In the recent edits somebody included an introduction an moved a lot of information from the lede to this section. This is very unconventional for a wikipedia article, and certainly not in compliance with the MoS. As such the article in the current format is unlikely to ever pass GA review. I'm curious what the editor's rationale was for this specific change. (TimothyRias (talk) 09:55, 26 March 2009 (UTC))

I would urge the other editors to read WP:LEAD. The current format with both a lede and an introduction section, does not comply with the layout guidelines set out there. As such it will be very hard to get the article through an review process such as GAN in its current format. Most of the information that is in the introduction should be reintegrated in the lead. (TimothyRias (talk) 06:41, 31 March 2009 (UTC))

I was unhappy too, at first. But upon reflection, the new lead was a clear definition and summary of main properties, although not of the detailed content of the article. It gets repetitive to say the same things twice, and if GA review does not like it, perhaps that says more about GA review than about the quality of this article.Likebox (talk) 16:15, 31 March 2009 (UTC)

Have you read WP:LEAD? It also gives arguments why a certain format is preferred. One of the guidelines is that the lead should be able to function as a mini version of the article. This something that readers expect from a wikipedia article, so an article that does not comply, will look unfinished and will put readers off. (especially since this tiny lead is followed by a humongous TOC.) An even bigger problem is that other applications that use wikipedia content will expect the lead to have this property. In the current form the BH article will be useless to such applications.(TimothyRias (talk) 08:10, 1 April 2009 (UTC))

I'm the one who made the structure and layout changes to the article. I'm trying to include as much sourced material as I can with the time I have, and I realize a lot of the changes I made can make other editors very upset. I myself was against the split but it grew on me over time. The opening can only be so long before readers become uninterested about the subject. I have been surprised that no body else said ANYTHING about the mass changes I made to the article. I'm happy to see the little bit of life this article has left but all of the sections still need to be enhanced. There is only so much a couple people can do. One issue I have with some of the new edits is the technicality of language used. The article needs to be accessible to all readers, not just those with background knowledge in physics. Also, sentence structure is very poor. Readability and information have to balanced as well as possible.--Ben Harkness (talk) 22:07, 31 March 2009 (UTC)

I don't think it's awfully unclear--- the "technical" parts are just the escape velocity, and it should be easy for anybody who reads it carefully, following the links. But please, make more specific complaints. Also, if you know a layperson willing to read this article, it would help identify what's confusing.Likebox (talk) 22:28, 31 March 2009 (UTC)

No, it is not just the escape velocity part that use technical language. The recent edits for example assume some familiarity with the role of coordinates in GR (and there ultimate irrelevance to observable physics). This is not necessary and makes it hard to follow for readers unfamiliar with GR. Things become even worse in the edits of the properties section which has become pretty much impossible to read for anybody unfamiliar with Hamiltonian mechanics.

That being said. I really welcome to new spurt of activity in this article. I haven't had time to give an in depth look to all the changes made, but the article has made good progress structure wise (other than my gripe with the lead/introduction situation). From my end I'll try to put some haste in the substantial rewrite of the observations section I have been working on. (see my sandbox) When finished it should integrate the current observation and candidates section, and also give a brief overview of alternative theories for these observations. (TimothyRias (talk) 08:39, 1 April 2009 (UTC))

Let me try to justify this. Often when people make claims for a general audience, the claims do not give an entry point into the literature which can be used to get the full picture. This means that it is hard for an outsider to learn the full content of the claims, and to evaluate them on their merits. I tried to give a reasonably complete picture, without detailed proofs or formulas, only using the names of the results. If you don't know anything about Hamiltonians, and you ignore the words that are unfamiliar, I think that the article still remain mostly readable, with only a few cryptic phrases. If the reader follows the links, those phrases then become less cryptic. Perhaps I'm wrong. But then to fix this, I think its best to leave the links and names of results in place, so that a reader interested in learning the whole thing can do so. Ultimately, the goal is to give all the information to everybody.Likebox (talk) 12:09, 1 April 2009 (UTC)

Although I agree with your last statement, I have to disagree on the method by which you are trying to achieve this. It is virtually impossible to give access to all information from this article alone. This article has quite a broad scope, which unfortunately means we have to be a little selective on what we focus on. The stuff you wrote about Lagrangians and Hamiltonians, in a part discussing the information loss paradox. This is quite a technical subject to discuss correctly, fortunately we have the Black hole information loss paradox article to do just that, it can refer to Hamiltonian mechanics, Liouville's theorem and its relation to quantum mechnical unitarity to quite some extent. This article on the other hand is subject to a much higher traffic from the general audience. IMHO, it should try to stay clear from all too technical details and jargon, and try to explain in normal English the gist of the physics of black holes. A more interest reader can follow the wikilink to the information loss paradox page, which should give more of the details. (TimothyRias (talk) 13:19, 1 April 2009 (UTC))

Yeah, I see what you're saying. I'll try to make it less technical. I'll tell you why I don't like moving this into Black hole information loss paradox. The reason is that the paradox was identified only with quantum effects, with Hawking radiation, and it seems to be a problem only with quantum mechanics, or at least in a semiclassical description. But what I'm trying to talk about here is purely classical stuff. The black hole horizon, because of its one-way character, behaves just like a dissipative system, when classical GR is Hamiltonian and should not have irreversible phenomenon. This was swept under the rug in the 70's by assuming that the Hamiltonian description needs new regions, the interior region, and possibly other sheets, other universes. Then the system is Hamiltonian only when you include the other regions. But this is now known to be an unsatisfactory resolution--- the thing had better be unitary in the exterior region only. In this case, the classical irreversible behavior is holographically linked to relaxation times in a gauge theory plasma. People could have noticed this already in the classical theory, and there are some people who were uneasy even in the 70s and 80s.Likebox (talk) 14:49, 1 April 2009 (UTC)

For a particular example of an uneasy person, there is an analysis of where the infalling information goes by 'tHooft in 86 in Nuclear Physics B, which is the paper that starts holography, where he analyses the entropy of fluctuating fields outside the black hole horizon in Schwartschild coordinates. He finds that the entropy is divergent at any fixed energy close to the horizon, and worries about this. This is the "frozen star" phenomenon, it's because the redshift factor goes to infinity at the horizon. All the information is getting "buried" in thin layers right near the horizon, from the point of view of Hamiltonian evolution in the exterior. He thinks this is nonsense, so he puts a cut-off which he called the "brick wall model" to make the entropy finite, then tries to figure out what that means, and eventually that turned into holography.Likebox (talk) 14:58, 1 April 2009 (UTC)

The reason I put in a short discussion of coordinates is because you can't say the "radius" of a black hole is 2M honestly without saying what coordinate system you are using for r. If you say the area of a black hole is 16piM^2, then you're OK. This is not just an esoteric point--- if you solve for the spherical black hole in isotropic coordinates, which is the way that allows for easiest generalization to the rotating case, the natural quantity which you would call the ”radius" of a black hole is M/2, but the area is the same.Likebox (talk) 12:17, 1 April 2009 (UTC)

That's interesting. So is the area a scalar? --Michael C. Price ^talk 12:26, 1 April 2009 (UTC)

I guess that's one way to put it. The reason it's invariant is because it's a null surface--- generated by light rays. If you define the area as the intersection of the horizon with a space-like surface, and then you push the spacelike surface forward in time, or tilt it, the total area of a cross section remains the same. That's true even when you would think it wouldn't, by geometry, because it's minkowski geometry. It's the reason that the Area theorem can be proved, because all the area theorem says is a local statement--- the area at each point goes up, but you need to have a well-defined global notion of area. That only works for null-surfaces.Likebox (talk) 13:18, 1 April 2009 (UTC)

To prove this, cut a null-surface with a horizontal t=0 plane in Minkowski space, and note that no matter how you tilt the t plane with boosts, the area of corresponding sections stays the same. It's elementary, but surprising.Likebox (talk) 13:25, 1 April 2009 (UTC)

That would be invariant under a Lorentz transformation, but not a Poincare transformation (which would include time translations)? If so does that imply that the BH's area is invariant under local lorentz transformations but not under a general coordinate transformation?--Michael C. Price ^talk 13:38, 3 April 2009 (UTC)

Think about the example of the volume of a region--- that's independent of the coordinates because you have a metric tensor, which allows you to define the volume of an infinitesimal parallelogram, and then you sum up the volume of all the parallelograms in a region, and the value of the sum doesn't depend on the coordinates. In Minkowski geometry, surprising as it is, the area of a null surface (but only a null surface) on a slice is independent of the coordinates you choose for that slice. That's weird if you think about Euclidean geometry, because there's no such thing as a null surface in Euclidean geometry, but this property makes the area of a black hole horizon a meaningful physical quantity.

If you go forward in time, the area goes up, because the null geodesics on a black hole horizon can only go away from each other, they can't come closer (classically, if you ignore Hawking radiation).Likebox (talk) 14:43, 3 April 2009 (UTC)

Sorry, I screwed up in the last comment above--- the area of a surface is always well defined once you are given the space-like slice which defines it, it's a coordinate invariant concept. I meant something else--- the area of a black hole horizon, as defined by intersecting it with a space-like surface, stays the same even when you deform the space-like slice infinitesimally. You can crumple it and crinkle it and the area of the black hole is still the same. That's not true in Euclidean geometry. If you cut a 3-d cylinder by a plane crinkled up like an accordion, the length of the intersection is bigger than if you intersect with a flat plane perpendicular to the axis of the cylinder. If you intersect a black-hole cylinder by a space-like surface, it doesn't matter if it's crinkled up like an accordion or not.Likebox (talk) 15:13, 3 April 2009 (UTC)

Presumably area is defined as $\int dS^{2}$ in which case I see what you mean about area being covariant (ignore my earlier comment about Poincare variance). If I've understood you correctly, the crinkle-invariance you mentioned only works if the surface remains space-like.--Michael C. Price ^talk 08:48, 7 April 2009 (UTC)

I completely agree that the Introduction section is superfluous. Everything that's currently in the Introduction section can be condensed and included in the lead. The current lead, on the other hand, is overly simplistic and doesn't usefully summarize the rest of the article. The Intro section spends several paragraphs explaining escape velocity, when the actual article on escape velocity already does a fine job of it. And in any case, defining a black hole in terms of escape velocity is naive at best, and the Intro doesn't state clearly the reasons why the escape-velocity explanation is insufficient. Dmitry Brant (talk) 15:40, 9 April 2009 (UTC)

I'd also like to point out the dubiousness of the claim that "black holes are often defined as objects whose escape velocity exceeds the speed of light". Outside of popular texts talking about dark stars I have never seen anybody do this. (TimothyRias (talk) 15:58, 9 April 2009 (UTC))

According to Thorne, in his popular book, that was how the "apparent horizon" was defined. Now people tend to use the "absolute horizon". --Michael C. Price ^talk 16:44, 12 April 2009 (UTC)

How is this for a lead section: User:Dmitry_Brant/temp1? I believe it can replace the current lead, and eliminate the "intro and terminology" section altogether. What do you think? Dmitry Brant (talk) 17:35, 13 April 2009 (UTC)

That already is much of an improvement over the current lead. There are some issue with it that need to be ironed out.

In the first paragraph it introduces the terms stellar mass- intermediate mass- and supermassive black hole without any indication of what these terms mean. Although this might be clear to a certain portion of the readers it might put readers that don't immediately get it off. Since this is only the first paragraph some softening of the technical jargon may be in order.

There are some minor factual issues with the second paragraph. First of all the singularity of a black hole is not always a point put may also be a ring. Second the curvature of a black hole at the horizon doesn't need to be extreme (it simply isn't for supermassive black holes) this has often been used as an argument that quantum gravity should not produce significant corrections at the horizon. (Although there are good arguments to suspect otherwise.

The third paragraph also isn't to accurate. Stellar mass BHs, for example, are typically found by studying accretion disks, etc.

Finally, I think the lead should say something about evaporation and the necessity for a theory of quantum gravity. This after all, is one of the reason why BHs are so important to modern physics.

As note, the article used to have a more elaborate lead. So you migth get some inspiration from looking at the old revisions of the article. (You have to go back to before the recent massive flux of edit to say back the beginning of this year. You can also look at the vasrious sandboxed versions of the article mentioned above. This might also be a good place to dig up some refs. (TimothyRias (talk) 10:19, 15 April 2009 (UTC))

Irreversibility and No Hair

One of the best references in this subject is Thorne's et al "Black Holes, The Membrane Paradigm" (sorry for placing an incomplete ref I'll fix it later). The important thing about this book is that it summarized the known physical properties of black hole horizons in a back-of-the-envelope sort of way, so that you could figure out what happens to a black hole from simple physical intuition derived from classical systems.

I think it is a great pity to not mention this up front, since it allows the reader to visualize how a black hole behaves from systems that make a relatively cheap demand on the reader's imagination. In particular, the black hole behaves with regard to charges like a resistive conducting sphere with a total resistance of some fixed number of ohms. With respect to deformations, the horizon behaves like a viscous stretchy membrane with high viscosity, like surface waves on molasses. These intuitions are extraordinarily powerful, since they are a classical version of holographic ideas which later became foundational. So I think this is a great reference to talk about up front.Likebox (talk) 20:04, 30 March 2009 (UTC)

Deletion of intro sources

These were terrific sources. Why were they deleted? It is a pain to restore them, because you have to find the text of the source in the earlier version. I am trying to do that. But please, be considerate. Somebody went to a lot of trouble to include the source! Do not just delete them without discussion. Also the old text was more accurate and more informative than the new text.Likebox (talk) 22:41, 30 March 2009 (UTC)

Oops-- I'm stupid. This was just moved elsewhere. Sorry, I'll undo what I did.Likebox (talk) 22:48, 30 March 2009 (UTC)

Sorry for the screwup. But I didn't completely revert, because I did a little rewording--- I hope it's ok. The previous discussion seemed a little confusing, but I might be totally off base.Likebox (talk) 22:52, 30 March 2009 (UTC)

Whether you inserted it I'm not sure, but the article currently talks about information loss in two places when one would do fine. Makes it rather rambling. --Michael C. Price ^talk 23:02, 30 March 2009 (UTC)

That wasn't me--- I removed the dup. I did insert a second discussion of quantum stuff in the information loss section, but I deleted it to leave that part completely classical. I think that's much better. Hope it is ok now.Likebox (talk) 01:30, 31 March 2009 (UTC)

Einstein denying BH

There's an infamous Einstein paper in 1940s or 1950s which denies black holes can form. The argument is that the angular momentum needed to stabilize a star diverges at the horizon, which he uses to conclude that the star will find some way to stabilize itself at some radius by spinning at the right rate. This argument is completely wrong, but it is notable, as are all things Einstein. I read the paper, but I can't remember the cite, and I couldn't find it on google. Hoping someone else knows about it.

I put a wordy ref that is about that paper, but I can't find the real citation. The ref reads like this: One of Albert Einstein's last published papers argues that as a collapsing star approaches the event horizon, it will spin faster and faster to stabilize itselfLikebox (talk) 13:13, 1 April 2009 (UTC)

In this it probably better to find a secondary source that not only explains what Einstein said, but also why he was wrong. Any statement on wikipedia that claims Einstein was wrong about anything is going to extract crackpots like flies to dog poop. (TimothyRias (talk) 13:30, 1 April 2009 (UTC))

You won't necessarily find a secondary source for that, but it's not really needed in this case. It's common knowledge that this particular paper is wrong. This is what I think Einstein's "biggest blunder" is, denying black holes. Not the cosmological constant. Einstein made many other mistakes--- he has a theory of superconductivity in the 1920's which is based on the Drude model, which is totally wrong (as he himself suspected when he published the paper, at the urging of others), a teleparallel modification of general relativity which is totally wrong (as he later recognized), and the entwurf theory, which he corrected himself with only a little outside prodding. His genius is not that he never made mistakes, but that he had the guts to imagine bold new things, and work out the consequences for years, sometimes in complete isolation, until they were either conclusively vindicated or refuted.Likebox (talk) 14:43, 1 April 2009 (UTC)

I don't think crackpots are attracted to a statement that a particular technical paper with the name "Einstein" on it is wrong. I think they are attracted to statements that say relativity is wrong. But if this becomes a problem it should be easy enough to find mainstream sources that talk about that particular paper. I know they exist, but I read them a billion years ago, and my memory is not so good.Likebox (talk) 14:54, 1 April 2009 (UTC)

Also, there's a book out there called "Einstein's mistakes", which I thought would be about this stuff. But that book is just a bunch of nonsense criticizing things which are both obviously correct and fully vindicated and well accepted, like synchronization of clocks.Likebox (talk) 15:06, 1 April 2009 (UTC)

You might want to look at List of scientific publications by Albert Einstein to figure out which article it was. Once we know the reference it shouldn't be to hard to find a rebuttal by search for stuff that references that article. (TimothyRias (talk) 15:18, 1 April 2009 (UTC))

That's the first place I looked. That list is incomplete.Likebox (talk) 15:27, 1 April 2009 (UTC)

Found it (thank you google scholar). The article is: "On A Stationary System With Spherical Symmetry Consisting of Many Gravitating Masses", by Einstein alone in Annals of Mathematics Vol 40 No 4 p 922 (1939). This is the linkLikebox (talk) 16:57, 1 April 2009 (UTC)

The analysis, as always with Einstein, is ingenious and very readable. The way he does it is by imagining a rotating dust, stable in orbit, and making a plot of the energy as the thing contracts. If you want to keep stable orbits, the dust has to go faster and faster, and it requires infinite energy to compress the thing this way beyond some point. This analysis is flawed, because the equilibrium orbit idea is violated when the orbits get too close to the Schwartschild radius. I don't think anyone published a "refutation", but it's obvious today.Likebox (talk) 17:06, 1 April 2009 (UTC)

It appears in the List of scientific publications by Albert Einstein--- my bad. I was looking past 1940.Likebox (talk) 17:29, 1 April 2009 (UTC)

Reference hunting

While trying to track down that Einstein paper, I ran across this great reference for the history section: this paper by Wheeler. It includes a comment about Oppenheimer, who seems to have been so upset about the way collapse was treated before the black hole revolution of the 60s that he leaves the room to sit on a bench when gravitational collapse was being discussed in a conference! The article gives some insight about how Oppenheimer viewed the infalling observer--- he had a modern point of view about this. It's a shame that his work was not taken seriously.Likebox (talk) 16:57, 1 April 2009 (UTC)

redundant wording

please revise the following sentence (first one of the article):

"In general relativity, a black hole is a region of space in which the gravitational field is so powerful that nothing, including light, can escape its pull. The black hole has a one-way surface, called the event horizon, into which objects can fall, but out of which nothing can come out."

the last word is redundant, either remove it or change the sentence. seems petty but its a simple mistake and takes away from the article.

thanks —Preceding unsigned comment added by 74.15.79.222 (talk) 00:25, 15 April 2009 (UTC)

Grammar

Could someone maybe revise this sentence? "It is called "black" because it absorbs all the light that hits it, reflecting nothing" the wording of it seems a bit off as light doesnt hit the black hole. it also seems slightly redundant because an earlier sentence say that not even light can escape. Indomei (talk) 09:39, 15 April 2009 (UTC)

shape of event horizon

The section on the event horizon states, "Rotating black holes have distorted, nonspherical event horizons." Elsewhere, however, the event horizon is described as spherical even in rotating black holes, such as the Ergosphere article, as well as the ergosphere graphic used in this article. Further, the Schwarzchild radius article says, "The surface at the Schwarzschild radius acts as an event horizon in a non-rotating body. (A rotating black hole operates slightly differently.)" Either this is a direct contradiction, or it's unclear enough that a non-dummy layman such as myself can't see why not. Mycroft7 (talk) 00:54, 28 April 2009 (UTC)

entropy

Shouldn't the discussion of entropy specify that it's the dimensionless entropy which is equal to 0.25*A in Planck units? As it stands, I doubt anyone who doesn't already know the relation would be able to decode the ambiguous units. —Preceding unsigned comment added by Mbwmbwmbw (talk • contribs) 03:47, 7 May 2009 (UTC)

radiation? Death of a blackhole?

First a caveat I'm not a physicist - and the last time I read A Brief History of Time was about 6 years ago - so I could be wrong, but don't blackholes die ... eventually? From reading the article the non-expert reader can almost be left with the impression that a black hole will exist permanently and will never cease to be a black hole. However, my understanding of the principles is that due to radiation (evaporation?) the mass of the black hole would eventually decrease to nothing. The problem here might be the standard of the article's language - it's too technical. What I'm asking might be explained in the article, or might be implied there, but it gets lost in the jargon--Cailil ^talk 21:50, 5 June 2009 (UTC)

I believe it's called Hawking radiation. -- œ^™ 13:09, 7 June 2009 (UTC)

Yeah I think so - but my point is that for the average reader the current section on it doesn't make clear that the production of this radiation is equivalent to the diminishing of the blackhole. Am I wrong in think that if enough Hawking radiation is emitted then the blackhole 'dies'? If this is so then the article might need to say this--Cailil ^talk 15:28, 11 June 2009 (UTC)

Nope, and yup I think you're right. And agree that the article should definitely reflect what's written at the Hawking radiation article. -- œ^™ 16:19, 11 June 2009 (UTC)

Isn't that exactly what it says in the evaporation subsection? (TimothyRias (talk) 17:10, 11 June 2009 (UTC))

Only if you can jargon-bust it. My only point is that it's not clear. The info may be there but that section actually doesn't explain what evaporation means (ie the death of a blackhole) in an accessible way, rather it assumes you understand what evaporation means (in terms of a blackhole) already. And since this teh parent article I just think it important that it explains clearly how blackholes cease to exist--Cailil ^talk 17:21, 11 June 2009 (UTC)

matter at the singularity

I just added a small pharagraph which is highly speculative, but satisfies hopefully the expectation of typically interested men on the street about reasonable idea for the scenario at the singularity. I hope this is okay, isn't it? —Preceding unsigned comment added by Achim1999 (talk • contribs) 17:12, 17 June 2009 (UTC)

Allright, TimothyRias, I accept the reason of your removal. But what mean the insider-abreviations WP and OR?

These are Wikipedia policies (usually links). Click to view: WP:OR --Christopher Thomas (talk) 20:41, 18 June 2009 (UTC)

black hole is a theoretical region

Some one removed this [1] on the basis of the following article http://news.bbc.co.uk/2/hi/science/nature/7774287.stm. But isn't this data 27000 light years old, how does this data co-relate to its current presence? It could have existed and then disappeared? How does one know it is currently present. Also, the confirmation is on the basis of one agency's study. Has anyone else verified this information? BalanceΩrestored ^Talk 07:12, 20 June 2009 (UTC)

The "region of space" is not theoretical. The explanation of the anomalous region involves theory ... that's normal science. Vsmith (talk) 23:11, 23 June 2009 (UTC)

Please post a relevant source citing it's presence. BalanceΩrestored ^Talk 02:50, 26 June 2009 (UTC)

Is this paper verified "Black hole confirmed in Milky Way" [2]BalanceΩrestored ^Talk 02:55, 26 June 2009 (UTC)

Again, is there a proof that the black hole is currently present? are the readings not 27000 light years old? BalanceΩrestored ^Talk 03:00, 26 June 2009 (UTC)

Proof? The news blurb you referenced above is about the specific black hole in the center of our galaxy ... not about black holes in general. The researchers presented solid evidence of stars in a specific region of space orbiting something not seen. The orbital evidence supports the presence of a black hole there. "Proof" - naw, science doesn't work that way. And yes for that specific "black hole" the research provides pretty good evidence of the existance of a black hole there 27000 years ago since the light analysed took that long to get here. So, is it still there? Who knows ... is the sun still there? Who knows, it takes roughly 8 minutes for sunlight to get here and in that 8 minutes it could have "poof" disappeared. Vsmith (talk) 03:30, 26 June 2009 (UTC)

Please see: Supermassive_black_hole#Milky_Way_galactic_center_black_hole and Sagittarius_A*#Supermassive_black_hole_hypothesis for the specific "black hole" you are referring to. Vsmith (talk) 03:40, 26 June 2009 (UTC)

(Removed section; no longer applicable)

Addition by User:Aranoff

I went to edit this for grammar and typos, and have ended up removing it as (a) I don't think it belongs in the lede of the article, (b) I am not convinced that what you said is correct.

I may be wrong about this - if you want to repost your text here, we can discuss it.--Elen of the Roads (talk) 16:38, 3 July 2009 (UTC)

I note from User talk:Aranoff talk page that there was some argument about a contribution to this article in 2004 - this may or may not be relevant.--Elen of the Roads (talk) 16:40, 3 July 2009 (UTC)

Black holes

I added the following, but someone removed it, saying it was not correct. It is correct. Here is what I added:

"In special relativity, first published by A. Einstein in 1905, this definition makes sense. The escape velocity from an object depends upon the mass. For a large enough mass, this velocity will approach and exceed the speed of light. Since special relativity forbids speeds greater than the speed of light, nothing can leave an object will large enough mass. This may be properly called a special relativistic black hole.

"In general relativity, which Einstein published a decade later, the explanation is different. Time on earth is measurably slower than time on a satellite due to the earth's mass, and this fact is critical for GPS. Imagine an object with mass large enough so that the time slows down to zero. It takes forever to approach. This is a general relativistic black hole. The correct definition of a black hole is an object that one can never reach.

"It is strange that almost a century after Einstein published his works, people still fail to understand the idea that mass changes geometry. I discuss this with my students this way. I ask them how many degrees a triangle has. Answer: 180. Then I say draw a line from the North Pole through New Jersey to the Equator, perpendicular to the Equator. Draw another line from the North Pole through London to the Equator, perpendicular to the Equator, and connect the lines. We now have a triangle with more than 180 degrees. This is because the geometry is the geometry of the surface of a sphere. The geometry near a black hole is also very different from Euclidean geometry. We must be willing to accept the idea that it takes forever to reach a black hole, and so we cannot discuss something leaving."

My comments. Do you disagree saying time on earth is not slower than time on a satellite? If we did not incorporate general relativity to GPS calculations, accuracy would be miles, not feel.

Another way of looking at this is curvature of space due to mass. Let's say I walk across the room. Let us measure the time it takes me. Now suppose there were a huge mass in the next room. My straight path across the room would be curved space, and so takes longer.

As the mass increases, the curvature increases to the point where it is a closed curve. Time stops.

We understand what it means to say time stops. When a person dies, time stopped for that person.

Since time stops at the black hole event horizon, it takes forever to reach the event horizon. That means several things. One is that one can never reach the event horizon. Secondly, we must not discuss the inside, as one can never get there.

There is a formal solution of GR from the point of view of the object falling into the BH. It takes finite time to reach the EV, and then continues to the center. Hawking proved that there is a singularity at the center. A singularity is division by zero. Tan x has a singularity at 90 degrees. This singularity proves that the formal solution is not correct.

I can say more, and have submitted a paper for publication on this topic.

Scary that people cannot understand a simple idea, which college physics students should understand, that space is so warped near a BH that one can never reach it.

In 1970 I had a similar idea with people not understanding a simple idea in special relativity. A right hand lever is at rest and at equilibrium. Viewed from a moving system, taking the Lorentz transformation of forces and torques at a time t', we find the system is not at equilibrium. The solution is that the equilibrium conditions must be transformed covariantly, which means in the moving system we cannot view equilibrium at a time t', but must use different times, covariantly transformed.

Please restore the material that you took out!

Thanks! —Preceding unsigned comment added by Aranoff (talk • contribs) 00:50, 6 July 2009 (UTC)

This is the same theory that you were proposing in 2004. It seems to me to be based on a misunderstanding on your part of the time effect. To an observer at a distance watching an object being drawn into the black hole, time (for the object) appears to slow down until at the gravitational radius it stops altogether - so a distant observer would not in their frame of reference ever actually see the spaceman being sucked into the black hole. However, in the frame of reference of the spaceman who has incautiously approached the gravitational radius, time does not slow down, and being sucked into the black hole is more or less instantaneous from the spaceman's perception.

Perhaps you might care to try falling into a black hole, courtesy of Colorado Uni [3]

BTW, your third paragraph is in my opinion totally unsuited to an encyclopaedia subject, even if you were right about the rest, which you evidently are not.Elen of the Roads (talk) 01:22, 6 July 2009 (UTC)

Maybe the third paragraph is unsuited. You said I am wrong?! I explained to you that the formal solution from the point of view of the observer is not valid due to the singularity! Why do you keep pushing it?

There is another reason this solution is not valid. The only meaning science has is for phenomena that can be independently observed. Since the crossing of the EV cannot be observed, it does not exist as a scientific statement. You said that I should try it. But how would I communicate with you? Once on crosses the EV, communication with the rest of the universe is impossible.

This false insistence on the formal solution probably has religious undertones, for people believing that dead souls go to Heaven. Science disagrees, as no one can communicate with Heaven. If general relativity states that someone can cross the EV and continue moving, it is just as false as someone saying the dead go to Heaven.

To summarize, the formal solution is false for the mathematical reason of the proven singularity (i.e., proving the solution false), and for the philosophical reason as to the meaning of science.

There is only one solution to GR, that which is observed by the external observers, who note that it takes forever to reach the EV.

You are doing a disservice to science by refusing to let my article appear.

I do think that due to the widespread confusion on this topic more needs to be said.

You do not like my arguments against the non-existence of the formal solution of the falling observer? Well, consider the issue of the non-existence of the advanced wave solution. Wheeler and Feynman proposed a solution. Einsten said that their solution is not valid in GR. There is no mathematical way to eliminate the advanced solution. We can only say that it violates a basic principle of science.

Ever hear of boundary conditions restricting wave solutions of a vibrating string? The singularity at the center of the BH is like a boundary condition saying that the solution is not valid.

Please stop talking about how the observer feels as he/she falls down the BH crossing the EV. This solution is not a valid solution to general relativity!

How much more do I need to say to convince you?

Everything I have said to you is contained within RELIABLE SOURCES. I can point you to half a dozen online sources from university physics professors which supports what I said. Please show me a reliable source that backs up what you are saying and we can consider how to word the article in its light.

ALSO, (and apologies if you did this by accident) please stop posting reruns of your original argument lower down the page. Elen of the Roads (talk) 15:23, 6 July 2009 (UTC)

Addition by user Aranoff Black holes

There is confusion about the definition of a black hole, BH. The idea of an object with gravity strong enough to prevent light from escaping was proposed in 1783. See http://library.thinkquest.org/25715/discovery/conceiving.htm. Reverend John Mitchell, an amateur British astronomer, proposed that gravity could affect light as well as matter. Mitchell showed that an object with the density of the sun, yet five hundred times larger would exert a gravitational pull so great that "all light emitted from such a body would be made to return toward it."

In special relativity, first published by A. Einstein in 1905, this definition makes sense. The escape velocity from an object depends upon the mass. For a large enough mass, this velocity will approach and exceed the speed of light. Since special relativity forbids speeds greater than the speed of light, nothing can leave an object will large enough mass.

In general relativity, a BH has an entirely different explanation. The basic equation of GR is G=T, where G is the geometry tensor, and T is the energy tensor. Geometry is not Euclidean. Time is slowed down in a gravitational field. A space shuttle will measure slowing of time as it descends from space to earth by a measurable amount. This is critical for the use of GPS, gravitational positioning systems.

Imagine the space shuttle landing on an object the size of a small planet, but the mass of a star. The gravitational field will be very large and so that the time dilation will be huge. Distant observers noting the descent will say that the shuttle slowed down.

In an extreme case where the mass of the star is large enough, the time dilation will be so large that the shuttle will never get to the surface. Time has stopped. This is what a BH is in GR.

GR admits two solutions of the motion of objects towards a BH. One solution is that seen by external observers, who see the approach taking an infinite time. The other is from the point of view of an observer moving towards the star, who sees the approach taking a finite time. There is an inconsistency between these solutions. This inconsistency is only for the BH case, not for objects falling towards massive objects that are not BH.

One solution is to say that a BH cannot exist, as do Abhas Mitra, "Non-Occurrence of Trapped Surfaces and Black Holes in Spherical Gravitational Collapse", Foundation of Physics Letters, 13, (6), pp. 543-579 (2000). As an object collapses under gravitation, the relativistic time dilation slows down the collapse so that it takes an eternity to reach the final state of a BH, an Eternally Collapsing Object.

Leonard Susskind,raises another point in The Black Hole War: My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics, amazon (2008). According to Susskind, Hawking claimed that the formal solution from the point of view of the observer falling into the BH is valid. All information about the object is lost. If information is lost, no information is available about the past , a paradoxial situation. Susskind's solution is his Holographic Principle, derived from quantum mechanics, which states that all the information about the "inside" of the BH resides outside. However, no quantum theory of gravity exists.

In summary, we define a BH as a region in the universe that it takes forever to reach. This is how we can currently define a BH using GR without quantum mechanics, and avoiding all conflicts.Aranoff (talk) 01:39, 7 July 2009 (UTC)

"we define a BH as a region in the universe that it takes forever to reach"

Who is we? Repeating a statement indefinitely doesn't make it verifiable. You better provide reliable references. JocK (talk) 02:44, 7 July 2009 (UTC)

Aranoff, this appears to be a completely original theory on your part, so unless you want to sound like Ann Elk (see Monty Python's Flying Circus) please provide references to where other physicists are saying what you are saying.Elen of the Roads (talk) 13:25, 7 July 2009 (UTC)

Metric

I understand that these articles are generally aimed at a non-technical audience, but shouldn't there be a technical section or at least a link to a technical article on wikipedia? As it stands, this page doesn't have the Schwartzschild metric written anywhere, or even a link to it. Njerseyguy (talk) 15:34, 4 August 2009 (UTC)

Well it does link to it, at least once. It is linked when the schwarzschild solution is first mentioned in the history section. It might be linked more than once though. (TimothyRias (talk) 21:13, 4 August 2009 (UTC))

black hole

does really a black hole exists in reality and how can one belive it —Preceding unsigned comment added by 220.225.211.171 (talk) 05:52, 13 August 2009 (UTC)

This is covered in detail in the article. The short version is that we've seen compact objects too heavy to be supported by any form of degeneracy pressure, with infalling matter that doesn't produce any form of impact heating when it spirals in and vanishes. Relativity predicts what happens under these conditions (a black hole forms, and matter passing through the horizon is absorbed). Observations match the predictions quite well, so it looks very much like black holes (or equivalent objects) do exist. --Christopher Thomas (talk) 05:59, 13 August 2009 (UTC)

Movement of Black Hole resulting from transfer of Kinetic Energy

Theoretically speaking, I suppose, a Black hole is incapable of moving as a result of transfer of kinetic energy due to a collision with another object. Iow, nothing can "push" it. It is only capable of changing position in space resulting from gravitation. If this is true, shouldn't we include this in the article? - neveos

It can have an inelastic collision, by absorbing an object that falls into it. The momentum the infalling object had (when it was far from the hole) is added to that of the black hole. You can consider gravitational interactions during close passes as producing the equivalent of elastic collisions, too. In this type of situation, the exchange of momentum is mediated by gravity rather than the electromagnetic force (which mediates it for normal matter), but the end result is the same. I'm not sure this really needs to be in the article (perhaps put a proposed phrasing here and ask for other editors' comments). --Christopher Thomas (talk) 19:21, 15 September 2009 (UTC)

Seems rather important since it dictates the only manner a black hole can change position. Meaning black holes really only change position, relative to everything else, only if there is a substantial degree of gravitational pull. Since most galaxies are facilitated by a central black hole, it seems to interesting, in the very least to mention it, since it implies the movement of galaxies. - neveos

It doesn't actually strongly influence the paths of galaxies. To put it more clearly, the galaxy's path would be the same whether the core was a supermassive black hole or a very-massive star cluster. Gravity is the only significant source of interaction over interstellar distances, so a black hole's lack of a tangible surface doesn't change how it interacts. --Christopher Thomas (talk)

Protect?

Can we protect (or at least semi-protect) the article? 90% of the recent history is "undo, revert, undo, revert", etc. That's a waste of everyone's time, as well as the server's resources. Dmitry Brant (talk) 17:59, 28 August 2009 (UTC)

I agree with Dmitry Brant. This "Black hole" article is an obvious target for idiotic juvenile jokers -- one recent anonymous 'edit' made a "your penis comes into contact with one" suggestion. --IVAN3MAN (talk) 04:14, 6 October 2009 (UTC)

Seconded. There were some useful edits made by 75.18.191.225 (talk), but I had to go pretty far back to find them. I've put in a request at WP:RFPP. --Christopher Thomas (talk) 06:56, 6 October 2009 (UTC)

Black hole forming behind a rocket accelarating at 1G

it true that a rocket accelerating continuously at 1G as perceived by the passengers would observe a black hole to form behind itself and that a photon emitted from this point would never reach the rocket? If so then what is this kind of black hole called? Is the math describing it much simpler than the math describing a regular black hole? Would it be appropriate to mention this somewhere in the article? If not then what article would it go under? Lemmiwinks2 (talk) 00:53, 30 August 2009 (UTC)

You're talking about relativistic rockets, the effect you're referring to is described in "Below the rocket, something strange is happening..." section of The Relativistic Rocket--Ben Harkness (talk) 01:27, 30 August 2009 (UTC)

The relevant math is at Rindler coordinates, which also discusses the Rindler Horizon, which is an event horizon arising in this spacetime. It is however not really a black hole. It does have a lot of technical similarities to a black hole, for example the Unruh effect is simply hawking radiation in the rindler spacetime. I, however see little reason to include in this (already overfull) article. (TimothyRias (talk) 11:19, 30 August 2009 (UTC))

Radius of Supermassive Black Holes

The following statement in the article could use further explanation: "The first estimates indicated that the central object contains 2.6 million solar masses and has a radius of less than 17 light hours. Only a black hole can contain such a vast mass in such a small volume."

It is not clear why an assumed sphere with a radius of 17 light-hours would have a small volume. Such a sphere would have roughly twice the radius of our entire solar system and could contain some 18 trillion of our suns (by my rough calulcations). I've always assumed black holes, even supermassive ones, to be much smaller than this. Probably a missunderstanding on my part, but like I said, it could use further explanation in the article. 75.57.252.245 (talk) 03:18, 3 September 2009 (UTC)

It's "small" when compared to the size of non-black-holes with similar mass (large star clusters, many tens of light-years in diameter). The important number here is the Schwarzschild radius, which can be thought of as the minimum possible size for an object with a given mass. Anything this compact collapses into a black hole with an event horizon of that radius. --Christopher Thomas (talk) 04:42, 3 September 2009 (UTC)

It is important to understand that the formation of a BH is not due to something mysterious that happens deep in the dark at the center, but is due to the global properties of the spacetime outside the event horizon. It does not depend either on exotic properties of matter at high density, as a black hole can be formed out of matter with only the density of water, or even less. Because the mass in a sphere of uniform density increases as the cube of the radius R, while the Schwarschild radius increases in proportion to its mass, it is inevitable that a BH can be formed if only R is made large enough. (A Schwarschild BH of 3 million solar masses would have a radius of around 10 million km, but a dense star cluster of that mass, with a radius of only 17 lt-hrs, would be unstable to "gravothermal collapse", even by Newtonian physics, on a time scale very short compared to the age of the Galaxy.) Wwheaton (talk) 17:50, 5 September 2009 (UTC)

Do Black Holes exist even theoretically ?

Surely all of you have read the original paper by Karl Schwarzschild. No?! You should! After that, please help me out here. Where in this original work did a black hole solution come up? As far as I can see, nowhere. There is no talk of a black hole radius whatsoever. Still the paper is correct, at least nobody has argued the contrary successfully ever since 1916. You see, that's my problem. Everybody uses Karl Schwarzschild's name and when you take a look in the fellow's original papers: nothing! Aoosten (talk) 19:56, 4 September 2009 (UTC)

Schwarzschild's original solution is correct but incomplete. This is true both for the original and the standard form of the metric named after him. (Note that the standard form is only a valid metric for r > 2m, just like the original form.) It has since been shown rigorously that this can be analytically extended across the horizon, until the point that a curvature singularity is reached. (TimothyRias (talk) 22:14, 4 September 2009 (UTC))

Could you point out what is incomplete about Schwarzschild's original paper? I do not see the restriction r>2m anywhere. In this paper r is simply the radial coordinate. There is a restriction on R=(r^3+a^3)^{1/3} namely R>a, but I presume this is not what you mean, do you? Aoosten (talk) 13:15, 5 September 2009 (UTC)

Scharzschild's R is now usually called r and his α is now usually called 2m. If you assume Schwarzschild's r > 0 then Schwarzschild's R > α, which corresponds to r > 2m in modern notation. That's what TimothyRias meant. In any case, coordinates aren't reality. Rindler coordinates only cover part of Minkowski space; that doesn't mean the rest of the Minkowski space doesn't exist. The coordinates in Schwarzschild's original paper didn't cover all of what's now called the "Schwarzschild geometry", but the geometry exists as a solution to GR and can be covered completely by other coordinates. If you're just trying to argue that Schwarzschild should be given less credit for discovering black holes then a case might be made for that, but we have to use terms like "Schwarzschild geometry" and "Schwarzschild radius" because they're standard. (Cf Stigler's law of eponymy.) -- BenRG (talk) 15:38, 5 September 2009 (UTC)

And as mentioned in the previous section, "Radius of Supermassive Black Holes", the formation of an event horizon is a consequence of the global geometry external to it, which Schwarzschild had right. As the history section of this article discusses, Oppenheimer and Volkoff explicitly discussed the formation of a BH in 1939; that this is inevitable was (generally accepted as) proved by the singularity theorems established in the 1960s. Schwarzschild found the first non-trivial solution to the Einstein Equations, which (appropriately, I think) bears his name; and that solution exhibits the global geometry external to r that is essential to the issue of the BH. Wwheaton (talk) 18:12, 5 September 2009 (UTC)

Schwarzschild warned against this explicitly in a letter to Einstein. He introduced $R=(r^{3}+a^{3})^{1/3}$ only for convenience, where r is the radial coordinate. If one insists on considering R a coordinate one should not forget that the region $R<a$ does not exist. That is what he meant when he wrote "R, ... sind keine ”erlaubten“ Koordinaten, mit denen man die Feldgleichungen bilden d¨urfte". Translation: "R, ..., must not be used as coordinates, with which one can form the field equations." Unfortunately, it is now "standard" to do precisely this. —Preceding unsigned comment added by Aoosten (talk • contribs) 19:23, 5 September 2009 (UTC)

First, it makes no difference what Schwarzschild thought or said. He has no authority. Your argument is like saying that we're wrong to ship computer games on CD because CDs were meant to be used for music. Second, the modern understanding of black hole geometry is not based on arbitrarily assigning meaning to R ≤ 2m in Schwarzschild's metric. It is based on analytic continuation of the R > 2m geometry. This does not take you "past R = 2m in Schwarzschild coordinates", it takes you to a region of the geometry that is not covered by Schwarzschild coordinates at all. R = 2m in Schwarzschild coordinates is not the event horizon, it's a coordinate singularity. Third, as Wwheaton said, all observable consequences of black hole behavior derive from the R > 2m part of the geometry only. That includes the horizon area, the Chandrasekhar limit, accretion discs, frame dragging, etc. It just doesn't matter if there's anything behind the event horizon. If you were right and spacetime magically ended just beyond the horizon, it wouldn't affect black hole phenomenology in the slightest. -- BenRG (talk) 20:15, 5 September 2009 (UTC)

So Karl Schwarzschild "has no authority" in this matter?! That is pure arrogance. Just because he can no longer review papers and grant applications? Show a little more respect for a dead genius, please. But back to contents. You state ".. and if spacetime magically ended beyond the horizon ...". Please don't spin my words around like a CD, I am not saying that at all. I am saying that spacetime ends beyond r=0, which is the same as beyond R=alpha or 2m. There simply IS no region R<alpha in Schwarzschild's solution as published in 1916. And as far as I know his solution is correct. —Preceding unsigned comment added by 84.194.184.127 (talk) 21:41, 6 September 2009 (UTC)

His solution is correct, but incomplete. You can show rigorously that his coordinates only cover a part of a manifold, and that you can reach the uncovered parts in finite proper time. You are correct to claim that Schwarzschild did not discover this, in fact, he may have thought that his solution was complete. But the rigour of mathematics does not lie, anybody with a decent grasp of differential geometry (which for odd reasons does not include everybody doing GR) can show you why the solution is in complete. If it makes you feel any better for a long time the general thought in the GR community was that the black hole interior must be unphysical, either because it is a mathematical pathological case induced by the symmetry requirements or because the laws of nature would prevent them forming. Bit since then it has both been shown that black holes are generic, and that there formation cannot be prevented without violating some of the more fundamental tenets of modern physics. (Which is exactly why the gather so much theoretical interest, because they are generally viewed as indicative of our laws of physics breaking down. (TimothyRias (talk) 09:33, 7 September 2009 (UTC))

TimothyRias: This thread is very interesting to me, both historically and your statements that "their formation cannot be prevented without violating some of the more fundamental tenets of modern physics. (Which is exactly why the gather so much theoretical interest, because they are generally viewed as indicative of our laws of physics breaking down.)" Could you undertake to get these ideas into the article? It would address many of the questions that are in the mind of the general reader, who is very likely to be skeptical of the sanity of thinking black holes exist, and to think that in fact they are simply the result of extrapolation of theory beyond the limits of applicability. Brews ohare (talk) 15:33, 8 September 2009 (UTC)

Oppenheimer/Snyder explained in doi:10.1103/PhysRev.56.455 page 459 "...; a cone within which a signal can escape has closed entirely." Bijection tells that the cone also closes for infalling signals. They also explain on page 456 "... from the point of view of a distant observer, an infinite time for this asymtotic isolation to be established, for an observer comoving with the stellar matter this time is finite and may be quite short." Bijection again tells that the comoving observer views infinite distant time in finite proper time -right?_{The trajetories are different, sorry.} And bijection is valid for all r>r0. --Swen (talk) 06:23, 9 September 2009 (UTC)

A better argument against the existence of "real" black holes

see here Count Iblis (talk) 15:07, 8 September 2009 (UTC)

paradox of irreversability

The article mentions the apparent paradox of the irreversability of objects entering the black hole. The reason its irreversable is because acceleration doesnt reverse when time reverses. A rocket accelerating in one direction will still be accelerating in the same direction when you reverse time. And of course gravity and acceleration are indistinguishable. Lemmiwinks2 (talk) 01:27, 16 September 2009 (UTC)

12.4mb "lensing" GIF on main page

Is this really a wise idea?

a) HUGE data load; it's a large amount even for broadband, someone viewing this page on dialup or a mobile device will take a MASSIVE hit (ironic, considering the subject). If for some reason I was checking it on my phone without an unlimited plan, and the entire image loaded, I would be charged more than £17 for the privelege without having even been given the option to skip. (Images turned on, but not expecting such a wedge from a wiki page, where the relevant info is in text)

b) GIF is a copyright-carrying file format, we should be moving towards open standards such as motion PNG (seeing as a lot of diagrams are shifting to SVG, etc). It's also inefficient on size and has colour limits. An embedded flash movie would probably be smaller and be just as copyright laden if not less.

c) it's a bit pointless, too; the phenomenon could be just as effectively demonstrated with four or five lower-resolution frames of this video-rez (CIF), high-frame file. Or even a quite small javascript effect (see: geocities from 1998 with "reflecting water" effects on graphics) would do it as well.

Also, can't it have a "preview" frame on the main page, with a link to the full thing, including the size in parentheses? Unfortunately I'm not au fait enough with wiki editing to know if this is even possible, let alone how to do it. In hope of solution, yours faithfully ;) 193.63.174.10 (talk) 11:46, 21 September 2009 (UTC)

The patent on GIF ran out several years back, but flash is still under patent, so switching to flash is probably not a good idea. I agree that the size of the GIF animation is a problem. It turns out that someone already made a scaled-down version of it (about 700k). I've modified the article to use that one, with a link to the larger one. --Christopher Thomas (talk) 16:14, 21 September 2009 (UTC)

Universe and "believe" or "presume/hypothesize".

Firstly, Elen of the Roads, the reason why I had chosen (with good intentions) to capitalize the term "universe" in this article was that, according to the International Astronomical Union (IAU), the names of all astronomical objects (the Sun, the Earth, the Moon, the Solar System, individual stars, planets, galaxies, etc.) should be capitalized, so I had assumed that it should apply to "the Universe" as well, to differentiate it from the "local universe"; however, on checking again, I found that they do not mention whether "universe" should be capitalized or not.

So, I apologize for any inconvenience caused -- I'm new here!

Secondly, Timothy Rias, I have a big bone to pick with you: "There is nothing religous [sic] about the word believed."

Obviously, sir, you have not engaged in 'debates' with creationist crackpots nor "Electric Universe" cranks -- the Wikipedia entry on "Crank (person)" is spot on!

Creationist crackpots often accuse evolutionists of having a 'belief' and that "evilution" is a religion. Likewise, "Electric Universe" cranks often state that "The Big Bang Never Happened" (quoting Eric J. Lerner's book), that black holes don't exist, and that the Big Bang theory and dark matter/energy are just "religious beliefs of modern cosmologists". In fact, one such crank, who frequents the comments section at Universe Today, pointed to this very article in Wikipedia and stated that it says astronomers 'believe' this and astronomers 'believe' that. So, you can see why I (and others like me) have an issue with the term "believe".

Now, may I refer you to the following definitions of the term "believe", according to the Oxford English Dictionary and Dictionary.com:

Oxford English Dictionary — believe

believe —verb: 1. accept that (something) is true or (someone) is telling the truth. 2. (believe in) have faith in the truth or existence of. 3. have religious faith. 4. think or suppose.

N.B. Definition #2 refers to "faith"; definition #3 refers to "religious faith".

Dictionary.com — believe

believe —verb (used without object): 1. to have confidence in the truth, the existence, or the reliability of something, although without absolute proof that one is right in doing so: Only if one believes in something can one act purposefully. —verb (used with object): 2. to have confidence or faith in the truth of (a positive assertion, story, etc.); give credence to. 3. to have confidence in the assertions of (a person). 4. to have a conviction that (a person or thing) is, has been, or will be engaged in a given action or involved in a given situation: The fugitive is believed to be headed for the Mexican border. 5. to suppose or assume; understand (usually fol. by a noun clause): I believe that he has left town. —verb phrase: 6. believe in, (a) to be persuaded of the truth or existence of: to believe in Zoroastrianism; to believe in ghosts. (b) to have faith in the reliability, honesty, benevolence, etc., of: I can help only if you believe in me. —Idiom: 7. make believe.

N.B. Definition #1 suggests having confidence in something "without absolute proof"; definition #2 implies having "faith".

Timothy Rias: "...(and in half the changes made presumed/hypothesized are flat out wrong)".

So, according to you, the famous phrase "Dr. Livingstone, I presume" is also flat out wrong as well, then?

Dude, I don't know which dictionary you're using, but according to the Oxford English Dictionary and Dictionary.com:

Oxford English Dictionary — hypothesize

hypothesize (also hypothesise) —verb: put forward as a hypothesis.

Oxford English Dictionary — hypothesis

hypothesis —noun (pl. hypotheses): 1. a supposition made on the basis of limited evidence as a starting point for further investigation. 2. Philosophy. a proposition made as a basis for reasoning.

N.B. Definition #1, in a scientific context, sounds better than the definition(s) for the word "believe".

Dictionary.com — hypothesis

hypothesis —noun (plural -ses): 1. a proposition, or set of propositions, set forth as an explanation for the occurrence of some specified group of phenomena, either asserted merely as a provisional conjecture to guide investigation (working hypothesis) or accepted as highly probable in the light of established facts. 2. a proposition assumed as a premise in an argument. 3. the antecedent of a conditional proposition. 4. a mere assumption or guess.

N.B. Definitions #1 and #2 are perfectly appropriate in a scientific context.

Oxford English Dictionary — presume

presume —verb: 1. suppose that something is probably the case. 2. take for granted. 3. be arrogant enough to do something. 4. (presume on/upon) unjustifiably regard (something) as entitling one to privileges.

N.B. Definition #1 states that "something is probably the case"; creationists 'believe' that "you're going to hell" if you don't agree with them.

Dictionary.com — presume

presume —verb (used with object): 1. to take for granted, assume, or suppose: I presume you're tired after your drive. 2. Law. to assume as true in the absence of proof to the contrary. 3. to undertake with unwarrantable boldness. 4. to undertake (to do something) without right or permission: to presume to speak for another. —verb (used without object): 5. to take something for granted; suppose. 6. to act or proceed with unwarrantable or impertinent boldness. 7. to go too far in acting unwarrantably or in taking liberties (usually followed by on or upon): Do not presume upon his tolerance.

So, Timothy Rias, where exactly am I wrong in the use of the terms "hypothesize" and "presumed", then? As far as I can see, those are appropriate scientific terms to me. --IVAN3MAN (talk) 21:44, 4 October 2009 (UTC)

Agree with IVAN about the "believes". The article contained 8 or 9 instances of the word. I've changed them in various ways - discuss each here rather than do such a blanket revert please. The word "believe" may not be a "religious term, but it has strong religious connotations and can be contentious. It is a "loaded" word and probably should be avoided in most science topics. Science works with evidence not beliefs. Now I'm well aware that scientists are human and impacted by their culture ... but that discussion belongs elswhere. Vsmith (talk) 23:39, 4 October 2009 (UTC)

As TimothyRias says, "believe" and "postulate" are not equivalent in meaning, and "believe" is correct here. It is standard usage. Replacing "believed" with "thought" would be acceptable (e.g., "it is thought that supermassive black holes exist in the centers of many galaxies"), but replacing it with "postulated" or "hypothesized" is wrong because these aren't postulates or hypotheses, they are tentative conclusions from the evidence. The capitalization of "universe" should also be removed because it does not match usage in the field. -- BenRG (talk) 10:52, 5 October 2009 (UTC)

"Believe" in the sense "to think to be true" is a verb that was perfectly fine in many of the places were it was used. In contrast to "presume"/"hypthesized" it does not actually imply any assumption about the truth of the statement for the basis of further reasoning. As such it perfectly describes the scientific opinion of Einstein and Eddington on black holes, they both believed that black holes couldn't exist and tried to find evidence for their position. They however did not presume that black holes did not exist nor did they make it a hypothesis. (They were in fact thrying to falsify to opposite hypothesis.)

Now, the word may have been over used in the article previously, some of the replacements made however still carry the wrong scientific connotation for the statement being made. I'll go over them more carefully later and correct were necessary.

N.B. IVAN3MAN you might want to read over WP:CIVIL and WP:AGF. (TimothyRias (talk) 07:02, 5 October 2009 (UTC))

Hi IVAN3MAN. No problems, I figured it was a style thing, but do read Wikipedia's manual of style. Names of the type "Solar System", "Sun" and "Moon" should not normally be capped up - although the names of astronomical items of the type "Venus" "Alpha Centauri", normally should be. Exceptions are listed in the MOS. Elen of the Roads (talk) 07:47, 5 October 2009 (UTC)

TimothyRias, after reading WP:CIVIL and WP:AGF, I realize now that I may have been a little arrogant towards you in my post above; that was due to me being somewhat wound-up as a result of recent 'debates' with "Electric Universe" cranks, creationist crackpots, and "Moon landing hoax" conspiracy theory nutters, as well as having a bit too much beer over the weekend. Consequently, some of my sarcasm, that I usually direct towards those individuals, may have spilled over in my post above, so I apologize if I've caused you any offence -- that was not my intention. Like I said above, I am new here! --IVAN3MAN (talk) 03:29, 6 October 2009 (UTC)

Welcome to wikipedia. No real harm was done, but for future reference remember that honey is more likely to get you anywhere then vinegar. I understand your frustration with dealing with cranks on the internet. These are however not the people we need/want to cater to. (TimothyRias (talk) 09:41, 6 October 2009 (UTC))

Acknowledged, and thanks! --IVAN3MAN (talk) 04:57, 7 October 2009 (UTC)

Discrepancy in "Black hole candidates": Sagittarius A*

In Black hole candidates - Intermediate-mass, Sagittarius A* is described as having an intermediate-mass black hole:

Section 6.2, line 1: "In November 2004 a team of astronomers reported the discovery of the first well-confirmed intermediate-mass black hole in our Galaxy, orbiting three light-years from Sagittarius A*. This black hole of 1,300 solar masses..."

Yet the following section, Black hole candidates - Stellar-mass, says the Sag A* black hole is supermassive (the size above intermediate-mass):

Section 6.3, line 1: "Our Milky Way galaxy contains several probable stellar-mass black holes which are closer to us than the supermassive black hole in the Sagittarius A* region."

Which size is the Sag A* black hole, supermassive or intermediate-mass? Or are the sections describing two different black holes near Sag A* (which is not how it reads)? 71.234.215.133 (talk) 02:54, 15 October 2009 (UTC)

section 6.1:Astronomers are confident that our own Milky Way galaxy has a supermassive black hole at its center, in a region called Sagittarius A*[65] since...the central object's mass is about 3.7 million solar masses and its radius no more than 6.25 light-hours. Lemmiwinks2 (talk) 03:29, 15 October 2009 (UTC)

OK, I got it. Sag A* is the supermassive black hole at the center of the galaxy, and the intermediate-mass black hole is orbiting it. Wow, what a confusion of cavities. 71.234.215.133 (talk) 04:14, 15 October 2009 (UTC)

As an aside, could the link to Sag A* be removed from section 6.3 (Stellar-mass)? It is already linked section 6.1 (Supermassive). 71.234.215.133 (talk) 04:50, 15 October 2009 (UTC)

Hawking Radiation

I was reading Discovering the Universe Seventh Edition by Neil F. Comins and William J. Kaufmann III, and I came upon the section about black holes, and Hawking Radiation. I would have cited information about Hawking Radiation; however, I do not know if this is a violation of copyright rules. Would it be a violation of copyright rules if one includes a citation?ItsJodo (talk) 01:00, 16 October 2009 (UTC)

If I understand correctly, the preferred approach is to add a sentence or paragraph describing in your own words the point the source makes with a <ref> tag giving the citation. Second choice would be a formal quotation, with a reference tag after it. Direct copying of material without making it very clear that it's a quote, and giving the source, is indeed a violation of copyright and of Wikipedia's policies.

What were you thinking of adding, by the way? Hawking radiation seems to be pretty well covered already. --Christopher Thomas (talk) 01:28, 16 October 2009 (UTC)