Talk:Black hole/Archive 5

This is an archive of past discussions about Black hole. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page.

Archive 1

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Archive 3

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Geodesics

Geodesics maximising proper time - this is the case inside a black hole, but is not precisely correct. Geodesics extremise the proper time (that is, an infinitesimal change in the path will result in no changes to the proper time). They could equally well minimise the proper time. Either way, it needs a slight word adjustment.

I can think of a few references, but the one on my lap is Gravity, Hartle (2003), Addison Wesley: San Fran.

Accretion is magnetically driven

Spectrographic studies indicate that accretion through the disk of a black hole is driven by magnetic forces, according to a paper (DOI 10.1038/441938a) in this week's Nature. See the Editor's Summary, and the full article (subscribers only). —The preceding unsigned comment was added by 62.49.224.52 (talk • contribs) on 09:44, 24 June 2006.

profanity in 'Black Hole' article

Please check the Black Hole article under 'Entropy and Hawking radiation' at the end, there is a curse that I am unable to edit. Thanks. Mona Balogh

This seems to have already been fixed. --Christopher Thomas 04:40, 31 May 2006 (UTC)

On a general note, this page seems to attract a quite astonishing number of vandalism attacks. It seems that many puerile adolescents are so scatalogically-minded that they cannot hear the word "hole" without sniggering. When they see it in print, they feel compelled to share their coruscating wit with the rest of us. Is it worth vandal-proofing the page? --Oscar Bravo 08:20, 2 August 2006 (UTC)

I've asked on previous occasions (this article and Big Bang seem to be popular for some reason). The admin who was handling semi-protection when I'd asked said that 10 reversions/day wasn't enough damage to justify it. --Christopher Thomas 13:19, 2 August 2006 (UTC)

Oh well... best let the kiddies have their fun then, I suppose. On the other hand, sometimes the edits are just a little bit funny :-) --Oscar Bravo 13:33, 2 August 2006 (UTC)

Blackhole Metrics

This is a relatively minor issue. Under Mathematical Theory, the following statment is made at the top of the section:

Black holes are predictions of Albert Einstein's theory of general relativity. In particular, they occur in the Schwarzschild metric, one of the earliest and simplest solutions to Einstein's equations, found by Karl Schwarzschild in 1915.

While it is true that Schwarzschild coordinates can yield black holes, those coordinates are not the only ones yielding singularities within an event horizon. A Kerr black hole is mentioned later but to be accurate, there really should be some mention along the lines of Schwarschild being one set of corrdinates that yield black holes rather than the occurance of a black hole. In fact, it is pretty easily shown that Schwarschild coordinates cannot represent spinning black holes because Schwarschild requires more symmetries than spin truly allows for.

Jason Grigsby

I fail to see why mentioning that the Schwarzschild metric describes a black hole should imply that it is the only metric which does so. As you point out, it is not the only one, but I don't see anything in the text which suggests it is. Nevertheless I've added a sentence which I hope addresses your concerns. -lethe ^{talk +} 22:49, 7 June 2006 (UTC)

Thanks. I think the edit makes it far more clear.

Jason

Jason, you mentioned "Schwarschild being one set of coordinates that yield black holes": I think you meant to say that Schwarzschild coordinate chart is one possible chart describing an ("the") exterior region of the one-parameter Schwarzschild family of vacuum solutions, the unique family of static spherically symmetric vacuum solutions, which can either be matched across a spherical surface to construct a spherically symmetric perfect fluid solution (as in typical models on non-rotating stars) or extended beyond the event horizon to make a black hole solution.

BTW, in addition to the rotating and charged generalizations, arguably an even more important generalization is to null dust solutions such as the Vaidya null dust, Kinnersley-Walker null dusts, and Robinson-Trautman null dusts. These include the gravitational effect of infalling incoherent massless radiation. The simplest, the Vaidya null dust, gives a beautiful way to understand the teological nature of an event horizon.---CH 04:59, 8 June 2006 (UTC)

Blackhole Section Is Dead Science

R. J.: Wikipedia is not the place to "debate" your claim; see WP:NOR, so I have moved your comment to your user talk page.

You can try the links suggested in the header above if you want to argue for your claim (which is far from original, BTW). TIA---CH 08:30, 26 June 2006 (UTC)

Entropy and Hawking Radiation: Mass of a "small" black hole

Can I query a couple of points in this paragraph,

Hawking radiation originates just outside the event horizon and, so far as it is understood, does not carry information from its interior since it is thermal. However, this means that black holes are not completely black: the effect implies that the mass of a black hole slowly evaporates with time. Although these effects are negligible for astronomical black holes, they are significant for hypothetical very small black holes where quantum-mechanical effects dominate. Indeed, small black holes are predicted to undergo runaway evaporation and eventually vanish in a burst of radiation. Hence, every black hole that cannot consume new mass has a finite life that is directly related to its mass.

in particular the implication that Hawking radiation is really significant only for "hypothetical very small black holes where quantum-mechanical effects dominate".

If I have understood correctly, for a Schwartzschild black hole, the formulae for the black hole temperature and the entropy (in nats) are:

T_{H}={\frac {T_{p}}{8\pi }}\left({\frac {m_{p}}{M}}\right);\qquad {\frac {S}{k}}=4\pi \left({\frac {M}{m_{p}}}\right)^{2}

where m_p and T_p are the Planck mass and temperature respectively, and M is the mass of the black hole.

In kelvins, that temperature works out to be about 1.2 × 10²³ / M. So a black hole needs to have a mass of approximately the planet Mercury (according to Orders of magnitude (mass)) if it is to be in equilibrium with the cosmic microwave background.

I was quite surprised by just how small the black hole entropy is for everday masses much smaller than this -- ie compared to the everyday state of matter around us, how very ordered matter would be in such a black hole. I wonder if it might be worth including the sums in one of the articles somewhere, to make it more concrete why the numbers might be expected overwhelmingly to push for a small black hole to evaporate.

If I have got the formulae above right, the information content of a black hole weighing 18 grams would be just 8.6 × 10¹² nats = 1.5 × 10¹² bytes. Compare that to the standard entropy of a mole of water, weighing 18 grams. S° = 63.34 J K^-1 mole^-1, so S/k = 630 × 10²¹ bytes.

That's why the temperature of an 18 gram black hole would be so high, about 0.67 × 10²⁶ K. It's true that the radius of such a black hole would be at a quantum mechanical scale of length; but the mass may not be what people would usually think of as a quantum mechanical scale of mass. Jheald 10:23, 4 July 2006 (UTC).

About escape velocity

The article introduction says "A black hole is a concentration of mass, the gravitational field of which is so strong that its escape velocity exceeds the speed of light. [...] This implies that nothing, including light, can escape from within the event horizon, since there is no known method to exceed light-speed."

While this is a very common definition, I think it's flawed.

Escape velocity only applies to objects with no other forces affecting them than the gravity of the larger body they are trying to "escape". In other words, escape velocity tells us how fast an object should be "hurled" from the surface of the planet (or whatever) for it to escape to infinity (assuming no air friction etc). For example, the escape velocity of Earth (about 11.2 Km/s) means that you would have to give an object an initial velocity of 11.2 Km/s on the surface of the Earth (and assuming there's no air friction) for it to escape to infinity.

However, that only applies when the escaping object is not affected by other forces after it has been given its initial velocity. It's perfectly possible for an object travelling at a constant 1 m/s (or any velocity for that matter) to escape the Earth (or any other planet for that matter) if that velocity is kept constant with a thrust force. There's nothing stopping the object. The escape velocity is not a "magical barrier" which would stop all objects travelling slower.

Thus the definition that a black hole is a concentration of mass with escape velocity > c is all by itself flawed. It would imply that it is possible to escape a black hole using constant thrust (because it is possible to escape any planet doing so). However, that is not the case. Even with constant thrust it is not possible to leave the event horizon, and this is due to the geometry or space inside it (all geodesics to to the center of the black hole and none goes outside).

I'm not saying it's not true that the escape velocity is larger than c (which it obviously is). I'm just saying that all by itself the escape velocity is not enough to explain why nothing can leave the event horizon.

-- Juha Nieminen

I agree, and raised the point a while back in the archives as part of a different discussion. The actual definition is that within the hole's horizon all forward light-cones are warped to point back into the hole. If you want to propose a major rewrite of the introduction, it's probably best to put a proposed new version here and get comments on it from other editors before swapping it in.--Christopher Thomas 15:54, 14 July 2006 (UTC)

So wait... does this mean there's a surface outside the event horizon where the escape velocity is c? What is that surface called? —Keenan Pepper 17:46, 14 July 2006 (UTC)

The escape velocity _is_ still C at the event horizon, for some definitions of "escape velocity". The problem is that people tend to think of "escape velocity" in Newtonian terms, while "escape energy" works better in this situation. A photon released from arbitrarily close to but outside the horizon will eventually escape, but one released from within the horizon won't. An example that gives you a better idea of what's actually happening is to consider firing a particle with rest mass from points arbitrarily close to, but outside, the horizon. The velocity needed for escape asymptotically approaches C, and the energy needed tends towards positive infinity. All of these velocities and energies are defined with respect to reference frames that are at rest with respect to a distant observer. Per the discussion at event horizon, the picture ends up looking very different in different reference frames, which is another reason that I don't like the "escape velocity" treatment of the problem. --Christopher Thomas 18:18, 14 July 2006 (UTC)

Cool, that makes sense. I think we should rewrite the introduction to discuss "escape energy" instead. —Keenan Pepper 18:37, 14 July 2006 (UTC)

Go ahead. Maybe throw in some of the light-cone explanation as an alternate view if you can think of a non-confusing way to phrase it (I tend to fail at this). --Christopher Thomas 19:29, 14 July 2006 (UTC)

I think that the easiest way of expressing the "light-cone" thing is simply by saying that the geometry of space (as predicted by GR) is so warped inside the event horizon that there is no way out. (Well, someone should come up with a little bit more scientifical, though understandable way of saying that, but keeping this simple idea.) -- Juha Nieminen

I've made a stab at it. But bear in mind I'm a biologist, so some of you physicist may want to check it over. Jefffire 13:12, 15 July 2006 (UTC)

That was an improvement, but it still mentioned "escape velocity", which is misleading. I rewrote it to use "the energy required to escape" instead. How does everyone like it now? —Keenan Pepper 17:41, 15 July 2006 (UTC)

Better technicaly, but inelegantly written. I've reworded, feel free to change it. Jefffire 17:44, 15 July 2006 (UTC)

External Link added

Added external link to Space.com Black Hole information page - which includes various features, news updates, but also some original video material primarily derived from the Starry Night Astronomy family of software and dvds. Starexplorer 13:05, 22 July 2006 (UTC)

History

The History section says "The concept of a body so massive that not even light could escape was put forward by the English geologist John Michell in a 1784 paper". Discover (magazine), August 2005, page 37 says that Mitchel was an astronomer. (It also lists the year as 1783, but that could be a normal publication lag.) So was he an astronomer or geologist? Bubba73 (talk), 04:13, 23 July 2006 (UTC)

Well, I should have read the WP article on him. Looks like Discover is not quite correct. Bubba73 (talk), 04:18, 23 July 2006 (UTC)

Theory or fact?

I'm wondering if the intro should be changed to let readers know that black holes are actually theoretical. As it stands now, there is a bare sentence that says their existence is well-supported by observation. If they are an actual fact, then it isn't necessary to say that at all! If they are theory, then that sentence makes sense and it also makes sense to say outright: they are theoretical. --Smithfarm 13:39, 28 July 2006 (UTC)

They're as close to fact as we're going to get. The existence of extremely compact objects without observable surfaces is proven by observations of accretion discs without associated glow from impact heating. The only open question is exactly how black holes are described mathematically. The description given by general relativity is usually assumed to be very close to being correct, due to GR surviving every test that's been thrown at it, and so it's the one discussed in the article. In practice, the GR description is expected to be inaccurate near the region where it predicts a singularity. This too is discussed in the article. So, to answer your question, black holes are fact. Our _model_ of them is a theory, that's believed to be mostly in line with reality. --Christopher Thomas 16:05, 28 July 2006 (UTC)

Actually, "New Scientist Space" just reported a high reliability observation of a MECO. This would mean black holes cannot exit, as the two theories are incompatible on magnetism. See here —The preceding unsigned comment was added by 195.70.32.136 (talk • contribs) on 18:53, 29 July 2006.

No, the observation was of an accretion disc that ended somewhere other than it was expected to, and that had a hot inner edge. Interpretations of that observation are open to debate. --Christopher Thomas 19:15, 29 July 2006 (UTC)

This is a prime example of people departing from Popper's sound definition of scientific principles and turning mainstream science into junk. All evidence of black holes is circumstantial, and they're based upon what amounts to nothing more than unverified guesswork. They very well may exist, but there is not enough hard science or observation to make this an assumption, or to treat is as so likely that doubts need not be expressed early in their definition. Black holes are speculative; by true scientific method they do not even amount to theory, since they cannot be experimentally verified to the elimination of any known alternative, as the MECO issue demonstrates. To say anything else is pseudoscientific; you might as well throw out any skepticism expressed in the beginning of the Loch Ness Monster article, while you're at it. --Kaz 16:59, 2 August 2006 (UTC)

...cannot be experimentally verified to the elimination of any known alternative? If you're going to bring in Popper, at least get it right. Popper proposed falsifiabilty as the test of whether something is science or not. In this case, the hypothesis "Black holes exist" would be falsified if (inter alia) x-ray bursts were observed from an x-ray binary whose compact partner had a mass greater than 3 solar masses (since that would mean a large, compact object didn't have an event horizon).

In the case of black hole theory (BHT):

Many observations would falsify BHT but, so far, none have been made.
BHT predicts many phenomena that have been observed.

Taken together, that usually means it's fairly safe to give BHT tentative acceptance but, as ever, to continue to test it (see [1]. BTW, falsification of BHT would be excellent news for physics since it would imply the existence of new physics to explain why matter doesn't collapse into a singularity! --Oscar Bravo 12:40, 10 August 2006 (UTC)

False claim in the article.

>if the Sun were replaced with a black hole of the same mass, the Earth would not spiral into the dark abyss, for the gravitational force would still be the same at distances larger than the radius of the Sun, based on the masses and distance between them. The Earth would rotate around the solar-mass black hole as though it was still a normal star.

This is false. The loss of solar pressure (as our current bright Sun fires lotsa photons and elementary particles at Earth, pushing it away, but a black hole does not emit that much radiation) would mean the Earth's orbit must change. The thrust effect of solar radiation on objects in space is NOT negligible. 195.70.32.136 18:48, 29 July 2006 (UTC)

Have you actually calculated the photon pressure? I get about 5e+8 N from the sunlight falling on Earth. By comparison, the Sun's gravitational force on Earth is about 3.6e+22 N. 14 orders of magnitude difference certainly fits my definition of "negligible". --Christopher Thomas 19:09, 29 July 2006 (UTC)

What's more, the point of the example was that any object of equal mass will have equal gravitational pull. The example could be worded carefully to clarify that this is what is being expressed, but it stands as absolutely true. It's like saying "which is heavier, a pound of feathers or a pound of gold?" The "claim" is an absolute truism, expressed for the obtuse. It solves the question of who is buried in Grant's tomb. --Kaz 17:06, 2 August 2006 (UTC)

Making this "claim" even worse, the solar wind is actually a drag term in the Earth's orbit and one many orders of magnitude more significant than photon pressure. 19:07, 6 August 2006 (UTC)

MECOs

The MECO article needs severe cleanup: am embarking on a bit of that, but by Lucifer some people need to learn what a bibliography is. I have added a link to MECOs in the article. There was apparently a New Scientist article about it, but New Scientist does have a tendency to blow new borderline discoveries out of all proportion with wow-gee-whiz reporting. Google gets about 30 000 hits for ("MECO" "black hole") but about 20 000 000 for "black hole": as yet they surely do not merit mention as a serious alternative to black holes? Byrgenwulf 19:36, 2 August 2006 (UTC)

Google's database is not exactly a great reference for determining the "seriousness" of a scientific theory either, though. :) Bryan 14:50, 3 August 2006 (UTC)

Agreed - see argumentum ad Googlem for a recent article on it. What I am saying, though, and none too eloquently, is that one New Scientist article, a handful of arXiv preprints and a smattering of actual publications do not mean that suddenly, just because New Scientist published an article a week ago, that an encyclopaedia has to portray MECOs as a major competitor at the moment, especially in such a way that it looks like black holes might just have been falsified (which is the problem with wow-gee-whiz reporting). As yet, MECOs are a possibility, but don't need anything more than a mention in the "alternative models" section, surely? Byrgenwulf 14:58, 3 August 2006 (UTC)

Nonetheless, it is pseudoscientific to report black holes as if they were pretty much fact. Perhaps you need to read up on the philosophy of Karl Popper, who pretty much established what valid scientific methodology is. No matter how many people like the dramatic, exciting story of the mythical black hole, including lazy physicists, there is ZERO hard or direct evidence of their existence, not even a proposed system for obtaining evidence which eliminates all known alternatives which might produce that evidence. The black hole "predictions" which come true are all of that type; they fail completely to eliminate other causes for the effect predicted. Black holes remain nothing more than speculation, outside of junk science. --Kaz 17:14, 3 August 2006 (UTC)

Please. No-one mentioned pseudoscience. MECOs are not pseudoscience. However, they do not have nearly the volume of scientific literature investigating them as black holes do, nor have they (yet) been subsumed by the greater body of theoretical analysis. As yet, they are still a new, largely uncorroborated (recent popular reporting notwithstanding) theory, which may or may not turn out to work. I take exception to being told to read Popper. If you had actually read what I wrote above, you would have seen that I referred to black holes as being falsified: if this is not a direct reference to Popperian ideas then I do not know what is. Nonetheless, Popper is by no means the last word on the matter. I think the idea is weird and bizarre, personally, but I have seen how they seem to be a natural consequence of general relativity. Anyway, black holes are a fact in as much as they drop out of relativistic equations. They are real constructs, so to speak, which may or may not exist "out there". The article makes it clear that no-one has found something that is definitely a black hole, etc. etc. But black holes, as theoretical constructs, do have certain properties, and the article describes those: same as the MECO article describes various properties that a MECO, as a theoretical construct, would have. Why are black holes junk science, though? I never knew they were tools of political manipulation or anything like that!

More importantly, what specifically in the article needs changing? Also, since this article is in fairly good shape at the moment, are you able to help clean up the MECO article? I have done a bit, but it needs a Hell of a lot of work. Byrgenwulf 17:32, 3 August 2006 (UTC)

GR predicts black holes. We observe objects matching what GR predicts. This argument is entirely anthrogenic. Just because you can't understand it, doesn't mean it can't be possible. That's religion, not science. 19:14, 6 August 2006 (UTC)

"MECOs are distinguished from black holes in that they do not possess an event horizon, but do possess a strong intrinsic magnetic field (which a black hole would not have)." Shouldn't the article add the "No magnetic field" as a property of black holes? Maybe even the reason behind. As I don't really know if it's true I let you guys decide if it should be added. At least to me that feature isn't obvious and would be appreciated. --eipipuz 22:25, 14 August 2006 (UTC)

I don't have any particular stand on the issue of whether or where to include mention of magnetic fields, but I do want to see people get the facts straight. It is possible for a black hole to be threaded by magnetic field lines. This will happen, for example, when an accretion disk surrounds the black hole. The plasma in the disk and the surrounding atmosphere will make sure that the field lines stay on the hole. The presence of the magnetic field is crucial to the Blandford-Znajek effect—a hypothetical, but broadly accepted method for producing jets from a black hole.

A lone black hole (no accretion disk) will tend to very rapidly shed any magnetic field. This seems to be the source of the confusion. The New Scientist article was clearly written in hopes of having something exciting to say, so that they could sell magazines—like the whole gravastar thing. Observation of a black-hole-like object with a substantial magnetic field does not mean that black holes don't exist. Indeed, such an observation would just be considered an observation of a black hole with an accretion disk by most astrophysicists.

The observation reported in New Scientist was an observation of an object with an accretion disk which happened to have some unexpected properties. Not evidence for a MECO; not evidence against black holes. I'm not saying that MECOs don't exist, or that black holes do. It's important to keep in mind that MECOs—while not really pseudoscience—are definitely very speculative science. Black hole theory is on nearly the same ground as evolution theory; it's pretty well supported by the facts, but we can still argue about the meaning of Truth. MOBle 00:20, 15 August 2006 (UTC)

Black Spheres?

I was wondering if it would be ok to include the concept that black "holes" are not actually two dimensional, but four dimensional, as the term "hole" might be confusing to some readers. The introductory paragraph contains a sentence about black holes being a concentration of mass that .... (sentence goes on), and I wonder if this would be enough to convey that it is a spherical object in our observable 3D universe; most pictures throughout the article show discharges and circular disks "orbiting" the black holes on its gravitational plane, but almost no reference or explination is made to counteract the impression made of only two dimensions...Just a thought, because the first time that I read it, it was a little confusing...

Doubtful statement tagged with {verify source}

http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/universe.html

FRW cosmologies are not black holes, according to the sci.physics.faq. Black holes have a singularity in the future, the cirtical density flat FRW cosmology has a singularity in the past. Thus "If the visible universe has a mean density equal to the critical density, then it is a black hole." is a doubtful statement. Pervect 07:32, 10 August 2006 (UTC)

Mathematical theory

Might it be worthwhile to explain in the math section why the given equatios lead to black holes? Namely, the divison by zero that occurs at the Schwarzchild radius and why this leads to the properties of a black hole from a mathematical perspective. Also, some more explanation of what the entropy equation tells us would be good. If needed, I can make these changes myself within the next few days, but I'd like to hear some feedback on the ideas first. Archmagusrm 07:14, 12 August 2006 (UTC)

I hope this isn't original research. WP:NOR There are several coordinate systems that are well behaved at the event horizon, for instance. You might also want to look at the material on the schwarzschild geometry page and the deriving the Schwarzschild solution page. Pervect 11:18, 12 August 2006 (UTC)

(add) you might find that what you are proposing has been done there, as one of the articles has a complete derivation of the Schwarzschild solution. I don't know if anyone has covered the Kruskal extension offhand, if they have it's not under that name. Pervect 16:34, 12 August 2006 (UTC)

This certainly wasn't original research. Just taking a mathematical perspective of the situation. Yes, there are other coordinate systems that are well behaved at the event horizon, which is why the event horizon is not a physical singularity. I apologize for not following the links quickly enough. I suppose a physics article isn't quite the place to approach the issue from the pure mathematics perspective. I apologize. Archmagusrm 03:36, 15 August 2006 (UTC)

Definition of a black hole

The previous definition I replaced was: A black hole is a region of space-time with such a concentration of mass that its gravitational field is extremely strong to the point that no material particle can escape from it.

I found this defintion EXTREMELY awkward, and felt compelled to revise it.

I was looking for an authoritative defintion, but couldn't find any I really liked. The one I finally decided on was a somewhat modified version of the Meriam-Webster defintion. Pervect 07:24, 18 August 2006 (UTC)

Well done, the original senttence was very cumbersome. What you've added looks fine to me. It is really difficult to define in one sentence to a non-physicist what a black hole actually is. The old one about escape-velocity > c doesn't pass muster these days (see section above) since it doesn't address the space-time warping effects of GR. Also, since any matter can be used to form a black hole and since all information about that matter is lost as soon as it crosses the EH, it's pretty hard to say what a BH is - object sounds fine. --Oscar Bravo 10:12, 18 August 2006 (UTC)

As an aside, I believe there is some question over whether all information about matter falling into a black hole is actually lost. It was originally predicted that a black hole could be described entirely in terms of its mass, charge, and rotation, but now at least Stephen Hawking has changed his mind about that and believes information about matter is conserved. I'm not adding this to the article since I don't have references, etc, but it might be something to consider for an editor with a better grasp of this than I have. Great article, by the way! Vpoko 14:37, 24 August 2006 (UTC)

For authoritative definitions, you need to look at sources like Hawking & Ellis and Frolov & Novikov. Such definitions are too technical to discuss here absent background articles. ---05:57, 29 August 2006 (UTC)

Edits by Danras (talk · contribs)

Sorry, guys, I can't help you combat even the most absurdly misleading edits anymore, but FWIW, alterations like this constitute utter hogwash, and any good student in an undergraduate gtr should be able to spot some specific howlers:

"For very large black holes the gravitational acceleration at the event horizon can be less than that at the surface of the Earth." Here, Danras has clearly confused the magnitude of typical components of the tidal tensor (measured by some free-falling or even accelerating observer who falls through the horizon), that is, the curvature of spacetime itself, with the magnitude of the acceleration vector, that is, the path curvature of a world line in said spacetime. It is true that the tidal forces on a human falling through a supermassive black hole's horizon can be mild, but this does not imply that photons can escape from inside the horizon (they can't, at least not according to gtr).
"Black holes are calculated from a hypothetical point that is distant from the black hole." This doesn't even make sense as stated. Danras probably misunderstood something like this: the easiest way to verify the physical interpretation of the parameter m which appears in the Schwarzschild vacuum solution as the mass of the central object is to compare the motion of distant orbiting particles with Kepler's laws.
"The calculated event horizon is not a fixed property of the black hole, but is dependant on the gravitational potential of the point from which it is calculated." Actually, "Newtonian gravitational potential" simply doesn't make sense in strong field gtr (where Newton's theory of gravitation breaks down entirely, which is why we need gtr).
"A black hole calculated relative to an observer at a high gravitational potential may not be a black hole to an observer at a low gravitational potential." Same comment; Danras plainly doesn't have a clue.
"By artificially positioning observers close to an alleged black hole, one can create successive frames of reference in which the observed black hole radius decreases to zero." This appears to rest upon some attempt to use special relativity over a large region in the strong field gtr, a procedure which is always doomed to fail badly (this is more or less what we mean by saying spacetime is curved; you know spacetime is curved because laws which would work if it were not do not work).
"Escape from a singularity would also only require a finite amount of energy, for while the gravitational acceleration there would be infinite, this fact would be counterbalanced by the fact that such a singularity would occupy zero volume." Actually, this is so bizarre I am beginning to think Danras is just trolling. He wouldn't be the first.

Well, you get the idea: absolutely every sentence Danrad added is a howler (which is in itself evidence he is just trolling). Apparently this user was also responsible, at least in part, for the recently deleted "symmetrical relativity" (no such notion is recognized in physics).

The sad thing is, based on his contribs, I suspect his motivation might not have been trolling after all, but performing some kind of extremely misguided "service" to his church. See Robert Sungenis, another "apologist" who just doesn't know what he is talking about when it comes to gtr, but note that Danras's wild misstatements appear too naive to suggest that he might be some kind of Sungenis sock. BTW, for all I know, Danras and Sungenis might be fine fellows when they are not going on about physics; I am just saying that they don't know what they are talking about when they talk about physics.---CH 05:57, 29 August 2006 (UTC)

Reply to Edits by Danras

Before I give detailed criticism to CH (Chris Hillman), I would like mention that the concept of black holes is a classical concept that was articulated in the 1700s. This was well before the general theory of relativity existed. I think the article should acknowledge this fact from the outset. The article title is Black Hole, not GR Black Hole. I acknowledge GR makes different specific predictions, but I do not believe GR can retain the concept of black holes, if the classical conception is shown to be false according to classical physics.

1. I addressed a condition of low gravitational acceleration. Black holes can be massive enough so that tidal forces at the event horizon are not even an issue. CH fails to address why he feels that the event horizon is an impenetrable barrier to outgoing travelers and objects. Local forces in the vicinity of the horizon do not make it impenetrable. To overcome these forces, an observer can possess anything from a jet pack to a rocket ship with a trillion-to-one fuel to payload ratio. If the interior of a black hole is in contact with the immediate exterior of the event horizon, then it is in contact with the universe as a whole. A path of escape then exists which cannot be rationalized away.

I do not have the reference handy, but the author of a Scientific American article admitted the event horizon was not impenetrable barrier. The author felt that an outgoing object could not escape to infinity, but he did not explain why. (Possibly he assumed that the object could not possess self-powered thrust, and/or could not be picked up by an outsider with self-powered thrust.)

The current version of the article states that as an object moves closer to a black hole, the energy required for it to escape continues to increase until it becomes infinite at the event horizon. According to this version, the event horizon is impenetrable since the energy required for escape is finite at a millimeter outside the event horizon. Thus, the energy required to move from the event horizon to a millimeter outside of it must be infinite.

2. CH fails to address how black holes are defined. Unlike stars and planets, which are defined locally, black holes are defined from a distance. Stars and planets have fixed radii and surfaces, but the radius and the event horizon of a black hole are not fixed. People accustomed to thinking about stars and planets imagine that black holes are similar, but such an assumption is flawed. The event horizon of a black hole moves away from an observer as he approaches, for this horizon is defined as the place from which light just fails to reach the observer.

3. CH claims that gravitational potential does not make sense. It is technically part of the definition of a black hole. The event horizon is defined as the gravitational potential from which light just fails to reach us (or reach the defining observer). Perhaps CH is clueless as to how a black hole is defined.

4. CH is unresponsive.

5. The curvature of spacetime does not prevent successive observers from communicating with each other and relaying allegedly hidden information about a black hole.

6. I added that statement in case someone asked, "Well, what about the singularity?" It is necessary only for completeness. The statement basically states zero times infinity is finite. CH finds it bizarre.

CH finds my knowledge of physics to be "utter hogwash" and says absolutely every sentence I added is a howler. Perhaps I should apologize to him for pretending to know something about physics. From the way he carries on, one would think CH is a professional and I am an amateur. I have a physics degree and graduate school credits in physics. According to his user page, CH is no physicist, but only a mathematician by training. I think if CH was especially concerned or interested in physical reality, he would have taken physics.

Juha Nieminen has already pointed out in the discussion section "About Escape Velocity" that one can escape from black holes at slow speeds by using powered thrust. One does not need to be shot out from it at an alleged speed in excess of the speed of light. His (or her) argument is fully sufficient to dispel the whole notion of a black hole. One can also give other arguments, but there is little reason to as misunderstanding is not the problem—bias is.

I believe that CH and many others have a bias that no amount of logic can dispel. As defined, a black hole is unknowable because it is surrounded an event horizon through which no information can pass. CH and others have a need to worship the unknowable. They have presumably done things in their personal lives they hope will remain forever unknown, and a belief in unknowable black holes vindicates their hope. CH seems to be a proud defender of ignorance. He wants everyone to know that he knows black holes are unknowable. I do not disagree with his personal claim of ignorance, only with his claim that everyone is ignorant. He does not speak for me.

In the past, many scientists were against witch doctors who promoted superstition. CH and like-minded others can embrace such witch doctors as fellow believers in unknowable superstitions. There has always been numerous people who believe in superstition, but I do not believe they should be censoring articles about science. --Danras 16:10, 1 September 2006 (UTC)

First of all, I'd like to address the zero times infinity question. Zero time infinity is actually undefined. Beyond that, I hesitate to argue these points, as I don't even have any undergraduate credits in physics, just knowledge from many readings. Instead, I'll just make a statement of principle.

Danras, you're edits (similar to your "symmetrical relativity" idea) are against all things I understand and have read about general relativity. As such, I actually am more interested in them since you seem to so adamantly defend such ideas. So, I'd be extremely pleased to see some references for them. However, you have failed to provide such sources. However, I do have solid references for CH's rebuttals and my own understanding agrees with CH. As a result, I stand behind him.

I do not doubt that have had quite a bit of training in physics and relativity. It only seems that your understanding of many ideas is not consistent with the theory. Perhaps your ideas are the "correct" understanding, but Wikipedia is not the place to make that case. Archmagusrm 22:56, 1 September 2006 (UTC)

Reply: You state zero times infinity is undefined. In a physical context where zero and infinity refers to something, it has to have meaning. However, this point is not important.

My criticism is that the explanation given of black holes is problematic. I have not directly criticized general relativity. You seem to claim that any criticism is original research and needs to be referenced. You call such criticisms "ideas" and write as though black hole criticism is a positive body of knowledge that I am adamantly trying to prove. Don't you think you are extremist in this attempt of yours to stifle criticism?

CH has gone through the motions of rebutting me, but his rebuttals are just assertions. A black hole does make sense from the standpoint of a distant observer, but we live in a universe where one can move around. If a second observer moves around to a higher or lower gravitational potential, he will observe that the same black hole has a different radius than that seen by the first observer. Some light or objects that can escape to one observer will not be theoretically able to escape to the other observer. Such a result contradicts known physics since we know that what can escape to one observer can escape to the other. The result is independent of whether classical theory or GR is used to calculate black holes. CH and the Black hole article fail to address this problem. The problem of multiple observers needs to be addressed. --Danras 07:53, 2 September 2006 (UTC)

User Danras needs to read wikipedia's policy on original research and wikipedia's policy on verifiability. To sum up the situation, one can easily find references that black holes cannot be escaped. One can also find references that the tidal forces at the event horizon are finite. These facts are not mutually inconsistent. It's unclear why user:Danras thinks they are. If user:Danras wants to convince the world that they are incosistent (in my opinion a waste of time, but it's his time) he needs to go somewhere else, per WP:NOT#Wikipedia_is_not_a_soapbox.

Uh... did you mean to say "not mutually inconsistent"? —Keenan Pepper 02:35, 2 September 2006 (UTC)

Yes, I just fixed it Pervect 19:16, 3 September 2006 (UTC)

Reply: General criticism is not original research. I think the unsigned critic is falsely trying to assert that my criticism of the current explanation of black holes is a soapbox for ideas unrelated to such criticism.

If you are the originator of the criticism, then the criticism is original research. Read the guidelines above, you need to find SOME OTHER PLACE to publish your criticism if you are the author. You need to find REFERENCES by OTHER PEOPLE in order to publish your criticism here. Pervect 19:16, 3 September 2006 (UTC)

If tidal forces at an event horizon are finite, then the gravitational acceleration there is finite. The distance from a black hole to nearby (or even far-away) stars is finite. Assuming powered flight, one only needs a finite supply of fuel to cross the event horizon and travel to such stars. The assertion that black holes cannot be escaped requires that there be an infinity in the calculations. Finite and infinite are mutually inconsistent. --Danras 11:45, 3 September 2006 (UTC)

The speed of light happens to be finite too, but we can't outrun photons. The event horizon is not a physical singularity in the sense of an infinity occuring at that point. It is only a coordinate singularity when using the Schwarzchild coordinates. However, this does not mean that it can be crossed by an actual physical spaceship or whatever you wish. To escape from the event horizon, you would need to exceed the speed of light. At this point, an infinity does occur due to the various basic effects of special relativity. Archmagusrm 16:11, 3 September 2006 (UTC)

Note that the tidal forces are finite only for an observer who actually falls through the horizon. Someone who tried to hover "at the horizon" would require an infinite acceleration at the horizon itself, and a very large but finite acceleration just outside the horizon, resulting in infinite tidal forces.

See http://en.wikipedia.org/wiki/Frame_fields_in_general_relativity#Example:_Static_observers_in_Schwarzschild_vacuum

for how the accelration required to hold station is computed (it's rather technical).

If you look at the references, such as MTW's textbook "Gravitation", or at the wikipedia, you'll see that the finite tidal forces are computed for an infalling observer, not a hovering one.

This sort of misunderstanding is why users with questions should ask questions in some other forum, not publish their original research to the wikipedia. The publication process serves to weed out these sort of mistakes. Pervect 19:16, 3 September 2006 (UTC)

Amended Reply to CH

The first paragraph of what I posted is apparently incorrect according to Pervect and others. It is wrong of me to assume that black holes exist when my second paragraph argued that they cannot exist as objective entities. If black holes exist objectively, then I accept that no escape is possible from them.

In point 2 of his post, CH appears to argue that black holes are objectively defined by their mass. Conventionally, if a black hole has a fixed mass m, then its radius and event horizon are rigidly defined. Such a definition ignores the fact that a black hole is fundamentally defined by its event horizon. By definition, the escape velocity from the event horizon of a black hole to an observer is equal to c, the speed of light. However, by moving closer to the black hole the observer can reach a position where the escape velocity from the same event horizon is equal to c – 10 m/sec. Light can now reach him from this event horizon.

If the same observer moves away from the black hole to a higher gravitational potential, possibly towards the outside the galaxy, the observer can reach a position where the escape velocity from the same event horizon is equal to c + 10 m/sec. At this position, light emitted just outside the event horizon cannot reach him.

Because of these facts, the observed radius of a black hole is subjective. The event horizon moves with the observer and always maintains an escape velocity of c, relative to him. For a black hole to objectively exist, the speed of light must decrease for observers who approach the black hole, and increase for observers who move gravitationally away from the black hole. This objective existence is impossible for according to special relativity, the speed of light is the same for all observers. --Danras 02:42, 5 September 2006 (UTC)

I've just read over some of your recent edits and comments and the debates you've had with others on this page. Assuming you are not trolling or trying to endorse a dogmatic viewpoint, I fear you have a crucial misunderstanding of the theory of Black Holes. I think the root of the problem is that you are attempting to apply Newtonian physics to the space-time around a BH. That won't work and is the reason Einstein invented GR in the first place. For instance, you keep going on about escape velocity as the defining metric for the event horizon - that's a limited view-point as it is a classical concept and applies to a ballistic object. In any case, the phrase "escape velocity to..." is incorrect. Escape velocity is always to infinity. It is more accurate to use the curvature of space-time as the defining point for the EH - you've crossed the EH when everything outside it lies outside your light-cone. --Oscar Bravo 08:29, 5 September 2006 (UTC)

Recent bad edits by anons over approximately the past 15 days

Again, I can't do this on a regular basis, but just took a quick look at recent bad edits by anons.

88.118.140.103 (talk · contribs · WHOIS) at 03:31, 27 August 2006 used the article as a sandbox. Self reverts in 2 min.
Several edits by Danras (talk · contribs · block log), already noted. Reverted in 11 min, 5 min, 75 min (too long!).
12.148.189.25 (talk · contribs · WHOIS) at [2] by 12.148.189.25 (talk · contribs · WHOIS) is bad (this IP may have been recently reassigned from the toolmaker Black and Decker in Lake Forest, CA). Fortunately, Brygenwulf's subsequent revert of Danas also reverted this anon.
124.47.166.87 (talk · contribs · WHOIS) at 06:19, 24 August 2006, henceforth the Bombay vandal, blanked most of the page. Reverted almost immediately.
169.199.155.65 (talk · contribs · WHOIS) at 14:21, 18 August 2006 blanked several articles (reverted by a registered user). This is the Contra Costa County Office of Education vandal who has been observed on several previous occasions here. Made several successive vandalisms and was promptly reverted by other users.
67.68.205.104 (talk · contribs · WHOIS) at 21:04, 16 August 2006 made a possibly intentional misstatement, promptly reverted. Several problem editors of physics-related articles happen to reside in the Montreal region. Reverted in 17 min.
195.89.27.198 (talk · contribs · WHOIS) at 09:18, 16 August 2006 blanks most of the article and replaces it with a bungled and misleading citation of Ted Bunn's cosmology FAQ. Reverted in 1 min.
84.77.143.12 (talk · contribs · WHOIS) at 03:11, 16 August 2006 replaces introduction with a version which exhibits substantially poorer English diction. Not reverted by time of next bad edit, by 195.89.27.198 (talk · contribs · WHOIS).
24.77.216.252 (talk · contribs · WHOIS) at 00:57, 13 August 2006 incorrectly "corrects" [sic] article to state that no radiation except gravitational radiation can escape from from a black hole. Reverted almost immediately, repeats, then not reverted for 280 minutes (much too long!!!!!) Many bad edits to physics-related articles from this ISP (Shaw cable) have been previously noted.
60.227.206.179 (talk · contribs · WHOIS) at 06:42, 12 August 2006 Silly blanking vandalism. Reverted immediately.

OK, so over the past 21000 minutes or so, this article was in an obviously vandalized state for something over 375 minutes, which would be suggest that this article typically exists in an anon-vandalized state just under 2% of the time. This is consistent with previous estimate from many months ago. Over this time period, only a handful of non-bad edits by anons were observed. This seems consistent with earlier estimates that less than one in five anon edits is acceptable. I didn't systematically check whether registered users made a comparable number of bad edits in this time period, but spot checks suggested that they did not. If we subtract out Danras, this article was anonvandalized for something like 285 minutes over about 21000 minutes, for a rate of about 1.4%. This also seems consistent with earlier estimates for anon vandalization rates, suggesting that contrary to the hopes of some, semiprotection does not seem to be ameliorating the anon vandalization problem; anons are just damaging easier targets, such as this article.

I think that finding this article in a vandalized state two percent of the time is too high a frequency to be acceptable. Even those who don't agree probably will agree that it is utterly unacceptable that this article should have existed in a vandalized state for 75 minutes on 25 August or 280 minutes on 13 August. Clearly, in such a popular article, that does not serve our readers well.

Once again, the inexorable conclusion is that Wikipedia must follow the model of all other large public forums (of which I am aware), by prohibiting all edits by unregistered users (by technical means, not social means). The figures above are consistent with my estimates from last fall suggesting that we could more than halve our problem with bad edits with no effort at all, simply by eliminating anon edits. ---CH 07:20, 29 August 2006 (UTC)

If this suggestion is seen as "too radical", I would like to at least propose that frequently vandalzied articles such as this "black hole" article be protected against unregistered edits.

My impression is that for frequent vandal targets, leaving the door wide open causes more trouble than what is gained. Hillman's data also supports this.

Things MAY be different for more technical articles. For instance, consider the following edit in gravitational radiation

16:32, 23 August 2006 130.209.6.40 (Talk) (I'm a GW physicist and had to remove the horribly glaring error about light. (It also has two polarizations by the way.))

Who knows if this would have been done if the user had to register? Locking up "vandal targets" should be a no-brainer, though. Pervect 19:11, 29 August 2006 (UTC)

We already have a mechanism for this: semi-protection. The problem is not the mechanism or the procedure, but the fact that last time I checked, the threshold for it being applied was too high (an average of ten vandalizations per day was considered too low to be worth shutting out anons). If you want a practical solution to the anon vandalization problem, focus on lobbying for the bar for semi-protection to be lowered. This has the virtue of not requiring changes in policy in order to be implemented. --Christopher Thomas 01:15, 30 August 2006 (UTC)

OK, I would like to see the bar on semi-protection to be lowered, and the program expanded in scope. Or possibly a new category of protection, micro-protection, that doesn't necessarily make people wait 4 days, but does make them at least register, in order to edit an article that is frequently vandalized. (Hmmm, I'm not sure how well this would work.) What exactly should I do to "provide input" to the administrators? Hopefully there are other people out there who share my feelings (otherwise the idea is dead). But I suspect that I am probably not alone. Thus the question in my mind is, where should I and people who share my opinion best direct out efforts to be heard? Pervect 02:09, 30 August 2006 (UTC)

On the semi-protection talk page, would be my best suggestion for the moment. The problem is that it seems to be implemented by only one administrator. Convincing this person that a moderate but constant level of vandalism _does_ warrant semi-protection is difficult, as they've presently stated that any level that _is_ being rolled back by hand is something they shouldn't bother with. I strongly disagree (it's unreasonable to force most of our edits here to be vandalism patrol rather than article improvement). If multiple editors make this case, it's possible they might be convinced to alter their thresholds. Failing that, a straw poll or RFC might convince them that there was support for this change, but that's a pretty slow process.--Christopher Thomas 04:22, 30 August 2006 (UTC)

Recent Edit by Stratadrake

"...as an object moves closer to a black hole, the energy required for it to escape continues to increase until it becomes infinite at the event horizon, the distance beyond which the escape is impossible."

To what point or points does escape become impossible? Is it impossible to escape from the event horizon to a millimeter outside of it? --Danras 16:35, 2 September 2006 (UTC)

The event horizon is not a spacelike surface, it's a lightlike surface. To an outside observer it appears stationary, but to an observer inside the black hole it's moving outwards at the speed of light, so it's impossible even to catch up to it. —Keenan Pepper 20:23, 2 September 2006 (UTC)

Singularity in future

As pointed out on this talk page (but not always clearly stated on the main black hole article), black holes have a singularity in the future -- there is no actual singularity within black holes in our Universe according to GR. As soon as the matter has collapsed to a volume smaller than the Schwarzschild radius, time has stopped for particles on the event horizon as far as observers in the outside world are concerned. This means that as far as outside observers are concerned, the matter falls infinitely slowly towards the center of the black hole, so the singularity doesn't actually form within the lifetime of our universe. The lower the mass of the black hole, the smaller the Schwarzschild radius, and so the closer the matter can get to the center of the black hole. A black hole would have to be infinitely low in mass for the matter to actually form a singularity, however. Rnt20 14:08, 8 September 2006 (UTC)

You're forgetting the matter that was already inside the black hole when it formed, aren't you? -- SCZenz 04:38, 12 September 2006 (UTC)

I took out Rnt20's edits because I think they muddy the waters a bit too much. When you say that the infalling particle takes an infinite time to cross, you are implicitly invoking the notion of simultaneity. You want the distant observer to be able to say where the infalling particle is at some point in time.

However, GR takes away any non-local notion of "now" -- even the relative notion from SR. We can define any spacelike slice we want, but this can't have the usual meaning as a surface of simultaneity. That is: In a very real sense, it's impossible in GR to say what is happening now, unless it is also happening here. If you want to talk about surfaces of constant Schwarzschild-like coordinate time, that's fine, but it's only a coordinate. You can make up some other coordinate system for which the behavior is different. (E.g., Finkelstein coordinates.)

Moreover, the usual "external observer sees the infalling observer stand still at the horizon" narrative has an important subtlety to take into account. It usually relies on signals sent from the infalling object. That is, it relies on what is on or in the past light cone of the distant observer, which is certainly not what you want for the concept of now.

In fact, the distant observer could -- at some point in time -- shoot a massive particle at the black hole. A well-aimed particle would arrive in a finite time (in its own frame). The shooter couldn't say where that particle is at a later time in any meaningful way.

It seems like there's probably a lot of cleaning up to be done on this article, but it is a big article. MOBle 06:44, 12 September 2006 (UTC)

Rnt20 raises an important point. Black hole singularities do not exist in relation to outside observers. The same argument can be extended to black holes themselves. Black holes and their associated event horizons do not exist to outside observers as they take an infinite amount of time to form. Rnt20 has not forgotten about material "in" a "black hole," as black holes form from the inside out and to outside observers an interior black hole also takes an infinite amount of time to form.

There is no absolute notion of simultaneity. However, all outside observers would agree that black holes take an infinite amount of time to form. No outsider observes an existing black hole in their present. Suggesting that outsiders observe a "past" light cone of an infalling observer has no reality. Physics is dictated by current observation and all currently observed events are, for all practical purposes, happening "now." The classical assumption that an event happens before light or a signal has an opportunity to travel from the event is false.

An infalling observer can, according to the mathematics of GR, descend into a black hole in a finite amount of time. However, to outside observers, the object that is collapsing to form a black hole may evaporate, due to Hawking radiation, before the infinite amount of time required to form an actual black hole. If so, then the observer will descend into empty space, or perhaps evaporate himself, before he reaches the interior of an alleged black hole. The possibility of interior descent involves speculation about the existence of the collapsing object in the infinitely distant future. --Danras 12:07, 3 October 2006 (UTC)

List of doomsday scenarios

Could use votes to save this article, thanks MapleTree 22:18, 28 September 2006 (UTC)

Recent edits by Nuclearfusion

Welcome to the wonderful world of Black Holes. I hope you don't mind, but I reverted your last two contributions: Here's why:

In section Gravitation: In astronomy, there are only four types of orbits: unbound hyperbolic, unbound parabolic, bound elliptical, and bound circular.

This is quite correct (though pedants might point out that a circle is an ellipse with an eccentricity of 0 - hence only three orbits. Or that all orbits are conic sections... but I digress.) The point is - what has this to do with black holes? Does it read well and fit in with the rest of the section? Not really, I think.

In section Singularity: Most astrophysicists predict that [a] singularity exists as a single point; however, in rapidly spinning black holes, [the] singularity could be stretched into an extremely thin and small circular ring.

There are lots of problems with this:

you missed out two articles (a and the), that detracts from the quality of the piece.
Weasel words: Most astrophysicists? which ones?, how many? Do you have a reference to a poll or review?
single point: is there any other type of point?
Then we have the qualifiers; how rapidly? how thin is extremely thin? How small?
Science: Are you really sure about all this? My understanding (and this is reflected in the article) is that the nature of the singularity is uncertain and the subject of ongoing research.

Please excuse my pruning and please have another go at adding to the article! BTW, fill in your userpage so we can learn about you. --Oscar Bravo 10:07, 29 September 2006 (UTC)

Requested references to common knowledge

Oysteinp wanted references to my first paragraph edit. I called it "common knowledge," but he said it was news to him. I could get outside references, but it is common knowledge even within the existing article. Admittedly, there seems to be some implicit belief that actual blacks holes exist relative to outside observers.

The article "History" paragraph starting with "In 1939" contains "Such objects for a while were called frozen stars since the collapse would be observed to rapidly slow down and become heavily redshifted near the Schwarzschild radius. 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." This description is vaguely correct, but more precisely the surface would continue to approach the "event horizon," slowing down as it gets nearer. There would actually be no event horizon (or Schwarzchild radius) to outside observers as the center of the star (even if invisible to outsiders) would slow down as well before reaching a black hole state.

The article "Space-time distortion and frame of reference" section contains "From the viewpoint of a distant observer, an object falling into a black hole appears to slow down, approaching but never quite reaching the event horizon."

Rnt20 brought up this point in his "Singularity in future" discussion above, although he applied it just to singularities. The basis of his point was common knowledge to me before I read it. I added a comment to his discussion. --Danras 03:29, 9 October 2006 (UTC)

In General Relativity there is no well defined notion as something happening elsewhere at a certain time for you. In case of a black hole, suppose I fall into it and you are in communication with me. The moment I pass across the horizon, I can still receive signals from you. But my signals will not reach you. The signals I send you just before reaching the horizon will take a very long time to reach you.

A short time after passing the horizon I'll be crushed to death by the singularity. But I can still receive signals from you till the very end, so there is a finite time T for you after which your signals won't reach me anymore.

If you take into account that the black hole formed from a collapsing star, then this discusion doesn't change qualitatively. From a distance the object still looks to be still collapsing, as the photons that were emitted just before the black hole formed will need an almost infinite time to reach you. This does not mean that there exists no black hole there. If some alien observer was falling with the material toward the forming black hole, you could, in theory, still receive signals from him, but you cannot send signals to the alien.

All of this is just a consequence of the presence of the horizon. Information about what happened before the formation of the horizon takes an infinite time to reach you and information about the things that happened afterwards will never reach you. Count Iblis 13:04, 9 October 2006 (UTC)

To an observer, events happen when he observes them. Distant events cannot be detected by any means until a signal from the event has a chance to reach him. Stating that an event occurred, but that the photons have not had a chance to travel to you implies that you possess a faster than light means of detection. We can use theory to predict when an event might occur. However, stating the event has occurred implies some form of detectable reality other than a person's affirmation.

No outside observer can observe the formation of a black hole. Unless black holes were created "as is" with the universe or began forming in the infinitely distant past, they cannot exist to outside observers today. Barring the existence of pre-existing black holes, black holes cannot be detected even by their absence. Electromagnetic radiation like gamma rays or radio waves from the center of frozen stars can still reach outsiders as long as such stars are transparent to such radiation.

As far as an infalling observer is concerned, he can fall into a frozen star, but he will not know whether such a star becomes a black hole with an event horizon until he finds himself crossing such a horizon. Signals and light falling in behind him reach him from our present. Assuming he can detect that he is in a black hole, he would conclude that the black hole exists simultaneously with our present.

However, infalling signals comes from outside and do not convey to him the existence of a black hole. He may not be able to detect that he is in a black hole simultaneously with observing our present. He is in free fall and may not be able to detect that he is in a gravitational field. Once he stops, he presumably would be crushed, but he would accelerating at infinite speed through time and in a mere instant he would observing (if alive) the infinitely distant future of outsiders (if such a future exists). If he can only observe the black hole when he ends his free fall, then his observations would not appear to contradict that of outside observers.

Whether or not an infalling observer can observe a black hole simultaneously with our present does not matter in regard to outside observers. There is little basis for claiming that actual black holes exist as outsiders cannot witness the formation of such objects. An infalling observer cannot report to outsiders his knowledge of such objects. --Danras 02:57, 11 October 2006 (UTC)

Danras edits and the three-revert rule

Recently, User:Danras has been expressing the same view over and over again, only to have his edits reverted by various, different people.

The purpose of this is to open a dispute resolution as per http://en.wikipedia.org/wiki/WP:3RR

First step: Talk to the other parties involved

While Danras may not have reverted three times within 24 hours, he has reverted the text many more than three times. The 3r rule is not a "license" to revert exactly 3 times per 24 hours. Other uses have already commented that Danras' edits are becoming "tiresome". As a first step before seeking any other action, the issue needs to be discussed.

There are several relevant points here, in my opinion.

First of all, the discussion in my opinion defintely does not belong in the lead paragraph, where Danras insists on putting it.

Secondly, Danras is going well beyond his sources when he makes statemnts such as

The eventual existence of black holes depends on speculation that the universe and objects in it will exist in the infinitely distant future.

This appears to be a highly personal opinion, i.e. WP:OR

I think that Danras needs to stick much more closely to his original source material, to interject less of his own personal opinions, and that the opening paragraph of the article is not the place to mention the issue.

I think that Danras also needs to show some respect to the other editors, and attempt to work out a compromise version here on the talk page, rather than to attempt to propagate his views by continuously reverting the article to his version. This is discussed in the WP:3rr webpage.

Persistent reversion remains strongly discouraged and is unlikely to constitute working properly with others. T

http://en.wikipedia.org/wiki/Wikipedia:Dispute_resolution

is also relevant here. Pervect 05:45, 12 October 2006 (UTC)

I quite agree. To Danras: Can you please provide a source to (for example) your claim that black holes can only exist at T=

\infty

? If you haven't read this somewhere, but have worked it out for yourself, then it's OR and doesn't belong here. (NB: I'm not even saying you're wrong - you just have to tell us where you found it and you can't have figured it out for yourself). --Oscar Bravo 14:58, 12 October 2006 (UTC)

John Baez? http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/fall_in.html --Pjacobi 15:12, 12 October 2006 (UTC)

Interesting page... This article is saying that BHs do form in real time and thus is denying Danras' claims. Is this what you intended? --Oscar Bravo 06:30, 13 October 2006 (UTC)

It depends on your frame of reference. Pls read carefully. I'd think, this is a point too fine and difficult to explain to throw it into the reader's face right in section 0. But it should be explained further down. --Pjacobi 14:41, 13 October 2006 (UTC)

I respect discussion and am more than willing to work things out. I think this talk page shows that I am more than willing to discuss things as I have written much. I have discussed the material in the first paragraph change on 3 Oct in "Singularity in Future" talk section. Since it received no comment or disagreement, I made the change on 8 Oct. The change was an article change, not a discussion like a talk page entry.

I do not like reverting but I have reverted when it was stated that my change is OR without specifically stating what in my change is OR. Perhaps I can clarify or eliminate some point that is alleged to be OR, but I did not think the entire change was OR. Byrgenwulf reverted my initial change and several reversions. Since his contributions are entirely administrative as far as I can tell, I felt his opinion did not reflect users who have the best technical understanding of black holes and GR.

"The eventual existence of black holes depends on speculation that the universe and objects in it will exist in the infinitely distant future." This statement is personal opinion in that I believe talk about the infinitely distant future is speculative. I wanted to be compromising in agreeing that black holes can form, albeit at T = infinity. GR theory is not speculative, but other factors like Hawking radiation might make collapsing stars disappear to outside observers before T = infinity. The statement itself is probably too speculative and should be replaced with something like "Black holes can never form."

The article already contained statements indicating that objects can never quite fall into a black hole. If such is the case, then a black hole could never quite form since it would have to contain initial material that has fallen into it. The statements assume black holes exist, but strangely deny the grounds for their formation. Is it OR to point out this inconsistency?

It is perhaps confusing to state that black holes can exist at T = infinity. There is a reason for saying so in relation to observations of possible internal observers. I see that my trying to address or relate to the internal observation problem has caused confusion. I think the internal observation problem has led to the belief among many that black holes exist for outside observers. The internal observation problem is speculative in that it can never be physically confirmed to outsiders. The link provided by Pjacobi primarily addresses internal observation.

I thought of a new way to explain the link between outside observers (who exist in the present) and internal observers (who have traveled to the future): An object collapsing towards a black hole state acts as one-way time portal to the infinitely distant future, where a black hole can theoretically exist. Because the present is linked to the future through this portal, some would argue that future black holes exist in the present. --Danras 04:02, 14 October 2006 (UTC)

Hello Danras. Your remarks are interesting, but the whole problem with relativity is that there are different times for different frames, so why our frame should be better than a frame of someone that falls to the black hole. If the black hole exists for some observer then it's enough, and we don't have to specify its time of existence - we will be always wrong for some observer.Melamed katz 05:06, 14 October 2006 (UTC)

Hello Melamed katz. Your claim that black holes exist is false. You can argue that if you just jump into a "frozen star" time portal, you can travel to the future and see one. In theory, observing a black hole is only a matter of some space travel. Such a black hole must exist in the present because one can presumably go and see it.

If such a black hole existed in the present, you would be able to report your observations to outsiders. GR does not allow you because your knowledge of the future would permit you to change the present and prevent the future you observed from occurring. The present according to relativity is that which you observe. One can use classical understanding to imagine that events occur outside your light cone. However, such understanding is not supported by relativity. --Danras 03:42, 15 October 2006 (UTC)

Note that according to the reference quoted earlier (which is a reasonably good one, BTW), an observer falling into a black hole will not in fact see the future of the universe. At least not for a Schwarzschild black hole.

http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/fall_in.html

"Will you see the universe end?

If an external observer sees me slow down asymptotically as I fall, it might seem reasonable that I'd see the universe speed up asymptotically-- that I'd see the universe end in a spectacular flash as I went through the horizon. This isn't the case, though. What an external observer sees depends on what light does after I emit it. What I see, however, depends on what light does before it gets to me. And there's no way that light from future events far away can get to me. Faraway events in the arbitrarily distant future never end up on my "past light-cone," the surface made of light rays that get to me at a given time.

That, at least, is the story for an uncharged, nonrotating black hole."

What happens with rotating black holes is very interesting, but rather technical. I'm not sure how much we want to get into this in the main article.

Pervect 05:18, 15 October 2006 (UTC)

Black holes lead to some paradoxes, and Danras is actually showing to us one of them, namely the one that is related to cosmic censorship. But the question is: do black holes exist?

I do not believe any paradox has been demonstrated. This is just another example of why it is said that "time is relative". Pervect 00:11, 16 October 2006 (UTC)

You are right. Actually he tells us: either you stay on earth and write about black holes in Wikipedia without seeing them, or you enter a black hole without being able to tell what you see to anyone outside the event horizon. He claims that one can be sure only about things that he measures.

Fortunately physics is more than that. We can get evidence in our frame for things that occur in other frames as well, and for that purpose we use a combination of experimental and theoretical tools.Melamed katz 02:14, 16 October 2006 (UTC)

I think yes, even if the only one that sees them is the one that enters the event horizon. Is there a future for that poor guy?

No, his time will end in the singularity and a big problem of causality is present in this analysis. It's a paradox, but it doesn't say that black hole doesn't exist for him. We are as sure as we can ever be that for some observers, in their time, black holes are real objects.Melamed katz 10:30, 15 October 2006 (UTC)

If an infalling person observes he is in a black hole, then he observes the future. Outside persons will never observe a black hole form in any finite time, so an infalling observer must see the future. If an infalling observer does not observe that he is in a black hole, then he is not in one.

Since "frozen stars," observable to outside observers do not presently have black hole centers, it is not clear than an infalling observer will cross an event horizon before traveling some distance to reach a singularity. The observer may cross an event horizon and reach a singularity at the same time. Material near the exterior of a "frozen star" collapses faster than material near the center. Perhaps everything that is ever going to enter a black hole, enters at its moment of formation.

Since he cannot observe a black hole from outside, an infalling person will enter one without observing it beforehand. Once an observer is stationary at the singularity, if he can avoid the impossibility of not being crushed, he should observe time in the outside world race by at infinite speed.

Black holes may be real in their time, but we use the term "exist" to mean existence in the present, unless use of the term is qualified. Extinct animals exist somewhere in spacetime as do inventions of the future, but they do not exist according to accepted and unqualified use of the term. I do not deny that "black hole" is an important concept. Its use is especially important to authors who want to sell books on the subject. However, I think the existent concept here is "frozen star" or a synonymous equivalent. We should be using this concept to talk about collapsed objects. We should abandon the idea that there is a classical, outside our light cone reality behind every distant object. Such an idea conveys nothing of physical relevance; it only conveys the notion that the speaker wants to make things complex so as to impress you with how sophisticated he is. --Danras 11:33, 16 October 2006 (UTC)

I have a queasy feeling that you are still expecting classical physics to work in the vicinity of a BH. A couple of points:

When an observer actually crosses an EH, all contact with the outside is lost since it now outside his light-cone. So he can't see time speeding up outside...
External observers do not see an infaller "freezing" on the surface of the EH; rather as he approaches it, his image becomes dimmer and more red-shifted before winking out entirely at the EH.

I think I understand your point - it is interesting, but I'm pretty sure it's wrong. Anyway, it's obviously your opinion and so not appropriate for the article, no matter how much fun it is to argue about it :-) --Oscar Bravo 15:02, 16 October 2006 (UTC)

Actually, only outside observers lose contact with the infalling observer. Photons, of course, can fall into the black hole too. So, theoretically, as you fall in, you can still see the outside world. It is an easy exercise to show that photons emitted later than some finite time after you fall in, won't be able to overtake you before you reach the singularity. So, although you can see the outside world, you don't see "infinitely far into the future" before you are crushed to death. Count Iblis 15:46, 16 October 2006 (UTC)

I understand nothing freezes on the surface (actually outside) of an EH. I use the historical name "frozen star," that is given is the History section of the article. I understand any frozen star is slowly collapsing asymptotically to a black hole state.

I think Count Iblis is right about infalling photons. I did not think one could see the outside world infinitely far into the future while still freefalling.

I presume a black hole exists for some observer, namely the infaller. In that case, if he could report the black hole to outsiders in the present, they could change the future that he reports. It would be very difficult, but outsiders could theoretically dredge a "frozen star" of material so that it was no longer capable of forming a black hole. GR does not allow such reporting presumably because such reporting would or could cause a causality violation. --Danras 03:21, 17 October 2006 (UTC)

To Danras: Returning to your comment: ...I have reverted when it was stated that my change is OR without specifically stating what in my change is OR...

Burden-of-proof is on you - you added the material, you have to provide sources if it's challenged.
Having said that, I gave you one specific challenge: ...black holes can only exist at T= $\infty$ .... This is not a statement that you will find in a mainstream text-book or university course on GR/Cosmology - those teach that the evidence to date indicates that BHs do exist here and now. So can you please tell us where you read it. If you didn't read it, but simply worked it out then that's great, but it's OR. So can't go in.

Note that an editor doesn't actually need to know anything about BHs or GR to mount this challenge - you added a statement and have been challenged to provide it's source. We don't actually care if you're right or wrong, only that you don't put in things you figured out for yourself. --Oscar Bravo 07:06, 16 October 2006 (UTC)

Added Talk Section

To Danras: Would you mind producing a reference for your statement that General relativity predicts the formation of frozen stars which asymptotically collapse towards a black hole state, but never quite reach it within any finite time.? If you read this somewhere, please tell us where. If you worked it out for yourself, go write a paper. Then, when it's been peer-reviewed and everyone has gone "ahhhh! how could we have been so wrong..." and the paradigm has shifted in your favour, come back and put in the statement.--Oscar Bravo 07:02, 18 October 2006 (UTC)

To Oscar Bravo: You seem upset or stressed out in recent days. Relax. Perhaps you should monitor or edit other articles if this one is too stressful. I do not personally believe in GR, so what I write is not my personal opinion as to reality, only what others show that GR predicts.

The article's "Space-time distortion and frame of reference" section contains "From the viewpoint of a distant observer, an object falling into a black hole appears to slow down, approaching but never quite reaching the event horizon." This statement paradoxically assumes the existence of a black hole, but confirms that GR does not allow such an entity to form as no object or material could fall into an initially forming event horizon.

You replaced my material in part with, "A black hole is an object predicted by general relativity." You make this statement without any qualification. We both are in basic agreement that stars collapse a certain amount. You write as though I am making a unique claim, as though by my conservatism I am instituting a paradigm shift. You are the one who maintains the collapse proceeds further.

Do you have any references that explain how general relativity leads to the formation of black holes? Theorists have applied general relativity to allegedly existing black holes and found that no objects can fall into them. However, such a result is very different from establishing that general relativity predicts black holes.

As it exists now, your replacement is original research. There does exist many references indicating belief in black holes, but none that I am aware of showing that general relativity leads to the formation of black holes in the universe today. I am reverting your edit pending the provision of a reference for your claim. --Danras 02:55, 20 October 2006 (UTC)

Good of you to care about my health, I thank you. I have a couple of points, however:

My "claim" was actually what was written before. I didn't put it there. It was put there by an earlier contributor and, as far as I am aware, is a reasonable statement of the current understanding of BHs. I simply put it back after you replaced it with a contentious statement. So it was you who made the first edit, not I.
You forgot to answer my first question: Would you mind producing a reference for your statement that General relativity predicts the formation of frozen stars which asymptotically collapse towards a black hole state, but never quite reach it within any finite time.?--Oscar Bravo 08:40, 20 October 2006 (UTC)

Removed unsourced statement

An object with mean density greater or equal to the critical density and with a radius equal to that of the observable universe is a black hole. Our visible universe does not have a singularity like the one associated with this kind of black hole.^{[citation needed]}

Nobody's found a source for this statement in over a month, and FRW cosmologies just don't have an event horizon, so I don't see in what sense our universe is a "black hole". There's a FAQ entry that points out that the universe is NOT a black hole as well. Therfore I'm moving this remark to the talk page. I've made another minor edit at the same time. Pervect 21:29, 19 October 2006 (UTC)

According to observable universe, the mass of the (observable) universe is 3.4e57 grams, and its radius is 46.5 billion ly. But the Schwarzschild radius for this mass is 2 6.7e-11 3.4e54 /9e16 = 5.1e27 m, or about 540 billion ly. So, if the Universe were static, it would be a black hole even at a tenth of critical density. But of course it is not. Ben Standeven 20:17, 27 November 2006 (UTC)

Original Research

The claim that a black hole is an object predicted by general relativity is unreferenced original research. It needs a reference that shows how black holes can form under general relativity. I do not think a credible reference is possible as: (1) GR prevents infalling matter from crossing a black hole's event horizon in any finite time. (2) To exist, black holes must contain matter. I do not see how matter could get inside a black hole without crossing its event horizon. --Danras 02:03, 23 October 2006 (UTC)

a black hole is an object predicted by general relativity is unreferenced original research See http://archive.ncsa.uiuc.edu/Cyberia/NumRel/EinsteinEquations.html and the section on Schwarzschild for a reference (but actually, it's all standard stuff that can be found in any number of books or online resources).

Regarding your other points above:

(1)GR prevents infalling matter from crossing a black hole's event horizon in any finite time. I think it's more that the light from the infalling object becomes increasingly time-dilated and red-shifted by the curvature of spacetime. The object itself doesn't actually freeze on the EH - it careers straight in. A distant observer sees the image of the infaller slow down and then stop on the EH, but then the image dims exponentially and vanishes. Also, don't forget that even if the infaller froze on the EH, it's mass would still contribute to the mass of the BH and this would have the effect of shifting the EH up/out a bit, beyond where the infaller has frozen and so moving the infaller into the BH.

(2)To exist, black holes must contain matter. I do not see how matter could get inside a black hole without crossing its event horizon. Let's assume, for a moment, that matter cannot cross the EH: You can still get BHs forming because the matter in a BH doesn't start off there - it is in a normal star to begin with. As the star collapses, its gravitational field becomes more and more intense as more matter is compressed into a smaller volume. As the field intensity rises, spacetime becomes more and more curved until all worldlines lead into the singularity - that's the EH. So the EH doesn't have to exist to begin with; it appears as the collapsing star reaches critical density and all the matter is already inside.

I hope that's helpful --Oscar Bravo 07:35, 23 October 2006 (UTC)

The reference you give is about black holes, but it does not address the specifics of how black holes form under general relativity. I will respond to your talk, but you still need a reference, as it is not my burden to disprove an unreferenced claim.

Bullet #1: Light from an infalling object becomes redshifted and eventually will become so redshifted that it cannot be detected even as radio waves, but such electromagnetic radiation never becomes infinitely redshifted. It is always observable in theory even if not by visible light. It never theoretically vanishes. If one alleged that matter fell towards a black hole at the beginning of the universe, we could still theoretically observe it falling in.

Under relativity, we do not see images of objects, we see the objects themselves. If a star 100 light years away explodes in 2006, we cannot say it exploded in 1906 and that we have been just seeing an image of the star as it used to be before 1906. Consider an observer near the star who sees perturbations of the star 10 minutes before the explosion. By traveling close to the speed of light, the observer can reach earth in 5 minutes of his time. His observations at the star five minutes ago in his time will correspond to ours 5 minutes ago in our time. The star will still explode 10 minutes after he observed the perturbations. His travel to earth will not make any difference.

One cannot allege that an observer has fallen into a black hole when we see him or his "image" outside of it. Theoretically, he can still escape to earth and talk to us. I doubt anyone would maintain that he has fallen into a black hole and we are just talking to his image.

Infallers do not freeze on the EH, they slowly approach it, even if the movement when viewed from afar becomes imperceptible. Since they are always outside the horizon, they do not contribute to the mass of the black hole.

From a Euclidean perspective, a black hole repels objects in that it slows down infalling matter. One cannot simply assemble concentrations of mass to achieve black hole density. The warping of space caused by such concentrations will actively repel such concentrations from concentrating further. If the spacetime vortex surrounding a concentrated mass expands, matter in the vortex walls will expand outward and cause matter from a Euclidean perspective to expand outward. An event horizon may be close by in space, but it is an infinite distance down the throat of any vortex in spacetime. There is no way for infaller to travel this distance, or for an expanding vortex wall to leave him outside of spacetime in the open center of the vortex.

In his own time frame an infaller can career (careen?) into a black hole, but only if one exists. Even if one exists, we cannot observe him cross an event horizon within any finite time. Predictions regarding infallers is speculative, not because of GR, but because infaller's observation of a black hole interior appear to correspond to a time beyond which the universe is expected to exist.

Bullet #2: Theoretically, ignoring GR, a collapsing star will reach critical density at one point first, usually in the center, forming a tiny black hole. The EH of this black hole will then expand in radius, perhaps rapidly, engulfing all infalling matter. GR does not distinguish between matter falling into an EH or the EH overtaking infalling matter. Matter cannot end up in the open center of a spacetime vortex. It makes no difference if matter is approaching the vortex or the vortex is expanding. --Danras 03:06, 24 October 2006 (UTC)

Oh, by the way, did you ever get around to posting your reference for your recent statement that, General relativity predicts the formation of frozen stars which asymptotically collapse towards a black hole state, but never quite reach it within any finite time.? --Oscar Bravo 07:38, 23 October 2006 (UTC)

I did give a reference (from the article) earlier (I think twice). From what I understand, you appear to agree with it, except that you regard frozen stars as images, which you know by faith to be black holes. I cannot obtain a reference that disproves your faith as GR only makes materialistic predictions. Rather than pursue the matter further, I saw how fond you were of demanding support for alleged OR and I took your hint and flagged your basic claim as OR. I do not think you will find a GR reference that supports your faith. --Danras 03:06, 24 October 2006 (UTC)

Re: I did give a reference (from the article) earlier (I think twice). I checked over the history and on 9 October, in the article, you directed the reader to the talk page for a reference. That took us to section 24 on this page (Requested references to common knowledge). The only "reference" I can find there is quotation from the History section in the main article. If we read this, it certainly goes along with your ideas, but clearly indicates that this was the state of theory in 1939 (History section , remember) and does not imply that this reflects the current understanding of BH theory. Aside from that, isn't it a little absurd that your reference comes from the article you're editing?

If you do happen to find a current reference (OK, any reference) that confirms that General relativity predicts the formation of frozen stars which asymptotically collapse towards a black hole state, but never quite reach it within any finite time, please don't hesitate to post it.--Oscar Bravo 08:13, 24 October 2006 (UTC)

The claim that a black hole is an object predicted by general relativity is unreferenced original research. - You're quite right, it was unreferenced. I have now added the reference to the paper by Stephen Hawking and Roger Penrose. Unfortunately, it doesn't seem to be available online, but you should be able to find it in a University Library. the abstract is at [3]. Have a read. If you can refute it, you'll be famous.--Oscar Bravo 07:35, 26 October 2006 (UTC)

Yay for science. :) --Kjoonlee 07:44, 26 October 2006 (UTC)

rfc for user Danras

I've opened a rfc on user Danras. It will need to be signed by at least one other person to become official. http://en.wikipedia.org/wiki/Wikipedia:Requests_for_comment/User_conduct#Candidate_pages Pervect 07:41, 23 October 2006 (UTC)

I'll second that...--Oscar Bravo 08:34, 24 October 2006 (UTC)

Two questions to Danras

1. Do you agree that there are different frames of reference, and their measurements shouldn't be always the same?

2. If the answer to 1 is yes: will someone in a frame of reference that moves in the direction of a black hole might pass the event horizon and enter the black hole?Melamed katz 20:47, 24 October 2006 (UTC)

My answer is yes to question 1. I would answer yes to question 2, as far as the mathematics of GR is concerned. However, I do not believe black holes exist for any outsider's frame. Nevertheless, an infaller into a frozen star will (in his own frame) quickly fall into a black hole if one makes assumptions generally assumed by outsiders.

However, from our frame, an infaller can always be theoretically rescued from infalling. We can never say he is never coming back. Using sufficiently advanced space propulsion we can swoop down and bring him back. We can also theoretically always rescue any matter alleged to have gone into the formation of a black hole. Processes in the universe can also inadvertently rescue infallers as they can theoretically destroy frozen stars at some point in the future.

Black holes and inside frames of reference exist as mathematical concepts. Specific claims regarding the actual existence of black holes (i.e., irretrievable objects), can always be disproved, at least theoretically. --Danras 03:48, 25 October 2006 (UTC)

Both of us agree that some observers enter the black hole. I have another question. It's more a question of definition. Do you think that we should define in Wikipedia things as "existing" only if they are present in our frame of reference?Melamed katz 10:41, 26 October 2006 (UTC)

Generally yes. Objects not in our frame of reference and which never interact with our frame should not be said to "exist" as their existence has no physical relevance to our frame. However, one-way interaction is probably sufficient. If one can travel one-way to another frame, objects in it can be said to exist as long as we are ignorant of any other information that refutes the existence of the frame. Since one cannot return from such a frame, its existence cannot be proved, but neither can it be disproved.

The existence of a black hole can be disproved. Material alleged to gone into the formation of the black hole can always be retrieved, at least theoretically. One cannot assign a black hole a time of formation, as such a time has not occurred. In our frame, an infalling observer has not fallen into any black hole, and does not observe one either, so we are not simply being prejudiced against his observations.

I think we should define in Wikipedia things as having only one "existence." Generally, this means being present in our frame as information about things in our frame trumps claims about existence in other frames. --Danras 15:58, 28 October 2006 (UTC)

Thank you Danras for your detailed answers. Now I understand better your point.

I would like to present here my point of view as well. I think that since black holes are explained only in the framework of relativity we should stick to this theory. In the relativity framework it is enough for something to "exist" even if it is not measured directly in all frames. In our case the situation is even better. Even though we don't see the black hole directly, we do measure some of its effects even in our frame of reference. Melamed katz 21:49, 29 October 2006 (UTC)

Thank you for your discussion. However, while relativity makes predictions, I do not think of it as a theory that defines existence. I would interpret its results using a theory such as Aristotle's law of non-contradiction that states that a thing cannot be A and non-A at the same time. I have some questions about your point of view:

1. The star Cygnus X-1 is alleged to be a black hole. Is Cygnus X-1 a black hole if I can retrieve all of the matter that accompanied its collapse? Is your definition of existence disprovable?

2. If a present day observer jumps into Cygnus X-1, and a future society dismantles it, perhaps out of a need for matter, does the observer in his own frame still fall into a black hole? --Danras 03:55, 30 October 2006 (UTC)

Here is my reply to your interesting thought experiments.

1.The time dilation of the infaller works only with respect to a static observer. If at some point you will start to move toward the black hole in order to rescue the infaller, then the time dilation will become smaller. If the infaller already crossed the event horizon (in his frame), then you will not be able to rescue him. If he is still on the safe side then you will meet (far in your future) and will be able to rescue him.

2.General relativity doesn't provide a way to dismantle black holes. This should be done by other means. Unfortunately, such a question can't be answered in the framework of general relativity.Melamed katz 10:24, 30 October 2006 (UTC)

Black Holes in String Theory

I have a suggestion which may be a little contentious - I think there should be more mention of black holes in string theory. Not a lot, just a little more in the section on entropy. It is one of string theory's triumphs that it provides a statistical mechanical accounting of the semi-classical Bekenstein-Hawking entropy for many supersymmetric black holes (as far as I know, all of them that can be considered compactifications of d=10 or d=11 supergravity) in terms of D-brane microstates. A great deal of theoretical research on black holes involves string theory and I would consider it a major thrust of string theory research as a whole. For those reasons alone, it should get a little more mention.

There is also supposedly a similar calculation in loop quantum gravity. I'm not an expert there, but I don't think it's as robust. Nevertheless, that may be worth a mention as well. A loop quatum gravity expert opinion would be helpful here. Joshua Davis 05:05, 26 October 2006 (UTC)

I agree. Black holes are indeed important in string theory. I think that it will be even better to devote a small section for this subject (Black Holes in String Theory).Melamed katz 10:46, 26 October 2006 (UTC)

So I got one response, but I was hoping for a little more input. Does anyone else think there should or should not be a string theory section? In particular, I'd like to get some opinions from the non-string inclined folks that are about. Any concerns/objections? Joshua Davis 03:54, 28 October 2006 (UTC)

My apologies if this is too late. I'd enthusiastically support a section on this page and/or a new article. --MOBle 23:05, 3 December 2006 (UTC)

Misnomer on micro-black hole at RHIC

The statement "The formation of micro black holes on Earth in particle accelerators has been tentatively reported" at the end of the Suspected Balck Holes section is a bit misleading. The problem is that the true nature of the relation between the RHIC fireball and black holes is hard to get across in the press but let me try to explain. Nastase's work is not claiming that an actual black hole was created at RHIC. It is extremely unlikely that gravity has any role in the phenomena there; rather the processes are govered by strong nuclear forces or more accurately, quantum chromodynamics. Rather, Nastase uses the AdS/CFT correspondence where a strongly coupled gauge theory, like QCD, can be mathematically expressed in terms of gravity in a higher dimension. This other description is called a dual gravity description and is argued through string theory although is probably valid even if string theory does not describe quatum gravity in the nature. You can think of it as a mathematical trick, if you like (or better, a mathematically precise analogy). Anyway, the point is that Nastase conjectures that the RHIC fireball corresponds to a black hole in the dual gravitational description. So, the RHIC experiments don't actually involve gravitational physics although there may be phenomena there which has a mathematical description which is identical to that of a black hole. This is a fairly subtle point but it won't do to misrepresent the observation. I think his first paper on the subject is at http://arxiv.org/abs/hep-th/0501068. Follow the link and read the abstract; you can see that he keeps calling it a dual black hole, not an actual black hole. Joshua Davis 06:11, 26 October 2006 (UTC)

So basically, he borrows the mathematics of black holes to describe the pion field soliton? Then you should probably remove the comment; the article is about gravitationally-bound BHs, it doesn't help to digress too far from the point.--Oscar Bravo 07:27, 26 October 2006 (UTC)

I agree. Thank you Joshua for that important comment. I'm afraid that the search for mini black holes will have to wait at least until the Large Hadron Collider will start taking data.Melamed katz 10:27, 26 October 2006 (UTC)

Oscar - yes, it is reasonable to say that he is borrowing the mathematics of black holes to describe a purely non-gravitational phenomenon. I don't want to belittle this idea, though. The AdS/CFT correspondence is incredibly deep physics and can also be used to describe quantum black holes in string theory in terms of more convential physics like quantum field theory. But the RHIC observation should be put in the proper context. I don't think the comment about RHIC should be deleted; maybe some other experts here can help me make it more accurate. Maybe it belongs in the proposed Black Holes in String Theory section. Any opinions?

I think that we have to be careful with a statement about mini black hole production in this article. Maybe we should have a short and informative section of String Theory and Black Holes, with redirection to a new, much bigger, and more theoretical article.Melamed katz 01:23, 27 October 2006 (UTC)

Abhas Mitra article deletion

I am trying to have the article on Abhas Mitra removed. Abhas Mitra is a contributor to the Magnetospheric eternally collapsing object or MECO theory, and apparently coined the term. In a nutshell, the MECO theory claims that radiation pressures prevent the collapse of an object into a black hole, and the you instead get an "eternally collapsing object" or ECO. If this ECO has a strong magnetic field, that further prevents collapse, and makes the object a MECO. As this is being thoerized in the context of GR, I see this a being a theory which claims that GR is not GR. Several such theories are noted in the a section of this article.

I won't contest that the MECO is encyclopedic, but I do not see that the notability extents to Dr. Mitra. Please see Wikipedia:Articles for deletion/Abhas Mitra. Thanks, EMS | Talk 03:40, 6 November 2006 (UTC)