Talk:Black hole/Archive 5

message: 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)
replies: text: 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)
thread_title:  profanity in 'Black Hole' article 

message: 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)
thread_title: Blackhole Section Is Dead Science

message: 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 mp and Tp 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)
thread_title: Entropy and Hawking Radiation: Mass of a "small" black hole

message: 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)
thread_title:  External Link added 

message: 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)
replies: text: 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)
thread_title:  History 

message: >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)
replies: text: 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)
replies: text: 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)
replies: text: 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)
thread_title:  False claim in the article. 

message: 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)
thread_title:  Doubtful statement tagged with {verify source} 

message: 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)
replies: text: 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)

text: (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)
replies: text: 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)
thread_title:  Mathematical theory 

message: 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)
replies: text: 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)
replies: text:  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)
replies: text: 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)
thread_title:  Definition of a black hole 

message: 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)
thread_title: Edits by {{user|Danras}}

message: 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)
replies: text: 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)
thread_title: Amended Reply to CH

message: "...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)
replies: text: 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)
thread_title: Recent Edit by Stratadrake

message: Could use votes to save this article, thanks MapleTree 22:18, 28 September 2006 (UTC)
thread_title: [[List of doomsday scenarios]]

message: 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)
thread_title: Recent edits by Nuclearfusion

message: 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.
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)
thread_title: Removed unsourced statement 

message: 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)
replies: text: I'll second that...--Oscar Bravo 08:34, 24 October 2006 (UTC)
thread_title:  rfc for user Danras 

message: 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)
replies: text: 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)

text: 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)
thread_title:  Black Holes in String Theory 

message: 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)
replies: text: 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)
replies: text: 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)

text: 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)
thread_title:  Misnomer on micro-black hole at RHIC 

message: 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)
thread_title:  Abhas Mitra article deletion