Talk:Black hole/Archive 1

stuff coming out
i don't think stuff can come out of a black hole could some one explane?(sorry about spell)

Without being "fed", black holes actually loose mass through its poles. Someone else can explain fully but as black holes spin, the centripetal force is enough to "throw" light and particals out its poles.

Quotes
Dear anonymous user


 * (Apologies for the anonymity .. I did put a page up, but looks as though it didn&#8217;t make it into the database, or I forgot to save it; it is there now (see: quota). Indented comments below are me, too.)

who put in the fancy quotes and apostrophes: kudos for the work, but unfortunately that is not the Wikipedia standard,


 * Since Wikipedia won&#8217;t render quotes and apostrophes correctly, how else does one get them to display?
 * Well, one doesn't, at least for now.

and it makes editing the page a lot harder.


 * It takes a few extra characters, it&#8217;s true, but with cut and paste it is only a couple of keystrokes. But it would be even easier if Wikipedia did them automatically, I agree!
 * It is not just a couple extra characters: those quotes show up as &amp;#8217; etc in browser editors (at least some of them). Editing text really becomes much harder if every "don't" and "it's" shows up as "don&amp;#8217;t" "it&amp;#8217;s". Jorge Stolfi 10:57, 31 May 2004 (UTC)

The Ascii character (') is an apostrophe, not a straight quote, so "Einstein's" is perfectly fine as is.


 * A (') is an apostrophe in Ascii text (as in a Wiki editing window), but it is not an apostrophe in the displayed text. And there are, one hopes, more readers than editors of Wikipedia&mdash;so we should optimise for the former.
 * I beg to differ: (') is defined to be apostrophe, the sign used in English language to indicate elision and posessives. A software or font that does not display ASCII (') as such is wrong and should be fixed.  Jorge Stolfi 10:57, 31 May 2004 (UTC)

As for double quotes: ideally we should use the opening and closing versions, yes; but those must be entered as Unicode entities and therefore would make the source file a mess, so it was settled to use (") instead.


 * In the current scheme, the displayed file is a mess, which is worse. Even a tabloid newspaper can get this right...
 * I agree, but I think that using (") was a wise compromise by the Wikipedia designers, and the solution to the aesthetics problem now is to wait for automatic conversion upon presentation. Jorge Stolfi 10:57, 31 May 2004 (UTC)

In any case most of the 200,000 articles in Wikipedia are written with (") quotes, so fixing only one page makes little difference


 * How else does one see how it looks?

and creates an inconsistency. It is better to wait until someone figures out how to do the conversion automatically, for all pages, when the text is displayed (without changing the source).


 * I agree, especially for double quotes. Single quotes and trailing/embedded apostrophes can also be automated easily.  Leading apostrophes need some kind of editing notation.


 * In the meantime, however, we have to put in the Unicode escapes (that&#8217;s the only way to have them work in all browsers). They can easily be reverted to the straight ASCII quotes automatically, in the source, once Wikipedia can display them correctly.

All the best, Jorge Stolfi 20:59, 22 May 2004 (UTC)

Jorge, thanks for taking the time to post an explanation. Would you mind reverting the quotes for now, please? They did take a long time to enter... quota
 * Well, they did take a lot of work to remove, too... All the best,Jorge Stolfi 10:57, 31 May 2004 (UTC)

Quota, Thanks for fixing my typos... and apologies for changing back your fancy double quotes again &mdash; that was just instinctive, I did not mean to start an edit war. However I will not put them back myself, since I think that simple quotes (") are better for now, for the reasons above. I offer no apologies for the apostrophe, though: as said above, (') is definitely the correct character to use, and fonts that do not render it as an apostrophe are bad fonts. Jorge Stolfi 22:33, 8 Jun 2004 (UTC)

I guess I put them in, instinctively, while making the other changes. There is really not much point when the whole article is in the unreadable sans serif font anyway. So I shall leave as-is for now... quota

Things falling in
I strongly disagree that objects fall to the center of a black hole. At least one recent referreed paper havs shown that no object can cross an event horizon (and thus fall to the center). I will go get my reprint later, and include a reference. Beyond that, most everything else is correct.

One item that I would really like to mention is that a gravitational field is equivalent to a region of space with excess volume (e.g. more than 4pi/3 r^3 inside a spherical surface of 4pi r^2.) This is one way of explaining how a wavefront of light is bent by the field. When falling into a black hole, space is observed to expand to an extreme, sometimes called "hyperexpansion". This is the same hyperexpansion which smoothed out many inhomogenieties after the "big bang". In fact, a black hole has a radius where the circumference is a minimum!

Joseph D. Rudmin


 * That would be surprising. Any general relativity textbook contains a discussion on how a falling object crosses the horizon and hits the singularity in a finite amount of proper time. I don't know what paper you're referring to, unless it's the so-called "gravastar" theory of black holes, which is highly speculative. -- CYD


 * Here is my reference to the article I mentioned:

"Does the Principle of Equivalence Prohibit Trapped Surfaces from Forming in the General Relativistic Collapse Process?", Darryl Leiter and Stanley Robertson, _Foundation of Physics Letters_, Volume 16 (2003), Number 2, pp 143-161.


 * I don't think that this correction to black hole theory will invalidate most other work, because most matter falling into a black hole never returns anyway.


 * I will now create the page for the Schwarzchild metric, and give further details there on how errors in that metric resulted in the myth that an object will fall to the center of a black hole.


 * Joseph D. Rudmin

I have edited this article because it was to much like prose writing, didn't realize the contraversy -Whoops! This is what was not retooled because Iw asn't sure how to handle it:

Once the gradient becomes large enough, close to the singularity, to tear atoms apart. The point at which the tidal forces become fatal depends on the size of the black hole. For a very large black hole such as those found at the center of galaxies, this point will lie well inside the event horizon (see also ), so the astronaut may cross the event horizon painlessly and live. Conversely, for a small black hole, those tidal effects may become fatal long before the astronaut reaches the event horizon.

This is my first attempt at a major edit, please advise.--Tznkai 01:10, 25 May 2005 (UTC) - In the first paragraph there was the idea, very common, that a black hole is very dense. We actually can not say anything of the structure inside the event horizon, but we can say the mean density inside it, and for large black holes is not so big, that is the reason of the numerical example. It probably could be moved inside the article.--AstroNomer 23:52, Jan 6, 2004 (UTC)

- Can anyone confirm/deny the recent edit made to this page by Plautus Satire? Looks like nonsense to me, but I don't really know cosmology. He also made an edit to Albert Einstein that looks pretty skewed... Isomorphic 19:33, 13 Feb 2004 (UTC) - As Plautus seems unwilling to discuss or step back from a rather extreme viewpoint, and the edit war is way past 3 reversions, will someone protect? I've had my fill of protecting for a while, but will if no one else is willing. Jwrosenzweig 20:48, 13 Feb 2004 (UTC)


 * The article is now protected, per request of Jwrosenzweig. --Modemac 20:55, 13 Feb 2004 (UTC)

Gamma Ray Burster Hole
Can you point me to a published paper in which a plasma cosmology paper comes up with GRB energies and spectra?

All I hear you saying is that GRB occur as a result of some process involving plasma. Well DUHHH!!!!! That's not particularly interesting or remarkable information. The unknown fact here is what sort of process can produce what is observed with GRB.

The GRB mystery is not solved by any means. Saying that it happens because of some sort of plasma process around some sort of star is not nearly specific enough.

(You appear to be vastly confusing plasma physics with plasma cosmology. Almost all of the observable matter in the universe is made of plasma, and saying that plasma physics is involved with GRB is not as earth-shaking as you think it is.  One should point out for example, that the black hole envelopes are made mostly of plasma.)

Roadrunner


 * You're the one who's confused here, Roadrunner. Your meaningless distinction between plasma physics and plasma cosmology is patently absurd and disingenuous to say the least. - Plautus

It's not a meaningless distinction. Plasma physics is taken seriously by most astrophysicists. Plasma cosmology is not.
 * I see. So the distinction you draw is based on who takes it seriously. Sounds meaningless to me. - Plautus satire 21:26, 16 Feb 2004 (UTC)
 * Synchotron radiation is emitted on Earth from particle accelerators. It is the result of charged particles (electrons or protons) passing near a magnetic field. Plasma physics is the study of behaviours and properties of plasma just such as these. Synchotron radiation is also observed in space, at energy levels that dwarf any terrestrial experimentation with plasmas. These synchotron radiation emissions are commonly referred to as "bursts". Any electromagnetic radiation of sufficiently high energy coming from space is called gamma radiation. - Plautus

Correct, and the prevailing theory of gamma ray bursts is that synchrotron radiation is responsible for gamma ray bursts. Synchrontron radiation also produces much (maybe even most) of the radiation around active galactic nuclei.
 * Plasma physics predicts that space, being filled with plasma (charged particles), will be filled with tremendous currents (passing through the plasma which generates magnetic fields that self-organize), will self-organize and will emit copious amounts of "gamma" radiation in bursts. - Plautus

O.K. here is where things get dodgy. The problem with synchrontron radiation is that you need a certain level of density and magnetic field to produce it. The interstellar and intergalactic medium don't have enough density or magnetic fields in order to produce gamma rays. So you need something that concentrates the gas, dust, and magnetic fields.
 * Yes, things are getting very dodgy, since it is the current not field density that is responsible for "high energy" radiation such as gamma ray bursts. The interstellar and intergalactic medium is a near-perfect conductor and it is VAST. Minute amperages over vast distances equals huge current. End of story. Plasmas also SELF-ORGANIZE in the presence of magnetic fields (which they GENERATE) and electromagnetic radiation (which they ALSO GENERATE). In short, plasmas self-organize, there is no need for any other "something" that concentrates the gas (Neutral gas? In interstellar space? No, plasms) and dust (Neutral dust? In interstellar space? No, charged particles) and magnetic fields. - Plautus satire 21:26, 16 Feb 2004 (UTC)

This isn't very hard math, and with basic algebra you can work it out for yourself. How strong a magentic field and how high a density do you need to emit gamma rays? Compare with the intergalactic and interstellar field, won't work. Compare with the fields near a black hole or neutron star. You get reasonable numbers. (Actually the fact that things are beamed helps a lot. You can get 1e+51 ergs of energy without too much trouble.  If it weren't beamed, we would be looking at 1e+54 ergs of energy and this is a big problem.)  So the prevailing idea is that something causes sychrontron radiation to be emitted.
 * Your "big problem" only arises if you use a flawed model to analyze the phenomena in question. In other words, it's only a "problem" for black hole hypotheses. It is completely consistent with predictions based on experimental study of plasmas. The energy levels you mention are derived from assumptions that redshift equals distance, therefore gamma bursts are extremely distant. This assumption is shown over and over again to be invalid. Given that plamas can self-organize into beams (check the highest-energy lasers on the planet, they do not use optics because the beams self-focus in the high-energy environment, they DESTROY any optics put in front of them), it's reasonable to assume we're looking at a plasma phenomenon here, not an unprovable magical black hole phenomenon. - Plautus satire 21:26, 16 Feb 2004 (UTC)


 * If that's not a prediction of gamma ray bursts I don't know what is. - Plautus

O.K. if it is a prediction. How long should the typical gamma ray burst last? Microseconds? Milliseconds? Seconds? Minutes? Hours? How many gamma ray bursts should we expect to see in a year? What is the energy of the gamma rays? What is the polarization? You see a gamma ray burst, and then you point your telescope at that spot. What should you see? If you can't use your theory to answer those questions, then your theory is incomplete. Not necessarily a bad thing, since no one has come up with a theory that answers those questions. They difference between you and the professional astrophysical community is that the latter is willing to admit that they don't know. Roadrunner 20:10, 16 Feb 2004 (UTC)
 * It's not my theory, it's theory based on observed, repeatable, verifiable high-energy plasma phsyics. Plasma physics predicts that naturally-occuring particle accelerators will occur in the universe, and that they will be ubiquitous, and that they will emit copious amounts of gamma rays due to the high energy densities capable at point loads in a plasma system with such vast physical dimensions. Plasma theory does not predict how long each burst will last, as this is dependent on many variables. Plasma theory is capable, however, of predicting some of the behaviours of known quantities of plasma, which black hole hypotheses fail at. Every prediction made by black hole hypotheses has been found starkly wanting when compared to observable reality. Plasma physics scores hit after predictive hit, and comparisons of predictions to previous unaccounted observations show more and more correlation between plasma theory and observed reality all the time. - Plautus satire 21:26, 16 Feb 2004 (UTC)

Once I've decided how I'm going to work this information into the black hole page I'm going to do it. If you want to take it down in contradiction of available evidence, feel free. - Plautus

The fundamental dishonesty of your actions is that you cite external pages which in fact do not support in any way what you write... then you sit back and pretend that they do. There is little point in arguing this further. Cite sources that actually support your text modifications, or else your modifications won't survive. It makes little difference whether I revert them, or any of a hundred other users do.
 * Your dishonesty doesn't surprise me at all. The sources I cited, and many more that I will cite in the future, back up what I say and demolish the black hole myth. Your willingness to ignore the facts does not make the facts irrelevant, nor does the willing ignorance of "a hundred other users" just like you. - Plautus

I would like to request that most of this section be edited to summarize it or re-written or re-organized or something, it's a jumbled mess and impossible to read for nearly everyone I'm sure. - Plautus satire 18:27, 18 Feb 2004 (UTC)

"The black hole myth"? It is true that black holes don't have to exist for GRBs to, but there are  numerous other observations that support (Note that I don't say prove) the black hole theories. That's all they are, theories. Disagree with them all you want, but I find it highly inaccurate to say "Myth" and use GBRs as the only reason of refuting the existence of black holes. Though there were ideas back in the eighteenth century (See the article), black holes were only brough into the light of physics after being found by the math. THe math wasn't used to support black holes; black holes were discovered in response to mathematical calculations. Observations then followed the mathematical discoveries and this is where the "mythology" as you call it began to come about. The core of the black hole theories, however, is completely reasonable, though if you have a reasonable way of refuting it, by all means present it.


 * You needn't worry about this old discussion, Plautus is long gone now. He had some highly dubious theories about how every sort of phenomenon you could name was caused by plasma in some way, and wound up in a large-scale battle across Wikipedia trying to rewrite articles based on that. IIRC he got banned for his antics. Bryan 20:00, 27 Nov 2004 (UTC)

The caption
The image is misleading. According to http://chandra.harvard.edu/photo/2004/rxj1242/ the bit at the top is just an illustration. Evercat 23:48, 18 Feb 2004 (UTC)
 * Yeah. NASA 00:03, 19 Feb 2004 (UTC)
 * Put the copyright credits on the image page. The caption should say what I'm looking at, and that is severely lacking at this moment. &rarr;Raul654 00:13, Feb 19, 2004 (UTC)
 * Just one quick comment here, pictures are meaningless without context. If we presume that the caption provides the fundamental context of an image, perhaps the source of the image should be included no matter what else is also included. Although properly credited on the "image page" would I assume be sufficient for any researchers to stumble across it if they really get a bee in their bonnet about it. - Plautus satire 05:19, 19 Feb 2004 (UTC)

I think the "supermassive" adjective should apply to the artist's conception too, since the black hole depicted is larger in physical dimensions than the star it's devouring and since the usual image of a stellar-mass black hole eating a companion star looks significantly different from what's pictured here. Bryan 06:38, 19 Feb 2004 (UTC)


 * The only reason I changed it was because I was editing the supermassive black hole page at the exact same time you were editing the black hole page, so I just changed yours to make it consistent. Go ahead and change both as long as they're consistent. The linked article makes it clear that the images are of a supermassive black hole, but it seemed a little less clear to me whether the artist's illustration was the same or just a regular black hole.  The optical telescope image certainly seems to show the star being closer than in the illustration. Curps

More things falling in
User:Aranoff just made a few comments about objects falling into black holes that I find difficult to believe or otherwise suspicious, but since I'm no expert I thought I'd explain my disagreement here. They are:


 * It is not possible to observe an object falling through the surface of a black hole, as it takes forever to get to the surface as viewed by an external observer. Since it is not possible to observe this effect, the effect is not part of physics.

This looks like a controversial statement at best, a gross oversimplification at worst. The laws of physics are being used to make these predictions in the first place, how can it not be a part of physics? Besides, the effect that Aranoff is claiming is unobservable seems quite observable to me; just drop a probe in and have it report on how things look from its frame of reference as it falls.


 * It cannot report on how things look! My statement is correct!


 * No, it can report on how things look, right up until the edge of the event horizon. As I mentioned below. And as far as the probe is concerned (according to theory), nothing will be slowing its fall. Bryan 15:23, 29 Apr 2004 (UTC)

You won't get any information from after it enters the event horizon, of course, but right up to that point things should look normal to the probe.


 * What happens inside the event horizon of black hole can be part of physics, of course. Just dump a physicist through it. (Or all of them, why not?) All the best 8-) Jorge Stolfi 11:07, 31 May 2004 (UTC)

Singularity

 * There is another point. The word singularity means a mathematical error, which means that the theory is not mathematically sound here.

Firstly, that's not at all what the word singularity means.


 * Yes, singularity means division by zero!


 * That is one way to get a singularity, yes (there are others, see mathematical singularity for more details). But that is not a mathematical error, and in any event the singularity only occurs right at the very center of the black hole; there's nothing implausable about the predictions general relativity makes about approaching or passing through the event horizon. Bryan 15:23, 29 Apr 2004 (UTC)

But more importantly, even granting that a singularity occurring here means that the theory is unsound near the center of a black hole, that doesn't mean it's unsound in the vicinity of the event horizon or most of the volume inside that. Bryan 15:00, 28 Apr 2004 (UTC)

Aronoff, I reverted the informal explanation of the singularity because Jorge Stolfi 21:30, 11 Jun 2004 (UTC)
 * 1) At that point, it is important to say that gravity (and gravity gradients) increases to infinity as one approaches the singularity.
 * 2) The mathematics is just a model for the physiscs, so the mathematical phenomenon (division by zero) is less important than its physical implication (gravity goes to infinity)
 * 3) The "division by zero" at the singularity would not be a problem per se, the problem is that the singularity is essential because of what happens in the immediate neighborhood (where there is no singularity).
 * 4) Your view -- that the singularity means that physics is incomplete -- is already mentioned at the end of that section.
 * 5) That view is just a view, not a proven thing. For instance, one could also argue that the singularity itself is not part of the universe, i.e. it "does not exist" -- in the same sense that &infinity; is not part of the real line.  In that case the equations would still be non-singular everywhere in the universe, the "division by zero" would not occur, and no new physics would be needed to "fix" it.

((begin relocated text Jorge Stolfi 02:38, 13 Jun 2004 (UTC)))

My name is Aranoff.

Sorry to disagree with you on the term "singularity". I stand by my statement. What you put in is wrong. I am disgusted with you and Wikipedia.

Again, it is very simple. I tell this to my high school students. Singularity means no more than an illegal division by zero, which means a breakdown of the theory. Using infinity is wrong. It is like saying 1/x is infinity when x=0. It is not infinity. It is not allowed.

If you wish to discuss this further, send me a private email: aranoff@analysis-knowledge.com.


 * Once again I direct you to read mathematical singularity; "division by zero" is only one particular way to get a singularity, there are many others, and in any event a singularity is not an indication of a "mathematical error." Furthermore, although 1/x is not infinite when x=0, it does approach infinity as x approaches 0. This is sufficient to produce the predictions that you've been objecting to. Bryan 21:25, 12 Jun 2004 (UTC)

((end relocated text))

Aranoff, my apologies for getting your name wrong.

I cannot understand your point. For one thing, I don't see how you can accept "zero" but reject "infinity". They are both mathematical abstractions; in physics one can only have "zero" or "infinity" or "pi" in some approximate or limiting sense. More generally mathematics is not physics, only a model for it; if the mathematical formula is not defined at some point, it does not follow that the physical theory is wrong. Thus, for example, one could patch the 1/x example by including ∞ in the set of real numbers and defining 1/0 = ∞. Algebraists don't like to do that because it is not a field any more, but geometers just love it (it separates the projective men from euclidean kids).

All the best, Jorge Stolfi 02:38, 13 Jun 2004 (UTC)

Just moved this in from the article, inserted by Aranoff:


 * The above statement is false. The center of the event horizon does not exist. 1/x is not defined for x=0.

Variations of which has been addressed here several times already, so I don't have anything new to add in response. Bryan 02:00, 16 Jun 2004 (UTC)

A singularity is an undefined mathematical operation, is it not? Division by zero falls into this category. However, it is simply undefined, not yet proven as impossible. Hence why we say that we don't yet have the math to determine what exists at the big bang or the center of a blackhole. But since we jumped to the "I am disgusted" part, I doubt you'll read this. I do hope you are disgusted because of other things besides just this, because it makes you look much worse than I would like to believe you are.

Things cannot interact with the interior?
I deleted the assertion that "nothing can interact with its interior". AFAIK, matter that falls into a black hole will cross the event horizon in a finite time (in its own frame) and once inside it can interact with other matter that is in there. Is this correct? 10:57, 31 May 2004 (UTC)

I deleted (again) a bougus explanation for the "hole" in the name:
 * In addition, particles cannot exceed the speed of light so nothing else can interact with the object, hence the term hole.

This is wrong physiscs (things can interact gravitationally with the black hole) and wrong etymology (it is called a "hole" simply because things fall into it &mdash; or, in Nerdish, because its gravitational potential is lower than that of surrounding space 8-) Jorge Stolfi 14:42, 3 Jun 2004 (UTC)

((Aranoff and Bryan entries relocated from here to Singularity section Jorge Stolfi 02:38, 13 Jun 2004 (UTC)))

Shape of black hole
A misconception some people have is that a black hole is a two-dimensional object on a flat surface, as a hole in the ground is a two-dimensional opening in the plane of the earth's surface. So, I added the text about ball-shaped.


 * Correction: a rotating black hole is probably an oblate (flattened) speroid rather than a prolate (enlogated) one, at least for reasonable rotation rates. In any case the American football is not an ellipsoid, it is technically a piece of a torus  -- a surface swept by a circle rotating about an axis on its plane. (Only that in this case the torus is degenerate because the axis cuts the circle.) Jorge Stolfi 10:57, 31 May 2004 (UTC)

Explanation of the name in first few lines
I think it is appropriate in an encyclopedic article to explain the two properties which define a black hole: it is black and (separately) it behaves from the interaction point of view as a hole in space. The explanation is given by the person who coined the name (Wheeler, see e.g. in Misner, Thorn and Wheeler, Gravitation, chapter 33, the section "Why black hole"). When a black hole is formed in a collapse it first forms a hole (for an external observer passing the surface of last influence) and later it becomes black (when in an exponentially decreasing intensity the last photon has come out so to say).

What do they look like?
What are the current views about the close-up appearance of a black hole?

Many yars ago I saw in some astrophyisics journal a "scientifically correct" picture of a black hole surrounded by a "low-calorie" accretion disk. The picture looked mostly like a coal-black Saturn, except that the portion of the ring that should have been hidden by the hole was visible above it, distorted by gravitational lensing from the hole.

Is that picture correct? If not, what is the current picture? Thanks...

Jorge Stolfi 00:36, 12 Jun 2004 (UTC)

Sounds about right, though it depends if it is an "active" black hole or not. If it has run out of material to accrete, the ring would not be present, just the distortion in the background...--AstroNomer 01:34, Jun 12, 2004 (UTC)


 * I'll briefly outline my perspective before making my comment: I have a layman's interest in space, I don't read sci fi books or watch sci fi TV or read academic literature on astrology. But big things like the creation of the universe and the existence of black holes excite me (as well they should, particularly not having any religious beliefs).


 * OK. Now, the thing is, as I understood it a black hole is a singularity, which - I thought - was the smallest area imaginable (to use my uneducated definition).


 * Now, NASA's concept pic (in the article) looks like a black planet.


 * I appreciate that even if something is smaller than pin prick tiny, a black hole is going to warp and effect large areas around it, but I'm still disappointed to see it looking like a large black globe. Shouldn't it really be more like...  Ah, cos they don't let light escape, do they?  Ah, hmmm, ah, er, OK.  [shuts up] --bodnotbod 18:44, Jul 22, 2004 (UTC)


 * Well you're really not going to see the singularity. What you would see would be closer to the event horizon. Outside the event horizon you would see mass in the accretion disk being pulled towards the event horizon.  So since there is a backround of stars, and the black hole lets no light outside the event horizon, that would be the black part, that could possibly be seen as a black globe. - Taxman 20:20, Jul 30, 2004 (UTC)


 * The main thing wrong with the NASA pic is that you should see some gravitational lensing (extreme magnifying glass distortions) around the edge of the event horizon (the black globe). But it looks fairly reasonable -- Solipsist 21:00, 30 Jul 2004 (UTC)
 * I recently saw a good supercomputer visualisation video of the gravitional waves generated during a merger of two black holes. The start of the video had a good representation of the black holes moving against a stellar background which would give you a good idea. Some stills from the video are here, but I can't find a link to the full mpeg. -- Solipsist 21:27, 30 Jul 2004 (UTC)


 * I think it is worth pointing out that the existence of a singularity in a black-hole is only *theoretical*. It might be the case that no singularity exists at all as some more recent theories postulate. (In my own opinion this would make sense: A singularity is a mathematical construct with an infinite property — something which reality has great difficulty in describing.) But like you I just have a laymans interest, so I'm sure a physisist will come along and put me in my place about it all. :)

The link listed no longer works. 24 Oct 2005

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

New Info
http://www.theglobeandmail.com/servlet/story/RTGAM.20040715.wholes0715/BNStory/specialScienceandHealth/

I didn't check the history to see when this was posted in here, but that information is already reflected in the article. - Taxman 12:08, Jul 31, 2004 (UTC)

"theoretical"
How can we say that black holes are "theoretical" and also that "The existence of black holes in the universe is well supported..."? Seems contradictory to me. I mean, their existence was first predicted in theory, so they WERE theoretical at one time, but if we're going to acknowledge the "dissenting minority" we shouldn't say "well supported". --Tothebarricades.tk 01:27, 23 Sep 2004 (UTC)


 * They are still theoretical because there is no *direct* proof that they exist in a state as described in theory. We have only indirect observations, albeit numerous and by most accounts conclusive to the existence of large-mass objects. But still, a black-hole has not been directly observed, and there is a minute chance that the evidence we have seen could be the result of something else - a collection of large-mass objects in close proximity perhaps. I'm not saying we need to land on one and stick a flag in it before we can say "Yes, they exist", but their nature and composition is still very much in the realm of theory.


 * There is no contradiction at all. It is a theory.  Good theories generally make predictions that can be substantiated or shown to be incorrect.  The best that can be hoped for for a physical theory is that it makes many strong predictions that are verified.  It can never be conclusively proven as true.  It can only be conclusively proven as false by finding an observation that could not occur if the theory were true (a counterexample).  Black hole theory is supported by many many observable facts and other theories.  But it is possible something else is responsible for the observations and explains the other theories.  Thus the minority position is unlikely to be correct, but possible.  So it needs to be noted too, and no contradiction is had. - Taxman 02:57, Sep 23, 2004 (UTC)


 * Notwithstanding this correct presentation of the nature of a theory, I find the word "theoretical" in the opening paragraph adds nothing to the understanding of readers, and in fact is actively confusing. By this measure, every Wikipedia article on a scientific phenomenon (such as atoms, electrons, sound) should preface it as being "theoretical" since there is no *direct* proof that it exists. The evidence for black holes is not yet completely convincing, but I think it is fair to say that it is the opinion of the majority of scientific practitioners that they represent a "real" phenomenon. I would move to strike the word "theoretical" from the introduction, since occuring so early in this discussion as it does, it falsely creates the impression in the average reader of black holes as not a "proper" part of the "real world". Apologies for the inverted commas! Bosmon 11:28, 23 Sep 2004 (UTC)
 * We're after accuracy here. It is a well supported theory, but very far from entirely understood.  It is still possible that something else explains the observations better or fits in with other theories or explains more.  Therefore the note as theoretical is important.  Other theories such as that of the atom are much more accepted and no serious scientist disputes them.  Most physicists that accept the theory of black holes accept the fact that it may not be the defiinitive theory.  Thats a big difference.  Without the word theoretical we're saying that it is an entirely true fact which is intellectually dishonest and potentially false. - Taxman 12:26, Sep 23, 2004 (UTC)
 * I don't mean to be unnecessarily flippant, but "entirely true facts" such as what? I would hope that no serious scientist considers any theory to be "definitive". In my view, accuracy is best served by not unnecessarily casting doubt on the validity of theories which are very wideley accepted. Anyway, I'm not sure we're really very much in disagreement - the rest of the page goes on to explain the current status of the theory in a quite satisfactory way, and I don't think anyone would leave it after reading it all in any way deceived. I just felt it was a rather off-putting opener. Someone else seems to have removed the word anyway, and I quite like the way the first paragraph reads now, not sure how much of it is your work at this point? Regards, Bosmon 21:31, 23 Sep 2004 (UTC)
 * Well without noting it as theoretical, then wikipedia is stating it as fact that black holes exist. Since the rest of the article is rightfully at odds with that, the article is inconsistent and incorrect.  Perception of the theory by readers may well be important, but it certainly does not serve to the accuracy of the article to omit the fact that their existence is only theoretical. We could fix the situation by moving the sentence about being a well supported theory up.  That improves the accuracy by covering your point and mine. Also, removing the reference to theoretical from the first sentence makes the last sentence in the first paragraph not flow as well.  The intro was very carefully worded for accuracy and to be a good overview, so be careful if you make adjustments. - Taxman 22:39, Sep 23, 2004 (UTC)
 * You seem to be using "theoretical" in a different way than it is usually used in science. There is a very good discussion at Theory which might resolve your objections. Fpahl 23:15, 23 Sep 2004 (UTC)


 * Hmm...I may have walked into a minefield here, but I blithely deleted the qualifying phrase, and then came across this discussion. I agree with Bosmon: there is absolutely no need to have the opening of the article sound so uncertain. Even without the qualifying phrase about dissent by a minority of physicists, it's actually very weakly worded. If there is a dissenting minority of physicists who think general relativity is incorrect when it comes to the basic facts of black holes, they must not publish in any of the journals I've ever read. Really, the concept hasn't been seriously in question for 50 years. Sure, there is real debate about certain issues (e.g., which observed objects might be black holes, the nature of the singularity, the information paradox), but these are not disagreements about whether black holes exist in our universe. The thing in the center of our galaxy is clearly a black hole, for example.--Bcrowell 20:43, 26 Sep 2004 (UTC)
 * Good news, black holes are no longer theoretical. See "Hubble measures first proven black hole in Andromeda" below. --Air 12:22, 22 September 2005 (UTC)

"Black holes have no hair"?
Quoted from the article:


 * According to classical general relativity, black holes can be entirely characterized according to three parameters: mass, angular momentum and electric charge, a principle summarized by the saying "black holes have no hair".

As someone with very little familiarity with physics, I have absolutely no idea how or why that principle is summarized in that saying. Perhaps someone could explain? (And maybe add some clarification to the article itself?) :) --Slartibartfast 09:01, 23 Sep 2004 (UTC)

Well, I found the no hair theorem article. That helps some... :) Added a link to that article from the black hole article. --Slartibartfast 13:04, 23 Sep 2004 (UTC)

Intro needs improvement
This excellent article merits a better opening.

A black hole is a theoretical concentration of mass with a gravitational field so strong that its escape velocity exceeds the speed of light.


 * Is there any need for the word "theoretical" in the 1st phrase ?
 * Absolutely, see above.
 * What does "its" refer to ? The mass ? The gravitational field ? The black hole ?
 * Good point. I tried to improve it, what do you think? - Taxman 12:26, Sep 23, 2004 (UTC)

-- Frau Holle 10:37, 23 Sep 2004 (UTC)

reference missing for black hole cluster
In 2004 a cluster of black holes was detected, - this is useless without a reference - there's a huge difference if they are stellar black holes are supermassive black holes. Maybe it's a reference to this expectation of bh's in the GC? 

Please look through http://arxiv.org to find the article if this is not the right one.

Boud 11:10, 23 Sep 2004 (UTC)

What happens if....
...you throw antimatter into a black hole? Since antimatter makes matter explode on contact, would the black hole explode, or does being a black hole somehow make things different?


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

This is a remarkable consequence of the no hair theorem. A more vivid example is the following. Consider some matter and an equal amount of antimatter located elsewhere. If the two collide, then they annihilate each other. If instead they both undergo gravitational collapse separately, one obtains two black holes of equal mass, each indistinguishable from each other by the no hair theorem. If the two black holes are then combined, one only obtains a bigger black hole rather than anihilation.

Black Holes Approached at the Speed of Light
My understanding of advanced physics, both astro- and quantum-, is relatively small, so bear that in mind as I ask my question. As I recall, as an object, X, approaches the speed of light, its mass increases infinitely. But due to relativity, since there is no "stand-still" object in the universe to base an object's speed off of, all objects' speed is only in relation to each other. Which means that while X is approaching the speed of light from the perspective of other objects, from X's perspective those objects are instead the ones approaching the speed of light.

So if X were approaching a black hole at or near the speed of light, what effect would that have on the black hole from X's point of view. Would the event horizon shrink? Is there a speed at which the black hole would no longer contain a singularity as, from X's perspective, it would be approaching the speed of light and therefore be expanding infinitely?

The problem of the radius
I am thinking about adding a section to the black hole article, but I would like to get people's opinions on whether this will produce any greater clarity or simply put people off. An almost universal (no pun intended) "problem" when people talk about black holes, this article included, is the rather casual way in which they talk about its "radius". For example, in the Schwarzschild metric that appears in the article, it is quite clear (apart from the coordinate singularity there) that the event horizon occurs where $$r = 2M$$. What is not usually mentioned is that this r coordinate is almost wholly *arbitrary*. The only special things about the coordinates for the metric in the form given are that i) the metric is independent of t (as well as being spherically symmetric), and ii) as space becomes asymptotically flat as $$r\to\infty$$, the length scale of the r coordinate agrees with flat space distance.

As the article on the Kerr metric points out, different r coordinates even though they agree at infinity, need not agree anywhere else. So this is where we have our problem - what can people actually mean to say when they say, "the radius of this black hole is 2M"? If we were to use the distance measured by a hypothetical traveller as he moves towards the event horizon, say from a distance of 100M where space is fairly flat, this is clearly false. The "proper distance" he measures on his way towards the event horizon is not 98M, it is infinite. This excellent page http://casa.colorado.edu/~ajsh/schwp.html illustrates the problem.

To cut a long story not terribly short, what we are doing descriptively when we make statements about black hole size is talking about places in flat space where there *would* be things other than a black hole if the space was flat there. So by saying the radius is 2M, what we actually mean is, there is definitely *no* black hole at places we can visit where we would have considered our distance from the putative black hole centre was *greater* than 2M had there been no black hole there. The black hole is actually a logical hole as well - we can only define its size in terms of where we know it isn't!

In fact, defining where the event horizon is in any case is actually problematic too. This other excellent article http://www.mathpages.com/home/kmath339.htm talks about a closely related issue, that a black hole cannot even be defined in a Universe that does not last forever. Loosely speaking, since anyone falling into what *would* be a black hole (i.e. no escape possible) sees infinite time passing in the exterior world as he crosses the horizon, he also sees the surrounding Universe end, if it is going to. Whatever else this means, it also means that he cannot cross any kind of horizon in this case.

All we can take away from this is that the "size" of a black hole, in terms of the region of what would be flat space if it weren't there that it affects, can only be said to be *proportional* to M, and not in fact precisely 2M or any other figure.

Anyway, I welcome (polite!) comments on what do with this info/discussion. Should I try to boil it down into a section on the black hole page, or another, or might it merit a separate page by itself, "The problem of the radius", or "Problems in talking about Black Holes" or so...? TIA. Bosmon 22:32, 23 Sep 2004 (UTC)


 * What you're saying is all true, but I'm not convinced it should go in the article. Essentially, there's a problem with the whole English language when we deal with relativity. The only solution is to use the language of mathematics, but this article needs to be intelligible to people who don't know differential geometry. It's also not just a problem with the radius. Well, just my 2 cents. BTW, the idea about seeing the end of the exterior universe is very interesting! Too bad we don't seem to live in a closed universe :-) --Bcrowell 03:03, 25 Sep 2004 (UTC)

Centrifugal Force and Black Holes
Some time ago I read an article by Marek Abramowicz in Scientific American about how the concept of centrifugal force can change radically when you approach a black hole.

It has some rather interesting effects and consequences and I believe it should be added to the main article. Perhaps not as explained below, but hey I'm trying my best...

We all know that light bends around a black hole, in fact there is a certain radius where the light would bend just the right amount so that it formed a perpetual 'ring'. It would neither fall into the black hole, or escape! (In reality it would be a sphere - but let's keep it simple.)

It gets very interesting if you were to build a donut shaped space-station at this radius with the black hole in the centre (or hole of the donut!). First, this radius is OUTSIDE the event horizon by some distance, so you're safe. Second, if you were an occupant of this station and there was a central corridor that ran the entire circumference of this station. You could look down it and see your own back!

The light would be bent in such a way that the circular corridor would appear to be straight, this has a direct influence on the so-called centrifugal force experienced by any object travelling around the black hole. You can check the corridor is still circular and which side is the inside or outside using a straight ruler - but that's another interesting aside.

Now imagine a space-ship doing laps around a black hole outside that radius. The centrifugal force the ship would experience would be similar to the everyday kind. However at the described radius it wouldn't experience any of that force at all - it could do laps without having to counter any sideways forces. For all purposes it would be going in a straight line.

Going closer to the black hole (again we're still outside the event horizon) the space-ship doing the circular laps would have to counter a force opposite to normal. To all purposes the space-ship would be doing laps of the universe, not the black hole!

I thought about this a bit more... this also creates interesting effects when you observe any light falling into the black hole - above and below this magical radius. Basically as you get closer to the black hole (and looking away from it) you'll see more and more of the universe, eventually the stuff behind you because it's being bent by, surprise, the black hole.

At the magical radius, if the black hole was truely black, it would look like a perfectly flat, black surface that extended forever, the universe above it. If you imagine this black surface as a 'lake', the universe would be the 'sky'.

Eventually you'll get to the point where the universe would appear to be a white hole as the light will get bent further and further, much like the view from a fish-eye lens.

I hope that isn't too confused and perhaps someone would do some research and add a reference.


 * I hope this doesn't burst your bubble, but as far as I knew Centrifugal Force does not exist in General Relativity, or many other recent competing theories. But regardless of the terminology used, you raise some interesting points. You may like to read this article for a similar take on what you've explained in your post.


 * I vaguely recall reading exactly this description of centrifugal forces in a popularization a while back. Hm... gives a nearly useless one-liner that indicates the magic radius where the force inverts is 1.5 times that of the event horizon... ah, here we are;  goes into detail about how the extreme gravity of black holes affects centrifugal force. Looks like the article I remember is the July 1995 Discover magazine, but it's only available online if you're a registered subscriber:  Bryan 05:33, 24 Sep 2004 (UTC)

this sentence is misleading
"But although the light from an infalling object crossing the event horizon will take an infinite amount of time to reach a distant observer, from the point of view of the object itself it will take a finite time to cross the event horizon and reach the singularity."

Seems to imply that the the light itself leaving the infalling object and going to the observer slows down. The light itself does not slow down, in fact light always travels at the same speed relative to every observer. This should say:

"But although it will appear to the distant observer that the infalling object, falling slower and slower, approaches but never reaches the event horizon (i.e. takes an infinite amount of time to reach it), from the point of view of the infalling object itself it will take a finite time to cross the event horizon and reach the singularity."


 * Sounds good to me. Make the edit! :)


 * Be careful in applying intuitions from special relativity to general relativity. The speed of light, measured locally, is always a constant, but there is no such thing as a global speed of light because spacetime itself is mutable. In this case, light from an infalling object does take an infinite amount of time to reach a distant observer as the object approaches the horizon. -- CYD

this is also wrong
"The closer he gets to the event horizon, the longer the photons he emits take to escape from the black hole's gravitational field. A distant observer will see the astronaut's descent slowing as he approaches the event horizon, which he never appears to reach." (emphasis mine)

This is wrong too. Light always travels at the same speed from the viewpoint of every observer. No time to change this right now, maybe someone else can do this.


 * I'm not so sure I agree with this one. From an observers point of view, granted, light appears to always be moving at a constant speed, due to time dilation. But surely the photon is still affected and its traversal slowed by gravity regardless of the presense of an observer?


 * The sentence in the article is essentially correct, perhaps a bit misleading. The distant observer will see the astronaut's descent slowing. The *traversal* of the photon is not slowed, photons always appear to move at the speed of light in every frame they pass through. However, you must bear in mind that the astronaut is passing through a lot more space (proper distance) than you think he is! I.e. imagine that by some miracle the astronaut could "pause" at each spot on his way towards the horizon, and get out a metre rule and lay it end to end, spot to spot. He would find much more "space" on his way than an exterior observer would measure if he didn't know the black hole to be there. By this interpretation, the photons do indeed "take longer to escape". However, since the astronaut and the exterior world have no common idea of simultaneity, the idea of "take longer" is meaningless - take longer than what? Now will someone please comment on my "problem of the radius" proposal! Bosmon 00:29, 25 Sep 2004 (UTC)


 * The statement in the article is correct. From the point of view of a distan observer, the photons do take longer and longer to arrive as the infalling object nears the horizon, assuming they are emitted at a constant (proper) rate. -- CYD

How can half of the mass-energy be converted into energy without violating baryon and lepton number? lysdexia 05:38, 16 Oct 2004 (UTC)

You have used a wrong picture
haha A black hole does not look like that. In the picture on top of the page, it seems as if the X-rays go up and then get back into the black hole (and have a shape of a magnetic field). The two jets should look like this (I took the picture from wikipedia itself - no copyright problem):

Would you mind if I change the picture?


 * I think they should both be in the article, with the current "top" image remaining at the top of the article. The current top image is focused on the black hole itself, this new image is good too but illustrates a special case situation where a black hole and normal star are orbiting each other closely. It's a very long article, I'm sure there's room to put another image in. Bryan 06:49, 21 Oct 2004 (UTC)
 * I guess you're right, but there's still the problen that black holes don't look like that. I don't really mind what picture would be there instead: the picture I used was just to illustrate what the jets should look like. But from the physical aspect, the image has a mistake. That's all.
 * I've uploaded a cropped version of the sme pic:



I don't know if editing the image is considered "fair use" or not, so I won't put it in myself. But if it's fine, I think it would still be better than the current one, and it doesn't have the problem with second star.


 * Image:Accretion_disk.jpg is marked as being public domain, so that means you can legally do anything that it's physically possible to do with it. :) Bryan 23:40, 22 Oct 2004 (UTC)


 * Hm. That new version has an accretion disk that isn't being "fed" by anything; it's my understanding that you only get them if there is a source of matter like a nearby companion to feed it. I've tried adding the original version of the image with a caption that points out all the various features, and since it's so picturesque I think I'll try moving it up to the top position. Let me know what you think. Bryan 23:48, 22 Oct 2004 (UTC)
 * oops, I didn't think about the accretion disk problem. I think it's fine like this. - Dan

Mass? Light?
The article currently states: "The whole idea gained little attention in the 19th century, since light was thought to be a massless wave, not influenced by gravity." But photons do have zero mass; that's what makes it possible for them to travel at the speed of light. The article doesn't seem entirely clear on why light is attracted by gravity, and how the current theory of light differs from the 19th century theory. --LostLeviathan 01:36, 21 Nov 2004 (UTC)

It isn't explained, but I'm guessing it has to do with general relativity and the gravity distorting spacetime theory.
 * Light always travels in a straight line, and (because photons are massless) light it is not attracted by gravity. However, according to general relativity, gravity bends space so that a "straight" line is actually curved. Light (which travels along those curved lines) appears to the observer to be attracted by the gravity, but it's really not. &rarr;Raul654 08:09, Nov 24, 2004 (UTC)
 * To use an analogy - think about rolling marbles on a mattress. Now, drop a bowling ball in the middle of the mattress. The depression formed by the bowling ball tends to attract marbles. Is the bowling ball attracting the marbles? No, but it is distorting their path so that they fall into its depression. &rarr;Raul654 08:10, Nov 24, 2004 (UTC)


 * RE: old theory: C18th theory (according to followers of Newton) did say that gravity bends lightbeams: Newton tried to put together a unified description in which waves and particles were deflected by variations in lightspeed, either because of a variation in the density of a material medium (e.g. air, glass), or because of the variations in density of the presumed light-aether due to the strength of the gravitational field. So, in 1784, you have the Royal Society publishing John Michell's paper on stars with an escape velocity near to or greater than lightspeed, which would be difficult or impossible for astronomers to see directly. We now tend to refer to these theoretical objects as "dark stars" to distinguish them from modern GR black holes. Anyhow, Newton accidentally messed up the speed relationships, and when they started testing real lightspeeds in the C19th, Newton's gaffe was realised, and everything written on the behaviour of light according to Newtonian arguments ended up considered as discredited. To save Newton's posthumous reputation, all the stuff about gravity bending light got quietly dropped from the English physics history books, "particle" and "wave-particle" concepts of light got dropped, and we got a lot of "wave-only" aether theories of light (C19th theory), where the authors no longer had an obvious reason to assume that their models had to agree with Newtonian logic or the principle of relativity. Then with Michelson, Lorentz and Einstein, everything lurched back to the idea of light being relativistic again and we came pretty much full circle, Einstein "rediscovering" the effect of gravity on light, apparently from scratch, in 1911. Michell's paper remained "lost" to modern physics histories until the 1970's.
 * Thorne's "Black holes and timewarps" covers a lot of this territory, its listed in the article as a university text, but IMO it's also readable enough to count as a "popular" book (casual tone, very few equations, lots of drawings) ErkDemon 15:17, 16 July 2005 (UTC)

A side point related to mass and light: the text reads, "an escape velocity equal to the speed of light", which is incorrect. I see that at some point previous it had read "an escape velocity greater than the speed of light", which is correct. If the escape velocity were equal to the speed of light, then light would escape from the confines of the hypothetical star. The point to the thought experiment is that it would not. Unless I'm missing a point here, the page should be reverted to the original text. --Harmil 20:56, 13 Apr 2005 (UTC)


 * Whoa! Of course light's affected by gravity. Saying that 'gravity curves spacetime and that light follows the curve' is the same thing, viewed in another way. It's not an illusion. After all, with this metaphor, a planet merely follows the curve of spacetime too. You wouldn't say the planet isn't affected by gravity! And this is the whole point of E=mc2: gravity is a mutual attraction between all forms of energy, mass included. Unless Raul's arguing that gravitation isn't attractive period, for mass or energy, which strikes me as probably more misleading than helpful.


 * Light-energy is supposed to have a gravitational effect, but only if you can make it localised, i.e. persuade the light to stay in one place long enough to allow measurement, and it should then appear as a conventional gravity-source. There've been papers saying that two constant, parallel, close, counter-propagating beams of light ought to mutually attract, and then there was the classic Wheeler paper on "geons", about lightenergies so intense that the light-complex gets trapped by its own gravity and starts looking like a particle with mass. The energy of light trapped in a box is supposed to contribute to the overall inertial and gravitational mass of the box, and light captured by a black hole is supposed to increase the mass of the hole. I think it's maybe easiest to say that to have "mass", something has to have energy and a persistent location, and since "free light" has energy (but moves at the speed of light), it normally fails the second criteria. But pop your photons into a mirrored thermos flask or slow them down with a gravitational field, or step back far enough to say that a region contains a persistent amount of concentrated energy due to light, and you should see associated mass-effects. ErkDemon 15:17, 16 July 2005 (UTC)


 * As for the escape velocity, I believe the event horizon is defined as the surface where v=c. I mean, where else would you put it? Where is "greater than"? With v=c, outgoing light would be infinitely redshifted, and no energy would be conveyed - it would effectively disappear from the external world. I think someone corrected the earlier version, and that they were probably correct in doing so. &mdash;kwami 07:59, 14 Apr 2005 (UTC)


 * I made this change, and it is correct. vc inside. Of course, the meaning of "escape velocity" in this case is not intuitive, because an idealized photon at the even horizon with v=c would never actually escape, but neither would it fall in. --Joke137 15:11, 14 Apr 2005 (UTC)


 * So right, gravity only bends the path of light and light can't have zero velocity, what happens to a ray emitted "straight up" (towards the closest point on the event horizon) from inside the horizon in a nonrotating bh? At some point it has to stop and go back, but can it do it without turning around? What kind of relatavistic weirdness do you get from allowing the photon to momentarily stop? Tzarius 07:49, 17 November 2005 (UTC)


 * The light always appears to be travelling at C. There are a couple of ways of explaining how this works. The more accurate is to say that the geometry of spacetime is such that any light generated inside the horizon has its path warped such that it ends up moving towards the singularity no matter which direction it's propagating in. The usual description of this is "the future light-cones in all reference frames inside the black hole point inwards" (no matter which direction you're going, it takes you farther into the hole). The horizon represents a limiting case where the path stays confined within the horizon but path length approaches infinity (it never strikes the singularity). This corresponds to the case where one ray on the edge of the future light-cone points neither inwards nor outwards. A more intuitive, but somewhat misleading, way of looking at it is to think of the black hole as sucking in space itself. The light at the horizon is propagating outwards, but it's running on a treadmill - it doesn't exit the hole. An observer in the same swatch of space would perceive the photon travelling at C, away from the direction of the singularity. I hope that at least one of these descriptions helps. --Christopher Thomas 22:25, 17 November 2005 (UTC)


 * Ah thanks, I'd forgotten to consider the 'space-as-moving-surface' viewpoint. But as a consequence, isn't the volume inside the black hole expanding FTL to an inside observer? Tzarius 06:18, 18 November 2005 (UTC)


 * My understanding is that the answer to this depends on what coordinate system you choose to use, but that you do at least get more volume inside the hole than you'd expect from Euclidean geometry. --Christopher Thomas 16:47, 18 November 2005 (UTC)

appearance of objects falling towards the event horizon
i do not mean to challenge established ideas of black holes, but it does not make sense that objects approaching the event horizon never seem to pass through the horizon from the view of a distant observer. if that is true, then nothing would ever appear to fall in, and all things near a black hole would appear to remain near it, which doesnt make sense. i understand the immense gravitational forces distort the rate of time, slowing it, however my understanding of GR is very superficial right now, and so i am not real confident that i understand what is going on exactly. perhaps this is a well countered argument and i just have not seen the answer yet. Quietly 04:27, 2005 Jan 3 (UTC)


 * That is exactly how it is. An old name for "black hole" was "frozen star", because a distant observer will continuously receive light from an instant before the star collapsed past the horizon. However, objects "stuck" at the horizon will become dimmer and dimmer as time progresses, because of the distortion of spacetime. (The long story: suppose a distant observer (Alice) is looking at the light emitted by an infalling body (Bob). Bob emits a certain amount of light over a period of one second (on his clock.) However, Alice receives this light over a period of, say, five seconds, because of the distortion of spacetime near the black hole. Later on, Bob has gotten even closer to the black hole. He sends out the same amount of light over one of his seconds. Alice now receives the light over a period of five million years. So it looks to Alice as though Bob is getting fainter and fainter. In fact, for all intents and purposes, Bob will very quickly become completely "black" from Alice's point of view.) -- CYD


 * Furthermore, the light being emitted by Bob will become increasingly red-shifted. Visible light has a wavelength of roughly 500nm (very roughly), but when Bob is at the point where one of his seconds is seen as taking 5 million years from Alice's perspective any visible light he emits will be seen as radio waves with a wavelength of approximately 80,000 kilometers (hope I got that calculation right). This is effectively undetectable. Bryan 08:42, 3 Jan 2005 (UTC)

Hawking reference
With all due respect to this brilliant physicist, aren't there a little bit too much references to him in the article? That no information will be lost and time evolution will still be a unitary operator was always the position of his contrahents on this topic. So at least for this point, references should be made to Susskind and Preskill, not to Hawking admitting defeat. --Pjacobi 18:54, 2005 Feb 1 (UTC)

Negative energy with complex-valued mass: Is it possible?
Question

Details of a black hole's structure are calculated from Albert Einstein's general theory of relativity: a “singularity” of zero volume and infinite density pulls in all matter and energy that comes within an event horizon, defined by the Schwarzschild radius, around it. In 1965, R. Penrose proved the singularity theorem, which says that a singularity must reside inside every imploding star, and therefore every black hole.

In general, a singularity is a point at which an equation, surface, etc., blows up or becomes degenerate.

At the singularity, though, the laws of physics, including General Relativity, break down. Enter the strange world of quantum gravity. In this bizzare realm in which space and time are broken apart, cause and effect cannot be unraveled. Even today, there is no satisfactory theory for what happens at and beyond the singularity. Singularity might also mean that our current physical knowledge is either beautifuly wrong (somewhere) or just incomplete.

This brings me to the questions: How could black holes form if there is singularity? What is exactly meant by infinite density? Is negative energy only possible when the mass is positive (meaning above zero)? What if the mass is also imaginary (or complex-valued)? Orionix 13:59, 8 Feb 2005 (UTC)


 * So, um, that's a nice summary of the history of black holes, but what are you asking, exactly? Is it possible to have a black hole with imaginary mass?  Short answer: no, not in classical GR.  The mass of the black hole is a parameter in the metric, which is strictly real-valued.  Lethe | Talk 08:37, Feb 10, 2005 (UTC)

Sorry, i didn't formulate my question correctly. The problem is the singularity. Classical GR incompletely describes gravity. Inflationary models are becomming the leading candidates now. Inflation also predicts that a characteristic pattern of long-wavelength gravitational waves would have been created in the early universe. These waves are literally gravitons - the hypothetical particles that carry the gravitational force - that have been stretched to macroscopic lengths by the cosmic expansion. The detection of these waves would provide a unique signature of inflation. -- Orionix 00:02, 9 Mar 2005 (UTC)

Proof
Is there any proof, besides X-rays, for the existence of black holes? What does Einstein's theory of Relativity have to do with black holes? And how would gravity affect light? Graham P.(user)


 * Black holes are predicted by general relativity (though there is doubt whether GR correctly describes the geometry of space and time). Basically, it's impossible to understand the structure of black holes without understanding GR theory.


 * Black holes will remain theoretical (at least for now) but there are good reasons to think they exist. The observation and analysis of supernovas and binary stars using radio telescopes is a good indication. Black holes could be the strong sources of gravitational waves (also predicted by general relativity), if these exist. -- Orionix 00:02, 9 Mar 2005 (UTC)

two points not answered in the article
I have two questions that I've been unable to find answers to, and which aren't addressed in the article:

(1) It's stated three times that, as far as the rest of the universe is concerned, it takes "an infinite amount of time for an object to approach the event horizon". How, then, can a black hole ever increase in mass, or even form in the first place, when the universe ends (if it does end) before anything crosses the event horizon? (Sure, the time is finite for the infalling object, but from its point of view the end of the universe is accelerated.) In other words, why aren't "frozen stars" really frozen? This problem is supposed to have been resolved in the 1960s, but I can't find a reference to the solution.


 * The object falls in in finite time. As it moves progressively closer to the "event horizon," however, the photons have a progressively harder time escaping from the vicinity of the black hole. For the observer a long distance away, this has two consequences. First, watching the object fall into the black hole, it becomes progressively redder: the photons have to do more work to get out of the hole, and consequently redshift. Second, the object becomes darker and seems to slow down until it is invisible. That is because the photons at the event horizon ideally take infinitely long to escape the black hole, so the photons emitted near the horizon take a very long time to escape the black hole. --Joke137 03:11, 20 Mar 2005 (UTC)


 * Sorry, I still don't get it. If what you say is true, then the article's three-time claim that it takes "an infinite amount of time for an object to approach the event horizon" is false. I thought time dilation was involved here, not just red shift, and that the dilation was infinite as stated in the article. If so, and the universe is finite, then a traveler would see the end of the universe before crossing the event horizon, and we would never see a black hole increase in mass (unless of course you consider the mass just outside the event horizon to be part of the black hole). If, on the other hand, the traveler does cross over in finite time per an outside observer, what is the time dilation factor at the event horizon? If time doesn't "freeze", how much it slows down? kwami 21:41, 22 Mar 2005 (UTC)


 * The article (at least since I got my hands on it) does not make such a claim. It takes an infinite amount of time to observe an object falling into the black hole. An object experiences falling in in only a finite time. Red shift and time dilation, in this case, are the same concept. Do we ever see the black hole increase in mass? No. Do we ever see the black hole? No. It's all part of the slippery notion of simultaneity in relativity. All an outside observer can tell is that there is a large amount of mass concentrated in a very small area.


 * Think of escape velocity from Earth. If I am falling towards the Earth's surface from space, and every second I launch a tennis ball at 11 km/s (escape velocity) away from the earth, then a distant observer would see (i) the frequency of the tennis balls arriving decrease (this corresponds to the light getting dimmer) and (ii) the velocity of the tennis balls when they arrive decrease (this corresponds to the photons redshifting and becoming less energetic). The very last tennis ball, launched at the Earth's surface, would take an infinite amount of time to reach the observer (because launching an object at 11km/s means it has EXACTLY enough energy to escape, and will have no kinetic energy left over when it is outside Earth's gravity). Now light, of course, always travels at the speed of light, but the effect, due to time-dilation, is similar. I hope this helps. --Joke137 23:43, 22 Mar 2005 (UTC)


 * Okay, that makes more sense, but what you seem to be saying at first is that nothing falls into a black hole during the lifetime of the universe. So the question of what happens to an object crossing an event horizon is purely speculative, because the universe that contains it ends before it ever gets there. But this recent edit is still weird: "it is a property of the light leaving from the vicinity of the hole that makes it seem as though the object never actually reaches the horizon." That makes it sound like it's some kind of optical illusion. Or is it that an intense gravitational field slows the speed of light, so that the object does cross the event horizon in finite time in the time frame of an external observer, but the light conveying that info is infinitely delayed? I just can't see that happening without the object itself being infinitely delayed per the external time frame as well.


 * Thanks for taking the time to go over all this, by the way! kwami 01:17, 23 Mar 2005 (UTC)


 * This all has to do with the slippery notion of simultaneity in general relativity. If I fell into a black hole that was big enough, I wouldn't even notice what was happening. But clocks near me would be going much slower than the clocks far away from the black hole, so it would seem to take much longer to an outside observer and the light (even though, according to local clocks, it is always going the speed of light) is incredibly redshifted and delayed.


 * The thing is, this formalism of clocks and light signals is the only way we have of understanding chronology in general relativity. If point A can send a signal to to point B, then you can say A is in the past of B (or B is in the future of A). If A is neither in the past of B nor B in the past of A, then it is not a sensible question to ask, whether A and B can communicate. So if Alice falls into a black hole, she'll no longer be able to send signals to Bob who stays outside the black hole. But Alice falling into the black hole is not in Bob's future, either (meaning, if Bob wanted to send Alice some last words of advice before crossing the event horizon, he couldn't).


 * If this is puzzling, don't worry. Whole monographs have been written about it, and it is related to all kinds of crazy ideas in physics, like the holographic principle. Hawking's new ideas about the black hole information paradox arose from it, so it's clear that physicists don't have it totally mastered either. --Joke137 18:07, 23 Mar 2005 (UTC)

(2) It's stated that "the collapse cannot be stopped by any physical force", or, equivalently, "all particles within the event horizon must move inexorably towards" the singularity. How do we know that? Is this based on the assumption that there's nothing denser than nucleonic (or quark) matter? That is, is the expectation of a singularity based on the Standard Model, which we know to be flawed? If, instead, compressing matter went beyond nucleons or quarks to forming ever more massive elementary particles, so that no matter how massive the black hole, there was a form of matter dense enough to prevent further collapse, would we avoid a singularity? (Collapse far enough, and there'd still be an event horizon, so the external effect would be quite similar.) Of course, this is pure speculation, being beyond the reach of our experiments, but are we merely assuming singularities form based on our current level of ignorance?


 * It has nothing to do with the standard model. It is a consequence of the so-called conformal structure of spacetime (Penrose diagram is unfortunately a stub). In Hawking's original work, the idea is this: if you have a sphere and the sphere suddenly "flashes" and light signals travel outwards from it, the signals travel outwards, on a sphere of ever increasing radius. On the other hand, when you shine light inwards, the light focuses at the center of the sphere. But if you put such a sphere inside the event horizon of a black hole, both the outward and inward moving flashes contract and eventually focus at the singularity: all paths lead there.


 * We are indeed assuming singularities form based on our current level of ignorance, and some people, like string theorists thing some effects will regulate the singularities and allow us to understand them. Another hope is that there are no naked singularities in nature (every singularity is behind an event horizon) which would mean the question is largely academic, as there would then be no physical way to probe a singularity.


 * It is useful to think of a black hole as an event horizon, though, and not as a singularity. The important thing is that general relativity predicts event horizons, from which nothing ever exits, and behind which we cannot probe. --Joke137 03:11, 20 Mar 2005 (UTC)

Thanks to anyone who can explain this to a poor slob like me, or can add it to the article! --kwami 02:40, 20 Mar 2005 (UTC)

I can just add an other puzzling question:

If you observe a target, falling into a BH, you will see it more and more red-shifted until the end of time/universe. Heisenbergs uncertainty principle tells me, that the location of the target will be more and more unknown. BH-theory tells me that the target is aproaching the event horizon (in infinite observers time but in finite target time) and his mass adds to the that of the BH. If the wavelength redshift exeeds the diameter of the event horizon EH, you can ask, "there is Schrödinger´s cat?", inside the EH? outside? far outside? If you can´t observe this "dark matter" with photons, will it still be possible to observe its gravitational contribution to the BH with more precision or does the uncertainty principle also effect the gravitationaly measured location? I remember a talk about infrared observations of the cluster surrounding Sgr A which showed excellent agreement with a mass concentration in a singularity and no deviation, which could support that Heisenbergs uncertainty principle also effect gravitation. But if the uncertainty principle would be valid for gravitation too, it would be a fine explanation for the nonbaryonic dark matter distribution observed in galaxies I think. --Swen 12. Juli 2005


 * I'll take a stab at some of this. First, the wavelength of the radiation emitted from the infalling object has little relation to the object's wavefunction's wavelength. Consider your cell phone; it emits microwaves with a wavelength of many centimetres, but you know where it is more precisely than that. The wavelength of the infalling object as measured (somehow) by a distant observer would appear to increase as it gained speed and mass (not sure what happens when it's close to the horizon, but it'll definitely still look very massive). Secondly, that close to the horizon, you'll also lose the ability to directly measure its wavelength (or anything else about it), as what you're seeing is closer to being an image of the object than the object itself. If you send a radar pulse in towards the hole, it won't bounce off the object, because by the time it arrives, the object is long gone. What you're seeing is light emitted that took quite a while to climb back out of the hole. Thirdly, if I remember correctly, what an external observer would see if they could observe the infalling object for a long time (which they can't, as noted by Joke137) would be the object wrapping itself around the entire surface of the hole. Once it appears to do this, gravitational measurements won't help you tell where the object fell from. You'd have to get Joke137 or another physicist to sanity-check this, though, as I may be either misunderstanding or misremembering the source I got that from. All of this only applies from the point of view of a distant observer; from the infalling object's point of view, it stays in one piece (subject to strong tidal forces) and hits the singularity very quickly. The odd effects that appear at the horizon are mainly artifacts of the coordinate systems you choose when analyzing what happens there (as described in black hole and in some of the reference links). --Christopher Thomas 16:21, 12 July 2005 (UTC)
 * Er, that should have read "The wavelength of the infalling object as measured (somehow) by a distant observer would appear to decrease as it gained speed and mass", but I'm sure you all already figured that out... --Christopher Thomas 15:27, 13 July 2005 (UTC)

I think the short answer is that for the object falling into the black hole (at least a big black hole), nothing seems unusual and there is no redshift. The redshift is occurring for the observer at infinity, because light has to climb out of a deep, deep well to get out to infinity. So as Christopher mentioned, it is not the location of the object that is necessarily becoming uncertain: in fact, as the object accelerates towards the black hole, it is picking up momentum, so its de Broglie wavelength is actually decreasing. At another level, though, this is related to the question: if you can never actually see that something has fallen into a black hole, how do you know it has happened? That's potentially a deep question with implications for quantum mechanics and gravity. I don't know the answer, but I sure wish I did. (See my comments above, also.)

The uncertainty principle should hold for gravity. One of the biggest problems in making a quantum mechanical theory of gravity is implementing the uncertainty principle for the microscopic "fabric" of space: the uncertainty principle suggests that at smaller and smaller scales, space becomes less and less like normal Euclidean space, or anything smooth, whereas general relativity in founded on the principle that there exist local inertial frames that look, at least on small scales, like Euclidean (rather, Minkowski) space. You can't just decide that the uncertainty principle and quantum mechanics don't hold for gravity, because uncertainties in the theory for matter must feed into uncertainties about the geometry of space, in order to have a consistent theory.

However, it is largely a moot point, because it has few direct implications. However, if you could measure the position of anything, using gravitation, to within the quantum mechanical limit, I'm pretty sure there would be a Nobel prize with your name on it. Another problem is Birkhoff's theorem (relativity), which states, slightly rephrased, that you can't tell how large a spherical mass distribution is from the gravitational field around it. In Newtonian gravity, this is just the law that a thin spherical shell has the same external gravitational field as a point mass. I don't know about the infrared measurements you're talking about. In fact, I don't really know much about how astronomers find black holes, although I wish I did. –Joke137 23:36, 12 July 2005 (UTC)


 * Good answers, really, but as I understand it, Birkhoff's theorem requires that you _know_ that the mass distribution is _inside_ the sphere there you measure the gravitational force. If you have a probability function for the mass distribution which extends beyond that sphere, I think the measured mass should be lower, still beeing consistent with Birkhoff´s theorem. One the other hand, the infalling observer will see something strange, then he approaches the EH. The outside universe will collapse in angular dimensions until he will see it as a single point in backward direction. I tried to find out what happens to the radial and time dimension which means "does the infalling observer see the _end_ of the universe?", but could not solve that question, neither in Schwarzschild- nor in Finkelstein-metric. If the radial dimension collapses too - like the angular dimensions - the infalling observer would see a collapsing universe (blue-shifted), even if it is ever expanding for the outside observer. A collapsed radial dimension means the infalling observer is at the end of time _anythere_ in the universe, then he reaches the EH in finite proper time. Swen 6:58, 13. Juli 2005 (MEST)
 * $$d\Omega^2$$ varies with $$r^2$$ in Schwarzschild metric, which means, its collapses at the singularity and not at the EH! Swen 08:58, 1 August 2005 (UTC)
 * There's an excellent page that describes the visual effects near and within a black hole. Google for "falling into a black hole" "movie" to find it. What appeared to happen there was that while the image of the outside universe shrinks, it only reaches zero apparent size when you reach the singularity. I'm afraid I don't recall which metric they used for the analysis. The page is also accessible via one of the resource pages under black hole, but I'm afraid I don't remember which one linked to it, so Google is probably the fastest search approach. --Christopher Thomas 15:24, 13 July 2005 (UTC)


 * Andrew Hamiltons movie used Schwarzschild metric and stopps bevor reaching the EH. (See also the virtual trips from Robert Nemiroff). My statement is based on a picture from Misner, Thorne, Wheeler: "Gravitation" (which I don´t have at hand unfortunately) and a seminar of Börner (MPIA München) on cosmology in 1992. If I remember that right, zero apparent size of the universe is reached at the EH for the infalling observer. Swen 8:25 14. Juli 2005 (MEST)
 * Click on the "singularity" link on the same page. There are several sets of movies, each intended to illustrate a different type of effect. --Christopher Thomas 06:45, 14 July 2005 (UTC)

Internal Link
"At that time, the Newtonian theory of gravity and the concept of escape velocity were well known."

Instead of having a link to the concept of gravity, would it be better if the words "theory of gravity" linked to the Law of universal gravitation (since the article is talking about a specific theory)? &#1090;&#601; z&#1108; &#1090;&#1110; 17:53, Mar 24, 2005 (UTC)

Black holes don't exist
http://news.google.ca/news?gl=ca&ned=ca&hl=en&ie=UTF-8&q=Black+holes+don%27t+exist&btnG=Search+News


 * Don't believe everything you read.--Joke137 14:26, 4 Apr 2005 (UTC)


 * I read one of the original papers. It seemed that the author was arguing from metaphor: He made an approachable metaphor for black holes, and then seemed to argue that whatever held true for the metaphor held true for black holes as well. I forwarded it to a physicist friend of mine, who it turns out had received another paper on the same topic from the same conference. He had this to say: "Both papers are mainly techno-gibberish with a lot of buzz words strung together without substance." --kwami 23:46, 7 Apr 2005 (UTC)

Black holes and stellar evolution
Can this article shed some light on the future of the universe and black holes? It seems to mainly concentrate on how black holes form and their effects, but not the consequences in the far flung future. Consequences for future generation of stars, coupled with Hawking radiation, etc. or galactic configurations, or maybe stuff like spawning new galaxies? The article sheds very little light on this. Maybe mention its significance to stellar evolution? Because black holes can cause stars to form as well as stars forming them, as well, right? -- Natalinasmpf 14:49, 16 Apr 2005 (UTC)


 * It seems unlikely. If they emit Hawking radiation, then they let tiny amounts of radiation into space until they evaporate completely (the total radiation emitted would correspond to the total mass & energy taken in). If they don't, then they accummulate mass until there's none left inside their Hubble distance. What happens to stuff between the event horizon and the singularity is undecided (and by definition, unknowable).
 * Ah, I just realized you mean they can act as a catalyst in nebulae and such. Yes they would affect the development of stars, but they do not create them. Tzarius 11:44, 17 November 2005 (UTC)

Request for references
Hi, I am working to encourage implementation of the goals of the Verifiability policy. Part of that is to make sure articles cite their sources. This is particularly important for featured articles, since they are a prominent part of Wikipedia. Further reading is not the same thing as proper references. Further reading could list works about the topic that were not ever consulted by the page authors. If some of the works listed in the further reading section were used to add or check material in the article, please list them in a references section instead. The Fact and Reference Check Project has more information. Thank you, and please leave me a message when you have added a few references to the article. - Taxman 17:29, Apr 22, 2005 (UTC)

V-404 Cygni
Shouldn't we put something in here about V-404 Cygni?

Black hole question (FTL)
Not aiming for a transfer to BJAODN, but what would happen if a Tardis went into a black hole? Could it escape?


 * Mu. Your question cannot be answered because the Tardis doesn't have well-defined capabilities - it's completely fictional and doesn't even behave consistently within the show. However, see The Three Doctors - in that episode the Tardis does go inside a black hole and later departs it, though whether this is in any way relevant to "real" black holes is questionable. :) Bryan 22:11, 12 Jun 2005 (UTC)

I know the Tardis is fictional (g). To rephrase the question slightly:

Could a spaceship (for want of a better term) capable of traveling through time in a non-linear manner (as the Tardis does) use that technology to escape from the black hole?


 * Sure. A time-reversed black hole (i.e. a black hole seen going backwards in time) is a white hole. The problem is moving backwards in time in the first place, which seems impossible. –Joke137 14:08, 13 Jun 2005 (UTC)

If anyone cares to develop a "real science for space fiction writers" book this could be included. I was using the term Tardis as shorthand.


 * Ah, any time machine would do? Well, in general relativity, travelling faster than the speed of light is equivalent to travelling back in time, so I suspect that yes, having this ability might enable one to get out of a black hole. It's a complicated issue, though, since it's not entirely clear yet whether it's possible for anything to go faster than the speed of light. Bryan 23:57, 13 Jun 2005 (UTC)
 * I don't think it's true that travelling faster than light gets you time travel in GR -Lethe | Talk 01:06, Jun 15, 2005 (UTC)


 * Faster than light indicates otherwise, and I know I've come across detailed explanations of why elswhere before, but unfortunately I don't know where one is offhand and I don't understand it well enough myself to start an article about it. Bryan 23:13, 15 Jun 2005 (UTC)


 * Under special relativity, spacetime is assumed to be flat, and "c here" is assumed to be the same as "c everywhere else". SR also declares that a light signal takes the same amount of time to cross from A to B as from B to A. So if you use a wormhole to jump to Mars, spend ten minutes sightseeing, and then jump back, and are reassured to find that your watch still agrees with earth time when you return, SR will nevertheless insist that you time travelled, because the "official" time of your arrival is the time that you are seen to arrive, through an earth telescope, using conventional light-signals, which will be some time after you already got back. In this example it might be sensible to suggest that the "time travel" part is just an optical artifact, an illusion ... but by assuming simple flat spacetime, SR rejects that idea and says that what you "observe" is what reality actually is.
 * In curved spacetime models, the rules about lightspeed constancy are more relaxed - lightspeed can vary across a region in lots of ways, as long as the measurable //local// speed of light isn't affected. If I see you apparently ageing at half my rate, with light in your region apparently moving at 0.5c, I just shrug, say that you must be in a more intense gravitational field than me, and change the nominal shape of the region's map until lightspeeds come out nominally okay again. So, if a spaceship somehow modified the local velocity of light around it away from nominal background c in a substantial way, it could then (in theory) move faster than background c without moving faster than its own local c. This is the basis of the warp drive idea. Modifying lightspeeds is ruled out under SR, and partly possible under GR. But since current GR is based on SR, it inherits a lot of SR conventions and relationships, making its attitude to some of these problems a little bit confusing. For instance - GR is generally supposed to forbid FTL, but if you drift into a black hole, GR also says that perhaps you normally ought to expect to be falling inward at more than a distant outsider's speed of light once you've fallen past the horizon, and be moving arbitrarily fast as you approach the centre. This feature isn't very useful for wizzy space travel, because the interior of a black hole is not a fun destination, but it does illustrate that perhaps certain "proofs" of the impossibility of FTL are not particularly realistic in their simpliying assumptions. Often the "small print" in these proofs assumes the absence of gravitational gradients, or that we are talking about a warpfield that creates bidirectional or omnidirectional increases in lightspeed, or imposes other limitations that aren't all that sensible in context. You don't need exotic matter to produce a gravitational gradient.
 * As with the case of black holes and Hawking radiation, QM also seems to suggest that some behaviour that is forbidden under SR&GR may still be possible in real life - consider the notion of information "tunnelling" through a classical SR lightspeed barrier - but SR-based theory can't offer a classical explanation of how they work, and will still tend to insist that these behaviours are physically impossible. ErkDemon 7 July 2005 03:31 (UTC)
 * I'd phrase comments about apparent changes in the speed of light differently. The best qualitative description I've heard was to describe it as light travelling at C, but the patch of space it's in appearing to move relative to the patch of space you're in. Among other things, this better illustrates how matter in the same vicinity seems to move, and why you can't climb out of a black hole (or fire a light beam out of it) (space within the hole is moving FTL relative to a distant observer, taking the light-cones of internal events with it). Other descriptions of course exist, and yours isn't incorrect; I just feel that it gives a misleading impression about the constancy of C to untrained readers. --Christopher Thomas 7 July 2005 04:41 (UTC)
 * No worries. :) I think some SR-oriented people might prefer to talk about "dragged coordinate systems" rather than "flowing space", but whatever. ErkDemon 7 July 2005 13:24 (UTC)
 * With regards to the original question, if you're using the "flowing space" mental model of the effects of gravity, it's straightforward to explain how time travel or an FTL drive would let you escape a black hole. Both allow you to move outside the past and future light-cones of the event corresponding to your starting point, and given this, moving to a swatch of space that's moving FTL relative to your starting point, in any given reference frame, is doable. --Christopher Thomas 7 July 2005 04:47 (UTC)
 * Yes-ish. Again, that's a very nice description that lets one visualise how ultrafast travel and information escape through a horizon probably /should/ work (and personally I do approve of the idea that physics /would/ work that way), but although these sorts of more "acoustic" metrics are very seductive and have been studied (e.g. Visser, gr-qc/9712010 [], I don't think that anyone has gotten them to work in the context of SR-based theory (like current GR). SR always seems to get in the way to spoil things. Hawking now seems to be trying to push GR towards more "acoustic"-looking behaviour, (see: his announcement in 2004, ->fluctuating horizons), but I don't personally see how he can finish it and get this sort of approach to work without discarding SR and reinventing GR to work as a freestanding model that applies curved spacetime princples "all the way down". IMO that sort of "GR mk2" would be a big improvement on what we have now, but a lot of researchers would not be happy to see SR downgraded in that way. Perhaps Hawking has enough nerve and a big enough name to be able to pull it off. Classical Hawking radiation has been an interesting topic for some time now, but an SR-based general theory doesn't seem to allow it. ErkDemon 7 July 2005 13:24 (UTC)

Perhaps the Wikipedia research project could include an area for such questions - useful (for SF writers and others).

I know that this topic sits between several stools (to quote someone I knew), but ftl spaceships appear to travel in approximately real time (ie time in ftl travel is roughly the same as time at the ordinary rate of things: a Tardis/Time machine however travels through time in a non-linear manner. (HG Wells' machine appears to operate differently to the Tardis, though. He does make an appearance in one of the Dr Who episodes - Timelash.

Unitarity
Aren't arguments for unitarity a bit older than the Hawking announcement? Not wanting downplay Hawking's achievements, but sometimes a "Hawking said this", "Hawking said that", sort of style is overused. --Pjacobi 18:34, July 19, 2005 (UTC)


 * I absolutely agree. I just moved the paragraph about Hawking down when I added the section about unitarity. I'm not sure who first pointed out the problem, or any of the history. In any case, the paper just came out today, a year after the conference for which his acheivements were plastered over every newspaper in creation. –Joke137 18:41, 19 July 2005 (UTC)

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

Black Hole in Picture
Hi, it seems that in this picture Cygnus Loop Supernova Blast Wave there is a black hole visible (Small black circle on blue band). helohe 23:18, 29 July 2005 (UTC)


 * Given how noisy that picture is, the black dot seems to me more likely an sensor or processing artifact. &mdash; HorsePunchKid &rarr; &#x9F9C; 00:30, July 30, 2005 (UTC)