Talk:Rotating black hole/Archive 1

stationary with angular momentum?

Peanut gallery here. Stationary BH needs to be explained/defined. What does it mean? -- non- orbiting? No discussion or references to non-stationary bh? Why not? embedded in flat space time? What? —Preceding unsigned comment added by 69.40.245.3 (talk) 22:58, 23 August 2008 (UTC)

Two event horizons

I think you misread it. The outer one moves in, the inner moves out. The move toward each other.

Two event horizons

The section "Two event horizons" is contradicting itself. As the spin increases, do they move father away from each other or towards each other? Also, it should probably state what the spin speed is where the even horizons merge, because clearly one of the two needs to change its direction at some critical spin speed. — Timwi 11:19, 21 Feb 2005 (UTC)

Ergosphere is not a horizon - a test particle can escape from there. This should be spelled out better in the article.

What I want to be explained is a transition of "Cauchy horizon". *That one* seems to be more like a "real" second horizon. As I understand it now, outer horizon is a lot like Schwarzschild hole's horizon (+ some rotation), but "Cauchy horizon" sits inside and it is basically a radius at which rotation is so strong that it actually allows you to STOP FALLING and enter an funny "orbit" where as you fly, you do not only return to the same place (just like the satellite of Earth) but to the same TIME too.

Can someone improve the picture in the article (one with ergospere and event horizon) and add Cauchy horizon? How big is it? Is it spherical or oblate or what? 89.103.91.47 (talk) 15:47, 29 June 2008 (UTC)

Question

Which event horizon is the ergosphere located outside of? тəzєті 18:07, May 14, 2005 (UTC)

I have read in other sources that the outer event horizon remains stationary while the inner one moves out. Which on is it?

Explanations needed

This article needs to be expanded quite a bit. Some questions that I as a layman with no real physics background do not understand:

• Why are there two event horizons?
• What is a photon sphere?
• What distinguishes the two event horizons?
• If I can only move closer to the singularity while inside the outer event horizon, why can I move away from it again once I pass the inner event horizon?
• Why would I (possibly) emerge in a different universe/part of the universe after passing the inner event horizon and then leaving it again?
• Why would I (no matter which universe/part of the universe I end up in) be able to leave the black hole's outer event horizon after using it for "wormhole travelling"? I.e., why would the black hole actually be *useful*?
• What does the image "Space-Time Diagram for a Spinning Black Hole" (Image:Space-Time Diagram for a Spinning Black Hole.gif) mean?

The image in particular is bugging me a lot - it's wholly unclear what is being displayed, I can't make sense of the labels (particularly "Our Universe" etc.), and there is no explanation of what it shows. -- Schnee (cheeks clone) 21:05, 18 August 2005 (UTC)

I don't think there are two event horizons, scince a Kerr Black Hole spins it has an ergosphere or ergoregion. Once an object is at the ergosphere it can't remain at rest whithout falling into the black hole.--Alex Arnold 16:51, 20 February 2006 (UTC)

If I've understood correctly, the static limit (which surrounds the ergosphere) is sometimes referred to as the "outer event horizon". I could be wrong, but if I'm not, I can't see how it is an event horizon according to definition. lotheac 21:57, 24 March 2006 (UTC)

I don't have a professional understanding of physics as I don't have a degree to back me up, but what I've derived rom this article and background knowledge is this:

• Two event horizons are made because the spinning black hole's singularity has a centrifugal force that turns it into a ring. The outer event horizon is where no contact with the outside universe would be made because the escape velocity is the speed of light, which cannot be exceeded. The inner event horizon is more mysterious: it is the area in which an incoming object would have no collision course with the singularity and therefore go through the ring-shaped singularity. As this is a place with infinite gravity, surrounded by a region of infinite density, though no friction is provided, space-time warps to such an extreme that an object passing through the ring would sail to another universe with the space-time deformity and near-lightspeed momentum. The inner horizon is the point of no return to any part of the known universe, including inside the black hole.
• I am not sure what a photon sphere is, but it might be in effect the same as the event horizon, though defined as the point in which light cannot escape a singularity's gravity. Another possibility is that light, inside an event horizon, is collected in great multituded by a black hole and the light that maintains an "orbit" around the singularity, though concealed by the event horizon, and makes a sphere of solid light. A photon sphere is a region that light orbits a black hole without falling in. Entering one would probably hurt your eyes.
• The outer event horizon is the point of no return to out of the outer event horizon, the inner event horizon is the one-way ticket to another universe, as described in my first response.
• The inner event horizon is where an object may pass through the ring instead of smashing into the infinitesimally small and dense singularity. Because of the lack of friction, an object could hypothetically continue its motion, only now away from the singularity, similar to how a roller coaster goes up a hill after a descent, as its momentum carries it forward despite gravity.
• The fabric of space-time is warped infinitely due to infinite density of the black hole, and its rotating, and therefore ring-shaped, singularity has a center of gravity not on the singularity, but right in the middle. At that point, where merging with the singularity isn't a consequence of presence but the gravitational force is as a zero-dimensional singularity, the universe may branch off into another dimension. On that matter, there is a bunch of slippery issues that are unsolved without a working quantum-gravity theory.
• If there is no object to slow the descent into the black hole, and the travelling object is not intercepted by the singularity, but passes through the ring, the momentum will carry the object out of the event horizon just as gravity brought it into it. Think of the roller coaster again, only without friction. A roller coaster, descending down a track with no friction, will manage climbing a hill of track with equal altitude as the original peak.
• The image is a crude diagram, (using squares?) which illustrates in a not-so-helpful detail which universe might be the destination upon entering the inner event horizon. (Depending on the angle relative to the plane of the ring-shaped singularity, I guess)

I am not sure of my speculations, but it is what I have derived from the article through a tad of research (equally unclear) and too much thinking for one day. If I am completely wrong, someone who knows what is completely right should define that in the article to avoid further confusion. C3PO the Dragon Slayer 00:49, 5 November 2006 (UTC)

question 2

if a massive donet shaped disk would rotate. Then from a distance it's centre could still have a mass of a blackhole. While in theory the disk mass would not have to be at a collapsing state. The ring can grow in mass and in size without a need of a colapse. And mass in centre me teared appart, while some mass will be jetsioned out like plume. It could have also have properties like magnetism (as seen in a MEC). Such theoretical rings of dense mass how are they called, and is there research in this direction as an alternative to blackholes ?

Why is Roy Kerr not given top billing?

Since these are Kerr blackholes is there any particular reason the Penrose process is given before explaining who Kerr and Newman are? Sophia 12:46, 5 September 2006 (UTC)

Nordström, not Nordstrøm

Gunnar Nordström was a Finn with a Swedish name, and thus his name had the letter ö, which is used in Finnish and Swedish. Ø is incorrect, because this particular letter is used only in Danish and Norwegian.195.16.202.19 10:26, 30 November 2006 (UTC)

Can we finally get rid of this "two event horizons" BS?

Apparently a few people don't understand what an event horizon is, in the context of a black hole. It's the surface at which, and within which, light cannot escape to future null infinity. It is conceptually nonsensical to talk about more than one per object, just as it's conceptually nonsensical to talk about two people of different heights that are both "the tallest person in the world". By definition, there can only be one such surface surrounding any given object. If there were an "inner" one of some sort, it would fail the criteria, and be a non-horizon of any fucking sort.

I think it's time this shit got deleted, instead of provoking more confused brow-furrowing from the peanut gallery, to be answered by people who took physics classes off the side of a fucking Crackerjack box.

--76.209.50.222 08:58, 20 February 2007 (UTC)

Your frustration I understand but your language needs to be toned down! I agree that the definition of an event horizon is nonsensical if there is more than one but I'm not up to date enough with this field to say for sure that there isn't some bizarre definition somewhere. If you are the most up to date person here then be bold and get rid of it. It's certainly the mainstream view and any editor who wants to keep the "2 horizons" version will have to explain why. Sophia 12:32, 20 February 2007 (UTC)

Actually, I see the sense of two or more horizons. An event horizon encloses a region which nothing can leave. If Kerr hole has two horizons, it just means that once you crossed outer horizon, you cannot leave the hole, and when you cross inner horizon, you cannot leave even inner horizon's enclosed area (and go into outer horizon's enclosed area). —Preceding unsigned comment added by 89.103.91.47 (talk) 15:39, 29 June 2008 (UTC)

I can't explain it. But I'm pretty sure I saw it on a legitamate scientific website. Might have been NASA's... I don't really have a stance here. Just mentioning it Kaloo (talk) 19:44, 14 May 2008 (UTC)

Two Horizons?

The "Two Horizons" section doesn't make sense. It also contradicts the previous section which implies that there is a maximum rotation rate of a BH. Also, the maximum rotation rate needs to be discussed directly. If I recall, it essentially means that the equator at the event horizon cannot move faster than light. There is a maximum % of energy that can be in the rotation vs the mass. —Długosz

Proposed changes

Unless I get feedback, I'm going to make some changes soon (Say by 3 November 2007). I'm not sure when and why this was split from Kerr metric, but I'll respect the decision. I believe most of this page should be moved back to the Kerr page. It is generally believed that there should be an enormous class of not explicitely known rotating black hole solutions which are neither Kerr or Kerr-Newman. The ergosphere and Penrose process should be moved to the Kerr page (with a link from the Kerr-Newman page), since it's not clear that a general rotating black hole will have such structures, whereas Kerr and Kerr-Newman do. The "two event horizons" section is a mess here and clearly (and correctly!) explained on the Kerr page, so it should be cut. The Kerr and Kerr-Newman section should stay. "Kerr black holes as wormholes" should be cleaned and then moved to the Kerr page. The "see Also" and "references" look OK, but Wald's book and the one by Hawking and Ellis should probably be added. Any suggestions before I do this? 91.37.231.178 22:44, 29 October 2007 (UTC)

You sound like you know what you are doing and we so badly lack knowledgeable editors in these areas. If it make more sense then please don't respect the split as it was likely done by someone who had just watched "The sky at night" or something! I only know a little of this area, having had the pleasure of taking Prof Kerr to dinner when he came to give a talk to our Astrosoc at Uni. As they say - "be bold" and thanks for the work you will be putting in. Sophia 06:45, 24 August 2008 (UTC)

I am also a physicist. 91.37.231.178 is absolutely right that this page has serious errors. If only he had completed his work. Nvf (talk) 23:23, 13 January 2011 (UTC)

frames of reference

The article doesn't mention frames of reference at all. If you are rotating at the same speed (angular velocity) as a black hole wouldn't you 'see' a non-rotating blackhole? —Preceding unsigned comment added by 77.99.150.12 (talk) 07:56, 9 December 2008 (UTC)

A rotating reference frame is non-inertial in every sense, and angular momentum measurements taken in a rotating reference frame must take this into account. Angular momentum measurements taken from a rotating reference frame are invalid unless the angular velocity of the frame is subtracted. Iameditingstuff (talk) 05:33, 1 May 2011 (UTC)

Deletion Nomination

It seems like this article should be deleted. It doesn't have any properly sourced information that isn't contained in any of the other black hole related articles (especially the "black hole" article, "Kerr metric" article, and "Kerr–Newman metric" article). It is also very poorly written... it doesn't seem to have been given any attention by somebody who has actually taken General Relativity in university. —Preceding unsigned comment added by 74.14.156.248 (talk) 04:54, 29 April 2011 (UTC)

This article serves as a simple introduction and a disambiguation page. From the chart one can choose which specific metric one wants to look at. People who do not already know the names of the specific metrics should appreciate this article for that reason. JRSpriggs (talk) 09:10, 29 April 2011 (UTC)