Talk:Black hole/Archive 2

message: I reverted an edit that said this:
:Some physicists, including Stephen Hawking, have recently concluded that black holes could act as natural supercomputers of incredible speed and power.
Is this true? I need someone to verify. — Ambush Commander(Talk) 15:44, August 16, 2005 (UTC)
::Sounds like high quality BS to me!--Light current 02:25, 30 September 2005 (UTC)
replies: text: It's a partly-correct statement, if I understand correctly. The actual idea came up in a number of "limits to computation" articles, with the idea being that a black hole's size was exactly small enough that if it was performing calculations at the maximum rate allowed by its contained rest energy, signals would be able to propagate completely across the hole between computations, making it the ultimate serial computer. Systems that weren't as compact (i.e., that weren't black holes) would have to have some degree of parallel computation if performing at their peak theoretical capacity, which limits the types of problems that you can solve. To the best of my knowledge, nobody's actually found a plausible way to make black holes perform useful computation. --Christopher Thomas 21:13, 25 November 2005 (UTC)
thread_title:  Black holes as supercomputers 

message: In a recent Horizon documentary the entropy formula of black holes was given, but their version did not feature the Boltzmann constant k. What is correct? ThomasWinwood 07:58, 16 September 2005 (UTC)
replies: text: In Planck units, the formula is A/4. In the article, it is not written in any particular system of units, hence all the constants (which go to one in Planck units). –Joke137 14:51, 16 September 2005 (UTC)
thread_title:  Horizon confusion 

message: * http://www.newscientistspace.com/article.ns?id=dn8025
* http://www.theregister.co.uk/2005/09/21/hot_stars/
* detail at http://www.universetoday.com/am/publish/blue_halo_stars_blackhole.html?2092005
This needs added to the Have we found them? section.
:Please note that there is nothing radically new here. Other black holes were proven to exist in the past -- this is just the latest of many, but it happens to have got a lot of publicity because the NASA Hubble public relations department have made a big deal (in order to help save Hubble). In particular, astronomers have been watching stars zipping close to the last stable orbit around the black hole in the centre of the Milky Way for two decades. Hubble doesn't have the resolution or wavelength coverage to look at that, so it doesn't get the benefit of the NASA Hubble public relations department ;-) Rnt20 14:39, 22 September 2005 (UTC)
thread_title:  Hubble measures first proven black hole in Andromeda 

message: I think Rnt20's edit about falling into a black hole is incorrect or at least incorrectly phrased. The three space-like directions do not become time-like, as general relativity is defined on a metric of a given signature (e.g. - + + +). That doesn't even make sense. What does happen is that in the usual Schwarzschild coordinates, the radial and time-like directions exchange. This is called a coordinate singularity, and has nothing to do with the physical situation being described. The Eddington-Finkelstein coordinates and Kruskal coordinates do not have this kind of behavior.
:In Schwarzchild coordinates the radial and time components of the metric change sign once past the event horizon, this implies that you cannot avoid moving in a direction of decreasing r no more than you could avoid moving in a direction of increasing t outside the black hole. r is now a timelike direction, and t is spacelike. Obviously nothing happens to the θ or φ directions. --Jpowell 21:42, 28 February 2006 (UTC)
replies: text: Ok that does sound better. My memory of General Relativity lectures is a bit hazy, I have to say. Rnt20 14:18, 23 September 2005 (UTC)
thread_title:  Falling into a black hole 

message: How can it be that a black hole is an infinite energy/mass sink? At some point the mass/energy must escape. Where does it go to?--Light current 00:48, 29 September 2005 (UTC)
replies: text: I'm not an expert in the area, but you might want to read about Hawking radiation. The idea there is that the black hole very, very slowly evaporates due to quantum effects. —HorsePunchKid→龜 02:12, 29 September 2005 (UTC)

text: Yes, until there's no more stuff outside the black holes.  Then they'll radiate themselves out of existance, but it takes something like 10^{50} years--at that point, the universe will be uniformly-distributed heat, entropy wins, and that's that.  So, what's wrong with that?  I admit it's a bummer, but other than that... -- SCZenz 11:09, 29 September 2005 (UTC)

text: It's hard to tell them apart sometimes. ;) -- SCZenz 08:37, 30 September 2005 (UTC)

text: Any charged object, including a black hole, that undergoes acceleration emits photons as a result of the interaction causing the acceleration. This is due the disturbance the object's charge makes in the field of virtual photons that carries the electromagnetic force. The energy comes from whatever potential energy source is being tapped to allow the acceleration. Two co-orbiting charged black holes would have their orbits degrade as a result of this emission, for instance. Even two non-charged co-orbiting black holes will emit orbital energy as gravity waves. Accelerating charges to produce photons just provides an additional loss mechanism in this context. --Christopher Thomas 18:39, 15 November 2005 (UTC)
thread_title:  Infinite energy sink 

message: Hawking's argument is not represented correctly here. 
see here for details.
Also his article has been accepted in Phys. Rev. D
see here
so it has been reviewed, but it is fair to say that many physicists are not convinced.
Count Iblis 15:16, 17 October 2005 (UTC)
thread_title:  Black hole unitarity paragraph needs to be corrected 

message: It is said that 
:In 1971, Stephen Hawking showed that the total area of the event horizons of any collection of classical black holes can never decrease.
If I understand this correctly, inclusion of Hawking radiation should modify this statement to say that "the sum of the entropy connected to the event horizon's area and the entropy due to the leaving radiation, can never decrease". Right? 
I think that ought to be mentioned, since it took at least me some time to figure out what happened to the (supposedly non-diminishing) event horizon area for a shrinking black hole... \Mike(z) 20:11, 25 October 2005 (UTC)
replies: text: I think the key word there is "classical."  A "classical" black hole does not radiate, because that's a quantum mechanical process.  All the sentence says is that black holes have a property (area) that's like entropy in that it never decreases; Hawking later showed that this quantity can decrease if entropy is produced elsewhere (i.e. in radiated particles), strongly implying that black hole area (times some constant) is entropy. -- SCZenz 22:18, 25 October 2005 (UTC)

text: A number of papers have been written about the subject. To the best of my knowledge, none of these are accepted as definitely correct, because without a complete theory of quantum gravity, many approximations are needed in order to model Hawking radiation, raising the possibility that the approximations mangle something important. See Hawking radiation and trans-Planckian problem for more information. Discussion of black hole entropy and Hawking radiation is probably best done at black hole information paradox. --Christopher Thomas 21:38, 2 March 2006 (UTC)
thread_title:  Hawking radiation and entropy  

message: This page appears to be long enough to create the black hole otherwise known as an archive.
:I agree to this anonymous and undated comment; but also the main article reaches a critical size. This is especially annoying if someone wants to edit the first lines which are not in a (sub)section, and for reverting (frequent!) vandalism (needs edit of the complete article). OK, it's already a featured article, so we can expect it won't grow much any more, but nevertheless this will happen.
: To reduce the article's size, I see as one possibility to move the FAQ to a separate page: In fact there is much redundancy (all "What is a ...." questions are explained previously, often with almost identical wording), and even if the FAQ makes up "only" about 2.5 pages in my browser, I think the density of text is so high that it represents a considerable part of the article's size.— MFH:Talk 13:44, 16 March 2006 (UTC)
replies: text: I'm not sure what the point of the FAQ is - it seems like a less precise summary of the information in the main article... Kjl 00:04, 17 March 2006 (UTC)
thread_title: Talkpage length

message: This is probably an elementary idea, but what if you sent something into a black hole with the sole purpose of destroying it? Say, an advanced bomb or payload of anti-matter. Nothing can escape the event horizon, but if you break it up from the inside, won't the gravimetric forces have to decrease as the chunks separate?
:Someone else here can probably provide a better answer, but the thing to remember is that what's inside the event horizon of a black hole is not just a really dense ball (like a neutron star, for example). The mass is actually concentrated at (mathmatically, anyway) a single point, called a singularity. How do you blow up a singularity? As a rough analogy, if I want to destroy a piece of string, I can cut it up into tiny pieces and scatter them around. But if the string has no length to begin with, how am I going to cut it apart? I'm not saying it's impossible, since that's far too strong a word, but I certainly can't picture how you'd go about it. —HorsePunchKid→龜 04:38, 29 October 2005 (UTC)
replies: text:  If it's really a singularity, you can't blow it apart.  Of course, we don't know what singularities really are without a theory of quantum gravity, but I find it pretty unlikely that anything could break up whatever it really is, or keep it from recollapsing if it did.  Certainly antimatter won't do it.  While I won't make a prediction about physics and technology thousands of years from now, this sounds even harder than making a stable macroscopic wormhole. -- SCZenz 05:45, 29 October 2005 (UTC)
replies: text: QG will probably allow for spontaneous break up of a black hole. Black holes can evaporate, so the amplitude for a black hole to emit a mini black hole is very likely larger than zero.Count Iblis 21:32, 29 October 2005 (UTC)

text: Apparently you'd see a reverse black hole (a "white hole") as you crossed the event horizon of a proper black hole, since everything entering the black hole behind you would appear to come from a single direction due to the intense curvature of space. As for the "edge" of the (possibly infinite) Universe, I'd put money on the Hubble distance as being the common answer. (where space expands faster than light can cross it). Tzarius 11:57, 17 November 2005 (UTC)
replies: text: This is covered at Talk:Big bang. The big bang could be argued to be the time-reversal of the _formation_ of a black hole, but is not a white hole (time reversal of a black hole post-formation). In practice, it's expected that our predictions of what happened break down past the time when the universe approached the Planck temperature, so the interpretation provided by general relativity doesn't completely hold. As for whether falling into a black hole sends you out a white hole at the other end, this turns out not to be the case. Some of the links at the end of the black hole article explain why. Short version is that if you include the original stellar collapse that created the black hole, you can't draw a consistent spacetime diagram of a black hole/white hole pair.--Christopher Thomas 22:11, 17 November 2005 (UTC)
thread_title: Break up a black hole from the inside?

message: Hello, I am not a user of wikipedia, however, I wished to address something. Because the gravitational pull of the black hole is so strong, if you were to "hover" just outside the event horizon of a black hole, then a form of time dilation would occur and, in essence, because time would be moving so much slower, it would be like traveling to the future, although in reality you would have just aged more slowly. I feel that this should be mentioned. —The preceding unsigned comment was added by 24.10.164.230 (talk • contribs) .
:Of course. It's easy to forget to include some of the many and strange effects of relativity. I guess that's why there's so many 'wrong' interpretations of black holes (among the general population). Tzarius 22:05, 20 December 2005 (UTC)
thread_title: Event Horizon and Time Travel