Talk:Black body/Archive 5

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Black holes (Part II)

Waleswatcher, sometime you should get round to explaining why black holes do not emit like black bodies. You appear, and forgive me if I am wrong, not to accept the conventional explanation for a black hole i.e. that the effect of its gravity is so great that internal radiation is trapped within the event horizon; no such trapping appears in the Kirchhoff/Stefan/Boltzmann/Planck theory, consequently emission as a fn(T⁴) is what is measured in the normal i.e. non black hole world. Stefan's theory was based on practical measurements made by John Tyndall. Now just tell us, who is getting it wrong and why? --Damorbel (talk) 13:45, 30 January 2012 (UTC)

Doesn't this discussion belong at Hawking radiation? Brews ohare (talk) 14:22, 30 January 2012 (UTC)

How many times do I have to say it? Black holes do emit Planck spectrum radiation (not that that's even necessary for them to be black bodies, as I've tried to explain to you at least three times now), and they do obey the Stefan-Boltzmann law (obviously, since that follows immediately from the first statement). If you don't believe it, go argue with Stephen Hawking, but your personal disbelief is not relevant to this article. Waleswatcher (talk) 20:43, 30 January 2012 (UTC)

"How many times do I have to say it?" Repetition has nothing to do with scientific logic, you destroy your own argument by claiming this kind of justification. Up until now I thought you did not grasp the 'blackbody' concept, now I am certain. Your defense of repetition seems more like a claim that you are somehow in possession of 'the truth'; you seem not to accept that any radiation coming from a black hole has a different source than black body radiation, that a black hole has a gravitational mass so enormous that radiation cannot escape from its interior, that Hawking radiation comes from the event horizon not the 'interior' (beyond the event horizon) The Wiki article has this-

" In layman's terms it is defined as "the point of no return" i.e. the point at which the gravitational pull becomes so great as to make escape impossible. The most common case of an event horizon is that surrounding a black hole. Light emitted from beyond the horizon can never reach the observer."

And you write "and they do obey the Stefan-Boltzmann law (obviously, since that follows immediately from the first statement). If you don't believe it, go argue with Stephen Hawking." Further on the Wiki article has:- " A black hole of one solar mass has a temperature of only 60 nanokelvins; in fact, such a black hole would absorb far more cosmic microwave background radiation than it emits" Do you agree?

Plus the fact that Hawking radiation has not yet been reliably detected [1]. Surely, if you are correct and Hawking radiation follows the Stefan-Boltzmann law, the hypothetical Hawking radiation would have been detected ages ago by means of a good old fashioned scientific experiment? I mean, what is your proof that Hawking's hypothesis is correct?

And why do you refer to ".... personal disbelief". Do you see this as a personal matter? I think you do and this is contrary to all the principles of Wikipedia. --Damorbel (talk) 12:45, 31 January 2012 (UTC)

Nothing in what you wrote addresses the issue of whether black holes are black bodies as defined in the article and by its sources. In fact, nothing in what you wrote addresses anything that is in the article. If you want to challenge what's written in the article, do so and be specific. Waleswatcher (talk) 12:54, 31 January 2012 (UTC)

'Nothing'...'nothing'------------ 'Nothing but nothing' you wrote Waleswatcher! What is this for an argument? Do you think I am disagreeing with you? Too bad! I have made substantive points about the difference between black holes and black bodies. You give the strong impression that you want the same explanation to do for both. What you have done is picked up a few snippets of information about the real connection between the two and are hanging onto them without realising that there is a much more coherent explanation than yours. But your desire to contribute a few 'bits' is concealing the much more interesting science. The Kirchhoff/Stefan/Boltzmann/Planck black body and associated blackbody radiation was a major triumph of 19^thC physics, leading directly to quantum theory and the glories of 20^thC physics and you appear to want to suppress the element of progress that took place, repeating again and again 'it's just the same'. Don't you want to make the differences clear instead of denying them? I think that attitude would be rather more creative. --Damorbel (talk) 14:15, 31 January 2012 (UTC)

I quite literally have no idea what you are talking about, so there's no point in me responding. Waleswatcher (talk) 02:33, 1 February 2012 (UTC)

Damorbel: As I understand your point, you say "any radiation coming from a black hole has a different source than black body radiation". Which I take as suggesting that black-body radiation must be thermal in origin, and other mechanisms that might lead to the Planck spectrum should be distinguished from black-body radiation. Is that your point?

If this is the point, then what needs to be done is to add a sentence to the black hole section to the effect that odd as it may seem the Hawking radiation that has a Planck's law behavior does not originate from thermal equilibrium radiation, but from something else, and provide a source to that effect.

I have not explored this matter carefully, but it does seem unlikely to me that Planck's law is going to originate in several ways. Brews ohare (talk) 16:26, 31 January 2012 (UTC)

I may be wrong about this: the Hawking radiation article says: "An important difference between the black hole radiation as computed by Hawking and thermal radiation emitted from a black body is that the latter is statistical in nature, and only its average satisfies what is known as Planck's law of black body radiation, while the former fits the data better." This statement doesn't seem to be sourced, however. Brews ohare (talk) 16:34, 31 January 2012 (UTC)

This appears to be the source of this remark. On the other hand, this appears to suggest it is not electromagnetic radiation that has a Planck spectrum, but the energy distribution of emitted particles, with an apparent "temperature" kT = κh/(4π²) with κ the "surface gravity". I'm not happy with this source. Maybe you can sort this out? Brews ohare (talk) 16:44, 31 January 2012 (UTC)

Black holes emit exactly like a black body at the Hawking temperature. Like all black bodies at thermal equilibrium, they don't only emit electromagnetic radiation with a Planck spectrum - they also emit every other species of particle with a thermal distribution. Does that clear it up for you? Waleswatcher (talk) 02:32, 1 February 2012 (UTC)

A complete expression for the temperature is $T={\frac {\hbar c^{3}}{8\pi Gk_{B}M}}$ with G=gravitational coupling constant and M the black hole mass parameter. Brews ohare (talk) 17:17, 31 January 2012 (UTC)

The most useful source appears to be here on p. 35 : §1.6.1 Is the emitted radiation exactly thermal? It looks possible to summarize something useful from this source. The 'Look inside' feature of Amazon provides the most complete access. Brews ohare (talk) 17:05, 31 January 2012 (UTC)

A very important point is that Hawking radation has seldom, if ever, been observed with sufficient resolution to see if it matches the hypothesis, however beautiful the hypo. is, it is falsified if it is not confirmed by diverse observations. Black body radiation may not often be observed in all perfection but the really great aspect is - the deviations from perfection are very well explained in enough cases to confirm the fundamental processes. --Damorbel (talk) 21:37, 31 January 2012 (UTC)

The importance of observation is to establish that theory fits reality. However, if it doesn't it means something is missing in our concepts. That is how black-body radiation led to quantum theory. So the lack of observation doesn't imply the theory is wrong, just that we haven't completed the cycle, eh?

You haven't said whether I understood your point in my comments above, and you haven't indicated whether you are interested in pursuing this matter with a more careful summary of the literature? Brews ohare (talk) 22:10, 31 January 2012 (UTC)

The issue of whether or not Hawking radiation is exactly thermal is a deep and subtle one, but it certainly does not belong here in this article. If it's not exactly thermal - which is my opinion and that of most specialists - the differences from a Planck spectrum are extremely small. In fact, they're of the same size as the deviations from a Planck spectrum in a standard black body cavity with equal entropy (which again are ridiculously small, but non-zero). I'll have a look at the Hawking radiation article, it sounds like a mess. Waleswatcher (talk) 04:03, 1 February 2012 (UTC)

"which is my opinion and that of most specialists" - Waleswatcher, nobody has the slightest interest in your opinion on this matter. And what is this about 'most specialists'? I imagine you are satisfied with the statement but can you think of any reason why anybody else should be? This kind of contribution is not proper to a Wiki discussion because it contributes, as I said before 'Nothing... ...nothing'!

I find this subject very interesting and some of your contributions have expanded my perception of the matter considerably, principally in relation to the concepts of a 'black body' - surely just Kirchhoff's abstract concept - and Planck's 'blackbody radiation; a manifestation that can, in principle, turn up anywhere, from whatever source, even a black hole! --Damorbel (talk) 10:05, 1 February 2012 (UTC)

Chjoaygame's edits regarding cavity walls

Chjoaygame, I don't understand the purpose of your sentence "These authors do not tell about the constitution of the walls under these conditions". First, the statement regarding e+/e- pairs is valid at all temperatures, so what is meant by "these conditions"? Second, I think it's an obscure phrase. Are trying to say that the walls will melt if they're made out of any ordinary material and the temp is high enough for there to be substantial numbers of e+/e- pairs? If so, you're right - but then why not just say that or something to that effect (that was the purpose of my edit). Waleswatcher (talk) 21:32, 30 January 2012 (UTC)

I see that further edits have been made while I was writing that. But now it makes even less sense - it refers to cosmology, and then to walls. It's also incorrect that "cosmological conditions" - whatever that means - are required for photon-photon interactions to occur and to thermalize a photon gas. I thought we had already settled that. Waleswatcher (talk) 21:34, 30 January 2012 (UTC)

The source speaks of cosmological conditions ("during the first few minutes after the Big Bang") but does not mention walls. The present text does not assert that cosmological conditions are necessary, just that they are sufficient. You have an argumentative habit, when someone states a sufficient condition, of gratuitously complaining that it is not necessary.

If you have a reliable source that deals with walls and these kinds of thermal equilibria, here is the place for you to cite it. I hope it will not be some kind of original research or synthesis, but will explicitly address the matter in hand.Chjoaygame (talk) 21:54, 30 January 2012 (UTC)

As currently written, that language implies that very high temperatures are required for photon-photon interactions to thermalize radiation. That's false (and unsourced), and it needs to be re-written so that it clearly doesn't imply that. Further, I don't understand why you've inserted a quote about the early universe in the middle of a section about a cavity with walls, particularly when you then add a comment that the authors didn't mention the walls. That's incoherent. Waleswatcher (talk) 22:06, 30 January 2012 (UTC)

As I mentioned, you have an argumentative habit, when someone states a sufficient condition, of gratuitously complaining that it is not necessary. The text does not imply, except to your mind, that the conditions are necessary. It is not right for you to impose your personal habit of reading things that are not there.

I agree that the idea is perhaps incoherent, and I would be comfortable omitting all mention of the extreme modes, but I know they are dear to your heart and that you would not allow their omission. PAR intelligently moderated the situation, when he introduced the distinction between "practical" conditions and "other" conditions. It is up to you to fill in the details of the other conditions, if Kondepudi and Prigogine do not satisfy you for that. You are trying to make me say that such conditions are not necessary, but I don't have a source for that; I have a source only for their sufficiency. It is up to you to provide a reliable source for your dearly beloved processes. If you want deletion of all mention of the photon-photon reactions, which seems to be one logical outcome that would putatively satisfy you (though I think it wouldn't really satisfy you), I will probably not oppose that.Chjoaygame (talk) 22:22, 30 January 2012 (UTC)

It makes no sense - logically or pedagogically - to mention cosmology in the middle paragraph of a section describing a cavity with a hole in it. I've removed that material and put it into its own section. I would prefer to remove it entirely and add it to the BB radiation article. As for sources for the statement that photon-photon interactions can thermalize radiation, if you want them please be very specific about what you want sourced. The Euler-Heisenberg article explains one type of photon-photon interaction, and the fact that any interaction will lead to thermalization can be easily sourced. Waleswatcher (talk) 00:44, 31 January 2012 (UTC)

I regard your action here as violent and unethical, but I know that you can get away with it with impunity.Chjoaygame (talk) 01:19, 31 January 2012 (UTC)

"violent and unethical". Perhaps you should take a wikibreak, Chjoaygame. Waleswatcher (talk) 04:27, 31 January 2012 (UTC)

You don't like me calling you on your action. I can understand that. I did above suggest the idea of playing a game by its rules.

The usual idea is that the writer of a sentence should himself provide a reliable source for it in context, which here is about practical conditions for thermalization. But since you haven't done that, and since you ask to be spoon-fed ("As for sources for the statement that photon-photon interactions can thermalize radiation, if you want them please be very specific about what you want sourced."), I will settle for a reliable source for a direct and explicit and empirically verified statement that photon-photon interactions can thermalize radiation in conditions that are not on cosmological time scales and that are not themselves cosmological. I would remind you that synthesis and own research do not constitute a reliable source.Chjoaygame (talk) 11:00, 31 January 2012 (UTC)

The article says nothing whatsoever about "cosmological timescales", so no, that's not necessary. I'll look for a reference that says that any interaction will lead to thermalization, that plus the (sourced) statement that photons interact is more than enough. Waleswatcher (talk) 11:04, 31 January 2012 (UTC)

By the way, you said earlier that you have already been involved in a dispute over this exact same issue with a different editor, who supplied some sources. What happened to those? Waleswatcher (talk) 11:06, 31 January 2012 (UTC)

It's not easy to find a reference that states something as elementary as this in so many words, but there are many, many sources that prove the H-theorem for weakly interacting gases. A good example is Tolman, "The principles of statistical mechanics" p 455 where he states that collisions lead to the decrease of H. Waleswatcher (talk) 11:37, 31 January 2012 (UTC)

Yes, I am not surprised to read that it is not easy to find reliable sources; I have noticed it myself too. That is why I was happy that you came along with new enthusiasm for it; there is much unsourced material that gets posted without criticism. PAR suggested that I Google for reliable sources, and offered a report of an experiment that detected photon-photon interactions under conditions very far from thermal. But he didn't provide reliable sources for the statement in question. It is all very well for you to deal with my request by dismissing it as asking for something elementary, obviously beneath the dignity of an omniscient fellow like yourself. I note that you here are offering a synthesis, perhaps hoping it will serve to dismiss. I await your finding a reliable source.Chjoaygame (talk) 11:50, 31 January 2012 (UTC)

Already done and in the article. Tolman (along with many other sources, but Tolman is reliable) proves the H-theorem (which says that a system not in equilibrium approaches it) using - wait for it - a weakly interacting Bose gas. The weaker the interaction, the better the approximation he uses is. So that part is done. Now, are you seriously disputing that photons interact? If you insist, I'll be happy to add the original Euler-Heisenberg paper (and there are plenty of other sources), but it's already linked to the wiki article. Waleswatcher (talk) 11:57, 31 January 2012 (UTC)

This is a direct admission that you are synthesizing, not offering a reliable source.Chjoaygame (talk) 12:00, 31 January 2012 (UTC)

Nonsense. I've just provided a reference to a proof that all collisional Bose gases thermalize. End of story. Regarding the scattering of light by light, I'll add yet another reference. Waleswatcher (talk) 12:03, 31 January 2012 (UTC)

You can dismiss my point as "nonsense". That isn't a refutation. You asked for a specification of what I thought was needed. I replied; "I will settle for a reliable source for a direct and explicit and empirically verified statement that photon-photon interactions can thermalize radiation in conditions that are not on cosmological time scales and that are not themselves cosmological. I would remind you that synthesis and own research do not constitute a reliable source." You are still offering synthesis, not even mentioning the key word cosmology.Chjoaygame (talk) 12:09, 31 January 2012 (UTC)

These is no synthesis. Tolman says explicitly in multiple places (and proves rigorously) that any weakly interacting Bose gase thermalizes. Your request is literally nonsense, since it aks for citations for statements (regarding cosmology) that don't appear anywhere in the article. Waleswatcher (talk) 12:14, 31 January 2012 (UTC)

So it turns out that you need to censor the truth stated in the reliable source, but can provide only synthesis for your beloved message.Chjoaygame (talk) 04:28, 1 February 2012 (UTC)

Please try to be more clear. Many of your comments are incomprehensible. You might be referring to "or even cosmological". I changed that because it's basically meaningless, both to non-physicists and physicists. Cosmology contains all sorts of time scales, including extremely short ones. As for your source, the quotes you gave to not support what was written. Waleswatcher (talk) 04:44, 1 February 2012 (UTC)

Your edit of the article on Black-body radiation, with a section header that you created, reads:

Cosmology

The cosmic microwave background radiation has a nearly perfect black body spectrum, because it is a "snapshot" of the radiation at the time of decoupling between matter and radiation in the early universe. Prior to this time, most matter in the universe was in the form of an ionized plasma in thermal equilibrium with radiation.

According to Kondepudi and Prigogine, at very high temperatures (above 10¹⁰K; such temperatures existed in the very early universe) significant quantities of electron-positron pairs appear and disappear spontanteously in thermal equilibrium with electromagnetic radiation. These particles form a part of the black body spectrum, in addition to the electromagnetic radiation.

Not supported? Not clear? The source has to support what is written, and it did. Did you check it? What do you mean by "quotes"?Chjoaygame (talk) 15:21, 1 February 2012 (UTC)

The statement that was in this article that thermalization timescales can be "cosmological" is not supported by any quote I've seen (you transcribed some) from that reference. Plus, as I already said it doesn't have a clear meaning. That's all. Waleswatcher (talk) 15:31, 1 February 2012 (UTC)

I'll take that as a "no". The requirement is not for support by quotes that Waleswatch has seen or had transcribed for him. It's for accurate reporting of the reliable source.Chjoaygame (talk) 16:10, 1 February 2012 (UTC)

Go ahead and provide a quote that says that thermalization timescales can be "cosmological". Once you do so, you're free to add that to the article, although I strongly suggest using a different term (like "very long" as it is now) that actually explains what you mean. Waleswatcher (talk) 17:08, 1 February 2012 (UTC)

It seems you are challenging me to find something more precise than that masterpiece of precision "a very long time"? How about Callen (1960/1985): "In actuality, few systems are in absolute and true equilibrium. In absolute thermodynamic equilibrium all radioactive materials would have decayed completely and nuclear reactions would have transmuted all nuclei to the most stable isotopes. Such processes, which would take cosmic times to complete, generally can be ignored." Would I be silly enough to try to post that, with you as a supervisor of the article? I think not. You would find some further objection, and so on, so that it might take a cosmological time to get your permission, which of course I need.Chjoaygame (talk) 18:18, 1 February 2012 (UTC)

History section

The history section placed here is a duplicate of the section in Black-body radiation, and it more properly belongs in that article as it is not the idea of a black body that is most important historically, but the nature of black-body radiation and its theoretical relation to Planck's law. So I removed it here. As noted on Black-body radiation's talk page, this history section needs a lot of work. Brews ohare (talk) 22:21, 31 January 2012 (UTC)

You complained at Talk:Black-body radiation/Archive 1#History that the history section as written was "a bit odd" for the article on Black-body radiation, where it "needs a lot of work" and perhaps "should be recast". Perhaps it is a bit odd and needs a lot of work there; that is perhaps because it was written as a history section for this article here on the Black body, and is focused on it. Now you find it not "proper" here in this article because this article is about a subject that is "not ... most important historically". Why bother with a separate article on a subject that is not most important historically? Are we seeing an instance of the law of unintended consequences, in the consequences of your arbitrary splitting of the article into two? Or what?Chjoaygame (talk) 23:57, 31 January 2012 (UTC)

The problems with the history section are related to its underemphasis upon the importance of black-body radiation in developing the idea of the quantum, and the use of headers for some scientists but not for Planck or for Einstein. The definition of the perfect absorber is small potatoes in this context. Brews ohare (talk) 01:43, 1 February 2012 (UTC)

Those are problems for an article on black-body radiation, for which it was not written, but whither it has been kidnapped. They are not problems for an article the black body, for which it was written, from which it has been kidnapped.Chjoaygame (talk) 04:21, 1 February 2012 (UTC)

Kirchhoff's perfect black bodies

This section refers to Planck's Theory of Heat Radiation available in English in its entirety here. This text is searchable. §14 refers to emission through an element of area being proportional to the cosine of the angle between the direction of radiation and the normal to the surface, which would appear to be Lambert's law, though Planck does not refer to it this way. This observation is used to derive some properties of pencils of radiation and later is used to determine some features of radiation in thermal equilibrium. I have found nothing here to suggest that Lambert's law militates against the existence of a black body as defined by Kirchhoff.

I have found on p. 10 several observations about the requirements upon a black body (i) entry without reflection (ii) requirement of a minimum thickness adequate to absorb the incident radiation an prevent its re-emission, (iii) severe limitations upon scattering to prevent radiation from entering and bouncing back out. These requirements may well be a challenge to any attempt to make a black body.

However, I find the text of this section needlessly obscure and not strictly in accord with the source cited. I've rewritten it in a more transparent manner, I hope. Brews ohare (talk) 06:19, 30 January 2012 (UTC)

Ah, diddums.Chjoaygame (talk) 08:33, 30 January 2012 (UTC)

One thing to bear in mind is that constructing a perfect black body is obviously impossible simply because it would have to be infinitely large. EM radiation with wavelength longer than the body is wide is simply never going to be absorbed completely no matter how the body is constructed. Black holes are a good example - you can't get any more absorbing than that, but they're not perfect black bodies for that reason. So however the article changes, it should make clear that the whole idea is an idealization. Waleswatcher (talk) 11:28, 30 January 2012 (UTC)

Waleswatcher: If you have a source for this observation, a remark about dimensions could be added as a final sentence to this section on limitations of "perfect" black bodies, don't you think? Brews ohare (talk) 13:32, 30 January 2012 (UTC)

I have added a source that explains the spectrum of radiation in a cavity becomes shape-dependent unless the cavity is large compared to wavelength. Maybe you want to elaborate upon this point? Brews ohare (talk) 16:24, 30 January 2012 (UTC)

If we are going to get into limitations of Planck radiation, there is also the fact that at low enough intensities we will be restricted to photon-counting and photon statistics. If the BB radiation is so low as to be one photon per hour, then any 1-second measurement will most probably give zero. PAR (talk) 03:03, 3 February 2012 (UTC)

Headers

I recast the headers as Idealizations, that is, conceptual notions of black bodies and some idealized properties with their criticisms, and Realizations, that is, physical objects that approximate black bodies, either man-made or found in nature. These divisions somewhat parallel metrology where one talks of an ideal or standard, like the metre, with principles behind it, and its realization, or its approximate embodiment in a lab or in nature. Brews ohare (talk) 01:55, 1 February 2012 (UTC)

So it turns out that this article really needs the stamp of the Kommissar.Chjoaygame (talk) 04:25, 1 February 2012 (UTC)

After a lot of thought (at first I did not understand what you meant by 'Idealizations'!), may I suggest 'Approximations' is a better word? A black body is already an 'idealization' (no reflection - no transmission). Most of the variations listed in the idealizations section are in fact less than ideal because they have some reflection, a refractive index greater than one or some transmission, some scattering and so forth.

When writing 'a black body may not transmit any radiation'; I know this is implied by the 'total absorption' requirement but so is the non-reflection requirement and I don't want the 'no transmission' freaks to feel left out. --Damorbel (talk) 14:15, 2 February 2012 (UTC)

Damorbel: In my mind, although "idealization" and "approximation" overlap, idealization is what I intended. The idea of a "black body" is an idealization, because it is understood that it cannot be realized. On the other hand, an approximation applies to a model of something extant. In the Idealization section are the topics of "infinitely thin" and/or perfectly absorbing surfaces which are explained to be unobtainable, and the pinhole in a cavity that never lets the entering light back out. In the realization section is the approximate treatment of the photosphere, an extant entity that is being approximated. Brews ohare (talk) 16:05, 2 February 2012 (UTC)

Brews ohare proposes that an approximation applies to a model of something extant, apparently meaning that a non-existent theoretical object cannot be approximated. Really? This is not the place for metaphysical investigations of the difference between "idealizations" and "realizations", however interesting that might be. These two terms are from metaphysics, and are scarcely informative for this present article about physics. In particular, they have here been used to expunge the distinction between "natural" objects and "artificial" ones, which I think does fall within the purview of ordinary physics.Chjoaygame (talk) 03:08, 3 February 2012 (UTC)

Chjoaygame: This article contains material that falls into two categories. On the one hand, discussions of abstractions like infinitely thin black body surfaces, perfect absorbers, and the limitations of concepts. On the other hand the behavior of things in nature, like carbon black and stars that can be approximately described by a black body. Characterizing the division as "natural" vs. "artificial" doesn't seem to me to capture the difference between concepts and their development, and the adequacy of their application to particular systems. Brews ohare (talk) 12:55, 3 February 2012 (UTC)

This statement.

This statement in the opening section "A black body in thermal equilibrium emits electromagnetic radiation called black-body radiation with a spectrum..." is incorrect. A black body at any temperature will emit blackbody radiation appropriate to its temperature; the requirement for equilibrium is too restrictive, e.g a black body cooling entirely by radiation to an environment which is also a black body but at a lower temperature is not in equilibrium but the radiation is still 'blackbody' radiation. --Damorbel (talk) 13:54, 2 February 2012 (UTC)

The statement is not incorrect. I think, as you also say, you are arguing it is too restrictive. The example you provide is for a cooling body, which as you note is not at thermal equilibrium. In fact, it also is not at a fixed temperature. For such a body to be described as a black body goes beyond saying it can be approximated as a black body over some time interval. Can you find a source that places the appropriate limitations on this situation? Brews ohare (talk) 16:10, 2 February 2012 (UTC)

"...for a cooling body...". Don't think so! The argument is equally as good for heating (temperature rising) as cooling.

I think the problem lies here:- it is the radiation that is in equilibrium (or not); 'Equilibrium' is not a property belonging to the 'nature' of a body.

Above 0K a black body produces 'blackbody' radiation whether it is in equilibrium or not. On the other hand, if the body is not black i.e. it reflects, scatters, transmits etc., it will only produce blackbody (Planck spectrum) radiation when the system is in thermal equilibrium i.e. with a uniform temperature throughout. --Damorbel (talk) 17:14, 2 February 2012 (UTC)

"Above 0K a black body produces 'blackbody' radiation whether it is in equilibrium or not" That's manifestly false; you've already been given several counterexamples. Waleswatcher (talk) 00:19, 3 February 2012 (UTC)

The topic you raise, of radiative cooling, is worthy of a section. Of course a black body defined as a perfect absorber need not be characterized by a single temperature even at a fixed time, as one could be imagined assembled of patches of perfect blackness at various temperatures. However, if a single temperature applies everywhere over the surface and also to some depth from the surface (the depth from which radiation can be emitted), it would appear that black body radiation will result. Inasmuch the lack of corresponding radiative input implies cooling, so to remain a black body it must cool uniformly to maintain a constant T over its surface and over the depth of emission. A close analysis of a cooling layer of any finite thickness will show a temperature gradient is needed to transport the heat through the region, so the requirement of a uniform temperature cannot be met exactly, but only to a degree of approximation. We know that even in principle a black body surface cannot be infinitely thin (as shown by Planck and discussed under "Idealizations").

So I believe the discussion of cooling is really one of approximating a cooling object by a succession of black bodies that have different temperatures at different times. An example is the discussion in the article of a photosphere.

Here is an interesting application of cooling of a black body that makes no nice distinctions like this. Brews ohare (talk) 16:37, 2 February 2012 (UTC)

This article discusses the heat balance between diffuse emitters each characterized at each time instant by an emissivity and a temperature. Brews ohare (talk) 16:53, 2 February 2012 (UTC)

A simple-minded approach considers emission by a body at temperature T and absorption from the surrounding ambient matrix at temperature T_a according to:

$P_{out}=eA\sigma T^{4}\ ,$ $P_{in}=eA\sigma T_{a}^{4}\ ,$ with σ the Stefan-Boltzmann constant and assumes a specific heat for the emitting body to determine its temperature behavior based upon the net $P_{in}-P_{out}$. Brews ohare (talk) 17:54, 2 February 2012 (UTC)

Brews ohare, I do think our discussion is converging. The title of the article is 'Black body which it a theoretical concept meaning it does not reflect, scatter, transmit etc. A body that does any of these things is less than 'black' and the behaviour becomes more complicated because there are so many different ways in which EM radiation can be reflected (diffuse, specular, Lambertian etc.); scattered (coherent, diffuse, forward, backward etc.) transmitted (refraction, diffraction, interference etc.). With all these possibilities and more the analysis must be made with care! --Damorbel (talk) 17:45, 2 February 2012 (UTC)

That is so. Brews ohare (talk) 17:54, 2 February 2012 (UTC)

I've added a section on Black body#Radiative cooling to reflect this discussion. Brews ohare (talk) 19:01, 2 February 2012 (UTC)

Brews ohare writes above: "so to remain a black body it must cool uniformly to maintain a constant T over its surface and over the depth of emission." I think he means 'for its radiation to remain black, the body must cool uniformly.'Chjoaygame (talk) 19:51, 2 February 2012 (UTC)

Writing "so to remain a black body it must cool uniformly to maintain a constant T..." Misses the point. Black is black, it can only emit 'blackbody thermal radiation' - perhaps with different parts of a 'real' body having diverse temperatures. But don't forget the term 'black body' is a theoretical abstraction, it doesn't have dimensions! For a 'real' (big!) body the radiation will be the appropriate 'blackbody radiation' for the temperature of the part being measured. --Damorbel (talk) 21:52, 2 February 2012 (UTC)

"Black is black, it can only emit 'blackbody thermal radiation'" - again, no. That's just wrong. Waleswatcher (talk) 00:23, 3 February 2012 (UTC)

"....- again, no. That's just wrong." Care to make a (scientific) justification for this? The basis of what I wrote is this, a black body emits 'blackbody radiation' according to its temperature; if the body is a thermodynamic system with a non-uniform temperature, then the emitted radiation will be characterised by the distribution of temperature. Of course the different contributions will not have the same spectrum because the temperature of the system is not uniform but each of the different parts are still 'blackbody radiation'. --Damorbel (talk) 08:29, 3 February 2012 (UTC)

We've already discussed this ad nauseam. Your example in the paragraph above is as good as any - black body radiation is homogeneous and isotropic by definition, so "Of course the different contributions will not have the same spectrum because the temperature of the system is not uniform but each of the different parts are still 'blackbody radiation'" is nonsense. Your example shows that back bodies do not always emit BB radiation. Another example I've give you at least three times now is a blackbody cavity stimulated to emit laser light. Another was the "magic wand" example, etc. etc. Waleswatcher (talk) 12:59, 3 February 2012 (UTC)

"We've already discussed this ad nauseam" I suppose if you consider writing - "is nonsense" - as a contribution to discussion then I suppose you are right!

Are you saying this happens - "a blackbody cavity stimulated to emit laser light" - or not? I'm awfully sorry but I didn't realise there was a question here. I suggest it is important to recognise that a laser is very far from equilibrium, most only works with population inversion. --Damorbel (talk) 16:47, 3 February 2012 (UTC)

"I suggest it is important to recognise that a laser is very far from equilibrium" - It's very far from thermal equilibrium, yes, that's the whole point. Note that the laser can perfectly well be held at fixed temperature, but it still emits a very non-BB spectrum. Thanks for acknowledging that thermal equilibrium is necessary for a black body to produce BB radiation. This conversation is over from my side, unless you're going to propose a change to the language in the article. Waleswatcher (talk) 17:00, 3 February 2012 (UTC)

Chjoaygame: I think it suffices that the region emitting the light have the same temperature. In practice, that may be impossible to achieve without the whole body at the same temperature, but the emitted radiation doesn't know that. Brews ohare (talk) 21:20, 2 February 2012 (UTC)

I don't like the wording now, it's convoluted. I think we should either say thermal equilibrium (perhaps qualified as internal TE, or approximate TE), or "at uniform, constant temperature". The problem with the latter is that it's actually incorrect - a black body cavity driven to emit laser light can be at uniform, constant T and yet doesn't emit BB radiation. So I think "internal thermal equilibrium" is the best bet if we must qualify it. Waleswatcher (talk) 00:23, 3 February 2012 (UTC)

Darmorbel was not happy because 'in thermal equilibrium' might be read to allow that the emission is determined not only by the state of the body itself, but in addition by factors other than the state of the body itself. Brews ohare is concerned that the body might be such as to emit only from some relatively superficial parts of itself, and that the condition of the other parts would not matter, being entirely hidden within the body, and so might be exempted from the requirement for spatially and temporally uniform temperature. Waleswatcher was concerned that 'that has a spatially and temporally uniform internal and surface temperature' which restricts the meaning of 'in thermal equilibrium' to the state of the body itself, is convoluted. Planck pointed out that the interface is a property jointly of both the body and its surroundings, and that the interface is a factor in the determination of the emission; for example a body might be black when surrounded by air but not when surrounded by water.Chjoaygame (talk) 02:59, 3 February 2012 (UTC)

Seems that way. Of course the interface is a factor so far as its being rough or being smooth. But your last point is not really about the interface, but about the surrounding material, perhaps how its refractive index compares with the balck body? Have you a suggestion to smooth out these objections? Brews ohare (talk) 04:03, 3 February 2012 (UTC)

Planck's view is that the interface is a joint property of the body and the surrounds. The roughness or smoothness is included in that.Chjoaygame (talk) 05:19, 3 February 2012 (UTC)

Practical materials

The article now says (at the beginning) "In practice, common applications define sources of infrared radiation with emissivity greater than approximately 0.99 as a black body" Why should an article about a theoretical abstraction introduce a 'practical approximation' at such an early stage? I see no purpose in introducing this wording here. Kirchhoff's purpose in introducing the concept of a 'blackbody' was to separate the function of absorption/emission of radiation (which results in temperature change) from reflection/scattering of the radiation which redirects it without any temperature change. The absorption process destroys photons generated by the emission process; reflection/scattering just redirects them just like a lens redirects the photons to a focal plane. Make no mistake, there are materials that have very high and very low absorptivity; 'approximately 0.99' is completely unscientific, as would be 'approximately 0.9' or 'approximately 0.00009'. --Damorbel (talk) 21:40, 2 February 2012 (UTC)

I rewrote this part and put the 0.99 in a footnote. See if you like that approach. Brews ohare (talk) 03:59, 3 February 2012 (UTC)

Cavity and the big bang

I don't understand why material about cosmology or the big bang or supernovae keeps getting inserted into the section about a realization of a black body as a cavity with hole in it. It makes no sense physically or pedagogically. Waleswatcher (talk) 13:17, 3 February 2012 (UTC)

By the way, at the moment the sentence reads "In the absence of any matter, photon-photon interactions [22] will thermalize the radiation, although the timescale for a photon gas near room temperature would be very long." That's last bit is not sourced. I'm not exactly sure what's in Kondepudi & Prigogine (I don't own it), but if all they say is that photon interactions are fast at 10^10K, we could reword to say that the timescale decreases with temperature (cite them) and can be very long at temperatures significantly below 10^10 K. Waleswatcher (talk) 13:28, 3 February 2012 (UTC)

The reason why material about cosmology and the big bang keeps getting inserted into the section about a realization of a black body with a hole in it is that you insist on bringing into that section factors which need explanation in terms of them. If you would desist from that, then the explanation would become unnecessary.

The relevant sentences at present, with your latest deletion of valid relevant reliably sourced material, read: "It is much faster with condensed matter than with rarefied matter such as a material gas, especially a Knudsen gas, but any interaction will accomplish thermalization. In the absence of any matter, photon-photon interactions will thermalize the radiation, although the timescale for a photon gas near room temperature would be very long."

There are several particular problems here. One is that the statement, that in the absence of matter, any interaction will thermalize the radiation, is synthesis (in your own words, a key part of it is "not sourced"), and should be removed until some non-synthetic reliable source is produced for it. Another is that it is not enough for you to delete reliably sourced material just because you have not bothered to check the source. In order to appear to be working in good faith you would need to check the source before deleting material that cites it. I note in this regard that you did not even bother to make a proper citation of you synthetic source, and left it to other editors to provide the citation details. Another problem is that the rate of approach to equilibrium has been raised and reliably sourced, and the matter of photon-photon interactions, which is the special message that you want to keep in, is relevant only because of that. Kondepudi and Prigogine explicitly say that the threshold for photon-interactions is at about 6 × 10⁷ K, as stated in the material that you have deleted while acknowledging above that you have not checked the source. The reliably sourced matter which you have deleted explicitly quotes the presence of a threshold for photon-photon interactions, and covers the question of time scales appropriately.

More generally, there is a problem here with your editing of this question. You say you don't understand why the material is introduced here; that can only mean that you have an intellectual blind spot in this, or that you are not working in good faith. Assuming that you are working in good faith, it would follow that you have an intellectual blind spot in this, presumably because you have a special point of view which, though you can support it only by synthesis, not reliable sourcing, you feel is so right and correct that it should exclude all others. It is unacceptable that you immediately delete reliably sourced relevant material while you acknowledge that you have not checked the source. I suggest you look at the special nature of your point of view with the hope that you may repair that intellectual blind spot. When you have done that, I suppose you will thereby make yourself fit to edit this area. Until then, assuming your good faith, it seems that your editing of this area is violent and unethical because you lack insight into your own actions.Chjoaygame (talk) 15:20, 3 February 2012 (UTC)

Material on cosmology is not relevant to a cavity with a hole in it, so the reliability of the source is irrelevant. That section as it was before you edited it was perfectly well sourced. Since you don't like this language, I will restore it as it was. Waleswatcher (talk) 16:48, 3 February 2012 (UTC)

Waleswatcher: I don't have access to the citation by Tolman. However, Planck and others have suggested in the case of insertion of a small body into the cavity that thermalization can occur only if emission and absorption are available at all frequencies. So a statement that any interaction will work is apparently exaggerated. For instance, if an interaction at only one frequency is allowed, and the system is linear, only that frequency will thermalize. As for light scattering by light, I simply do not know if it is satisfactory to achieve complete thermalization, or if it is too restricted. I doubt that Tolman discusses this point, as to my memory he was not into such matters. Brews ohare (talk) 17:10, 3 February 2012 (UTC)

Perhaps this reference can lead to something useful? However, it requires the presence of atoms to catalyze the photon-photon equilibration.Brews ohare (talk) 17:26, 3 February 2012 (UTC)

I'm not sure what you mean by a linear interaction - that's more or less a contradiction in terms. The only known (even theoretically) interactions that don't lead to thermalization are those in very special mathematical models that are called "integrable", with an infinite number of conserved charges. Light-by-light scattering is not in that class. In any case, Tolman proves that in any weakly collisional Bose gas of massless particles, the system evolves towards the equilibrium state (the Planck spectrum). That's pretty standard, versions of which can be found in any text on stat mech. It's usually done treating the photons as particles with some probability of colliding, and Tolman does it both in the classical approximation and quantum mechanically. The reference on the scattering of light by light says that photons are (weakly) collisional, and that's precisely the input to Tolman's proof.

I don't know what Planck thought about this, but the physics is far better understood now than in his time, so it's relevant only to history. Waleswatcher (talk) 18:43, 3 February 2012 (UTC)

"...the physics is far better understood now than in his time...". Perhaps by some but reading the contributions here I suggest your statement exaggerates rather a lot!

BTW, which of Planck's works have you read, or Einstein's or Kirchhoff's or Fourier's for that matter? I recommend Fourier, he had a wonderful sense of humour; being a good friend of Napoleon Bonaparte he probably needed it! --Damorbel (talk) 19:50, 3 February 2012 (UTC)

It is good to have your contribution here, Brews ohare. Waleswatcher has a point to make, and perhaps it may help if I say here something about his point.

Waleswatcher is pointing out that in Planck's day, say about 1900, there was not the least inkling of knowledge of photons, let alone belief that they could interact. The whole of Planck's thinking was built around the idea thoroughly accepted in his day, that light rays do not interact with each other. The idea of photon-photon interactions arises only in quantum electrodynamics, which was invented more than half a century after Planck's day. Experimental observations that are believed to be explained by them are even later.

Waleswatcher is very keen to put in the quantum electrodynamical or even more advanced ideas such as a more general quantum field theory. He thinks it thoroughly supersedes Planck's nineteenth century level of understanding. He is right that in general quantum field theory as a theory supersedes Planck's understanding. Waleswatcher is so impressed by this that he wants to minimize or de-emphasize Planck's viewpoint. Waleswatcher is very familiar with the theory of statistical mechanics, so familiar that he writes it as stat mech because it would be too repetitive for him to write the words in full. He is indeed familiar with many texts on the subject, it seems. This is evidence that he is a very learned and clever physicist.

The physical point that Waleswatcher wants to make is not only that photon-photon interactions (or some other similarly advanced physical concepts) can explain thermalization. He wants to go significantly further, and to say that it actually does explain in the context of the present article.

There are several difficulties for a Wikipedia entry about this here. One is that other mechanisms, such as you have noted that Planck thought of, are important in many physical applications, such as for example atmospheric physics. Another is that such photon-photon interactions such as electron-positron production require extreme conditions or extremely long times.

The extremities are enormous: for example Kondepudi & Prigogine 1998 say that the electron-positron production events do not occur thermally below a threshold temperature of 6 × 10⁷ K, such as are believed to have occurred a few minutes after the Big Bang, and they say that present day stars reach temperatures only significantly less, about 10⁷ K. Waleswatcher has not offered any reason for us to believe that Kondepudi & Prigogine are wrong about this. What K&P say is intuitively plausible. For electron-positron production of this kind, gamma rays are needed. At atmospheric temperatures, thermal production of gamma rays is fantastically slight, indeed, according to Goody and Yung 1989 who are accepted authorities and have studied the matter, completely negligible. The expected time for such production would be on the order of the currently believed age of the universe or longer. This kind of timing is not a problem for Waleswatcher; for example, he is happy to write about radiation from black holes; a black hole the size of the sun would take (if the Wikipedia article is to be trusted, 10⁶⁷ years to evaporate, enormously far beyond the currently believed age of the universe. Waleswatcher has produced no evidence about this, but he wants to include it in the article. He is basing his case on two things, one the theory thermalization of quantum fields, and two the experimental evidence from non-thermal experiments that can be interpreted as gamma-ray photon-photon interactions. He does not seem to understand that he is here engaging in synthesis, which is not allowed as reliable sourcing, and he is trying to deny that he is so engaging in synthesis.

For our article, your point, that Planck offered a mechanism with ordinary matter, is valid and useful and important. Very much present-day natural phenomenological and laboratory physics is concerned with such a mechanism. Such a mechanism is the basis of the Einstein theory of A and B coefficients and the more pedestrian physical discussions of Kirchhoff's law and Planck's law. The Einstein A and B coefficient theory is widely used today in the physics of interaction between light and matter, and anyone who does not know it is not in touch with modern quantum optics. I am sure that Waleswatcher himself knows it perfectly well, but he seems to want very much that it be thoroughly de-emphasized so as to highlight the more modern, if exotic, quantum field theory understanding that he prefers

Waleswatcher feels that such mechanisms as Planck's are out of date and should be much de-emphasized. Part of his way of expressing this feeling is that he tries to hide the exotic nature of the photon-photon interactions by his referring to abstract physical theory, such as Tolman, theory that is so abstract that one cannot really tell its direct physical meaning in laboratory terms; and by his avoiding telling the reader about the high temperatures or long times needed. I think Tolman had no idea of photon-interactions by way of electron-positron pair production when he was writing; if I am mistaken about that and he did have such an idea, he certainly had no experimental evidence for it such as is now available and is being called upon in Walewatcher's synthesis.

Waleswatcher is making the intellectual mistake of inferring, from a proposition that photon-photon interactions can explain, to a proposition that they do actually explain in the present circumstances. It is the latter that Wikipedia policy requires to be reliably sourced, and that Waleswatcher is manifestly unable to source at all; and he seems even unaware of the need for him to do so.

Waleswatcher seems to think that the mere mention of a widely familiar and revered text such as Tolman, and some loosely related experimental evidence, can make his case; he is synthesizing, not reliably sourcing.Chjoaygame (talk) 20:43, 3 February 2012 (UTC)

Chjoaygame,you write "Waleswatcher is pointing out that in Planck's day, say about 1900". Max Planck was very active in the highest levels of 20th C physics until at least the 1930s (see [[2]]) it is quite ridiculous not to recognise his contributions at the highest level. This remark (Planck's dayy..... 1900....) is precisely why I asked Waleswatcher if he had read anything written by Planck, have any of you? Taking such a patronising attitude makes it rather obvious that most of the contributions here are coming from the poorly informed, no wonder you can't agree! --Damorbel (talk) 22:01, 3 February 2012 (UTC)

Thank you for your comment, Damorbel. I have over some years read plenty of Planck's work and plenty of commentary and historical studies interpreting his work; I am well aware that he published long after 1900, and have read with admiration a fair amount of his later work. (By the way, the Wikipedia article that you cite on Planck is not right up to the mark with the best researched historical work on Planck.) Waleswatcher told us at the beginning of this present controversy that he had not read Planck. My just foregoing comments were intended to give Waleswatch the benefit of the leeway in Planck's opinions. Many physicists today disagree strongly and fundamentally with Planck's physical philosophy and he disagrees with theirs. Planck is generally at odds with the currently prevailing trends in physics; that does not make Planck wrong. Nevertheless it does provide some solace for Waleswatcher, that he is diligently following the majority opinion. The knowledge that he is following the majority attitude to Planck's philosophy is probably a main factor in making Waleswatcher feel that his own personal opinion and viewpoint are so incontrovertibly correct and right that he is entitled for that reason to ride roughshod over all opinion that differs from his. My point is that even giving Planck the least credit within the bounds of reason, Waleswatcher's utter rejection of Planck's thinking is misguided and comes from a very specialized and narrow perspective. Planck's philosophy of physics does not bear on the main questions here. The main thing here is that for much of physics, Planck happens to be right about the need for a material to transduce radiation to thermal equilibrium. At best for Waleswatcher, his viewpoint is exotic and far from ordinary basic physics such as a Wikipedia reader needs to know; Waleswatcher lives in a world of specialized theory. I am here not seeking to downplay Planck's work, which I very much respect. I am seeking to give as sympathetic as possible reading to Waleswatcher's viewpoint; even treated sympathetically, it lacks breadth of physical understanding such as is desirable for editing Wikipedia.Chjoaygame (talk) 23:04, 3 February 2012 (UTC)

The article as it is now is adequately sourced and physically correct. It states that interactions with matter are the main thermalization mechanism practically speaking, but that any interaction - including photon-photon - will suffice. It also says that thermalization can take a very long time, which is true. It is true that at low temperatures the time due to photon-photon is very, very long. It's also true that at high temperatures it's very, very short. I don't think the article needs to go into detail on that, because it's quite peripheral to the subject of black bodies. But if it belongs anywhere, it would fit best in the cosmology section. As for Planck, I've read a little - but I'm a physicist, not a historian of science. Waleswatcher (talk) 23:27, 3 February 2012 (UTC)

By the way - I read the pages of Kondepudi & Prigogine Chjoaygame referred to. They never mention thermalization at all, let alone a time scale for it. I found nothing in there I disagree with even slightly (on the contrary, it's well written). .Chjoaygame seems to be under the misapprehension that electron-positron production has to be possible or likely in order for photons to interact. That's false, because photons interact through virtual e+/e- pairs, and that interaction is non-zero at all photon energies (although as I've said, it is weak at low energies). Waleswatcher (talk) 23:36, 3 February 2012 (UTC)

• Waleswatcher writes that Kondepudi & Prigogine 1998 do not mention thermalization. Indeed they do not use that very word. But they are writing explicitly about the processes by which systems reach thermodynamic equilibrium and thermal equilibrium, words which they do use. The level of intellectual honesty of Waleswatcher in saying that they "never mention thermalization at all" tells it all. Assuming good faith, one has to infer that Waleswatcher is unable to be intellectually honest with himself about this particular matter. It is ok for him to deceive himself to protect his special message, but when he tries to convince Wikipedia readers of his special point of view, then it's time to try to resist him. But, sad to say, the process of Wikipedia, in practice, allows a determined self-deceiver like Waleswatcher to get away with overemphasizing his special point of view in a Wikipedia article without effective constraint.Chjoaygame (talk) 15:20, 4 February 2012 (UTC)

• Some selected bits of the final paragraph of Karplus & Neumann 1951 may be of interest to some. It goes:

   Finally, we may discuss the experimental significance of the results. ... For presently attainable values of the experimental parameters, therefore, it would seem that N is too small to be detected in the presence of the probable background radiation.

Oh! In ordinary language, that means that the synthesized photon-photon interaction thermalization could not be detected empirically. The proposed Planck distribution derived from the synthesized photon-photon interaction thermalization would not be empirically detectable for the writers of the proposed source.Chjoaygame (talk) 22:43, 5 February 2012 (UTC)

Thermalization and photon-photon scattering

Is the very long thread on this topic only about the parenthetic remark "(for instance photon-photon interactions[15])". That seems to be more attention than this remark deserves, and I'd say the question of whether photon-photon interaction without matter present can thermalize the photons themselves is a fine point that need not be included here inasmuch as its accuracy is challenged. The references provided to support this point are not easily accessible (I can't access them), and could be classified as unintelligible to the non-expert, so I think this parenthetic remark should be deleted from the article.

Is the point instead whether a black body can be made as a tiny hole in a cavity with perfectly reflecting walls? Then the temperature of the photons is not related to the temperature of the walls. What is it? By definition, the perfectly reflecting wall does not emit. Does a perfectly reflecting wall have a temperature?

Suppose the radiation inside the perfectly reflecting cavity were compressed by moving the cavity walls. The radiation exerts pressure on the walls so work is done compressing the radiation, and the energy of the radiation increases. We know the spectrum of the radiation changes with the cavity dimensions, so the population of the spectrum will be non-thermal initially even if it began in thermal equilibrium. What brings the photons to equilibrium now? Do we need Planck's small piece of absorbing and radiating matter? Is it photon-photon interaction? Is there a discussion available somewhere?

The full discussion of thermalization of photons via its many mechanisms and its role in the evolution of the Big Bang belong in an article devoted to that topic. Brews ohare (talk) 16:29, 4 February 2012 (UTC)

Question: "Is the very long thread on this topic only about the parenthetic remark "(for instance photon-photon interactions[15])"." Chjoaygame's answer: No. It's not only about that parenthetic remark.

Comment written above: "That seems to be more attention than this remark deserves, and I'd say the question of whether photon-photon interaction without matter present can thermalize the photons themselves is a fine point that need not be included here inasmuch as its accuracy is challenged. The references provided to support this point are not easily accessible (I can't access them), and could be classified as unintelligible to the non-expert, so I think this parenthetic remark should be deleted from the article." Chjoaygame's reply. It depends what you mean by a fine point. The editor who wants to make his point (that the statistics of quantum field theory indicate thermalization of photons) has offered the photon-photon interaction as part of his synthesis to justify including his point. Take away that part of his synthesis, and his synthesis collapses entirely. Doubtless he will offer some replacement for the photon-photon mechanism if it is removed, and will continue to synthesize. He is determined to make his point without being able to source it properly. I do not foresee his becoming reasonable about this. If the parenthetic remark were to be deleted, the whole sentence would logically need to be deleted with it. I think he would probably find a way to put it back in again.

Comment written above: "Is the point instead whether a black body can be made as a tiny hole in a cavity with perfectly reflecting walls? Then the temperature of the photons is not related to the temperature of the walls. What is it? Does a perfectly reflecting wall have a temperature?"

Chjoaygame's answer:

It seems to be agreed by all that a black body surface can be modeled by a tiny hole in a cavity with perfectly reflecting walls under some conditions. The argument is as to what conditions.

Waleswatcher is of the view that there need be no matter in the cavity; just put some definite number of photons in the cavity with perfectly reflecting walls, and the statistics of quantum field theory will see to it that the photons become thermalized according to Planck's law. He does not have a definite statement to make about the time it will take for this to happen. He is content to say that it can be very long. This is because he does not have a source about the time. Since the time is important, some information is needed about it. Without clear information about the time, the idea is synthesis and not reliably sourced. Prigogine & Kondepudi 1998 give clear information about the time, but Waleswatcher won't allow it to be stated in the article. Goody & Yung state that for atmospheric thermal physics, it is "completely negligible" but Waleswatcher won't allow it to be stated in the article. There is a physical problem with Waleswatcher's story. If a definite amount of light is put into the cavity with no matter present, then on the time-scale of the hypothesized thermalization, according to the quantum field theory, some of the light will be converted to matter and the remaining light will be of less energy than was originally put into the cavity; Planck's law for a given definite amount of light, the amount originally put in, prescribes a final equilibrium temperature; if some of the original amount of light is converted to matter, then the full temperature prescribed by Planck's law for the original amount of light will not be achieved in the eventual equilibrium. It takes a considerable amount of light to create particles of matter; this means that Waleswatcher is not describing a pure thermalization according to Planck's law, and is contrary to his claim to be doing so.

In practice, and traditionally in many textbooks of quantum optics, it is customary to put in some condensed matter to bring about thermalization of the light field. It is established and sourced (Milne 1930 and later confirmations) that the nature of the matter determines how long it takes to thermalize, and that it happens within laboratory time frames. This much is now in the article and is not a problem.

Above comment: "Then the temperature of the photons is not related to the temperature of the walls." Chjoaygame's answer: This a practical viewpoint and is explicitly accepted by ordinary quantum optics textbooks, and is implicitly accepted in the article as a practical viewpoint. But Waleswatcher has a message that it's not the end of the question. He thinks that the light will find its own temperature without interacting with the walls, and this needs to be made clear. But he can't provide a reliable source for it, and has to rely on synthesis.

Above comment: "What is it? Does a perfectly reflecting wall have a temperature?" Chjoaygame's answer: Perhaps the wall has a temperature, but if it is perfectly reflective, that will not be conveyed to the light field in the cavity. I think that is agreed by all here.

Above comment: "The full discussion of thermalization of photons via its many mechanisms and its role in the evolution of the Big Bang belong in an article devoted to that topic." Chjoaygame's answer: Yes. Yes. Yes. (Such an article in Wikipedia would not constitute a reliable source for a quick comment about it here, because Wikipedia is not a reliable source.)

It is not good practice to selectively delete a key part of a story with the practical effect of allowing unsourced material to stay in Wikipedia, the deletion being made mainly for that purpose. Deleting the parenthetic remark would in practice probably serve to hide the fact that the whole story of thermalization is not reliably sourced. The proper Wikipedia way is deletion of the whole story of thermalization without matter till it is reliably sourced. Reliable sourcing for this matter must include clear statements of the mechanisms, temperatures and time-scales. Without them, statements would be just speculative unsourced chatter or synthesis.Chjoaygame (talk) 17:49, 4 February 2012 (UTC)

Chjoaygame: This article already points out that thermalization of photons in a cavity with perfectly reflecting walls can be made to occur by including matter. I'd guess that is acceptable to all.

If a source can be found, it would be germane to note that EM radiation in thermal equilibrium with matter inside a cavity need not be in thermal equilibrium with the cavity walls if they are perfectly reflecting, simply because no energy exchange with the walls takes place.

The question could be posed as to whether thermalization of photons will occur in a cavity with perfectly reflecting walls when no matter is present. There may be no definitive answer to this question, and if so, the question can be raised and just left that way, if that seems to be useful.

As for the status of theory on this subject, I don't see that there is any definitive statement in the literature in this regard. We know that in QED vacuum light scatters from light via vacuum fluctuations. I don't know if this process allows energy transfer. I suppose that a careful examination of this mechanism could determine whether an entire population of photons at all frequencies could participate in such interactions, in which case thermalization might occur if the interactions allow energy transfer between all photons, and not simply a subset. Maybe there are other mechanisms. I don't think this article is the place to discuss this matter. Brews ohare (talk) 18:19, 4 February 2012 (UTC)

Above comment: "This article already points out that thermalization of photons in a cavity with perfectly reflecting walls can be made to occur by including matter. I'd guess that is acceptable to all." Chjoaygame's response: Agree.

Above comment: "If a source can be found, it would be germane to note that EM radiation in thermal equilibrium with matter inside a cavity need not be in thermal equilibrium with the cavity walls if they are perfectly reflecting, simply because no energy exchange with the walls takes place." Chjoaygame's response: There are plenty of reliable sources for that. The sources already given say it. But Waleswatcher will say that it is misleading to leave it at that because he wants to say that eventually the light will thermalize by mechanisms proposed by quantum field theory without matter in the cavity even with perfectly reflecting walls. There is of course no possibility of an experimental test of this claim of Waleswatcher, if only because no sufficiently perfectly reflective walls could be made. But Waleswatcher will say that it is necessarily true because of the quantum field theory. He then offers some experimental support for the quantum field theory. But such experimental support does not bear directly on the thermalization question that concerns us here, and so to use it, in composition with the theory, is synthesis, and does not give any hint about the key question of temperature scales and time scales, for which direct sources are necessary. For example, Waleswatch is not too worried about talking about a process that might take 10⁶⁷ years, but we should hardly give such a process space, or even mention, in a section about black body simulations, at least at the present stage of its development.

Above comment: "The question could be posed as to whether thermalization of photons will occur in a cavity with perfectly reflecting walls when no matter is present. There may be no definitive answer to this question, and if so, the question can be raised and just left that way, if that seems to be useful." Chjoaygame's response: That is a very reasonable suggestion. I have no finally definite feeling about it right now.

Above comment: "As for the status of theory on this subject, I don't see that there is any definitive statement in the literature in this regard. We know that in QED vacuum light scatters from light via vacuum fluctuations. I don't know if this process allows energy transfer. I suppose that a careful examination of this mechanism could determine whether an entire population of photons at all frequencies could participate in such interactions, in which case thermalization might occur if the interactions allow energy transfer between all photons, and not simply a subset. Maybe there are other mechanisms. I don't think this article is the place to discuss this matter." Chjoaygame's response: Agree. Thermalization requires transduction between frequencies, and scattering is not enough for that. As you note, an important relevant question is as to the sufficiency of mechanisms for full thermalization (I would add, within a specified and physically relevant energy- and time-frame). Without prejudice as to the appropriateness or not of quantum field theory's putative mention here, if Waleswatch wants something about it in this article, he should provide reliable sources.Chjoaygame (talk) 19:14, 4 February 2012 (UTC)

If in fact the QED vacuum fluctuations lead to thermal equilibrium all by themselves, that would seem to suggest the equations of QED involve irreversibility, which seems unlikely. Brews ohare (talk) 19:18, 4 February 2012 (UTC)

There is debate over this. Brews ohare (talk) 19:23, 4 February 2012 (UTC)

Let us never, never doubt

What nobody is sure about!

Relevant to this debate is the statement by Anastopoulos, C. (2008), Particle or Wave: the Evolution of the Concept of Matter in Modern Physics, Princeton University Press, Princeton NJ, ISBN 978–0–691–13512–0, as follows: "There has been so far not even the outline of a proof that the macroscopic properties of matter arise from the principles of quantum field theory.⁶ We cannot employ our theories to demonstrate conclusively that quarks lump together to form protons and neutrons. [The ⁶ note is extensive with references and I will not copy it here.]"Chjoaygame (talk) 19:43, 4 February 2012 (UTC)

I have to say that I am not convinced that EVERY interaction will cause thermalization. I also have to admit that I can find no source that says that QED photon-photon interactions (which certainly exist!) will lead to thermalization, although I suspect that they do. I can find no source that says that gravitational interactions between photons (again, which certainly exist!) will lead to thermalization. I am even suspicious that inverse-square interactions for massive particles lead to thermalization, although we should be able to get some good references on this by looking at the theory of globular clusters of stars. That leads me to be suspicious of gravitational photon-photon interactions. Unless someone can come up with a reference that says that Planck's speck of carbon, or matter of some sort is IN PRINCIPLE necessary, then we should not ever imply that it is. I doubt if there is any reference that says that matter is needed in principle.

Elastic collisions between massive particles absolutely DO cause thermalization. The fact that QED fluctuations lead to thermal equilibrium does not imply that QED is irreversible any more than the fact that classical collisions are reversible implies that classical physics is irreversible. Irreversibility only occurs when you have limited information about the system, and because of this, as time goes by, you lose information that cannot be recovered by the same techniques that you acquired the original information. (Those techniques being measurement of classical thermodynamic parameters like temperature, pressure, etc.) PAR (talk) 18:18, 5 February 2012 (UTC)

Effect of size of hole in cavity

The section Black_body#Cavity_with_a_hole contains the statement:

"The fact that not all radiation incident on the hole will enter into the cavity — particularly if the wavelength is longer than the diameter of the hole — makes the hole not quite a perfect black body.

So far as I can determine, no source discusses this point. It also would affect the photons emitted from the cavity.

It would seem the relevant discussion would belong in diffraction. Diffraction still allows transmission, but I don't know how it affects the number of photons transmitted through the hole. Presumably some photons won't make it. However, this source suggests diffraction has little effect upon transmission. Anybody know of a source? Brews ohare (talk) 18:07, 5 February 2012 (UTC)

Here's a possibility. Brews ohare (talk) 18:13, 5 February 2012 (UTC)

My feeble intuition tells me that if the incident light has a long wavelength compared to the diameter of the hole, a wave approach works, and the fields across the hole are spatially constant, and the transmitted power is just proportional to the area of the hole. On the other hand, if the wavelength of the light is small, a particle picture works, and again the number getting through is just proportional to the area. So the complicated case is when the two are very similar in size, and suggests if there is an effect of hole size upon transmission it occurs over a rather limited frequency range. Any comments? Brews ohare (talk) 18:22, 5 February 2012 (UTC)

the point

The whole point of mentioning photon-photon interactions is not to imply that they are a practical means of thermalization. The point is to stress that thermalization is a fundamental requirement for equilibrium and that the idea that matter must be present for photon equilibrium does not have this fundamental standing. It is not a physical law or principle, it is just something that happens to be true over a wide range of conditions found in the universe, but not all. Thermalization, by any means, is absolutely necessary. The need for matter in photon equilibrium is NOT absolutely necessary and mentioning the unlikely case of thermalization by photon-photon interactions drives this point home. Arguing that it hardly ever happens misses the point. PAR (talk) 03:18, 5 February 2012 (UTC)

• PAR: I have no doubt that photon-photon scattering occurs, and sources can be found that say so. However, I have found no accessible source that says photons can achieve thermal equilibrium this way. On the other hand, it is easy to find sources that say photons can achieve thermal equilibrium when there is matter present. At a minimum this lack of discussion indicates that photon-photon interaction in the absence of matter is a very minor mechanism. At worst, it may not occur. The Big Bang discussions suggest that thermalization of photons stopped as soon as matter became charge neutral.

So, for this article, I'd say this mechanism is not worth bringing up, and may be incorrect, and certainly is hard to support. Brews ohare (talk) 14:48, 5 February 2012 (UTC)

PAR is correct. The text as it is now is adequately sourced (any interaction thermalizes, and photons interact), and as PAR says it makes an important point of principle. Photons ARE in equilibrium today for the most part (at 2.7K), it's the matter that isn't. Waleswatcher (talk) 15:02, 5 February 2012 (UTC)

• Waleswatcher: Your discussion here is inadequate in several respects: first, it is demonstrably false that any interaction thermalizes. An obvious example is elastic scattering. That is an overstatement. The mere fact of photon interaction does not establish energy exchange and does not establish that it occurs at all energies. The 2.7K equilibrium observed today does not imply that this equilibrium temperature was achieved by photon-photon interaction, and in fact sources say the contrary, it is due to photon-photon interaction catalyzed by interaction with matter at an earlier epoch. What is called decoupling occurred when the Universe reached a size about ≈10⁻³ its present size. A more detailed discussion suggests these photons are not scattering at all at present.

If you can provide accessible documentation, that would be helpful. Brews ohare (talk) 15:24, 5 February 2012 (UTC)

• Waleswatcher is here showing us the nature of synthesis: he writes "any interaction thermalizes, and photons interact" from which he synthesizes that 'photon interactions thermalize'.Chjoaygame (talk) 15:46, 5 February 2012 (UTC)

• It seems that there are several points that PAR wants made, even driven home. One such point of PAR is that for equilibrium to occur, there must be a process by which it is approached; in this case the process is called thermalization. For this, it seems agreed that one actually occurring form of thermalization of light is in the presence of matter. Another such point of PAR seems to be that sometimes, even if only "hardly ever", it actually happens that a mass of light thermalizes in the absence of matter; perhaps, with a reliable source, he can point to a particular definite actual instance?Chjoaygame (talk) 15:32, 5 February 2012 (UTC)

Elastic scattering does thermalize, Brews ohare. Your "obvious" counterexample isn't a counterexample at all. Chjoaygame, there is no synthesis. Every statement is sourced except "very long time" - shall we remove that? And for your future reference, wiki's guidelines do not forbid synthesis. You might want to review them. Waleswatcher (talk) 18:11, 5 February 2012 (UTC)

• Waleswatcher: Maybe you could elaborate a bit on elastic scattering. In my mind elastic scattering involves no energy transfer. Maybe that isn't accurate. But it's what I meant. In that case, the incoming and outgoing particles just change direction, and the energy distribution isn't affected. Brews ohare (talk) 18:38, 5 February 2012 (UTC)

Elastic scattering means the total kinetic energy of the particles after the collision equals the total KE before. It certainly does lead to thermalization (that's a classic result, actually). By the way, the simplest photon-photon interaction is elastic scattering, it occurs at all energies/frequencies of the photons, and as I said before that's the input to (some of) Tolman's calculations. Waleswatcher (talk) 18:55, 5 February 2012 (UTC)

• Synthesis of published material that advances a position

Do not combine material from multiple sources to reach or imply a conclusion not explicitly stated by any of the sources. If one reliable source says A, and another reliable source says B, do not join A and B together to imply a conclusion C that is not mentioned by either of the sources. This would be a synthesis of published material to advance a new position, which is original research. "A and B, therefore C" is acceptable only if a reliable source has published the same argument in relation to the topic of the article.

Oh?Chjoaygame (talk) 19:05, 5 February 2012 (UTC)

Exactly. Synthesis is just fine so long as it doesn't advance a position not explicitly stated in one of the sources. There's nothing in the article that does that. In fact there's no synthesis at all. One source says interactions (specifically, weakly collisional interactions in a Bose gas) thermalize. Another source says photons are a weakly collisional Bose gas. That's all the article states. Waleswatcher (talk) 19:11, 5 February 2012 (UTC)

Oh! The juxtaposition of the statements seeks to advance the position that photon-photon interactions thermalize, which is, as you show, not stated explicitly in a reliable source. Without that, the statements are irrelevant.Chjoaygame (talk) 19:44, 5 February 2012 (UTC)

It "seeks to advance". I see. Just a few days ago you were accusing me of reading something into what was written that wasn't there. The facts are very clear - everything written there is sourced, and if the reader draws the obvious (and completely correct) conclusion from that, so be it (and all the better). Waleswatcher (talk) 00:29, 6 February 2012 (UTC)

Waleswatcher:

A: interactions (specifically, weakly collisional interactions in a Bose gas) thermalize.

B: photons are a weakly collisional Bose gas.

The sources you have provided haven't said exactly A & B. Maybe they say something like that with some caveats. I think you have to dwell upon the sources further to show that is the situation, that the caveats are satisfied, and the summary of the sources has not been oversimplified. If you do that, it will become a multiparagraph discussion just to establish a parenthetic remark of little consequence. Brews ohare (talk) 21:50, 5 February 2012 (UTC)

There aren't any subtleties - the sources are very clear. It's not like there's anything the least bit controversial about this - the whole reason thermodynamics is interesting and this article exists is that all interactions thermalize. As for photon-photon scattering, it's a completely standard part of modern physics that's been understood at least since Euler and Heisenberg in the 1930s. I really have no idea why you guys are arguing over it - it's obviously correct, it's adequately sourced, and as it's written now the article makes clear that it's not the origin of thermalization "practically" speaking (meaning in an oven, for instance). Waleswatcher (talk) 00:28, 6 February 2012 (UTC)

As you make clear, it seeks to advance.Chjoaygame (talk) 04:09, 6 February 2012 (UTC)

It simply states facts, all of which are reliably sourced, and all of which are obviously relevant (as agreed on by at least two editors). If from those facts you draw a conclusion, that's your own business. Indeed, it's the entire purpose of an encyclopedia to put relevant facts in front of the reader, allowing her to draw her own conclusions. Waleswatcher (talk) 04:20, 6 February 2012 (UTC)

Waleswatcher: You might take into consideration that not all readers have your background, and myself in particular. For such an audience, more care is needed to explain the obvious, as it may not be obvious to the unversed. In this particular matter, none of the sources you have brought forward are particularly transparent, and you have not provided any guidance as to what to look for in the pages of discussion identified, some of it very technical. I believe we all understand the formal logic that if A → B and B → C then A → C. However, it is not clear the sources fall into this pattern. You need to provide further assistance. Imagine yourself addressing a high-school student curious about this matter, but with little notion of what thermodynamics and scattering are about. If you compose an argument on this Talk page, I'd be happy to tell you what seems to me to be too much detail and what is too little. Brews ohare (talk) 04:29, 6 February 2012 (UTC)

Cavity with a hole

Among other statements, the last paragraph in the section Black body#Cavity with a hole states:

"This characteristic, continuous spectrum of thermal radiation depends only on the body's temperature, and not upon its shape,..."

I believe this paragraph to be misleading. The occurrence of black-body radiation depends upon the temperature of the radiation, and the cavity itself is instrumental only if it plays a role in setting the temperature of the radiation. That is made clear, for example, by Planck's consideration of a cavity with perfectly reflecting walls containing a small piece of absorbing and emitting matter. The thermalization of the photons occurs because the matter catalyzes energy exchange among all the photons in the cavity. The temperature achieved at equilibrium has no relation to the temperature of the cavity because no energy is exchanged when the walls are reflecting.

Likewise, the radiation emitted from the small hole depends upon the size of the cavity because that affects the energy levels of the modes in the cavity, and these are spaced in a manner strongly dependent upon the shape of the cavity for wavelengths comparable to the cavity dimensions, or longer. Consequently, even if these modes are populated according to a temperature T, the emission fails to be quasi-continuous when the level spacings are not close together. If you like, the sampling is coarse.

This paragraph needs to be rewritten. Brews ohare (talk) 15:18, 5 February 2012 (UTC)

Please don't forget that photons are not conserved in a cavity unless the cavity is 100% reflecting. Photons are generated by the emission process and they remain in existence until absorbed.

I suggest the whole article needs extensive revision, it contains far too much about 'non-black' bodies (gray etc.) so much so that the rather restricted properties of the concept are almost undetectable. --Damorbel (talk) 16:18, 5 February 2012 (UTC)

I made some changes to emphasize the temperature of the radiation is critical, and that the size of the cavity plays a role for long enough wavelengths. I did not bring up the point you mention, and I am unsure of its significance. I imagine that thermal equilibrium is not possible unless the number of photons at each energy can change to achieve a Planck spectrum. That can be achieved by means other than the walls of the cavity, of course. So I am unsure that the reader will entertain some misconception about conservation of photon number. Brews ohare (talk) 17:17, 5 February 2012 (UTC)

A black body absorbs all frequencies by definition. The caveats you are writing about do not apply to black bodies. It's true that a finite cavity or finite hole won't emit a Planck spectrum, but that's because it's not a black body. Your rewrites have confused that. Waleswatcher (talk) 18:16, 5 February 2012 (UTC)

Waleswatcher: I agree with your first statement. The rest of this comment is simply not true. Black bodies don't have to be in thermal equilibrium. So the radiation they emit doesn't have to have a Planck spectrum. In the case where it does, the radiation is called black-body radiation, but black bodies that have nothing to do with black-body radiation are feasible. I thought this discussion was settled a long time ago. Brews ohare (talk) 18:43, 5 February 2012 (UTC)

I'm the one that made the point that thermal equilibrium is required. What I'm telling you is that finite size effects, which do in fact prevent the body (even at thermal equilibrium) from emitting a Planck spectrum, also prevent it from absorbing perfectly. If it doesn't emit a Planck spectrum at thermal equilibrium it doesn't absorb perfectly, and vice versa. So it's incorrect to say that black bodes in thermal equil. do not emit a Planck spectrum because of finite size after just defining black bodies as perfect absorbers. Waleswatcher (talk) 18:51, 5 February 2012 (UTC)

So, the correct statements are "No finite size cavity with a geometry not specially contrived can be a black body. Likewise, thermal radiation inside a finite sized cavity cannot have a Planck spectrum. However, ignoring such wavelength effects, radiation in thermal equilibrium inside a cavity is well approximated by a Planck spectrum." ?? Brews ohare (talk) 20:34, 5 February 2012 (UTC)

No finite sized object can be a black body, full stop. I have no objection to adding that to the article if you can find a source for it (it's obvious, which makes it one of those things you might have trouble finding in a reliable source). The important physical point is the absorption and emission are intimately related, which is why imperfect absorbers do not emit Planck spectra even in equilibrium. Waleswatcher (talk) 00:34, 6 February 2012 (UTC)

Waleswatcher: I understand that when wavelengths approach a cavity dimension the frequency of that excitation is very sensitive to the boundary, having few if any oscillations within the cavity. So the spectrum exhibits peculiarities related to the cavity size and shape. That being so, I can understand that the population of this state in thermal equilibrium might depart somewhat from Planck's law. I further can understand that emission and absorption in equilibrium are connected by this discrepancy.

However, I confess that I do not understand why the cavity's ability to be a perfect absorber is so affected. The explanation of the cavity with a hole provided by many and by this article is simply that the cavity is so-constructed that any entering photon becomes lost inside and cannot return. Perhaps "entering" the cavity amounts to exciting the internal modes of the cavity, each to a degree determined by the field oscillation at the aperture. Now, supposing the cavity is finite, perhaps it is inevitable that the long wavelength excitations have a likelihood for re-emission not shared by the shorter wavelength components. Is that the sort of explanation to look for? Brews ohare (talk) 04:42, 6 February 2012 (UTC)

If the photon has a wavelength that's longer than the hole size, it's unlikely to enter at all. The hole cannot be bigger than the cavity, so photons with wavelength longer than the cavity are unlikely to be absorbed, and hence that's not a black body. Black holes might actually be the best example of this, as they are otherwise ideal black bodies and their "grey body factors" have been calculated in detail. Waleswatcher (talk) 04:47, 6 February 2012 (UTC)

Waleswatcher: I believe this argument could be phrased as follows: A low frequency incident wave causes a slow variation of the EM fields at the aperture (the supposed black body). The coupling to the modes in the cavity is determined by a Fourier expansion of the excitation at the aperture in the modes of the cavity. So for instance, assuming the slowly varying excitation to cause a spatially uniform field at the aperture that varies in time, we have to find a Fourier expansion of the aperture in cavity modes at any particular time. And this has to be possible again at a later time when the fields have changed. So the absorption by the "black body" seems to translate into whether the cavity wavefunctions are capable of representing a disc-like excitation on the cavity wall. If we have a continuum of wavelengths and frequencies in the cavity that isn't a problem, but the smaller the cavity the fewer wavefunctions we have to accomplish the Fourier representation, and so the less of the incident excitation will enter the cavity.

Does that accord with how you see the matter? I haven't seen a careful analysis, have you? Brews ohare (talk) 05:04, 6 February 2012 (UTC)

That sounds like a correct intuition for the classical problem. It would be modified somewhat if the incident radiation is a single photon, but it would still be more or less correct. I've never seen a careful treatment for a cavity with a hole, which is why I suggested black holes. You don't get any blacker than a black hole, and yet neither absorption nor emission is perfect for wavelengths longer than the radius of the hole. Waleswatcher (talk) 06:01, 6 February 2012 (UTC)

Photon-photon interactions again

Waleswatcher, your revision has reinstated the following remark.

Any interaction (for instance photon-photon interactions[15]) will accomplish thermalization,[16] but the time it takes to do so depends on the strength of the interaction and may be very long.[17]

To begin with, the statement that any interaction is sufficient is not supportable, and none of the sources say anything like this. That can be fixed by minor rewording.

More difficult is the reference to photon-photon interactions. The sources are

[15] Robert Karplus* and Maurice Neuman ,"The Scattering of Light by Light", Phys. Rev. 83, 776–784 (1951)

[16] R.Tolman, "The principles of statistical mechanics", p. 458

[17] Kondepudi & Prigogine 1998, pp. 227–228; also Section 11.6, pp. 294–296.

Reference 15 is found here and appears from its abstract to discuss only the nature of photon-photon scattering, without any regard for thermalization.

Reference 16 appears to refer to a section of Tolman discussing "Change of H with time as a result of collisions" This discussion is very general and makes some assumptions about the collisions involved that may, or may not, be true of photon-photon collisions. I don't know and Tolman is concerned with matter, not radiation.

Reference 17 I would take as referring to Modern thermodynamics: from heat engines to dissipative structures. From the Amazon "look inside" feature I see that pp. 227-228 refer to "Transformation of matter" and pp. 294-296 to "Matter, radiation and chemical potential". Unfortunately it is not possible to access these pages.

My conclusion is that the first of these references is related to some aspects of photon-photon interactions, and the second to some aspects of the approach to equilibrium, but the dots are not connected. The last reference might have something to say, and if it does perhaps you could quote the relevant remarks. On the face of it, this last discussion is a general overview and will not describe directly the case of photon-photon interactions in achieving thermal equilibrium.

So, in sum, I believe I have made a good faith effort to follow up with your documentation and have not been successful. Moreover, I have looked myself using Google books for some discussion without success. I conclude that further support is necessary to include these remarks in the article. Brews ohare (talk) 21:01, 5 February 2012 (UTC)

Ref 16 proves that weakly interacting Bose gases thermalize. He also states that any interaction should do the same. You're incorrect that he is concerned with matter and not radiation; for one thing, he derives the Planck distribution. Waleswatcher (talk) 04:27, 6 February 2012 (UTC)

By the way - do you realize that in arguing that photon-photon interactions will not thermalize a gas of photons in a perfectly sealed container, you are claiming that the H-theorem and the second law of thermodynamics are wrong? That's precisely the type of system the 2nd law applies to - it's an isolated system, so its entropy must increase according to that fundamental law. Indeed, it is the claim that they will not thermalize that is extraordinary and requires extraordinary evidence. The second law itself is a sufficient citation for this. Shall we add one to Kittel&Kroemer, with the appropriate page number? Waleswatcher (talk) 04:39, 6 February 2012 (UTC)

Waleswatcher: You misconstrue my objection, which is not that I believe your statement to be wrong, but that it is inadequately supported. To fix this problem, perhaps you could quote the sources where they have supported your thesis so that can be remedied. Please excuse my unwillingness to accept that you have understood the sources correctly, but my experience on WP has been that many contributors read what they want to find, and not what the author has said. Brews ohare (talk) 04:48, 6 February 2012 (UTC)

The second law of thermodynamics - the entropy of a closed system increases until it's in thermal equilibrium. If a law of physics won't satisfy you, what will? Waleswatcher (talk) 05:58, 6 February 2012 (UTC)

"...what will?" Perhaps quantum effects? The whole point of quantum theory is that reactions (at the relevant quantum level) between particles, both photons and massive particles, do not take place in an energy continuum but in energy related quantum steps, this is why Planck's black body radiation formula was able to match experimental evidence. Should the energy of the radiation be always proportional to frequency, the classical mechanical analysis, this would have produced what Ehrenfest described as 'an ultraviolet catastrophe' - the energy of the waves would have increased without restriction, thus waves had to be replaced by photons.

Your description of the 2nd law is for reversible systems e.g. thermal systems where the energy is characterised by temperature; gravitational systems are also reversible. Non reversible systems, where the energy is not free to transfer backwards and forwards smoothly, do not violate 2nd law. There are very many processes of this kind, e.g. chemical energy, few chemical processes are fully reversible; similarly the energy in electron orbits changes in quantum steps. Do I need to continue? --Damorbel (talk) 07:30, 6 February 2012 (UTC)

If you find it satisfactory to cite the second law in this connection, it is hardly necessary to bring up the very minor contribution of photon-photon interactions. Just cite the second law and be done with it. It appears that no attempt will made here to illuminate the specifics of photon-photon interactions in achieving thermal equilibrium, and I take this lack of engagement in elaboration as an indication that all assembled here have failed as miserably as myself in tracking them down. Brews ohare (talk) 13:39, 6 February 2012 (UTC)

On what basis are you deleting relevant, sourced material? I'm trying, but I honest cannot understand your position. You now agree that the material there is correct. You've been unable to point to any statement that isn't sourced. The second law of thermodynamics is what the Tolman reference proves for the case of a weakly collisional Bose gas (as well as other cases), which is precisely what a gas of photons in a sealed container is. Your position - that photon-photon interactions will not thermalize such a Bose gas - is in direct conflict with that proof, and it's in direct conflict with the fundamental laws of physics, specifically the 2nd law applied to that system. In the absence of interactions with any other material or particles, photon-photon interactions are the only interactions, and - as Tolman says, as every other book says - those photons will thermalize. Waleswatcher (talk) 14:14, 6 February 2012 (UTC)

Waleswatcher: I don't agree with your characterization of this material either as relevant (it is not) or as sourced (it is not). As to the first, photon-photon interaction as a significant contributor to thermal equilibrium has not been sourced at all, and as for the second, your best effort is to refer to the second law, which of course, hardly depends upon photon-photon interaction. If you insist upon inserting this material, perhaps an RfC is necessary? Brews ohare (talk) 14:19, 6 February 2012 (UTC)

Brews ohare: As you wish. For the fourth (?) time: the Tolman reference proves explicitly, both in quantum mechanics and in classical mechanics, that a gas of photons (or any other Bose particle) will thermalize. The only conditions for the proof are the existence of interactions. Of course photons interact, but in case there's any doubt, there are two references for that too. Note that the Tolman proof is simply the second law of thermodynamics applied to the specific case of a sealed box of photons, so if you dispute it, you are disputing a law of physics. There are many, many other sources that do the same derivation.

Why you think this is remotely controversial and are wasting both of our time on it is beyond me. The only argument you could make is that it's not relevant, because photon-photon interactions aren't the dominant interaction under "practical" circumstances. That's already mentioned in the article, and as PAR and I agree, it's important as a point of principle to mention that interactions other than with matter will also thermalize the photons. That shows that matter is not necessary, it highlights that any interaction will thermalize, and it's both relevant and important in some situations of great interest to physicists (such as very high electric or magnetic fields or the very early universe). It's relevant, important, and might teach the reader something new and interesting.

If you're simply going to insist that photon-photon interactions cannot be mentioned, this dispute will go on for the reasons explained above. If instead you would like to modify the language, go ahead and suggest such a modification and we can discuss it. Waleswatcher (talk) 15:17, 6 February 2012 (UTC)

Waleswatcher: Perhaps photon-photon interactions could be mentioned if there were some source that indicated they were of any importance. Otherwise, one could add to the "second law text" in the RfC:

"The approach of electromagnetic radiation to equilibrium is governed primarily by interactions mediated by matter, but photon-photon scattering (see [15]) by itself may play a minor role."

How's that? Brews ohare (talk) 16:26, 6 February 2012 (UTC)

Waleswatcher:BTW, I don't understand why it is important to you to emphasize that matter is not essential to the equilibrium of radiation, most particularly when there is no example to cite where photon-photon interactions without matter have been of any significance. Even QED vacuum scattering of photons by photons requires the presence of virtual charged particles. Brews ohare (talk) 16:40, 6 February 2012 (UTC)

Compromise text

Below is a proposal for the text involving photon-photon scattering:

According to the second law of thermodynamics, any closed system eventually will reach thermal equilibrium; but notwithstanding this law, the time it takes to do so may be very long. Although in practice the role of photon-photon scattering usually is negligible, in principle, even in the absence of matter, radiation will come to thermal equilibrium eventually.

Brews ohare (talk) 18:07, 6 February 2012 (UTC)

I don't object seriously to that. I'd edit it for wording, re-insert several references, and perhaps clarify "in practice" by adding something like "(at temperatures below billions of Kelvin)". Waleswatcher (talk) 18:28, 6 February 2012 (UTC)

OK, please format the references so they have links and try to find some that are intelligible. Brews ohare (talk) 18:36, 6 February 2012 (UTC)

How's this:

According to the second law of thermodynamics, any closed system will approach thermal equilibrium; although the time it takes to do so may be very long. At temperatures below 10^9K the role of photon-photon scattering CITE PHOTON SCATTERING PAPER is usually negligible compared to interactions with matter. In principle, even in the absence of matter, radiation will come to thermal equilibrium eventually. CITE TOLMAN Waleswatcher (talk) 18:42, 6 February 2012 (UTC)

I think the purpose of the sources should be more clear. I'd take it that your purpose is two-fold: (i) photon-photon scattering does occur, and (ii) it can lead to thermal equilibrium of radiation.

To establish (i) is not difficult, and I don't know why you wish to refer to a source from 1951 that is not available without subscription. How about this source, for example? As for (ii), no direct source involving actual photon scattering appears available, so we have to appeal to these photons behaving like a Bose-Einstein gas. So I'd suggest something like this:

According to the second law of thermodynamics, any closed system eventually will reach thermal equilibrium; but notwithstanding this law, the time it takes to do so may be very long. Although in practice the role of photon-photon scattering usually is negligible, photons do interact with one another, and like other interacting bosons are expected to reach thermal equilibrium eventually, even in the absence of matter.

Brews ohare (talk) 21:07, 6 February 2012 (UTC)

I'd say we're close enough. I'll add my version to the article. As for the references, the 1951 paper has 100s of citations. I don't think the fact that it's hard to access on the web for free has much if any bearing on whether we should use it. Tolman is an excellent book that goes into detail on precisely this point, so I'm going to insist we keep them both unless you have a superior proposal (and of course we can have multiple references). Waleswatcher (talk) 03:27, 7 February 2012 (UTC)

Knudsen gas

Why is this example brought up? First, the rest of the sentence is about practical matters, and a Knudsen gas is a hypothetical construct. Second, the Knudsen gas as I understand it is collisionless, so any equilibrium is achieved only by interaction with the cavity walls. The rest of the sentence suggests the Knudsen gas is a form of "rarefied matter" and is about role in thermalization effects due to this matter, not to wall effects. So on both counts, this example should be dumped, and I did that. Brews ohare (talk) 18:11, 16 February 2012 (UTC)

Oh, dear! The earth's upper atmosphere (above say 100 km altitude) is considered to be an example of a Knudsen gas. Radiative events outweigh molecular collisional events. The result is that the Maxwell-Boltzmann distribution does not prevail, and Kirchhoff's law and Planck's law do not apply. It is a real-life example of failure of the conditions for the source function to be Planck, and the failure of the condition of local thermodynamic equilibrium. It is a good example of how the presence of matter quantitatively affects the rate of establishment of the approach to the Planck source function. True, the Wikipedia article on the Knudsen gas does not tell you this, but tells only an idealized story without reference to real Knudsen gases. Nevertheless the Milne reference does so; its relevant section is focused on it. It seems you might have been relying on the Wikipedia article, not the cited reliable source. Wikipedia articles are not reliable sources.

Please undo your unconsulted deletion of this good and relevant information that was supported by a reliable source.Chjoaygame (talk) 22:47, 16 February 2012 (UTC)

GO ahead and change this. I'd suggest that you put the Knudsen gas in a separate sentence, reference something that has a google books link so it can be read on line, and refer to the example of the upper atmosphere, with a source. Brews ohare (talk) 23:27, 16 February 2012 (UTC)

Dear Brews ohare, with respect, you are not a kind of supervisor of this site, to delete valid entries because they are about things unfamiliar to you, and then to tell other editors how to revert your deletions. I do not think it is the right emphasis to put in here a whole lot of detail about the atmosphere. It is not required that a source have a google book link so that it can be read on line. It is not a criterion of reliability that a source be readable on-line. It is not required that entries spoon-feed readers. A few words are enough for a keen reader at this point. The entry was good as it was. Remember, eventually, we will need to persuade Waleswatcher to repair the damage he has done; he will likely continue to exploit the weakness of the Wikipedia process and will likely not allow others to make the necessary repairs. I think it is up to you to repair the damage you did.Chjoaygame (talk) 23:57, 16 February 2012 (UTC)

I agree with deleting the reference to Knudsen gases - it's both an obscure concept and one that's particularly ill-suited to discussions of thermalization. Waleswatcher (talk) 00:49, 17 February 2012 (UTC)

Knudsen gases may be a concept obscure to some, but that is not a reason to exclude it from a relevant place in the Wikipedia. Knudsen gases are important to those studying the upper atmosphere, and are relevant here because they exemplify the factors that govern the rate of approach to the Planck source function. But I can understand why you would like it deleted! Hiding facts of physics under smooth talk is dear to your heart.Chjoaygame (talk) 02:06, 17 February 2012 (UTC)

Chjoaygame: I am under no delusion about "running" this page; I was simply saying that it's OK with me to revisit this entry, and have suggested some details that would make it generally useful and acceptable. If you prefer to argue about it, I'm not interested. Apparently you'll get lets of flak from other quarters. Brews ohare (talk) 02:54, 17 February 2012 (UTC)

May I pose a simple minded question? What possible reason is there for including a concept as restrictive as a gas (i.e. massive particles) in an article about a 'black body'?--Damorbel (talk) 06:57, 17 February 2012 (UTC)

The topic where Knudsen gas cropped up was roughly "mechanisms that may bring about thermalization of radiation". As pointed out in this source and many others, a Knudsen gas does not fit into this topic because it is collisionless, and reaches equilibrium only by interaction with the walls. Nevermind how it achieves its own equilibrium, it is unclear how it interacts with radiation, if at all. So to demonstrate relevance of this topic Chjoaygame has to find sources that discuss both of these matters. He claims Milne (1930) is such a source, but it is not available on-line and he has not provided any quotation to indicate it actually covers these issues. Another issue is that the topic is real mechanisms, not hypothetical ones. Chjoaygame argues that the Knudsen gas is not an idealization, but a good model for the Earth's upper atmosphere. Inasmuch as the reference I have linked suggests the Knudsen gas exists only as a idealized limiting case, it seems debatable to suggest the Knudsen gas has application as a real mechanism for radiation to achieve equilibrium. Brews ohare (talk) 15:22, 17 February 2012 (UTC)

The more general topic of achievement of thermal equilibrium by interaction with a rarefied gas does fit in to the discussion of mechanisms: it is simply one example of interaction with matter. It may be that some caveats should be attached to this example. For instance, if the gas is of a single element, say hydrogen initially at a very low temperature, is the spectral response of hydrogen gas sufficiently broad in spectrum to allow radiation of all wavelengths to exchange energy via hydrogen interaction? I don't know. Brews ohare (talk) 15:28, 17 February 2012 (UTC)

Could it be that Chjoaygame intended to introduce the Knudsen gas as a model for the photons in the upper atmosphere? Maybe he can shoehorn that topic in here, where equilibrium of the photon gas is the direct topic? It still looks like a boundary effect however. Brews ohare (talk) 16:03, 17 February 2012 (UTC)

Tolman reference

This recent edit removed the phrase "like any interacting gas" preceding the Tolman citation. Inasmuch as the Tolman reference discusses an interacting gas of molecules and does not mention at all the case of a photon gas, removal of this phrase incorrectly leaves the reader with the idea that Tolman explicitly suggested that a photon gas would achieve equilibrium by itself. That is not the case. I tried to come up- with a relevant statement that Tolman supports. Tolman's H-theorem discussion does not even mention radiation. It would seem that one would have to say something like: "Photons are an example of an interacting boson gas[Ref1] and , as described by the H-theorem{Tolman], under very general conditions any interacting boson gas will achieve thermal equilibrium." That is what I did. Brews ohare (talk) 15:53, 17 February 2012 (UTC)

Photons in the presence of matter are in effect "weakly interacting bosons"; that was how they started their career in theoretical physics. A question is whether they are such in the absence of matter; Waleswatcher and PAR say that they are so for reasons proposed by quantum field theory. Another question is, if they are such, how weak is the interaction, for that will determine the time-scale of the approach to thermodynamic equilibrium. Looking at Tolman 1938, I have not found him considering these questions. I found no fully explicit mention of "quantum field theory" in so many words. A single sentence on page 19 might be construed as a hint at it; but the matter is not pursued.

On page 383, Tolman writes; "The treatment of electromagnetic energy in a hollow enclosure, which has come to thermal equilibrium, as an Einstein-Bose system in which photons are regarded as taking the place of more ordinary particles, has proved quite interesting."Chjoaygame (talk) 19:05, 17 February 2012 (UTC)

Black holes (again)

Since Damorbel brings it up yet again:

We we saw that any body that absorbs all the light that falls on it is a black body...Now, a black hole certainly absorbs all the light that falls on it, so it is a black body.

Gravity from the ground up, p.304, my bold for emphasis. Waleswatcher (talk) 12:36, 17 February 2012 (UTC)

"We we saw that any body that absorbs all the light that falls on it is a black body.. " Not so Waleswatcher, not only must energy be seen to be conserved it has to be thermal energy (i.e. the absorbed photon energy has to show up as thermal energy) otherwise Kirchhoff's thermal equilibrium condition cannot arise, that is why gravitational energy; chemical energy etc. etc. have to be excluded. --Damorbel (talk) 17:13, 19 February 2012 (UTC)

Your comment is wrong for multiple reasons. Regardless, that's a quote from a reliable source (there are two now in the article), and your unsupported opinion is not relevant. Waleswatcher (talk) 17:35, 19 February 2012 (UTC)

"...and your unsupported opinion..." Not too sure what you are aiming at here, Waleswatcher. My argument is that an isolated (Kirchhoffian) black body absorbing (all) radiation falling on it will undergo a change in temperature until it is emitting the same power as it is absorbing, this being an example of the thermal equilbrium condition. Now if you see anything wrong with this please say so because I would like to provide clarification and even some supporting reliable sources. --Damorbel (talk) 18:18, 19 February 2012 (UTC)

Reliable sources support both the definition of black body given on this page and the fact that black holes are black bodies. If you want to edit the article, go for it - but be sure to back up your edits with reliable sources, otherwise they'll be reverted. That's really all there is to it. Waleswatcher (talk) 22:20, 19 February 2012 (UTC)

Response to Damorbel: Dear Damorbel, I think that Waleswatcher is right to say that a black hole is a perfectly absorbing body, and so falls within our definition of a black body.

The definition of a black body as a perfectly absorbing body is silent on the question of the gravity exerted by the body; it neither requires nor forbids the body from exerting its own gravity. That means that the definition of a black body permits it to exert its own gravity, and indeed, to be a black hole.

I should say that I am not remotely expert in black hole theory. Not remotely. What I am about to say might very easily be quite wrong. But here goes. So far as I can see, a black hole with the mass of the sun will not come to thermal equilibrium in a short time. The Wikipedia article on Hawking radiation proposes that the temperature of a black hole with the mass of the sun is 60 nanokelvin. Since the radiative temperature of space is about 2.7 K, space will continue to pass heat to such a black hole, and this will increase its mass and thus lower its temperature even more. A very small black hole would be unstable because it would radiate more than it absorbed. For all I know, there might be a size of a black hole that would be radiatively metastable — practically unstable — with a space background temperature of 2.7 K : a little extra energy supplied to it would lead to its getting bigger and colder, a little less energy supplied to it would lead to its getting smaller and hotter. The Wikipedia article on Hawking radiation says that a black hole of about the mass of the moon would be in thermal equilibrium, though it does not say in so many words that such an equilibrium would be metastable.

In summary, one should distinguish between a black body as such and its condition in whatever adventures it might encounter.

Kirchhoff considered the rather unadventurous case in which thermal equilibrium had been established. At least so far as I can see, as you say, it will be very rarely that the adventures of a black hole will include a situation of established thermal equilibrium. That doesn't make the black hole not a black body, it just makes it a black body not in thermal equilibrium.Chjoaygame (talk) 23:08, 19 February 2012 (UTC)

• " I am not remotely expert in black hole theory. Not remotely." That is how it appears from what you write. I wrote before that 'a black body' has got nothing to do with 'bodies that appear to be black (or nearly black)'. What you write above shows that neither you nor Waleswatcher appreciate the distinction which actually lies at the heart of the question of the defintion of a 'Black body'.

I haven't the slightest objection to explanations about 'grey bodies' etc. but the mechanism that makes them grey etc. has exactly zero relationship with a Kirchhoffian 'black body' - far better put them in an article about the 'grey' mechanism (scattering) or gravitation or perhaps quantum tunneling. But as it stands this Black body article is just confused and confusing, clearly written without expertise. - Sorry! --Damorbel (talk) 07:56, 20 February 2012 (UTC)

• A blackbody absorbs all energy that falls on it and emits energy with a blackbody spectrum based on the actual temperature. Also, if the energy absorbed is greater than the energy emitted, then the temperature of the blackbody increases until equilibrium is reached. Therefore, anything that gets colder as it absorbs energy would not be a blackbody. In addition, it is my understanding that black holes have energy jets associated with their poles. As a result, the article should make it very clear that black holes are not blackbodies. Q Science (talk) 07:01, 20 February 2012 (UTC)

It seems that there is a divergence of views about how to define a black body. The definition in the article seems to be pretty close to Kirchhoff's, and to the generally accepted one: a body that absorbs all the radiation that falls on it. The definition in the article does not say anything about its emission, or about its gravity. It seems to me that Damorbel and Q Science are in effect proposing a divergent definition, or perhaps two divergent definitions, one(s) that say(s) also something about the emission of the body, as well as about its absorption. Such a divergent definition is perfectly reasonable, but it is not the one given in the article as it stands. And it seems to me also to be unhistorical, though that may not matter too much.

The question about a black body, as defined in the article at present, as to how it radiates, seems to me to be a question not of the definition of the body, but a question about how it behaves during its adventures, a question about the analysis and development of the properties specified in the definition.

Damorbel, I have to say that I think you are a bit high-handed telling me that I don't appreciate certain distinctions. With respect, I think I can see clearly enough what is going on. It seems to me that you have an idea of what constitutes a definition different from the usual idea of it, but that you are not making that difference clear. It seems necessary at this point to be explicit in distinguishing what constitutes a definition from what is considered to be an analysis and development of the properties specified in the definition. The question of how to define a black body is not the same as a discussion of the nature of black holes. It is the latter that I am saying I am ignorant of, but you are accusing me also of being ignorant of the former. I think you are being a bit rough there. The definition of a black body, as usually accepted, is rather easier to grasp than is an account of the properties of black holes.

I am not here to defend the presence of the black hole story in the article. Far from it. (Indeed it seems to me a bit out of place, but would I dare to say such a thing out loud where I might be heard by the new masters of this article? These fellows are very much in charge here now.) Nevertheless, without prejudice, if it is granted that the article mention a black hole then it seems logical to me that it should admit a black hole as a black body, though one that for practical purposes can hardly ever be anywhere near thermal equilibrium. One might argue that a body that is not anywhere near thermal equilibrium cannot have an ordinary thermodynamic temperature, and, without prejudice, I would not immediately dismiss such an argument out of hand. If Q Science is right (no opinion by me) that black holes have jets coming out of them, then indeed one would have to say that they are quite likely not in thermal equilibrium and will quite likely not obey Kirchhoff's law and will not be like the ordinary black bodies that happen by adventure to be in or nearly in thermal equilibrium with their environment.Chjoaygame (talk) 09:04, 20 February 2012 (UTC)

try this

Chjoaygame, try this: Kirchhoff's thesis is that absorption = emission (a = ε) when the temperature is uniform i.e. in equilibrium. How can this possibly apply to a black hole when its emission temperature can be well below 2.7K - CMB? The equilibrium condition is fundamental to the argument Kirchhoff uses for cavity radiation. If the interior of the cavity is not in equilibrium and the interior material is is not 'black' then the neither is the cavity radiation blackbody radiation. All this can be read in Kichhoff's original paper, it is very, very far from a black hole, grey bodies and other 'approximations'. Although it may appear so, I am not being condesending. My problem is that the article seems to ignore the significance of Kirchhoff's contribution. If you follow his arguments you will see that a black body cannot be 'almost black', that is to miss the whole point of his work. --Damorbel (talk) 10:45, 20 February 2012 (UTC)

• Black holes in our universe are not in thermodynamic equilibrium. The definition of a black body does not require that it be in equilibrium. The definition of a black body - the one given in many, many reliable sources and at the head of the article - is very simple, that it absorb all incident radiation. Black holes do that better than anything else in the universe. Not only are they black bodies, they are the most perfect black bodies in existence. We have multiple reliable sources that say exactly that.

As for jets etc. they occur only for some back holes, and they originate from the matter that is inspiraling into them, not from the hole itself (which cannot emit anything other than quantum radiation, which is very faint). Waleswatcher (talk) 11:44, 20 February 2012 (UTC)

Waleswatcher,"The definition of a black body does not require that it be in equilibrium". Of course not! You are right. Surely it is obvious that neither a (Kirchhoffian) black body nor a (Kirchhoffian) cavity are obliged to be in equilibrium? I have made this clear more than once; so why you are you trying to stick it on me? Kirchoff's definition of a black body, and he invented the name, is a body that not only absorbs all incident radiation but also 1/ whose temperature changes according to the amount of radiation it absorbs, 2/ that emits radiation proportionational to its temperature and consequently has an equilibrium temperature where the incoming (absorbed) energy is equalled by the outgoing (emitted) energy; these are important parts of the definition cut out 'black holes'. You seem to have a problem with these 3 'temperature change' parts, care to explain why? --Damorbel (talk) 12:47, 20 February 2012 (UTC)

"why are you trying to stick it on me"

Because you keep bringing up the fact that astrophysical black holes aren't in equilibrium.

"Kirchoff's definition of a black body, and he invented the name, is a body that not only absorbs all incident radiation but also 1/ whose temperature changes according to the amount of radiation it absorbs, 2/ that emits radiation proportionational to its temperature and consequently has an equilibrium temperature where the incoming (absorbed) energy is equalled by the outgoing (emitted) energy"

(1) is true for a black hole in equilibrium. The first phrase of (2) isn't true for any black body. Black bodies in equilibrium emit a total intensity that's proportional to T^4, not to T, and that goes for black holes too (for obvious reasons). The second part of (2) is part of the definition of equilibrium, it's true for any body including black holes. Waleswatcher (talk) 13:12, 20 February 2012 (UTC)

"(1) is true for a black hole in equilibrium" Oh boy! That's a very interesting idea! Where did you find it? A black hole in thermal equilibrium? I relly would like to have a reference for that. The Kirchhoff black body is about absorbing and emitting radiation, black holes swallow matter as well as radiation, Kirchhoff has nothing about matter, neither Planck nor Einstein had anything about matter being absorbed by a black body either. Is this a new theory?

A Kirchoff black body does not require a gravitational field as does a black hole, I've mentioned this before but you do not appear to see this as a distinguishing requirement. This is a serious difference; please take my discussion points seriously.--Damorbel (talk) 13:50, 20 February 2012 (UTC)

I see you've abandoned your points and moved on to some new/old ones. That's happened many times now - every time one of your points is disposed of, you bring up one that was disposed of previously, as if we never discussed it before. Cycle, rinse, and repeat.

" A Kirchoff black body does not require a gravitational field as does a black hole, I've mentioned this before but you do not appear to see this as a distinguishing requirement."

Of course I don't take it seriously, it's nonsense. A Kirchoff black body does not require that the body be located or produced in Chicago, Illinois. If a unique facility located in Chicago produces a black body, would you reject it on that basis? A Kirchoff black body does not require that the body be made out of carbon nanotubes. If some lab produces a black body made out of nanotubes, would you reject it on that basis?

"neither Planck nor Einstein had anything about matter being absorbed by a black body either"

Are you joking? The single most common example of a black body is a hole in a large cavity.

"I relly would like to have a reference for that."

I could provide you with thousands, there's an entire subfield of theoretical physics devoted to studying such black holes (which turn out to be extremely interesting). But you know what? I'm not going to waste my time. This exchange is a black hole, and it's absorbing a lot more than it's emitting. Until you propose or make a specific edit to the article, we're done. Have a nice day. Waleswatcher (talk) 14:23, 20 February 2012 (UTC)

Waleswatcher, you write "The single most common example of a black body is a hole in a large cavity." Neither a cavity nor a hole in its side form any part of the definition of a 'black body'; all that is required for a 'black body' is that it absorbs all radiation falling on it, it emits radiation according to a function of its temperature and neither reflects nor transmits any radiation, there is no requirement for a cavity or a hole. The purpose of the hole in the wall of a cavity is to detect the spectrum of the radiation inside the cavity without disturbing its equilibrium. --Damorbel (talk) 10:05, 21 February 2012 (UTC)

• Dear Darmorbel, we have here a matter of logic, not just physics.

I think you are not making a distinction between the substantial physics and the logic of its presentation. I don't think there is a conflict here between (a) your view of the physics and (b) mine and, without intending to preempt or prejudice his views, those of Waleswatcher on this point; but I think we differ about the logical structure of the presentation, the argumentation if you like. This distinction between logic and substance is important, but I think you do not recognize it as it needs to be recognized.

Logic is about style of presentation of reason, not primarily about the substance of the subject matter. Logic is a trivial subject, as it was taught in mediaeval days, one of the trivium, the three subjects, grammar, rhetoric, and logic, taught in the very earliest part of the educational curriculum. Substantial physics was kept till much later in the curriculum, where it was known as 'natural philosophy'. With respect, it seems to me that you never studied logic as such. Many modern educators think they are very clever, avoiding "wasting time with trivia". But they are mistaken. The trivia are very important and valuable, and their study repays the time spent on it; lack of their study is responsible for the futility of many exchanges of words.

The logic here is that we have an article about a black body, not one about Kirchhoff's thesis. You seem to be working on the premise that the article is about Kirchhoff's thesis. I have long studied Kirchhoff's several papers, and literature criticizing them. I don't think I am in conflict with his idea of the logic of the situation.

It is in the nature of a definition that it says nothing substantial. It is just an argumentative preliminary that sets down a convention for the use of a word. The substantial statements of physics follow the definition, using it to structure the presentation of the substantial physics. They analyze and develop the substantial implications of the definition.

It is substantial physics that a black body may or may not be thermal equilibrium with its environment. It is neither a logically necessary consequence of the definition (a black body is one that absorbs all radiation that falls on it) that a black body is in thermal equilibrium with its environment, nor that is is not. Whether it is in thermal equilibrium with its environment is a question of its adventures. Another way of saying this, in the language of classical Aristotelian logic, is that the presence of thermal equilibrium is not essential, but is accidental to a black body. These logical terms are examined in the trivial study of logic, the beginnings of philosophy.

As a matter of detail, Kirchhoff does not define a black body as emitting radiation. He defines it as a body that absorbs all the radiation that falls on it; that is trivial. He finds that it is a consequence of his definition that in thermal equilibrium a black body must emit radiation. That finding is substantial physics, not mere definition and very far from trivial.

If your problem is that the article seems to ignore the significance of Kirchhoff's contribution, then I think this particular approach that you are making on this point is not a good way to advance your cause. Very seriously, I think that your ability to present your views is currently labouring under a significant disadvantage, that you seem not to have studied logic. In the circumstances, and with all respect to your personal dignity, I do not think I am out of line in making what seems to me a constructive suggestion, that you make a study of logic as a way of giving yourself a better platform from which to present your views. I regret that I cannot right now suggest particular resources which you might use for this purpose, but I suppose you will find them.Chjoaygame (talk) 15:27, 20 February 2012 (UTC)

"The logic here is that we have an article about a black body, not one about Kirchhoff's thesis." I suggest that there is no conflict here. I have already mentioned that the article does not distinguish between 'a black body', a physical concept introduced by Gustav Kirchoff, and 'an object that is black', a general concept that includes objects coated with black carbon, black holes, deep space etc. The reason why this article should be about Kirchoff's concept only is the success it had in leading to further development in science i.e. the relation between matter and radiation, a substantial part of quantum physics. As the article stands this connection is not mentioned, are you happy about this? I am astonished that a Wiki article could be so deficient.

Because of this I suggest my logic is not deficient, I do not understand why you abuse my contribution(s) in this way.--Damorbel (talk) 09:48, 21 February 2012 (UTC)

• I do not agree that a hole in the side of a cavity is a black body. It is a model of a part of the surface of a black body.Chjoaygame (talk) 15:27, 20 February 2012 (UTC)

"I do not agree that a hole in the side of a cavity is a black body." The argument about a 'hole in the side etc. ..." is about the radiation inside the cavity. In modern parlance it would be equivalent to a test probe, a device to measure a vaue of something with minimum disturbance. The 'hole in the side' is a device to measure the spectrum of radiation inside the cavity with minimum disturbance. --Damorbel (talk) 09:48, 21 February 2012 (UTC)