Talk:Black body/Archive 4

Archive 1

Archive 2

Archive 3

Archive 4

Archive 5

Archive 6

Archive 7

time for equilibration to Planck spectrum

I agree with Waleswatcher that a material body is not necessary to establish a Planck spectrum, due to e.g. photon-photon interactions. The point is that ANY randomizing interaction will do to establish equilibrium, material interaction being the quickest. I have made this point in the article, but it was deleted for lack of reference. I think something as elementary as Feynman's "QED" would reference it, but I don't have a copy easily accessible. Waleswatcher, can you find a reference for photon-photon interaction so that this point can be entered? I had not thought of gravitational attraction between photons, but this is an excellent example of a randomizing interaction that does not require a material body. That should be entered as well, as long as we have a reference. These interactions are extremely weak, but that only means that the equilibration time is extremely long. Its a matter of principle, not speed of equilibration.

Regarding the RFC, I cannot get a clear picture of the problem. Can someone give a clear picture of the problem, preferably, as Waleswatcher has suggested, a specific statment which can be discussed. I'm thinking of an optically thin plasma at a particular material temperature (T) which emits line radiation. The radiation field is not in equilibrium, cannot be characterized by a temperature, and consists of a series of lines specific to the emitting material. Nevertheless, the radiance of these lines can never exceed the Planck spectrum (at T) that would exist if the radiation field were in equilibrium with the material. I wonder if this example is helpful. PAR (talk) 20:08, 23 January 2012 (UTC)

I fully agree with all of the above. PAR, maybe you would be willing to restore the material on randomizing interactions? That's important, and I'd be happy to help find references. For the existence of photon-photon interactions I can easily do so (most texts on quantum field theory or particle physics, for example). Waleswatcher (talk) 20:15, 23 January 2012 (UTC)

Oh, dear! I wrote above "I hope this does not lead to the article on the black body becoming an article on quantum optics or quantum electrodynamics." But that outcome seems to be imminent. I will have to urge the bureaucrats to hurry with my grant request to watch the process. I have only a few lifetimes of the universe left to wait.Chjoaygame (talk) 20:30, 23 January 2012 (UTC)

What? Waleswatcher (talk) 20:40, 23 January 2012 (UTC)

Here's yet another source (provided by Dicklyon) that explicitly states that the material of the cavity walls is not relevant. "College Physics", by Serway and Faughn, p 871: "The nature of the radiation...depends only on the temperature of the cavity walls and not at all on the material composition of the object, its shape, or other factors." Waleswatcher (talk) 22:35, 23 January 2012 (UTC)

It is good to have PAR on board here. By the way, the RFC below is not about this problem, but about another. I am requested by the requester for that comment to refrain from saying more about it.

PAR proposes that Richard Feynman's QED might supply reliable sourcing about Planck equilibrium. In particular PAR refers to the gravitational effect for photons. I failed to find a suitable statement in Feynman's QED for the Planck equilibrium story. As for gravity, Feynman writes on page 151: "In all of these lectures I did not discuss gravitation. The reason is, gravitational influence between objects is extremely small; it is a force that is weaker by 1 followed by 40 zeros the the electrical force between two electrons (parhaps it's 41 zeros). ... Because the gravitational force is so much weaker than any of the other interactions, it is impossible at the present time to make any experiment that is sufficiently delicate to measure any effect that requires the precision of a quantum theory of gravitation to detect it."

Goody & Yung 1989 may be a reliable source for some matters. Whether they are reliable for the present matter must of course be considered debatable. Here's what they have to say on page 21: "The fundamental law of extinction is that of Lambert.² It states that the extinction process is linear, independently in the intensity of radiation and in the amount of matter, provided that the physical state (i.e. temperature, pressure, composition) is held constant. The possible processes non-linear in the light intensity have been fully explored. The scattering of light by light is a tractable theoretical problem and nonlinear scattering from bound electrons can be observed in the laboratory with the aid of coherent light amplifiers. The photon densities required to exhibit nonlinear effects are greatly in excess of those occurring in planetary atmospheres, and deviations from Lambert's law on this account are completely negligible. On the other hand, the optical properties of individual molecules are strongly influenced by the proximity of other molecules; the problem of pressure broadening of spectral lines, which will be discussed in Chapter 3, is one example. ²The name of Bouguet is more commonly used in European literature."Chjoaygame (talk) 10:35, 24 January 2012 (UTC)

P.T. Landsberg (1978), Thermodynamics and Statistical Mechanics, Oxford University Press, Oxford UK, ISBN 0–19–851142–6 has a Chapter 13 headed Bosons: black body radiation. Table 13.1 is headed The elementary particles. It lists leptons, ... , graviton, ... , quarks, gluons. This is evidence that Landsberg is aware of quantum field theory. Section 13.1, headed Temperature and black-body radiation is about equilibria in a cavity with perfectly reflecting walls. Landsberg writes on page 209: "Also, by adding a (small) piece of matter, capable of absorbing and emitting all frequencies, to radiation in an enclosure with perfectly reflecting walls, this particle will by absorption and emission enable radiation to come into thermal equilibrium with it. The initially arbitrary radiation is thus converted to isotropic radiation corresponding to equilibrium with a body at some temperature T. The particle acts as a kind of catalyst for this transformation. It is seen, therefore, that, given a body (not a perfect reflector) at a temperature T, radiation which is in equilibrium with it can have only one type of spectral distribution, whatever the nature of the body and whatever the nature of the radiation enclosure." Landsberg could be criticized here for using the word 'catalyst'; the literature notes that this is not quite the right word; Planck's word was "auslösend"; 'transducer' would be better in English. Also, Landsberg might reasonably be accused of contradicting himself because he mentions only twice in a few lines that the small body must have special properties (capable of absorbing and emitting all frequencies, and not a perfect reflector), when more logically he should have mentioned it three times. His last sentence could be quoted out of context to make it seem that he does not require those special properties for his physical reasoning. I have not chased up the quotes from other sources offered here to check for this, and for the present I may be forgiven for supposing that those quotes are made in good faith. Apparently, Landsberg is not so proud that he feels a need to contradict Planck on this.Chjoaygame (talk) 11:06, 24 January 2012 (UTC)

In Physics Today of Aug 2011 which arrived in hardcopy yesterday, on page 10 I read a letter from Mariano Bauer, of the National Autonomous University of Mexico, to the editor. Bauer says he wanted to know what was energy. He writes: "I finally found the answer, from Max Planck. ..." He cites the third edition of Planck's Treatise on Thermodynamics.Chjoaygame (talk) 12:20, 24 January 2012 (UTC)

I am a believer that quantum field theory is for practical purposes the right theory for present-say physics. I do not dispute the validity of quantum field theory. I do not dispute the validity of quantum mechanics. I say this here to head off potential objections along the lines "Oh, Chjoaygame is such a fool, he does not accept ...".

Here it is being argued by some editors that we can for the Wikipedia dispense with old fashioned physics and rely solely on quantum field theory. One of those editors is arguing for 'derivations' that are purely mathematical, without need for physical interpretation, and at the same time is laudably championing the good policy of 'reliable sourcing'. For reasonable consistency, one would expect the availability of a fully rigorous chain of mathematical reasoning from postulates to Wikipedia conclusion. That editor has deleted reliably sourced material from another article, there making a challenge to prove him wrong about this, demanding that another Wikipedia editor do calculations in quantum field theory. That demanding editor does not offer any such calculation of his own, nor even a reliable source for his viewpoint. It is not usual for Wikipedia editors to be required to do own research and synthesis in order to defend posted material that has reliable sources. I mention this here instead of there in order to focus the debate rather than scatter it. 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.]"

It is a pain that I am driven to write so much, "walls of text" indeed, by the fact that another editor can at his whim simply delete reliably sourced material without so much as a reliable source of his own, and have the support of rules against edit wars.Chjoaygame (talk) 11:52, 24 January 2012 (UTC)

This comment is too long, and I don't have time to comment point by point. I will simply say this - there are multiple reliable sources for the text as it is now listed here on the talk page. Just about any text on stat mech or modern physics derives the Planck spectrum for an empty cavity, often with perfectly reflecting walls. All that matters is that the radiation is in thermal equilibrium - it doesn't matter how it got that way. Several examples of such sources are listed above. The assertion that some material is needed, or that reflecting walls are impossible, is unsourced by any modern book I know of. If it's stated anywhere it could be given due weight as a dispute, but we have no such source. Planck doesn't count, because he died before big chunks of relevant physics were understood. Still, if you like we could include that material in a historical paragrap somewhere ("Planck believed that X, but the modern understanding is Y"). Waleswatcher (talk) 14:29, 24 January 2012 (UTC)

Why is this so difficult to understand? The bottom line is that, in order for equilibrium of ANY system to occur, there must be an process which redistributes energy among the particles of the system. In other words, there must be a "thermalizing process". Any one who says that for a photon gas to thermalize, there must be material (i.e. massive particles) to mediate the exchange, is saying that there is absolutely, in principle, no process by which photons may exchange energy other than those involving ponderable mass as a mediator. This is a very strong statement, and the slightest counterexample will refute it. It may seem like grasping at straws to mention QED and gravitation as counterexamples, but as I said, the slightest straw will totally refute the argument. The idea that there MUST be mass in order for photons to exchange energy is NOT a fundamental physical principle and should not presented as such.

A gas of non-equilibrium photons in a perfectly reflecting container will not equilibrate ONLY if there is ABSOLUTELY no photon-photon interaction. The minute you admit of even the tiniest possibility of photon-photon interaction, then you must admit that the gas will equilibrate, in principle. The fact that the equilibrium state does not contain the same amount of photons is irrelevant. The fact that the time scale for equilibration is very large is irrelevant. I can state with certainty that a 1 kilogram mass can never be accelerated to $(1-10^{-300})$ times the speed of light because it would require more energy than that contained in the entire universe. This does not mean that $(1-10^{-300})$ times the speed of light is, in principle, an upper bound on the speed of a 1 kilogram mass. It is a practical limit, but not a limit in principle. If a photon-photon interaction requires the age of the universe to acquire equilibrium, this is only a practical limit, not a limit in principle, and we need to make that distinction. PAR (talk) 16:41, 25 January 2012 (UTC)

The lead should summarize the article

The lead is not the place to introduce new complications. Let settle on good sourced content first, summarize it second. I have a hard time imagining that the gray scotopic appearance mentioned by a few guys in the 19th century needs to be in the lead. Nor does Kirchoff's law, which goes way beyond black bodies. Dicklyon (talk) 18:07, 21 January 2012 (UTC)

The paragraph about cavity radiation is probably too technical for the lead, I wouldn't object to trimming it or moving it. Kirchoff's law is actually quite central to this topic, but I agree with you that it doesn't necessarily need to be mentioned in the lead. The visual appearance of blackbodies is highly relevant and should be there. I don't know why you think the date of the source is important - neither human eyes nor hot materials have changed much in the last century. Anyway I don't object to editing that, but if you do you need to source it with something reliable. You can't just replace cited content with some sequence of colors that sounds right to you. Waleswatcher (talk) 18:42, 21 January 2012 (UTC)

The date of the ref is relevant because no modern writers on the topic have thought that the distraction of mentioning the "ghostly gray" appearance is worth it. It can only be understood by appeal to scotopic vision, which is quite a distraction from the topic at hand. Dicklyon (talk) 20:21, 21 January 2012 (UTC)

I've modified it to refer to chromaticity, a physical colorimetric concept, and cited sources for that and for the red appearance starting at the Draper point. Dicklyon (talk) 20:44, 21 January 2012 (UTC)

In limiting the description to chromaticity you have stripped it of important physical information that was present in the observational reports that you have removed. There is another dimension to visibility besides the two in chromaticity: brightness. You have thrown that away. But of course it is physically important.

There is no need to go into the details of visual physiology as you allege. It is enough to report the facts as they are observed. A keen student can then search out the explanation. All he has to do is to look around his bedroom one night long after lights out, perhaps if the curtains are drawn, and he will see shades of grey all round, and work out that this is because it is dark, and there is hardly any light to see by. Though you are keen to expunge a physiological consideration, curiously you still seem to want to name the colours in terms of subjective qualia, though a strictly exclusivist physicist would be fully satisfied simply with the Planck spectral numbers. It is of course important that the Wikipedia be thoroughly modern and make no mention of the fools of the nineteenth century.Chjoaygame (talk) 07:36, 23 January 2012 (UTC)

You misunderstand my point. I have no objection to you covering such sourced material in detail. But it's a subtlety that doesn't belong in the lead. Dicklyon (talk) 07:44, 23 January 2012 (UTC)

I agree with Chjoaygame. I think the previous description was better, because it was both more detailed and more accurate. Personally when I first saw "grey" I reacted with incredulity, an as a result I learned something of interest. That's exactly what encyclopedias are for, and excluding such facts from them is completely backwards. Waleswatcher (talk) 13:54, 23 January 2012 (UTC)

Yes, I learned something, too. And I'm not advocating excluding such facts. Please review the heading of this section. Dicklyon (talk) 18:28, 23 January 2012 (UTC)

Actually, from wiki's manual of style: "The lead serves as an introduction to the article and a summary of its most important aspects. The lead should be able to stand alone as a concise overview. It should define the topic, establish context, explain why the topic is interesting or notable, and summarize the most important points—including any prominent controversies." The perceived color and intensity of glowing hot objects is a very important fact about BB radiation, it belongs in the lead, and it's there. What's the problem? Waleswatcher (talk) 20:57, 23 January 2012 (UTC)

Necessity of thermal equilibrium

Dicklyon asserts that "no equilibrium is necessary" for black body radiation. That is incorrect for several reasons. First, "temperature" is only defined in thermal equilibrium, and the BB spectrum depends on the temperature of the body, so obviously the body must be in equilibrium to emit BB radiation. Second, the prototypical BB is a cavity full of radiation with a small hole in it. The state of the cavity radiation obviously affects the radiation emitted by the hole, and it's only when the state is thermal that the emitted radiation is black body. That derivation can be found in any text, or you can trivially check it for yourself. Waleswatcher (talk) 21:06, 21 January 2012 (UTC)

Nonsense; sort of true, yet nonsense. The cavity is clearly not in equilibrium with what it's emitting radiation to. Any hot black object that can be approximately described by a temperature will emit approximately as a black body. In particular, is an object is black (all-absorbing), and has a temperature gradient behind the surface to supply power to be radiated, such that the surface is at an approximately constant temperature, then the radiation will be approximately blackbody. So, to the extent that an object can be described as having a temperature, it can emit as a blackbody. If you really want to claim that only objects in thermodynamic equilibrium have temperature, then you're off into a corner of pickiness that's quite irrelevant here. Dicklyon (talk) 21:13, 21 January 2012 (UTC)

Of course only objects in equilibrium have a temperature, Dicklyon. That's part of the definition of "temperature". Sure, you can be in approximate thermal equilibrium and emit approximately black body radiation at the approximate temperature. But in all such cases the body is in equilibrium or close to it, just as the article says.

Go and look at any derivation of the Planck distribution, and you'll see that every single one uses thermal equilibrium in an essential way. Go and look at any experimental evidence for the black body spectrum, and you'll see that all of them involve objects that are in something close to thermal equilibrium.

Can you make a rational argument for why you don't want thermal equilibrium mentioned? I'm open to that, but "no equilibrium is necessary" isn't one. Waleswatcher (talk) 21:22, 21 January 2012 (UTC)

There are many perfectly reliable sources using "temperature" in the absence of thermal equilibrium, saying things like "Thermal equilibrium is obtained within a system when all parts of the system have reached the same temperature and there is no further flow of energy." They discuss a flow of energy between bodies at different temperatures. Thus, the notion of temperature is common, useful, and meaningful outside of the strict definition of temperature in thermal equilibrium. If we only discuss blackbodies in thermal equilibrium, a whole lot of blackbody-radiation-based calculations and examples become off limits. That's not what's done. Actually, in searching books that mention "temperature" and "thermal equilibrium" on the same page, I don't see any that support your definition. It's just too limiting for normal use. I have no objection to mentioning thermal equilibrium, as it's a key part of the understanding of the blackbody spectrum. But it was mentioned where it was inappropriate. Dicklyon (talk) 21:59, 21 January 2012 (UTC)

Sorry, but you're flat-out wrong on this. Every single derivation of black body radiation or the Planck spectrum makes essential use of thermal equilibrium. It's absolutely crucial to the derivation, and if you used some other assumption (the microcanonical ensemble for example, or something truly non-thermal) you'd get a different spectrum. Just one example is Huang's Statistical Mechanics, which uses thermal equilibrium in the derivation on p 279. As for "my" definition of temperature, it's absolutely standard stat mech. It's even explained (slightly quirkily) on the wiki page on temperature.

Anyway, I've just provided a cite that uses thermal equilibrium in the derivation of black body radiation (and can provide hundreds more, literally). Until you provide cites for your assertion that it's not necessary, your edits are invalid and should be reverted. Waleswatcher (talk) 04:20, 22 January 2012 (UTC)

Here's another source, Eisberg and Resnick, "Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles" p.3: "...all blackbodies at the same temperature emit thermal radiation with the same spectrum. This general fact can be understood on the basis of classical arguments involving thermodynamic equilibrium." They go on to describe the classic example of a black body, a cavity with the walls "uniformly heated to a temperature T", i.e. in thermodynamic equilibrium. Waleswatcher (talk) 04:35, 22 January 2012 (UTC)

Your source says ""...all blackbodies at the same temperature emit thermal radiation with the same spectrum." It doesn't say only those in thermal equilibrium. Why can't we do the same? Dicklyon (talk) 08:32, 22 January 2012 (UTC)

"It doesn't say only those in thermal equilibrium" Yes, it does - in the next sentence. Thermodynamic equilibrium is essential - the black body spectrum is almost synonymous with it. The only thing that bugs me slightly is that when physicists talk about a black body, they almost always mean a body in thermal equilibrium (or very close). So saying "a black body in thermal equilibrium" sounds slightly redundant. On the other hand, readers of the article won't know that, and it's an essential fact that thermal equilibrium is needed for deriving or producing a BB spectrum, so I think it's fine as written (but I'm open to suggestions on how to reword it if anyone has any). For now I'll link that phrase to the wiki article. Waleswatcher (talk) 16:53, 22 January 2012 (UTC)

That next sentence is not a restriction on the first. It says "This general fact can be understood on the basis of classical arguments involving thermodynamic equilibrium." What sense does it make to deny blackbody radiation for objects not in thermal equilibrium? No source does that. It's OK that they bug you, not OK for you to decide that WP has to do it more to your liking. And if it's slightly redundant, as you say, to add "in thermal equilibrium", then we might as well leave it out, as you claim it's implied by mentioning temperature (though in your scheme, the idea of different objects having different temperatures seems to be ruled out, if they can radiate or otherwise exchange energy). Dicklyon (talk) 18:10, 22 January 2012 (UTC)

It's not a "restriction", it's the explanation. The black body spectrum is a result of thermal equilibrium plus the special properties of an ideal black body (perfect absorptivity). That's what every single source says, that's how every single source derives the result, and those are indisputable facts with hundreds (probably thousands) of very reliable sources, several of which you've been provided. You cannot and do not have blackbody radiation from objects not in thermal equilibrium - that's impossible, and you have no source for it. It's your own false and unsourced opinion, and it's not allowed in wiki. As for redundancy, it's only redundant for people that already understand this, which presumably doesn't include most readers of the article. But like I said, I'm open to suggestions for alternative wordings. I'm sure this one isn't perfect.

As for multiple temperatures, there's no problem with that when the objects or systems involved are isolated from each other. Each individually is in thermal equilibrium at some temperature. If you put them in contact they are no longer in equilibrium, and heat will flow until the combined system equilibrates at some intermediate temperature. This is all basic thermodynamics. Waleswatcher (talk) 18:19, 22 January 2012 (UTC)

By the way - the most perfect BB spectrum ever observed is the cosmic microwave background radiation. That is radiation that last scattered in a plasma - a plasma that was in nearly perfect thermal equilibrium - that existed everywhere in the universe. No walls at all, but rather like the infinite volume limit of a thermal cavity. Then the universe "suddenly" became transparent, and the radiation we see today is a near perfect replica of the radiation around then (except redshifted by a factor of about 1100). Waleswatcher (talk) 21:42, 21 January 2012 (UTC)

Waleswatcher writes above: "Sure, you can be in approximate thermal equilibrium and emit approximately black body radiation at the approximate temperature. But in all such cases the body is in equilibrium or close to it, just as the article says."

Waleswatcher has the physics right there. And plenty of reliable sources to support him.

A list of sources may not be the best way to get things clear here, if the list turns out to be too long. Real understanding of the physics is needed, with selected reliable sources in support.

Let us look for the physical understanding. While Waleswatcher is right to insist that temperature in its strictest thermodynamic sense is defined for strict thermodynamic equilibrium, Dicklyon is right to want some access to a concept of temperature outside strict thermodynamic equilibrium.

The concept of local thermodynamic equilibrium was introduced by Edward Arthur Milne in 1928 [1], specifically for the purpose of understanding the extension the domain of applicability of Kirchhoff's law of thermal radiation from its strict statement for strict thermodynamic equilibrium. Goody & Yung 1989 explain on page 31: "Einstein demonstrated that Planck's source function results if (2.45) is obeyed for the levels under consideration. We may, therefore, regard Planck's and Boltzmann's laws as interchangeable; conditions leading to one lead to the other, and vice versa."

[ ${\frac {n(1)}{n(2)}}={\frac {g_{1}}{g_{2}}}\mathrm {exp} \left(-{\frac {E_{1}-E_{2}}{k_{\mathrm {B} }\theta }}\right)$        (2.45)

             $={\frac {g_{1}}{g_{2}}}\mathrm {exp} \left({\frac {-h\nu _{1,2}}{k_{\mathrm {B} }\theta }}\right)$ ]

For local thermodynamic equilibrium the usual thermodynamic relations hold in each infinitesimal element of volume of the system and it is usally taken that the usual thermodynamic variables are continuous functions of time and spatial position (see de Groot & Mazur 1962, page 23; Glansdorrf & Prigogine 1971, page 14; Kondepudi 2008, page 119; Lebon, Jou, Casas-Vázquez 2008, page 39; etc.). For systems that are near enough to thermodynamic equilibrium that local thermodynamic equilibrium is near enough, the usual no-flow variables of equilibrium thermodynamics are the basic descriptors of processes, with the thermodynamic flows as dependent variables derivable from them.

For systems further from thermodynamic equilibrium, it has been found helpful to admit also the thermodynamic flows themselves as further basic descriptors, on the same rank as the no-flow flow thermodynamic variables; such an extended description is called extended irreversible thermodynamics (see Lebon, Jou, Casas-Vázquez 2008, Chapter 7). For extended irreversible thermodynamic systems, for which strict thermodynamic temperature is undefined, there are specially and carefully defined non-equilibrium temperatures. For such systems, when the Maxwell-Boltzmann distribution does not fit, then it may be expected that Kirchhoff's law of thermal radiation and Planck's law are not precisely obeyed.

An example, of a system far enough from local thermodynamic equilibrium that those laws are not precisely obeyed, is the earths' upper atmosphere, where radiation processes occur frequently in comparison with intermolecular collisions, because the number density of molecules is small. Another example is in the operation of a laser, where Kirchhoff's and Planck's laws are flagrantly violated. The violations of the laws are due to the absence of local thermodynamic equilibrium.

Thus the fundamental physics is that Kirchhoff's law of thermal radiation and Planck's law refer to thermodynamic equilibrium, as stated above by Waleswatcher. Approximations may apply as stated above by Waleswatcher.

Again, the fundamental physics is that black bodies are strictly defined for strict thermodynamic equilibrium. Approximations may apply, as stated above by Waleswatcher.

It is important for a Wikipedia article that the fundamental physics get a good exposition, prior to approximations and developments. It is an important part of physics to make explicit how approximations and developments are made and applied.Chjoaygame (talk) 12:59, 22 January 2012 (UTC)

Thanks for the explanation. I understand your principled approach. But it is not the usual approach to describing blackbody radiation; local equilibrium, in the sense that the object has a temperature at its surface, is all that's considered in the usual approach. The blackbody does not have to be in equilibrium with a radiation field around it to emit with a blackbody spectrum. Not even approximately. To suggest that blackbody radiation comes only from bodies in thermal equilibrium is very misleading, and does not follow any treatment of blackbody radiation in sources. In particular, I think the article is correct where it states "The principle of detailed balance says that there are no strange correlations between the process of emission and absorption: the process of emission is not affected by the absorption, but only by the thermal state of the emitting body." That is, blackbody radiation is emitted from a blackbody even when it's not surrounded by a radiation field in equilibrium with it. No? Dicklyon (talk) 18:16, 22 January 2012 (UTC)

The black body radiation IS a radiation field that's in equilibrium with the body. To be a little more precise, if the body is emitting but not absorbing, then it cannot really be in equilibrium. It will cool, and in that case the radiation it emits will not be exactly BB (because the temperature is changing with time, and you can prove that that means the spectrum at any given time isn't exactly BB). If the object has a large heat capacity relative to the power it is emitting, the cooling is very slow, and the radiation is very close to BB. But to have exact BB radiation the object must be in exact equilibrium. That's why the canonical example is a cavity - with walls in thermal equilibrium and held at fixed temperature - with a small hole cut in it that lets a little radiation out for you to observe. It's supposed to be a small hole, so the radiation being emitted doesn't substantially disturb the equilibrium of the radiation inside the cavity. Similarly, the best BB ever observed is the cosmic microwave background, which is a (redshifted) snapshot of the radiation field as it was in the early universe, at a time when it (the radiation) was in thermal equilibrium with the plasma (it's a snapshot because the plasma "suddenly" recombined to form transparent neutral hydrogen, so the radiation has propagated without scattering pretty much ever since).

In all cases, both theoretically and experimentally, thermodynamic equilibrium (or a good approximation to it) is required for a BB spectrum. That's a pretty important fact, and I think it really must be mentioned. Waleswatcher (talk) 20:48, 22 January 2012 (UTC)

That's not what sources say. It's easy enough to address the problem you cite by providing energy to the body (e.g. by a heater coil or by internal fusion processes) to keep the surface temperature constant. That makes it steady state, but not thermodynamic equilibrium. And even if the temperature is slowly changing, the deviation from black body spectrum is so small as to be not worth a mention. It's still true that "A black body emits a characteristic, continuous spectrum of incandescent thermal radiation that depends only on the body's temperature" even if the temperature is slowly changing. Live with it, or produce a source that does it your way. Dicklyon (talk) 21:22, 22 January 2012 (UTC)

It's exactly what the sources say. I've given you multiple cites now, all of which use thermodynamic equilibrium as an essential ingredient in deriving or producing the BB spectrum. The burden is on you to find a source that says any body can produce a BB spectrum when it's not in thermal equilibrium. Until then, it stays as is. Waleswatcher (talk) 21:47, 22 January 2012 (UTC)

I am not questioning that sources "use thermodynamic equilibrium as an essential ingredient in deriving or producing the BB spectrum." I'm questioning your unnecessary and misleading narrowing of the true statement that "A black body emits a characteristic, continuous spectrum of incandescent thermal radiation that depends only on the body's temperature". If you gave a source about that, I've missed it. But I think you didn't. Dicklyon (talk) 22:47, 22 January 2012 (UTC)

Surely Waleswatcher's case is better than Dicklyon's here. Waleswatcher is saying what most characterizes the physics, citing precisely what the reliable sources say, while Dicklyon is asking for the statement to be vague in a way that admits and even invites massively wrong readings, with no explicitly cited sources, and reading others' sources in flaky ways, on the grounds that his way is the "usual" one. Dicklyon seems to be able to dismiss reliable sources simply by not reading them.

Thermal equilibrium is a term that I find open to risky readings. One common reading of it is that the temperature of the body is uniform. It is hard to see how a body that is heated inside and is cooling itself to a cooler environment can avoid being cooler at its most superficial surface than at some slight optical depth, and thus not in thermal equilibrium by that reading. The non-uniformity of temperature will lead to non-Planckian, i.e. non-black, emission. That of course is assuming that one can find a material that is truly black, and do away with the usual need for tricks like the hole in the cavity that experimentalists have found they need to get a good Planck spectrum.

Dicklyon's assertion is that it is true without reservation or condition that "A black body emits a characteristic, continuous spectrum of incandescent thermal radiation that depends only on the body's temperature." Dicklyon explicitly wants to heat the body from the inside and let it cool to the environment; consequently he can hardly claim that the unqualified use of the word 'temperature' somehow implies uniformity of temperature. Dicklyon is conceding a little when he requires that the temperature change only slowly, but his assertion fails to indicate that, and unqualified as it is, could reasonably be read as admitting rapid temperature change. Because it lacks a qualification that makes the body's temperature uniform, or even nearly uniform, the falsity of that assertion is obvious, and it is puzzling that Dicklyon still sticks to it.

Dicklyon seems to be pushing for an approximation to be passed off as the genuine article without any hint of it, while Waleswatcher is telling the physics as it is.Chjoaygame (talk) 05:03, 23 January 2012 (UTC)

Dicklyon writes above:"In particular, I think the article is correct where it states 'The principle of detailed balance says that there are no strange correlations between the process of emission and absorption: the process of emission is not affected by the absorption, but only by the thermal state of the emitting body.' That is, blackbody radiation is emitted from a blackbody even when it's not surrounded by a radiation field in equilibrium with it. No?"

No. The use of the wording "there are no strange correlations" may be poetic but is only imprecisely and not categorically physical. Dicklyon is trying to use the imprecision to make an invalid argument. It is true that the emission is fully logically determined by the thermal state of the emitting body. That was first fully recognized by Prevost. But the thermal state of the emitting body is causally affected by the absorption which in turn is causally affected by the surrounding radiation. It is a matter of distinguishing logical determination from causal affection. Contrary to the rather poetic but imprecise statement that Dicklyon is quoting, the process of emission is indirectly though not directly causally affected by the absorption. That is not contrary to the full logical determination of the emission by only the thermal state of the emitting body. (All the while of course we are assuming that we have a really black material at the temperatures and wavelengths of interest, sometimes a heroic assumption.)

The physical reality that Dicklyon is trying to hide or deny is that it is only in thermodynamic equilibrium that the thermal state of the body is such as to make Kirchhoff's and Planck's laws strictly applicable to the body as a whole. Local thermodynamic equilibrium usually means that the source function is Planckian in every part of the body, but the temperature is in general not uniform; uniform temperature and local thermodynamic equilibrium combine to be thermodynamic equilibrium. It may be true that near thermodynamic equilibrium, Kirchhoff's and Planck's laws are near enough true to satisfy Dicklyon, and that he is making a reasonable or even a usual approximation; but it is still an approximation and should be labeled as such.Chjoaygame (talk) 05:31, 23 January 2012 (UTC)

source for definition of a black body

According to the present cited source (Massoud 2005) at 2.1 on page 568: "A blackbody is an ideal surface, which satisfies three conditions. First it is a perfect emitter. Thus, for a specified temperature and wavelength, a blackbody emits more radiant energy than any other surface at the same temperature. Second a blackbody is the best absorber of energy. Therefore it absorbs all energies incident on it from all directions and at all wavelengths. Third, a blackbody is a diffuse emitter." Kirchhoff defined it by its infinitesimally thin surface properties, and was criticized by Planck for that. This has all been discussed at length here already. This present cited source of definition is a very very poor source, for reasons that have been long discussed here. This present source does not agree with the lead definition. I could go on criticizing it further, but would elicit complaints that I write too much. Why do we find ourselves offered poorer and poorer sources? The reason is that editors ignore the policy that for scientific articles, sources should be compared and considered (I have to admit that having looked I can't find again where I read that in the mass of policy articles, but it feeld to me that I didn't make it up, I think I did read it somewhere there; or am I imagining it as a happy dream?). Or other editors write as if they know so much that they themselves are ultimate reliable sources.Chjoaygame (talk) 09:23, 26 January 2012 (UTC)

I agree with you - it's not a great source, and that definition is non-standard. Here's a book about heat and radiative transfer that spends much of its pages analyzing black body radiation: "Thermal Radiation Heat Transfer" by Robert Siegel, John R. Howell. It defines black body on page 7: "An ideal body is now defined, called a blackbody. A blackbody allows all incident radiation to pass into it (no reflected energy) and internally absorbs all the incident radiation (no energy transmitted through the body). This is true of radiation for all wavelengths and for all angles of incidence. Hence the blackbody is a perfect absorber for all incident radiation." That's a careful and complete definition, and I think the source is reliable. Waleswatcher (talk) 11:58, 26 January 2012 (UTC)

Thank you for your thougthful response. I will presumably take some time to get a copy of this book. I get some useful snippets from it on the internet. Special points in its favour as it is quoted by you are:

(1) it admits that a black body is a body; this is convenient for logical discussion;

(2) it distinguishes the surface from the interior points at which absorption takes place; this is to some extent realistic.

With fear of being ridiculed, I would point out that Planck (perhaps the first to clarify the advantages of this analysis, though I have not studied the literature enough to be sure of that and am not hanging anything on it) made these points in his monograph on his law, and that his monograph is cited by many current reliable sources and that it is available (in pdf downloadable searchable form) on the internet in full for free and can be bought as a printed copy.Chjoaygame (talk) 15:18, 26 January 2012 (UTC)

Good luck

This article was a big mess, with an active dispute between two physicists of odd viewpoints, who continue to make it overly complicated. My attempt to help attracted Brews ohare, who has his own idiosyncracies, but it this case is nearly a voice of reason. He moved almost all the content out to Blackbody radiation, leaving a reasonable-looking stub here, which he and the others have now bloated up to nearly as big as what was here before. There is no hope that on a limited time budget I can influence the direction of things here, so I'm taking it off my watchlist. Good luck to you all. Dicklyon (talk) 16:23, 27 January 2012 (UTC)

black hole

It seems to me that a black hole is an important concept in its own right, but not quite bullseye for an article on a black body.Chjoaygame (talk) 15:36, 26 January 2012 (UTC)

I agree, but it is a good example of a BB, so I think it's worth keeping. I de-emphasized it by making a list of BBs in nature of which that's one example. Waleswatcher (talk) 16:13, 26 January 2012 (UTC)

This will be one of those 'black holes' with a measurable temperature and that emits thermal radiation according to the Planck law, I suppose? --Damorbel (talk) 16:55, 26 January 2012 (UTC)

My remarks above are (supposed to be) ironic. A 'black hole' has got nothing to do with a 'blackbody'. A 'black hole' is an extremely massive object that has gravitational attraction is sufficient to prevent even light escaping, thus it cannot possibly emit radiation (Hawking radiation is not 'thermally emitted').

The presence of such a subsection, particularly with the remark "Physicists believe that black holes have a non-zero temperature and emit radiation". If no good reason for keeping it appears I shall remove it - or replace it with a 'not to be confused with...." kind of statement. --Damorbel (talk) 16:09, 27 January 2012 (UTC)

--Damorbel (talk) 18:55, 27 January 2012 (UTC)

Agreed. There's little or no sensible connection of the quantum concept of a black body to the GR concept of a black hole. How did that even get in there? Remove it. Dicklyon (talk) 16:19, 27 January 2012 (UTC)

All this agreement is impressive in the context of WP. However, as indicated in the book Lectures on Quantum Gravity Chapter 1: The thermodynamics of black holes, the predictions are that black holes are well described by the model of black-body radiation at a given temperature. I take it that you all feel that this is a superficial analogy, but this author doesn't think this way. Brews ohare (talk) 17:46, 27 January 2012 (UTC)

I think there are essential differences between a super massive body that can't be seen because radiation cannot escape from its gravitational field and a body which has no defined mass, in fact has none of the properties associated with mass, and still manages to emit radiation according to the 4^th power of its temperature. --Damorbel (talk) 18:55, 27 January 2012 (UTC)

• Quite the contrary. First of all, the definition of a BB is a perfect absorber, which black holes nearly are. They are probably the best black bodies in the universe. Second, Hawking radiation is (almost) exactly BB radiation. I'd be happy to provide plenty of sources for that if anyone doubts it. I'm putting that back in. Waleswatcher (talk) 20:59, 27 January 2012 (UTC)

• I think this is Waleswatcher's comment on his restoration (at 21:01) of the section that Damorbel removed. I agree with Waleswatcher's restoration of that section, and I would have restored it myself if Waleswatcher had not done it already. I think Damorbel needed more consensus than he had to justify his removal. That is not to say I am fond of the section! Indeed I would prefer it out, but not by violence. It is my impression that Brews ohare is very fond of it.Chjoaygame (talk) 23:22, 27 January 2012 (UTC)

Waleswatcher undid (here) [2] the deletion of the reference to a black hole as an example of a blackbody, making the statement "They are probably the best black bodies in the universe". And what about the T⁴ dependency? I have already pointed out that a 'black hole' exists because of its (very large mass), whereas there is no requirement for mass of any sort in the definition of a blackbody according to Kirchhoff's (and Planck's) definition, in fact any mass below that critical for the formation of a black hole would automatically make it 'none black' because it would have a refractive index, causing ot to reflect and refract accoding to the Fresnel equations.

But a black hole's gravitational aspect alone is sufficient to put it in the 'disambiguation' category with a link for those who do not (yet) understand the difference.

By the way, I have checked Brews ohare's link to 'Lectures on Quantum Gravity' where it says (on p2) "Classically, black holes ar perfect absorbers but do not emit anything, their physical temperature is absolute zero". The authors go on to discuss Hawking radiation and, if you read the full description, you will find that Hawking radiation comes from the event horizon, just outside the BH itself. The fact that Hawking radiation has a Planck spectrum is neither here nor there, to be 'blackbody' radiation it also needs the T⁴ dependency, the two requirements are not seperable! --Damorbel (talk) 07:41, 28 January 2012 (UTC)

First of all, as I already said, the definition of black body is simply that it is a perfect absorber, and that's the case for black holes. T^4 etc. depends on the BB being in a thermal state, whether it holds is a separate statement from whether a black hole is a black body (read the first sentence of the lead of this article). But it is in fact the case for black holes in vacuum, at least according to theory - they in fact radiate a very precise BB spectrum, including (of course) the T^4 dependence. I'm not sure what you're referring to when you say "any mass below that critical for..." - if you have a black hole, it's a near-perfect black body. If you don't, it's a star or something else. Waleswatcher (talk) 19:51, 28 January 2012 (UTC)

(Outdent) This discussion says to me that (i) the claim is made that black holes emit like black bodies, and (ii) there are differences between the origins of black body radiation in the two cases, although the radiation is itself indistinguishable.

I draw these conclusions about this: (i) modeling the radiation from a black hole is not different than modeling a star; the star also is far more complicated than any black body. (ii) a reader may well run into this example, and if there are significant distinctions to be drawn, they should be made here. (iii) a reader may legitimately look at the black body article in response to reading about black holes, so at a minimum there should be a brief intro here with a link to a more complete discussion elsewhere, if that discussion is too long to appear in this article. Brews ohare (talk) 16:33, 28 January 2012 (UTC)

Dicklyon's inquiry: Black body, thermal equilibrium & uniform temperature

This Google book search shows the term thermal equilibrium is commonly used in describing electromagnetic radiation. The application of of the term temperature to electromagnetic radiation may be less intuitive, or possibly more difficult to establish, than its equilibrium with a surrounding cavity at a given temperature.

This Google book search indicates that for material bodies, uniform temperature usually is taken to mean thermal equilibrium, but some authors add the caveat that the body is at thermal equilibrium at a uniform temperature only if it also is at the same temperature as its surroundings. Uniform T means zero grad T and so no heat flow. That implies thermal equilibrium within the material where temperature is uniform.

So Dicklyon's one-line edit summary regarding a black body:

"the key thing is a uniform temperature of its emitting surfaces; if this is what's meant by thermal equilibrium OK; if not, explain"

is answered in part by saying for a material realization of a black body, a uniform temperature throughout is the same thing as thermal equilibrium within the black body, but if body is emitting more than it absorbs, it is not in thermal equilibrium. If the surface is at a constant temperature and the body is emitting more than it absorbs, the temperature within the body eventually will exhibit a gradient transporting heat from its interior, so it will not be internally at thermal equilibrium.

Thus, a star may be a black body in the sense that the reflection of incident radiation is completely negligible compared to that emitted, but it certainly is not in thermal equilibrium and is not a black body in thermal equilibrium by a long shot. This source is one of many that makes clear it is the surface temperature of a star that decides the radiation from it, and is approximated as a black-body radiation. It would seem (pure conjecture) that the mechanisms transporting heat to the surface have very little to do with the emission of energy from the star. Surface emission is the limiting step in cooling the star, not the mechanisms transporting the heat to the surface. Maybe these transport mechanisms are so efficient, or the generation of heat is so uniform within the star, that it really is nearly at internal thermal equilibrium?? See Star#Temperature for a discussion of this approximate modeling as a black body that suggests the contrary. Here is a discussion. Here is an interesting figure.

DickLyon, perhaps you could suggest what should be put into the article? Brews ohare (talk) 17:13, 26 January 2012 (UTC)

"uniform temperature usually is taken to mean thermal equilibrium". There is more to equilibrium than 'uniform temperature'. There are many possible distributions with 'uniform temperature' that are not in equilibrium, the most obvious (but rather unlikely) has all the particles at the same temperature. One other requirement is for the entropy to be at a maximum, a condition where the energy of the particles has a Maxwell-Boltzmann distribution. This Maxwell-Boltzmann distribution is, of course, the energy distribution of blackbody radiation. --Damorbel (talk) 10:54, 27 January 2012 (UTC)

I understand there is a distinction between "thermal equilibrium", which refers to uniform temperature, and "thermodynamic equilibrium", which implies all processes are in equilibrium: mechanical, chemical, etc. I am unclear how one establishes the temperature of a single particle; it is probably an abnormal use of "temperature"? Brews ohare (talk) 15:06, 27 January 2012 (UTC)

This 'single particle' question is an old one. One approach is the Brownian motion analysis where the pollen particle, being large relative to the gas or liquid molecules does not undergo proportionally large energy fluctuations, its large mass has a filtering effect so its temperature does not fluctuate much. For the atomic/molecular particles, their energy fluctuations are violent and possibly in the region of their total energy but each particle wil also have an average energy (over time) that approaches a mean, thus each particle will have a mean energy which will be the same as all the other particles, thus all particles have the same temperature. This is (I think!) what is called the 'Ergodic hypothesis'--Damorbel (talk) 15:28, 27 January 2012 (UTC)

For the article, perhaps a description is needed of the circumstances under which a body with surface temperature T is well described as emitting black-body radiation. Apparently it is not essential that the body be in thermal or thermodynamic equilibrium, but all that is required is that the radiation be emitted entirely from a surface layer that is entirely at one temperature not changing in time. So, the energy lost by the surface in radiating has negligible effect upon the temperature of the emitting layer. How does that sound? Brews ohare (talk) 15:19, 27 January 2012 (UTC)

For Planckian emission, it is essential that the part of the body that does the emitting be in local thermodynamic equilibrium at a unifrom temperature. The concept of local thermodynamic equilibrium provides a systematic way of describing "a surface layer that is entirely at one temperature not changing in time", when neighbouring layers are at different temperatures. For the present requirements, it is also essential that the layer that does the emitting must be optically of infinite depth if it is to do all the emitting. It seems a mighty stretch to reconcile these requirements without approximation; it would take pages of detail to set it up and I think it has to be accepted that it will in general be an approximation.

The aim here seems to be to find or sanction a way to say that a body not in thermodynamic equilibrium can emit Planck radiation, and not to say at the same time that one is making an approximation. When is a door not a door? When it is a-jar.

That may be a convenient way of speaking, but it is really a statement of approximation without saying it explicitly that it is an approximation. It is normal physics to make approximations, but it is also normal when doing so to say that one is doing so; it not proper to make an approximation and to speak as if one is not doing so.Chjoaygame (talk) 15:51, 27 January 2012 (UTC)

Chjoaygame: Maybe that is so: looking for a qualitative explanation that leads naturally to the practical use of the black body, though perhaps non-rigorously.

The article says that a black body is one that absorbs all electromagnetic radiation. It also says that black-body radiation emerges from a pin hole in a cavity with electromagnetic radiation in thermal equilibrium with its walls.

The article doesn't say:

1. What connects a black body according to the "perfect absorber" definition to black-body radiation from a pin hole?
2. Can a body not at a uniform temperature T have a black-body surface at a temperature T?
3. Is the only connection of real objects to black bodies that black bodies are an idealized maximum possible emitter?

The literature (IMO) is in two camps: very sloppy popularized treatments that are hopelessly non-rigorous, and very technical treatments that assume the intuitive aspects don't require any explanation at all. Brews ohare (talk) 17:29, 27 January 2012 (UTC)

Dear Brews ohare, you accuse me of being murky. What about the pot calling the kettle black? To be more serious, as noted in my murky entry, a surface cannot emit or absorb. Only a material body can do so; a small or even infinitesimal one is still a three-dimensional body. True, the present article doesn't tell how absorption is connected to emission: that used to be there till it was recently removed; the connection is through thermodynamic equilibrium.Chjoaygame (talk) 23:11, 27 January 2012 (UTC)

Chjoaygame: I'm glad you take my comment about murky text in good spirit, and I accept that it applies to text by myself as well.

About surfaces, I wasn't aiming at a mathematical surface, but a physical region like the photosphere that originates emission. The discussion of stars often tries to explain why the photosphere deviates from emitting like a black body by providing some argument such as the presence of a severe temperature gradient through the photosphere. Or they try to explain why it should be a black body because the photons in the photosphere have a short mean free path and so they thermalize in this region. So it would appear that some conditions could be provided that would make a photosphere a perfect balck body emitter, maybe?? Brews ohare (talk) 16:48, 28 January 2012 (UTC)

Section: Theoretical models of a black body - - what is it for?

This section is hard to follow.

Title: Theoretical models of a black body

This title does not appear descriptive of the section, which is very qualitative and murky.

Section:Opaque poorly reflective bodies

1. Uses a lot of jargon: refraction, reciprocity, contiguous medium, mathematical surface
2. The black body has been defined as perfectly absorbing, so why the "poorly reflective" description?

Section: Kirchhoff's black body:

1. It uses concepts like Lambert's cosine law that are not explained, and whose significance is unclear.
2. It suggests that a "perfect black body" must incorporate an infinitely thin surface layer responsible for its properties. That seems to be only Kirchhoff's approach, not a necessity.

What are the objectives of this section? Is it historical? Is it clarification of the concepts? Is it technical?

Brews ohare (talk) 16:54, 27 January 2012 (UTC)

I can see you don't like this section. Reference [2], to Masoud, cited as a "reliable source", says that "A blackbody is an ideal surface, which satisfies three conditions" That looks like a definition to me, and indeed rather like Kirchhoff's. Kirchhoff in 1860 can be excused for such a thing, but not a modern source. So the basic ideas need explicit statement, I think. If you want to do a calculation about a near-black body, you need some kind of model. This section gives a prototypical one that is quite practical to use. And, (dare I say it?) is reliably sourced. The imperfect black body is poorly reflective, while the perfect one is strictly non-reflective; showing an approximative approach. I will put it down at the bottom in the hope of mollifying your dislike of qualitative, murky, and jargonical things.Chjoaygame (talk) 19:10, 27 January 2012 (UTC)

Chjoaygame: I'd go along with your idea that "the basic ideas need explicit statement". However, I don't understand what you have written or how it achieves this goal. To take this piece by piece, let's discuss the section Kirchhoff's perfect black bodies.

The topic sentence of this paragraph is that Kirchhoff's perfect black bodies don't exist. Inasmuch as the reader has not encountered Kirchhoff's perfect black bodies up to this point, they may be very happy they don't exist because they can simply avoid troubling about them. But that is not really the point of the paragraph, apparently, which is to describe what Kirchhoff's perfect black bodies really are. That is done by invoking Lambert's cosine law, to which the uninitiated reader might reasonably say "Huh?".

So, focusing upon this paragraph for now: Why do we care about Kirchhoff's perfect black bodies? Do we need to understand Lambert's cosine law in some way to grasp the purpose and value of Kirchhoff's perfect black bodies? Inasmuch as they don't exist, what is their value? What was Planck's purpose in pointing out they don't exist?

In short, what is going on here?? Brews ohare (talk) 19:41, 28 January 2012 (UTC)

Separately, let's discuss the section Opaque bodies. The topic sentence suggest this paragraph is about the value of opaque and poorly reflective bodies in the study of thermal radiation. However, their value actually never is discussed. Instead, the next sentence for some reason brings up multiple such bodies immersed in a rarefied medium with a contiguous interface. There follows a rather obscure discussion of properties and caveats about these objects that seems to assume some discussion is underway about nuances of the modeling of these structures.

Again, what is going on here?? What is the point?? What is the follow-on that discusses the topic of the lead sentence - the value in study of thermal radiation?? Brews ohare (talk) 19:52, 28 January 2012 (UTC)

Dear Brews ohare, Kirchhoff invented the theoretical idea of a black body, but he didn't get it into the best theoretical shape, and Planck improved on Kirchhoff's model. It still happens that sources cited above (e.g. Masoud [2]) mix up ideas of bodies and surfaces, when Planck sorted that out long ago. I think you yourself have been led into just those muddles by those sources. I think if you calm down a bit and carefully read the section, and (dare I suggest it?) check the references, you will see the point of the section, which is to help understand the physics. Of course Planck was also well aware of the cavity model. I will shortly do some re-wording.Chjoaygame (talk) 21:59, 28 January 2012 (UTC)

The uninitiated reader, after he has said "Huh?", if he were very adventurous, could go to the wild and bizarre extreme of following the link to Lambert's cosine law.Chjoaygame (talk)

Chjoaygame: My comments were intended less to invite an explanation from you for myself than to suggest that the sections be rewritten. Maybe if you put yourself in the mindset of a junior high school student whose knowledge is limited, it would help to set up the objectives of the section for easy consumption? I think being forced to look up Lambert's cosine law, when there is nothing in the article to motivate that search beyond confusion, is not good writing. Brews ohare (talk) 23:51, 28 January 2012 (UTC)

table moved here

I have moved a table to this article because this article seems to have some place for it alongside the subsections on planets and stars and black holes and because I don't like the table being where I moved it from, which I think is an inappropriate place for it. I think the table really belongs where it came from and should not be propagated like topsy, as it seems some editor wants to happen. But here at least I think it may get a thorough consensus about its presence or otherwise. I couldn't seem to get enough consensus just to delete it from the article on Thermal equilibrium. It really belongs where it originally came from in an article about extrasolar planets, and for me that is where it should stay. I have put it here to stimulate a response, not because I like it being here. The real problem is to find a target for a redirect from Equilibrium temperature. I would prefer to see the latter simply deleted.Chjoaygame (talk) 00:02, 28 January 2012 (UTC)

The table appears to be more information than necessary here. However, a bigger objection is that no explanation accompanies it, so it is hard to say if the temperatures are the black-body effective temperatures or not. Also, the effective temperature for a planet is not discussed in sufficient detail in the article nor sources provided. Brews ohare (talk) 16:22, 28 January 2012 (UTC)

I dropped this table because it seems to be mainly a list of properties that have no connection to the topic here. Brews ohare (talk) 19:29, 28 January 2012 (UTC)

Glad to see the table out. We will see if someone puts it back; I hope not. I have altered the re-direct to the appropriate target. Why didn't I think of that before?Chjoaygame (talk) 21:38, 28 January 2012 (UTC)

new edit of the lead

The new edit of the lead strikes me as an exercise in free-running homespun philosophy. The lead is no longer a summary of the article, but indeed is more like a mini-article expounding one aspect of the matter in a way that would perhaps be suitable for the body of the article.Chjoaygame (talk) 19:00, 28 January 2012 (UTC)

Right; it is an explanation of the concept, not an outline of the article. It presents an easily understood example of a black body and its connection to black-body radiation that doesn't require much background. This example is ubiquitous in texts, suggesting that most authors have failed to come up with a better introductory example.

The intro could be extended a bit to outline the following sections. If I knew how to do it, I'd give a bit of a preamble on how one might come up with the idea that a star's photosphere can be approximated by a black body other than through strictly empirical observation. Brews ohare (talk) 19:27, 28 January 2012 (UTC)

I agree with Chjoaygame - I think the lead is too verbose. In my opinion it should mention only the most important concepts of the article that follows, and use simple language that any reader can understand. I'd support reverting it back to what it was yesterday or whenever, and merging the new material (which is good, don't get me wrong) into the body. Waleswatcher (talk) 19:57, 28 January 2012 (UTC)

I moved that paragraph down into the body, and tightened up the lead a bit. Hope that's OK. Waleswatcher (talk) 20:12, 28 January 2012 (UTC)

Kondepudi & Prigogine 1998 on matter and radiation

Kondepudi, D., Prigogine, I. (1998), Modern Thermodynamics. From Heat Engines to Dissipative Structures, Wiley, Chichester UK, ISBN 0–471–97393–9, have their Chapter 11 on Thermodynamics of Radiation. Section 11.6 on pages 294–296 is headed Matter, Radiation and Zero Chemical Potential. Kondepudi & Prigogine cite Max Planck's Theory of Heat Radiation (first and third editions) four times in that chapter. Ilya Prigogine was awarded a Nobel Prize for his work on non-equilibrium thermodynamics.

On page 296, Kondepudi & Prigogine write: "When we consider interconversion of particles and radiation, as in the case of particle-antiparticle pair creation and annihilation, the chemical potential of thermal photons becomes more significant (Fig 11.4 [shows collision of gamma rays to generate electron and positron, as proposed above]). ... The physical meaning of these equations can be understood as follows. ... In this state [of thermal equilibrium] in which μ = 0 [μ denotes the chemical potential of photons], there is no distinction between thermal radiation and matter; just as for thermal photons, the particle density is entirely determined by the temperature. ... At ordinary temperatures, this thermal particle density is extremely small. But quantum field theory has now revealed the thermodynamic importance of the state μ = 0. It is a state of thermal equilibrium that matter could reach; indeed matter was in such a state during the early part of the universe. Had matter stayed in thermal equilibrium with radiation, at the current temperature of the universe the density of protons and electrons, given by (11.6.5) or its modifications, would be virtually zero. The existence of particles at their present temperatures has to be viewed as a nonequilibrium state. As a result of the particular way in which the universe has evolved matter was not able to convert to radiation and stay in thermal equilibrium with it."

We are talking for ordinary physics about a black body, which is not characterized by an extremely small particle density. I read this section of Kondepudi & Prigogine as a nearly explicit statement that quantum field theory equilibrium of photons is not relevant to laboratory physics at ordinary temperatures. I read this as leaving this game to ordinary physics.Chjoaygame (talk) 13:46, 24 January 2012 (UTC)

You've misunderstood. mu=0 means that photons are massless - photon pairs can be created and destroyed without any energy cost. What they are saying there is that at very high temperatures (higher than the mass of whatever particle is important), the effective mu is zero for that particle species too, because there is plenty of energy around to create and destroy pairs.

Relativistic QFT is always relevant for photons (strictly speaking) because of this. You need it to derive the quantization of mode occupation number which is the modern understanding of the resolution of the UV catastrophe. And that's obviously relevant, since it's what is responsible for the BB spectrum in the first place. Waleswatcher (talk) 14:34, 24 January 2012 (UTC)

And by the way - observations of the CMB are certainly "ordinary laboratory physics", as well as being the most perfect BB spectrum every observed in any experiment. That should play an important part in this article. But those photons come directly to us from the early universe (although not quite as early as what's referred to in that passage). Waleswatcher (talk) 15:32, 24 January 2012 (UTC)

Kondepudi & Prigogine 1998 write on page 293: " ... we conclude that the chemical potential μ = 0."Chjoaygame (talk) 18:14, 24 January 2012 (UTC)

Yes. Your point?? As I explained just above, mu=0 means particles can be created and destroyed with no thermodynamic cost (i.e. no energy change with all else held fixed). Physically, that's either because the particles in question are massless (photons, for instance) or because the temperature is so high that the mass is not relevant (electrons etc. in the very early universe, for instance). Waleswatcher (talk) 18:55, 24 January 2012 (UTC)

Kondepudi & Prigogine 1998 section 11.6 is quoted above. The theory of the black body was introduced by Gustav Kirchhoff as an auxiliary to his law of thermal radiation. We are here interested in black-body radiation at temperature T. The question has been raised of the time for equilibration of the kind of black body radiation known as cavity radiation. The time for equilibration will depend on the mechanisms available. Kondepudi & Prigogine are comparing (a) ordinary electromagnetic radiation in equilibrium with matter such as was considered by Kirchhoff, and (b) electromagnetic radiation that reaches equilibrium by way of quantum field theory mechanisms such as gamma ray collision to produce electron-positron pairs, as is proposed by two editors here. Kondepudi & Prigogine make that point that the quantum field theory mechanism has in the current state of evolution of the universe not nearly reached thermodynamic equilibrium. This gives rise, as is typical in thermodynamics, to a separation of time scales. According to this separation, (1) ordinary laboratory thermal radiation can reach thermodynamic equilibrium with ordinary matter in the short-time scale regime. But, starting from the same initial non-equilibrium state, the equilibrium composition (1) of the system will be entirely different from (2) what would be reached at the very much longer time scale that allowed the quantum field theory mechanisms to reach equilibrium. In the latter equilibrium (2), the matter is present but is very dilute. The quantum field theory equilibrium is nevertheless an equilibrium of radiation in the presence of matter. Thus there is no mechanism offered here for pure radiation in the absence of matter to reach thermodynamic equilibrium. According to the Wikipedia article cosmic microwave background radiation, the CMB was formed in the presence of a white-hot fog of hydrogen plasma. The idea that pure radiation without interaction with matter can reach thermodynamic equilibrium is fantastic and scarcely relevant to the behaviour of a black body that is considered in the discussion of Kirchhoff's law of thermal radiation. This leaves the need for radiation in a cavity to have, in the walls or interior of the cavity, matter that can absorb and emit radiation at every wavelength, in order to reach thermal equilibrium on a laboratory time scale, as considered by Max Planck.Chjoaygame (talk) 22:21, 24 January 2012 (UTC)

" In the latter equilibrium (2), the matter is present but is very dilute. The quantum field theory equilibrium is nevertheless an equilibrium of radiation in the presence of matter. Thus there is no mechanism offered here for pure radiation in the absence of matter to reach thermodynamic equilibrium. " What?? Look - let me ask you two specific questions:

(A) Take a perfectly reflecting, completely empty, perfectly sealed cavity. Put some number of photons (and nothing else) in it, with some total energy E. It doesn't matter how many, or how E is distributed between them. Wait, possibly a very long time (the time depends very strongly on E). Do you or do you not agree that the state of the photons in the cavity will eventually be well-described by a thermal ensemble (and hence the photons will have a Planck spectrum) at the temperature corresponding to that energy E?

• If it is perfectly reflecting, then the spectra will not change. Q Science (talk) 00:49, 25 January 2012 (UTC)

That is true if and only if there are no interactions at all between photons. However, there are interactions between photons (from virtual charged particles and from gravity, among other things). Therefore, the photons will equilibrate. Waleswatcher (talk) 01:05, 25 January 2012 (UTC)

One of the "proofs" that photons are waves is that they don't interact. How about a reference? Also, if they did interact, where is the theory that the resulting spectrum would be the exact same as a blackbody spectrum? Q Science (talk) 09:10, 25 January 2012 (UTC)

Here for example Euler-Heisenberg Lagrangian. To prove it's a BB spectrum, you prove that maximizes the entropy, and then the usual statistical arguments apply. But in any case you'd need to do the same for interactions with the walls or with matter inside the cavity. Waleswatcher (talk) 12:26, 25 January 2012 (UTC)

• Dear Waleswatcher, it seems that you intend to cathechize me. Or to waterboard me? To make that work, you need to know how to ask the right questions. You seem not to know how to do that.

It will turn out that you are catechizing me with a question of the form "Have you stopped beating your wife?" You are trying to lure me into using slipshod quick and dirty arguments such as you use for yourself, and thereby get led astray. Not today, thank you kindly. Your questions reveal more about your own thinking than if taken literally they would reveal much about physics. But I will play your game, up to a point.

You are appealing to quantum field theory. For that you need to ask questions in the forms proper for quantum field theory. This first question is not even nearly asked in such a form. As the most obvious reading of your question, I take it that you are asking about a pure photon number state.

According to Gerry, C., Knight, P. (2005) Introductory Quantum Optics, Cambridge University Press, Cambridge UK, ISBN 0–521–82035–9, page 25:

2.5  Thermal fields

As is well known, quantum theory originated with Planck's discovery of the radiation law that now bears his name. We refer, of course, to the law describing the radiation emitted by an ideal object known as a black body – a perfect emitter and absorber of radiation. A black body can be modeled as a cavity (or actually a small hole in the cavity) containing radiation at thermal equilibrium with its walls. The radiation is thus coupled to a heat bath and so is not, unlike in the preceding sections of this chapter, a truly free field.

According to Loudon, R. (2000) The Quantum Theory of Light, third edition, Oxford University Press, Oxford UK, ISBN 0–19–850177–3, page 148:

The multimode number states defined in eqn (4.4.6) form a complete set of basis states for the electromagnetic field. A general pure state of the electromagnetic field is expressible as a superposition of these basis states of the form

$|\mathrm {pure\,\,state} \rangle =\sum _{\{n_{\mathbf {k} \lambda }\}}c\left(\{n_{\mathbf {k} \lambda }\}\right)|\{n_{\mathbf {k} \lambda }\}\rangle$ ,    (4.61.)

where the summation runs in general over all sets {n_{k λ}} of photon numbers n_{k λ} = 0, 1, 2,..., one for each mode k λ in the quantization cavity. ...... ......

    However, not all excitations of the radiation field are expressible as linear superpositions of the basis states defined in eqn (4.4.6), and they cannot be represented by pure states of the form of eqn (4.6.1). .... .... where the nature of a chaotic light source excludes the possibility of a description of the emitted field in terms of pure states with specified relations between the different photon-number states in the output beam. When all that can be specified is a set of probabilities that the radiation field is found in a range of states, each corresponding to one of a complete set of basis states, then the state of the field is a statistical mixture ....

One of the rules of the game of quantum theory is that to get the right answer, one has to ask the right question. You are asking me to let a pure number state drift into a statistical mixture state. Sorry, wrong question. Try again.

Dear Waleswatcher, you are, however, really asking about an equilibrium reached not by interaction of a light field with matter such as is considered in textbooks of quantum optics which I have quoted above. Nevertheless, you have to ask your question about a properly specified quantum state of the field; it seems that you have not done that.

You are really asking about an equilibrium reached in the absence of matter. As you note, this takes a long time to be reached. For example, for a field at laboratory temperatures, you will need to wait a long time for a few thermal gamma rays to appear so as to collide to form an electron-positron pair. That long time is off the scale of laboratory physics, and makes yours more of a cosmological question than a question about elementary thermal radiation theory. You want to insist that the time doesn't matter. But it does, as mentioned above. The pure light that you introduced will generate some matter at equilibrium. That is what you are relying on to get to equilibrium. So your final state will not be free of matter as was your initial state. But your intent was to produce a pure radiation field as your final state as well as your initial state. Your thinking didn't deliver the goods for you.

So I agree with the above answer of Q Science: "If it is perfectly reflecting, then the spectra will not change." I learnt that by reading Planck 1914. You presumably think I am a fool because of that. Maybe I am a fool, but not because of that ! Chjoaygame (talk) 13:33, 25 January 2012 (UTC)

I'm not trying to "waterboard" you, how absurdly hyperbolic. You should assume good faith. My question is not "wrong",. and I will not "try again". The initial state can be a pure photon number state, that's fine. The question I asked you is what you think it evolves into. (By the way, with that initial condition it will not evolve into an exact thermal state, that's provably impossible. But it will evolve into an absurdly good approximation to one.)

I have no idea why you think the time scale is relevant, I did not say it will take a long time (I said it depends on E), I don't know what you mean by "laboratory temperatures", and as to whether it will generate matter, that's true in a sense - but I didn't ask that, I asked whether a pure photon state at t=0 in a perfectly reflecting cavity does or does not evolve into something very close to a Planck distribution for the photons. If you're saying no, you're wrong. If you're saying yes but it will also include matter because the equilibrium ensemble has support on configurations with non-zero numbers of electron-positron pairs, that's correct and fine - but you don't need to put anything into the cavity except electromagnetic energy, that matter gets generated spontaneously. No specks of carbon are needed.

In any case, I don't think this is a fruitful discussion any more (if it ever was). This talk page is not intended for physics lessons. If you want to edit the article, do it or suggest a change here and we'll discuss the language you add. Waleswatcher (talk) 16:00, 25 January 2012 (UTC)

Not waterboard me; just catechize me, it seems. From your exalted position of perfect knowledge, you know that I am radically and fundamentally wrong. That tells me that I would be wasting my time to post things that I think are right, because your knowledge that I am wrong would justify your continuing to delete my posts, as you have already done, and dictating your own viewpoint; I do not engage in edit wars. So I have tried to reason with you. But now I know that nothing will alter your uniquely correct and perfect knowledge, I can desist even from that. You clearly feel unshakably justified in dictating what is right, so I am wasting my time to go on.Chjoaygame (talk) 18:24, 25 January 2012 (UTC)

Clearly you're upset, and I'm sorry for that. As far as I know I've never reverted any change you made, and you've never reverted any change I made, so we're about as far from an edit war as one can get. All I can say is to ask yet again that you propose language you'd like in the article (or simply edit it in) so that we can discuss it. If you want to mention the speck of carbon, it's fine with me so long as it's clear that that's a historical account, and that all or at least the great majority of modern treatments never mention it and do not require it or regard it as necessary (which is a verifiable fact). Waleswatcher (talk) 20:27, 25 January 2012 (UTC)

(B) Do you agree that photons in a perfectly reflecting etc. cavity, IF they are in thermal equilibrium (i.e. a canonical ensemble, and for this question I'm not saying - because it makes now difference - how they got in that state), will have a Planck spectrum, and a small hole cut in the side will emit a black body spectrum? Waleswatcher (talk) 23:09, 24 January 2012 (UTC)

Please, what is a 'perfectly reflecting cavity'?. Like a 'real' black body, it would have to have completely unrealisable physical properties; perhaps a superconductor would do it, but a what temperature? It is all very well discussing these theoretical concepts, it is just about as much fun as trying to work out how many angels can dance on a pin head. --Damorbel (talk) 14:30, 25 January 2012 (UTC)

Formally, it's a Dirichlet boundary condition on the electromagnetic field at the walls. In more physical language, it's a cavity with walls that are perfect mirrors. It's a standard concept. I agree with you that it's not physically realizable, but neither is a black body or a perfect black body spectrum. There is an important physical point that I'm trying to get at, which is that any interaction - no matter how small, and no matter of what form - will lead to a state that is almost a perfect Planck spectrum. That's because the Planck spectrum is the maximum entropy state, and so any perturbation or interaction will with overwhelming likelihood push the system towards it. That's the fundamental principle that underlies all of thermal and statistical physics. Planck might have thought he needed a speck of carbon to provide those interactions, but if so he was wrong, because they exist even for a pure photon state (which he couldn't possibly have known, it wasn't understood until long after his death). But he did understand the basic point here, which is that ANY interaction will do - even a speck of carbon, or a weak photon-photon force. Waleswatcher (talk) 16:06, 25 January 2012 (UTC)

For me I find it difficult to accept that 'perfect' mathematical approximations are anything more than that. To a great extent the mathematics merely conceals the physical reality and 'standard concepts' are no better. In this category come photon-photon interactions. Photons originate and terminate at electric charge, mainly electrons; they travel at the speed of light and do not interact, as in pair production, in the absence of some other (massive) particle i.e. something traveling at less than the speed of light. The converse is not true, pair annihilation takes place without the presence of any other material. --Damorbel (talk) 17:19, 25 January 2012 (UTC)

Photons may interact without the presence of matter to form virtual electron-positron pairs, or even other things, which then annihilate to produce more photons, so the above is incorrect. PAR (talk) 03:01, 26 January 2012 (UTC)

I am interested in this. Up until now I thought this was the explanation - [3] of pair production - and here [4]; summarized here - [5] The wiki article on pair production has it like this - Pair_production#Examples where it says this :-

"This means that the pair production must take place near another photon or the nucleus of an atom since they will be able to absorb the momentum of the original photon. In other words, since the momentum of the initial photon must be absorbed by something, pair production by a single photon cannot occur in empty space; the nucleus (or another particle) is needed to conserve both momentum and energy"

I am not at all sure about "near another photon" (above) since the problem of momentum remains (you cannot modify the momentum of a photon without destroying the original photon - photons start and finish with charge). --Damorbel (talk) 16:08, 29 January 2012 (UTC)

New stars and planets subsection

The material in there looks good. But I have a question - aren't we in danger of having three articles on almost exactly the same topic? We've got black body, black body radiation, and Planck's law, all of which overlap substantially. The original idea here was that the black body article would just be a stub, with most material in BB radiation, but it seems that is likely not to remain for much longer (if it's even true now). Waleswatcher (talk) 21:28, 26 January 2012 (UTC)

Heh, heh. You said it. It's not necessarily redundancy, so much as overlap. The Planck's law article looks pretty devoted to the mathematics, more than the applications. The Black-body radiation more toward the electromagnetic field in thermal equilibrium (maybe, It's pretty long and I haven't read it all). The Black body is aimed at the perfect absorber.

I think Black body needs some help in relating its property as a perfect absorber to the subject of Black-body radiation and the ideas brought up by Dicklyon. Brews ohare (talk) 21:36, 26 January 2012 (UTC)

How can the "New stars and planets subsection" be relevant in an article on a 'Blackbody'? A star is about as far from equilibrium as you can get, there is absolutely nothing uniform about a star's temperature; in reality a star is a (very large) source of energy. Planets also are far from black, even Mercury (albedo = 0.068) is bright against the depth of space, so it isn't black, is it? It seems we are again into this business of assuming something with a Planck spectrum is therefore a black body; not true!--Damorbel (talk) 08:24, 28 January 2012 (UTC)

Damorbel: Maybe you can elaborate on this idea that black bodies have nothing to do with stars and planets. So far as I have seen, almost every discussion of stars bring up the effective temperature. Although you don't mean it that way, one could say that no discussion of civil engineering should talk about Newton's laws because rigid bodies don't exist. Brews ohare (talk) 16:39, 28 January 2012 (UTC)

Elaborate? Have I not already stated that a star is far from equilibrium, that should be enough. But to take it further, a star has a whacking great thermal gradient inside it, from somewhere near 10¹²K at the core to 6.0x10³K at the surface; this is very, very far from a cavity with a uniform temperature!

Again, a planet reflects light, a black body reflects no light, bodies that reflect light are not black by any definition! It matters not if they reflect light, they cannot emit like a black body does, such a non-black body will always have an emissivity lower than a truly black object.

There seems to be massive confusion here; some editors seem to think if an object emits, however feebly, with a Planck spectrum, that lets it be classified as a black body; nothing could be further from the truth. To qualify as a black body an object must emit with the (theoretical) efficiency given by the Stefan-Boltzmann T⁴ formula and with a Planck spectrum, both conditions are required, in reality they never exist together. I have now explained this twice, I will happily explain it further if you are more precise in where you find shortcomings in what I have written. --Damorbel (talk) 20:35, 28 January 2012 (UTC)

The definition of black body is not at all what you've asserted there. The definition is that the body is an ideal absorber, full stop. Stars are very good absorbers of incident radiation at just about any frequency, therefore, stars are near-ideal black bodies. They also emit a pretty clean BB spectrum, although you're correct that their lack of internal equilibrium leads to some features in it (but you're incorrect that this makes them not good black bodies, as I just explained). Planets are significantly less close to the ideal in all respects, but it's still an approximation that's often used.

As for T^4, the confusion seems to be yours. Any object that emits a Planck spectrum obeys that law, you can derive it directly from the Planck spectrum. Waleswatcher (talk) 22:58, 28 January 2012 (UTC)

"As for T^4, the confusion seems to be yours. Any object that emits a Planck spectrum obeys that law". Er, I don't think so. I did not mention, because you write as though you are familiar with these things, that the power of emission of a black body is not only a function of T⁴ but also the Stefan–Boltzmann constant σ, so the Planck spectrum is not sufficient condition for a black body, it must emit according to the Stefan-Boltzmann law $j=\varepsilon \sigma T^{4}$ where ε is the emissivity. If the body is not entirely black its emitted power is reduced by the factor ε (ε always 1 or less). ε may well be a function of wavelength in which case the emitted radiation is coloured; if ε is not a function of wavelength i.e. it is a 'grey' body, then the emitted power will still have a Planck spectrum, but its radiated power will be reduced from the classical black body power by the factor ε.--Damorbel (talk) 08:39, 29 January 2012 (UTC)

No, that's wrong. The emission is not a "function" of sigma - sigma is a combination of fundamental physical constants that cannot ever change (under the known laws of physics at least). The emissivity is 1 by definition for a black body. Any body with an emissivity not equal to 1 does not emit a Planck spectrum and isn't a black body. In all real cases the emissivity depends on temperature, which means there's a frequency dependence and the object cannot possibly emit a Planck spectrum. But what you said is wrong even for a "perfectly grey" body, a phrase that refers to an object that reflects and absorbs all frequencies equally (no such objects really exist, but never mind). Even perfect grey bodies don't emit a Planck spectrum - they emit a Planck spectrum multiplied by the emissivity, which is not a Planck spectrum. Waleswatcher (talk) 12:36, 29 January 2012 (UTC)

"not a "function" of sigma" Why ever not? 'Function of....' is just a mathematical expression, if you posit the speed of light or some such 'physical constant' as a variable, all you have to do is say so.

But aren't we going too fast here? Just a while ago for you a black body was just an ideal absorber ("....the body is an ideal absorber, full stop."). My position is that it has both absorption and emission properties, both of which are important. Do you agree? --Damorbel (talk) 15:25, 29 January 2012 (UTC)

On the first issue: do you or do you not now agree with me that an object that emits a Planck spectrum satisfies the Stefan-Boltzmann law? And do you agree that the Planck spectrum times a constant not equal to 1 is not a Planck spectrum? If not, I suggest reviewing the derivation of the S-B law from the Planck spectrum, and the Planck spectrum from thermal equilibrium of a photon gas. If yes, you're welcome.

"Just a while ago for you a black body was just an ideal absorber ("....the body is an ideal absorber, full stop."). My position is that it has both absorption and emission properties, both of which are important. Do you agree?" No. As I said - a black body is an ideal absorber, full stop. That's the standard definition, it's in hundreds of references, and it's the first sentence of this article. That such a body emits a Planck spectrum when it's at thermal equilibrium internally is derived from the ideal absorber property. Emitting that spectrum is manifestly logically independent and not part of the definition, because the exact same body (hole in a cavity, say) can under some circumstances (internal thermal state) emit a BB spectrum, and under other circumstances (acting as maser or laser, say) emit a totally different spectrum. Waleswatcher (talk) 15:37, 29 January 2012 (UTC)

"it's in hundreds of references" Um, really, at equilibrium? I have never seen a reference that does not say that emission and absorption must be equal at equilibrium.

There is no requirement for a system to be in equilibrium, actually it is a very rare condition, this is the value of Kirchoff's black body definition since it defines what happens at equilibrium, enabling non-equilibrium conditions to be analysed also. The laser is a good example. Lasers work in a non-equilibrium state, equally the 'box with a small hole' comes nowhere near representing a black body, only the form of the spectrum. The spectrum emerging from a 'box with a small hole' is only 'blackbody spectrum' because the multiple internal reflections reduce the wavelength dependence of the internal spectrum caused by reflections.

It is entirely necessary for a body to be described as 'black' for it to emit according to $j=\sigma T^{4}$, or do you still say that absorption like that of a black hole is the only requirement? --Damorbel (talk) 18:17, 29 January 2012 (UTC)

I'm sorry, I can't make sense of your comment, so I'll just state some facts. The definition of BB used in this article and given in all the references I've checked is that it be a perfect absorber, full stop. I'm pretty sure if forced I can find hundreds of references that agree on that; I've got perhaps ten on the shelf behind me. Whether or not such a body emits a thermal spectrum is dependent on the state of the body. The Planck spectrum implies Stefan-Boltzmann, so any object that emits a Planck spectrum obeys S-B, and objects that do not obey S-B do not emit a Planck spectrum. If you want to continue this thread, please propose specific language you want added or removed or changed so that I know what you are talking about. Waleswatcher (talk) 19:45, 29 January 2012 (UTC)

Waleswatcher's grammar

Waleswatcher has "corrected" my 'was done neither by A nor by B' to his 'was not done by either A or B'. I had till now thought that his omniscience was in the area of physics, but now I see it is wider, and covers grammar as well. I would be grateful for his source for this valuable "correction". I failed to find such a source in my limited available range.Chjoaygame (talk) 03:55, 29 January 2012 (UTC)

Two things. First, please assume good faith and try to stop being so touchy. Second, I described my edit as "minor grammar edit", I meant (and should have typed) "minor usage edit". I think it's cleaner like this, because "not been tested" makes it more clear immediately where the sentence is going. But it's fine with me if you prefer it the other way revert my change. Waleswatcher (talk) 12:48, 29 January 2012 (UTC)

First I am "touchy"; since we are in black-body country, what about the pot and the kettle? You had just reverted an edit of mine in particular circumstances. Second, I do not go around 'fixing' you minor stylistic preferences. I put it that way following a fine literary example (from Alfred North Whitehead) that struck me, and I intended it to strike the reader too; evidently it struck you. And you called it a 'grammatical' correction. Would it be touchy to respond to something like that? I think it fair that you revert your own edit instead of asking me to do it, since you are admitting that you were mistaken, at least in your labeling, if not in your substance.Chjoaygame (talk) 13:18, 29 January 2012 (UTC)

Good call

Good call PAR. Well worded. As I write now, the score for Raffa and Novak is two all!Chjoaygame (talk) 13:26, 29 January 2012 (UTC)

Chjoaygame: This kind of cheerleading on the Talk page is out of order. It tends to cause division of the contributors into "camps" and encourage rhetoric instead of discussion. Please confine yourself to substantive contributions about the subject and its exposition. Brews ohare (talk) 17:40, 29 January 2012 (UTC)

Dear Brews ohare, PAR's comment was evenly mediating and not one-sided or divisive. It constructively offered a way for a resolution of the conflict that it was responding to. I was congratulating him on that.Chjoaygame (talk) 18:20, 29 January 2012 (UTC)

Chjoaygame: Your comment could be construed differently, especially as it suggests you are keeping a score card. Brews ohare (talk) 18:37, 29 January 2012 (UTC)

Since you are persistent, I will say more. I was gently suggesting that the game has rules.Chjoaygame (talk) 18:41, 29 January 2012 (UTC)

accurate citation

It is permitted within the rules of the Wikipedia to actually check one's cited sources. For example, currently standing in our article is a "quote" of Kirchhoff's 1860 paper in Poggendorff's Annalen, but that is not what is found in the paper. What is found in the paper has already been cited in a section of our article, and that citation has been criticized one way or another, but apparently without the cited source having been read by the criticizer.Chjoaygame (talk) 19:39, 29 January 2012 (UTC)

The quote is not from Kirchhoff, but from the source capsulizing Kirchhoff's definition. Both were footnoted. Brews ohare (talk) 20:46, 29 January 2012 (UTC)

I think it is wrong to eliminate Kirchhoff's definition simply on the basis that it is not a literal translation. There are other sources that take the same stance that Kirchhoff was the first to introduce the idea of a perfect absorber. The source cited has a more detailed resume of Kirchhoff's papers than most, and is why I picked this one. This observation provides an historical background for this view of a black body, which as I understand, you personally do not agree with and wish to replace with the idea of a black body in thermal equilibrium. As pointed out in the reply to PAR, that is not the decision underlying the construction of the various articles Black body, Black-body radiation, Planck's law, and if that idea is to be superseded, an alternative orientation to these articles should be discussed rather than unilateral editing of the main page. Brews ohare (talk) 20:56, 29 January 2012 (UTC)

Dear Brews ohare, I have read the above comment from you.Chjoaygame (talk) 02:44, 30 January 2012 (UTC)

Thermalization is necessary

I reverted the statement that the radiation must not escape the interior as the defining aspect of a black body. The whole point of a black body is that the incident radiation is absorbed AND thermalized. If you have an interior of perfectly reflecting walls and shoot a laser beam into the cavity, then without thermalization, the radiation will just bounce around in there forever, and the black body will emit its thermal radiation, plus a small amount of radiation at the laser frequency through the hole. PAR (talk) 16:42, 29 January 2012 (UTC)

PAR: As you will discover by reading the discussion on this page, the definition of a black body adopted here is a perfect absorber. "It was first defined by Kirchhoff in 1859 as an object that absorbs all radiation falling upon it." See, for example, Hoffmann. On the other hand, black-body radiation is characterized by temperature and follows Planck's law.

This approach is not universal, as it happens, and some authors restrict a black body to objects that emit black-body radiation. For example, these authors state that "a black body is a cavity of any shape and of any material, empty except for electromagnetic radiation at some temperature". That would appear to be the definition you prefer.

The decision to use the Kirchhoff definition here in Black body was accompanied by a separation of the material on black-body radiation. This arrangement makes it easier to accommodate the variety of views on this subject. For example, it makes a statement like "consider a black body at a uniform temperature" have some actual content, while if a black body is always in thermal equilibrium such a statement would be a clumsy way to repeat the obvious. It makes clear the role of thermalization, which requires a clear discussion about emission and absorption, and thermalization may not occur for cavity walls with arbitrary properties. A discussion of thermalization can be found here.

PAR, you may find this choice not to your taste, but if you wish to change matters, some consensus must be found on this Talk page first. Brews ohare (talk) 17:27, 29 January 2012 (UTC)

Brews ohare, thank you for these useful references. I read you as saying that you think PAR is trying to alter the strategy of defining a black body as a complete absorber. I don't see him as trying to make such an alteration of strategy.Chjoaygame (talk) 18:37, 29 January 2012 (UTC)

I have added to the introduction to reinforce the emphasis upon the black body as an absorber, and added several sources to this effect. I've also added back the reference to the black-body radiation that makes some sense of the figure. The separation of black body and black-body radiation should be clearer now.

My comments to PAR are intended to point out that the literature is divided over the definition of black body, and one way to clarify the matter is to separate the thermal equilibrium behavior in the article black-body radiation. Brews ohare (talk)

Dear Brews ohare, you seem to believe that there is some importance in your idea that "the literature is divided over the definition of black body". I think you may be the only editor who is concerned with that idea, and I think it unlikely that you will convert others to what seems to be your belief in its importance. In other words, I don't think any other editor but you is in the least interested in any definition other than that the black body absorbs all the radiation that falls on it. No one needs to have it emphasized that the black body is a complete absorber; no one here but you has entertained any other competing idea. From there, people go on to say that if such a body is brought to thermodynamic equilibrium it must emit radiation, and that there is just one unique character for such emitted radiation, the same for every black body.Chjoaygame (talk) 03:06, 30 January 2012 (UTC)

PAR - several of us discussed this at length already and I think reached a consensus. In fact, I thought you and I were in agreement on it, but I might be mis-remembering your involvement. In any case what you wrote is correct, but it's logically distinct from the notion of a black body. The definition that's given by every source I've looked at is the minimal one - that a black body is a body that absorbs incident radiation. As usual with a definition you don't want to include anything in it that isn't logically necessary. A mirrored cavity with a hole works just fine for that, at least so long as the cavity is large compared to the hole so that there is very little probability of the radiation escaping (and you can formalize that as a limit if you want). So regardless of the internal state of the radiation, specks of black carbon, and the reflectivity of the walls, a large cavity with a small hole is a black body, full stop.

Dear Waleswatcher, with respect, I am happy enough to say that the small hole behaves as a small part of a surface of a black body, but I am not so happy to say that the cavity plus the small part of the surface is a black body. The rest of the large cavity is not specified to have a completely absorbing surface. I think the usual idea of a black body is that it is a body that is black however you look at it, black all over. So I would prefer to say that the small hole is a model of a black surface.Chjoaygame (talk) 03:18, 30 January 2012 (UTC)

I agree - it's really the surface of the hole that's the black body, so long as the hole leads into a large cavity. That's all I meant to say above (sorry if it wasn't clear). Waleswatcher (talk) 03:28, 30 January 2012 (UTC)

Whether it emits radiation with a Planck spectrum through the hole is a logically distinct issue. You are correct that if left alone it eventually will, because radiation always thermalizes eventually. But at any given moment it might not - the example I gave earlier was a laser cavity, the hole in which might be close to an ideal BB because it absorbs incident radiation almost perfectly, but emits light with a totally non-thermal spectrum. Waleswatcher (talk) 19:35, 29 January 2012 (UTC)

The idea, proposed by Brews ohare, that the literature is divided over the definition of a black body, is a new one here. I agree with the comment of Waleswatcher, that takes it that we have till now not had any concern about such an alleged division.Chjoaygame (talk) 19:50, 29 January 2012 (UTC)

Waleswatcher you write ".....I think reached a consensus". I understand from what I read that your 'consensus' includes an assertion that Kirchoff's 'black body' does not necessarily emit radiation when its temperature is above 0K. I would like to know, in your consensus, what is the effect on the black body of the radiation it absorbs. --Damorbel (talk) 21:20, 29 January 2012 (UTC)

Your understanding is not accurate. Black bodies certainly emit radiation when their temperature is above 0K, and there's been no suggestion otherwise. As for the effect of radiation that the BB absorbs, it will eventually thermalize and heat up the BB - after at time that depends on many factors, and assuming nothing else is going on that might cool it off or otherwise affect it. Waleswatcher (talk) 22:17, 29 January 2012 (UTC)

Thank you for the explanation of your argument. The problem is here you stated:-

"Quite the contrary. First of all, the definition of a BB is a perfect absorber, which black holes nearly are. They are probably the best black bodies in the universe... ... I'd be happy to provide plenty of sources ... ... I'm putting that back in. Waleswatcher (talk) 20:59, 27 January 2012 (UTC"

Because of your 'undo-ing' of my deletion you clearly see an astrophysical 'black hole', caused by massive gravitational collapse and Kirchhoff's model for perfect absorption/emission of EM radiation, despite tha fact that a black hole does not emit thermal radiation and doesn't (ever) have a temperature.

Do you claim a consensus for this? Black bodies, according to Kirchhoff, lose energy by their thermal (T > 0K) emission; that is what 'thermalisation' is all about! How does this apply to black holes? Why is it that a black hole depends a massive gravitational field for its operation and Kirchhoff's black body does not? --Damorbel (talk) 07:42, 30 January 2012 (UTC)

Black holes do have a temperature, they do emit a Planck spectrum of radiation, they do thermalize any radiation they absorb, and they do lose energy from their radiation. There's only one thing that makes them strange in that regard, which is that they cool off when the absorb energy and heat up when they emit it (i.e. they have negative specific heat) - but the same can be true for certain stars and other gravitationally bound systems. "Why is it that a black hole depends a massive gravitational field for its operation and Kirchhoff's black body does not?" I don't understand your question. First of all the article doesn't quite use Kirchoff's definition, so it's not relevant. Second, nowhere in Kirchoff's definition or in ours do we say anything about the presence or absence of a gravitational field, any more than we say anything about the presence of absence of lamp black. It's a general definition, that's the whole point and why it's so powerful. Waleswatcher (talk) 11:24, 30 January 2012 (UTC)

I guess its true, I haven't followed the discussion on this in enough detail. I will look more closely at it. PAR (talk) 14:15, 30 January 2012 (UTC)

Waleswatcher you write "same can be true for certain stars". I really do not understand why you think this is relevant. 'Can be true' is not really a testable scientific hypothesis. A star has a (very) approximate Planck spectrum; so what? A dog has four legs and a tail, so does a cow; in your logic does a dog approximate a cow? Does a biped halfway approximate a dog?

Further - "and other gravitationally bound systems"- Kirchhoff's black body is definitely not 'gravitationally bound! Do you think therefore the differences are of no interest to a reader of a Wikipedia article? It is the difference between the concepts that is important. My objection to what you write is the attempt to place the similarities above the differences in their rank of importance, surely the complete opposite of what an encyclopedia should be doing. --Damorbel (talk) 15:31, 30 January 2012 (UTC)

Are you disputing that a black hole absorbs incident EM radiation nearly perfectly? If so, you're in a tiny minority and your views do not belong in the article. If not, you've agreed it's a black body by the multiply-sourced definition used in this article. End of story. Waleswatcher (talk) 20:47, 30 January 2012 (UTC)

Robitaille

The series of papers by Robitaille is the product of considerable research and is interesting, but it is not a suitable source for the lead, as you will discover by carefully and fully reading the series of papers.Chjoaygame (talk) 19:59, 29 January 2012 (UTC)

Robitaille is cited in connection with the statement "Black bodies continue to be highly specialized objects constructed from absorbers which are nearly perfect over the frequency range of interest."

In this sentence Robitaille refers to several papers about construction of black bodies. There is nothing controversial in this statement or in the sources cited, so I see nothing inappropriate here, whatever your opinion of Robitaille's body of work. Can you really take issue with this remark? 20:50, 29 January 2012 (UTC) [This comment was written by Brews ohare but something seems to have gone wrong with the signature process.Chjoaygame (talk) 08:40, 30 January 2012 (UTC)]

I think it would be better if you chased up the relevant papers cited by Robitaille and cited them directly; you could then credit Robitaille with having assembled the citation list. The general reliability of Robitaille himself as a source is open to significant question, and simple citation of him directly does not give a caveat about that. Robitaille has been previously discussed on this talk page. He even replied in person at one stage.Chjoaygame (talk) 03:29, 30 January 2012 (UTC)Chjoaygame (talk) 03:30, 30 January 2012 (UTC)

For example, in the cited place, Robitaille writes: "True black bodies [13–25] are extremely difficult to produce and testify against Kirchhoff's universal formulation. [1, 2, 5]." This statement needs careful interpretation and is not a suitable one to offer to a beginner.Chjoaygame (talk) 03:38, 30 January 2012 (UTC)

I googled that name and clicked the first link (http://www.ptep-online.com/index_files/2008/PP-14-07.PDF). Having read the abstract and the first two pages (the paper is "published" in a crackpot journal), I can say with some force that we should NOT be citing anything by that author in this article. It sounds very, very far out of the mainstream, and wiki's policy of due weight then forbids giving more than a very brief mention to such stuff. Waleswatcher (talk) 04:40, 30 January 2012 (UTC)

Opaque poorly reflective bodies

My impression of this section is that its goal is to establish that according to Planck, black bodies have perfectly non-reflective and transmitting interfaces with adjacent media. I personally find this paragraph completely opaque (excuse the pun). I propose to replace this paragraph in its entirety with a brief discussion clearly pointed in this direction, possibly with some more modern developments. What say you all? Brews ohare (talk) 15:10, 30 January 2012 (UTC)

Aye, aye, Cap'n!Chjoaygame (talk) 16:50, 30 January 2012 (UTC)

According to Planck, there is no such thing as an "absorbing interface", such as now has been introduced into the section. That is the point of distinction between Planck's model and Kirchhoff's. The interface is not a material body and so can neither emit nor absorb. Only material bodies can emit and absorb. The new version utterly expunges Planck's meaning. Perhaps that is the new intention?Chjoaygame (talk) 17:54, 30 January 2012 (UTC)

Chjoaygame: I suspect I have used words too loosely: I meant by an absorbing interface one that only accepted radiation, and did not reflect. Maybe some rephrasing would fix things? I believe it is made clear in the Kirchhoff black body section that Planck insisted upon a finite thickness for a black body and rejected the infinitely thin interface idea. Brews ohare (talk) 18:13, 30 January 2012 (UTC)

I reworded this section. Brews ohare (talk) 18:49, 30 January 2012 (UTC)

Quotes in the lead

In defense of the quotes in the lead, I'd suggest a reprise of what I understand to be the course of events on this talk page. Although I think positions have evolved here, there existed at first a strong opinion that the notion of a black body included a reference to temperature or thermal equilibrium. That view has been advanced several times, and will continue to arise throughout the history of this article. Brews ohare (talk) 13:47, 30 January 2012 (UTC)

• This alleged but I think non-existent "strong opinion that the notion of a black body included a reference to temperature or thermal equilibrium" touted by Brews ohare seems to me to be a fiction of the fertile imagination of Brews ohare. I don't recall anyone on this page expressing it. And I don't think anyone is likely to do so. Perhaps his attacks on it are because they provide the "great fun there is in defending an article against every boat-load of new arrivals to the scene"?Chjoaygame (talk) 16:38, 30 January 2012 (UTC)

The quotes have the merit of indicating beyond any reasonable doubt that the definition of a black body is only that of perfect absorption. Any assault upon the definition will have to deal with these two very clear statements about what a black body is. Brews ohare (talk) 13:47, 30 January 2012 (UTC)

• This alleged but I think non-existent "assault upon the definition ... of a black body [as] only that of perfect absorption" seems to me like another fiction of the fertile imagination of Brews ohare. One begins to wonder whether his wholesale reconstruction of the set of articles is based on Brews ohare's imaginative mis-readings of what was there before his arrival.Chjoaygame (talk) 16:38, 30 January 2012 (UTC)

If the quotes are removed or downplayed in the interest of an aesthetic that opposes quotations, the price will be an unending defense of the definition of a black body, which can be viewed as the price of this aesthetic, or as another example of the great fun there is in defending an article against every boat-load of new arrivals to the scene. Brews ohare (talk) 13:47, 30 January 2012 (UTC)

• We are terrified by the thought of "an unending defense of the definition of a black body", and because of our terror, we will gladly submit to any whim or fancy that Boat Captain Brews might like to dictate. In particular, we acknowledge the inviolable sacredness of the "quotes".Chjoaygame (talk) 16:38, 30 January 2012 (UTC)

I am impressed by your reactions. Have you forgotten already this discussion and this, and this? Brews ohare (talk) 18:20, 30 January 2012 (UTC)

Yes.Chjoaygame (talk) 20:03, 30 January 2012 (UTC)

I agree with Brews ohare, there have been multiple posters here that confuse "black body" with "black body radiation" and want to include thermality in the def. of BB. I'm fully on board with the idea of having those quotes in the article, for that and other reasons. But I think in the interests of brevity and clarity it might be better to have them in the section immediately below the lead (titled "Formal definition" or just "Definition" perhaps) rather than in the lead itself, since we should care more about a typical reader than the people commenting here. Waleswatcher (talk) 20:50, 30 January 2012 (UTC)

I suppose, if such posters exist, I have dismissed and forgotten their comments because I could hardly take them seriously. Has their point of view actually found its way into the article?

But I have reported you to the Kommissar of Korrect Thinking for entertaining the diabolically wicked idea of demoting the sacred texts from the lead.Chjoaygame (talk) 21:46, 30 January 2012 (UTC)

I am glad to see that you have defied the fatwa and have demoted the nineteenth century definition from the lead. I note, nevertheless, that not so long ago there were moves to expunge the silly nonsense talked by the fools of the nineteenth century.

Now going back to read the talk-page comments that the Kommissar is worried about, one in particular will be enough: "I reverted the statement that the radiation must not escape the interior as the defining aspect of a black body. The whole point of a black body is that the incident radiation is absorbed AND thermalized. If you have an interior of perfectly reflecting walls and shoot a laser beam into the cavity, then without thermalization, the radiation will just bounce around in there forever, and the black body will emit its thermal radiation, plus a small amount of radiation at the laser frequency through the hole." The writer there was correctly saying that a hole in the wall of a cavity with perfectly reflecting walls will not make the grade as a perfect black body because some of the incident radation will find its way out again. He was at the same time driven by a need to express the apparently fascinatingly attractive (to certain mentalities) idea that thermalization will occur by photon-photon interaction in the absence of matter. He did not say that the definition of a black body requires thermalization, but was emphasizing that the point of the exercise is that thermalization is the important consequence of blackness. I don't think PAR opposes the definition of a black body as a perfect absorber. I think he was just focusing on the physical reason for considering the concept of a black body, not the same thing as demanding a new definition. That he reverted a statement that he thought was loosely worded is not evidence that he wants to reject the definition. The problem here is that PAR was hung up on the idea that photon-photon interactions will thermalize, not that he didn't accept perfect absorption as defining. It is true that his hang-up muddled him there, but his muddle didn't go so far as to make him post a new definition. I think that now PAR has intelligently worked out an admirable moderation of the photon-photon problem. I think Brews ohare is here inventing a straw man, and that such straw men are a main source of his recent re-arrangement of this set of articles.Chjoaygame (talk) 11:38, 31 January 2012 (UTC)

manufactured better than artificial

It is good to see that you have made the mighty advance of changing (after some steps) 'artificial approximations' into 'manufactured'.Chjoaygame (talk) 12:04, 31 January 2012 (UTC)

No, actually, I didn't. I changed "Practical approximations to a black surface" - which doesn't do a good job of expressing the fact that it's discussing physical realizations made by humans - to "Manufactured BBs" - which does a slightly better job.

I suggest you take a break from editing wiki, Chjoaygame. Your comments are hostile, don't assume good faith, and often aren't constructive. Waleswatcher (talk) 12:09, 31 January 2012 (UTC)

Originally the section was headed 'artificial approximations'. The word 'artificial' was deleted. Then the word 'practical' was added. Now it has been changed to 'manufactured'.

I assume good faith until persistent and unresponsive breaking of the rules indicates otherwise. You would like me to shut up.Chjoaygame (talk) 12:15, 31 January 2012 (UTC)

I have no idea what the history of that section title was. I don't own the article or what is written in it. Neither do you. What was written there wasn't very clear in my opinion, so I changed it. You wrongfully accused me of doing something I didn't. Now instead of apologizing, you accuse me of "breaking of the rules" and wanting you to shut up.

I don't want you to "shut up". I would like you to follow wikipedia's guidelines, try to be civil, and try to be constructive. OK? Waleswatcher (talk) 12:20, 31 January 2012 (UTC)

RFC on recent "thermal equilibrium" edit war

Two or three editors disagree about whether it makes sense to include "in thermal equilibrium" in this sentence in the lead:

• "A black body in thermal equilibrium emits a characteristic, continuous spectrum of incandescent thermal radiation that depends only on the body's temperature."

The issue is discussed in the section mmediately above, without resolution, so I request comments from uninvolved editors who understand the physics. Dicklyon (talk) 22:53, 22 January 2012 (UTC)

• I am sorry, but how can something be at the thermal equilibrium and emit energy at the same time? vvv^t 23:55, 22 January 2012 (UTC)

By being surrounded by other stuff at the same temperature, for example; not hard to imagine, but not what's usually discussed for black bodies radiating. Dicklyon (talk) 00:18, 23 January 2012 (UTC)

Dicklyon, at the moment, the article states that a body that absorbs perfectly, when in thermal equilibrium, will emit a black body spectrum (i.e. a Planck spectrum) of radiation.  That's a claim with many sources, and so meets wiki's requirement of verifiability.

You edited the article to state that a perfectly absorbing body will emit a Planck spectrum even if it's not in thermal equilibrium.  That assertion was challenged, and you have provided no sources for it.  Therefore, it cannot be included in wikipedia because it isn't verifiable (independently of the fact that it's also false).  That's really all there is to it as far as the article goes.

As far as the physics goes, let me give you one more example.  I assume you agree that a cavity with a small hole is a black body, because any radiation that's incident on the hole will enter it and is very unlikely to come back out (hence, the hole is a near-perfect absorber).   If the walls of the cavity are heated and the whole cavity is in thermal equilibrium, the hole will emit radiation with a Planck spectrum.  All of those statements are explained in every text on the topic.  (It's also a fact that the properties of the cavity and its walls are not relevant apart from the conditions I've given.)

Now, have you heard of lasers?  Lasers are cavities with a  small hole.  Just as before, that small hole is a black body in exactly the same sense - it absorbs incident radiation.  But if the right state of electromagnetic radiation is set up in the cavity, the radiation that comes out of the hole is laser light, which is extremely far from a Planck spectrum.  The laser state in the cavity actually is a kind of equilibrium (in the sense that it doesn't change with time, although it is unstable), but it's very non-thermal.  Because it's non-thermal, the radiation that gets emitted isn't Planck.

That example proves unequivocally that exactly the same perfectly absorbing body will emit Planck/BB radiation only if it's in thermal equilibrium.  Waleswatcher (talk) 18:44, 23 January 2012 (UTC)

You forgot to login. Can you please come back and sign your comment? Dicklyon (talk) 18:33, 23 January 2012 (UTC)

I couldn't login due to some technical problem, but I did sign with my name. Anyway, the comment above is mine. I'll add a regular sig now. Waleswatcher (talk) 18:44, 23 January 2012 (UTC)

OK, thanks for verifying that it's you. I was hoping to attract comments from someone else who understand physics, rather than just continue our argument here, so I won't respond to your specifics. Dicklyon (talk) 21:14, 23 January 2012 (UTC)

• Editor Dicklyon is asking for the topic to be treated in "the usual way". Perhaps he would very kindly say usual for whom, and how he recognizes reliable sources for it?Chjoaygame (talk) 01:37, 23 January 2012 (UTC)Chjoaygame (talk) 04:26, 23 January 2012 (UTC)

The "usual overview" sort of treatment of a blackbody says what it is (something that absorbs all the radiation that falls on it) and says it emits with a characteristic spectrum that just depends on its temperature, as in this college physics book or this one. Of course, one can argue that if it has a temperature it must be in thermal equilibrium, but if that's the point then it's redundant to say so, especially in the lead. Of course, many sources also explain that you can get blackbody radiation, not only from a black body, but from any body in thermal equilibrium with its surroundings, as this book does; but we not confuse these concepts by saying that a black body needs to be in thermal (or thermodynamic as was sometimes claimed above) equilibrium to emit a blackbody spectrum. I still haven't seen any source that makes such a claim. Dicklyon (talk) 21:12, 23 January 2012 (UTC)

• I think part of the confusion over what this RFC is about arises from the fact that Dicklyon's position has changed completely in the last day or so. At first, s/he asserted that the requirement that the BB be in thermal equilibrium was "nonsense", and that the "bb radiation spectrum does not require any kind of equilibrium". Now, s/he seems to have come around to agree that thermal equilibrium is necessary, and simply objects that mentioning thermal equilibrium in that sentence is "redundant" because it also mentions temperature (a notion that I myself introduced into this discussion, incidentally).

As far as I'm aware, I haven't agreed that any kind of equilibrium is necessary. What's necessary is to have a temperature for the black body. Dicklyon (talk) 23:59, 23 January 2012 (UTC)

I see - so when you said "Of course, one can argue that if it has a temperature it must be in thermal equilibrium, but if that's the point then it's redundant to say so" you didn't mean it? Or have you changed your mind again? Waleswatcher (talk) 00:23, 24 January 2012 (UTC)

As I've said several times, the language that's currently in the article is not perfect, and people are perfectly free to suggest changes to it. What isn't ok is to re-write it so that it implies that the Planck spectrum can be produced by a BB that isn't in thermal equilibrium, because such a statement is both false and unsourced. But if the re-write makes it clear (even implicitly) that thermal equilibrium is required, and if it's an improvement on the language there now, I'd be fine with it. Waleswatcher (talk) 22:31, 23 January 2012 (UTC)

• Dicklyon writes: "I still haven't seen any source that makes such a claim." The classic sources work always in terms of thermodynamic equilibrium, but I agree that doesn't constitute a statement that thermodynamic equilibrium is necessary. But it surely waves a flag that thermodynamic equilibrium is to be presumed importantly relevant till otherwise proved. The classic sources are simply silent on the non-equilibrium situation, until Edward Arthur Milne in 1928 at [6]. He was considering the Planck distribution not as a description of emission from a black body, but as a source function for other kinds of emission. He showed that local thermodynamic equilibrium is a sufficient condition for the source function to be Planck, and mentioned conditions when it wasn't Planck; such conditions hold in the earth's upper atmosphere. This is not a treatment of a black body and doesn't answer your requirement. But it gives a hint about what one might find for a black body. For a serious experimental test of the Planck distribution for emission from a black body, one needs a body with a uniform temperature. One common reading of the term 'thermal equilibrium' is that it refers to a body with a uniform temperature. (Local thermodynamic equilibrium is intended to deal with bodies with non-uniform temperature, but still not so far from thermodynamic equilibrium as to destroy the Maxwell-Boltzmann distribution and the other ordinary near-equilibrium thermodynamic relations.) If a body is heated from within and cools to the outside cooler surrounds, its near-surface temperature will be non-uniform. Some experimenters have observed effects that they attribute to such non-uniformity of near-surface temperature. I think most experimenters will not think of the question that you ask "Is the radiation from a body not in thermodynamic equilibrium Planck?" because if it is not in near-enough thermodynamic equilibrium it will most likely have a non-uniform temperature and this will obviously make the emission a mixture from the various different temperatured parts, and so non-Planck. May I make a simile. A water supply company may say "Our pure water is good for you." They may not add "Dirty water is bad for you" because they vaguely think that is obvious enough. Your demand for a statement that thermal equilibrium is required is like that, I think; like a demand that the sources explicitly add "Dirty water is bad for you." If the thermally radiating body is significantly not in thermal equilibrium then it has a significantly non-uniform temperature and thus a significantly non-Planck spectrum; it might be worse: there could be a non-equilibrium temperature but still so far from equilibrium that the non-uniformity of temperature is not as damaging to the posssibility of Planckness as is the departure from the Maxwell-Boltzmann statistics.

The local thermodynamic equilibrium condition is recognized as sufficient for each infinitesimal volume element to have a Planck source function. I do not know of anyone who has established a precisely defined necessary and sufficient condition for it. There is a small literature about such matters. Experiments are quite difficult and people don't expect to find much surprising in this area.

It is important physics here that when there is a significant departure from local thermodynamic equilibrium, then there will be a significant departure from a Planck source function. Some indication of this is needed. Otherwise people tend to get the idea that there is no need for some nearness to thermodynamic equilibrium. They reason to themselves the following homespun argument: "Oh, the emission depends only on the quantum mechanical properties of the molecules. That means it doesn't depend on the radiative environment." They are forgetting that the real situation is that the emission is logically determined by only the state of the emitting material, but causally, in general, the state of the emitting material depends not only on the quantum mechanical properties of the molecules, but also on the distribution of excitation of the quantum mechanical states, which in general is causally dependent on the incident radiation. When the incident radiation is thermal of the same temperature as the body's, then, and only then, the distribution of excitation of quantum mechanical states is just right to make the Kirchhoff law and the Planck law hold exactly. This is the burden of the Einstein theory of A and B coefficients, and the basis of the Milne paper cited just above. This invalidates the homespun argument, and makes it advisable to make it explicit right from the start that thermodynamic equilibrium is crucial for this work. True, if there is a small departure from thermodynamic equilibrium, then the departures from Kirchhoff and Planck will be small. But for a beginner it is best to make that explicit, so as to avoid the above-mentioned homespun argument that is otherwise likely to spring into existence and corrupt the innocent mind.Chjoaygame (talk) 23:47, 23 January 2012 (UTC)

I wish to contibute to this RFC. The question of temperature and thermal equilibrium is not just a matter of radiation, it applies to all thermal mechanisms that allow the free exchange of energy, important among these is kinetic theory where the equilibrium for a defined temperture is also a requirement. The matter is perhaps easier to understand when considering that a thermodynamic system, not in equilibrium, will have locations that may be assigned different tempertures if they are in local equilibrium. If this is the case (locations with different temperatures) then, according to the 2nd Law, all of these locations will exchange energy until the temperature becomes uniform and thus the entropy of the (whole) system will be maximised. I do hope that the contributing editors do recognise that systems with a non-uniform temperature distribution have an entropy below the maximum. HND (have a nice day)!--Damorbel (talk) 12:14, 23 January 2012 (UTC)

• May I repeat my request to editor Dicklyon? Please would you very kindly be willing clarify 'the usual way'. Usual to whom? How to recognize sources for it?Chjoaygame (talk) 19:48, 23 January 2012 (UTC)

I have provided part of an answer above. I'm going to wait and see if we can get comments from others now. Dicklyon (talk) 21:15, 23 January 2012 (UTC)

Hello! I would like provide an initial comment to this RfC before I read the extensive arguments above. I think the sentence, as worded now, is ambiguous and needs some qualifications before we can really talk about it. "In thermal equilibrium" can mean two things:

• "A black body in thermal equilibrium with itself emits a characteristic, continuous spectrum of incandescent thermal radiation that depends only on the body's temperature."

This is what I thought you meant. It is a pleonasm, like saying "John is a single bachelor." Can a blackbody not be in thermal equilibrium with itself? No. Imagine a box filled with a photon gas. You have a magic wand and you partition the photons in the box such that half are of energy E₁ and the other half are E₂, then you set set things going. At t=0, the photons are not in thermal equilibrium, so when you poke a hole in the box and measure the spectrum of those that escape, you won't observe a blackbody spectrum. You'll have to wait for them to interact with each-other to reach equilibrium, after which time you will see a blackbody spectrum. That the radiation in the box is at a uniform temperature is implicit in “blackbody”.

• "A black body in thermal equilibrium with its environment emits a characteristic, continuous spectrum of incandescent thermal radiation that depends only on the body's temperature."

This is also true. Suppose you have a hole-in-a-box (blackbody "object") and some surface ("environment"), both at the same temperature, facing each other. Each emits photons, some of which are absorbed by the other. There is no net transfer of energy, because that would violate the Second Law (both are at the same temperature). Even if the blackbody is not in thermal equilibrium with its environment, it will continue to emit radiation in a Planck spectrum. Because there is a temperature difference, it will either emit more power or less power than the environment, resulting in a net transfer of energy. Either way, it is still emitting blackbody radiation.

Here is an alternative that I think would clear things up:

• "A perfectly non-reflecting surface at uniform temperature emits a characteristic, continuous spectrum of incandescent thermal radiation that depends only on the body's temperature." Best Regards. Braincricket (talk) 00:30, 24 January 2012 (UTC)

Thanks for your comment. The definition of "black body" given in the first sentence of the article is perfect absorber, which is the definition given in every source we've discussed here so far. You gave a good example in your comment of a black body (by that definition) that does not emit a Planck spectrum because it is not in thermal equilibrium (with itself, in your words), so it's manifestly not a pleonasm. Another example I have earlier is a laser or maser, in which the radiation is not in thermal equilibrium at all (although it is in a kind of equilibrium), and the radiation is nothing close to Planck.

So unless we change the definition of "black body" to include thermal equilibrium - which I think wouldn't be justified by any source we've discussed at least so far - your example shows that we need to mention thermal equil., or explicitly describe it in some other words. Waleswatcher (talk) 00:44, 24 January 2012 (UTC)

I had taken "blackbody" to mean that which emits a Plank spectrum, which implies thermal equilibrium. My clumsy example with the magic wand was meant to illustrate that the initial setup was non-uniform, non-steady-state, non-thermal, and that a blackbody spectrum arises after the system has thermalized. Only then, thought I, could the system properly be called a blackbody. I think I put the cart before the horse.

Regarding the point you made with the laser, it was well-taken—if we stick to the definition in the first sentence of the article, then a blackbody can emit non-thermal radiation under certain circumstances, e.g. when the walls of the cavity and the photon gas are not thermalized, as in a laser, or during that first nanosecond in my magic wand example. In that case, I think it is important to include some phrase which mentions "thermal equilibrium" or "uniform temperature" or something along those lines.

I think part of the confusion in the pre-RfC discussion is illustrated in the first two paragraphs of the Necessity of thermal equilibrium section, and was due to the ambiguity I pointed out in my previous post. It seemed like one author was talking about internal thermal equilibrium (between the photon gas and the walls of the cavity) while the other was talking about TE between the hole and the environment outside. As for temperature, Schroeder's Thermal Physics (p.85) offers a simple definition without using statistical mech: temperature is that property which is the same for two objects when they are in thermal equilibrium.

How about "A black body at uniform temperature emits a characteristic, continuous spectrum..."? Regards. Braincricket (talk) 08:08, 24 January 2012 (UTC)

It would appear from sources that a black body is a perfect absorber. For example, this source says that a black body is one that reflects no light, regardless of what it emits. So, for example, a black hole is the ultimate black body. On the other hand, the present controversy could be resolved by introducing the term black body radiation which appears unequivocally to refer to radiation from a black body that is in thermal equilibrium. According to this source black body radiation is radiation in equilibrium with matter. According to this source it is radiation emitted by an object in thermal equilibrium with its environment. According to Suskind, linked above, there is an interesting discussion about whether a black hole emits black body radiation, ultimately evaporating. Brews ohare (talk) 14:38, 24 January 2012 (UTC)

Radiation from a black hole is discussed in detail here. See also Black_hole#Evaporation. Brews ohare (talk) 14:44, 24 January 2012 (UTC)

A discussion of black body radiation more complete than most is found here. Brews ohare (talk) 15:30, 24 January 2012 (UTC)

At present black body radiation is a redirect to black body. Inasmuch as these two topics are distinct, the introduction to black body should introduce the term black body radiation, or the redirect should be removed and a separate article for black body radiation constructed. In fact, the article black body is almost entirely devoted to black body radiation, so the sensible course is to move the present article to black body radiation and to make black body a stub with a For...see directing attention to black body radiation. Brews ohare (talk) 15:49, 24 January 2012 (UTC)

That sounds pretty sensible. Regarding the physics, the standard technical usage is that a black body is a perfect absorber, and black body radiation is the radiation emitted by such a body when it's in thermal equilibrium at some temperature T. The spectrum of that radiation is called the Planck spectrum, and the fact that it has that spectrum is called Planck's law. One wrinkle is that there's already a wiki article called "Planck's law," which is also about black body radiation. The phrase "Planck's law" is a lot less common than "black body radiation", so I think it should just redirect to the article on BB radiation (whatever it ends up being called). So - on possibility is to merge the current Planck's law article with this one, call it BB radiation, and have a stub as you say for BB itself. Waleswatcher (talk) 16:33, 24 January 2012 (UTC)

I have undertaken to move the contents of this article to Black-body radiation and have rewritten Black body as a stub, adding a few references. Brews ohare (talk) 17:18, 24 January 2012 (UTC)

break

"in thermal equilibrium" adds nothing. Blackbody radiation is related to temperature. If there is a thermal gradient, then the emitted spectra will depend on the local temperature where it is generated. Q Science (talk) 00:57, 25 January 2012 (UTC)

Black body radiation is related to temperature, yes. But a black body is defined by all the sources we've looked at as a perfect absorber. Several examples are given above of perfect absorbers that are not in thermal equilibrium and do not radiate with a Planck spectrum. That shows that "thermal equilibrium" (or equivalent) adds something necessary. Waleswatcher (talk) 01:07, 25 January 2012 (UTC)

Please be explicit. I am not sure what "perfect absorbers" you are talking about. A search for that phrase did not help. Q Science (talk) 09:20, 25 January 2012 (UTC)

A cavity with a small hole for example. See what I wrote above about lasers, or what Braincricket wrote (search "magic wand", and read through that comment thread). Waleswatcher (talk) 12:26, 25 January 2012 (UTC)

Well, a laser is not a perfect absorber, so that does not apply. The "magic wand" stuff does not apply either. The "small hole" example needs thermal equilibrium to approximate a blackbody. This gives an experimental setup to check the results. Once the theory is validated, then the concept of a blackbody no longer requires that qualification. Q Science (talk) 18:03, 25 January 2012 (UTC)

The hole is a black body by definition, regardless of what's inside of the cavity (well, as long as radiation can enter it). But as you say, you need thermal equilibrium to get a BB spectrum out. That's the whole point. Waleswatcher (talk) 19:00, 25 January 2012 (UTC)

RFC going nowhere

The point of the RFC is to get comments from others. The insistence of Chjoaygame and Waleswatcher in filling it up with the same old stuff from the same two guys makes it hard to see if there is any other input. So far, there is not much. But I'd still like go get some input on the question. I will refrain from commenting in this section if the other two will. Dicklyon (talk) 23:59, 23 January 2012 (UTC)

Dicklyon, surely it is illogical for an editor to define who are acceptable contributors to an RFC? --Damorbel (talk) 07:03, 24 January 2012 (UTC)

(Comment from uninvolved editor) I believe that if a black body is not in thermal equilibrium, the emitted radiation will not be a single characteristic spectrum. Therefore the caveat "in thermal equilibrium" is required for the statement. Axl ¤ [Talk] 13:11, 1 February 2012 (UTC)