Talk:Black body

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Copying from Archive 4 at Talk:Black body/Archive 4#Kondepudi & Prigogine 1998 on matter and radiation:

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)

Now I see more clearly how Waleswatcher was mistaken. He was presuming that the formula μ = 0 was from his own context in quantum electrodynamics in which the symbol μ denotes the virtual mass of the virtual photon, when in the context in which I cited it from Kondepudi & Prigogine 1998 as they say it refers to the chemical potential, an entirely distinct concept. An example of the use of the symbol in Waleswatcher's context is as follows, from A. Zee (2101), Quantum Field Theory in a Nutshell, second edition, Princeton University Press, Princeton NJ, ISBN 978–0–691–14034–6, on page 150: "The resolution of course is that as the coupling of the longitudinal mode vanishes as μ → 0 the time it takes for the longitudinal mode to come to thermal equilibrium goes to infinity. Our crafty experimentalist would have to be very patient." This is the relaxation time for the photon-photon interactions beloved of Waleswatcher and PAR.Chjoaygame (talk) 14:17, 23 February 2012 (UTC)

I wasn't mistaken, and I was referring to the chemical potential when I said "mu=0 means particles can be created and destroyed with no thermodynamic cost". The chemical potential divided by the temperature is the energy cost to create a particle (as can be easily seen from the form of the grand canonical ensemble). For a photon, that energy cost depends on the frequency, and it goes to zero for zero frequency because photons are massless. As for your quote from Zee, you'll need to provide more context for me to be sure what he's talking about, but it sounds like the longitudinal mode of the photon - which doesn't exist on-shell and is irrelevant to the arguments regarding thermalization we had (it's the transverse modes that thermalize). Waleswatcher (talk) 14:29, 23 February 2012 (UTC)

Good to know you weren't mistaken. Not so clear about what you mean by "no thermodynamic cost". It might mean "with no entropy production". We are hoping that thermalization will indeed be a process with entropy production. But it seems from what you write that you mean "no energy consumption"? Indeed photons are massless as to their rest mass, but they have an energy, and that energy usually has to come from somewhere; things can be transduced with no energy consumption. So why were you worried about what Kondepudi & Prigogine 1998 had so say? As to Zee, indeed he is, as he says in the quote, talking about the longitudinal mode of the photon, but all virtual photons have that, being as you say off-shell. You need to show that your thermalization by photon-photon interaction does not require thermalization also of virtual photons, if you want to avoid what Zee is saying.Chjoaygame (talk) 17:07, 23 February 2012 (UTC)

I meant at no energy cost, not no entropy production. In the limit of zero momentum photons have zero energy, and that's what zero chemical potential corresponds to. As for Kondepudi&Prigogine, I don't recall being "worried" about what they had to say.

"You need to show that your thermalization by photon-photon interaction does not require thermalization also of virtual photons, if you want to avoid what Zee is saying."

I can't make sense of what you are saying. Virtual particles aren't thermal or non-thermal, physical modes are. Thermality is tested by (for example) putting a thermometer into the system and seeing what it says. Because the thermometer will never absorb or emit a longitudinally polarized photon, the state of such photons (to the extent that phrase even makes sense) is irrelevant. In any case, none of this has anything to do with the particular interactions that thermalize the photons. If we had to worry about the thermality of longitudinal photons (we don't), we'd have to worry about it even when the approach to equilibrium is brought about by interactions with matter. Waleswatcher (talk) 18:53, 23 February 2012 (UTC)

edit of insulated enclosure

The enclosure is said in this caption to be 'insulated' for good reasons. You may read the article for a fuller explanation of why the enclosure is 'insulated'. In a nutshell, the walls must not be transparent to thermal radiation and must not allow matter or energy to flow in or out so as to destroy the steady state of the contents of the enclosure. The word 'insulated' in the caption intends to briefly indicate these necessary things.Chjoaygame (talk) 05:15, 3 March 2012 (UTC)

photon–photon interaction in the absence of matter

For the purpose of sourcing for photon–photon interactions in the absence of matter, the present Wikipedia article currrently contains the following: "direct photon–photon interactions".

The 1951 article by Karplus and Neuman starts: "In an earlier paper¹ the nonlinear interactions between electromagnetic fields were expressed in terms of the polarization of the electron-positron vacuum.

¹ R. Karplus and M. Neuman, Phys. Rev. 80, 380 (1950)."

The 1950 article by Karplus and Neuman starts: "It has long been recognized that higher order corrections in quantum electrodynamics include non-linear interactions between electromagnetic fields.¹

¹ For a summary of the literature on this subject the reader is referred to A. Pais, "Developments in the Theory of the Electron" (Institute for Advanced Study and Princeton University, 1948), pp. 21–26."

Thus the 1951 article is about higher order correction calculations in quantum electrodynamics, not about direct physical photon–photon interactions in the absence of matter. It is a mistake to suppose that higher order interactions can be simply transferred to real physical processes. The non-linearities arise in the higher order calculations because there are several different fields with actual quanta present. There is no experimental evidence that the light field in the absence of matter has a non-linear character such as is considered for higher order correction calculations. The absence of such evidence is clear from the non-production of it on this talk page. It is true that speculations are made that in 2015 a facility will become available to test a presently untestable speculation that there may be non-linearity in the pure light field, but it is pure speculation without present-day experimental support. It is also true that it is planned at massive experimental facilities in future to make empirical studies of collisions of gamma rays in the absence of matter, but as is clear from the non-production of empirical evidence about it on this page, at present, non-linear gamma-gamma interaction in the absence of matter is also pure speculation.

According to the second edition (the first edition was cited above on this page) of Mandl, F., Shaw, G. (2010) Quantum Field Theory, Wiley, Chichester, ISBN 978–0–471–49683–0, on page 1: "The interactions between these particles are brought about by other fields whose quanta are other particles. ... These and other processes of course only occur through the interactions of fields." They clarify this on page 9 by saying: "For anything 'to happen' requires interactions with charges and currents so that photons can be absorbed, emitted or scattered." They are saying that, other than propagation of itself, a free field (in the absence of actual quanta of other fields) does not support any processes such as scattering. In particular this means that they are ruling out non-linear interactions of photons between each other in the absence of matter. The quantum vacuum implies the presence of matter. For example, the Casimir forces occur in a cavity so small that the distance between the material walls is of the order of the wavelength of the relevant light waves; that is to say, the quantum vacuum is derived from the presence of matter. Milonni, P.W. (1993) The Quantum Vacuum, Academic Press, Boston, shows how the quantum vacuum effects on an atom arise from the self-fields of the atom itself. Statements of principle like that of Mandl & Shaw 2010 just cited may be found in other reliable textbooks.

The article by Karplus and Neuman 1951 states on page 776: "The processes to be considered are the scattering of light by light,² two-quantum pair creation,³ the scattering of light in an external field,⁴ and the creation of pairs in an external field.⁵

^{2 a} H. Euler, Ann. Phys. 26 398 (1936). ^b A. Achieser Physik Z. Sowjetunion 11, 263 (1937).

³ G. Breit and J.A. Wheeler, Phys. Rev. 46, 1087 (1933)

The article by Karplus and Neuman 1951 in their section VI. FORWARD SCATTERING states on page 782: "This result is identical with that of Breit and Wheeler.^3,9 [reference ⁹ is to a then unpublished article.]"

Breit and Wheeler are both well recognized physicists. The title of their cited article is 'Collision of two light quanta'. They are writing about "calculations for the production of positron electron pairs as a result of collision of two light quanta".

On page 1087 they start their article: "Two simultaneously acting light waves with vector potentials

$\mathbf {A} _{j}=\mathbf {a} _{j}^{*}\ \mathrm {exp} \{-i(\omega _{j}t-\mathbf {k} _{j}\mathbf {r} )\}+\mathbf {a} _{j}\ \mathrm {exp} \{i(\omega _{j}t-\mathbf {k} _{j}\mathbf {r} )\}$   (1)

are considered as acting on an electron. Under the influence of the waves a single electron wave function ψ⁽⁰⁾ is changed, ..." [I have inserted an $i$ which I think was omitted by misprint from the formula in the original Breit and Wheeler 1934 paper.]

Breit and Wheeler 1934 also remark: "It is also unnecessary to use quantized light waves in the pair production problem, since the results with quantized waves are known to be identical with those obtained by means of ordinary waves."

Breit and Wheeler 1934 are referring to a physical electron being present for their collision of two light quanta. Considering the general principle cited above as stated by Mandl & Shaw 2010 and considering the "identical" coincidence of results stated by Karplus and Neuman 1951, it seems hard to believe that this was not also implicit, though unstated, in the result of Karplus and Neuman 1951. Thus it seems that the paper of Karplus and Neuman 1951 is about photon–photon interactions in the presence of matter and is not suitable as a source for article entries about such interactions in the absence of matter.Chjoaygame (talk) 15:02, 3 March 2012 (UTC)

"Thus it seems that the paper of Karplus and Neuman 1951 is about photon–photon interactions in the presence of matter and is not suitable as a source for article entries about such interactions in the absence of matter." What in the world are you talking about? In the abstract it says very clearly what they do: "The differential cross section for the scattering of light by light is calculated as a function of energy and angle..." Do you know what a differential cross section is? Do you know what "light" means? They are computing the amplitude for scattering of photons by photons, not by matter, and (as I've tried to explain to you many times now) it's not zero even when the center of mass energy is below the threshold for e+/e- pair production. Their calculations are done in a perfect vacuum, with an in state of two photons, and an out state of two photons. Waleswatcher (talk) 16:21, 3 March 2012 (UTC)

Here's a freely available recent paper that calculates some high order corrections to 2-->2 photon scattering. The first two paragraphs of the introduction give a nice overview. I suggest you read them. Waleswatcher (talk) 16:59, 3 March 2012 (UTC)

Dear Waleswatcher, it seems you haven't read the body of the cited "source" article itself or its references, since you address only its abstract. Yes, they are computing the amplitude for scattering of photons by photons, but virtual photons by virtual photons, not as you mistakenly suppose actual photons by actual photons (perhaps though not explicitly in the presence of the quantum vacuum), as you would have noticed if you had done your homework by reading your supposed source and its references. Your "repeated" "explanations" are no counter to an actual reading of the article and its references. Your statement that your cited Karplus and Neuman 1951 calculations are done in a perfect vacuum (not actually explicitly stated in their article) might seem plausible if their results were not identical with results for the same problem done in the presence of matter as made explicit in the 1934 article by Breit and Wheeler to which they refer. It does not advance your argument to suggest that I don't know what a differential cross section or what light mean. To try to justify your supposed source, you need something a lot more convincing than a handwaving attempt at distraction such as that. The substantial matter here is not what I know, but what the sources say. The question is not as to what the scattering is by, as you try to suggest; it is as to whether it takes place in the presence of matter. Your suggestion that higher order corrections are relevant only compounds the fact that you haven't noticed that the article is about virtual photons in interactions between different fields. Well may you ask "What in the world are you talking about?" You have not attended to the physics, but have read only the mathematical formulas without their physical meaning. You have misread the abstract, and apparently left unread the main body and references, of your supposed source, instead of which the statement in the article needs a suitable and reliable one.Chjoaygame (talk) 17:17, 3 March 2012 (UTC)

Total and complete nonsense. I read the article, and unlike you, I understand what it says. Equation (5) on page 777 is the differential cross section for 2-->2 elastic photon scattering, exactly as I said and the abstract says. These are obviously not virtual photons, because this is (a) a cross section, and (b) they say so explicitly. Waleswatcher (talk)

I have now read the arXiv article to which you refer. It doesn't come near addressing the problem of your not having read the Karplus and Neuman 1951 article or its references. In referring to an arXiv research article, a primary source, you are clutching at straws instead of producing a reliable source, which for a substantial matter of principle like this should be an established secondary one, or preferably several, since you claim that everyone knows it.Chjoaygame (talk) 17:26, 3 March 2012 (UTC)

"It doesn't come near addressing the problem of your not having read the Karplus and Neuman 1951 article or its references." You're obviously not posting in good faith, and I will ignore your future comments until you retract that and apologize. Waleswatcher (talk) 18:17, 3 March 2012 (UTC)

You refer to the first two paragraphs of the arXiv research article to which you give a link. The first paragraph has nearly the same old references as that of Karplus and Neuman 1951. The arXiv article references are (1) to 1935, 1936 papers by two authors with Euler, (2) to a 1951 paper by Schwinger not cited by Karplus and Neuman though they cite one by him of 1950 (3) to Karplus and Neuman 1951, and (4) the Akhiezer 1937 paper cited above. The second paragraph of thge arXiv research article to which you link confirms my above comment on the lack of adequate empirical evidence for photon–photon scattering. The arXiv authors write: "The direct experimental evidence for γγ → γγ scattering is still scant, ..." Thus, your citation of this arXiv article does not advance your case, but just emphasizes that you have no Wikipedia-acceptable reliable source for your synthesized original research claim that in the absence of matter light will interact with itself to reach a Planck distribution.

You are avoiding the substantial issues by accusing me of bad faith and demanding an apology. If I thought you would really refrain from further comment I would expect to be able to edit the article without your intervention. Would I be so naive?

Your refusal to respond is just a word-game to cover your inability to produce reliable sources, which is what you need to do.Chjoaygame (talk) 18:43, 3 March 2012 (UTC)

Experimental Apparatus - Cavity with a Hole

Since the experimental apparatus of Lummer and Kurlbaum is central to real world attempts to create a blackbody, I feel this section should be expanded. Inclusion of a brief description of how the apparatus works and a figure would go a long ways to aiding reader comprehension. I am not knowledgeable enough about experimental physics to do this myself, so could someone with the background please expand this section? --BBUCommander (talk) 15:25, 19 August 2012 (UTC)

Definition of a white body

Why is a white body necessarily rough? Seems like this statement is not true and includes unnecessary detail. The roughness or smoothness of an ideal black body is not mentioned. neffk (talk)