Talk:Photon/Archive 2

message: The lead says "has three continuous parameters, the components of its wave vector."  Doesn't this imply a particular state with no uncertainty of energy or momentum?  And what about phase and helicity parameters? Isn't this statement too narrow to be true in general and to be part of the lead? Dicklyon 18:40, 26 August 2006 (UTC)
replies: text: The sentence in question is "A photon can have two different helicities (polarization states) and has three continuous parameters, the components of its wave vector.". As you see, it already mentions helicity (the spin projection in the direction of motion, i.e. polarization). Individual photons do not have a definable phase (see the section on "Wave–particle duality"). The closest one gets to that is the probability amplitude that the photon is in that basis state. And the alternative to talking about a  "particular state" (i.e. a pure basis state) is to talk about non-basis states or mixtures of states which is too difficult for the introduction. JRSpriggs 09:32, 27 August 2006 (UTC)
thread_title: Wave number question

message: I tried to finesse these issues by rewriting the section "Energy, momentum, angular momentum and mass" to avoid mentioning the time component of momentum. I hope that it is satisfactory to you-all. JRSpriggs 10:05, 27 August 2006 (UTC)
thread_title:  Momentum's time component and the units of the Metric tensor 

message: Hi, I was thinking of putting us up for peer review.  We might get some good insights that would help us get to Featured article status.  What do you all think — is the article ready for that kind of attention? Willow 14:52, 28 August 2006 (UTC)
replies: text: I fully support the idea.  I am a physicist working within the Wikipedia Physics Project and I would gladly support a peer-review of this article.  I'll put in the template and get the ball rolling... Astrobayes 19:41, 28 August 2006 (UTC)
replies: text: Willow, the peer review is now complete and can be accessed through the template links at the top of this page.  Additional comments will be made about the historical dates, if warranted, but thus far all appropriate review comments can be found on the now-archived review page.  Please let me know if I can be of any further assistance.  I'll keep an eye on this article.  Cheers, Astrobayes 20:46, 1 September 2006 (UTC)
thread_title: Time for Peer review?

message: Melchoir, Willow, and even Astrobayes (in the peer review), all mention that there is a "2 polarization" usage of virtual photons as well.  I have never heard of this (and the article currently doesn't include this as a possibility), and the gauge condition seems like it could only eliminate one polarization (although the peer review seems to state otherwise). So how does this work?  And how can the Coulomb interaction work with only 2 polarizations, as it seems to suggest virtual photons can only have "real/physical" polarizations? I would very much be interested in hearing more about this. -- Gregory9 21:26, 4 September 2006 (UTC)
replies: text: I'm no expert, but here's my (limited) understanding.  In basic textbooks about quantum electrodynamics (there's an oxymoron! ;), they seem to adopt both the Lorentz \partial_{\mu} A^{\mu} = 0 and Coulomb \boldsymbol\nabla \cdot \mathbf{A} = 0 gauge conditions for quantizing the pure electromagnetic field (i.e., no charges are present), which is possible because photons are massless.  These two conditions reduce the four components of A^{\mu} to the two independent polarizations observed physically; the electrostatic potential \phi is zero (no charges present) and the Coulomb gauge condition makes the polarizations transverse since its Fourier transform is \mathbf{k} \cdot \tilde{\mathbf{A}} = 0.  The coefficients of the Fourier-transformed vector potential \tilde{\mathbf{A}} are plane-wave solutions of the corresponding wave-vector \mathbf{k}, and the Hamiltonian for the pure EM field can be written as a set of simple harmonic oscillators, one for each possible wave-vector.  Upon quantization, these coefficients become creation and annihilation operators for a photon with that wave-vector.  Treating the charge-field interaction term in the Hamiltonian as a perturbation, one can derive Einstein's A coefficients and first-order corrections for radiative processes.  I guess, strictly speaking, these processes involve real photons, not virtual photons.  However, higher-order corrections can involve processes in which virtual photons temporarily have nonzero 4-momentum magnitude (invariant mass, gasp!), as Melchoir pointed out.  This is where my understanding begins to dissolve into a grey fog peppered with Feynman diagrams. ;)  Hoping this helps, and hoping that more knowledgable people will step in, Willow 11:31, 5 September 2006 (UTC)
thread_title: Two-polarization treatment in QED

message: I've tried to explain the physics behind some experiments that have been done to establish the limits on the photon mass. The reference list at the end of the article should be moved to the ref. section at the end of the article. But it would be good to first incorporate the results of these articles. E.g. in the article by Roderic Lakes a limit of 2 10^(-16) eV is derived using the torque exerted on a magnetized ring in the presense of the galactic vector potential. This is less controversial than the limit of 10^(-27) eV. Count Iblis 01:12, 2 September 2006 (UTC)
replies: text: Rather than continue to lump all the references together, we should be going in the direction of citing them to specific facts (and by "we" I mean not I, since I haven't read them). Melchoir 17:07, 2 September 2006 (UTC)
thread_title:  Partial rewrite of photon mass section 

message: The article currently seems to have a mix of reference systems - some as footnotes, most only in the References section. I would recommend that some sort of inline references are used throughout the article, with possibly the exception of general references used throughout the whole article. I would recommend either using footnotes throughout, or using Harvard referencing with footnotes reserved for notes rather than references. Thoughts/comments? Mike Peel 08:42, 3 September 2006 (UTC)
replies: text: I think that the in-line references with   are very hard to read when editing. Therefor, I think that it would be better to avoid them. I have to make an effort to see where the normal text ends and begins again in some cases. JRSpriggs 09:51, 3 September 2006 (UTC)
replies: text: Hi, Mike, thanks for all your work on the references!  I also favor Harvard referencing, although I haven't always been consistent about it.   I noticed that other FA's sub-section their References by subject — should we do likewise?  Willow 09:57, 3 September 2006 (UTC)
replies: text:  Personally, I'd lean towards not subsectioning - I would expect that at least some of the references will overlap into several sections, which would cause problems. If Harvard referencing is used, then inline tags should be used to show where the references apply, for example (Einstein 1616b) would be put near/after the text where Einstein's second paper from 1916 has been cited. Mike Peel 10:05, 3 September 2006 (UTC)

text: Having now tried the inline Harvard references, I'm beginning to think that we should change to the footnotes system throughout.  The Harvard approach makes me either be redundant (e.g., "In 1909, Einstein showed...(Einstein 1909).") or stilt my writing uncomfortably.  I agree with JRSpriggs that the footnotes are difficult for editing, but they seem now to be better than the alternatives.  Thoughts from the other editors here? Willow 16:07, 5 September 2006 (UTC)
replies: text:  I think we should use footnotes for the moment. But I would suggest that wikipedia should work on automatically numbered inline refs, similary to current footnotes. Footnotes are not refs, most journals distinguish between the two and footnotes appear on the bottom of the page while the refs end up in the references section. I also agree with JRSpriggs about the markup text in the main text. This should be improved. The markup text should mostly be in the References section.  Another thing that is missing in wikipedia are equation numbers. Count Iblis 17:14, 5 September 2006 (UTC)
thread_title:  References 

message: I've been thinking about how to offer different levels of presentation to our readers, e.g., to let interested readers actually see the derivation or formula from which some assertion in the main text follows.  Perhaps we could do that with subpages such as Photon/Semiclassical radiation theory?  One could even have subpages of subpages, for readers who really wanted to delve deeply.  I suspect that the idea won't work, but I thought I'd at least float the idea.  What do you all think? Willow 12:22, 9 September 2006 (UTC)
replies: text: That's a good idea! Count Iblis 13:21, 9 September 2006 (UTC)
replies: text:  It would probably be better to create new pages, rather than subpages, as that seems to be the way people do things on Wikipedia. For example, use Derivation of the Semiclassical radiation theory for photons (or something snappier). I would only do this if you can justify a wikipedia article on the derivation, though - i.e. it's a notable derivation, and not something that can simply be referenced. Mike Peel 08:41, 16 September 2006 (UTC)

text: Subpages in article space are contrary to policy (not sure where to find it at the moment; take a look at WP:ASR, though). The idea is that, since each article can be freely copied by mirrors under the GFDL, it should stand on its own, and not be dependent on other WP articles. Of course that doesn't mean you shouldn't use wikilinks to other articles, but it does mean that the article shouldn't say something that's wrong or misleading to someone who can't find the link, and it's also interpreted to militate against any hierarchical structure for articles. --Trovatore 20:21, 16 September 2006 (UTC)
thread_title: Subpages?

message: Are we ready for a non-scientific peer review?  Most of the outstanding issues seem to have been addressed, although some may have been overlooked.  Perhaps Mike Peel will be so good as to put all the new references into that  format?  If we're all agreed, we can go for peer review, and afterwards FA status.  Something to look forward to! :) Willow 18:16, 11 September 2006 (UTC)
replies: text: Yes, easy.  Delete that sentence and change the first one to: "In modern physics, the photon is the elementary particle responsible for electromagnetic interactions and light (that is, electromagnetic radiation of all wavelengths, not just those visible to humans)."  And then a paragraph break before the properties.  I tried, but my editing is messed up, maybe due to the article length or something. Dicklyon 03:05, 13 September 2006 (UTC)
thread_title: Ready for Non-scientific Peer Review?

message: see here and here. The reason is that there is no well defined theory of a charged photon, so you cannot translate experimental/observational results and derive limits on the photon charge. What you can say is that all experiments are consistent with the standard model in which the photon has no charge. So, I think that this article should at least say that limits on photon charge are disputed.
Limits on the photon mass are also disputed but that's because there are different models for the photon mass, see here.  Count Iblis 15:42, 12 September 2006 (UTC)
replies: text: As always, I advocate going with what the Review of Particle Physics says, which is <5×10-30e with some comments, but no serious reservations.  See , page 2. --Strait 16:36, 12 September 2006 (UTC)
thread_title:  Limits on photon charge make no sense 

message: I have a hard time interpreting this section.  How can a photon travel slower than c?  How can a scattered photon have differnet energy than it started with?  The way I've seen these handled before is to admit that the matter involved is absorbing and re-radiating, simultaneously.  In that case, it all makes sense.  But for particles to change their identity or their speed this way is somewhat nonsensical, isn't it? How can the photon with a different energy be the same photon that came in? Dicklyon 00:06, 13 September 2006 (UTC)
replies: text: Each photon is indistinguishable from any other photon in the same state. So if there is some amplitude for an incoming photon to be absorbed and re-emitted and some amplitude for it to pass thru unalterred, then the two cases can interfere with each other to produce the outgoing photon. One cannot say whether the outgoing photon is the same photon or not. But its phase (the complex argument of its probability amplitude) will be shifted and the photon thus delayed (slowed). If it is reflected or deflected, then its frequency and energy can change due to the Doppler shift caused by the motion of the entity which reflected or deflected it. JRSpriggs 09:38, 13 September 2006 (UTC)
thread_title: Photons in matter

message: Since Willow poked some of us working on the protein article to have a look at this one, here's a few suggestions from a scientist who hasn't thought about photons in years:
* For FAC purposes, the lead will almost certainly need to be expanded - I'd suggest putting a more general/non-technical explanation in the first paragraph and putting the current information in a second paragraph. (FAC seems to also have a strange obsession with putting footnote numbers after punctuation, even though as far as I know it's nonstandard.)
* This is stretching the limits of dumbed-down-ness, but I'm sure I'm not the only lazy one who writes "hv" (not ν) - worth mentioning this as an alternative?
* There's quite a lot of historical information, which is very well-written, but the chronological organization of the article is potentially confusing without an initial summary of the modern consensus view (otherwise the reader gets a sense of repeatedly being told "close but wrong" information). I thought of this when reading the "energy, momentum, angular momentum and mass" section, which occurs late in the article but contains most of the information that a reader generally acquainted with the subject would be familiar with.
* It should be made clearer that the "gamma ray microscope" is a thought experiment, not an actual device. (Also that the required "collision" isn't really.)
* There's a couple of quotes that appear somewhat arbitrarily without direct citations - they stand out because the rest is very well cited. I'm thinking mostly of 'honorable funeral' and 'all the world is quantized' - are they actual quoted words, or just turns of phrase?
* The section on the Bose-Einstein model is a bit confusing - it reads like it used to be longer and got chopped too much. "In 1926, Paul Dirac showed that the number of photons is not conserved." - this sentence seems to come out of nowhere. Not conserved in the formation of a Bose-Einstein condensate? Similarly, the subsequent discussion of whether photons are bosons or fermions doesn't have an obvious relationship with the preceding paragraph.
* I don't understand what is meant by "the density of radiation at frequency ν within the cavity". Is it assumed that the density is not time-dependent? Is there any significance to the decomposition of Bij into its two component rates?
* "...photons automatically obey Bose–Einstein statistics." - somehow automatically seems like an odd word choice here. Something more like inherently? (Also, because I didn't understand the statement about Dirac's photon-conservation result earlier, I'm not clear on how photons can obey B-E statistics, and yet shouldn't they by definition, because they're bosons?)
* "The unification of the photon with W and Z gauge bosons..." - the concept of "unification" hasn't been explicitly defined. The gauge boson section also mentions that "photons are predicted to be massless and chargeless", but these are prerequisites for falling under gauge theory, so is this a prediction or an assumption?
* The "invariant mass" section mentions the necessity of counting virtual photons in QED, but it's not previously explained whether virtual photons add to invariant mass.
* The mass of citations (hurr, bad pun) for the masslessness of the photon are nice to have, but kind of distracting in a big string. I don't know how this would look formatting-wise, but maybe just one number to link all the references would work better.
Erm, I think that's it. Nice job, everyone. Opabinia regalis 05:34, 13 September 2006 (UTC)
thread_title:  Suggestions from a non-physicist 

message: Thanks to the efforts of everyone here and our friends from biochemistry, I think we might be ready for non-scientific peer review.  I'm going to look at a few other encyclopedias to see whether we missed anything, but the article is respectable, don't you think?  If we're agreed, I'll add the peer-review template late tonight or tomorrow morning.  The present length of the printed article is ~8.5 pages. Willow 16:01, 14 September 2006 (UTC)
replies: text: I don't know how you are doing your counting, but the article is 14 "printed pages"; I'm sure it will hit the 20-page mark soon. --Sadi Carnot 13:11, 20 September 2006 (UTC)

text: I've followed this process from the beginning and it's going great.  As far as my last commitment to check the historical accuracy of the various sections, my delayed but final response is this: they are well-written, the dates and players match well with the sources I've consulted, and the only thing that remains is, per comments above in previous sections and in my SPR of the article: the chronology and organizational layout might benefit from a little restructuring.  The SPR page just linked contains my suggestion for the layout based upon the content of the individual sections at the time.  WillowW, I think it's great the initiative you've taken here and I've just been glad to have been one of the grains of sand riding this wave.  I look forward to the FA!  Cheers, Astrobayes 17:13, 22 September 2006 (UTC)
thread_title: Non-scientific Peer Review

message: Sadi Carnot has added some excellent elements to 
our article which were sorely missing, such as the retinal example and mentioning the wave-particle duality in the lead.
However, the new lead section needs a little work, since it no longer 
conforms to Wikipedia's guidelines for the lead, which the experienced editor Peta recently called to our attention on this page.
The lead section should provide a clear and concise introduction to an article's topic, establish context, and characterize the terms. It should contain several paragraphs, depending on the length of the article, and should provide an overview of the main points the article will make, summarizing the primary reasons the subject matter is interesting or notable, and including a mention of its notable controversies, if there are any. The lead should be capable of standing alone as a concise overview of the article, should be written in a clear and accessible style, should be carefully sourced like the rest of the text, and should encourage the reader to read more.
A table later in the guideline article suggests that we should have 3-4 paragraphs in the lead.  Are there any objections to returning the two "Overview" paragraphs back into the lead?
Another issue is that, as written, some of the new material in the lead
might be worded confusingly.  For example, the expression "force carrier for all forms of electromagnetic radiation" might confuse readers between two different aspects of photons, their conveyance of light energy/momentum and their mediation of electromagnetic interactions.  Also, the sentence "The photon, essentially, is a 'quantum' or packet of electromagnetic radiation measured in proportion to its ability to move an electron between atomic orbitals in atoms and molecules." seems somewhat vague and, at the same time, overly specialized to absorption/electronic transitions in atoms and molecules.  Finally, readers might become confused when we say in the lead unqualifiedly that photons move at the speed of light; perhaps, we should add in vacuum or in empty space?
By the way, we haven't received any more criticisms of our article at Wikipedia:Peer review so, if we're all agreed, I'll submit the 
FA candidacy tomorrow. Willow 17:16, 21 September 2006 (UTC)
replies: text: I think it's ready for FA. -- SCZenz 22:05, 21 September 2006 (UTC)
thread_title: Edits of 20-21 September 2006

message: Hi all, 
The FA candidacy seems
to be going well; three Supports (four, if we count Count Iblis) and no Opposes so far. Please add your Support votes to help make this article a FA — thanks! :D Willow 22:51, 23 September 2006 (UTC)
replies: text: OK, we're at 5 Supports (Thanks, Count! :), 1 "nice" Comment and no Opposes.  We'll get at least one more review  from a physicist, since Laura Scudder agreed to review the article for us.  She always has good insights and seems fair so, either way, it's a win for us and for the article.  Keep the fingers crossed! :D Willow 15:22, 25 September 2006 (UTC)
thread_title: We're now a Featured Article Candidate

message: Hi, the info table at the start of the article has a dark grey background. Could it be changed to something lighter? I'd fix it myself, but I don't know if the colour was chosen on purpose, to match other articles on particles. --Kjoonlee 06:15, 26 September 2006 (UTC)
replies: text:  Hi, Kjoon, I agree, it was too dark to read easily.  I lightened it just now, but we may have to Talk to the other particle pages, to see if we all can be consistent — they're still in the dark ;).  Thanks for pointing that out! :) Willow 06:40, 26 September 2006 (UTC)
replies: text: Thank you. :) I edited one boson article, and it seems fermions need to be edited next. (Unless someone objects, that is.) --Kjoonlee 06:50, 26 September 2006 (UTC)
replies: text: Done. :) List of articles I edited is at Wikipedia talk:WikiProject Physics#Background colour of info tables. --Kjoonlee 08:27, 26 September 2006 (UTC)
thread_title:  Background of info table 

message: At the moment, most sections have  links at the top of them. I would suggest that  is only used where the article being linked to covers the material in the section it's being linked from in more depth - for example, the main article for the "Physical properties of the photon" section would be something that discusses the physical properties of the photon in more detail than the section does. If the page being linked to covers something related, but off-topic, then use , or if it gives a broader scope to a topic then use . I'd make the change myself, but I thought it might be better if someone more familiar with the details of the topic made the change. Mike Peel 07:31, 26 September 2006 (UTC)
replies: text: Thanks, Mike! I hadn't realized that there were different types of templates for those sorts of links.  I think it's fixed up now. If you'd like to weigh in at the FA candidacy, that'd be great. :)  Willow 14:28, 26 September 2006 (UTC)
thread_title:  {{[[Template:main|main]]}} links