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Photon

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Hi, the editors at Photon believe that it is ready to be a Featured Article. It has had fruitful scientific and non-scientific peer reviews, and seems to be stable and complete. Willow 17:02, 22 September 2006 (UTC)[reply]

  • I think this is a good article. I have one comment. This article fails to mention photon-photon interactions in vacuum. This is a technical subject and cannot be treated in detail in this article, however it should be mentioned that due to QED effects photons do interact with themselves (and the electromagnetic field). One can refer to the wiki pages on QED, the Euler Heisenberg Lagrangian etc. etc. I do think this is important, because a lay person reading this article will get the idea that because photons have zero charge, they don't interact with the EM field. Count Iblis 18:32, 22 September 2006 (UTC)[reply]
Response Hi, Count, that's a good suggestion; I hope you like how the article has been amended. I had been nervous about including photon-photon interactions before, since it really is in the Himalayas of theoretical physics and the article is already a "tad technical". However, it fits in neatly with other types of virtual particles, which had not been mentioned earlier. Thanks muchly for improving the article! :) Willow 09:46, 23 September 2006 (UTC)[reply]
Hi Willow, yes I like amendment! Needless to say, I support this article becoming a featured article! Count Iblis 19:52, 24 September 2006 (UTC)[reply]
Response Hi, CG, thanks for catching that! I've added a brief section near the end to cover the applications mentioned in the lead. Please let me know if it should be improved somehow. Willow 02:50, 25 September 2006 (UTC)[reply]
  • I just have a few comments:
Lead
The prose is a little awkward in places. Things like the aside about light here meaning all EM radiation breaks the flow by raising unaswered questions for the reader about what else it could mean. A simplier replacement like maybe light and all forms of electromagnetic radiation, although I always prefer to read "light" as meaning all EM radiation, would be an improvement I think.
Response The wording is finicky, because the word "light" is used to refer to "all forms of EM radiation" throughout the text, for both beauty and brevity. Unfortunately, some people understand "light" only as "visible light", so we have clarify the definition for them early on. How do you like the present wording?
Much better than my suggestion!
My personal preference is to minimize the use of parentheses in favor of commas.
Response Me, too, although you'd never know it. ;) Fixed excess paren's throughout the article.
Nomenclature
Why do we use γ for photons? Carry over from gamma rays?
Response Umm, not sure. It's probably nuclear gamma rays, as you suggest, but I don't know who used it first. I'll try to track it down.
Yes, it's because of gamma rays.--24.52.254.62 03:31, 26 September 2006 (UTC)[reply]
Great! Could you give us a reference to a book or a journal article that says so? Ideally, it would be the first article to make the definition, or the paper of a scientific nomenclature committee. Thanks! :) Willow 04:10, 26 September 2006 (UTC)[reply]
I looked around a bit for one before asking, but it's one of those tricky set of keywords that grabs all sorts of other junk. If there's no source, it's not a big deal. Just a question that occurred to me while reading. — Laura Scudder ☎ 16:05, 26 September 2006 (UTC)[reply]
Wave-particle duality
Where are the see belows at the ends of the first second, and final paragraph supposed to be directing me?
Response Added specific intra-article links; are these allowed? Alternatively, I could just delete them, although it is a nice way of connecting different parts of the article and useful for readers who would like an immediate link for more information.
So far as I know, there's no problem with intra-article links.
Second quantization
I agree with Opabinia regalis that Image:Photon_waves.png doesn't add anything, and could confuse a lay reader (e.g., as to color-wavelength correspondences).
Response Agreed, but this is delicate. I'll try to make an improved image that satisfies everyone.
OK, a new image has been uploaded; although it might benefit from fuller explanation, it's at least as clear as the previous image and pretty accurate on the correspondence between wavelength and color (450, 550 and 750 nm for blue, green and red).
Although not as pretty, this is much clearer and informative. — Laura Scudder ☎ 16:05, 26 September 2006 (UTC)[reply]
Bose–Einstein model of a photon gas
The last sentence is out of place, introducing helicity without further explanation.
Response That sentence is now gone, its Helicity definition moved up earlier to the "Physics" section.
Fits much better now. The extra context of what the two values correspond to makes it more obvious that there wasn't simply a mistake where zero got left out.
The photon as a gauge boson
This section is a little too free with i.e.s
Response <Blush> What can I say? I like Latin. ;) OK, they're gone, too.
My physics is rusty already. What's the reason for no 0 spin angular momentum along the direction of the momentum?
Response It's totally not obvious. Photons are the quanta of the Fourier modes of the electromagnetic four-potential , which are indexed by their angular frequency and wave vector . The Fourier transform of the Coulomb gauge, is , so there can be no component of the vector potential along the wave-vector of an EM mode. Hence, choosing this gauge kills all longitudinal modes and all longitudinal photons with them. And we have to choose a gauge so as not to overcount the possibilities in the Feynman summation.
Ah, I see now.
Response Thank you so much for your thoughtful review, Laura! The article is definitely better for it. :D Willow 02:34, 26 September 2006 (UTC)[reply]
Thanks, Laura, your praise is the best gift. Willow 17:09, 26 September 2006 (UTC)[reply]
  • Comment: I think this article does a poor job of introducing the topic to people who don't have a lot of physics background. (I have a PhD in physics, and teach it for a living.) It gets to be heavy going immediately when you get to the section titled "Physical properties of the photon." The average person is not going to be able to make heads or tails of this. There are lots of easy, intelligible ways to introduce this topic to the general reader. You could, for example, start with the photoelectric effect, or with the Bohr model of the hydrogen atom. Another example of a section that's not going to be intelligible to the typical reader is the one on "Stimulated and spontaneous emission," with all the matrix notation. Also, I could be wrong, but I think the "Main article" links are being used incorrectly here; for instance, the Squeezed coherent state isn't the main article on wave–particle duality, it's an article on a related topic. A more minor nitpick is some of the notation. It's not necessary to use both f and nu for frequency, or to give two versions of the same equation, one using nu and one using omega; show some editorial judgment and just pick a notation and stick to it. Also, hf isn't a notation for a photon, it's a notation for the energy of a photon.--24.52.254.62 03:06, 26 September 2006 (UTC)[reply]
Response Hello, 24.52.254.62, and welcome to Wikipedia. :) Let me address your minor concerns first, and then tackle the major ones.
  • The link to Squeezed coherent state is indeed correct, as it is meant to clarify the uncertainty principle ΔnΔφ for photons as well as the uncertainty principle ΔxΔp for material particles. I have amended the section title here to clarify the section content better — thanks for the tip!
  • "hf" is a symbol used for the photon by some chemists, albeit quite rarely; it's written over the reaction arrow in photochemical reactions, such as in free radical formation of chlorine. It's rare enough that I would've preferred not to include it, but we are striving to be encyclopedic and this symbol has its partisans among this article's editors.
  • This "hf/hν" symbol thing is the only use of f for frequency in the article, and it's well-explained, so I think that usage is innocuous.
  • Regarding the usage of both ν and ω, here's the rationale. We must have ν in the article, because of the ubiquitous E=hν equation and the corresponding photon symbol. It is indeed the dominant symbol, with ω appearing in only two sections ("Physical properties" and "Photons in matter"). However, it would be similarly hard to leave out the wave-vector , as you may see from all the formulae that employ it. However, the wave-vector is part of a four-vector that includes ω, just as E and form a four-vector. We would like to relate these four-vectors in this and other articles, so we likewise need ω. Besides, it's useful for defining the group velocity in a compact way. So we have included both frequency ν and angular frequency ω to serve an educational purpose, not because we're undecided or careless.
  • I personally believe that the basic results of the "Stimulated emission" section will be intelligible to an interested reader. They'll need to know what "emit", "absorb", "rate" and "equilibrium" mean, as well as the concept of an energy transition. The editors and reviewers here and in the earlier peer review are not all physicists and still manage to understand that section.
  • The photoelectric effect is mentioned pretty prominently in the text, as you recommend.
  • I think the Bohr model approach to photons is ill-advised. (1) First and foremost, you cannot derive E=hν or any other photon properties from the Bohr model. Bohr himself rejected the reality of photons for over 12 years after he published his model, as is clear from the BKS model (1924,1925). (2) Even in its own day, the Bohr model was never considered to be a scientific theory, since it was founded on no physical principles, and did not succeed in fully accounting even for the spectrum of hydrogen, to say nothing of higher elements. It was instead appreciated as a thought-provoking model that stimulated further research. But we should definitely not present it as a valid physics model from which other results can be derived with confidence.
  • I agree that the "Physical properties" section might be less daunting if it came later, and I had arranged it so myself. But there are strong educational reasons for presenting it early, as argued by several editors here. I've added a trial "skip this section" flag to warn readers that they might want to skip directly to the History. I'm not sure if I like it, though, and would appreciate it if the other reviewers would comment on this flag. I'm also open to re-arranging the order of that section, which was also recommended by Astrobayes in his scientific peer review.
Please write again if some of your concerns have not been addressed adequately. Willow 05:02, 26 September 2006 (UTC)[reply]
Well, frankly, I don't think you've addressed my concerns at all. The article is not comprehensible to the general reader. The general reader, for instance, doesn't know a matrix from a hole in the ground, and therefore will have no idea of what's going on in the section on "Stimulated and spontaneous emission."--24.52.254.62 03:17, 29 September 2006 (UTC)[reply]
  • Comment: As I have already tried to correct, and as noted above by another reviewer, the following intro sentence is grammatically incoherent and contradictory:
It mediates electromagnetic interactions and is the fundamental constituent of all forms of electromagnetic radiation, that is, light.
Response I'm not sure why you call this sentence "grammatically incoherent". The "that is" is an English equivalent of "i.e."=id est, and part of a 12-step program for my Latin addiction. ;)
The average reader is not going to know, according to some esoteric definition (which is not sourced), that light = all forms of electromagnetic radiation. Moreover, according to the majority of views, e.g. Gribbin’s Encyclopedia of Particle Physics, Oxford’s Dictionary of Science, “light is usually regarded as the range of electromagnetic radiation which human eyes are sensitive to [380 nm – 750 nm]”. People are not going to assume that x-rays, for example, are a type of light, but rather a type of electromagnetic energy or radiation.
Response I'm surprised that you think that this identification of light with electromagnetic radiation requires a reference, given that Wikipedia's own article on light states
Light is electromagnetic radiation with a wavelength that is visible to the eye (visible light) or, in a technical or scientific context, electromagnetic radiation of any wavelength.
It does not make sense for Wikipedia to replace a familiar, one-syllable word like "light" with a less familiar, 10-syllable phrase like "electromagnetic radiation". However, I am open to improved re-wordings that clarify this identification without interrupting the flow of the article too much, as Laura points out above.
Also, the “reader warning” comment is unnecessary:
Some readers may wish to skip this section on first reading. The definitions and descriptions are clearly written, and necessary for a quantitative understanding of the photon. However, their interpretation may require some prior knowledge of physics.
Response I don't like this warning, either, but consider the criticism just above yours. If it is assumed that some readers cannot understand the "Physical properties" section (which I do not agree with), then we should either guide such readers to the next section that they will understand, or re-arrange the order of the sections, which has been suggested by some editors and deprecated by other editors, both for good reasons.
That is why we use key-work links. Next, the phrase “law of photon energy E = hν” sounds a bit pompous; I don’t necessarily think it is Wikipedia’s place to be defining new laws.
Response I eliminated the pompous phrasing; thanks!
Also, I would cut all the "See Below" notes; it's not like we're writing a book. Moreover, articles will change over time, and those "see below's" may not actually match up down the road.
Response A good point. However, as pointed out above, it's also handy for readers to have a link to further information readily available. We can perhaps hope that future editors will respect these intra-article links.
Lastly, the article seems to give the impression that everything is known about the photon and its relation to the electromagnetic field, as though it were a solid theory, said and done. This view contradicts with Feynman who was famous for talking about the “screwy behavior of the photon”; or, for example, how no one is able to give an accurate description of the whereabouts of a photon on its trajectory from source to screen in the double-slits experiment and how we are told by Nobel Prize-winning physicists, such as Martinus Veltman, to "Forget about it, it's daydreaming. What counts is what you see on the screen. Do not ask if the particle did follow some continuous path. We do not know about that. Forget about it." --Sadi Carnot 10:24, 26 September 2006 (UTC)[reply]
Response The photon is described by the Standard Model theory, which has been able to accurately predict experimental observations such as the magnetic moment of the electron to roughly 16 decimal places. There is no other scientific theory that can make testable predictions of comparable accuracy. We may discover new physics beyond the Standard Model (CP violation hints at this), but this is not certain at present. Unlike the electron and other material particles, the photon's properties can be derived entirely from its gauge symmetry. Therefore, the photon is arguably the best understood object in all of science.
Well then, why is it when light is shined on ultracold liquid helium, while it is subjected to a voltage, that the effect is to increase the current of negative charges, presumably electrons, in the fluid. From what I have read, this has been a long-standing "puzzle" since the 1960s.
Response2 This is not a mystery pertaining to photons per se, but rather to ultracold liquid helium. If you wish to discuss this further, please provide a reference to a recent scientific journal article, e.g., from Physical Review Letters.
Moreover, I don't see anything in the article on how, in the mid 1990s, scientists at Geneva induced a single atom to emit two photons and sent them down separate fiber-optic cables 6.2 miles in length and recorded how the two photons remained "entangled" thus acting like one particle. This is taken to mean that the photons that had once been in contact somehow remained "aware" of each other when far apart.
Response2 Entanglement pertains more to quantum mechanics than to photons per se, but such effects were treated at length in the "photon correlation experiments" and, less directly, in the "quantum cryptography" and "quantum computing" parts of the article.
Lastly, of course, no one really understands how photons interact in the double-slits experiment. My point is that there are still some photon-puzzles still to be solved, and I don't see this reflected in the article. --Sadi Carnot 16:55, 26 September 2006 (UTC)[reply]
Response2 Admittedly, quantum mechanics is strange, but it is internally consistent and it's the way the world works. We might try to derive it from a more fundamental and more intuitive principle, but so far, no one has succeeded in that. If we accept quantum mechanics (and its infinite-dimensional counterpart, quantum field theory) and gauge symmetry, the intrinsic properties of the photon are completely understood.
Response Feynman's "screwy" quote reflects the fact that quantum mechanics and quantum field theory are counter-intuitive to most people; it does not mean that physicists do not understand the photon.
Quote from Feynman: "I think I can safely say that no one understands quantum mechanics."
Response2 True; but given quantum mechanics, physicists understand photons probably better than anything else.
Response Finally, I am aware of your profound respect for Nobel laureate Martinus Veltman but, as I have said elsewhere, he did not contribute significantly to our understanding of the photon, but rather to the electroweak interaction. No doubt, he is a good expositor of basic physics, but so are countless other scientists such as, say, Feynman, whom we also do not quote, despite his contributions to quantum electrodynamics, a much closer topic to photons. I do not think that the Photon article will benefit significantly from quotes taken from MV's popular works.
The light = EM radiation is a contentious issue around here. Lots of optics people like me regularly use light to mean X-rays, or any EM radiation. Some other scientists like to use light as meaning only visible light. I rather like the current phrasing; it quickly and painlessly establishes that light will be used as shorthand for any EM radiation throughout the article. — Laura Scudder ☎ 16:05, 26 September 2006 (UTC)[reply]
I am going to guess that more than just "optics people" are going to read the article; and that the smart reader is not going to see the Latin "i.e." and think to themselves, oh yes this means "light will be used as shorthand for any EM radiation throughout the article." This may be common jargon to some, but certainly not to all. Unless someone wants to provide a source for this, then I think it would be best to be technically correct and wise not to push our own personal biases or shorthand notations on people. --Sadi Carnot 16:55, 26 September 2006 (UTC)[reply]
I don't know what you're talking about. There is no i.e. in that sentence. As for remaining "technically correct", there is no consensus on what that would be. "Technically correct" for my entire physics department was that any statement about "light" was a generic statement about EM radiation. I'm not interested in encumbering the reader with a multitude of lengthy "electromagnetic radiations" just to prop up some silly distinction that's arbitrarily based on human sight. — Laura Scudder ☎ 17:43, 26 September 2006 (UTC)[reply]
Response2 As the Wikipedia article on light makes clear, this identification light=EM radiation is the mainstream in scientific writing and teaching, presumably because of its brevity. We are not inventing shorthand notations or pushing any biases — just improving the writing of the Photon article.
Lastly, I don't see a discussion of gamma-ray photon generations via proton-proton chain reactions in the sun and how this is the major driving force of evolution:
In the second step of the chain, a single proton fuses with a deuterium nucleus, resulting in 3He and a gamma ray
Response2 Nuclear transitions are indeed interesting, although we mention them several times already. Perhaps it might make a good example, though; what do other people think? I don't see how it pertains directly to evolution, though; that discussion might be "off-pathway" for an article on photons.
I don't see photosynthesis represented in the article:


Response2 Having illustrated molecular absorption in retinal at your suggestion, do we need another example?
I also don't see a reference to common facts such as 1,000 billion photons fall on a pinhead each second? These are just a few points to note. --Sadi Carnot 17:21, 26 September 2006 (UTC)[reply]
Response2 Your example brings one particular pinhead to mind. However, since Wikipedia is not a collection of facts, we probably don't need to mention this common fact. Serenely yours, Willow 17:54, 26 September 2006 (UTC)[reply]
  • Overall Support, but a few minor points.

:Most theories up to the eighteenth century hypothesized that light was composed of particles.

Theories describe, not hypothesise.

:However, before Compton's famous experiment[15] in 1922,

As a non-physicist I've never head of it, tell us what it was.

:On the one hand, the photon displays wave phenomena..

The phrase "on the other hand" is lacking later on, seems to hang as a incomplete statement as a result. This section also needs a short and clear statement of what wave-particle duality is at the beginning (state the obvious).

:Overuse of wiki links, with the same link (such as quantum mechanics) linked several times in one paragraph. I removed a few of these, but some more need to go.

Overall, a very good article, reasonably clear even to an beginner. TimVickers 15:14, 26 September 2006 (UTC)[reply]
All my suggestions have been addressed. TimVickers 22:08, 27 September 2006 (UTC)[reply]
Response Thank you very much for your review, Tim! Those are all excellent suggestions. I've changed the wording on "theories", described the Compton experiment more fully (it was also mentioned in the previous section), and took your hint about a better topic sentence for "Wave-particle duality". I felt a pang when removing all those extra wiki-links, but the article's better for it as well. ;)
Thanks also for the your deft editing and formatting of Photon itself — it's easy to see how you produce so many Featured articles. Thanks for the catalysis, Willow 17:09, 26 September 2006 (UTC)[reply]
  • Support Willow's done an impressive job over the last month or so bringing this article up to the standard it's now at (for comparison, the article before she started editing it was [1]). There are small things that need to be fixed (e.g. the reference that I just requested), but nothing major that I can see. I just hope that Willow will continue editing the article throughout and after the deluge of random edits that will happen when this article hits the main page. Mike Peel 17:42, 26 September 2006 (UTC)[reply]
(glowing) Thanks, Mike! I'll try to stay true to Photon and nurture it whatever happens. Hopefully, it'll never make Main Page because of its obscure figures. ;) But what was the reference you asked for? I don't see it. :( Willow 18:13, 26 September 2006 (UTC)[reply]
It's in the "Historical development of the photon concept" section, "wave theories of light were proposed by René Descartes (1637), Robert Hooke (~1665), and Christian Huygens (1678); [citation needed]". Unless that's from Newton's Opticks? Mike Peel 20:02, 26 September 2006 (UTC)[reply]
OK, I added the references for those three theories. Enjoy! :) Willow 20:46, 26 September 2006 (UTC)[reply]
Thanks. :) Mike Peel 20:47, 26 September 2006 (UTC)[reply]
  • Support This article is very thorough and yet very accessible to a wide range of audiences. My primary comment echoes some of those above, that care be taken on the more technical issues where the physical properties of the photon are concerned, and my second comment would be to ensure that the layout of the article has been reviewed so that transitions between sections are smooth and the background and historical information preceeds the technical details. It's a great article! Cheers, Astrobayes 21:23, 26 September 2006 (UTC)[reply]
  • Comment This is unsightly in the article lead:
    • The modern concept of the photon was developed gradually (1905–1917) by Albert Einstein[1][2][3][4]
  • The desired referencing can be accomplished by combining notes about all four sources inside one set of ref tags: for an example of how that was done very effectively in a recent and exemplary FA, see Daniel Boone. Sandy 17:55, 28 September 2006 (UTC)[reply]
  • Those references are also used in other places, though, so merging them would mean that their content would have to be duplicated. Personally, I'd recommend that things stay as they are. Mike Peel 18:20, 28 September 2006 (UTC)[reply]
  • Comment is there any reason the "particles" articles don't have a standard infobox? They all seem to have HTML tables which seems like bad practice to me. Also isn't there any photo that can be placed in the upper right hand corner? Photons are light so any cool light picture should work, maybe a cool laser picture. -Ravedave (help name my baby) 07:24, 30 September 2006 (UTC)[reply]
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