Wikipedia:Scientific peer review/Photon

Review Request

In response to Willow's 14:52, 28 August 2006 request, I volunteer to peer-review this article. I am a physicist working within the Wiki Physics Project and I would like to collaborate with any other experts in the field who wish to co-review this article with me. Let's begin the review process with what the most frequent editors of this article perceive as the highest priorities for review. Where do you want to begin? Astrobayes 19:47, 28 August 2006 (UTC)

Thanks very much, Astrobayes! I've never gone through this before, though, so I hope you'll be patient with me.

Let's begin with a technical question about virtual photons that has been vexing us:

• How many polarizations do virtual photons have? My reading of a few quantum field theory books suggested that virtual photons have four possible polarizations: the two physical transverse polarizations, one unphysical longitudinal polarization and one unphysical "time-like" polarization. By "unphysical", I mean "not seen in real photons". These four polarizations seemed to correspond to the four components of the relativistically covariant vector potential $A^{\mu }$. However, User:JRSpriggs maintains that virtual photons can only have three polarizations, since a photon has spin 1. Can you figure out which is true, and why? Maybe it depends on the choice of gauge?

More generally, here are the questions we'd like to have answered, I think:

• Is the article complete — are there other historical or scientific topics that we should cover in Photon? So as not to overlap with Light, we've agreed to stick to topics that correspond to the quantum nature of light, not topics that pertain to light generally, such as gravitational bending. I was thinking of adding something about Brioulloin scattering (phonon-photon scattering), nonlinear (multi-photon) processes and other interactions of photons with matter, but I was a little concerned that they could also be explained by a simple models of "normal" (Maxwellian, wave-like) light, e.g., a nonlinear material oscillator.

• Are the formulae accurate and the units consistent?

• Is the order of the presentation logical?

• Is the historical development of concepts accurate?

• Could the article benefit from additional Figures or Tables?

• (Non-scientific) Is the writing intelligible to the non-expert?

Thanks very much for your help, Astrobayes! :) Willow 10:12, 29 August 2006 (UTC)

This is a great place to start and I'll get to work on these right away. I have a few standard sources, and many great additional sources to consult for specific questions like number of polarizations of virtual photons (with a tie-in to quantum field theory). However, I share your desire to avoid repeating information that would be found in the QFT and Light articles. This will be easier to do with light (since light can be perceived differently by different people for the same photon) but not as easy to do for QFT since a photon is the fundamental quantum of the electromagnetic field (which therefore dovetails right into QFT). And I will pay special attention to the accessibility of the article for general Wiki readers. After all, we don't have to all have experience as physicists to appreciate the richness and beauty in an article about so fundamental a particle of Nature. So, give me about 24-36 hours to go through all of these great bullets you have above. I work during standard hours (8-5pm) so I'll start the bulk of my work after I get home. Thanks for your quick response. Cheers, Astrobayes 18:46, 29 August 2006 (UTC)

As I posted on the main article's talk page, I'm still working on this. The virtual photon question is a good one and I'll post specific selections of sources to support what I find. Aside from that there is the less technical review of the other aspects mentioned above. I've had a few long work days recently so I'm still reviewing the article but I plan to post my review by the weekend. I welcome any others with experience in this field to post a peer review of the article focusing on the criteria above as well. Cheers, Astrobayes 17:38, 31 August 2006 (UTC)

Formal Peer Review

Completeness, Accessibility, and Overall Impression

The article is very accessible to non-specialists, with its emphasis on the major and "minor" historical players, its inclusion of pictures that illustrate the sections appropriately, and its fair treatment of the mathematical formalism as complementary to the article rather than weighing it down as its focus. The article is very complete in that it presents the very broad role that photons play as elementary particles in the Standard Model and as the quanta in electromagnetism (without simply restating the Light article), but some of the more recent developments - which consequently are also the most technical if not obtuse to the average reader - are less accessible than the more historical and mathematical sections of the article. As a result, the flow of the article is a little rough. A reorganization of the layout would help the article flow better, and would allow a better focus on improving sections that discuss recent developments in this field. According to how each section is currently written, the article would flow more smoothly if it was organized by section as follows:

• Introductory remarks
• Nomenclature
• Historical Development
• Wave-Particle Duality
• Early Objections to the Photon Hypothesis
• Second Quantization (this section is largely historical)
• Energy, Momentum, Angular Momentum, and Mass
• Bose-Einstein Model of a Photon Gas
• The Photon as a Gauge Boson
• Experimental Limits on Mass
• Contributions of photons to the invariant mass of a system
• Photons in Matter
• the See Also, References, External links, and Footnotes are fine as they are

And it would be nice to have an additional section covering the most recent research in this field. Cheers, Astrobayes 20:30, 1 September 2006 (UTC)

Historical Accuracy

The historical sections of the article are quite complete and accurate, however it would benefit the integrity of the article to cite sources for some of the major events and statements such as (Lewis 1926), Heinrich Hertz's detection of radio waves (1888), and Bose's 1924 derivation; three examples of many. There are so many scientists involved in the development of the concepts of electromagnetic quanta, as well as their measurement, that the task of confirming every single date and author will take more than the day or two of review I've done on the article. Therefore, after my preliminary review of the more technical aspects of the article is written here I will set about confirming every date in the article over the next week. But from the initial players right down to Bose and beyond, the major events are presented as they should be, in context, and with dates that appropriately match events. However, it would be nice to add a section on recent developments in the field since experiments done by particle physicists all over the world are constantly revealing new insight into the nature of the elementary particles of the Standard Model. Cheers, Astrobayes 20:30, 1 September 2006 (UTC)

Formulae, Units, etc.

The formulae are correct in form, the units appear to be consistent, and the most common mistakes such as mishandling $\pi$, Planck's constant and Planck's reduced constant, as well as the speed of light, c, are all avoided. However, this section is in dire need of good sources and there are plenty. In fact, the virtual photon polarization section on the talk page lists great sources such as texts by Lewis Ryder and J.D. Griffiths that would support the text in this section. I am not disputing the substance of this section but having cited sources for such an important development in the history of the photon is very important.

The only other major concern I have is this section on radiation, and while the formulae given are in great shape, a bit of care should be taken with the discussion to make it more accessible to non-specialists. I do understand that quantum mechanics can only be broken down so far but the article presents, up to that point, solely classical mathematical terms - and then out of nowhere the casual reader is confronted with the bra-ket notation for quantum mechanical probability. A trained eye has no problem with the transition but the flow of the article is a bit stilted for the layperson by such formalism. The mathematics are important and should therefore be left in that section so I offer that a rewording of some of the language (using analogies familiar to all of us from quantum 101), and/or the addition of any diagrams, or illustrations of some kind, might add a lot of substance to this important section. Cheers, Astrobayes 20:30, 1 September 2006 (UTC)

Virtual Photon Polarization

We now have several peer reviews of the virtual photon question and the consensus appears to be that virtual photon polarization depends upon your choice of mathematical gauge. It is possible, by choice of gauge to have as many as four virtual photon polarizations, and as few as two. In the peer discussion, several resources were cited to which Wiki readers may refer to learn more. This part of the review should help shape the appropriate section in the article now. Cheers, Astrobayes 20:30, 1 September 2006 (UTC)

Additional Peer Review or Remarks

This initial review covers the bulk of the initial concerns of Willow's request. Any additional scientific peers in the field are encouraged to offer their own additional comments in the sections above or alternately in additional sections below. Now that my initial technical review is completed, I will set about over the next several days to verify all of the dates given - they appear correct according to the sources I've consulted so far, and with what I remember from my education, but I would like to get this article featured so it will be worth the time. If the initial requestor or any other individual wishes to make a request, please do not hesitate to post on this page. It is on my watchlist. It has been a pleasure contributing to this article and I look forward to doing so again soon. Cheers, Astrobayes 20:30, 1 September 2006 (UTC)

In addition, it wouldn't hurt us to get some fresh illustrations and eliminate these overused images in the photon article. As many users point out, they are actually better suited for the Light article, and even though they are aesthetically pleasing they do not give a great depiction of the photon as a quantum. Something to think about anyway... Cheers, Astrobayes 23:13, 1 September 2006 (UTC)