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The following is an archived discussion of a featured article nomination. Please do not modify it. Subsequent comments should be made on the article's talk page or in Wikipedia talk:Featured article candidates. No further edits should be made to this page.

The article was promoted by Laser brain 04:35, 20 December 2010 [1].

Speed of light

Speed of light (edit | talk | history | protect | delete | links | watch | logs | views) {{Wikipedia:Featured article tools|1=Speed of light}}

Nominator(s): TimothyRias (talk) 15:58, 6 December 2010 (UTC)

I am nominating this for featured article because, most of (if not all) of the issues identified in past FAC and PR have been dealt with. Moreover, the article has proved stable since the last bout of Wikidrama this summer. TimothyRias (talk) 15:58, 6 December 2010 (UTC)

Have you sought concurrence to nominate from the three other significant contributors, were they consulted in advance, and should they be co-noms? If they weren't consulted in advance, please withdraw; if they were, please ask if they want to be co-noms. SandyGeorgia (Talk) 16:02, 6 December 2010 (UTC)
I've left messages with all users with more 100 edits on the article (with the exception of user:Brews ohare who is currently banned from all physics related discussions and articles, which I assume includes FACs on these articles), informing them of the FAC. This should not be a problem, since there basically was consensus that the article was read for FAC this summer, but that we should wait until the dust of the drama this summer had settled down. If they do not chime in here within the next 24 hours, I'll be happy to withdraw the candidacy.TimothyRias (talk) 16:22, 6 December 2010 (UTC)
No objection to this going forward to WP:FAC: if others want little stars for Christmas, I'm not going to stand in their way ;) Physchim62 (talk) 17:57, 9 December 2010 (UTC)
  • Support per TimothyRias. A. di M. (talk) 16:28, 6 December 2010 (UTC)
Note to closing admin: User is major contributor to the article. wackywace 19:33, 6 December 2010 (UTC)
  • Support with only a couple of minor reservations.—RJH (talk)
  • Comment—I've done reviews of this in the past, but the article continues to undergo changes and each time through I seem to find new issues. Overall it remains in pretty good shape. Some of the wording seemed a little awkward, however, so we may want to have somebody with good language skills give it a proofing.
    • There are a pair of sentences needing citations:
      • The Lunar Laser Ranging Experiment, radar astronomy and the Deep Space Network determine distances to the Moon, planets and spacecraft, respectively, by measuring round-trip transit times. <<-- No cite. I took care of this.—RJH (talk)
        ThanksTimothyRias (talk) 21:20, 9 December 2010 (UTC)
      • Robert Hooke explained that the results of Galileo's experiment did not establish that the speed of light was infinite, but only that it must be very fast. <<-- No cite.
        • Note: I'm not sure that this statement is even correct. The "exceedingly quick" statement made by Hooke was probably in reference to the rate of vibrations (frequency) of light, rather than the velocity of light.—RJH (talk)
          I took out that sentence, since it wasn't that relevant to the big picture anyway.TimothyRias (talk) 21:20, 9 December 2010 (UTC)
          • Looking at the sentence that Timothy took out, I'm not sure it was even accurate! Hooke was a player in the discussions about the speed of light in the 17th century, but that particular "idea" doesn't seem right to me compared with what he complained about later. Thanks for spotting it. Physchim62 (talk) 21:46, 9 December 2010 (UTC)
    • I would like to see a citation for the following: "c, being the upper limit of the speed with which signals can be sent, provides a theoretical upper limit for the operating speed of microprocessors". Does this apply to processors using quantum entanglement? If you can continue to reduce the size of the microprocessor, doesn't that negate this assertion (in which case the limit is size-based)?
    I've added a ref to support that statement. Note that it is in fact a combination of limits on size and signalling speed. Size limits would be irrelevant if there was no limit on signalling speed and vice versa.TimothyRias (talk) 08:28, 7 December 2010 (UTC)
    The statements referenced by the current references 66 and 70 are essentially the same (it looks like the first paragraph is supposed to be a summary of the rest of the section), so should they both cite both sources? A. di M. (talk) 12:10, 7 December 2010 (UTC)
    Good point. I actually forgot we had paragraph just down the section that went into more detail. Reference 70 goes into more detail, but reference 66 is more recent (in the world of computers 70 is basically ancient). Having both references makes sense, I guess.TimothyRias (talk) 12:35, 7 December 2010 (UTC)
    I've merged the two refs.TimothyRias (talk) 22:02, 9 December 2010 (UTC)
    • The discussion of the relation between c and the vacuum permittivity constants in the section titled "Propagation of light" is somewhat redundant with the discussion at the start of the "Measurement" section and the "Electromagnetic constants". I'm not sure that information is needed in the "Propagation of light" section, so perhaps it should all be consolidated under "Electromagnetic constants"?
    I think the relevant point to make in the "propagation of light" section is that Maxwell theory provides a connection between the propagation of light and the electromagnetic forces. In particular, it relates c to other physical constants. Giving the exact relation makes this point clearer. I don't think it is too redundant to mention this relation again, when we discuss using it to measure c, since the the two mentions are quite far apart.TimothyRias (talk) 21:56, 9 December 2010 (UTC)
    • Okay, well then the description in the "Propagation of light" section seems unhelpful. Yes it's related to two seemingly arbitrary constants, but the lay reader has no idea what that signifies. Even reading "Electromagnetic constants" doesn't really help; neither does the lead section of Vacuum permeability. I just don't find that sentence in the propagation section satisfying in its current form. Sorry to be difficult, but that information is important and I think it needs to be elaborated upon.—RJH (talk)
      I think the point is that you can write the speed of light in vacuum as "cee equals" followed by an expression which doesn't contain the velocity of the source, the velocity of the observer, the price of tea in China, or the phase of the Moon. A. di M. (talk) 01:41, 14 December 2010 (UTC)
      If that is the point, then I think you need to spell it out for the lay reader. Right now it leaves the reader guessing about the significance of the information. Thanks.—RJH (talk)
    —RJH (talk) 18:34, 6 December 2010 (UTC)

Query - What is Extremes of motion referenced in the article? Fasach Nua (talk) 19:19, 7 December 2010 (UTC)

You mean Template:Extreme motion at the bottom? A. di M. (talk) 20:06, 7 December 2010 (UTC)
It's the term I have never come across before, the first google result is that template though. Fasach Nua (talk) 19:09, 8 December 2010 (UTC)
The template was removed by Physchim62, anyway. A. di M. (talk) 14:33, 10 December 2010 (UTC)
Indeed, I didn't think the comparisons in the template added anything to the speed of light article. Physchim62 (talk) 14:57, 10 December 2010 (UTC)

Support - this is a superb synopsis. For a subject that could be riddled with technical expressions, all the editors have done well to make it so accessible. It is well-written and engaging. I am not qualified to comment on the mathematical formulae, (the editors have been kind, there aren't that many) but the article is comprehensive and well-structured. The illustrations are particularly helpful and the references are sound. I have read the FAC delegate's comments about procedure above and I know an image review is still needed, but understanding the amount of effort that goes into producing articles of this high standard gets my support. Graham Colm (talk) 17:47, 9 December 2010 (UTC)

Comment The statement "the local speed of light is constant and equal to c, but the speed of light along a trajectory of finite length can differ from c" isn't as specific as the article it links to which says that the local instantaneous coordinate speed of light does change. It says that it is only the local instantaneous proper speed of light that is always c. And coordinate speed/coordinate velocity is a red-link. 89.241.227.10 (talk) 14:16, 10 December 2010 (UTC)

In normal coordinates, the local speed of light is c; but I can't find a way to say that appropriate to the level of the article. A. di M. (talk) 14:31, 10 December 2010 (UTC)

Comment Could the section on the finite speed of light include a discussion of retardation effects that effect microscopic forces. And in chemistry "relativistic effects" can be defined as anything arising from the finite speed of light c, as compared to c=infinity. Relativistic effects in structural chemistry. 89.241.227.10 (talk) 15:42, 10 December 2010 (UTC)

Query - Who is this article aimed at? Fasach Nua (talk) 16:35, 11 December 2010 (UTC)

The lead should be quite accessible even to high-schoolers, but some of the material in the sections down below will be interesting for senior undergrad students as well. This is a feature, not a bug: if it were entirely targeted to a very specific audience (say, physics freshmen), those with a lower ‘level’ would understand pretty little of it and those with a higher level would find in it pretty little they didn't already know. See WP:MTTMP. Do you think it fails these aims? A. di M. (talk) 16:51, 11 December 2010 (UTC)
I think it is a good well written article, I did enjoy reading it, I think you have set yourself a difficult task, one can compare this article to another here on the same subject, and they are worlds apart, and I do prefer the one here over the other. I have no issue stating Criteria 1, 2, & 3 are met, I however don't know about four ... but I'm not expert on these things Fasach Nua (talk) 17:13, 11 December 2010 (UTC)
Indeed, it'd be the 408th longest FA, though it wouldn't be easy to trim it while keeping it comprehensive. Does anyone think it's excessively long? A. di M. (talk) 17:53, 11 December 2010 (UTC)
On the other hand it is shorter than or similar to many other physics FAs. (atom is #304,big bang is #332, electron is #180, etc.)TimothyRias (talk) 19:14, 11 December 2010 (UTC)
I don't think it is long, I just find it curious that if the article I referenced above was presented here it could pass FAC and god willing, so too will this one, they are both on the same topic, yet their contents are worlds apart. Fasach Nua (talk) 22:15, 11 December 2010 (UTC)
[Blushes] A. di M. (talk) 02:19, 12 December 2010 (UTC)

Query - This article contains a serious mistake. On one side the article says:

The speed of light in vacuum is a dimensional physical constant, so its numerical value depends on the system of units used.

On the other side it says:

In branches of physics in which the speed of light plays an important part, such as in relativity, it is common to use systems of natural units of measurement in which c = 1

The last equation should be: {c} = 1. It is only the numerical factor that is one. Since c is a dimensional physical constant, it needs a unit. The unit is, btw, c. Kehrli (talk) 20:33, 12 December 2010 (UTC)

When using natural units, you're essentially using the same dimensions for space and time (e.g. calling t what's usually called ct, and so on), so that velocities are dimensionless (i.e. you're calling v what's usually called v/c). If you do that, the speed of light is obviously 1, and you don't even write that in the equations. An extensive discussion of how that works wouldn't belong here, but it does belong to the natural units article which is linked. A. di M. (talk) 21:18, 12 December 2010 (UTC)

Support A fine article. No quibbles with the physics, which should be comprehensible to high schoolers. The history section is particularly good. Added a link to something I thought required further explanation. Some of the paragraphs, particularly in the Faster-than-light section have only two sentences and seem a little short. You might consider consolidating them. Hawkeye7 (talk) 19:47, 13 December 2010 (UTC)

  • Support, one mini-quibble I've read this twice now, it's a clear exposition, pitched at the right level. I don't like the Fizeau diagram on the left., it's the only left-aligned image and it breaks up the headings. Jimfbleak - talk to me? 07:06, 14 December 2010 (UTC)
    Is it better now? (Alternatively, we could just place all of them to the right. Maybe I'll start a straw poll on the talk page or something.) A. di M. (talk) 13:30, 14 December 2010 (UTC)

Sources comments:-

  • Refs 25, 28 and possibly others (PhD theses): I would add information beyond the link, identifying the receiving institution in each case
  • Ref 42: Who publishes MathPages?
  • Refs 54-56: Who is Mikhail Polyanskiy? The pages carry disclaimers: "No guarantee of validity"
  • Ref 68: What makes "Royal Pingdom" a reliable source?
  • Ref 69: What makes "Fourmilab" a reliable source?
  • Ref 99: JSTOR requires subsrciption for access; add (subscription required)
  • None of the "Historical references", and only Mackay et al of the have citations to them. In what way are they references for this article?
  • Minor nitpick: "retrieved" or "Retrieved"? There should be consistency.

Otherwise, sources and citations look OK. Brianboulton (talk) 17:18, 14 December 2010 (UTC)

Some replies:
  • MathPages is a self published website by Kevin Brown.
  • I have no clue Mikhail Polyanskiy is. The site "refractiveindex.info" simply collects refractive index data from various sources with provided references. We could in principle directly cite those sources, but I think readers are generally better served by being linked to a site that is free to access.
  • Pingdom is "a service that tracks the uptime, downtime, and performance of websites." Royal Pingdom is a blog by some of its employees. It is about as reliable a source as your going to get for typical real world ping values.
  • Fourmilab is a self-published website by John Walker. At appears twice as a source. Once for an English language translation of Einstein's 1905 SR paper, which is provided as a service anyway. (The real ref is the German original paper). The second time it is used as a source for the length of the communication delay with the Apollo missions. This could maybe use a better source.
I'll fix some of the other stuff. Thanks for the thorough comments.TimothyRias (talk) 11:23, 15 December 2010 (UTC)
I've add the granting institution to the thesis of Mota (ref 25). Ref 28, is not a thesis but a review article that has not been published. Nonetheless it has 52 citations according to SPIRES, making it a reasonable ref to support the claim that the subject of the review is an area of active research.
I'm not sure what you intention is with the remark about adding a "subscription required" to ref 99. There are loads of references in the article which have, DOI, BIBCODE, or JSTOR identifiers attached, most of which will link to a site requiring subscription. Are you suggesting that all of these should have a "subscription required" warning? TimothyRias (talk) 15:21, 15 December 2010 (UTC)
I've replaced the Fourmilab and mathpages refs with more suitable and more easily recognizable as WP:RS sources. I also fixed the retrieved consistency issue (which was due to the {{MacTutor}} template using {{citation}} instead of {{cite web}}.)TimothyRias (talk) 10:32, 16 December 2010 (UTC)

Dab/EL check - no dabs, 1 dead external link- (http://dbserv.ihep.su/hist/owa/hw.move?s_c=VAVILOV+1934&m=1) which is used as a convenience link to an offline citation so no biggie, though I expect it will come back up. I fixed three links that were not broken, but were redirecting somewhat. --PresN 00:02, 15 December 2010 (UTC)

Support: This is an article that Wikipedia should be very proud of. I haven't worked on or looked at this article for several months; it has substantially improved in (by?) my absence. The entire article is as accessible to the non-specialist as the subject matter permits. Even the more technical parts have been rewritten to be more accessible, but without dumbing down the content. Some previously awkward spots in the organization have been solved, and the writing has been refined, with a consistent overall tone. The article is long, but no longer than necessary for a comprehensive treatment. I do not see how it could be significantly shortened without either losing important content or sacrificing adequate explanation.—Finell 11:35, 15 December 2010 (UTC)

Support: I agree strongly with Finell. In terms of content, organization, and length it is ready to be a FA now. It has drastically improved and continues to improve. It is stable. Over all, kudos to all involved. Assuming that the figures and references, etc.. are all in order it should be promoted to FA. TStein (talk) 18:27, 15 December 2010 (UTC)

Comment In the history section, there is a sentence which reads,

In the early 1860s, Maxwell showed that according to the theory of electromagnetism which he was working on, that electromagnetic waves propagate in empty space - - - -

This sentence is misleading in that Maxwell believed that light propagated in a physical medium, and the famous equations which bear his name were derived by him using that medium. The follow up paragraphs deal with the manner in which that physical medium, often referred to as theluminiferous aether, was eventually shelved from modern physics.

This matter was raised in October, but certain people argued that 'empty space' is an adequate term to use even when referring to the luminiferous medium. And this was despite the fact that the next paragraph actually states that at that time, people believed that empty space was filled with a medium called the 'luminiferous aether'. I would hold to the point of view that the above sentence is highly inaccurate. David Tombe (talk) 23:27, 15 December 2010 (UTC)

David: Your comment does not indicate support of or opposition to the FA nomination. The proper place this discussion is on the article's talk page. Addressing your point directly, this would be appropriate to cover in other articles that, e.g., are devoted to Maxwell, his beliefs and theories, and the history of pre-20th physics or science philosophy. In Speed of Light, however, it is an unnecessary distraction, in my opinion.—Finell 01:07, 16 December 2010 (UTC)
David, your concerns have already been addressed on the article talk page and the project talk page. Raising them again in exactly the same way at yet another venue after consensus has been arrived at could be seen as disruptive.--JohnBlackburnewordsdeeds 01:15, 16 December 2010 (UTC)

Comment Section Faster-than-light observations and experiments, final para "In models of the expanding universe..."

  • Clearly difficult to summarise this judging by the two linked articles, but the bit about "apparent recessional velocity becomes greater than the speed of light" sounds wrong: those objects will I think not be observed at all? --Mirokado (talk) 00:10, 16 December 2010 (UTC)
About your first point. You are right that points beyond the Hubble sphere will not be observed. The reason is that the distance between us and these objects is becoming greater at a speed greater than c. The apparent (to an omnipresent observer) recessional velocity of these object is thus greater than c. The sentence as it stands is correct, but I see how the use of the word "apparent" may be confusing. I'll try to think of a better alternative.TimothyRias (talk) 10:42, 16 December 2010 (UTC)
I've replaced "apparent" with "relative". Is it better? A. di M. (talk) 15:14, 16 December 2010 (UTC)
I actual think that is more confusing, since we are not talking about an actual speed, but a change in distance. This is what the word "apparent" was trying to relate. An alternative would "fictitious" or something like it. Although that doesn't sound right.TimothyRias (talk) 17:13, 16 December 2010 (UTC)
"apparent" is confusing, "relative" is redundant since all velocities are relative to something. How about: "Beyond a boundary called the Hubble sphere, this recessional velocity becomes greater than the speed of light and the objects are unobservable." (or "... cannot be observed.") This not only clarifies the sentence, but answers the question, inevitable from some readers, "so what happens then?" --Mirokado (talk) 00:53, 17 December 2010 (UTC)
Not saying "apparent" is also confusing, because a reader may be left wondering whether this is a real velocity or not. Also note that an object being beyond the Hubble sphere does not mean that it cannot be observed. (See the Hubble sphere article or arXiv:astro-ph/0310808.TimothyRias (talk) 12:51, 17 December 2010 (UTC)
  • No mention of the period of inflation just after the big bang, which I think is another expansion-of-space effect? --Mirokado (talk) 00:10, 16 December 2010 (UTC)
About your second point. Expansion during inflation is (what the remarks in this article are concerned) no that different from the current expansion. As such, I see no reason to mention it separately.TimothyRias (talk) 10:42, 16 December 2010 (UTC)
I promise not to go on about this, but the reason for mentioning inflation is that most readers who have heard of it will think of it as a spectacularly different event from the current expansion of the universe and thus may, as I did, assume it has been omitted. A couple of words now will save well-meaning changes from other editors in the future, which would probably need further tidying up. --Mirokado (talk) 00:53, 17 December 2010 (UTC)
I'm not sure if this article is the place to clear up misconceptions that may exist about inflation. I don't think anything useful can be said about inflation in the limit space we have in this article. With respect to the effect described here, expansion is expansion, and whether it is constant or exponential makes very little difference. A reader looking for more detail should look at one of the linked articles like metric expansion of space and Hubble sphere.TimothyRias (talk) 12:51, 17 December 2010 (UTC)

Comment Just on a purely trivial note, there is a sentence in the history section which reads,

The following year Gustav Kirchoff calculated that an electric signal in a resistanceless wire travels along a wire at this speed.

I would have thought that we should have been using the definite article 'the' before the second mention of the word 'wire' in this sentence, since we have already mentioned the wire earlier in the sentence. I did correct this but Martin Hogbin reverted the correction. It's something that would need to be addressed before any decision is made about FA status, because it doesn't look good if an FA article contains bad grammar. David Tombe (talk) 17:00, 16 December 2010 (UTC)

 FixedTimothyRias (talk) 17:15, 16 December 2010 (UTC)

Support, with one comment regarding prose. I think this article generally does a very good job of explaining a highly technical subject without baffling the lay reader, and also without losing sight of scientific detail. However I would like someone to rewrite this sentence - it is also my view that the word 'anisotropy' should be burned with fire.

"Observations of the emissions from nuclear energy levels as a function of the orientation of the emitting nuclei in a magnetic field and of rotating optical resonators have put stringent limits on the possible anisotropy" ...
Another comment; there was a dash character being used in the term "light-year" which displayed incorrectly for me (and I suspect others). I have replaced it with a normal -. Great work! The Land (talk) 22:46, 19 December 2010 (UTC)


The above discussion is preserved as an archive. Please do not modify it. No further edits should be made to this page.
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