Talk:Speed of light

Template:WP1.0

WP:V, WP:RS, and WP:NPOV

My recent reverts, discussed in the sections above, are not based on any point of view on the topic, but on the idea that in an article as contentious as this one, it is important not to make assertions that are not verifiable in a reliable sources, and that questionable assertions be accompanied by something about the alternative point of view on them. Otherwise, what basis do we have for driving to any kind of convergence and improvement if people just keep adding their favorite tangents and whatever logical extrapolations come to mind? I have no objection to the concepts presented in the edits I reversed; but they need decent support in sources. Dicklyon (talk) 03:33, 17 October 2009 (UTC)

The article is not that contentious at all. You have to look at what the articles are actually saying, also in what journals they are published, if they are cited a lot etc. etc. Count Iblis (talk) 03:46, 17 October 2009 (UTC)

The contentiousness tends to be among editors of speed of light; that's why we need a solid policy-based process here. Dicklyon (talk) 03:49, 17 October 2009 (UTC)

I would suggest everyone to take the time to actually read the articles. E.g. what does Heidi in her article (http://arxiv.org/abs/0706.0553 ) actually say? After she makes her claims that some nontrivial assumptions were made that she claims may not be valid (without proving that they are not valid) what does she say on page 4 about the opinion of other people with whom she discussed this at a conference? Count Iblis (talk) 04:00, 17 October 2009 (UTC)

My worries about the inclusion of this material are more that it is being given WP:UNDUE weight than anything else. The Scharnhorst effect is verifiably the subject of discussion and theoretical study in physics. It is also ludicrously small, one part in 10³⁶ for plates separated by 1 µm. It has never been measured, and one is quite justified in asking if it ever could be measured: in fact, Milonni & Svosil (1990) have claimed that the Heisenberg uncertainty in any measured velocity of propagation "will always be enormously larger than the correction to c associated with the Scharnhorst effect," in other words, the Scharnhorst effect can never be measured (as quoted in Prof. Fearn's article, can't bring myself to call her Heidi, I've never met the lady).

So, should we be saying that the Scharnhorst effect is a "known effect allowing information or energy to travel faster than c," as Count Iblis would like? I would prefer that we didn't do so on this article, although I wouldn't object to a more thorough discussion on the page devoted to the Scharnhorst effect. Should we be saying that "it is impossible for information or matter to travel faster than c," as states the version of the article that Dicklyon is reverting to? I can see Count Iblis' problem with this statement. I would be tempted to add a "measurably" in there somewhere, but it wouldn't really resolve things. Suggestions for a new wording? Physchim62 (talk) 12:29, 17 October 2009 (UTC)

Such as this one? (As far as I'm concerned, the parenthesis might as well go away: the "normally" makes the first sentence not false even if the Sch. effect were true, and the "it is thought" makes the sentence in parentheses not false even if the Sch. effect were false.) --___A. di M. 14:02, 17 October 2009 (UTC)

Yes, that's also a good solution. I'm ok. with this edit. Count Iblis (talk) 14:18, 17 October 2009 (UTC)

ec:

We could also have a more neutral statement, saying that faster than light propagation of information is problematic because it leads in general to causality violations, and then we can give a footnote in which we say that the Scharnhorst effect is a posible exception to this rule, i.e. that in this case you have faster than light propagation of information but without the possibility of using it to create a causal paradox.

About measurability, that's approached from a practial point of view in http://arxiv.org/abs/0706.0553 The article says that in so many words. The uncertainty in the velocity being much larger than the effect does not imply that you cannot see the effect in principle. It means that you would need to perform such a huge number measurements to average out that huge uncertainty that in practice you could never do that on any reasonable time scale. Count Iblis (talk) 14:15, 17 October 2009 (UTC)

The paradox is resolved by showing that even if FTL matter exists (e.g. tachyons or casimir effect photons) they can't be used to transmit FTL information. FTL energy, yes; FTL information, no. I know this sounds nonsensical, but see Gerald Feinberg's mainstream citations at the tachyon article. So the Scharnhorst effect allows FTL photons without allowing FTL information transfer or causality violations, which is what Visser et al show. Visser's work is essentially an extension of Feinberg's SR/QFT work into the domain of GR.--Michael C. Price ^talk 10:18, 19 October 2009 (UTC)

Hmmm, I thought that perpendicular to the plates you do get faster than c propagation of information but you then cannot use it to create a closed timelike loop. At least that is what I read in section 3.2 of this article (page 13 and furhter). Count Iblis (talk) 14:38, 19 October 2009 (UTC)

They (Visser et al) say that the photons move FTL, but information is not mentioned. Hawking's chronology protection conjecture is invoked to save causality, which is compatible with Feinberg.--Michael C. Price ^talk 15:06, 19 October 2009 (UTC)

Yes, but while they can use the analogous result from GR (due to the structure of the effective metric given in Eq. 3.4), in this case you do have faster than c signals. That's very clear from the proof they give that does not invoke this result. See page 16, just before the start of section 3.2.2 where they say that it is because the speed of light has a unique value for each observer with some four-velocity. It is then the fact that to violate causality you need the same larger than c signal speed in two different frames that saves causality.

Eq. 3.4. is only the effective metric, the ordinary Lorentz metric still defines speeds. Count Iblis (talk) 15:38, 19 October 2009 (UTC)

You're right, perhaps signals travel FTL as well as photons. I think all the SoL article has to do is link to chronology protection conjecture (something that it does not do at the moment), where it can be explored in more detail.--Michael C. Price ^talk 21:41, 21 October 2009 (UTC)

Signals would only travel FTL between the plates, where causality is well behaved. You would only get a problem if you could detect the FTL signal outside the plates, and uncertainty seems to cloak the system in such a way that you could never actually measure an FTL signal. Consider this extract from page 17 of Visser's paper: "The problem in this case is edge effects: the effective metric given in (3.4) is only expected to be a good approximation far away from the edge of the plates, while well outside the plates the effective metric should approach that of Minkowski space. Near the edge of the plates the effective metric is impossible to calculate, and the situation only gets worse when two pairs of half-infinite plates pass each other with a grazing not-quite collision. It is certainly clear that the simple naive result of equation (3.14) should not be trusted. The present arguments do not guarantee the total absence of causality violations, but they do demonstrate that the most naive estimates of the causality violating regime are likely to be grossly misleading." Physchim62 (talk) 08:25, 22 October 2009 (UTC)

For the record

Wikipedia:Arbitration/Requests/Case/Speed of light#Final decision

This arbitration case has been closed, and the final decision may be viewed at the link above.

• All editors are reminded to be civil at all times and seek consensus where possible, and encouraged pursue dispute resolution when necessary.
• Brews ohare (talk · contribs) is warned for his conduct in this dispute, and placed under a general probation for one year, under which any uninvolved administrator may impose sanctions if Brews ohare fails to adhere to the purpose of Wikipedia or general editing and behavioral guidelines, policies, and expectations, despite warnings.
• David Tombe (talk · contribs) is also warned for his conduct in this dispute and during the course of the arbitration case, and is placed under the same general probation but for an indefinite duration. David Tombe may not appeal his probation for one year, and is limited to one appeal every six months thereafter.
• Both Brews ohare and David Tombe are banned from all physics-related pages and topics, broadly construed, for twelve months.
• Violations of the topic bans or general sanctions may be enforced by blocks of up to a week in length for repeated violations, to increase to one year after the third block. All blocks and other sanctions applied should be logged on the case page here.

They're not my words above, but I thought they might be of use or interest to editors at this page. Physchim62 (talk) 23:58, 20 October 2009 (UTC)

To clarify, the above is the Clerk's summary of the remedies, only. The entire decision is recommended reading for editors of this page. Finell (Talk) 00:45, 21 October 2009 (UTC)

Now the dust has settled

After the excitement and stress of the arbitration it is now time to get back to improving the article. I found it hard to know where and how to start.

Previously, because of the bad feeling that abounded, it was important to discuss things on the talk page before making significant changes. Now that a general assumption of good faith has returned (I hope) it should be possible to return to normal cooperative editing in the best traditions of WP.

I have started the ball rolling with some editing to the 'Light as EM radiation' section. This is not an attempt to make any kind of point or to dominate the article and I think it still need plenty of improvement to that section is needed. I look forward to trying to get this article back to an FA by working together to improve it. Martin Hogbin (talk) 10:18, 24 October 2009 (UTC)

I've already nominated the article for peer review. (There's a link at the top of this talk page.) ___A. di M. 11:05, 24 October 2009 (UTC)

I think that the article should be looked at as a whole, rather than as edits to individual sections. The basic question is: what should we be discussing, given that we can't hope to discuss everything? Several editors have already given their opinion in the various userspace pages that cropped up (and also here, for specific points): maybe we could attempt a synthesis here. Physchim62 (talk) 13:33, 24 October 2009 (UTC)

Perhaps you could start by listing the issues that you consider important. Martin Hogbin (talk) 15:44, 24 October 2009 (UTC)

I think the article as it is now is all in all pretty good, So I don't think an "incremental" approach, in which we start from what we already have and tweak it, is that bad. (Is that what you meant by "as edits to individual sections"?) ___A. di M. 10:20, 25 October 2009 (UTC)

Yes, the article is in standard shape, so let's just treat it like any other article in such. Let's make a mod here, a mod there, an add here, an add there, a rem here, a rem there...

That should give no problem, as Hare and Tombe are no longer here to make standard wiki life impossible. DVdm (talk) 10:39, 25 October 2009 (UTC)

Astronomical units per day

I had removed the infobox expression of light speed in astronomical units per day because (1) I think it's a rather silly measure (i.e. why don't we also take parsecs per week?), and (2) it was listed in high precision, as opposed to the 3/4 digit precision of the other expressions. Physchim62 seems to have "ordered" this expression and reverted my removal, suggesting discussion on talk page. So here we are. Any thoughs? DVdm (talk) 14:16, 24 October 2009 (UTC)

Re (2) why didn't you just round it off then? --Michael C. Price ^talk 14:55, 24 October 2009 (UTC)

I had rounded it (to zero precision) to accomodate (1) and (2) at the same time :-) DVdm (talk) 15:04, 24 October 2009 (UTC)

It's no "sillier" a measure than natural units, and it is a measured value known to high precision (as opposed to the others, which have defined values). Physchim62 (talk) 10:54, 25 October 2009 (UTC)

I tend to think that 'Astronomical units per day' should stay. I can see why people are concerned, it may encourage some to add all sorts of weird units just for the sake of it. However, as I understand it, these units are not based on SI units and thus the speed of light does not have a defined value in that system of units (not that I want to revisit that argument). Martin Hogbin (talk) 12:12, 25 October 2009 (UTC)

Why not just respect policy, and include only those values and units that we can back up with a reliable source? Why should we convert to odd units of our own that don't appear in sources? By that criterion, astronmical units per day is OK (per this 1976 proceedings). Dicklyon (talk) 20:08, 25 October 2009 (UTC)

"speed+of+light"+"furlongs+per+fortnight"+-wikipedia Er... ___A. di M. 20:25, 25 October 2009 (UTC)

Yes, absolutely. And let's use Googlebooks. It's more than properly sourced. Let's take it. Why not? Because it could make someone smile? DVdm (talk) 21:43, 25 October 2009 (UTC)

Hard to say who is serious and who is not, but I agree that there's nothing wrong with quoting the value in furlongs per fortnight, a commonly used humorous unit for the speed of light. Also, let's put a book link on each value, and limit ourself to books, as opposed to random web sources and such, OK? Dicklyon (talk) 21:52, 25 October 2009 (UTC)

I don't think we need a source for each value. Having seen one for au/day, I'm already satisfied. I'd only provide one (or two) for the furlongs, sort of to "scare off" the really serious ones. DVdm (talk) 22:10, 25 October 2009 (UTC)

Selected astronomical constants in the current Astronomical Almanac gives the speed of light to a few more digits than that given in the IAU (1976) Proceedings, to wit: 1/τ_A = 173.1446326847 AU/d. The units AU/d are natural in astronomy because its fundamental measurement units are the astronomical unit, the day, and the mass of the Sun; not the metre, second and kilogram. — Joe Kress (talk) 05:36, 26 October 2009 (UTC)

I would prefer to give this in Au/hour. You get a smaller number; the time scale of hours is more relevant when thinking about communicating with spacecraft in the solar system. Also, if we use Au/day, then "day" has to be defined, there is more than one definition in astronomy. Count Iblis (talk) 17:47, 26 October 2009 (UTC)

We have a source using au/day. Do you have one using au/hour? DVdm (talk) 17:51, 26 October 2009 (UTC)

And you don't have that same problem with hour? In astronomy that is usually 1/24th day.(be it solar,siderial or SI). Let's stay with what the sources are using au/day.(TimothyRias (talk) 09:42, 27 October 2009 (UTC))

The value I put in is for a day of 86,400 TDB seconds, that is for the mean day measured as 86,400 seconds on the surface of the geoid. You do get slightly different values if you use different time scales, as there is no absolute consensus as to how to account for gravitational time dilation in astronomical measurements and the SI system is fairly silent on the matter. The TDB value (the one I put in the infobox) is the one that's used practically. Physchim62 (talk) 15:02, 27 October 2009 (UTC)

Modern astronomical units

I have removed this part of the above section because it is unreferenced and because it makes no mention of the uncertainty units or, failing units, a ratio? Surely a bald number mean nothing in this context?

"The relative uncertainty in these measurements is 2 × 10−11, equivalent to the uncertainty in Earth-based measurements of length by interferometry." Abtract (talk) 17:21, 26 October 2009 (UTC)

Relative uncertainty is dimensionless. See for instance here. Want more? Here's some more. DVdm (talk) 17:39, 26 October 2009 (UTC)

Thanks that was very useful. I have two points to make: 1) It would be better written as 20 parts per trillion as one of your sources suggests (per million actually but ...) as this would make it clear to simpletons like me that it was per something. 2) It is uncited and atm is original research even though it is a simple calculation. Abtract (talk) 18:32, 26 October 2009 (UTC)

You are welcome. Concerning your remarks "...simpletons like me..." and "...original research even though it is a simple calculation.", perhaps this is useful as well: "...Any relatively simple and direct mathematical calculation that reasonably educated readers can be expected to quickly and easily reproduce...". :-) DVdm (talk) 18:42, 26 October 2009 (UTC)

Thanks again ... I will be sooooo knowledeable soon. Abtract (talk) 18:50, 26 October 2009 (UTC)

No problem. I have removed the fact-tag, and got rid of the treacherously ambiguous trillions as well. - DVdm (talk) 19:01, 26 October 2009 (UTC)

Actually, nowadays the "short scale" is the only one used in modern English, even in countries where the long scale was formerly used. This article itself is written in British English but consistently uses "billion" to mean 10⁹. Anyway, the trillion is much less familiar than the billion, so I vote for "2×10⁻¹¹" (or for "0.02 parts per billion"). ___A. di M. 19:45, 26 October 2009 (UTC)

Obviously anything larger than "million" should not be used for any article used worldwide.Wdl1961 (talk) 20:46, 26 October 2009 (UTC)

C'mon, even the BBC by billion now mean 10⁹ (and by now I mean "for the last couple of decades at least")... ___A. di M. 20:54, 26 October 2009 (UTC)

what's BBC ? There are other areas than gob and even places where english is not spoken. I know i have been there. By the way billion equals million x million. Milliard equals thousand million.Wdl1961 (talk) 21:16, 26 October 2009 (UTC)

That is generally regarded as outdated even in the UK. When I was at school a billion was unambiguously 10⁹ and that was 15 years ago. In any case we have an official policy on the topic (see WP:ORDINAL) which says which approach should be taken. CrispMuncher (talk) 21:27, 26 October 2009 (UTC)

Huh? This article is written in English, so what do places where English is not spoken have to do with this? ___A. di M. 21:44, 26 October 2009 (UTC)

I tend to agree with Wdl1961 on this. Anything beyond "million" invariably makes at least some readers wonder. Whenever we can avoid that, I think we should :-)

Indeed, from WP:ORDINAL#Numbers as figures or words we can use bullet_2 ("Comparable quantities should be all spelled out or all figures: we may write either 5 cats and 32 dogs or five cats and thirty-two dogs, not five cats and 32 dogs.") and bullet_11 ("Measurements, stock prices, and other quasi-continuous quantities are normally stated in figures.") DVdm (talk) 22:07, 26 October 2009 (UTC)

Yeah, and the following bullet says "When expressing large approximate quantities, it is preferable to write them spelled out, or partly in figures and part as a spelled‑out named number". "13,000,000,000 years" looks like it has eleven significant figures and is slightly less "immediate" to read; maybe "1.3×10¹⁰ years", but I think that the intended readership of the article includes people unfamiliar with scientific notation. As for the countries using long scale, none of them is English-speaking, so it's no more confusing than "actually", whose cognate in most other European languages means "now". ___A. di M. 22:19, 26 October 2009 (UTC)

May i suggest 13 billion (13×10⁹) years. Wdl1961 (talk) 23:45, 26 October 2009 (UTC)

Ah... Whatever Works, right? :-) DVdm (talk) 09:41, 27 October 2009 (UTC)

Wdl1961's suggestion below is what works best, IMO. (I didn't apply it when "billion" was used several times in the same section because I expect their attention span to be longer than they take to read it, and that they be able to figure out that no-one'd use "billion" with two different meanings in the same section unless explicitly discussing these meanings. ___A. di M. 09:53, 27 October 2009 (UTC)

I find it ironic that we can go from discussing the arcane depths of quantum electrodynamics (see the discussion of the Scharnhorst effect above), to suggesting that readers (who would have gotten past all the relativity in the article to reach the end) cannot understand the scientific notation of small numbers! Physchim62 (talk) 13:19, 28 October 2009 (UTC)

Even more ironic is suggesting that they cannot understand "parts per billion". And, FWIW, Abtract did fail to understand what "relative uncertainty of 2×10⁻¹¹" meant (see above). (Anyway, now that the article shows both forms, this discussion is moot.) ___A. di M. 14:19, 28 October 2009 (UTC)

Learning to use english "after getting of the boat" I was more confused or a long time , there was no wikpedia! Therefore I believe in keeping it simple to correlate quantities . I am sure the english wikpedia is used by an higher percentage non native speakers than any other language wikpedia. Wdl1961 (talk) 15:08, 28 October 2009 (UTC)

General notes

First sentence: "electromagnetic radiation, such as *visible* light..."

"Light" can both mean EM radiation and visible light, when used in a sentence that already contains the term EM radiation it is clear that the latter meaning is meant. Adding "visible" would feel a tad pedantic to me. (TimothyRias (talk) 15:59, 27 October 2009 (UTC))

Third paragraph: the physical meter rod was no longer used in the early 1960's. At that time ("1975" as stated), a large-but-finite number of wavelengths of characteristic radiation were used to define the meter, and the uncertainty derived from the uncertainty in the wavelengths... the meter itself was not uncertain.

Actually, I think there was an uncertainty in what exact wavelength was to be used to define light making the definition of the metre in it self fuzzy. (TimothyRias (talk) 15:59, 27 October 2009 (UTC))

Fourth paragraph: the speed of light of X-rays an gamma radiation in air is c. More energetic radiation does not "propagate" so much as "penetrate". The "index of refraction" is wavelength dependent, and the values / ranges cited apply to *visible* light.

This paragraph could indeed use some improvement. (TimothyRias (talk) 15:59, 27 October 2009 (UTC))

Section "The nature of light", subsection "Light as photons", a nod to the photoelectric effect would be a really good idea here.

In what sense? Please by bold if you have a specific idea. (TimothyRias (talk) 15:59, 27 October 2009 (UTC))

The photoelectric effect was what suggested Einstein that light might be discrete. (Planck did that before, but he considered that to be just a mathematical trick, with no direct physical significance.) So if we needed to have a one-paragraph explanation of the history of QED in "Light as photons", the photoelectric effect would have to be mentioned; but I think the current content of that section is sufficient for the scope of this article. ___A. di M. 21:33, 27 October 2009 (UTC)

Personally, I think it would be a mistake to mix up photons with speed of light. Light propagates as a wave, which has a speed; the quantized or particle aspect has not so much to do with the propagation or the speed; we speak of photons traveling at the speed of light, but that really is mixup of the wave/particle properties. QED is all about the waves, leading to event probabilities. Dicklyon (talk) 23:35, 27 October 2009 (UTC)

Section "Fundamental importance in physics", subsection "Variations with time and frequency", a massive photon will violate charge conservation, and the mass is not "taken to be zero", but its rest mass has been experimentally verified to be 18 orders of magnitude smaller than its energy... inclusive of zero.

In (almost) any calculation the value for the photon mass *is* taken to be zero. i.e. Zero is used as the preferred value consistent with experimental bounds. (TimothyRias (talk) 15:59, 27 October 2009 (UTC))

Section "History", subsection "Redefinition of the meter", the 1972 values established c as 299792462 (Bay/Luther/White) or 299792460 (NRC/NBS), with less accuracy than established by the US-NBS in 1983. The US-NBS value of 299792458.6 in 1983 was truncated, and carried a smaller uncertainty. 98.165.4.177 (talk) 02:16, 27 October 2009 (UTC) David A. Smith

Reply from David A. Smith (transplanted from User_talk:TimothyRias)

This is David A. Smith from a different IP address.

Re: third paragraph. There was no uncertainty in which wavelength of light was to be used for measuring c between early 1960s and 1983. One wavelength was chosen, and then another (shorter) was chosen later as being better because it was sharper. But wavelengths do not derive from classical equations, and mapping from quantum processes to the macroscopic world leaves uncertainty.

Re: Section "The nature of light", subsection "Light as photons" Peer review implies that the text is controlled in some sense, to be edited by others. So my 'bold' contribution might be to add this sentence: The [photoelectric effect] shows that light is comprised of discrete packets of energy, that do not merge to fewer, more energetic, packets as waves would.

Re: Section "Fundamental importance in physics", subsection "Variations with time and frequency" What is made is a blanket statement, implying that we did not look for the mass of a photon. We did. I suggest that if the wording stays as is, it is preceded by a statement referring to experimental testing of a photon's rest mass.

David A. Smith 216.161.188.207 (talk) 16:32, 27 October 2009 (UTC)

Article organization

I think that the overall organization of the article can be improved. Since any such improvement will involve some major changes to the article I think it better to discuss before being bold. (I'm also unsure what the best way forward will be.) The issues: (it might be useful to respond to each separately.)(TimothyRias (talk) 15:13, 27 October 2009 (UTC))

• Measurements and History Currently, all information about measuring techniques for the speed of light are contained in the history section. Now, this made a lot of sense when these sections discussed the increasing accuracy of measurements ultimately leading up to the redefinition of the metre, but some of the recent expansions have brought it beyond that scope. Might it not be better to have a separate section discussing various techniques for measuring the speed of light. (It would be nice if we could have one that does not use light, but I'm unaware of anything of sufficient notability in that extent.) This might be more in line with what a reader will expect from a history section.(TimothyRias (talk) 15:13, 27 October 2009 (UTC))

  I think that just promoting the last two subsections (which I'm going to do) will fix that. All of the rest of "History" deals either with very early measurements, or with measurements in metres per seconds which are now moot. ___A. di M. 21:28, 27 October 2009 (UTC)

  That is at least marginally better, but leaves us with a second level section of only a single sentence. (Lab demo). Is there some useful way we can expand on that? (TimothyRias (talk) 08:45, 28 October 2009 (UTC))

• Propagation of Light I think it might be a good idea to merge all the section that deal with the propagation of actual light. This would mainly involve the sections Nature of Light and Light in Transparent Media, but could also absorb the "variations with frequency" part of the variations with time and frequency section. This will help the article discriminate between the propagation speed of light and the fundamental spacetime constant speed of light. As it stands I fear that some readers may still come away from the article with the impression that the properties of spacetime depend on the speed with which light travels and not the other way around.(TimothyRias (talk) 15:13, 27 October 2009 (UTC))

• Fundamental Importance in Physics I am not sure about the title and content of this section. Anybody already familiar with the mentioned material will know that the treated subjects are of fundamental importance, but I'm not convinced that a reader with hardly any knowledge of physics will recognize them as such. This might be helped by including a nice introductory paragraph at the beginning of the section explaining that that the speed of light pops up all over physics. But I can't help to think that there might be a better way of organizing the material covered in that section.(TimothyRias (talk) 15:13, 27 October 2009 (UTC))

Any thoughts? (TimothyRias (talk) 15:13, 27 October 2009 (UTC))

Agree with the other two points. ___A. di M. 21:28, 27 October 2009 (UTC)

That might all be OK. To me, what's important is more the "fundamental importance" stuff than the details of the measurement, the history, and the exact value. These all need to be in there, but the reader really probably wants to know what's so special about "the speed of light". And as many physics book do, I'd introduce it in the opening paragraph as "about 3 time 10^8 ms" instead of the distracting and relatively useless exact value, which can follow in a subsequent paragraph. Dicklyon (talk) 23:46, 27 October 2009 (UTC)

I'm not sure how that is a reply to the points on organization of the article I brought up. Lets leave discussing the lead for another thread, and until after we have hammered out the main content. (TimothyRias (talk) 08:40, 28 October 2009 (UTC))

Another issue:

• The overall length of the article The article has recently breached the 100K mark. Now, this isn't a disaster per se, but if we are going to attempt to get the article through FAC we need to be able to be able to argue that these length is really needed for this article. I'm not sure that it is. Currently, the article strays a lot into topics that are related to the speed of light, such as the nature and behaviour of light, and the effects of special relativity. But special relativity and light already exist. I think we should try to get the article more into focus on the subject matter and more reliant on the existence of other articles. What is really essential to this article?

Some places where I think we can conserve space:

1. The article currently spends 4 paragraphs to discussing Ole Christensen Rømer's calculation of the speed of light. This by itself is probably notable enough to have its own article. Moving the material to its own article and expanding it further, we can replace the mention here with a single summary style paragraph saving quit a bit of space, and improving the flow of the article.
2. The current Fundamental importance in physics section strays a lot towards the implications of special relativity. Can we reduce this to only a couple of paragraphs which succinctly state the role of the speed of light in SR (and consequently GR and QFT)?
3. Do we really need to mention photons? And if so, can we do this in a more organized manner?

(TimothyRias (talk) 14:34, 28 October 2009 (UTC))

1. I agree. While we're at it, "Ancient, medieval and early modern speculation" can be merged into [some article which I'm sure already exists, but can't tell where it is], and "Redefinition of the metre" can be merged into Metre, retaining two paragraphs for each of these.
2. "A couple paragraphs" is too little for the relevance of c in SR; that's essentially the reason why c is so important. (I'm not saying that all the five subsections of three paragraphs each in average which exist now are strictly necessary; but I wouldn't trim "Fundamental importance in physics" by more than 40% the current size.)

Fair enough, my main point was to increase the focus on what we really need to say. (TimothyRias (talk) 16:29, 28 October 2009 (UTC))

1. We should mention them; of course an extensive discussion about them wouldn't belong here, but I don't think that the one paragraph in "Light as photons" and the third paragraph of "Variations with time and frequency" are that excessive. What do you mean by "more organized manner", exactly? ___A. di M. 16:03, 28 October 2009 (UTC)

More structured in the sense that is probably best to sweep the discussion of photons and photon mass together in one section. The main reason to mention photons is that the photon being massless is the reason light travels with the speed of light. Once we mention photons are massless that would also be the best location to mention the experimental limits on the photon mass, and the implications of a non-zero photon mass for the propagation of light. (TimothyRias (talk) 16:29, 28 October 2009 (UTC))

Free space

For some reason, links to Free space do not seem to be working and lead instead to: 404 error: File not found. Brews ohare (talk) 16:51, 28 October 2009 (UTC)