Talk:Speed of light/Archive 8

Speed of light exactly
Wikipedian64, I have reverted your removal of italics and your change of 'exactly' to 'precisely'. The use of italics is as per WP guidelines in that they are used sparingly for emphasis.

The word 'exactly' is a better word in this context because 'precisely' could be taken to mean that there is some, very small, room for error or change, which there is not. Martin Hogbin (talk) 16:54, 26 July 2009 (UTC)

The fact that the value of physical quantity, when stated in SI units, should have an exact value is surprising to most people. This makes the point worthy of emphasis in the lead in my opinion. Martin Hogbin (talk) 16:22, 28 July 2009 (UTC)

Introduction
In view of the conventional nature of 299,792,458 m/s, it seems misplaced to make it the central point of the introduction. Instead, the importance of the physical nature of the speed of light in setting the maximum rate of information and of matter transfer, and its connection to the speed of EM radiation should be the main focus. The numerical value in SI units is subsidiary. Brews ohare (talk) 16:18, 28 July 2009 (UTC)


 * Comment by Martin from above:
 * The fact that the value of physical quantity, when stated in SI units, should have an exact value is surprising to most people. This makes the point worthy of emphasis in the lead in my opinion. Martin Hogbin (talk) 16:22, 28 July 2009 (UTC)

It's surprising that Sarah Palin quit midterm, but that doesn't mean it's important. I'm inclined to think the placement of an accidental value in the lead, however surprising, tends to make it look like it is more significant than it is, and sets the article up with unnecessary explaining to do. A primary position in the lead also diverts attention from the actual importance of the speed of light to physical theory. Brews ohare (talk) 16:39, 28 July 2009 (UTC)


 * The decision to base the metre on the speed of light was, no doubt, influenced by the importance of the speed of light to physics. Martin Hogbin (talk) 16:47, 28 July 2009 (UTC)

Martin: this is an incomplete thought at best. It does not approach the subject, which is the diversion from what is important to highlight the marginally relevant. Brews ohare (talk) 17:01, 28 July 2009 (UTC)

299,792,458 m/s is a conversion factor
Apparently there is resistance form Martin to the notion that 299,792,458 m/s is a conversion factor from time to length. However, it will be noted that in support of this notion was a verbatim quote from (just prior to the quote, the NIST decision to "define" the value of "c" is discussed), and two other sources:

and

.

Previously I provided JA Wheeler and Mendel Sachs, not exactly slouches in the realms of physics. Here's three more sources: Jespersen; Brzeziński; ND Mermin.

In the discussion with Softvision this conversion factor role was very extensively described, and Martin himself suggested the arbitrariness of 'c' was evidenced by the "natural" units where c=1.

So what is the problem here? Use of λ = c/f (or ℓ = c t) means c is a conversion factor. Stating that makes it very clear that any value of c may be used (in principle), and that 299,792,458 m/s has nothing to do whatsoever with the physical entity known as the speed of light. That realization defuses 98% of the controversy around the "speed of light by definition". Brews ohare (talk) 02:21, 28 July 2009 (UTC)


 * A conversion factor is purely numerical. It has no system of units. 299,792,458 is a conversion factor. When the system of units "m/s" is attached, it is related to a physical entity and is no longer a conversion factor. Lestrade (talk) 04:25, 28 July 2009 (UTC)Lestrade

The term "conversion factor" is not mine. It appears in the cited sources. Brews ohare (talk) 11:17, 28 July 2009 (UTC)


 * When you're thinking about special relativity and 4-dimensional Minkowski spacetime, 299,792,458 m/s is a conversion factor from time to length. But in other contexts, I wouldn't call it a conversion factor. It's a velocity. Any velocity can be used to convert lengths into times and vice-versa.
 * I'd rather say: "In the context of the theory of relativity, it makes sense to think of the speed of light as a conversion factor between time dimensions and space dimensions within 4-dimensional spacetime." (or something like that). --Steve (talk) 07:11, 28 July 2009 (UTC)

I agree that there are two usages involved here, of which one is strictly the space-time notion that the metric ds2 involves c2dt2, so c converts the time component to a distance allowing the computation of ds.

However Gelʹfand very clearly is describing a second usage: “This definition was replaced in 1983 by the current definition which, using the speed of light as a conversion factor, is expressed in terms of inverse frequencies rather than wavelength …” Likewise, “By an international agreement, made in 1983, 'the metre is the length of the path travelled in vacuum by light during a time 1/299,792,458 of a second'. From the physicist's point of view, the speed of light has become a conversion factor, to be used for turning distances into times… ”ABC's of Relativity; Wheeler mentions conversion factor: “The conversion factor between seconds and meters is the speed of light, c = 299,792,458 m/s. … The speed of light is the only natural constant that has the necessary units to convert a time to a length.” John Archibald Wheeler. Jespersen is very clear about the arbitrariness of the value for c: “One fallout of this new definition was that the speed of light was no longer a measured quantity; it became a defined quantity. The reason is that, by definition, a meter is the distance that light travels in a designated length of time, so however we label that distance – one meter, five meters, whatever – the speed of light is automatically determined. And measuring length in terms of time is a prime example of how defining one unit in terms of another removes a constant of nature by turning c into a conversion factor whose value is fixed and arbitrary.″Jesperson

I bring up all these quotes from the already cited sources (of which there are more above) to persuade that the arbitrariness of the value for c is not my idea, and has multiple published occurrences. Likewise, the term "conversion factor". Of course, the quotes should be unnecessary because the idea is a logical consequence of the definition, as NIST has pointed out, and as Jespersen states in so many words.

In sum, the realization that the choice of 299,792,458 m/s is simply a practical choice of number and it could equally well be 1 m/s defuses the controversy around this numerical value, by showing any number can be taken when using the "speed of light by definition". Naturally, it is the very arbitrariness of 299,792,458 m/s that raises people's eyebrows, because they think of c as a definite value that can be measured. Introducing the term "conversion factor" places the emphasis where it belongs: on human conventions that are human choices, and not in nature outside man's grasp. Unlike the famous quip about a senator introducing a bill to define π to be a round number, a senator could introduce a bill to make c a round number. Brews ohare (talk) 11:17, 28 July 2009 (UTC)


 * Origin of the numbers in SI meter and time units definitions is in interconnection of the natural physical effect of the caesium 133 radiation with the existing meter and time measuring devices, not to invalidate existing devices. Metre and time SI units definitions are bound to the same natural physical effect - the radiation of the caesium 133 atom. The metre unit is derived from the wavelenght of the radiation. The time unit is derived from the period of the radiation. This is the reason why speed of light is numericaly constant. Wave lenght and frequency used are unseparable components of one and the same natural effect of radiation - the speed of light propagation. If the stability of this natural effect is guaranteed, the definitions are valid, along with the outcome of speed of light numerical constantness. According to this model of the units definition, the real space-time ratio is not given by the numerical constant of the speed of light, but by the physical realizations of these units. This is specific outcome of this model of units definition. That means, that we cannot percieve exactly the space-time ratio of the speed of light by means of the speed of light numerical constant. Symbols c and 299792458 are now equivalent, because both are exact. If you think about 299792458, you can think about c, and vice versa.


 * The problem of the current metre/time/speed of light SI definitions is, that we are psychologicaly bound to numbers as representing quantity of property. In this sense, the concept of property is question, and the concept of numerical value is the answer. If property c and quantity 299792458 are equivalent, such perception can be problematic. More exactly, if the number of the speed of light originated in well established experiments, the space time ratio expressed by the number 299792458 is very good answer, but not exact. If you want to percieve the real physical quantity of speed of light, you must think, that the speed of light is somewhere near this numerical constant. You cannot add +/- uncertainity to the speed of light constant, because it will be not consistent with metre/time definitions. But in the sense of perception, the uncertainity of the speed of light constant exists, because the units of metre and time are uncertain in the sense of perception - measurement. This is specific outcome of the current metre/time units definitions.


 * If we assume, that natural effect of the caesium 133 atom radiation is exact, because transition effect is exact, that means allways the same, all three definitions of meter/time/speed of light are exact. If we assume, that any experiment can be performed only under some environmental conditions, all three definitions of meter/time/speed of light are uncertain, regardless of the precission of the measuring devices. Uncertainity of the speed of light constant is not numerical, but physical. Softvision (talk) 11:51, 28 July 2009 (UTC)


 * The speed of light is fundamental reality. If we calibrate the metric and time device using the caesium 133 source, the new calibration is physical expression of the speed of light constant. If we want to know what is the difference of new calibration, we must perform physical comparsion, because numerical comparsion is not possible. Therefore we must measure old physical calibrations using new physical calibrations. By means of this we can compare new speed of light constant with old speed of light constant. Even though the numerical value did not changed, we have performed valid and comparable speed of light measurement. This is the proof of physical uncertainity of the speed of light constant. Softvision (talk) 14:33, 28 July 2009 (UTC)

It is not easy to determine your goal in the above remarks. My take is that you wish to explain how it is that the speed of light cannot be taken as a fixed value. There is some ambiguity in such a remark, which may be contained in your observations, but eludes me. The distinction to be made is that the physical speed of light may in fact change over time (as suggested by some cosmologies), or may be observed with greater precision in the future (resulting in more precise numerical values for wavelengths of selected atomic transitions). However, the numerical value of the speed of light in SI units will never change, short of an agreement by the CIPM (Comité International des Poids et Mesures) that the definition should be changed. The numerical value in SI units simply is one of man's conventions, and is not an experimental matter, but a matter of convenience. Do we agree? Brews ohare (talk) 15:13, 28 July 2009 (UTC)


 * The point about the speed of light being a conversion factor between space and time is already made in the article in the section 'Spacetime constant'. We do not need it twice. Martin Hogbin (talk) 16:29, 28 July 2009 (UTC)


 * The role of c in spacetime is different from the implications in the definition in SI units. It needs to be mentioned in both contexts. Brews ohare (talk) 16:34, 28 July 2009 (UTC)


 * Once again Brews, the place to make your point is here, not by ruining the presentation of the article to make your point stand out (like this). Are you doing this on purpose or do you really not understand what you are doing wrong?
 * Martin Hogbin (talk) 16:29, 28 July 2009 (UTC)


 * There is no "ruination" taking place here. A bit of re-organization to put first things first is all. Brews ohare (talk) 16:34, 28 July 2009 (UTC)


 * Every time you want to make a point you add a quotation to the article surrounded by white space. If they had all been left in place, the article would be a complete mess. Martin Hogbin (talk) 16:45, 28 July 2009 (UTC)

Have we abandoned logic here? First, we say that c = 299,792,458 m/s. Then we say that 299,792,458 m/s is a mere conversion factor, not related to physical entities. This is a logical contradiction because "m/s" means "meters per second" or, in general, "movement through space in a certain time." What moves through space in a certain time? A physical entity.Lestrade (talk) 19:51, 30 July 2009 (UTC)Lestrade


 * Hi Lestrade: I don't think a logical contradiction is involved here. Of course, you are right: something moves through space. That would be the light. There is a certain time: 1/299,792,458 s. There is a certain distance covered by the light in that time. That is defined to be a metre. Thus, we don't actually have to have a physical length as a measuring stick to find the distance between point A and point B: we just find how long it takes light to travel A to B, and compare that with 1/299,792,458 s. The ratio is the length in metres. What say you? Brews ohare (talk) 20:04, 30 July 2009 (UTC)
 * For example, it takes 8.3… minutes for light to reach us from the Sun. So the Sun is 8.3… min ÷ (1/299,792,458 s) ≈ 1.49 × 108 km away from Earth in metres, with an accuracy dependent upon how good the 8.3… min estimate might be. Compare with ABC's of Relativity. Brews ohare (talk) 20:08, 30 July 2009 (UTC)


 * The velocity of a car functions as a conversion factor between how far it has gone and how long it has been driving. This is true, but a confusing way to state a simple point. :-)


 * I'm very happy with the current incarnation of the article, where the "conversion factor" phrase is used in the context of 4-dimensional spacetime but not in the context of SI conventions. The SI thing is stated better without using the term "conversion factor". :-) --Steve (talk) 05:31, 31 July 2009 (UTC)

deletion of sourced material
Please explain deletion of this quote:

“One fallout of this new definition was that the speed of light was no longer a measured quantity; it became a defined quantity. The reason is that, by definition, a meter is the distance that light travels in a designated length of time, so however we label that distance – one meter, five meters, whatever – the speed of light is automatically determined. And measuring length in terms of time is a prime example of how defining one unit in terms of another removes a constant of nature by turning c into a conversion factor whose value is fixed and arbitrary.″ Brews ohare (talk) 16:59, 28 July 2009 (UTC) I find this quote to be (i) authoritative (ii) very clear and (iii) important to have in the article in view of the "very surprising" [Martin's words] nature of an exact value for the speed of light in SI units. Brews ohare (talk) 16:59, 28 July 2009 (UTC)


 * Firstly, we do not need it as a direct quote, surrounded by whitespace. Secondly it is pretty much what is said anyway. For example we already have, 'Defining the metre as the distance light travels in a specified time simply has the effect of setting the speed of light to a definite numerical value when measured in the SI units of m/s'.  We do not need or want to make every point twice. Martin Hogbin (talk) 17:09, 28 July 2009 (UTC)

If this point is as "surprising" as you said it is, the point bears some amplification, and this text does it very well. Also, your own resistance to this viewpoint indicates the need for a direct quote, because nothing less proves convincing. The point must be made in a way that very clearly is not a WP editor's flaky opinion, or it will be the source of unending battles on this page. Brews ohare (talk) 17:18, 28 July 2009 (UTC)


 * We have references to show that a statement is not just 'a WP editor's flaky opinion'. What point do you think your quotation makes that is not covered by my quote from the article?  Martin Hogbin (talk) 17:27, 28 July 2009 (UTC)

Martin: I don't know what "quote from the article" you are talking about. First, the article doesn't presently even cite Jespersen, never mind quote him. Second, the only item in quotes is from NIST. Third, the removed quotation uses the phrase "by turning c into a conversion factor whose value is fixed and arbitrary", which I find a very eloquent statement of the situation compared to the existing text. IMO you have deleted the quote precisely because it is so clear, and you would rather preserve an element of mystery about this subject. Brews ohare (talk) 17:41, 28 July 2009 (UTC)


 * I mean this quote, which I have mentioned just a few lines above, 'Defining the metre as the distance light travels in a specified time simply has the effect of setting the speed of light to a definite numerical value when measured in the SI units of m/s'. What point does your quotation make that this does not make?


 * Your comment about preserving an air of mystery is so bizarre that I cannot respond to it. Martin Hogbin (talk) 17:48, 28 July 2009 (UTC)

This "quote" appears to be the result of your slight rewording of an entry of my own words 21:46, 24 July 2009. So, it lacks the clout of a real verbatim quote from a published source.

As I've said, the removed quotation from Jespersen uses the phrase "by turning c into a conversion factor whose value is fixed and arbitrary", which I find an eloquent statement of the situation compared to the existing text. In addition, as I'm sure you are aware, this subject of an exact speed of light comes up over and over here, the most recent exchange being Softvision, but prior to that Venus10041, Physchim62 and others. It's worth a try to see whether a clearer statement would improve matters. It is a commentary on the present wording that readers don't find the explanation satisfying. Brews ohare (talk) 18:03, 28 July 2009 (UTC)


 * Yes, you are quite right the quote is the result of my slight rewording of an entry of your words. That is because I am trying hard to work with you.


 * I do not find "by turning c into a conversion factor whose value is fixed and arbitrary" particularly eloquent or a particularly good explanation.  I think what we currently have (mainly your words) is better. Either way we do not want it in the article as a direct quote.Martin Hogbin (talk) 22:18, 28 July 2009 (UTC)

You have stated your preference, without arguments in support, and I disagree with it, for substantial reasons provided by me, and not challenged by you. Brews ohare (talk) 23:54, 28 July 2009 (UTC)


 * 'Conversion factor' is a vague term; your quote does not make clear what is converted to what by means of this factor. Our current statement, '...definite numerical value when measured in the SI units of m/s', makes quite clear that we are talking about the SI units of the metre and second.


 * The more philosophical aspects of this discussion would be better addressed by mentioning the fact that the speed of light is a dimensionful constant, but please, let us talk about this before you go round looking for sources and adding more quotations to the article.Martin Hogbin (talk) 13:01, 29 July 2009 (UTC)

Where do you get the notion that "conversion factor" is used vaguely when all the verbatim quotes you have deleted from several different sources all define what is meant in absolutely starkly clear and unambiguous terms, and include specific examples? Brews ohare (talk) 14:44, 29 July 2009 (UTC)

I have added Jespersen as a reference (no quote), and a sentence stating that the value of c is not a property of nature, but a convention. Brews ohare (talk) 15:19, 29 July 2009 (UTC)

Incorrect description of QED
The article as revised by Martin now states: A more complete theory of light that describes the interaction of photons with matter is given by quantum electrodynamics (QED) in which c plays the role of a fundamental constant. Brews ohare (talk) 17:14, 28 July 2009 (UTC) The problems with this are: Brews ohare (talk) 17:14, 28 July 2009 (UTC)
 * 1) QED is not designed to provide the interaction of photons with matter, as explained at length earlier by Steve, inasmuch as matter is far too complicated to be approached by QED.
 * 2) The footnote made very clear what the limitations of QED are in terms of the original text, but unfortunately is now garbled in Martin's version because it doesn't fit the modified text.
 * 3) The above problems were pointed out previously, and have been ignored again by Martin.


 * QED deals with the electromagnetic interaction. Steve has not said anything like you claim and he is quite happy with my version. Your statement is quite pointless, it is not even clear what you is trying to say.  As I have explained, the interaction of photons with ordinary matter, is pretty well exclusively with electrons, so we could say, 'with the electrons in matter' if it makes you happy.  Positrons do not hang around for long in ordinary matter and are irrelevant. Martin Hogbin (talk) 17:22, 28 July 2009 (UTC)

Martin:
 * Here is part of what Steve said, which does not agree with your notion of previous discussion: "The photon does not directly feel the strong or weak force, but the photon feels quarks and quarks feel the strong force. The way Feynman diagrams work means that that's enough for the photons to feel the strong interaction a little bit. Here's an example: The question 'What is the electron magnetic moment' appears to be a purely electromagnetic question. And it can be answered to 99.9999999999% accuracy within QED. But to get to the parts-per-trillion level, you need to include strong interactions (see Figure 2 of [3], 'hadronic' means strong force) despite the fact that neither the electron nor photon feels the strong force. :-) --Steve (talk) 03:55, 24 July 2009 (UTC)"

In addition: Brews ohare (talk) 17:33, 28 July 2009 (UTC)
 * 1) The source in the footnote does not agree with the text you have modified.
 * 2) The footnote doesn't make sense with the text you modified.
 * 3) Positrons are included because the source included them for completeness in describing QED, not because they were a major factor.
 * 4) We do not have to change this text in the way you have attempted. It was perfectly fine and well-sourced previously. You are simply trying to rewrite QED to suit yourself.

Let us ask Steve what he thinks. Martin Hogbin (talk) 17:45, 28 July 2009 (UTC) This is what was said above:


 * Are you happy then for me to restore my original statement of, A more complete theory of the interaction of light and matter is given by quantum electrodynamics'? Martin Hogbin


 * Yes. Although as Brews points out, it would be appropriate to have photons mentioned somewhere around here. Steve

I have no objection to mentioning photons. Martin Hogbin (talk) 17:53, 28 July 2009 (UTC)


 * Mentioning photons would be fine, but the major issue is the impression given that QED attempts to be a theory of the interaction of photons with matter. The source (and the footnote) doesn't agree with that characterization. There isn't any point in being 99.9999999999% accurate numerically, when you can be 100% accurate conceptually without any effort. Brews ohare (talk) 18:05, 28 July 2009 (UTC)


 * Stating that QED deals only with the interaction of photons electrons and positrons is not 100% accurate. Martin Hogbin (talk) 22:08, 28 July 2009 (UTC)

It also is not what is said in the deleted statement. Brews ohare (talk) 23:53, 28 July 2009 (UTC)


 * Most of the interaction between light and matter involves only electrons, positrons and photons to a very good approximation. Therefore, QED is a "more complete theory of the interaction between light and matter". It's not 100% complete, but it's certainly more complete.


 * The footnote is getting pretty far off-topic. Why not just the book reference? People who want to know about QED and its limitations and successors can read the book or the wikipedia article. The standard model doesn't need to be mentioned here, QED was the important conceptual advance, and the standard model just fills in more details (as far as light is concerned). I say the footnote should be just the book reference. :-) --Steve (talk) 06:42, 29 July 2009 (UTC)


 * It is because I do not claim to to be an expert on the subject that I restricted myself to adding the simple and basic statement, "more complete theory of the interaction between light and matter", with a link to the QED article. I think that the best reference would be Feynman's non-technical introduction to the subject, 'QED - The strange theory of light and matter'.  This is an excellent book and one of the few that will be accessible to anyone who does not already know what QED is. Anyone who wants to write more on the subject should be an expert rather than trying to make it up by searching the net for online publications. See my comments below. Martin Hogbin (talk) 09:26, 29 July 2009 (UTC)

The source is written by an expert, There is no argument that the source has misstated things. It is clear and unambiguous. Martin apparently doubts that he understands it, but nobody else will have that problem. Possibly Martin's statement can be interpreted as he would like it to be, but it is ambiguous and casts QED in a somewhat odd light, as "more complete", as though we just ain't got there yet. Why not stick with the source, which is "more complete" yet? And why make the reader dig through an entire text on the subject when all that is wanted is a sentence that makes the entire matter clear? Is that Steve humoring Martin, or some real effort to be useful to the reader??? Brews ohare (talk) 14:27, 29 July 2009 (UTC) I have re-written the note to agree with Martin's text. Brews ohare (talk) 15:17, 29 July 2009 (UTC)

Photons in vacuum travel at the speed of light, c
I have just removed the following phrase about photons from the article, "which in vacuum travel at the speed of light, c".

Photons are quantum entities and to talk of them travelling with a particular speed is not a good idea, although it is often done. Also what speed do photons travel at when they are not in a vacuum? Martin Hogbin (talk) 22:27, 28 July 2009 (UTC)

I believe you should answer these questions, which are natural, and source your answers to a reference available on line. BTW, the article already states (and sources) c as the speed of rest-mass-zero particles. Brews ohare (talk) 23:51, 28 July 2009 (UTC)


 * The trouble is that that statement was about virtual photons. Real photons travel at the speed of light, but I don't think virtual photons do. --Steve (talk) 06:45, 29 July 2009 (UTC)

What statement is "that statement" that you are referring to?? Brews ohare (talk) 15:44, 29 July 2009 (UTC)


 * The real problem comes from trying to treat quantum entities as little balls with positions, trajectories, and speeds. Over long distances photons can be treated as having a speed but on a quantum level this is not a good description, even though the constant c may enter into the equations. The main reason that I removed this bit was that it talked about photons 'in a vacuum'.  To use this terminology clearly shows a lack of understanding of the subject.


 * Attempts to write sections such as this without a background knowledge of the subject reminds me of this book English as she is spoke. You cannot write about every subject in physics by simply looking up online references and extracting bits from them to add to the article.  I think that several sections have been written in this way by should be deleted completely or at least replaced by fairly general statements with links to the relevant subject articles. Martin Hogbin (talk) 09:39, 29 July 2009 (UTC)

You are arguing among yourselves here. What Martin's comments "clearly show" is a lack of civility. Brews ohare (talk) 14:21, 29 July 2009 (UTC)


 * Brews, in my comment above, when I said "this statement", I was referring to this, the sentence Martin said was dubious. Sorry that wasn't clear. --Steve (talk) 06:27, 30 July 2009 (UTC)


 * Brews, are you suggesting that it is a good idea to write about physics by simply looking up online references and extracting bits from them to add to the article with no background knowledge of the subject? Do you claim that "[photons] in vacuum travel at the speed of light, c" is a valid statement? Martin Hogbin (talk) 17:01, 30 July 2009 (UTC)

I have not made any suggestions like that. I understand that the juxtaposition of the speed of photons in vacuum next to a description of their role as force carriers was unfortunate, allowing virtual photons to be confused with photons moving freely in space. However, it easily could be corrected without a diatribe on background, and "little balls". I won't engage in argument over the speed of photons - do whatever you like. Brews ohare (talk) 17:57, 30 July 2009 (UTC)


 * If you will not discuss the speed o photons you should not be writing in WP about it. Martin Hogbin (talk) 14:49, 1 August 2009 (UTC)

Constant speed in inertial frames
There was no need to change any of this. The cited reference on the independence of the speed of light on this source velocity gives examples of many experiments, including light from double stars, rotating mirrors, light from limbs of the sun and more.

The relativity postulates were modern and concise ones from a reliable source. Martin Hogbin (talk) 22:44, 28 July 2009 (UTC)


 * First, it is difficult to argue that your source is better than Einstein himself. You could add your source, but to delete Einstein's statements and his own names for the postulates is ludicrous, contrary to history and unsupportable.
 * Second, the WP article on Special relativity states the matter this way, and uses Einstein as a source. Consistency is a virtue.
 * Third, your link to relativity is to a disambiguation page instead of the correct link that you erased.
 * Fourth, your sub-section title: Constant speed in inertial frames is not a satisfactory main heading for its sub-sections, which are properly subsections of Relativity not of Inertial frames.
 * Martin, you are not thinking about what you are doing. Brews ohare (talk) 23:47, 28 July 2009 (UTC)


 * At least you are now discussing the subject before rushing in and changing everything.


 * Einstein's statements are not regarded as the best statements of the postulates of SR today. Not that his work is in any way wrong but the terms and explanations he used are now over 100 years old and people have find more concise ways of saying the same thing.  If you look in any modern relativity text book you will see the two postulates phrased differently from Einstein. You may find his original statements for historical interest.  The two statements that I use are from a modern well know and respected text book (d'Inverno) and have the advantage of being concise.


 * The article Special relativity has much more space to discuss the subject and therefore starting with Einstein's postulates may be a better idea.  We do not need to duplicate everything in every article.


 * Why is 'Constant speed in inertial frames' not satisfactory for a subsection within 'Fundamental importance in physics'? This is what the section is about.  What does 'Speed in relativity' mean.  As opposed to speed in what?


 * I will change the link. Thank you for pointing this error out.


 * There are a few other changes that you made that I notice that you have not decided to defend, I assume because you can see that you were wrong. Other editors have complained that your approach to any issue is to make a series of rapid-fire changes to the article rather than discussing things to arrive at a consensus.  If you took the consensus approach there would be far less friction on this page and we could get on with improving it.


 * If you want to change any of this section can I suggest the we discuss it and come to a consensus first. Martin Hogbin (talk) 09:04, 29 July 2009 (UTC)

If you look at the table of contents, you will see that the sub-sub-sub-sections I mention are sub-sub-sub-sections of the Inertial frames sub-sub-section, not of the sub-section Fundamental importance. Hence my observations about inappropriateness of the heading. Brews ohare (talk) 14:18, 29 July 2009 (UTC)

"What does 'Speed in relativity' mean?" As a subsection title, it means "the role of the speed of light in relativity". Previously you chastised me about this, saying that the article was about the "speed of light" and it need not be repeated. Brews ohare (talk) 15:39, 29 July 2009 (UTC)

I've changed the sub-title to a more complete description of its subject that fits with its own sub-sub-sections. Brews ohare (talk) 17:30, 29 July 2009 (UTC)

We are hardly "repeating everything in every article" by listing Einstein's statements of his postulates and maintaining some consistency. In addition, his names for the postulates are helpful in this context because they are divided nicely and are labeled exactly as is needed. I do not find your statements more succinct, you just dumped the labeling and chose a formulation that requires some logic to attach to the text instead of being direct statements of the postulates. I hope you have enough objectivity to see that. Brews ohare (talk) 14:18, 29 July 2009 (UTC)

I find little support for your statement that Einstein's statements of the postulates have been superseded in modern times. For example: Chow (2008) states them virtually verbatim as in Einstein's papers, and I find many other recent texts that do likewise. Moreover, the statement in terms of the "principle of relativity" and separately as "the principle of the constancy of the velocity of light" is a formulation that (i) suits the article, and (ii) is logically preferable to the conglomeration you presented, which fails to separate these two ideas, and does not state the invariance of all physical laws in inertial frames.

If a reformulation of Einstein's postulates exists, it is a stress upon the identification of inertial frames as those where the physical laws are not only invariant, but have their simplest form, a notion Einstein put forward in other writings, but not relevant here. Brews ohare (talk) 14:18, 29 July 2009 (UTC)

I have modified the presentation of the postulates more in keeping with your text, but using a reference accessible on-line, and a more accurate enunciation. Brews ohare (talk) 17:33, 29 July 2009 (UTC)

Proposed change
I would like to change the sentence:

'[The speed of light] has also been confirmed by the Michelson-Morley experiment and the Kennedy-Thorndike experiment (an improved version of the Michelson-Morley experiment) that the two-way speed of light; between a source and a mirror, and back again; is the same, independent of the common constant velocity of the observer, source and mirror (all moving together), regardless of its magnitude or direction'.

Which I consider to be long winded and containing a lot of unnecessary and irrelevant detail.

To

'The two-way speed of light, that is from a source to a mirror and back again, has also been confirmed by the Michelson-Morley and later, more accurate, experiments to be constant'.

The MM experiment is by far the best known and should be mentioned by name but there are at least 14 others that we need not mention specifically. I think that giving detail on some of the experiments is not just unnecessary but undesirable. I would add the summary page from Zhang as a reference as this the most comprehensive source on the subject. I would suggest leaving just one online reference. Martin Hogbin (talk) 10:51, 29 July 2009 (UTC)


 * We discussed earlier that Zhang is not available on line. I added two references that are on line and one of these is another Zhang, probably similar to the one not available. The other is less technical and therefore probably more understandable. So I'd retain the references.


 * Sorry, I misremembered this; deleted the two journal references. I'd keep the on-line reference, because Zhang is not on-line. Add Zhang if you like. Brews ohare (talk) 20:10, 29 July 2009 (UTC)


 * Beyond that, the statement about the two-way speed of light is not an accurate summary of the MM experiment, which also is about the isotropy of space. Light travels at the same speed in all directions. Brews ohare (talk) 14:09, 29 July 2009 (UTC)


 * Yes, but the isotropy of the speed of light can only be experimentally verified for the two-way case. Martin Hogbin (talk) 23:54, 29 July 2009 (UTC)


 * I have modified the statements along the lines you suggest. Brews ohare (talk) 17:26, 29 July 2009 (UTC)

Once again, you response to the suggestion that we discuss changes before making them has been to make yet another series of rapid-fire edits to the article. Please stop this. Martin Hogbin (talk) 00:20, 30 July 2009 (UTC)


 * Well, Martin, at least I explained my reasoning, which is a far forward step beyond your edits without comment, which in fact are nonsensical, as I have explained at length without the generosity of any repsonse from you. Brews ohare (talk) 02:28, 30 July 2009 (UTC)

Replacement
"It is important to separate the definition of a physical quantity from the definition of the unit in which it is measured.[96]Defining the metre as the distance light travels in a specified time simply has the effect of setting the speed of light to a definite numerical value when measured in the SI units of m/s:[97][98] Said in different words, the exact value, c = 299 792 458 m/s, is an international convention, not a property of nature.[99]"

I suggest that the following using a quote from Jespersen is better: "Defining the metre as the distance light travels in a specified time has the effect of setting the speed of light to a definite numerical value when measured in the SI units of m/s.[97][98] “One fallout of this new definition was that the speed of light was no longer a measured quantity; it became a defined quantity. The reason is that, by definition, a meter is the distance that light travels in a designated length of time, so however we label that distance – one meter, five meters, whatever – the speed of light is automatically determined. And measuring length in terms of time is a prime example of how defining one unit in terms of another removes a constant of nature by turning c into a conversion factor whose value is fixed and arbitrary.″[99]" Brews ohare (talk) 17:01, 29 July 2009 (UTC)

Please explain deletion of this sourced paragraph
“Also verifying the postulates, the speed of light has been shown experimentally to be independent of the motion of the source, as shown by experiments on the speed of γ-rays emitted during the decay of rapidly moving pions. Although the speed of propagation is independent of motion of the source, the observed frequency can change due to the Doppler effect.” Brews ohare (talk) 02:32, 30 July 2009 (UTC)

This statement is well sourced and also of interest in providing some details of how the experiment was done, an experiment that is neither obvious nor easy to do. I represent one class of readers interested in the speed of light who would find this information about it interesting and would appreciate some sources where details could be found. Brews ohare (talk) 02:48, 30 July 2009 (UTC)


 * As I have explained before, the first reference, which is still there, refers to a number of different experiments, some of which use light and some gamma rays. It is neither necessary nor desirable to attempt to give experimental details for just one of the experiments.  Zhang is a secondary source which itself cites many experiments and draws the stated conclusion that the speed of light is independent of the motion of the source. Martin Hogbin (talk) 14:58, 30 July 2009 (UTC)

As you know, Zhang is not available on-line. I have read his co-authored book that is on line about this topic, and he refers to the gamma ray experiment. At a minimum here: (i) Provide an on-line available source ( I provided three, which you deleted) (ii) Mention the nature of these tests or provide links where they may be found (The present state of this matter appears to be represented in §3.2 of Will, and on p. 23 of Field, although the binary star observation may be less striking than normally stated because of gravitational (curved space-time) effects neglected in its interpretation. Baird). Brews ohare (talk) 15:06, 30 July 2009 (UTC)

Please explain this mis-statement of the postulates of relativity
"“Einstein (who was aware of this fact) postulated that the speed of light should be taken as constant in all cases, one-way and two-way. This postulate, that the speed of light is the same in all inertial systems together with the postulate that all inertial observers are equivalent forms the basis of Einstein's theory of relativity.″" As carefully pointed out earlier, inertial systems are not identified by the fact that the speed of light is c in such frames. Rather, these frames are identified by the fact that all physical laws take on the same and simplest forms in such frames. In addition, the postulate of invariant speed of light is more appropriately phrased for the discussion of the text.

As also was pointed out, the cited source is not available on line and should be supplemented by one that is. Two such sources were provided (and deleted), both of which contain the correct statement of the postulates, which are in accord with the statements by Einstein cited in Special relativity. Examples are Chow and Banerjee. Brews ohare (talk) 02:37, 30 July 2009 (UTC)


 * You're reading that sentence as a definition of what is an inertial system...it doesn't read that way to me. To me it seems like the sentence presupposes that people know what an inertial system is, and is discussing Einstein's postulates. So I wouldn't call it a mis-statement. I'm sure it could be rewritten to be clearer and more eloquent, like anything... --Steve (talk) 06:18, 30 July 2009 (UTC)

The statement says: "This postulate ... together with the postulate that ..." .There is no way to read this as anything short of a statement of the two postulates of relativity, which they are not. In addition, the first of the true postulates does not assume an inertial frame is understood, it defines an inertial frame. Moreover, various correct statements of the two postulates have been summarily deleted and replaced with this misinformation twice, along with Martin's claims on this Talk page that his so-called "modern" statement of the postulates is more succinct and accurate than Einstein's own version. See Martin's posture here and here, and an explanation that had no impact upon Martin here.

Again, modern examples of correct statements of the postulates are Chow and Banerjee, which are very similar to Einstein's version cited in the WP article Special relativity. Brews ohare (talk) 07:39, 30 July 2009 (UTC)


 * As Steve has said you are misreading what I have written. There are two postulates the, second (which I give first as it is more relevant to the subject of the article) is given in the modern and well-respected text book by d'Inverno as 'The velocity of light is the same in all inertial systems'.  I have changed 'velocity' to 'speed' as, for some reason, many relativity text books the word 'velocity' is used to refer to the scalar quantity that is usually called 'speed'.  The first postulate is given by d'Inverno as, 'All inertial observers are equivalent'.  I do not insist that these exact statements are used but think that it would be much better to use modern and reasonably concise statements rather than Einstein's originals. Martin Hogbin (talk) 08:52, 30 July 2009 (UTC)

As you know, d'Inverno is not available on line. I cannot check the accuracy of your reading of this source, which reading may well have selected a few notions out of context in an ill-fated attempt to be brief. In any event the text as placed in the article is incorrect; the most recent, deleted, correct version, which followed your sentence construction, and is supported by on-line available sources, is hardly longer than your incorrect version, which cannot be supported as it stands. Brews ohare (talk) 14:55, 30 July 2009 (UTC)


 * My version can be and is supported by a very reliable source. In a section called 'The principle of special relativity' of the quoted source there is a display box which has in it 'Postulate I. Principle of special relativity: All inertial observers are equivalent'.  In the next section called 'The constancy of the velocity of light' there is a display box with, 'Postulate II. Constancy of the velocity of light: The velocity of light is the same in all inertial systems'.  These are exact quotations from the source.  As explained, I changed 'velocity' to 'speed'. Martin Hogbin (talk) 16:51, 30 July 2009 (UTC)

The quotation of a boxed text highlight without the context it refers to has led you astray. However, this argument can be avoided entirely as we really don't need to refer to inertial frames at all. Brews ohare (talk) 18:00, 30 July 2009 (UTC)

Please explain the insertion of this sentence as lead to sub-section
"The speed of light has been shown experimentally to be independent of the motion of the source.[14"

First, this source is not available on-line, and several that are available were simply deleted.


 * Sorry for deleting the other sources. As discussed, one on-line source would be a good idea. The reason for this action is that your rapid fire edits make it difficult to do anything but replace the whole section with a previous version in order to get it back into shape.  As you will see above I have been suggesting that we now proceed slowly with the editing of this article, discussing points before changing the text.  This is what I was trying to do by stating my proposed change above.  I was dismayed to find that your response was the usual series of rapid fire edits.

Second, this lead sentence properly belongs with the last sentence of the section, which treats the subject, and which follows the necessary introduction of this idea via the postulates.


 * The most logical order to me seems to be experimental evidence followed by the second postulate, as this can then naturally lead on to the next section on relativity and spacetime.

This sentence along with the last sentence of this subsection should be replaced by the deleted subsection noted above. Brews ohare (talk) 02:41, 30 July 2009 (UTC)


 * The sentence is fine, technically correct, well sourced, and in the right place.

Martin Hogbin (talk) 09:02, 30 July 2009 (UTC)

Nonresponsive to misplacement of sentence when topic has not been introduced and is not subject of the paragraph it serves as topic sentence for. Subject is brought up again at end of subsection as a dangling sentence. Poor construction. Source not available on line, and supplementary sources that are available deleted. Brews ohare (talk) 14:49, 30 July 2009 (UTC)

Please explain change of sub-section heading
The heading Role of the speed of light in relativity was replaced summarily as Constant speed in inertial frames, with Martin's complimentary comment " Restored sensible version ", despite the carefully worded observation that the sub-sub-sections of this sub-section fit better under the deleted heading, being in fact about relativity, not about inertial frames. Apparently relevance to the topic is not sensible. Brews ohare (talk) 02:45, 30 July 2009 (UTC)


 * Your complaint fails to note that you previously changed the heading from Constant speed in inertial frames to Role of the speed of light in relativity with logical justification.


 * Referring to the speed of light in relativity or the Role of the speed of light in relativity suggests that there is some part of physics in which the speed of light is not constant. There is no part of current physics in which the macroscopic speed of light is not constant.

Constant speed in inertial frames is a statement of the experimentally observed facts (and a generally accepted postulate) concerning the subject of the article. I might add that it is similar to the wording in the article when it originally became an FA.


 * Nonresponsive to fact that it does not fit the titles of its own sub-sections. Nonresponsive to the proposed title being more suitable than the one selected for this reason. Brews ohare (talk) 14:47, 30 July 2009 (UTC)

Please explain the rest of your edits
Now that I have explained my actions, perhaps you can explain you recent series of edits Brews. I count 23 of them in the last day.

My suggestion was that we should now discuss changes before making them. I do not think that there are any serious problems with the content of this page. If you disagree let us discuss what you think is wrong before you make substantial changes.

Martin Hogbin (talk) 09:18, 30 July 2009 (UTC)


 * You have not responded to all my queries about your actions, and your few actual responses need amplification.
 * As you well know, most of my edits you refer to are completely minor things like fixing punctuation, adding a template and so forth. I don't think anything I have done that remotely bothered you was left standing, and virtually none of your reverts are supported on the talk page.
 * So, you maintain a standard of discussion when it suits you and not otherwise. I see no reason to do differently, do you? Brews ohare (talk) 15:00, 30 July 2009 (UTC)
 * I do see serious problems with this page, as you know from my requests for clarification of your edits above, and requests for on-line available documentation. Please respond to these requests with greater seriousness. Brews ohare (talk) 15:01, 30 July 2009 (UTC)


 * You seem to have a misconception about what sources are acceptable to be used in the article. WP:RS is the relevant guideline on wikipedia, which does not recommend anywhere that the source be available on the internet.  If you wish to verify the cited sources say what the article claims they say, you may have to do a little work and go to a library, which would be especially encouraged as someone who edits the article.  If an online source is also self-published (and many are), the following would apply: "Self-published sources are largely not acceptable, though may be used in limited circumstances". Pecos Joe (talk) 17:30, 30 July 2009 (UTC)

It is not an issue of what is acceptable to WP, but of convenience for the reader. To suggest that every reader be prepared to go to a technical library (hardly " a little work") to follow up on sources is impractical. It also is common for editors to misread sources or abridge them without retaining the sense of the source, so citing a source that is not readily checked adds to WP's greatest problem: readers' assessment of credibility. Brews ohare (talk) 17:52, 30 July 2009 (UTC)


 * You are wrong. The cited guideline says to use a reliable source, without restricting it to be available online.  Further, anyone seriously studying the topic will be prepared to go to a library to research.  Finally, from a credibility standpoint, citing and using sources which are not standard to the academic community will do more harm to your credibility than citing a source that isn't immediately available. Pecos Joe (talk) 18:06, 30 July 2009 (UTC)

I have not disputed the text of the guidelines. I have made an observation. Please re-read my remarks.

My own experience with technical libraries, both in industry and in universities, is that the book you want is often not present (either lost, on loan, or never acquired (a situation more and more prevalent)) and must be ordered in from a remote source with a delivery time of several weeks. That applies even to common textbooks, never mind specialized sources. Working on line is the most common method of acquiring information both in universities and in industry.

Of course, the nonacademic, or the person not employed by the corporate host, is denied access to the technical library.

I don't know about sources that are "not standard". I'd guess that sources from reputable publishers are frequently a cut above WP, and citing them adds to WP's credibility far beyond the unsupported assertions from WP editors, including their assertions about the contents of these sources. Brews ohare (talk) 18:30, 30 July 2009 (UTC)


 * You have claimed that some source is not suitable to use (as the only source of a statement) in the article. I have pointed out that, in fact, there is a guideline to help decide its utility, which states the source is fine (because it meets all reliability criteria).  (I guessed that standard may cause a problem, so let's amend that to well-cited, which fits in with the guideline.)  As to access; then buy or borrow the book; if it is out of print, then we should try to find a different source, but I think it would be better to point the reader to useful and well-cited sources than to free ones. (Also, do you mean that a free source is always better than one only available in print?) Pecos Joe (talk) 21:13, 30 July 2009 (UTC)

The guideline may be a bit Utopian if it entails that a reader or an editor purchase a source to find out whether it has been accurately represented as supporting a statement in the WP article. (Typically these sources are about $100 apiece.) My view is that if the book is published by, say Cambridge University Press, and it is available in its relevant portions on-line at, say Google books, that is a useful source that could be cited as an aid to the reader, especially where some alternative sources are not available in this manner. Not every Cambridge University Press book is definitive, but I'd probably be more inclined to accept the editor's stated views if a Cambridge University Press title could be seen to support their viewpoint. That is even more the case if I happen to disagree with an editor who resorts only to inaccessible sources in a continuing debate. Brews ohare (talk) 21:26, 30 July 2009 (UTC)

299,792,458 m/s is a conversion factor - supplement
I am adding this ony as supplement to logicaly enclose my discussion. - - - The calibration of physical SI metre unit implements the number 299792458 as a space/time factor in relation with the speed of light. When performing measurements with SI calibrated devices, all measurements will, according to the precission of the measurement, converge to the implemented value 299792458. The question is, why to perform speed of light measurements using the space/time units independent of the speed of light physical phenomenon ? If the results of independent measurement are expressed in SI units, the speed of light quantity will allways converge to the implemented value 299792458 in SI units, according to the precission of the measurements. Because constant 299792458 is exact numerical value, the uncertainity of this value has changed to the physical uncertainity, that is, uncertainity what is the physical meaning of this value. We can obtain the physical meaning of this value only by calibration of the physical metre/second units. The more precisely we calibrate the units, the more precisely we now the physical meaning of the number 299792458. The measurement of the speed of light is therefore in the context of SI definitions made nonstandardly by calibration of the physical metre/second units, instead of measurement of the numerical quantity of property. But the number itself is not only relation of SI units - saying : "our space/time units are this way exactly related". If c is maximal speed of interaction - propagation of cause - the number itself is quantitative expression of this phenomenon, expressed in SI units. That means, the number itself has substantial significance when implementing apropriate physical devices. If the number itself has substantial significance, the valid calibration of the metre/second units in apropriate physical devices has substantial significance, because without that the number may generate errors, because the meaning of the number 299792458 (exact constant) is fully bound to the physical metre/second units realizations. In this sense the meaning of the number must be physicaly realised, and therefore measured by units calibration. In this sense the speed of light is allways measured when calibratinng the metre/second units. In this sense the meaning of the constant 299792458 is physicaly uncertain. Physical uncertainity is not related to the number itself, but it is related to the precission of the physical realizations of the metre/second units. Physical uncertainity of the constant 299792458 can be quantified in context of individual metre/second units realizations. Therefore the lowest possible uncertainity can be evaluated. Anybody can start to do this considering the ruler and the wristwatch. Softvision (talk) 13:49, 30 July 2009 (UTC)
 * I believe we are all on the same page here. Any measurement of a length automatically involves experimental uncertainties, among them the uncertainty in the accuracy of the realization of the time interval used. Brews ohare (talk) 15:19, 30 July 2009 (UTC)


 * Yes. According to my consideration, I have calculated just for myself physical uncertainity of the SI speed of light constant related to the uncertainity of metre ux = +/- 0.5mm and uncertainity of second ut = +/- 0,5s, as approximate human perception space/time uncertainities. According to these uncertainities, the SI speed of light constant can be interpreted as physicaly meaning the value somewhere between $$299792458 \frac{(1 - ux) \left[m\right]}{(1 + ut) \left[s\right]} ... 299792458 \frac{(1 + ux) \left[m\right]}{(1 - ut) \left[s\right]}$$. That means the physical uncertainity to be $$\frac{1}{2} \left( \frac{1+ux}{1-ut} - \frac{1-ux}{1+ut} \right) = \frac{ux+ut}{1-ut^2} = 0.6673$$. That means that in the context of approximate human perception space/time uncertainities the SI speed of light constant has physical meaning of $$299792458 \pm 0.6673 \left[\frac{m}{s}\right]$$, and that is beyond numerical misconception. The SI speed of light constant is no more a problem for me. However I think that the physical uncertainity of synthetic constants is reality and should be considered. Softvision (talk) 20:54, 30 July 2009 (UTC)

Tests of light speed
There is no need to state both postulates of relativity, so I changed this section to avoid that issue. That requires a title change for the subsection, which no longer mentions inertial frames. Some sources are added and the paragraphs rearranged for better flow. Brews ohare (talk) 16:53, 30 July 2009 (UTC)


 * Once again the article is changed to reflect Brews' personal and somewhat idiosyncratic opinion on the subject. This is just plain silly:


 * 'The independence of the speed of light from the motion of its source has two experimental confirmations.[19][20] One is the observation of X-rays from binary stars, one of which moves away from us and one towards us.[21] The difference in arrival times of light from the two stars is experimentally unobservable, setting an upper limit upon the dependence of c upon the movement of the source. The second is the measurement of the speed of γ-rays emitted by moving pions, where again the dependence upon the speed of the moving pion is unmeasurable, setting a limit.[22] These tests are "spot checks" as they cover only a limited range of source velocities and a limited region of the electromagnetic spectrum.


 * I have cited the reliable secondary source (Zhang) as confirming the independence of light speed on source velocity. This source includes a table giving 18 experiments confirming the independence of the speed of light from source motion.  It doe not mention 'spot checks'!


 * Please somebody revert this nonsense. It is completely pointless trying to patch it up, the original statement was perfectly correct.  Martin Hogbin (talk) 20:06, 30 July 2009 (UTC)

Hi Martin:
 * 1) What is idiosyncratic about this text? It states exactly what has been done, with sources. Evidently a check using γ-rays and a check using X-rays is not a check at any other wavelengths, is it? Isn't a random check at a few points in the spectrum a "spot check"?
 * 2) Your source still is present. I just added some sources that are available for all to see, whether they have access to Zhang or not.
 * 3) I have even included a different on-line source for which your favorite, Zhang, is a co-author.
 * 4) I have done an extensive search, including the last mentioned Zhang article, and there are two and only two measurements considered definitive, the two cited here directly.

Please respond to the points raised instead of reiterating your viewpoint without addressing the objections to it. Brews ohare (talk) 20:26, 30 July 2009 (UTC)


 * It was clear that the text misrepresented the statements in the cited source (article said two experiments, source said several), and since there was no reason to use those two as examples, I deleted that paragraph. I liked the order of the presentation (theory, then experiment), so I kept that.  I may have inadvertently missed adding in some relevant sources, though, so feel free to check it.  Also, the summary of the edit may have understated the magnitude of the change; I forgot to alter it before hitting save.  The text itself still seems mostly clear to me, though the prose could use some work. Pecos Joe (talk) 21:07, 30 July 2009 (UTC)

Joe: Your edit far exceeded your stated goal. Regarding the stated goal, the references provided (and many more general discussions of this work) refer only to the two experiments cited as being decisive. Of course there were earlier experiments that established weaker bounds, or that were later found to be misinterpreted. The source Zhang & Hsu has a long discussion. Brews ohare (talk) 21:12, 30 July 2009 (UTC).


 * In my cursory search, the sources expressed no comment that the two experiments were decisive, only factual statements like they found this bound on the speed. At any rate, the implication there were only two experiments must be removed from the article. Pecos Joe (talk) 21:28, 30 July 2009 (UTC)

Is your objection met if it is restated that there are "two experiments primarily cited in this connection"? See, for example,Zhang & Hsu; Baierlein; Will (§2.2);Field, p. 29; etc. etc. Brews ohare (talk) 21:37, 30 July 2009 (UTC)


 * I think I would like a better description of the results of the experiments. Such as "SomeScientist measured the SomeHypothesis to be accurate to within SomeError.[ref to SomeScientist]"  I'm not sure if others opinions on this type of statement vary, however. Pecos Joe (talk) 00:03, 31 July 2009 (UTC)

Incidentally, I note that you reverted to "This postulate, that the speed of light is the same in all inertial systems, together with the postulate that all inertial observers are equivalent, forms the basis of Einstein's theory of special relativity." which is an incorrect statement of the postulates. I had hoped to diffuse this debate by stating only Einstein's postulate about the behavior of light, which is all that is necessary. For example Banerjee states this postulate as: "The velocity of light in empty space is a constant, independent not only of the direction of propagation but also of the relative velocity between the source of light and the observer". What are you aiming at here?? Brews ohare (talk) 21:43, 30 July 2009 (UTC)

Brews ohare's conduct
The only way forward that I can see for this page now and the only way it will ever get back to being an FA is for Brews ohare to stop editing it to make points and to state his own opinions and private research on the subject. I have tried to get him to discuss things before making changes but his response is always the same, a bunch of rapid-fire and ill conceived edits to the article.

I suggest that we need to raise an RfC on his conduct to save this article from becoming Brews' personal soapbox. Martin Hogbin (talk) 20:28, 30 July 2009 (UTC)


 * Martin: I have invited you to discuss matters, but have yet to see any discussion. Only complaints that I don't agree with you. On the matter of the above subsection, just respond to the points raised instead of reiterating your viewpoint without addressing the objections to it. Brews ohare (talk) 20:34, 30 July 2009 (UTC)


 * I am happy to discuss things as much as you want, provided that while an item is under discussion you leave it alone.Martin Hogbin (talk) 21:03, 30 July 2009 (UTC)

Determining article content
I think a good way to determine which points should be in the article would be to follow the lead of published sources that treat the subject speed of light in detail. Are there any books or other lengthy publications which we could compare to determine whether a particular subject should be covered here, and to what extent? Also, would this method be acceptable to the editors of this article? Pecos Joe (talk) 21:17, 30 July 2009 (UTC) edited to clarify at Pecos Joe (talk) 21:26, 30 July 2009 (UTC)


 * On the subject currently under discussion I happen to have a copy of the book that it widely regarded as the definitive word on the subject. It is a review by Zhang of all the experimental evidence regarding the speed of light with clearly stated conclusions as to what this evidence means.  This is what I want to put into the article but Brews would rather have his own version based on his own searches of online sources. Martin Hogbin (talk) 21:22, 30 July 2009 (UTC)


 * The source Zhang & Hsu have a pretty long discussion and is available via Google books. This is the same Zhang that Martin likes, and is a more recent text. Martin is not inclined to put the evidence before us, and I can produce two or three general reviews in the technical literature that agree upon the two papers cited in the WP text I have written. So I am inclined to think that with the exception of the two papers cited there is not much more than cases of strictly historical interest in Martin's list and discussion. Brews ohare (talk) 21:33, 30 July 2009 (UTC)
 * See, besides Zhang & Hsu, these sources: Giulini; Baierlein; Will (§2.2);Field, p. 29; etc. etc. Brews ohare (talk) 22:04, 30 July 2009 (UTC)

I hoped this would have conversation unrelated to the dispute about Zhang, in hopes to address what I think is a more persistent problem - "Should we include stuff about this particular topic, and if so, in how much detail?" I hoped that by following the lead of other publications, we could arrive at well-supported answers for these questions. Maybe I am trying to solve a problem that doesn't really exist? Pecos Joe (talk) 00:09, 31 July 2009 (UTC)


 * Sorry, I misunderstood what you were asking. I do not know of any books that are just about the speed of light. Martin Hogbin (talk) 20:58, 31 July 2009 (UTC)

Incorrect statement of postulates of relativity (again)
"'This postulate, that the speed of light is the same in all inertial systems, together with the postulate that all inertial observers are equivalent, forms the basis of Einstein's theory of special relativity.'"

The above statements are an incorrect statement of the postulates, unlike a correct statement that provides a careful definition of "inertial frames". This situation cannot be left like this.

I had hoped to diffuse this debate by stating only Einstein's postulate about the behavior of light, which is all that is necessary. For example Banerjee states this postulate as: "The velocity of light in empty space is a constant, independent not only of the direction of propagation but also of the relative velocity between the source of light and the observer".

The statement of the postulates due to Einstein are:
 * The Principle of Relativity – If a system of coordinates K is chosen so that, in relation to it, physical laws hold good in their simplest form, the same laws also hold good in relation to any other system of coordinates K′ moving in uniform translation relatively to K.
 * The Principle of Invariant Light Speed – Every ray of light moves in the "stationary co-ordinate system" with the same velocity c, the velocity being independent of the condition whether this ray of light is emitted by a body at rest or in motion.

The first postulate defines inertial frames. It appears that this will have to go to RfC. Brews ohare (talk) 22:19, 30 July 2009 (UTC)


 * I made a slight change to the article that I think you will be happy with. Feel free to reword it.  The part you objected to was more a throw-away clause than anything else, and I am generally in favor of not requiring knowledge of special relativity to understand the bulk of this article. I think there is no need for rfc, just a little more careful editing. Pecos Joe (talk) 23:58, 30 July 2009 (UTC)


 * If only we would get some response from the RfC. The statement at the top of this section is based on the wording in a modern textbook on the subject (d'Inverno,'Introduction to Einstein's relativity').  This is a reliable secondary source.  Your personal  reinterpretation of Einstein's original postulates is neither desirable nor necessary.Martin Hogbin (talk) 09:15, 31 July 2009 (UTC)

Apparently Martin believes the statements of Einstein's postulates by Einstein himself at A Einstein and at §2 On the relativity of lengths and times are a “personal reinterpretation”. An indication of Martin's state of mind, perhaps. Brews ohare (talk) 14:10, 31 July 2009 (UTC)


 * As the wording of the relevant section is now back to a slightly mangled version of the original (read it and see) I will put it back how it was. Then we can discuss what the problems are and, if necessary, change the section.Martin Hogbin (talk) 09:15, 31 July 2009 (UTC)

It is an interesting form of cooperation to refer to Pecos Joe's attempted compromise as a "mangled version" and take that as an excuse to re-insert exactly the form causing the difficulties. These difficulties have been pointed out repeatedly, with documentation from published sources, including Einstein. Brews ohare (talk) 15:08, 31 July 2009 (UTC)

An alternative lead is proposed in the section located here. Brews ohare (talk) 15:18, 31 July 2009 (UTC)

Uniformity of the speed of light
The speed of light is postulated to be constant in all directions, regardless of motion. According to this postulate, the SI speed of light constant is postulated to be exact numerical value. According to this, noone can talk about speed of light as comparable numerical quantity, in context of speed of light measurement. Therefore the concept "measurement of the speed of light quantity" is obsolete. According to current postulates, comparable quantities are physical metre and second units. If you are talking about comparsion of the speed of light quantity in different directions, you are talking about physical comparsion of the physical metre/second units calibrated in different directions. If space is physicaly contracted in the direction of motion, and time is proportionaly dilated, calibration of the physical metre/second units in different directions, and their physical comparsion, must lead to conformity of physical metre/second units, confirming the uniformity of the speed of light in apropriate directions. It is hard to imagine, how to turn the time device carrying the physical second unit. Time units calibrations in different directions must be compared simultaneously. But comparing the time units simultaneously, we must compare the space units simultaneously, to perform valid comparsion. If difference in physical space units is proportional to the difference in physical time units, the speed of light is uniform in appropriate directions, as measured in SI units. (...) This is not my observation, or my own research. This is consequence of current physical postulates. I am submiting this to help the editors to solve the article formulation. Softvision (talk) 14:23, 31 July 2009 (UTC)
 * I'd like to think that an analysis of the situation would help, but what we have here is more a matter of editor squabbling. Brews ohare (talk) 14:43, 31 July 2009 (UTC)
 * We are talking about light versus a meter stick in that occupies the same reference frame as the observer. This should clear up the issues. Awickert (talk) 01:03, 1 August 2009 (UTC)

Proposed lead paragraph for debated section
Proposed lead-in: It is not possible to measure the one-way speed of light (for example from a source to a distant detector) without some convention as to how clocks at the source and detector should be synchronized. Einstein postulated both that the speed of light should be taken as constant in all cases, one-way and two-way, and that the speed of light was independent of the motion of its source. Brews ohare (talk) 14:34, 31 July 2009 (UTC) Rationale:

The goal is first to introduce the two topics of two-way light speed, and of motion of the source, and in follow-on discussion describe the experimental supporting data. This order is better than the reverse order in providing orientation as to the point of the entire subsection.

The above statement of Einstein's postulates concerning these matters is accurate, and does not go into undue detail. The footnotes provide opportunity for the reader to pursue matters further. The Banerjee source is on-line at Google books, and states the second postulate in a form very suitable for the purpose of this subsection. Brews ohare (talk) 14:34, 31 July 2009 (UTC)


 * Where would you propose to put the experimental evidence concerting the two-way speed of light. This really needs to come after the first sentence, otherwise the second sentence, which refers to the two-way speed, does not make much sense.


 * What exactly is your objection to the current order? Martin Hogbin (talk) 15:51, 31 July 2009 (UTC)

Experimental evidence concerting the two-way speed of light would appear in the following paragraph. I find the present lead two sentences fit each other.

The objections to the current order are (i) no basis is laid for introducing the selected experimental data from the (possibly) myriads of things that could be discussed. The revised order introduces the subject of Einstein's postulates about light and the experiment then fits as substantiation. (ii) the present order tends to force one into an unduly elaborate statement of the postulates, which so far has proved a stumbling block. The revised order allows introduction only of what Einstein said about the speed of light, which is all that is germane. Brews ohare (talk) 16:05, 31 July 2009 (UTC)


 * (i)The experimental facts (for measurements made in an inertial frame and in vacuo) are as follows: The two-way speed of light has be shown to be constant, always; that is to say regardless of the motion of the source or the observer or anything else. The one-way speed of light has be shown to be independent of the motion of the source.  There are no other important experimental results to report regarding the speed of light in an inertial frame. Therefore there is nothing selective about the experimental data introduced, it is that required to verify the statements made above.


 * (ii)I do not understand what you mean by elaborate statements of the postulates. My proposed statements are concise and from a modern well-respected text book. Einstein's second postulate is simply that the speed of light (in an inertial frame and in vacuo) is constant. That is to say, both the one-way and the two-way speed of light are independent of the velocity of the source or the (inertial) observer.  Einstein did indeed specifically mention that it is independent of the motion of the source but there is no need to say that if you just say it is constant (regardless of anything). Martin Hogbin (talk) 16:53, 31 July 2009 (UTC)


 * I, too, think the previous version is clearer. This argument over the proper wording of the second postulate need not continue - this is an article about the speed of light, not special relativity.  I see no reason the second postulate of special relativity needs to be in this article, so I removed it and left a less mangled statement in its place (and my previous edit did result in mangled prose).  Am I missing the reason the statement should be here? Pecos Joe (talk) 18:09, 31 July 2009 (UTC)


 * Pecos Joe, my hope was that we could discuss this subject without changing anything until we reached a consensus. There are only two postulates and it only takes a few word to describe the principle of relativity (according to a good reliable source) so in my opinion, it is worth including, but if the consensus is not to have it that is fine with me. Martin Hogbin (talk) 18:21, 31 July 2009 (UTC)


 * I also happen to think the article should not grow in size from its current version if it can be helped, so I think restricting the facts in the article to be only the ones necessary for understanding the speed of light would be helpful. Pecos Joe (talk) 18:31, 31 July 2009 (UTC)


 * Let us see what others think. Martin Hogbin (talk) 18:37, 31 July 2009 (UTC)


 * Joe, I have just noticed that in your latest edit you removed all mention of inertial frames. I assume that this was a mistake and have added it in, although I still prefer the original version. Martin Hogbin (talk) 10:13, 1 August 2009 (UTC)

Martin: (i)The experimental facts: There is no argument here about the experimental facts. The point is that stating the Einstein assumptions about light speed automatically raises the question in the reader as to whether they are true. Therefore this sentence order placing the Einstein assumptions first is natural, while the reverse order is less natural. Brews ohare (talk) 04:37, 1 August 2009 (UTC)


 * Einstein's statements are postulates, the equivalent of axioms in mathematics. His second postulate goes beyond what can be experimentally verified and thus remains a postulate, as Einstein originally understood it to be.


 * It therefore makes more sense to have all the experimental evidence together, followed (or maybe preceded) by Einstein's second postulate. Martin Hogbin (talk) 09:32, 1 August 2009 (UTC)


 * The portion of this article describing the experimental results is intended to follow the proposed lead, and therefore is not part of the present discussion.


 * The subject here is not the role of postulates in a physical theory. The subject is (i) a succinct and accurate statement of the assumptions regarding speed of light and (ii) whether the proposed introduction achieves this goal. It does do this, and does so more accurately, more understandably and more succinctly than the present WP text, which is longer due to an inaccurate statement of material extraneous to the presentation. Brews ohare (talk) 09:54, 1 August 2009 (UTC)

Martin: (ii) "I do not understand what you mean by elaborate statements of the postulates." I can see very well that you have no understanding of this point, and after a week of unending effort on my part, I am convinced that you never will. That does not mean that you are right, it means just that you can understand only your own view. My most recent attempt is this subsection. An earlier attempt is this subsection. Brews ohare (talk) 04:37, 1 August 2009 (UTC)
 * This is a very unhelpful reply. How are concise postulates taken from a modern, well-respected, reliable source elaborate?  You still have not explained.Martin Hogbin (talk) 09:32, 1 August 2009 (UTC)

Martin: Of course, the answer to this is this subsection. To summarize: the present text in the WP article appears to claim to state the two postulates of the special theory. However, (i) that is not the case, and (ii) it is not necessary to undertake some (inadequate) summary of the two postulates; a simple statement of the assumptions concerning speed of light is all that is needed.

The proposed lead above provides a succinct statement of these assumptions appropriate for this section, expressed in a similar fashion by, for example, Banerjee, as cited in the lead. See the proposed lead here. Brews ohare (talk) 09:46, 1 August 2009 (UTC)


 * Brews, for your above claims to have any credibility you need to explain in what way a statement of the second postulate of SR taken from d'Inverno is inadequate. All we need say is that the speed of light is constant in all inertial frames.  The word 'constant' is self-explanatory and does not need clarification; we no do need to list all the thinks a constant does not depend on such as motion of the source, frequency, direction, or the colour trousers that I am wearing.Martin Hogbin (talk)

The first postulate of relativity, as explained carefully above, and in greater detail in inertial frames establishes what an inertial frame is. The statement you've attributed to d'Inverno assumes this definition is understood from elsewhere (possibly the main text, instead of the boxed highlight you chose to quote). Clearly, omission of this definition is an emasculation of the postulates that do indeed include this definition. It is, therefore, incorrect to parade the emasculated version as the postulates of relativity. However, of greater interest, we do not need any of this for this discussion, as is evident form the proposed lead. Brews ohare (talk) 11:18, 1 August 2009 (UTC)


 * On the other hand I see nothing to commend the statement given in Banerjee, it contains unnecessary amplification of the word 'constant' but, much more seriously, fails to state that it refers to an inertial frame; this is absolutely essential. Martin Hogbin (talk)

An inertial frame does enter the postulates of relativity, and could be included by making a full statement of the postulates as originally proposed a long, long time ago and reverted by you. That is what Banerjee does. However, for the purposes of introducing the experimental work, that does not seem necessary, especially as the experimental work does not take place in an inertial frame, and makes little or no reference to it. Brews ohare (talk) 11:18, 1 August 2009 (UTC)


 * The term inertial frame is in general use in physics and I see no reason not to use it here, especially as we can link to the relevant article where it can be explained properly and in detail. To attempt to have a set of concise self-explaining postulates is not possible.


 * I do not understand your bizarre use of the term 'emasculated version' to refer to what are clearly stated as the postulates of relativity on a good reliable source. Martin Hogbin (talk) 11:41, 1 August 2009 (UTC)

Martin: You are not engaging here. The postulates themselves define what an inertial frame is. Your attribution to d'Inverno does not. Therefore, your attribution to d'Inverno is not equivalent to the postulates. OK? Brews ohare (talk) 11:51, 1 August 2009 (UTC)


 * Once again you are trying to promote your own idiosyncratic view over that of a modern and well-respected reliable source. D'Inverno clearly gives the two statements I have quoted as the postulates of relativity.  The fact that they do not include what you think they should include is irrelevant.  Martin Hogbin (talk) 12:01, 1 August 2009 (UTC)

No Martin. There is nothing idiosyncratic about a literal quotation from multiple sources including Einstein. What is a bit odd is that the simple logic that A includes B and C does not include B; therefore A and C are not the same, eludes you. Brews ohare (talk) 12:13, 1 August 2009 (UTC)


 * Your logic certainly does. Martin Hogbin (talk) 12:54, 1 August 2009 (UTC)

Let me elaborate: A=Einstein's Postulates; B=Definition of inertial frame; C =Your attribution to d'Inverno. A includes B, C does not include B so A is not C and C is not A. Brews ohare (talk) 15:35, 1 August 2009 (UTC)


 * I do not think that the term 'inertial frame' was in common use when Einstein wrote his paper. He therefore described what he meant in the language of the time.  Now we can just say 'inertial frame', with a link to the relevant article in the case of WP. Martin Hogbin (talk) 16:12, 1 August 2009 (UTC)

Which still does not address the point that introduction of "inertial frames" is just distracting in terms of introducing the experiments about two-way speed of light and independence from motion of the source. The experimental papers barely mention if they do at all, the role of "inertial frames", which is not a major point to be made in this WP subsection. Avoidance also allows deletion of the present incorrect WP text that suggests the idea of "inertial frames" is a matter outside the original postulates. It is not. Brews ohare (talk) 17:23, 1 August 2009 (UTC)


 * Your point about 'inertial frames' being inside or outside the postulates is meaningless. Originally there may have been a description of the term 'inertial frames' in one of the postulates, now we just have just 'inertial frames'.Martin Hogbin (talk) 18:58, 1 August 2009 (UTC)

Martin: It is not the issue of  'inertial frames' being inside or outside the postulates. The issues are (i) the WP phrasing of the postulates is incorrect and (ii) for purposes of framing the role of the experiments, we don't need to bring up inertial frames.

The purpose here is to introduce the experimental work, and sufficient understanding of the role of the experiments can be gained without inertial frames. If you insist that inertial frames are needed, then use the real statement of the two postulates instead of a bastardization that misrepresents them. Brews ohare (talk) 03:50, 2 August 2009 (UTC)


 * Regarding the experimental evidence, it might be a good idea to make clear that they all refer to measurements of the speed of light in an inertial frame. Martin Hogbin (talk) 18:58, 1 August 2009 (UTC)

If you insist that inertial frames are needed, then use the real statement of the two postulates instead of a bastardization. Brews ohare (talk) 03:50, 2 August 2009 (UTC)


 * Inertial frames are central to SR and relevant to all measurements and calculations of the speed of light. The sooner this point is raised in the article the better, that is one reason that I wanted the section title to include  'inertial frames'.  Whether some brief description of what an inertial frame is would be beneficial to the article is a matter for discussion.  I think not, as it is a somewhat philosophical subject that is not easily covered in a sentence or two.  If we decide to include a description we should take one from a good introductory book to SR such as ('Spacetime Physics' by Taylor and Wheeler).  Like free space and absolute zero, inertial frames are idealizations that can never be perfectly achieved. On balance I think that we are best with just a link to the inertial frames article. Martin Hogbin (talk) 09:03, 2 August 2009 (UTC)

The simplest way to handle this is to leave inertial frames out of this paragraph altogether. The more precise approach is to put in both postulates (correctly, not in some elliptic version), for example as done by Banerjee. I'd call this approach pedantic for the purpose of this section, and suggest the role of inertial frames is better discussed under other sections. 22:45, 2 August 2009 (UTC)

I am OK with the present version. Brews ohare (talk) 00:47, 3 August 2009 (UTC)

Bacon in history, moved from Comments
I happened to notice this from the comments section above, I moved it here so it would eventually be archived with the rest of this talk page: "Francis Bacon argued that the speed of light was not necessarily infinite..." No. That was Roger Bacon. However I hesitate to edit a page that is the subject of so much heated discussion about the scientific method, philosophy of science, and definitions of simple words, merely to correct a fact. —Preceding unsigned comment added by 67.9.148.203 (talk) 13:36, 29 July 2009 (UTC)


 * That seems plausible from a quick reading of their wikipedia pages, so I will delete it and note that it should be cited if someone wants it back in the article. Pecos Joe (talk) 18:13, 31 July 2009 (UTC)

Possibly this passage can be construed as saying the speed of light is not infinite? "The difference between light, sound and odour can be expressed in another way. Light travels far more quickly in air than do the other two; thus when somebody far away strikes a blow with a mallet or stick, we see the blow before we hear the sound it generates...Therefore when Aristotle claims that there is a difference between light and the other sensibles, this should be understood as a difference not between an instant and time, but between less time and more time..." Lindberg Brews ohare (talk) 10:33, 1 August 2009 (UTC) I've reinstated a remark here about Roger Bacon with this source. Brews ohare (talk) 11:00, 1 August 2009 (UTC)


 * Apparently Dicklyon does not accept the above verbatim quote from the source I tracked down and quoted, and so has mangled the Bacon discussion. I have no intention of embroiling myself over this minor point, although I'm annoyed that no attempt at discussion (never mind appreciation) of my efforts was made here, and instead a diatribe about "my editing style" presented at Lyon diatribe. Brews ohare (talk) 04:39, 2 August 2009 (UTC)


 * Perhaps, Brews, if you changed your editing style we would be able to have better quality discussion of the content. Martin Hogbin (talk) 08:26, 2 August 2009 (UTC)


 * Brews, the change I made did not involve a quote. I did read the cited source before I made it.  There's no doubt that Bacon argued for a finite speed of light, but to say as you did that he "argued that the speed of light in air was not infinite, on the basis that we see a distant blow before it is heard" makes no sense and does not begin to capture the argument as in the source, since what you said would be also consistent with infinite velocity.  Rather, he has to appeal to more subtle points by Alhazen and Aristotle to arrive at his conclusion; it's still not very logical, but we don't have to frame it as if it is.  OK?  As to the "diatribe", I think it's a fair characterization of the problem here.  Dicklyon (talk) 09:06, 2 August 2009 (UTC)

Dicklyon: The verbatim text indicates that Bacon thought the speed of light was finite in air, and that he based that view on analogy between the propagation of light and sound. One of the two examples he used is exactly the one stated in the text you reverted. Yes, it is illogical. No, it is not different from his statements. Yes your explanation for your reverts is annoying and unjustifiable, as well as non-factual. Brews ohare (talk) 14:45, 2 August 2009 (UTC)

Brews rewrite of the lead section
I notice that, without discussion, Brews has decided to rewrite and reorganise the lead section. There is a change of order and emphasis to make the fundamental spacetime aspect come first. This is an major change to the article for which it would have been a good idea to seek a consensus first.

As it happens, I do not disagree with this change of emphasis, but if we are going to do it it needs careful thought and needs to be written very carefully. This is not the case at present. Brews has also taken the opportunity to insert the chatty line, 'Said in different words, the exact value, c = 299 792 458 m/s, is an international convention concerning the metre, not a property of light', that he could not get in elsewhere, into the lead. This point is fully discussed in the relevant section. Martin Hogbin (talk) 10:30, 1 August 2009 (UTC)


 * This "chatty line" is sourced. This Bureau of Standards document explains the adoption of the definition in 1983 by the General Conference on Weights and Measures, which I'd say is an "international agreement", and goes on to say its value "is a conversion factor whose value is fixed and arbitrary", which I'd say is indicative that this number is not a property of light itself. Brews ohare (talk) 10:35, 1 August 2009 (UTC)


 * Yes, and it is correct, but it is not suitable for verbatim inclusion in an encyclopedia, especially in the lead section. As usual, you were trying to make a point about a subject we were discussing in more detail in the appropriate section.


 * You might like to look at some of the wording of your first paragraph as you are now an expert on the subject. Martin Hogbin (talk) 11:10, 1 August 2009 (UTC)

There is nothing "verbatim" about this, it's just an English sentence, and it fits here perfectly; as it says, it is a restatement of the preceding sentence in words that emphasize the meaning of the preceding sentence, which requires more emphasis. You seemingly prefer the meaning of the previous sentence to go unnoticed. Brews ohare (talk)


 * I would prefer to have one well though out sentence in the lead and to discuss the subject in more detail in the appropriate section. Martin Hogbin (talk) 11:30, 1 August 2009 (UTC)

I prefer to make the point that is being made where it is introduced, and where the reader will find it most appropriate. That takes two sentences, it seems... Brews ohare (talk) 11:32, 1 August 2009 (UTC)


 * We cannot take the view, as you have tried to do in the past, that everything must be fully explained in the lead section. The lead is intended to be a summary of the article as a whole with more detailed explanations being given in the main text.  We should try to make the point clearly, concisely, and in an appropriate style in the lead. Martin Hogbin (talk) 11:54, 1 August 2009 (UTC)

"We" are not taking that view. It is preposterous to say a one line restatement for emphasis is an exhaustive discussion. Brews ohare (talk) 12:00, 1 August 2009 (UTC)


 * Let me just repeat, clearly, concisely, and in an appropriate style. Martin Hogbin (talk) 12:04, 1 August 2009 (UTC)

The sentences in question are "In SI units, the magnitude of the speed of light in vacuum is exactly 299,792,458 metres per second (m/s)[6] because of the way the metre is defined. Said in different words, the exact value, c = 299 792 458 m/s, is an international convention concerning the metre, not a property of light.[7]". I don't like having two sentences, it's redundant and unhelpful. The way it's written makes it come across a deep and important and complicated insight, when it's really a very simple point. I suggest instead:

"In SI units, the magnitude of the speed of light in vacuum is exactly 299,792,458 metres per second (m/s), because of the way the metre is defined."

OK for me to make that change? --Steve (talk) 13:17, 1 August 2009 (UTC)


 * Any reason to ignore this source? I regret to say that I think the resistance to stating simply that the numerical value for c is arbitrary is because that idea grates on Martin's nerves, as he is not very anxious to relinquish the view that it is a "magical" number reflecting some property of the real universe. Whether or not that is Martin's view, it is a common misconception, and your proposed statement does not go far enough in countering this superstition. Brews ohare (talk) 17:13, 1 August 2009 (UTC)


 * Brews, I am sure that you know by now that I am well aware that c is not a magical number reflecting some property of the real universe and I have always agreed that we should make this clear. However the lead is intended to be a summary with things stated, as I have said before, clearly, concisely, and in an appropriate style.  Some people will understand straight way others can look at the body of the article where I hope this point is well made.  If you do not think that this is the case then let us work on making it clearer in the relevant section.  Please note that 'clearer' does not mean saying the same thing over and over again. Martin Hogbin (talk) 17:25, 1 August 2009 (UTC)


 * Well, how about humoring me and rephrasing this sentence in the introduction to include Jespersen and the idea of an "arbitrary value, not a property of light" using words you deem to express this fact clearly, concisely, and in an appropriate style? Brews ohare (talk) 17:35, 1 August 2009 (UTC)


 * [Reply to Steve]That would be OK with me. Normally I would have tried to rewrite it myself to avoid things like 'Said in different words' and repeating the same number in two consecutive sentences.  However, in this case my worry would be that Brews would show his disapproval by making another wave of rapid fire edits throughout the article.


 * From an English point of view, I would prefer, 'The metre is defined such that the the speed of light in vacuum is exactly 299,792,458 metres per second (m/s)', but this has the disadvantage of putting the more important speed of light at the end of the sentence. Martin Hogbin (talk) 13:42, 1 August 2009 (UTC)

If we are talking about the quantity c = 299 792 458 m/s, we are talking about the property of reality expressed in SI units. Because we know what is second and what is metre, according to the approximate human perception space/time uncertainities, the property c is well explained by this quantity. Both, property c and quantity 299 792 458 m/s, can be considered as real, because our senses are real and we have real perception of the metre and second units, limited by the approximate human space/time perception uncertainities. Theese uncertainities of the speed of light quantity perception can be minimized by expressing the speed of light in following context : Around Earth's equator 0.13 seconds. This is very well done at the begining of the atricle. In the era of intercontinental flights, this is the best expression of the speed of light quantity, in relation to the human perception. The numbers 299792458 and 9192631700 itself, are the international agreement, that interconnects the Caesium 133 etalon with historical metre/second units, not to invalidate them, and to extend their precission.

Quantity 299792458 m/s is the speed of light physical phenomenon in vacuum expressed in SI units. Nothing less, nothing more. We know what is speed, we know what is light, we know what is metre, we know what is second. The perception of these realities is very good and intuitive, even without exact technical SI definitions. It could be good to stress the "Around Earth's equator 0.13 seconds" - click time scale - context. Afterwards, the fact, that the current speed of light constant is synthetic constant (exact) should be mentioned, and the subject of the interconnection of the SI metre unit with the speed of light physical phenomenon should be explained, including the explanation of the meaning of the word "exact". Therefore the motive for this interconnection should be explained, probably before the metre interconnection subject details. In context of this, the consequences of this interconnection should be explained, including positive and negative consequences. I have mentioned some of the negative consequences in my previous submissions. I think this is the way how to put everything together. Softvision (talk) 14:34, 1 August 2009 (UTC)


 * How about:

"The speed of light in vacuum in SI units has an exact value: 299,792,458 metres per second (m/s). In fact, the metre is officially defined as the distance light travels in vacuum in exactly 1/299,792,458 seconds."


 * Is this clear enough? Jespersen and NIST can be references.


 * I hope we can get the point across without saying anything too extreme, for example that the number 299,792,458 is purely 100% arbitrary convention and nothing else. Remember, the "metre" has meaning and substance in it independent of its official definition...there's a historical definition that preceded the current one, there are billions of people who have a sense of roughly how long a metre is, etc. Given all that baggage, BIPM had to pick something like 299,792,458 m/s, they couldn't have picked 1 m/s or 10100 m/s, or it would have been wildly inconsistent with people's experience and with the previous definition. It would have been internally consistent but not externally consistent, so to speak. The average reader has prior knowledge of roughly how long a metre is and always has been and always should be, and would get confused if we tell them that BIPM is at liberty to define the speed of light as any number of metres per second that it chooses. BIPM is not really at liberty to tell people that the numerical value of c has changed to 1 m/s by a new international agreement, so starting tomorrow morning you are only 5 nanometers tall, please update your driver's license and have a nice day! :-) Anyway, I'm just saying, let's not get carried away in making too much of a simple point. I hope my suggested text above is suitable, or at least on the right track. :-) --Steve (talk) 19:25, 1 August 2009 (UTC)


 * My suggestion above (slightly amended), 'The length of the metre is defined such that the the speed of light in vacuum is exactly 299,792,458 metres per second (m/s)' says much the same thing without the repetition of the number. If read carefully it makes it clear that it is the the length metre that has been defined rather than the speed of light.  Of course there is more to say on the subject, and we should say it, but not in the lead. Martin Hogbin (talk) 09:13, 2 August 2009 (UTC)


 * Beautifull. :-)) Softvision (talk) 19:35, 1 August 2009 (UTC)


 * I think it is necessary to explain the subject of exactness. This is the key problem. If you write about the speed of light, you must put down the quantity of 299,792,458 m/s. If so, you must explain the meaning of the exactness of this quantity. Formulation "not to be measured again" is very confusing. I have explained how the speed of light can be measured. This have serious implications. There are many reasons to measure the speed of light. At minimum to obtain the refraction indexes. But this is not the point of the article introduction. Softvision (talk) 19:37, 1 August 2009 (UTC)


 * Supplement : Of course, the speed of light physical phenomenon is not just a matter of speed. Speed of light, or better maximal causal speed, is fundamental physical cause. Therefore the real physical effects of this fundamental cause should be considered (truthfully balanced) in the article content. This matter has at least the same importance as the matter of the speed. And finally, as the distinct fact not related to the effects of the maximal causal speed, the speed itself is not enough to make effects. This is physics. Softvision (talk) 22:09, 1 August 2009 (UTC)

Steve: You say I hope we can get the point across without saying anything too extreme, for example that the number 299,792,458 is purely 100% arbitrary convention and nothing else. I am more concerned that the point that comes across is that the number 299,792,458 is (from a strictly scientific stance) something more than 100% arbitrary convention. Of course, all conventions have their social context, and do not exist in limbo created from thin air. But the hazard here is not that; the hazard is that people will read that the speed of light is exactly 299,792,458 m/s and say "What? No experimental error? What, no need for future measurement? Are these guys nuts? " Maybe more significantly, they may say: "Look at that; one more example of how whacked out WP is - they even allow stuff like this to go unchallenged."

The way to cope with this reaction is to make clear at the outset that the reason an exact value is possible, and that no measurement is necessary is precisely because this number is not the physical speed of light, it's just a conversion factor. As Jespersen says: its value is a matter of convention, its value is entirely arbitrary. Once the reader understands that we are not dealing here with a physical property of light, the amazement over how it can be exact and independent of measurement goes away. Then the question arises: "Why this particular value?" and that discussion is the one you have presented above. Your discussion cannot be absorbed until the "conversion factor" idea has taken hold. Brews ohare (talk) 03:59, 2 August 2009 (UTC)

In this connection, a discussion by David Mermin about the "foot" and the "phoot" (the distance light travels in 1 ns) might be fun to read and lead to some good ideas on this subject. Brews ohare (talk) 04:24, 2 August 2009 (UTC)


 * The current SI definitions are the cause of the fact, that speed of light constant is exact (100%) conversion factor between metre and second unit. Now consider the case, when the metre unit is not calibrated using the speed of light physical phenomenon. Current physical knowledge confirms, that there is a causal speed limit. "In mathematics, the concept of a limit is used to describe the behavior of a function as its argument or input either "gets close" to some point...". In this case the function is the speed of light quantity and the input is measurement precission. In this sense, the speed of light quantity is distinct, regardless of units used, and can be incorrectly considered as a conversion factor between the space and time entities. The valid conception of this reality is, that the limit is relation in context of the speed of light physical phenomenon, not conversion. The distance in context of the maximal causal speed, can be correctly expressed as space distance or time distance. This is intuitively consistent with our spacetime perception. Manytimes the time distance is better expression than the space distance. But, when you express the distance as time, you are implicitly considering the speed of motion. You are converting space into time using the speed estimation. Conversion of the space into time is not bound to any speed. There is no way to compare space and time directly, and conversion of the space into time exactly is possible only in context of the speed of light physical phenomenon, as the fundamental limit, regardless of units used, because of assumption, that the speed of light quantity is distinct and universal, and the speed of light is fundamental causal limit. Softvision (talk) 10:40, 2 August 2009 (UTC)

Softvision, this is just an example of the very kind of thing that I was talking about. Nobody ever analyses the speed of sound in such philosophical depth, in connection with the system of units. This is all a product of the fact that by removing the medium of propagation of light from the textbooks, they have removed all rational physical basis for the speed of light. The end result is the endless philosophical arguments that we can see above, while everybody tries in vain to make sense out of nonsense. David Tombe (talk) 15:08, 1 August 2009 (UTC)


 * Brews, I put in my proposed edit above here. Is this clear enough? Do you think that someone could read these two sentences and not understand why the value is exact? If so, how do you think might they misinterpret it? --Steve (talk) 12:40, 2 August 2009 (UTC)


 * Brews, I re-read your text above, and it still seems like you think this is a deeper issue than it really is. Ponder this: The second is the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. Therefore, the caesium atom hyperfine radiation period is an exact measurement when expressed in seconds, exactly 1/9192631770 seconds. This period is a physical quantity, it's definitely not a "conversion factor", yet it still has an exact value. Therefore, a physical quantity can have an exact value in a given unit system, despite what you're saying above. Whether a quantity is exact or not in certain units has everything to do with how those certain units are defined and nothing to do with whether the quantity is a "conversion factor" or not. --Steve (talk) 12:58, 2 August 2009 (UTC)


 * As you might guess, I am happy with the wording that Abtract has put in (it is pretty much what we started with). Martin Hogbin (talk) 13:03, 2 August 2009 (UTC)


 * To be clear, I'm happy with either Abtract's one sentence or my two sentences, but not Brews's two sentences. --Steve (talk) 13:08, 2 August 2009 (UTC)

Two items are not addressed here by you all:

(i) I've explained a reader's astonished reaction to reading the speed is exact, no measurement necessary, no error bars, because the reader takes the numerical value to refer to the physical speed of light, which seems intuitively to be a measurable quantity. A number of editors here share this view that it is measurable, or at least have not fully abandoned it. If you read the sentence in the lead, it is technically correct, but will not avoid this reaction or dispel the misconception.

(ii) The treatment of c as a conversion factor is beyond dispute, and is the only viewpoint from which an exact value seems reasonable: its exact because we could define it to be any value whatsoever were it not for the widespread use of the foot rather than the phoot (to use David Mermin's example). Steve has said above that defining the second in terms of a particular atomic transition (under a variety of conditions like 0 K, zero gravity, zero linewidth etc. that cannot be realized) is exact, but it is not a conversion factor. Of course, that is not the case. Instead of 1/9192631770 s, this transition could equally be chosen to be 100 ps or 100 s. The physical entity atomic transition must be kept separate from the unit second. The fraction 1/9192631770 s is a conversion factor from the physical standard to units of seconds, just like converting the speed of light to  299,792,458 m/s could equally be 1 ft/ns.

The resistance to these facts, and the desire to hide behind a form of words that is technically correct, but not terribly obvious to the non-legal mind, indicates an unwillingness to accept the real status of 299,792,458 m/s and a continuing belief that 299,792,458 m/s is somehow a real, physical, constant of nature. It is a failure to separate 299,792,458 m/s from the physical entity speed of light. The resulting lack of clarity in the WP article will cause a reader reaction that the lead is paradoxical and the article is not authoritative. Brews ohare (talk) 13:44, 2 August 2009 (UTC)
 * I agree with Brews, but to be fair, one has to mention in this article the fact that some physicists take the (i.m.o) flawed POV that the speed of light is more than just an irrelevant conversion factor. I think this article explains the core of the dispute quite well. Perhaps one can devote a section in this article to discuss if the speed of light can be time dependent and the opposing views on why that's (im)possible. Count Iblis (talk) 14:37, 2 August 2009 (UTC)


 * In this particular point there is no disagreement about the physics. We all understand that the the metre is defined in such a way that the speed of light in m/s is constant and that the number 299,792,458 has no special significance in physics (because it is a dimensionful constant).


 * The argument is simply about how much we need say about this in the lead section of the article. The current statement (the metre is defined such that the speed of light in vacuum is exactly 299,792,458 metres per second) is concise, correct and not misleading as it makes clear that it is the way that the metre is defined that gives rise to this particular numerical value.


 * There is a whole section devoted to the speed of light being set by definition where the issue can be discussed fully. Martin Hogbin (talk) 15:48, 2 August 2009 (UTC)

Martin: Your comment doesn't address one key point: The WP single-sentence treatment will cause a reader reaction that the lead is paradoxical and the article is not authoritative. That is the natural reaction to reading that the speed of light in vacuum is exactly 299,792,458 metres per second, which is exactly the reason you italicized "exactly" - your stated purpose in doing that is to produce that reaction. Brews ohare (talk) 16:02, 2 August 2009 (UTC)


 * Brews, how do you feel about my two-sentence suggestion? Do you think most readers would read these two sentences and say "An exact quantity? Of course it can't be exact! That's a bunch of baloney!" Or would the two sentences together make it clear? If you're OK with those two sentences, then maybe we're on the road to something everyone can agree on. --Steve (talk) 16:09, 2 August 2009 (UTC)


 * The current single sentence makes the situation clear if you read it carefully. I do not think that saying what is essentially the same thing twice will help. Martin Hogbin (talk) 17:06, 2 August 2009 (UTC)

If this sentence is so clear, why the insistence on italics with: 16:48, 26 July 2009 Martin Hogbin (The italics are in accordance with WP guidelines to add emphasis.); and earlier 18:29, 21 July 2009 Martin Hogbin (Restore  emphasis);  and the first insertion of these italics according to: 16:32, 28 July 2009 Martin Hogbin (Surprising fact (to many people) needs emphasis.)?

It appears very clear that you, Martin, wish to startle the reader with this statement, rather than explain why this statement should be only a ho hum minor observation as Steve suggests it to be. Brews ohare (talk) 20:19, 2 August 2009 (UTC)


 * I cannot understand what you are making all the fuss about. Most people would not expect the numerical value of the speed of light to be exact but when it is explained that the metre is defined to make it exact then they can understand.  The italics are there to emphasize something unexpected but not mysterious or incomprehensible.  Martin Hogbin (talk) 21:38, 2 August 2009 (UTC)

The revised version is OK with me, so I guess I can graciously ignore assault and hair-splitting. Brews ohare (talk) 00:24, 3 August 2009 (UTC)

Underlying differences of view?
I'm still trying to focus on what is the real underlying cause for this ongoing argument. I had a look at the article entitled speed of sound for ideas as to how an introduction might look. The first sentence in the speed of sound article reveals the entire problem as regards the 'speed of light' article. The speed of sound article begins by explaining the rational physical basis behind sound waves. In modern texbooks, no such rational physical basis exists to explain light waves. This is of course a perfect recipe for endless argument. The ongoing arguments here must surely be about the differing points of view and opinions that exist amongst the different editors as regards trying to make sense out of something that does not make sense.

That's why we are seeing the sword fight straying far away from the original starting point. It has now entered into the terrain of the definition of the metre which is hardly in anyway connected to the actual speed of light. I think that in order to bring this dispute to a swift conclusion, all parties need to clearly state in simple terms what they would like to add to the introduction and why, and what they would like removed from the introduction and why. There also needs to be a more explicit statement of the individual points of view in order to avoid all the tip-toeing around the core issues. David Tombe (talk) 14:28, 1 August 2009 (UTC)


 * I have moved my submission up, where it belongs. Sorry. Some editing conflict caused the invalid placement. Softvision (talk) 19:20, 1 August 2009 (UTC)

Softvision, this is just an example of the very kind of thing that I was talking about. Nobody ever analyses the speed of sound in such philosophical depth, in connection with the system of units. This is all a product of the fact that by removing the medium of propagation of light from the textbooks, they have removed all rational physical basis for the speed of light. The end result is the endless philosophical arguments that we can see above, while everybody tries in vain to make sense out of nonsense. David Tombe (talk) 15:08, 1 August 2009 (UTC)


 * I have moved my submission up, where it belongs. Sorry. Softvision (talk) 19:20, 1 August 2009 (UTC)

OK, I'll move my reply up to join it. David Tombe (talk) 10:48, 2 August 2009 (UTC)


 * David, the speed of light is completely different from the speed of sound. You say, 'In modern texbooks, no such rational physical basis exists to explain light waves'.  That is why we do not want your 'rational' explanation of light waves here. Martin Hogbin (talk) 09:21, 2 August 2009 (UTC)

Martin, I'm merely trying to ascertain the underlying cause of the argument between yourself and Brews. Brews has suggested that it is to do with the fact that you see the speed of light as being something in the realms of a platonic constant like π. Would you agree with that suggestion? David Tombe (talk) 10:46, 2 August 2009 (UTC)


 * Brews knows this to be incorrect.Martin Hogbin (talk) 16:46, 2 August 2009 (UTC)

Martin, I've made a suggestion on Brew's talk page that he moves matters relating to the first line of the introduction (regarding 'c' being a physical constant) to further down. The idea of this is to maintain a historical chronology in the introduction and to separate the issues of dispute. David Tombe (talk) 18:29, 2 August 2009 (UTC)

Is exactness real?
Much is being made in the article (in the lead the word exactly is stated thus) and in this talk page of the exactness of the speed of light (SoL). It seems to me that this has been overdone and indeed may even be incorrect. The SoL is only exact because we have defined it in reference to the metre and the second. The second has been defined separately but the metre has not, it owes its definition to the relationship ("conversion factor") with the SoL and the second. This was not always so; the metre used to be defined, by the French Academy of Sciences, as the length between two marks on a platinum-iridium bar, which was designed to represent 1⁄10,000,000 of the distance from the equator to the north pole through Paris. It was only defined in relation to the SoL in 1983. I find it difficult to accept that this redefinition made the speed of light accurate when it clearly wasn't before 1983. Where did I go wrong? Abtract (talk) 17:09, 2 August 2009 (UTC)


 * This "exactness" is conditional on the validity of special relativity (and the photon being exactly massless). If you accept this, then the unit for length and time satisfy an exact relation. It actually does not make sense to have a separate unit for length and time, but we didn't know that before the advent of relativity, so we still use the old units and that makes the discussion a bit confusing.


 * You may have read about about "flatland" analogies to explain 4 dimensional space time. Suppose e.g. that on the surface of a neutron star some small almost flat intelligent creatures live. Due to the huge gravitational force they can't move perpendicular to the surface. But if there is a very small slope in the surface they can detect the effects of that, but it appears to them as some new physical quantity Z and they assign to that some physical dimension independent of length.


 * Then later, their version of Einstein appears and he explains that these physical effects are caused by small distances in a new direction perpendicular to the known two directions. But since a unit has already been defined for the physical quantity Z, what happens is that formulas like s^2 = c^2 Z^2 + X^2 + Y^2 appear where c is constant with the dimensions of (length/units of Z). Then there were old inacurate measurements of c, but knowing what is really going on at a fundamental level, it makes more sense to put c = 1 and accept the fact that space is 3 dimensional. Count Iblis (talk) 17:39, 2 August 2009 (UTC)


 * Definitely overdone by Brews, but not inaccurate. :-)


 * The kilogram is defined as the mass of the international prototype kilogram (IPK). Therefore the mass of the IPK is exactly 1 kg, with no error bars. There's nothing complicated here, and the lack of error bars says nothing profound about the IPK, it's just a statement about how the SI chooses to define its units. How exactly the SI chooses to define its units is something that's important in the BIPM boardroom, in a few precision physics labs, and is not terribly important anywhere else in the universe. :-)


 * When you define a unit in terms of a quantity, that quantity ends up being exact in that unit, with no error bars. That's all there is to it. It's not a deep or complicated or philosophically-interesting point. When Brews writes text describing this, he makes it come across as something profound. In fact it's a simple point, and it shouldn't sound profound or deep or complicated, because it's not. That's why I generally haven't liked Brews's descriptions on this, despite the fact that his descriptions are not technically incorrect. 1 metre is currently defined by BIPM as how far light travels in exactly 1/299792458 seconds, and therefore the speed of light is exactly 299792458 m/s. That's all there is to it, why say any more? :-) --Steve (talk) 17:52, 2 August 2009 (UTC)


 * Abtract, firstly, I agree with what Steve has just said.


 * Secondly, have you read the section on this subject in the article? If not it the perhaps you could read it and give us some feedback on whether it makes sense to you and explains the situation.  If not then it need to be improved so that it makes clear to everyone what Steve just said.  Martin Hogbin (talk) 18:23, 2 August 2009 (UTC)


 * Exactness of the SI speed of light constant is synthetic. Synthesis of this exactness is done by the SI definition of metre. SI speed of light constant is numericaly exact but physicaly uncertain, because if we want to know what is the real physical meaning of the quantity of the SI speed of light constant, we must calibrate the physical metre and second units. The calibration of units is measurement of the Caesium 133 etalon, and therefore it is dependent on the measurement precision.


 * Steve : Can you create the exact copy (in the sense of physical unit calibration) of the IPK etalon ? No. Is kilogram exact under any conditions ? No. The speed of light in free space can be "copied". In free space speed of light can be exactly (...) physicaly realized. Caesium 133 ethalon - not under any conditions. This is reality.


 * Exactness of the SI speed of light constant should be considered and explained. Softvision (talk) 19:12, 2 August 2009 (UTC)


 * Martin yes I have read the section and I think I understand it but, imho, it evades the underlying truth. The SoL may be fixed and exact but the metre is not; the only thing that is exact is the relationship between them ... measure one more accurately and the other automatically changes even though it will appear to be the same. I don't want to engage too much in OR here so I won't say anymore. With the changes I have made to the section and to the lead, it reads ok imho though I like Steve's ideas a lot, maybe he should rewrite the section? Abtract (talk) 19:32, 2 August 2009 (UTC)


 * Abtract, yes, your first sentence is quite right. There once was something in that section along the lines of what you added but it has been messed around with so much recently that it seems to have got lost.  It is better to say the the meter is more precisely delineated rather than measured. You cannot measure the metre you can just determine how long it is.  Martin Hogbin (talk) 21:30, 2 August 2009 (UTC)


 * Imagine, that you are sending information about your space/time units to the "flatland". How do you need to describe your units ? You must describe : That your etalon is caesium 133 and its appropriate hyperfine levels, for both space and time units. Than you must describe, that the time unit is derived from the period of the radiation, and that the space unit is derived from the wavelength of the radiation. Than you must describe, that the radiation must be measured in vacuum. Finally you will describe your factors of the time unit (integer 9192631770 periods) and space unit (two integers ratio - 9192631770/299792458 wavelengths). The "flatland" scientists know the speed of light according to their measurements, but they cannot use this quantity, to realize the metre unit, if they want to know something about our speed of light quantity, or they are not sure, that the speed of light quantities difference is negligible. If they now the period and wavelength of the corresponding caesium 133 radiation, according to their measurement precision, they just use the factors. To convert their units to metre and second, they just express their units as periods and wavelength of the caesium 133 and apply our time and space factors. What's wrong ? What if they think, that their measurement precision is less than ours ? They will know, that our technology can measure the corresponding caesium 133 radiation. They will know, that our measurement precision is at least on the level : To count the 9192631770 periods of radiation. Their lowest estimation of our time unit precision will be 1/9192631770sec. The estimation of our metre unit precision will be greater problem, because they will don't know, what is our method of the metre unit calibration. But they will surely assume, that we are using the best method available on our technological level. Finally they estimate our metre unit precision, that will be related to our time unit precision. However, they can only estimate the lowest precisions, in context of their scientific knowledge. What if they want to know better the quantity of the speed of light physical phenomenon ? From our information, they will know the integer quantity, which we use for the speed of light. They will know our metre and second expressed in their units, with the precision based on their lowest precision estimations. That means, they will know the speed of light quantity with this precision, that will be not greater than their precision. If you want to transfer your precision in this units model, you must know your best units precision, and send it as information, with consequence of the speed of light constant precision. If "flatland" scientists know the speed of light quantity as 100 000 000 m/s (expressed in our units), and their technology is not sufficient, than receiving our information, or physical units, they will realize, that their speed of light quantity is not valid, if they consider uncertainty of our quantities, physical or numeric, as sufficient. If "flatland" scientists are unable to physically realize our units based on our information, the only way to consign the speed of light quantity is to deliver physical space and time units and the ratio number (conversion factor ???).


 * This example shows, that SI units definitions are valid, and more, they are universal, in context of universality level of the caesium 133. They can be transferred, without transferring the physical units, and afterwards physically realized, on local technological level. The model of definition can be adapted according to the future technological progress. However, the precision of this model is virtual, dependent on the methods of physical realization. Nevertheless, this is relative disadvantage and the etalon is on the universal high technology level with implicit minimum precision. There are other, more serious disadvantages of this model of units. Disadvantages of this model of units are not a secondary matter.


 * Exactness of the SI speed of light quantity is virtual and dependent on the metre and second units calibration. Softvision (talk) 21:47, 2 August 2009 (UTC)

The Historical Section
Martin, you removed some edits of mine in the historical section on the grounds that Maxwell's molecular vortex theory has no role in modern physics.

It was of course the history section. Secondly, the way that you have left it is now totally inaccurate. You have got the contents of Maxwell's 1861 paper totally mixed up with the contents of his 1865 paper.

I assume that you overlooked the fact that these edits were in the history section and so I will restore them. David Tombe (talk) 22:37, 2 August 2009 (UTC)

My rewrite of the first paragraph of the lead.
The current lead has too many problems to be left as it is. Let me start with the first sentence:The term speed of light generally refers to a fundamental physical constant of spacetime that limits the rate of transfer of matter or information.

The first problem is that it would appear from this construction that the only effect of this fundamental constant is to limit the speed at which matter or information can travel. (I know that it is possible to derive SR from this kind of premise but I do not think that is particularly relevant here). The implications of c as a fundamental constant of spacetime are much wider that indicated by this statement. It is better to show this speed limit as a result of the spacetime in which we live

More importantly limits the rate of transfer of matter or information is ambiguous. The rate of transfer of information could refer to bytes per second for example. Better to say something along the lines of maximum speed at which the can travel. Martin Hogbin (talk) 15:58, 2 August 2009 (UTC)


 * I agree, it's a very hoaky definition/lead. It doesn't even mention light or EM radiation, and it oddly couples massless information carrying waves that travel at c in free space with matter, which can never quite do so.  How about something sourced instead?  How do typical good sources introduce this topic?  Dicklyon (talk) 16:05, 2 August 2009 (UTC)

The basic point of c is the spacetime connection, given prominence in the lead, which is fundamentally associated with information transfer, as that sets up the clocks. Perhaps the information transfer needs a separate sentence to get it more correctly stated. The EM connection is the next line, which hardly is burying this aspect. However, it is more or less accidental that EM radiation travels at c - many formulations of relativity simply posit a maximum speed of transfer, as Martin points out. (In fact, c does not actually refer to an extant physical speed of light in any realizable medium; it is fundamentally a theoretical construct.) This is a key point, and not at all one that is "not particularly relevant". Brews ohare (talk) 16:20, 2 August 2009 (UTC)


 * If you have a source that introduces the topic this way, why not tell us what it is? Dicklyon (talk) 16:23, 2 August 2009 (UTC)

I'm not sure what "this way" refers to. Every source agrees there is a limiting speed. Most take it as the "speed of light" but do not examine what that phrase actually means: light is medium for the transfer of information [http://books.google.com/books?id=CDoFxUJWcLgC&pg=PA112 the maximum speed of interaction ... is equal to the speed of light] By and large the view is that the Lorentz invariance of the Maxwell equations in classical vacuum carries over to all of physics, and implies a limiting speed of information transfer. The inverse procedure is more fundamental in that Maxwell's equations are just one "law of nature" and so are subsumed under "the laws of nature" in the postulates of relativity. In short, historically the Maxwell symmetry "discovers" the Lorentz transformation, but then the Lorentz invariance is posited, and so subsumes the Maxwell relations as a special case. Brews ohare (talk) 16:43, 2 August 2009 (UTC)


 * I've made a new opening sentence that I think contrasts that way with a better way. Basically, don't talk about "the term", but say what the topic is, more simply at first, still with spacetime prominent.  See if you like it.  Dicklyon (talk) 21:35, 2 August 2009 (UTC)

Looks OK to me. Brews ohare (talk) 21:45, 2 August 2009 (UTC)


 * I think the explicit mention of spacetime would be confusing for new readers. Is it really necessary to say that in the lead? Pecos Joe (talk) 00:33, 3 August 2009 (UTC)


 * Bring us an alternative proposal with a source that represents a good way to introduce the concept of speed of light, so we can consider it. Dicklyon (talk) 04:12, 3 August 2009 (UTC)


 * Actually, I was wondering which source the current text is based on. I couldn't find any that used 'spacetime' when initially introducing the concept of the speed of light, and so I think it should be removed from the lead section. Pecos Joe (talk) 21:15, 3 August 2009 (UTC)


 * Brews, if it looked OK, what compelled you to re-complexify it with alternate terminology, parentheticals, complex footnotes, etc.? Is that what an ideal opening sentence looks like to you?  Dicklyon (talk) 04:11, 3 August 2009 (UTC)

Sorry, I was trying to consolidate it with the same thing said in the second paragraph. You did it better. Brews ohare (talk) 04:55, 3 August 2009 (UTC)

Molecular vortices
We have a mini edit war going on the appropriateness of talking about Maxwell's "molecular vortices." I think this needs to be resolved in terms of what's in secondary sources. Providing an interpretation of, and assessment of the important of, of Maxwell's original primary writings, in the history section, is inappropriate per WP:NOR. If we find such interpretion and assessment in secondary sources that connect to the topic of the speed of light, then maybe it's OK. So, David, please cite a source if you want this material in. Dicklyon (talk) 23:12, 2 August 2009 (UTC)


 * The information that I added is almost exactly as it reads in the James Clerk-Maxwell chapter in the Dictionary of Scientific Biography (Charles Coulston Gillispie). That was the entry that first got me interested in Maxwell's 1861 paper. See also, particularly page 153.


 * I can't see what the problem is here because it is only a matter of history, and nobody is disputing the accuracy of the contents. I could understand your concern if I had written this in the introduction as representing the modern position on the speed of light.


 * My main reason for making that edit was to rectify the gross inaccuracies that were present in the original version. Somebody had got the contents of two of Maxwell's original papers all mixed up. David Tombe (talk) 23:19, 2 August 2009 (UTC)


 * There are hundreds (literally) of good secondary sources on this subject, they are called physics text books. None of them describe molecular vortices as forming a significant part of the history of this subject.  Maxwell and others did indeed study them but it turned to to be a dead end. Martin Hogbin (talk) 13:05, 3 August 2009 (UTC)

I don't think it is our place to determine whether molecular vortices played a significant part in the history. Obviously it was important at the time of Maxwell, and may have played an important role in how thought evolved. History of science is not just about what survives in today's textbooks of physics. It also is about the dynamics of the evolution of thought and the context in which it arose. For example, it seems the molecular vortices were the underlying intuition behind the curl operation. See Siegel and for vortices in fluid mechanics see Durand-Vidal et al.. Brews ohare (talk) 13:47, 3 August 2009 (UTC)


 * The sources we need here, for the history, are physics texts, but books on the history of physics. There are plenty; we should find one that talks about how Maxwell came up with the speed of light, and if it talk about vortices as part of that, then we can cite it for that.  Otherwise, let's don't mention it.  Dicklyon (talk) 14:47, 3 August 2009 (UTC)

This has all been a bit of a storm in a tea cup over what was just a correction edit in the history section. But the subject matter of that correction has clearly triggered off sensitivities in relation to the ongoing dispute regarding the modern day quagmire. Within the history section, there is nothing at all controversial about mentioning the point in history when Maxwell first discovered the connection between the speed of light and the electric and magnetic constants. And there is nothing wrong with giving a brief description as to how he did it. That brief description has been made, fully in line with primary, secondary, and tertiary sources. We have plenty of sources, so what exactly is the problem? Is anybody challenging the accuracy of the description of how Maxwell arrived at his conclusion?

My guess is that the controversy over this historical edit is based on the contrast between the logical approach that Maxwell used in the 19th century as compared to the total nonsense situation that we have reached in the 21st century. I opted to keep out of the dispute in the modern sections of the article because I have nothing useful to contribute to those sections. But while I was watching the dispute, I decided to correct some fundamental errors that I noticed in the historical section, and that seems to have ruffled some feathers. Martin has revealed his total disdain for Maxwell's approach, and so it seems that he would prefer to have Maxwell's approach reported inaccurately, rather than accurately.

If Maxwell's approach was the joke that Martin is suggesting that it was, then it should hardly be of any concern to Martin that it is described down in the history section. But I suspect that the real fear surrounding any mention of Maxwell's approach is that his old fashioned classical values put the modern nonsense to shame, and as such it must either be distorted or brushed out of view. David Tombe (talk) 17:18, 3 August 2009 (UTC)


 * I haven't looked at the sources, but the old version did look like it confused the contents of those two papers. The 'molecular vortices' part could be removed by combining the first two sentences in your revised paragraph, if that is the sticking point.  Is there any relevant information from Maxwell's 1864/5 paper (which you have removed all mention of) to put in this section? Pecos Joe (talk) 21:13, 3 August 2009 (UTC)

Pecos, I examined the situation very carefully before making the correction. The original paragraph was totally confused and it needed to be re-worded. The first thing that I saw wrong was the use of the term 'electromagnetic field' in conjunction with Maxwell's 1861 paper. That is clearly wrong terminology in the context. So what term should we use in its place? We cannot use the term 'aether' because Maxwell clearly states that he sees his sea of molecular vortices to be partly aether and partly ordinary matter. So why not use the exact term that Maxwell used. Maxwell's concept was a sea of molecular vortices, so why not call a spade a spade? We cannot have a situation in which we have to play down a historical term in a historical context in a historical section simply because one editor holds such a prejudice against Maxwell's work. Martin deleted my corrections twice, and both times the grounds stated were specious. The first time, he declared that molecular vortices have no place in modern physics. Nobody was claiming that they do. The second time, he declared that he was removing crackpot physics. It is not for him to decide that Maxwell's work is crackpot physics and then to proceed to eliminate an accurate description of it from the history section and replace it with a distorted version.

I'm glad that you agree that my correction was valid, and I can see that you are looking for a compromise. I also tried to consider a compromise, knowing that the very words 'molecular vortices' seem to severely aggravate Martin. But I simply can't think of what other term to use for Maxwell's sea of molecular vortices, and on reflection, I don't see why we should be bothering to pander to these kind of prejudices. David Tombe (talk) 21:30, 3 August 2009 (UTC)

The speed of light as a conversion factor
There is a line in the introduction that reads,

The speed of light can be viewed as simply a conversion factor between space and time in spacetime

Can anybody elaborate on the meaning of this sentence? Having already stated that the speed of light is a speed, and that it is the speed at which EM radiation moves, then how can the speed of light suddenly become a conversion factor? It is not sufficient to refer to the 1983 definition of the metre in terms of the speed of light. The statement in question is much more profound than anything to do with systems of units. It needs some kind of elaboration. By the same token, could we say that the speed of sound is a conversion factor between air and time?

As regards the 1983 definition of the metre in terms of the speed of light, it is somewhat tautologous. It neither tells us what a metre is nor what the speed of light is. I would blame this entire edit war on the person that introduced that definition of the metre in 1983. They were clearly too fascinated with their belief that the speed of light is a universal constant to be able to think rationally about the underlying purpose of what they were doing. The end result has been to sew additional confusion into an already badly confused scientific community.

On reading the entries above, perhaps this is what editor Abtract was also driving at. David Tombe (talk) 09:25, 3 August 2009 (UTC)


 * Something like that indeed.Abtract (talk) 10:53, 3 August 2009 (UTC)

David, you complain "It neither tells us what a metre is nor what the speed of light is." But that's the point, sort of. The meter and the speed of light are not independent of each other; the speed of light is a physical constant that anyone can measure, and the metre is an arbitrarily defined distance defined in terms of it; this definition ended the redundancy of having separate standards for time and distance, when the standard for the second and the physical constant c are enough. The definition recoginzes that there are just two degrees of freedom: you choice of time unit and you choice of conversion factor. What other way would tell you more about what a meter is, or what c is? For practical purposes, to make you comfortable, take the meter as the length of bar in a vault, and then you'll know pretty well what the measurements of c come out to. Dicklyon (talk) 14:52, 3 August 2009 (UTC)


 * Dicklyon: You are mistaken to say that the "speed of light is a physical constant that anyone can measure". In the SI units, the magnitude of the speed of light is a number that no-one can or needs to measure: it is defined. Consequently, this numerical value is not a physical constant, although the physical entity speed of light is thought to be a physical constant. (See Sydenham: "Thus, the speed of light, as a numerical value, is not a fundamental constant".) The use of a defined numerical value for c converts all length measurements to time-of-flight measurements. This approach is practical because we have an elaborate set of conventions about how to correct times-of-flight to refer to "vacuum", and in "vacuum" so far as we can tell, the physical entity speed of light is constant (although its numerical value is not an issue in this determination). Brews ohare (talk) 15:25, 3 August 2009 (UTC)

Dick, The 1983 definition is a total abomination. The man who concocted this definition chose a time fraction based on the most up to date speed of light as measured using classical values in line with the normally understood concept of speed. He then turned the whole thing upside down and defined the metre as the distance that light travels in this fraction of time. This had the effect of fixing the value of something whose value could yet be subjected to more refined meausurements. It is a fraud.

And why did he do it? In 1983, we were already languishing in the doldrums of a relativistic era in which the speed of light was supposed to be a universal constant. This man obviously wanted to impress the high priests of relativity. The relativistic priests probably indicated that they would like to see the speed of light being elevated to the sainthood, and this was his opportunity to oblige. The 1983 definition is tantamount to a standing proclamation of 'Amen!' to relativity. It firmly locked the door to any future variations in the speed of light using an elaborate conjuring trick.

I can detect that Brews knows that something needs explaining here. Although he claims to be comfortable enough with the 1983 definition, he can still see that the actual speed of light is something different than what this definition is implying. He can see that this definition is about something that has to be constant by definition, yet light will still nevertheless move in conjunction with a traditional style speed that might be subject to ongoing revisions and perfections of its value.

Maybe I've picked it up wrongly, but is Brews pointing to the fact that the 1983 definition allows a divergence between the classical speed of light and " Saint Speed of Light" as per this definition? And are his opponents trying to argue that no such dichotomy exists? Have I got the basis of the dispute correct? David Tombe (talk) 18:35, 3 August 2009 (UTC)

How to make progress
I have been involved in this page for only a few days but I am sorry to say I must now leave it ... there is a major problem - Brews ohare. I have looked back over the edit history on this page and the article and it is clear that he is exactly the type of editor that has caused me much anguish in the past. I suggest that those of you who know the position more intimately than I do refer him for comment with a view to a topic ban; that's the only way progress will be made imho. Good luck. Abtract (talk) 10:58, 3 August 2009 (UTC)


 * Abtract, I don't think that it's quite as simple as that. Brews is definitely a unique kind of editor. In the last two years he has been working his way through many theoretical physics topics that are not properly taught in the schools and universities. He is trying to make sense of these topics and then allow the readers to benefit from his own learning.


 * You must surely realize that the problem on this page is not so much Brews as it is the person who decided in 1983 to define the metre in terms of the speed of light. Brews has merely got himself tangled up in a sterile argument with other editors over how to interpret nonsense.


 * I had some serious arguments with Brews last year based on what in my view was his inability to see something that was relatively basic. At that time, he had virtually everybody else on his side. But Brews differed from the rest of them in that he continued to research the topic until he discovered Lagrangian mechanics. That broke the deadlock to a large degree and everybody learned alot form the ensuing discussions.


 * I see Brews as a net asset to the project. You do need to consider the role of the other editors in digging in to nonsense in the particular manner in which they were taught it. David Tombe (talk) 12:13, 3 August 2009 (UTC)


 * Abtract, I completely agree with you. Also, if you look at David Tombe's background and contributions, as well as his comments here, you will see that he supports many non-standard views about physics.  WP is meant to represent the current mainstream opinion on the subject.


 * The point I was just about to make is that the physics relating to this page was settled many years ago, what we need to do here is decide how to best present it, clearly, unambiguously, and in good English. Of course there is ongoing research into physics but there have been no relevant significant changes that have become part of established physics in the past few decades.  There is therefore no need to look at research papers on the subject and quote them here.  All we need to say is contained withing reliable secondary sources. Martin Hogbin (talk) 13:01, 3 August 2009 (UTC)

Some of the physics was settled years ago. However, as quantum gravity, doubly special relativity and cosmology show, there is ongoing activity, even about the basics. Brews ohare (talk) 13:39, 3 August 2009 (UTC) And the role of c as a defined standard of comparison for speed contrasted with its role in the metric of spacetime may be understood by some, but is hard to explain nonetheless. Brews ohare (talk) 13:42, 3 August 2009 (UTC)


 * The physics is mostly not the issue here, I think. The problem is how to collaborate on an encyclopedia article when one editor focuses his full-time attention on it, pushing a complexifying and often idiosyncratic view, cramming details on his favorite hangups into the lead, etc. Dicklyon (talk) 15:03, 3 August 2009 (UTC)

The issue is really communication. Dicklyon and Martin share the trait they they are always right about (i) their views, and (ii) about what is appropriate in a WP article vis-à-vis level of presentation and topics. These two editors simply know no response to resistance to their "authority" other than reversion and reiteration of their personal standards in the face of sources beyond dispute. Brews ohare (talk) 15:29, 3 August 2009 (UTC)


 * The issue is principally about your style of editing Brews. The way you do things makes it virtually impossible for other editors to work with you. Martin Hogbin (talk) 19:31, 3 August 2009 (UTC)


 * No Martin, I have no doubt whatsoever that it is you that is the problem here, and not Brews. You have revealed your prejudices by distorting a paragraph in the history section. You have undone a perfectly valid correction on the specious grounds of removing crackpot physics. You removed a correction that was fully sourced and accurate because you are opposed to Maxwell's point of view, and as such it appears that you are not even prepared to allow his point of view to be accurately recorded in the history section. It took me a while to work out what was going on in the argument between yourself and Brews. I have now got it figured out. You are the one that is trying to push a point of view. Brews was trying to point out that the matter was not as simple as you were suggesting. You clearly do hold a very strong point of view about the speed of light and it is clear that you do not want anything to appear in the article which might cause a passing reader to doubt that point of view. Even Maxwell's point of view in the history section is too much for you to cope with. As it stands now, it is inaccurate. It confuses the contents of two of Maxwell's papers. Maxwell does not talk about the electromagnetic field in his 1861 paper. He talks about a sea of molecular vortices. That's how we therefore have to report that aspect of history. We cannot avoid calling a spade a spade simply because you hold so much disdain for Maxwell's approach. I intend to correct that section once again. David Tombe (talk) 19:57, 3 August 2009 (UTC)


 * Brews, I've seen no problem with Martin or Dicklyon's edits, justifications, etc. The sources you believe to be beyond dispute are in fact sometimes legitimately disputable, as I've explained in great detail. Martin and Dicklyon and Abtract, I agree with you that Brews's editing is annoying and often makes articles worse. But making many poor edits in good faith with discussion is not against any wikipedia rule, for better or worse. The only thing I can think of is I guess we could argue that Brews refuses to acknowledge that he's been overruled by other editors and back off. But even here there's not a clear-cut case...for example, you'll notice Brews (to his credit) has stopped pushing his goals of using the notation c0 instead of c and "free space" instead of "vacuum", and he now seems OK with not making any of the strong statements about free space that we were disputing ("Maxwell's equations are exactly true in free space", etc). To summarize, if Brews is going to be forced administratively to change, it would be because administrators are willing to follow common sense rather than strict wikipedia guidelines, and I haven't seen that happen and don't expect to. You're welcome to try. --Steve (talk) 20:37, 3 August 2009 (UTC)


 * Well Steve, I am thrilled by your view of my efforts on WP, especially when you are well aware of my numerous laudatory and non-controversial contributions. The simple fact is, when controversy arises, certain readily identifiable editors simply will not accept that they have a misconception of the situation, even in the face of sources. Their responses to controversy are to reiterate their opinion, to revert changes with one-liner Edit summaries (usually with some put-down; like revert nonsensical edit), to delete uncomfortable sources, and not to evolve the argument. In short, the model is that of an American primary debate. My reaction to a sourced opposition is to relate to the source, not to ignore it, and to deal with points raised through discussion, not obfuscation. Brews ohare (talk) 16:04, 4 August 2009 (UTC)

Addition to superluminal light description.
The accepted equation for the dispersion of light traveling though a plasma medium is ω2 = ωp2 + c2k2, which leads to a group velocity less than c and a phase velocity greater than c. I thought someone might be interested in knowing this, because it not even alluded to in the section on superluminal light. —Preceding unsigned comment added by Odellus (talk • contribs) 22:55, 5 August 2009 (UTC)

Request for Comment
Should the notion that one can measure a defined property be deleted from speed of light? Presently stated in the article speed of light:"'According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. This has been verified to a high degree of accuracy by experiment.'" Comment: It is a defined property of free space that its electromagnetic properties εo and μo are frequency-independent. To claim that this defined lack of frequency dependence is experimentally verifiable is a logical error, as one cannot (in principle) measure a defined property: its value is, after all, what the definition says it is. All that experiment can do is confirm whether some realizable medium, like outer space say, has this property. Such confirmation serves to support the notion that "free space" is a useful model, but it cannot change the model. It can only support its utility. These statements in the article should be replaced with something like:"'According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. This behavior has been verified by experiment to a high degree of accuracy for media such as outer space or ultra-high vacuum, showing that in this respect these media are good approximations to free space.'" The present statement should be replaced. Brews ohare (talk) 20:41, 15 July 2009 (UTC)

Comments
 * 1) The issue is one of misuse of a definition by suggesting that a defined value is the same as a measured value. That incorrect view should be amended, as was attempted by LouScheffer and summarily reverted by Martin Hogbin. Brews ohare (talk) 20:44, 15 July 2009 (UTC)

My original statement was: 'According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. This has been verified to a high degree of accuracy by experiment'.

Two of the quoted sources are entitled: 'Severe limits on variations of the speed of light with frequency' and 'Probing the Speed of Light with Radio Waves at Extremely Low Frequencies'.

I am not sure how it could be any simpler. Theory claims that the speed of light is independent of frequency and experiment verifies this. Is there a physicist in the house????Martin Hogbin (talk) 23:34, 15 July 2009 (UTC)


 * Theory regarding properties of real media can be confirmed by measurement. Measurement cannot confirm the defined properties of a hypothetical entity, free space. Brews ohare (talk) 00:09, 16 July 2009 (UTC)


 * It may not be clear from Brews's description: The real dispute is over how the term "free space" is defined. If you accept that Brews's definition of free space is correct, then obviously what he's saying here is right. (Pure logic!) But if you reject Brews's definition, as I do, it's not so obvious. --Steve (talk) 00:21, 16 July 2009 (UTC)


 * Brew has repeatedly failed to find any accepted definition which states that in free space the speed of EM radiation is independent of frequency. It is, of course, true that classical EM theory predicts this and as far as I know, so do all accepted theories on the subject.  But, as you know, all theories are subject to experimental falsification which is what several experimenters have tried to do, but their results have been consistent with accepted theories on the subject and thus the experiments verify the theories.  This is all I want to say.  Martin Hogbin (talk) 00:35, 16 July 2009 (UTC)


 * The properties of vacuum are specified by NIST (reporting for CODATA) as μ0 ε0, and | c0 all of which have exact numerical values independent of frequency. Accordingly, "vacuum" has no dispersion. Inasmuch as the values are exact, they cannot be measured values. No theory exists that predicts these numbers (e.g. field theory or QCD) - they are postulated independent of any theory. As the article Free space points out, "vacuum" refers to no known real medium. As a defined, hypothetical medium, it may serve as a model for some real media (to be established within some error bounds), but it is not itself amenable to observation and measurement. It is defined to have the defined properties.


 * It's fine for Steve to reject this definition, but that is a personal matter, independent of WP or physics until some basis for this odd view is provided. As for Martin??? Brews ohare (talk) 00:42, 16 July 2009 (UTC)


 * This does not definethe speed of EM radiation to be the same at all frequencies in free space. Martin Hogbin (talk) 08:24, 16 July 2009 (UTC)

I hesitate to enter into "define". However, NIST does post | c0 as a unique, exact number for "vacuum". I take "vacuum" as free space. Thus "vacuum" exhibits no dispersion. Moreover | c0 is exact, so there is no doubt that it is not a measured value. Do you agree?


 * Your claim is that, 'It is a defined property of free space that its electromagnetic properties εo and μo are frequency-independent'. Show me the definition that says this. Martin Hogbin (talk) 17:57, 16 July 2009 (UTC)
 * I've pointed out the links μ0 ε0, and | c0. I believe you have agreed that the number c0 is not subject to experiment. As I understand you, you think somehow the meter is the point where experiment intrudes. I have pointed out that the meter can be measured only in real media. You do not grasp that it is not possible to realize a physical example of free space in which to make such a measurement. I have invited you to engage this statement, but you have not done so. Brews ohare (talk) 18:46, 16 July 2009 (UTC)
 * It is not a definition it is a theory which says this and, as such, it is subject to experimental verification. Martin Hogbin (talk) 17:57, 16 July 2009 (UTC)
 * What theory is this? It is not QED or QCD. Brews ohare (talk) 18:46, 16 July 2009 (UTC)
 * Our currently accepted theories on the subject say that EM radiation of all frequencies travels at the same speed, but you claim that it does so by definition. Please show me that definition. Martin Hogbin (talk) 20:39, 16 July 2009 (UTC)

In addition, NIST posts this remark: "The effect of this definition [of the meter] is to fix the speed of light in vacuum at exactly 299 792 458 m/s." The notion of a definition being able to fix the speed of light appears to rule out measurement, and a value independent of frequency rules out dispersion. Brews ohare (talk) 14:30, 16 July 2009 (UTC)


 * You're reading way to much into that comment. For example, NIST defines the kilogram as the mass of the IPK. The effect of that definition is to fix the mass of the IPK. But the IPK has a real, slightly-varying mass. Do you see what I'm saying? Let me be absolutely explicit: "If NIST defines a unit U as a quantity Q, then one cannot automatically infer that the NIST believes that Q is an absolute constant." Do you agree with the previous sentence? --Steve (talk) 16:04, 16 July 2009 (UTC)
 * I hesitate to broaden the discussion to other standards. My uninformed view of what you have said is simply that the standard kilogram is related to a material object, and therefore may change with the history of that object. I see no relevance to the specification of the speed of light, except possibly that in this case no such accidental history is involved. Brews ohare (talk) 16:22, 16 July 2009 (UTC)


 * As a practical matter, if vacuum is shown to have a measurable dispersion, I suspect NIST and other agencies would simply specify the wavelenght to be used.  I don't think they are saying no dispersion exists, or that the meter depends on the measuring wavelength - it's just that the dispersion, if any, is too small to matter to the current accuracy.   LouScheffer (talk) 16:14, 16 July 2009 (UTC)


 * I'd disagree with you Lou. The hypothetical here should be stated differently. Here is an attempt: if it proves that using a free space definition that exhibits no dispersion is cumbersome in practical use, the BIPM might conclude that a different standard would be advisable, and adopt a different definition that employs some "standard dispersion". The effect of this change simply would reflect the accumulation of experience that showed all material media were subject to a universal dispersion correction, and rather than be burdened with subtraction of this universal dispersion in all calculations, it would be simpler to include it in the baseline.
 * Notice that such a change in the definition of free space does not alter its role as a defined, unrealizable medium, not a real medium. Consequently, the possibility in principle of such shifts in definition does not impact the present argument in any way. Brews ohare (talk) 16:22, 16 July 2009 (UTC)
 * And, for Martin's benefit, notice that the impetus for such changes in definition stem from measurements of real media, not from "observations" of free space. Brews ohare (talk) 16:34, 16 July 2009 (UTC)

Brews, I don't know how I could make it any more clear, so I'll just repeat myself: "If NIST defines a unit U as a quantity Q, then one cannot automatically infer that the NIST believes that Q is an absolute constant." Do you agree with the previous sentence? Yes or no? I say yes, with the kilogram as proof. What about you? --Steve (talk) 17:01, 16 July 2009 (UTC)


 * You can make it more clear by saying what you are driving at instead of trying the cat and mouse game. As I have said (clearly): the kg case is not comparable. So to answer your question: the answer is "no" for the speed of light, and "yes" for the kg. Have to be careful about the double negatives here.


 * Let me explain why the kg case is not comparable: BIPM "fixes" an equality between the kg and an object; in the case of light BIPM fixes c to a numerical value. The object can change by historical accident, like accretion of some rust. The number cannot change except by formal international agreement. For example, see above dialog with LouScheffer on adoption of a new standard.


 * Relevance?  Brews ohare (talk) 17:13, 16 July 2009 (UTC)


 * Brews, I'll restate it to make the analogy even more clear. It seems that you believe the following:


 * "NIST/BIPM state that the IPK is exactly 1kg, but they obviously don't believe that the IPK has a constant mass. Meanwhile, NIST/BIPM state that "the distance that light travels in vacuum in 1/299792458 s" is exactly 1 meter, and this statement is 100% absolutely clear-cut evidence that NIST/BIPM believe that "the distance light travels in vacuum in 1/299792458 s" is a universal constant independent of light-frequency, direction, date, or anything else."


 * This is your point of view, correct? You really don't see anything wrong with this? --Steve (talk) 17:49, 16 July 2009 (UTC)


 * That is consistent with my view. You have not responded to my reason for separating these two cases. Brews ohare (talk) 18:10, 16 July 2009 (UTC)


 * I thought my restatement would help. BIPM "fixes" an equality between the kg and the IPK. BIPM "fixes" an equality between the meter and the distance light travels in vacuum in 1/299792458 s. These are analogous. The object might change by the accumulation of rust. The distance light travels in vacuum in 1/299792458 s might change by using a different light frequency. These are analogous. There is no new formal international agreement involved in either case. --Steve (talk) 20:33, 16 July 2009 (UTC)

Steve: There is no analogy here. The weight of the object may change. BIPM is aware of that. Work is ongoing to replace the specimen with a better standard. There is no specification of light frequency in the BIPM | c0; do you think that is simply an oversight? It is an exact number for all frequencies. It took a resolution to adopt this number: why do you think it might change without need for another such resolution? Brews ohare (talk) 21:01, 16 July 2009 (UTC)


 * Let me repeat my statement: "The distance light travels in vacuum in 1/299792458 s might change by using a different light frequency." You'll notice that I did not say that the number 299792458 will ever change. You seem to have a perpetual confusion: "The speed of light in vacuum as measured in meters per second is a constant" is an obviously-true statement (it's 299792458). "The speed of light in vacuum is a constant" is an entirely different statement. This is the statement under dispute, not the other one. This statement has nothing to do with the number 299792458.Steve (talk) 21:40, 16 July 2009 (UTC)
 * Well, yes, it's a different statement. So we might measure the speed using different system of units; is that the point? And which units might you choose? Units in which c=1?? Because "free space" is defined in the SI units, I don't think a switch of units is going to help because everything has to be translated anyway. Brews ohare (talk) 22:00, 16 July 2009 (UTC)


 * You also asked why isn't the frequency specified in the definition of the meter. This is explained in Jackson's Classical Electrodynamics: We're using a definition of the meter which is informed by the experimental fact that, as accurately as we can measure today, the speed of light is independent of frequency. If that experimental fact changed, the definition would presumably be made more specific. --Steve (talk) 21:40, 16 July 2009 (UTC)


 * Independent of frequency in what medium?? No, the corrections of "good practice" would change, to bring the measurements in that medium back to free space, that is all. Brews ohare (talk) 22:00, 16 July 2009 (UTC)

OK, where are we? BIPM "fixes" an equality between the kg and the mass of the IPK. BIPM "fixes" an equality between the meter and the distance light travels in vacuum in 1/299792458 s. BIPM is not implying that the mass of the IPK is fixed. BIPM, according to you, is implying with 100% certainty that the distance light travels in vacuum in 1/299792458 s is fixed. How do you know? First you said something obvious and irrelevant: The number 299792458 will not change. Yes, this is true and irrelevant. Next, you said that BIPM explicitly says that they're aware that the mass of the IPK is not fixed, and didn't give a similar warning about the meter. OK, fair enough. So to summarize, you believe that NIST/BIPM makes it 100% clear and unambiguous that the distance light travels in vacuum in 1/299792458 s is an absolute constant independent of frequency, date, direction, etc., and you believe this because of the absence of an explicit warning to the contrary. Is that really enough to base your entire argument on? Is that really the basis for 100% certainty? Or can we agree that NIST/BIPM don't quite make it 100% clear whether vacuum dispersion is possible in principle? --Steve (talk) 23:13, 16 July 2009 (UTC)
 * The whole issue boils down to "vacuum". I think "vacuum" is an unachievable medium with μ0 ε0, and | c0. Real media may approximate "vacuum", but that can be established only within error bars (notably absent in the specification of "vacuum"). Brews ohare (talk) 00:04, 17 July 2009 (UTC)

Although a clear definition of free space would aid in this discussion, I propose that the aforementioned change be made anyway. Even if free space is not in fact defined as possessing the property that all electromagnetic radiation travels at the same speed, the proposed change,

"'According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. This behavior has been verified by experiment to a high degree of accuracy for media such as outer space or ultra-high vacuum, showing that in this respect these media are good approximations to free space.'"

is still a more accurate, and less ambiguous statement.SemitoneSonata (talk) 15:10, 22 July 2009 (UTC)

Perhaps the following statement would be sufficiently accurate, without depending on too many other definitions. It should also be more accessible and easy to understand. "'According to the classical theory of electromagnetism, the speed of electromagnetic radiation in the absence of matter is the same for all frequencies. This has been experimentally confirmed to a high degree of accuracy in outer space and ultra-high vacuum.'" This dispenses with the task of defining free space, and implies only (rather obviously) that outer space and ultra-high vacuum are good approximations to the absence of matter. Fizyxnrd (talk) 13:56, 5 August 2009 (UTC)


 * I would regard that statement as a bit weak. It is generally accepted that the experiments in real vacua verify the theory that the the speed of light in free space is independent of frequency.   One editor here does not agree with that view.  A slightly stronger statement might be

"'According to the classical theory of electromagnetism, the speed of electromagnetic radiation in the absence of matter is the same for all frequencies. This has been confirmed to a high degree of accuracy by experiments in outer space and ultra-high vacuum.'"


 * What do you think of that? Martin Hogbin (talk) 14:25, 5 August 2009 (UTC)


 * Hi Fizyxnrd: Your statement is accurate. Of course, "absence of matter" is one version of "classical vacuum". I'd be happy with your statement if it used the link absence of matter to free space. You will recognize that the version by SemitoneSonata is more explicit, and is not subject to any misinterpretaion that "absence of matter" is the same as "outer space" or "ultrahigh vacuum". Brews ohare (talk) 14:45, 5 August 2009 (UTC)
 * Martin: Your thought process is the same as before: you state It is generally accepted that the experiments in real vacua verify the theory that the the speed of light in free space is independent of frequency. Not only is this not generally accepted, it is patent nonsense: no experiment in a real medium can "confirm' the behavior of a model medium like free space. Experiment can only confirm how well the model fits the real medium. So an accurate version of your statement is It is generally accepted that the experiments in real vacua verify that the the speed of light in in these media is independent of frequency to within experimental error. Brews ohare (talk) 14:45, 5 August 2009 (UTC)
 * I am unclear about where this sentence is supposed to show up. The article has changed over time. Maybe somewhere in the section Speed_of_light? Brews ohare (talk) 16:39, 5 August 2009 (UTC)


 * @Martin Hogbin: I would (personally) prefer the phrase "experimentally confirmed" to "confirmed... by experiment", although the difference is mostly semantic. The first informs the reader that experiment implies this conclusion.  The second states that we know a truth, and that we arrived at it by experiment.  I feel that the first statement is a little more modest and consistent with the care with which scientific statements should be made.
 * @Brews ohare: I think the statement

"'According to the classical theory of electromagnetism, the speed of electromagnetic radiation in the absence of matter is the same for all frequencies. This has been experimentally confirmed to a high degree of accuracy in outer space and ultra-high vacuum.'"
 * is satisfactory.
 * I agree that the sentence suggested by SemitoneSonata is technically quite accurate. However, it is a somewhat complicated statement to make a relatively simple point.  I think that the editing standard on NPOV:Undue may have something to contribute here.  Since the important concept here is that all light travels through the vacuum at the same speed (which is a fact well known to science, despite the possible exceptions that may exist) this is the statement that should be made.  Highly technical definitions obscure this point unnecessarily for all but a relatively small group of researchers and scientists who are interested in the more fundamental nature of light that may arise as a consequence of non-zero vacuum energies (or the non-existence of vacuum).  I suggest that this angle, while not precisely non-neutral, represents an argument of very narrow appeal, and to obscure the basic point with exacting precision is counter-productive.  If a further discussion of the (possibly) dispersive nature of the vacuum is desired, it should be included in another location. Fizyxnrd (talk) 21:18, 6 August 2009 (UTC)


 * You are, of course, quite right Fizyxnrd. Unfortunately, one editor has not grasped this simple fact yet. Martin Hogbin (talk) 09:10, 7 August 2009 (UTC)

The Edit War at The History Section
Martin, it seems that you are not prepared to discuss this issue. You have now reverted my correction three times, leaving a short bogus excuse on each occasion, and without coming to the talk page to explain yourself. You have taken it upon yourself to deem Maxwell's work in 1861 to be 'crackpot physics'. We have a history section which touches on the subject of the aether ideas of the 19th century. That of course necessarily touches upon the very important work that Maxwell did in 1861. He linked the speed of light to the electric and magnetic constants. He did this by modelling Faraday's lines of force in terms of a sea of molecular vortices.

As the section now stands subsequent to your reversions, it contains gross inaccuracies. How do you propose that we fix those inaccuracies? Is there any particular aspect of my corrections in particular which you find to be inaccurate? Or is it the use of the term 'sea of molecular vortices' that you object to? What name would you propose to use instead?

You cannot simply impose distortions into the history section just because it is an aspect of history which you clearly don't like to be reminded about. Your actions are totally contrary to wikipedia's rules. You are censoring history in order to favour your own prejudices about the nature of the speed of light.

I've noticed that you have just removed the link to the 1861 paper claiming it to be irrelevant in the context of the 1861 paper, yet you have left the link to the 1865 paper which is not being discussed. You are clearly messing this whole paragraph up deliberately. It is clear that you hold Maxwell's 1861 paper in total contempt, and you are simply trashing the paragraph that discusses this paper. I think that we are going to have to get administrator intervention here, because your actions are starting to appear like vandalism. David Tombe (talk) 21:53, 3 August 2009 (UTC)


 * Martin, I'm watching your ongoing edits. You're simply making the matter worse. You are now attempting to change the subject to Maxwell's 1865 paper instead. But if you do that you will have two questions to answer.


 * The first will be why you are trying to skip the chronology? Maxwell first used the results of Weber and Kohlrausch in 1861 when writing the 1861 paper. He looked those results up when he was down in London on a break from his home in Galloway. So why are you insisting on changing all this to his 1865 paper?


 * Secondly, the method that paralled the speed of sound is in his 1861 paper. It is not in his 1865 paper.


 * You have clearly realized that your reversions were wrong, and supported by nobody, and you are now trying to wriggle out of it without losing face. David Tombe (talk) 22:15, 3 August 2009 (UTC)

It was in his 1864 paper that Maxwell came up with his famous wave equation that predicted EM radiation and allowed its speed to be calculated. He used the results of Weber and Kohlrausch in that paper (see link) to calculate the speed.

The sentence about the speed of sound is just a general comment about the way in which values for the properties of the transmitting medium can be put into a wave equation to calculate the wave speed speed. It was not intended to relate directly to Maxwell's work. (unsigned comment by Martin Hogbin)


 * Martin, Maxwell's 1865 paper came after his 1861 paper. Maxwell first made the link between the speed of light and the electric and magnetic constants in his 1861 paper. In 1861 he returned from Scotland to London to look up the 1856 results of Weber and Kohlrausch regarding the ratio of the two kinds of units of charge. He applied those results to an equation in his 1861 paper that then linked this ratio to the magnetic permeability and the dielectric constant. He also equated these constants with the density and transverse elasticity of his sea of molecular vortices and then applied the result to Newton's equation for the speed of sound. It was then, in 1862, that he first saw the link between electricity, magnetism and the speed of light.


 * You have, for whatever reason decided that nobody is allowed to know about the 1861 paper. You have decided to jump ahead to the 1865 paper where some things have been carried forward from the 1861 paper, such as the 1856 Weber/Kohlrausch ratio. In doing so, you have got the contents of the two papers mixed up. And in doing so, you have distorted historical truth by trying to imply that it was in the 1865 paper that Maxwell first discovered the link between the speed of light and electromagnetism.


 * So why are you so keen to suppress all references to the 1861 paper? David Tombe (talk) 22:46, 3 August 2009 (UTC)


 * Here is Martin's re-written section, complete with the bad grammar,

''In 1864, Maxwell derived the electromagnetic wave equation. This allowed the speed of light to be calculated in much the same way as the speed of sound can be calculated in normal matter.'' ''Maxwell use this theory to calculate the speed of light from ratio of electrostatic to electromagnetic units by Weber and Kohlrausch. This was in good agreement with the measured value of the speed of light and supported the view that light was a form of electromagnetic radiation. ''


 * He deliberately skips the fact that the link between light and EM had already been made in Maxwell's 1861 paper. He then makes a false statement to the extent that the electromagnetic wave equation allowed the speed of light to be calculated in much the same way as the speed of sound is calculated in normal matter. He is getting totally confused with equation (132) in Maxwell's 1861 paper.


 * And all this seems to be because he doesn't want to have any mention of the sea of molecular vortices which was the entire basis of the important calculations in the 1861 paper. So he jumps ahead to the 1865 paper and attempts to re-write history. David Tombe (talk) 23:03, 3 August 2009 (UTC)

Hi David: I believe you are learning how Martin does things. It is a mistake to think it has something to do with the subject, with your edits, or with WP. It is all about Martin. Brews ohare (talk) 23:11, 3 August 2009 (UTC)


 * Martin, I've elaborated the paragraph in question in order to clearly segregate matters which belong in the 1861 paper from matters that belong in the 1864 paper. As a compromise, I've explicitly mentioned about Maxwell's own reservations about the molecular vortex model. I believe that it is the very mention of this model that is the source of your determined opposition. Nevertheless, it is part of history and it led Maxwell to a landmark result in relation to the speed of light. We cannot brush that aspect of history under the carpet just because you have decided that Maxwell's 1861 paper was crackpot physics. David Tombe (talk) 11:26, 4 August 2009 (UTC)


 * David, It is better how it is now but I would still ask the question of what molecular vortices have to do with the speed of light.  It is quite true that they formed part of Maxwell's initial thinking on the subject but he later dropped the idea and it most certainly forms no part of current physics.


 * In the road leading to our current understanding of the subject there were may dead ends and the molecular vortex model is just one of them and, in my opinion, there is reason why we should mention it, however, I am not going to start edit warring over the subject but I would like to make just one small change so that it is clear that molecular vortices for no longer relevant to the speed of light. Martin Hogbin (talk) 10:11, 5 August 2009 (UTC)

Martin, Well at least that has cleared up what the problem was all about. You do of course know that this is the history section and that nobody has been saying that molecular vortices form any part of modern electromagnetism. Nevertheless, the fact that Maxwell arrived at the link between electromagnetism and light using that method is very interesting, and it is certainly not crackpot physics. It's true that Maxwell didn't use the 1861 method in his 1864 paper but he did nevertheless retain a less explicit luminiferous medium with all the characteristics of his sea of molecular vortices. The characteristics of that luminiferous medium are clearly described in the 1864 paper. Maxwell still talks about an aethereal medium filling all of space (section 4 part I) with a rotatory nature such that the axes of rotation are along the magnetic lines of force (section 8 part I), and an elastic nature (section 15 of part I). He may not have mentioned a sea of molecular vortices explicitly but the concept is still implicit in the 1864 paper. Maxwell never gave up the vortex sea idea. He merely played it down because he was uncertain about the details. Maxwell's luminiferous medium remained a major part of physics right up until the arguments that followed in the wake of the Michelson-Morley experiment in 1887, after Maxwell's death.

So it is somewhat misleading to state so boldly, as you have done, that Maxwell did not use the sea of molecular vortices concept in his 1864 paper. He had already done the calculation in his 1861 paper. In 1864, he merely transferred the result into a wave equation using the well known 'Maxwell's equations' of which the most important ones can also be found throughout his 1861 paper.

I’m going to re-word it again along the lines that in his 1864 paper, Maxwell used a less explicit luminiferous medium and that he never mentioned the specific idea of molecular vortices. David Tombe (talk) 17:54, 5 August 2009 (UTC)


 * David: I may be all wet on this, but is it possible that the curl operation is an outgrowth of vortices? For example, Siegel, Barrow, The second coming of vortices and some other history texts might help with this. I know that vortices still show up in fluid mechanics, see Durand-Vidal and I would not be surprised to see a lot of mathematical similarities. (For example, mutual attraction of vortices in fluids, etc.) The point is this: the derivation of the wave equation involving the speed of light requires the curl equation, and so a case can be made that the molecular vortices, whatever their residual intuitive power today, served their purpose in getting the math right in Maxwell's time. Another point to be made here is that Maxwell was only one of many physicists of his time to talk about vortices: it wasn't the brain child of only one genius. And finally, I think Maxwell used the vortices because his colleagues were all about mechanical models, not trusting purely math manipulations (who can blame them). Today we are less inclined to mechanical intuition, but at that time to sell the notion of waves such a model was needed. The modern reaction to molecular vortices is that it is really weird, but if some social context is provided it may seem less so.
 * Oh, and one more thing, vortex theory still has its interest: see The fifth coming of vortices and Baker & Steinke. Brews ohare (talk) 22:16, 5 August 2009 (UTC)

Brews, I'll get back to you on this in more detail later. But just off the top of the head, yes, Maxwell wasn't the only one to be playing about with vortices at that time. The young John Bernoulli seems to have been the one who first came up with the idea of a sea of tiny vortices pressing against each other with centrifugal force. I do re-call reading a long time ago that it was Maxwell who actually coined the terms curl, div, and grad. And yes, curl and vorticity are intricately linked. If a curl is non-zero, then we have vorticity in the sytem. Magnetism has got plenty of non-zero curl. David Tombe (talk) 07:52, 6 August 2009 (UTC)


 * Brews, Maxwell's molecular vortices played a crucial role in the 1861 paper in his derivation of the two curl equations. The two curl equations are of course Ampère's circuital law at equation (9) and Faraday's law at equation (54). When it came to the wave mechanics and Newton's equation for the speed of sound in part III, the emphasis was more on linear elasticity, ratios, and analogies.
 * The 1865 paper produced a wave equation using the equations that had been derived in the 1861 paper using the molecular vortices, and of course the same numerical ratios led to the same conclusion as in the 1861 paper. What is interesting is the insistence by some that the molecular vortices are not necessary because they are not mentioned in the 1865 paper.
 * The natural thing to do would therefore be to examine the 1865 paper in order to see how Maxwell derived the two curl equations if molecular vortices were not being considered. You'll probably notice that Faraday's law does not appear in the 1865 paper. There will of course be the equation (D) which corresponds to the modern Lorentz force equation, and hence to Faraday's law. Maxwell used equation (D) to derive the EM wave equation, where modern textbooks use the clipped (partial time derivative) Faraday's law.
 * At any rate, I think that you all ought to carefully study how Maxwell derived Ampère's circuital law and the Lorentz force equation in his 1865 paper without using molecular vortices. That might be quite an eye opener. David Tombe (talk) 23:33, 7 August 2009 (UTC)

should c be considered nothing more than purely a human construct
This sentence is entirely misleading in this context, as it can be construed as meaning the exact number 299,792,458 m/s is considered by some to be a fundamental constant of nature, which absolutely no-one believes.

Duff himself doesn't think this way: Duff says "Asking whether c has varied over cosmic history (a question unfortunately appearing on the front page of the New York Times [7], in Physics World [8]4, in New Scientist [10, 11, 12], in Nature [3] and on CNN [13]) is like asking whether the number of liters to the gallon has varied." Unfortunately, however, in saying this, Duff confuses the units 299,792,458 m/s with the physical entity the speed of light. Naturally, 299,792,458 m/s is arbitrary (as all agree; for example Jespersen), but that has nothing to do with whether the physical entity speed of light has fundamental meaning. Of course, relativity shows that it does. So this is a bit of a straw man.

Duff also suggests, however, that actually determining whether the physical entity speed of light has, for example, changed over time, has to be based on dimensionless ratios involving c. In that regard he has a point, as suggested by Smolin, who points out the role of ratios. Brews ohare (talk) 20:12, 3 August 2009 (UTC)


 * Which sentence are you talking about? Your argument seems confused about this, and you can't claim a source is both right and wrong.  I'm also unclear on the apparent distinction between c and the speed of light.  At any rate, the speed of light is a fundamental constant in the SI system, and should be introduced as such. Pecos Joe (talk) 21:22, 3 August 2009 (UTC)

Pecos, I'm not sure, but I think that Brews is trying to say that the pre-1983 speed of light was a physical thing whereas the post 1983 speed of light is just a number by human definition. Maybe I've picked it up wrongly. David Tombe (talk) 22:08, 3 August 2009 (UTC)


 * The sentence occurs in Speed_of_light and is as follows: Whether the speed of light can be considered to be a fundamental constant of Nature, or whether only dimensionless combinations of constants such as α can be considered fundamental, while dimensionfull constants such as c should be considered nothing more than purely a human constructs fixing our unit system, is disputed among theorists.


 * It's a confusing subject. Here's how I see it; see if you agree:


 * In Nature there is light, and light travels. It travels at a constant rate, at least in free space. This rate can be called c, but we don't know what it's actual numerical value is (there is no theory that calculates it from other facts). However, we can compare this speed with other speeds, e.g. the speed of sound in some specified medium, and say it is x times faster than that speed of sound. We can take a yardstick and measure how long it takes light to traverse it, and measure it again and again. If the time of traversal does not change, we can say the speed of light hasn't changed enough to notice during our series of tests (or the yardstick and the speed of light conspired to keep the transit time the same.) Likewise, we can check on the frequency of an atomic transition on earth and compare it with the transition emitted by an atom in the outer universe to see if the frequency changed. Any change in c could affect that frequency. The comparison now is more complicated because some things could have happened to apparently change the frequency even if c did not change, but still we can say: Hey, as far as we can tell, c hasn't changed over the life of the universe. So there are lots of things we can say about c that have nothing to do with its actual value, and they all have to do with comparisons.


 * So the point is, there is a physical something we can call the speed of light, without involving ourselves in its numerical value. See Okum's contribution to Trialogue.


 * Then there is the question of its numerical value. That is arbitrary, because it involves comparison with a "standard" which is itself arbitrary. We could pick the length of some king's arm and ask how many of those does light cover in a second. That gives the speed as (king's arms)/second. Or, we can say the speed of light is 299,792,458 (king's arms/s) and then the king's arm is not his arm any more but the distance light travels in 1/299,792,458 s. This last method is preferable if the king doesn't want to travel a lot, to bring his arm around for people to compare to.


 * Duff confuses these two things: the physical speed of light and the numerical value of the speed of light. However, though this confusion is unfortunate, his point that real physical quantities have properties discovered by comparisons, by ratios, not by looking at the numerical values that depend upon the choice of units, makes sense. He carries the question a bit further by saying unless you can discover a property of c by looking at the behavior of a catalog of various dimensionless ratios, you actually cannot say anything experimentally verifiable about that property of c. He may go too far, however, because I don't think he admits a dimensionless ratio like c(f) / c(f1) tells you anything about the dispersion of c.


 * So ratios matter, units don't. Ratios determine physical properties, units facilitate commerce.


 * I don't think the WP sentences above convey the point at all well. Brews ohare (talk) 22:56, 3 August 2009 (UTC)


 * Yes, I agree that there can be some confusion about this. I think one should just expand that paragraph a bit and explain things better. You could say that as soon as one chooses units for space and time, the speed of light is also fixed, assuming the validity of Special Relativity and that electromagnetism is described by Maxwell's equations (so, the photon is massless). This choice of units is arbitrary, albeit that the natural choice would be to choose the same units for space and time.


 * In his other article on this subject, where the authors Duff, Okun and Veneziano disagree about this issue, Duff makes the point on page 23 that you could just as well define three different units for the three spatial directions in some coordinate system and define a unit "xylophone" for distances in the x-directions, a unit "yacht" for distances in the y-direction and a unit "zebra" for distances in the z-direction. Then we have a line element:


 * ds^2 = -c^2 dt^2 + c_x^2 dx^2 + c_y^2 dy^2 + c_z^2 dz^2


 * Of course, c is the speed of light and t is time, but x, y, and z are not lengths, they are xylophones, yachts, and zebras respectively and these are physically not the same as length :). c_x, c_y and c_z have dimensions of length/xylophone, length/yacht and length/zebra, respectively. Then, as silly as all this sounds (given the fact that we have rotational symmetry so there is no preferred x, y and z direction), it is no more silly than defining time to have a different dimension from space given Lorentz invariance. So, Duff argues that c is no more fundamental than the superfluous c_x, c_y or c_z that you could introduce. Count Iblis (talk) 23:47, 3 August 2009 (UTC)


 * I don't think there's much dispute that c is a "fundamental physical constant" in the colloquial sense. The dispute is whether or not c is so fundamental that shame on us for even bringing it up, rather than incorporating it implicitly into our thought process.


 * If we did measure xylophones and yachts, the conversion from xylophone to yacht would sure as heck be a fundamental physical constant (insofar as it's "fundamental" in importance, it's related to "physical" measurements, and it's "constant"). :-)


 * Maybe we can say "From the standpoint of special relativity, space and time are so closely intertwined (with c as the conversion between them) that some physicists say c isn't really such a "fundamental constant", but rather c is just an artifact of our naive inclination to measure space and time in different units. Indeed, in "natural units", space and time are measured in the same units, and light travels at a speed of 1, so here the speed of light is not taken as a new and meaningful physical constant."...or something like that... :-) --Steve (talk)

To say c dt is on the same level as dx is perfectly fine, and completely consistent with saying c is a conversion factor to make dt into an element of length. That is said in several of the cited sources. However, to say the revolution caused by this revelation is nothing to talk about would be silly. Brews ohare (talk) 05:27, 4 August 2009 (UTC)


 * Brews, on thinking more about it, I can see now that what has happened is that the physical significance that was implicit in the speed of light prior to 1983 was transferred into the definition of the metre in 1983. The speed of light subsequent to 1983 is therefore merely a tautology. All the phyiscal significance of it lies in the definition of the metre. The 1983 definition of the metre was mischievous because it was bound to cause the kind of confusion that you are currently trying to unravel. David Tombe (talk) 11:30, 4 August 2009 (UTC)


 * I recall that prior to 1983 the meter was defined in terms of the wavelength of a certain atomic transition. That so, the numerical value of c was based upon how many wavelengths were traveled in a second: c = λf. The argument was made that f was known very precisely, but λ wasn't. That made c imprecise. So they figured λ = c/f fixed the problem, because (whatever its value) c was, so far as theory and experiment were concerned, a constant of nature, and could be counted upon to always have the same value (whatever that might be). At that point, one could pick a value for c with impunity. The error in λ still is not the same as the error in f, however, because there are some experimental difficulties in assessing corrections to the measurement to bring it into accord with "vacuum", which is where c has its fundamental value. Brews ohare (talk) 14:21, 4 August 2009 (UTC)


 * If we use the symbols, they are allways "impune". Impunity of SI speed of light constant is in fact, that we don't need to perform very precise measurement of c, published and then refined, that will be used in devices. According to definitions, the number is constant. All we need to do now, is same as before - correctly calibrate our devices. Now we don't see uncertainity of our speed of light constant, because it is individual to every device and every single device calibration. Now, every device calibrated directly by caesium 133 etalon measurement, is performing the speed of light measurement, because it says - this is the second and this is the metre. Valid motivation for this model of units definitions is technical simplification and independence of technical realizations, based on the speed of light physics. That means, that the valid motivation is related to units, not to equations. Softvision (talk) 00:51, 5 August 2009 (UTC)

Quantity 299,792,458 m/s is not arbitrary. Arbitrary are the units of space and time. If the space and time units are correctly derived from the stable effects of reality, than the speed of light quantity is the real quantity expressed in these units, with uncertainity determined by the measurement precision and the units definition.

Current metre/second units are not arbitrary in the sense, that they are based on continuity of the historical units. Historical units are not arbitrary in the sense, that they are based on human perception of the space and time and on human intelligence. Human perception and human intelligence in this context certainly does not mean arbitrarity. Thanks to this nonarbitrarity this internet discussion about arbitrarity of the speed of light constant is possible. I am sure, that the result of this discussion will be nonarbitrary.

The meaning of the speed of light is essentially dependent on the concept of the time. According to current SI units definitions, the time unit is fundamental independent unit. It could be very dangerous, to undermine the concept of time. (already partially undermined)

"In mathematics and its applications, a coordinate system is a system for assigning an n-tuple of numbers or scalars to each point in an n-dimensional space." If the n-dimensional space is the real space-time, then you can assign as "numbers" in a coordinate system only real dimensions. Universe has three same dimensions and one specific dimension. That is the reason, why we have two types of units. There is no way to constitute the real coordinate system on one type of unit. Softvision (talk) 00:51, 5 August 2009 (UTC)


 * I think I can see what Brews is getting at. The value 299,792,458 m/s is totally arbitrary because it is a consequence of the choice of the definition of the metre in 1983. It doesn't tell us anything about the speed of light as such. It is merely a knock-on tautology from the definition of the metre. Any physical significance that is associated with the speed of light is now firmly devolved to the definition of the metre. The fraud is that prior to 1983 the constancy of the speed of light was a belief held by relativists, whereas since 1983, the constancy of the speed of light is now locked into arbitrary definition so that even if they did perchance perform experiments that showed up a variation in the speed of light (in the traditional sense), this would instantly be masked out by the modern definition of the metre. David Tombe (talk) 08:56, 5 August 2009 (UTC)


 * It is the same, like when you are speaking. The meaning of what you say is nonarbitrary. The language you use is arbitrary. In the sense, that you must use the language that is intelligible, the language you use is not arbitrary.


 * The SI units are the language, that we have not arbitrarily choosed, that the Nature will be speeking to us. The Nature can speek many languages, but the meaning is allways the same. This is the fundamental assumption of scientific research. When the Nature speeks about the speed of light in our SI units, the meaning is the same as in any valid space/time units. The better language we choose for the Nature to speek to us, the better is the meaning of what the Nature has spoken.


 * The language of current SI speed of light-metre definitions is a little bit confuzing. Nevertheless, the current SI definitions are valid, even though problematic. If we assume, that the caesium 133 etalon is valid and stable on the level of precision expected, the physical meaning of the the metre, time and speed of light units are according to the measurement precision converging to the real meaning of the SI definitions, given by the caesium 133 etalon. An absolute precision is not possible, and therefore the exactness of the SI speed of light quantity is virtual, not the meaning of the quantity in SI units. Current technological level is sufficient, to consider the question of precision as nonproblematic. There are other, more serious disadvandages of this model of units, especially when used in relation with the speed of light, under scientific level, that did not solved important questions regarding the speed of light physical phenomenon and causality. Validity of devices calibration should be verified. Softvision (talk) 10:51, 5 August 2009 (UTC)


 * David: Much of what you say I agree with. However, experiments to measure the variation of light with time and with frequency are ongoing to test theories in cosmology and in quantum gravity where such variations are predicted. Likewise measurements are ongoing to detect field-amplitude dependence to test QCD. They are not masked out by the definition of the metre, because these experiments are measurements of ratios, for example, c (tnow) / c(tthen) or c(f1) / c (f2) in which the functional variation of c can be detected because the ratio is independent of the definition of the metre in terms of the speed of light: they are basically time-of-flight ratios. Brews ohare (talk) 14:27, 5 August 2009 (UTC)

Brews, yes I see your point about the ratios. But getting back to the main point, the number 299,792,458 was arbitrarily chosen because it related to the most accurate measured value of the speed of light in 1983. The effect of that choice was to keep the metre close in size to the already existing metre. They could however have chosen any number. They could have decided that a metre is the distance that light travels in 1/ 400,000,000 of a second. That would have yielded a different number for the speed of light.

So, just as you said, the speed of light, post 1983, is a consequence of the definition of the metre and it has got no physical significance in its own right. It is just a tautology. It is just a number, and that number doesn't even have any of the mysterious qualities that are possessed by e or π. The physical significance of the traditional speed of light, post 1983, has been delegated to the definition of the metre.

Is that the point that you have been trying to make in the introduction? And have others been trying to say that the speed of light is about the speed of light? It would seem to me that since 1983, the speed of light is no longer about the speed of light. I can't even quite see it as a conversion factor, although I can see why others might. I see it as a tautology, plain and simple. We are in a new era in which Saint 'Speed of Light' is not only a constant by belief in relativity, but also as an unalterable fact embalmed in our international system of units. David Tombe (talk) 18:18, 5 August 2009 (UTC)


 * Yes - "The physical significance of the traditional speed of light, post 1983, has been delegated to the definition of the metre." When you ask the Nature "What is the quantity of the speed of light in vacuum in SI units ?", the answer will be allways 299,792,458 m/s. More exactly - you don't need to ask the Nature, you just ask your definitions. However, when you want to implement the speed of light based device, you must calibrate it using the real speed of light effect. Than you are asking the Nature what is the real speed of light quantity.


 * Generally, the physical concepts are the units of the language, that we have nonarbitrarily choosed, that the Nature will be speeking to us. What is constant in one "language" can be variable in other "language". What is flat in one "language", can be round in other "language". Nature is not simple, nature is complex. If the words of what the Nature is saying to us are contradictious, it is the matter of invalid language we have choosed, not the contradiction of the Nature. The fundamental scientific requirement is to understand, what is constant and what is variable, what is flat and what is round - curved. Therefore selection of the "language" cannot be arbitrary. Softvision (talk) 12:53, 6 August 2009 (UTC)


 * Attempts are made to define c as a mere conversion factor or simply a human construct or merely the unit "1" because it is meaningless to square meters per second with the result being meters2/second2.Lestrade (talk) 13:07, 6 August 2009 (UTC)Lestrade


 * Speed of light, or better maximal causal speed, is the physical phenomenon, that according to its nature, or better - essence, can be considered as one of the fundamental causes. That means, the speed of light phenomenon should be considered as fundamental cause, event propagation, not just as an effect or just as quantity.


 * Energetical balance of valid experimental data confirms rapid increase of causal energy needed to create kinetic effects near the speed of light. This is most important evidence of existence and character of the causal speed limit. That means, the causal speed limit is not only the separate limit of the "light" propagation, but it has deep fundamental physical context. The constantness of the speed of light phenomenon should be considered in this context.


 * The speed of light phenomenon is not human construct, it is essential fundamental physical cause that should be considered completly, in causal context of event propagation, not just like an effect, speed or quantity. The quantity 299,792,458 m/s is expression of the speed of this physical phenomenon in vacuum in SI units. SI units are not arbitrary in the sense, that they are the language that we have nonarbitrarily choosed, that the Nature will be speeking to us. Selection of the "language" of units cannot be arbitrary. The "language" of units can affect the interpretation of measurements. Softvision (talk) 16:47, 6 August 2009 (UTC)


 * Softvision, It's all pretty simple. Prior to 1983, the speed of light referred to the speed of light. It was a quantity that had physical significance. Relativists believed it to be a universal constant.
 * Since 1983, the speed of light is simply a human construct. It is a universal constant by definition. It has got nothing to do with the pre-1983 understanding of the speed of light. It is a tautology. David Tombe (talk) 23:39, 7 August 2009 (UTC)


 * And that statement is an example of why this talk page and article are such a mess. It's absurd to say that "It has got nothing to do with the pre-1983 understanding of the speed of light" just because we recognized it as such a good constant that we take c, instead of the length of a bar in a vault, as the basis for length standards.  The speed of light still means what it always did, but its expression in meters per second has subtly shifted meaning.  Dicklyon (talk) 23:48, 7 August 2009 (UTC)

Dicklyon is mistaken in his remark above. It is true that the modern value is the value that it is because of the existence of the metre in its earlier forms. Convenience dictates that the new measure be close to the old one to avoid conflict where possible, and for purposes where great accuracy is not an issue, the old and new metres are the same. However, from a logical standpoint there is a big difference. Today c no longer means what it did. It is now, from a logical standpoint, an entirely arbitrary number, and is divorced completely from measurement. That means that the "meaning" of the speed of light is no longer what it used to be. Dicklyon has not absorbed the WP article nor the references in it. In particular, Dicklyon should read the article cited in the WP article by Jespersen. Brews ohare (talk) 03:12, 8 August 2009 (UTC)


 * I've read the article and Jespersen; they seem OK. But I don't see where they back you up.  I'm talking about "the speed of light"; perhaps you and David are talking about the numerical value that represents it, or the logic of international standards.  I don't think the changes in the latter make such a fundamental difference in the meaning of the former.  Jesperson says they "removed a constant of nature by turning c into a conversion factor whose value is fixed and arbitrary."  OK, we all agree there.  But c is still how fast light travels; we just have different standards for what to define in terms of what; you can still measure how light slows down in different media, for example; and if we ever see light of different wavelengths moving at different speeds through whatever you want to call empty space, then we'll probably just revise the standards definitions again, not argue that it's impossible.  I know you like to belabor this point, but to say that "It has got nothing to do with the pre-1983 understanding of the speed of light" is a bit extreme (shall I say idiosyncratic); are there sources that agree with this view?  Dicklyon (talk) 05:33, 8 August 2009 (UTC)


 * The phrase "the speed of light" may refer to the "numerical value" or to the physical concept and it makes statements unclear when no distinction is drawn. Thus, "the speed of light still means what it always did" is ambiguous. The "it" in "It has got nothing to do with the pre-1983 understanding of the speed of light" refers to the numerical value and is a correct statement, as pre-1983 the numerical value was still found by experiment, while post-1983 it was a defined (arbitrary) value. Maybe a little more precision in language would fix all this debate?? Brews ohare (talk) 19:35, 8 August 2009 (UTC)


 * Can anyone tell me what this line means, 'The speed of light can be viewed as simply a conversion factor between space and time in spacetime', and why we need it in the lead. For some reason, Brews now wants to include the term 'conversion factor' throughout the article. In the next line of the lead we have, '...the numerical value of this conversion factor is fixed...'.  We already have, '...In Einstein's theory of special relativity space and time are viewed as a four dimensional unification of space and time, known as spacetime,[24] with c playing the fundamental role of a conversion factor between the units of space and time'.


 * The sentence Martin cannot understand: The speed of light can be viewed as simply a conversion factor between space and time in spacetime.  is made in several sources that have appeared and disappeared from this article, (e.g. ABC's of Relativity) and has only the simple meaning that the metric in spacetime is ds2 = dx2 − (c dt)2 so c "converts" time to units of length in the metric. Not too wild a statement, I guess?? It should be in the intro somewhere because it is a fundamentally important property of the physical entity "speed of light", not a minor point. Brews ohare (talk) 14:55, 8 August 2009 (UTC)


 * What we do not need is an individual editor's personal philosophy scattered throughout the article. The arbitrary nature of the numerical value of c is already summed up in this statement,  'As the speed of light is a dimensionful constant its numerical value is dependent on the system of units used. In the SI system, the metre is defined as the distance light travels in vacuum in 1⁄299,792,458 of a second. The effect of this definition is to fix the speed of light in vacuum at exactly 299 792 458 m/s.'  Anyone who wants to delve into the philospohy of the subject is better to follow the dimensionful constant link where there is a fuller and better though out discussion of the subject. Martin Hogbin (talk) 09:01, 8 August 2009 (UTC)


 * This reference to "an individual editor" might refer to me, but I am not the creator of the third paragraph in the intro. Brews ohare (talk) 15:08, 8 August 2009 (UTC)


 * I believe I wrote that bit, as an attempt to simplify a rather more drawn out explanation by Brews into something that nobody would object to. Sounds like I didn't succeed.  I'm not strongly attached to "simply"; really it's the explanation that needs to be simple.  There are tons of sources about the conversion factor concept, here.  I'm not sure what to think of the dimensionful constant link, as there's nothing sourced there; is it just someone's opinion or OR? Dicklyon (talk) 22:20, 8 August 2009 (UTC)


 * OK, blast from the past. The dimensionful constant section was created by the crank User:Rbj back in 2005-2006; he was my first arch-nemesis on WP, for his wedged position on the Nyquist–Shannon sampling theorem back in 2006; he's been blocked over 20 times, and is now blocked indefinitely, so I think it would be OK to clean up that section, deleting unsourced stuff and finding sourced stuff to replace it with, if anyone cares, but to point to it as some kind of authoritative section is a bit absurd.  Dicklyon (talk) 22:33, 8 August 2009 (UTC)

This discussion seems to ignore two properties of the speed of light which definitely are not simply human constructs. Firstly, the speed of light is not infinite: it it were infinite, it would be infinite in any set of units. Secondly, the mainstream view is that the speed of light in free space is constant (although there is also a respectable minority hypothesis that it has changed over cosmological time, see Variable speed of light). This is equivalent to saying that $dc/dt$ = 0. Again, either c is constant (the time-derivative is exactly zero) or it isn't, regardless of the units you use to measure it. Physchim62 (talk) 10:18, 8 August 2009 (UTC)


 * All this philosophical musing is fine here. I only object when it begins to find its way into the article, with lines like, "The speed of light can be viewed as simply a conversion factor between space and time in spacetime" [my italics].


 * A discussion on which science and maths is, and which is not, simply a human construct, is way beyond the scope of this article. Let us stick to stating the established facts on the subject, as given in reliable sources. Martin Hogbin (talk) 12:38, 8 August 2009 (UTC)


 * The distinction between the properties of the physical entity speed of light and the numerical value attached to this speed in the SI system of units, is a major emphasis of the article. Physchim62 appears to have missed this distinction, so perhaps some change in wording or organization is warranted? I don't see Martin's comment as germane to this issue, unless he feels that the distinction is a "philosophical issue", which it may be, but it also is a scientific issue and a major point that cannot be ignored. 19:22, 8 August 2009 (UTC)
 * A time depend c has no operational meaning, as pointed out here. Dimensionfull constants are simply conversion factors that relate physical quantities that were assigned different dimensions by humans. Count Iblis (talk) 20:03, 8 August 2009 (UTC)
 * Any speed can be used as a conversion factor between space and time. The physical meaning of this speed-space-time conversion is - the space distance overcomed per time, or time needed to overcome the space distance. The space needed to overcome the time is nonsense.


 * To overcome the distance you need time and motion. To overcome the time you don't need motion or space distance, you need only time. In this sense, to overcome the space you need causal energy, but to overcome the time, you don't need causal energy, just stability. Stability, equilibrium, is the cause of time overcoming. In this sense, the speed (motion) is not time overcoming, but space overcoming.


 * Space and time dimensions are not reciprocal. Space and time dimensions are fundamentaly not the same type.


 * Space and time units can be "converted" only in appropriate physical context, typically in causal context of field propagation. In the sense, that speed of light is the real causal limit, the speed of light is very specific and constant speed, that can be under specific conditions used as conversion factor, considering all consequences, and the fact, that this is contextual operation, that can be (does not need to be) misleading. However, the physical context of the speed of light is much deeper. Speed of light is not speed of time. Softvision (talk) 22:12, 8 August 2009 (UTC)


 * I'm unsure what the claim made by Count Iblis is. Is it a simple statement about time dependence of c, or is it a statement about any property of c? As regards the source, it argues actually that physically observable variations are variations in dimensionless constants, as these are independent of any choice of units. The source illustrates this point using the expression for entropy, which is shown to be expressible in terms of such dimensionless constants.


 * I'd have two issues with drawing a wide conclusion about the observability of properties of c: first, c may not appear in the same manner in every dimensionless constant, so observing a catalog of such constants might make possible observation of some properties of c. Perhaps this is not possible, but an illustration based on a limited example requires more support. A second issue is that there is no theory expressing the properties of c, such as its time or frequency dependence. Thus, we do not have a formula expressing the properties of c in terms of other fundamental constants. In particular, it seems entirely possible to measure the wavelength in SI units of an atomic transition repeatedly, and to determine whether this wavelength changes. Already ongoing are experiments to track whether the standard second changes. It would seem that putting these two types of observation together one could determine whether c varies over time. Moreover, these observations do not involve the choice of units, as the measurements all are ratios independent of the units chosen. So in a way these observation do not differ with Duff on the use of dimensionless quantities, but extends the catalog of dimensionless constants that could be used. Brews ohare (talk) 22:21, 8 August 2009 (UTC)

Since 1983, the speed of light has meant that light travels 299 792 458 times the distance that light travels in 1/299 792 458 of a second, in one second. It is a total tautology based on the definition of the metre. The number, 299 792 458, that is connected with the new definition of the metre was specially chosen so as to create a smooth and unnoticeable transition between the old real physics and the new nonsense physics. The new nonsense physics was inspired by a desire on the part of some, to elevate the speed of light to the sainthood because of its role in Einstein's theories of relativity. David Tombe (talk) 09:18, 9 August 2009 (UTC)


 * Once there was a good definition of the second, reprodicuble in most labs, and a bad definition of the meter, reproducible in no lab. There also was a good and highly successfull theory which used the osbserved fact that light speed was invariant, fact that could be experimentally verified in most labs, and without using the bad definition of the meter. Thus, we had a good second, a good light speed and a bad meter. So what did we do? We did the right thing and redefined the meter in terms of second and light speed, allowing most labs in the world to reliably reproduce it. Now, people who, due to some kind of allergy against "Einstein's theories of relativity", think this was the wrong thing to do, should know that there are plenty of forums available to vent their frustration, but that an article's talk page in Wikipedia is not the place to do it. Please take your frustrations elsewhere? Thanks. DVdm (talk) 11:33, 9 August 2009 (UTC)
 * The only people are trying to "elevate the speed of light to sainthood" are those commentators in this discussion who seem to believe that you can change the speed of light in free space by a humble choice of units. Nobody makes all this fuss about the frequency of the hyperfine transition of caesium-133 (definition of the second) or the wavelength of light from a certain transition in the krypton-86 atom (definition of the metre, 1960-1983), and yet these physical constants are (or were) also "fixed" in SI units by the very definition of the units themselves. It's metrologically useful to measure distances in terms of the speed of light: just get over it. Physchim62 (talk) 12:12, 9 August 2009 (UTC)

Sloppy editing
Dicklyon: In response to the turd you left on my user page, you can't read, and are too lazy to look at the source. The source explains how the history of Roemer's work has been mangled, not that the WP article has a mangled version of history. Wake up, and get that chip off your shoulder. Brews ohare (talk) 05:32, 4 August 2009 (UTC)


 * I already followed up on this on your talk page. What was mangled was your brain; I mean your edit. Dicklyon (talk) 06:02, 4 August 2009 (UTC)

In response to your second reversion, this source, Roemer: a cautionary tale, if read, explains how Roemer's work was misreported in a series of historical discussions. It often was said that Roemer calculated the speed of light, when in fact he did not, and it was Huygens who used Roemer's work to estimate a speed (and using his data incorrectly, at that). Inasmuch as this widespread misreporting is out there, a reader might well wonder if the WP treatment is indeed accurate, and this reference provides the necessary discussion to put everything straight.

All this would be completely understood by anyone who spent a minute reading this source, which is available at google books at the provided link. To remove this source once, without proper examination of the source and without taking the time to understand its purpose is pretty sloppy. To do it twice, when reminded about its purpose is really sloppy. And to accompany the first reversion with a turd on my user page about it??? What is that???.

Sloppy and impolite and intemperate editing. Brews ohare (talk) 06:14, 4 August 2009 (UTC)


 * Brews if you are going to challenge the generally accepted views on Roemer's measurement of the speed of light you will need very good quality and convincing evidence. One page in a newly published book is not enough.  Does the book cite its sources and what are the credentials of its author? Martin Hogbin (talk) 07:52, 4 August 2009 (UTC)

Well Martin, your research into this mater is flawed. The WP article has a carefully documented version of this history that agrees with the version in this source. This source also explains very carefully with additional sources that your notion of the "generally accepted views" (which you apparently feel are being challenged) is incorrect, and just how that view came to be promulgated. As I said, this account of misinformation is helpful in supporting the WP account. All this entry amounts to is a note supporting what has been in this article for ages; suddenly this minor addition becomes a cause célèbre. As I am sure you are perfectly capable of understanding my summary of the source (and checking its accuracy), of reading the WP article, and perfectly capable of assisting with this entry instead of being obstructive, I wonder what drives your actions? Could it be because a person on your hit list made the addition? Or, could it be that your action is based upon sycophantic acceptance of Dicklyon's sloppy assessment? Or, is it just Martin's usual Not Invented Here attitude? These traits combined make a mighty force! Brews ohare (talk) 13:10, 4 August 2009 (UTC)


 * Brews, I have no problem with that version of the history, and sm wondering why Martin jumped in to object; maybe he's the one that didn't read the source. My problem was with your mangled edits; the latest one is OK, though not really necessary.  Dicklyon (talk) 15:18, 4 August 2009 (UTC)


 * In my opinion, the account of Roemer's discovery currently given in the article is not satisfactory. Among several problems with it, the following statements constitute the most serious:
 * "On the basis of those observations, Rømer concluded that it took light 22 minutes to cross the distance the Earth traversed in 80 orbits of Io."
 * " Christiaan Huygens ... misinterpreted Rømer's value of 22 minutes to mean the time it would take light to cross the diameter of the orbit of the Earth."
 * According to several reliable source&mdash;including I. Bernard Cohen's definitive account, Carl Boyer's widely cited paper, as well as the account given in French's Roemer: a cautionary tale (p.121), already cited by Brews ohare&mdash;Roemer gave the value of 22 minutes as the time it took light to cross the diameter of the Earth's orbit, and not the time it took it "to cross the distance the Earth traversed in 80 orbits of Io." Moreover, on my reading of the original French report of Roemer's findings (available on-line here), as well as the 1677 English translation from the Philosophical Transactions of the Royal Society (also available on-line here), Cohen, Boyer and French's interpretations are clearly correct while the article's is clearly not.


 * The source cited for the article's account appears to be in Danish (which I do not understand). In view of the inconsistencies between the article's account and those of the sources I have cited above, I am sceptical that the article's interpretation of this Danish source is accurate. Is ((Brews ohare)) the editor responsible for the article's account see below sufficiently confident of his fluency in Danish to assure us that his interpretation of the source is accurate?  A google search brings up a couple of web-sites whose interpretation of the source appears to be the same as the article's, or at least very similar to it.  Could ((Brews ohare)) the responsible editor please let us know whether he relied on information from any such web-sites to arrive at his interpretation, or whether he arrived at it independently of any them?.
 * The article's account of Roemer's work was put in its current form with this edit, by an editor who appears to be fluent in Danish. I have therefore struck out my above questions as inappropriate.
 * &mdash;David Wilson (talk · cont) 16:10, 6 August 2009 (UTC)


 * This post appears to support the point that I made above, which is not a criticism of any particular editor or view, it just a reminder that we should stick to standard WP practice.


 * Many reliable sources, such as physics text books, indicate that Roemer measured the speed of light. I have no knowledge or strong opinion on this matter but, if we are going to state a different view, we need to cite good quality and reliable sources.  It is not acceptable to base the article on an individual editor's assessment of one or two online sources.  We need to find out what current, established and expert opinion on the subject is. Martin Hogbin (talk) 10:59, 7 August 2009 (UTC)


 * While I generally agree with these sentiments, there is one point I would take issue with. Physics textbooks are reliable sources for physics.  They should not be regarded as reliable sources for the history of science, or even for the history of physics.  Their accounts of Roemer's discovery of the finite speed of light are generally quite poor, and this is well-documented in Carl Boyer's paper, already cited above.
 * On the issue of the article's current treatment of Roemer, I have now stumbled across further evidence that the article has not interpreted the cited Danish source (Ole Rømer og den bevægede Jord – en dansk førsteplads? (2004) by Jan Teuber) accurately. This web-cite claims to be quoting comments directly from the source.  On the interpretation of a crucial part of the 1667 English report of Roemer's work, Teuber is quoted as saying:
 * "This cryptic part is difficult to understand but an interpretation could be that the proportion between the velocity of light and the velocity of the Earth is like (40 revolutions of Io seen from one side of the Earth orbit + 40 seen from the other)*42½*60 to 22, this gives 9300 very near today's value 10000."
 * It is clear from this wording that Teuber is not at all certain of his interpretation, and is only proposing it as one possibility. The article, however, presents it as if there were no doubts at all about the interpretation.
 * But in fact it seems to me that Teuber has exaggerated the difficulty of understanding the report of Roemer's work. I. Bernard Cohen's interpretation (in the article I have already cited above) seems to me to make perfectly good sense, while Teuber's own doesn't.  It's not clear whether Teuber has explicitly rejected Cohen's interpretation, or whether he was simply unaware of it.  But in any case, Cohen's article has been widely cited in the scholarly literature, and no-one else seems to have raised any problems with his interpretation.  I therefore propose to amend the article accordingly.
 * &mdash;David Wilson (talk · cont) 13:08, 7 August 2009 (UTC)

David Wilson has looked into this matter carefully and assembled several sources. The article French cites a number of reputable sources and, being a mea culpa, does not appear to be grinding any axes. I have read other accounts that support the view that Huygen's did the calculation (not Roemer) and got the wrong answer. Just why that happened, I don't know. I confess to not having carefully read the WP account, and no, I cannot read Danish. So I'd suggest that David put together what appears to him to be a suitable version. Brews ohare (talk) 13:18, 7 August 2009 (UTC) BTW I am not responsible for posting the WP account: I simply sought to add a footnote with a source. Brews ohare (talk) 13:18, 7 August 2009 (UTC)
 * "BTW I am not responsible for posting the WP account: .... "
 * Grovelling apologies for jumping to the unwarranted conclusion that you were. I should have read the preceding discussion more carefully.
 * &mdash;David Wilson (talk · cont) 13:28, 7 August 2009 (UTC)

Yeah, if the current version is wrong you should probably blame me, not Brews. And having read a bit more on the topic since I last edited this article I have got a bit of doubt on the "22min/80 orbits of Io" version myself. It's something I've been meaning to get back to. Regarding Teuber's article his wording doesn't actually leave that much room for doubt (in my own translation: "The only explanation of the numbers that make even the slightest bit of sense is..."), but in my view the article has a significant weakness in that it only seems to use the 1676 article and the correspondence between Rømer and Huygens. If the 1676 article is so cryptic why not use the other key article on the topic, Meyer, Kirstine (1915) Om Rømers opdagelse af Lysets Tøven (in Danish), The Royal Danish Academy of Sciences. Kirstine Meyer discovered Rømer's observation tables at the University of Copenhagen Library in 1913, documented that they were the basis of the value of 22 minutes and published them with extensive commentary in 1915. I haven't yet gotten hold of a copy of her article, which is why I haven't gotten back to this yet, but I've read summaries of it in Pedersen, Kurt Møller (1976) Ole Rømers opdagelse af lysets tøven (in Danish), Astronomisk Tidsskrift, vol. 9, pp. 160-66 and in Friedrichsen, Per; Tortzen, Christian Gorm (2001) Ole Rømer - Korrespondance og afhandlinger samt et udvalg af dokumenter, C. A. Reitzel, and neither of the two seem to question the "22min/diameter of the Earth orbit" explanation. So in conclusion I think I have to agree with David that Teuber's explanation appears not to be a widely held view and the "22min/diameter of the Earth orbit"-version is more appropriate here. Hemmingsen 07:42, 9 August 2009 (UTC)