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What the introduction says (or doesn't)

While I agree with Brews that it would be better to start with the approximate number, I don't agree with his reasoning or logic for why. The present lead says "In physics, the speed of light is a fundamental physical constant, the speed at which light and all electromagnetic radiation travels in vacuum. It is usually denoted by the letter "c". In the International System of Units, the metre is defined so that c has the exact value of 299,792,458 metres per second." which certainly does not suggest to anyone that 299,792,458 is a fundamental constant. And I agree that this number has to be in the article, and almost certainly in the lead; I just don't think that the somewhat distracting fact of the defined value should appear in the first paragraph. Many readers would be better served the way many books do this (not like Brew's sources, but some gentler sources), starting with the approximation that a reader can remember and use. Dicklyon (talk) 03:53, 26 August 2009 (UTC)

Let me understand this carefully: WP says: the speed of light is a fundamental physical constant ... usually denoted by the letter "c". Moreover, in the SI units, "c" has the exact value of 299,792,458 metres per second.

However, these WP statements (according to Dicklyon) in no way: suggest to anyone that 299,792,458 is a fundamental constant. Well tell me about the way this conclusion flows from the WP statements, please. Will the reader who is not a Philadelphia lawyer tease out this conclusion from the premises? The WP intro presents incorrect information. Brews ohare (talk) 05:38, 26 August 2009 (UTC)

So you're saying a speed being a fundamental constant of nature implies that its numerical value in SI units is a fundamental constant? Yes, I can see why that would confuse you, since you think of it differently. Oh, well, we'll have to live that that duality. Dicklyon (talk) 05:44, 26 August 2009 (UTC)

Dick: Not only does your statement completely misstate my stance, which I will explain further below if you wish, but you have not dealt with your incorrect statement about the implications of the WP introduction. The WP intro presents incorrect information, despite your remarks. Brews ohare (talk) 13:32, 26 August 2009 (UTC)

To spell out the misinformation: WP says by implication: 299 792 458 m/s is a fundamental constant. The sources Wheeler; Jespersen; Sydenham say explicitly the contrary. Such errors certainly will impede attainment of featured article status for this page. Brews ohare (talk) 13:57, 26 August 2009 (UTC)

h is a fundamental constant of nature. 6.62606896×10 isn't. Should we rewrite the Plank constant article because we state that h is a fundamental constant equal to (6.62606896±33)×10 J·s in SI units, out of fear that readers will think that 6.62606896×10 is a fundamental number? h and c are as fundamental as the other, both are scaling factors. Accept that you are the minority view here Brews, and move on. Headbomb {_{κοντριβς} – WP Physics} 15:03, 26 August 2009 (UTC)

Headbomb: There is no majority or minority opinion involved here: WP states that 299 792 458 m/s is the exact value of a fundamental constant in contradiction to sources Wheeler; Jespersen; Sydenham. That is neither opinion nor conjecture. Brews ohare (talk) 15:23, 26 August 2009 (UTC)

These sources say that the numerical value itself is arbitrary and not "fundamental", not that the physical concepts upon which they are based on cease to be "fundamental". You can define c or h to be whatever you want (other than 0 or infinity), all you're doing is scaling units ('cause you know, these are fundamental constants). Quantization doesn't stop because you've set h to be 10×10 instead of 10×10. The only fundamental constants which aren't scaling factors are dimensionless constants such as G_F and α, and we have a name for those: dimensionless constants. Headbomb {_{κοντριβς} – WP Physics} 17:25, 26 August 2009 (UTC)

Of course there is opinion involved. Brews believes the sound bytes carelessly quote-mined from the sources support his fringe viewpoints/conclusions, and most others do not. The Wheeler quote, for example, comes from a book where the predominant unit system used has c=1. Such a system having c=1 uses c as an exact value, without error bars. The idea that Brews is the only one using logic or arguing from sources is a gross violation of good faith. Brews' opinion is a minority view, is against current consensus, and his continuing tendentious editing is the problem here. Tim Shuba (talk) 18:07, 26 August 2009 (UTC)

I've yet to see more than private opinion on your side. The only sources you quote are repeating the 1983 committee decision, which cannot settle the matter as it is confined to the SI units system.

Apparently it is hairsplitting for you guys. Of course the underlying physical entity "speed of light" that occurs in physical theory such as the theory of relativity is fundamental. That was never an issue. It also happens to be true that this speed of light has a real physical value in (say) wavelengths of some transition /s, and that this value can be measured and does have an error bar. In contrast, as I am sure you agree, the so called speed of light in SI units is a convention, can have any value whatsoever. It is not measured, and has no error bar. The metre is adjusted to make this value of so-called speed of light to whatever you choose. The only experimental input to the metre is just a transit time of light. There is no fundamental length unit in the SI system. In no way can this arbitrary number for so-called speed of light be taken as indicative of the physical entity "speed of light" because the metre definition precludes ever getting a value different from the defined value. The units system is "elastic". It cannot express the concept of length traversed per unit time, as there is no length input in this system, only time input.

To put it differently, the speed of light in (wavelengths of some atomic transition) / (transit time) contains more physical information than does the statement of the speed of light in m/s. Brews ohare (talk) 18:39, 26 August 2009 (UTC)

I think missing from all this argument is *why* the meter is defined this way - it's precisely *because* the speed of light appears (by all experimental evidence) to be a real, universal, physical constant. So the two uses of c, the fundamental physical constant and the 'conversion factor' are not independent - the second was a direct result of the first. For example, a similar definition of the meter would not work at all if the speed of sound was used instead of the speed of light. LouScheffer (talk) 02:37, 27 August 2009 (UTC)

I'd agree with that, Lou. The fact one is permitted to use a conversion factor to relate length to time of transit is a result of the confidence with which the speed of light can be realized, even if you do not know what that speed is, and make no attempt to measure it. (One complication is realizing "vacuum".) However, the numerical value of this conversion factor is arbitrary, and is independent of whatever the actual speed the speed of light may have in vacuum. Brews ohare (talk) 05:09, 27 August 2009 (UTC)

I agree with Lou's too. We could say more about why the metre was defined the way that it was, but anything we say must reflect current mainstream scientific thinking, as described in reliable sources, in the subject. We do have the reasons that the standard was changed in 1983, from the people who actually made the change, see my reply to Edward below. Martin Hogbin (talk) 11:37, 27 August 2009 (UTC)

You are of course correct that "the speed of light in (wavelengths of some atomic transition) / (transit time) contains more physical information than does the statement of the speed of light in m/s". With definitions expanded, the former is a comparison of two different physical phenomena, the latter is a statement of a conventional scaling factor.

Given the post-1983 definitions, the physical content that is contained in the first statement is now expressed in the statement "the wavelength is x m", which previously would have been an exact statement of a conversion factor, with no physical content. EdwardLockhart (talk) 05:37, 27 August 2009 (UTC)

Actually, Edward, the pre-1983 definition of the meter contains exactly the same information as the later one, or at least the 17th Conférence Générale des Poids et Mesures thought so (See point 7 in this link ]). The specified atomic transition of krypton emits light of a particular frequency. The wavelength of this light depends on the speed of the light, thus the specified krypton transition was actually being used as a frequency standard, with the meter effectively being defined as the distance light travels in a specified number of periods of the krypton radiation. Later advances in technology meant that the frequency of the krypton radiation could be measured against the existing caesium frequency standard. By stating the time of travel in seconds rather as a number of periods the krypton radiation it became possible to use other, more stable, sources of light, and the existing caesium time (frequency) standard to define the metre. You can read about it in this link kindly provided by Brews ]. Martin Hogbin (talk) 11:15, 27 August 2009 (UTC)

I think you've misread ]. Point 7 says that the various *proposed* standards mentioned in point 6 are equivalent, not that the new standard is equivalent to the old one. I believe the following two pairs of statements are equivalent:

Pre-1983 - the speed of light is approx 299,792,458 m/s, the wavelength of is exactly 1/1,650,763.73 m.

Post-1983 - the speed of light is exactly 299,792,458 m/s, the wavelength of is approx 1/1,650,763.73 m.

In each case, one statement is a definition of our length unit (but phrased in a curious way), and the other statement is a physical observation using that unit. The 1983 definition change flipped the roles of these two physical phenomena. And in so doing, it made a philosophical (if not a practical) difference to the statement of the speed of light in SI units. EdwardLockhart (talk) 12:04, 27 August 2009 (UTC)

I was referring to this statement, 'that these various forms, making reference either to the path travelled by light in a specified time interval or to the wavelength of a radiation of measured or specified frequency, have been the object of consultations and deep discussions, have been recognized as being equivalent and that a consensus has emerged in favour of the first form'. They are clearly saying here that using the wavelength of a specified radiation and using the time travelled by light in a specified time are equivalent. Martin Hogbin (talk) 13:50, 27 August 2009 (UTC)

I agree with EdwardLockhart that equivalent to the BIPM means equivalent within the context of length comparisons using the SI units, not physical equivalence. As the actual, physical speed of light simply cancels out in any length comparison:

${\displaystyle {\frac {\ell _{1}}{\ell _{2}}}={\frac {c\ t_{1}}{c\ t_{2}}}={\frac {t_{1}}{t_{2}}}\ ,}$

any length comparison is "equivalent" to a "time-of-transit" comparison. The "fundamental discussion" for the BIPM is simply whether the length ratio or the time ratio has the greater error bar. Brews ohare (talk) 12:43, 27 August 2009 (UTC)

BIPM statement

Martin has brought up some sources for discussion. One of these sources is BIPM, which states:

that these various forms, making reference either to the path travelled by light in a specified time interval or to the wavelength of a radiation of measured or specified frequency, have been the object of consultations and deep discussions, have been recognized as being equivalent and that a consensus has emerged in favour of the first form

The metrology advantage of replacing length comparisons with time-of-transit comparisons is that the measurement of time-of-transit involves only a time measurement error, and this error is small compared to length comparison based on fringe-count (interference pattern) comparisons.

Of course, the BIPM is a metrology organization concerned with the SI system of units, that is, with practical criteria of convenience and upon accuracy sufficient within the SI system of units, and unconcerned with "equivalence" of methods in the more strict sense of physical law. They are concerned with only length comparisons and are not concerned at all with the actual speed of light, which is unnecessary to their chosen time-of-transit methodology.

In other words, use of a conversion factor, chosen arbitrarily as 299 792 458 m/s but equally any other number, simply reduces length measurement to being identically the same as measurement of transit time. The actual speed of light is not necessary, and indeed is not even a knowable (measurable) within this approach. It is simply a defined value, arbitrary, chosen for convenience. Brews ohare (talk) 12:32, 27 August 2009 (UTC)

NIST document

Martin has brought up the source NIST paper for discussion. Martin has suggested that this source supports this view expressed by Martin: "By stating the time of travel in seconds rather as a number of periods the krypton radiation it became possible to use other, more stable, sources of light, and the existing caesium time (frequency) standard to define the metre."

This statement of Martin's is not (to me) terribly clear. What tipped the scale to comparisons of "time-of-transit" for comparing lengths, rather than comparison of "counted interference fringes" was that time measurement errors became much smaller than fringe-counting errors as technique improved.

As the actual, physical speed of light simply cancels out in any length comparison:

${\displaystyle {\frac {\ell _{1}}{\ell _{2}}}={\frac {c\ t_{1}}{c\ t_{2}}}={\frac {t_{1}}{t_{2}}}\ ,}$

any length comparison is "equivalent" to a "time-of-transit" comparison.

It is totally irrelevant what the actual value of c might be, so long as one can be sure that the same value appeared in both lengths. Consequently, one is free to choose any value for c whatsoever. In addition, one never needs to measure the value of c .

A consequence of this

definition is that the speed of light is now a defined

constant, not to be measured again. NIST paper

The definition of the metre by BIPM and NIST is really not a length definition at all, but the identification of a standard time of transit namely t_standard = 1/299 792 458 s. When one says a length is so many metres, one really is saying the length has a transit time in vacuum of so many t_standard units. Brews ohare (talk) 12:55, 27 August 2009 (UTC)

When you add the unit of metre you are putting the definition of c back into the value for distance.

Thanks for the quote from NIST; it shows that one credible body, at least, believes that c is defined as exactly 299 792 458 m/s. This is a credible enough source that I think we can include that value in the lead now without any objections. TStein (talk) 14:12, 27 August 2009 (UTC)

TStein: A bit more care is needed here before rushing ahead with this quote. This whole mess is just a result of the confusion engendered when the literature uses the same exact wording for different concepts. It happens again and again. Examples are centrifugal force and Faraday's law and electromotive force. In this case, BIPM refers to an arbitrary conversion factor with absolutely no physical significance as the speed of light, while the structure of spacetime also uses the notion of speed of light in an entirely different way.

By introducing wavelength, a true length is invoked. On the other hand, the present-day metre is actually not a length, but a new name for a standard transit time of 1/299 792 458 s. This time is converted to a length by a fictitious dimensional conversion factor.

The key to this jumble is that the actual, physical speed of light simply cancels out in any length comparison:

${\displaystyle {\frac {\ell _{1}}{\ell _{2}}}={\frac {c\ t_{1}}{c\ t_{2}}}={\frac {t_{1}}{t_{2}}}\ ,}$

any length comparison is "equivalent" to a "time-of-transit" comparison. It is totally irrelevant what the actual value of c might be, so long as one can be sure that the same value appeared in both lengths. Consequently, one is free to choose any value for c whatsoever. Obviously, the same is not true of the relation:

${\displaystyle \ell =c\ t\ ,}$

so length (e.g. atomic wavelength) depends critically upon the actual speed of light, a matter no longer of any concern to NIST or BIPM, at least wrt the SI units. Brews ohare (talk) 14:50, 27 August 2009 (UTC)

Lead picture

This seems to add nothing to the article except maybe confusion by talking of light travelling at  % of the SoL. IMHO we would be better with no pic than this one. Abtract (talk) 21:55, 26 August 2009 (UTC)

We need a picture of some sort. I see nothing wrong with the current one. Maybe it is the caption that you do not like. This is also the cause of the problem above. Martin Hogbin (talk) 22:53, 26 August 2009 (UTC)

I think the caption is good, as it clarifies that, in air, light travels at less than the speed of light. Or maybe it should say 99.97% of c if that's more clear. Dicklyon (talk) 23:54, 26 August 2009 (UTC)

I see no reason why we need a picture of some sort (Speed of sound has none in the lead) but, even assuming we do, why on earth would we choose one that showed light travellling at less than c? I much better picture imho would be of a celestial object with the statement that "light takes xx years to travel from the zzz galaxy to earth" or maybe of the moon or sun with a similar caption. An alternative would be a picture of the equipment used in one of the historic experiments to measure the SoL. Abtract (talk) 08:35, 27 August 2009 (UTC)

I think we need a lead image for FA status. I am not fighting for this particular picture, if you can fine a better one or some candidate images the please present the here where we can argue endlessly about them ;-) Martin Hogbin (talk) 11:20, 27 August 2009 (UTC)

I have replaced it with a picture of the sun, the source of our natural light. Abtract (talk) 12:59, 27 August 2009 (UTC)

I like that. However, in order to hint at the article's subject (a speed), lets replace the caption "Light takes 499 seconds to travel from the Sun to the Earth" with "Light takes about 500 seconds to cover the 150 million Km distance from Sun to Earth". DVdm (talk) 13:10, 27 August 2009 (UTC)

Why not, Travelling at 299,792,458 m/s, light takes about 500 seconds to cover the 150 million km distance from Sun to Earth, This get round the problem that Edward noticed. Martin Hogbin (talk) 13:42, 27 August 2009 (UTC)

Excellent. I gofered it. DVdm (talk) 14:36, 27 August 2009 (UTC)

By the way, an even better picture would be one where we see Earth on the foreground and a distant Sun on the background, like this... DVdm (talk) 14:48, 27 August 2009 (UTC)

Excellent; I like that useful picture and the meaningful caption but while we are rounding we may as well do a complete job. Abtract (talk) 15:28, 27 August 2009 (UTC)

I'd vote for the exact value, but I wouldn't have a fight over it ;-) - DVdm (talk) 15:35, 27 August 2009 (UTC)

The new "L2" illustration is terrible here, as it's dominated by graphical information that's off topic and goes unmentioned. Dicklyon (talk) 16:23, 27 August 2009 (UTC)

Let's edit the L2 part out of it or find a better one. Here is the edited version. I don't know how to add it as another alternative version at - DVdm (talk) 16:52, 27 August 2009 (UTC)

I edited the picture (originally from a govt source) and enlarged the titles. See what you think. Brews ohare (talk) 14:15, 28 August 2009 (UTC)

Exactly what I had in mind :-) DVdm (talk) 14:17, 28 August 2009 (UTC)

introduction to the speed of light

may a make the same observation i made some time ago in the torque article . this reads like a thesis for a physics degree and is not very interesting to most people . i would suggest to make the introduction readable and understandable for a ten year old , pretend it is your child who wants some information and keep the highly sophisticated stuff for the rest of the article .

Wdl1961 (talk) 15:03, 27 August 2009 (UTC)

May I suggest that you use proper capitalisation and punctuation in your talk page messages? Imagine the influence your way of writing might have on these kids. DVdm (talk) 15:13, 27 August 2009 (UTC)

Wdl1961:How about some concrete suggestions on how to simplify the lede.

Remember that this article is primarily about the constant c which relates space and time and is not simply the speed at which light travels. For that reason the inherent difficulty level of this article is akin to that of special relativity; I am not quite sure how advisable it would be to write the lead of special relativity so that it is understandable by a 10 year old. The same is true for this article. If you can do it or make a suggestion that works then more power to you, though. TStein (talk) 15:26, 27 August 2009 (UTC)

The best approach so far to simplifying the lead is that of Dicklyon: introduce the speed of light with an approximate value; avoid startling terms like exactly 299 792 458 m/s and the concept of defining the metre to make this so; and follow the lead with the "Speed of light by definition" subsection where matters can be discussed carefully without severe length limitations. Brews ohare (talk) 15:54, 27 August 2009 (UTC)

I am trying a rewrite on my name page.

One suggestion is to rename it as "C" the speed of light Abtract (talk) 16:00, 27 August 2009 (UTC)

You might find it more convenient than replacing your user page to create your draft article by making a new page User:Abtract/Speed of light which you can link to your user page by inserting this link at the top of your user page. Brews ohare (talk) 16:09, 27 August 2009 (UTC)

Proposed new lead section

My suggested rewrite can be found here (I don't use my user page for anything else but thanks for the idea). IMHO this is more inline with wp:lead, is more user friendly (tells a story) whilst still being scientific, and brings all(?) the salient points to the fore. Whether it will bring together the two sides of the divide remains to be seen but it clarifies the circularity problem. Naturally it needs refinement even if you have some liking for it. Abtract (talk) 17:11, 27 August 2009 (UTC)

It looks acceptable to me. I'd suggest using something like "In physics, the speed of light 'c' " as being more in keeping with a standard intro line, and allowing the use of a lower case c. Brews ohare (talk) 17:32, 27 August 2009 (UTC)

Personally I prefer to leave out "In physics" because that somehow seems to diminish the importance of the SoL, but let's see what others say. I do agree with the lower case c so I have reordered the beginning and maybe the article might be better named The speed of light ... "c"Abtract (talk) 08:36, 28 August 2009 (UTC)

"For many years thought to be infinite," - I think it would be more accurate to say it wasn't known if it was infinite or simply very fast. Or even for that matter whether it made sense to talk of a speed at all. EdwardLockhart (talk) 05:55, 28 August 2009 (UTC)

I have changed it along those lines. Abtract (talk) 08:36, 28 August 2009 (UTC)

Your proposal now says that the speed of light was previously assumed to be instantaneous. If you read the relevant section of the article| it should be clear that this is not the case - from Empodocles onwards, some thinkers on the subject have thought that light takes some time to travel. Equally others, such as Aristotle, didn't believe there was any movement going on, meaning there would be no such thing as a speed. EdwardLockhart (talk) 08:46, 28 August 2009 (UTC)

OK it is quite difficult to get the idea across in a few words but I will try again. Abtract (talk) 10:26, 28 August 2009 (UTC)

No way The current lead is the result of pages of discussion, cooperative editing, and a considerable degree of consensus amongst editors here. There is no possible benefit to our readers in not giving the exact speed of light at the start of the article, and this seems to be a plan to avoid doing that. The only reason for not wanting to give the exact speed is that, despite extensive discussion, some editors still do not understand why the speed is now exact. I do not think the style of the proposed lead is terrible encyclopedic and it contains a number of dubious or unclear statements. No doubt it could be improved but that would then start the whole cycle of editing and arguing all over again.

I am not against careful rewriting of the lead to improve the English and style and but the content has been discussed in great detail and I suggest that it should not be changed dramatically like this. The content should evolve slowly as a result of discussions between editors to reflect consensus views. Martin Hogbin (talk) 09:30, 28 August 2009 (UTC)

I don't know how to respond to Hogbin's accusatory dismissal of my proposal except to say I thought it was a "careful rewriting". There is no "plan" just a reflection of my, and I thought others', opinion that starting with a very close approximation would be much easier on the eye and therefor more reader friendly, and by then taking the reader gently through a brief history we could lead them to the exact figure and explain how it arose. The final para explains the importance of c. All this imho is what a lead should do and seems to accord with wp:lead. I await other comments with interest. Abtract (talk) 10:26, 28 August 2009 (UTC)

Sorry Abtract that my remarks appeared accusatory. Let me explain the individual points that I was trying to make.

There was a very long discussion on what numerical value we should show in the lead followed by an RfC. There was there a clear consensus to show the exact value first (I believe that this same consensus should apply to the picture caption). The two editors who were against doing this took that view because they believed that the exact value was, in some way wrong, a position clearly not in accordance with the generally accepted scientific view. I proposed adding that 300,000,000 m/s was often used as a convenient approximation, but it seemed that most editors were against this. For many readers there is a simple question, 'What is the speed of light', and to this question there is a simple answer, '299,792,458 m/s'. I cannot see how anything is made simpler by saying it is about 300,000,000 m/s followed some while later with 'well actually it is '299,792,458 m/s'. For those interested in why the value is exact (many may have no interest at all in this fact) we have a section devoted to that subject in the article.

We should be trying to improve this article by cooperative editing. This cannot be achieved by every editor rewriting the lead in their own words. We all have different talents and it is better to try to use them together to produce something better than any individual could have done. Martin Hogbin (talk) 14:16, 28 August 2009 (UTC)

I'm in broad agreement with Martin - the exact value of c in m/s should be at the top. This was the concensus of lengthy previous discussion, which I think you missed. I'm not convinced either that so much history of mistaken ideas and imprecise estimates should be in the lead - other articles on scientific topics don't generally take this approach. EdwardLockhart (talk) 13:11, 28 August 2009 (UTC)

The proposed lead by Abtract in its original version contained the SI units conversion factor exactly, as seems to be desired by EdwardLockhart & Martin Hogbin, and contained the wording:

For technical reasons in 1983, the metre was redefined using this value as a conversion factor. This had the effect of 'fixing' c at exactly 299,792,458 metres per second in the International System of Units.

This wording is accurate. The present wording:

In physics, the speed of light is a fundamental physical constant, the speed at which light and all electromagnetic radiation travels in vacuum. It is usually denoted by the letter "c". In the International System of Units, the metre is defined so that c has the exact value of 299,792,458 metres per second.

juxtaposes two different usages for the term "speed of light" without distinguishing between them, and is in fact incorrect on that account. More detail on this point is found at NIST document and at Some math.

The first form of wording by Abtract should be adopted. Brews ohare (talk) 14:13, 28 August 2009 (UTC)

We cannot pander to the views of those who do not understand the consequences of the current definition of the metre. The page must reflect current mainstream scientific thought on the subject. Martin Hogbin (talk) 14:16, 28 August 2009 (UTC)

Martin Hogbin: The word "pander" is, of course, inflammatory. You make the further unsupported claim that you understand the implications of the SI units conversion factor.

The problems with your view of this factor have been carefully explained to you here, and on the page you created for discussion of this point, and you have not succeeded in supplying any source or argument that supports your views.

What you have done is continually reiterate your unsupported view, namely, that the SI units conversion factor is the numerical value of the physical constant called the speed of light throughout special relativity and the rest of physics. You have never advanced argument or sources, not attempted to meet sourced objection. You do not understand the 1983 BIPM definition of the SI units conversion factor, nor its implications, nor its limitation to the SI units. You have not addressed the three sources by Wheeler, Sydenham or Jespersen that refute your viewpoint . Brews ohare (talk) 14:38, 28 August 2009 (UTC)

I did indeed miss it and, although I have looked at some of the more recent discussions, there seems to be so much heat generated that I have tried to put together a lead which, as it should be, is a summary of the article, brings out the salient points and makes the reader want to know more. IMHO the salient points are: 1) how we reached the counterintuitive conclusion that light has a speed 2) some idea of how the SoL is measured 3) what the measured speed is (300,000 km/s will be a good degree of accuracy for 90% of readers and is much simpler to read), 4) how we came to have a fixed exact figure 5) why c is so important and in what fields. All these points are to be found in the article in much more detail ... I don't think I have missed any important points. One more point on the use of 300 early in the lead with the exact figure saved for after the story has unfolded ... it isn't as though 300,000 km/sec is incorrect, it is the correct figure stated to three significant figures. I quite fail to see why showing c to nine significant figures in the first paragraph should arouse such high feelings. Pi is approximated in the lead as 3.14159 but given much more accurately later in the article; interestingly e is not shown as a number at all until the 4th paragraph and the Gravitational constant is first shown in numeric form at the end of the 3rd paragraph; Planck constant is not mentioned numerically in the lead, being relegated to a side box; the Fine-structure constant doesn't really count because the lead is only one very short paragraph; there are two with a high degree of accuracy early in a decent sized lead Elementary charge (1st para) and Vacuum permittivity (2nd para). All this demonstrates that other editors in other similar articles have taken a pragmatic approach and used approximations and more accurate figures where they fitted the particular story ... that is what I have tried to do, hoping it would satisfy the warring factions here. This is not pandering. Abtract (talk) 14:36, 28 August 2009 (UTC)

Your present proposed lead is acceptable to me. You appear to have dropped a reference for the speed of gravitational waves: Wei-Tou Ni (2005). "Empirical foundation of the relativistic gravity" (PDF). Intl J Modern Physics D. 14: 901–921. Brews ohare (talk) 15:41, 28 August 2009 (UTC)

So why not just restore the reference? Martin Hogbin (talk) 16:12, 28 August 2009 (UTC)

Gentlemen, according to current, generally accepted, mainstream scientific thinking there is only one speed of light and it is exactly 299,792,458 m/s. Pi and e are transcendental numbers and must always be approximated in any finite numerical representation. Martin Hogbin (talk) 16:10, 28 August 2009 (UTC)

Martin Hogbin: Abtract's present lead is accurate and appears to me to meet all your requirements. Why, specifically & in exact detail, do you not wish to adopt it? Brews ohare (talk) 16:16, 28 August 2009 (UTC)

Since no-one seems to have any objection tomy proposed lead except for the rounding in the first para, I am going to be bold put it in. The only change I will make is that I will put the exact figure in the first para but without the word "exactly" which I hope may 'satisfy' both sides (although why there are 'sides' in this beats me). Abtract (talk) 09:41, 29 August 2009 (UTC)

I have a strong objection to your lead and I have clearly stated it above. It starts with No way! Martin Hogbin (talk) 10:22, 29 August 2009 (UTC)

Yes so it appeared at first sight but, having overcome my shock at the tone, I could only find one actual point - the use of an approximation - I have altered that. What more do you require? Abtract (talk) 10:57, 29 August 2009 (UTC)

As I have just said in reply to Tim Shuba it is not that I think your lead is that bad but there are a few things about it that need changing. However, there is absolutely no point in me or any other editor doing this because the likelihood is that someone else will rewrite it from scratch again. If this is going to be the ethos of editors here than we will never reap the benefits of cooperative editing. What I would like is for you to discuss exactly what was wrong with the original lead (which was far from perfect) so that we can all work together to improve it.

It is generally true that a lead written by one person is likely to read better than one edited by several people over a period of time. I would therefore suggest that we should finalise the content and order of the lead and then, when that is agreed, one person should rewrite it to improve the style and presentation. Martin Hogbin (talk) 11:06, 29 August 2009 (UTC)

Apologies, I thought I had made it clear previously ... the previous lead was not bad but it failed in a couple of key aspects imho 1) it did not bring out the history suffieciently well (I have expanded it), 2) it overemphasised the "exactness" too early and failed to explain in properly later (this is all tied up with my 'circularity' point and ohare's point too I believe), 3) it did not 'read' very well (maybe your point about one editor v many). If you compare the two versions, I haven't actuallt changed it much just sharpened it up , reordered slightly and extended the history including clarifying the exactness point ... and, maybe important to you,I have delayed using the word exact until later in the piece. Abtract (talk) 11:40, 29 August 2009 (UTC)

So what exactly is you 'circularity point'? Martin Hogbin (talk) 12:30, 29 August 2009 (UTC)

As it was the last time we discussed it: defining the metre in terms of the SoL and then stating the SoL in units of m/s says absolutley nothing about the SoL (except that we all have in our mind a vision of the metre as previously defined independently of the SoL). I don't think we can escape this circularity but we can 1) not bother the reader with it too soon, and 2) explain it at the end of the historical story so that it makes some sort of sense in context. I have tried to do both. I have no axe to grind in this matter except producing a readable, useful and scientifically correct article ... Abtract (talk) 13:27, 29 August 2009 (UTC)

If the metre is defined in terms of the second and the speed of light, an obvious and inevitable consequence of this definition is that the speed of light will have a fixed numerical value when measured in metres per second. If this is what you meant by 'circularity problem' then yes, this is true, get over it! This is not a world shattering revelation or a dark secret revealed only to the chosen few, it is a fact that is quite obvious to nearly everyone.

If you meant that nobody knows roughly how long the meter is, it is about this long , so from an intuitive perspective giving the speed of light in metres per second is no different from giving it in furlongs per second; you have to know roughly how long a furlong is for the value to have any intuitive meaning.

If you mean that nobody knows exactly how long a meter is then you are right. All issues of experimental error and imprecision manifest themselves in the practical realization of the meter. Mind you, they can get pretty close these days.

If you are going to assert that there is a 'circularity problem' with the exact speed of light you must find a reliable source that states, in plain English, that there is a circularity problem (or indeed a problem of any kind) with the current definition of the metre. You have seen the statements from the world and US standards authorities stating the exact value of the speed of light and they are quite clear. If you want to argue with this you must find a source that plainly and simply disagrees with what BIPM and NIST say. Bizarre interpretations of sentences plucked from good sources are not acceptable. Martin Hogbin (talk) 13:52, 29 August 2009 (UTC)

I am really sorry that you do not understand my point or indeed the topic, as evidenced by "If the metre is defined in terms of the second and the speed of light, an obvious and inevitable consequence of this definition is that the speed of light will have a fixed numerical value when measured in metres per second" which would only be true if you substituted "stated" for "measured". I think, under these circumstances, we had better stick to discussing content. What changes would you like to see to the current lead, and why? Abtract (talk) 14:21, 29 August 2009 (UTC)

You are quite right, 'measured' was a poor choice of words. Change it to 'stated'. Now, what was your problem? Martin Hogbin (talk) 14:25, 29 August 2009 (UTC)

I don't have a problem ... what changes would you like to see? Abtract (talk) 14:33, 29 August 2009 (UTC)

I would like to see it restored to its earlier consensus version which said, 'In the International System of Units, the metre is defined so that c has the exact value of 299,792,458 metres per second'. How does your 'circularity' point apply to this?

Imho it is best not to burden the reader with the counter intuitive "exact value" until it can be placed in context later in the lead. According to wp:lead "The first paragraph of the introductory text needs to unambiguously define the topic for the reader, without being overly specific". I believe the words "exact value" are "overly specific". Perhaps you could explain why it is important to you to include those words in the first para?Abtract (talk) 16:28, 29 August 2009 (UTC)

within the SI system of units

User Headbomb removed the phrase within the SI system of units from the following: "The effect of this definition is to fix the speed of light in vacuum at exactly 299,792,458 m/s; thus, within the SI system of units, the speed of light is now a defined constant and can no longer be measured."

with the cryptic comment "untrue". However, the source supports the statement, and the use of a defined value for c certainly applies within the SI system, and not necessarily for other systems of units (for example, not for the pre-1983 system of units). Brews ohare (talk) 17:17, 27 August 2009 (UTC)

Mathematical elaboration

User Headbomb removed this mathematical follow up of the references by Boyes and Smolin with the comment "untrue"

"Measurement is basically comparison between the quantity being measured and the standard unit; it is a matter of ratios. Setting the speed of light to a definite numerical value when measured in the SI units of m/s simply means comparisons of length become comparisons of transit times of light. Mathematically, comparison of two lengths ℓ₁, ℓ₂ with times-of-transit of light, t₁, t₂ may be expressed as the ratio:

${\displaystyle {\frac {\ell _{1}}{\ell _{2}}}={\frac {c\ t_{1}}{c\ t_{2}}}={\frac {t_{1}}{t_{2}}}\ ,}$

which is independent of the the speed of light c."

This mathematical elaboration explains further what these sources have said, and is certainly "true". Thus, at a minimum Headbomb's deletion is poorly justified. Brews ohare (talk) 17:25, 27 August 2009 (UTC)

No it is not true, because you are not comparing transit times, you are comparing a length (of say a table) with another length (the distance covered by light in vacuum in 1 / 299972458 seconds). That l_table/l_meter = t_{light across table}/t_{light across meter} only illustrates that is it equivalent to compare transit times. Looking back reverting may have been overdramatic. Headbomb {_{κοντριβς} – WP Physics} 17:49, 27 August 2009 (UTC)

Headbomb: If t₂ = 1 / 299972458 s then ℓ₁ is in metres. Brews ohare (talk) 18:28, 27 August 2009 (UTC)

Some math

The wavelength of a given frequency of light is exactly equal to the period of that radiation multiplied by the actual, physical speed of light, which is not the BIPM arbitrary conversion factor. BIPM is interested only in length comparisons, which for a fixed speed of propagation c are transit-time comparisons (See footnote ):

${\displaystyle {\frac {\ell _{1}}{\ell _{2}}}={\frac {c\ t_{1}}{c\ t_{2}}}={\frac {t_{1}}{t_{2}}}\ ,}$

where c can be any arbitrary speed, so long as it is the same speed for both transit-time measurements. In the SI units, for example, if t₂ = 1 / 299 792 458 s then ℓ₁ is in metres.

At least insofar as the SI units are concerned, BIPM is not interested in lengths per se :

${\displaystyle \ell =c\ t\ ,}$

which length requires the actual, physical speed of light (preferably measured using 2009 techniques, rather than a 1983 estimated value), not an arbitrary conversion factor set by committee.

Footnote: Measurement is basically comparison between the quantity being measured and the standard unit; it is a matter of ratios. See Smolin Brews ohare (talk) 14:58, 28 August 2009 (UTC)

We have a consensus

I am not going to get involved in yet another edit war about the same subject - whether the speed of light is exact or not. There was a clear consensus to quote the exact sped at the start of this article and I see no reason why this does not apply to the caption of the lead image. Martin Hogbin (talk) 16:47, 28 August 2009 (UTC)

It simply is bad practice to state a rounded-off distance in the figure, indicate a rounded-off time, and then relate the two in the caption to a speed calculated to nine figures. It is stupid. Brews ohare (talk) 17:01, 28 August 2009 (UTC)

Martin Hogbin: Abtract's present lead is accurate and appears to me to meet all your requirements. Why, specifically & in exact detail, do you not wish to adopt it? That would replace your so-called "consensus" (actually a cramming down of your own wording) with unanimity. Brews ohare (talk) 17:06, 28 August 2009 (UTC)

I see the speed has now been removed from the caption. There was a reason that it was there, which is Edward's observation that this caption is the text that appears when you do a Google search on the speed of light. It was agreed that it might be useful to have the value of the speed of light in this text. I am not particularly that fussed about that but my real concern is this: The structure and wording of this article are still being influenced by a minority of editors who, despite the clear statements of impeccable sources to the contrary, insist that there is something wrong with stating the exact speed of light in m/s. I would like some support in taking a stand against this nonsense, which is still taking up pages of discussion and causing silly arguments about the wording of the article. Martin Hogbin (talk) 20:26, 28 August 2009 (UTC)

And the speed disappeared from the caption long ago. And cooperative discourse does not involve terms like "nonsense" and "silly arguments" to characterize what, in fact, are solid arguments supported by sources, not to be dealt with by rounding up a lynch mob. And we have an acceptable proposal in front of us at User:Abtract for a lead that Martin Hogbin refuses to discuss at all. Brews ohare (talk) 00:13, 29 August 2009 (UTC)

Aggressive editing spree now over

Ah, that was refreshing. I am calm now, and I will expect and enjoy any and all excoriations and reversions. Tim Shuba (talk) 04:05, 29 August 2009 (UTC)

Commentary for Tim Shuba: In the first line of your lead is the phrase: "In physics, the speed of light is a fundamental physical constant, the speed at which light and all electromagnetic radiation travels in vacuum."

I agree with that.

The second sentence is: "It is usually denoted by the letter ‘c’."

I agree with that.

The third sentence is: "In the International System of Units, the metre is defined so that c has the exact value of 299,792,458 metres per second."

From this concatenation, I'd take this 'c' to be the same as the 'c' from the first two lines. However, that is not quite so. One could say: "In the International System of Units before 1983, the metre is defined so that ‘c’ has the approximate measured value of 299,792,458 metres per second."

Or, one could say: "In the International System of Units following 1983, the term ‘speed of light’ is used to refer to a conversion factor denoted by ‘c’ that has the exact value of 299,792,458 metres per second." One could elaborate upon that point, to be sure.

However, the key here is that the ‘c’ referred to in the 1983 definition is not the ‘c’ referred to in the first two lines of your intro. In 1983, length was replaced in the SI units by "time of transit" and conversion of such times is accomplished by an exactly defined conversion factor 299,792,458 metres per second. This conversion factor, being defined, clearly lies outside any kind of measurement.

One might argue that because the metre is defined elastically to make this so, you cannot argue about the value of the conversion factor. True. However, this elastic metre that makes ‘c’ unmeasurable and exact, also removes this meaning of ‘c’ from referring to the "fundamental physical constant" of the first two sentences, which most certainly is measurable, and most certainly is not known exactly. The SI conversion factor is not the numerical value of the "fundamental physical constant", even in SI units. It is just a conversion factor.

The fundamental speed of light is made measurable by introducing a length (like a wavelength) that is not elastic.

These points have been argued more carefully on this page, and sources supplied. Feel free to delve in further.

I find User:Abtract's leading sentences to be technically correct and yet convey all that you want to say unambiguously. Brews ohare (talk) 05:31, 29 August 2009 (UTC)

I've been unable to locate any recent sentences by Abtract that differ in any way that I can discern from the ones you're complaining about. Can you be more clear about what you would prefer to see that would "be technically correct and yet convey all that you want to say unambiguously"? Dicklyon (talk) 06:11, 29 August 2009 (UTC)

Dicklyon: The wording in Abstract's lead is

by the mid-20th century, the speed of light was known to be approximately 299,792,458 metres per second. For technical reasons in 1983, the metre was redefined using this value as a conversion factor. This had the effect of 'fixing' c at exactly 299,792,458 metres per second in the International System of Units.

Brews ohare (talk) 14:20, 29 August 2009 (UTC)

Say what? You won't find "my lead" anywhere. I copyedited and united two paragraphs in the lead. That's all. I didn't add a single word or concept of my own to it. Please don't interpret that to mean that I want to get bogged down in your personal obsessions about some purported problems with some particular definition of the speed of light. As I intimated in the first post to this section, you or anyone else should feel completely free to revert or change anything in my recent edits. That would certainly be preferable to polluting yet another section of this talk page with your useless "commentary". Tim Shuba (talk) 06:43, 29 August 2009 (UTC)

Tim Shuba Your suggestion that I dive into the article and rewrite it without discussion indicates little familiarity with what has been going on with this article. Likewise, you show no familiarity with the discussion on this Talk page. Calling my comment to you on the Talk page "pollution", rather than providing any response to it beyond pejorative characterization, is impolite and underscores your lack of understanding of decorum and of the use of a Talk page. Brews ohare (talk) 14:31, 29 August 2009 (UTC)

Brews ohare provides, in a direct response to me, a "commentary" in this section which has nothing, zero, nada, zilch to do with any edits I've made to this article or talk page, and expects me to respond and get involved in the tendentious editing that he has kept up for weeks? What a joke. Tim Shuba (talk) 15:58, 29 August 2009 (UTC)

Thanks for you contribution Tim. Although I think that we should say something about the aether and the MMX, the section that you removed had clearly been hijacked by crackpot science. Best to start again from scratch rather than argue endlessly about what exactly is wrong with each word.

Your work in improving the lead has been completely wasted. User:Abtract has decided that to rewrite it from scratch again. It is not that the new lead is that bad it is just that there is absolutely no point in trying to improve it as some other editor will then decide to rewrite it yet again.

I am beginning to think that trying to get this article back to being a FA by cooperative editing is a lost cause. There seem to be plenty of editors around who support this idea in principle but not may who are prepared to act in a way necessary to achieve this aim.

In my opinion a few more aggressive editing sprees by the right people would be most welcome. Martin Hogbin (talk) 10:56, 29 August 2009 (UTC)

In my opinion Abstract's lead should be adopted. If you differ, why not say why specifically, instead of urging your own form of words, which I find inaccurate for reasons explained at length here? It will not interfere with FA status, while your formulation will do so. Brews ohare (talk) 14:20, 29 August 2009 (UTC)

Approximate 300,000,000 m/s in lead paragraph?

Apparently we don't have as much consensus as it had appeared to start with the exact value in the opening paragraph. LouScheffer, Brews ohare, Abtract, and myself (Dicklyon) have all expressed a preference to open with 300,000,000 m/s and maybe also 186,000 mi/s approximation in the lead paragraph, and then introduce the complexity of a defined fixed value in a later paragraph, still in the lead. I don't think any of us are denying the fixed exact value, nor wanting to hide it, though we may have differing opinions on the deep philosophical implications of defining the speed of light and a system of units this way.

I understand that Martin Hogbin and some others are firmly set on wanting the exact value in the lead paragraph; but I'm not sure I understand why. Can we have some discussion on this again, without all the angst, but directed toward what makes a better wikipedia article? If each person with an opinion could limit themselves this weekend to one not-too-big well-thought-out position paragraph, we might start to collect some points of view and begin to understand each other. Please hold off a day or two on responding to each other and starting to argue the points, so that we can have the luxury of seeing who stands where on a simple issue for a change. Dicklyon (talk) 16:42, 29 August 2009 (UTC)

• As before, I've no objection to mentioning approximate values, as long as the exact value is also mentioned. Philosophicallly speaking the problems seem to be caused by different interpretations of the word distance. If you define distance between two points as the time it takes for light to travel (measured in seconds) then speed of light defined as d (s) /t (s) = 1 with no dimensions. If you define distance as the number of metres then speed of light defined as d (m) / t(s) = 299.... m/s exactly. Charvest (talk) 16:57, 29 August 2009 (UTC)

• Approximate in first para followed by 'exact' later in the lead. Imho wp:lead is clear in not advising too much detail too early. When that detail (nine significant figures and the word 'exact') may sidetrack readers before the context is known, I believe it should not be shown. I strongly favour not showing the word 'exact' until the history/context paragraph (of the lead) and I weakly favour using 300... in the first para because it is more user friendly (easier to read and all that most viewers need/want to know) whilst still being accurate (to three s.f.). This approach is aimed solely at helping the reader. Abtract (talk) 17:24, 29 August 2009 (UTC)

• I support the use of an approximate value at the outset, and an exact value, properly introduced, as in Abstract's Abtract |lead, later on. My reasoning is that early introduction of a nine or ten place number called "exact" for a speed of light that intuitively (and over centuries of history) would be thought of as a measurement, with a measurement error bar, along with the seemingly solipsist statement that the metre is "defined" to make it exact, is all just too much for a reader to absorb without more extended discussion. Arguing on the Talk page that a more extended discussion does appear in a later section "Speed of light by definition" is not too helpful, as that section is not mentioned in the intro. There is the further difficulty that the present formulation in the intro is in flat contradiction with the more correct presentation in the later "Speed of light by definition" section. Brews ohare (talk) 18:30, 29 August 2009 (UTC)

• If I might poke my head in here, I find it extraordinarly awkward that the opening paragraph on an article about the speed of light concludes by defining the length of a meter. I would suggest that the speed of light in m/sec simply be given, and then later in the article explain why and how this definition came to be.Clayhalliwell ([[User talk:

• Approximate first, "exact" and "defined" value later, still seems to me to be the right compromise; no need to complexify the opening with the idea of an exact defined value. As Millikan says, "It is sufficiently correct to remember it as 300,000 kilometers or 186,000 miles ." Dicklyon (talk) 22:12, 29 August 2009 (UTC)

• I think everyone else has given up the will to live but please remember there was a strong consensus not to do this. Martin Hogbin (talk) 21:53, 31 August 2009 (UTC)

• Speaking only for myself, in my opinion the exact value and its connection to the metre's definition is sufficiently important to include in the lead. Again in my opinion, an approximation in SI units is unnecessary in addition to the exact figure because the exact figure is comprehensible to those who think metric; unlike Martin, though, I don't having a strong objection to adding "(approximately 300,000 kilometres per second)" to the exact figure. On the other hand, I do favor adding "(roughly 186,000 miles per second)" for those general readers who think in US customary units; WP:MOSNUM ¶ 4 supports this, as does the more fundamental principle of making the lead, especially, friendly to the broad spectrum of general readers. —Finell (Talk) 23:09, 31 August 2009 (UTC)

• I think the fact that the speed of light is used as a standard to define the meter, and therefore has an exact value, is such an important fact that it has to be mentioned in the first few lines in the lead. Similarly, the article about the kilogram, mentions the artifact that defines it in the first few sentnces of the lead. The approximate "value" (the mass of one liter of water) is mentioned afterwards. Count Iblis (talk) 01:53, 1 September 2009 (UTC)

Discussion of positions stated above

• We have just had a poll on this very subject (18 August), the votes were:

• A. Show only the approximate speed 1 Vote

• B. Show the approximate speed, followed by the exact speed 2 Votes

• C. Show the exact speed after the current first sentence 8 Votes Martin Hogbin (talk) 17:52, 29 August 2009 (UTC)

• Thoughts move on ... why not just give us your views? Abtract (talk) 17:59, 29 August 2009 (UTC)

Martin, as Abtract suggests, can you give us your reasons above, as we've re-opened this question and apparently the old vote is no longer representative? Dicklyon (talk) 22:12, 29 August 2009 (UTC)

If you want to assert that after just 11 days the old vote is not representative, I guess you should contact all those who voted last time and get their votes again. Martin Hogbin (talk) 22:49, 29 August 2009 (UTC)

I'd guess that Martin Hogbin might provide his notions as to why the exact value should appear and any thoughts he might have as to why the intro should begin with a startling conundrum. Appealing to a majority vote in the past does little to flesh out the reasons for Martin Hogbin's support. Brews ohare (talk) 22:58, 29 August 2009 (UTC)

• The discussion in this article is hard to follow .

Not being an expert i do know .

The speed of light can be measured to 10^-11.

Time can be measured to 10^-11.

Distance or length can not be measured with that accuracy except for the Kaisers foot when he was still alive.

What is the problem with by definition m=l/t ?. Wdl1961 (talk) 18:08, 29 August 2009 (UTC) p.s. Pi has been calculated to at least a million decimal places so you are way behind.

In response to Brews:

• • The statement in the lead section is in no way in contradication with the longer section that appears later. The problem is that Brews simply cannot accept that there is no problem with fixing the speed of light in SI units. S/he seems to feel that it is better to fix in terms of the length of some arbitrary bit of metal. This argument has gone on over many pages, and should stop forthwith. Brews, if you don't shut up on the question, I will ask for you to be banned from all pages relating to the speed of light in any way. Consider yourself warned. Physchim62 (talk) 19:47, 29 August 2009 (UTC)
    • Physchim62 might do well to reflect on the fact that threats rarely work and often rebound on the threatener ... time spent reaching the best wording, on the other hand, is rarely wasted. Abtract (talk) 20:15, 29 August 2009 (UTC)
      • I am in no position to do anything except ask, so that can hardly be construed as a threat. I would not be the first editor to complain about Brews' behaviour. In the meantime, editing this article (and others like it) is wasted effort while we pander to the pseudoscience spouted by a couple of vociferous soapboxers. Physchim62 (talk) 21:34, 29 August 2009 (UTC)

Physchim62, I'd say your contribution on my user page is a threat: see Physchim62 threat. Your summary of my views in that threat as “You wish to advance a point of view which is quite obviously held only by an extreme minority, that is that most of physics was destroyed by the decision of the CGPM to fix the speed of light in SI units in 1983.” is complete balderdash. I defy you to cite any statement of mine that even borders on such a view. It is nuts! I do want their views correctly stated, as per solid sources: Wheeler; Jespersen; Sydenham, etc. I do not want incorrect inferences about their position, such as presently provided in the WP intro. Moreover, nothing said here by me is disruptive: my contributions all are a plea to deal with these sources directly, which pleas have resulted in repetitive harangue & hectoring, without sources or reasoning. What are your motivations for that behavior, eh? A better article, or just a chance to vent inner hostility?? Why do you attribute to me statements made up by yourself?? Brews ohare (talk) 22:47, 29 August 2009 (UTC)

In response to Dicklyon:

• • A value can only be exact if it is defined. It's really no different from saying that the circumference of the Earth used to be exactly 40,000 kilometres (at last if you measured it along the Paris meridian). The speed of light is exactly 299 792 458 m/s, by the current definition of the metre. We're not helping anyone by pretending otherwise. Physchim62 (talk) 22:26, 29 August 2009 (UTC)

I think I already stipulated my agreement to that. Dicklyon (talk) 22:33, 29 August 2009 (UTC)

Physchim62: Enter the world of this discussion. Your comment is totally off the subject, and a point no-one disputes. Brews ohare (talk) 22:49, 29 August 2009 (UTC)

According to Dick, 'the old vote is no longer representative'?? Martin Hogbin (talk) 22:57, 29 August 2009 (UTC)

Right; in the sense that we now have at least 4 people pushing to start with the approximate value. Dicklyon (talk) 23:31, 29 August 2009 (UTC)

In what way is giving the wrong answer less complex than giving the correct one? Martin Hogbin (talk) 23:16, 29 August 2009 (UTC)

There's a difference between "wrong" and "widely used approximation". Dicklyon (talk) 23:33, 29 August 2009 (UTC)

But you ware claiming above that giving the actual (rather than approximate) value complexified things. Why is this? Martin Hogbin (talk) 23:40, 29 August 2009 (UTC)

In response to Clayhalliwell:

Thanks for your interest. The meter has been defined using the speed of light. As a direct result of this definition the speed of light is fixed at an exact value when expressed in m/s. There have been endless discussions on how we should present this information (dramatically complicated by a couple of editors who do not accept or understand the effect of the definition). For the majority who do accept the definition of the metre we have these options:

1. Just give the figure
2. Give the figure stating that it is an exact value
3. Give the figure and say that it is exact due to the definition of the meter (current wording)
4. Give the figure with a more detailed explanation as to why it is exact.

What do you suggest? Martin Hogbin (talk) 23:14, 29 August 2009 (UTC)

Martin Hogbin, this list of options leaves out the subject under discussion, namely: putting an approximate value first, and filling in the exact value later, so as to avoid a startling and (at first glance) solipsist statement of the defined exact value. Brews ohare (talk) 23:35, 29 August 2009 (UTC)

One doesn't have to agree that the exact value is startling or solipsist to be in favor of starting with the approximate value. We did that long before the 1983 change, too, you know, like in the 1963 Feynman lectures on physics. Using an approximation is in no way a denial of the true value, nor have we heard from anyone who can credibly be interpreted as outside "the majority who do accept the definition of the metre". Dicklyon (talk) 23:41, 29 August 2009 (UTC)

That is true. Earlier on the talk page is a google search link that shows a ton of sources that begin with an approximate value. Some never provide the exact value at all. Brews ohare (talk) 23:46, 29 August 2009 (UTC)

Many books may use the approximate value, because it is convenient and makes calculations simple, not because they think their readers will find the number 299,792,458 harder to understand than the number 300,000,000. This is an article about the speed of light. Many of our readers will want to know what the speed of light is, and there is no reason not to tell them. Martin Hogbin (talk) 23:49, 29 August 2009 (UTC)

You'll get no argument there; also many want an approximate value they can remember and use, and there's no reason not to tell them that, too; and first, like many books do. Or is there a reason? Dicklyon (talk) 23:56, 29 August 2009 (UTC)

We've already got spacetime and gravitational waves up there in the lead, and yet there is a desire to make the actual value of the speed of light simpler! Erm, who are we writing this article for? Physchim62 (talk) 00:06, 30 August 2009 (UTC)

It's not clear what you're suggesting; can you state your position on the topic in the section above? Dicklyon (talk) 00:33, 30 August 2009 (UTC)

It is perfectly clear what Physchim62 is suggesting; that we keep the exact number because it is really not that hard to understand. Martin Hogbin (talk) 11:42, 30 August 2009 (UTC)

Physchim62: Um, were we discussing these topics?? Or, perhaps you are proposing, like these topics, a link to a separate article Speed of light (1983 definition)? Brews ohare (talk) 00:18, 30 August 2009 (UTC)

• Heh... honestly, I can barely tell what the point of contention is. The only thing I see wrong with the intro is that it includes the definition of a meter, which belongs in the meter article, not here. If I were feeling sufficiently bold, I'd rewrite the intro as follows--

The speed of light is a fundamental physical constant, the speed at which light and all other electromagnetic radiation travels in a vacuum. It is usually denoted in physics by the letter c. The speed of light is precisely 299,792,458 metres per second, though it is commonly approximated as either 300,000 kilometers per second or 186,000 miles per second.

• Yeah, it's kind of heavy on links, but this is a scientific definition after all. Clayhalliwell (talk) 01:20, 30 August 2009 (UTC)

I think that many editors here would be happy with that but for some reason that I have never been able to divine, Brews strongly objects to stating the exact value first and Dicklyon and Abtract have some lesser objection to doing this. Eight other editors have given their support with stating the exact value first. There still would be some polite discussion as to which of the options I have stated above we should use but I do not think that there are any strong feelings in that respect. If Brews goes off to set up his own article, the test of us could get on with improving this one by cooperation and consensus. Martin Hogbin (talk) 09:19, 30 August 2009 (UTC)

Martin has chosen to misinterpret my actions as "setting up a separate article" and planning to "go off". My objectives were clearly stated in my opening remarks upon creating the article that appear below.

I've introduced a new article Speed of light (1983 definition) which I hope will have two effects: first, it presents a very clear statement of the situation, and second, it may draw the venom expressed on this page away to that one where hopefully the greater detail will introduce a real discussion of sources and contents to replace harangue and hectoring. Brews ohare (talk) 01:33, 30 August 2009 (UTC)

• Ummm... shouldn't that article be Metre (1983 definition)? It's the length of the meter that's changed, not the speed of light. The speed of light is kind of a, wossname, universal physical constant. The last time it changed was sometime around the Big Bang, not 1983. Clayhalliwell (talk) 01:42, 30 August 2009 (UTC)

Clayhalliwell: It is as you say that the definition of the metre changed, but that in turn changed the role of the speed of light from a measured value of a constant of nature to a defined exact conversion factor, whose only relation to the actual speed of light is historical accident. (In the SI system of units, that is, not in nature proper.) If the 1983 committee convened a century ago, the defined speed of light might be 300,000,000 m/s today. Brews ohare (talk) 02:23, 30 August 2009 (UTC)

So after an extended period of tendentious editing during which Brews ohare came up on the short side of consensus, his answer is to create a content fork. That's a rather obvious candidate for a deletion discussion and I'd be surprised not to see it there shortly. Tim Shuba (talk) 03:20, 30 August 2009 (UTC)

It seems bizarre to me, given that the discussion here hasn't been about whether any of that material would be acceptable here – almost all the talk has been about the lead. Has he had a problem getting material into the article other than in the lead? Dicklyon (talk) 04:34, 30 August 2009 (UTC)

This material in Speed of light (1983 definition), which is a rewrite of material in the "Speed of light by definition" section, has so far not proved contentious (never mind the incorrect claim by Tim Shuba that it "came up on the short side of consensus"). However, I do not think the editors battling over a short summary for the intro have digested this material, so I hope creation of this article will bring it to their attention, and that understanding of the issues will replace hectoring and harangue. Brews ohare (talk) 06:18, 30 August 2009 (UTC)

Before a deletion (or a merge) discussion occurs, it would be nice to have a hiatus during which we might see how or whether matters settle. As said earlier, it may be that the longer and more complete discussion of the situation in Speed of light (1983 definition) might lead to a more calm and accurate assessment of the very much shorter version to be put into the lead. For example, it provides a basic understanding for why the lead proposed by Abstract is preferable to the present lead. In any event Tim Shuba is on record as saying: “Primarily, I use wikipedia for enjoyment rather than attempt to edit seriously. Since I am aware of how and why so much blatantly bogus information gets into articles, and why a large number of articles are highly unreliable, it doesn't affect me adversely as a user. Therefore, whether the speed of light article gets better or gets worse isn't too important to me.″ so he should have no difficulty putting his plans on hold. Brews ohare (talk) 05:25, 30 August 2009 (UTC)

I think your article is an excellent idea, Brews. Here is the deal, you stick to editing your own article and leave this one alone and I will agree not to nominate yours for deletion. I cannot speak for others, however. Martin Hogbin (talk) 09:22, 30 August 2009 (UTC)

I have an even better idea ... we all discuss content in a way that will produce an article helpful to readers of all 'standards', consistent with guidelines (in the current context wp:lead) would be a good one to look at), well sourced (and consistent with those sources), and formulated in such a way that brings to an end this unseemly arguement. ownership, name calling and veiled insults have no place in this debate. Abtract (talk) 10:05, 30 August 2009 (UTC)

I would agree with you if it were not for the fact that that was exactly what we were trying to do before you decided to rewrite the lead. As I have already said, the current lead is not perfect (neither was it written by me) and I am happy to discuss ways of improving it by incremental changes. The one thing we must draw a line under is the consensus to show the exact speed of light in m/s as the first numerical value and in the first paragraph. If we can stop the endless argument about that point then I am all for it, are you? Martin Hogbin (talk) 11:36, 30 August 2009 (UTC)

If I recollect, the marginally re-written lead that I put in a couple of days ago that has been reverted out had just that ... not my ideal because I favour using 300 first to help readers not because I have a hangup about it. I left out the word 'exact' in the first para but included it in the second after due explanation and context building. My slightly rewritten lead was designed to be a compromise between Hogbin and ohare that should (I thought) satisfy you both ... so far ohare has agreed it but Hogbin has simply dismissed it as unencyclopedic with no detail as to what needs changing despite many requests to provide such. Please look again at my proposed lead and tell me what is wrong with it. Forget your entrenched position and look at it from the point of view of an average reader.Abtract (talk) 11:54, 30 August 2009 (UTC)

First you say let us stop arguing and get on with improving the article and when I agree to do this the first thing you do is start arguing about the original topic. In what way is the exact figure a problem for the average reader? Martin Hogbin (talk) 12:28, 30 August 2009 (UTC)

Dicklyon's efforts above have established support for the approach taken by Abtract. This whole matter could be settled at once by acceptance of Abtract's lead here, to which you have provided no detailed objection, but only harangue. Brews ohare (talk) 12:46, 30 August 2009 (UTC)

New lead ... further refined

I have made two edits to the lead; please look at them separately. 1) I have moved the first part of para 2 into the start of para 3 and compressed paras 3 and 4 into one; this is to keep all history together and to keep all usage/importance together ... no content was changed in this edit. 2) I have added content to the history para (not as much as I would like but we can discuss adding more when we have agreed the format) in order to make the struggle (for an accurate measurement) clearer and to clarify/expand the explanation of the 1983 decision which after all was quite important. You will see that I have left the first para unchanged though I still favour change ... when discussions above have concluded maybe change will come, maybe not. I have made these edits because the only objections to my full re-write seem to concern the use of 300 in the first para. I hope this improved and slightly expanded lead will enable editors to see the first para in context and perhaps we can move forward to agree a compromise that will satisfy us all and help the reader. Abtract (talk) 13:38, 30 August 2009 (UTC)

I still agree with this introduction of yours. You did not reinstate the missing gravitational wave reference. Brews ohare (talk) 14:27, 30 August 2009 (UTC)

Sorry could u do it pls. Abtract (talk) 15:11, 30 August 2009 (UTC)

Done. Brews ohare (talk) 15:24, 30 August 2009 (UTC)

I'm not a fan of either of those two edits. The second adds too much detail to the lead. The first I think makes it less logical than it was before. A better fix would be to just remove the sentence "Exceptions are when precise time measurements or very long distances are involved," thereby letting the sentence before it lead to the sentence after it better. Mixing this sentence with the actual importance of c in spacetime makes a mess and a nonsequitur, I think. Dicklyon (talk) 16:39, 30 August 2009 (UTC)

I would be happy if you made that edit; I have never thought that sentence worthy of the lead. Abtract (talk) 17:05, 30 August 2009 (UTC)

Done. It wasn't correct, anyway, since we routinely use delta-t measurements well below a nanosecond to resolve range in time-of-flight rangefinders down to cm and less. And even when we're not measuring time, we need it to design antennas and such. Dicklyon (talk) 17:24, 30 August 2009 (UTC)

You took it far further than I had thought you were suggesting ... you also removed the useful extra content I had added as well as removing the sentence in question - I guess I didn't notice the word "just" in your suggestion. I am going to put it back, hopefully without starting another war, as it adds considerably to the ease of understanding 1983 imho. Abtract (talk)

I attempted to put the image before the table, but couldn't get the formatting to work. I think the article would look much nicer if it started with the picture. Dicklyon (talk) 17:29, 30 August 2009 (UTC)

Finell has made some good improvements to the lead. So far, there seems to be little opposition to also doing the change to start with the approximate value (that, the few who are against that idea have so far declined to start a position to explain why). Dicklyon (talk) 20:28, 30 August 2009 (UTC)

Removing the history and emphasising relativity is surely a backward step and most certainly is not an improvement. I am giving up until sanity returns. Abtract (talk) 23:06, 30 August 2009 (UTC)

Dear Abtract: Please don't assume, or charge, that everyone who disagrees with your very substantial rewrite of the lead is insane. I did not have a problem when there was a little bit of history in the lead, but your last version introduced too much redundancy; a chronology of various estimates or measurements of c is too much detail for a lead section. The following sentence was always problematic: "Throughout much of human history, whether light traveled at infinite speed, or simply very quickly, was unknown." Infinity was not a well developed concept throughout most of the undefined historical (or pre-historical) period to which that sentence referred, and "infinite speed" is better described as "instantaneous", but the subject can be better treated in the history section than in the lead. Relativity was always a main subject of the lead, but without mentioning it by name; I added the name. The following sentences made assertions that are head-scratchers to anyone who doesn't already understand the topic fully (that is, the general reader): "For technical reasons in 1983, the metre was redefined using this value as a conversion factor. This had the effect of 'fixing' c at exactly 299,792,458 metres per second in the International System of Units." Thank you for your contributions to this article; I hope that you will continue to try to improve the article. —Finell (Talk) 04:17, 31 August 2009 (UTC)

I too am puzzled by Abtract's outburst about "sanity"; and the concept of it returning presumes that it was once here, so one has to wonder what part of recent history he figures that was. Anyway, Abtract, I'd be interested in understanding your POV is you're up to it. Personally, I think that the role of the speed of light in spacetime and relativity is the main reason why it's an important concept, and that ought to be in the lead. I like history section, but don't like to see so much of it in the lead. Dicklyon (talk) 05:00, 31 August 2009 (UTC)

Abstract's words "For technical reasons in 1983, the metre was redefined using this value as a conversion factor. This had the effect of 'fixing' c at exactly 299,792,458 metres per second in the International System of Units." were what I liked most about his proposed intro, and IMO is preferable to the present intro, which at best is ambiguous and more probably misleading. Brews ohare (talk) 05:04, 31 August 2009 (UTC)

I don't believe that's what he was referring to in his sanity comment, but please can you quote the part of the present lead that you think is ambiguous or misleading? Dicklyon (talk) 05:32, 31 August 2009 (UTC)

Hi Dick: I'm surprised you ask. The main issue is that the discussion of the redefinition of the metre and its implied redefinition of the speed of light is complex, and simply stating that c has the exact value of 299,792,458 m/s is misleading because the reader is likely to take the same view as centuries of history prior to 1983, namely, the reader will take it that this new SI units c refers to the measurable speed of light that underlies EM and relativity. That is not so, and that is what the entire argument with Martin and his associates has been about.

From your question, I take it that either you have not been following things here at all, or perhaps you don't see that the changed definition of the metre also changed the meaning of c within the SI units. (Of course, the concept of c in general physics is not affected by what happens in the SI units.) All this is discussed very exactly in the section on "Speed of light by definition" that you just finished editing. I'd guess that you edited for style, but did not absorb its content, eh? You and ten other people.

So there is the issue, not acknowledged and not discussed despite what I consider amazingly patient attempts to get the issue out in view in the face of dramatic violation of WP protocol by the "exact-value fanatics" in innumerable ways. I take it that nothing will be done here.

Martin's formulation is now back just as it was at the outset. The notion you advanced, that the combination of an "exact value" and a seemingly solipsist accompanying definition of the metre were a lot for the reader to swallow in one sentence, now has been dropped altogether, without argument or resistance. Abstract's attempts to make it right have been walked over.

As Brew-ohare's corollary to Peter Jackson's law Number 275 says: "Where a preponderance of editors share a common view, no amount of contrary sources or opposing logic will affect the WP article."

When the idea of FA status rises later on, I expect all this to resurface, probably with no more civilized or reasonable reception. It is my hope (if not my expectation) that when the FA review is made, the "exact value fanatics" will be seen for what they are, and will be sent to hard labor camp. Brews ohare (talk) 06:07, 31 August 2009 (UTC)

Quite frankly, I'm surprised I asked, too. It seems sometimes that I never learn. I'll take your answer to mean that you believe that "the metre is defined so that c has the exact value of 299,792,458 metres per second" is ambiguous or misleading. Dicklyon (talk) 06:31, 31 August 2009 (UTC)

Hi Dick: Remember this? Whatever happened to this idea? As for what I think? I think the 1983 redefinition of the metre accompanies a 1983 redefinition of the speed of light to place it beyond measurement, and simply stating that “c = 299,792,458 m/s exactly by definition of the metre” hardly constitutes an understandable presentation, particularly as the 1983 c is not the real, physical c that was measured circa 1972 to obtain the number 299,792,458 m/s. Abstract's intro made this clear. Maybe WP could draft language for sub prime mortgages too? Brews ohare (talk) 15:03, 31 August 2009 (UTC)

I believe I asked you to show a source to support your POV that "the 1983 c is not the real, physical c that was measured circa 1972". I don't argue with the sources you've shown us, but if they say that, I've missed it, so maybe you can provide a quote along those lines? Dicklyon (talk) 15:30, 31 August 2009 (UTC)

Well, the imprecision of language is hard at work here. The 1960 definition of the metre is cited. It is in terms of wavelength. I'd guess that you'd agree that with this old definition it was found by measurement (counting of fringes and dividing by measured time) that c = 299,792,458 m/s, approximately. I'm sure you'd agree as well that post-1983 a conversion factor is defined of 299,792,458 m/s, which is used to convert times-of-transit to lengths. That is well sourced as well. So the issue seems to be, is the c that can be measured the same thing as the conversion factor that is exact and cannot be measured? How say you to this formulation? Brews ohare (talk) 16:07, 31 August 2009 (UTC)

The problem here is finding a source that uses the term speed of light so carefully. Although the distinctions above are made, and the differences between a conversion factor and the earlier speed of light is sourced already, I don't know if a source can be found that says simply "a conversion factor is not the same thing as the real, physical speed of light". Brews ohare (talk) 16:20, 31 August 2009 (UTC)

Does an example help? If the real speed of light increases as the universe evolves, say, the metre will lengthen because it is based upon a fixed time of 1/299,792,458 s. If the separation between A and B is given,the greater speed of light means light gets from A to B sooner. But in SI units the longer metre means the distance from A to B is fewer metres. Thus, the speed of light in SI units hasn't changed (it's still 299,792,458 m/s) but it is the case that the light takes less time to transit, because A & B are fewer metres apart. Brews ohare (talk) 16:44, 31 August 2009 (UTC)

As for putting the approximation in the lead paragraph, I was hoping to hear some arguments against it first, before deciding; but if Martin and others aren't going to state their positions, maybe we should go ahead. Dicklyon (talk) 15:32, 31 August 2009 (UTC)

I think everyone else has given up the will to live but please remember there was a strong consensus not to do this. Martin Hogbin (talk) 21:53, 31 August 2009 (UTC)

Martin: Dick made a reasonable request for your reasons for not putting an approximation in the lead, not merely an assertion of consensus. Speaking only for myself, in my opinion the exact value and its connection to the metre's definition is sufficiently important to include in the lead. Again in my opinion, an approximation in SI units is unnecessary in addition to the exact figure because the exact figure is comprehensible to those who think metric; unlike Martin, though, I don't having a strong objection to adding "(approximately 300,000 kilometres per second)" to the exact figure. On the other hand, I do favor adding "(roughly 186,000 miles per second)" for those general readers who think in US customary units; WP:MOSNUM ¶ 4 supports this, as does the more fundamental principle of making the lead, especially, friendly to the broad spectrum of general readers. —Finell (Talk) 23:09, 31 August 2009 (UTC)

Finell, I don't think anyone has proposed to NOT put the exact value in the lead; just not in the first paragraph. As for the approx. value 3x10 m/s, it's so commonly found in physics books that I can hardly imagine omitting it. Dicklyon (talk) 05:46, 1 September 2009 (UTC)

I think the fact that the speed of light is used as a standard to define the meter, and therefore has an exact value, is such an important fact that it has to be mentioned in the first few lines in the lead. Similarly, the article about the kilogram, mentions the artifact that defines it in the first few sentnces of the lead. The approximate "value" (the mass of one liter of water) is mentioned afterwards. Count Iblis (talk) 01:53, 1 September 2009 (UTC)

I copied your statement, and Martin's, and Finell's into the section above where I asked for them, so that it will be easier to see where we stand; if you object, undo. Dicklyon (talk) 05:41, 1 September 2009 (UTC)

Real physical versus conversion factor

These changes still do not implement the formulation of Abstract's lead, and in my opinion, the present WP lead sloughs over the change in the meaning of c that accompanied the change in definition of the metre, as very clearly described in Speed of light (1983 definition).

For those unwilling to consult that article, here is a summary: The change in definition of the metre made c a mere conversion factor within the SI system of units (a very well-sourced statement), reducing c in the SI units to a mere matter of definition. Actual measurement of the real, physical, speed of light is no longer do-able within the SI units and requires introduction of a unit of length different than the metre (e.g. a wavelength of some transition), inasmuch as the metre simply expands or contracts to maintain the defined value of c. If your metre adjusts to maintain your c, changes in c due to measurement improvements become undetectable using the metre by itself (another well-sourced statement). Brews ohare (talk) 17:41, 30 August 2009 (UTC)

I think we have a pretty good consensus that none of that needs to be addressed in the lead. Why don't you work on putting it into an appropriate section, and get rid of your content fork? Dicklyon (talk) 17:48, 30 August 2009 (UTC)

Well, I can simply replace the existing section. That done, inasmuch as no-one seems to get the point, the lead will be readily interpreted as in contradiction with the article. C'est la vie, I guess. Abstract's lead gets the point across correctly without going into undue detail. Brews ohare (talk) 17:53, 30 August 2009 (UTC)

I wouldn't do that. The existing section is probably close, and the new article is huge, which would give your obsession undue weight. Can't you fix the existing section? What's wrong about it at present? Dicklyon (talk) 20:28, 30 August 2009 (UTC)

Hi Dick: Well, I thought that was your advice, so now it is done. I think what is said in the subsection has to be said. Maybe it could be said better or more briefly. However, some length can be attributed to CYA and adding sources to that end. Also, so far as I can tell, nobody has read it, probably a good thing, as nobody appears likely to take the time to understand the subject or read the sources. Otherwise, the conflict over the lead would have been settled long ago. Brews ohare (talk) 20:33, 30 August 2009 (UTC)

I toned it down a bit. I didn't see a source supporting your idea that there's a "real, physical" speed of light that's different from what everyone else calls the speed of light, which was part of what seems to frame your POV and your writing. I have no objection to the notions of "conversion factor" and "real, physical"; it's just that if we introduce such notions they need to be sourced; yes, the actual terminology, esp. if emphasized in the article, needs to be sourced. Dicklyon (talk) 21:44, 30 August 2009 (UTC)

Hi Dick: The real physical speed of light is the speed of light that was measured pre 1983 to obtain the measured estimate 299,792,458 m/s. Now (post 1983) that the metre adjusts to force a conversion factor between transit time and length of 299,792,458 m/s, within the SI system this conversion factor is fixed, and not measurable. I believe all these points to be tied down by multiple published sources, a subset of which is found in the WP article here. Do you? Brews ohare (talk) 15:27, 31 August 2009 (UTC)

Brews: You've just put your finger on the nub of the problem, if there is one. Please show us when the length of the metre fluctuated since the 1983 redefinition due to a change in the physical speed of light. I believe that your statement reveals the source of your misunderstanding. The speed of light was chosen as the standard of measure because it is constant in fact (at least that was one of the reasons); it is not constant because it is the standard, notwithstanding the definition. We have a very good theory, which is consistent and well confirmed by experiment, that tells us that the speed of light is constant. Further, extensive measurements of high accuracy confirm that the speed of light is constant. As has been pointed out to you several times before, the issue would be the same with any standard of measure.

• If the metre were defined in terms of the wavelength of a particular atomic transition, then the length of the metre would fluctuate with the wavelength (if the wavelength fluctuated) and the wavelength would become unmeasurable in the standard unit.
• If the metre were defined in terms of the length of a particular metal bar in a particular vault, then the length of the metre would fluctuate with the bar (if the bar's length fluctuated) and the bar would become unmeasurable in the standard unit.

Please try to understand this once and for all. If you cannot show us an instance when the length of the metre fluctuated due to a change in the physical speed of light, please drop this issue and stop repeating the same argument that you have been repeating over and over and over again. If you can't drop the issue, then find a more receptive audience elsewhere. It isn't fair to you to keep banging your head against a wall, and it isn't fair to the rest of us to be battered by you. Thank you. —Finell (Talk) 17:12, 31 August 2009 (UTC)

Finell, as apparently I hurt your feelings when I did not respond to your remarks about metre fluctuating with wavelength, let me try here. This fluctuation issue is not part of my argument about a "conversion factor" vs. the "real speed of light", but a completely different topic. This may be more clear to you if you look at the example I suggested to you below once more. Bear in mind, this is a hypothetical example to illustrate the logical difference between these two concepts and their connection. It is not a comment upon how the universe works or might work. It is just to clarify the difference between terms. Brews ohare (talk) 05:41, 1 September 2009 (UTC)

Finell: Your impatience is born of frustration in understanding the issues, not in my behavior beyond a lack of eloquence. I have tried several different approaches: mathematical equations, cited sources, logic. Most recently I have tried this example to distinguish between the real speed of light and the conversion factor; please have the patience to let me know if this example makes sense to you. I'll review it below:

Take two points A & B. If the real speed of light increases as the universe evolves, say, the metre will lengthen because it is based upon a fixed time of 1/299,792,458 s. (The metre is the distance traveled by the real speed of light in 1/299,792,458 s.) If the separation between A and B is fixed, the greater speed of light means light gets from A to B sooner. But in SI units the longer metre means the distance from A to B is fewer metres. Thus, the speed of light in SI units hasn't changed (it's still the conversion factor of 299,792,458 m/s) but it is the case still that the light takes less time to transit, because A & B are fewer metres apart.

This example shows (to me at least) that the real speed of light (for example the number of wavelengths traveled per second) is different form the conversion factor of 299,792,458 m/s. The real speed of light can change (in principle), the conversion factor is defined and lies outside measurement, and is not a property of nature.

I hope you will take the time to explain to me whether you have any difficulties with using this example to illustrate that there are two different conceptions involved here: the real speed of light (which can be measured in wavelengths/s), and a fixed conversion factor in the SI units. Brews ohare (talk) 20:03, 31 August 2009 (UTC)

Brews: I think that I have a decent grasp on the issue, actually, and I understand what you are saying (I understood it the first time I read it several reiterations ago). I don't accept your premise that it is reasonable to expect that "the real speed of light increases as the universe evolves". I am not prepared to assume something that is contrary to both well established physics and all reliable measurements of the speed of light. Likewise, as I said before (to Tombe or you), if the real speed of light were to surprise us all and change, the consequence would be immediate re-definition of the meter would upon a different, more stable standard; the worlds' standards organizations would not, as you and Tombe assume, simply accept a changed real metre. Now, please answer my question: Has the length of the metre changed since the 1983 redefinition due to a change in the physical speed of light? Yes or no, please. If yes, please cite a reliable source. —Finell (Talk) 20:50, 31 August 2009 (UTC)

PS: At a deeper level, I suspect that a change in the real c would be undetectable by any means (or, equivalently, that it is meaningless to talk about a change in the real c) because c is the thread from which spacetime is woven. But that is not part of my objection to the Tombe-ohare thesis. —Finell (Talk) 20:50, 31 August 2009 (UTC)

If I recall correctly, only certain dimensionless parameters like the fine structure constant are directly detectable. In any case, the value of the meter is continually checked against other standards by people who know a lot more about this then we do. If the assumptions that Brews is worrying about turns out to be incorrect in any way it would be BIG news and everyone would know about it. The fact that the meter is defined using a defined value for c makes it MORE likely, not less that any variance of c with time or location, if it exists, will be detected. TStein (talk) 21:11, 31 August 2009 (UTC)

Finell, TStein you amaze me that you are off discussing whether the speed of light does change, can be observed to change, blah, blah, blah. The example is entirely hypothetical and it isn't necessary that it actually happen. The point is that if it happened it illustrates the difference between the real speed of light and a conversion factor. It is an exercise of illustration, for Pete's sake, not a physics theory, not a study of what BIPM might do if it happened, and not a question of assumptions about the real universe. Brews ohare (talk) 21:45, 31 August 2009 (UTC)

Finell, I also am amazed that you connect me to "Tombe-ohare thesis" whatever you think that is. There is no such thing. Wake up please. Brews ohare (talk) 21:45, 31 August 2009 (UTC)

In any event, to your great relief (and mine) I wish to discuss this matter with you all no longer. Brews ohare (talk) 21:45, 31 August 2009 (UTC)

Brews, please stop putting your nonsense about this subject in the article. Martin Hogbin (talk) 22:10, 31 August 2009 (UTC)

Martin: I have put absolutely no nonsense in the article. If you don't like this section, plese indicate why. Brews ohare (talk) 22:51, 31 August 2009 (UTC)

I'm willing to help with this dispute, either by asking for the article to be protected and the two of you blocked, or by trying to get some convergence between your warring positions. The latter would require that Brews stop adding stuff not explicitly supported by cited sources, and the Martin not remove stuff that cites sources without first explaining why the sources are not adequate to support such. This opinion-based edit warring is just getting old. Dicklyon (talk) 05:49, 1 September 2009 (UTC)

Excellent solution. I second the motion. —Finell (Talk) 06:02, 1 September 2009 (UTC)

This is not opinion-based edit warring it is the idiosyncratic views of one editor compared with the established scientific views on the subject. The current definition of the meter has been made clear by the world's standards authorities, who have found no problems or difficulties with it. personal opinions on the subject should form no part of this article

I have made three one-off deletions of unsourced and innacurate material. Martin Hogbin (talk) 08:53, 1 September 2009 (UTC)

It seems to me if a paragraph is going to talk about spacetime and conversion between mass and energy, it should mention Einstein's theory of special relativity. If my statement of the history is inaccurate in some way, please fix it and cite the source that you believe is necessary, rather than simply deleting the statement. I don't doubt your command of physics and I am not siding with fringe viewpoints. Thank you. —Finell (Talk) 16:18, 1 September 2009 (UTC)

Please explain the purpose of this section

Rather than making threats to try to get me blocked, perhaps Dick and Finell would like to try to explain the purpose of the section that I deleted and have shown below.

Although much of what is stated is indeed true, the purpose of the section is clearly to suggest that the previous definition of the meter is in some way superior to the current one because it (so it is claimed) lets us measure the speed of light.

Note that the two references given do not support the assertions made one might like to 'measure the speed of light, not just use the SI system with its defined relationship between the metre and the speed of light, that can be done by introducing a unit of length other than the metre'. The first just supports the undisputed fact that the pre-1983 definition of the meter was based on the wavelength of a specified radiation and the second the undisputed fact that interference fringes were counted in the measurement of distance.

Martin, you removed a whole section, which cited two sources, because you don't like what you believe to be its purpose. It would be better to say what's wrong about it, make it conform better to what the sources say, add more sources if you find different points of view on the topic, or say why you think that topic is not relevant in this article. I already did a pass or two of taking out some the outrageous Brews ohare POV; if some is left, and not supported by the sources, do the constructive thing and help fix it. Don't ask us to explain the "purpose" of a section; it has no purpose but to explain some stuff. Dicklyon (talk) 16:24, 1 September 2009 (UTC)

Dick, I believe that I, or any other editor, are entitled to get a bit heavy handed with two things. The first is the continued attempts by Brews to get his fringe views into this article in some way. Have a look at the title that Brews originally gave this section. The second (which is not relevant here) is the attempt by some to ignore a very recent poll of editors as to what value should be given first in the lead.

The section I deleted had no purpose other than to get Brews fringe views into the article. If we want to say something more about the subject we would be better giving more detail as to how the meter is actually realized using the current standard, although that would probably be better in the metre article. The section I deleted was essentially about the way the metre was realized over a quarter of a century ago followed by a claim that the pre-1983 definition allowed the speed of light to be measured.

Finally, I should point out that I deleted just once and did not engage in edit warring. Your threat to try to get me blocked is completely unjustified. Martin Hogbin (talk) 18:17, 1 September 2009 (UTC)

Martin: I don't remember doing anything with that section. If I have forgotten, please point out what I did. I have not suggested that the prior definition of the metre was superior. Please do not confuse me with those who have been pushing fringe views, which I have consistently argued against. Further, I never threatened to have you blocked. I would, however, appreciate it if you would try a more collaborative mode of editing and discussion. That might help to achieve a more readable and accurate article, with broad consensus (although not necessarily unanimity) of the participating editors, within our lifetime. —Finell (Talk) 16:37, 1 September 2009 (UTC)

Finell, I was referring to this conversation above:

I'm willing to help with this dispute, either by asking for the article to be protected and the two of you blocked... Dicklyon (talk) 05:49, 1 September 2009 (UTC)

Excellent solution. I second the motion. —Finell (Talk) 06:02, 1 September 2009 (UTC)

I am perfectly willing to collaborate with anyone except Brews while he continues to push his fringe opinion. Martin Hogbin (talk) 17:15, 1 September 2009 (UTC)

Measuring the speed of light

If one wishes to measure the speed of light, not just use the SI system with its defined relationship between the metre and the speed of light, that can be done by introducing a unit of length other than the metre, for example the wavelength of an atomic transition, as was done in the earlier definition of the metre before 1983:

"On October 14, 1960 the Eleventh General Conference on Weights and Measures redefined the International Standard of Length as 1,650,763.73 vacuum wavelengths of light resulting from unperturbed atomic energy level transition 2p10 5d5 of the krypton isotope having an atomic weight of 86. The wavelength is

λ = 1 m / 1,650,763.73 = 0.605,780,211 µm

At different times some national laboratories used light sources other than krypton 86 as length standards. Mercury 198 and cadmium 114 were among these and they were accepted by the General Conference as secondary length standards."

Because a length such as the wavelength of an atomic transition is determined from the transit time using the relation:

\ell = c\ t \ ,

where c is the speed of light to be measured, the speed of light can be determined in units of wavelengths per second by counting interference fringes and dividing by the measured period of the transition. This speed of light in terms of an independent length standard would be not an exactly known value, set by definition, but a measured value subject to experimental error.

Martin Hogbin (talk) 10:40, 1 September 2009 (UTC)

And there are independent length standards used in scientific measurements against which the SI metre can be compared. I've spoken about them "at length" above (excuse the pun). There are a couple of dozen recommended wavelengths for the "realization" of the metre (apart from the independent length standards such as the astronomical unit): the ratio of the observed speed of light between any two of these secondary standards can be compared (and is compared in calibrating high-precision apparatus). Astronomical measurements can give us an indication as to whether the speed of light varies with time (it doesn't appear to, to the fourteenth significant figure, over the period of accurate observations). If you want to measure the speed of light, you can do, you just have to define your unit system, as has always been the case. Physchim62 (talk) 12:47, 1 September 2009 (UTC)

Yes, of course. As I said, most of what was written was correct and as you say it is quite possible to define a system of units in which length is not defined using the speed of light and of course in these unit systems the speed of light can be measured. However that was not the reason this section was added. It was started by Brews under the heading 'Real, physical speed of light' as yet another attempt to sneak in something about his usual rant.

I would have no objection to something being added to the article about the speed of light using other definitions of length, such as the astronomical unit but I see no point on referring to the previous BIPM standard, which is over a quarter of a century old, especially as it uses the wavelength of a specified light and is thus also dependent on the speed of light. Current realizations of the meter do indeed use recommended sources of light but the frequencies of these sources must be known or measured so that the current length standard can be implemented.

This is not a subject on which we should compromise. The current exact SI speed of light is the actual speed of real light and any suggestion that this may not be the case does not belong in this article. Martin Hogbin (talk) 17:09, 1 September 2009 (UTC)

How many people of the six thousand a day that look at this article will remember the sol is 299,792,458 whatever /sec.Just put the approximate speed is 300 k km per sec or 186 k ml /s and then get into the arguments and the reason for whatever is decided to be the standard. All of this is way more than anybody needs in their daily lives and profession. It can be an interesting thinking experiment for most people and also useless except for physicist in a laboratory. I believe the speed of light was there before anything in this world (Gen "let there be light" ) so the arguments are merely about invented quantities like second and meter labels . Wdl1961 (talk) 18:21, 1 September 2009 (UTC)

Some may want the exact speed. Anyone who wants the approximate speed can look at the table or work it out for themselves. Why not give the answer to the most obvious question, 'What is the speed of light?'. Martin Hogbin (talk) 18:28, 1 September 2009 (UTC)

Section restored!!

A previously uninvolved editor (look at their user page) has just restored the section that I deleted. I wonder why.Martin Hogbin (talk) 18:32, 1 September 2009 (UTC)

Let's start new discussions with Brews (and others)

I also posted this on the wikiproject physics talk page:

"I think it would be helpful to discuss this with Brews again (he asked me to get involved on my talk page a few days ago, but I was too busy then), but this time with one new rule: Citing from sources is not allowed. So, we have to discuss from first principles and explain everything when challenged from first principles. This removes the freedom to interpret what some source says in some arbitrary way. Because most contributors are experts in physics, this can work. If someone is not an expert and makes mistakes he/she will be disqualified more easily (precisely because you can't hide behind sources)." Count Iblis (talk) 20:46, 2 September 2009 (UTC)

A discussion of conversion factor vs. speed of light

I have placed an argument at User:Brews ohare/Speed of light (Example). It does have sources, but I believe they can be ignored for the purposes of this discussion, because all that is needed is velocity = distance/time. The key sources are to the original definitions from the BIPM and NIST. If there is a sourced point that requires some first-principles support, that certainly can be looked at. That discussion page can be used to present comments. Brews ohare (talk) 21:08, 2 September 2009 (UTC)

A few months ago I might have been willing to engage in this discussion, but we were talking about this same subject on my talk page but you seem to have lost interest. Let us see what others have to say about your argument. Martin Hogbin (talk) 16:16, 3 September 2009 (UTC)

Brews: Several editors, a substantial consensus, have clearly stated why your fundamental thesis is unsound and that the sources you cite do not support your position. There is no point in our repeating ourselves, or for you to repeat yourself again. It is your prerogative to disagree. However, unless there are other editors who agree with your position and have reliable sources that, when read correctly, support your position, further discussion on this page is counterproductive, and we should consider this topic closed. Absent support from other editors, your continuing to press your POV would constitute disruptive bahavoir. —Finell (Talk) 22:23, 3 September 2009 (UTC)

Good contributions

A. di M. made several improvements to the article. Thanks, and welcome aboard! Please continue the good work. Also, feel free to participate the talk page discussions about how to improve the article further, with the goal of bringing it back to FA quality. —Finell (Talk) 09:48, 3 September 2009 (UTC)

And TimothyRias too. Thanks, guys. —Finell (Talk) 13:20, 3 September 2009 (UTC)

The implication being that the rest of us have not "made several improvements to the article"? Abtract (talk) 15:20, 3 September 2009 (UTC)

Of course not! However, given the way things have been going and the battle weariness that has set in, having knowledgeable new editors willing to help out is especially welcome. —Finell (Talk) 16:31, 3 September 2009 (UTC)

I knew that ... I just felt like the prodigal son's sibling for a minute. Abtract (talk) 22:07, 3 September 2009 (UTC)

The lead section - one more time

This has been the start of the lead for some time as a result of a consensus of editors here. It is not my words.

In physics, the speed of light (usually denoted c) refers to the speed at which light travels in vacuum. In the International System of Units, the metre is defined so that c has the exact value of 299792458 metres per second.

The speed of light is a fundamental physical constant that connects space and time in a unified structure of spacetime. As such, it defines the conversion between mass and energy

and is an upper bound on the speed at which matter and information can travel. It is the speed of travel of all electromagnetic radiation in free space, and is believed to be the speed of gravitational waves.

It was recently rewritten by one editor to the current version, which in my opinion is considerably worse. When I reverted it I was admonished by Finell for spoiling the hard work of others. Whilst I do not think the above is perfect, it is, in my opinion, a better place to start than the current text. What is the general opinion here? Martin Hogbin (talk) 17:03, 3 September 2009 (UTC)

I like the one proposed my Martin much better. It gets to why it is important much earlier, and much more specifically ("plays a role") is way too vague. LouScheffer (talk) 17:12, 3 September 2009 (UTC)

Thanks Lou. Please note that this version has not been proposed by me it is just the one that has been here for some time. It is already the result of the work of several editors and in my opinion is more encylcopedic. All I am trying to do is to move improve the text incrementally rather than as a series of complete rewrites. Martin Hogbin (talk) 17:45, 3 September 2009 (UTC)

Half a dozen editors have refined the lead to its current version ... rather than simply reverting that I suggest we start from here and now and progress smoothly to the 'perfect version' we all seek. Clearly it's my fault for not understanding but I never quite get what it is Hogbin actually wants to change and why, except back to the old version. Abtract (talk) 17:24, 3 September 2009 (UTC)

I have strengthened "plays a role". Abtract (talk) 17:36, 3 September 2009 (UTC)

There was no consensus for this version. I could explain what I do not like about it but let us wait and see what others think first. Martin Hogbin (talk) 17:45, 3 September 2009 (UTC)

The present version has the merit of not passing off tautology as if it conveyed objective information. I am opposed to any version that seems to say we know exactly what the speed of light is. The case of c = 299 792 458 m/s is a tautology because, although it says how many metres/s c is, in fact we have no idea of what the metre is, because it is defined itself in terms of the speed of light. Maybe the BIPM definition helps here: “length is obtained from the measured time t, using the relation ℓ = c_o·t and the value of the speed of light in vacuum c₀ = 299 792 458 m/s” In other words, one cannot determine speed as ℓ/t, because ℓ is defined in terms of c₀ so ℓ/t is always c₀, no matter what the real speed of light may be. Brews ohare (talk) 18:21, 3 September 2009 (UTC)

But there are many readers who have an intuitive grasp of how long a metre roughly is, while they have no idea of how it's officially defined, or of how fast light is. To these readers, saying that the speed of light is 299,792,458 metres per second is a non-useless piece of information, even if it's true by definition. --___A. di M. 19:00, 3 September 2009 (UTC)

And this is exactly the problem with the current version (or one of the things), it panders to Brews' misconceptions on this subject. Martin Hogbin (talk) 19:02, 3 September 2009 (UTC)

I've no problem stating 299,792,458 metres per second as the speed of light you two; my problem is with claiming the fundamental physical quantity is exactly known as 299,792,458 metres per second, which parades tautology as objective fact. The intro as of this moment doesn't do that, so it's OK. Brews ohare (talk) 19:40, 3 September 2009 (UTC

Of course, I also have problems with words "pandering", "Brews' misconceptions" and other rot that has no basis in fact and is impolite and inflammatory. Brews ohare (talk) 19:41, 3 September 2009 (UTC)

It is exactly known as 299,792,458 metres per second, much like the mass of the International Prototype Kilogram is exactly known as one kilogram, or the triple point of water is exactly known as 273.16 K. At worst, we might not exactly know how long one metre is, how heavy one kilogram is, or how warm one kelvin is. --___A. di M. 20:01, 3 September 2009 (UTC)

It is exactly known in an odd sense of the words, inasmuch as the metre is not exactly known. The uncertainty in the metre is a bit unlike that in the kg, as the actual kg is a physical object with mass to compare against, while the metre is not even a length in principle, but a transit time. If you go back to transit time, the whole matter becomes circular. Also, it is misleading to suggest that the fundamental physical constant speed-of-light can be known exactly by making the units invisibly absorb the error bar instead of the 9-place "exact" value. Brews ohare (talk) 20:12, 3 September 2009 (UTC)

And why is it any more problematic than with any other definition of unit of measurement? Whichever way you define any unit you absorb some error bars: before 1983 it invisibly absorbed the error bar in the measurement of the wavelength of a particular transition of krypton-86, before 1960 it invisibly absorbed the error bar in the measurement of the distance between two lines on a piece of metal in Paris, before 1799 it invisibly absorbed the error bar in the measurement of the half-meridian from the north pole through Paris to the equator, and before that it invisibly absorbed the error bar in the measurement of the length of a seconds pendulum. Of course, the narrowest error bars we absorb the better it is, and so the one we absorb right now are very narrow indeed. --___A. di M. 20:45, 3 September 2009 (UTC)

Proposal for a method of seeking conssensus

(after several edit conflicts)

Abtract's last version, which Martin reverted over in full, actually incorporated other several editors' past attempts to improve the lead, and there have been more refinements by other editors since—including my attempts to restore some things from earlier versions of the lead that Martin preferred, and to restore some others' work that Martin reverted even though they are not my personal preferences (my edit comments indicate some of these)—and some recent edits by Project Physics participants who are new, and very welcome, to this article (who may have come here in response to Martin's plea for help). So I am not wedded to everything in the current version; it includes some things that I would do differently, and that I have commented about on this talk page and in my edit comments.

However, these wholesale reversions back and forth is not the way to progress, even thought they are the product of understandable frustration. Specific aspects of the lead need to be addressed individually, in an effort to reach a reasoned consensus (not necessarily unanimity) supported by reliable sources. To try to begin this process, I am adding several level 3 topic headings below for issues that are in some dispute. I suggest that anyone who believes that there are additional issues that need resolution add an additional level 3 heading for each one. NOTE: My comments about the "current version" refer to this version of the lead; I have not idea what to expect the next time I look at the article. —Finell (Talk) 20:19, 3 September 2009 (UTC)

I agree entirely. We have done this before but if it helps come to a decision and stick to it then I am all in favour. I have added one of the most highly contested questions (in two parts) at the start. Who said this? Probably Martin, yes?

Should the lead give the exact value in m/s in the first paragraph and state that the value is exact

For example: 'c has the exact value of 299,792,458 metres per second'.

Of course we should, because it is true and what many readers may want to know. Martin Hogbin (talk) 21:51, 3 September 2009 (UTC)

Of course we should not, because it is a fiction hiding in a tautology ... the speed of light has not been, will not be, cannot be measured exactly. State the figure 299.. if you must (I favour using ~300) but do not use the word exact. Abtract (talk) 23:01, 3 September 2009 (UTC)

I agree with Martin. There is no real tautology, only an apparent one at best (or worst). —Finell (Talk) 23:13, 3 September 2009 (UTC)

If the speed of light is not given as exact, it logically follows that it should not be presented as a fundamental constant either. After all, the photon mass has error bars, and may not be zero (c.f. Proca lagrangian). So either mention that the speed of light is exact or get rid of the notion that it is a fundamental constant. Obviously, the evil bastards who met in 1983 with the purpose of presenting a tautology and a fiction didn't make a distinction between "the speed of light" and "c", but considered them the same. What they effectively did is define the metre as a derived unit of the second. In the subsequent 26 years, no experimental nor theoretical considerations have invalidated this choice. In fact, the next possible changes in SI base units are likely to involve completely different units, perhaps the ampere . The actual physics is far more interesting than any contentious gibberish in a typical wiki-battle. Assuming the goal is to represent the current best understanding of the speed of light, the article should unquestionably give the exact value and explicitly mention that it is exact. In the lead. At the top. Tim Shuba (talk) 00:57, 4 September 2009 (UTC)

Should a brief reason for the exact vale be given in the first paragraph

For example: 'The metre is defined so that c has the exact value of 299,792,458 metres per second'

Yes, it should explain the situation to most people. I suggest an internal link from 'exact'.Martin Hogbin (talk) 21:51, 3 September 2009 (UTC)

I agree, yes. —Finell (Talk) 23:07, 3 September 2009 (UTC)

No, the word 'exact' should not appear in the first para because it has connotations to the average reader which are confused by the tautology of the self-defining exact figure. Abtract (talk) 22:47, 3 September 2009 (UTC)

Our "average reader" doesn't know what a tautology is. However, we should be able to come up with some satisfactory formulation to dispel any concerns that the "above average reader who is not a scientist" may have, perhaps with an explicit cross-reference to the speed set by definition section. By the way, in my opinion, after the lead, that is the next section that needs real help. For starters, beginning by saying that the metre was re-defined based on the speed of light for reasons stated in the resolution, which has only bullet points, in not sufficient. I hate to reopen this can of worms, but we do need to address that topic with exceptional accuracy and clarity, and excluding quotations that may be misleading out of context (or else giving the quotations sufficient context). —Finell (Talk) 23:07, 3 September 2009 (UTC)

Should the lead include some approximations of c for the general reader?

In my opinion, it should have approximation in US customary units per MOS, and the most common one is 186,000 miles/sec. However, I personally care less about this than some other editors who favor or oppose approximations (Dicklyon gathered editors' views on this above), and we do have the table of approximations adjacent the lead. —Finell (Talk) 20:19, 3 September 2009 (UTC)

I like the 186,000 mps, and also 1 ft/ns which has some currency in EM and in IC packaging where people like a rule of thumb to relate dimensions to delays. Brews ohare (talk) 20:29, 3 September 2009 (UTC)

I think the values in km/h and mph should be there, because each one is by far the unit of speed in more widespread everyday use among laymen in some part of the world. 1 ft/ns is fine too if written in symbols so as not to take up much space. OTOH, once you write 299,792,458 m/s there's no point in also writing 3×10 m/s: you can expect the reader to just ignore the last figures if they're not interested in them. --___A. di M. 20:37, 3 September 2009 (UTC)

1 ft/ns is going to far in the lead; we have the table for that. Brevity is important in the lead. Also, MOSNUM prescribes that units should be written out in text. —Finell (Talk) 20:58, 3 September 2009 (UTC)

No, except for possibly 300,000,000 m/s so that readers know that this is an approximation. We have a table for all the others. Martin Hogbin (talk) 21:53, 3 September 2009 (UTC)

For my money, I favour just 300.. (plus 186.. if you must) in the lead since this is more than accurate enough for most readers. However, since this seems unlikely to win the day my second best is 299.. (but definitely without the word "exact" since this is 1) unecessary and 2) potentially confusing) ... whether you then add 300.., 186.. etc is imho overthetop but not the end of the world. Abtract (talk) 22:44, 3 September 2009 (UTC)

How much history should the lead include?

In my opinion, expressed above, the current verskon has too much and too repetition of material in the history sections. Also, the attempt in the lead to summarize what humans thought about instantaneous versus finite speed since the beginning of the species is futile; when Abtract restored it after I deleted it (that's not a complaint), I attempted some refinements, but I still don't like it and don't know how to write a version that I would consider to be satisfactory. —Finell (Talk) 20:19, 3 September 2009 (UTC)

In many treatments of the speed of light aimed at the general reader, the history is described in depth (see Bodanis' book, E=mc2). I don't believe the attempt is futile; it is fine to say people didn't really know something until Roemer came along. Pecos Joe (talk) 21:15, 3 September 2009 (UTC)

I like Bodanis's book and believe that a thorough history belongs in the article. My only reservation is how much belongs in the lead. Also, what I think is futile is trying to summarize what humans thought about instantaneous versus finite speed since pre-history in one sentence. A well sourced exposition of that subject belongs in the body of the article. —Finell (Talk) 21:37, 3 September 2009 (UTC)

The lead is meant to be a summary of the article. I would like to see a paragraph starting with a brief mention that some considered it infinite, some not, proceeding through the fact that it was measured with increasing accuracy and ending with the fact that it is now fixed by the definition of the meter. This follows the body of the article. I suggest that no one is mentioned by name in this section. Martin Hogbin (talk) 21:56, 3 September 2009 (UTC)

Which is what we have in the base version with the exception that two people are mentioned by name. Abtract (talk) 22:21, 3 September 2009 (UTC)

The current version gives a succession of measurements, which is too much for the lead in my opinion. More importantly, one cannot accurately to refer to any ancient view as infinitely fast travel. Aristotle, and therefore his many followers, maintained that light was not motion; light was perceived immediately upon opening one's eyes because it was always there, not because it traveled instantaneously or that it traveled at all (this is his idea, and he was a pretty bright fellow in his day; don't blame me for how it sounds a couple millenia later). Also, I don't believe there was a well developed concept of infinity then to justify saying that any of them believed in infinitely fast travel. —Finell (Talk) 22:42, 3 September 2009 (UTC)

I am surprised that the phrase '"whether it was instantaneous or simply travelled very quickly," doesn't satisfy your last few points. As to the measured values, my purpose in showing them was twofold 1) to demonstrate just how long a road it was and 2) to bring out the point that 299.. was originally a measured value but that it then became a defined conversion factor. I was trying to bring together the two strands of thought. Abtract (talk) 23:11, 3 September 2009 (UTC)

Relativity in the lead?

In my opinion, the lead should not talk about c in connection with spacetime and as the conversion factor between mass and energy without saying that this comes from Einstein's special theory. When I weaved that into the beginning of the paragraph, Martin reverted the whole paragraph with the edit comment that it included an inaccurate statement, but I don't know what was inaccurate (I don't quesiton Martin's superior knowledge, but I asked what was inaccurate and he didn't answer). Abtract restored this in his last major revision, but not in exactly the same form. I'm OK with this version, but perhaps Martin is not. Martin? —Finell (Talk) 20:19, 3 September 2009 (UTC)

Of course we should mention c as a constant of spacetime (I do not particularly like the term 'conversion factor'), this is the central role that c plays in physics. I stated in my edit note that it was Minkowski (not Einstein) who came up with the concept of spacetime and that this was not in 1905. I do not think we should mention anyone by name here. Martin Hogbin (talk) 22:10, 3 September 2009 (UTC)

If you said that in your edit summary, I missed it. I am aware that Minkowski coined the term spacetime, but isn't the concept implicit in Einstein's 2 special relativity papers? —Finell (Talk) 23:15, 3 September 2009 (UTC)

I am beginning to warm towards the idea of no names in the lead ... sometimes who came up with what is a tad complex and needs the wordy explanations available in the body of the article. However we surely must mention, in the lead, the major role played by the SoL in these fields. Abtract (talk) 22:27, 3 September 2009 (UTC)

Perhaps just Einstein, who is a household word? And include special relativity in the conceptual mix? —Finell (Talk) 23:15, 3 September 2009 (UTC)

Free space or vacuum?

This applies to the lead and the rest of the article. Free space is the correct term. If we use that term consistently throughout the lead and the article, and do not pipe the wikilink of its first use in the lead, that would solve the problem. The current statement, deep in the body of the article, that when the article says vacuum it means free space, is too late to be helpful. Piping free space to vacuum, as the earlier version of the lead did, is contrary to MOS because Misplaced Pages has an article on vacuum (I'll find the specifically guideline if necessary). Piping it at all leads to an undesirable double link to Free space, since the first use of free space in the lead must also be wikilinked. Since free space is not a familiar term to the general reader, perhaps a brief parenthetical explanation after the first wikilinked use would be helpful. Or, perhaps the wikilink is enough; that is the reason the wikilink is there. —Finell (Talk) 20:19, 3 September 2009 (UTC)

Actually, vacuum is the correct term. See a lengthy discussion in one of the recent archives for more detail, but basically, vacuum is the official term used by BIPM and it is the more widely used term overall. Best would be to link to vacuum and put a satisfactory explanation of the scientific use of vacuum in that article. Pecos Joe (talk) 21:09, 3 September 2009 (UTC) (The end of it was in archive 7, under heading free space) Pecos Joe (talk) 21:27, 3 September 2009 (UTC)

Most important is that we pick one and stick to it consistently. Physicists tend to use 'vacuum' and electronics and radio engineers use 'free space'. The Free space article gives a much better description of what is meant here than the Vacuum article does. 'Vacuum' on the other hand is used by BIPM. My preference in order is Vacuum (regardless of the MoS), Free space, Vacuum but I am not going to fight over it. I can live with any of them provided we stick to it and stop arguing about it and changing it. Martin Hogbin (talk) 22:03, 3 September 2009 (UTC)

Imagine a non-scientist reader faced with vacuum and free space ... it is no contest imho ... vacuum is known and 'understood' by pretty well all educated people and surely by most readers of this article. In comparison, how many even educated people will have a reasonable grasp of 'free space'? not many I would guess. Both should be in the article but only vacuum in the lead. Abtract (talk) 22:35, 3 September 2009 (UTC)

Another possibility often found in the literature is classical vacuum. This has the plus that it includes the word "vacuum", does not allow confusion with other meanings of "vacuum", and clearly demarcates what is meant from its two top likelihoods for confusion: Quantum vacuum and QCD vacuum. Brews ohare (talk) 00:47, 4 September 2009 (UTC)

Physical quantities and their units

I have connected this section back to the topic 299 792 458 m/s. Brews ohare (talk) 19:36, 3 September 2009 (UTC)

I think the section is useful in showing how the conversion factor 299 792 458 m/s is intimately connected to the 1983 switch to time-of-transit comparisons instead of the length comparisons previously used with the pre-1983 definition. Brews ohare (talk) 20:08, 3 September 2009 (UTC)

1. Smolin, L (2007). The Trouble with Physics: The Rise of String Theory, the Fall of a Science, and What Comes Next. Houghton Mifflin Harcourt. p. 215. ISBN 061891868X.
2. Cite error: The named reference Boyes was invoked but never defined (see the help page).
3. Uzan, J-P; Leclercq, B (2008). The Natural Laws of the Universe: Understanding Fundamental Constants. Springer. pp. 43–44. ISBN 0387734546.
4. Greene, G (2003). The Elegant Universe. WW Norton & Co. pp. 55–56. ISBN 0393058581.
5. Davies, PCW (1979). The Forces of Nature. Cambridge University Press. pp. 127–128. ISBN 052122523X.
6. Duke, PJ (2000). "Electromagnetic waves in free space – no electric charges or currents". Synchrotron Radiation: Production and Properties. Oxford University Press. p. 53. ISBN 0198517580.
7. Schwinger, JS (2002) . "Gravitational waves". Einstein's Legacy: The Unity of Space and Time (Reprint ed.). Courier Dover. p. 223. ISBN 0486419746. {{cite book}}: External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help)
8. Wei-Tou Ni (2005). "Empirical foundation of the relativistic gravity" (PDF). Intl J Modern Physics D. 14: 901–921.

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