Talk:Speed of light/Archive 14

Is this article ready for FAC?
I've cleared the last maintenance templates on the article. Are there any remaining issues to clear up before putting the article up for FAC? Regular editors please chime in. TimothyRias (talk) 14:59, 7 May 2010 (UTC)

The lead
The lead is not, in my opinion up to the required standard. The problems are all minor ones and mostly issues of style rather than content. I know we have been through the lead countless times before I will list here what I see as the problems with it. Martin Hogbin (talk) 15:15, 7 May 2010 (UTC)
 * Good, on that note can everybody check if they feel that the lead satisfies WP:LEAD, in particular, does accurately reflect the content of the rest of the article? TimothyRias (talk) 15:21, 7 May 2010 (UTC)
 * Would it better for the lead to more or less follow the same structure as the main article? This come down to moving the current second paragraph to the end. TimothyRias (talk) 15:24, 7 May 2010 (UTC)
 * Yes I think that is a good idea. Martin Hogbin (talk) 15:30, 7 May 2010 (UTC)

First paragraph
The speed of light (usually denoted c) is a physical constant. It is the speed at which electromagnetic radiation (such as light) travels in vacuum, the speed of massless particles, and the fastest speed at which energy or information can travel. Its value is exactly 299,792,458 metres per second, often approximated as 300,000 kilometres per second or 186,000 miles per second.

1 Brackets do not look good in the lead, maybe one pair could be replaced with a pair of commas but not both, I think

2 I think normal idiomatic English is 'denoted by'?

3 We mention fastest speed of information transfer later in the lead, this should be there only once.

4 Do we need the references here? We have them both later on. Martin Hogbin (talk)

How about:

The speed of light, usually denoted by c, is a physical constant. It is the speed at which light and all other electromagnetic radiation travels in vacuum, the speed of massless particles, and the fastest speed at which energy or information can travel. Its value is exactly 299,792,458 metres per second, often approximated as 300,000 kilometres per second or 186,000 miles per second.
 * It looks good. ― _ _ _ A._di_M. (formerly Army1987) 16:30, 7 May 2010 (UTC)

But we still need to decide where to have the max information transfer bit. Martin Hogbin (talk)
 * I'm not sure it's a problem. "It is the speed at which light and all other electromagnetic radiation travels in vacuum, the speed of massless particles, and the fastest speed at which energy or information can travel" serves as a quick definition of the topic (though it might be trimmed somehow), and each of the points is expanded in the paragraph about SR. BTW, I also agree about moving the paragraph about history to the end of the lead. ― _ _ _ A._di_M. (formerly Army1987) 16:30, 7 May 2010 (UTC)
 * Now that I think about it, if the second paragraph is moved to the end, the first and third ones can be merged:
 * The speed of light, usually denoted by c, is a physical constant, the speed at which light and all other electromagnetic radiation travels in vacuum. Its value is exactly 299,792,458 metres per second, often approximated as 300,000 kilometres per second or 186,000 miles per second. According to the theory of special relativity, c connects space and time in the unified structure of spacetime, and its square is the constant of proportionality between mass and energy (E = mc2).  In any inertial frame of reference, independently of the relative velocity of the emitter and the observer, c is the speed of all massless particles and associated fields, including all electromagnetic radiation in free space, and it is believed to be the speed of gravity and of gravitational waves.  It is an upper bound on the speed at which energy, matter, and information can travel, as surpassing it "would lead to the destruction of the essential relation between cause and effect." Its finite value is a limiting factor in the operational speed of electronic devices.
 * ― _ _ _ A._di_M. (formerly Army1987) 16:43, 7 May 2010 (UTC)


 * I think we are getting there but we need to move slowly and let other editors comment. In the past the lead has been rewritten several times, often from scratch to suit a particular editors style or emphasis.  The paragraph as you have it is a bit long for the opening paragraph.  Perhaps we should keep it as two paragraphs for the moment and concentrate on the second. Martin Hogbin (talk) 16:59, 7 May 2010 (UTC)


 * I like the first sentence as it is in Martin's version. It gives a more complete picture of what the constant c is about. (Since it is not just the speed at which light propagates.) I would also keep the two paragraph separate as a sort of two stage introduction. TimothyRias (talk) 17:55, 7 May 2010 (UTC)


 * I'm fine with both of them, personally. A. di M.'s draft goes into just enough detail about SR, I think, without being overdone (although any more would probably be too much). Looks good, guys! Sebastian Garth (talk) 19:25, 7 May 2010 (UTC)

I still have quite a few style issues.

1 Why 'according to the theory of special relativity'?

2 'Unified structure' is not a term commonly use in reference to spacetime.

3 The formula E = mc2 is superfluous and formulae are not recommended in the lead.

4 The bit about constant speed in inertial frames might be better elsewhere in the lead.

5 'as surpassing it "would lead to the destruction of the essential relation between cause and effect." is superfluous, and why the quotes?

6 The last sentence belongs elsewhere, maybe not in the lead at all.

Attending to all these points results in:


 * The speed of light, usually denoted by c, is a physical constant, the speed at which light and all other electromagnetic radiation travels in vacuum. Its value is exactly 299,792,458 metres per second, often approximated as 300,000 kilometres per second or 186,000 miles per second. In relativity, c connects space and time in  spacetime, and its square is the constant of proportionality between mass and energy.  The speed of light is the speed of all massless particles and associated fields in free space, and it is believed to be the speed of gravity and of gravitational waves and an upper bound on the speed at which energy, matter, and information can travel.

but I think this could be improved. Martin Hogbin (talk) 21:37, 7 May 2010 (UTC)
 * Still have the issue, that c is not defined as the speed at which light travels. (It is a constant that appears in various contexts, one of them being the speed at which light travels.)TimothyRias (talk) 22:29, 7 May 2010 (UTC)
 * E = mc2 is one of the most famous formulae in the world (there are also T-shirts with it*), so if we are going to say "its square is the constant of proportionality between mass and energy", we might as well show it. This is one of the very rare cases when a formula is more likely to be familiar to a lay reader than its English translation. (Not that mentioning mass–energy equivalence at all at this point is that vital.)
 * * For that matter, there are also T-shirts with the formula for the volume of an n-ball (and I have one), but you know what I mean. ― _ _ _ A._di_M. (formerly Army1987) 23:21, 7 May 2010 (UTC)
 * I take your point, it is a very famous formula. If we are going to state it then we should do it in the narrative rather than have it in brackets after a verbal description.  For example how about something like " c is the constant in the famous equation E = mc2 connecting mass and energy.' Martin Hogbin (talk) 09:22, 8 May 2010 (UTC)
 * Yes, that would be fine. ― _ _ _ A._di_M. (formerly Army1987) 09:36, 8 May 2010 (UTC)
 * Or else "and it appears in the famous equation of mass–energy equivalence E = mc2", as repeating "connecting" so closely would feel awkward. ― _ _ _ A._di_M. (formerly Army1987) 09:42, 8 May 2010 (UTC)
 * Yes, anything that reads well. Martin Hogbin (talk) 09:50, 8 May 2010 (UTC)

So how about?


 * The speed of light, usually denoted by c, is a physical constant, the speed at which light and all other electromagnetic radiation travels in vacuum. Its value is exactly 299,792,458 metres per second, often approximated as 300,000 kilometres per second or 186,000 miles per second. In relativity, c connects space and time in  spacetime, and appears in the famous equation of mass–energy equivalence E = mc2.  The speed of light is the speed of all massless particles and associated fields in free space, an upper bound on the speed at which energy, matter, and information can travel, and it is believed to be the speed of gravity and of gravitational waves. Martin Hogbin (talk) 12:55, 8 May 2010 (UTC)
 * "In relativity" is too generic, as in Galilean relativity c doesn't connect anything at all. :-) If you feel "special relativity" is too narrow, try "special and general relativity". (Then there's the TimothyRias mentioned about "the speed at which light and all other electromagnetic radiation travels in vacuum" not being the definition of c, but since its wider significance is explained two sentences later that doesn't seem a deal-breaker to me.) ― _ _ _ A._di_M. (formerly Army1987) 18:59, 8 May 2010 (UTC)
 * I think 'relativity' is generally taken to refer Einstein's theory but I would not object to your suggestion. Alternatively, we might drop the reference altogether and say something like, 'The constant c connects space and time in spacetime'.
 * Relativity is a disambiguation page, but theory of relativity is what we want here. "In the theory of relativity, c connects..." should be OK. ― _ _ _ A._di_M. (formerly Army1987) 21:25, 9 May 2010 (UTC)
 * The point of the discussion that I started below was to show that there is no single correct definition of the speed of light. In one current theory of physics, classical electromagnetism, 'the speed of light' is literally the speed light travels at in a vacuum.  It therefore seems sensible to start with this intuitive description and then immediately proceed to the descriptions appropriate to other theories of physics. Martin Hogbin (talk) 20:47, 9 May 2010 (UTC)
 * Even, in classical electromagnetism c is more than just the speed of EM waves. For example, it is also the (root of) conversion factor between electrostatic and electromagnetic units. In my view, the speed of light is a constant/parameter that appears in various physical theories, with the most prominent claim of classical EM that EM waves in vacuum travel at that speed.
 * The pedagogical problem I have with starting out with saying that "c is the speed of EM waves" is that it sounds like a definition, and leads readers on the confusing path of trying to figure out, why the speed of EM waves in vacuum should effect things like causality. I'll try some more alternatives. TimothyRias (talk) 08:12, 10 May 2010 (UTC)

Alternatives
Martin's last suggestion:
 * The speed of light, usually denoted by c, is a physical constant, the speed at which light and all other electromagnetic radiation travels in vacuum. Its value is exactly 299,792,458 metres per second, often approximated as 300,000 kilometres per second or 186,000 miles per second. In relativity, c connects space and time in  spacetime, and its square is the constant of proportionality between mass and energy.  The speed of light is the speed of all massless particles and associated fields in free space, and it is believed to be the speed of gravity and of gravitational waves and an upper bound on the speed at which energy, matter, and information can travel.

Alternative suggestion:
 * The speed of light, usually denoted by c, is a constant featuring in various theories of physics. Its name derives from the fact electromagnetic waves such as light travel at this speed according to electromagnetism. Its value is exactly 299,792,458 metres per second, often approximated as 300,000 kilometres per second or 186,000 miles per second. In the theory of relativity, c connects space and time in  spacetime, and determines the relationship betweenmass and energy through E = mc2.   The speed of light is the speed of all massless particles and associated fields in free space, and it is believed to be the speed of gravity and of gravitational waves and an upper bound on the speed at which energy, matter, and information can travel.

I'm not particularly married to the precise formulation. In particular, I think the phrase "... derives from the fact that" can do with improvement. The underlying thought is that if there is no single correct definition, then the article should not appear to give one. The first line says what c is. It stays rather vague to avoid becoming false from some POV. The second line tells us why it is named the way it is, and immediately gives its most important function, without suggesting that this is the definition. In the rest I just tried to implement the other points being discussed. In particular, I think having E = mc2 in its formula form is more helpful then spelling it out, due to the iconic nature of this formula. TimothyRias (talk) 08:33, 10 May 2010 (UTC)
 * I like the idea, but there's something about the phrase "various theories of physics" I don't like (I'm not sure what, so I can't give a better alternative). Also, what about "The speed of light, usually denoted by c, is a physical constant [insert something here], so named because it is the speed at which light and all other electromagnetic radiation travels in vacuum. Its value is ..." followed by the rest of the lead as it exists right now? (IMO, mentioning vacuum is essential, and mentioning electromagnetism isn't.) ― _ _ _ A._di_M. (formerly Army1987) 09:31, 10 May 2010 (UTC)

Martin's latest suggestion:


 * The speed of light, usually denoted by c, is a physical constant having a value of exactly 299,792,458 metres per second, often approximated as 300,000 kilometres per second or 186,000 miles per second. It is the speed at which light and all other electromagnetic radiation travels in vacuum, it connects space and time in spacetime, and it appears in the famous equation of mass–energy equivalence E = mc2.  The speed of light is the speed of all massless particles and associated fields in free space, an upper bound on the speed at which energy, matter, and information can travel, and it is believed to be the speed of gravity and of gravitational waves.

I think just using the term 'physical constant' is sufficient, thus we can drop various theories etc.
 * (sorry to interupt your reply, but this seems most efficient) I agree. TimothyRias (talk) 10:10, 10 May 2010 (UTC)

I have changed the sentence order so that it is first defined as a constant then is significance in various theories is described.
 * Hmm. We could do that, but it might open some cans of worms wrt to other discussions we've had here before. TimothyRias (talk) 10:10, 10 May 2010 (UTC)

I have tried to avoid using 'theory'. This word causes much confusion to many people.
 * Point taken. TimothyRias (talk) 10:10, 10 May 2010 (UTC)

There is a fundamental problem in attempting to define the 'speed of light' to definitively in that we do not know what terminology would be used if if it turns out that the constant has different values in different theories of physics. I think this is the best we can do in that respect.Martin Hogbin (talk) 09:55, 10 May 2010 (UTC)
 * Agree. I think, I like AdiM's suggestion above best, where we can just not inserst anything at the [insert something here]. The first sentence would become: "The speed of light, usually denoted by c, is a physical constant [insert something here], so named because it is the speed at which light and all other electromagnetic radiation travels in vacuum.
 * This actually fairly close to what we have now. The "so named" removes the possibility of misinterpretation of the last part. TimothyRias (talk) 10:10, 10 May 2010 (UTC)
 * I think the reason the constant has the name it does will be obvious to most people. Whether this nomenclature is 'correct' is not, in my opinion, a discussion we should provoke in the lead, although we might talk about it in the body of the article. Martin Hogbin (talk) 10:24, 10 May 2010 (UTC)
 * Stating the obvious is what a lead inevitably does to some extend. I don't see the proposed formulation as provoking the discussion whether the nomenclature is correct. It just doesn't exclude the thought that the constant my have a wider context than just light. TimothyRias (talk) 11:13, 10 May 2010 (UTC)
 * Yeah, what I wrote without anything in the [insert something here] could also do it. A way to address Hogbin's point could be: "... is a physical constant. It was originally [verb] ed as the speed at which light and all other electromagnetic radiation travels in vacuum, but it has a much wider significance in special relativity and theories depending on it." But the second sentence is way too soon to frighten readers away, so... "...but it has a much wider significance in modern physics"? That could do it, but we need a suitable choice of [verb] (defined? introduced?), and it might be misleading (by originally I mean "before 1905", but that's far from obvious). ―  _ _ _ A._di_M. (formerly Army1987) 13:38, 10 May 2010 (UTC)
 * The problem with 'so named because' is that it has several connotations which we cannot support. The first is that the constant existed in physics before it was called 'the speed of light', then at some time, a decision was made by someone to call the existing constant 'the speed of light'.  Secondly we have no source for your (undoubtedly true) assertion, simply because it is obvious.  I think the point that the term is related to several areas of physics is well made by having all the relevant areas together in the opening paragraph.  I suggest that this point would be better expanded on in the body of the article under 'Fundamental role in physics' where there is room to discuss it more fully. Martin Hogbin (talk) 13:48, 10 May 2010 (UTC)
 * Well, Weber did use that constant (modulo a factor of $\sqrt{2}$, IIRC) before electromagnetic radiation had even been theorized, let alone hypothesized to be light... ;-) (And I don't think "named" has that connotation, anyway.) ― _ _ _ A._di_M. (formerly Army1987) 14:01, 10 May 2010 (UTC)


 * 1)About expanding on this in 'Fundamental role in physics' I think this is currently expand on sufficiently in that section.
 * 2)About sourcing the statement that it is "named/called so, because ...". That should not prove to hard. I think there is a good chance of Griffith's EM book or something similar yo say this out loud. These books typically come to the conclusion that this parameter c in their equation is the speed of EM waves.
 * 3)I agree with AdiM that it does not have that connotation for me at all. (And also that even if it did, it would not be false, since the constant was used in EM before the discovery of EM waves.) However, if it would help we could replace 'named' with 'called'. TimothyRias (talk) 14:25, 10 May 2010 (UTC)


 * 1) I cannot find anything in 'Fundamental role in physics' that makes this point, which I thought was the one you were making: There are several parts of physics which have a constant in them often referred to as 'the speed of light'. As far as we know, these constants all have exactly the same value but it is possible that they might turn out to be different.
 * They could only differ only if Lorentz invariance were broken, and the assumption of Lorentz invariance is the reason for c to be relevant to those theories in the first place... ― _ _ _ A._di_M. (formerly Army1987) 15:56, 10 May 2010 (UTC)
 * As AdiM mentions, the whole upshot of SR is that there is only one such constant. TimothyRias (talk) 08:07, 11 May 2010 (UTC)
 * 2)and 3) You are rather missing my point. The term 'speed of light' was is use long before classical electromagnetism with its obvious meaning of 'the speed that light travelled at'.  To say that the 'speed of light' was originally named because it was just that, 'the speed of light' is not really necessary.
 * Anyway, I am not going to fight over this, we are still on the first paragraph! Martin Hogbin (talk) 15:37, 10 May 2010 (UTC)
 * The point here is that the idea of c as a physical constant only arose in the 19th century in the context of EM, where it was used before it had been shown that this constant was equal to the speed of light waves. Anyway, the main connotation I get from the "named so because" phrase is that when the constant c was first (widely) used it was just that the "speed of light" but that since then it has be come more than just that. Which was exactly what we need to convey. That it is just a name related to the historical origin of the constant rather than its current physical use. (If the constant were named in 2009 it would probably be called "spacetime anisotropy quotient" or something due to the Horava-Lifshitz gravity hype.)
 * I guess the other piece of information the "named so because" phrase conveys, is why that use of c is mentioned before the value and the other equally important uses are mentioned later.
 * I do agree that we have argued enough about this. My suggestion is that we move forward with AdiM's last suggestion. TimothyRias (talk) 08:07, 11 May 2010 (UTC)
 * Agreed Martin Hogbin (talk) 10:52, 11 May 2010 (UTC)
 * Just so we are clear this is the version we are agreeing on?
 * The speed of light, usually denoted by c, is a physical constant, so named because it is the speed at which light and all other electromagnetic radiation travels in vacuum. Its value is exactly 299,792,458 metres per second, often approximated as 300,000 kilometres per second or 186,000 miles per second. In the theory of relativity, c connects space and time in  spacetime, and determines the relationship betweenmass and energy through E = mc2.   The speed of light is the speed of all massless particles and associated fields in free space, and it is believed to be the speed of gravity and of gravitational waves and an upper bound on the speed at which energy, matter, and information can travel.
 * It required some cutting and pasting so I just wanted to check to make sure. TimothyRias (talk) 12:50, 11 May 2010 (UTC)
 * I still think explaining that it is called 'the speed of light' because it is 'the speed of light' is crazy but let us go with it. It could also still do with a little style improvement. Martin Hogbin (talk) 15:41, 11 May 2010 (UTC)
 * I like it, but I'd also keep Hogbin's "it appears in the famous equation of mass–energy equivalence E = mc2". If I were naming it, I'd probably call it the Lorentz constant or something like that. :-) ― _ _ _ A._di_M. (formerly Army1987) 15:57, 11 May 2010 (UTC)

I've gone ahead and implemented a version. It can be adjusted for style issues in the article. TimothyRias (talk) 08:11, 12 May 2010 (UTC)

Vacuum vs. Free space
Why do you say "vacuum" the first time and "free space" the second time, JIC? —Preceding unsigned comment added by 165.212.189.187 (talk) 13:52, 10 May 2010 (UTC)
 * That should indeed be fixed. Using the different terms may lead to wonder if we are try to make a subtle distinction, which we are not. TimothyRias (talk) 14:27, 10 May 2010 (UTC)
 * I've fixed that. There's still the issue that the free space article focuses too much on a particular, er..., non-standard definition, but there are tags at the top of it which ought to dissuade readers from taking it too seriously. ― _ _ _ A._di_M. (formerly Army1987) 15:12, 10 May 2010 (UTC)
 * In the new version I've opted for vacuum over free space, since that is what the rest of the article is using. Should there be a note that clarifies that vacuum in this context is to be read as free space? TimothyRias (talk) 08:09, 12 May 2010 (UTC)

Will the real 'Speed of light' please stand up
TimothyRias has raised a very important question (again) as to what exactly is the 'speed of light'. In discussing this I think it is important to separate the current theories of physics into those that have been experimentally verified and those respectable theories that are incomplete or have no experimental verification.

Relevant accepted and experimentally verified theories are:
Classical electromagnetism - in this, the 'speed of light' is the speed that light travels at in free space.

Relativity (SR and GR) - in this c is a constant linking space and time. According to GR c is the speed of gravitational waves.

QED c is a constant of this theory but it should not be treated as a classical speed.

I think the role of c in these theories should be given in the lead without qualification. The question is how to combine these roles into a paragraph that is accurate, has good style, and does not repeat facts.
 * Right (except that gravitational waves have never been directly observed, let alone experimentally verifying that their speed is c; OTOH the "believed" in the current lead might be too weak, what about "predicted"?). ― _ _ _ A._di_M. (formerly Army1987) 11:14, 8 May 2010 (UTC)
 * I have no objection to leaving it as 'believed'. Martin Hogbin (talk) 12:46, 8 May 2010 (UTC)

Incomplete or unverified theories might include:
QCD

String theory

The role that c plays in these theories should be discussed in the body of the article but, I suggest, not in the lead.
 * If you can propose anything relevant, go ahead. As far as I am concerned, what we have in the article now (search "Lorentz invariance has become") is quite enough. ― _ _ _ A._di_M. (formerly Army1987) 11:14, 8 May 2010 (UTC)
 * I agree. Martin Hogbin (talk) 12:46, 8 May 2010 (UTC)

The standard model
I do not know much about this but I think it might be best for someone who understands the exact role of c in this theory to add a sentence to the lead along the lines of In the standard model c'' is....'. Martin Hogbin (talk) 09:50, 8 May 2010 (UTC)
 * I suspect no-one is so foolish as to formulate the SM in any other system of units than those with c = 1; the role of c in it is the same as in SR, which is a fundamental assumption of the SM. ― _ _ _ A._di_M. (formerly Army1987) 11:14, 8 May 2010 (UTC)
 * Agreed also. Martin Hogbin (talk) 12:46, 8 May 2010 (UTC)

It looks like we agree on the principles. Martin Hogbin (talk) 12:46, 8 May 2010 (UTC)

Does the lead adequately cover the content of the article?
Currently, I would say that the first paragraph covers the main points of section 1 and 2, the second paragraph cover section 3 and the last paragraph covers section 5 and 6 (with a heavy emphasis on the last section). So, this leaves section 4 about the practical effects of c being finite uncovered in the lead. Is this a problem, or do we think this is OK? TimothyRias (talk) 08:18, 12 May 2010 (UTC)

Talk:Alfvén wave-faster than speed of light?
Alfvén wave is faster than speed of light? Newone (talk) 10:11, 11 May 2010 (UTC)

Many waves travel faster than the speed of light, this was once mentioned in the 'Faster-than-light observations and experiments' section and is discussed in the Faster-than-light article. In no case does matter, energy, or information travel faster than light. I suggest that a sentence on FTL waves is added to this article. Martin Hogbin (talk) 10:35, 11 May 2010 (UTC)

I have added a sentence to the article on this subject. Martin Hogbin (talk) 10:47, 11 May 2010 (UTC)
 * This was indeed mentioned in the "in a medium" section. I linked your remark to that section.TimothyRias (talk) 12:34, 11 May 2010 (UTC)
 * Sorry, I had not noticed that. However I think it is better as it is now. Martin Hogbin (talk)

History table
I've thought about adding a table like here on the right to the history section to summarize to progress in measurement of c. Is this a good idea? If so, what should the format be? TimothyRias (talk) 12:37, 11 May 2010 (UTC) to my sandbox, IMO it looks less bad that way. ― _ _ _ A._di_M. (formerly Army1987) 14:13, 12 May 2010 (UTC)
 * What about this? This way, it will depend on the window width whether "Author" and "Method" are on one or two columns (though the code looks awful with all those nbsp's). ― _ _ _ A._di_M. (formerly Army1987) 11:47, 13 May 2010 (UTC)

That table rocks! It is a really great idea.TStein (talk) 06:20, 22 May 2010 (UTC)

Speed of light in a medium
The current opening paragraph in this section seems rather confused to me and surely does not represent Feynman's words on the subject as the reference suggests. We currently have:

''When light enters materials, its energy is absorbed. In the case of transparent materials, this energy is quickly re-radiated. However, this absorption and re-radiation introduces a delay. As light propagates through dielectric material it undergoes continuous absorption and re-radiation. Therefore the speed of light in a medium is said to be less than c, which should be read as the speed of energy propagation at the macroscopic level. At an atomic level, electromagnetic waves always travel at c in the empty space between atoms. Two factors influence this slowing: stronger absorption leading to shorter path length between each re-radiation cycle, and longer delays. The slowing is therefore the result of these two factors''

This seems to mix up two models into one confused explanation. In classical electromagnetic theory, the original wave is extinguished by interaction with the bound electrons in the atoms of the substance and replaced with another wave moving at a lower velocity. This is a bulk phenomenon, representing the interaction of an EM wave with the huge number of electrons in the material.


 * Where is the confusion? The EM-wave encounters a field of bound-charges. You then have a simple case of a field of driven oscillators. The new wave, travelling at c, is in turn extinguished like the original. As with any driven oscillation (with a mass) each generation undergoes a phase-delay - thus the phase-velocity of the propagation is reduced.


 * You suggest that the 2nd generation wave propogates through the field of bound electrons without further interaction - which is not possible.


 * I think your interpretation of the text above is somewhat charitable.  I think that whoever wrote it was  trying to explain the quantum behaviour of light in a medium in layman's terms, as TimothyRias says below. The text has been changed now anyway. Martin Hogbin (talk) 23:10, 15 June 2010 (UTC)

In QED photons are repeatedly absorbed and emitted by atoms resulting in a delay in the signal propagation through the material. As photons are quantum entities, I would stop short of saying that they travel at c between the atoms although this may be the mental model that many people have. Martin Hogbin (talk) 14:08, 13 May 2010 (UTC)


 * Stopping short implies that photons might be travelling at some other velocity...


 * I think the text is trying to explain the quantum behaviour of light in a medium in laymen's terms. Talking about virtual processes as if they were real always makes me cringe a little, but it is exactly the sort of thing Feynman does all the time. Someone with quick access to the Feynman lectures should check that this paragraph accurately reflects his explanation, though. TimothyRias (talk) 08:34, 17 May 2010 (UTC)
 * No, IIRC, Feynman gives a purely classical explanation in that section (treating each electron as a harmonic oscillator, essentially; he only mentions at the end that for quantum reasons each one actually has several frequencies). He assumes an incoming wave on the left of a plane layer of transparent material, and describes the outgoing wave at the right a superposition of the incoming one and a wave generated by oscillating electrons, and the result has (at sufficiently low densities) almost the same amplitude but different phase than the original wave. If it's exp(ikx - i\omega t + i\phi_1) on the left, it's exp(ikx - i \omega t + i\phi_2) on the right, then he shows that \phi_2 - \phi_1 is proportional to the thickness, so that inside the material it can be written as (i(k+k')x - i\omega t + i\phi_1) and calls \omega/(k+k') + c/n. (When I get back home, I'll check whether it's actually as I remember it.) A. di M. (talk) 09:35, 17 May 2010 (UTC)
 * Yes, I checked this out and Feynman is discussing the classical model in which the original wave is extinguished and a new one created.
 * As I suggest above, I am not against giving a layman's QED answer but we must be careful not to describe photons as classical particles. Martin Hogbin (talk) 22:19, 18 May 2010 (UTC)
 * I definitely agree on the last bit. Any ideas for a clear exposition? TimothyRias (talk) 07:46, 19 May 2010 (UTC)

I wish I had more insight. I don't know if I like any qualitative description of how the speed of light varies in a material before the article tells me which speed of light it is talking about. The mechanism for the phase speed is different then for the group speed and the front speed. My first instinct is that description fits the description of the front velocity and not the phase velocity since phase velocity can be and is for certain circumstances greater than c. The explanation for phase velocity almost certainly needs to involve waves since it is a wave phenomenon.

This article has progressed a lot since I last saw it, but this section is still its Achilles's heal. (The only other significant item is the amount of physics speak in the relativity section.) I will see if I can find something better somewhere. TStein (talk) 06:37, 22 May 2010 (UTC)


 * As this article is not about light, it is somewhat out of the scope of the article to provide an explanation of why light travels at a different speed in a medium. I've been bold and rework the section to omit this, and focus on relaying the facts about light propagation in a medium. I've also added a good source discussing in detail the bussiness of group and phase velocities being superluminal or negative, and showing that the front velocity is equal to c. TimothyRias (talk) 15:48, 25 May 2010 (UTC)
 * Looks good. I've copy-edited it a little. A. di M. (talk) 18:21, 27 May 2010 (UTC)

Am I interpreting this wrong?
'From the observation that the periods of Jupiter's innermost moon Io appeared to be shorter when the earth was approaching Jupiter than when receding from it, he concluded that light travels at a finite speed, and was able to estimate that would take light 22 minutes to cross the diameter of Earth's orbit.'

Light would take about 1/23 of a second to cross the diameter of the earth. What's this 22 minutes all about? —Preceding unsigned comment added by 91.108.4.98 (talk) 14:02, 29 May 2010 (UTC)
 * The time needed to cross the diameter of the Earth's orbit. Not the time needed to cross the diameter of the Earth.TimothyRias (talk) 20:04, 29 May 2010 (UTC)

Misusing of refs
is one of the main contributors to Wikipedia (over 67,000 edits; he's ranked 198 in the number of edits), and practically all of his edits have to do with Islamic science, technology and philosophy. This editor has persistently misused sources here over several years. This editor's contributions are always well provided with citations, but examination of these sources often reveals either a blatant misrepresentation of those sources or a selective interpretation, going beyond any reasonable interpretation of the authors' intent. Please see: Requests for comment/Jagged 85. The damage is so extensive that it is undermining Wikipedia's credibility as a source. I searched the page history, and found 39 edits by Jagged 85 (for example, see this series of edits). Tobby72 (talk) 20:45, 14 June 2010 (UTC)
 * From the edits above, the information that needs to be checked is the following text that is currently in the History section:
 * Early Islamic philosophers initially agreed with the Aristotelian view that light had no speed of travel. In 1021, Islamic physicist Alhazen (Ibn al-Haytham) published the Book of Optics, in which he used experiments related to the camera obscura to support the now accepted intromission theory of vision, in which light moves from an object into the eye.
 * This led Alhazen to propose that light must therefore have a finite speed, and that the speed of light is variable, decreasing in denser bodies.
 * Also in the 11th century, Abū Rayhān al-Bīrūnī agreed that light has a finite speed, and observed that the speed of light is much faster than the speed of sound.
 * I have seen the discussion of this editor's work, and there is wide consensus that enormous misrepresentations of sources have occurred, so this text needs to be confirmed. Johnuniq (talk) 23:55, 14 June 2010 (UTC)
 * Well, the MacTutor and other sources quoted support the claims made almost verbatim. Since those sources are generally reliable the facts seem OK. The only issue remaining is the somewhat undue emphasis of the Islamic nature of these philosophers. TimothyRias (talk) 08:24, 15 June 2010 (UTC)
 * Agreed. Are there any reliable sources suggesting that the religion of the philosophers was relevant to their scientific theories.  If not the reference religion should be removed. Martin Hogbin (talk) 23:14, 15 June 2010 (UTC)


 * The point is the culture, not the religion. People of this culture are often referred to as Islamic (or Muslim) or Arabic (that is, by religion or by language) to distinguish them from other cultures that existed at about the same time in the same region.  This should not be taken as an indication that the religion had much to do with the science.  The early islamic philosophy article refers to Alhazen as an "Arab polymath"; some sources call him "Persian"; he was born in what's now Iraq and lived mostly in Egypt. Dicklyon (talk) 02:46, 16 June 2010 (UTC)

Measurement
The introduction says about the speed of light: “Its value is exactly 299,792,458 metres per second”. Sydenham is cited in this connection, as is Jespersen “turning c into a conversion factor whose value is fixed and arbitrary” (p. 280). I'd suggest that some reconciliation of this viewpoint be attempted in the section Measurement which explains many methods for determining the speed of light and cites values like 299,710±22 km/s with error bars, as is appropriate only for an uncertain quantity, not an exact value.

I do not feel comfortable in pursuing a discussion of these matters, which caused me a great deal of difficulty with ArbCom in the past, and brought the most extreme invective and vituperation upon me that I have experienced in my 71 odd years of life.

However, it still seems to me that a naive reader is likely to wonder what is going on here. I hope that some enterprising soul can brave this wilderness to bring some clarity to the Measurement section. That involves at most the addition of a few sentences of reconciliation. Brews ohare (talk) 20:49, 15 July 2010 (UTC)
 * As for the introduction, the reader is going to understand what is going on by the time they get to the end of the third paragraph. Maybe a paragraph could be added after the first paragraph of "Measurement" stating that today measuring c in metres serves no purpose (other than verifying that your measuring instruments are properly calibrated and properly working) and that measuring it in some other unit is equivalent to measuring the length of other unit in metres ... but I'm not sure of how to word it. What would you propose? —Preceding unsigned comment added by A. di M. (talk • contribs) 08:06, 16 July 2010 (UTC)


 * A. di M.: I wouldn't hazard an attempt at wording given the history of conduct among editors of this article.
 * The underlying difficulty with the Speed of light article is short shrift given to how a system of units based upon replacing ‘distance’ with ‘time-of-flight’ compares with a different system where distance and time are kept separate. That comparison could be done in a general manner for any speed standard, explaining the need for reassurance that the "standard" speed has been realized in any given measurement, and the role of definitions in making that speed "exact". That presentation could then be narrowed to describe why light-speed is a good choice. Brews ohare (talk) 11:36, 16 July 2010 (UTC)
 * But there aren't two or more systems: modern physics is based on special relativity, from which we get that the speed of light is fixed, so distance can be defined in terms of time. Historically there were many different ways to understand it, all covered in the article, but the current system has been settled science for about a hundred years. The definition changed more recently, but standards bodies are much more conservative than scientists, and have to take account of how easily a definition is to use.-- JohnBlackburne wordsdeeds 13:53, 16 July 2010 (UTC)
 * The idea of a fixed, finite speed of light dates back to Rømer (who also used time of flight as his measure of distance, as astronomers still do to this day). This subject has been discussed ad nauseam on this page, there is no need for any additional clarification, and certainly no need for philosophical ponderings of what might have been had physcics been different. Physchim62 (talk) 14:52, 16 July 2010 (UTC)
 * FWIW, the idea of a fixed speed of light only dates back to special relativity, actually. Before then, it was believed to depend on the frame of reference, which left Michelson and Morley scratching their heads. A. di M. (formerly Army1987) (talk) 02:07, 17 July 2010 (UTC)

"I do not feel comfortable in pursuing a discussion of these matters, which caused me a great deal of difficulty with ArbCom in the past, and brought the most extreme invective and vituperation upon me that I have experienced in my 71 odd years of life." — well don't do it then! Physchim62 (talk) 14:52, 16 July 2010 (UTC)

John, and Physchim62: Your remarks are not at all responsive to the matters raised. To recapitulate, the section on Measurement describes many attempts to measure the speed of light and quotes the results with error bars appropriate to an inexactly known quantity, which appears to be in conflict with the statement of the Introduction that the speed of light is an exact value. That is confusing, and could be clarified with some remarks reconciling these two views in the Measurement section. This is a matter of making the logic clear, and is not a question about historical development or modern practice. I'm sorry to see your intemperate reaction, Physchim62. Brews ohare (talk) 15:10, 16 July 2010 (UTC) -


 * And I'm sorry that you have chosen to ignore Coren's good advice. The logic is clear: after a couple of centuries of ever-decreasing the error bars on measurements of the speed of light against standards of the metre, the uncertainty in the definition of the metre became the limiting factor. Rather than stop improving measurement techniques, the definition of the metre was changed so that the speed of light became the defining factor. This is no different from, say defining the ampere in terms of the magnetic constant and not in terms of a number of electrons which passes a given point per second, or saying that the mass of a carbon-12 atom is exactly 12 atomic mass units. All of these are definitions which have changed over the years, without any logical inconsistency. Physchim62 (talk) 15:33, 16 July 2010 (UTC)

Perhaps the last sentence of the lead "...the numerical value of c in metres per second is now fixed exactly by the definition of the metre.[1][2]" could be moved up... Count Iblis (talk) 15:31, 16 July 2010 (UTC)
 * And again, Physchim62, your are beside the point. There is no question of logical inconsistency in the historical record or in modern practice. The question is simply one of clarity of presentation in a WP article called Speed of light of two different approaches: the “time-of-flight” approach that uses a definition with an exact value for c, and the approach that uses separate ‘time’ and ‘distance’ units and therefore a measurement of c as ‘distance/time’. Brews ohare (talk) 15:50, 16 July 2010 (UTC)
 * But there aren't two different approaches, there's only one, the one given in the article. In what we think of as modern physics it's one of the best established facts, that the speed of light in a vacuum is a constant, which fixes the definition of length in terms of time. Any other approach is historic (and covered already) or wrong (and so has no place here).-- JohnBlackburne wordsdeeds 16:46, 16 July 2010 (UTC)
 * John, you are mistaken: please re-read the article itself carefully. The standards methodology changed in 1983 to achieve greater reproducibility and accuracy. Read this section and take a look at the references, which provide a more detailed discussion, in particular, Sydenham is cited in this connection, as is Jespersen. That is why error bars appear on the numbers in the Measurement section. I'd say that if a person can come away from the Speed of light article without this awareness, the article is lacking. Brews ohare (talk) 17:19, 16 July 2010 (UTC)
 * Brews, you have just plunged headlong into the same discussion that, 'brought the most extreme invective and vituperation upon me that I have experienced in my 71 odd years of life'. I am sure that you have been right about many things in your life but in this particular case please just consider the possibility that you might be wrong before going any further. Martin Hogbin (talk) 17:32, 16 July 2010 (UTC)
 * Thank you Martin, for that advice. Frankly, I see nothing at all controversial in what I have said here. My view of the reaction to these rather simple remarks is that in the minds of several (among whom Physchim62 clearly is one), old, ingrained arguments that have not been brought up here at all, dominate their thoughts and prevent them from reading what is before them. Like Pavlov's dogs, certain sounds cause salivation, even when no food is present. So, the Speed of light article is free to continue in its present confusing form, that even an erudite editor like John Blackburne will misinterpret. I will follow your advice, which I take as urging my withdrawal. Brews ohare (talk) 17:48, 16 July 2010 (UTC)
 * Well, distances could be and were measured through transit times before 1983, and they still can be and are measured by other means today. The only thing that changed was which of the conversion factors needed to express one's measurement in metres is exact. Section 6.3 of the current version already clearly explains the purpose of the redefinition; what is confusing with it? A. di M. (formerly Army1987) (talk) 02:07, 17 July 2010 (UTC)
 * BTW, the "the approach that uses separate ‘time’ and ‘distance’ units and therefore a measurement of c as ‘distance/time’" was not quite what the most precise measurements in the 1970s were about: the experiments described in "Laser interferometry" were essentially measuring the ratio of two frequencies. A. di M. (formerly Army1987) (talk) 10:29, 17 July 2010 (UTC)

Gentlemen, the sentence should state, by international agreement, for purposes of measurement in the SI system of units, the value of the speed of light is taken to be exactly... This is a rather simple solution to the difficulty and Brews was correct to raise the point.72.64.55.156 (talk) 21:26, 16 July 2010 (UTC)
 * It already does say that, except in more detail and more clearly, in the third paragraph. But most of the time the history of the definition is inimportant - when you use c in a formula like E=mc2 for example you should know it's a constant, and to do calculations you need to know it's value, but that's all. A curious or attentive reader might wonder why it's exceptionally an exact value, and can find the theory behind and the practical reasons for this later in the article.-- JohnBlackburne wordsdeeds 21:46, 16 July 2010 (UTC)
 * The suggested sentence gives the impression that there is a 'real' definition of distance that has somehow been ignored by the definition of the metre. Such notions have long since been abandoned by physicists.  Even so, the metre is still defined as a unit of proper length. Martin Hogbin (talk) 08:34, 17 July 2010 (UTC

You gentlemen seem to be throughly confused. There is no constant of physics that is known exactly, except by a definition as to the agreement of the value to be used. All of this talk is beside the point. You have said that a constant is known exactly through measurement and that is impossible. Please stop putting impossible physics into Wikipedia articles, we have enough of that already.
 * Who said it is "known exactly through measurement", exactly? A. di M. (formerly Army1987) (talk) 14:04, 17 July 2010 (UTC)


 * A. di M.: Your comment above about "measuring the ratio of two frequencies" is the kind of detail that requires more attention in this article. Statements like Blackburne's “aren't two different approaches, there's only one, the one given in the article” are indicative of the confusion generated around this topic. Replies like Physchim62's “there is no need for any additional clarification, and certainly no need for philosophical ponderings of what might have been had physics been different.” are total irrelevancies.
 * I hope you can clear up and explain better how the measurement situation led to the decision to change the basis of the units from a ‘time and space units’ approach to a ‘time-of-flight’ approach requiring only the second, and connect that to today's definition that doesn't employ error bars like 299,710±22 km/s, but an exact value 299,792,458 m/s. It's a big challenge, given that so many want to believe that they understand perfectly what happened in 1983 but cannot tolerate any attempt to explain it more carefully. Brews ohare (talk) 16:48, 17 July 2010 (UTC)
 * It may be that a separate article should be written to discuss at length the comparison of “time and space” units approaches and “time-of-flight” approaches using only the second. That would allow for a more detailed discussion of measurement techniques, historical developments, and the difference in logical basis allowing the use of a defined speed of light. It also would allow for the treatment of issues such as how we can check that c is a universal constant even though we use a defined value for it. And above all, it would allow referral to this article from Speed of light and thereby remove all this furor to a different venue. Brews ohare (talk) 16:58, 17 July 2010 (UTC)
 * If we move this to a different venue it will be to return to ArbCom; all your points have been discussed at great length before, and the discussions can be found in the archives of this page. That you choose to to return here with exactly the same arguments once your topic ban from physics articles was lifted is a sign that you do not wish to improve this page, simply to find an outlet for your minority views. Physchim62 (talk) 17:25, 17 July 2010 (UTC)
 * Physichim62: Threats of ArbCom action as a response to a mild suggestion to create a separate article for more in-depth treatment is exaggerated and hostile. Brews ohare (talk) 17:49, 17 July 2010 (UTC)
 * Again there is no "difference in logical basis", there is only the current correct and universally accepted approach. Everything else is wrong and/or historical, and is already covered at length in the article. There's certainly no need for a separate article, especially not to explore alternative (i.e. incorrect) interpretations of the science.-- JohnBlackburne wordsdeeds 17:07, 17 July 2010 (UTC)
 * John: An approach using both length (metre) and time (second) units is not "incorrect". It is the system in use prior to 1983. It requires a speed to be established by measuring a distance traveled and the time of travel, and as both of these measurements are indeed measurements, the speed has an experimental error bar. An approach using a standard speed also is perfectly viable. It could be the speed of sound in a specified medium, for example. Then a distance is determined as a "time-of-flight", that is, how long it takes the sound wave to travel the length. Such a system requires only a time unit, the second, and all lengths are determined in terms of times-of-flight in seconds. In choosing between these two systems, it is not their "correctness" that is at stake. What is involved is practical matters, most notably, how much trouble it is to insure that the standard speed has actually been realized. In the case of a standard speed in terms of the propagation of sound, the issue is how readily and accurately one can ascertain that the standard medium has been realized. It is when considering this matter that the speed of light becomes so highly recommended, because experiment has shown the speed of light to be readily realized in a great variety of circumstances, available to all observers without undue concern and preparation. Brews ohare (talk) 17:23, 17 July 2010 (UTC)

Suggestion
Might I suggest that this discussion is continued on a separate talk page where those that wish to discuss the subject can do so and this page can continue to be used for improving the article in other respects. I have started a page at Talk:Speed_of_light/Definition_of_the_metre for those interested and have copied Brews paragraph above there. Martin Hogbin (talk) 18:52, 17 July 2010 (UTC)


 * That's a good idea. I think whether Brews is right or wrong about how to present the definition of the meter etc. can be gauged better by improving the article on other points to make the article ready for FA review. Then we'll get comments from univinvolved editors who may or may not notice the same issues as Brews is raising. Then, if the FA review is successful and the article appears on the main page, we can expect a lot of comments from a much larger audience. So, if people are really surprised that c has an exact value and want the article to explain this in a clearer way, then we'll get that feedback. Count Iblis (talk) 19:03, 17 July 2010 (UTC)


 * I like this idea myself. I make this suggestion: in a day or so I'll set up a user page along the lines of Speed of light pre- and post-1983 and present what seem to me to be the salient topics. On its discussion page all are invited to comment. How does that sound? Brews ohare (talk) 19:23, 17 July 2010 (UTC)


 * Fine. Martin Hogbin (talk) 20:12, 17 July 2010 (UTC)


 * I suggest that the discussion be discontinued, here and everywhere, because it will accomplish nothing. The article's present treatment of this issue is consistent with mainstream treatment. And, since it takes at least two to argue, nothing is gained by responding to remarks about an issue that was argued, long past death, for over a year. Anyone who addresses the subject here is just asking for a replay of that argument. Let's just stop it now, please.—Finell 10:10, 20 July 2010 (UTC)

Exact value for c in miles per second.
In SI units the mile is defined exactly in terms of the meter, which means c has an exact rational value in units of miles per second:

$$ \textstyle c= \frac{ 18737028625 }{ 100584 } $$ mi/s.

It is impossible to convert meters/sec to miles/second using a decimal conversion factor. —Preceding unsigned comment added by NOrbeck (talk • contribs) 11:13, 17 July 2010 (UTC)
 * This is already mentioned in footnote 1. A. di M. (formerly Army1987) (talk) 14:06, 17 July 2010 (UTC)

Proposal for a new article concerning time-of-flight and length-and-time standards
A rough, preliminary draft of a proposed article can be found here. Comments are invited on its discussion page.

This article is intended for the naive reader to help them understand the change in SI units that occurred in 1983. The article is far from final form, and some help in its construction would be appreciated. Brews ohare (talk) 16:50, 18 July 2010 (UTC)
 * Aside from the way it is written - after the first paragraph is seems largely devoid of encyclopaedic writing, and reads more like an essay than an article making arguments largely unsupported by the references - I don't see the need for such an article. Anything that needs to be said of the definition of the metre should be in the article Metre, or on the speed of light in this one. The history of it is interesting, as it tells us a lot about how our understanding of the universe has developed, but that's already covered.-- JohnBlackburne wordsdeeds 16:59, 18 July 2010 (UTC)


 * The article is not about the metre per se, but about two definitions of the metre, a ‘time-of-flight’ definition and a ‘length’ definition. The idea is to contrast the approaches and point out why one provides better reproducibility and accuracy than the other. Improvements are expected and recommendations and comments are invited. Brews ohare (talk) 05:22, 19 July 2010 (UTC)


 * Re the 1st sentence of the draft, "A change in the meaning of the term speed of light as used in the SI system of units occurred in 1983." – Perhaps you meant the physical constant c, which changed to have a specific fixed value because of the change in the definition of the meter? My feeling is that the speed that light travels in a vacuum is what it is, regardless of what humans do, whereas the value and units of the physical constant c are derived from definitions made by humans. (BTW, I'm not sure if this view of mine is in the consensus of opinion here.) I recognize that you might have had this distinction in mind when you wrote "term" but this subject can easily get confusing if it isn't made as clear as possible.


 * Also, assuming this article isn't a POV fork, it seems that the material in it could be covered in existing articles, if it isn't already. Regards, --Bob K31416 (talk) 08:13, 19 July 2010 (UTC)


 * This is nothing but a WP:FORK. I also note that you are again try to push the same idiosyncrasies as you did before you got banned. You'd think that after the Arbcom kurfuffle you would have learned your lesson and stop beating deadhorses, but I guess this is impossible for you (as we all feared). You yourself said this was the worse experience of your life, and yet you're back asking for more. Brews, please drop the stick and find yourself a less controversial article for you to edit. Because I'm really not looking forwards to ARBCOM/Speed of Light 2, which will inevitably happen if you keep at it. Headbomb {talk / contribs / physics / books} 10:25, 19 July 2010 (UTC)


 * I don't think Brews is behaving in a disruptive way anymore. As long as discussions are taking place elsewhere so that the editors here can continue working on the other issues and as long as Brews won't actually create or edit articles unless he gets the necessary consensus for that, there is really no problem.


 * Note that I wrote earlier that making this aticle ready for FA review should be a priority and that this will actually help to deal with the issues Brews has brought up. If the definition of the speed of light really does require more explanation, then we'll get that feedback in the FA review or when the article is featured on the main space. If no one sees a problem here, then Brews argument that it is not clear, is not something that is widely shared by people who are not involved in the writing of this article.


 * Also, note that the ArbCom case was a disaster, the Arbitrators had to reverse their ruling over their objections after intervention by Jimbo Wales. I don't think they will consider to hear a new case in the absense of evidence of clear disruption, e.g. a clear pushing of fringe views in articles.
 * ArbCom is right now considering a case on climate change. If you want to see a example of real disruption right now, see here. Now if Cla68 and the other sceptics were to decide to just go about their business as Brews is doing now (simply write up something on their own userspace and then ask for comments, instead of editing against consensus), what a relief that would be. Count Iblis (talk) 14:59, 19 July 2010 (UTC)
 * Can you give a link to the intervention by Wales? BTW, I agree that as long as the discussion is only in Ohare's own user space, there's no disruption. Also, I think that the article as it is now is ready for FA; what would you think about nominating it? A. di M. (formerly Army1987) (talk) 15:20, 19 July 2010 (UTC)
 * I also think the article is ready for FA review. I do have some issues about the section on the role in modern physics. It is not mentioned that in modern physics you can see that c is trivial scaling constant as it now only appears in equations that express equivalences like that mass and rest energy are equivalent. So, c = 1 is a natural choice for that reason. But this is not so important for FA review.


 * About the intervention by Wales, Brews send Wales an email and Wales replied saying that he would ask ArbCom to reconsider the case. A few days after that, ArbCom accepted a motion that let the topic ban expire after 90 days. I can send you that email (I don't think Brews would object to that). But this was not done in the open, presumably to reduce the drama, as that was seen to be a major issue as well. I and a few others were also restricted from advocating for Brews in a separate motion (which has now expired). Count Iblis (talk) 16:50, 19 July 2010 (UTC)

Are we ready for FA?
When I returned to this article last week, after about a month I hadn't significantly edited it, I found it even better than I remembered. I am quite positive that it is ready for FA status, but can anyone find any issue with it before I nominate it again? ― A._di_M.3rd Dramaout (formerly Army1987) 18:41, 10 July 2010 (UTC)


 * I have an issue with the 'Fundamental role in physics section. In this section there important facts about the subject of the article have been relegated to footnotes. Why are they not in the main text? Martin Hogbin (talk) 23:40, 10 July 2010 (UTC)
 * The Doppler effect and the Terrell rotation are quite irrelevant to the point being made, so I would keep them where they are. The Scharnhorst effect is so small that it might well be never observed during the lifetime of anyone around here, so it was agreed that it was undue weight to put it in the main text.
 * Anyway, I'm going to move the reference to the relativity of simultaneity and the tachyonic antitelephone back into the main text (being sent to a footnote only comprising one link distracts more than seeing it in the text, IMO). As for the note on one-way vs two-way speed, I think it should be in the main text too, but I seem to remember there once was an opposition to that and was moved to the footnote as a compromise. ― A._di_M.3rd Dramaout (formerly Army1987) 13:56, 11 July 2010 (UTC)


 * I thought that the article was ready for FA last time, and it is somewhat better now than it was then. The FA review did not bring out significant criticism, and the points that were raised were dealt with (mostly by A. di M.). The review simply died with a whimper from lack of comment. Would it useful to put the article up for peer review before re-nominating it for FA?—Finell 10:24, 20 July 2010 (UTC)
 * The "Natural sciences and mathematics" section of WP:PR appears to be stagnating right now: I submitted the Physics article eleven days ago and so far only an editor saying "" popped up. A. di M. (formerly Army1987) (talk) 15:38, 20 July 2010 (UTC)

Does light always travel at c, or does it never travel at c?
These are not rhetorical questions. I'm curious, if c in is the speed of light in a vacuum, and pure vacuums do not exist in nature, why is it claimed that that physical light travels at c? Does light always travel at c as claimed in section 4, or does it never travel at c as claimed in section 5? Isn't it common knowledge that c is a concept distinct from "the speed of light", and that it's just called that for historical reasons? Is anyone willing to argue that c is in fact "the speed at which light travels"? The question is, does light travel at c (y/n)? NOrbeck (talk) 07:32, 19 July 2010 (UTC)
 * I think the article deals with the issues that you have raised. In current models of physics c is a fundamental constant representing, among other things, the speed of light in a vacuum.  You might say, as you suggest, that there is no such thing as an absolute vacuum, indeed there can be no such thing except in a completely empty universe. However, most of physics is based on concepts that do not actually exist so the speed of light is no different in that respect.


 * Bear in mind also that the article is entitled 'The speed of light' and should therefore have some basis in the natural meaning of these words. The philosophy of this subject has been discussed at some length and we could continue to discuss it ad infinitum but I think the article in its current state reflects the current state of physics regarding the subject reasonably well.  That is not to say it could not possibly be improved.  Do you have any suggestions? Martin Hogbin (talk) 08:12, 19 July 2010 (UTC)


 * Re "The question is, does light travel at c (y/n)?" – y --Bob K31416 (talk) 08:24, 19 July 2010 (UTC)


 * Yes. We 'know' that light travels at c in a vacuum as well as we 'know' most other things in physics. Martin Hogbin (talk) 08:34, 19 July 2010 (UTC)

Hey this is a really good question Bob. The answer is, NO. The physicists are just hoping that their opinion in the matter is the right one. But, lets get real. Most of the time I calculate the speed of light I don't use the value as the physicists give it in their hopeful definitions. That is because I live in the real world and it isn't a vacuum. The statement that it is a constant of physics is simply wishful thinking. In my world I have to figure out what its velocity is really and account for that fact. —Preceding unsigned comment added by 72.64.52.188 (talk) 20:24, 19 July 2010 (UTC)


 * Re "In my world I have to figure out what its velocity is really and account for that fact." - Regarding having to "figure out" what the speed of light in vacuum "is really", could you give an example of what you mean? Thanks. --Bob K31416 (talk) 20:34, 19 July 2010 (UTC)

I'm unsure what the reader is after myself. On the one hand, it seems that the concern is that the speed of light is c only in free space, and free space is not real. I think the article deals with that issue in discussing the role of a medium upon the speed of light. On the other hand, the concern may be that c is a "defined" value, not a measured one. So saying c is 1 city blocks/ ns isn't too sharp when we don't know what a city block is. It doesn't help much to say a city block is c × 1 ns. That is what is going on with the definition: we don't know what the metre is. It's c × 1/299792458 of a second. Brews ohare (talk) 15:11, 20 July 2010 (UTC)

The way out of this dilemma is to specify circumstances under which the "standard speed of light" is realized. These circumstances are established by careful experiments. That amounts to specifying a "standard medium" and the appropriate correction to convert measurements to their equivalent in free space. Of course, the standard speed can be assigned any value, because all that does is establish the arbitrary unit for speeds. Brews ohare (talk) 15:39, 20 July 2010 (UTC)

The lead section needs expansion
Alright, so there are a few people getting ready to resubmit this to the featured article crowd. Cool, I hope it works out well. I suggest working on the lead section more before renomination, and doing so with a slightly different focus. There are several knowledgeable editors who have reworked the text and debated all kinds of technical minutiae, and the result is a nice, reasonably comprehensive and stable presentation of the important aspects of the topic. However, the lead doesn't do its job of providing an adequate summary, and more importantly fails to reach out and grab the reader in an entertaining way. At the moment, the lead has three paragraphs. The first has been through some bloody hand-to-hand combat, and looks pretty good. The second paragraph starts out okay, but quickly descends into equations and values for refractive indexes, which are unneeded in an intro and not particularly entertaining for a general reader. There are more interesting details from the  'propagation of light'  section that should be included instead. The third, currently final paragraph of the lead, gives an overview of the history. It's not terrible, but is far too sketchy and misses out on several chances to grab the reader. This paragraph should be expanded. And, though admittedly a personal pet peeve which I tried to eliminate months ago, the first sentence of the paragraph is one of the cheesiest lead-ins to a history section possible — "For much of human history, it was debated..." — give me a break, that is unsourceable and virtually meaningless. Besides revamping those last two paragraphs, there is additional material conspicuously missing from the lead. The article sections  'practical effects of finiteness'  and  'measurement'  are poorly represented, if at all, and those sections contain a lot of good material. The lead needs expanding with emphasis on a fun presentation, rather than a dry recitation of facts. Think of Feynman in his book QED, where he says something like "now you know that light not only does not move in straight lines, but doesn't go at the speed of light either". The basic facts are in the article, but it needs more evocative details in the lead, such as providing a mental image of a 17th century cannon firing in order to measure the speed of light. The ingredients are in the pot, now add the spice. Tim Shuba (talk) 09:46, 20 July 2010 (UTC)


 * In my opinion, the lead is in the best shape that it has been since I have been following this article. This is an encyclopedia article, not a paperback book. Expanding the lead would not be an improvement.—Finell 10:20, 20 July 2010 (UTC)
 * I agree with Tim that the lead is intended to be a summary of the article, that is one reason why I have previously suggested leaving it until the rest of the article is complete. Once the body of the article is complete, writing the lead should be relatively straightforward.  One of the requirements for FA is 'brilliant prose'.  I do not think the lead meets this standard yet.  Ideally we need a copyedit (maybe rewrite) from a good writer of English under supervision from the techies here. Martin Hogbin (talk) 10:42, 20 July 2010 (UTC)
 * I disagree - the lead is a summary of the article, but very often articles, especially mathematical ones, cover a lot more ground in a lot more detail and depth than should be in the lead. The lead is meant to be an accessible introduction that encourages readers to read on. Trying to summarise all the content from a technical article will make it less accessible and readable and probably too long. Looking at it I can't see anything seriously wrong with it. -- JohnBlackburne wordsdeeds 11:00, 20 July 2010 (UTC)
 * The article (including the lead) might not be perfect, but IMO it is in very good shape, and it's not like featured articles are required to be perfect, nor like it is forbidden to improve an article once it is listed at FA. A. di M. (formerly Army1987) (talk) 15:42, 20 July 2010 (UTC)
 * BTW, Shuba, if you have ideas about how to improve the lead clear enough to write a draft in a sandbox, please do so... A. di M. (formerly Army1987) (talk) 15:55, 20 July 2010 (UTC)

Remove animated graphic?
Forgive me if this has been thoroughly hashed out before, but in my opinion the animated graphic doesn't really contribute to the article. Most people don't have any real sense of the distance between the Earth and Moon, so showing how long it takes light to travel from one to the other doesn't inform.—Finell 10:40, 20 July 2010 (UTC)
 * I agree. Martin Hogbin (talk) 10:47, 20 July 2010 (UTC)


 * I don't like most animations, but I like this one. It's hard to illustrate the speed of light, and while the general reader will have no idea how far the light is travelling in this animation, they know it's a long way. I think the animation provides a good human scale: the reader sees that light takes a bit over a second to travel to the Moon which they have some passing familiarity with. Johnuniq (talk) 11:16, 20 July 2010 (UTC)


 * I like it. It gives me a good feel of the speed of light for a distance that is just the right size to display the info. --Bob K31416 (talk) 13:50, 20 July 2010 (UTC)
 * But most people will have at least a rough sense of how big the Earth is, and the picture is to scale. A. di M. (formerly Army1987) (talk) 15:19, 20 July 2010 (UTC)
 * Agree with A. di M. Also, if people really think that more comparison is needed, why not add the fact that it took the Apollo spacecraft three days to cover the distance that light travels in 1½ seconds. Physchim62 (talk) 15:32, 20 July 2010 (UTC)

Too much personal criticism
There's too much personal criticism on this article's talk page IMO. It can be disruptive. That's why there is the policy WP:NPA which says,
 * "Do not make personal attacks anywhere in Wikipedia. Comment on content, not on the contributor'''."

I feel that an article talk page should be used for discussing the article, rather than discussing editors personally. If an editor has some useful criticism to make of another editor, perhaps it is best to make it on the subject editor's talk page. Also, if an editor doesn't like another editor's change of the article, simply be bold and revert it, rather than criticize an editor personally on this talk page. Regards, --Bob K31416 (talk) 14:13, 20 July 2010 (UTC)


 * Hi BobK: A most temperate observation, and in keeping with the objectives of the Talk page to be a forum to improve the article, not its contributors. Thank you. Brews ohare (talk) 14:18, 20 July 2010 (UTC)
 * Brews is known for his disruptive editing on this and related subjects, including engaging in tendentious debates and soapboxing: so sayeth ArbCom. He has only recently come back from a topic ban covering all physics-related topics. Any sign of a resumption of his disruptive behaviour on the very page that was the nucleus of his previous activities is of great concern. It is up to Brews to demonstrate that he can edit constructively on the subject, not the other way round. Physchim62 (talk) 15:20, 20 July 2010 (UTC)
 * Hey, some tolerance is called for here. This particular article is a tough one, and involves a lot of nice points and subtle distinctions. It is natural for editors to become a bit frustrated over attempts to express things right, because it isn't simple, and iteration is required. However, I hope that suggestions for clarifications can be taken as exactly that. Brews ohare (talk) 15:46, 20 July 2010 (UTC)
 * It would help if you were to discuss things first though rather than make a whole bunch of edits in one hit. Martin Hogbin (talk) 16:54, 20 July 2010 (UTC)

Brews'edits
Brews, since 12:00 UTC you have made 9 edits to this article. I have not looked at them all but they seem to be a mixture of uncontentious edits and ones making more of a point. This was the kind of thing that got you into trouble before. In the light of the fact that a lot of work has gone into this article to try to get it ready for FAR, can I suggest that you slow down a bit to let others review your edits. Idealy, I would suggest that, at this stage, all edits are discussed first. Martin Hogbin (talk) 13:35, 20 July 2010 (UTC)


 * Hi Martin: I think the edits are all uncontentious, involving a bit of added detail from the sources already cited. I don't think any of them are "making a point".


 * I haven't made any edits on one point that might be considered. There is some lack of clarity about just how an interferometer can be used to "measure a wavelength". Basically, the interferometer can compare path lengths. Thus, to measure a wavelength, a known length has to be compared against.


 * A tricky issue is this: let's say you start with a metre bar as the standard, and you want to establish that the metre is 1 650 763.73 wavelengths in vacuum of the krypton-86 atomic transition. The standard metre is not known to the kind of accuracy suggested by this number. Thus, the standard metre is being replaced by a more precise standard. Evidently, the new standard can be selected at any value within the precision with which the old standard is known. I don't know the actual error bars on this one, but suppose the old standard is between 1 650 762 wavelengths and 1 650 765 wavelengths, a wavelength being about 605 nm. Then I'd say the committee is free to pick any value in this range. That automatically decides the speed of light too, given λ = cf.


 * The article hasn't mentioned these issues of upgrading a standard. Brews ohare (talk) 14:03, 20 July 2010 (UTC)
 * The point here is that you can't measure c to greater precision than the standard allows, and a more precise value for c implies a more precise standard of length. Of course, this all refers to the "old" methodology, as is discussed in the section on interferometry. Brews ohare (talk) 14:29, 20 July 2010 (UTC)
 * What do you mean by 'the precision with which the old standard is known'? At the time, it was the standard. Martin Hogbin (talk) 14:45, 20 July 2010 (UTC)

HI Martin: The idea is this: the standard meter bar is the distance between a couple of scratches on a bar kept under "standard conditions". That definition has an inherent vagueness related to the role of the "standard conditions" on the separation of the marks (are the marks reproducibly at the same separation at all times) and also the marks are physical scratches, let's say, and so just where they are located is a ± issue too. Of course the krypton source has its problems that way too, but they are on a finer scale. The modern laser has its problems too, but they are on an even finer scale. Each time the standard is refined, we have a more precise version of the metre. Consequently, we also have a more precise value for c when c is thought of as a distance/time or as λf. Is this responsive to your query? Brews ohare (talk) 15:00, 20 July 2010 (UTC)


 * OK, you mean the precision with which the old standard can be transferred to the new system. (I am not sure that I have used the right technical terms there.) I guess this is always the case, when the equipment is upgraded there is some judgment to be used when setting the new standard, right up to and including the SoL standard that we now have. I am not sure that this is all relevant to a SoL article. Martin Hogbin (talk) 16:52, 20 July 2010 (UTC)


 * The relevance is apparent in discussion of the evolution of technique: one cannot obtain a more precise value for the speed of light without introducing a more precise standard for the metre. Of course, the modern approach is to define λ =c/f so the metre automatically is refined as the frequency measurement is improved. That was not so previously. Brews ohare (talk) 17:57, 20 July 2010 (UTC)
 * You mean apart from the sentence "Improved experimental techniques do not affect the value of the speed of light in SI units, but do result in a more precise realisation of the SI metre."? Never mind, if you're happy all the better! Physchim62 (talk) 08:28, 22 July 2010 (UTC)

Bold, revert, discuss
As this article is hoping to be ready for FAR soon, can I suggest that everyone here adopts the edit, revert, discuss rule. That is to say make your change, but not too many at once, if your change is reverted, discuss it before putting it back. Martin Hogbin (talk) 17:18, 20 July 2010 (UTC)
 * And can we try not to discuss too many points at once, especially if they all come from the same editor? All editors have the right to comment on the article, but there are also procedures availble for ensuring that single individuals do not crowd out others. Physchim62 (talk) 00:30, 22 July 2010 (UTC)

Paragraph in interferometry
Present paragraph: "The main difficulty in measuring c through interferometry is to measure the frequency of light in or near the optical region; such frequencies are too high to be measured with conventional methods. This was first overcome by a group at the US National Institute of Standards and Technology (NIST) laboratories in Boulder, Colorado, in 1972."

Suggested replacement: "A major difficulty in measuring c through interferometry is to measure the wavelength. The accuracy of this measurement improves as the wavelength is shortened, but shortening the wavelengths places higher demands upon the frequency determination. As the the frequency of light is increased from the microwave region (Froome used 72 GHz) into or near the optical region, such frequencies are too high to be measured with conventional methods. This was first overcome by a group at the US National Institute of Standards and Technology (NIST) laboratories in Boulder, Colorado, in 1972."


 * Reference

Purpose:

The point of this text is that accuracy is improved by going to shorter wavelengths because the interferometer fringes are clearer at short wavelengths, allowing greater precision. The earlier measurements were made at microwave frequencies, and it is pointed out here that the Boulder group was able to extend frequency determinations to the optical region, allowing the use of short wavelengths. IMO the present paragraph inadequately conveys the main point about short wavelengths, and jumps to the frequency issue without the necessary preamble, thereby missing the entire point of the discussion. Brews ohare (talk) 17:44, 20 July 2010 (UTC)


 * Issues concerning the accuracy of optical interferometry, which was used for accurate length measurements, were around for years before the Boulder measurements These were all well understood and dealt with in the best way known. The real breakthrough referred to in this paragraph was the ability to measure the frequency of optical lasers and thus relate optical frequency interferometric measurements directly to the speed of light. Martin Hogbin (talk) 17:55, 20 July 2010 (UTC)


 * Martin: Here again we have differences that are largely semantic. The source “Speed of Light from Direct Frequency and Wavelength Measurements of the Methane-Stabilized Laser” (Evenson, Wells, Petersen, Danielson, Day, Barger and Hall) reads as follows:
 * "At shorter optical wavelengths the accuracy of the wavelength determination increases. ... The 100-fold improvement in the presently reported measurement comes mainly from the increased accuracy possible in the measurement of the shorter wavelength."
 * As you know the speed of light is c=λf. The error is thus the sum of the errors in λ and in f. That error is dominated by the error in λ at longer wavelengths, and by the error in f (prior to this work) at shorter wavelengths. So the authors prefer to compare the two cases in terms of the errors in each factor, and see that the error in the product is now dominated by the error in λ, just as it was before, but of course this error is less at shorter λ.
 * The authors' view, and the one I express in the above text replacement, is that the breakthrough in frequency measurement would be useless if it were not that the error in λ is much less. This reason for the success now is stated, which is omitted entirely in the present paragraph, and the nature of the breakthrough is described just as it is in the present paragraph. Brews ohare (talk) 18:23, 20 July 2010 (UTC)
 * I cannot read that paper from the link. What year was it and what wavelength was the laser. Your quoted sentence is ambiguous.  Martin Hogbin (talk) 21:33, 20 July 2010 (UTC)
 * Phys Rev Lett vol 29 No 19 pages 1346ff Nov 1972 methane stabilized laser at 3.39um. It is the paper cited in footnote 93 of the article. Brews ohare (talk) 00:34, 21 July 2010 (UTC)
 * Thanks, footnote 94 in the article gives the whole story. It seem to be as I thought. At the time of the Boulder measurements, which were a few years before the change in definition of the metre, the situation was that the speed of light had been most accurately measured by using microwaves of known frequency and measuring their wavelength. The main accuracy limitation was the measurement of the 72 GHz microwave wavelength.  Calibration of distance would have been against a physical standard, using (visible) optical interferometry before 1960 and by interferometry using the specified krypton line after that time.


 * The point is that, long before the change to the SoL standard for the meter, distance was routinely being accurately measured by optical interferometry. The big step forward was the ability to measure the frequency of the light used in interferometry against the caesium frequency and time standard. The fact that the wavelength of light could be determined much more accurately than that of microwaves was well-known long before the Boulder experiments. Martin Hogbin (talk) 11:21, 21 July 2010 (UTC)
 * To put it another way, if you're working with microwaves, the problem is to measure the wavelength because measuring the frequency is easy; if you're working with visible (or near-visible) light, the problem is to measure the frequency because measuring the wavelength is easy. I think we should really change the title of the section to something like "Heterodyne frequency measurements" to make that even clearer. Physchim62 (talk) 14:39, 21 July 2010 (UTC)

Gentlemen: Everybody is on the same page that the measurement of frequency was a roadblock that was lifted. What you also both know is that fringes are clearer at short wavelengths. Martin says that was well known long before this paper, and of course that is why the push was on to improve frequency measurements. Everybody agrees about that too. Without the improvement in frequency measurement a better value for the speed of light would not result. The error in c reduced because it became possible to take advantage of the lower error in fringe counting and that advantage was opened up because of the advance in frequency measurement.

Now, I think we all know these two facts. What I think is a problem is that the WP paragraph assumes the reader knows how these two things work together, and doesn't say all this. The article stresses that frequency measurement improved, but doesn't bother to say that the significance of this improvement was that it opened the door to the better regime for wavelength measurement. The quote I provided from the paper is the authors' expression of this fact, and it is missing from the WP article. Brews ohare (talk) 15:03, 21 July 2010 (UTC)


 * I reworked it a bit. It was not clear why the paragraph was speaking about microwave wavelength measurement being problematic, since there was no statement that microwave interferometry had been attempted to measure c. I tried to get the whole idea into the text, and moved the quotation to the footnote as background. I also corrected some transcription errors in the quotation and added the last sentence of the quoted paragraph, which puts the whole idea together. If someone wants to add more history, more could be said in the text about the 1958 experiment that Brews's source discusses; this addition is not necessary, in my opinion, but I wouldn't oppose a succinct sentence or three about it. I also added the page number, which the MOS requires (when there is one) for all direct quotations.—Finell 02:58, 22 July 2010 (UTC)
 * Finell: Looks good to me; thanks. Brews ohare (talk) 05:47, 22 July 2010 (UTC)

Lead sentence
Present lead: "The speed of light, usually denoted by c, is a physical constant representing the speed at which light and all other electromagnetic radiation travels in vacuum. Its value is exactly 299,792,458 metres per second (approximately 186,282 miles per second)."

Proposed replacement: "Light and all other electromagnetic radiation travels in an idealized vacuum at the speed of light, usually denoted by c. In the ideal vacuum, the speed of light is defined to have the value c = 299,792,458 m/s exactly (approximately 186,282 miles per second), setting the unit of speed against which all other speeds are compared. In practice, the ideal vacuum is only approximated by real-life media, and measured speeds are corrected to refer to the ideal vacuum. Thus, the speed of light observed in a real-life medium never is known exactly."

Purpose: The speed of light having a defined value is a bit hard to swallow. The notion that it can have an exact value in an ideal medium is easier to swallow, and in real life measurement is always involved. Brews ohare (talk) 17:24, 20 July 2010 (UTC)


 * No, the current version is much better: mostly as it's grammatical and clear, unlike your suggested replacement. And please read WP:LEAD for why it should start "The speed of light is...".-- JohnBlackburne wordsdeeds 17:35, 20 July 2010 (UTC)

Proposed replacement: "The speed of light, usually denoted by c, is the speed of travel of light and all other electromagnetic radiation in an idealized vacuum. In the ideal vacuum, the speed of light is defined to have exactly the value c = 299,792,458 m/s (approximately 186,282 miles per second), setting the unit of speed against which all other speeds are compared. In practice, the ideal vacuum is only approximated by real-life media, and measured speeds are corrected to compare with the ideal vacuum. Thus, the speed of light observed in a real-life medium never is known exactly." Brews ohare (talk) 17:51, 20 July 2010 (UTC)


 * You seem to be the only person who finds the speed of light having a defined value is a bit hard to swallow. It is not at all clear what, 'measured speeds are corrected to compare with the ideal vacuum' means.  This is not an article about vacua. Nothing that is not a defined quantity is ever known exactly. Martin Hogbin (talk) 18:07, 20 July 2010 (UTC)


 * I see we have reached the point of diminishing returns. The problem remains that the use of the term "physical constant" contradicts the usage in the article physical constant and the sources I have quoted. Brews ohare (talk) 19:01, 20 July 2010 (UTC)
 * I see no contradiction. Martin Hogbin (talk) 21:38, 20 July 2010 (UTC)


 * For clarity, would changing the existing text to read "Its value is defined as exactly..." to emphasize that this is a defined, exact quantity be helpful? I am inclined to argree with Martin and John's concerns that the proposed changes offered are confusing and unnecessarily wordy.  The lede is not a good place to attempt to introduce subtle points about definition versus measurement.  TenOfAllTrades(talk) 19:18, 20 July 2010 (UTC)
 * I don't think so: I think the fact it's defined rather than measured is too subtle a point for the first paragraph. After all it's defined to the exact value to nine significant figures that was found by measuring it, so that's also the measured value. And it's as much a definition of the metre as of the speed of light. So I would leave it as it is. Very smart readers might pick up on the "is exactly" as unusual, and then read on (to the third paragraph initially) to find out why, but most will probably ignore the "exactly" and pick up the first two things everyone should know: it's a constant and it's a very big number.-- JohnBlackburne wordsdeeds 19:48, 20 July 2010 (UTC)
 * Also it is not, strictly speaking, defined, it is fixed by definition, of the metre. Martin Hogbin (talk) 21:38, 20 July 2010 (UTC)


 * The notion that one should let slide a point like this because only "smart readers" will notice and everyone else will be stunned by the fact it is a "very big number" is patronizing and also a poor basis for content. Brews ohare (talk) 20:57, 20 July 2010 (UTC)
 * The detail is given on the body of the article, as always. Martin Hogbin (talk) 21:38, 20 July 2010 (UTC)
 * Everyone else can read on: the details are there, starting in the third paragraph. But the lead is meant to be accessible, and it certainly won't be if we try and load too much into the first sentence. The relationship between the definitions of light speed and the metre is of only historic interest to most readers and so much less important, and is anyway well covered already in the lead.-- JohnBlackburne wordsdeeds 22:08, 20 July 2010 (UTC)
 * Brews's version would suggest that the ideal vacuum is defined in terms of the speed of light (as David Tombe claimed), rather than the other way round. A. di M. (formerly Army1987) (talk) 13:37, 21 July 2010 (UTC)
 * Brews is still flogging his dead donkey of a hypothetical standard state – note how he uses "ideal" or "idealized" four times in a single paragraph! The definition of the metre simply says "in vacuum", and so should we. The important point is that it is not the speed of light in air. It is the speed of light in outer space, as near as can be measured, given that outer space is non-dispersive. It is also the physical constant relevant to the interaction between a proton and an electron in a hydrogen atom. Or maybe Brews thinks that a few pesky air molecules might get between the electron and the proton... Physchim62 (talk) 14:30, 21 July 2010 (UTC)

Regarding the part of the proposed change, "In practice, the ideal vacuum is only approximated by real-life media, and measured speeds are corrected to refer to the ideal vacuum." - In the most precise measurement of the speed of light, could anyone give the excerpt from the corresponding journal article that discusses that work's correction for not having a perfect vacuum? Thanks. --Bob K31416 (talk) 15:09, 21 July 2010 (UTC)
 * Physchim62 & A di M: Gentlemen: My wording is less simple and straightforward than it might be.


 * Physchim62, I thought we were on the same page and I actually quoted you word for word. You pointed out that the exact definition of c and its removal from the sphere of measurement came at the expense of specifying the circumstances where this standard defined speed could be realized, and the measurement error of the older method was replaced by an uncertainty in how accurately these ideal, (excuse the word), ideal circumstances were realized in practice.


 * Now how about forgetting about nitpicking, and finding some way to get this point across in a simple fashion in the introduction? Eh? Brews ohare (talk) 15:52, 21 July 2010 (UTC)

BobK: It would indeed be helpful to find some such statement. If one looks at the NIST literature you can find discussions of corrections made for measurements in air. They take the form of determining the refractive index of the air. As for measurements made in partial vacuum, it appears that the errors introduced by fringe counting are dominant, and they are discussed in the literature on interferometers. These errors due to imperfections in the optical paths, imbalance in the paths, mirror problems, and other errors due to the failure to obtain a plane wave, failure to achieve a monochromatic source. These issues are a bigger concern than the inadequacy of the vacuum. Brews ohare (talk) 15:26, 21 July 2010 (UTC)


 * Does that mean that you aren't aware of any discussion of a correction for not having a perfect vacuum, in the journal article on the most precise measurement of the speed of light? --Bob K31416 (talk) 15:52, 21 July 2010 (UTC)

Hi BobK: That is the case. I haven't done an exhaustive search. The paper by Evenson et al. from 1972 is the only one I've looked at closely. It doesn't mention the error due to the non-ideality of the vacuum. What it does mention in calculation of the errors is: (i) the asymmetry of the Kr line: modeling of this asymmetry is needed to interpret the fringe pattern (ii) the incoherence of the Kr source. They refer to a paper detailing the error analysis to be published by two of the authors (Barger & Hall), but so far as I can determine this error analysis paper never was published. Brews ohare (talk) 17:12, 21 July 2010 (UTC)

Follow-up: The only explicit reference I've found to the quality of the vacuum is in spectroscopy, where a line is observed as the vacuum is pumped down, and the vacuum is considered adequate when the line properties cease to depend on the partial pressure. The inferred speed of light is then compared to the standard as a reassurance that the vacuum is good, but no effort is expended to account for the discrepancy: all that is wanted is that it is ‘small’. Brews ohare (talk) 19:06, 21 July 2010 (UTC)


 * I'm getting the impression that there is negligible effect when the vacuum is "adequate" for the measurement of the speed of light with laser beams. Perhaps the concentration of photons in a laser beam is so much greater than the concentration of residual atoms in the space occupied by the beam, that only a negligible percentage of photons in the laser beam interact with the residual atoms, and to the precision of the measurement has no effect on the result for the measured speed of light. --Bob K31416 (talk) 20:26, 21 July 2010 (UTC)
 * In effect, what Brews is saying is that there is no evidence that the speed of light as measured in a laboratory vacuum is different from the limiting value. That is that the limiting value will be within the measurement uncertainty of the determinations. And so there is no justification for his tirades about ideal vacua and the such like. Brews should stop his soapboxing and tendentious editing, on this aspect and others. Physchim62 (talk) 20:43, 21 July 2010 (UTC)
 * Please see Too much personal criticism. Thanks. --Bob K31416 (talk) 21:02, 21 July 2010 (UTC)
 * And please see my reply to your previous comments there. There are a remarkable number of editors replying to Brews' even more remarkable number of "queries". Physchim62 (talk) 21:24, 21 July 2010 (UTC)
 * It is not the concentration of photons relative to the density of atoms that is important. If you have a certain density of atoms, then you'll have a certain index of refraction that determines the speed of light. I think that in ultrahigh vacuum you'll typically have a residual plasma and then you can treat the situation as presented here. Count Iblis (talk) 21:03, 21 July 2010 (UTC)
 * Well Count Iblis, I suppose I could elaborate on my point and further discuss your point, but for the purpose of developing this article we need to use reliable sources. Do you know of any reliable source that says that the most precise measurement of the speed of light made a correction for not having a perfect vacuum? Regards, --Bob K31416 (talk) 21:16, 21 July 2010 (UTC)
 * If you have a certain number density of atoms you will, in theory, have a refractive index which is not equal to one. On the other hand, below a certain number density of atoms, you will have a refractive index that is experimentally indistinguishable from one. Brews' reading of interferometer literature suggests that this latter number density is attained in practical laboratory vacua, not to mention in outer space. I hardly see how the ionosphere is a good model for a vacuum in terms of its interaction with electromagnetic radiation (as the Count seems to suggest) – quite the opposite, in fact! Physchim62 (talk) 21:24, 21 July 2010 (UTC)
 * Physchim62 is sort of correct. The point is that the errors due to various problems with interferometry make the error in measuring wavelength so large that the added error due to imperfect vacuum is secondary. However, that is not to say that because of practical difficulties one should simply discard the point that one wants to have a perfect vacuum in principle. The logical basis for the whole matter is based upon Physchim62's earlier point that the speed of light is defined to appear in vacuum, and so one should try to obtain a vacuum, or make theoretical corrections for the medium actually used so as to compare the results with the standard speed. I'm left with the notion that Physchim62 wants to back away from this view he expressed earlier.


 * In any event, if one wishes to understand the statement of an exact speed of light, it is inescapable that one has to accept that it occurs under specific ideal circumstances that can be ensured only imperfectly. The fact of imperfection has nothing to do with its being small: the logic of the situation is separable from the practice. Just like geometry: Geometry is the science of correct reasoning on incorrect figures. Brews ohare (talk) 22:06, 21 July 2010 (UTC)
 * I'm not "backing away" from my previous position at all. Brews has admitted that, in a decent laboratory vacuum, the corrections due to the presence of residual gas are insignificant compared to the other measurement uncertainties. Note that this can be tested: you can flush the apparatus with helium or with sulfur hexafluoride prior to the experiments and then take it down to the same pressure as you did from air to see if you get the same results. In whatever case the experimenter has two options:
 * if the effect cannot be completely removed, to get as many data points as possible and extrapolate to zero interference; or
 * if, after extrapolation as above, it is possible to work in a régime of negligeable interference, to work in said régime.
 * Brews' own comments admit that working in a decent laboratory vacuum is working in a régime of negligeable interference for length measurements, and hence for SoL measurements. Again, this is simple to test: you measure the wavelengths of various sources (against an internal standard) and see if the different ratios of wavelengths differ from the ratios of their frequencies.
 * Brews seems to wish that "vacuum" in the definition of the metre means something different from "limit of zero pressure": it seems obvious to me that it doesn't. Physchim62 (talk) 00:21, 22 July 2010 (UTC)

I have been quite clear. Physchim62 is addressing nothing of what I have said. Brews ohare (talk) 05:44, 22 July 2010 (UTC)


 * Physchim62 has addressed everything that you have said that is relevant to the lead sentence. Martin Hogbin (talk) 08:21, 22 July 2010 (UTC)


 * Brews, Drop it. There's a clear consensus against your changes, you're never going to change that, your continual efforts simply waste everyone's time, including your own.-- JohnBlackburne wordsdeeds 08:43, 22 July 2010 (UTC)

Martin & John: The unaddressed issue is: If one wishes to understand the statement of an exact speed of light, it is inescapable that one has to accept that it occurs under specified ideal circumstances (measurement in vacuum) that must be approximated. The approximation is still an approximation, no matter how accurate it may be: the logic of the situation is what underlies the use of an exact definition. I will insist no further, but I am dismayed at your unwillingness to come to grips with this simple point (despite Physchim62's clear enunciation of it), and instead to digress upon irrelevant details of the implementation of the practical approximation to vacuum. Brews ohare (talk) 14:38, 22 July 2010 (UTC)

Reasons for the CGPM decision
The section Speed of light says the following: "“This made its wavelength, and hence the length of the metre, uncertain, because the definition did not specify what point on the line profile (e.g., its maximum-intensity point or its centre of gravity) it referred to.[Note 8] To get around this problem, in 1975, the 15th Conférence Générale des Poids et Mesures (CGPM) recommended using 299,792,458 metres per second for 'the speed of propagation of electromagnetic waves in vacuum'.”" This paragraph suggests that the main problem addressed was the lineshape of the krypton source. Although this was certainly a problem, the CGPM resolution (which should be linked, as it is the full statement of the decision) itemizes a number of other considerations. In particular, the improvement in stability and coherence of laser sources made the krypton discharge lamp obsolete.

Supposedly, the CGPM could therefore simply have replaced the krypton source with a more coherent and monochromatic laser source, thereby avoiding the problems with the krypton lamp. Instead of simply paralleling the old definition of the metre and substituting a better source, they avoided specifying any particular source or transition. Instead in their Mise en pratique (which also should be linked) they specified a “CIPM list of approved radiations for the practical realization of the metre, 1997: frequencies and vacuum wavelengths”. These various sources were related to one another “exactly by the relation λf = c0, with c0 = 299 792 458 m/s, but the values of λ are rounded.”

The better accuracy of frequency measurements compared to wavelength measurements meant that relating the sources through their frequencies was much more accurate than an attempt to relate their wavelengths. This simple and accurate method for interrelating sources was the main driver behind the switch to a new definition. It was recognized that the most accurate source would change over time, and that different applications would be better served by different sources. This accurate comparison coupled with its flexibility in selecting a source is the underlying reason for the change to a defined value for c.

Of course, I expect a lot of debate over this matter. However, we should understand the issues involved. Brews ohare (talk) 13:47, 22 July 2010 (UTC)

Perhaps the link to the mise en pratique should be to the more recent version: CI-2002. It contains this language indicative of the wide range of considerations behind the standard: "considering
 * that science and technology continue to demand improved accuracy in the realization of the metre;
 * that since 1997 work in national laboratories, in the BIPM and elsewhere has identified new radiations and methods for their realization which lead to lower uncertainties;
 * that there is an increasing move towards optical frequencies for time-related activities, and that there continues to be a general widening of the scope of application of the recommended radiations of the mise en pratique to cover not only dimensional metrology and the realization of the metre, but also high-resolution spectroscopy, atomic and molecular physics, fundamental constants and telecommunication;
 * that a number of new frequency values with reduced uncertainties for radiations of high-stability cold atom and ion standards already listed in the recommended radiations list are now available, that the frequencies of radiations of several new cold atom and ion species have also recently been measured, and that new improved values with substantially reduced uncertainties for a number of optical frequency standards based on gas cells have been determined, including the wavelength region of interest to optical telecommunications;
 * that new femtosecond comb techniques have clear significance for relating the frequency of high-stability optical frequency standards to that of the frequency standard realizing the SI second, that these techniques represent a convenient measurement technique for providing traceability to the International System of Units (SI) and that comb technology also can provide frequency sources as well as a measurement technique;" Brews ohare (talk) 16:30, 22 July 2010 (UTC)

At a minimum, this section should be modified to include links to the decision and the mise en pratique, and the discussion of the krypton source modified to avoid the impression that its lineshape was the sole factor behind the switch in CGPM's approach to the speed of light. Brews ohare (talk) 14:50, 22 July 2010 (UTC)
 * No, I think those are fair points, and your summary of the reasons behind the change is pretty much the same as my understanding of the matter. We should certainly cite the two CGPM resolutions (1975 and 1983), and I'll do that in a moment if nobody beats me to it ;) I'm a bit more wary of citing the mise en pratique, because it changes regularly and because it is the sort of technical detail that would be better in the metre article.
 * As for other changes, I'll see what others have to say. I think the article already covers the most important point, which is to explain why the krypton source was considered no longer satisfactory. The stability issue is actually quite secondary – there are other ways of getting round a definition which is no longer a good practical method for measuring the quantity concerned, such as the International Temperature Scale or conventional electrical units. My first choice for something to add (if we're going to add anything, we also have to consider the weight of the various topics in the article) would be the comment in the 1983 Resolution "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 recognized as being equivalent". Physchim62 (talk) 17:00, 22 July 2010 (UTC)


 * The mise en pratique of [2002 recapitulates the earlier resolutions and standards, and adds to them. Therefore, it is a better source than the more recent versions that are simply lists of additions to the standard radiations or revisions of accuracies. I believe a link to the 2002 version of the mise en pratique is suitable because (i) it provides details of what the standards are, (ii) provides a very detailed rationale, (iii) alerts the reader to where such info can be located, and (iv) provides a more up-to-date perspective than the 1983 resolutions. It is therefore a useful link and adds to the value of the WP article. [[User:Brews ohare|Brews ohare]] (talk) 17:31, 22 July 2010 (UTC)

Table heading
I just reverted the change to the table heading as it is the history of measurements: from the first to the decision to fix it to match the then current value: that put an end to needing to measure it, as the text makes clear, so ends the sequence of measurements, while a table of values should include other earlier estimates. Also the change wasn't grammatical and for no obvious reason broke a perfectly good link.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 18:50, 22 July 2010 (UTC)


 * John Blackburne has made this reversion of a change in title of this table. His reasons are: “it is the history of measurements, not values. Rv ungrammatical change and bad edit)”


 * Unfortunately, not all the items in this table are measurements. The last entry is a definition. A more correct title is therefore: History of values of c . I don't see much quarrel about this. If it an ungrammatical title, at least it is accurate. Referring to a ‘definition’ as a ‘measurement’ is not accurate. Brews ohare (talk) 18:48, 22 July 2010 (UTC)

Measurement section intro
The section by this name states:
 * “The techniques described here are therefore no longer needed to measure it, though they can be used to confirm its value.”

The difficulty with this statement is that it suggests the defined value of c=299 792 458 m/s can be confirmed by measurement. That is an incorrect statement, as a defined value can be changed only by changing the definition, which is the prerogative of the CGPM, not of measurement.

Perhaps what could be said instead is:


 * “The techniques described here are therefore no longer needed to measure it; it may be noted, however, that they produce values the same as the defined value to within the present-day accuracy of such measurements.”

Brews ohare (talk) 19:59, 22 July 2010 (UTC)


 * Wouldn't it though be confirming it's value, measuring it though you know the answer? The same way I can confirm the value of π by comparing the diameter of a wheel to the distance travelled as it rotates once, or confirm the date of Christmas this year by looking on a calendar, but I cannot change either π or the holiday. I did when writing it think it simplifies it somewhat. You might rather be measuring the speed of light to gauge the errors in your time and distance measurements, or to test your methods or equipment. The point is there's no need to measure the speed of light today, as at all you can do is get the existing value and confirm it - or get a different value which indicates a problem in your methods.


 * And this is the second section you've started titled "Measurement" on this page, so I've appended "again" to make it appear with a different name in edit summaries: otherwise it will be difficult to find on a very long talk page.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 20:22, 22 July 2010 (UTC)

-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 20:22, 22 July 2010 (UTC)


 * Well, John, I have a lot of trouble understanding the need for convoluted meaning in what is a simple matter. The CGPM chose to define the SI c as a value near the value used at the time so as not to cause too much dislocation. But that wasn't a logical requirement, it was just expediency. And if it turns out one day that the last 3 decimal places in c determine whether the universe obeys quantum gravity or not, the defined value of c won't change. It just won't be confused with the "speed of light" any more, it'll be obvious to all that it is just a conversion factor. So I totally object to "confirming" a definition by means of measurements. It is a Humpty Dumpty usage, and turns the meaning of words upside down. Brews ohare (talk) 23:37, 22 July 2010 (UTC)
 * The wording at present is awkward in the very least. What are we trying to say here? IMO, we want to say something like "we don't actually measure c any more, but here are some of the methods that have been used in the past." Physchim62 (talk) 00:07, 23 July 2010 (UTC)


 * Just checking something regarding the views here. Is it only the physical constant c that is fixed and not the result of a measurement by someone in some lab for the speed at which light travels in a vacuum? Thanks. --Bob K31416 (talk) 00:50, 23 July 2010 (UTC)
 * Well, to measure the speed at which light travels in a vacuum, you need a length standard. If your working in SI units, the speed of light is your length standard, so the measurement becomes rather pointless. You're free to choose another length standard, of course, but it wouldn't be normal to express your results as a "speed of light" in that case: you might be measuring the length of your standard in metres, or measuring the ratio of the speeds of light at two different frequencies, for example. Physchim62 (talk) 01:13, 23 July 2010 (UTC)

Not, I hope, to disagree with Physchim62 but to say things in my own words: To answer your question directly: the "physical constant" is not fixed. (It is to get mired down to explore what that term actually means.) Rather, the standard unit of of speed is fixed. The standard unit is not established by measurement but by fiat by the CGPM. The standard unit of speed can be realized approximately in a lab by creating an approximate vacuum (a "real" vacuum is beyond realization, but the CGPM will help you set up a good approximation). However, the purpose in doing so is not to "measure" the standard; it is to get a measuring stick with which to compare other unknown speeds to the standard. Brews ohare (talk) 02:10, 23 July 2010 (UTC)

Once you have the standard speed set up in your lab, you also can compare two lengths by simply comparing the two times-of-flight of your approximate standard speed along the lengths. Brews ohare (talk) 02:17, 23 July 2010 (UTC)
 * Another point to remember is that when we say that c is "fixed" by the definition of the metre, we're usuing a sort of shorthand: c is "fixed" as long as relativity, quantum electrodynamics and a whole range of other physical theories are correct. If any of those theories are not correct, then our equations won't add up, although it may take a long time before our measurement techniques are good enough to realize that the equations don't add up. If the equations don't add up, we will have to either introduce a correcting factor and then figure out what that correcting factor actually means physically, or we will have to come up with a whole new theory: such is the stuff of scientific revolutions, as Thomas Kuhn pointed out. But, at the moment, the equations add up to within experimental uncertainty, at least most of the time! Physchim62 (talk) 03:13, 23 July 2010 (UTC)

I think the latest fix is lame. All the techniques still work; they just measure the meter instead of the speed of light. If you start with a meter bar, the measurement in the end gives you a correction to it. The only thing that has changed is what you take as fixed in the algebra. Dicklyon (talk) 03:24, 23 July 2010 (UTC)

Physical constant
The lead says light is a physical constant. It is interesting that that article says: "“A physical constant is a physical quantity that is generally believed to be both universal in nature and constant in time. It can be contrasted with a mathematical constant, which is a fixed numerical value but does not directly involve any physical measurement .”" That contrast can be compared with the last statement in the article: "“As a result, in the SI system of units the speed of light is now a defined constant.[7] Improved experimental techniques do not affect the value of the speed of light in SI units, but do result in a more precise realization of the SI metre.”" An even clearer statement comes from one of sources cited: "“One fallout of this new definition was that the speed of light was no longer a measured quantity; it became a defined quantity. The reason is that, by definition, a meter is the distance light travels in a designated length of time, so however we label that distance - one meter, five meters, whatever - the speed of light is automatically determined. And measuring length in terms of time is a prime example of how defining one unit in terms of another removes a constant of nature by turning c into a conversion factor whose value is fixed and arbitrary.”" (my underscore). There appears to be a lack of clarity here. Brews ohare (talk) 11:52, 20 July 2010 (UTC)
 * Not at all, you are being deliberately obtuse again Brews.
 * Let's take another example. We have strong reason to believe that all atoms of carbon-12 have exactly the same mass. We can say that the mass is exactly 12 atomic mass units, or we can say that the mass is about 11.996186 "old atomic mass units": the mass of carbon-12 atoms didn't change in 1960 when carbon-12 became the basis of the atomic mass scale in place of oxygen-16! Our units of measurement are mere human constructs, we can choose whatever suits us: the speed of light is the same if you express it in metres per second or in furlongs per fortnight. Physchim62 (talk) 12:08, 20 July 2010 (UTC)


 * I read that quote when looking at your new article, and I'm not sure what Mr Jespersen is getting at. Nothing has been removed, or changed. The older less accurate ways of doing things are still there if people want to use them. All that's happened is we've through better understanding unified length and time, replacing two arbitrary scales (the second and metre) with one. But he's written a popular science book not a text book or research paper, so hardly a reliable academic source, and I don't think it's of use as a reference here.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 12:32, 20 July 2010 (UTC)
 * John, this source is a NIST publication, and so I'd hesitate to discount it as "a popular science book". It is that, but it is written by an authority on the subject. You are right that the old methods can be resorted to, and maybe they will one day if some revolution in technique occurs to make wavelenght a more accurate measurement than frequency, though it's hard to imagine at the moment. However, the logical basis of the units has to be made clear, and there is a shift in this regard. Brews ohare (talk) 14:21, 20 July 2010 (UTC)

A somewhat more careful wording is this, from Hall and Ye: "redefinition of the International Meter in 1983 in terms of an adopted value for the speed of light, thus reducing by one the number of independent, base physical standards." Brews ohare (talk) 13:48, 20 July 2010 (UTC)


 * Yes, that's better, and agrees with what's in the article. Again you can think of instead as reducing the number units, i.e. scales, which we have to define to do physics, i.e. one less arbitrary thing. That was my objection to the first quote: the only thing removed is one independent and so arbitrary measure. Now there's only one, the second, with the metre defined in terms of it. That definition also includes c and so fixes it in terms of our units. We could if we wanted to do away with the metre altogether and measure everything in seconds, but the metre is far more convenient and familiar so we keep it. But c is also physical constant - it's a property of our universe, not a purely mathematical constant like π or e. We can hypothesise about a universe with a different c, or that c might change over time, but we can't do so with π. That I think is the difference.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 14:55, 20 July 2010 (UTC)

In response to Physchim62: The issue here is not what scientists are free to do about standards. The issue here is the choice of wording in a WP article. The term "physical constant" is irretrievably related to measurement in the linked article physical constant. Jespersen says the redefinition has "removed a physical constant". Hall and Ye say it "reduced by one the number of base physical standards". Personally, I like Hall & Ye's terminology better. But whatever the approach chosen, we need a terminology that is clear and consistent, and that does not seem to be the case here. Brews ohare (talk) 14:40, 20 July 2010 (UTC)
 * The dimensionless number 299,792,458 "does not directly involve any physical measurement", but the dimensionful quantity 299,792,458 m/s does involve some measurement (for example, how long a second lasts). Can you see the difference? A. di M. (formerly Army1987) (talk) 15:22, 20 July 2010 (UTC)
 * A physical constant has an existence independent of any attempt to measure it. The speed of light is still the same as it was in Rømer's time: or are you denying that, Brews? Physchim62 (talk) 15:29, 20 July 2010 (UTC)
 * Response to Physchim62. No, not at all. I would hope you would understand this matter yourself, particularly in view of your very apt summary of the situation when you said “Basically, you exchange a measurement uncertainty in the value of a physical constant for a realization uncertainty in the corresponding unit.” To me, this statement of yours very neatly sums the situation where one defines c as an arbitrary value, setting the unit for speeds, and then specifies the circumstances under which the unit will be realized. Brews ohare (talk) 15:51, 20 July 2010 (UTC)
 * Response to A di M: I do not believe you present the situation usefully by making a distinction between the number 299,792,458 and the definition of c = 299,792,458 m/s. There is no measurement involved in setting c = 299,792,458 m/s, and, for example, Jespersen and also Sydenham say this definition is independent of measurement and arbitrary. I believe Physchim62 has explained the matter very well: one sets up the circumstances under which c is realized; under those circumstances the unit is produced, and what value is assigned to the unit is a matter for the CGPM. Brews ohare (talk) 15:58, 20 July 2010 (UTC)

I believe that a simple formulation like Physchim62's should appear in the lead and that would go a long way to clarify what is meant by an "exact" value for c and the removal of c from the list of "independent, base physical standards". Brews ohare (talk) 16:09, 20 July 2010 (UTC)
 * I can't see what you mean, i.e. what formulation, and what precisely "like". Don't expect us to work it out for ourselves, post what you think should be changed or inserted if you want other editors' opinions on it.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 16:20, 20 July 2010 (UTC)


 * John: Physchim62's formulation is spelled out in the previous remark. Here it is again: “One sets up the circumstances under which the standard speed of light is realized; under these circumstances the standard speed of light is produced, and what value is assigned to this unit is a matter decided by the CGPM. This procedure exchanges a measurement uncertainty in the value of a physical constant for a realization uncertainty in the corresponding unit. Specifically, c = 299,792,458 m/s exactly, and this standard unit of speed is realized in free space, which as a practical matter, can be realized only approximately.” Brews ohare (talk) 16:43, 20 July 2010 (UTC)
 * The definition says "vacuum", not "free space". Physchim62 (talk) 17:06, 20 July 2010 (UTC)
 * I'm not sure how that would fit in - it more an answer to a question, and does not add anything as that information's already in the article, i.e .the article already covers this very well.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 16:52, 20 July 2010 (UTC)


 * You are quite right Brews, the metre standard can only ever be realised approximately, for all the usual 'measurement' error reasons. This point is probably more appropriate for the metre article. Martin Hogbin (talk) 16:59, 20 July 2010 (UTC)

The problem remains that the use of the term "physical constant" contradicts the usage in the article physical constant and the sources I have quoted. Brews ohare (talk) 15:50, 21 July 2010 (UTC)
 * I agree here, this can be clarified in the section about moden physics. My opinion is that in that section it should be made clear that from the point of view of relativity, c is a conversion factor, see e.g. here
 * "The possible time variation of dimensionless fundamental constants of nature, such as the fine-structure constant $\alpha$, is a legitimate subject of physical enquiry. By contrast, the time variation of dimensional constants, such as $\hbar$, $c$, $G$, $e$, $k$..., which are merely human constructs whose number and values differ from one choice of units to the next, has no operational meaning." Count Iblis (talk) 18:06, 21 July 2010 (UTC)
 * And, yet again, we get the Duff paper coming out, dispite the fact that it fails WP:RS (it was rejected from a peer-reviewed journal) and without any mention of the refutations that the author honourably includes as appendices to the preprint. We could simply look at standard usage of the term "physical constant" as in, for example, the list of values regularly published by CODATA (the 2006 list is available at and here): the speed of light is not only included in the list, but it is even the first on the list in most printed summaries! To pertend that the speed of light is not a physical constant would be to give undue weight to a minority point of view, regardless of the intellectual merits of that point of view. Physchim62 (talk) 19:07, 21 July 2010 (UTC)
 * I agree. Martin Hogbin (talk) 19:12, 21 July 2010 (UTC)


 * I don't agree. It is a reliable source despite being rejected from Nature. To see this, you have to look at the citations of the paper, how it is cited, by whom it is cited and how much those citing papers themselves are cited. What Duff was saying in his paper is now something most people who look at time variation of constants take very serious; they take great care to make sure the quantity they are sudying is dimensionless. In contrast, if you look at the paper by Paul Davies, and evaluate that in the same way, you see that his argument which also the Referees made who rejected Duff's paper, have little support in the physics community. People are not looking at whether c or e is changing (if alpha is changing).


 * So, I would say that to not mention that c is regarded to be a rather irrelevant conversion factor, leaves out the very important POV of many theoretical physicists. But I won't make a big issue out of this, for a FA review this is not a stumbling block. Count Iblis (talk) 21:24, 21 July 2010 (UTC)


 * B.t.w., I think there is another paper by someone else that makes a simlar argument as Duff did that did get published in Nature... Count Iblis (talk) 21:24, 21 July 2010 (UTC)


 * I agree with Physchim62 and Hogbin on this point.—Finell 23:01, 21 July 2010 (UTC)


 * Then you are wrong as well. I'm not saying that the article should not mention that c is a physical constant (as that is what many sources say), but simply that it is a wideheld opinion in the theoretical physics community that it isn't, i.e. that it is nothing more than an irrelevant re-scaling constant that has no operational meaning whatsoever. There are tons of articles that say this in so many words. My opinion is that the section on modern physics is misleading because it doesn't mention this. It gives the (sort of) high school POV on units that actually causes students at university to unecessarily struggle a bit when they have to learn Natural Units. Sometimes a mental block prevents them from seeing that c = 1 is not "wrong", "inconsistent", "paradoxical", "just a trick as you ultimately have to put back c to its correct value", or whatever.


 * I'm not going to make a big issue out of this; I won't change the article on this point, but the editors here have to consider that the irrelevance of c as a genuine physical constant is a wideheld opinion, whether they agree with it or not, and that the article does not discuss this properly. Count Iblis (talk) 14:42, 22 July 2010 (UTC)
 * It's a respectable opinion that held by a significant number of physicists, but it's not the only opinion and so we shouldn't present it as if it is. This is an article that made it past the journal referees and that expresses both Duff's opinion on the matter and two other contrasting points of view. Physchim62 (talk) 23:25, 22 July 2010 (UTC)

The problem remains that the use of the term "physical constant" contradicts the usage in the article physical constant, is not used consistently throughout Speed of light, and contradicts the sources I have quoted, for example, Jespersen. Brews ohare (talk) 12:08, 22 July 2010 (UTC)
 * Maybe it's the physical constant article that needs tweaking on this point (and others) ;) Physchim62 (talk) 23:25, 22 July 2010 (UTC)
 * Whatever our physical constant article claims, the standard usage of the phrase doesn't exclude values which are exact in a particular system of units, or conversion factors. See for example. A. di M. (formerly Army1987) (talk) 16:01, 23 July 2010 (UTC)

Count Iblis has accurately summarized the situation. Perhaps the underlying problem here is that (IMO) there is in reality a phenomenon called ‘light’ and it travels at fixed speed (according to relativity, anyway). That phenomena underlies the set-up of the SI units. However, it is easy to confuse the usage of c in the SI units, which is simply a conversion factor for that system of units, with the speed of travel of the fundamental phenomenon ‘light’. The ability of the mind to cope with the same name for different things is very limited. Brews ohare (talk) 15:01, 22 July 2010 (UTC)
 * Unfortunately, this article has to cope with the fact that the speed of light is much more than, well, the speed of light! It's a fair point that the role of the speed of light in defining the metre is physically insignificant, as the metre is simply a human construct that we can define as we wish. On the other hand, we do have to give the numerical value for c, as that's what many readers will have come looking for. And once we give the numerical value we have to explain that it is fixed by definition and so has no measurement uncertainty in SI units. And we have to say a lot of other things about the speed of light, so we cannot give undue weight to its role in the SI.
 * But it remains that there is this phenomenon called 'light', and that it travels at a fixed speed (as far as we can tell). In that sense, the speed of light is a "physical constant". The speed doesn't depend on what day of the week it is, nor on what unit system we choose to express it in. On the other hand, we have to express it in some unit system to be able to compare it with anything else, and SI units are the conventional choice. Physchim62 (talk) 23:25, 22 July 2010 (UTC)

In a ‘time-of-flight’ system of units based upon a standard speed, there is no requirement that one know what that speed is, or whether it is fast or slow. The only requirement is that one be able to identify situations where that standard speed is actually realized. The choice of units decides the numerical value assigned to the speed in that system, and that value is “fixed and arbitrary”, to quote Jespersen. Brews ohare (talk) 15:21, 22 July 2010 (UTC)

"most used reference length scale"
The beginning of the following sentence seems awkward and unnecessary:
 * "Since the most used reference length scale in modern experiments (the SI metre) is determined by the speed of light, the value of c is fixed when measured in metres per second."

Perhaps it should be changed to:
 * "Since the most used reference length scale in modern experiments (the SI metre ) is determined by the speed of light, the value of c is fixed when measured in metres per second."

--Bob K31416 (talk) 20:29, 24 July 2010 (UTC)
 * Agree. Physchim62 (talk) 21:14, 24 July 2010 (UTC)
 * ✅. A. di M. (formerly Army1987) (talk) 10:54, 25 July 2010 (UTC)

Last two sentences of section Increased accuracy and redefinition of the metre
In the section Increased accuracy and redefinition of the metre, the last paragraph is,


 * The effect of this definition gives the speed of light the exact value $220,000$, which is nearly the same as the value $301,000$ obtained in the 1972 experiment. The CGPM chose this value to minimise any change in the length of the metre. As a result, in the SI system of units the speed of light is now a defined constant. Improved experimental techniques do not affect the value of the speed of light in SI units, but do result in a more precise realisation of the SI metre.

Could the last two sentences be trimmed in the following way?


 * The effect of this definition gives the speed of light the exact value $315,000$, which is nearly the same as the value $298,000$ obtained in the 1972 experiment. The CGPM chose this value to minimise any change in the length of the metre. As a result, in the SI system of units the speed of light is now a defined constant. Improved experimental techniques do not affect the value of the speed of light in SI units, but do result in a more precise realisation of the SI metre.

--Bob K31416 (talk) 12:51, 23 July 2010 (UTC)

I'd say that is sticking your neck out. The statement with SI units in it is correct. The statement without it is very broad, and appears to encompass all systems of units. It may be that some of these are not based upon a standard speed, but instead continue to use a standard length. In any event, one can imagine such systems. Brews ohare (talk) 13:56, 23 July 2010 (UTC)


 * Re "one can imagine such systems" - Could you give an example of an existing system where the proposed change would be incorrect ? Thanks. --Bob K31416 (talk) 15:31, 23 July 2010 (UTC)
 * I assume by "existing system" you mean one that is used today. In that case, the astronomical system of units, as Brews mentions in the next section. If we're allowing any conceivable system, then it would be any system of units (including SI) from before 1983. Physchim62 (talk) 15:43, 23 July 2010 (UTC)
 * I don't see the point, but I won't pursue this question because I don't think I could make any progress. Thanks anyhow. --Bob K31416 (talk) 16:57, 23 July 2010 (UTC)
 * It's metrology-wonkishness, I agree, but there are plenty of subject-wonks at WP:FAC as well ;) Physchim62 (talk) 17:11, 23 July 2010 (UTC)
 * I don't think it's a matter of wonkishness. It doesn't appear that these points are made in reliable sources. If they were, there wouldn't be any difficulty. I could just read the reliable source. Cheers, --Bob K31416 (talk) 17:29, 23 July 2010 (UTC)

Light doesn't give a damn about what metre means, so whatever the speed of light in itself was in 1982 still is now. It was the value of the speed of light in metres per second which changed status. A. di M. (formerly Army1987) (talk) 18:07, 23 July 2010 (UTC)
 * And similarly the value of the speed of light in any units that metres per second can be converted to, which changed status? --Bob K31416 (talk) 18:40, 23 July 2010 (UTC)
 * Only in units that metres per second can be exactly converted to. This includes miles per hours (for example), but excludes astronomical units per day. The value of c in AU/day had to be measured before 1983 and has to be measured now. A. di M. (formerly Army1987) (talk) 20:14, 23 July 2010 (UTC)
 * The value of c in AU/d wouldn't be exact if the conversion between m and AU isn't exact. Nevertheless it seems that, along with the status change of the value of c in m/s, the status changed for the value of c in AU/d that is obtained when m/s is converted to AU/d (using for example, 1 AU = 149,597,870,691 meters), because the value of c in AU/d would also not be changed  by any future measurement of c.   Regards, --Bob K31416 (talk) 22:58, 23 July 2010 (UTC)
 * I don't follow you. The value of c in AU/d is not exact and continues to be measured, and it is $299,788$ where the digit in parentheses is the uncertainty; the length of 1 AU in metres can then be computed (neglecting nitpicking about the metre being supposed to be a unit of proper length) as $299,796$×$299,792$/$299,792.5$, and the result is $299,792.457$. What the metre has to do with measurements of c in AU/d I can't understand. A. di M. (formerly Army1987) (talk) 01:20, 24 July 2010 (UTC)


 * I think that you are saying that the speed of light c in units of AU/day is not derived from c=299,792,458 m/s. Is that what you're saying? --Bob K31416 (talk) 10:08, 24 July 2010 (UTC)
 * What do you mean by "derived from"? Certainly it can be measured without reference to the metre... A. di M. (formerly Army1987) (talk) 10:31, 24 July 2010 (UTC)
 * By "derived from" I meant c = 299,792,458 x 86,400 / 149,597,870,700 AU/d, using the inexact value for AU in metres that you gave and is given here. I may have to rethink this. What I just wrote, considering your comments, looks  kind of circular. I'm open to any helpful comments from you and any reliable sources that you may know of that discusses this. --Bob K31416 (talk) 12:17, 24 July 2010 (UTC)
 * I don't think there's any fundamental difference between computing the value in AU/day and then converting the result into AU/metre or the other way round; our Astronomical unit article suggests it's the former, but ultimately (AFAICT) is just a matter of conventions. A. di M. (formerly Army1987) (talk) 12:54, 24 July 2010 (UTC)

A di M & BobK: Isn't the underlying issue here whether a change in the phenomenon of light propagation (due some mysterious cause) would change the speed of light in AU/day, but will not change the speed of light in SI units? And for the same reasons, the AU conversion to metres would change, because the metre changed length. Brews ohare (talk) 14:54, 28 July 2010 (UTC)
 * It depends on which mysterious cause does that. In any event, that's wild speculation. (Also, for the length of the AU to change it would suffice for the Sun to change mass.) A. di M. (formerly Army1987) (talk) 15:47, 28 July 2010 (UTC)

A di M: You have introduced a red herring in mentioning the change in AU due to other reasons. Of course, it is known that the AU does increase with time, and it is speculated that is due to the changing mass of the Sun. However, the subject here is not an error analysis to determine whether an observed effect is due to a change in the travel of light or due to other causes, nor is it a discussion of how large a change in travel of light would have to be to be clearly identified. Nor is it about what coauses the change in the way light travels. Rather, a question of logical issues in principle is at hand here.

I would assume that you would agree that, for example, should it happen that the travel of light doubled in speed, the metre would half, the AU wouldn't change, and the speed of light in AU/day would double, while c in SI units would stay exactly the same. Brews ohare (talk) 16:00, 28 July 2010 (UTC)

Astronomical units
The AU apparently is a measured quantity given by:
 * $$A^3 = \frac {G\ M_S}{k^2} \, $$

where A = AU, k=0.01720209895 AU3/2 d-1 exactly, and d is a day of 86400 s, and MS is the solar mass, G the gravitational constant. G and MS are measured quantities, introducing uncertainty in the AU. Its value in metres at the present time is 149597870700 ± 3 m (same source). It increases at a rate of (15 ± 4) cm/yr. The origin of the increase is unclear, but attention is given to trying to determine what is happening to the mass of the Sun.

For the purpose of this article, there are two issues. One is that a system of units based upon a length is not a system based upon a standard unit of speed.

The second issue is whether one can attach any significance to the speed of light as measured in this system of units based upon the uncertainty in the AU. In other words, is the error bar here too big to make such a determination interesting?

What should the article say about these matters? Brews ohare (talk) 14:49, 23 July 2010 (UTC)

I'd say the article hits the main points at the moment, but fails to clearly contrast this approach to the SI units approach as logically separate methods. Brews ohare (talk) 14:54, 23 July 2010 (UTC)
 * I'm glad you think it hits on the main points, because I think I wrote that paragraph! I'll see if I can add a footnote to cover the points you raise, but I'm not sure it merits more than that. As for the accuracy of the measurement, I think it's impressive (possibly even a little optimistic, but probably order-of-magnitude correct): we couldn't do much better on Earth for the moment, even if we defined a length scale based on the best practical wavelength for laser interferometry. Physchim62 (talk) 16:49, 23 July 2010 (UTC)


 * I added the Nieto source, and a few words of emphasis. Brews ohare (talk) 14:34, 28 July 2010 (UTC)

Folk relativity withdraw syndrome. Struggling only makes it worse.
Relativity: "light is always propagated in empty space with a definite velocity c" -- Einstein Wikipedia: "light is always propagated with a definite velocity c". Does anyone (else) object to eliminating this discrepancy? Before you answer, recall your fifth year of college when you were informed that condensed matter physics is not the domain of special relativity.

Also, this statement needs to be justified with references: "The physical photon is the bare photon plus all sorts of (formally infinite) corrections and this physical photon travels at c". To the contrary: A wave packet containing a single dressed photon travels at the group velocity through the medium "In the spirit of Einstein’s original model of light quanta in the vacuum, each dressed photon carries energy ℏω (k) according to (19). Furthermore, one can show that the appearance of the group velocity in the normalization factor in (20) guarantees that a single-photon wave packet, propagating at the group velocity, carries the energy ℏω (k) associated with the carrier wave.", "the dressed photon might be regarded as a relativistic particle with velocity vgr(k)"

"In general terms, a photon always propagates in a medium at less than the speed of light. In the language of modern quantum field theory it can be considered a 'dressed' photon

"The photon in plasma (dressed photon) has rest mass = wp/c.

Photons in matter (Wikipedia) "light that travels through transparent matter does so at a lower speed than c, the speed of light in a vacuum. X-rays, on the other hand, usually have a phase velocity above c ... The factor by which the speed of light is decreased in a material is called the refractive index of the material. In a classical wave picture, the slowing can be explained by the light inducing electric polarization in the matter, the polarized matter radiating new light, and the new light interfering with the original light wave to form a delayed wave. In a particle picture, the slowing can instead be described as a blending of the photon with quantum excitations of the matter (quasi-particles such as phonons and excitons) to form a polariton; this polariton has a nonzero effective mass, which means that it cannot travel at c. Alternatively, photons may be viewed as always traveling at c, even in matter ... a light wave made up of these photons does travel slower than the speed of light. In this view the photons are "bare", and are scattered and phase shifted, while in the view of the preceding paragraph the photons are "dressed" by their interaction with matter, and move without scattering or phase shifting, but at a lower speed."

A bare photon plus the corrections is called a dressed photon, dressed photons do not travel at c.

Next on the agenda, more heresy! The speed of light on Earth (as compared to light in interstellar spare) is slower in January than in July. Physical light always travels slower than c, etc, etc. Unless of course, Einstein was wrong when he said all of these things. —Preceding unsigned comment added by NOrbeck (talk • contribs) 08:05, 24 July 2010 (UTC)


 * The resolution of this problem is simple. Light as a classical wave travels at c in (the idealised medium of) free space and more slowly in real media.  Photons are a quantum entities and it is not valid to apply classical concepts of velocity, trajectory, or position to them, as this leads to paradoxes, problems, and arguments such as we have here.


 * The answer is never to talk of the speed or path of a photon or photons. Whatever the original intention may be, someone will always imaging the photon as a little ball travelling from A to B down a fixed trajectory in a precise time. Martin Hogbin (talk) 09:13, 24 July 2010 (UTC)
 * I agree. The article at 100 KB is already long enough, there's no point in adding tons of esoteric discussions which would be better suited for (e.g.) Metre, Interferometry, or Quantum electrodynamics which would only interest (and be understood) by about 1% of the readers of this article. :-) A. di M. (formerly Army1987) (talk) 10:38, 24 July 2010 (UTC)

It is not correct to say that dressed photons do not travel at c. This is only the case when you define dressed photons by taking into account interactions with a medium (which is not Lorentz invariant and thus leads to the dressed photon travelling at a speed different than c). But the phrase "dressed photon" is also used to take into account interactions the electromagnetic field has with the fields of other paticles in vacuum. So, the dressed phton is then simply the ordinary physical photon in vacuum which travels at c. Note that in the Scharnhorst effect the physical photon travels faster than c, which happens because the Casimir vacuum between the two plates is not Lorentz invariant. Count Iblis (talk) 15:36, 24 July 2010 (UTC)
 * Great, I wholeheartedly agree. In the change log I wrote: "dressed particles do not travel at c, except in a vacuum.". Above I meant to say "dressed photons do not necessarily travel at c". I also agree that we should avoid mentioning photons whenever possible.

This statement from the article is is egregious: "c is the speed at which all massless particles and waves, including light, must travel."

The tone is classic pseudo-relativity, no theory dictates how reality "must" behave. Of all the relativity myths that cause confusion and frustration, this is one of the worse.

I would have been satisfied to append "in a vacuum" to the end of the sentence, but that still would have been a lie. The truth is real physical light never travels at c. I don't think this fact is controversial or contestable. Nevertheless, despite two attempts, the claim is still there.

The article often seems to be intentionally misleading the reader. Is this lying to children? I wonder, what do people really believe?

Prove your faith. The questions are: Does real physical light travel travel at c? (No) Is c is the limiting speed approached, but never reached, in partial vacuums. (Yes) Is the local instantaneous speed isotropic and invariant in inertial frames? (Yes!) Does real physical light actually move from point a to point b at a constant speed? (No) Is the real physical (coordinate) speed slowed by the everpresence of matter; so that even in a vacuum, the speed of light is still not c. (Yes) Is the speed of light slowed by proximity to other light? (Yes) When viewed from outside, does the speed of light in a room speed up when the lights are dimmed? (Yes) Is the speed of light equal to c in a universe with no matter? (No) Is the speed of light equal to c in a universe with no matter and no light? (Yes!)

Please vote, but let's not waste effort debating this. I'm just trying to see to what extent my expertise will be useful here. NOrbeck (talk) 03:37, 25 July 2010 (UTC)


 * What is real physical light?
 * What is the 'instantaneous' speed?
 * What is the 'real physical' speed?
 * How do you know what the speed is in a universe with no matter?
 * Is this philosophical speculation appropriate for this article or even this talk page? Martin Hogbin (talk) 09:08, 25 July 2010 (UTC)


 * 1) Special relativity dictates that all real massless particles and waves have light-like momentum, which is an obvious consequence of the definition of massless; thus, their velocity (if defined as c2p/m) does equal c in magnitude. (It doesn't dictate that light must be massless, though.)
 * 2) It is true that "travel at" isn't supposed to mean "have a well-defined position at every single instant", but such nuances are better suited to such articles as photon, quantum electrodynamics, etc. than here: c is the fundamental conversion factor of special relativity, and special relativity doesn't depend on quantum mechanics. (Matter of fact, it was formulated before quantum mechanics).
 * 3) The answer to the last of Hogbin's questions above is "No, it isn't." A. di M. (formerly Army1987) (talk) 10:50, 25 July 2010 (UTC)

Re Martin Hogbin's remark "Is this philosophical speculation appropriate for this article or even this talk page?" - I would add to this remark that Wikipedia is an encyclopedia which summarizes existing published information, and is not meant to be a source of information that has not already been published. I think that the article should only include material that appears in reliable sources and this should be demonstrated by citing them. If there is a question as to whether a cited reliable source contains the material, then the relevant excerpt from the reliable source should be given here on this talk page where editors can decide whether the reliable source supports the material.

So with this in mind, what reliable source or sources are we trying to summarize here? On the other hand, is there a reliable source for the material in the article that is being questioned? If not, perhaps at least a Citation needed template should be placed. I'll leave that for the other editors here to decide. --Bob K31416 (talk) 12:10, 25 July 2010 (UTC)


 * My point was not so much, 'Are there any sources which support these views?', but 'Do the questions asked have any meaning?. Martin Hogbin (talk) 13:37, 25 July 2010 (UTC)


 * BobK has the right approach to this matter, especially for this article that has so many editors with intransigent positions of their own. And the questions raised, whether meaningful to Martin or not, are certainly meaningful to a naive reader and should be dealt with. Brews ohare (talk) 13:29, 28 July 2010 (UTC)

Gravity and gravitational waves
The sentence in the introduction about the relation of the speed of light to gravity and gravitational waves appears to go beyond established ideas on this subject to adopt a particular viewpoint. See Fabio Cardone, Roberto Mignani. I believe this sentence distorts the record. Brews ohare (talk) 15:27, 28 July 2010 (UTC)


 * "Established ideas"? You mean like "... u_grav >= 2.5 10^10 c, in astonishing agreement with the astronomical estimate by van Flandern"? Duh. DVdm (talk) 15:38, 28 July 2010 (UTC)
 * It says c is believed to be the speed of gravitational waves. That much is true. A. di M. (formerly Army1987) (talk) 15:40, 28 July 2010 (UTC)

See also: Part 3: On the speed of gravity controversy. Brews ohare (talk) 15:47, 28 July 2010 (UTC) BTW, a belief is something held without adequate justification, and probably signifies that a different belief is entertained by some. WP is supposed to be even-handed in such matters, and not present only one side. Brews ohare (talk) 15:49, 28 July 2010 (UTC)


 * Yes, but then we have to give a detailed overview of this issue to present the correct context. If we just mention that there are alternative points of view without explaining in any depth what the standard point of view is, that would amount to giving a huge weight to a contrarian point of view. Count Iblis (talk) 15:54, 28 July 2010 (UTC)
 * So, where some honest difference of opinion occurs among experts, a reader of WP should assume that one view will be presented and others ignored with a warning flag (if you're lucky) that the stated position is "believed" (by the editors in charge of the WP article at this moment in time)? Brews ohare (talk) 16:05, 28 July 2010 (UTC)
 * A preferable approach would be to state the preferred position with some sources, and say that there is some controversy over the point and cite those sources. If the matter is so subtle that only an extended discussion will cover the matter, and I believe that is the case, the reader is left with places to look. That methodology is preferable to introducing the tactic of "beliefs". Lord help us if the "belief" approach makes it to your favorite topic of global warming. Brews ohare (talk) 16:17, 28 July 2010 (UTC)


 * Lord help us if Van Flandern is considered to be an "expert" in this matter and cited as a source. DVdm (talk) 16:31, 28 July 2010 (UTC)

It appears that Schafer and Brugmann think the subject is up for grabs experimentally, and Cardone & Mignani feel the same way. Brews ohare (talk) 16:48, 28 July 2010 (UTC)
 * General relativity is by far the most widely accepted theory of gravitation among physicists (to the point that discussing alternatives would be WP:UNDUE except in very specialized articles), and it predicts that gravity travels at c. The reason for saying "is believed to be" rather than just "is" is that no-one has yet detected gravitational waves, though there's a lot of indirect evidence that they actually exist. A. di M. (formerly Army1987) (talk) 16:54, 28 July 2010 (UTC)
 * If the situation is ‘theoretically’ as stated, the position should be labeled that way. A statement that is open to the interpretation that it has experimental support where there is none is unacceptable. And to claim theoretical support is a ‘belief’ is a smack-down of theoretical arguments, which are after all much better established than some credo. Brews ohare (talk) 16:59, 28 July 2010 (UTC)
 * I could only read Schafer and Brugmann book (presumably because my IP address is different) and they make clear that the issue is purely one of experimental verification. They point out that Kopeikin's argument that you could measure the speed of gravity using the Shapiro delay effects when Jupiter passes in the direction of a quasar is flawed, as the effect does not depend on c_g. Count Iblis (talk) 17:05, 28 July 2010 (UTC)
 * That is exactly correct, and applies to the other source as well. The point is that the WP article should make it clear that the predictions of theory in this regard are unsupported, and that this is not a question of general "belief", which is a most unfortunate choice of wording. There are some apparently contradictory experimental observations, whose apparent conflict with theory has yet to be resolved, and some observations that appeared to support the theory that have been found to be erroneous calculations.


 * It is a bit mystifying to me why it is thought that what is proposed here is an alternative theory. Perhaps I phrased things incorrectly at the outset. Brews ohare (talk) 17:31, 28 July 2010 (UTC)

While on this subject, do we need to say the speed of "gravity and of gravitational waves"? Just "gravitational waves" would be simpler and maybe more accureate imho (speaking with the store of knowledge available to one who is just progressing from year 1 to year 2). Abtract (talk) 17:55, 28 July 2010 (UTC).
 * It appears from the sources that a distinction is drawn; Fabio Cardone, Roberto Mignani p. 222 say: “Gravitational radiation does admit retarded potential solutions of electromagnetic type. It therefore describes propagation of the perturbations of a static or near static gravitational potential field. ...On the contrary, how much is the propagation speed of the gravitational force? It answers the question of how much time a target body will take to respond to the acceleration of the source mass.” Brews ohare (talk) 18:24, 28 July 2010 (UTC)
 * For me, the changes by A di M fix the problem. Brews ohare (talk) 18:46, 28 July 2010 (UTC)

minimize the change in length of the metre
this change by MC Price has introduced the sentence:
 * "The CGPM chose this value to minimise any change in the length of the metre."

This last sentence is a bit obscure, as it leads to the natural query: Why didn't they make the change in the metre zero? That would be about as minimal a minimum as one could get.

The missed point is that metre was known only to be within a certain range of values (stated as 299,792,456.2±1.1 m/s), and the issue was to choose a value, ostensibly within this range. The chosen value 299,792,458 m/s actually appears to be outside this range (the upper limit is 299, 792, 457.3 m/s), leaving the reader wondering how a "minimal" change criterion could lead to a value outside the measurement error. Brews ohare (talk) 17:24, 28 July 2010 (UTC)
 * Mmmm, I wondered about that too. Abtract (talk) 17:32, 28 July 2010 (UTC)

The answer is given in a footnote. If you take the definition of the metre to apply to the weighted mid-point of the krypton line, NIST found a speed of light of 299,792,456.2(11) m/s; if instead you take the definition to be based on the maximum-intensity point of the line, the speed of light would be 299,792,458.7(11) m/s. The CGPM decided chose a value in between the two, but nearer to the maximum-intensity point. The quoted uncertainties are given as one estimated standard deviation, so you have to double them (at least) to get a 95% confidence range. Physchim62 (talk) 17:57, 28 July 2010 (UTC)
 * This explanation is a bit unsatisfactory inasmuch as the CGPM was at liberty to choose any definition, but chose this one. Why? Or, is it simply: Why not? In other words, there is no "minimal change" criterion, it is just six of one, half a dozen of the other. Brews ohare (talk) 18:12, 28 July 2010 (UTC)
 * Of course, given that there were an infinite number of idiotic redefinitions available to the CGPM, one wonders how they managed to chose an non-idiotic redefinition, does one? Physchim62 (talk) 02:05, 29 July 2010 (UTC)
 * The original choice of the 299,792,458 in 1975 is at http://www.bipm.org/en/CGPM/db/15/2/ and it seems to suggest that several such measurements were taken in account; does anyone have access to the references cited in it? (In any event, I have removed the "minimise any change" wording.) A. di M. (formerly Army1987) (talk) 19:36, 28 July 2010 (UTC)
 * I doubt those two references would shed much light on the matter in any case. Physchim62 (talk) 02:05, 29 July 2010 (UTC)
 * Physchim62: I wasn't suggesting that CGPM made a random choice between all possible options, but that they had a number of equally practical choices and so could just flip a coin. The removal of the wording about minimizing change fixes things anyway. Brews ohare (talk) 03:05, 29 July 2010 (UTC)
 * Indeed, I managed to get a PDF of the Metrologia article (good ol',   and  !) and it just has the text of the resolutions. Anyway, I'm trying to say in the article that several such measurements were available in 1975. A. di M. (formerly Army1987) (talk) 11:05, 29 July 2010 (UTC)

Excessive detail in "Increased accuracy and redefinition of the metre" section
IMHO, the "Increased accuracy and redefinition of the metre" section now has an excessive amount of detail on the realization of the metre. This article is about the speed of light, not the metre. The article as it is, is already very long for its limited scope. The "see also" link to metre is there for a reason.

The info relevant to this article is that the metre was redefined and that the with the new definition the value of c in SI (based) units is exact. Any more information about the mise en practique of the realisation of the metre, is fluff to this article, better discussed in the metre article.

Effectively, the part starting with "The difficulty with..." and ending with mise en practique can be omitted here. (With some additional rephrasing of the surrounding text to reflect this rephrasing.) At least that is my humble opinion. TimothyRias (talk) 09:46, 27 July 2010 (UTC)


 * I agree.Martin Hogbin (talk) 14:21, 27 July 2010 (UTC)
 * I agree that it could use some trimming, but perhaps we need to be careful. For example, the part that you suggest omitting contains the sentence "In 1975, the 15th Conférence Générale des Poids et Mesures (CGPM) recommended using $299,792.458$ for "the speed of propagation of electromagnetic waves in vacuum". ", which seems essential, unless it's somewhere else in the article that I didn't notice. Maybe the section title is misleading, since it might appear that the section  is mainly about the metre, rather than mainly about the increased accuracy of the speed of light, if I understand the purpose of the section correctly. --Bob K31416 (talk) 14:53, 27 July 2010 (UTC)


 * The 1975 decision by the 15th CGPM, simply was to establish what value should be accepted for the 1972 result in face of the ambiguity of the definition of the metre. The decision is again more a matter of the definition of the metre rather than an increase in the accuracy of the knowledge of the speed of light. This is also that the 1975 decision was not included in the table. But I do agree it is not simply a matter of deleting the suggested part the surrounding material material will need some modification as well. TimothyRias (talk) 16:38, 27 July 2010 (UTC)


 * I agree with Timothy and Martin.—Finell 06:39, 28 July 2010 (UTC)

This subsection is the only place in the article Speed of light that attempts to make clear the issues surrounding the introduction of a defined value for c. In view of the tremendous blather that has occurred on this Talk page for years, mainly in an attempt to install a quasi-religious belief in a magic number, it is evident that this topic requires explanation here. It should not be removed on the basis that it "belongs elsewhere"; it belongs here because it relates to the meaning of the SI version of c. It belongs here because it relates to the switch to 'time-of-flight' units which many editors on this page have yet to appreciate. It belongs here to clarify the distinction between a phenomenon, the measure of that phenomenon, and the measurement of that phenomenon. If this section is to be shortened, another entire article should be introduced on this subject and referred to here as the Main article on this subject. Brews ohare (talk) 13:40, 28 July 2010 (UTC)


 * Longer is not always clearer. By straying from the subject in to excessive detail (like the fact that the BIPM keeps a list of recommended sources for realising the metre) the main issues are obfuscated rather than clearified. As the section is written now the fact that the metre was redefined comes as an after thought, rather than as the main message of the relevant paragraph.TimothyRias (talk) 14:09, 28 July 2010 (UTC)


 * One can quarrel over details of the wording. For example, what is the purpose of mentioning a mise en pratique? Is it just useless baggage? Or, does it provide a useful contrast with the earlier method of keeping a hard-to-access specimen metre under standard conditions in Paris? Or, the specification of a more available single source, a krypton discharge lamp with various serous problems? Instead, the modern mise en pratique is very accessible, is continually updated and refined, provides numerous sources of standards, and provides the associated uncertainties (although the notion of uncertainties may be an anathema to some). This development of context has some interest, I'd say, and points out that although c has a defined value, its realization is only approximate. Brews ohare (talk) 14:32, 28 July 2010 (UTC)


 * I'd is not the realisation of c that is approximate, but the realization of the metre. As interesting as this subject is. It is about the metre and not the speed of light. TimothyRias (talk) 15:34, 28 July 2010 (UTC)


 * A review of the wording doesn't require drastic changes in this section. This subsection is the only place in the article Speed of light that attempts to make clear the issues surrounding the introduction of a defined value for c. Brews ohare (talk) 15:16, 28 July 2010 (UTC)
 * Timothy: As I have pointed out, the subject of the defined value for the speed of light is contingent upon the change of definition of the metre, as you well know. To say the topic is therefore about the metre and not about the speed of light is disingenuous, and suggests an agenda is being forced for reasons unstated. Brews ohare (talk) 16:10, 28 July 2010 (UTC)
 * Brews, my only agenda here is to make this article as clear as possible without unnecessary digressions. (btw, your suggestion that I'm trying to push some other agenda is: 1)extremely insulting 2)a violation of WP:AGF 3)a violation of your general probation, I suggest you try harder to keep this discussion civil.) Your saying that the mention of the the current mise en practique is a useful contrast against the practice for the realisation of the metre before that, but since the practice of the realisation of the metre before 1983 is also not discussed in this article (because it is about the metre not c), the sentecne about the mise en practique for reasilisation of the current definition is also out of place.
 * Again, my opinion here is that the only points truely relevant to this article are, that the metre was redefined and that the result of this redefinition was that the value of c in SI units is now a defined exact quantity. (Like the freezing point of water is a defined quantity on the Celsius scale (or at least was, I'm not sure how degrees centigrade are currently defined)).TimothyRias (talk) 17:10, 28 July 2010 (UTC)
 * Indeed. The maximum density of water at 1 atm used to be 1 kg/L exactly by definition of the litre, but an extensive discussion of that doesn't belong to the water article; the same applies to whichever other way to define a unit of measurement. How c would be different is beyond my grasp. A. di M. (formerly Army1987) (talk) 17:31, 28 July 2010 (UTC)

A di M: You question: “How c would be different is beyond my grasp.” Now, what is c in this observation? Is c the value 299, 792, 457.3 m/s? I'd guess you have little trouble imagining this number could be chosen to be any number whatsoever, although making it 1 m/s might cause some expense as all existing metre sticks went to the dump. On the other hand, is c the maximum speed of information transfer viewed as a limit, without particular regard to its expression in units? I can see where you'd have trouble imagining it to depend upon a choice of units.

What Timothy and yourself appear to me to overlook is that these two things are readily confused by a reader, and that a discussion of the switch to "time-of-flight" units in this subsection is a necessary attempt to straighten these matters out for the reader. Brews ohare (talk) 17:51, 28 July 2010 (UTC)
 * The same thing applies with any other way of defining anything. The temperature at which liquid water, water ice and water steam can coexist doesn't depend upon a choice of units, but it exactly equals 273.15 K by definition of the kelvin; but the water article spends no more than a paragraph to discuss this point, which is (rightly) more fully discussed at kelvin. In which way the speed of light differs from the triple point of water, and the metre differs from the kelvin? A. di M. (formerly Army1987) (talk) 18:20, 28 July 2010 (UTC)
 * BTW, I would rather remove the mention of mise en pratique, so that it just read "Based on this recommendation, the 17th CGPM in 1983 redefined the metre as ..."; more detail should go at Metre (which right now doesn't even make clear that there are several radiations used for that purpose). A. di M. (formerly Army1987) (talk) 18:20, 28 July 2010 (UTC)

Can you explain to me why the subject of how the "time-of-flight" definition of the metre has been implemented is irrelevant and a gross distraction here? It is plain that the implementation of this change underlies the use of the defined value for the speed of light, and so is pertinent to a reader's normal curiosity about how this change (that so fundamentally alters the system of units) is accomplished. It also links the reader to the mise en pratique which is a valuable source for further explanation. It also makes clear that the realization of the standard speed of light is subject to uncertainty by virtue of the uncertainty in the source radiations, and the notion that such uncertainty is present is a valuable antidote to the mystique associated with an "exact" speed of light. Can you respond to each of these points? I simply don't see that there is either (i) any pressure due to lack of space in the article, (ii) any distraction from the article as it is a subsection clearly devoted to this topic. Brews ohare (talk) 18:34, 28 July 2010 (UTC)
 * Any change in the definition of a unit does the same, but that is normally discussed in the article about the unit, not in that about the quantity used to measure it. The article water doesn't discuss arcane details of the redefinition of the kelvin which are discussed at Kelvin, the article Isotopes of caesium doesn't discuss arcane details of the redefinition of the second which are discussed at Second, so why should arcane details of the redefinition of the metre be discussed here rather than at Metre? A. di M. (formerly Army1987) (talk) 19:29, 28 July 2010 (UTC)


 * A di M: It is not an "arcane detail about the definition of the metre" that changes the speed of light from a measured quantity with an associated experimental error bar to an exact number beyond the reach of measurement. The effect of the definition of the metre  upon the value for the speed of light is profound, affects methodology, measurement practice, and reduces the standard units of time and distance to a standard for time alone. There is no parallel with the other units you mention. Brews ohare (talk) 02:23, 29 July 2010 (UTC)


 * Brews, that is simply not true for reasons repeated to you a thousand times. If you want to be nitpicky then you can say the decision changes the standard units of time and distance to standard units of time and velocity, which is equivalent. This is analagous to what happend with the introduction of the international prototype for the kilo, (change from fundamental density to a fundamental mass), or the type of change that will happen with possible future redefinitions of the Coulomb, Ampere and kilogram. Nor the the changes is methodology and measurement practice as profound as you claim. TimothyRias (talk) 08:24, 29 July 2010 (UTC)
 * Indeed, in what way does it change methodology and measurement practice at all? Before 1983, precise lengths were measured by interferometry; since 1983, precise lengths are measured by interferometry. Big change, huh? Physchim62 (talk) 08:36, 29 July 2010 (UTC)
 * I don't object to stating that "changes the speed of light from a measured quantity with an associated experimental error bar to an exact number beyond the reach of measurement"; I object to use several paragraphs to discuss that including obscure details about interferometry and metrology which will interest about 1% of the readers (who could always read them at interferometry and metre anyway). That'd be akin to expand the section Water by a factor of two by discussing how densities and temperatures are measured and what the mise in pratique of the kelvin and the kilogram are. A. di M. (formerly Army1987) (talk) 10:49, 29 July 2010 (UTC)

Well folks, I'd say what all this amounts to is that you want to compress this section. Not for reasons of clarity or usefulness of the article, but because you don't want this material here. As space is hardly at a premium, and as the topic is difficult, I don't see any advantage in curtailing the discussion other than to please you all. Why that would please you is unclear. Brews ohare (talk) 15:20, 29 July 2010 (UTC) The real issue with this section is that it is poorly organized and so appears to wander about. It is not its content that is the problem. Brews ohare (talk) 15:34, 29 July 2010 (UTC)
 * The reason is the article is very long, too long to be easily read by many readers, so it is indeed about clarity. See e.g. WP:LENGTH, a link to which appears whenever the article is edited, especially WP:SIZERULE.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 15:39, 29 July 2010 (UTC)

This argument is sounding a lot like the one that led to the arbitration. As I said above, rehashing this argument is not going to persuade those who hold the opposite view, is not contributing to the article, and is a waste of time and energy. Enough is enough.—Finell 16:28, 29 July 2010 (UTC)


 * Finell: You exhibit an unfortunate exasperation with the proceedings here. In fact, unlike the previous episode that you unnecessarily draw attention to, there has been a largely constructive revision of the article in this round of editing. I am confident that improvements will continue to be made. Please do not despair, and please do not resort to gloom and doom pronouncements suggesting that the dark ages are at hand. Brews ohare (talk) 16:44, 29 July 2010 (UTC)

Section title
Suggest changing the title of the section from
 * Increased accuracy and redefinition of the metre

to
 * Second half of the 20th century.

--Bob K31416 (talk) 15:21, 28 July 2010 (UTC)


 * Object, since the first is much more descriptive and to the point. TimothyRias (talk) 15:35, 28 July 2010 (UTC)
 * Thanks. BTW, does "Increased accuracy" refer to c, the metre, or both? --Bob K31416 (talk) 19:23, 28 July 2010 (UTC)
 * "increased accuracy is supposed to refer to c. (At least that was my intention when a chose the heading.) TimothyRias (talk) 22:46, 28 July 2010 (UTC)
 * Maybe add "of c"? --Bob K31416 (talk) 01:04, 29 July 2010 (UTC)

"To resolve the ambiguity in the definition of the metre"
In the section Increased accuracy and redefinition of the metre, suggest deleting the first part of the first sentence of the third paragraph:
 * To resolve the ambiguity in the definition of the metre, t The 17th CGPM in 1983 decided on a new definition of the metre: "the length of the path travelled by light in vacuum during a time interval of $299,792,458 m/s$ of a second".[120]


 * 120.

The reference doesn't appear to list that as a reason. --Bob K31416 (talk) 01:46, 29 July 2010 (UTC)
 * It's not an "ambiguity", its an "uncertainty in the realization" (first reason given in the resolution). Physchim62 (talk) 02:07, 29 July 2010 (UTC)
 * It's an "insufficiently precise" realization, a wording that doesn't suggest the problem is cured, just ameliorated. Brews ohare (talk) 03:09, 29 July 2010 (UTC)
 * Of course the "problem" isn't "cured"! You can never have an exact realization of a unit because, by definition, a realization relies on a real-world measurement. The new definition gives lower uncertainties than the old one, I thought we were agreed on all that. Physchim62 (talk) 03:51, 29 July 2010 (UTC)
 * What would you like to see done with the phrase in question? --Bob K31416 (talk) 14:39, 29 July 2010 (UTC)

BobK: Certainly the words struck out are worthy of deletion. However, their role is to suggest a motivation for the change, and striking them out leaves no motivation. So instead, one could place:
 * “To obtain a more precise realization of the metre, the 17th CGPM in 1983 decided on a new definition: "the length of the path travelled by light in vacuum during a time interval of $299,792,456.2 m/s$ of a second".[120]”

Of course, that raises the immediate question: Why did that result in a more precise realization? The answer isn't addressed at this point in the subsection. A rearrangement of the text in this subsection to deal with this question directly would be desirable. At the moment this subsection is disjoint and its object (the answer to this question) is not made its focal point. Brews ohare (talk) 15:32, 29 July 2010 (UTC) The new wording simply avoids this question and its answer. Brews ohare (talk) 15:42, 29 July 2010 (UTC)

Update: The beginning of the 3rd paragraph has been changed by another editor to: so that the new version of the subject sentence in the 3rd paragraph seems OK now. --Bob K31416 (talk) 16:20, 29 July 2010 (UTC)
 * "In 1975, considering that similar measurements of c agreed with each other and their uncertainty was comparable to that in the realization of the metre, the 15th Conférence Générale des Poids et Mesures (CGPM) recommended using $299,792,458 m/s$ for "the speed of propagation of electromagnetic waves in vacuum". In 1983, the 17th CGPM decided on a new definition of the metre: "the length of the path travelled by light in vacuum during a time interval of $299,792,456.2 m/s$ of a second". "

BobK: Please see my suggestions in the following subsection of this page. Brews ohare (talk) 16:46, 29 July 2010 (UTC)


 * Thanks. I looked and commented, albeit on another editor's change, which I thought addressed your main point. Things are moving kind of fast and furious here! I'll try to keep up. : ) --Bob K31416 (talk) 17:13, 29 July 2010 (UTC)

Increased accuracy and redefinition of the metre
The subsection Speed of light now has a discussion of increased accuracy in measuring the speed of light, which is related to the title of the subsection on "increased accuracy". It also has a paragraph on the redefinition of the metre. What is does not have is any attempt to connect these two matters. That is, it does not answer the focal question: Why did this change result in an “increased precision in the realization of the metre”, which was the espoused purpose of the change in definition as stated by the CGPM.

In an unseemly haste to rid this section of anything about the metre as being somehow too far afield in an article on the speed of light, this subsection whose entire motivation is to connect these two explains nothing, and has become a simple laundry list. Brews ohare (talk) 15:51, 29 July 2010 (UTC)

The explanation of the increased precision in realization has two parts:
 * First, the replacement of the krypton discharge lamp as a source. The subsection does detail some of the failings of this source related to line shape. In addition it lacked sufficient coherence requiring a two-step measurement. This source was replaced not with a single better laser source but with a methodology that allowed a variety of sources, anticipating that the "best" source would evolve over time and would differ depending upon the measurement length needed for a particular task. Clearly, these steps improved the precision of realization. They did not, however, require a change in the definition of the metre.


 * Second: The change in definition of the metre to a "time-of-flight" definition. The purpose behind this change was that the introduction of many sources required a methodology for comparison. The wavelength of the sources could be compared, but due to errors like the failure to obtain a plane wave and errors of compensation in balancing the two paths in an interferometer, using interferometry to measure wavelengths was error prone. On the other hand the measurement of frequencies had become very good. As all believed that frequency and wavelength were related by the speed of propagation, it was evident that a comparison of the frequencies of sources was tantamount to a comparison of wavelengths, but more accurate. The only issue was to insure that the speed of propagation was the same in all such comparisons, which led to the adoption of the speed of light in vacuum as the standard of speed of propagation. Here the improvement in realization of the metre was related to flexibility in choice of source to match the measurement requirements at hand.

The remaining uncertainties in the realization of the metre were now (i) the uncertainties in the radiations, now tabulated and kept current in the mis en pratique; (ii) the uncertainties in the realization of "vacuum" (or, equivalently, uncertainties in the corrections implemented to account for the medium employed) and (iii) the uncertainty in interferometry in establishing a wavelength. This last is no better than use of a better source and the old definition of the metre as a number of wavelengths of a specific transition. However, the flexibility in choice of source and the ability to let the standard evolve with improved precision of the sources are the determining factors in making the change in definition.

Something along these lines is what is needed in this section. Brews ohare (talk) 16:35, 29 July 2010 (UTC)
 * I've added " in order to allow a more precise realization of it" to the sentence about the 1983 redefinition. A. di M. (formerly Army1987) (talk) 16:47, 29 July 2010 (UTC)
 * That's OK, but how about tweaking to this version, in order to explicitly mention the motivation of CGPM, which is what was stated in the reference?
 * In 1983, because the definition of the metre did not allow a sufficiently precise realization of it for all requirements, the 17th CGPM decided on a new definition: "the length of the path travelled by light in vacuum during a time interval of $173.145$ of a second".
 * --Bob K31416 (talk) 17:01, 29 July 2010 (UTC)
 * Yes; I'm going to do that. A. di M. (formerly Army1987) (talk) 17:16, 29 July 2010 (UTC)

I have implemented some of the above suggestions of mine in the subsection. I hope that you all might agree with these changes, or some minor modifications of them. Brews ohare (talk) 18:29, 29 July 2010 (UTC)


 * Your changes seem to me completely unnecessary, as they say nothing about the speed of light, only about the metre, probably in far more detail than's appropriate even for that article, and are largely unsourced. As I noted above the article is already very long, so any extra content should only be added if closely related to the topic.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 19:33, 29 July 2010 (UTC)
 * I agree. Perhaps a separate article on the 1983 redefinition of the metre is the answer.  Martin Hogbin (talk) 20:21, 29 July 2010 (UTC)
 * I agree, the detail brews is trying to add here should be added on the metre page. Here the discussion of this matter should be brief at the very most a couple of paragraphs in total, and refer to the metre page for further details on how the change result in a more precise realization of the metre, because that question is completely off-topic in this article. The only thing relevant here is the consequence of the change for the speed of light. To martin:maybe, but I'd vote for first trying to add some discussion of this to the metre page.TimothyRias (talk) 20:25, 29 July 2010 (UTC)


 * John, Martin and Timothy: Do we agree on the historical facts? I'd say they include these items: (i) There was a change in the definition of the metre (ii) This change resulted in the switch to a defined value for the speed of light. Then, given the foregoing, the change in the definition of the metre is pertinent to this article on the speed of light. One could merely note that the change occurred and was pertinent. However, that approach clearly leaves dangling the question of why that change was made. There is no doubt that this switch appears perplexing upon first exposure. (For example,the New Scientist stated “Despite powerful scientific arguments, the idea of expressing the metre as a fraction of a second may still seem a bit bizarre. ...do not expect to get away with complaining to your local timber merchant that he has been supplying you with wood that is 10-9 seconds too short.”   This reaction is why the discussion ought cover these details. Brews ohare (talk) 20:32, 29 July 2010 (UTC)
 * The facts were already in the article in more than enough detail, in that section and elsewhere in the article. In particular it was clear why the decision was made. Your changes have confused the matter by introducing irrelevant detail, almost entirely unsourced, that adds nothing to our understanding of the speed of light.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 20:49, 29 July 2010 (UTC)
 * My latest revision (copied and pasted from Bob K's proposal above) did address "the question of why that change was made". Please re-read it more carefully. What the coherence length of the krypton gas discharge is and similar stuff are way beyond the scope of this article. (Also, the source you cited about the coherence length does not mention at all the definitions of the metre and their realization and the Resolution 1 of the 17th meeting of the CGPM doesn't mention coherence lengths at all, so all that smells like WP:OR to me. A more detailed discussion of the redefinition of the metre should go at Metre, but even there please stick to what the CGPM actually said.) A. di M. (formerly Army1987) (talk) 21:57, 29 July 2010 (UTC)

I'd add to this that links to the decisions of the CGPM are pretty much mandatory, and the link to the 2002 mis en pratique provides a good summary. These documents cry out for some additional explanation such as that given here. The espoused purpose of the change in definition as stated by the CGPM is to improve the precision of realization of the metre, and that seems to make the metre a central issue in understanding what happened to the speed of light. Brews ohare (talk) 20:47, 29 July 2010 (UTC)


 * John, perhaps you could point out to me what specifically is said in the article that "made clear why the decision was made". If that assessment is accurate, I'd reconsider. Brews ohare (talk) 20:55, 29 July 2010 (UTC)
 * That section before your changes was clear to me. In addition the third paragraph of the lead sets out the facts of the definition, while the section immediately after it goes into more detail of the constancy of lightspeed.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 21:03, 29 July 2010 (UTC)
 * The section before changes, as I explained above, was a laundry list making no connections between the items listed. The facts of the definition are not at issue here. The constancy of light also is not at issue. None of these items at all supports the claim that the reasons for the decision have been made clear elsewhere in the article. Brews ohare (talk) 21:49, 29 July 2010 (UTC)

A di M: I doubt that you really disbelieve the coherence length difficulties, clearly implicated by the definition of the coherence length. I'll dig up sources from interferometry texts if you like. My intention however, was simply to indicate that the linewidth problem was only the tip of the iceberg. There is no need that the CGPM specifically state a coherence length issue. Brews ohare (talk) 22:44, 29 July 2010 (UTC)
 * The linewidth problem (strictly, the asymmetry of the line) was not "the tip of the iceberg", it was a fundamental problem with the pre-1983 definition of the metre. All the rest is secondary. Physchim62 (talk) 22:48, 29 July 2010 (UTC)

A di M: Not to be argumentative, but to describe my problems with your revision, here is my synopsis of it. The first two paragraphs describe difficulties in obtaining an accurate metre. That's fine, but the implication is only that some new method might be better. The last paragraph then makes a jump to the definition of the metre and the resulting defined value of the speed of light. It states that the motivation for the switch is that the old definition was insufficiently precise, but there is no clue here as to why the introduction of the new definition has any effect upon this problem. It is left to the reader to connect the dots in this non-sequitor. That is my difficulty with this version. Brews ohare (talk) 22:58, 29 July 2010 (UTC)
 * "Because the definition of the metre did not allow a sufficiently precise realization of it for all requirements, the 17th CGPM decided on a new definition" makes it quite obvious that the new definition allows for a more precise realization. If I told you that I replaced the strings I had on my guitar because they were too thick for my tastes, wouldn't you be able to understand that the new ones are thinner than the old ones, even though you might never have played a guitar? A. di M. (formerly Army1987) (talk) 23:18, 29 July 2010 (UTC)

BTW, the logical fix following the description of the first two paragraphs is simply to switch to a different transition that has a narrower and more symmetric line, or simply to state specifically what position on the Kr line to use in the measurement, instead of leaving it uncertain. These solutions were indeed considered, and yet the change in definition was made regardless of these more obvious solutions. Why? That has to be answered. Brews ohare (talk) 23:03, 29 July 2010 (UTC)
 * That's what they did. Before 1983 the metre was 1,650,763.73 krypton-86 transition wavelengths, after 1983 it is $299,792,458$ caesium-133 transition wavelengths. A. di M. (formerly Army1987) (talk) 23:18, 29 July 2010 (UTC)
 * Brews, we have repeatedly told you "why", both last summer and this summer, yet you refuse to listen: "This way of defining the meter has proven to be particularly robust, since unlike a definition based on a standard such as the krypton lamp, length measurement can be continuously improved without resorting to a new definition." Physchim62 (talk) 23:41, 29 July 2010 (UTC)


 * A di M: Is your reply above to be interpreted as saying the responsibility of the article is to say what CGPM did, and not to provide any of the reasons behind it? That's what they did, period? Brews ohare (talk) 00:32, 30 July 2010 (UTC)
 * Are you dumb or what? Again, I don't oppose to stating in the article that the definition was changed in order to allow a more precise definition of the metre. What I oppose is giving too much detail about that, when 1) this article is already 100 KB long, 2) such detail would be more on-topic in the Metre article which right now is 26 KB long, and 3) some of the details were not even mentioned by the Resolution 1 of the 17th meeting of the CGPM. (It might well be obvious to you that the short coherence length was one of the problems with the old definition, but if it's not obvious to "any educated person without specialist knowledge" you need a source explicitly stating that. BTW, it's not obvious to me either: if that one was the issue, could you just use the krypton lamp to measure a length much smaller than the coherence length e.g. 2 cm and then measure the ratio of that length and a longer length by whatever means is the most precise? ) A. di M. (formerly Army1987) (talk) 10:09, 30 July 2010 (UTC)
 * Sorry, A di M, I didn't get your point, which was that the Cs line was better. However, they did not institute the Cs line as a new replacement source for the Kr lamp. They went far further and defined the speed of light, opening the door to the multiplicity of radiations listed in the mise en pratique. Don't you agree? See the wording summarized in the mise en pratique Brews ohare (talk) 03:15, 30 July 2010 (UTC)
 * That's just a wording issue; see the seventh bullet of the resolution which says "these various forms [...] have been recognized as being equivalent". A. di M. (formerly Army1987) (talk) 10:09, 30 July 2010 (UTC)


 * Physchim62: Your quote above simply documents exactly what is said the addition I made to the subsection: “However, the flexibility in choice of source and the ability to let the standard evolve with improved precision of the sources are the determining factors in making the change in definition.” Brews ohare (talk) 00:58, 30 July 2010 (UTC)


 * Brews, although the question why this particular definition was chosen has indeed to be answered but not in this article. This should be discussed in some detail in the metre article. Here it is enough to state that it was changed and what the consequences of this were for the value of the speed of light. A reader wondering why this particular choice was made is bound to have enough wits to click on the wikilink to the metre article linked at the top of the section. Since that seems to have the preference of all but one of the editors here, I suggest we revert to a more concise version and move some of the detail to the metre article. TimothyRias (talk) 09:28, 30 July 2010 (UTC)

I'm not sure what's being discussed here. Is it this series of edits ? If so, I don't see why there is all this discussion. Just go by the reliable source for the sentence which doesn't support the additions in these diffs regarding Krypton etc. For that reason, I have again reverted it. Please do not add similar material that is not supported by the source for the sentence without first getting consensus here. --Bob K31416 (talk) 01:19, 30 July 2010 (UTC)


 * BobK: I really don't care if this sentence makes sense, so I'll leave it alone. However, it is perfectly clear that the 1960 definition is based upon the Kr transition, and to revert this phrase as unsourced is ludicrous. Brews ohare (talk) 02:31, 30 July 2010 (UTC)
 * As for the earlier version referring to a Kr discharge lamp, look at This way of defining the meter has proven to be particularly robust, since unlike a definition based on a standard such as the krypton lamp, length measurement can be continuously improved without resorting to a new definition. Here's another one. Here's another one. BobK, you have to become more familiar with the subject, and not go by the Talk page debates. Brews ohare (talk) 02:55, 30 July 2010 (UTC)
 * I've just restored the version prior to your lengthy additions.There's little point wasting our time debating whether individual sentences were factually correct or properly sourced; as a lengthy discussion of the metre they were simply irrelevant to this article, which already covers the relationship of the metre definition and speed of light very well. The article is already too long and does not need content adding which is not on the topic, the speed of light.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 09:39, 30 July 2010 (UTC)

Restoration of the original form of this subsection without regard to anything that has transpired here is just arrogance. Once again, this form of the subsection fails to connect the topic of "increased accuracy" with the topic of "redefinition". This presentation is just a juxtaposition of disjoint topics. Brews ohare (talk) 14:46, 30 July 2010 (UTC)

Separate article
This is an interesting subject, maybe more interesting to some than others, but it is holding up progress towards FAR for this article. Like the One-way speed of light this subject contains an interesting mix of the practical and philosophical that could be debated endlessly. Nevertheless, it is an important part of physics that should be explained in WP somewhere. I propose moving all the detail to a new article where those interested can discuss it.

By the way, I suggest that there should be a link to the One-way speed of light from this article. Martin Hogbin (talk) 09:49, 30 July 2010 (UTC)


 * Don't give a fig about FAR, but I agree this subject needs no more than a line or two here. Stuff it in another article "Exact metric speed of light", link to it and be done.--Michael C. Price talk 09:57, 30 July 2010 (UTC)


 * I have started Redefinition_of_the_Metre_in_1983 by copying the section here to a new page. The refs need copying too. Martin Hogbin (talk) 10:34, 30 July 2010 (UTC)
 * I would try to expand the Metre article first (currently it's just 26 KB), and then to split it off should it become too large. A. di M. (formerly Army1987) (talk) 11:13, 30 July 2010 (UTC)
 * That is an idea, but let us move the detail from here. Martin Hogbin (talk) 11:50, 30 July 2010 (UTC)
 * Indeed. Redefinition_of_the_Metre_in_1983 riskes becoming a PoV fork for those who think it's important. Physchim62 (talk) 11:53, 30 July 2010 (UTC)
 * Agree that would be the usual "best practice".TimothyRias (talk) 12:07, 30 July 2010 (UTC)


 * But the "exactness of the SoL" does lie in the intersection of the SoL and the metre. That justifies its own article. --Michael C. Price talk 12:20, 30 July 2010 (UTC)
 * "That justifies its own article."??? Not if we don't have anything to say about it! The big problems here is that some editors are giving the redefinition of the metre far more attention than it merits. What is wrong with just merging things into Metre? Physchim62 (talk) 12:31, 30 July 2010 (UTC)
 * So, do the "exactness of the International Prototype Kilogram", the "exactness of the period of the ground state hyperfine transition of caesium-133", the "exactness of the magnetic permeability of free space", the "exactness of the triple point temperature of water", the "exactness of the luminous efficacy of 540 THz light" and the "exactness of the molar mass of carbon-12" all justify their own articles because they do lie in the intersection of their respective constant and the relevant units? A. di M. (formerly Army1987) (talk) 12:38, 30 July 2010 (UTC)
 * The big problems here is that some editors are giving the redefinition of the metre far more attention than it merits. That isn't a problem if the subject has its own article. And the constancy of the SoL is more interesting than the other examples listed, so your analogies break down. --Michael C. Price talk 20:01, 30 July 2010 (UTC)
 * I might agree about the definition of the kilogram (after all, that's just some old piece of metal) or the candela (it's a semi-arbitrary function intended to model the frequency response of the human eye); but as for (e.g.) the ampere, I fail to see how &mu;0 is any more or any less interesting than c: after all, of the constants &mu;0, &epsilon;0, Z0 and c any two can be expressed in terms of the other two, so I'd say none is more fundamental than another. A. di M. (formerly Army1987) (talk) 00:08, 31 July 2010 (UTC)
 * From a physicist's POV you are correct, but the general public are more likely to find the constancy of the speed of light interesting than the constancy of &mu;0. --Michael C. Price talk 06:18, 31 July 2010 (UTC)
 * However, the constancy of the speed of light has nothing to do with our choice of units, nor should we suggest that it has. The speed of light is not constant because it has a defined value in SI units, that's getting the argument the wrong way round! Physchim62 (talk) 06:40, 31 July 2010 (UTC)
 * An excellent point which must be emphasized. --Michael C. Price talk 07:10, 31 July 2010 (UTC)
 * And is emphasized in the article. The constancy of the speed of light is discussed in completely different section than the exactness of the value of c in SI units. (And to be nitpicky, just for shits and giggles: Strictly speaking because the metre defined in terms of an exact speed of light is a unit of proper length, anything that previously would be interpreted as a change in c now has to be interpreted as a change in the spacetime metric. In this sense the modern definition of the metre may actually be interpreted as defining c to be constant over spacetime. This, of course, is over interpretation of the definition since no such effect was intended and any evidence that c has changed over time (or space) is bound to lead to renewed evaluation of the current definition which assumes c is constant.) TimothyRias (talk) 09:40, 31 July 2010 (UTC)
 * AFAIK, general relativity says you can always define things in such a way that the local speed of light is c, so do you mean "any evidence that GR is broken"? BTW, the CIPM in 2002 explicitly stated that "in the context of general relativity, the metre is considered a unit of proper length. Its definition, therefore, applies only within a spatial extent sufficiently small that the effects of the non-uniformity of the gravitational field can be ignored (note that, at the surface of the Earth, this effect in the vertical direction is about 1 part in 1016 per metre). In this case, the effects to be taken into account are those of special relativity only." A. di M. (talk) 10:31, 31 July 2010 (UTC)

(outdent) I meant, it must be emphasized in the separate article - which it now is. --Michael C. Price talk 09:52, 31 July 2010 (UTC)

The move of a paragraph or two to a new venue without attempting to make a stand-alone article out of it is just stupid. To testify to that, the article is tagged with AfD before anything has been done about it, indicating that this move is no more than a ploy to avoid rewriting the section on this subject in Speed of light. Tsk tsk. Brews ohare (talk) 14:46, 30 July 2010 (UTC)
 * The consensus is that the section does not need rewriting in such a way. It certainly did not need the lengthy off-topic and largely unsourced additions that were removed earlier. But yes, just because the content does not belong here, that does not mean a new article is needed for it - hence the PROD and AfD.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 15:08, 30 July 2010 (UTC)
 * The Speed of light subsection is now a juxtaposition of topics, not an explanation of their interconnection. A new article would be great, but it should be more detailed and broader in scope. Brews ohare (talk) 15:24, 30 July 2010 (UTC)
 * No it is a simple statement of fact (with sources), which is very appropriate for an encyclopidia. TimothyRias (talk) 15:32, 30 July 2010 (UTC)
 * Timothy: The first two paragraphs describe difficulties in obtaining an accurate metre. That's fine, but the implication is only that some new method might be better. The last paragraph then makes a jump to the definition of the metre and the resulting defined value of the speed of light. It states that the motivation for the switch is that the old definition was insufficiently precise, but there is no clue here as to why the introduction of the new definition has any effect upon this problem. It is left to the reader to connect the dots in this non-sequitor. That is my difficulty with this version. Brews ohare (talk) 15:49, 30 July 2010 (UTC)
 * (ec)Brews, the first two paragraphs are not describing difficulties in obtaining an accurate metre. The first paragraphs relates improvements in the accuracy of measurements of the speed of light in the second half of the 20th century (the increased accuracy part of this subsection), up to the point that no further improvements could be made do to limitations in the way the metre was defined. The second paragraph then explains what the definition of the metre was at this time and what the trouble was with it. (This may already be digressing further from the subject of the speed of light than necessary.) The last section then relates that the metre was redefined to the modern definition and what the effect of this on the value of the speed of light was. This is a simple chronological statement of facts as is appropriate in the history section of an encyclopedic article. The fact that you read the first two paragraphs as being about the metre rather than the speed of light, explains a lot about our difference of opinion and may indicate that something in these paragraphs needs to be clarified. TimothyRias (talk) 16:15, 30 July 2010 (UTC)
 * Do you really think that an interested reader can read the current version of the article and be unable to figure out that the 1983 definition allows a better realization of the metre than the 1960 definition does? A. di M. (formerly Army1987) (talk) 16:04, 30 July 2010 (UTC)
 * Now I've edited it to state "in order to allow a more precise realization of the metre". This is as clear as it gets; a reader who can't get the point now wouldn't be helped by obscure details of the interferometric techniques at all. A. di M. (formerly Army1987) (talk) 16:14, 30 July 2010 (UTC)

A di M: The addition of words like "in order to allow" serve to join sentences together, but they don't explain how the goal is accomplished. To suggest that the reader is well served by being forced to digest the entire article in order to understand the connections between a few paragraphs is just laziness in editing. Add the bridge. Brews ohare (talk) 16:27, 30 July 2010 (UTC)
 * Brews, editors are not stupid or lazy just because they disagree with changes you would like to make. The consensus is clearly against you on this, I suggest you stop trying to argue it now, especially given the tone of your recent comments.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 16:45, 30 July 2010 (UTC)
 * John, no comments about editors were made. It was suggested that the action of transferring a few paragraphs into a stand-alone article without taking the time to reshape them was stupid. It was said that avoidance of a few words of explanation connecting paragraphs was lazy editing. Perhaps if you wish to adopt full and sole responsibility for those actions or inactions, you can then take these comments personally. However, that was not my intention. Brews ohare (talk) 17:11, 30 July 2010 (UTC)

Please see my comments on this subject in the 'articles for deletion' page and the 'redefinition' talk page. Martin Hogbin (talk) 10:20, 31 July 2010 (UTC)

What else can be included before FAR
Scharnhorst effect. What other not widely known things or technical issues could we mention in the speed of light article before going to FAR. If not in the main article then as footnotes or see alsos. 89.240.130.248 (talk) 14:31, 1 August 2010 (UTC)
 * The Scharnhorst effect is  already mentioned in this note. --Bob K31416 (talk) 14:43, 1 August 2010 (UTC)

What other not widely known things or technical issues could we mention in the speed of light article before going to FAR. If not in the main article then as footnotes or see alsos. 89.240.130.248 (talk) 14:31, 1 August 2010 (UTC) —Preceding unsigned comment added by 89.242.128.249 (talk)

Notation
It seems that the international standards organization symbol c0 for the numerical value is not mentioned in the article. Brews ohare (talk) 01:09, 2 August 2010 (UTC) Sorry, I found it. Brews ohare (talk) 01:11, 2 August 2010 (UTC)

My objections to this article
This is best explained by writing up what I think the article fails to mention. I just wrote that up here (not 100% done yet, though, but you can see what my point is). Comments welcome on the talk page of that page, I don't want to derail the FAR of this article. Count Iblis (talk) 03:55, 1 August 2010 (UTC)
 * The main omission seems to be that the classical/Galilean limit may be obtained by expanding in 1/c. I agree that this should be mentioned somewhere, although briefly (not more than a paragraph) linking to another article with a more detailed explanation. (Classical limit would be a good candidate to add such an explanation. Any suggestion for such an introduction?TimothyRias (talk) 10:12, 1 August 2010 (UTC)
 * The material mentioned above seems to be Count Iblis's thoughts on the subject based on SR. There is nothing of that nature missing from here. Martin Hogbin (talk) 10:48, 1 August 2010 (UTC)
 * The role of c in QM/GR and the classical limits seems to be worthy of mention here IMO. As far as the FA goes, it'll fail on grounds of article stability, so don't worry about getting the star. Headbomb {talk / contribs / physics / books} 14:00, 1 August 2010 (UTC)
 * Yes, the classical limit is an important aspect. I've just expanded my argument and added a section. Note that I derive everything starting from natural units, which is different from how most textbooks treat this. Then, it should be clear that c really is a scaling constant. I also explain how this leads to incompatible physical quantities in the scaling limit and thus why dimensionful quantitities are invented by classical physicists. So, this is sort of a first principles derivation of c. Count Iblis (talk) 00:15, 4 August 2010 (UTC)