# Talk:Speed of light/Archive 15

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## In 1983, in order to allow a more precise realization of the metre

In this section, this thought is inadequately followed through. Why did the definition accomplish this goal? To make it clear just why, the point must be made that a "time-of-flight" definition makes the metre have the same error as the measurement of time (or frequency). I believe that is mentioned somewhere, but it is needed here to connect the dots. Brews ohare (talk) 05:30, 1 August 2010 (UTC)

Re "the point must be made that a "time-of-flight" definition makes the metre have the same error as the measurement of time (or frequency)." - Seems like a digression. I looked at the Metre article and I didn't notice this point there. If this point appears in a reliable source, perhaps you may want to try to add it to the Metre article instead. Just a suggestion. --Bob K31416 (talk) 06:20, 1 August 2010 (UTC)
1. No further explanation is required. Wikipedia is not a textbook, but an encyclopedia and can suffice with sourced statements of fact, especially when the explanation would be a digression away from the main subject of the article. For this article it is simply not needed for the reader to understand why the change of definition allowed a more accurate realization, nor will a reader be interested in this, and if the reader is wondering about this he/she will look up the metre article.
2. The new definition does not automatically give the realization of the metre the same error as the realization of the second.TimothyRias (talk) 09:49, 1 August 2010 (UTC)
Now we have an article on the specific topic, I suggest that much of excessive detail can be removed. We want one short paragraph at the most, more or less the last paragraph as it is now. Martin Hogbin (talk) 10:44, 1 August 2010 (UTC)
Timothy: I've exaggerated the situation. The time-of-flight definition for length would have a lower bound of accuracy that is the same as that for the time interval involved. The interferometric method would have a larger error due to the added errors of interferometry.
The point you all choose to ignore, however, is that WP claims that the change in definition enabled higher accuracy in the realization of the metre. In the case of a time-of-flight measurement, that is possibly true; I don't know much about errors in time-of-flight measurements, and so far as I can tell they are seldom used in a laboratory. However in the normal lab implementation of part (b) using an interferometer, it was not the change in definition that allowed greater precision, but the change away from the krypton discharge lamp to better laser sources. From the perspective of a wider variety of applications, like surveying, the definition enabled better selection of a source for the purpose, which in a broad sense enabled greater precision of realization in practical pursuits. The requirements in these situations are less precise than a scientific context, of course.
So the bottom line is that the statement in WP is misleading, and largely wrong. The accuracy improvements are due to better sources, not due to the choice of definition, which has mainly the advantage of allowing accurate comparison between sources. That capacity enables evolution of the standard with evolution in sources. Brews ohare (talk) 00:32, 2 August 2010 (UTC)
The whole point of the new definition is that it is no longer linked to any particular source, but allows a realization of the metre with any radiation source of which the frequency is accurately known. So, yes the new definition did enable higher accuracy in the realization of the metre. A claim that the article doesn't even make, even the old text only claimed that the intent of the new definition was to allow for a more precise realization of the metre. TimothyRias (talk) 09:11, 2 August 2010 (UTC)

TImothy: The definition of the metre made it measurable in seconds. That change was completely unnecessary so far as improving the accuracy was concerned. The old definition would work just as well as the new one if a source better than the krypton lamp were selected and the old definition changed to refer to wavelengths of the new source. So accuracy and the new definition are unrelated. As you point out, what the new definition did was to allow selection of a variety of sources, which meant as a practical matter that the CGPM didn't have to introduce a new definition every time a better source came along. I suspect we both agree about this; the issue is just a bit better precision in expressing matters. Brews ohare (talk) 13:53, 2 August 2010 (UTC)

Well, the new definition did allow a better realisation didn't it? A discussion why this definition was chosen over other definitions that would have allowed a better realisation of the metre, seems out of place here since that requires much more detail. Such an explanation is much better suited for the new Redefinition of the metre in 1983 article or the metre article. At the present, the article simply gives the reason they chose a new definition at all, without going into the reasons why they chose that particular definition. This seems fine to me.
(As to your first sentence: the 1960 definition made the metre measurable in seconds times a ratio of energy levels of krypton and caesium so I don't see there be that much change except the elimination of the dependence on an unnecessary quantity.)TimothyRias (talk) 14:22, 2 August 2010 (UTC)

Timothy: Well, if you wish to avoid discussion of why the definition was adopted, fine. But if you wish to enter upon why it was adopted, it is best not to present a distorted version. The conversion of lengths to seconds is by far the most striking aspect of the new definition and the one that attracts the most curiosity. So to say by implication that the purpose of the new definition was greater accuracy is tantamount to saying that the switch to lengths as times led to greater accuracy. That interpretation would be incorrect. Because that interpretation is the likely outcome of the present WP wording, that wording should be changed. The full list of eight reasons for the switch is found here. Brews ohare (talk) 14:45, 2 August 2010 (UTC)

Seems like further details beyond what is already in the article, regarding why the definition of the metre was adopted, would be too much of a digression from the topic of the article Speed of light, IMO. --Bob K31416 (talk) 15:20, 2 August 2010 (UTC)
The wording is now: “In 1983, considering that the definition of the metre did not allow a sufficiently precise realization of it for all requirements,” a wording probably meaningless to the reader (although employed in the resolution), who cannot presume to understand what "all requirements" includes. So why not just link again to the CGPM Resolution that itemizes the eight reasons and let it drop: “In 1983, for various reasons detailed in Resolution 1,” eh? Brews ohare (talk) 17:53, 2 August 2010 (UTC)
If more detail is desirable, this sentence could be added to, for example, to point out the desire to avoid annual redefinitions of the metre based upon ever newer & better sources. Brews ohare (talk) 18:04, 2 August 2010 (UTC)
Compared to the first item that you were suggesting, I think it's more informative while still concise, and the source is still available in the present version for the reader to look up further details. The second item that you are suggesting to be added would be a needless digression IMO. --Bob K31416 (talk) 21:55, 2 August 2010 (UTC)

## Constancy of the speed of light and units

On the Talk page above, it is said:

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)

That statement is accurate. However, the ease and accessibility to realization of the unit of speed is certainly an important factor. That is why the speed of light in vacuum was chosen instead of (say) the speed of sound in water under standard conditions.

This point is not made in the article. Probably it belongs in the redefinition section. Brews ohare (talk) 18:15, 2 August 2010 (UTC) I made an attempt at doing this. Brews ohare (talk) 19:12, 2 August 2010 (UTC)

You have got it the wrong way round Brews, as Physchim62 says. Everything we know about the universe tells us that the speed of light (in appropriate conditions) is a fundamental constant. It is therefore a good idea to incorporate it into our standards system. Martin Hogbin (talk) 19:21, 2 August 2010 (UTC)

Martin: I have agreed entirely with Physchim62, and made a quite separate point that agrees entirely with yours. Brews ohare (talk) 19:37, 2 August 2010 (UTC)

Sorry Brews, I am still puzzled over the exact point that you are trying to make in this article. Martin Hogbin (talk) 10:42, 3 August 2010 (UTC)

Let me explain: suppose we set up a standard speed of sound in water by specifying as carefully as need be the circumstances of the water, the exact pitch of the sound, its amplitude and a myriad of complex details that must be established by an experimenter to insure that the standard speed is realized. Call this speed s0. Then the metre could be defined as the distance traveled by standard sound in a ms (say). The speed of light in vacuum would be measured to be some multiple of s0. Everything would work the same way it does now in the SI units. For instance, s0 would be a defined constant, s0 = 1000 m/s, outside measurement. Improvements in measuring sound or refinements in defining the standard water medium would not change s0, but would refine the metre.

What are the defects of this approach? Mainly they are the complexity of realizing the standard speed, not the least of which is the preparation of the standard water medium and insuring that it is in fact a good representation of the standard medium.

What are the advantages of choosing instead the speed of light? Viewed entirely from an empirical observational viewpoint, and not from the viewpoint of relativity (which only explains these observations), the advantages of choosing the speed of light are that it is readily realized because it is independent of almost every parameter one might think of: orientation, speed of the emitting source, frequency of the radiation, intensity of the radiation, polarization of the radiation, blah, blah, blah. The hooker remains that the standard medium, vacuum, still has to be obtained, but the preparation of vacuum is thought to be understood, especially if you steer clear of things like quantum field effects by sweeping them under the rug (or, if you like, by virtue of their being so very small that for measurement purposes they are way down the list of errors). Gravity is a bit of a problem.

So what this means for the article is to point out that there is a general choice available in choosing base units, namely to choose a standard of speed to replace the standard of length. As a standard of speed, the above properties of light recommend it above any other such standard one might propose. Those advantages are not based upon theory (although supported by theory, of course, as that is the function of theory) but based upon empirical observations. Brews ohare (talk) 13:11, 3 August 2010 (UTC)

When you say that the advantages of light are not based on theory you are wrong. It would be better to say that the advantages of using light are based on theory, which is itself based on empirical observations.
I think by theory you really mean theories that you do not like the look of. Pretty well every experiment and observation in physics has to be based on theory. Martin Hogbin (talk) 13:28, 3 August 2010 (UTC)
Martin: Spoken as a true theoretician. Of course, the theoretician believes experimentalists are a bunch of unimaginative lackeys who must be told where to look or they wouldn't ever observe the important things. However, the experimenters may object to this characterization, and point out that Planck came up with the quantum trying to explain black body data, and Maxwell stumbled upon the em origin of light by observing a coincidence in the ratio of units. In any event, your view of matters is debatable. And in this particular context, as a practical matter in lauding the speed of light as a standard of speed superior to all others, theory is not the issue: the issue is the observed advantages, which will not go away even if relativity goes down the drain. Brews ohare (talk)
You seem to have a dislike of some theories. The problem is that all physics is based on theory. It would be impossible to attach any meaning to the term speed of light without some underlying theory of what light was and what it did. How would you measure the speed of light using Ptolemy's emission theory? The question would have no meaning. Martin Hogbin (talk) 13:47, 3 August 2010 (UTC)

What is the advantage in pointing these matters out in the article? A major advantage is that it removes the mystique of the speed of light from the formulation of a standard of speed. By lifting this baggage, the adoption of a speed standard becomes a simple idea. It is not confounded with all the brouhaha surrounding the speed of light and its multiple roles in physics over the millenia: the aether, relativity, em propagation etc. The adoption of the speed of light as a standard is just a super convenience, not a magical event. Brews ohare (talk) 13:24, 3 August 2010 (UTC)

I think it is something between those two. No one claims that the speed of light has magic properties or even that it is a fundamental part of nature. How could we tell? It is, however, a fundamental part of our currently accepted and verified theories of physics and, as such, it is a much better basis on which to build a system of units than an arbitrary measurement such as the speed of sound in water. The trend in metrology is towards using fundamental (according to our current understanding) constants of physics. It would be better if we could use units based on our fundamental concepts of the future but unfortunately nobody know how to do this. Martin Hogbin (talk) 13:47, 3 August 2010 (UTC)

Martin, I am just being a devil's advocate here. Of course theory and experiment have both played a role. I don't agree with your assessment that the speed of light is a great basis for metrology because of its fundamental role in theory. For example, if it happened that the speed of light was very dependent upon polarization, field intensity, direction of propagation etc. etc. , then it would not be suitable for metrology, regardless of its fundamental importance. And if the propagation of sound in some 0 Kelvin superfluid had super reproducibility, it would be a great candidate, even if it was a negligible theoretical curiosity. The advantage of the speed of light for metrology is its super convenience, which comes from its easy realization regardless of almost any parameter you can think of, except gravity. That capacity of light is a fact, not a theory. Brews ohare (talk) 14:07, 3 August 2010 (UTC)

Apart from this religious digression, the above advantages for the article of first pointing out the general possibility of setting a standard of speed rather than length, and then followed by the advantages of light for this purpose, remain the point. Brews ohare (talk) 14:07, 3 August 2010 (UTC)

The point is that theory tells us that the speed of light does not depend on any of the things that you mention. Of course the theory could be disproved tomorrow but it is still better to build a measurement system on what we confidently expect to be the case rather than some arbitrary assumption or artifact.
I still do not think you have understood how much physics depends on theory of some kind. We have to start with a theory that tells us that light even exists. Martin Hogbin (talk) 14:20, 3 August 2010 (UTC)
Martin: Theory is not pertinent here. What matters for metrology is reproducibility and accuracy, and these are experimental matters, whatever their explanation. For the article, the point is that the logical possibility of using a standard speed exists, and the exemplary empirical reproducibility of the speed of light (whatever its explanation) eminently suits it for this role, and thereby weights the decision about units strongly in favor of adopting a standard speed as a basis for the SI system. That is what the article should point out. It should illuminate the metrology argument, not the importance of theory. Brews ohare (talk) 14:31, 3 August 2010 (UTC)
You still have not understood what theory is. Even the fact that light exists at all is a theory. Read this article! Martin Hogbin (talk) 14:38, 3 August 2010 (UTC)

Martin: It doesn't matter whether I understand what theory is, nor whether you do. Please address the metrology. Brews ohare (talk) 14:46, 3 August 2010 (UTC)

Metrology is about repeatability, reproducibility and accuracy. Brews ohare (talk) 14:56, 3 August 2010 (UTC)

If you do not have a theory that tells you light exists then you cannot measure it in any way at all, let alone one that is repeatable, reproducible and accurate. You are using the word 'theory' to mean stuff you do not like. Martin Hogbin (talk) 16:48, 3 August 2010 (UTC)

(ec, I also agree with Brews latest comment on metrology) I think that Brews has a point here about the reproducibility of the experiemntal conditions one uses to define the metre, but one has to add here that small deviations around this ideal state don't lead to significant errors, partially because they produce small effects and also because such effects are predictable to a high degree of accuracy and can thus can be compensated for. If you measure the same length over and over again, you get some distribution around an average value, and then what matters is that the spread around the average is smaller than what you would get using another method, otherwise you would have to think about using that other method to define the metre.

Another issue is the experimental limits on any deviations of the theory that describes the experimental setting that renders the metre. In general, it is not the case that if you can render some standard with an error of sigma, that therefore the theory that describes the experiemntal setting for this can have deviations that fall just within this area. In case of special relativity, this certainly is not the case. Any violation of Lorentz invariance that are not yet ruled out, cannot influence the metre standard, because these would produce effects that are many orders of magnitude smaller than the other known effects. Count Iblis (talk) 14:58, 3 August 2010 (UTC)

Regarding the comment by Brews ohare that "...the above advantages for the article of first pointing out the general possibility of setting a standard of speed rather than length, and then followed by the advantages of light for this purpose, remain the point." - In what reliable source do these ideas appear? --Bob K31416 (talk) 15:12, 3 August 2010 (UTC)

BobK: I don't think we need any additional sources to point out that in the SI units the speed of light is a defined constant. Likewise, no additional sources are needed to point out that length is now a derived unit based upon the defined speed of light. Perhaps it is unclear that any choice for a standard speed has the same effect, namely lengths are measured in seconds? Assuming that point of logic is understood, it follows that there exists any number of choices for that standard speed, and one can ask: Upon what basis is a selection made? The answer to that question, according to the precepts of metrology, is based upon repeatability, reproducibility and accuracy. Brews ohare (talk) 15:40, 3 August 2010 (UTC)
Re " I don't think we need any additional sources..." - Please give the excerpt from the reliable source that expresses the idea of "the general possibility of setting a standard of speed rather than length, and then followed by the advantages of light for this purpose". --Bob K31416 (talk) 15:58, 3 August 2010 (UTC)
BobK: The SI units have done exactly that: they choose a standard of speed rather than length. It hardly needs a source to point out the possibility when we have an existence proof in front of our eyes. As for the advantages of light as a choice, they have been described in detail. I could ask you for a counterexample that would be better; I don't think you can find one. Brews ohare (talk) 16:04, 3 August 2010 (UTC)
Please note that I will have to see the excerpt before I can support any addition to the article related to the subject ideas and consensus will be needed for any such additions. --Bob K31416 (talk) 16:25, 3 August 2010 (UTC)

OK, BobK: How's this for a paragraph outline:

• SI units use a defined c. [Source CGPM resolution]
• Lengths are measured in terms of times of transit, t by ℓ = c t. [Source CGPM resolution]
• If instead of c one chose some other speed s, the analogous thing would be to set ℓ = s t. [Obvious]
• What considerations would enter a choice for s other than the speed of light? [Source on metrological considerations of accuracy reproducibility ease of access etc.]
• Observation that the speed of light is independent from variation in a host of parameters [Source: texts on experimental verification of relativity]
• Conclusion: Speed of light satisfies the metrology requirements pretty well. [Source: Sydenham on accuracy]

Would a paragraph along these lines meet with your approval, or why not? Brews ohare (talk) 16:45, 3 August 2010 (UTC)

Before I can support this addition to the article I would have to decide whether it is consistent with WP:NOR, so please give the wording of the conclusion that you would like to put in the article and give the excerpt from the reliable source that makes that conclusion. --Bob K31416 (talk) 17:54, 3 August 2010 (UTC)
Hi BobK: I'd be happy to make an attempt at drafting a few lines, but Blackburne has decided to make a federal case out of my participation here and here, so I'm quitting for the moment. Brews ohare (talk) 19:19, 3 August 2010 (UTC)
Brews, I have to say that I do agree with Blackburne's action. You seem to have come back to this article to carry on where you left off. I would have thought that to discuss and try to get a consensus before making widespread edits would have been a good idea. You could still try backing off a bit while you have the chance. Martin Hogbin (talk) 21:05, 3 August 2010 (UTC)

Terminological details aside, Brews does have a point. While it's true that SR says x and ct are as fundamental as each other so that it makes completely sense to base a measurement system of c, it does not say whether such a system would be practical enough: if lenghts could be compared to each other more easily than to times, it'd still make sense to have separate standards. By comparison, in quantum mechanics the reduced Planck constant is a natural unit, but a redefinition of the kilogram in terms of it hasn't been done yet, and while it would be more attractive theoretically (and solve the problem of standard weights drifting wrt each other with time), it could also have practical problems as comparing a macroscopical mass with the Planck constant has been much harder than comparing two macroscopical masses to each other.[1] And in the theory of GR it makes sense to use units based on the gravitational constant G, but such a system would be terribly ill-suited for practical purposes as almost all human-scale quantities cannot be measured wrt it any better than to within one part in 10,000. A. di M. (talk) 23:07, 4 August 2010 (UTC)

The question is: Is this pertinent to this article? And if so what would you like to add? TimothyRias (talk) 07:14, 5 August 2010 (UTC)
It's not pertinent to the current content of the article, but I just wanted to point out that Hogbin's claim that "a fundamental part of our currently accepted and verified theories of physics [...] is a much better basis on which to build a system of units" is not always correct (the gravitational constant clearly being a counterexample). (The pertinence of this to the article is that no statement claiming that should be added.) A. di M. (talk) 22:56, 5 August 2010 (UTC)

## Does this article have space to attempt to illustrate why c is c ?

I have just been wondering this for sometime and have not found any full answers via whitepapers on the subject.

What is the fundamental reason the maximal speed of anything in the universe is 3x10^8 m/s ? I found an article on Scientific American here that hypothesized that it comes about due to objects residing in the dimensions of space and time needing a factor of conversion to truly be facets of the same coin (of 4 dimensional space-time). Is this argument completely off the wall or are there any respectable scientific journals that debate this concept and bring light(pardon the pun) to the origins of this upper limit of traversal of any mass-less object from one point in space to another...

It probably doesn't deserve a place in the article unless there are some papers discussing the origin of fundamental constants giving rise to this idea. But i'd like to see it explored if it has merit. Does anyone have any opinions or data they could add to this query?

TL:DR I understand that there couldn't exist a universe with no upper limit, but i don't understand what drives the value for the limit as it stands today. —Preceding unsigned comment added by 109.78.64.45 (talk) 17:19, 3 August 2010 (UTC)

The actual value has no meaning whatsoever. You can choose units to give it any value you like. The value 3x10^8 m/s is an historical relic related to how units were defined in the past. Many theorist prefer to work in units were c has the value 1. TimothyRias (talk) 07:20, 4 August 2010 (UTC)
See this: roughly, "it isn't the light that is fast, it's us who are slow". A. di M. (talk) 23:54, 4 August 2010 (UTC)

## Relation of c to fundamental units

Here is an excerpt describing the role of fundamental constants:

“In one common usage the fundamental constants are a set of quantities, the knowledge of which is sufficient to predict, using appropriate theory, all the properties of matter and radiation at both a macroscopic and a microscopic level. Candidates for such a set are G, the gravitational constant; h, the Planck constant; c, the speed of light; e, the electron charge; me, the mass of the electron; mp, the mass of the proton; k, the Boltzmann constant; θw, the Weinberg angle, which relates the charged and neutral weak currents; GF, the coupling constant for the weak nuclear interaction; θC, the Cabibbo angle, which relates the strangeness changing and nonstrangeness changing weak nuclear interactions; and Λ, the quantum chromodynamic coupling parameter, which characterizes the strong nuclear interaction.”
“[For example] quantum electrodynamics and the Dirac theory of the electron [enabled] the calculation in 1947 of the magnetic moment of the electron in terms of e, h, me and c. There is as yet [1983] no such precise calculation of the magnetic moment of the proton.”

Question: What should this article say about how a definition of c = 299,792,458 m/s unrelated to measurement affects its role in such a context? Shouldn't the article discuss the modification of the relationship of c to fundamental properties of nature when the definition of the units precludes alteration of c by measurements, no matter how refined, and no matter what path the evolution of theory may take? Brews ohare (talk) 15:29, 3 August 2010 (UTC)

I think the article's fine on this, and don't see that anything else needs adding. Of course there is more to it than is in the article, but for deeper theoretical and and mathematical underpinnings the reader is better served by e.g. the article on special relativity. As for 'what path the evolution of theory may take' that is simply an unknown about which little useful can be said until it actually happens and we know what new form theory takes.--JohnBlackburnewordsdeeds 15:43, 3 August 2010 (UTC)
(edit conflict) Regarding the title of this section "Relation of c to fundamental units", it doesn't appear that the quoted excerpt discusses that but rather its topic is described in the sentence, "In one common usage the fundamental constants are a set of quantities, the knowledge of which is sufficient to predict, using appropriate theory, all the properties of matter and radiation at both a macroscopic and a microscopic level." --Bob K31416 (talk) 15:46, 3 August 2010 (UTC)
(edit conflict)Nothing really. The only thing that should be made clear as the redefinition of the metre does not affect c at all, in any way. The constant c does not care in what units you express it. This also means that its value does not have any predictive value whatsoever. The quote above is somewhat naive in this respect. It is widely accepted that only dimensionless constants can be fundamental in the sense described in that paragraph.TimothyRias (talk) 15:50, 3 August 2010 (UTC)
There are three responses above: (i) It's too complicated, let's forget about it. (ii) I don't understand the issue. (iii) It doesn't matter because I choose to define the term “fundamental constant” differently, and so I can duck the question. Brews ohare (talk) 15:55, 3 August 2010 (UTC)
Well, you can write down the complete theory with all these constants and then using scale transforms map it to a theory of the same form but with different values for c, hbar, k, and G. Any set of values for these constants can be obtained from any other via scale transformations. What does that tell you about these constants? Count Iblis (talk) 16:06, 3 August 2010 (UTC)
As for (iii), since different reliable sources use the term "fundamental constant" with different meanings, I have always argued that it should not be used except in direct quotations. A. di M. (talk) 14:56, 9 August 2010 (UTC)

The speed of light as a conversion factor contains absolutely no information. The speed of light as a phenomena limiting the transfer of information has significance in separating our universe for other hypothetical ones. There is a difference here: info vs. no info. It may be that additional info is needed, but there is still some info here that is not present in a conversion factor set in a conference room debating easy and accurate lab procedures. I'd guess that the correct answer to the question is that various expressions involving c are not to be evaluated setting c = c0 but by reforming all such formulas so c doesn't appear any more. Brews ohare (talk) 16:21, 3 August 2010 (UTC)

Or, by interpreting c as not a reference to the SI c0 but to the real phenomena; that is, in principle, cc0 but given by some formula based upon a theory of em propagation. Brews ohare (talk) 16:32, 3 August 2010 (UTC)

This is my favorite answer to the question: in such formulas involving c, where the real speed of light in the sense of something measurable is meant, c can be replaced in SI units by c0/n where n is the refractive index of the medium, probably referring to a form of vacuum such as the quantum vacuum or the quantum chromodynamic vacuum. Brews ohare (talk) 17:22, 3 August 2010 (UTC)

c as a conversion factor is relative to some standard normalization. The conversion factor would not be needed in the first place had the units be chosen in a "natural way" w.r.t. the fundamental laws. Then because classical physics is invariant w.r.t. rescaling of time, there was no way we could have defined the units for distances and times relative to each other in the "correct way" in previous centuries. But the relativistic equations are not invariant w.r.t. to rescaling the time coordinate. Then because we still want to rescale the time coordinate, even when the laws are not invariant under such a rescaling (for hystorical reasons, for practical reasons or whatever), the constant c appears. It's role is merely to compensate for any rescaling of the time coordinate. Count Iblis (talk) 18:06, 3 August 2010 (UTC)

## Shorter wavelength

I've reverted this edit. For two reasons, 1) The reasons that the method used by Evenson is more accurate and precise is already explained in the interferometry section. 2) The use of the comparative "shorter wavelength" is potential confusing since no wavelengths were mentioned earlier in the section. This leaves the reader with the question "shorter than what?". TimothyRias (talk) 08:34, 9 August 2010 (UTC)

I disagree. It was a good addition but I'm not going to do anything about it without support from another editor. --Bob K31416 (talk) 11:22, 9 August 2010 (UTC)
Why? I'd help the constructive discussion a lot if you would actually explain why you think a certain edit is good or bad. I've explained why I disliked the addition. Is there any of those points you disagree with?TimothyRias (talk) 11:32, 9 August 2010 (UTC)
Sorry. I don't care to discuss it without first getting support from another editor because it's a matter of style and opinion which I don't think can be settled in a discussion with just you and I don't want to get involved in a long unproductive discussion. --Bob K31416 (talk) 11:44, 9 August 2010 (UTC)

## Light and the speed of sources and observers

I've reverted this edit, as it significantly alters the meaning of the sentence. Both the facts that it is independent of the speed of the source and of the observer are very relevant. Not it in modern relativistic context, then the latter statement is equivalent to the former, but when relativity needs to be motivated both are observed and tested separately (and they were historically). There is the (slim) theoretical option that if the speed varies with the speed of the source and of the observer, that it still is invariant with regard to the relative speed of the to.

I do however agree that the current phrasing is a little awkward. A suggestion:

"The speed of light, usually denoted by c, is a physical constant appearing in many areas of physics. Light and all other electromagnetic radiation is always observed to travel at this speed in vacuum, regardless of the speed of the source or the observer."

This has a number of advantages. First it is a more compact phrasing of the current statement about c being constant. Second it makes immediately clear that the context of c is not always light.TimothyRias (talk) 08:21, 9 August 2010 (UTC)

Your statement is open to misinterpretation. The facts given in reliable source are that:
1. The (two-way) speed of light is c when measured in any inertial frame. (There is no need to mention the 'speed' of the frame)
2. The speed of light is independent of the motion of the source (including non-inertial motion of the source)
If these facts cannot be clearly and fully expressed it is best to say nothing about the subject. Martin Hogbin (talk) 18:14, 9 August 2010 (UTC)
Re "Your statement is open to misinterpretation." - What misinterpretation did you have in mind? --Bob K31416 (talk) 19:57, 9 August 2010 (UTC)
'always observed' could mean in a non-inertial frame. Why 'speed' of source - it is any motion, including acceleration, what is the speed of the observer relative to? Martin Hogbin (talk) 22:14, 9 August 2010 (UTC)
Well, maybe we should be satisfied with being editors instead of authors and use material from a source. Here's a source, an excerpt from it, and a possible version for the article.
Source: Reucroft S, S; Swain J (1998-01-19). "Why isn't the speed of light infinite?". ScientificAmerican.com. Scientific American. Retrieved 2010-08-09.
Excerpt: "The speed of light is strange in that it has the same value independent of the relative velocity between the source and the observer."
Version: "It is constant regardless of the relative velocity between the source and the observer."
Everyone should feel free to find a better source and excerpt to use, or compose a better version of this one. Note the opening sentence from the Wikipedia policy WP:V,
"The threshold for inclusion in Wikipedia is verifiability, not truth—whether readers can check that material added to Wikipedia has already been published by a reliable source, not whether editors think it is true."
--Bob K31416 (talk) 00:37, 10 August 2010 (UTC)
To Bob, this is the lead, it summarizes the main text. This process of summarizing is by its nature editorial. If additional sourcing is deemed required it should be done in the main text to avoid swamping the lead with references. However as it is the statement in the main text is referenced fine by Einstein 1905 since, the first one is (part of) his first postulate, the second is his second postulate. TimothyRias (talk) 07:36, 10 August 2010 (UTC)
To Martin: First a remark, this sentence is in the lead, it echoes a more careful statement in the body of the article. In the lead we can afford to allow a little bit of room for misinterpretation (as long as it is easily rectified by reading the body) in exchange. If we want to stay close to the body text the sentence could read:
"Light and all other electromagnetic radiation is always travel at this speed in vacuum, regardless of the motion of the source or the inertial frame of the observer."
This would indeed be more correct than the current phrasing. However for sake of accessibility I would like to avoid using the term inertial frame of reference in the lead. Any suggestions on how to avoid this and stay correct?TimothyRias (talk) 07:27, 10 August 2010 (UTC)
The sentence above looks OK to me. To get it any shorter or simpler than this risks being misleading. You could maybe have, 'its speed is the same to all inertial observers'. The problem with dropping 'inertial' is that there are some quite well known instances of light travelling at different speeds in non-inertial frames, such as the laser giro. Martin Hogbin (talk)
I'll add that then, it is at least better than the current phrasing. We can can tweak it some more of we come up with something better. TimothyRias (talk) 09:06, 10 August 2010 (UTC)

## recent changes to Increased accuracy of c and redefinition of the metre

I'm not sure why this was necessary. Looking at the versions before and after I think restoring the version of a few hours ago makes that section notably worse: more clumsily written and less clear, as well as less well referenced. The edit summary says it "much seemed better before", but I think that's simply the wrong way around. And since when do editors need to discuss edits on the talk page before making them? Unless the article is under protection, or the editors under some sort of sanction, that is not normal WP policy even for an article on the front page.--JohnBlackburnewordsdeeds 13:15, 4 August 2010 (UTC)

I've reverted the blanket revert. I purposefully split the edit in several separate edits so that if somebody found any of the edits discutable he/she could challenge the specific edit in question. Each edit has an edit summary with what was done, so it should be fairly easy to figure out. In total the change is much less severe than pretty much any of the edits done to that section in the last month so I don't see what the problem is. TimothyRias (talk) 13:43, 4 August 2010 (UTC)
I think this line is dubious, 'Because the previous definition was deemed inadequate for the needs of experiments'. It is only one of the reasons given for the change. The rest are shown in the Redefinition of the metre in 1983, which now shows my summaries of the reasons given by the BIPM for the change. Martin Hogbin (talk) 14:41, 4 August 2010 (UTC)
The reasons given there, are why that particular definition was adopted (above other available definitions). The reason that a new definition was discussed in the first place, was because the old one was deemed adequate. If the old definition was adequate there would be no discussion at all. The sentence quite effective summarizes the first paragraph of the ref given as a source.TimothyRias (talk) 15:23, 4 August 2010 (UTC)
It is hardly a summary of anything. To say that a new definition was needed because the old one was no good is uninformative and superfluous. Martin Hogbin (talk) 15:36, 4 August 2010 (UTC)
I agree that the statement is somewhat empty. (Note that this also goes for the "for various reasons" statement you suggest below. The main function of the phrase is to lead in the paragraph. I'm fine with not stating an reason at all, if we can find a proper alternative to lead in that paragraph.TimothyRias (talk) 18:42, 4 August 2010 (UTC)
(edit conflict)Re the change you made from this
"In 1983, considering that the definition of the metre did not allow a sufficiently precise realization of it for all requirements"
to this
"Because the previous definition was deemed inadequate for the needs of experiments"
1) Why did you leave the date out?
2) I think "metre" should be in the phrase.
3) "inadequate for the needs of experiments" suggests for experiments in general, which isn't the case.
4) Re your edit summary "Rephrase to be less jargony and more accessible" - I like that motive. Using some of Martin's wording from elsewhere, how about this:
"In 1983, considering that the definition of the metre was not precise enough for all purposes "
--Bob K31416 (talk) 15:59, 4 August 2010 (UTC)
The point is that we should not be making up a reason at all. The BIPM give the 7 reasons for the change. We should either try to summarise them simply, say something general like 'for various reasons', or say nothing at all. Anything else is OR. Martin Hogbin (talk) 16:37, 4 August 2010 (UTC)
I think the issue you are bringing up was already settled when Brews ohare brought it up previously. --Bob K31416 (talk) 18:58, 4 August 2010 (UTC)
The issue has nothing to do with Brews, it has to do with WP policy, which that what we say should not be based on our own opinions but on what is said in reliable sources. In this case we have access to the source which tells us exactly why the change was made. To claim anything else is wrong. Perhaps the best solution is to give no reason at all. Martin Hogbin (talk) 21:12, 4 August 2010 (UTC)
1)The year was left out by accident it should have been added after 17th CGPM.
2)The word "metre" is in the second part of the phrase, the advantage here is that the piped wikilink to the "Redefinition of the metre in 1983" article is less of an Easter egg.
3)But it was inadequate for a fairly wide variety of experiments as indicated in the new scientist article. You could add the word precision before experiments.
4)The phrasing with "considering" is very stiff and legalistic. (The kind of language you'd expect in a resolution of the CGPM :) ) This was (in part) what I was trying to remedy. The other reason is that the sentence as it was does not quite work as the start of a paragraph.
I'm not married to this phrasing though.TimothyRias (talk) 18:38, 4 August 2010 (UTC)
Would you care to rewrite it based on our discussion? --Bob K31416 (talk) 18:55, 4 August 2010 (UTC)
Why have anything? Martin Hogbin (talk) 21:12, 4 August 2010 (UTC)
Well you need some sentence to connect the previous paragraph (about speed of light measurements hitting the precision limit caused by the previous definition) to this paragraph about the redefinition of the speed of light. Without such a sentence the section makes little sense. The clear link is that the process of redefining the metre was triggered by experiments being limited in precision by the previous definition. TimothyRias (talk) 07:38, 5 August 2010 (UTC)
The precision limit you mention had already been overcome by adopting a standard speed for light some years earlier. If I had to give a single reason for the change, it would be the desire to move towards a system of standards based on fundamental physical constants.
The point is, however, it does not matter what I think or you think was the reason, we have a very reliable source that tells us exactly what the reasons were. The only question is how best to reflect those reasons in this article. Martin Hogbin (talk) 08:26, 5 August 2010 (UTC)

──────────────────────────────────────────────────────────────────────────────────────────────────── We have a reliable source telling us what the reasons were for adopting the new definition (that particular one). I think we all agree that discussing those in detail here would be to big of a digression. The sentence you have objections to is not trying to give a summary of those, it merrily relates why a new definition was needed in the first place. To see the difference consider the following (fictional) example.

"Because my computer broke down I went to the store to buy a new one. I decide to buy the latest model MacBook pro, because (insert washlist of reasons why a choose that particular model.)"

The first part relates why a new computer was needed, the second why a particular model was chosen. I think we can safely leave out the second part in this article. The first part (why the CGPM went to the "store") is needed IMO for the flow of the article. Of course you can disagree on this, in which case I would like a concrete example of how you would change the article.TimothyRias (talk) 09:22, 5 August 2010 (UTC)

(offtopic) The 1975 decision was a fix for nothing, it was merrily a recommendation to use a particular value (since there is considerable advantage in everybody using the same number). It did not solve the pressing issue that in various fields lengths could be more accurately compared to each other than they could to the metre.TimothyRias (talk) 09:22, 5 August 2010 (UTC)
You are wrong. CGBM did not propose a new standard because the old one was 'broken', the committee has the task of continually improving standards both for practical and philosophical reasons. As I have already said, and as the CGPM have confirmed, an important reason for changing the standard was the desire to move towards standards based on fundamental constants.
The current wording, 'Because the previous definition was deemed inadequate for the needs of various experiments' is somewhere between meaningless and plain wrong.
Why not say 'for a number of practical and theoretical reasons' with a reference to the CGPM paper. Alternatively, you could just give no reason, starting 'In 1983 the CGPM made the decision ...', or someone could try to write a very brief summary of the real reasons the change was made. Which would you prefer? Martin Hogbin (talk) 09:38, 5 August 2010 (UTC)
(On your first remark, the CIPM tends to be very reluctant to change standards, unless there is a pressing need. This becomes very apparent if your read the reports on the meetings of the CCDM (for example in the "News from the BIPM article in Metrolgia"), discussion about a new definition basically started the moment it was realized that the one in 1960 was not as precise as assumed, decision on the new definition was however put off for a decade or so, because there was no pressing need for a better standard from the POV of experiments.)
More on the point. How about a compromise:
"Because of the previous definition was not accurate enough for all the needs of experiments as well as various other practical and theoretical reasons, the 17th CGPM in 1983 decided to redefine the metre."
This has the advantage of providing a link with the precision problems all ready touched upon in the preceding paragraph, while still mentioning that it was not the only reason to do so. Preferably this sentence should refer to an article with a more in depth discussion of the choice, for example the "News from the BIPM" preceding the 17th CGPM, which has a slightly more detailed discussion of the arguments made in de CCDM etc., but I can also live with refing it with the 17th CGPM decision.
Not saying anything feels like an weird continuity break in the section, almost requiring a complete section break, but that would lead to two one paragraph sections.TimothyRias (talk) 14:12, 5 August 2010 (UTC)

I think this discussion came about because of the significant change in the section that was recently made by one editor. The previous version that I had reverted to,[2] had been discussed and worked on by a number of editors incrementally, and it hung together. With the new version, old issues that were settled with the previous version, are being raised again. --Bob K31416 (talk) 10:24, 5 August 2010 (UTC)

No this discussion came about because of one change I made, and even then I assume that Martin had the same objections to the version before that, since the meaning was almost unchanged. TimothyRias (talk) 10:36, 5 August 2010 (UTC)
I don't think so, for the reasons I mentioned. --Bob K31416 (talk) 11:00, 5 August 2010 (UTC)
Bob, most of the edits you tried to blanket revert were unrelated, having to do with cleaning up referencing and adding details to this section that were to be left out from another section. Blanket reverting them without adressing why each of those edits is inappropriate. If you want to revert all those edits, address the issues dealt with in all those edits. Thus far there has only been made issue with the changes I made to a single sentence. And most of that commentary (i.e. martin's part) apply equally to the previous version of that same sentence. The issue being raised had not been settled by the old version at all.TimothyRias (talk) 11:42, 5 August 2010 (UTC)
I do not think the reasons for this discussion are that important. Do neither of you not like any of the suggestions that I have made above? Martin Hogbin (talk) 11:49, 5 August 2010 (UTC)
I have a problem with this discussion and I have difficulty defending the new version for the reasons mentioned above. But your suggestions of more detail or no detail I don't support. Also, since I feel these issues were settled before with the previous version, and since this has become a long discussion, I don't feel like continuing. (BTW Timothy, I have no intention of reverting again, so I hope that eases any concerns you have in that regard. I'll let others decide whether they want to put back any part of the previous version.) --Bob K31416 (talk) 13:55, 5 August 2010 (UTC)
You cannot settle on a version where you just make stuff up. The exact reasons for the change are known and recorded in a most reliable source. The only decision to be made here is how best to express those reasons, if at all, in this article. Martin Hogbin (talk) 17:20, 5 August 2010 (UTC)

### lengths comparison

Re the sentence of the present version, "The precision of the experiment was limited by the accuracy with which lengths could be compared to the definition of the metre." - This suggests to me that the limitation was with the experimental technique of that particular experiment, rather than the imprecision of the definition of the metre. This may not have been the intended meaning by the editor since it doesn't support motivation for the redefinition of the metre. --Bob K31416 (talk) 11:10, 6 August 2010 (UTC)

Yes, the relevant source actually says, 'The main limitation is asymmetry in the krypton 6057-Å line defining the meter', so the problem was in determining exactly how long the meter was under the old definition. How about "The precision of the experiment was limited by the uncertainty in the definition of the metre." This is literally correct. The problem was not in comparison to, or even realisation of the meter but in deciding exactly what the definition meant. In other words the old standard did not make clear exactly which part of the krypton line was to be used. Martin Hogbin (talk) 11:30, 6 August 2010 (UTC)
1)Your suggestion is an improvement but I think the version that was in the article before the present version would be more informative, "Almost all the uncertainty in this measurement of the speed of light was due to uncertainty in the length of the metre", since it comments on the relative amount of the contribution of the imprecision of the definition to the total uncertainty of the measurement. A further improvement might be, "Almost all the uncertainty in this measurement was due to the definition of the metre."
2)re Krypton - Could you give the relevant excerpt from a source that makes the claim that the limitation was because the old standard did not make clear exactly which part of the krypton line was to be used. (Not meant as an argument but rather a request for information since I suspect that you have a source in mind. Thanks.) --Bob K31416 (talk) 11:45, 6 August 2010 (UTC)
1)How do you justify 'almost all'? The source actually says 'The main limitation...'. How about, "The main uncertainty in this measurement of the speed of light was in the definition of the length of the metre"?
2)I thought I had a source in mind but I cannot find it at the moment. It said that some groups had used the peak wavelength of the line and others its 'center of gravity'. Because of the asymmetry these two are not the same. Martin Hogbin (talk) 12:51, 6 August 2010 (UTC)
1) "Main" is fine with me if that's what the source says. How about "The main uncertainty in this measurement was due to the definition of the metre."
An afterthought, what's the rest of that excerpt 'The main limitation...' ? It might help us with the wording and with being true to the source.
2)BTW, re "some groups had used the peak wavelength of the line and others its 'center of gravity'" - that sounds like a determinate error rather than a random error or uncertainty which is expressed with ± . --Bob K31416 (talk) 13:48, 6 August 2010 (UTC)
I guess it is a insufficiency of the definition. Whoever wishes to use the definition has to decide which exact part of the line is meant. Two logical options are peak and CG. Martin Hogbin (talk) 15:42, 6 August 2010 (UTC)
I think I found the excerpt from the source and the sentence that preceded it.[3]
"Multiplication yields the speed of light c=299792456.2(1.1) m/sec, in agreement with and 100 times less uncertain than the previously accepted value. The main limitation is asymmetry in the krypton 6057-Å line defining the meter."
Perhaps the limitation they are referring to is a limitation related to the krypton line asymmetry that prevents them from getting the uncertainty lower than ±1.1 m/s , which seems different than the question of whether to choose the peak or mean (CG), which seems like a determinate error rather than an uncertainty. --Bob K31416 (talk) 16:35, 6 August 2010 (UTC)
No, the asymmetry is the reason that the peak and the mean are different. If the line was symmetrical the peak and the mean would be the same. Martin Hogbin (talk) 16:41, 6 August 2010 (UTC)
I understand that.
When they refer to "limitation", do you feel that they are referring to something other than the limitation in getting the uncertainty (a random error) lower than ±1.1 m/s ? --Bob K31416 (talk) 16:55, 6 August 2010 (UTC)

<outdent> If you read pp. 1347-8 of http://prl.aps.org/abstract/PRL/v29/i19/p1346_1 you will see that the asymmetry of the Kr line affected the measurement result c=299792456.2(1.1) m/sec in two ways.

1) The value 299792456.2 was affected by choosing the 'center of gravity' of the asymmetric Kr line profile.
2) The uncertainty of ±1.1 m/s was affected by the asymmetry of the Kr line profile because the asymmetry caused a small shift of effective wavelength with order of interference and the measured wavelength depended on the mirror spacing which contributed a random error, i.e. uncertainty, to the measurement.

--Bob K31416 (talk) 12:54, 7 August 2010 (UTC)

I do not see how such a small wavelength change could affect the uncertainty so much but I do not have the full article where this is all explained. Martin Hogbin (talk) 15:32, 7 August 2010 (UTC)
On p. 1347 there's mention of a two-component model of the Kr asymmetry that is used to analyze the data and reduce the standard deviation in the wavelength measurements from 6.4 to 2.7 parts in 109. And here's an excerpt from p. 1348.
"The fractional uncertainty in our value for the speed of light, ±3.5x10-9, essentially arises from the interferometric measurements with the incoherent krypton radiation which operationally defines the international meter."
--Bob K31416 (talk) 14:02, 8 August 2010 (UTC)
Here's an excerpt that is the last paragraph on p.1347 which continues on p. 1348. It gives more details of the effect of the Kr 6057 Å line asymmetry on both the value and uncertainty of the measured wavelength of the methane-stabilized laser at 3.39μm.
"In view of the (small) intrinsic asymmetry of the Kr standard line, it is necessary to specify the point on the line profile to which the defined wavelength (6057.802105 Å) is applied. At present there is no universal convention for this choice. Thus if the defined value is applied to the maximum-intensity point of the Kr line, we find λ = 33922.31404 Å; if the defined value is applied to the center of gravity of the Kr line, λ = 33922.31376 Å. Detailed consideration20 of random and known systematic effects, along with uncertainties in the krypton asymmetry model, leads to an estimated 68% confidence interval of δλ = ±1.2x10-4 Å or δλ/λ = ±3.5x10-9 for both of these results."
--Bob K31416 (talk) 17:10, 8 August 2010 (UTC)
The trick is to find a phrasing that accurately summarizes this. I was thinking something along the lines of
"The remaining uncertainty was mainly due to limitations of the definition of the metre."
If deemed necessary the note at the end of the current sentence giving the definition of the metre at the time can then be edited to says something about the limitations of that definition. (Line asymmetry and incoherence of the reference source.) TimothyRias (talk) 08:40, 9 August 2010 (UTC)
I'm not sure I communicated my points to you. Was there any part of what I wrote or excerpts from the source that might need to be clarified? --Bob K31416 (talk) 10:54, 9 August 2010 (UTC)
I'm afraid that you haven't communicated your points at all. I've read the paper (and a bunch of other one regarding this subject.) and it is (somewhat) clear to me what the issues were with the old definition and why that limited the accuracy of the experiment. One of the gripes I have with the current phrasing is that it is unclear what is meant by precision of a definition. The phrase "limitations of the definition" I suggested above has the advantage that it makes clear that the cause of the uncertainty lay in the way the metre was defined, while leaving in the middle what exactly the issues were. If needed the exact nature of these "limitations" can be expand upon in the note.TimothyRias (talk) 11:41, 9 August 2010 (UTC)
(And to answer the question you asked but deleted, the incoherence comes from the fact that a Krypton discharge lamp is a incoherent light source. Unlike a laser.)TimothyRias (talk) 11:47, 9 August 2010 (UTC)
My main point was that the asymmetry of the Kr line affected both the value 299792456.2 and uncertainty ±1.1 m/s of the measurement. I'm not sure if we agree on that. --Bob K31416 (talk) 12:03, 9 August 2010 (UTC)
We do. The source claims the the uncertainty is mainly due to the asymmetry of the Kr line. As I understand it, normally line width and incoherence do not effect accuracy much if the line is perfect symmetrical. However, if the there is an asymmetry, these effects do enter. My proposal is to use the unspecific phrase "limitations of the definition of the metre" for the asymmetry of the Kr line and all related effects, leaving the description of these limitations to the source. The main thing we want to get across is that the experiments hit a wall in uncertainty that could not be improved upon by just better measurement techniques.TimothyRias (talk) 12:53, 9 August 2010 (UTC)

────────────────────────────────────────────────────────────────────────────────────────────────────There's some more items that we have to see whether or not we agree on before I can discuss the wording for the article that you are suggesting.

The previously accepted result was c=299792500(100) m/s and the newer result was c=299792456.2(1.1) m/s, which is 100 times less uncertain (i.e. ±1.1m/s vs ±100m/s). They got the value 299792456.2 by arbitrarily choosing the center of gravity for the Kr line and this choice did not affect the uncertainty ±1.1m/s . The main limitation in getting the uncertainty lower than ±1.1 m/s was due to the Kr asymmetry's effect on the interferometric measurements. I'm not sure if you agree with this. --Bob K31416 (talk) 14:31, 9 August 2010 (UTC)

yes.TimothyRias (talk) 15:10, 9 August 2010 (UTC)
OK. Exactly what change would you like to make? --Bob K31416 (talk) 15:19, 9 August 2010 (UTC)
The currently the sentence in question reads:
"The precision of the experiment was mainly limited by the precision of the definition of the metre."
Above I suggested changing this to:
"The remaining uncertainty was mainly due to limitations of the definition of the metre."
There are two main changes
First, the use of uncertainty vs precision (of the experiment). This stays closer two the wording of the source. Moreover, I don't think it is not the precision of the experiment that was limited, but the accuracy. (Doing the experiment again with the same setup would not decrease the uncertainty) Using the word uncertainty avoids any debate on the technical meaning of accuracy and precision.
Second, the use of limitations vs precision (of the definition of the metre). First it is not the definition of the metre that was imprecise but its realization (a distinction not always made). Moreover, to a lay reader it would be unclear how a definition could even be imprecise, most people would interpret this as that the definition was ambiguous, which it was but which is not the point. The word limitations signals to any reader that the limit in precision was due to short comings of the definition without actually going into the details of these short comings. (The relevant short coming was of course that due to the asymmetry of the Kr line, the definition did not allow a sufficiently precise realization through interferometry) If other editor feel that it is necessary to include more detail of these limitations, they can be discussed in the note to that sentence.TimothyRias (talk) 07:53, 10 August 2010 (UTC)
Your use of the term "uncertainty" seems to include the issue of the arbitrary choice of cg vs peak in the Evenson et al article. It doesn't appear that they used the term "uncertainty" or "uncertain" in that way. It's normally used in articles about scientific measurements to refer to items like the uncertainty ±1.1m/s. In the source, the term uncertainty was not used for the arbitrary choice between cg and peak. And in the source when they speak of the "limitation" they are referring to the limitation of not being able to reduce the uncertainty ±1.1m/s any more. It appears that we aren't in agreement on this point that I thought you had agreed to previously. Since we don't seem to be able to communicate on this subject, I'm ending my participation in this conversation. And I Oppose your change because the "limitation" described in the source was regarding the uncertainty ±1.1m/s, i.e. precision, not the cg/peak issue, and there is no reason to be non-specific about it. If you want to make the change you will need consensus from other editors. --Bob K31416 (talk) 11:42, 10 August 2010 (UTC)
There used to be a note in the article which said: "The value of 299,792,456.2±1.1 m/s quoted above assumes that the definition of the metre is to be applied to the centre of gravity of the krypton line. The NIST group reported that using the maximum-intensity point of the line instead, the result would be 299,792,458.7±1.1 m/s." That gives you the the Type B uncertainty resulting from the 1960 definition of the metre as ±1.3 m/s in the speed of light (as of 1972): in other words, the ±1.1 m/s uncertainty which is quoted does not take the problems of definition into account. The NIST group were able to quote an uncertainty of ±1.1 m/s only by clarifying the definition of the metre to specify either cg or peak for the krypton line. That to me is a "limitation" in the 1960 definition of the metre – twelve years later it had become less precise than the most precise measurements. Physchim62 (talk) 12:24, 10 August 2010 (UTC)
The limitation that the source refers to is the limitation due to the asymmetry of the Kr line that prevents them from further reducing the uncertainty ±1.1 m/s because the asymmetry affects their interferometric measurements. The choice of cg vs peak doesn't enter into the uncertainty ±1.1 m/s. All I can suggest is that you carefully reread the Evenson et al source, particularly pp. 1347-8 if you disagree with this. If there is anything unclear to you in the source, feel free to ask me about it on my talk page and I will try to help you. Timothy, that offer is extended to you too, but please note that it is an offer to clarify the article for you as best as I can, and not an invitation to argue points, and if I see that I'm not helping you, I will terminate the discussion. Regards, --Bob K31416 (talk) 13:33, 10 August 2010 (UTC)
Bob, maybe you should reread my proposal, because you seem to misinterpreting it. We are in full agreement on the interpretation of the article. What I'm objecting to is the current use of the word precision in the article in an in correct way. In the first case it is the accuracy rather than precision of the experiment that is limited by the insufficient precision of the realization of the metre. Anyway my proposal side steps this semantic issue by calling it uncertainty, inline with the abstract of the quoted article. In the second case the word precision should apply to the realisation of the definition of the metre rather than the definition of the metre. (This distinction was not always made because these are the same when using artefacts to define the metre.) Again this issue is avoided by using the term limitations. (also don't be partonizing, it is rude and insulting.)TimothyRias (talk) 13:53, 10 August 2010 (UTC)
Bob, no my use of the term uncertainty has nothing to do with the ambiguity of using the cg/peak intensity of the line for the definition of the metre. It simply refers the total accuracy/uncertainty of 1.1 m/s in the measurement of the speed of light. This is referred to in the Evenson article both as uncertainty and as accuracy very explicitly at several stages and clear distinguished from the ambiguity associated with the choice of point of the spectral line to apply the definition to. Since we both confirmed that we agree to that, am a bit puzzled why you would interpret my suggestion as referring to that ambiguity.TimothyRias (talk) 12:49, 10 August 2010 (UTC)
To support my point that the quoted uncertainty (1.1 m/s) in the article is an accuracy rather than a precision I quote directly from the article (pg 1348):
"The uncertainties quoted are 1-standard-deviation (68% reliance) estimates and include both random and residual systematic uncertainties. This result is in agreement with the previously accepted value of c=299 792 500(100) m/sec and is about 100 times more accurate." (emphasis mine)TimothyRias (talk) 13:01, 10 August 2010 (UTC)

Considering that:

1. The current sentence "The precision of the experiment was mainly limited by the precision of the definition of the metre." is inadequately support by the given reference. In particular the source those not use the word "precision", which has a very particular meaning in this context, is not supported by the given source.
2. My previous proposal is considerably closer to the wording of the given references.
3. BobK's opposition of to that proposal is based on an interpretation of that proposal that is neither implied or intended.

I will make the following change to the article:

1. The sentence "The precision of the experiment was mainly limited by the precision of the definition of the metre." is changed to" The remaining uncertainty was mainly due to limitations of the definition of the metre.".
2. To still concerns by Bob that the word limitations is to vague, the text of the note following the sentence will be changed to "Since 1960 the metre was defined as: "The metre is the length equal to 1,650,763.73 wavelengths in vacuum of the radiation corresponding to the transition between the levels 2p10 and 5d5 of the krypton 86 atom." In practice in turned out that this spectral line was not perfectly symmetric, which put a limit on the precision with which the definition could be realized in interferometry experiments." with the last part supported by a reference to Barger and Hall 1973.

This at least implements a version that is fully supported by sources. If Bob still disagrees then he can propose a different version that is also supported by sources. (Which the current version was not.) TimothyRias (talk) 09:11, 11 August 2010 (UTC)

• Oppose - For the reasons I mentioned above and the present version is supported by the source for the reasons I mentioned above. Also, the issue of not digressing into details has been already discussed and settled, IMO. --Bob K31416 (talk) 11:45, 11 August 2010 (UTC)
Just realized that the details were put in a footnote so that part is OK with me, although it might not be expressed in the best way, but I won't go into that now. --Bob K31416 (talk) 12:23, 11 August 2010 (UTC)
• If you feel that the previous version was supported by the sources then give a quote where the sources support that it was the precision rather than the accuracy that was limited by the asymmetry of the Kr line. Otherwise the statement that the precision was limited is not supported by the sources. (Note that I have actually given a direct quote from the article that supports the fact that the article is reporting that the accuracy of the experiment was limited, by the Kr line asymmetry.)TimothyRias (talk) 11:59, 11 August 2010 (UTC)
Suggest reading the article Accuracy and precision and asking yourself the question what the difference is between the measurement terms precision and uncertainty. See especially the figure at the top of the article. BTW, I agree with your use of the term precision in the phrase "put a limit on the precision with which the definition could be realized" but I suspect that for the Evenson et al source, in your mind you think that precision, which they discuss when they use the term uncertainty, includes the peak/cg choice, which is incorrect. As I mentioned before, the Kr asymmetry affected the c measurement in more ways than just the peak/cg choice, as discussed in the source and which I think I previously quoted from the source, and that the source does not include the peak/cg choice as part of ±1.1 m/s, which is the uncertainty, i.e. precision, of the measurement but acknowledges that the value for c depends on whether peak or cg is chosen. Regards, --Bob K31416 (talk) 12:14, 11 August 2010 (UTC)
(edit conflict) to bob:
1) I suggest you stop making incorrect assumptions about what I think, and actually read what I have said above. Thus far you have been opposing solely on the basis of incorrect assumptions of what I think, which quite frankly I find frustrating and a tad insulting.
2) The cg/peak ambiguity enters neither the precision nor the accuracy of the measurements according to standard definitions.
3) To quote the Evenson article directly: "The uncertainties quoted are 1-standard-deviation (68% reliance) estimates and include both random and residual systematic uncertainties." That is they included (residual) systematic errors hence calling the uncertainty quoted precision is flat out wrong, since the precision is the random non-systematic part of the uncertainty. To see exactly what is included in the uncertainty please read the Barger and Hall reference reffed from the note.
4) Please review my reasons for objecting to use of the term "precision of the definition of the metre". This a short-hand used by metrologists to refer to "the precision of the realisation of the definition of the metre" (which admittedly is a mouth full). However, it is likely to be misunderstood by a lay reader as to include something like the ambiguity between cg/peak point. Which (and cannot stress this enough before you start making wrong assumptions again) we both agree should not be included in the reasons for the limitation on the uncertainty. My suggestion has the advantage of avoiding such misinterpretation, by avoiding technical jargon.TimothyRias (talk) 13:32, 11 August 2010 (UTC)
Accuracy indicates proximity of measurement results to the true value, precision to the repeatability or reproducibility of the measurement
In my previous message I wrote "Suggest reading the article Accuracy and precision and asking yourself the question what the difference is between the measurement terms precision and uncertainty. See especially the figure at the top of the article." - So, here's the figure. What do you think is the difference between the terms uncertainty and precision? --Bob K31416 (talk) 14:43, 11 August 2010 (UTC)
The difference is very simple. The uncertainty (as used in the relevant articles) consists of both systematic and random errors, while precision is just the random error. TimothyRias (talk) 14:54, 11 August 2010 (UTC)
Not for argument, but rather to let you know what my understanding is, I feel that the terms precision and uncertainty in the context of measurements are referring to the same thing and only differ in their inverse relationship, i.e. more precision means less uncertainty. They both can refer to uncertainties from Type B evaluation of systematic errors that are not well quantified but are only known to lie within a range. --Bob K31416 (talk) 14:02, 12 August 2010 (UTC)

I do not understand your point in relation to the krypton line. The abstract you quoted does not explain how the line asymmetry causes a reduction in measurement precision as far as I can see. Martin Hogbin (talk) 13:22, 11 August 2010 (UTC)
Perhaps you forgot a previous comment that you wrote[4] where you seemed to acknowledge that I was also referring to some of the rest of the full text article too. You might want to reread the part of this discussion beginning around that part, which contains excerpts from the rest of the full text article. --Bob K31416 (talk) 13:49, 11 August 2010 (UTC)
To martin, this is explained in the Barger and Hall article I reffed to the note. This article contains a detailed explanation of the error calculations of the experiment. The assymmetry of the spectral line causes the effectively measured wave length to depend on the total path length of the interferometer. This introduces a systematic error into the measurement. This can be reduced by modelling the asymmetry of the line. The result is the quoted uncertainty. (Which I repeat should not be called a precision because it is not random, redoing the measurement with the same apparatus with the same settings would yield the same result with a precision quite a bit higher than the quoted uncertainty.)TimothyRias (talk) 13:44, 11 August 2010 (UTC)
As you basically mentioned, the error from the Kr asymmetry comes in through the mirror spacing. But please recognize that in the real world, mirrors cannot be set exactly and thus a random error arises from the Kr asymmetry. Because the mirrors are inadvertently positioned slightly differently each time they are set up, the Kr asymmetry causes a different error for each setup, which results in a random error. Hmmm. On second thought I'm not so sure that this is what they mean and I should get and read the Barger article to check this. Or you could give an excerpt from the Barger article that might help. Thanks. --Bob K31416 (talk) 14:03, 11 August 2010 (UTC)
(ec) The relevant part of the Barger and Hall article is about a page long (it starts on the bottom of page 197 and continues for the most of page 198). Anyway, I'll summarize: what they did in reality is measure with a variety of mirror spacings and fit the dependence on the path length to a model parametrizing the asymmetry. One of the parameters of that fit is the "true" wavelength which using this model can be obtained with a total uncertainty of 2.7×10−9. However the choice of the model to model the asymmetry of the line introduces and extra systematic error of about 1.2×10−9. Combined these yield the total quoted uncertainty of 3.5×10−9, which since it contains systematic errors is not a precision.TimothyRias (talk) 14:25, 11 August 2010 (UTC)
Thanks but that didn't answer for me the question I mentioned in my above message that was in my mind. I'll get the Barger article and look at it. Regards, --Bob K31416 (talk) 19:24, 11 August 2010 (UTC)
Judging from Timothy's comments just above, he seems to be using the terms "accuracy" and "precision" in a different sense than I would use them (and, I suspect, from the sense that Bob is using them). To say that The distance from London to New York is 47.1564746168(6) km is very precise but completely inaccurate; to say the distance is 5½ thousand kilometres is accurate but much less precise. To get a figure which is significantly more precise than 5½ thousand kilometres while still being accurate, you need to specify what you mean by "London" and "New York": if you take "London" as meaning Heathrow airport and "New York" as meaning John F. Kennedy airport, the distance is 5539 km; from Trafalgar Square to Grand Central Station, the distance is 5565 km. If two separate observers are using different definitions of "London" and "New York", they will consistently get different measurements of the distance. This is a special case of a Type B measurement uncertainty, and one which is relevant to the cg/peak problem of the krypton line.
That's not to say that the precision of the definition of the metre is the only measurement uncertainty – indeed, it is so much larger than the other uncertainties that the authors chose to separate it out completely be unilaterally deciding on a cg-definition for the metre. There are, of course, other measurement uncertainties (both Type A and Type B) which are included in the quoted ±1.1 m/s (later upped to ±1.2 m/s by CODATA on the grounds that the frequency measurement was a new technique a so needed a more conservative estimate of uncertainties). But that says nothing about the accuracy of the quoted result, that is its relationship with the "real" value. The precision of the definition of the metre posed a fundamental limit as to the accuracy of any measurement of the speed of light in metres per second. Under the 1960 definition of the metre, the speed of light was both 299,792,456.2±1.1 m/s and 299,792,458.7±1.1 m/s, just as the distance from London to New York can be both 5539 km and 5565 km. To limit the use of "precision" to simply Type A measurement uncertainties is somewhat naïve, as it assumes that the experimenter has been bright enough to correct for all the Type B uncertainties (and experience warns us to be cautious about such claims). Physchim62 (talk) 14:51, 11 August 2010 (UTC)
The total uncertainty (+1.1 m/s) thus is not a precision. The Evenson article explicitly calls it accuracy. My proposed change simply side steps this technical but for this article completely irrelevant distinction by following suit with the abstract of the article and just calling it "uncertainty".TimothyRias (talk) 15:09, 11 August 2010 (UTC)
Terminology changes! "Uncertainty" is definitely the more modern term, and I doubt that Evenson at al. would get away with a phrase like "100-times more accurate" these days. Physchim62 (talk) 15:25, 11 August 2010 (UTC)
I know which is why I've advocated the use of the term "uncertainty"over the inappropriate use of the term precision. At very last the quoted uncertainty is due to a systematic error due to the way that the line asymmetry was modeled.TimothyRias (talk) 21:29, 11 August 2010 (UTC)
I do not have access to the whole papers, unless you can provide me with a copy or link. Martin Hogbin (talk) 14:22, 11 August 2010 (UTC)
Thanks for the info everyone. I still cannot see what this is all about. I agree now that there were two issues, the asymmetry increased the uncertainty of realisation and caused a (slightly larger by my calculation) ambiguity in the definition of the metre, but what is all the argument about? If it is just the words used, why not use those in the abstract and say the limitation in determining the wavelength was due to the krypton line asymmetry. Martin Hogbin (talk) 19:05, 11 August 2010 (UTC)
Well, the problem with saying that is that a general reader cannot be expected to know how the metre was defined at the time. With my suggestion I've tried to do the next best thing and replace the words "krypton line asymmetry" with "limitations of the definition of the metre" (which I think we all agree the krypton line asymmetry is) and use the note to expand upon what the definition was and what its limitation was for the interested reader.TimothyRias (talk) 21:35, 11 August 2010 (UTC)
I think I will withdraw from this discussion as it seems to be about a very minor point, "limitations of the definition of the metre" vs "limitations of the definition and realisation of the metre", considering other issue with the article. Martin Hogbin (talk) 08:08, 12 August 2010 (UTC)

#### lengths comparison Break 1

A) Current version:

"This was a factor of 100 improvement in accuracy over that of the previously accepted value. The precision of the experiment was mainly limited by the precision of the definition of the metre."[Note 9]

B) Timothy's proposal:

"This was a factor of 100 improvement in accuracy over that of the previously accepted value. The remaining uncertainty was mainly due to limitations of the definition of the metre."[Note 9]

C) Timothy's proposal with the following modification would be acceptable to me:

"This was a 100 times less uncertain than the previously accepted value. The remaining uncertainty was mainly related to the definition of the metre."[Note 9]

For all three versions, Note 9 refers to:

9. Since 1960 the metre was defined as: "The metre is the length equal to 1,650,763.73 wavelengths in vacuum of the radiation corresponding to the transition between the levels 2p10 and 5d5 of the krypton 86 atom."[122] In practice in turned out that this spectral line was not perfectly symmetric, which put a limit on the precision with which the definition could be realized in interferometry experiments.[123]

The phrase "a 100 times less uncertain than the previously accepted value" was in the abstract of the Evenson et al article.

The wikilink uncertain goes to an article that includes mention of the two types of uncertainty evaluation that correspond to the random and residual systematic uncertainties in the measurement of c.

--Bob K31416 (talk) 09:19, 12 August 2010 (UTC)

That would be completely acceptable to me. TimothyRias (talk) 10:41, 12 August 2010 (UTC)
Done. --Bob K31416 (talk) 13:09, 12 August 2010 (UTC)

### Evenson

In the recent changes to the article, Evenson seems to be given undue credit and weight, compared to his collaborators. It appears to be undue weight compared to the weight that mention of his name is given in the sources. Here are two instances to be considered for change.

1) "This technique is due to Ken Evenson and his group at the US National Institute of Standards and Technology (NIST), who used in 1972 to measure the speed of light with an accuracy of 3.5×10−9.[1][2]"
2) "In 1972, using the latter method, a team at the US National Institute of Standards and Technology (NIST) laboratories in Boulder, Colorado led by Ken Evenson determined the speed of light to be c = 299792456.2±1.1 m/s."

Suggest the following removals of "Evenson", and incidentally changing "team" to "group" in (2), and adding "it" (typo) in (1).

1) "This technique is due to a group at the US National Institute of Standards and Technology (NIST), who used it in 1972 to measure the speed of light with an accuracy of 3.5×10−9.[1][2]"
2) In 1972, using the latter method, a group at the US National Institute of Standards and Technology (NIST) laboratories in Boulder, Colorado determined the speed of light to be c = 299792456.2±1.1 m/s.

--Bob K31416 (talk) 14:03, 13 August 2010 (UTC)

Well, the reason I added the name was mostly so that the link with the table of historical results was more explicit, but I also see your point. Looking more closely at the papers, it appears that Evenson is mainly to credit with linking the frequency of the methance laser to the Caesium frequency, with others responsible for the precision measurements of the methane laser wavelength. Not sure what the best solution is.TimothyRias (talk) 14:15, 13 August 2010 (UTC)
Agree with Bob. If there's a problem with the table, we could always change the table to say "NIST" instead of Evenson et al. Also, in (1) it should say "to a relative uncertainty of 3.5×10−9"; and in (2), the location of the laboratory is not really relevant. Physchim62 (talk) 14:19, 13 August 2010 (UTC)
Made above changes (1) and (2) re Evenson.
Agree with Physchim62 that "uncertainty" seems to be the correct term. Re location, in some sources there is mention of a "Boulder group" so I'm not sure what to say about it. --Bob K31416 (talk) 17:32, 13 August 2010 (UTC)
OK, if other sources talk of the "Boulder group" then we're fine mentioning it (needs a comma after "Colorado" though ;) Physchim62 (talk) 19:23, 13 August 2010 (UTC)
BTW, I found this phrase at the bottom left of p. 1348 of the Evenson et al article,
"The fractional uncertainty in our value for the speed of light, 3.5×10−9 ..."
if that's of any help. --Bob K31416 (talk) 21:22, 13 August 2010 (UTC)

## NBS or NIST

I don't know if this has been discussed and settled before, so stop me if it has. : )

The source Evenson et al describes work that was done at the National Bureau of Standards (NBS), as noted in the source. Later the name of NBS was changed to the National Institute of Standards and Technology (NIST). It seems like in the article we should say the work was done at NBS with a footnote indicating that the name was later changed to NIST. --Bob K31416 (talk) 14:39, 14 August 2010 (UTC)

I just noticed the same problem while trying to write up a History of the metre article! Yes, we should say "U.S. National Bureau of Standards (which later became NIST)" – no need for yet another footnote if it's only a short thing we can put in parentheses. Physchim62 (talk) 15:31, 14 August 2010 (UTC)
This appears in two places in the article, so maybe it would be more efficient to have one footnote that serves both places. Also, in the explanation in the footnote there would be the full name of NBS and NIST, and the location Boulder, Colorado, instead of having the location in the main text. Also, in this way the acronym NBS might work better for the second appearance of NBS in the article. However, I like the parentheses since it is easier for the reader. I'll give this some more thought. --Bob K31416 (talk) 15:55, 14 August 2010 (UTC)
Present versions:
1) This technique is due to a group at the US National Institute of Standards and Technology (NIST), who used it in 1972 to measure the speed of light with an accuracy of 3.5×10−9.
2) In 1972, using the latter method, a group at the NIST facility in Boulder, Colorado determined the speed of light to be c = 299792456.2±1.1 m/s.
Proposed versions:
1) This technique was due to a group at the National Bureau of Standards (NBS) (which later became NIST). They used it in 1972 to measure the speed of light in vacuum with a fractional uncertainty of 3.5×10−9.
2) In 1972, using the latter method, a group at NBS in Boulder, Colorado determined the speed of light in vacuum to be c = 299792456.2±1.1 m/s.
--Bob K31416 (talk) 04:07, 15 August 2010 (UTC)
Or with just "now" in place of "which later became". A. di M. (talk) 08:53, 16 August 2010 (UTC)
Just "now" doesn't seem as clear. --Bob K31416 (talk) 10:41, 16 August 2010 (UTC)
I replaced the present versions with the proposed versions. Feel free to edit it as with any part of the article. --Bob K31416 (talk) 11:08, 16 August 2010 (UTC)

## Is this statement relevant about phase velocity?

Is this statement relevant about phase velocity?

"The requirement that causality is not violated implies that the real and imaginary parts of the dielectric constant of any material, corresponding respectively to the index of refraction and to the attenuation coefficient, are linked by the Kramers–Kronig relations.[6] In practical terms, this means that in a material with refractive index less than 1, the absorption of the wave is so quick that no signal can be sent faster than c."

It is my understanding that signals generally don't travel at the phase velocity but typically at the group velocity. For example, the confined EM waves inside of a wave guide have a phase velocity that is greater than c. Further, a signal does not need to travel through the material for the speed to be relevant. For example X-ray telescopes work because of total external reflection which depends on the speed of light inside of the mirror being greater then c.

I would like to replace, move, or remove this statement because it seems to muddy the waters unnecessarily more then it illuminates.TStein (talk) 04:25, 12 August 2010 (UTC)

Well, considering that this article is not about light, I'm hesitant to add more detail about the propagation of light to the article. The main message of the subsection is that light does not always travel at a velocity c. This used to be the opening line of the subsection, but your attempt to clarify things has removed this.
This particular statement is relevant as it explains under what conditions the phase velocity can be larger than c. (i.e. such a material cannot be transparent.). Maybe the last sentence starting with "In practical terms" should just say that.TimothyRias (talk) 08:21, 12 August 2010 (UTC)
I will easily defer to those who spend more time on this article, without making too much fuss. I can understand your hesitation to add more detail about the propagation of light. (In my view the index of refraction section can be pared quite a bit for that reason.) Personally, I think of 'speed of light' as a constant denoting the speed limit of the universe and that its name is an unhappy historical artifact. BUT, since the article is named 'speed of light' and not 'c' it is inevitable that the discussion of 'speed of light' in a material will come up.
You say that your main message was that 'light does not always travel at a velocity c'. If I cut that, it was unintentional. What I remember cutting is stuff that made no sense except to some small fraction of experts. My 'attempt to clarify' was to accomplish these goals:
1. say that 'the speed of light in a material cannot be described by just one 'speed of light; (Different 'parts' move at different speeds).
2. say that 'none of these speeds need to be the same as c and will generally be different from c and from each other in a material'
3. Explain clearly what the phase and group velocities are in correct and physical terms that anyone can understand. How is the phase velocity a 'speed of light'? What part of the wave is it the speed of?, etc.
4. I didn't like the fact that the article stated that there are 'different speeds of light in a medium' then made the readers wait to find out what they are.
5. Get rid of all information that isn't relevant to 'speed of light'. (I didn't complete this, since I know I would be stepping on people's toes in doing so.) There is a lot of stuff in this article that is completely irrelevant to the 'speed of light', such as most of the relativity section, IMO.
I may or may not have failed at that; you and the other dedicated editors to this article will sort that out. Finally you state that:
"This particular statement is relevant as it explains under what conditions the phase velocity can be larger than c. (i.e. such a material cannot be transparent.). Maybe the last sentence starting with "In practical terms" should just say that."
I know that this is true for the group velocity simple oscillator models predict that for the group velocity (see figure on p. 403 of Griffith's). I didn't know that this was also true for the phase velocity. (I know for a fact that the phase velocity is always greater then c inside of a wave guide in a region where it IS transparent but that isn't 'inside' a material.) The sentence as it stands is gobbledygook to me but I only teach optics at the college level. (For one thing the index of refraction is proportional to the squareroot of the dielectric constant which has a different real and imaginary parts then the dielectric constant.) If the statement is true that the phase velocity of light cannot be greater then c except in regions of absorption, in a material at least, then can you please reference it. (I would prefer if you could explain it here as well, but that is only to help me understand it better.) TStein (talk) 17:51, 12 August 2010 (UTC)
(Very roughly speaking) a monochromatic wave of frequency ω propagates like A exp(iωt−ikx) where k = ω/ε(ω); so a unit pulse (which contains all the frequencies) propagates like the sum of all of these waves, which is the Fourier transform of some function of ε(ω). (A signal with a finite duration propagates like the convolution of the propagator of a unit pulse and the original signal.) Now this Fourier transform must vanish for all t < 0 because the signal cannot travel backwards in time, which under certain very reasonable hypotheses is equivalent to the condition that ε(ω) verify the Kramers–Kronig relations. As Rias said, the point of saying this is showing that some stringent conditions must be verified if the phase velocity has to be larger than c. A. di M. (talk) 08:46, 12 August 2010 (UTC)
I was so excited that you were going to explain why the 'phase' velocity (and not just the group velocity) must be in an absorption region to be greater then c. I am following you until you get to "the condition that ε(ω) verify the Kramers–Kronig relations" and then you end it as if I was suppose to know this proves that the phase velocity cannot exceed c without being in an absorption region. I am not an expert on this, unfortunately, I have some vague knowledge about that the Kramers-Kronig relations are but that is about it. If you have the time to explain it (or give a reference) on the level of someone who teaches one semester of E&M and one of optics, I would greatly appreciate it.TStein (talk) 17:51, 12 August 2010 (UTC)
I remember the book by Milonni having a very good exposition on this subject. Unfortunately, it is no longer on preview on google books, but maybe your local university library has a copy. TimothyRias (talk) 08:06, 16 August 2010 (UTC)
The Toll reference in the article has a fairly technical treatment of the subject. I think I understand the origin of your objection though. Although the Kramers–Kronig relations imply that if the phase velocity at some frequency is to be greater than c, then there has to be absorption at some frequencies, they do not require those energies to be the same. The paragraph in question simply discusses the conditions under which the refractive index can be smaller than 1, in which context the made statement is true and relevant: The refractive index can only be smaller than 1 if there is sufficient absorption to prevent superluminal signalling. TimothyRias (talk) 08:55, 16 August 2010 (UTC)
Thanks for explaining that. Hopefully, I will have the time to look up that reference. My knowledge, in that area of optics is, sadly, superficial. My impression had always been that the phase velocity does not carry information. So it never bothered me if the phase velocity is "superluminal". TStein (talk) 16:08, 16 August 2010 (UTC)
The thing is more about mathematics (in particular, Fourier analysis) than optics. Any signal can be analysed as a superposition of many monochromatic waves of different frequencies; in a Dirac delta they all have the same amplitude and, at t = 0, x = 0, they all have the same phase: at t = 0 they cancel out everywhere except at x = 0. As the signal propagates, each frequency changes its phase in a different way so that the shape of the signal changes, but all the waves must cancel out for all t′ < 0 (where t′ is tx/c), because the "full" signal can't travel backwards in time. Now, the shape of the signal as it propagates is just the Fourier transform of some function of the phase velocity evaluated at t′, and the Fourier transform of a function vanishing for all negative arguments (i.e. a function written as $\hat f(t)=\int_{-\infin}^{+\infin}f(\omega)\exp(i\omega t)d\omega$ where f(t) vanishes at all negative t so that the lower bound of integration can be taken to be 0 can be shown to verify Kramers–Kronig relations, by some theorems of complex analysis. (I've probably messed up with the signs, which anyway depend on the notation used.) A. di M. (talk) 11:46, 17 August 2010 (UTC)
In some textbooks this is introduced to students by asking if by wearing sunglasses you can see the future. Count Iblis (talk) 15:22, 22 August 2010 (UTC)

## Quantum mechanics FTL

In some interpretations of quantum mechanics, certain quantum effects may seem to be transmitted faster than c It sounds like this is some sort of minority or controversial view, but instantaneous quantum wave collapse is part of the orthodox, or Copenhagen, interpretation. Kauffner (talk) 13:34, 21 August 2010 (UTC)

Actually, what is orthodox is to shut up and compute :) .Count Iblis (talk) 13:57, 22 August 2010 (UTC)
I agree with you both... A. di M. (talk) 14:53, 22 August 2010 (UTC)
I think the "In some interpretations of quantum mechanics" part of the sentence can be left off. It is misleading anyway, since in the case of given example, nonlocal correlations exist no matter the interpretation of QM you use.TimothyRias (talk) 15:37, 22 August 2010 (UTC)

A requirement of WP:LEAD is that the lead should provide an accessible overview of the rest of the article. Does the current lead do that adequately?TimothyRias (talk) 09:44, 10 August 2010 (UTC)

### First paragraph

"The speed of light, usually denoted by c, is a physical constant important in many areas of physics. Light and all other electromagnetic radiation always travel at this speed in vacuum, regardless of the motion of the source or the inertial frame of the observer. Its value is exactly 299,792,458 metres per second[1] (approximately 186,282 miles per second). In the theory of relativity, c connects space and time, and appears in the famous equation of mass–energy equivalence E = mc2.[2] The speed of light is the speed of all massless particles and associated fields in vacuum, and it is predicted by the current theory 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."

In addition to introducing the topic this gives a fairly accessible overview of the first two section ("notation" and "fundamental role in physics"). IMHO this paragraph is currently fine.TimothyRias (talk) 09:44, 10 August 2010 (UTC)

I agree. Let's not mess with this paragraph. I am certain that it can be tweaked to improve it, but any given tweak is much more likely to make it worse rather than making it better. TStein (talk) 18:25, 12 August 2010 (UTC)
"Inertial frame" is not exactly the kind of phrase I would use in the second sentence of a lead, per WP:MTAA. I'm going to link it, but I'll try to find a more accessible way to word the "regardless" clause. A. di M. (talk) 15:01, 22 August 2010 (UTC)

The measurements concerning the determination of the velocity are those involved in the determination of an unknown variable, and the determined value is given in the discussion as 299,792,458.2+- (1.1). Then this value is rounded to 299,792,458, and declared to be an exact limiting value. This creates a problem in that the precision related to the determination of a variable value and the tolerance related to meeting limit values do not involve the same statistical analysis test procedures, and might give different (although minute) results.WFPM (talk) 03:03, 26 August 2010 (UTC)

I'm not sure what point you are trying to make with respect to the whether the lead covers contents of the rest of the article.TimothyRias (talk) 20:40, 30 August 2010 (UTC)

What I am getting at is that there are statistical mathematics differences between the determine of the value of a variable and its accuracy confidence limits as compared to that of determining a limiting value of a variable. Statistically, it is impossible to determine the exact value of what is considered to be a variable.WFPM (talk) 16:50, 30 September 2010 (UTC)

They did not take one value declare that to be exact. The changed the definition of the metre (i.e. changed its length, although within the range it could be realized) with the effect of giving c an exact value. Within the SI c is no longer a variable.TimothyRias (talk) 18:01, 30 September 2010 (UTC)

I appreciate the problem and the way they handled it. It's just that the statistical method of determining the best estimate of a variable, as would be used for the velocity of light would never arrive at an exact value. So using a 9 digit (base 10) number as a constant related to the length of the meter is OK until the same value is declared to be an exact determination of the velocity of light in a vacuum. I'm surprised that they haven't done that to the value of Pi.WFPM (talk) 03:13, 1 October 2010 (UTC)

c is a dimensionful constant so its numerical value can be changed by changing the units of measurement, whereas π is a dimensionless constant so its numerical value doesn't depend on anything. A. di M. (talk) 12:21, 2 October 2010 (UTC)

Fascinating!! Well the French were the people who dreamed up the idea of the meter in the first place. So I guess they can define it anyway they want. But I'm a Missourian, and therefor don't like the idea of being told that a variable is a constant, so there!! And I like the word "dimensionful" as opposed to dimensionless, but I couldn't find it in my Webster's.WFPM (talk) 18:02, 2 October 2010 (UTC)

c is a constant according to special relativity, and it was a constant even before the metre was defined this way. A. di M. (talk) 18:09, 2 October 2010 (UTC)

Got nothing against constants as tools and even as the limiting values for the approximations of variables. But as the actual value of the variable, Nah!! Violates my quality assurance mathematics credo, which is never to take anything for granted. That feeling gives me trouble with some of the paradoxes of special relativity, like the twin paradox. But don't have the time to worry about that.WFPM (talk) 17:53, 3 October 2010 (UTC)

### Second paragraph

"The speed at which light propagates through transparent materials, such as glass or air, is less than c. The ratio between c and the speed v at which light travels in a material is called the refractive index n of the material (n = c / v). For example, for visible light the refractive index of glass is typically around 1.5, meaning that light in glass travels at c / 1.5 ≈ 200,000 km/s; the refractive index of air for visible light is about 1.0003, so the speed of light in air is very close to c."

This basically repeat the first paragraph of the "in a medium" subsection of the "propagation of light section". I feel it should be more inclusive of the other things discussed in that section. Moreover, I feel that it currently is too detailed and uses too many numbers and equations making it less accessible. This paragraph could definitely use some work.TimothyRias (talk) 09:44, 10 August 2010 (UTC)

I agree. I would try something on the order of
"The speed at which light travels through transparent materials is less than c and varies from material to material. For example, light travels at around 0.67c in glass and 0.75c in water. In materials, though, the movement of a light-waves cannot be described by just one 'speed of light'. The phase velocity, vp, is most often used to represent the 'speed of light' in a material and represents the speed at which a particular 'ripple' travels. The group velocity, vg is the speed at which a pulse of light travels and in most cases represents the speed that information carried by light-wave travels as well. The phase velocity in a material is most often represented in terms of an index of refraction n which equals c/vp. Both the group and phase velocities are typically different then c in a material and can even be greater then c. In no case where either of these velocities are greater than c is causality violated, since no 'information' is being carried at these velocities in these circumstances."
That is definitely better than the current paragraph. I do think it puts a little too much emphasis on the concepts of group and phase velocity. We need to explain what these are in the main text in order to be able to express the point relevant to this article: light does not always propagate with velocity c. For example, I don't think it is necessary to mention the refractive index at all in the lead. A condensed form:
"The speed at which light travels through transparent materials is less than c and varies from material to material. For example, light travels at around 0.67c in glass and 0.75c in water. In some materials different parts of light waves travel at different speeds. The phase velocity represents the speed at which individual 'ripples' travel, whereas the group velocity is the speed at which complete light pulses travel. Both the group and phase velocities are typically different then c in a material and can even be greater then c. In no case where either of these velocities are greater than c is causality violated, since no 'information' is being carried at these velocities in these circumstances."
TimothyRias (talk)
I like it. I only wish I could have come up with a better way of explaining the phase velocity. I hate using the term 'ripple' since it is non-technical. Using the term 'wave' is worse though since it refers to the entire wave and not just one ripple. TStein (talk) 16:15, 16 August 2010 (UTC)
IMO, mentioning the distinction between phase velocities and group velocities in the lead is unnecessary. I don't think we need to explicitly say that it depends on the frequency, either: if it didn't we wouldn't be saying "for visible light", after all. If the problem is too many numbers and equations, what about:
The speed at which light propagates through transparent materials, such as glass or air, is usually less than c, and depends on the refractive index of the material. For example, for visible light in glass it is typically around 0.67c ≈ 200,000 km/s, and in air it is about 0.9997c.
A. di M. (talk) 15:13, 22 August 2010 (UTC)
I think it is relevant to mention that speed at which light can propagate in materials can be both smaller and larger than c.TimothyRias (talk) 15:41, 22 August 2010 (UTC)
Larger than c?.WFPM (talk) 18:30, 30 August 2010 (UTC)
Yes. See relevant section of the article and the refs therein.TimothyRias (talk) 20:37, 30 August 2010 (UTC)
I was under the impression that light involved an electromagnetic carrier wave of an appropriate frequency, and didn't have to be modulated.WFPM (talk) 02:24, 31 August 2010 (UTC)And that the refractive index was related to the frequency of the carrier wave as well as the involved material. Like in the refraction of sunlight through Newton's prism.
Again, I'm not sure what point you are trying to make with respect to whether the lead adequately covers the content of the article. (Which is what is being discussed in this section. If you think that the article contains errors it is better to open a new thread.) TimothyRias (talk) 08:03, 31 August 2010 (UTC)
It depends by what you mean by "have to", but an unmodulated wave would fill the whole spacetime with constant frequency and wavevector, and I haven't ever seen such a thing yet. A. di M. (talk) 09:22, 31 August 2010 (UTC)
I thought that each individual light spectrum consisted in an unmodulated constant frequency carrier wave, which was propagating (transporting) the energy being dissipated at that frequency.WFPM (talk) 09:43, 31 August 2010 (UTC)
A wave which doesn't last forever is modulated, even if the modulation might be a trivial rectangle function equalling zero before the light is turned on and after it's turned off and a constant in between. A. di M. (talk) 11:31, 31 August 2010 (UTC)
Granted that the carrier wave can be turned on and off intermittently, but that system of communication hasn't been utilized since the Morse code days. And when it's on its a constant frequency carrier wave, which can be modified by the two methods, 1:Group modulation of amplitude, (with FM modulation not under discussion), or 2:Associated with additional EMR waves as per the principles of the Fourier series analysis methods.WFPM (talk) 17:01, 31 August 2010 (UTC)
So what? A. di M. (talk) 17:06, 31 August 2010 (UTC)
Very good question!! But the subject matter is the speed of light. And I'm the first atom of a new media of propagation and I'm trying to figure out what it is that's going to go by me at a speed faster than the velocity of the incoming carrier wave/particle. And I can't think of anything tangible.WFPM (talk) 18:43, 31 August 2010 (UTC)SeeTalk:Refraction

### Third paragraph

"Ole Rømer first demonstrated in 1676 that light travelled at a finite speed (as opposed to instantaneously) by studying the apparent motion of Jupiter's moon Io. After centuries of increasingly precise measurements, in 1975 the speed of light was known to be 299,792,458 m/s with a relative measurement uncertainty of 4 parts per billion. In 1983, the metre was redefined in the International System of Units (SI) as the distance travelled by light in vacuum in 1⁄299,792,458 of a second. As a result, the numerical value of c in metres per second is now fixed exactly by the definition of the metre.[3]"

This paragraph effectively summarizes the contents of the final section ("History"). I think it is probably adequate.TimothyRias (talk) 09:44, 10 August 2010 (UTC)

I almost agree. It would be nice to have a sentence that explained how Einstein showed that c was not just the speed that light travels in a vacuum, but more importantly is a property of space time; light happens to travel at that speed because it is mass-less. TStein (talk) 18:40, 12 August 2010 (UTC)
"In 1905, Albert Einstein postulated that the speed of light in vacuum was independent of the source or inertial frame of reference, and explored the consequences of that postulate deriving the Special theory of relativity and showing that the parameter c had relevance outside of the context of light."
Or at least something like this.TimothyRias (talk) 09:08, 16 August 2010 (UTC)
I don't think that it could be said any better. TStein (talk) 16:20, 16 August 2010 (UTC)
I like it too. I'm going to add it (modulo minor tweaks). A. di M. (talk) 15:32, 22 August 2010 (UTC)

### Subjects not covered in the lead

As it stands the basically the entire contents of sections 4 and 5 ("practical effects of finiteness" and "Measurement") are not touched upon. (in addition to some of the points from the third section mentioned above). Something should be said about the contents of these sections, though I'm not sure what. The lead is allowed to be at most 4 paragraphs so we could add an additional paragraph between the current second and third paragraphs that summarizes these sections. Any ideas? TimothyRias (talk) 09:44, 10 August 2010 (UTC)

Personally, I don't think that either of those sections need to be in the lead. Measurement is at least touched on in the history paragraph. If you wanted a paragraph similar to what was there before could be added:
"In most practical cases, light can be thought of as moving instantaneously, but for long distances and very sensitive measurements the finite speed of light can have noticeable effects. In communicating with distant satellites it can take minutes to hours for the message to get from Earth to the satellite and back. The light we see from stars left the star years to centuries ago, even millions of years ago for the most distant object visible to the naked eye. Large telescopes, can see light that left distant objects billions of years ago. The finite speed of light also limits the theoretical maximum speed of computers since information must be sent within the computer chips and from chip to chip. Finally, the speed of light can be used with time of flight measurements to measure distances to high precision."
This is only a quick example. It is probably too wordy, but it is the kind of thing that can be stated in that fourth paragraph.TStein (talk) 19:03, 12 August 2010 (UTC)
It should be "large distances" in the last sentence, to avoid giving them impression that time-of-flight methods are the preferred method to measure distances of the order of one metre or smaller. Physchim62 (talk) 19:39, 14 August 2010 (UTC)
I think some paragraph along those lines would be in order. I agree that this might be a little too wordy though.TimothyRias (talk) 09:11, 16 August 2010 (UTC)
I agree it is too wordy. It was only a first iteration. TStein (talk) 16:20, 16 August 2010 (UTC)
The two sentences starting with "The light we see.." might be shortened to: "The light we see from stars left them many years ago." A. di M. (talk) 15:46, 22 August 2010 (UTC)

How about "The light we see from the stars left each of them many years ago", for emphasis as to the individuality of the source of light energy from each star?WFPM (talk) 17:01, 30 September 2010 (UTC)SeeWhirlpool Galaxy

#### Fourth paragraph draft

"In most practical cases, light can be thought of as moving instantaneously, but for long distances and very sensitive measurements the finite speed of light can have noticeable effects. In communicating with distant space probes it can take minutes to hours for the message to get from Earth to the satellite and back. The light we see from stars left them many years ago, allowing us to study the history of universe by looking at distant objects. The finite speed of light also limits the theoretical maximum speed of computers since information must be sent within the computer chips and from chip to chip. Finally, the speed of light can be used with time of flight measurements to measure large distances to high precision."

How about this as a draft for a fourth paragraph. If anybody has further tweaks please do not hesitate to add them to the above text.TimothyRias (talk) 07:29, 23 August 2010 (UTC)
Looks good. Let's add it to the article if no-one objects in 24 hours... A. di M. (talk) 09:02, 23 August 2010 (UTC)
Much better then mine. Lets get it in. Any further tweaks can be done in the main article. To be honest, though, any further tweaks are more likely to harm it then help. TStein (talk) 16:25, 23 August 2010 (UTC)

## Speed of light in a medium

Some time ago user:TStein made the following edit to the "In a medium" section in an attempt to make it clearer. I've been looking at it, and am not sure that he succeeded. This may be just me, but he basically looses me at "Imagine shining a light continuously with a bright pulse of light in the middle." It is not clear to me what I'm asked to imagine.

Do other editors think this is clear? I do sort of like the structure of first briefly explaining what the phase and the group velocity are, before giving details on how these can vary. (Although it does sort of obscure the third relevant velocity of light in a medium, the front velocity, which is the only one to actually equal c.)TimothyRias (talk) 08:14, 24 August 2010 (UTC)

I agree that it needs a lot of work. My goal was to give a physical picture of what group and phase velocity is without obscuring it in too much mathematical detail. I agree with you that in at least some respects I failed at that. It is a hard task, though. Personally, I think we need a "movie" that plots how a wave pulse moves in time with locations marked which represent the phase, group, and front velocities. What I tried to describe is a AM signal where the information carried is a single 'pulse'. (For example, the pulse might be where the amplitude is doubled for a few periods of the carrier wave or so.) In this example, the phase velocity is essentially the speed of the carrier wave and the group velocity is how fast the center of the pulse travels. (The pulse will broaden and flatten, though due to dispersion.)
I also agree that the last sentence in the first paragraph is misleading and mixes in front velocity which isn't described later. I am simply too swamped now to do too much on any of this, unfortunately. TStein (talk) 14:38, 24 August 2010 (UTC)

By movie I expect you mean something like this? It is group and phase velocity only. Making one with front velocity as well might be a bit hard, though.TimothyRias (talk) 15:06, 24 August 2010 (UTC)

That movie is pretty close to what I was looking for. I find it curious that that is uses a peak for the phase velocity and a node for the group velocity. Personally, I would prefer to use this over my kludged together description. In order to include the front velocity you would probably need more horizontal space and repeat the movie. TStein (talk) 15:22, 24 August 2010 (UTC)

I tried to fix the first paragraph. I made it a little too wordy I think, but it is easier to cut words then it is fix the concept. I also found a better description of what I was trying to say with the example I use in the second paragraph. The following quote is from front velocity.

"For definiteness, consider an amplitude modulated electromagnetic carrier wave. The phase velocity is the wave speed of the carrier. The group velocity is the wave speed of the modulation or envelope. Initially it was thought that the group velocity coincided with the speed at which information traveled. However, it turns out that this speed can exceed the speed of light in some circumstances, causing a conflict with the theory of relativity. That observation led to consideration of what constitutes a signal."
This is understandable! Except that it limits the speed of the "phase velocity" to be that of the carrier wave, and doesn't explain how anybody can change the modulation system such as to cause the (AM?) modulation envelop to advance along the carrier wave.WFPM (talk) 09:57, 31 August 2010 (UTC)Note that an AM modulator is a FIFO (first in first out) device that processes each wave of the carrier as it goes by, in some time based modulation frequency manner. Also note that the red dot, which is the phase velocity, is always in the same carrier wave, and therefor moving at the same velocity, and is moving twice as fast as the groups. And the key thing to notice is that at the end of the moving images display, everything is associated with the advancement of the last cycle of the carrier wave off the board, so that point everything is advancing at the velocity of the carrier wave! And can you do it some other way?WFPM (talk) 03:23, 2 September 2010 (UTC)
Not being a great expert in signal processing I can't understand the point you're trying to make, but I think it strays faaar away from the scope of this article. A. di M. (talk) 11:34, 31 August 2010 (UTC)
WFPM, I still fail to see what your point is with relation to improving the article. TimothyRias (talk) 08:55, 2 September 2010 (UTC)
Well okay! I was just pointing out that if I'm sitting at the delivery end of your moving image, I note that everything that is coming off the end is moving at the velocity of the red dot (Phase Velocity?) or slower, and that the carrier cycles, which are moving at the same speed as the red dot are having to crawl through the nodes of the variable amplification groups in order to advance at the same velocity of the red dots. And I thought that was supposed to be an explanatory moving image.

Obviously, not all of this needs to be said here. Also, I prefer not to just dump a lot of technical words on the reader at once. Terms like 'amplitude modulated', 'carrier wave', 'modulation', 'envelope', and 'information' make a lot of sense to us but need appropriate explanation in a non-technical article like speed of light. TStein (talk) 15:24, 24 August 2010 (UTC)

I agree that using a wall of technical terms is generally unhelpful. The risk of avoiding the technical terms is becoming overly vague, especially when appealing to intuition about a subject that might not be present with the reader. I have the feeling we are entering the territory of the latter here. I mean, is all this real that much clearer than just saying: "The phase velocity is the speed of a plane wave of constant frequency and the group velocity is the speed of a pulse." (or some variant thereof)
Since this article is not really about phase and group velocities, I sort of lean towards keeping it simple and concise and move towards something in the spirit of the latter short description. (Maybe supported by the image above.) This has the advantage of not burdening a reader that he should now understand the difference between phase and group velocity, he can just go on and take the message that there are two different speeds of waves for granted. In the end that is the only message that is real relevant to this article. (And of course that both these velocities can be smaller or greater than c.)TimothyRias (talk) 08:56, 25 August 2010 (UTC)
The main issue is that if the reader doesn't know that any signal can be decomposed as a sum (or integral, if you take spacetime to be unlimited) of monochromatic waves, there's no way they could get the point. So I would propose something along the lines of:
In a medium, there are several possible definition for the speed of wave. The speed at which a plane wave (a wave filling the whole space, with only one frequency) propagates is called the phase velocity vp. An actual physical signal with a finite extent (e.g. a pulse of light) can be taken as the sum of many plane waves with different frequencies, cancelling out everywhere except in the region of the signal. In materials where the phase velocity depends on the frequency, these plane waves travel at different speeds and the pulse undergoes dispersion (smears out) as it propagates. The largest part of the pulse travels at the group velocity vg, and its earliest part travels at the front velocity vf.
A. di M. (talk) 10:53, 25 August 2010 (UTC)
Sounds sensible to me. Might be a good idea to start of by saying that light in a medium does not always travel at c. This helps make clear why this article is discussing this in the first place.TimothyRias (talk) 11:29, 25 August 2010 (UTC)
But it never travels faster than c!!WFPM (talk) 12:54, 2 September 2010 (UTC)
I like the spirit of A. di M. proposal. I am certainly ok with dumping a good portion of the description of phase and group velocities. Too, I am not wedded to any of the stuff I wrote. I was merely trying to start a conversation. My main difficulty with A. di M. proposal is that the concepts of 'plane waves' and 'adding them up to cancel everywhere except where the pulse of light is' is, in my opinion unneeded and confusing to the average reader. I think that the ideal situation is where you have a animated diagram where the group, phase, and maybe front velocities are shown. You would only need a short description plus two or three sentences explaining what the phase and group velocities are used for. One or two sentences can be used to introduce the index of refraction, since that is the most common way to indicate the speed of light. Another couple sentences can be used to state that both the group and phase velocities can be greater then c in certain situations where information is not carried at those velocities. All the rest can safely be excised from this article as being covered in the main articles of phase velocity, group velocity, and front velocity. TStein (talk) 18:09, 25 August 2010 (UTC)
Would the version in this sandbox be acceptable to everybody? If not please feel free to make edits to that sandbox. I've tried to take the suggestion above and condense the rest a bit. The basic structure now is:
• First paragraph: intro of different types of wave velocity with image.
• Second paragraph: Reflective index with some common values.
• Third paragraph: Conditions under which phase velocity may exceed c
• Fouth paragraph: slow light, superluminal group velocities
• Last paragraph: Front velocity is the limit of information transfer.
TimothyRias (talk) 08:10, 30 August 2010 (UTC)
It looks all right. A. di M. (talk) 19:18, 30 August 2010 (UTC)

I've made an attempt at an animation that shows the front velocity as well as the group and phase velocities. It still needs to be tweaked (I don't think it looks too good at the moment). But I'd like the opinion of other editors if this is conceptually clear.TimothyRias (talk) 09:05, 2 September 2010 (UTC)

Maybe the line representing the wave itself should be thicker, and you could use only one dot for the phase velocity. A. di M. (talk) 09:32, 2 September 2010 (UTC)
I've updated the image.TimothyRias (talk) 12:53, 2 September 2010 (UTC)
Now you've introduced a variable group size factor into your variable amplitude modulation system. And You've introduced two non-tangible points related to the modulation profile. The first is the point of beginning or "front", and which is a non-entity, because it has to be deduced, and the second is evidently a time based notation of the modulated group midpoint, which is also deduced. But neither of these involve the passage of a tangible entity past the end point at a velocity greater than that of the carrier wave.WFPM (talk) 13:49, 2 September 2010 (UTC)
You are very much mistaken. The front is the first point of the wave you could possibly detect. Before this point has reach you, you cannot know anything about the wave.The speed at which this point moves is the front velocity and for electromagnetic waves this is always equal to c.
The maximum of the envelope is indeed not observable as such, but its speed is representative for the speed of the pulse, the group velocity. The speed of the pulse, is relevant for signaling purposes since it is often effectively the speed at which a signal is sent. It is after all, the maximum of a pulse on which receivers typically trigger.
But please don't take my word for it. This can all be found in the sources linked in the article.TimothyRias (talk) 14:11, 2 September 2010 (UTC)
Okay. So the red point in this instance is the front (first) point of detection of the Carrier wave, and the green point is a lesser velocity and non-detectable center point of the modulation wave, and the blue is the last point of the modulation wave (which is also undetectable) And nothing is going by except the material/energy content of the carrier wave. Is that correct?WFPM (talk) 16:42, 2 September 2010 (UTC)
The red point is indeed non-detectable because any detector will have a finite threshold, but it is the upper limit to the velocity at which the wave could be detected. The energy content of the wave is proportional to its square magnitude, so the bulk of it moves at the speed of the green dot. A. di M. (talk) 17:49, 2 September 2010 (UTC)
But you're implying that there's something more going by than the contents of the modulated carrier wave, and I cant see anything else going by; just that the modulation program causes more or less of it to be going by (at the carrier velocity) at any instant. Therefor the volume has been changed by the modulator, but the speed hasn't.WFPM (talk) 19:21, 2 September 2010 (UTC)
Imagine the detector hasn't sufficient resolution to distinguish the individual peaks (or that the wave is circularly polarized and the detector isn't sensitive to polarization direction), which is pretty much always the case for visible light and higher frequencies: then all you can detect is the envelope, which is moving at the speed of the green dot. A. di M. (talk) 09:27, 3 September 2010 (UTC)
That's what detectors do, detect the modulation envelope. Okay, so you can detect/deduce the modulation envelope. But nothing went past you faster than the speed of the carrier wave particles that made up the (modulated) carrier wave.WFPM (talk) 13:12, 3 September 2010 (UTC)
And another interesting thing to note about this discussion is that whereas we have time to wait and note the midpoint and end of the modulation envelope, I, as the first atom of the refractive medium, only have the time until the second carrier wave to decide at what angle to refract the propagation direction of the wave, based on its frequency of occurrence. And I assume that the modulation of light does not effect its index of refraction.WFPM (talk) 17:31, 2 September 2010 (UTC)
The refracted wave is a collective phenomenon, the sum of the original wave and all what the atoms do, which add up in a way that the normal-to-the-constant-phase-surfaces (averaged over a mesoscopic volume) is bent wrt the original one as described by Snell's law. A. di M. (talk) 17:49, 2 September 2010 (UTC)But the refraction process has to be able to discriminate and differentially bend the lights of the different spectrum colors as it did through Newton's prism, which I assume is a multi-atom process, since I don't think that a single atom bent them all.WFPM (talk) 13:12, 3 September 2010 (UTC)
So what? A. di M. (talk) 14:32, 3 September 2010 (UTC)

Well I wont discuss refraction any more here. And I didn't understand what you said in the previous, But I don't see anything in an EMR transmission that moves faster than the frequency carrier beam.WFPM (talk) 14:49, 3 September 2010 (UTC)

## Lorentz Model?

The discussion of light propagating through a medium lacks any mention of the Lorentz model.

http://nlo.optics.ucf.edu/hagan/OSE5312spring2008/OSE5312%20slides%20class%2015%20-%20Dipole%20radiation.ppt#944,1,Microscopic picture of refractive index and absorption

I found this classic description of light propagation very useful and powerful during my Optics MSc course. 85.189.92.106 (talk) 14:09, 25 August 2010 (UTC)

## "Historical references" and "modern references"

The references section currently has to sections ("Historical references" and "modern references") with general references. This seems to be fairly incoherent selection of links. IMHO the inline citations give adequate references about just about any topic covered in this article. As such, I think these sections can be safely deleted. Do others object?TimothyRias (talk) 15:45, 13 October 2010 (UTC)

## Is there too much detail about relativity in the article?

Above user:TStein remarked that he (or she?) thinks that there is too much detail about relativity in the current article. Since this article is longish this is a concern we should look at seriously, the issue of length is bound to appear at FAC anyway. Most of the relativity stuff is mentioned in the Fundamental role in physics section. Are all the topics discussed here essential to this article?

The essential point to make (IMHO) are:

1. Speed of light is invariant. (and experimental limits on any variance)
2. Invariance of c leads to special relativity.
3. Special relativity implies that massless objects move with velocity c and massive objects move slower than c.
4. The role of c in the structure of spacetime/Lorentz invariance leads to c showing up in theories that assume Lorentz invariance in particular general relativity and quantum field theories.
5. Relation of c to causal structure. (i.e. moving faster than c implies going backwards in time in some other frame.

Looking at this list I'm not convinced that there is too much detail, but some trimming might be possible. For example, we could maybe save some space by merging the paragraph about relativity of simultaneity with the last paragraph of the "upper limit on speeds" subsection. But I'm more interested to know what other users (especial TStein) think.TimothyRias (talk) 09:28, 2 September 2010 (UTC)

I agree with all you said. BTW, it's a pity that the article Introduction to special relativity sucks that much. A. di M. (talk) 09:36, 2 September 2010 (UTC)
For what my opinion is worth, it looks good to me. A little organization (possible subsections), etc. might not hurt. But overall I am excited about how far it has come. TStein (talk) 18:46, 22 October 2010 (UTC)

## Fizeau's experiment and the Weber constant

Timothy, I notice that you inserted the word 'approximately' into the sentence relating the equivalence of the measured speed of light to the Weber constant. We all know that the equivalence wasn't exact. But don't you think that perhaps it might be more accurate to use the term 'close to' rather than 'approximately'? The word 'approximately' in the context tends to somewhat play down the significance. Have a look at the exact words that Maxwell uses when making the comparison. You can find these words on page 49 of the pdf file here [5]. Maxwell uses the Weber constant in the Newton equation (132), and then he cites the Fizeau result further down. Maxwell's exact words are,

The velocity of transverse undulations in our hypothetical medium, calculated from the electro-magnetic experiments of M.M. Kohlrausch and Weber, agrees so exactly with the velocity of light calculated from the optical experiments of M. Fizeau, that we can scarcely avoid the inference that light consists in the transverse undulations of the same medium which is the cause of electric and magnetic phenomena.

Don't you perhaps think that your use of the word 'approximately' in the context is somewhat of an understatement? David Tombe (talk) 15:57, 22 October 2010 (UTC)

Actually "approximately" sounds stronger than "close to" to me. Anyway, I'm replacing it with "equal within measurement uncertainties"; is it OK? A. di M. (talk) 16:23, 22 October 2010 (UTC)

A.di M. That's fine now thank you. David Tombe (talk) 20:10, 22 October 2010 (UTC)

## Note 1

I have removed the words 'Strictly speaking' from note 1 note 3 in 'Fundamental role in physics' as unencylcopedic. I still question why this well-accepted and referenced fact is hidden away in a note. It is an important point about the very subject of the article and should, in my opinion, be in the body of the text. What is the rationale for hiding it away? Martin Hogbin (talk) 10:07, 23 October 2010 (UTC)

To be clear, you are refering to note note 3 (the second note in the 'Fundamental role in physics' section) which says:
"It is only possible to experimentally verify that the two-way speed of light (for example from a source to a mirror and back again) is frame-independent, since it is impossible to measure the one-way speed of light (for example from a source to a distant detector) without some convention as to how clocks at the source and at the detector should be synchronized. However, by adopting Einstein synchronization for the clocks, the one-way speed of light becomes equal to the two way speed of light by definition."
Sorry, note number corrected above. Martin Hogbin (talk) 09:35, 24 October 2010 (UTC)
I think the reason this was moved to a note was accessibility for a general audience. The point made in the note is fairly technical and needs quite a bit of explanation. Putting this in the main text would significantly disrupt the narrative there. And, with the amount of extra explanation it would need there, would borderline issues with undue weight.TimothyRias (talk) 15:40, 23 October 2010 (UTC)
I cannot see how two sentences on a fact directly concerning the subject of the article could be called undue weight. The subject is no more technical than much of the rest of the article although some might consider it philosophically provocative. I do not think we should duck such issues in the interests of 'accessibility for a general audience'; this is an unnecessary form of self-censorship. Martin Hogbin (talk) 09:35, 24 October 2010 (UTC)

## The Pierre Duhem Error

Regarding FyzixFighter's insertion of a recent Dan Siegel reference, there has indeed been an allegation made by some that Maxwell fudged his results in his 1861 paper. But if he did, and I'm not altogether sure whether he did or not, then he will have omitted a factor of 2 on two different occasions in the elasticity section in part III, such that the two errors mutually cancelled. But if we are going to draw attention to the Pierre Duhem error in the reference section, it should be worded in such a way as to merely draw the matter to attention rather than to make out that the allegation is undoubtedly true. I can assure you, that having looked into this matter in the greatest detail, that the 'factor of 2' issue is not an easy issue to resolve and it involves having to second guess the nature of the elastic solid. And since the critics never knew that either, it's hard to figure out how they are so certain that Maxwell made an error. And if Maxwell was wrong, it was only a trivial error in relation to wider body of that particular piece of work. As regards stating categorically that Maxwell's numerical match was a lucky coincidence, that is a matter that can be left for the readers to judge, because the coincidence actually occurred at the moment when Weber and Kohlrausch did their Leyden jar experiment in 1856, and not when Maxwell used that result in the elasticity section in his 1861 paper.

At any rate, in case there are readers here would like to know more about the Pierre Duhem allegation, have a look at equation (81) in Maxwell's 1861 paper which you can see here [6]. The allegation by Pierre Duhem is that Maxwell should have included a factor of 2 in equation (81) which relates to the coefficient of rigidity (which is taken as equivalent to the transverse elasticity in Newton's equation at equation (132)). If anybody agrees with Pierre Duhem's allegation, I'd be interested to hear their reasoning.

FyzixFighter's footnote also talks about Maxwell's 'aether independent' paper of 1864/65 which is apparently all correct. But let's see if Maxwell's 1864/65 paper really is 'aether independent'. At the top of page 498 (page 40 of the pdf file),[7] we can see how the displacement current is central to the derivation of the EM wave equation. And Maxwell writes,

If the medium in the field is a perfect dielectric - - -

from which he then draws the conclusion that we can drop the 'free current' terms and work only with the displacement current terms. I see no evidence whatsoever that Maxwell's 1864 paper was aether independent. This again highlights the importance of comparing primary sources with modern secondary sources. David Tombe (talk) 23:04, 23 October 2010 (UTC)

## A suggestion about the faster than light section.

Before I say anything else, I would like to say that I am excited about how much this article has improved. I wish I had more time to contribute seriously to the peer review.

One small thing that I thought was a little tedious was all of the sentences in the faster than light section saying 'but it does not carry information[link].' Don't get me wrong I like the list all of the various ways that things go faster then light and we definitely need the links to the evidence that it doesn't go faster than c. One possible way to simplify this section is to condense the exceptions into one or two paragraphs eliminating the superfluous 'it doesn't carry information' then create a table with the name of the exception, very brief description, and reason with link for not violating information traveling at c or slower. I am not quite sure how well it would work without trying it. It may make things worse. (If I was 100% certain it would be better I would just do it myself.) TStein (talk) 21:44, 23 October 2010 (UTC)

You can do that in a sandbox, I guess. A. di M. (talk) 23:11, 23 October 2010 (UTC)
There is definitely room for improvement there. If you could work out your idea in a sandbox somewhere that would be very helpful.TimothyRias (talk) 10:56, 24 October 2010 (UTC)

## Weber and Kohlrausch 1856

Timothy, was your inclusion of the equation 1/√ε0μ0, in connection with the 1856 experiment not slightly premature? It's true that the 1856 experiment is the pivotal point in this entire topic, but it wasn't until Maxwell's 1861 paper that a physical explanation was given for the result in terms of the equation 1/√ε0μ0, with Maxwell of course thinking in terms of Newton's equation for the speed of sound (equation 132). It's very hard to get definitive information on the chronology of events surrounding that period. I have heard totally conflicting stories. Some say that Weber and Kohlrausch didn't even notice their result because it was masked behind a factor of the square root of two. Others say that their result caused them the utmost excitement. Some say that Maxwell knew as soon as he heard about it, that he needed to start working towards a wave equation, while others say that Maxwell was not aware of the result until late 1861 when he was nearly finished part III of his paper. At any rate, we do know that in late 1861, Maxwell travelled from Scotland to London and looked up the details of the Weber/Kohlrausch experiment and equated the result with the ratio of the dielectric constant to the density of his sea of vortices. The Pierre Duhem controversy surrounds the manner in which he linked that ratio to classical elasticity. The equation 1/√ε0μ0, is the modern carry over from that part of history, a skeleton of its former self. So perhaps you maybe ought to just drop that equation altogether in the particular historical context. David Tombe (talk) 16:02, 25 October 2010 (UTC)

Mentioning 1/√ε0μ0 helps explain what quantity Weber and Kohlrausch were measuring to the audience even though the notation may be a little anachronistic.TimothyRias (talk) 19:35, 25 October 2010 (UTC)

Timothy, Yes it does help. But 'anachronistic' is indeed the correct word. What about an explanatory phrase something like, 'which in modern day language translates to 1/√ε0μ0' David Tombe (talk) 19:49, 25 October 2010 (UTC)

## History section

Timothy, You are writing in a history section. You cannot modernize history. Modern teaching is that light is a wave which propagates in empty space, but Maxwell believed that light propagated in an elastic medium. It is totally false to claim that Maxwell believed that light propagated in empty space. Besides, the rest of the section continues by dealing with the controversy surrounding attempts to measure the speed of the Earth through the aether. But you have already written the aether out of history before we even get to that point. It needs to be restored to the fact that Maxwell's theory relates to an elastic medium and not to empty space. David Tombe (talk) 16:54, 25 October 2010 (UTC)

Maxwell believed that empty space WAS (filled with) an elastic medium. What he calculated was the speed of light waves in the absence of the influence of other matter, i.e. empty space or vacuum. His assumptions about the nature of the vacuum later proved to be false, but his theory also proved to be independent of them.TimothyRias (talk) 19:29, 25 October 2010 (UTC)

Timothy, Maxwell calculated the speed of light in a sea of molecular vortices. It is pushing it too far to use the terms 'vacuum' or 'empty space' to describe this sea of molecular vortices. The sea of molecular vortices is no longer part of modern physics but that doesn't mean that it has become equivalent to the vacuum in retrospect. On your latter points, I don't want to get drawn in too much here. It's true that Maxwell's equations today are still used in the absence of the medium that he used to derive them. It's true that modern physics applies Maxwell's equations to free space (or vacuum). But it's pushing it to state in a history section, which is charting a chronology of events involving the aether, that Maxwell's theories in the 1860s were theories in empty space. That comes over very much as historical revisonism. By all means talk about Maxwell's equations in empty space elsewhere in the article. And by all means talk about the modern precision instruments that are used nowadays to measure the speed of light elsewhere in the article. But the history article is supposed to be charting the role of the aether in Maxwell's work and then the events which led up to Einstein's theories and the abandoning of the aether, and that involves both the Michelson-Morley experiment and the Lorentz contraction hypothesis. You cannot write Lorentz out of the history section in preference to the fact that we nowadays have modern precision instruments for measuring the speed of light. You must keep focused on the fact that it is a history section. David Tombe (talk) 20:00, 25 October 2010 (UTC)

See also this and this. Please consider stopping this kind of multiposting. Thank you. DVdm (talk) 20:04, 25 October 2010 (UTC)
I tend to agree with DVdm [8].Biophys (talk) 21:51, 25 October 2010 (UTC)

## Speed of light in a medium

Sorry for joining too late. Two quick and possibly naive comments/questions. (1). It's probably worth noticing that particles can move faster than the speed of light in a medium, as in the case of famous Cherenkov effect, but I did not see this in the article. (2) The equation c^2 = 1/(εμ) tells that if ε and μ are very high (which is hypothetically possible), then relativistic effects could be observed as a speed of moving car (see E. N. Parker, Elementary explanation of Lorentz-Fitzgerald contraction, Am. J. Phys., 36, 156-158 (1968), for example). Is it so? If so, that would be interesting for readers. Biophys (talk) 21:24, 25 October 2010 (UTC)

There was a short paragraph on the Cherenkov effect but it was deleted with this comment,'Might as well do without that paragraph then, because it that form it does not add anything useful' on 2010-01-10. I cannot see why. Martin Hogbin (talk) 08:36, 26 October 2010 (UTC)
The paragraph, that was there did not really explain its relevance to the article. It merely noted that there was such a thing as the Cerenkov effect. Since attempts to rewrite the paragraph in a way that was sensible failed, we (I in particular) decided to get rid of the paragraph altogether.
I would not be opposed to remark in the "in a medium" section of the sort: "The speed of light in a medium does not act as an upperbound for objects moving in that medium. In the case of charged particle this gives rise to the Cherenkov effect." The biggest hurdle to take is to explain what "speed of light in a medium" the remark refers to as the rest of the section makes a big point of different ways in which that can be defined. (And the front velocity IS an upperbound for the speeds at which particles move.TimothyRias (talk) 09:29, 26 October 2010 (UTC)
It is an interesting subject relevant to the 'speed of light' in a general sense. It is also a subject that may cause confusion to readers as it is often described as being due to particles travelling faster than light. I agree that we should take care to ensure that what we write is technically correct. Martin Hogbin (talk) 09:53, 26 October 2010 (UTC)
If you have time, could you draft something?TimothyRias (talk) 09:55, 26 October 2010 (UTC)
(edit conflict) IMO, if we're going to say "In the case of charged particle this gives rise to the Cherenkov effect", we'd have to spend a dozen words or so to say what the Cherenkov effect is, too. A. di M. (talk) 09:56, 26 October 2010 (UTC)

A charged particle (such as an electron) can travelthrough an insulator at a constant speed greater than the phase velocity of light in that medium, but still less than c. In doing so it will emit Cherenkov radiation.[3] Martin Hogbin (talk) 00:28, 4 November 2010 (UTC)

Maybe. Some issues: 1) it suggests that only charged particles can do this. 2)That they can only travel at a constant velocity. Both are not true, any particle can travel at speeds greater than the phase velocity of light in the medium. Only charged particles will emit cherenkov radiation.TimothyRias (talk) 12:25, 4 November 2010 (UTC)

Objects can travel through a medium faster than the phase velocity of light in that medium but still slower than c. When a charged particle (such as an electron) does so through an insulator, Cherenkov radiation is emitted.[4] Martin Hogbin (talk) 13:27, 4 November 2010 (UTC)

Something like that, yeah. I tend to agree with AdiM's remark above that if we are going to say something about cherenkov radiation, we should say something about what it is. TimothyRias (talk) 16:34, 4 November 2010 (UTC)

AdiM says that we should talk about the Cherencov effect, which is what I do. It is the emission of radiation when a charged particle travels faster than the phase velocity of light in an insulating medium. We might say that it is EM radiation often including visible light. Apart form that we have a link. Martin Hogbin (talk) 18:56, 4 November 2010 (UTC)

Well as it currently reads a reader might be left wondering "what is cherenkov radiation". I guess this could be helped by writing something like "...does so through an insulator, an electromagnetic shockwave known as Cherenkov radiation is emitted." This is slightly more descriptive, and at least mentions that is EM radiation and not something else.
Also, if you add this to the article please check for overlinking. I'm pretty sure stuff like electron has already been linked in the article. Per MoS only the first occurance of terms in the main text should be linked.TimothyRias (talk) 20:43, 4 November 2010 (UTC)

## Sayanacharya,

I have removed this value from the 'Measurements' table because it refers to an latter day interpretation of a religious work not a measurement. Martin Hogbin (talk) 09:32, 7 November 2010 (UTC)

## Fundamental basis for understanding the speed of light

We should actually talk about the slowness of matter, not the speed of light. From the perspective of light, there is no time, and it takes just as long to reach any point in the universe. In other words, to light, the universe is infinitely small. As you approach the speed of light, ironically it would appear that the stationery matter around you is speeding up. As a matter of fact - when light travels, by definition it travels through time from an earthling observer's perspective. In this view, what is light in a stationary state? Is a better definition for light not matter that is travelling through time? Ie. matter can not travel at the speed of light unless it is converted into photons or "matter travelling through time" - User:Tunasashimi 196.215.118.171 (talk) 23:50, 23 November 2010 (UTC)

In Arthur Eddington's 'Space, Time and Gravitation', he proposed a model in which time is not uniformly distributed in space. In this model, a point B one light year distant from A is also one year ahead in time. As a result, a wave propagated instantaneously from A to B is observed to arrive one year later. Since it would not be possible to be faster than instantaneous, the observed speed of the wave can't exceed 'c'. The time differential would be based on a local frame of reference, so for a wave reflecting from a mirror at B, A is now one year ahead of B, and the round trip appears to take two years. This model accounted for the fact that a particle with mass requires infinite energy to be accelerated to the velocity of light, since the 'real' velocity of the particle would be infinite in order to appear to move with the velocity of light. It would also account for the observation that 'c' is constant regardless of the velocity of the source.96.54.32.44 (talk) 23:51, 30 November 2010 (UTC)
This view makes much more sense, I think it should be taught and covered more widely. - User:Tunasashimi
The point you make is covered by one of the notes, which mentions that the one-way speed of light is arbitrary, and varies according to the convention used to synchronise distant clocks. As you (and Eddington) point out, the one-way speed of light can be considered infinite with an appropriate clock synchronisation scheme. On the other hand the two-way speed of light (say to a mirror and back again) can be measured using just one clock. The currently used convention is to make the one-way speed equal to the two-way speed (Einstein synchronisation) but this is not the only way of doing things. See One-way speed of light. Martin Hogbin (talk) 18:37, 1 December 2010 (UTC)
Interesting indeed - No reference/easily understandable mention of this in the article either! User:Tunasashimi

## Overwhelming Evidence

There is so much overwhelming evidence cited here that the speed of light is an absolute limit, that I can't help but think that the "speed of light" limit can't possibly be true. Usually human beings pile up overwhelming evidence when deep down they don't believe something and have to convince themselves otherwise.

98.245.150.162 (talk) 23:56, 22 November 2010 (UTC)

## FAC nomination

I've nominated this article for Feature Article status today. Please note that user:SandyGeorgia has asked that the other major contributors co-nominate. I suppose this means user:Martin Hogbin and user:A. di M.. (Although see probably was also referring to Brews, not knowing that he is banned.TimothyRias (talk) 16:36, 6 December 2010 (UTC)

I'll try to give feedback over the week-end. I stayed out of this article for the last year and a half so I would be able to review it impartially. 00:05, 10 December 2010 (UTC)
I have also kept a low profile on this article since the arbcom case. If you need me to do anything please let me know what you want. Martin Hogbin (talk) 18:21, 11 December 2010 (UTC)
Martin: You were a very major contributor, qualitatively as well as qualitatively, and a lot of your content remains in the article. You deserve to be proud of your contributions, and there is no reason whatever for you to keep a low profile. The arbitration solved a problem, and that problem was not you.—Finell 11:53, 15 December 2010 (UTC)

Good work and good luck with the FA. I strongly support it. I have four minor concerns that I feel should be looked at by someone more acquainted with the article then me. None of these are important enough to affect FA, in my opinion but are nice to have.

1. The lead is too fat with detail. The most blatant example is the use of an example in the lead. But every paragraph has at least one sentence that can be removed.
2. (nitpick) I don't think that Einstein was motivated by the 'lack of evidence of ether'. He seemed to think the theory was sufficient, iirc.
3. The 'Upper limits on speed' section needs a rewrite (and/or a different diagram). It is confusing.
4. Cavity resonance is an example of interference and therefore should properly be placed after the interference and include a note that it is a type of interference.

Again good work. TStein (talk) 18:32, 15 December 2010 (UTC)

## "Practical effects" -> Astronomy

I don't believe that the "Astronomy" section should be under the "practical effects" section. A "practical effect" is something that changes something that occurs in everyday life. Saying B is a practical effect of A, when B and A are both abstract sciences, seems to be a bit of a non-sequitur. Magog the Ogre (talk) 19:49, 10 December 2010 (UTC)

But nor A, nor B are abstract sciences here. A is "the fact that the speed of light is finite", and B is "the fact that we (i.e. astromers) use A to infer the evolution of stars and galaxies". I think that B qualifies as a "Practical effect" of A, or even better as a "Practical consequence", or, if you like, as a "Useful consequence", or as a "Useful effect", or as a "Useful application". I don't object to changing that section header, but I do object your removal of the astronomy subsection, so I fully agree with Physchim62's revert. DVdm (talk) 20:50, 10 December 2010 (UTC)

Wikt:practical: "Being likely to be effective and applicable to a real situation; able to be put to use." I cannot fathom how Astronomy fits that definition, or will fit that definition until humanity gains an advantage from it aside from the theoretical (theory being the antonym). I would much prefer a section on the relationship between energy and mass; e.g., "were c twice as fast, then every energy reaction would involve four times as much energy, greatly altering existence as we know it. Even a small change in c would cause large changes - for example in how cells function." Magog the Ogre (talk) 04:04, 11 December 2010 (UTC)

But, the finiteness of the speed of light is indeed "likely to be effective and applicable to a real situation", namely to the creation of theories about stellar and galactic evolution. What you propose is not related in any way to (1) the finiteness of the speed of light, nor (2) to the point that is made in the current Astronomy section. DVdm (talk) 11:30, 11 December 2010 (UTC)
Would "Consequences and applications of finiteness" cover the entire section adequately (for all practical purposes)? Hertz1888 (talk) 04:24, 11 December 2010 (UTC)
Fine with me. Webster lists the following synonyms of practical: actionable, applicable, applicative, applied, functional, practicable, serviceable, ultrapractical, usable (also useable), useful, workable, working. Useful is there, applicable is there... — it's a all there, in fact. DVdm (talk) 11:30, 11 December 2010 (UTC)

I guess this is what happens when you come to a page frequented by science geeks (not an insult: I'm one too). The ability to measure a distant star is considered "practical" on the same level as everyday usage of a computer or a GPS. Magog the Ogre (talk) 21:20, 15 December 2010 (UTC)

## Upper limit on speeds

The section "Upper limit on speeds" says "According to special relativity, the energy of an object with rest mass m and speed v is given by γmc2". Shouldn't the "c" be a "v"? It talks about velocity approaching the speed of light in the following text.

I remember very little about physics, so I don't feel right correcting formulas I don't know anything about. However, the text around that formula seems to indicate that it is wrong.

Cowgod14 (talk) 18:32, 22 December 2010 (UTC)

Nope. For small v, γ ≈ 1 + v2/2c2 so Emc2 + mv2/2 i.e. the rest energy plus the non-relativistic kinetic energy. A. di M. (talk) 18:56, 22 December 2010 (UTC)

## FA

Congratulations, and thanks, to all of you who helped to win back this article's star. It definitely deserves that honor now.—Finell 00:44, 25 December 2010 (UTC)

1. ^ a b Evenson, KM; et al. (1972). "Speed of Light from Direct Frequency and Wavelength Measurements of the Methane-Stabilized Laser". Physical Review Letters 29: 1346–49. doi:10.1103/PhysRevLett.29.1346.
2. ^ Cite error: The named reference NIST_pub was invoked but never defined (see the help page).
3. ^ Cherenkov, Pavel A. (1934). [Visible emission of clean liquids by action of γ radiation] |trans-title= requires |title= (help). Doklady Akademii Nauk SSSR 2: 451. Reprinted in Selected Papers of Soviet Physicists, Usp. Fiz. Nauk 93 (1967) 385. V sbornike: Pavel Alekseyevich Čerenkov: Chelovek i Otkrytie pod redaktsiej A. N. Gorbunova i E. P. Čerenkovoj, M.,"Nauka, 1999, s. 149-153. (ref)
4. ^ Cherenkov, Pavel A. (1934). [Visible emission of clean liquids by action of γ radiation] |trans-title= requires |title= (help). Doklady Akademii Nauk SSSR 2: 451. Reprinted in Selected Papers of Soviet Physicists, Usp. Fiz. Nauk 93 (1967) 385. V sbornike: Pavel Alekseyevich Čerenkov: Chelovek i Otkrytie pod redaktsiej A. N. Gorbunova i E. P. Čerenkovoj, M.,"Nauka, 1999, s. 149-153. (ref)