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Revision as of 08:28, 22 July 2010 editPhyschim62 (talk \| contribs)Autopatrolled, Extended confirmed users, Pending changes reviewers33,631 edits →Brews'edits: r← Previous edit		Revision as of 08:43, 22 July 2010 edit undoJohnBlackburne (talk \| contribs)Extended confirmed users, Pending changes reviewers, Rollbackers30,799 edits →Lead sentence: drop itNext edit →
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	:] has addressed everything that you have said that is relevant to the lead sentence. ] (]) 08:21, 22 July 2010 (UTC)		:] has addressed everything that you have said that is relevant to the lead sentence. ] (]) 08:21, 22 July 2010 (UTC)

			:Brews, ]. There's a clear consensus against your changes, you're never going to change that, your continual efforts simply waste everyone's time, including your own.--<small>]</small><sup>]</sup><sub style="margin-left:-2.0ex;">]</sub> 08:43, 22 July 2010 (UTC)

	== Paragraph in interferometry ==		== Paragraph in interferometry ==

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Are we ready for FA?

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

I have an issue with the 'Fundamental role in physics section. In this section there important facts about the subject of the article have been relegated to footnotes. Why are they not in the main text? Martin Hogbin (talk) 23:40, 10 July 2010 (UTC)

The Doppler effect and the Terrell rotation are quite irrelevant to the point being made, so I would keep them where they are. The Scharnhorst effect is so small that it might well be never observed during the lifetime of anyone around here, so it was agreed that it was undue weight to put it in the main text.

Anyway, I'm going to move the reference to the relativity of simultaneity and the tachyonic antitelephone back into the main text (being sent to a footnote only comprising one link distracts more than seeing it in the text, IMO). As for the note on one-way vs two-way speed, I think it should be in the main text too, but I seem to remember there once was an opposition to that and was moved to the footnote as a compromise. ― A._di_M. (formerly Army1987) 13:56, 11 July 2010 (UTC)

I thought that the article was ready for FA last time, and it is somewhat better now than it was then. The FA review did not bring out significant criticism, and the points that were raised were dealt with (mostly by A. di M.). The review simply died with a whimper from lack of comment. Would it useful to put the article up for peer review before re-nominating it for FA?—Finell 10:24, 20 July 2010 (UTC)

The "Natural sciences and mathematics" section of WP:PR appears to be stagnating right now: I submitted the Physics article eleven days ago and so far only an editor saying "

Doing..." popped up. A. di M. (formerly Army1987) (talk) 15:38, 20 July 2010 (UTC)

Measurement

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

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

However, it still seems to me that a naive reader is likely to wonder what is going on here. I hope that some enterprising soul can brave this wilderness to bring some clarity to the Measurement section. That involves at most the addition of a few sentences of reconciliation. Brews ohare (talk) 20:49, 15 July 2010 (UTC)

As for the introduction, the reader is going to understand what is going on by the time they get to the end of the third paragraph. Maybe a paragraph could be added after the first paragraph of "Measurement" stating that today measuring c in metres serves no purpose (other than verifying that your measuring instruments are properly calibrated and properly working) and that measuring it in some other unit is equivalent to measuring the length of other unit in metres ... but I'm not sure of how to word it. What would you propose? —Preceding unsigned comment added by A. di M. (talk • contribs) 08:06, 16 July 2010 (UTC)

A. di M.: I wouldn't hazard an attempt at wording given the history of conduct among editors of this article.

The underlying difficulty with the Speed of light article is short shrift given to how a system of units based upon replacing ‘distance’ with ‘time-of-flight’ compares with a different system where distance and time are kept separate. That comparison could be done in a general manner for any speed standard, explaining the need for reassurance that the "standard" speed has been realized in any given measurement, and the role of definitions in making that speed "exact". That presentation could then be narrowed to describe why light-speed is a good choice. Brews ohare (talk) 11:36, 16 July 2010 (UTC)

But there aren't two or more systems: modern physics is based on special relativity, from which we get that the speed of light is fixed, so distance can be defined in terms of time. Historically there were many different ways to understand it, all covered in the article, but the current system has been settled science for about a hundred years. The definition changed more recently, but standards bodies are much more conservative than scientists, and have to take account of how easily a definition is to use.--JohnBlackburne_deeds 13:53, 16 July 2010 (UTC)

The idea of a fixed, finite speed of light dates back to Rømer (who also used time of flight as his measure of distance, as astronomers still do to this day). This subject has been discussed ad nauseam on this page, there is no need for any additional clarification, and certainly no need for philosophical ponderings of what might have been had physcics been different. Physchim62 (talk) 14:52, 16 July 2010 (UTC)

FWIW, the idea of a fixed speed of light only dates back to special relativity, actually. Before then, it was believed to depend on the frame of reference, which left Michelson and Morley scratching their heads. A. di M. (formerly Army1987) (talk) 02:07, 17 July 2010 (UTC)

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

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

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

Perhaps the last sentence of the lead "...the numerical value of c in metres per second is now fixed exactly by the definition of the metre." could be moved up... Count Iblis (talk) 15:31, 16 July 2010 (UTC)

And again, Physchim62, your are beside the point. There is no question of logical inconsistency in the historical record or in modern practice. The question is simply one of clarity of presentation in a WP article called Speed of light of two different approaches: the “time-of-flight” approach that uses a definition with an exact value for c, and the approach that uses separate ‘time’ and ‘distance’ units and therefore a measurement of c as ‘distance/time’. Brews ohare (talk) 15:50, 16 July 2010 (UTC)

But there aren't two different approaches, there's only one, the one given in the article. In what we think of as modern physics it's one of the best established facts, that the speed of light in a vacuum is a constant, which fixes the definition of length in terms of time. Any other approach is historic (and covered already) or wrong (and so has no place here).--JohnBlackburne_deeds 16:46, 16 July 2010 (UTC)

John, you are mistaken: please re-read the article itself carefully. The standards methodology changed in 1983 to achieve greater reproducibility and accuracy. Read this section and take a look at the references, which provide a more detailed discussion, in particular, Sydenham is cited in this connection, as is Jespersen. That is why error bars appear on the numbers in the Measurement section. I'd say that if a person can come away from the Speed of light article without this awareness, the article is lacking. Brews ohare (talk) 17:19, 16 July 2010 (UTC)

Brews, you have just plunged headlong into the same discussion that, 'brought the most extreme invective and vituperation upon me that I have experienced in my 71 odd years of life'. I am sure that you have been right about many things in your life but in this particular case please just consider the possibility that you might be wrong before going any further. Martin Hogbin (talk) 17:32, 16 July 2010 (UTC)

Thank you Martin, for that advice. Frankly, I see nothing at all controversial in what I have said here. My view of the reaction to these rather simple remarks is that in the minds of several (among whom Physchim62 clearly is one), old, ingrained arguments that have not been brought up here at all, dominate their thoughts and prevent them from reading what is before them. Like Pavlov's dogs, certain sounds cause salivation, even when no food is present. So, the Speed of light article is free to continue in its present confusing form, that even an erudite editor like John Blackburne will misinterpret. I will follow your advice, which I take as urging my withdrawal. Brews ohare (talk) 17:48, 16 July 2010 (UTC)

Well, distances could be and were measured through transit times before 1983, and they still can be and are measured by other means today. The only thing that changed was which of the conversion factors needed to express one's measurement in metres is exact. Section 6.3 of the current version already clearly explains the purpose of the redefinition; what is confusing with it? A. di M. (formerly Army1987) (talk) 02:07, 17 July 2010 (UTC)

BTW, the "the approach that uses separate ‘time’ and ‘distance’ units and therefore a measurement of c as ‘distance/time’" was not quite what the most precise measurements in the 1970s were about: the experiments described in "Laser interferometry" were essentially measuring the ratio of two frequencies. A. di M. (formerly Army1987) (talk) 10:29, 17 July 2010 (UTC)

Gentlemen, the sentence should state, by international agreement, for purposes of measurement in the SI system of units, the value of the speed of light is taken to be exactly... This is a rather simple solution to the difficulty and Brews was correct to raise the point.72.64.55.156 (talk) 21:26, 16 July 2010 (UTC)

It already does say that, except in more detail and more clearly, in the third paragraph. But most of the time the history of the definition is inimportant - when you use c in a formula like E=mc for example you should know it's a constant, and to do calculations you need to know it's value, but that's all. A curious or attentive reader might wonder why it's exceptionally an exact value, and can find the theory behind and the practical reasons for this later in the article.--JohnBlackburne_deeds 21:46, 16 July 2010 (UTC)

The suggested sentence gives the impression that there is a 'real' definition of distance that has somehow been ignored by the definition of the metre. Such notions have long since been abandoned by physicists. Even so, the metre is still defined as a unit of proper length. Martin Hogbin (talk) 08:34, 17 July 2010 (UTC

You gentlemen seem to be throughly confused. There is no constant of physics that is known exactly, except by a definition as to the agreement of the value to be used. All of this talk is beside the point. You have said that a constant is known exactly through measurement and that is impossible. Please stop putting impossible physics into Misplaced Pages articles, we have enough of that already.

Who said it is "known exactly through measurement", exactly? A. di M. (formerly Army1987) (talk) 14:04, 17 July 2010 (UTC)

A. di M.: Your comment above about "measuring the ratio of two frequencies" is the kind of detail that requires more attention in this article. Statements like Blackburne's “aren't two different approaches, there's only one, the one given in the article” are indicative of the confusion generated around this topic. Replies like Physchim62's “there is no need for any additional clarification, and certainly no need for philosophical ponderings of what might have been had physics been different.” are total irrelevancies.

I hope you can clear up and explain better how the measurement situation led to the decision to change the basis of the units from a ‘time and space units’ approach to a ‘time-of-flight’ approach requiring only the second, and connect that to today's definition that doesn't employ error bars like 299,710±22 km/s, but an exact value 299,792,458 m/s. It's a big challenge, given that so many want to believe that they understand perfectly what happened in 1983 but cannot tolerate any attempt to explain it more carefully. Brews ohare (talk) 16:48, 17 July 2010 (UTC)

It may be that a separate article should be written to discuss at length the comparison of “time and space” units approaches and “time-of-flight” approaches using only the second. That would allow for a more detailed discussion of measurement techniques, historical developments, and the difference in logical basis allowing the use of a defined speed of light. It also would allow for the treatment of issues such as how we can check that c is a universal constant even though we use a defined value for it. And above all, it would allow referral to this article from Speed of light and thereby remove all this furor to a different venue. Brews ohare (talk) 16:58, 17 July 2010 (UTC)

If we move this to a different venue it will be to return to ArbCom; all your points have been discussed at great length before, and the discussions can be found in the archives of this page. That you choose to to return here with exactly the same arguments once your topic ban from physics articles was lifted is a sign that you do not wish to improve this page, simply to find an outlet for your minority views. Physchim62 (talk) 17:25, 17 July 2010 (UTC)

Physichim62: Threats of ArbCom action as a response to a mild suggestion to create a separate article for more in-depth treatment is exaggerated and hostile. Brews ohare (talk) 17:49, 17 July 2010 (UTC)

Again there is no "difference in logical basis", there is only the current correct and universally accepted approach. Everything else is wrong and/or historical, and is already covered at length in the article. There's certainly no need for a separate article, especially not to explore alternative (i.e. incorrect) interpretations of the science.--JohnBlackburne_deeds 17:07, 17 July 2010 (UTC)

John: An approach using both length (metre) and time (second) units is not "incorrect". It is the system in use prior to 1983. It requires a speed to be established by measuring a distance traveled and the time of travel, and as both of these measurements are indeed measurements, the speed has an experimental error bar. An approach using a standard speed also is perfectly viable. It could be the speed of sound in a specified medium, for example. Then a distance is determined as a "time-of-flight", that is, how long it takes the sound wave to travel the length. Such a system requires only a time unit, the second, and all lengths are determined in terms of times-of-flight in seconds. In choosing between these two systems, it is not their "correctness" that is at stake. What is involved is practical matters, most notably, how much trouble it is to insure that the standard speed has actually been realized. In the case of a standard speed in terms of the propagation of sound, the issue is how readily and accurately one can ascertain that the standard medium has been realized. It is when considering this matter that the speed of light becomes so highly recommended, because experiment has shown the speed of light to be readily realized in a great variety of circumstances, available to all observers without undue concern and preparation. Brews ohare (talk) 17:23, 17 July 2010 (UTC)

Suggestion

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

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

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

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

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

Exact value for c in miles per second.

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

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

It is impossible to convert meters/sec to miles/second using a decimal conversion factor. —Preceding unsigned comment added by NOrbeck (talk • contribs) 11:13, 17 July 2010 (UTC)

This is already mentioned in footnote 1. A. di M. (formerly Army1987) (talk) 14:06, 17 July 2010 (UTC)

Proposal for a new article concerning time-of-flight and length-and-time standards

A rough, preliminary draft of a proposed article can be found here. Comments are invited on its discussion page.

This article is intended for the naive reader to help them understand the change in SI units that occurred in 1983. The article is far from final form, and some help in its construction would be appreciated. Brews ohare (talk) 16:50, 18 July 2010 (UTC)

Aside from the way it is written - after the first paragraph is seems largely devoid of encyclopaedic writing, and reads more like an essay than an article making arguments largely unsupported by the references - I don't see the need for such an article. Anything that needs to be said of the definition of the metre should be in the article Metre, or on the speed of light in this one. The history of it is interesting, as it tells us a lot about how our understanding of the universe has developed, but that's already covered.--JohnBlackburne_deeds 16:59, 18 July 2010 (UTC)

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

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

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

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

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

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

Also, note that the ArbCom case was a disaster, the Arbitrators had to reverse their ruling over their objections after intervention by Jimbo Wales. I don't think they will consider to hear a new case in the absense of evidence of clear disruption, e.g. a clear pushing of fringe views in articles.

ArbCom is right now considering a case on climate change. If you want to see a example of real disruption right now, see here. Now if Cla68 and the other sceptics were to decide to just go about their business as Brews is doing now (simply write up something on their own userspace and then ask for comments, instead of editing against consensus), what a relief that would be. Count Iblis (talk) 14:59, 19 July 2010 (UTC)

Can you give a link to the intervention by Wales? BTW, I agree that as long as the discussion is only in Ohare's own user space, there's no disruption. Also, I think that the article as it is now is ready for FA; what would you think about nominating it? A. di M. (formerly Army1987) (talk) 15:20, 19 July 2010 (UTC)

I also think the article is ready for FA review. I do have some issues about the section on the role in modern physics. It is not mentioned that in modern physics you can see that c is trivial scaling constant as it now only appears in equations that express equivalences like that mass and rest energy are equivalent. So, c = 1 is a natural choice for that reason. But this is not so important for FA review.

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

Does light always travel at c, or does it never travel at c?

These are not rhetorical questions. I'm curious, if c in is the speed of light in a vacuum, and pure vacuums do not exist in nature, why is it claimed that that physical light travels at c? Does light always travel at c as claimed in section 4, or does it never travel at c as claimed in section 5? Isn't it common knowledge that c is a concept distinct from "the speed of light", and that it's just called that for historical reasons? Is anyone willing to argue that c is in fact "the speed at which light travels"? The question is, does light travel at c (y/n)? NOrbeck (talk) 07:32, 19 July 2010 (UTC)

I think the article deals with the issues that you have raised. In current models of physics c is a fundamental constant representing, among other things, the speed of light in a vacuum. You might say, as you suggest, that there is no such thing as an absolute vacuum, indeed there can be no such thing except in a completely empty universe. However, most of physics is based on concepts that do not actually exist so the speed of light is no different in that respect.

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

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

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

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

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

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

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

The lead section needs expansion

Alright, so there are a few people getting ready to resubmit this to the featured article crowd. Cool, I hope it works out well. I suggest working on the lead section more before renomination, and doing so with a slightly different focus.

There are several knowledgeable editors who have reworked the text and debated all kinds of technical minutiae, and the result is a nice, reasonably comprehensive and stable presentation of the important aspects of the topic. However, the lead doesn't do its job of providing an adequate summary, and more importantly fails to reach out and grab the reader in an entertaining way.

At the moment, the lead has three paragraphs. The first has been through some bloody hand-to-hand combat, and looks pretty good. The second paragraph starts out okay, but quickly descends into equations and values for refractive indexes, which are unneeded in an intro and not particularly entertaining for a general reader. There are more interesting details from the 'propagation of light' section that should be included instead. The third, currently final paragraph of the lead, gives an overview of the history. It's not terrible, but is far too sketchy and misses out on several chances to grab the reader. This paragraph should be expanded. And, though admittedly a personal pet peeve which I tried to eliminate months ago, the first sentence of the paragraph is one of the cheesiest lead-ins to a history section possible — "For much of human history, it was debated..." — give me a break, that is unsourceable and virtually meaningless.

Besides revamping those last two paragraphs, there is additional material conspicuously missing from the lead. The article sections 'practical effects of finiteness' and 'measurement' are poorly represented, if at all, and those sections contain a lot of good material. The lead needs expanding with emphasis on a fun presentation, rather than a dry recitation of facts. Think of Feynman in his book QED, where he says something like "now you know that light not only does not move in straight lines, but doesn't go at the speed of light either". The basic facts are in the article, but it needs more evocative details in the lead, such as providing a mental image of a 17th century cannon firing in order to measure the speed of light. The ingredients are in the pot, now add the spice. Tim Shuba (talk) 09:46, 20 July 2010 (UTC)

In my opinion, the lead is in the best shape that it has been since I have been following this article. This is an encyclopedia article, not a paperback book. Expanding the lead would not be an improvement.—Finell 10:20, 20 July 2010 (UTC)

I agree with Tim that the lead is intended to be a summary of the article, that is one reason why I have previously suggested leaving it until the rest of the article is complete. Once the body of the article is complete, writing the lead should be relatively straightforward. One of the requirements for FA is 'brilliant prose'. I do not think the lead meets this standard yet. Ideally we need a copyedit (maybe rewrite) from a good writer of English under supervision from the techies here. Martin Hogbin (talk) 10:42, 20 July 2010 (UTC)

I disagree - the lead is a summary of the article, but very often articles, especially mathematical ones, cover a lot more ground in a lot more detail and depth than should be in the lead. The lead is meant to be an accessible introduction that encourages readers to read on. Trying to summarise all the content from a technical article will make it less accessible and readable and probably too long. Looking at it I can't see anything seriously wrong with it. --JohnBlackburne_deeds 11:00, 20 July 2010 (UTC)

The article (including the lead) might not be perfect, but IMO it is in very good shape, and it's not like featured articles are required to be perfect, nor like it is forbidden to improve an article once it is listed at FA. A. di M. (formerly Army1987) (talk) 15:42, 20 July 2010 (UTC)

BTW, Shuba, if you have ideas about how to improve the lead clear enough to write a draft in a sandbox, please do so... A. di M. (formerly Army1987) (talk) 15:55, 20 July 2010 (UTC)

Remove animated graphic?

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

I agree. Martin Hogbin (talk) 10:47, 20 July 2010 (UTC)

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

I like it. It gives me a good feel of the speed of light for a distance that is just the right size to display the info. --Bob K31416 (talk) 13:50, 20 July 2010 (UTC)

But most people will have at least a rough sense of how big the Earth is, and the picture is to scale. A. di M. (formerly Army1987) (talk) 15:19, 20 July 2010 (UTC)

Agree with A. di M. Also, if people really think that more comparison is needed, why not add the fact that it took the Apollo spacecraft three days to cover the distance that light travels in 1½ seconds. Physchim62 (talk) 15:32, 20 July 2010 (UTC)

Physical constant

The lead says light is a physical constant. It is interesting that that article says:

“A physical constant is a physical quantity that is generally believed to be both universal in nature and constant in time. It can be contrasted with a mathematical constant, which is a fixed numerical value but does not directly involve any physical measurement.”

That contrast can be compared with the last statement in the article:

“As a result, in the SI system of units the speed of light is now a defined constant. Improved experimental techniques do not affect the value of the speed of light in SI units, but do result in a more precise realization of the SI metre.”

An even clearer statement comes from one of sources cited:

“One fallout of this new definition was that the speed of light was no longer a measured quantity; it became a defined quantity. The reason is that, by definition, a meter is the distance light travels in a designated length of time, so however we label that distance - one meter, five meters, whatever - the speed of light is automatically determined. And measuring length in terms of time is a prime example of how defining one unit in terms of another removes a constant of nature by turning c into a conversion factor whose value is fixed and arbitrary.”
— James Jespersen, Jane Fitz-Randolph, From sundials to atomic clocks: understanding time and frequency, p. 280

(my underscore). There appears to be a lack of clarity here. Brews ohare (talk) 11:52, 20 July 2010 (UTC)

Not at all, you are being deliberately obtuse again Brews.

Let's take another example. We have strong reason to believe that all atoms of carbon-12 have exactly the same mass. We can say that the mass is exactly 12 atomic mass units, or we can say that the mass is about 11.996186 "old atomic mass units": the mass of carbon-12 atoms didn't change in 1960 when carbon-12 became the basis of the atomic mass scale in place of oxygen-16! Our units of measurement are mere human constructs, we can choose whatever suits us: the speed of light is the same if you express it in metres per second or in furlongs per fortnight. Physchim62 (talk) 12:08, 20 July 2010 (UTC)

I read that quote when looking at your new article, and I'm not sure what Mr Jespersen is getting at. Nothing has been removed, or changed. The older less accurate ways of doing things are still there if people want to use them. All that's happened is we've through better understanding unified length and time, replacing two arbitrary scales (the second and metre) with one. But he's written a popular science book not a text book or research paper, so hardly a reliable academic source, and I don't think it's of use as a reference here.--JohnBlackburne_deeds 12:32, 20 July 2010 (UTC)

John, this source is a NIST publication, and so I'd hesitate to discount it as "a popular science book". It is that, but it is written by an authority on the subject. You are right that the old methods can be resorted to, and maybe they will one day if some revolution in technique occurs to make wavelenght a more accurate measurement than frequency, though it's hard to imagine at the moment. However, the logical basis of the units has to be made clear, and there is a shift in this regard. Brews ohare (talk) 14:21, 20 July 2010 (UTC)

A somewhat more careful wording is this, from Hall and Ye:

redefinition of the International Meter in 1983 in terms of an adopted value for the speed of light, thus reducing by one the number of independent, base physical standards.
— John L. Hall and Jun Ye (2003), Optical Frequency Standards and Measurement

Brews ohare (talk) 13:48, 20 July 2010 (UTC)

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

In response to Physchim62: The issue here is not what scientists are free to do about standards. The issue here is the choice of wording in a WP article. The term "physical constant" is irretrievably related to measurement in the linked article physical constant. Jespersen says the redefinition has "removed a physical constant". Hall and Ye say it "reduced by one the number of base physical standards". Personally, I like Hall & Ye's terminology better. But whatever the approach chosen, we need a terminology that is clear and consistent, and that does not seem to be the case here. Brews ohare (talk) 14:40, 20 July 2010 (UTC)

The dimensionless number 299,792,458 "does not directly involve any physical measurement", but the dimensionful quantity 299,792,458 m/s does involve some measurement (for example, how long a second lasts). Can you see the difference? A. di M. (formerly Army1987) (talk) 15:22, 20 July 2010 (UTC)

A physical constant has an existence independent of any attempt to measure it. The speed of light is still the same as it was in Rømer's time: or are you denying that, Brews? Physchim62 (talk) 15:29, 20 July 2010 (UTC)

Response to Physchim62. No, not at all. I would hope you would understand this matter yourself, particularly in view of your very apt summary of the situation when you said “Basically, you exchange a measurement uncertainty in the value of a physical constant for a realization uncertainty in the corresponding unit.” To me, this statement of yours very neatly sums the situation where one defines c as an arbitrary value, setting the unit for speeds, and then specifies the circumstances under which the unit will be realized. Brews ohare (talk) 15:51, 20 July 2010 (UTC)

Response to A di M: I do not believe you present the situation usefully by making a distinction between the number 299,792,458 and the definition of c = 299,792,458 m/s. There is no measurement involved in setting c = 299,792,458 m/s, and, for example, Jespersen and also Sydenham say this definition is independent of measurement and arbitrary. I believe Physchim62 has explained the matter very well: one sets up the circumstances under which c is realized; under those circumstances the unit is produced, and what value is assigned to the unit is a matter for the CGPM. Brews ohare (talk) 15:58, 20 July 2010 (UTC)

I believe that a simple formulation like Physchim62's should appear in the lead and that would go a long way to clarify what is meant by an "exact" value for c and the removal of c from the list of "independent, base physical standards". Brews ohare (talk) 16:09, 20 July 2010 (UTC)

I can't see what you mean, i.e. what formulation, and what precisely "like". Don't expect us to work it out for ourselves, post what you think should be changed or inserted if you want other editors' opinions on it.--JohnBlackburne_deeds 16:20, 20 July 2010 (UTC)

John: Physchim62's formulation is spelled out in the previous remark. Here it is again: “One sets up the circumstances under which the standard speed of light is realized; under these circumstances the standard speed of light is produced, and what value is assigned to this unit is a matter decided by the CGPM. This procedure exchanges a measurement uncertainty in the value of a physical constant for a realization uncertainty in the corresponding unit. Specifically, c = 299,792,458 m/s exactly, and this standard unit of speed is realized in free space, which as a practical matter, can be realized only approximately.” Brews ohare (talk) 16:43, 20 July 2010 (UTC)

The definition says "vacuum", not "free space". Physchim62 (talk) 17:06, 20 July 2010 (UTC)

I'm not sure how that would fit in - it more an answer to a question, and does not add anything as that information's already in the article, i.e .the article already covers this very well.--JohnBlackburne_deeds 16:52, 20 July 2010 (UTC)

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

The problem remains that the use of the term "physical constant" contradicts the usage in the article physical constant and the sources I have quoted. Brews ohare (talk) 15:50, 21 July 2010 (UTC)

I agree here, this can be clarified in the section about moden physics. My opinion is that in that section it should be made clear that from the point of view of relativity, c is a conversion factor, see e.g. here

The possible time variation of dimensionless fundamental constants of nature, such as the fine-structure constant $\alpha$, is a legitimate subject of physical enquiry. By contrast, the time variation of dimensional constants, such as $\hbar$, $c$, $G$, $e$, $k$..., which are merely human constructs whose number and values differ from one choice of units to the next, has no operational meaning.

Count Iblis (talk) 18:06, 21 July 2010 (UTC)

And, yet again, we get the Duff paper coming out, dispite the fact that it fails WP:RS (it was rejected from a peer-reviewed journal) and without any mention of the refutations that the author honourably includes as appendices to the preprint. We could simply look at standard usage of the term "physical constant" as in, for example, the list of values regularly published by CODATA (the 2006 list is available at Template:CODATA 2006 and here): the speed of light is not only included in the list, but it is even the first on the list in most printed summaries! To pertend that the speed of light is not a physical constant would be to give undue weight to a minority point of view, regardless of the intellectual merits of that point of view. Physchim62 (talk) 19:07, 21 July 2010 (UTC)

I agree. Martin Hogbin (talk) 19:12, 21 July 2010 (UTC)

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

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

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

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

Brews'edits

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

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

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

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

The article hasn't mentioned these issues of upgrading a standard. Brews ohare (talk) 14:03, 20 July 2010 (UTC)

The point here is that you can't measure c to greater precision than the standard allows, and a more precise value for c implies a more precise standard of length. Of course, this all refers to the "old" methodology, as is discussed in the section on interferometry. Brews ohare (talk) 14:29, 20 July 2010 (UTC)

What do you mean by 'the precision with which the old standard is known'? At the time, it was the standard. Martin Hogbin (talk) 14:45, 20 July 2010 (UTC)

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

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

The relevance is apparent in discussion of the evolution of technique: one cannot obtain a more precise value for the speed of light without introducing a more precise standard for the metre. Of course, the modern approach is to define λ =c/f so the metre automatically is refined as the frequency measurement is improved. That was not so previously. Brews ohare (talk) 17:57, 20 July 2010 (UTC)

You mean apart from the sentence "Improved experimental techniques do not affect the value of the speed of light in SI units, but do result in a more precise realisation of the SI metre."? Never mind, if you're happy all the better! Physchim62 (talk) 08:28, 22 July 2010 (UTC)

Too much personal criticism

There's too much personal criticism on this article's talk page IMO. It can be disruptive. That's why there is the policy WP:NPA which says,

"Do not make personal attacks anywhere in Misplaced Pages. Comment on content, not on the contributor."

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

Hi BobK: A most temperate observation, and in keeping with the objectives of the Talk page to be a forum to improve the article, not its contributors. Thank you. Brews ohare (talk) 14:18, 20 July 2010 (UTC)

Brews is known for his disruptive editing on this and related subjects, including engaging in tendentious debates and soapboxing: so sayeth ArbCom. He has only recently come back from a topic ban covering all physics-related topics. Any sign of a resumption of his disruptive behaviour on the very page that was the nucleus of his previous activities is of great concern. It is up to Brews to demonstrate that he can edit constructively on the subject, not the other way round. Physchim62 (talk) 15:20, 20 July 2010 (UTC)

Hey, some tolerance is called for here. This particular article is a tough one, and involves a lot of nice points and subtle distinctions. It is natural for editors to become a bit frustrated over attempts to express things right, because it isn't simple, and iteration is required. However, I hope that suggestions for clarifications can be taken as exactly that. Brews ohare (talk) 15:46, 20 July 2010 (UTC)

It would help if you were to discuss things first though rather than make a whole bunch of edits in one hit. Martin Hogbin (talk) 16:54, 20 July 2010 (UTC)

Bold, revert, discuss

As this article is hoping to be ready for FAR soon, can I suggest that everyone here adopts the edit, revert, discuss rule. That is to say make your change, but not too many at once, if your change is reverted, discuss it before putting it back. Martin Hogbin (talk) 17:18, 20 July 2010 (UTC)

And can we try not to discuss too many points at once, especially if they all come from the same editor? All editors have the right to comment on the article, but there are also procedures availble for ensuring that single individuals do not crowd out others. Physchim62 (talk) 00:30, 22 July 2010 (UTC)

Lead sentence

Present lead:

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

Proposed replacement:

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

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

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

Proposed replacement:

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

Brews ohare (talk) 17:51, 20 July 2010 (UTC)

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

I see we have reached the point of diminishing returns. The problem remains that the use of the term "physical constant" contradicts the usage in the article physical constant and the sources I have quoted. Brews ohare (talk) 19:01, 20 July 2010 (UTC)

I see no contradiction. Martin Hogbin (talk) 21:38, 20 July 2010 (UTC)

For clarity, would changing the existing text to read "Its value is defined as exactly..." to emphasize that this is a defined, exact quantity be helpful? I am inclined to argree with Martin and John's concerns that the proposed changes offered are confusing and unnecessarily wordy. The lede is not a good place to attempt to introduce subtle points about definition versus measurement. TenOfAllTrades(talk) 19:18, 20 July 2010 (UTC)

I don't think so: I think the fact it's defined rather than measured is too subtle a point for the first paragraph. After all it's defined to the exact value to nine significant figures that was found by measuring it, so that's also the measured value. And it's as much a definition of the metre as of the speed of light. So I would leave it as it is. Very smart readers might pick up on the "is exactly" as unusual, and then read on (to the third paragraph initially) to find out why, but most will probably ignore the "exactly" and pick up the first two things everyone should know: it's a constant and it's a very big number.--JohnBlackburne_deeds 19:48, 20 July 2010 (UTC)

Also it is not, strictly speaking, defined, it is fixed by definition, of the metre. Martin Hogbin (talk) 21:38, 20 July 2010 (UTC)

The notion that one should let slide a point like this because only "smart readers" will notice and everyone else will be stunned by the fact it is a "very big number" is patronizing and also a poor basis for content. Brews ohare (talk) 20:57, 20 July 2010 (UTC)

The detail is given on the body of the article, as always. Martin Hogbin (talk) 21:38, 20 July 2010 (UTC)

Everyone else can read on: the details are there, starting in the third paragraph. But the lead is meant to be accessible, and it certainly won't be if we try and load too much into the first sentence. The relationship between the definitions of light speed and the metre is of only historic interest to most readers and so much less important, and is anyway well covered already in the lead.--JohnBlackburne_deeds 22:08, 20 July 2010 (UTC)

Brews's version would suggest that the ideal vacuum is defined in terms of the speed of light (as David Tombe claimed), rather than the other way round. A. di M. (formerly Army1987) (talk) 13:37, 21 July 2010 (UTC)

Brews is still flogging his dead donkey of a hypothetical standard state – note how he uses "ideal" or "idealized" four times in a single paragraph! The definition of the metre simply says "in vacuum", and so should we. The important point is that it is not the speed of light in air. It is the speed of light in outer space, as near as can be measured, given that outer space is non-dispersive. It is also the physical constant relevant to the interaction between a proton and an electron in a hydrogen atom. Or maybe Brews thinks that a few pesky air molecules might get between the electron and the proton... Physchim62 (talk) 14:30, 21 July 2010 (UTC)

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

Physchim62 & A di M: Gentlemen: My wording is less simple and straightforward than it might be.

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

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

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

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

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

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

I'm getting the impression that there is negligible effect when the vacuum is "adequate" for the measurement of the speed of light with laser beams. Perhaps the concentration of photons in a laser beam is so much greater than the concentration of residual atoms in the space occupied by the beam, that only a negligible percentage of photons in the laser beam interact with the residual atoms, and to the precision of the measurement has no effect on the result for the measured speed of light. --Bob K31416 (talk) 20:26, 21 July 2010 (UTC)

In effect, what Brews is saying is that there is no evidence that the speed of light as measured in a laboratory vacuum is different from the limiting value. That is that the limiting value will be within the measurement uncertainty of the determinations. And so there is no justification for his tirades about ideal vacua and the such like. Brews should stop his soapboxing and tendentious editing, on this aspect and others. Physchim62 (talk) 20:43, 21 July 2010 (UTC)

Please see Too much personal criticism. Thanks. --Bob K31416 (talk) 21:02, 21 July 2010 (UTC)

And please see my reply to your previous comments there. There are a remarkable number of editors replying to Brews' even more remarkable number of "queries". Physchim62 (talk) 21:24, 21 July 2010 (UTC)

It is not the concentration of photons relative to the density of atoms that is important. If you have a certain density of atoms, then you'll have a certain index of refraction that determines the speed of light. I think that in ultrahigh vacuum you'll typically have a residual plasma and then you can treat the situation as presented here. Count Iblis (talk) 21:03, 21 July 2010 (UTC)

Well Count Iblis, I suppose I could elaborate on my point and further discuss your point, but for the purpose of developing this article we need to use reliable sources. Do you know of any reliable source that says that the most precise measurement of the speed of light made a correction for not having a perfect vacuum? Regards, --Bob K31416 (talk) 21:16, 21 July 2010 (UTC)

If you have a certain number density of atoms you will, in theory, have a refractive index which is not equal to one. On the other hand, below a certain number density of atoms, you will have a refractive index that is experimentally indistinguishable from one. Brews' reading of interferometer literature suggests that this latter number density is attained in practical laboratory vacua, not to mention in outer space. I hardly see how the ionosphere is a good model for a vacuum in terms of its interaction with electromagnetic radiation (as the Count seems to suggest) – quite the opposite, in fact! Physchim62 (talk) 21:24, 21 July 2010 (UTC)

Physchim62 is sort of correct. The point is that the errors due to various problems with interferometry make the error in measuring wavelength so large that the added error due to imperfect vacuum is secondary. However, that is not to say that because of practical difficulties one should simply discard the point that one wants to have a perfect vacuum in principle. The logical basis for the whole matter is based upon Physchim62's earlier point that the speed of light is defined to appear in vacuum, and so one should try to obtain a vacuum, or make theoretical corrections for the medium actually used so as to compare the results with the standard speed. I'm left with the notion that Physchim62 wants to back away from this view he expressed earlier.

In any event, if one wishes to understand the statement of an exact speed of light, it is inescapable that one has to accept that it occurs under specific ideal circumstances that can be ensured only imperfectly. The fact of imperfection has nothing to do with its being small: the logic of the situation is separable from the practice. Just like geometry: Geometry is the science of correct reasoning on incorrect figures. Brews ohare (talk) 22:06, 21 July 2010 (UTC)

I'm not "backing away" from my previous position at all. Brews has admitted that, in a decent laboratory vacuum, the corrections due to the presence of residual gas are insignificant compared to the other measurement uncertainties. Note that this can be tested: you can flush the apparatus with helium or with sulfur hexafluoride prior to the experiments and then take it down to the same pressure as you did from air to see if you get the same results. In whatever case the experimenter has two options:

if the effect cannot be completely removed, to get as many data points as possible and extrapolate to zero interference; or
if, after extrapolation as above, it is possible to work in a régime of negligeable interference, to work in said régime.

Brews' own comments admit that working in a decent laboratory vacuum is working in a régime of negligeable interference for length measurements, and hence for SoL measurements. Again, this is simple to test: you measure the wavelengths of various sources (against an internal standard) and see if the different ratios of wavelengths differ from the ratios of their frequencies.

Brews seems to wish that "vacuum" in the definition of the metre means something different from "limit of zero pressure": it seems obvious to me that it doesn't. Physchim62 (talk) 00:21, 22 July 2010 (UTC)

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

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

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

Paragraph in interferometry

Present paragraph:

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

Suggested replacement:

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

Reference: K D Froome (1958). "A new determination of the free-space velocity of electromagnetic waves". Proc R Soc London Ser A. 247: 109–122.

Purpose:

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

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

Martin: Here again we have differences that are largely semantic. The source “Speed of Light from Direct Frequency and Wavelength Measurements of the Methane-Stabilized Laser” (Evenson, Wells, Petersen, Danielson, Day, Barger and Hall) reads as follows:

At shorter optical wavelengths the accuracy of the wavelength determination increases. ... The 100-fold improvement in the presently reported measurement comes mainly from the increased accuracy possible in the measurement of the shorter wavelength.

As you know the speed of light is c=λf. The error is thus the sum of the errors in λ and in f. That error is dominated by the error in λ at longer wavelengths, and by the error in f (prior to this work) at shorter wavelengths. So the authors prefer to compare the two cases in terms of the errors in each factor, and see that the error in the product is now dominated by the error in λ, just as it was before, but of course this error is less at shorter λ.

The authors' view, and the one I express in the above text replacement, is that the breakthrough in frequency measurement would be useless if it were not that the error in λ is much less. This reason for the success now is stated, which is omitted entirely in the present paragraph, and the nature of the breakthrough is described just as it is in the present paragraph. Brews ohare (talk) 18:23, 20 July 2010 (UTC)

I cannot read that paper from the link. What year was it and what wavelength was the laser. Your quoted sentence is ambiguous. Martin Hogbin (talk) 21:33, 20 July 2010 (UTC)

Phys Rev Lett vol 29 No 19 pages 1346ff Nov 1972 methane stabilized laser at 3.39um. It is the paper cited in footnote 93 of the article. Brews ohare (talk) 00:34, 21 July 2010 (UTC)

Thanks, footnote 94 in the article gives the whole story. It seem to be as I thought. At the time of the Boulder measurements, which were a few years before the change in definition of the metre, the situation was that the speed of light had been most accurately measured by using microwaves of known frequency and measuring their wavelength. The main accuracy limitation was the measurement of the 72 GHz microwave wavelength. Calibration of distance would have been against a physical standard, using (visible) optical interferometry before 1960 and by interferometry using the specified krypton line after that time.

The point is that, long before the change to the SoL standard for the meter, distance was routinely being accurately measured by optical interferometry. The big step forward was the ability to measure the frequency of the light used in interferometry against the caesium frequency and time standard. The fact that the wavelength of light could be determined much more accurately than that of microwaves was well-known long before the Boulder experiments. Martin Hogbin (talk) 11:21, 21 July 2010 (UTC)

To put it another way, if you're working with microwaves, the problem is to measure the wavelength because measuring the frequency is easy; if you're working with visible (or near-visible) light, the problem is to measure the frequency because measuring the wavelength is easy. I think we should really change the title of the section to something like "Heterodyne frequency measurements" to make that even clearer. Physchim62 (talk) 14:39, 21 July 2010 (UTC)

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

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

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

Finell: Looks good to me; thanks. Brews ohare (talk) 05:47, 22 July 2010 (UTC)

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