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

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

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

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

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

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

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

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

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

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

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

As opinions here are about evenly divided, we perhaps need another approach, like including both the exact and the approximate in the lead. We do this via the table already, so maybe it's OK. Dicklyon (talk) 20:30, 6 September 2009 (UTC)

User:NotAnIP83:149:66:11 has pointed out on WP:ANI that a previous formulation of the lead used the wording: "The speed of light in a vacuum is exactly equal to 299,792,458 metres per second (approximately 186,282 miles per second). This exact speed is a definition, not a measurement, as the metre is defined in terms of the speed of light and not vice versa." Wouldn't something very close to that wording solve everything? Brews ohare (talk) 20:59, 6 September 2009 (UTC)

Discussion of positions stated above

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

• A. Show only the approximate speed 1 Vote

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

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

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

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

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

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

• The discussion in this article is hard to follow .

Not being an expert i do know .

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

Time can be measured to 10^-11.

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

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

In response to Brews:

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

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

In response to Dicklyon:

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

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

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

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

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

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

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

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

In response to Clayhalliwell:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

I also posted this on the wikiproject physics talk page:

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

A discussion of conversion factor vs. speed of light

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

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

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

Good contributions

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

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

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

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

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

I just have to say: This is an amazing article. Excellent work everyone! --Kvng (talk) 18:25, 4 September 2009 (UTC)

The lead section - one more time

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Pre-1983 error bars were stated expicitly; post-1983 we have the so-called exact value with no error bars, because the error bars are buried in the metre itself. Brews ohare (talk) 01:40, 4 September 2009 (UTC)

Who, between 1960 and 1983, ever stated error bars in the measurement of the wavelength in vacuum of the radiation corresponding to the transition between the 2p and 5d quantum levels of the krypton-86 atom? --___A. di M. 10:51, 4 September 2009 (UTC)

Proposal for a method of seeking conssensus

(after several edit conflicts)

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

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

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

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

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

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

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

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

There isn't a real tautology?? The BIPM says “length is obtained from the measured time t, using the relation ℓ = c_o·t and the value of the speed of light in vacuum c₀ = 299 792 458 m/s”. Speed is ℓ/t so the only c you can get is c₀ = 299 792 458 m/s. That is tautology. Brews ohare (talk) 05:14, 4 September 2009 (UTC)

Brews: If the real physical speed of light varied and the length of the meter varied with it as a result of the 1983 definition, to keep the value of the varying speed of light at a constant 299,792,458 m/s, and we all had multiple metre sticks of varied lengths and chose among them based on the defined length of the metre that day (as reported on an official web site, e.g., "It's 13:30 UTC and metre is 31.5 inches; next update in 15 minutes."), you would have a real tautology. We all agree, I think, that light's rate of travel is constant, so there is no real tautology. Further, the real speed of light can be and is measured very accurately all the time in non-SI units, which confirms our expectation that the real speed of light is constant, so there is no real tautology. Further still, as has been explained repeatedly by several of us, a similar philosophical-only tautology arises from whatever standard is used to define the metre and the second; if either standard varies, the length of the metre or the second changes and the value of c changes with it even though the real physical speed of light remains constant. Fortunately, the standards organizations have the good sense to choose standards that remain constant, so there is no real tautology. Further still, no physicist here agrees that there is a tautology. Most important of all for this Misplaced Pages article, no WP:RS says that there is a tautology; your conclusion that reliable sources imply a tautology or lead to a tautology is WP:OR and cannot be used in the article, regardless of who among us is right or wrong. As far as publication in Misplaced Pages is concerned, there is no tautology, which in this instance happens to correspond with the reality that there is no tautology, so we get the bonus of verifiability and truth. (I know, you think I don't understand the science, and I think you don't understand the science, so there in no point in arguing that again.) —Finell (Talk) 07:11, 4 September 2009 (UTC)

Finell: All that is meant is that c₀ = 299 792 458 m/s regardless of any observation, and, in fact, regardless of what the speed of light may be and regardless of how accurately it is measured. To produce a nine-figure number and make statements as though it were a property of nature, not merely a product of committee decision, is misleading.Brews ohare (talk) 14:47, 4 September 2009 (UTC)

It may be misleading, but it's convention. The speed of light is commonly described in the way the article describes it now, so it makes sense to keep it that way. To invalidate this convention would be original research. The current lead of the article does mention that this value is a consequence of the definition of the meter. Evil saltine (talk) 14:58, 4 September 2009 (UTC)

Sorry, I mean to say that it is commonly described as being exactly 299 792 458 m/s. Evil saltine (talk) 15:05, 4 September 2009 (UTC)

That said, I don't think we necessarily have to use the word "exact" either. Evil saltine (talk) 15:35, 4 September 2009 (UTC)

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

There are many fundamental constants for which no exact value is known, like the electron charge, for example. What is Tim Shuba's logic that ties presentation of c as a fundamental constant to its (actually unknown) exact value?? Brews ohare (talk) 05:19, 4 September 2009 (UTC)

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

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

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

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

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

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

A definition is not a tautology it the normal sense of the word. Martin Hogbin (talk) 13:43, 4 September 2009 (UTC)

Internal links from the intro apparently are frowned upon. Such a solution was reverted earlier. The word "exact" should not appear in the first para because it requires explanation that needs more space as is found in "Speed of light by definition". Use of "exact" is contrary to WP:ASTONISH. Brews ohare (talk)

I have a couple suggestions (note the word "quantity" to differentiate the physical constant from how it is measured):

1. "The quantity c is exactly 299,792,458 metres per second in the International System of Units" - I prefer this one
2. "The quantity c is defined to be 299,792,458 metres per second" - Sounds a bit awkward but is shorter

Evil saltine (talk) 13:18, 4 September 2009 (UTC)

The point is that, strictly speaking, the speed of light has not been defined. The length of the meter has been defined in terms of the speed of light and the second. This has the effect of fixing the speed of light when expressed in meters per second. Martin Hogbin (talk) 13:43, 4 September 2009 (UTC). I do not understand how the use of 'quantity' helps. I presume that by 'how it is measured' you mean 'how it is expressed', the speed of light can no longer be measured.

How about,'The speed of light is exactly 299,792,458 metres per second because the metre is defined as the distance light travels in a 1/299,792,458 of a second'. How many people cannot understand that? Maybe one. Martin Hogbin (talk) 13:43, 4 September 2009 (UTC)

In the first paragraph just keep it like it is now. It simply states that "(the speed of light) is 299,792,458 metres per second ..." without any reference to it being exact or not. It this point in the article any such qualification must necessarily come without further explanation, and thus can be confusing and at worst misleading. The lead later comes back to say that in the SI the metre was defined to make this value exact. The value of c being exact in some systems of measurement is not among the most important things to convey in this article anyway. It is exact in some systems (one of those being the dominant scientific standard) and not exact in others. That it was chosen to be exact in the SI is an interesting factoid, but actually not that central to the physics. (TimothyRias (talk) 14:07, 4 September 2009 (UTC))

That's a good idea. I didn't realize that it was already mentioned. Evil saltine (talk) 14:24, 4 September 2009 (UTC)

It is more confusing and misleading in my opinion not to say that the SI value is exact. Readers may wonder if a more precise figure might be available somewhere else. The speed of light having an exact value is not peculiar to the SI system (although this system is the world's standard) inches and feet are defined exactly in metres so the speed of light is also exact in these units. In most natural systems of units the speed of light is defined to be exactly 1. I agree that, from a fundamental point of view, 299,792,458 is an arbitrary number but that will always be the case with any dimensionful constant, to give it a numerical value we have to define a system of units and this system determines the numerical value of the constant. Martin Hogbin (talk) 15:43, 4 September 2009 (UTC)

The precise figure is presently in the lead as Martin wishes and the use of the term "exact" is simply contrary to WP:ASTONISH. The term "exact" flies in the face of history: the speed of light, which has been measured by astronomers and laboratories with increasing accuracy for centuries, has ceased to be an experimentally determined quantity. Brews ohare (talk) 15:50, 4 September 2009 (UTC)

I think the current version of the lead (the one giving the exact value in the first sentence, but which doesn't say either that it's exact or why it is until the paragraph about history) is fine, as far as this point is concerned. --___A. di M. 16:29, 4 September 2009 (UTC)

What an encyclopedia should say

We all know that the number 299,792,458 (unlike the value of a dimensionless constant) has no fundamental significance in physics but it is, by virtue of the way that the meter is defined, the exact speed of light in the SI units of metres per second. Whether we should show it as an exact figure is not a matter of physics but of what an encyclopedia should say.

Any modern encyclopedia, including WP, should clearly should give the speed of light in the world standard SI units early on in the lead. It should also give some idea of how precisely that value is known. In the case of the speed of light that value is exact because of the way the meter is defined and that is what we should say. Here are some examples:

This is how Britannica starts, 'speed at which light waves propagate through different materials. In particular, the value for the speed of light in a vacuum is now defined as exactly 299,792,458 metres per second'.

This article, when it first became an FA started, 'The speed of light in a vacuum is exactly 299,792,458 metres per second'.

We have allowed ourselves to be influenced in our thoughts on the subject by the persistent fringe views of one editor in particular and the attempts of some others to promote a compromise. A compromise is not what is needed here. What is needed is a clear, concise, and encyclopedic statement of the facts, which were settled long ago. Martin Hogbin (talk) 10:11, 5 September 2009 (UTC)

First, I have changed the title of this section to enable me to read it. Second, I have a problem with Hogbin's assertion that "It should also give some idea of how precisely that value is known." Why do we need to do that in the first para? My guess is that there will be three types of readers: 1) 'Non-scientists' for whom any discussion of accuracy is way too much info and indeed, when coupled with a nine digit number, could side-track them from reading the article. They come to the article seeking general reading on a topic that for whatever reason has taken their interest 2) 'Scientists' who already know the definition of a metre and come to the article looking for something specific, maybe a reference or link or connection. 3) 'Students' who could be at any stage and are seeking help with an asignment etc. They will be quite capable of reading to the end of the lead before discovering the exactness and they will need a good explanation not simply "defined". For all these groups, the number 299,792,458 has a clear degree of accuracy without the need to say it; imho it is more encyclopedic for the lead to simply introduce the topic in para 1 and expand on history/definition/significance in subsequent paras. Abtract (talk) 10:39, 5 September 2009 (UTC)

No problem with your change of title, mine was a touch confrontational but the point needs to be made clearly that, despite what one editor may think, the physics was settled long ago and there is nothing to discuss in that front.

I cannot understand how giving the answer to the question, 'What is the speed of light' as 'exactly 299,792,458 metres per second' will confuse a non-scientist. It also does not simplify anything the give the wrong answer, followed by the right one. Let us just give the right answer once at the start. Martin Hogbin (talk) 10:57, 5 September 2009 (UTC)

In what way is "the speed of light ... is 299,792,458 metres per second" wrong? As to whether 'exactly' could confuse, well it confused me when I first read it I don't mind admitting ... and it still makes me feel uncomfortable although I now understand it perfectly and know it to be true (by definition!). Maybe confuse is the wrong word for what it does to ohare but it's not a pretty sight. Why take the chance? Abtract (talk) 15:31, 5 September 2009 (UTC)

If we say just 299,792,458 metres per second then it could be that the speed is really 299,792,458.0865 metres per second but we have rounded it to the nearest whole number. That is not the case and we should inform our readers so. Martin Hogbin (talk) 21:58, 5 September 2009 (UTC)

In fact I am coming round to the view that we should not even mention the word 'exactly' anywhere in the lead; it is a definitional nicety and really has little to do with the speed of light per se. Whether (due to the definition of the metre) it is exactly 299.. or whether (due to it being measured by reference to a metre defined independently of the speed of light) it is approximately 299.. is actually just administration. What it certainly isn't is a big deal in terms of the speed of light since it is the metre that has been defined not the speed of light. Let's drop all reference to exactly in the lead. Abtract (talk) 15:42, 5 September 2009 (UTC)

If you are interested in discussing why the metre was defined in the way that it was then I am happy to do so, somewhere else, but it is not up to you or me to decide exactly how the metre has been defined or whether that has anything to do with,'the speed of light per se'. The metre has been defined in such a way that it makes the speed of light exact when expressed in SI units. As editors of an encyclopedia it is our job to inform our readers of that fact. Martin Hogbin (talk) 09:34, 6 September 2009 (UTC)

Why not just say 300,000,000 m/s that is close enough for most purposes. In fact why not just say that it goes really fast, no one but scientists need to know the actual speed and they can look it up in a proper encyclopedia. Martin Hogbin (talk) 21:58, 5 September 2009 (UTC)

Abtract is on the money here. Martin's support for using the word "exact" contravenes WP:ASTONISH, as indicated in part by the frequent use of !! in sources that discuss this point. Also the wording "exact" is misleading inasmuch as the SI units conversion factor achieves its exactitude simply via the metre absorbing all measurement error. This burying of the error (it's not explicit anymore) and the use of a nine-figure integer value, conspire to produce a false impression in the reader that a fundamental constant of nature is known exactly, distracting the reader from the realization that only the conversion factor is defined exactly. (See section "Speed of light by definition".) Brews ohare (talk) 17:12, 5 September 2009 (UTC)

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

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

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

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

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

Not in parenthetical notes. --___A. di M. 11:22, 4 September 2009 (UTC)

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

No point in writing down nine digits if they're not exact. If you must round, write "300 million". --___A. di M. 11:22, 4 September 2009 (UTC)

I agree. In fact, on reflection, I think it is best to have no approximate values in the lead. This at least ends the argument as to which ones we have and in what format. We still a have a comprehensive table. The lead should be clean and clear. Martin Hogbin (talk) 12:25, 4 September 2009 (UTC)

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

This is the bit I do not understand. You say that stating it is exact is not necessary. Why is it any less necessary that giving the speed at all. It is an important fact that we should communicate to our readers.

Why it it confusing? I accept that one editor here seems permanently confused by the fact but is it really that hard for most people to understand ? Martin Hogbin (talk) 12:25, 4 September 2009 (UTC)

"(about 1,079 million kilometres per hour or 671 million miles per hour). " is better for the non expert than 300.000 km/s ?? Wdl1961 (talk) 14:15, 4 September 2009 (UTC)

Anybody who knows what a kilometre is also knows how to convert metres to kilometres at a glance (they are taught to do this in elementary school); and anyone who has ever paid something costing £19.99 using a 20-pound note knows how to round numbers up. So writing "about 300,000 km/s" right after "299,792,458 m/s" is pointless. (BTW, I'm not a fan of writing non-significant zeroes down, so if I really had to do that I'd rather write "300 thousand".) "1,079 million km/h" is useful to give a sense of scale because everyone measures speeds in either km/h or mph everyday, but then it's not vital. (Anyway, I'd vote to keep it.) --___A. di M. 16:26, 4 September 2009 (UTC)

This "important fact" is just as important and on exactly the same level as saying the dollar is 100 cents - just like that the speed of light is 299,792,458 m/s, by definition; there is no physics or fact of nature here at all. According to von Baeyer, "When one uses the modern definitions of the meter and the second, measuring the speed of light is as pointless as counting pennies in a dollar." Brews ohare (talk) 19:17, 5 September 2009 (UTC)

Yes, we all know that there is no fundamental physical significance to the number 299,792,458 but just as the article on the United States dollar tells us that 'It is divided into 100 cents' in the first paragraph of the lead we need to say that the speed of light is exactly 299,792,458 m/s in the lead. Why? Because it is! Martin Hogbin (talk) 22:08, 5 September 2009 (UTC)

Sorry, "Because it is!" will butter no parsnips here. Abtract (talk) 22:17, 5 September 2009 (UTC)

Are you telling be that you believe that the speed of light is not exactly 299,792,458 m/s? Martin Hogbin (talk) 22:27, 5 September 2009 (UTC)

You seem reluctant to answer this question. Martin Hogbin (talk) 11:26, 6 September 2009 (UTC)

Martin Hogbin: If the "exact speed of light" is not a fact of nature (undeniable) but the result of a committee decision (undeniable), then an article on not "the SI units definition of the speed of light" but upon the real physical fundamental physical speed of light, should not appear to identify these two things by setting up easy confusion for the reader. WP:ASTONISH. As you freely admit, 299,792,458 m/s is not exact in the sense of telling what the exact real physical fundamental physical speed of light might be, because the error bar is buried in the metre, and is not explicitly stated. That makes the word "exact" ( and a nine-figure integer value with no error bar) WP:ASTONISH. Explaining this detail is deferred as it is too complex for the limited space in the intro. Therefore, "exact" also should be deferred. Brews ohare (talk) 00:53, 6 September 2009 (UTC)

What do you exactly mean by the question of "what the exact real physical fundamental physical speed of light might be"? How would we detect that experimentally? (Remember, we're talking about physics not philosophy, so if the answer is "you can't" that's not a valid question.) --___A. di M. 11:35, 6 September 2009 (UTC)

My position is much simpler: When using SI units, the speed of light is exactly 299.. m/s; of this there can be no doubt because the metre has been defined to make it so (I know this because Roger Penrose, an unempeachable source, makes a point of explaining it very clearly in The Road). However, the speed of light is not known exactly, cannot be measured exactly and never will be known exactly in any units than SI (and related or similarly self defining units) or 'natural units' (where c = 1). There can also be no doubt about this because all measurements have an inherent error factor due to the inadequacies of equipment and researchers (I know this because everyone I have spoken to, on the subject of experimental measurement, reminds me to state the error; and because there is a wp article on the topic). It is because of the seeming conflict between these two statements (both equally true) that imho the word exact should not appear anywhere in the lead; the lead is a summary of the article's major contents and this exactness is a small part of the main body of the article; it is an administrative nicety to be explained later, not shouted in the lead. Abtract (talk) 12:00, 6 September 2009 (UTC)

Abtract, you are creating a problem where none exists. All your talk of, 'measurements have an inherent error factor due to the inadequacies of equipment and researchers' is well understood and accepted but in the case of the SI system this translates quite simply into the statement that we will never be able to realize the metre exactly. Martin Hogbin (talk) 12:46, 6 September 2009 (UTC)

I know that ... and I know that you already knew all that I wrote ... my intention was to be quite uncontroversial while indicating that there is a problem created by these two correct statements. This is a problem that we should not allow to dominate the lead (or the talk page) because it is indeed a minor administrative matter, not a significant principle of physics. Abtract (talk) 15:05, 6 September 2009 (UTC)

What problem? Martin Hogbin (talk) 16:21, 6 September 2009 (UTC)

Circularity and confusing the average reader .. I am not going to repeat myself, please refer to my previous explanations. Abtract (talk) 17:28, 6 September 2009 (UTC)

The reason that I ask is that you seem unclear as to whether there is a problem or not. If there is a problem, it is one created by you, of not then let is just state that the speed is exact. I am sure that the poor old average reader will understand. Martin Hogbin (talk) 19:09, 6 September 2009 (UTC)

I think that the redefinition of the metre automatically changed the meaning of "0.9144 metres" in the definition of the yard and consequently those of the inch, the foot, etc., so I disagree that the speed of light "never will be known exactly in any units than SI". --___A. di M. 14:14, 6 September 2009 (UTC)

If you look at my statement again you will see that I continue "... (and related or similarly self defining units) or 'natural units' (where c = 1)." I hope this satisfies your point? Abtract (talk) 15:05, 6 September 2009 (UTC)

Earlier formulation

User:NotAnIP83:149:66:11 has pointed out on WP:ANI that a previous formulation of the lead used the wording: "The speed of light in a vacuum is exactly equal to 299,792,458 metres per second (approximately 186,282 miles per second). This exact speed is a definition, not a measurement, as the metre is defined in terms of the speed of light and not vice versa." Wouldn't that wording solve everything? Brews ohare (talk) 14:15, 6 September 2009 (UTC)

I would leave out km/s approximation from the lead paragraph. Rounding and converting m -> km are skill taught in elementary school in most places (at the risk of sounding Russian). To pretty much anybody any of these numbers are just going to register as a very long distance per counting second. I don't think there is anybody that is going a better idea of how fast this is from the km/s value than the m/s value. I'd of course leave miles per second value for those people used to even more unnatural units than the metric system. ;) Note that the value will still be in the table next to the lead.(TimothyRias (talk) 12:47, 7 September 2009 (UTC))

Timothy, I'd be inclined to agree with you, although I'm not sure if it's for the same reasons. Within the new SI system, the defined speed of light is an exact number. I would have assumed then that conversions to kilometres and hours would merely have introduced a factor of 1,000 and/or 3,600 to this exact number. It doesn't make any sense to put in an aproximate value in connection with the new post-1983 SI metric system. I'd of course leave in the miles per second value, because 186,000 miles per second is the commonly held value for the speed of light, and of course we know that it is only an approximation. David Tombe (talk) 13:47, 7 September 2009 (UTC)

" I'd of course leave in the miles per second value, because 186,000 miles per second is the commonly held value for the speed of light, and of course we know that it is only an approximation." There are some places in this world that are already using the metric system and are used to 300.000 km/s. Some even use english.Wdl1961 (talk) 14:26, 7 September 2009 (UTC)

Folks: The key point in the formulation

"The speed of light in a vacuum is exactly equal to 299,792,458 metres per second (approximately 186,282 miles per second). This exact speed is a definition, not a measurement, as the metre is defined in terms of the speed of light and not vice versa "

is the second sentence, not which approximation to 299,792,458 metres per second should be mentioned. The second sentence clarifies the status of the number 299,792,458 metres per second. Brews ohare (talk) 15:50, 7 September 2009 (UTC)

How much history should the lead include?

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

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

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

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

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

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

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

My point is simply that the lead should reflect what the article says. Martin Hogbin (talk) 12:28, 4 September 2009 (UTC)

Naturally I agree with that. Abtract (talk) 15:55, 5 September 2009 (UTC)

Relativity in the lead?

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

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

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

Well I guess it is, but nobody, including Einstein, noticed that fact until Minkowski pointed it out later. Why give unnecessary detail in the lead where there is not space to expand and explain? Just state the facts plainly.

My main point was that, as we state that c is a fundamental constant of physics, we should explain that it is a spacetime constant. Martin Hogbin (talk) 09:26, 4 September 2009 (UTC)

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

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

The reason that I suggest no names in the lead is that it prevents a whole new class of arguments, 'Why Einstein and not Minkowski?', 'Why only western scientists and not Muslim ones?' In the body of the article we have the opportunity to make clear the contribution of each person. Martin Hogbin (talk) 09:26, 4 September 2009 (UTC)

OK, those are good reasons. I agree. —Finell (Talk) 09:43, 4 September 2009 (UTC)

I have to slightly disagree with Martin here, so let's go with what we should agree on first!

The current sentence in the lead is simply wrong. I quote:

According to Albert Einstein's special theory of relativity, the speed of light is an important constant connecting space and time in the unified structure of spacetime.

The errors are left as an exercise for the reader (at least for the time being!). My personal PoV is that concentrating on spacetime is to miss a more fundamental point, that is that every observer will measure the same value (in local units) for the speed of light, regardless of relative motion between observer and source. In metrological terms, it is not the first attempt at a "fundamental" constant but it is one of the first, and certainly the most enduring. In physical terms, it is simply a restatement of the results of the Michelson–Morley experiment, with rather more theoretical deduction all the same!

The speed of light is certainly of vital importance in spacetime, but I think it's wrong to say that's its only importance in physics, especially not for non-specialists to whom the lead should be primarily directed. Physchim62 (talk) 11:41, 4 September 2009 (UTC)

All the results in special relativity, including the constancy of the speed of light, follow from Minkowski spacetime. Therefore my preferred construction would be along the lines of 'The speed of light is important constant of Spacetime and as such is constant in every inertial frame, connects mass and energy etc '. The constant speed of light is discussed in some detail in the body. Martin Hogbin (talk) 12:42, 4 September 2009 (UTC)

Maybe you should spend your time at Hermann Minkowski then, not at this article. All that is needed in the lead section (IMHO) is that it is believed (by the generally accepted theories of physics and metrology) to be a fundamental physical constant. Physchim62 (talk) 14:30, 4 September 2009 (UTC)

Free space or vacuum?

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

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

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

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

Yes but the Vacuum article refers mainly to partial vacuums. Making up non-standard terms like 'ideal vacuum' is not the answer either. I agree that 'vacuum' is a more widely used term and it is the term used by BIPM which is why I favour the piped link. Maybe this is the time to ignore all the rules as we have a compelling reason to pipe the link to the Free space article. But I really do not care that much so long as we pick a term and use it consistently, with maybe a brief explanation using both terms in an appropriate place in the article (not the lead). Martin Hogbin (talk) 09:40, 4 September 2009 (UTC)

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

On Misplaced Pages, classical vacuum redirects to the Free space article; the article does not give classical vacuum as a synonym for free space in the lead. So I don't think that solves the issue of nomenclature in Misplaced Pages. —Finell (Talk) 01:12, 4 September 2009 (UTC)

Yes, why not confuse the issue beyond all measure with Brews' understanding of Quantum physics? Martin Hogbin (talk) 09:40, 4 September 2009 (UTC)

Martin: I provided a google link to classical vacuum to show the term has widespread use. That is all. Now you are inventing disputes that have not arisen to provide your distorted mind with a platform for invective. Tut, tut, old man. Brews ohare (talk) 14:33, 4 September 2009 (UTC)

Physical quantities and their units

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

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

If Finell believes this discussion is duplicated in another article, he may bring that forward here, of course. Brews ohare (talk) 01:45, 4 September 2009 (UTC)

I do not believe this section is useful. It seems irrelevant and makes the article worse. I'm surprised to see a book on cryogenics used in an article on the speed of light. This is WP:OR and WP:SYNTH and should be removed. Quale (talk) 06:17, 4 September 2009 (UTC)

I removed it before, Brews reverted me, and I just removed it again. I agree with Quale's reasons. At least one other Project Physics editor said in an edit comment that the section doesn't belong in this article. The first part, which is sourced, is about metrology, not the speed of light. When others earlier challenged the relevance of this section to the speed of light, Brews added the second part, an example involving the speed of light that he made up for the express purpose of making the section "relevant". Brews's example is his WP:OR, and must be deleted for that reason. Further, adding an example to tie a section about another topic to the present article is just an artifice to bring in matter that properly is the subject of a different article. What Brews did is like a time-reversed content fork: it brings irrelevant matter in, instead of sending relevant matter out to another article in positive time. —Finell (Talk) 07:41, 4 September 2009 (UTC)

Well done! You may have to do it again though. Martin Hogbin (talk) 09:41, 4 September 2009 (UTC)

I agree with Quale & Finell. Headbomb {_{κοντριβς} – WP Physics} 14:38, 4 September 2009 (UTC)

Quale and thinkers above: The BIPM definition of length is WP:OR and WP:SYNTH? All this section does is present the way transit times are used as equivalent to lengths. This presentation of this use of transit times is not irrelevant to the topic "Speed of light by definition" inasmuch as it is the basis for the 1983 definition of c₀. Your arguments do not apply here. Brews ohare (talk) 15:04, 4 September 2009 (UTC)

I've rewritten this section to make the relevance clear and to emphasize this contribution is neither WP:OR nor WP:SYNTH. If you disagree, I'd appreciate identification of which statements are thought to be ] or WP:SYNTH. Brews ohare (talk) 16:43, 4 September 2009 (UTC)

Martin has reverted this section in its entirety without even an Editorial Comment line, and with no guidance how this rewritten and well-sourced material might be improved to meet his approval. I have placed a request for a detailed (not philosophical) discussion on his talk page. Brews ohare (talk) 23:06, 4 September 2009 (UTC)

The reasons are all discussed above. As usual, you insist on adding material that no other editors supports. This is what "idiosyncratic" means (see defs). You are the individual who is causing all the trouble. Try to drop back, understand, and cooperate instead. Dicklyon (talk) 02:48, 5 September 2009 (UTC)

Actually, Quale deleted this section that Brews ohare re-inserted contrary to clear consensus, and Brews reverted Quale. Martin acted properly in conformity with the consensus. Brews: Do not add new material to this article without first discussing it on this talk page. Your record here of pushing your fringe POV and disruptively editing the article contrary to consensus requires that we take this step to uphold Misplaced Pages's content policies and guidelines. Thank you for understanding. —Finell (Talk) 07:58, 5 September 2009 (UTC)

This material was rewritten, as pointed out, and merits a detailed re-examination before rejection. In looking at the previous version, Quale objected that the material was not relevant - relevance is now very clearly stated. Quale objected to what he called WP:OR and WP:SYNTH, although no specifics were offered about what exactly he referred to. In the rewrite, all statements are sourced and anything remotely conjectural has been removed. Thus, Dicklyon's statement that earlier objections apply is not the case.

Finell's statement that somehow this material is fringe POV is not supported by any actual evidence, and as just remarked, everything in the present version is completely sourced and entirely non-controversial. This submission deserves re-examination and some detailed response about just what it is that might support its rejection. Simply stating non-specific generalities and making personal attacks upon myself does not constitute critique of the subsection. As said in WP:NPA, comment should be upon the the content, not upon the contributor. And based upon the item at hand (the content), not upon impressions based upon other submissions.

My feeling about these actions is that they are not based in any way upon this submission itself, which is relevant, non-controversial, sourced. Brews ohare (talk) 12:51, 5 September 2009 (UTC)

Brews, let me give you the my crackpot test from my days on Usenet. Have you considered the possibility that you could be wrong? Martin Hogbin (talk) 22:10, 5 September 2009 (UTC)

Et tu Brute. Abtract (talk) 22:20, 5 September 2009 (UTC)

Firstly, are you asserting that I might be wrong and that the actual speed of light is not exactly 299 792 458 m/s?

Secondly every other editors here agrees with me (unless you do not) that the speed of actual light is exactly 299 792 458 m/s. If the reverse were the case and everyone else asserted that I was wrong then I certainly would be considering that possibility. Martin Hogbin (talk) 22:26, 5 September 2009 (UTC)

I was attempting, with rather weak 'humour', to suggest that some editors might consider the possibility that they might be wrong in their beligerent attitude to other editors and their unwillingness to give reasons other than "because it is". That's all. Abtract (talk) 22:32, 5 September 2009 (UTC)

Martin: Have you even looked at this material?? It does not say anything yea or nay about your pet idea that the speed of actual light is exactly 299 792 458 m/s. Can you just read the thing and make a specific assessment of its content (not philosophical ramblings about my crackpot ideas), cite verbatim anything you disagree with, and not keep reverting to this refrain, which insofar as this subsection is concerned, is completely off-topic?? Brews ohare (talk) 00:34, 6 September 2009 (UTC)

Proposed topic ban of Brews ohare

A proposed topic ban of Brews ohare is being discussed now at AN/I Misplaced Pages:Arbitration/Requests/Case#Speed of light. Very few of the editors here have commented there. As a result, the supports and opposes there do not reflect the consensus here. —Finell (Talk) 07:33, 6 September 2009 (UTC)

I have added my comments. We have an excellent demonstration of the damage that Brews' editing style causes to this article in the minor edit war over ''. This was one of Brews' typical references to support some obscure point that he was making. It is not clear what the text that he added to the article is trying to say, it has two references (to make a point) where only one is needed and the references given are from an obscure books that Brews found from a web search of some kind. This is not how to write an encyclopedia. The section needs to be rewritten. Martin Hogbin (talk) 11:23, 6 September 2009 (UTC)

Martin: An "edit war" over ? You must be joking. The insert was made in the expression “where the estimated uncertainty is ±4 × 10” to explain this was a relative error, and hence had no units attached. There was no "war". There is no "obscure point". Brews ohare (talk) 13:55, 6 September 2009 (UTC) 299 792 458

For confused readers this gives a low of 299 792 45x.xxx m/s and a high of 299 792 45y.yyy m/s ??Wdl1961 (talk) 14:12, 6 September 2009 (UTC)

Why not then put it in m/s? Saros136 (talk) 18:24, 6 September 2009 (UTC)

Yes, I understand that, but my point was that the whole issue of whether we have or not is completely pointless. Discussion of details of the pre-1983 definition of the metre is not that relevant to this article.Martin Hogbin (talk) 16:17, 6 September 2009 (UTC)

This is an endless circular discussion not going anywhere. The speed of light has been around before us and all the labels we used to describe the sol.. How about little history , measurement is comparing a known A versus an unknown B .After a while it is readily apparent that some B(i) is superior (easier to measure, duplicate , less repeatable error ,etc. ) so B(i) becomes the primary standard . So we go from X to the Kaiser's foot to a yard , meter , n cycles and more to describe the sol. Now sol. and time is the standard. If this is the last final standard nobody can know for sure. Ref Conférence Générale des Poids et Mesures is perfectly clear and understandable to anybody with some technical education. How hard can it be for some good writer (i is a injuneer) to make an interesting introduction for the average reader. Wdl1961 (talk) 19:09, 6 September 2009 (UTC)

Arbitration request filed

Admin Jehochman filed a request for arbitration concerning the disputes here: Speed_of_light Misplaced Pages:Arbitration/Requests/Case#Speed_of_light. In addition to the parties that he named, other editors have been involved in these disputes and should make statements in the arbitration request. —Finell (Talk) 23:18, 6 September 2009 (UTC)

More thoughts on the introduction

A few possibly interesting points for the introduction:

• The use of c in connecting space and time into spacetime is not limited to special relativity. See What is General Relativity for an explicit example of how C figures into the metrics of general relativity. So I think it was better when it just said relativity, not special relativity.
• Showing E=mc does not need relativity at all. Given that the momentum of a photon is E/c, then E=mc follows. The proof is simple - start with a box in free space, emit a photon from one end and absorb it at the other. The recoil shifts the box; figure the photon weight that keeps the center of mass unchanged.
• The so-called problem that you cannot measure the speed of light in SI did not begin in 1983; the same problem existed since 1960. Suppose the real, physical speed of light changes, with no other physical constant changing. Post 1983, this cannot be expressed in SI since the meter changes to match. But this was true from 1960 on - the changed speed of light will affect the wavelength of the krypton transition, and in exactly the manner that will cancel the change in SI units. (If the speed doubles, the wavelength doubles, the meter is twice as long, and the numerical value of c is unchanged). You'd need to go back to the old scratches-on-a-metal bar to express this (rather unlikely) change in SI units.
• Of the readers of this article, more will be interested in *why* the speed of light is important than will be interested in the history. So the "why" part should come before the history part.

Suggestions and comments are expected, of course. LouScheffer (talk) 03:07, 7 September 2009 (UTC)

I have no disagreement with anything you said. I have some questions on your third point: how important is this in terms of the article? and the lead? In other words how strongly should this be explained? Do you think that using the word exactly in the lead is necessary? Advisable? Or do you think it begs more questions than can sensibly be answered in the lead? Abtract (talk) 07:34, 7 September 2009 (UTC)

Sorry, this was just an observation. In my opinion, this should not be in the lead at all, except a brief mention that the meter is defined in terms of the speed of light. LouScheffer (talk) 12:19, 7 September 2009 (UTC)

LouScheffer, Regarding your second point, this was indeed demonstrated by the American scientist Gilbert N. Lewis in 1908. The derivation involves Maxwell's equation for radiation pressure. David Tombe (talk) 08:36, 7 September 2009 (UTC)

Lou, I agree with point 1. In fact there is no real need to mention relativity at all. The whole concept of spacetime is what relativity is all about. So we might just say that it is a constant of spacetime.

Regarding point 2, it was really relativity that put E=mc into a sound theoretical context although the formula had been proposed by other physicists before Einstein.

Regarding point 3, I have been trying to persuade Brews of that fact for some time but he seems to have lost interest.

I agree with point 4, that is how it used to be.

I suggest that we wait to see the results of arbitration before we make any major changes now. Martin Hogbin (talk) 16:45, 7 September 2009 (UTC)

Regarding point 3, since the particles making up the metal bar are held together by electromagnetic forces, I s'pose that a change in c would resize the bar, too (provided that no dimensionless constant such as α changes). IIRC, something like that was the standard interpretation of the Lorentz–FitzGerald contraction before Einstein's 1905 paper. --___A. di M. 22:04, 7 September 2009 (UTC)

(BTW, I think we could link Planck_units#Planck_units_and_the_invariant_scaling_of_nature from somewhere in this article, but I'm not sure of where. --___A. di M. 22:12, 7 September 2009 (UTC))

I disagree with points 1–3 as well, but I think the most important for the planning of the article is point 4. While we most definitely shouldn't make the article simplistic, we cannot forget that we are dealing with a topic which is introduced in junior high school. The first point that needs to be made is that the speed of light is finite and not infinite: the history section demonstrates how we know that it is finite. I would then go on to say that the speed of light – given a few disclaimers like "perfect vacuum" – is constant, at least over the period of human experience: again, it is important to show that historical measurements converged on the same value. Only then can you really explain why the majority physical theory says that it must be constant. Finally, to be fair, one should mention that there is a minority cosmological theory which doesn't assume that c has always had the same value that it has today. All the same, I think the history section is at least as important as the high brow theoretical physics and philosophy, if not more so for the majority of readers. Physchim62 (talk) 02:11, 8 September 2009 (UTC)

Hi Lou: I am interested in your statement repeated below:

(If the speed doubles, the wavelength doubles, the meter is twice as long, and the numerical value of c is unchanged). You'd need to go back to the old scratches-on-a-metal bar to express this (rather unlikely) change in SI units.

If ℓ = c t assuming the time of transit is fixed, then ℓ doubles, which I think is what you are pointing at. Of course, the time of transit actually will half when c doubles, so ℓ = 2c t/2 does not change. However, the situation post-1983 is, I think, a bit different. Then ℓ = c₀ t with c₀ = 299 792 458 m/s exactly. (That is the BIPM definition of length). The doubling of the speed of light means the transit time halves, so ℓ = c₀ (t/2) and the length halves too, even though the real length is exactly the same. Of course, those interested in GR will say that the doubling of the speed of light is necessarily accompanied by other changes that make this analysis simplistic. That is why I prefer the example where the measured speed of light changes due to improvements in measurement technique, rather than postulating changes in the actual speed of light. Any comments? Brews ohare (talk) 23:07, 7 September 2009 (UTC)

Is there any proof the speed of light has changed in the last billions of years?Wdl1961 (talk) 00:53, 8 September 2009 (UTC)

Wdl1961: Whether it has changed, or might change, or might be impossible to change, one can speculate about what would happen if it did change, just in order to test one's ideas. Brews ohare (talk) 02:05, 8 September 2009 (UTC)

There seems to be little evidence the speed of light has changed. How many times have the human definition of length and speed changed ? For the little time i got left i will bet on a steady speed of light. We can only do the best with what we know reasonably well now. I am all for thinking exercises but they should be kept in perspective. Personally i have no problem to turn back thirty or more years ago and remember my understanding at that time. On government inspections i only was required to know the calibration equipment was about ten times more accurate than the measuring equipment. It is always a question what is relative to what. The most available, accurate and repeatable will win for the time being, sometimes i go from fingertip to fingertip for length (not for sol.). Wdl1961 (talk) 03:24, 8 September 2009 (UTC)

Brews ohare: What would happen if the metre is defined in terms of c and c changed is not only the same thing that would happen is the metre were defined in terms of the length of a piece of metal and the the length of that piece of metal changed (i.e. the metre would change); but also, since particles in your fingertips are held together by electromagnetic forces, probably their size would change, too, so that light would still take the same time to travel across your fingertips. A varying speed of light would have much funnier consequences to worry about than changing the length of metre. My own bet is that stuff would resize so that we would have no way to even realize that it happened. --___A. di M. 10:14, 8 September 2009 (UTC)

To expand on that an alternative way of describing the expansion of the universe is not that the universe is expanding, but that the speed of light is decreasing (this simply is the redefinition c = c/a(t) with a(t) the scale factor, and using comoving position and momentum as 'true' position and momentum). This would also describe the universe equally well, the interpretation would not be that the universe is expanding but that everything in the universe is shrinking. In the end it comes down to the samething.(TimothyRias (talk) 10:36, 8 September 2009 (UTC))

I think the point here is not about deep things. It is about the functional dependence upon c of the equation for length ℓ₀ in the SI system ℓ₀ = c₀ t, where the time of transit is t = ℓ / c, with ℓ the hypothesized actual spatial separation of two points. One could ask whether these equations are embedded in a more complex system of equations that cause the dependence of the time of transit upon c, say t = t (c) to be different. However, maybe the simpler case already is too complicated? Brews ohare (talk) 14:41, 8 September 2009 (UTC).

May i suggest the following addition in the introduction: "1⁄299,792,458 of a second. As a result, c is fixed at exactly 299,792,458 metres per second." This value falls between the upper and lower accuracy limits of the Paris standard meter bar.Wdl1961 (talk) 15:44, 8 September 2009 (UTC)

I don't understand LouScheffer's point 3:

"The so-called problem that you cannot measure the speed of light in SI did not begin in 1983; the same problem existed since 1960. Suppose the real, physical speed of light changes, with no other physical constant changing. Post 1983, this cannot be expressed in SI since the meter changes to match. But this was true from 1960 on - the changed speed of light will affect the wavelength of the krypton transition, and in exactly the manner that will cancel the change in SI units. (If the speed doubles, the wavelength doubles, the meter is twice as long, and the numerical value of c is unchanged)."

But why would it be the wavelength rather than the frequency (or both) which would change? If the frequency of the radiation were to change in proportion to the change in the speed of light, then the wavelength (and therefore length of the metre) would remain the same. If the period of the radiation from the same krypton transition had been used in 1960 as the basis of the time standard, then in that case the numerical value of the speed of that particular radiation would have been precisely fixed by virtue of the definitions of the length and time standards. But even then, this would not have been true for the speed of radiation of any other frequency, and in any case the time standard wasn't based on the period of any electromagnetic radiation at all in 1960—it was the astronomically based ephemeris second.

Even the redefinition of the time standard in 1967 to be based on the period of radiation corresponding to a certain transition of the cesium atom still doesn't seem to me to entail that the speed of light of any frequency was then fixed by virtue of the definitions of the standards, since it is logically possible for the speeds, c₁, c₂, wavelengths, λ₁, λ₂, and frequencies, ν₁, ν₂, of the two radiations to vary independently of each other, subject only to the constraints c₁ = λ₁ ν₁ and c₂ = λ₂ ν₂.
—David Wilson (talk · cont) 15:14, 9 September 2009 (UTC)

Incidentally, for much the same reason, I don't understand why a change in the "real" speed of light (whatever that means) would necessarily imply a corresponding change in the length of the SI metre. Talk of the SI metre changing can only make sense if one has in mind some other standard of length with respect to which it changes, even if this is only an imaginary canonised sacred length unit (cslu) in some idealised Platonic heaven.

Brews ohare's discussion seems to presume that the wavelength of the radiation from some atomic transition is less likely to change with respect to such a cslu than the SI metre. But why should we presume that this is so? And if we assume that the SI metre might be changing with respect to the cslu, why should we not also assume that the SI second might be changing with respect to its own canonised sacred time unit (cstu)? Ultimately, the "real" speed of light would have to be expressed in cslu/cstu. If it were to be changing, then one or both of the SI metre and the SI second would also have to be changing with respect to their corresponding canonised sacred units, but I don't see why we should presume that it would necessarily be the SI metre (or only the SI metre) that was changing. If the SI second were to be changing (relative to the cstu) in exact inverse proportion to the change in the "real" speed of light (as measured in cslu/cstu), for example, then the length of the SI metre would in fact remain fixed (relative to the cslu).

—David Wilson (talk · cont) 23:12, 9 September 2009 (UTC)

Since writing the above, I have realised that if one uses radiation from the transition beteeen the two hyperfine levels of the cesium 133 atom's ground state to establish the length of your SI metre (which it seems you are permitted to do by the standard, since it doesn't specify that radiation of any specific frequency must be used) then it will be exactly 9,192,631,770/299,792,458 wavelengths of that radiation long, by definition. So if you were to choose the wavelenth of that radiation as the basis for your alternative standard of length, then it would be impossible for the SI metre to change relative to it (although its definition would of course give rise to inconsistencies if there were radiation of another frequency whose speed was different relative to that alternative length standard and the SI time standard).

—David Wilson (talk · cont) 10:38, 10 September 2009 (UTC)

The view that the speed of light is not a fundamental constant at all

This view, which is widely held in the theoretical physics community, is presently not at all represented in the article. It is not an authorative view, many physicists disagree with this view. I.m.o., we have to address this in a section of the article. Then, since the dispute with Brews about the 1983 definition is actually closely related to the dispute about the fundamental nature (or not) of the speed of light, it is easier to give this a place in such a section.

A relevant source we can use is this article. It is an article, Michael Duff argues that there are no dimensionful fundamental constants at all, Okun argues in favor of 3 fundamental dimensionful constants and Veneziano argues in favor of 2 constants. Count Iblis (talk) 13:55, 7 September 2009 (UTC)

Such speculative views might be mentioned in the article in a appropriate section but the majority of the article should be about accepted and verified mainstream opinion on the subject. Such views are not relevant to the dispute involving Brews regarding the 1983 definition of the metre because what we must report here was what was actually done by the BIPM rather that what you think that they might or should have done. Regardless of what anyone here thinks, the metre was fixed in terms of the speed of light. Martin Hogbin (talk) 14:18, 7 September 2009 (UTC)

It is not a "speculative view" that the speed of light is not a fundamental constant. In fact, it is a pretty mainstream view in the theoretical physics community. I agree that this has to be discussed in an appropriate section. And this is relevant to the dispute with Brews, because you can just as well put c = 1 instead of assigning to c the SI value, and then Brews will essentially play the role of Lev Okun in the article I cited above. Count Iblis (talk) 15:54, 7 September 2009 (UTC)

Speculative does not imply that the view is not held or discussed by reputable physicists just that it is not part of any established and verified theory. The link you gave refers to an article concerning superstring theory. As far as I know, no such theories are even theoretically complete yet. For them to become established theories, they need to make some predictions that are different from the current theories and those predictions need to be experimentally verified. Even if and when that is done it will not effect the SI definition of the metre until the relevant authorities decide to change the standard. Martin Hogbin (talk) 17:22, 7 September 2009 (UTC)

I'm not sure what Martin means by "it's not a speculative view". But also I'm not sure what Martin means by "verified mainstream opinion"; this sounds like Science Apologist's way of thinking, that the current dominant paradigm is the one true way. Per WP:NPOV, we should represent all viewpoints, appropriately weighted. If there's a verifiable viewpoint about the speed of light not being constant, or not being fundamental, we need to include that, but not in a way that mixes it in and confuses things. A minor viewpoint should get a minor section. But I'm not convinced based on an article in arXiv.org; can't we do better than that? Dicklyon (talk) 17:23, 7 September 2009 (UTC)

Well I am not sure what I meant by, "it's not a speculative view", mainly because I did not say that. Also note that I did not insist that this view should not be mentioned in the article just that it does not influence the 1983 definition of the metre and thus the speed of light in SI units, for the reasons given above. Martin Hogbin (talk) 17:39, 7 September 2009 (UTC)

I understand, yes, that you're in favor of inclusion; I also agree that "it does not influence the 1983 definition of the metre", but I don't see why you brought that up. Count Iblis's point that "the 1983 definition is actually closely related to the dispute about the fundamental nature (or not) of the speed of light" is sensible enough, but as a rejoinder, yours seems to miss his point. Dicklyon (talk) 17:44, 7 September 2009 (UTC)

The basic argument/dispute does not really depend on superstring theory, it is more about the meaning of natural units, you can use string theory as a setting to debate things as the authors are doing, but it is not necessary. The arXiv article is, I think, the only one that exists in which peple with opposing views write about this issue. It can be used as a source for the claim that a dispute exists at all.

Now, about Brews, his arguments are essentially similar to what has been raised w.r.t. natural units, it is not something that has been a big issue in the physics community w.r.t. the 1983 redefinition of the metre. If you forget about 1983 redefinition of the metre (let's pretend that this did not happen) you could have exactly the same dispute with Brews about natural units in which we simply put c = 1. But then Brews would more or less argue Okun's point and that is something that can be sourced much better. Count Iblis (talk) 18:06, 7 September 2009 (UTC)

Quoting myself from a comment at Talk:Electron:

The fact is that no-one seems to agree about what "fundamental constant" means. Guys such as John Baez and John Barrow use it to mean "dimensionless constant" (because the numerical value of the fine-structure constant doesn't depend on the choice of units of measurement, unlike the vacuum permittivity, for example); the NIST uses it in a much broader sense, including the Sackur-Tetrode constant at 1 K and 101.325 kPa, the {220} lattice spacing of silicon, or the atomic mass constant. The electron-to-proton mass ratio is "fundamental" in both those senses, but it isn't in senses which would make more sense. (Too bad that no-one seems able to clearly explain which these senses which would make more sense are.) But "derived" isn't much better; I'd just say that it is constant.

Do we really need to use the phrase "fundamental constant"? --___A. di M. 21:59, 7 September 2009 (UTC)

I think it doesn't need to be there, but I realize there are potentially good arguments to keep it, so in case others disagree I've added some specific concerns below. Pecos Joe (talk) 22:09, 7 September 2009 (UTC)

On google books you can see pages 165-168 of the book A broader view of relativity where they say c is not fundamental, including a quote from Dirac who said it was. Charvest (talk) 04:55, 8 September 2009 (UTC)

Concerns about the source

After having read the source linked above, I have a few concerns. First, the source was published in JHEP, and that is the version I read. I believe it is available to all. The source is a discussion among three people - the first two (Okun and Veneziano) agree that c is a fundamental constant. Duff argues there are no dimensionful constants; thus, Duff is the only argument against the 'c is fundamental' idea.

Above it is claimed that c is not fundamental is common within theoretical physics, but that assertion is not supported by the article. Duff agrees that the mainstream physics opinion is that c is fundamental, but appears to cite no one who agrees with him that it is not. He says he came to the conclusion during discussions with others at lunch time, and that there is some guy he works with that agrees with him. Based on this, it appears that 'c is not fundamental' is an idea that only he holds, or at least he is unaware of any publications that agree with him. Also note that all authors make a clear distinction between dimensionful and dimensionless constants - Duff said there were 19 of those. If other authors do the same implicitly, they could simply omit the bit about whether any dimensionful constants are fundamental (which may confuse discussions about this matter).

So, to me, the source says that Duff holds the view that c is not fundamental. It is not clear to me why Duff's opinion should be mentioned in the article, but if there are other sources indicating this view is widely held/debated, then this article may provide interesting points to add to the wikipedia article. Pecos Joe (talk) 22:09, 7 September 2009 (UTC)

This issue is not debated a lot in the sense that people write a lot of peer reviewed articles about this issue. I know that Paul Davies has writen an article about changing c and then Duff wrote a comment to that article. And that may be all there exists in the form of peer reviewed articles. For most purposes, it doesn't matter what view you have, so the debates stays limited to the university lunch rooms (no surprise then that, as you can read, the HEP article by Duff et al. was the result debates at the CERN Cafetaria).

If you read the acknowledgments section of the article you can, I think, read between the lines that Warren Siegel supports the view of Duff. If you do some searching on the internet, you can find other examples. E.g., Lubos Motl had a blogposting not so long ago about this issue.

Anyway, this issue is not all that different from other metaphysical debates, like e.g. the interpretation of quantum mechanics. How many physicists believe in the Many Worlds Interpretation? Something has to be mentioned about this on the wiki page about this subject, so we've quoted an article by Tegmark in which he gives the result of a privately held poll. Is that a very reliable poll? Of course not, but the point is simply that there is some sizeable fraction of physicists that believe in the MWI (which I and the other editors on that page know from personal experience) and you have to somehow source such a statement.

Lacking other sources, one can e.g. simply say that: "some physicists are of the opinion that c is not a fundamental constant, it's role being nothing more than a conversion factor." This is more or less what Duff quotes in the article from his private communication with Siegel (Siegel says that the 1983 redefinition of the meter emphasizes this view). Count Iblis (talk) 23:05, 7 September 2009 (UTC)

There seems to be a confusion about what is meant by "fundamental". c in fundamental in the way that all the other dimensional physics constants such as the k_B, the h, ε₀, μ₀, and so on are fundamental. It's not the particular value that is fundamental to physics, but rather the link they make (c is the link between length and time, k_B is the link between energy and temperature, and so on). Not in the way that dimensionless physical constants are, such as α or G_F. Headbomb {_{κοντριβς} – WP Physics} 16:38, 8 September 2009 (UTC)

Indeed. Different people use the phrase "fundamental constant" with different meaning. According to NIST, the molar volume of silicon is a "Fundamental Physical Constant" (whatever they mean by that), too. --___A. di M. 16:55, 8 September 2009 (UTC)

A. di M. has a point here. I gather that Duff views all physics as expressible in terms of a few dimensionless ratios of physical parameters, and proposes that, supposing that being the case, only these dimensionless ratios are fundamental, and not the constituent parameters themselves, in part because only changes in the ratios have observable consequences. Changes in the individual parameters are not observable unless they result in changes in the ratios. I am unclear how Duff would view theories like quantum gravity or MOND that appear to me (an unsophisticated observer) to introduce additional physical parameters into the basic theories. Another issue never to be resolved is whether a "fundamental" parameter has to be observable, or maybe has only to play a theoretical role in the precepts of the theory. Brews ohare (talk) 17:26, 8 September 2009 (UTC)

Charvest's source quotes a rather nice summary by Dirac. The authors also disagree with Dirac, not a good sign. This source contains the authors' conclusions, using Wheeler and Taylor as a source, that demonstrates a misunderstanding of that source and of the 1983 definition. They quote Taylor and Wheeler's remark that "299,792,458 m/s is the work of two centuries of committees" as meaning that c is not a universal and fundamental constant, when all that Taylor and Wheeler mean is that the defined value of the conversion factor in SI units is not a fundamental constant. Brews ohare (talk) 16:59, 8 September 2009 (UTC)

The question of what is 'really' fundamental is beyond the scope of physics but c is certainly an important constant of the theory of relativity. I do not have a particularly strong opinion on whether we should use the word 'fundamental' in the article. There is a general feeling that dimensionless constants may be more fundamental than dimensionful ones but I still think that the speed of light important enough in the current established physics of spacetime (relativity) that we should call it fundamental. Perhaps we should look at articles on other physical constants to see which ones are described as fundamental. Martin Hogbin (talk) 17:27, 8 September 2009 (UTC)

Constancy of speed of light is just a convention

Can this subsection be made to say something? The way it is, I'd say it suggests there is a topic there, but provides so little one cannot fathom it. Is about generalizing relativity? Is it about quantum gravity? Brews ohare (talk) 05:27, 8 September 2009 (UTC)

It is not about those things. It is about alternative mathematical descriptions of special relativity which have different postulates concerning the speed of light, but which nonetheless make exactly the same predictions as special relativity. The existence of alternative ways of describing reality demonstrates that the the speed of light is not a fundamental constant in any way, but a human convention which depends on the way we describe things. Charvest (talk) 05:46, 8 September 2009 (UTC)

Chapter 8 of the cited source describes what the authors call "common relativity". They use this version because it is mathematically more convenient when dealing with certain calculations. In this formulation the speed of light is not constant in all inertial frames, which one might think is a problem, but not in the framework of "common relativity" it isn't. Charvest (talk) 05:51, 8 September 2009 (UTC)

The work you are citing gives an extremely fringe view. I've been unable to find any major publication in independent journals that support it. This means that including it in the wording you gave would and best a major case of WP:UNDUE. I'm not excluding the possibility of the article mentioning the existence of this view in some way, but it would need some kind of secondary or tertiary source to gauge its place in scientific discussion. (TimothyRias (talk) 08:33, 8 September 2009 (UTC))

Well the section has been deleted by two different editors, so I won't re-add it without getting agreement. For the record it said: Concepts such as force, mass and speed are just conventions. Just as epicycles or ellipses are descriptions of the solar system from different points of view, so the whole of relativity is just a mathematical description from a particular point of view. There can be other equally valid descriptions starting from different conventions. One description might be more convenient in certain circumstances, but other descriptions can be more convenient in other situations. In some descriptions the speed of light is not constant. It was deleted with the reason that it was a fringe view. I would say "common relativity" is more acccurately described as a not very well known formulation rather than a fringe view. Charvest (talk) 08:30, 8 September 2009 (UTC)

I would add that making changes to the postulate about the speed of light is far from fringe - Test theories of special relativity is all about making such changes. And the "Constant speed in inertial frames" section does say that Einstein was aware that his postulate was just a convention. Charvest (talk) 08:41, 8 September 2009 (UTC)

My remarks were mainly aimed at the fringeness of the source used (and the section title). The paragraph in the article itself was so vague it was meanless, and appeared in an odd context for that discussion. (TimothyRias (talk) 10:21, 8 September 2009 (UTC))

Fair enough. I've split the discussion of test theories into its own section as that is a separate topic. Charvest (talk) 20:07, 8 September 2009 (UTC)

Charvest, I understand the point that you are making. Some people are surprised that the (one-way) speed of light should be merely a convention but ,as you have pointed out, so is much of physics. This is a subject on which we need to tread very carefully so as not to stir up pointless heated debate here and confuse the general public. See the heated, and in my opinion, rather pointless debate at Evolution_as_theory_and_fact.

I think that there is a case for expanding carefully on the constant speed of light but we should probably tackle the more general philosophical issues by directing the reader to other articles where they can be discussed more fully. Martin Hogbin (talk) 09:31, 10 September 2009 (UTC)

Test theories of special relativity

Different conventions about the one-way speed lead to test theories of special relativity some of which differ from special relativity in their physical predictions.

Charvest, you seem not to understand the purpose of Test theories of special relativity. I suggest that you read the article before making any suggestions for change to this article. Martin Hogbin (talk) 10:48, 8 September 2009 (UTC)

I am surprised that this topic has not come up before. Contrary to what some might suggest, there is nothing new or controversial in this subject. If anyone really wants to understand this topic then I recommend that they obtain a copy of 'Special Relativity and its Experimental Foundations' by Yuan Zhong Zhang and read it. Otherwise, the situation is summarized in the relevant section of this article.

The two-way speed of light, that is to say from a source to a mirror and back again has been experimentally confirmed to be constant within tight limits (as always, when measured in a vacuum and an inertial frame). Also the one-way sped of light has also been shown experimentally to be independent of the motion of the source. However, the one-way speed, for example, from a source to a distant detector, cannot be measured without some convention as to how the clocks at each end should be synchronized. There was a short time when some reputable scientists may have believed that there were experiments that could measure the one-way speed of light independently of any clock synchronization scheme (for example it might at first sight appear that Romer's original measurement does this) but Zhang carefully re-analyzed these experiments and showed that, as Einstein has originally supposed, the determination of the one-way speed always requires a convention on how to synchronize distant clocks. His book is now widely recognized as the definitive work on the subject.

So, what are the choices when synchronizing clocks. One is to use Einstein's scheme, however this uses the postulated constant one-way speed of light to synchronize the clocks. Clearly, using clocks synchronized in this way to measure the one-way speed of light would be a pointless exercise as it would obviously give the expected value. Alternatively we could use slowly transported clocks (we know that moving them quickly produces strange results). The only problem is that we have know way of knowing that the clocks do not lose their synchronization whilst they are apart. However, experimentally and according to Einstein's theory this produces the same results as Einstein's scheme.

It is possible devise a theory using an additional parameter relating to the way that distant clocks are synchronized. This is essentially what was done by Lorentz in his aether theory, which was published the year before Einstein published his paper. There is an additional parameter that appeared in the equations but it could be set to any value (less than c) without affecting any experimental prediction. Zhang refers to this theory as Edwards theory in his book. One could liken this position the one that pertains to electric potential. There is no way to measure an absolute electric potential, only a potential difference, although we can set an arbitrary zero if we wish, such as the potential of the Earth. Einstein's solution was to drop the parameter completely, knowing that this would make no difference to anything physically measurable or detectable, and that is the solution universally adopted today. Anyone who wishes can add in a parameter that would, in principle, affect the one-way speed of light but as there is no way to measure the value of this parameter it serves no real purpose. In any real experiment the value makes no difference whatsoever and thus, by convention, it is ignored.

Finally, we might ask what the relevance of all this is to things like the realization of the metre. The answer is 'none'. All delineations of this kind are done using interference techniques which essentially rely on the two-way speed of light. If we really wanted to be fussy, we could point out that the two way speed of light is fixed by the definition of the metre and the one-way speed in fixed by this definition and a further convention. However, we already have a section that explains this.

If have no objection to saying more about this subject in the relevant section of the article (please not the lead), provided that absolutely no suggestion is made that there is any confusion, conspiracy, lack of understanding, cover-up, or uncertainty amongst physicists or other scientists. Martin Hogbin (talk) 10:02, 8 September 2009 (UTC)

Martin: Thanks for the explanation. When you have time, please consider doing some work on the Test theories of special relativity article, which is clearly in need of expert help. —Finell (Talk) 15:38, 8 September 2009 (UTC)

On this talk page I've split off the discussion of Test theories into its own section as it is a separate topic from the other one. I only used Test theories in my reply to Timothy Rias to emphasize that using a different postulate about light has been done before. It was not to suggest that the purpose of test theories was the same as the purpose of "common relativity".

When I added "Different conventions about the one-way speed lead to test theories of special relativity some of which differ from special relativity in their physical predictions." to the article, I did not mean to imply that these were potential alternative theories, and when I said some I was also thinking of Edward's theory which isn't actually a test theory at all, so I accept that the wording of that sentence is not suitable. In fact the article test theories of special relativity is rather short and should really be greatly expanded before linking to it. Charvest (talk) 20:04, 8 September 2009 (UTC)

OK but in that case we have to ask whether those test theories have any relevance to this article. They would be more relevant to any pages discussing the experimental verification of special relativity. Martin Hogbin (talk) 21:37, 8 September 2009 (UTC)

Possible dependence on frequency

This section is unclear. Perhaps it can be combined with "Cosmology and quantum gravity" which certainly looks at frequency dependence? Brews ohare (talk)

I question whether this material is sufficiently well established to belong in this encyclopedia. If it is, I agree that it requires a more encyclopedic treatment. —Finell (Talk) 21:48, 8 September 2009 (UTC)

A Google scholar search turns up papers like W. Bednarek and R. M. Wagner that do not attribute this time delay to different speeds of propagation of light, but to a delayed emission model. Therefore, this sub-section is not pertinent to this article. See also Martinez & Errando. Brews ohare (talk) 22:26, 8 September 2009 (UTC)

Brews's sources show an alternate explanation of the observations. That, plus the speculative nature of the hypothesis itself, which bets against relativity, and the lack of quality secondary or tertiary sources, persuades me that the observations are not sufficient for an encyclopedia (as distinguished from a paper) to include this material as support for the proposition that c is frequency dependent. I deleted it. —Finell (Talk) 00:43, 9 September 2009 (UTC)

I think we need to make a much stronger distinction between established and experimentally verified theories on this subject, which are classical EM and relativity (with QED at the quantum level) and current physics research, experiments, and observations. In the currently accepted and verified theories, c is fixed and an important constant.

There is much ongoing research into cosmology and quantum theories of gravity but we should not try to write a general review of such theories or pick out specific ones for mention, unless we have a real expert on such matters amongst us. We should either make a simple statement along the lines of 'There is much ongoing research into... in some of these theories the speed of light may play a less important role or may vary with time, frequency etc...', or we should look for a reputable and accessible general review book or paper on the subject to base our comments on. In my opinion, it is far better to say nothing at all than to say something wrong, misleading, or even correct but not particularly important. We should not base our article on the odd paper plucked from arxiv or quotation from a popular science magazine. Martin Hogbin (talk) 09:18, 10 September 2009 (UTC)

Martin: What have your remarks to do with deletion of this section? Are you supporting it or not? Did you put your remarks in the wrong place? What are they about? Brews ohare (talk) 13:11, 10 September 2009 (UTC)

Hypothetical Questions

something i don't get; if earth was moving through space at 99% the speed of light, and someone emitted a beam of light in the opposite direction, would it appear as though the beam is moving twice the speed of light to us, or would it appear still to any bystanders?Chocolog (talk) 13:22, 9 September 2009 (UTC)

This is not really the place to ask questions of that nature but the short answer is 'no'. There is no such thing as motion 'relative to space' but the earth could be moving relative to some other object. Have a look at the section 'Composition of velocities'. Martin Hogbin (talk) 13:30, 9 September 2009 (UTC)

Neither, it would appear to move with the same speed for both. I know that sounds ridiculous but it can be verified experimentally. (TimothyRias (talk) 13:32, 9 September 2009 (UTC))

Constant speed in inertial frames.

I am not sure why it was necessary completely rewrite this section, a copyedit is one thing but this was more than that. The sense of the original has now been lost and the text contains a number of errors in the physics. Where now? Martin Hogbin (talk) 19:13, 9 September 2009 (UTC)

I've reverted it. Charvest (talk) 19:31, 9 September 2009 (UTC)

Corrections to the modifications that the copyediting introduced:

Non-inertial frames result from acceleration, not just gravity.

Observers do not have motion with respect to their inertial frame.

The speed of light has been shown experimentally to be independent of the motion of the source, full stop. Charvest (talk) 20:08, 9 September 2009 (UTC)

Thanks, I think this was the best option. If there are problems with the English or style I am sure they can be addressed but at least the physics is now correct. Martin Hogbin (talk) 21:37, 9 September 2009 (UTC)

Sorry, I obviously didn't mean to muddle the physics. The statement about spacetime being curved in a non-inertial frame due to gravity, without mention of acceleration, was there before me. I knew it was incorrect, but I didn't change it and didn't look at the cited source. There was a recent awkward edit that drew me to the section, and my only intent was to try to improve the way it read (replace passive with active voice, etc.), eliminate some redundancy, and eliminate a few things that were a bit off the topic of that section. I may try a more limited copy edit, but not now. —Finell (Talk) 00:01, 10 September 2009 (UTC)

Finell, I am very happy to work with you to improve that section if you think in is not clear of if you feel the style could be improved. I also wonder if the section should be expanded, it is quite central to the subject of the article. Martin Hogbin (talk) 08:30, 10 September 2009 (UTC)

Metre bar in lead?

There is a new, unsourced sentence in the lead, at the end of the second paragraph: "This value falls between the upper and lower manufacturing accuracy limits of the Paris standard metre bar." "This value" referred to the 1983 redefinition of the metre. I don't believe that the metre bar had anything to do with this, and had been abandoned as the standard metre long before 1983. Should this sentence be deleted? —Finell (Talk) 00:16, 10 September 2009 (UTC)

Whether true or not (and given the 'distance' between the 1983 definition and the old platinum standard metre, I'm quite wary of this unsourced statement), this seems like too minor a point to need to be in the article lede. If a suitable source is found, the fact might belong elsewhere within the article. TenOfAllTrades(talk) 01:01, 10 September 2009 (UTC)

Wdl1961, who inserted the sentence, now added a web reference (although with an incorrect URL). The source is a web page on an anonymous site, so it is not a WP:RS. Further, the source says nothing about the "manufacturing accuracy limits of the Paris standard metre bar." I deleted the sentence until there is some consensus that (1) it is accurate, (2) it is supported by a WP:RS, and, most importantly, (3) it is sufficient importance to be in the lead. —Finell (Talk) 02:18, 10 September 2009 (UTC)

I agree with removing this sentence for the reasons given but I have generally stayed away from editing the lead as I think it needs completely rewriting. I originally suggest that this was done after the content of the main body of the article was stable. The reason for this is that the lead should be a summary of the article. It is not the place to introduce new content, or discussions that are not dealt with more fully elsewhere.

When things have settled down after the arbitration decision I am going to suggest that we get the content of the article stable then rewrite the lead from scratch to make it a clear and concise summary of the overall article in a good style of English. It should then only be changed to reflect changes to the article. Martin Hogbin (talk) 08:22, 10 September 2009 (UTC)

I think this is a clear case of the editor not quite understanding the source: the editor seems to be confusing the old International Prototype Metre with the national copies: the national copies couldn't be exactly the same length because of manufacturing difficulties, so there was a correction applied to each of them (eg, 1 m + 0.2 µm, a hypothetical example in its numbers). There's also a confusion between precision and accuracy: one can be precise without being accurate, and vice versa. The International Prototype Metre was defined as being exactly one metre long, so there were no manufacturing defects! Well, actually, there were, but these should be discussed in Metre on in an even more specialized metrology article, not here IMHO. I agree that the comment should be removed. Physchim62 (talk) 10:49, 10 September 2009 (UTC)

Doppler

In the box under "Doppler Effect", the words "right" and "left" seem to be mistakes for "top" and "bottom". —Preceding unsigned comment added by 217.44.109.137 (talk) 08:56, 10 September 2009 (UTC)

Sorry, forgot to adjust that as well when I introduced the more compact version of this picture. (TimothyRias (talk) 09:51, 10 September 2009 (UTC))

Speed of light set by definition section

Currently, this section drifts somewhat off-topic in to the finer points of metrology concerning the metre, which is not the subject of this article. What is it that we really need say in this section. The first paragraph no covers the when, how and why of the redefinition of the metre. (Should the why part be expanded more upon?)

(The gist being that we could measure the speed of light more accurately than we could measure the then current metre and thus decided to use it as a new (more precise) definition.)

What else should we discuss? There should be at least something about the value of the speed of light become exact when expressed in m/s (and any units derived from those). IMHO there should also be a few lines explaining that even though the value of the speed of light is fixed, that this does not change anything of the involved fixes. It does not change anything about the physical act of measuring the speed of light (= comparing it to some other speed). All this, of course, with an adequate source.

Is the tag on paragraph about the word vacuum in the definition really needed? This pertains much more to the definition of the metre than to the speed of light. If it is thought that this is really needed, it may be much more functional in a footnote to the definition, instead of at the end of the section where it really feels like something that was tagged on. {Unsigned contribution by User:TimothyRias 10:16, 10 September 2009}

Hi Timothy: Your statement that "we could measure the speed of light more accurately than we could measure the then current metre and thus decided to use it as a new (more precise) definition.)" is not quite so. The difficulty in measuring the meter was the error in fringe counting. By switching from a length (fringe count) to a time-of-flight definition, fringe counting was eliminated and only time had to be measured. Naturally, with only time to measure, the error was only that of time measurement. The switch in definition is not about improved accuracy in measuring the speed of light. In fact, the speed of light became a defined not a measured quantity. Take a look at Sydenham and Jespersen & Randolph. Brews ohare (talk) 13:28, 10 September 2009 (UTC)

Timothy: Your remark "even though the value of the speed of light is fixed, that this does not change anything of the involved fixes. It does not change anything about the physical act of measuring the speed of light (= comparing it to some other speed)." Is worth some discussion here. Of course, you are right that one can compare the speed of light with any other speed just as before, because that is a dimensionless ratio and is independent of units. However, that does not measure the speed of light except as a multiple of some other speed. The speed of light in units of m/s cannot be measured any longer, because it is a defined value of 299 792 458 m/s and that is it. Brews ohare (talk) 13:44, 10 September 2009 (UTC)

Timothy, That's it in a nutshell. The new SI units speed of light is beyond measurement. It is a definition with an arbitrarily chosen number. The speed of light in all other systems of units is a physical quantity that can be measured. This article is about the speed of light, and so it should be concentrating on the latter concept. Physics as a topic is bigger than any particular system of units. We cannot subordinate the physical speed of light to the SI system. David Tombe (talk) 13:58, 10 September 2009 (UTC)

Timothy: Your statements: "Is the tag on paragraph about the word vacuum in the definition really needed? This pertains much more to the definition of the metre than to the speed of light." also deserve some discussion here. This section is about "the speed of light by definition". That topic is inextricably bound to the definition of the metre, as that is how the defined speed of light arises. Thus, it is difficult to support the notion that somehow the two definitions (metre and SoL) are separable. In the same vein, the definitions of both of these are inextricably related to the 'vacuum', and it seems to me pertinent to point out that the 'vacuum' in the sense meant here of 'classical vacuum' is not your garden variety terrestrial vacuum. In effect, the "defined value" of the speed of light applies nowhere in the universe but measured values of the speed of light in this universe can be referred to 'vacuum' by implementing the proper corrections of standard practice (as approved by the BIPM). Brews ohare (talk) 13:53, 10 September 2009 (UTC)

Timothy: Another point you have raised is "the physical act of measuring the speed of light". Inasmuch as the speed of light is defined in SI units, such a measurement must go beyond SI units. That is, a unit of length must be introduced that, unlike the metre, is not defined in terms of the numerical value of the speed of light, but that is independent of the numerical value of the speed of light. So, for example, one could use as a unit the wavelength of some transition, and count fringes. (The wavelength approach assumes the speed of light is a constant, but does not prejudge its exact value.) That would produce the speed of light in wavelengths/s, and would introduce the errors of fringe counting. So the measured speed of light in wavelengths/s would have similar error bars to that using the old pre-1983 metre, which after all was used in exactly this way. Perhaps something about this should be added? Brews ohare (talk) 14:17, 10 September 2009 (UTC)

Timothy: Not to be cute about it, but if one imagines the physical act of measuring the speed of light, and one entertains that the 'exact' speed of light is 299 792 458 m/s, what exactly is one trying to measure? My point is that there are two concepts here (a point that Dicklyon will fight to his dying breath), namely the SI units defined value and the value one seeks when doing "a physical act of measuring". Brews ohare (talk) 14:32, 10 September 2009 (UTC)

• Well, yet again, we have three comments from Brews ohare (talk · contribs · deleted contribs · nuke contribs · logs · filter log · block user · block log) and one comment from David Tombe (talk · contribs · deleted contribs · nuke contribs · logs · filter log · block user · block log) that serve only to promote their own idosyncratic views and not – in any way – to answer the editorial question posed at the top of this section. Physchim62 (talk) 14:37, 10 September 2009 (UTC)

Oops!

I've made two or three little edits which I don't think are contentious in their context, but I've just realised that I've used ν (Greek nu) for frequency, while elsewhere other editors have used f. It doesn't really matter to me which we use, as each one is used almost exclusively in some contexts and almost never in others, we will never please everyone! What do other editors think? I guess the "default" position is to standardise on f, but I wanted to check before changing (or not changing) anything. Physchim62 (talk) 10:24, 10 September 2009 (UTC)

1. Uzan, J-P; Leclercq, B (2008). The Natural Laws of the Universe: Understanding Fundamental Constants. Springer. pp. 43–44. ISBN 0387734546.
2. Greene, G (2003). The Elegant Universe. WW Norton & Co. pp. 55–56. ISBN 0393058581.
3. Davies, PCW (1979). The Forces of Nature. Cambridge University Press. pp. 127–128. ISBN 052122523X.
4. Duke, PJ (2000). "Electromagnetic waves in free space – no electric charges or currents". Synchrotron Radiation: Production and Properties. Oxford University Press. p. 53. ISBN 0198517580.
5. Schwinger, JS (2002) . "Gravitational waves". Einstein's Legacy: The Unity of Space and Time (Reprint ed.). Courier Dover. p. 223. ISBN 0486419746. {{cite book}}: External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help)
6. Wei-Tou Ni (2005). "Empirical foundation of the relativistic gravity" (PDF). Intl J Modern Physics D. 14: 901–921.
7. Jong-Ping Hsu, Leonardo Hsu (2006). A Broader View of Relativity (specifically chapter 8). World Scientific. ISBN 981-256-651-1.

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