Talk:Speed of light/Archive 9

The use of a definition for c replaces the length standard with the time standard
Martin removed this explanation from the article without discussion on this Talk page:"The first effective measurements of the speed of light were made in the 17th century, and these were progressively refined. Modern time standards are more reproducible than length standards, so in 1983 the standard metre was defined in terms of the speed of light in a vacuum, replacing the length standard with the time standard."and replaced it with"The first effective measurements of the speed of light were made in the 17th century, and these were progressively refined, until in 1983 the metre was defined in terms of the speed of light in a vacuum."Unfortunately the replacement sentence contains a non sequitor. Why should improvements in measurement of the numerical value of c lead to use of c as a defined value, thereby making no use of this improved measurement skill at all, but making it moot? The earlier text has the merit of making sense out of this change as a means of improving the accuracy with which the metre can be used.

To quote Sydenham "If the speed of light is defined as a fixed number then, in principle, the time standard will serve a the length standard." Brews ohare (talk) 14:14, 9 August 2009 (UTC)


 * Brews, I'm still trying to figure out what the edit war is all about. I'm now clear on the fact that the year 1983 radically altered the meaning of the speed of light. Is the edit war about the fact that some editors are refusing to acknowledge that the re-definition of the metre had any bearing on the meaning of the speed of light? David Tombe (talk) 14:23, 9 August 2009 (UTC)


 * David: I'm in accord that the replacement of the length standard with the time standard in 1983 is a change of approach. I'm unclear what the resistance to a simple statement of the change is here. Two possible sources are: (i) Confusion over the distinction between the physical entity "speed of light" that enters into special relativity and the numerical value of the speed of light in SI units, and (ii) Lack of understanding about how the prerequisite properties of c (as postulated by the special theory) may be experimentally verified given the SI units definition. Brews ohare (talk) 14:35, 9 August 2009 (UTC)

Brews, I am not trying to start an edit war, I am just trying to make the wording a little more concise. My wording is just intended to indicate the historical order in which things happened, first measurement of the speed of light improved, then the speed was set by definition, no causation was implied. I will try to rewrite it to be clearer.

About time and length standards, to say a length standard has been replaced by a time standard is not correct; one type of length standard has been replaced with another length standard that is based on a time standard. The question is how to say this neatly. I will try to find a way to do so. Martin Hogbin (talk) 15:53, 9 August 2009 (UTC)


 * The length standard has not been replaced by a time standard. The speed of light is only "fixed" in SI units, and the different SI base units are independent of one another (under the current definitions, notwithstanding GR). Length measurements are based on multiples of a given wavelength of light, and the speed of light comes in as the "conversion factor" between wavelength and frequency. The second, of course, is defined in terms of a frequency standard. Physchim62 (talk) 16:36, 9 August 2009 (UTC)


 * It is a bit of hairsplitting to say the length standard is not replaced by a time standard but is "replaced by another length standard based upon a time standard." Sydenham does not find it necessary to split this hair, for example. The underlying issue is that length is now proportional to time with a fixed proportionality factor, so the accuracy of time measurement is reflected in the length standard. That was the purpose of the change to a defined value for c: to improve the accuracy or the length standard by taking advantage of the accuracy in the time standard. That is what has to be said, in place of the non-sequitor. Instead, the new wording removed the non-sequitor (a step forward) and deleted the entire discussion of the reasoning (a step backward). Brews ohare (talk) 19:27, 9 August 2009 (UTC)

Physchim62, What I am beginning to witness here is a denial on the part of most editors, that something fundamentally changed in relation to the concept of the speed of light with the 1983 definition of the metre. It changed from a speed to a tautology. Few seem to want to admit that fact. David Tombe (talk) 16:42, 9 August 2009 (UTC)
 * Not at all, you are getting mixed up between the speed of light, a physical quantity which most people believe is unchanging, and its value in a given set of units of measurement. The distance between Paris and Marseilles (for example) hasn't changed significantly dispite the various redefinitions of the metre! There is only a tautology if you believe that we can somehow choose the speed of light to be whatever we want it to be. We can't, of course, but we can choose the metre (the human construct) to be whatever we want it to be, which is exactly what happened in 1983 (and several times previously, for that matter). Physchim62 (talk) 17:39, 9 August 2009 (UTC)


 * Physchim62, I know this is not the place to discuss this, but the second of course is most certainly not, as you say, defined "in terms of a frequency standard". It is defined in terms of a count, and that is what makes the difference.
 * David Tombe, what you need, is a soapbox. Try for instance Usenet. You'll feel much better at ease there, and the place was created for people like you in the first place. Good luck. DVdm (talk) 17:48, 9 August 2009 (UTC)


 * DVdm, these suggestions about a soap box are uncalled for, contrary to WP policy, and irritating. They detract from the atmosphere of this discussion. Brews ohare (talk) 19:29, 9 August 2009 (UTC)


 * These are not suggestions but facts. There is a wiki policy against soap boxing. David Tombe is clearly abusing these talk pages to vent his frustration over relativity theory. That abuse is what you can rightfully find irritating. DVdm (talk) 20:01, 9 August 2009 (UTC)

The statement by D Tombe "'What I am beginning to witness here is a denial on the part of most editors, that something fundamentally changed in relation to the concept of the speed of light with the 1983 definition of the metre. It changed from a speed to a tautology.'" is not "soap boxing", but an observation upon editor conduct (or misconduct). It is not an abuse of Talk pages. Brews ohare (talk) 00:20, 10 August 2009 (UTC)


 * Don't be silly! David Tombe is using this talk page to promote his own very idiosyncratic view of metrology. His comments have no bearing on the article, or even on mainstream science. DVdm is quite right to ask that he desist. Physchim62 (talk) 00:48, 10 August 2009 (UTC)


 * It does seem like vintage Tombe. It's kind of amusing to see him teamed with up with Brews.  Poles apart usually, but feeding on each other...  Dicklyon (talk) 01:05, 10 August 2009 (UTC)

I hate to amuse. However, Tombe is perfectly right about the change to tautology, and the sources agree with him. Just what leads to the desire among editors to pussyfoot around this with elliptic statements too short to get the point across may not be denial of the facts, but it is denial of the need to call a spade a spade in clear language that doesn't require a legalistic reading for understanding. Brews ohare (talk) 05:13, 10 August 2009 (UTC)


 * So let me "call a spade a spade" three times:
 * Spade 1: The second is defined as a count.
 * Spade 2: David Tombe is soap boxing his anti-relativity POV.
 * Spade 3: Either David and Bruce don't understand what a count is, or they don't understand what a tautology is.
 * There you go, spades galore. DVdm (talk) 07:06, 10 August 2009 (UTC)

Physchim62, I wasn't getting mixed up between "the speed of light, a physical quantity which most people believe is unchanging, and its value in a given set of units of measurement", to use your precise quote. If you read what I have said above, you will see that I was pointing out this very distinction and stating that the pre-1983 definition relates to the former, whereas the post 1983 not only relates to the latter but that it is also a tautology.

The tautology is clear for all to see. Since 1983, the speed of light has come to mean "the factor of k times the distance that light travels in 1/k seconds, in one second". It doesn't matter what numerical value we substitute for k because ultimately the statement tells us nothing. The value, 299 792 458, that was chosen was merely such as to ensure that the transition to the new system went smoothly and unnoticed.

My objective here was merely to try and ascertain what the edit war is about. I am beginning to realize that the edit war is rooted in the fact that most other editors here don't want to admit that the post 1983 definition of the speed of light is merely a tautology. The actual physical significance of the speed of light, since 1983 has been devolved into the definition of the metre.

Brews has raised the question as to why this simple fact is meeting with such resistance and I am inclined to agree with him that the reason is rooted in a gross misunderstanding of the theory of relativity amongst those who support that theory. David Tombe (talk) 08:48, 10 August 2009 (UTC)


 * The SI metre/speed of light definitions are problematic, however they are valid. The SI metre definition should be understood as : The metre is the 9 192 631 770 ⁄ 299 792 458 x wavelength of the radiation in vacuum corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. That means, the metre is derived from the wavelength of real and distinct physical effect, independent of any SI unit. The wavelenght effect is physical space distance. Calibration (measurement) can be theoretically preformed using interference, without any time device. The fact, that both second and metre are derived from the one and the same effect - radiation - can be considered as not problematic, if valid technical solutions are available. The problematic consequence of definitions are complications regarding the speed of light measurement and comparsion. The numbers are from the physical point of view arbitrary. I think, that SI definition of metre shoud be at least corrected, not to express the time unit dependence, because it is not true, it is just misleading, and not representing the units model correctly. All three definitions are entangled using the simple equation x=vt and shared etalon. The same problems will emerge when using any other speed - in relation to that speed. Entanglement of physical phenomena into units is not a secondary matter and should be considered seriously, both when constituting the units and when using the units. However, it is not elementary to constitute high-precision units of space and time, and physical consequences cannot be absolutely avoided. When SI units cannot be used, due to physical reality of the experiment, measurements have to be performed in experiment units, convertible to SI units. Whether any physical reality can be converted to current model of SI units is the question. Softvision (talk) 08:50, 10 August 2009 (UTC)

Softvision, I'll reply here to your note on my talk page. I think that you are making this whole issue far more complicated that it needs to be. I can't see why we need to bring caesium atoms into this. Speed is the rate of change of distance with respect to time. Distance and time are the two fundamental dimensions. Hence if we have a standard of length, such as one of those wooden yard rulers that primary teachers use on blackboards, and a standard of time, such as the second (based on the year), then we will have a basis upon which to understand the concept of speed. As it turns out, light travels at 186,000 miles per second. That's all there is to it. There is no need to turn it all upside down by using speed as the starting point for understanding distance. Doing so leads to the tautology that we have had since 1983 as regards the speed of light. Since 1983, light travels k times the distance that light travels in 1/k seconds, in one second. k can be any number and they have chosen the number 299 792 458 so as to maintain continuity with the old order. It is this tautology which has confused so many people and led to all this computer storage space being wasted here in relation to this never ending debate. David Tombe (talk) 19:47, 10 August 2009 (UTC)


 * David, I don't want to complicate things futher. My conclusion can be expressed as follows : We cannot define absolute space and time units, because reproducibility requires physical dependence. Current (corrected) definitions are valid in the sense, that they are derived from distinct time effect (period) and distinct space effect (wavelength). My recommendation is to correct SI metre unit definition to reflect this fact. Without that, SI metre definition can be considered as tautology, because $$1 [m] = 299792458 [m/s] \frac{1}{299792458} [s] = 1 [m]$$. I am sure, that intension of current metre definition is not this. Correcting this error, we obtain an entangled definitions, hiding the physical significance of the speed of light quantity, as regular consequence of this model of definitions. Whether any physical reality can be converted to current model of SI units is the question. I think (know) that not. We must think, if there is a better modus of space and time units definitions, more suitable for the experiments and measurements that we need to perform, or to use existing units the best we can. Softvision (talk) 21:30, 10 August 2009 (UTC)

Softvision, I think that we're just going to have to accept that we're never going to get an exact standard of length. There will always be some variable in whatever standard we choose. We just have to adopt as best a system as we can and stick to it. We shouldn't allow unnecessary concerns about accuracy cause us to become blinkered about the higher purposes for which we use our systems of units. David Tombe (talk) 21:44, 10 August 2009 (UTC)


 * Finally I would like to explain my sentence : "Whether any physical reality can be converted to current model of SI units is the question." I think, that there are experiments, which measurements in experiment units, convertible to SI units, when converted to SI units, will display zero delta, but in experiment units will display nonzero delta. It is not hard to understand, that the experiments I am talking about are related to speed of light. (...) I don't want to complicate things futher. Softvision (talk) 22:11, 10 August 2009 (UTC)

Softvision, Are you saying that there are experiments in which physical changes in the speed of light are detected but that these changes don't show up in modern SI units?

New lead paragraphs
I've tried to organize the lead section more systematically. In particular:

First paragraph, basic info accessible to any educated person (no jargon at all)

Second paragraph, how it's used as a fundamental constant in physics.

Third paragraph, measurement history.

Fourth paragraph, practical consequences.

Fifth paragraph, it is not the speed of all light, but just in a vacuum, and in other cases it may be different.

I think this is a more coherent introduction; but of course other's opinions may differ. Also, I think we should get rid of the footnotes, and move them down to the detailed sections. This is standard practice in Wikipedia, and makes the intro read more smoothly. LouScheffer (talk) 13:27, 10 August 2009 (UTC)


 * Looks great. I hope we'll be able to resist the temptation to complicate it again by quibbling over details like "vacuum".  Dicklyon (talk) 14:45, 10 August 2009 (UTC)


 * I don't like the changes. They again tend to emphasize the unimportant number given the speed of light in SI units, and underplay the importance of the real physical speed of light and its properties established by confirmation of the special theory of relativity. The link is made to vacuum, which is inaccurate, referring to real media (partial vacuum), which is not where c has this numerical value. The speed of light in general relativity of course includes special relativity in a local region, but the wording suggests the role is the same in both theories. Still no indication is given of why a defined value of c was chosen. This very confusing point should be dealt with up front in simple terms.Brews ohare (talk) 14:57, 10 August 2009 (UTC)


 * I think this is a point well understood. F=MA is not true, either, in the presence of any drag, which is always there.  When you refer to physics in a vacuum, it's well understood the explanation applies to a hypothetical perfect vacuum, and experimentalists need to correct for the imperfect nature of any real vacuum.   This correction is obviously important to NIST, etc,, but is correctly described in these sections, not in the first sentence.   How about add 'perfect vacuuum"? LouScheffer (talk) 15:15, 10 August 2009 (UTC)


 * Lou, it looks like you forgot who is the real owner of this article. Good luck ;-) DVdm (talk) 15:18, 10 August 2009 (UTC)


 * In this case I agree with Brews. It would be better if we all made comparatively small changes to the article at a time, especially to the lead, so that they could be discussed as we go along.  For a complete rewrite of the lead, I would suggest that it is discussed first. The style of the lead is now rather unencyclopedic.  I would suggest that it is reverted to is former version then its deficiencies are discussed. Martin Hogbin (talk) 15:44, 10 August 2009 (UTC)


 * Hi, Martin! I think these are pretty small changes (almost all is re-ordering), then mostly copy-editing.  The previous version was quite scattered, in my view.  As for the tone, I personally think the new tone is more in line with Wikipedia expectations, where most of the readers will not be physicists, or even technically oriented.  (As a result, I think the lead paragraphs in particular should be as simple as possible without misleading the reader.)  Compare, for example, the lead in Hubble Space Telescope or Cosmic Microwave Background Radiation, other leads I've worked on for quite a while.  My suggestion is to leave the new lead up for a few days and see what others think.  In particular, I'd love to know the opinions of those who are primarily readers, and not editors of this fairly contentious article.  Clearly it's a topic where opinions differ.   LouScheffer (talk) 16:29, 10 August 2009 (UTC)


 * Well, Lou, we could debate the F=ma allusion, which I feel is incorrect. But to stick to basics: (i) Why not make the link to free space instead of the generic vacuum, as in vacuum? (ii) Why repeat the magic number 299,792.458 twice in the intro? That is definitely overkill for what is, after all, an arbitrary number determined by international convention, and a matter of some practical but zero theoretical importance. (iii) Along this line, I'd suggest deletion of the entire third paragraph about "The speed of light is great" because the topics raised are inadequately presented, particularly the matter of the interpretation of the defined value for c. It is a case of take the time to make a sensible presentation, or leave it to the article proper. Brews ohare (talk) 15:46, 10 August 2009 (UTC)


 * I have no trouble with changing/removing some of the exact figures. Perhaps the first one should be approximate, as in introductory physics where 300,000 km/sec is often used?  LouScheffer (talk) 16:29, 10 August 2009 (UTC)


 * You are joking, right? I have restored th exact value. Good grief... DVdm (talk) 17:04, 10 August 2009 (UTC)


 * Obviously he's not joking; for most purposes, the conventional approximation is all the reader wants. The problem here is simply too many cooks; everyone has their favorite spice they want right up front, and the result is a foul-tasting broth.  Dicklyon (talk) 17:42, 10 August 2009 (UTC)

Somebody called Chez37 put in the speed of light in miles per hour and it got deleted. That's a sign that things are going too far and that all sight has been lost of what the article is supposed to be about. Why shouldn't we be allowed to mention the fact that light travels at 186,000 miles per second? What about reducing the whole article to a single sentence stating the fact that the speed of light is 186,000 miles per second? David Tombe (talk) 20:41, 10 August 2009 (UTC)


 * Chez37's input got reverted because he tried to express light speed exactly in miles per hour. You can't do that with decimal numbers. Check the definition(s) of mile and try it. Go ahead.
 * What about reducing the whole article to "Light goes pretty fast and relativity is all wrong". Would that make you happy?
 * DVdm (talk) 21:30, 10 August 2009 (UTC)

Well I suppose that if Chez37 was claiming the value in miles per hour to be exact, then you have a point. We're not likely to ever get an exact value in any system of units, not counting the post 1983 tautology of course. David Tombe (talk) 21:36, 10 August 2009 (UTC)


 * Yes yes, 1983 was the year when your world came to an end. If you can't manage the article to reflect that horrible fact, make sure it stands out in its talk page. Mentioning it 4 times per day would do the job I estimate. DVdm (talk) 07:50, 11 August 2009 (UTC)

What would an encyclopedia writer do?
The books that mention "speed of light" and vacuum and have encyclopedia in their title generally start out with approximations, don't belabor the definition of vacuum, don't refer to 1983 or the SI standards, etc. They just talk about the speed of light. Later, they may get into these subtleties. I recommend we do similarly. Keep the broth bland to start with, as not everyone likes that much spice in their encyclopedic leads. Dicklyon (talk) 17:58, 10 August 2009 (UTC)


 * I notice that Lou reverted back to the vageness. Ok with me. Let's keep the simple minds happy :-) DVdm (talk) 21:34, 10 August 2009 (UTC)

this speed is measured to find a more exact definition of the metre
This sentence in the intro is wrong. The numerical value in SI units of the speed of light is not measured, and its value is a defined constant beyond all measurement. This constant is known exactly, and the definition of the metre does not depend upon any refinement in this numerical value of c, which is now fixed until some future international convention decides it has to change. By statement of the BIPM, refinements in the measurement of the metre will have no effect upon this numerical constant. Please read the section Speed_of_light. This horribly erroneous statement must be removed. Brews ohare (talk) 02:09, 11 August 2009 (UTC)


 * It's not wrong, you just didn't understand it. Measuring the speed of light will not affect the numerical value, as you note.  Rather, as the sentence says, it will only affect a refinement of the length of a meter.  You'd need some absolute length standard if you wanted to put numbers on it.  Perhaps there's an alternative concise way to say it that people could agree on?  I doubt it.  Dicklyon (talk) 02:49, 11 August 2009 (UTC)


 * I think one should mention what the main experimental challenges here are (e.g. you can imagine that the fact that you can't make a perfect vacuum and thus the actual speed of light is not "c" plays a role). Without such information, a reader may be confused, as it seems that as c is already fixed, measuring lengths has become the same as measuring time. Count Iblis (talk) 02:59, 11 August 2009 (UTC)

I am glad to see this erroneous statement was removed. Brews ohare (talk) 03:26, 11 August 2009 (UTC)

For most purposes, the speed of light has been defined... as exactly 299,792,458
Exactly what are the "other" purposes suggested here, and should they not be mentioned explicitly? My feeling is that the words "for most purposes" should be deleted, inasmuch as the metre always is defined so the numerical value of the speed of light is always 299,792,458 m/s. These words are just nonsensical filler. Brews ohare (talk) 03:32, 11 August 2009 (UTC)
 * Astronomical measurements (within the inner solar system) don't use the SI definition: in the astronomical system of units, the speed of light is still an empirical constant with an associated measurement uncertainty. Physchim62 (talk) 03:39, 11 August 2009 (UTC)


 * Physchim62, fine but 299,792,458 m/s applies to the SI units, which is the topic, and has nothing to do with AU. If an intro is to be written so as to have no leaning toward any particular set of units, then the SI units should be omitted in the intro and put in a later discussion. The 299,792,458 m/s figure should be dropped from the intro altogether. Brews ohare (talk) 03:43, 11 August 2009 (UTC)


 * It was simple enough to excise that complicating phrase from the lead; feel free to go into the distinction later in the article. Dicklyon (talk) 05:35, 11 August 2009 (UTC)

Pre 1983 vs. post 1983
It appears that the intro wishes to point out that something is different following 1983. It says that prior to 1983 "progressive refinement" in the measurement of the speed of light took place. It says following 1983, the speed of light has been defined (and though not stated, unfortunately, the speed of light is placed outside the realm of measurement altogether).

Now, don't you think this change of stance is unexplained here, leaving a huge gap in the reader's mind? Is this gap really preferable to a one- or two-sentence explanation about replacing the length standard with a time standard (for which sources already have been provided)?

If you don't agree, I'd say your only explanation can be that lack of clarity on this point is just inevitable, a point that cannot be explained in the intro. Why?

Perhaps, again, the whole matter of SI units should be postponed to its own section and 299,792,458 m/s dropped outside of that section? Brews ohare (talk) 03:41, 11 August 2009 (UTC)


 * The intro has no wishes of its own. Some of us editors want the lead to introduce the topic without much hinting at its complexities.  But I have to disagree that in 1983 "the speed of light is placed outside the realm of measurement altogether."  Its measurement just can't be expressed in SI units any more, that's all.  Dicklyon (talk) 05:36, 11 August 2009 (UTC)

Dick, it's your last sentence "Its measurement just can't be expressed in SI units any more, that's all." which is what it's all about. I'm all in favour of simplification. But why is Brews not allowed to emphasize this important shift of meaning that occurred in 1983? David Tombe (talk) 14:04, 11 August 2009 (UTC)

Brews, I was just reading further up the page and I see that Dicklyon would be happy enough for this matter to be highlighted further down the main article. Would that be a satisfactory compromise? He is not objecting to the content matter as such. In other words, have a very simple introduction to the extent that the speed of light is 186,000 miles per second and then elaborate on all the details surrounding the change of approach in 1983 in a separate section further down? David Tombe (talk) 14:09, 11 August 2009 (UTC)


 * Dick: As the above points out, the pre-1983 - post-1983 discussion concerns only the SI units, no others so, as you and others have said, the speed of light can be measured in other units, but "So what?" It's an SI units issue.


 * As David points out, and as I said myself, if you don't want to deal with these "complexities" (requiring all of one or two sentences, and already done in earlier versions of this intro, with sources), then the way out is to dump all this 1983 "stuff" into an "SI section" including the magic number 299,792,458 m/s and all reference to a "defined value". Brews ohare (talk) 14:36, 11 August 2009 (UTC)

The Light-Year
The 1983 definition of the metre is a similar concept to that of the light-year. Therefore, this same problem would have existed before 1983 should we have wished to state the speed of light in terms of light-years per year. The speed of light has always been one light-year per year. But that hardly tells us very much about the speed of light. The post-1983 situation in SI units is just as useless. David Tombe (talk) 11:07, 11 August 2009 (UTC)
 * The definition of the light-year tells us all we need to know about the light-year. The definition of the metre tells us all we need to know about the metre. Your comments tells us all we need to know about about you knowledge of metrology and you usefulness as an editor on this article. Please desist from soapboxing your nonsense here. Physchim62 (talk) 12:33, 11 August 2009 (UTC)


 * No Physchim62, this is all about the fact that Brews ohare has been encountering unnecessary obstruction in his attempts to clarify the distinction between the pre-1983 situation and the post-1983 situation. It is obvious to me that those who have been obstructing Brews haven't realized the significance of the concept of the SI speed of light in relation to the post-1983 definition of the metre. My comments about the light-year were designed to illustrate that distinction to those, such as yourself, who haven't noticed the tautology that is associated with the post-1983 situation. David Tombe (talk) 13:49, 11 August 2009 (UTC)

I don't see the tautology here. As a thought experiment, suppose we get a message from an alien civilization that says "Consider the hyperfine transition of hydrogen. 1,000,000,000 of these cycles is a gnorf, our unit of time.  Our unit of length, the fnotz, is the distance traveled by light in 1/200,000,000 of a gnorf." From this we know the gnorf and the fnotz, and we know the speed of light, in their units, is exactly 200,000,000 fnotz/gnorf. This seems a very un-ambiguous system of units. Where's the tautology? LouScheffer (talk) 14:08, 11 August 2009 (UTC)


 * LouScheffer, It is a tautology to state that light travels at one light-year per year. The same applies to the post-1983 definition of the metre. The post-1983 definition of the metre is a 'speed of light' based length, just like the light-year. David Tombe (talk) 14:15, 11 August 2009 (UTC)

So where's the tautology? The second is defined without reference to the speed of light. Then you can define the meter as the distance needed to make the speed of light come out any value you want (they choose the particular one to minimize the transition, but you could pick any value in principal). Then *of course* the speed of light has a fixed value - that's the way you defined the meter. This is not a tautology at all, just a choice of how to define your unit of length. LouScheffer (talk) 14:48, 11 August 2009 (UTC)


 * Lou: it is a tautology in the sense that the speed of light must come out to be the magic number because λ = ct means that c=λ/t for any length λ and any time-of-flight t. The measure of the length "adjusts" to insure the result. If you prefer, c becomes a conversion factor. If you can't measure the thing, its value is what it is defined to be, it's tautological.
 * See Woit "Setting the speed of light equal to one determines the choice of units used to measure time in terms of the units used to measure space and vice versa"; Sydenham "the time standard will serve as the length standard" "The speed of light as a numerical value is not a fundamental constant"; Jespersen "measuring length in terms of time is a prime example of how defining one unit in terms of another removes a constant of nature by turning c into a conversion factor whose value is fixed and arbitrary"; Russell "By an international agreement made in 1983 ... the speed of light has become a conversion factor for turning distances into times"; Singer et al. "this definition [1960] was was then replaced in 1983 with the current definition which, using the speed of light as a conversion factor..." I assume that the role of c as a conversion factor and the nature of the tautology is clear. The point is that if λ = c t, with c given, then c = λ/t for any length λ and time-of-transit t (in vacuum, of course). I hope you will not confine yourself to these snip-it quotes, but will pursue these sources (all available at the provided links) in more detail. Brews ohare (talk) 15:12, 11 August 2009 (UTC)


 * Are you saying that these sources that talk about "removing a constant" and make a "conversion factor" are support for David's interpretation of "useless" or of your interpretation of "tautology"? That seems like a stretch.  Let's just stick with what they say, instead of putting our interpretive words on top of it. Dicklyon (talk) 15:32, 11 August 2009 (UTC)


 * Dicklyon: The sources are quoted in my response so they can "speak for themselves". However, the editors appear unwilling to allow them to do that, and instead simply juxtapose the pre- and post-1983 approaches with no explanation (previously with incorrect explanation) of the difference. The key decision here is whether this change is to be presented in the intro or presented later. In the last case, the entire matter should be dropped from the intro as it explains nothing and introduces confusion.


 * Here is a source that actually uses the word "tautology": Harper "With this definition...it is built into the definition of the units and so has become a tautology." Brews ohare (talk) 16:29, 11 August 2009 (UTC)


 * Yes, that's a good source for that POV; thanks. It makes the same stupid mistake that you do:  to go from the "tautology" that makes the speed of light constant in SI units to the false conclusion that "one does not need to perform any experiment to prove the constancy of the speed of light".  Like you, they've mixed up constancy of the conversion factor in SI units with constancy of the actual underlying physical concept.  I don't agree with you that the standard setters have the power to redefine physics -- just units.  Dicklyon (talk) 17:16, 11 August 2009 (UTC)

Brews, I can now see that this entire edit war has arisen because you have seen through a case of The Emperor's New Clothes. There are some who will never accept the fact that the authorities could actually have done something like this in 1983, and so they will paper over cracks at all costs. They will forever make themselves believe that the 1983 definition of the metre had no bearing on the traditional concept of the speed of light, even in SI units. They will forever deny the tautology. You have rocked the boat, and that is why you are being obstructed from inserting a simple clarifying line into the introduction. David Tombe (talk) 15:37, 11 August 2009 (UTC)


 * Here is a quote from the wikipedia article about The Emperor's New Clothes,


 * ==Colloquial use as metaphor==
 * The story has given rise to its common reference as a metaphor in numerous situations. Most commonly, the statement "the emperor has no clothes" is used to refer to a situation in which (at least in the opinion of those using the phrase) the majority of people are unwilling to state an obvious truth, out of fear of appearing stupid, unenlightened, sacrilegious, or unpatriotic, or perhaps out of "political correctness". In such cases it is often implied that the motive and rationale for not seeing the obvious truth has become so ingrained that the majority do not even realize that they are perpetuating a falsehood.


 * This is exactly what is going on here as regards the post 1983 concept of the speed of light. David Tombe (talk) 15:43, 11 August 2009 (UTC)

David, this is Wikipedia. Perhaps you would like to find a reliable secondary source that explains how the 1983 definition of the speed of light was all a dastardly plot to confuse the general public. No doubt this source will also tell us exactly who the conspirators were and how they gained out of this cunning plan. Martin Hogbin (talk) 16:44, 11 August 2009 (UTC)


 * Martin, all I said was that it is a tautology. The 1983 definition of the metre has got absolutely nothing to do with actual measurements of the speed of light. This has been an enormous source of confusion. Some of the editors had actually been trying to link the 1983 definition of the metre with ongoing refinements in the measurement of the speed of light. That is a classic sign of the confusion that has been created. I don't know what the motive was in 1983, but I suspect that it had something to do with somebody wanting to do a rousing 'Amen!' to the role of the speed of light in relativity. David Tombe (talk) 16:50, 11 August 2009 (UTC)


 * Ah yes, relativity! That other wicked conspiracy to achieve some unexplained end. Martin Hogbin (talk) 17:00, 11 August 2009 (UTC)


 * Martin: Sarcasm is not the way forward. Brews ohare (talk) 12:05, 12 August 2009 (UTC)

Dick, Neither Brews nor myself have mixed up constancy of the conversion factor in SI units with constancy of the actual underlying physical concept. But the 1983 definition of the metre very conveniently allows the two to get easily mixed up. Brews has been making stringent efforts to make sure that the readers don't get mixed up about something that could very easily be mixed up. The other editors have been trying to prevent him because they are the ones that have already got mixed up. They are so mixed up that they are trying to write the lead in such a way as if the 1983 definition of the metre made no fundamental change to the concept of the speed of light in SI units. Only today, we saw how one editor actually believed that the 1983 definition of the metre was actually inspired by refined measurements of the 'physical concept' of the speed of light. The evidence is that you yourself can actually see the tautology in the post-1983 situation. So why not act on your understanding and support Brews? David Tombe (talk) 19:15, 11 August 2009 (UTC)


 * It's hard to support Brews, because it's hard to understand what his point is, even when the underlying issues of physics are clear; the typical effect of his efforts, however, is often clear, which is to add distractions and complexity where none are needed. This behavior seems to be the main point of agreement between the two of you; I have undone your move of this complex distration into the lead paragraph; I'm not sure it belongs in the lead at all, but certainly not in the first paragraph; it is my impression that we have broad consensus on that, but if anyone besides you and Brews disagrees, they should please let me know here.  Dicklyontalk) 20:39, 11 August 2009 (UTC)


 * Dicklyon: It is hard for yourself to support Brews_ohare because you have not read what he has said, nor the sources provided, and have absolutely no idea of his position on this subject. How do you find the chutzpah to say these things when you have not made minimal effort to understand what is going on here??? Brews ohare (talk) 02:06, 12 August 2009 (UTC)


 * I've made an effort, yet I admit I do fail to understand why you and David keep supporting each other, and why you cite a source that makes a stupid mistake if not to support your position. Dicklyon (talk) 04:10, 12 August 2009 (UTC)

It makes the same stupid mistake that you do
Dicklyon is being polite again. Nowhere do I say or advocate the view that the numerical value for the speed of light is the same thing as the physical entity the spreed of light. A little reading of this talk page will demonstrate this point. Moreover, I have said time and again that the properties of the physical speed of light are amenable to measurement, while the numerical value in SI units is tautology (no measurement can discover the value, because the value is defined). The entire discussion here has been about the proper place for, and treatment of, the numerical value and the SI definition of the numerical value of the speed of light as 299,792,458 m/s.

Instead of "getting off" on some wild misinterpretation of the discussion, why not address the issues raised, Dicklyon? Brews ohare (talk) 02:02, 12 August 2009 (UTC)


 * You cited that source in support of your "tautology" idea; sorry if I misinterpreted you as meaning that you agreed with it. Dicklyon (talk) 04:12, 12 August 2009 (UTC)


 * In view of your failure to follow a very, very careful exposition of my position here, by myself and by D Tombe, it would be a surprise if you were to understand the sources. Brews ohare (talk) 11:58, 12 August 2009 (UTC)

avoid confusion between 299,792,458 m/s and physical constancy of the speed of light
It does not fix confusion engendered by one sentence to suggest that the reader "should not be confused" by that sentence. This paragraph:"Currently the speed of light is used to define the metre and hence the speed of light in a vacuum in SI units is now fixed at 299,792,458 metres per second.[8][9][10] This fact should not be confused with the physical constancy of the speed of light that was used as a postulate in the original formulation of the special theory of relativity, and which is independent of the choice of units." is a disaster and should be rewritten.

Here are some things wrong with it:
 * 1) It is unclear what is meant by saying "the speed of light is used to define the metre"; a normal person would ask how that was done.
 * 2) It is unclear how the speed of light can be "fixed"; a normal person would think it should be measured, and if it is "fixed", then that should be an experimental observation.
 * 3) What is this 299,792,458 m/s? A normal person would expect some experimental error bar attached, not some "fixed" number.
 * 4) Given that this is WP, where erroneous opinion flourishes, one would like a well-documented statement. This presentation is overly short, and the sources tacked on are inherited from an earlier, more cogent presentation that they really supported. Now the reader is best advised to read the sources and figure out what they say for themselves, because the WP presentation is useless.

IMO, the best course is to delete all mention of the defined value of 299,792,458 m/s from the introduction and confine it to the section on "speed of light by definition". That section should carefully present the replacement of the length standard with a time standard, and why that was done. Brews ohare (talk) 11:50, 12 August 2009 (UTC)


 * Despite LouScheffer's good faith rewrite of the lead section, I think that our best option is to revert it to an earlier version and then try to address any problems incrementally, discussing disagreements that may arise as we go. Martin Hogbin (talk) 12:00, 12 August 2009 (UTC)


 * Martin: Your suggestion puts us back where we started. Perhaps the second time around would work out better, but it would be a long process as we can see. What about my suggestion above, that the defined value of c be removed from the intro? I think that would remove almost all controversy over the introduction, don't you? Brews ohare (talk) 12:11, 12 August 2009 (UTC)

OK, I removed the constancy part from the intro, and added it to a paragraph below, where I tried to not confuse the 4 forms of constancy - constancy with wavelength, constancy with time, constancy with motion of the observer/source, and constancy by definition in SI units. LouScheffer (talk) 12:23, 12 August 2009 (UTC)


 * The new paragraphs moves the location of the difficulty, but not the difficulty. This paragraph appears to be an addition to the introduction, placed apart, and not an exposition of any one of the many topics raised. That is not a good structure, because the issues cannot be discussed adequately in the form of an introduction. Brews ohare (talk) 12:31, 12 August 2009 (UTC)


 * Here are some problems:


 * 1) The claims are true only in vacuum, but this point is not made.
 * 2) The phrase "due to relativity" sounds like theory can dictate to nature.
 * 3) Why is the "light bouncing" experiment singled out from all others for exposition?
 * 4) "The speed of light is the only speed for which this is true" seems to me to confuse the definition with observation.
 * 5) The "somewhat confusingly" presented matter of SI units still is confusing.
 * 6) No citations are provided for many of the statements made.


 * Brews ohare (talk) 12:40, 12 August 2009 (UTC)

Constancy of the speed of light
Sorry Lou but there are far too many problems in this section - see below

The speed of light is constant in many ways.


 * Unhelpful and chatty start

It is the same for all frequencies and wavelengths of light, within very tight experimental limits.


 * Frequencies and wavelengths? Anyway we already say this, with refs.

It does not change with time - by bouncing pulses of laser light off the reflectors on the moon, scientists have shown the variation in the speed of light must be less than one part in 1013 per year.


 * As the speed of light is a dimensionful constant there is some doubt as to whether a change in the speed if light could be detected but anyway you give no reference for this claimed result.

Next, according to special relativity, the speed of light is the same in any inertial frame, and independent of any motion of the observer or the source. The speed of light is the only speed for which this is true, and this too has been verified experimentally to within very tight limits.


 * This is not entirely true, the one-way speed of light is only true by convention. See the section on the subject below, which this is a less accurate and unsourced repeat of.

Finally, and somewhat confusingly,


 * Is it really that hard to understand?

the speed of light is numerically stable when expressed in SI units.


 * 'Numerically stable' it is fixed by definition.

This is because the speed of light is used to define the metre and hence the speed of light in a vacuum in SI units is now fixed at 299,792,458 metres per second.[8][9][10] Therefore if the speed of light physically changed, the meter would be adjusted as well to keep the speed of light the same when expressed in SI units.


 * 'Speed of light physically changed', what does this mean and when are we expecting it to happen?

Most of this is either wrong or a repeat of what we already have. Martin Hogbin (talk) 17:09, 12 August 2009 (UTC)

Making points in the article
The arguments in the few sections above are essentially covered by the statement that the speed of light is a dimensionful constant. The interested reader is able to follow up on the philosophical implications of this statement by following the link. These philosophical issues are not ones that are going to be settled on this page and we should restrict ourselves to stating the currently accepted views on the subject as given in reliable sources.

What we do not need is the argument spilling over into the article itself, especially the lead section as, for example, in:

'This fact should not be confused with the physical constancy of the speed of light that was used as a postulate in the original formulation of the special theory of relativity, and which is independent of the choice of units'. Martin Hogbin (talk) 10:00, 12 August 2009 (UTC)


 * Martin, Well the argument certainly spills over into the permittivity article, never mind this article. Electric permittivity, denoted ε, is a measured quantity. It's a matter of interest that the measured value of permittivity combines with permeability, denoted μ in the equation,


 * c^2 = 1/(εμ)


 * However, since 1983, are we supposed to stop measuring ε and just automatically accept a value as dictated by this equation? We now have an extended tautology. Previously it was a case of making an interesting physical observation. The fact that these two quantities, ε and μ, combine together in the above equation to yield a figure that was close to the speed of light was clearly telling us something. In fact, it was telling us that very thing that Maxwell showed us in 1861, and which you didn't want to be seen in the history section of the article. But nowadays, Newton's equation for the speed of sound, as applied to EM has been reduced to a definition through the very tautology which you don't want to be reported on these pages.


 * Now if I were advocating the return of phlogiston or a flat Earth theory, I'm sure that you would have no objection to these views being aired in the history section. There would be no fear that exposing such archaic and discarded theories might lead to a revival. But clearly you do seem to have some kind of a fear over matters to do with the speed of light and the above equation. The physical significance of this equation is nowadays all masked behind 'definition' and 'tautology' and you clearly don't want the full significance of the 1983 definition of the metre to be exposed here.


 * It took me a while to figure out what this edit war was about. I asked both Brews and yourself what it was about. You weren't prepared to tell me your side of the story. But now I can see that there is clearly something about modern physics which you want to hide. Your edit above shows exactly what that 'something' is.


 * And meanwhile, you went around other people's talk pages sabre rattling about Brews ohare's editing style, and Dicklyon went to the wikiphysics project page trying to stir up another Uncle Tom's Cabin.


 * Your edit above has clearly revealed exactly what the edit war has been about. It is about the fact that you want to gloss over a very crucial point which is well sourced and relevant to the topic of the speed of light. And in the process, you have been trying to denigrate Brews ohare for wanting to put this point into the article. David Tombe (talk) 10:29, 12 August 2009 (UTC)


 * You are trying to conjure up allies for your extremely unconventional views on this subject. My complaint against Brews concerned his over-active editing style which was not conducive to cooperative editing or the development of a good style of English and presentation.  As far as I know, neither he nor any of the other editors here support your conspiracy theory that physicists are trying to hide some dark secret. Martin Hogbin (talk) 10:54, 12 August 2009 (UTC)


 * Martin: Whatever your opinion of D Tombe's underlying notions about relativity, he has made some perfectly reasonable arguments here, and trying to duck them by throwing mud does not reflect well upon you. Brews ohare (talk) 12:07, 12 August 2009 (UTC)

Martin, Never mind physicists in general. You yourself have blatantly tried to keep certain important facts away from the readership. You tried to keep Maxwell's 1861 paper out of the history section, and now you are trying to prevent Brews from doing an exposition of the change in the definition of the metre which took place in 1983. If you had carefully read what I wrote above, you would have seen that the 1983 definition of the metre has had a profound significance. Its repercussions bear heavily on both this article and the permittivity article. I don't know whether or not Brews shares my contempt for the 1983 definition, but he is at least trying to clarify the situation for the readers while you are trying to sweep it under the carpet.

According to Maxwell, c^2 = 1/(εμ) is Newton's equation for the speed of sound, applied to light. If we disregard that fact as being merely history, we are still nevertheless left with an amazing equation that requires some kind of physical explanation. But the 1983 definition of the metre has rendered that equation into one nice big definition. We can now throw away the electric capacitor circuit that we used to use to measure ε, because we don't need to measure it anymore. It follows automatically from the definition of the metre.

There is obviously alot of explaining to do here, so why is everybody so keen to hide the explanation? David Tombe (talk) 12:24, 12 August 2009 (UTC)


 * I would be happy to include an intelligent discussion of why the standard was changed, based on reliable sources and in the right place (which is not the lead section). We already have a section on the subject, what do you claim is missing from it? Martin Hogbin (talk) 13:00, 13 August 2009 (UTC)

What are the problems with the lead?
Is there still a problem with stating the exact SI speed of light in the lead. We currently have the explanation shown below with my notes added in the main text, which seems to me to make the situation quite clear.

As the speed of light is a dimensionful constant


 * The dimensionful constant link takes care of the philosphical issues that we do not want to discuss here

its numerical value


 * This is quite clear, we are just talking about a number.

is dependent on the system of units used.


 * The value of that number depends on how we define our units

In the SI system, the metre is defined as the distance light travels in vacuum in 1⁄299,792,458 of a second.


 * We say what system of units we are talking about and how the metre is defined in it.

The effect of this definition is to fix the speed of light in vacuum at exactly 299 792 458 m/s.


 * This does not seem too hard to grasp by now. What is there left not to understand? Martin Hogbin (talk) 17:48, 12 August 2009 (UTC)

The exact speed of light is presently not in the lead, and that is how matters should remain because the speed of light is not exact in all systems of units, and so bringing an "exact" value into the intro is asking for a huge digression upon systems of units.

The "exact" value is introduced in the subsection Numerical value, notation and units, where it is at least stated correctly. However, the morass surrounding the use of an exact definition is avoided simply by not raising the issue. If a reader happens to question how an exact value might arise, they are left in the air unless they proceed to the last section on Speed of light by definition. I don't think this arrangement is optimal, and believe that it has been left this way only because of editors' disputes, and not because anyone likes it. Brews ohare (talk) 21:23, 12 August 2009 (UTC)


 * Yes, I think this is much better, though I question the value of the link to unsourced essay on "dimensionful" constants. Dicklyon (talk) 04:34, 13 August 2009 (UTC)


 * It is customary in wikipedia leads to start with a statement of the context, the field of study; but Martin removed "In physics" without saying why. I think it's not a big deal, but I'd generally prefer the stylistic consistency of leaving it in.  Other opinions?  Dicklyon (talk) 04:40, 13 August 2009 (UTC)

Although "dimensionful" shows up, I think it is more common and understandable to use the words "dimensional" and "dimensionless". for example see this google search. I agree with Dick about following the format "In physics" Brews ohare (talk) 05:18, 13 August 2009 (UTC)


 * "Dimesional" is fine with me but I think 'in physics' is unnecessary. The study of speeds, times, and physical constants is physics.  It is like saying, 'In geography the city of Paris is the capital of France'. Martin Hogbin (talk) 08:21, 13 August 2009 (UTC)


 * I cannot understand how the discussion about the speed being a defined value in SI units has got to such a state that we cannot now even mention in the lead the numerical value of the subject of the article in the units that used as a standard throughout the world (feet and inches are officially defined in terms of the metre). Right from the start, this article has done that, up until now.


 * Why not simply say, 'The length of the metre is defined such that the speed of light is exactly 299 792 458 m/s', and leave the details to the main text? I agree that from a fundamental physics point of view the number is somewhat arbitrary, but it is still quite an important arbitrary number.  The, 'morass surrounding the use of an exact definition' is a creation of editors here. Martin Hogbin (talk) 08:34, 13 August 2009 (UTC)


 * Martin: The reasons for not saying this have been put forth several dozen times on this page. Instead of lamenting, if you wish to do this, confront the objections already out there. IMO the most serous one is that a statement of an exact value for c automatically raises the issues of "How can that be?" which you, for one, do not wish to engage in the intro. Included in this issue is the point that the numerical value of the speed of light in the SI units is not, unfortunately for simplicity, the physical speed of light that a naive reader would expect it to be, but an arbitrary value set by international convention. Therefore, the "exact value" issue has to be left out, and placed where it can be handled properly. Brews ohare (talk) 14:37, 13 August 2009 (UTC)


 * A statement of the exact speed of light in SI units may raise questions in some readers, that is why we have the rest of the article. As the lead section is a summary of the article as a whole it is quite possible that the full implications of any statement it may not be  obvious.  Those who are interested or puzzled can read the relevant section in the main body of article to find out more.


 * You still seem to have some hang-up about the speed of light being set by definition. It is not that complicated, in the SI system we no longer measure the speed of light because it is a defined value, instead we do experiments to delineate the metre more precisely.  Perhaps you could explain what you mean by,'the numerical value of the speed of light in the SI units is not, unfortunately for simplicity, the physical speed of light that a naive reader would expect it to be'.Martin Hogbin (talk) 19:32, 13 August 2009 (UTC)

Martin: I wonder if you dispute Jespersen's statement that the numerical value of the speed of light in SI units is a defined number set by international agreement, and from a logical standpoint, an arbitrary value? In effect this numerical value freezes c at a number close to the value as measured at one point in history, with one specific technology. Therefore, to call this number "the exact speed of light" in the introduction is basically misleading, as the reader has no idea that all this is going on "behind the curtains", because it is not mentioned. To say the reader cannot complain because there is something about it in the "fine print" later on puts the intro in the same position as a balloon mortgage broker (except the mortgage agreement advises reading  the fine print, and the intro contains no such warning). Brews ohare (talk) 21:29, 13 August 2009 (UTC)


 * I cannot work out what it is that you think I do not understand. The meter is defined to make the speed of light fixed, exactly as Jespersen, and everybody else, says. That means that, from now on, the numerical value of the speed of light in m/s is fixed but the metre can be delineated by experiment, hopefully more precisely as experimental methods improve.  My proposed sentence for the lead says exactly that, The length of the metre is defined such that the speed of light is exactly 299 792 458 m/s.  I cannot see why I need to spell it out for you but let me try:


 * The length of the metre is defined such that This means that we define the length of the metre in a particular way. As we are in the process of defining the metre, we can make it any length that we like. So how shall we determine its length?


 * the speed of light is exactly 299 792 458 m/s This seems pretty clear to me, we define its length to be whatever makes the speed of light to be 299 792 458 m/s. So just to recap. How do we determine the length of a meter?  We take some light and see how far it goes in 1/299 792 458 s.  That is a metre.  Any time somebody wants to know how long a metre is they do that same experiment.  In ten year's time, when someone has, a more stable laser, and they want to delineate the metre more accurately they do the same experiment.  The metre is then delineated more precisely.


 * Not much of a conjuring trick, and anyway we tell our readers how it is accomplished. Enough of this nonsense, just because a couple of editors think that there is some plot by physicists and standards authorities to confuse the public does not mean that we should not state the exact speed of light (which is the subject of the article) in international standard units in the lead.  I propose to do just this.  Martin Hogbin (talk) 22:43, 13 August 2009 (UTC)

Martin walks right into the trap
Martin has just boldly stated the inevitable nonsensical consequence of the 1983 definition of the metre. He has informed us all that since the magnetic permeability (μ) and the speed of light (c) are both fixed by definition, then it must necessarily follow that the electric permittivity (ε) must also be fixed. He is of course using the famous equation c^2 = 1/(εμ).

This is exactly the kind of situation that arises when somebody fiddles with the international system of units without thinking through what all the repercussions are going to be elsewhere in physics. And in this case the mess lands right on the plate of electric permittivity. The famous equation c^2 = 1/(εμ) has never at any stage in history had anything to do with measurements of the speed of light. That equation follows from an experimental determination of (ε) using an electric capacitor circuit (or in days of old by the Leyden jar method used in 1856 by Wilhelm Eduard Weber and Rudolf Kohlrausch). These experiments led to a numerical value that was close to the speed of light, hence pointing to a physical connection between light and electromagnetism.

But now we are seeing one of the greatest scientific discoveries of all times being absorbed into the SI definition of the metre, and hence nullified. Electric permittivity is no longer a measured quantity associated with dielectric polarization. It has now become a defined quantity in relation to two other defined quantities through a totally meaningless equation. And Martin has fallen for it lock, stock, and barrel. David Tombe (talk) 14:57, 13 August 2009 (UTC)


 * Maybe the whole physics community has fallen for it. If you accept that standard physics of EM in free space, there's no alternative.  This number is not, and never has been, "a measured quantity associated with dielectric polarization," since there's no dielectric in free space.  Of course, one can still make measurements, and if it turns out that the theory of EM in free space is not consistent with this definition, one can propose to change it.  I doubt that we'll see that happen, since I expect the theory to hold up, but the 1983 standards don't logically prevent such steps.  Presumably you have a minority POV on this; if your POV is represented in sources, show it to us; if not, please just drop it. Dicklyon (talk) 16:54, 13 August 2009 (UTC)
 * As I have said before, David seems to be reading a different book from the rest of us. Martin Hogbin (talk) 19:35, 13 August 2009 (UTC)


 * Martin: You're right and you're wrong. You're right to say If you accept that standard physics of EM in free space, there's no alternative. You're wrong to say of course, one can still make measurements, and if it turns out that the theory of EM in free space is not consistent with this definition, one can propose to change it.


 * The latter is wrong because no-one ever can measure the properties of free space. It is a theoretical construct, a chimera built of postulates. All that one can do is to compare some real medium, say outer space, with the predictions for free space and decide how closely the real medium is to a behavior like that of free space. If no medium ever is found to behave like free space, free space can remain as it is, and the discrepancies between real and hypothetical media quantized.


 * Perhaps D Tombe thinks free space is a real medium, as you do. If so, you both are mistaken. Brews ohare (talk) 21:37, 13 August 2009 (UTC)


 * If only I had met you before then I would not have fallen into the trap of believing all that nonsense pumped out by the world's physicists. Martin Hogbin (talk) 22:05, 13 August 2009 (UTC)


 * Martin: Sorry, I don't know how your comment is responsive to my remark above. It doesn't answer the questions posed. Brews ohare (talk) 22:14, 13 August 2009 (UTC)


 * It was hard to respond to a remark that tells me what I am thinking and then attempts to teach me physics. Martin Hogbin (talk) 22:48, 13 August 2009 (UTC)


 * Martin: Pardon me: I am asking you whether you think "free space" is a medium with measurable properties, and if so, how come its electric constant and magnetic constant have exact values with no error bars (see links to NIST provided here), postulated values that are not measured?
 * It appears that instead, free space is a model medium with postulated exact properties, to which measurements upon real media may be compared. Do you agree with that?? Brews ohare (talk) 00:02, 14 August 2009 (UTC)


 * We have been over this point many times before. We agree that free space is reference state.  In it, the magnetic constant has a defined value and the electric constant an exact value fixed  by virtue of the definition of the speed of light, the definition of the magnetic constant, and the equations of classical electromagnetism.  As you say all actual measurements are made in real media, such as laboratory UHV.  So far I think we agree.


 * Suppose we want to delineate a meter, that is to say create two points that are exactly one meter apart. We cannot set up our experiment in free space because there isn't any.  I am not an expert in metrology but I expect that modern delineations of the metre are done in a good laboratory vacuum.  We then use light of a known frequency and count the requisite number of wavelengths to get our metre.  As the experiment was not done in free space, we have to take account of the fact that the actual speed of light in our experiment will not be exactly the defined value of c.  To get round this problem we might make some assumptions, but most people will consider these very reasonable ones.  We could assume that our understanding of the way that light interacts with matter (the residual gas in our system) holds down to an arbitrarily low pressure, in other words we can extrapolate from measured values of the refractive index of gasses at low pressure to a pressure of zero.  Alternatively, if we do not wish to make such assumptions, we could define free space to be a limiting case of a real medium as the pressure is reduced towards zero, much as is done in the case of absolute zero.  Either way, the very small correction is the same.  Do we still agree? Martin Hogbin (talk) 14:04, 14 August 2009 (UTC)


 * Martin: I believe we agree so far. I imagine that you would agree that the extrapolation to "free space" needn't be made from vacuum, but could be (for example) from air. There is this theoretical aspect, that the corrections may be based upon formula. Or, the corrections may be based upon extrapolation of a fitted expression beyond the realm where measurement is available. In any case, the corrections correspond to present generally agreed upon notions of "good practice". Brews ohare (talk) 15:59, 14 August 2009 (UTC)
 * Yes. Of course good practice would be to try to get the experimental conditions as close to free space as possible before making any corrections. Martin Hogbin (talk) 17:07, 14 August 2009 (UTC)


 * No, every medium has its "good practices" to reduce measurements in that medium to a free space equivalent. Brews ohare (talk) 22:12, 14 August 2009 (UTC)


 * I'd like to add that the use of an extrapolation procedure in no way implies that the extrapolation limit corresponds to a "real" medium, and that calculations of real media such as quantum vacuum or QCD vacuum suggest that these media are not free space. Brews ohare (talk) 16:38, 14 August 2009 (UTC)


 * Free space is a real medium just as much as absolute zero is a real temperature, they are both unobtainable ideals. The 'Speed of light' is essentially a classical quantity so I think we should stay well away from any discussion about quantum physics where uncertainty rules. Martin Hogbin (talk) 17:07, 14 August 2009 (UTC)


 * I suppose we could argue that "absolute zero" is not real either, if it is not obtainable in our universe. I don't understand why "speed of light" is only a classical quantity, any more than "speed of transfer of information" is a classical quantity. And I don't understand why any real medium is off limits just because it can be modeled quantum-mechanically: most media can be so modeled. Brews ohare (talk) 22:08, 14 August 2009 (UTC)

The point is that the equation c^2 = 1/(εμ) has got nothing to do with any measured value for the speed of light. The measured value of ε leads us to the speed of light through this formula. That is telling us something, and it should be a matter of interest to physicists as to exactly what it is telling us.

We cannot then use this formula in reverse, in connection with a defined value for the speed of light, in order to obtain a value for ε. It doesn't matter what the individual opinions are regarding the nature of space. The tautology that is associated with the 1983 definition of the metre has spilled right over into electric permittivity and killed off the historic experiment that was performed by Weber and Kohlrausch in 1856 with the Leyden jar. That is how ridiculous the situation has become. A crucial experiment in history has been abolished by a definition.

I looked up sources to confirm that ε is a measured quantity. This fact was confirmed, and the capacitor circuit that is used in the experiment was explained. But these sources were all pre-1983. The only post-1983 sources that I read were wikipedia itself. And all I read were unconvincing attempts to try and explain how two precisely defined constants in the above formula could only manage to lead to an approximate value for the third variable. Somebody was actually trying to justify it all on the basis that π is an irrational number! It is obvious that those who defined the metre in 1983 had forgotten the origins of the equation c^2 = 1/(εμ). Its origins are even earlier than Maxwell's equations. Its origins are in Newton's equation for the speed of sound, and in Weber and Kohlraush's Leyden jar experiment of 1856.

And because of this oversight, density (ρ), elastic constant (k), and speed (c) in the equation $$c = \sqrt{\frac{k}{\rho}}$$ have all been reduced from physical variables to defined constants, and the equation has lost all of its physical meaning to become a pointless excercise in numerical algebra. David Tombe (talk) 22:20, 13 August 2009 (UTC)


 * David, your views are not those currently held by the world's physicists. Why this is this is something you are going to have to work out for yourself. Martin Hogbin (talk) 22:50, 13 August 2009 (UTC)


 * David: I believe your concerns are mitigated if you think of "free space" as an idealized medium with permittivity given by the electric constant, and permeability by the magnetic constant. Being an idealized model medium, of course the speed of light also is an idealized value. Thus, there is no contradiction with experiments or observations made upon real vacuums. What do you think about that? Brews ohare (talk) 01:07, 14 August 2009 (UTC)

Brews, we know that both the speed of light and the magnetic permeability are defined quantities. The magnetic permeability has been a defined quantity since Gregorio de Giorgi of Rome proposed the current value in 1904 at the sixth International Electrical Congress in St. Louis, Missouri. And we know all about this recent re-definition of the metre in 1983.

Now if we accept the equation c^2 = 1/(εμ), as I do, then there are some who might mistakenly think that it follows automatically that ε will then also become a defined quantity. But it's not as simple as that. The only reason that we have the equation c^2 = 1/(εμ) in the first place is because experiments yield a value of ε which when put into this equation, yields a value that is very close to the speed of light. Hence, we cannot use that equation in conjunction with the defined speed of light. That equation has to sit aside from any matters relating to either direct measurements of the speed of light, or defined speeds of light.

Originally, until 1983, we had two totally independent measurements. We had a meausured value for the speed of light and we also had a measured value for ε. The close correlation of these two measurements through the above equation was telling us something. We cannot brush that message under the carpet simply by bringing this equation under the jurisdiction of the 1983 definition of the metre. That would be a case of being wise after the event and missing out on something that is very important.

The only way out of the mess that was created in 1983 is to dissociate the equation c^2 = 1/(εμ) with the post-1983 speed of light, and to continue to treat ε as a measured quantity that produces a value that is very close to the speed of light when it is put into this equation. We cannot directly associate the modern defined value of c with the equation c^2 = 1/(εμ). The two must sit side by side independently of each other, as an interesting comparison which requires a physical intrepretation. David Tombe (talk) 11:20, 14 August 2009 (UTC)


 * David: Once we reach the realization that EM radiation satisfies Maxwell's equations, there is nothing to prevent the postulation of an ideal medium with permittivity equal to the electric constant and permeability equal to the magnetic constant. That medium is imaginary, and has the defined value for the speed of light. The experimental issue is then whether any particular real medium behaves approximately like this. The present theoretical situation is that in fact there is no such real medium, in particular, quantum vacuum is not free space. The present experimental situation is that the theoretical discrepancies are too small to measure just now. The earlier work you refer to is experimental in nature, and therefore does not impact the hypothetical behavior of free space. Could you agree with this picture?? Brews ohare (talk) 16:07, 14 August 2009 (UTC)

Brews, The deductions about the nature of space come afterwards. I had not moved on to that stage yet. All I am trying to say is that the new post-1983 defined speed of light cannot be used in conjunction with the equation c^2 = 1/(εμ) in order to give a defined value of ε. To do so would amount to cooking the books. It is the fact that the experimentally measured value of ε leads to the equation c^2 = 1/(εμ), where c is very close to the speed of light, that is the real point of interest for physicists. Electric permittivity is a measured quantity irrespective of what deductions we make about the nature of space. We cannot work backwards to ε by using an equation that came about in the first place from the experimental measurement of ε. David Tombe (talk) 19:02, 14 August 2009 (UTC)


 * Hi David: We are not on the same page yet. My take is that one is free to posit a medium with any properties one wishes, although that may not be a useful thing to do. One such posit is a medium with a given value for ε and a given value for μ. Plugged into Maxwell's equations, one necessarily then arrives at c = 1/√εμ, which sets c for this medium. If ε = ε0 and μ = μ0, then c=c0 = 299 792 458 m/s. Do you have any problems with this?  Brews ohare (talk) 21:52, 14 August 2009 (UTC)

We must have the value of speed of light in the most common, internationally recognised units in the lead.
The article is called 'The speed of light'. The first thing that many readers want to know is is probably, 'How fast does light go?'. What possible reason can there be for not giving them the answer to this question in SI units. Why would we want to give an approximate answer, in SI units, followed by the exact answer, in SI units?

This article has had the exact value of the speed of light in SI units since the day it was created. During the time it was an FA this value remained there, early in the lead section.


 * Yet you reverted my attempt to put it into the lead in a more acceptable position than the opening paragraph. Did you forget how much trouble we've already had with bloat when the exact value is in the lead, due to the complexity of the notion of how it can be a defined fixed value?  I thought we had settled on an alternative, but am willing to try it in the lead if others can avoid expanding it again.  I think the value of giving an approximate answer is clear; the 3 times ten to the eighth answer is the one that everyone actually uses, since it's easy to remember and is accurate enough for most purposes. Dicklyon (talk) 15:29, 14 August 2009 (UTC)
 * There is nothing remotely unacceptable about giving the correct and exact value of the speed of light, in the only widely used, international system of units in the world, early on in the lead section.


 * I completely agree with you about the bloat. We need to prevent this from happening again. Martin Hogbin (talk) 15:35, 14 August 2009 (UTC)
 * I cannot see what there is to discuss about this subject. In 1983 the 17th Conférence Générale des Poids et Mesures defined the metre to be the length of the path travelled by light in vacuum during a time interval of 1 ⁄ 299 792 458 of a second.  We cannot pander to the whims of those who do not like this decision.  Our job in WP is to report the established mainstream scientific views.  Martin Hogbin (talk) 15:41, 14 August 2009 (UTC)


 * I have suggested that we expand the 'Speed of light set by definition' section to explain why this decision was taken and what its consequences are but we must be careful only to make statements on the subject that are supported by high quality reliable sources. This section must not become a soapbox for speculation and crackpot scienceMartin Hogbin (talk) 15:45, 14 August 2009 (UTC)


 * Martin: We are at an impasse. You feel that stating matters that raise questions without posing the questions openly and dealing with them is just fine, provided there is some unnoticed further discussion somewhere else in the article. There is a contrary opinion. I don't see a middle ground, and simply exhorting that you are right, without proposing any kind of alternative or compromise is abortive, given your present capabilities at eloquent exhortation. Brews ohare (talk) 16:16, 14 August 2009 (UTC)
 * There is no need to put the "exact" value in the lead: there is a table of values, including the useful 1 ft/ ns. The "exact" value is not at all "exact" if one focuses upon the physical entity speed of light: it is exact because of a choice of definitions peculiar to the SI units, and logically could equally well be 1 ft /ns. Brews ohare (talk) 16:29, 14 August 2009 (UTC)
 * There is no need to put the "exact" value in the lead: there is a table of values, including the useful 1 ft/ ns. The "exact" value is not at all "exact" if one focuses upon the physical entity speed of light: it is exact because of a choice of definitions peculiar to the SI units, and logically could equally well be 1 ft /ns. Brews ohare (talk) 16:29, 14 August 2009 (UTC)


 * The inability to see middle ground is a big part of the problem, and is not unique to Martin. We could, for example, state the exact value in the lead (not in the opening paragraph) and have a simple forward reference to the section where the questions are raised and answered.  I would support doing there, or in a later section, something like what Martins suggests, reviewing the reasons for the change (and the implications for other units and constants, too, if we can agree on a sensible source about that and not let David inject his own opinions).  Dicklyon (talk) 16:36, 14 August 2009 (UTC)
 * There is no 'middle ground' here. On the one side we have the world's standards authorities telling us what the speed of light is and on the other we have some WP editors who do not like it.  The official, generally (pretty well universally) accepted value for the subject of the article should obviously be in the first paragraph of the lead, where it has been right from the start until some editors started finding problems with it.
 * For those who want to delve deeper into the subject, I am all on favour of expanding the relevant section. Martin Hogbin (talk) 16:46, 14 August 2009 (UTC)


 * And Martin, to say that we "must" have the exact value up front goes contrary to hundreds to reputable books that either give only the approximate value, or start with it before going into the 1983 defined value (like here). Dicklyon (talk) 16:41, 14 August 2009 (UTC)
 * They are not books just about the speed of light.Martin Hogbin (talk) 16:47, 14 August 2009 (UTC)
 * The excellent book that you quote is essentially about one of the first things that we say in the lead, which is that the speed of light is an important physical constant. As we say later on in the article on the, relatively specialist, subject of relativity it is common to use a system of units in which the speed of light is 1.Martin Hogbin (talk) 16:54, 14 August 2009 (UTC)


 * I am not going to argue forever about whether the defined speed of light should be in the first or second paragraph of the lead. My main points are that it should be in the lead, the first numerical value that we present, in a position decided by style and context, and its position should not be influenced by spurious objections to the standard. Martin Hogbin (talk) 16:58, 14 August 2009 (UTC)


 * Martin: You are not addressing the issues here: the "exact" speed of light is not exact, except by virtue of a rather intricate sequence of events peculiar to the SI units. Hence, the statement of the "exact" value is misleading without explanation. I had originally put in an internal crosslink following a statement of this value in the intro, somewhat as Dicklyon suggests, and it was removed according to some WP policy about "teasers" in the lead. Brews ohare (talk) 16:59, 14 August 2009 (UTC)


 * Brews, I am quite happy to discuss the subject with you but the SI standard is that which the world uses and it is therefore the one that we should present in the article, regardless of whether you or I approve of it.


 * The decision to fix the numerical speed of light is not particular to the SI system, it is exactly what has been done for years in most natural systems of units. The only difference is that in these systems the value chosen for the speed of light is 1 whilst in the SI system it is a different number.  In natural systems of units the speed of light is 1, by definition.


 * Regarding a crosslink in the lead I would not personally be against that but we are better off not doing it if it is against WP policy. I really do not think that most (non-crackpot) readers are going to get confused or annoyed by the brief explanation in the lead.  As I have said before, you could probably take other statements in the lead and criticise them for not being totally clear, that is why we have the rest of the article. Martin Hogbin (talk) 17:22, 14 August 2009 (UTC)

Hi Martin: It is not about approval, of course. And it is not about a variety of possible systems of units. It is about stating an "exact" value that has a tricky meaning for those not versed in the replacement of standards of length by standards of time. If you think this is an obvious or easy matter, look back at the last year of discussion on this talk page which is largely about confusion over what a "defined" value for a speed can mean when speed "should" be a measurable quantity, and was in fact, until 1983. So, Martin, despite your belief that iteration of your wishes will ultimately prove revealing to us all, it just ain't so.

Moreover, your remarks in the section lead me to believe that the real situation here has yet to sink into your mind, which may account for your lack of ability to understand the objections to your actions. Brews ohare (talk) 21:43, 14 August 2009 (UTC)


 * Of course some people will be interested, confused, or surprised that the speed of light can be set by definition, but the short statement in the lead explains how this is done, the length of the meter is defined to make it so. Even after reading this, some people may still be interested, confused, or surprised, in which case they can read the section on the topic where I hope we can give a clear and accurate explanation of all the issues involved based on current scientific thinking as supported by reliable sources.


 * Rather than discussing why we should not put one of the the most notable and supported facts about the subject of the article in the lead section we could be discussing how to illuminate the situation in the relevant section.


 * Regarding your last remark, your objections are irrelevant. The speed has been set by definition and that is what we need to say in the article. Martin Hogbin (talk) 22:07, 14 August 2009 (UTC)


 * Martin, you simply are mistaken. The speed of light in SI units is not a "most notable and supported fact": it is an arbitrary international convention. The speed of light as a physical entity is not set by convention: the conversion factor between the time standard and the length standard in SI units is so established.


 * I will revert any reintroduction of this material again, and if you re-revert you will violate the 3 reversion rule. Brews ohare (talk) 22:19, 14 August 2009 (UTC)

Page protected
I've protected the page in its current revision to forestall an edit war. Please discuss contentious changes on the talk page and come to agreement before committing controversial edits. If agreement cannot be reached among page editors, dispute resolution should be sought. Protonk (talk) 22:55, 14 August 2009 (UTC)

Arbitrary break, settle down all
Okay, all editors who have been been involved on the talk page and (especially) on the article page. Please step back from the keyboard and take a deep breath. Do you feel better now? If not, repeat until you do.

I've warned all (non-blocked) parties about 3RR; you are all experienced editors and should know better. Thanks. Now, get an outside opinion, pursue WP:DR, or do something different than what you've been doing. tedder (talk) 22:58, 14 August 2009 (UTC)

Gravitational time dilation
I have moved the added section and dropped the quote. It could still do with some rewriting. Martin Hogbin (talk) 19:53, 12 August 2009 (UTC)

To get discussion started we should ask how much sense it makes to talk of the speed of light in GR, it is one of the constants of the theory. We can talk about the time it takes light to travel a given path, but how do we define the path length. Martin Hogbin (talk) 19:56, 12 August 2009 (UTC)


 * Surely there are plenty of sources that can guide us to appropriate language on such issues. Dicklyon (talk) 04:35, 13 August 2009 (UTC)


 * Yes, of course. My question was somewhat rhetorical. I suggest that we put something quite simple in.  I think we did once have something it got lost in one of the edit storms.


 * The problem is that intuitive concepts of distance and time are not applicable to GR, so to talk of the speed of light in terms of such concepts is not very helpful. As the section says, the speed of light is always constant in an inertial frame, however, in GR, inertial frames, only exist locally (strictly only in an infinitesimal region of spacetime).  Martin Hogbin (talk) 08:57, 13 August 2009 (UTC)


 * This statement (of mine) was once on the lead, 'In general relativity c remains an important constant of spacetime, however the concepts of 'distance', 'time', and therefore 'speed' are not always unambiguously defined due to the curvature of spacetime caused by gravitation'. I think this is the best that we can say here.  I am sure that there will be a supporting statement in 'General Relativity from A to B' by Robert Geroch somewhere but I cannot find my copy. Martin Hogbin (talk) 09:34, 13 August 2009 (UTC)


 * Let's just don't put it in the lead, and make sure whatever we say is backed by a source, and it probably won't be something people will want to argue about. Dicklyon (talk) 00:08, 14 August 2009 (UTC)


 * Your book source is here. Dicklyon (talk) 00:12, 14 August 2009 (UTC)


 * Thanks. I was intending to replace the last paragraph in the 'Constant speed in inertial frames', which I suspect was added somewhat mischievously. I will quote the book as a reference, without a page number.


 * As the section is about inertial frames, should I change the sentence to say, 'In general relativity c remains an important constant of spacetime, but inertial frames can only be defined locally. Over extended regions of spacetime the concepts of 'distance', 'time', and therefore 'speed' are not always unambiguously defined due to the curvature of spacetime caused by gravitation'? Martin Hogbin (talk) 09:00, 14 August 2009 (UTC)


 * I did not add the section mischievously - I firmly believe that an article about the speed of light should include the gravitational time delay. This is not about SR vs GR. This is about real world measurements, and the Shapiro effect is absolutely pertinent to the subject of this article. Of course the article should also mention that using proper time resolves the situation. All this is a refinement of the issues involved and we would be remiss not to mention it. Charvest (talk) 16:30, 14 August 2009 (UTC)

Can somebody explain what this means:

A light signal travelling near the sun can be delayed by gravitational time delay. See the Shapiro effect. Although the speed of light to an outside observer would thus be changed, the speed as measured at any given point using proper time remains constant.

What is an 'outside observer'. Proper time along what worldline?

If we are going to add something about the speed of light and GR we need someone who understands the subject to write it. Martin Hogbin (talk) 21:20, 14 August 2009 (UTC)


 * At the very least, this article should have a see also link to gravitational time dilation where it explains that this doesn't contradict constancy of light but refines the notion. Charvest (talk) 10:59, 15 August 2009 (UTC)


 * Unless we are lucky enough to get an editor who is a real expert on GR I think that the less we say the better. There may be some areas of physics and astronomy where it is convenient to treat the effects of gravitation as corrections to Newtonian physics but in an article on the speed of light this not a good approach.  If we take this approach to the case of gravitational time delay we raise the question of what 'really' happens.  Does the light slow down, the distance get longer, or time speed up.  I am no expert, but in GR I suspect it could be described as any of these depending on what definitions of time and distance we use.  Martin Hogbin (talk) 15:25, 15 August 2009 (UTC)

Shapiro effect and gravitational time dilation are different effects (Shapiro effect, as defined in its wiki article, is about a light ray approaching a mass from infinity then being deflected and then moving away to infinity. Then that detour leads to a time delay). I don't think it is useful to mention these things in this article. As Martin has already pointed out, in GR distances and time intervals are defined locally, and speed of light is a local concept anway.

Now, I accept that you could argue that while along the path of a light ray, you have ds = 0, so the speed of light is locally always c, integrating the time like part of ds and the space like part of ds separately and then comparing the two does not, in general, yield an effective average speed of c. But this argument fundamentally isn't anyting different from considering light being bounced off a mirror in a perpendicular direction and then arguing that the distance traveled by the light is less than c times the time it has travelled. And before you can let the reader clearly understand what is going on in the GR, case you need to explain GR, the definitions of lengths and time intervals in GR, that light travels along null geodesics (ds = 0) etc. etc.. Count Iblis (talk) 16:11, 15 August 2009 (UTC)


 * My point of view is that an encyclopedic article should mention relevant interesting facts. I don't see that we have to explain the whole of GR within this article before we say that there are circumstances in which light can appear to slow down. We then link to the appropriate articles to explain why, despite this, we can still talk about a constant speed of light. Charvest (talk) 17:12, 15 August 2009 (UTC)
 * I think we did once have a section on non-inertial frames and GR but it got lost in one of the edit storms. I was proposing to add this to the end of the 'Speed constant in inertial frames section' in place of my earlier words, In general relativity c remains an important constant of spacetime, but inertial frames can only be defined locally. Over extended regions of spacetime the concepts of 'distance', 'time', and therefore 'speed' are not always unambiguously defined due to the curvature of spacetime caused by gravitation.   Perhaps we could have a section on non-inertial frames and GR which started with the above statement followed by something like, 'examples of cases where the speed of light might be thought to be other than  c, due to the effects of gravitation or measurements made in non-inertial frames, are', followed by some links.  My main point is that it is better to say nothing than something wrong, misleading, or half-baked.  Martin Hogbin (talk) 17:57, 15 August 2009 (UTC)

Charvest, "light appearing to down down" which may be interesting to mention is a different effect that what Martin is talking about. If you consider a photon traveling in the radial direction toward a black hole, then the effect Martin is talking about is absent. Count Iblis (talk) 18:15, 15 August 2009 (UTC)

Reasons the speed of light was set by definition.
As the fixing of the speed of light by definition still seems to be causing problems, with some editors claiming that things are going on "behind the curtains", I thought it might be useful to include the reasons for the change in the article.

Taken directly from 17th Conférence Générale des Poids et Mesures - Resolution 1, but in my words, the reasons are:


 * 1) The older definition did not allow the meter to be sufficiently precisely realised for all purposes.
 * 2) More stable sources of light (stabilized lasers) than the krypton 86 lamp used in the previous definition became available.
 * 3) Measurements (delineations of the metre) made with the new light sources agreed well with those made by the old ones
 * 4) Realizations of the metre are more reproducible using the newer light sources
 * 5) The new defined value is the numerically the same as the older (1975) recommended value
 * 6) The new definition agrees well enough with older realizations that there will me no sudden jump in the length of the meter for all practical purposes.
 * 7) After consultations and deep discussions on the subject it was agreed that this definition is equivalent to the older one, which was based on the wavelength of a defined light source.
 * 8) The new definition still allowed the older krypton 86 source to be used for realization of the metre but also newer and better sources.

Will it help to add these reasons to the relevant section in the article? Martin Hogbin (talk) 09:53, 14 August 2009 (UTC)


 * Martin: All these points are just great, but they do not clarify the reasoning, which is well described by Sydenham: the length standard has been replaced by the time standard using λ = c/f. The remaining issue in setting up the length standard, having settled upon the frequency (or time) standard, is the realization of "vacuum". The error introduced in setting up "vacuum" is added to the error in the frequency standard to find the error in the length standard. The consensus is that this error in length measurement is less than the previous method based upon counting fringes. The argument is all about accuracy. That is the point. How the accuracy is improved is the point. Brews ohare (talk) 16:46, 14 August 2009 (UTC)


 * Martin, the fixing of the speed of light is a problem with me, but it's not the cause of the edit war. The edit war has been caused because of attempts by some to gloss over the full significance of the 1983 definition of the metre in relation to the speed of light. But at any rate, those reasons given above are rubbish. They never considered the mess that was going to land on the plate of electric permittivity. Also, I see that one of the reasons talks about 'consultations and deep discussions on the subject' leading to an agreement that the new definition was equivalent to the older one? So why did they do it? Who did they consult? Those reasons leave us no wiser about anything. David Tombe (talk) 11:35, 14 August 2009 (UTC)


 * It makes no difference whether you like the reasons or not. Martin Hogbin (talk) 13:20, 14 August 2009 (UTC)
 * I was not a delegate at the 17th General Conference on Weights and Measures in 1983, so I cannot give you an exact answer as to "Why did they do it?" Maybe the assembled diplomats (the CGPM is a body under international law, established by the Metre Convention) just thought "Hell, you know, if we just change the definition of the metre this way it will really piss off some idiot in sixteen years time! Let's go for it!" It's more likely that they realised what David Tombe is incapable of realising; that the new definition allows more accurate length determination at the laboratory scale than the old one, and that it is also less likely to need changing in the future. After all, that's what they actually said in the resolution… but conspiracy theorists will never be satisfied with mere evidence. Physchim62 (talk) 13:55, 14 August 2009 (UTC)

Physchim62, I have got a modern physics textbook (Nelkon & Parker 1979) which describes the experiment that is used to measure the numerical value of electric permittivity (ε). It uses the same principles that were involved in the famous Leyden jar experiment of 1856, as performed by Wilhelm Eduard Weber and Rudolf Kohlrausch.

If you are trying to tell me that the 1983 definition of the metre renders this experiment unnecessary, then can you please tell me exactly where the equation c^2 = 1/(εμ) came from in the first place. Unless you can explain the origins of that equation, you are not in a position to be defending the men that defined the metre in 1983. David Tombe (talk) 14:11, 14 August 2009 (UTC)
 * And the permeability of free space (µ0)? Where does that value come from then? Physchim62 (talk) 14:59, 14 August 2009 (UTC)


 * Physchim62: You must ask yourself whether you are really interested in reaching some detente with D Tombe, or if your main interest is in psychological combat. Brews ohare (talk) 16:11, 14 August 2009 (UTC)

Physchim62, If you had read further up the page, you would have seen that I had stated that the permeability of free space (µ0) was chosen by Gregorio de Giorgi of Rome at the sixth international electrical congress at St. Louis, Missouri, in 1904. It is a defined quantity. That is not a problem, because it is only ever the ration of ε and μ that we are interested in. That ratio was determined experimentally by Weber and Kohlrausch in 1856.

The problem arises when we try to make ε a defined quantity also, by reversing a defined value for the speed of light into the experimental results of Weber and Kohlrausch. You didn't answer the question that I asked you. I asked you 'where does the equation c^2 = 1/(εμ) come from?' It comes from the experimental results of Weber and Kohlrausch in 1856.

The only way out of this dilemma is to divorce the equation c^2 = 1/(εμ) from the defined speed of light and to return to comparing the two numbers and asking ourselves what the similarity is telling us. David Tombe (talk) 18:52, 14 August 2009 (UTC)


 * David: My view is that the issue is not an experimental matter at all. Rather, it is about making a model medium with model properties. One is then free to judge experimentally whether an extant medium is well-modeled by this "model medium", but there is no point arguing over whether some "model" is "the model". Brews ohare (talk) 22:04, 14 August 2009 (UTC)
 * The permeability of free space is fixed (in SI units) by the definition of the ampere. If we had a different definition for the unit of electric current (as Weber and Kohlrausch did), say for example defining the ampere as a given number of elementary charges passing a given point in a given time, then µ would not be fixed and would have to be determined experimentally. As is stands, µ is fixed and the elementary charge is an empirical constant, but that is simply a choice of units, not a physical constraint.
 * The equation $$c^2 = 1/(\epsilon\mu)$$ will hold for any choice of units. Indeed, it is not restricted to free space, but also holds for light travelling thorugh a medium. You can repeat Weber and Kohlrausch's experiment if you wish, to show that $$\epsilon = 1/(c^2\mu)$$, there is no definition of ε in the SI except for this equation. The only tautology is to suggest, as David Tombe does, that becsue we assume it is true then it must be true and therefore lacking in physical significance. Do we suggest that Newton's second law has no physical significance simply beacuse the units we use to measure force are defined in such a way that it must be true? Of course not! It is the physical significance of Newton's second law (at least on everyday scales) which allows us to define units to measure force!
 * Any system of units is a human construct, even so called "natural units", and any measurement is dependent on theory. You can choose to have a unit of length that is equal to the length of a bronze measuring stick, but that depends on the theory that the length of the bronze measuring stick doesn't change with time. The United States realised in the late 19th century that the length of bronze measuring sticks does change with time (it took another 0 years for the British to admit this), so chose a new definition for the unit of length with the Mendenhall Order (based on the theory that the length of platinum–iridium measuring sticks doesn't change with time, which is closer to reality).
 * The intellectual vacuity of David Tombe's arguments is perhaps best shown by the fact that he admits that the ampere is defined in terms of a fixed value of a physical constant, as is the second, as the metre has been since 1960. Yet he only wishes to vilify the current definition of the metre, not any of these other definitions. It is David Tombe who has an unhealthy obsession with the speed of light, wishing to giving it some special status over and above that of other physical constants, exactly the sin which he so loudly accuses others of committing. This is worse than hypocrisy, it is plain stupidy. Those who relish in their own ignorance have no place on an encyclopedia project. Physchim62 (talk) 12:11, 15 August 2009 (UTC)

Physchim62, You seem to have totally misunderstood my position. I am the one that has been strongly advocating that the equation $$\epsilon = 1/(c^2\mu)$$ does have physical significance. As regards the actual content dispute at 'speed of light', my only point has been that the 'speed of light' has been a tautology since the 1983 definition of the metre. Brews wants to elaborate on this fact and I have been supporting his right to do so. The issues surrounding the equation $$\epsilon = 1/(c^2\mu)$$ relate to how this tautology spills over into electric permittivity. David Tombe (talk) 12:52, 15 August 2009 (UTC)

Britannica's take on the speed of light
Their article begins by saying the speed of light is the ''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.'' Saros136 (talk) 06:22, 15 August 2009 (UTC)


 * Of course it does, and so did this article from its inception, through the period when it was an FA right up until this month when one editor, who has some kind of hang-up about this, decided that it was confusing or something to give this standard defined value. It looks as though a bit of clever maneuvering has been engaged it to get the page locked in its current state.


 * Wikipedia should reflect current, established, mainstream scientific thought on a subject not the idiosyncratic fews of a few. Martin Hogbin (talk) 08:14, 15 August 2009 (UTC)

There is also an issue here that Lou Scheffer should take note of. Britannica didn't state that the 'speed of light' is specifically the 'speed of light in a vacuum'. Britannica introduces the issue generally for all materials and then goes on to talk about the speed in the vacuum in particular. A few days ago, I tried to correct this error in the main article, but Lou Scheffer reverted it. Unfortunately this can't be rectified until the page becomes unprotected again. David Tombe (talk) 12:00, 15 August 2009 (UTC)


 * Most sources treat "speed of light" as the speed in a vacuum. Britannica is in a distinct minority in this case; it wouldn't hurt to mention that POV, too, but probably it isn't needed in the lead.  Dicklyon (talk) 16:35, 15 August 2009 (UTC)
 * I think Saros136 was trying to bring this bit, the value for the speed of light in a vacuum is now defined as exactly 299,792,458 metres per second to our attention. It is the simple answer to a simple question, 'What is the speed of light?', that we should obviously retain in the lead of this article.  Martin Hogbin (talk) 16:55, 15 August 2009 (UTC)


 * Well, it's in the lead now, and nobody was fighting or arguing to take it out, so how's that? Dicklyon (talk) 16:59, 15 August 2009 (UTC)


 * We currently have, '[The speed of light] is normally denoted by c, and is approximately 300,000 kilometres (or 186,000 miles) per second', followed nine lines later by, 'In the International System of Units, the length of the metre is now defined such that the speed of light is exactly 299 792 458 m/s'. What purpose does giving the first approximate value serve? Martin Hogbin (talk) 18:05, 15 August 2009 (UTC)


 * I think I already answered that above. Dicklyon (talk) 01:43, 16 August 2009 (UTC)

Behavioral change is needed
Both points in Britannica's take on the speed of light above are reasonable and can be accommodated in the article with productive, collaborative editing. The above quotation from Britannica certainly reads better than this article's lead does now. On the other hand, the lead should also distinguish between (1) the literal meaning of speed of light and that substances or circumstances may affect it, and (2) the fundamental constant of the speed of light in a vacuum and its physical consequences. Further, I doubt that anything in the Britannica article suggests that c is a tautology or "human construct" because of the way the meter was redefined.

"Unfortunately", to quote Tombe, the article had to be protected, which temporarily prevents improving the article, because of the manner in which several editors, Tombe and others, conducted themselves in edit warring in the article and in discussing the issues on this Talk page. Improving the article requires a change in that behavior—an attitude adjustment would also help, but constructive, productive, civil behavior is essential, and is required by Wikipedia's policies and behavioral guidelines.

Thus far, the constructive discussion of the issues that page protection was intended to encourage has not even begun. The article cannot be improved, nor the present stalemate resolved, without that constructive discussion beginning and continuing to consensus. Yes, consensus is possible and will be reached if Wikipedia's policies and guidelines are followed. The system really does work if editors cooperate constructively. All of the participants in this dispute, even those of you who know that you are correct—that is, all of the participants in this dispute—should read Disruptive editing and learn to comply with the Wikipedia behavioral guidelines in it.

Page protection will not continue indefinitely. Indefinite page protection is unfair to non-disruptive editors and to Wikipedia's readers. If the current temporary protection does not lead to constructive, collaborative editing, appropriate measures will be taken to prevent those who continue to behave disruptively from impeding progress in improving this article. None of you is indispensable. There are other editors who can improve the article if you won't cooperate sufficiently to do so. —Finell (Talk) 17:49, 15 August 2009 (UTC)


 * Finell, Before we consider any of the points that you have raised above, we need to correct an important fact. You mentioned my name alone in connection with edit warring. This edit war has been going on for months. I am only a newcomer here. I have made remarkably few edits to the main article, and I have not been involved in edit warring. When the page was protected due to an edit war yesterday, I doubt if you will even see my name on the history page without having to turn over to the second page. On the talk page I have pointed out that the post-1983 speed of light is a tautology. That is a true fact and there are sources to back it up. So far I haven't attempted to write this fact into the main article. So please get your facts straight before naming names in relation to who has been edit warring.David Tombe (talk) 23:27, 15 August 2009 (UTC)


 * My facts are straight. Your behavior, and the behaviour of several others here, including others who disagree with you, needs to change. And no, the speed of light is not a tautology, although it is understandable that someone who reads the definition of the meter in isolation from the real world could reach that conclusion through pure "logic". If (1) those experimenters who continually measure the speed of light in various circumstances observe a fluctuation or change in the physical speed of light, and if (2) the only consequence of that discovery is that everyone around the world dutifully and silently re-calibrates all their measuring devices, then you would be correct that the speed of light had become a tautology. That is not, however, how things work in the real world, definitions notwithstanding. I promise you that, no matter what happens to the observed speed of light, you will not have to buy a new set of metric wrenches on that account. Furthermore, no matter what happens to the observed speed of light, Olympic-size swimming pools will not be redesigned to accommodate that development. So I urge you and everybody else who has been arguing over this to behave according to Wikipedia's polices and guidelines. The alternative, for those who don't, is that you won't be arguing about it on Wikipedia. I won't address the merits of this issue further. I will be observing editors' behaviour and I will take appropriate action to stop disruptive conduct. Speed of light, which ought to become an FA again, will not be held hostage by disruptive editing. Editing against a consensus that is founded upon the most reliable sources will not be tolerated. Arguing against the reliable sources is pointless. Being tendentious even if reliable sources are on your side will not insulate you from sanctions for misbehavior. Enough is enough! —Finell (Talk) 00:21, 16 August 2009 (UTC)

Finell, The speed of light, since the 1983 definition of the metre, reads "k times 'the distance that light travels in 1/k seconds' per second, where k can be any number, and we have chosen the number 299 792 458 in order to smooth over the transition from the old system". That is a tautology if ever there was a tautology. And the spill over into electric permittivity is even worse.

I haven't tried to write this in the article. I was merely mediating in the edit war in order to try and establish what the edit war was about. It seems that it is about the right to elaborate on this tautology in the article, but I'm not even altogether sure about that. David Tombe (talk) 00:38, 16 August 2009 (UTC)
 * The Contributors statistics for this article are quite instructive as to the problems faced by new editors. Physchim62 (talk) 05:53, 16 August 2009 (UTC)


 * Thanks for pointing that out; I didn't know about that tool. It sure does support what I've been trying to say about Brews, as well supporting my contention that Martin is part of the problem.  Here is another article where I had big problems with Brews.  And another.  And another.  And another.  Of course, I realize this can make me look like part of the problem for standing up to him, like Martin is doing.  Oh, and here's Brews's really big one, an article that he and everyone abandoned after I started the summary-style article Centrifugal force to defuse it.  (ps. by way of full disclosure, there are a lot of articles on which I'm the number 1 editor; with over 40,000 edits and a focus on certain technical areas, how could there not be?  But very few where I have over 100 edits, and some of those are where Brews is involved, like Wavelength.  My biggest of probably this one where's is almost all vandalism reverting.)   Dicklyon (talk) 06:13, 16 August 2009 (UTC)


 * Finel, you seem to mixing up two roles here. Firstly as an admin you are quite rightly suggesting that we all behave in a better Wikipedian manner.   An addition, you are attempting to address the content  of the page directly.  What you say about wrenches and swimming pools changing size is exactly what will happen in principle as the meter becomes more precisely delineated but it is very confidently expected that changes will be less than the equivalent of changing the last digit in the speed of light.  In practice, therefore, no one is going to be chiseling extra length on swimming pools or filing their wrenches down, however, if there were a technology that required exceptionally high precision of length then it would need to keep abreast of any changes (or more likely refinements) in the length of the metre as they occurred. In practice this would be probably achieved by realisation of the meter at the point of use.  The ability to do this is one of the great advantages of the current definition.


 * Can I ask your opinion, as a disinterested admin, on what would be the best way to resolve the issue of what value or values for the speed of light in m/s we have in the lead section. Martin Hogbin (talk) 09:50, 16 August 2009 (UTC)


 * I'm not sure exactly what that statistics link is supposed to prove. What do the two figures in brackets refer to? There is a total figure that is followed by two figures in brackets? David Tombe (talk) 15:13, 16 August 2009 (UTC)

How do we move on?
OK, the class is now quiet. I would like to proceed with improving this article. How do we do it? I suggest that we start with a poll of current editors on the relatively simple question of what numeric value or values for the speed of light we show in the lead section Martin Hogbin (talk) 12:10, 17 August 2009 (UTC)


 * I am opposed to the notion that the number 299 792 458 m/s be billed in the introduction as the exact speed of light in SI units. This number is not an experimental result, and cannot be changed by appeal to measurement. A clear discussion may be found in Jespersen and in Sydenham. Therefore, attributing 299 792 458 m/s to a physical "speed of light" is simply a logical or semantic error. Whatever this number is chosen to be in the SI units, it is the conversion factor that converts a time-of-transit across a length (in seconds) to an actual length (in metres). This conversion factor is determined by international convention, and is in fact an arbitrary number selected to be close to the speed of light as measured prior to 1983 (before the new definition was implemented). Of course, the speed of light prior to 1983 was not 299 792 458 m/s exactly, but only approximately (because under the old rules c was a measured quantity, not an exact, defined conversion factor). Brews ohare (talk) 12:42, 17 August 2009 (UTC)


 * Something along these lines: According to our current theories the speed of light is a fundamental physical constant. The meter and the second are defined such that this speed is exactly 299 792 458 m/s. DVdm (talk) 13:13, 17 August 2009 (UTC)
 * Dirk, a minor point, the speed of light became a defined value only when the metre was defined by the speed of light, the second was already defined at that point . If you want to keep the link to second it would therefore be better to have,'The meter is defined such that this speed is exactly 299 792 458 metres per second. But this is just a detail. Martin Hogbin (talk) 13:29, 17 August 2009 (UTC)


 * I don't fully agree. Indeed, as you say, the second was already defined, and its definition is independent of everything else, but its definition has an influence on the SI value of the speed of light. So it is indeed a fact that both meter and second are defined such that the SI value gets that particular value. The SI value is a direct consequence of these definitions. DVdm (talk) 13:39, 17 August 2009 (UTC)


 * I agree that if the second 'got longer' then so would the metre, but the definition of the meter is the only one that makes use of the speed of light. It was only when the metre was defined that the speed of light became fixed by definition, and those who made the choice to define the meter the way that they did knew that this would be the case. Martin Hogbin (talk) 14:16, 17 August 2009 (UTC)

I was just writing the bit below while you added this. Perhaps you could add your votes below.
 * Again, the definition of the metre does not define the speed of light (in the sense of the physical speed of propagation of light). What it does do is define the conversion factor from a time-of-transit in seconds to a length in metres. The difference in view is this: the conversion factor is arbitrary (and for practical reasons happens to have been chosen as a number close to the measured value prior to 1983) while the actual physical speed of light is a a fact of the universe that has nothing to do with man's conventions. Brews ohare (talk) 13:20, 17 August 2009 (UTC)


 * So far, I agree with everything that Brews has said above. I will now read on. David Tombe (talk) 19:03, 17 August 2009 (UTC)

gravitational effects
This reference appears useful in this connection Gaposchkin Brews ohare (talk) 16:45, 17 August 2009 (UTC)

Poll on what numerical value or values we show in the lead section
There are three basic options on what value for the speed of light in SI units we should show in the lead section. Note that in all cases further values and explanations can be provided in the main body of the article. Options are:

It is normally denoted by c, and is approximately 300,000 kilometres (or 186,000 miles) per second It is normally denoted by c, and is approximately 300,000 kilometres (or 186,000 miles) per second in the first paragraph, with:
 * A. Show only the approximate speed, as in:
 * B. Show the approximate speed, followed by the exact speed as in:

In the International System of Units, the length of the metre is now defined such that the speed of light is exactly 299 792 458 m/s., in the fourth paragraph. The length of the metre is defined such that the speed of light is exactly 299 792 458 m/s. optionally followed by something like: For convenience, the approximate value of 300,000 km/s is often used.
 * C. Show the exact speed after the current first sentence (optionally followed by an approximation, I do not think this is a contentious issue) as in:

Votes
Please state here your preference with only one or two lines of explanation


 * Support C It states an impeccably supported fact that is likely to be what many readers actually want to know. This approach has been used through the life of this article until this month. There is no valid reason not to do this.Martin Hogbin (talk) 13:18, 17 August 2009 (UTC)


 * Support C. I would open with the sentence like "According to our current theories the speed of light is a fundamental physical constant." The next sentence can mention the definition of the meter, together with the exact value for the speed of light in SI units. DVdm (talk) 13:25, 17 August 2009 (UTC)


 * Support C. I agree with DVdm's suggestion. The fact that c is a fundamental constant makes it possible to use it to define the meter in terms of the second in a useful way. So, such a sentence may be helpful to readers. Count Iblis (talk) 13:30, 17 August 2009 (UTC)


 * Support A Quite aside from this poll, which attempts to set up majority rule on this topic, the matter should be decided on the basis of what approach actually is logically, historically and semantically correct. In stark contrast to the assertion that C is an "impeccably supported fact", it is a misuse of language to say the SI units provide an "exact" number for the physical speed of light, as that is clearly not the case. The SI units provide an exact conversion factor from time-of-transit to length in metres, as described carefully by Jespersen and Sydenham. Brews ohare (talk) 13:37, 17 August 2009 (UTC)


 * Support B – While either A or C would also be acceptable, the history shows that they are polarizing alternatives that lead to a lot of fighting. The important thing is that IF we include the exact value in the lead, we defer the complex and polarizing discussing of the philosophical implications of having a defined fixed value for c until a later section.  Dicklyon (talk) 15:07, 17 August 2009 (UTC)


 * Support C – c is defined as a specific number in the SI system, the rest is ajusted in consequence. The article should reflect this from the get go, and not wishy-wash it down at first to introduce the real definition later. Mentionning that it is often approximated to 300 000 km/s (or 300 000 000 m/s, whichever pleases people) immediatly following the exact definition will address any familiarity concerns of laymen. Headbomb {{{sup|ταλκ}}κοντριβς – WP Physics} 15:37, 17 August 2009 (UTC)


 * Support B - While I fully sympathize with Brews's reasons for supporting A, there is a problem with wikipedia's rules and regulations that needs to be considered. It would be expected that the up to date SI unit value would appear somewhere near the top of the article. As far as I am concerned, the up to date value in SI units is a tautology, but if that's what they decided at that 1983 conference, then we're stuck with it for the time being. I further support the right of Brews to elaborate on this tautology later on in the article. David Tombe (talk) 19:14, 17 August 2009 (UTC)


 * Support C - C is concise and scientifically correct. The metre has been defined to give c the above value, but that does not make c a mere conversion factor. Distance and time are separate physically observable quantities, and c is a measurable physical quantity. Ehrenkater (talk) 20:01, 17 August 2009 (UTC)
 * Ehrenkater these entities all are measurable in some systems of units, but not in SI units. Brews ohare (talk) 20:21, 17 August 2009 (UTC)
 * This is a poll - Please do NOT comment. Thank you. DVdm (talk)
 * Brews ohare It only requires them to be measurable in any ONE system of units to demonstrate that they are separate observable quantities. SI units are a man-made construct which does not alter the physical realities. Your contributions are just a long string of one non sequitur after another. Ehrenkater (talk) 18:05, 18 August 2009 (UTC)
 * This is a poll - Please do NOT comment here. Rather use the space below. Thank you. DVdm (talk) 18:10, 18 August 2009 (UTC)


 * Support C - but include somewhere in the article an explanation that while the speed of light is a fundamental constant, its numerical value is not fundamental as this depends on the choice of units, and we can define unit of length in terms of speed of light and therefore give the speed of light any value we find convenient. Also if the speed of light changed we would notice if we measured it using a stick physically created prior to a change rather than the SI definition - we obviously cannot use the SI definition to measure the speed of light. Charvest (talk) 20:27, 17 August 2009 (UTC)
 * Charvest: your statement is accurate, but is it really adequate to bury the important caveat about units while stating a so-called (misnomer) exact value??
 * The question of whereabouts in the article to explain all this is a different question than whether the value 299 792 458 should be mentioned in the lead. Charvest (talk) 20:31, 17 August 2009 (UTC)
 * This is a poll - Please do NOT comment. Thank you. DVdm (talk) 20:49, 17 August 2009 (UTC)


 * Support C - The statement is correct as written, and all the philosphical implications follow from the explicitly stated definition. LouScheffer (talk) 03:24, 18 August 2009 (UTC)


 * Support C - I would like to see the exact value in m/s as high up the article as possible. It is surely what many readers to the article are looking for. Philosophical questions of definition and measurement should be deferred (without a teaser in the lead). EdwardLockhart (talk) 11:40, 18 August 2009 (UTC)

Discussion
I agree that the first sentence should state that the speed of light is a fundamental constant, then we state its exact value in SI units. In fact it might be a good idea to say 'dimensional/dimensionful' constant, although this is covered later. Martin Hogbin (talk)


 * Majority rule does not decide matters of logic and accuracy. See Jespersen and Sydenham. WP uses sources, not majority rule. Brews ohare (talk) 13:38, 17 August 2009 (UTC)


 * Nor Jespersen, nor Sydenham, nor any majority, nor a minority like Bruce will be able to show there is something illogical or inaccurate in a statement like
 * "According to our current theories the speed of light is a fundamental physical constant. The meter and the second are defined such that this speed is exactly 299 792 458 m/s".
 * DVdm (talk) 13:54, 17 August 2009 (UTC)

To replace opinion with sources:
 * Jespersen: "c [is] a conversion factor whose value is fixed and arbitrary"
 * Sydenham: "Thus the speed of light as a numerical value is not a fundamental constant...if the speed of light is defined as a fixed number then in principle the time standard will serve as the length standard."

The problem is in the last phrase of DVdm's remark: "defined such that this speed is exactly 299 792 458 m/s" The problem is that the "speed" of light is not defined this way; it is the metre that is defined this way. The number 299 792 458 m/s is not the speed of light; it is a conversion factor selected by international agreement (not measurement) to be close to the speed of light as determined prior to 1983 when c was a measured quantity. That international choice was dictated by convenience, not nature, and could have been chosen instead to be 1 ft/ns (for example) if they felt like it. It was just the pain of conversion that decided continuity with the past was more important than numerical simplification. Brews ohare (talk) 14:12, 17 August 2009 (UTC)


 * So what is the numerical value of the speed of light in m/s? Martin Hogbin (talk) 14:19, 17 August 2009 (UTC)


 * The speed of light cannot be determined by measurement within the SI system of units as presently set up. Of course, you can compare the speed of light with other speeds (e.g. with the speed of sound in specified circumstances). As Jespersen says: its "value is fixed and arbitrary". You might find the discussion by Bondi "the units of distance" interesting. Brews ohare (talk) 14:23, 17 August 2009 (UTC)


 * You are deliberately avoiding the question that I asked. I simply asked what the numerical value of the speed of light was in SI units, I did not ask if it could be measured. What is your answer? Martin Hogbin (talk) 14:36, 17 August 2009 (UTC)


 * I'm not ducking you: it is not measurable, has no operational meaning in the sense of some operations that lead to its determination. Assuming a genuine interest in sorting this out, try to explain to yourself how Jespersen and Sydenham and Bondi arrive at their positions. Brews ohare (talk) 14:39, 17 August 2009 (UTC)


 * Yes you are ducking me. I just asked what its numerical value is.  The value is 'fixed and arbitrary', what is it? Martin Hogbin (talk) 14:44, 17 August 2009 (UTC)

All right. I take it you have no intention of reconciling with the sources I mention. The physical speed of light as the speed of light in the universe (an extant real speed) is not known numerically within the SI system of units; its numerical value is not defined (though it has properties, like isotropy). What is defined is the conversion factor between time-of-flight and distance. I gather that you would like to state that this conversion factor is the speed of light in SI units, but of course, that is a play on words, and suggests that somehow man can dictate to nature what the real speed of light is. Brews ohare (talk) 14:55, 17 August 2009 (UTC)


 * You are still refusing to answer my simple question, which is, 'What is the speed of light in SI units?'. I am making no claim that I or anyone else can dictate anything to nature. Please answer the question asked.  Martin Hogbin (talk) 15:01, 17 August 2009 (UTC)

The physical speed of light as the speed of light in the universe (an extant real speed) is not known numerically within the SI system of units, and in fact, cannot be determined in principle. For example, if the true speed of light changed tomorrow, the meter would simply expand or contract, and the numerical conversion factor would still be the same. Thus, the conversion factor is not the speed of light. Brews ohare (talk) 15:06, 17 August 2009 (UTC)


 * I think you have made your position quite clear, The physical speed of light as the speed of light in the universe (an extant real speed) is not known numerically within the SI system of units. I will say no more. Martin Hogbin (talk) 15:20, 17 August 2009 (UTC)

Dick, There are times to compromise and times not to, and this is one of the times where there should be no compromise, especially just appease one editor who makes comments like the quote above. We need to make a decision. Do you yourself actually believe that there is a problem in stating the numerical value of the speed of light in SI units at the start and, if so, what is the problem? Martin Hogbin (talk) 15:25, 17 August 2009 (UTC)


 * Martin: Don't you see a certain intransigence on your part: you will not address the sources, you repeat yourself over and over with no evolution of thought, you take a clear statement backed by sources as somehow a reductio ad absurdum in your mind anyway??? How do you respond to this observation: If the true speed of light changed tomorrow, the meter would simply expand or contract, and the numerical conversion factor 299 792 458 m/s would still be the same. Thus, the conversion factor is not the speed of light. Brews ohare (talk) 15:29, 17 August 2009 (UTC)


 * Brews: You are no less intransigent. —Finell (Talk) 18:05, 17 August 2009 (UTC)


 * I use sources and logic, Martin uses reiteration with no evolution of thought. Brews ohare (talk) 19:58, 17 August 2009 (UTC)

Finell, Brews can see the tautology. The others can't. David Tombe (talk) 19:21, 17 August 2009 (UTC)


 * The 'tautology' as you call it, is obvious, seen by everyone, and intentional. Martin Hogbin (talk) 08:51, 18 August 2009 (UTC)


 * If you take the laws of physics and change c into 2 c while leaving all the dimensionless constants the same, you still have the same laws of physics. All you're doing is rescaling your time coordinate. Although someone simulating the laws of physics using a computer could see a difference when looking at his computer screen, that difference amounts to a simple rescaling. He could have left c the same and apply the rescaling to the results of the original computations instead. This means that an observer located inside the universe cannot see the difference between c and 2 c. Count Iblis (talk) 17:00, 17 August 2009 (UTC)
 * Yes, for this reason I think we should consider stating at the start that the speed of light is a fundamental dimensionful/dimensional physical constant. To many people this may be unimportant but those who are interested could follow the link to a discussion like yours above.  Martin Hogbin (talk) 17:54, 17 August 2009 (UTC)


 * The sources quoted above state explicitly that the numerical value of the speed of light in SI units is not, repeat not, a fundamental physical constant. The actual, physical speed of light is a physical constant of course. Can you see the difference??? That is why we don't need to know what its numerical value is when we set up a standard: the assumption is that the value of c will be a fixed value for any lab that sets things up, and they actually don't have to measure it to be sure that this is the case. They do have to check they have a "vacuum" or know how to correct to refer to "vacuum". Brews ohare (talk) 20:05, 17 August 2009 (UTC)


 * I might add that this fact is not critical to this discussion. If a change in the speed of light should turn out to observable from within this universe there is no experimental evidence that it has changed. Martin Hogbin (talk) 18:32, 17 August 2009 (UTC)


 * To reply to Brews' question. It will always be true that if it is found that the means of defining any standard is detectably unstable then there would have to be a rethink or refinement of the standard, depending on the degree of instability.  Such a potential problem is not limited to the current standard however, there would have been exactly the same problem if it had been found that the frequency of the krypton light used in the earlier standard was unstable or if it had been found that the length of rods made of the the particular platinum alloy used for the original standard was unstable.  Of course the people who design standards generally have a good idea that such things will not happen.  This problem is not specific to using light, it is a problem with having standards, see [] Martin Hogbin (talk) 19:07, 17 August 2009 (UTC)


 * Martin: The point is not whether action would have to be taken if a change were discovered. The point is that any such action is a matter of international agreement, not physics. The conversion factor is a defined not a measured value. So, for example, if the speed of light changed every year by 10−50%, it possibly would be decided that no action need be taken for the next century. The conversion factor would stay at its present value, and for those applications of extreme accuracy some "good practice" correction would be specified. Who knows what the decision would be, the point is that it is simply a decision, and has no bearing upon what "really" is the speed of light. The conversion factor itself is not the "speed of light". That is why the conversion factor is an exact value: it's only a convention. Brews ohare (talk) 19:49, 17 August 2009 (UTC)

Martin, The ultimate extrapolation of all this is where Headbomb tried to tell us that when you measure the separation distance between the plates of a capacitor, to quote him exactly,

you are not measuring the separation of the plates, but rather are making sure that your ruler is calibrated

He said that on the wiki-physics project page earlier today. When it gets to statements like that, then you need to stand back and seriously consider what is going on. At the 1983 conference, they tried to turn physics upside down by playing around with the fundamental dimensions of length and time and making a compound dimension, L/T, into a simple dimension. When it all came unstuck over the issue of measuring electric permittivity, they tried to wriggle out of it by sacrificing the original meaning of a basic electrical experiment involving a capacitor and a vibrating reed switch. Headbomb's quote above is an example of the Alice in Wonderland world of physics that the 1983 conference has led us into. David Tombe (talk) 19:35, 17 August 2009 (UTC)
 * You can dislike the "Alice in Wonderland" physics all you want, this is the definition and what it implies. You can lobby the BIPM to change its definition if you want, but Wikipedia is not the place for this. Headbomb {{{sup|ταλκ}}κοντριβς – WP Physics} 05:23, 18 August 2009 (UTC)
 * To add my tuppence worth: The way to escape the tautology, is to distinguish between an ideal metre as 1/299792458th of a light second and a physical metre stick that has been physically created based on this definition. 299792458 m/s may be the exact conversion figure between the speed of light and an ideal metre, but 299792458 m/s is the approximate measured speed using an actual stick, since all real world measurements are approximate. When it is said that 299792458 m/s is the exact speed of light, that phrase should be taken as shorthand for explaining that this means ideal speed defined with ideal metres in a circular way so that the numerical value is close to the measured value using sticks that were otherwise created prior to 1983. The article could say something to this effect but probably not in the lead. Charvest (talk) 19:49, 17 August 2009 (UTC)

Charvest, I could put it even more simply. The statement from the 1983 conference was "The speed of light is k times the distance that light travels in 1/k seconds, per second, where k can be any number, and we have chosen the number 299792458 in order to disguise the transition from the old system". It's a tautologist's picnic. David Tombe (talk) 20:00, 17 August 2009 (UTC)


 * I suggest you phrase it as A meter is one cth of the distance traveled by light in vacuum in 1 second, and see if you have a problem with that one. It's completely and totally equivalent to the previous definition. c is simply a scaling factor, exactly like hbar is. This is why they are often set to 1. The only difference is that we choose the scale for c while we didn't for hbar, although we certainly could (the unit of energy would then be ajusted in consequence to give the proper slope in photovoltaic effect experiments). Headbomb {{{sup|ταλκ}}κοντριβς – WP Physics} 05:38, 18 August 2009 (UTC)

No Headbomb, I have described the tautology exactly and I don't intend to re-describe it in line with your suggestions. David Tombe (talk) 12:14, 18 August 2009 (UTC)


 * Headbomb, I take it that c in your remarks is shorthand for 299 792 458 m/s? Or, is it short for the physically real speed of light that is in principle not measurable in SI units? Brews ohare (talk) 12:22, 18 August 2009 (UTC)

Brews, Headbomb has merely tried to sew confusion on top of my elucidation of the tautology. What he writes above does not appear to make any sense. David Tombe (talk) 12:35, 18 August 2009 (UTC)

Reliable sources
The resolution to this dispute probably lies in what the most reliable sources say on the matter in dispute. How do Encyclopaedia Britannica and Encyclopedia Americanna treat the speed of light in view of the re-definition of the metre? How do tertiary scientific reference works (scientific encyclopedias and dictionaries) treat it? Do they say that the speed of light is now a tautology?

Perhaps I missed it, but I don't see where any of the other editors have directly addressed the sources that Brews cited above. What are the relevant qualifications of the authors and publishers? Are they reliable sources under Wikipedia's stated criteria? (The authors are not the subject of Wikipedia articles, but that is not determinitive.) Are they primary, secondary, or (probably not) tertiary sources? Do their views on this subject represent today's consensus mainstream scientific thought, minority views recognized within the scientifc community, or fringe views? The answers to these questions determine whether and how these sources and what they say should be treated in the article under Wikipedia's content policies and guidelines.

With due respect to the editors addressing this dispute, Wikipedia's content should be determined by what the reliable sources, and the weight of reliable sources, say. Wikipedians' opinions on how Wikipedia should treat the subject, and Wikipedians' conclusions drawn from what the reliable sources say (as opposed what the sources expressly say), fall within Wikipedia's prohibition of original research. Wikipedia follows the sources. Generally, Wikipedia places primary reliance on the conclusions reached by reliable secondary sources. Generally, Wikipedia should be consistent with reliable, up-to-date tertiary sources. —Finell (Talk) 04:31, 18 August 2009 (UTC)


 * Jespersen is a Bureau of Standards publication recently updated, so has the authority of NIST, approximately. Sydenham appears also to be very reputable. These sources are preferable to Encyclopedia Britannica, for example, because they are expert opinion, not popularized versions. Popularized versions may use simple formulations, but in this case the distinctions needed are sharper than the demands placed by the Encyclopedias. Brews ohare (talk) 06:02, 18 August 2009 (UTC)


 * They are all very good sources, Brews, unfortunately they do not support your claims.


 * Finell, welcome to the discussion. The question is whether or not we say that the speed of light is exactly 299 792 458 m/s.  It is quite clear from the sources quoted, such as NIST that this is the case.  I am not sure whether NIST would be classed as a primary or secondary source but their job is clearly to disseminate reliable information to the public and scientific community.  We do, as it happens, also have Britannica, giving this same exact value.  Brews himself has found many sources which all state that the speed of light has a fixed numerical value in SI units because of the way that the metre is defined.  We are not going to find a source to specifically say that this is not a tautology, they will say that it is true because of the definition of the metre.


 * Regarding editors here, we are not just giving our personal opinions but stating the views of every reliable source. There is no up-to-date reliable source anywhere that says that the the speed of light is anything but 299 792 458 m/s.  Brews views and claims have been addressed endlessly here before, but no one has been able to convince him. We need to move on.Martin Hogbin (talk) 09:31, 18 August 2009 (UTC)


 * Just to make the situation absolutely clear this [] is what a reliable sources says. Please look at the very clear statement in the centre of the page, It follows that the speed of light in vacuum is exactly 299 792 458 metres per second.  This is an impeccable a secondary source.  The purpose of the BIPM is for experts to study and collate reputable research and understanding on subjects relating to measurements and the definition of units and having done this to to set and disseminate international standards.  Martin Hogbin (talk) 12:55, 18 August 2009 (UTC)


 * This source defines the meter in the SI system of units. It states that because of this definition the speed of light in vacuum is exactly 299 792 458 m/s, and says that this value is a consequence of the definition of the metre; i.e. is not a measured result. The term "exact speed of light" in this connection clearly refers to a conversion factor (that is, length = 299 792 458 m/s × transit time, a restatement of the definition for an arbitrary length instead of one metre). The number 299 792 458 m/s does not refer to the physical speed of light, which cannot be established by a "definition", of course, but has to be established by observation, and always will have experimental error attached to it. As pointed out in the discussion on this page and by many cited sources (including the BIPM itself), the consequence of this definition of the metre is that the number 299 792 458 m/s will not change, no matter what refinements in measurement technique may occur in the future, simply because such refinements alter the standard metre, not the factor 299 792 458 m/s. That independence of 299 792 458 m/s from measurement is why the number 299 792 458 m/s is exact: because it has no relation whatsoever to measurement, nor to the exact physical speed of light. It is a number that was selected for convenience as a pretty close approximation to the value of the speed of light as measured prior to 1983 when the metre was "1,650,763.73 vacuum wavelengths of light resulting from unperturbed atomic energy level transition 2p10 5d5 of the krypton isotope having an atomic weight of 86" (a speed certainly never considered an exact value). This value of 299 792 458 m/s was adequate to avoid practical issues of accommodation to the new metre. Apart from the need for such accommodation, the value 299 792 458 m/s could have been selected as any number whatsoever. Brews ohare (talk) 15:32, 18 August 2009 (UTC)

Hi Martin: You have yet to address the difficulty that your "exact" speed of light is not measurable within the SI units system, neither in practice nor in principle, as is stated explicitly in many sources. (A partial list provided at the end of this paragraph.) Instead, you provide your own opinion on the matter, and never address these sources. You say "they do not support" my "claims" (although I am quite unsure that you know what these claims are), and I have yet to see any of your reasoning behind your statements. A partial list of sources is: NIST publication, Jespersen, Sydenham and Bortfeldt ("Since 1983 any measurement of the speed of light is neither necessary nor possible"); Brush ("Thus the speed of light ceased to be a measurable constant.") Brews ohare (talk) 12:09, 18 August 2009 (UTC)


 * Finell, You are correct in principle about the issue of 'weighting and reliability of sources'. But my experience has shown that groups of editors who push unweighted samples of sourced material are generally not pushing a fringe viewpoint, but rather they pushing their own attachment to a popular fad within the current orthodoxy, to the exclusion of a wider knowledge of the topic.


 * Where the issue of weighting becomes particularly acute is in relation to reporting matters to do with the history of science. For example, there has been a recent case in which the views of the 18th century Daniel Bernoulli, arguably one of the greatest scientists ever, has had his views overstamped by the modern orthodoxy on the basis of an opinion expressed by the author of a book written in 1990. These issues will eventually have to be addressed at somewhere like the wiki-physics project page. A clear directive will eventually be needed from the top. As it stands now, I often get suspicious of determined editors who hide behind the party line and claim that their only interest in the matter is to ensure that all edits are backed up by reliable sources. Unless they are countering defensively with this strategy, I generally suspect that the editor in question has an agenda and a point of view which they are not willing to openly expose. It is this kind of silent 'point of view' pushing that causes most of the edit wars. And the edit wars are never won 'de jure' on sources, despite the rallying cry. They are always won 'de facto' on mob rule, by tactical use of the 3RR rule and by abuse of the policy of consensus. David Tombe (talk) 12:31, 18 August 2009 (UTC)


 * David: You seem to be pushing every bit at hard as anyone else for your particular point of view. —Finell (Talk) 19:15, 18 August 2009 (UTC)

Finell, Can you please elaborate on that. What is the point of view that you have in mind? On the talk pages here, I have made my opinion on the matter quite clear. But for the purposes of the main article, I have merely been advocating that a clear distinction is drawn between the speed of light as the concept is traditionally understood, and the new post-1983 defined speed of light. This is new territory in physics, and I don't think that the consequences of the 1983 conference have been as yet fully realized, even by many physicists. The non-physics public at large will not understand the 'new speed of light' concept. Brews is correct to want to clarify the difference in the article. This is a case where it is far too presumptious to dominate an introduction with the new Alice in Wonderland physics at the expense of the non-physics readership's expectations. I reluctantly voted B above, because I acknowledge wikipedia's obligations to report the official position. But this is a case where the official position is very new, and that it is so ludicrous that it needs to be complimented with a non SI position on the matter. In principle, I really wanted to vote A like Brews. The lead at vacuum permittivity is one of the most presumptious cases of pushing the new physics that I have ever encountered. It completely deletes an important topic in experimental physics that is of major historic importance, and which was taught at least up until recent times David Tombe (talk) 20:19, 18 August 2009 (UTC)

The initial definition
The initial definition of the speed of light in the first sentence of the article is misleading. For example, should one read "the speed of light in glass" to mean "a fundamental constant in glass"; or as "299... m/s in glass"? The first sentence should define the general concept of the speed of light, and only after that mention any specific examples (like in vacuum). Each point in the article should be clear about whether it refers to the general concept of speed of light, or the speed in a particular medium (such as vacuum). Is there agreement with these statements? Pecos Joe (talk) 21:00, 17 August 2009 (UTC)


 * The article's lead defines the term as used in the article. The fact that there is a "speed of light in glass" does not mean that "the speed of light" can't be a fundamental constant.  They are different terms, related as discussed in the article. Dicklyon (talk) 23:24, 17 August 2009 (UTC)

Pecos Joe, I made that very same point myself last week. I noted how the initial definition limited the speed of light specifically to the speed of light in the vacuum. I corrected this error, but the correction was immediately reverted by Lou Scheffer. Then a few days later, somebody produced an example lead from Encyclopaedia Britannica. It adopted the correct and generalized approach. This Britannica lead was hailed by some editors until I also hailed it in the specific respect that we are now talking about. Dicklyon then immediately adopted a negative approach to that particular aspect and informed us all that Britiannica was taking a minority point of view in that respect.

Everybody knows that the lead is wrong as it stands at the moment. But nobody wants to correct it because it would be going against Lou Scheffer's reversion of my correction. If I ever correct anything, it is a guaranteed way of ensuring that it will be made wrong again and kept wrong as a crucial matter of principle over and above every other principle. David Tombe (talk) 23:42, 17 August 2009 (UTC)


 * Look, I enjoy pedantic nitpicking as much as the next guy (gals are normally more sensible) but there are excellent reasons phrase "the speed of light" refers to the speed of light in a vacuum. If you go up to any physicist (not involved here) and ask how you equate mass and energy, they say "multiply by the speed of light squared".  They will also tell you that you can't go faster than the speed of light; you can't send information faster than the speed of light; you use the speed of light to swap space for time in spacetime; and the speed of light is part of the metric in GR.  Any normal physicist would agree with these statements, but all are wrong unless the speed of light is used to mean the speed of light in vacuum.  Yes, it is *possible* to use the speed of light to mean the speed of light in any medium, then add "in vacuum" or c when the fundamental constant is meant, but that's not the normal usage.  Since one particular speed of light is spectacularly important (involved in almost all of physics) and the others are measured values of interest only to specialists, it's very rational for "the speed of light" to refer to the important one.  This is just like "the white house", which in theory could refer to any white house, but in normal usage refers to a specific one on Pennsylvania Avenue.   LouScheffer (talk) 02:53, 18 August 2009 (UTC)


 * When a physicist hears speed of light, the physicist assumes it means the speed of light in a vacuum (unless otherwise stated), because that is the particular speed of light that is fundamental to physics. When most non-scientific, general readers hear speed of light, they make no assumption at all, and that the term means simply how fast light travels. That is probably why general encyclopedias do not limit speed of light to mean its speed in a vacuum, and we should follow suit. Wikipedia's article should address both readerships, should make the distinction in the lead, and explain that it is light's speed a vacuum (without getting into what vacuum means) that is a fundamental physical constant. I may be repeating what Dicklyon proposed awhile ago. Further, I believe that it is more productive to say that the lead needs work than that it is "wrong", since there are opposing opinions on what is wrong with it. —Finell (Talk) 05:05, 18 August 2009 (UTC)


 * The vast majority of sources seem to treat "the speed of light" as c. Other uses can be discussed, to, but not such that they dilute this main point.  If you want to propose improvements, or repositioning, of the section "Light in transparent media", that would be worth a consideration.  But to come in and suggest changing the one thing that all the arguing parties to date have agreed on is just disruptive. Dicklyon (talk) 05:20, 18 August 2009 (UTC)


 * I agree, but we should make this clear at the start with something like: 'The term speed of light normally refers to a fundamental physical constant, the speed that light travels in vacuum'. I do not think there is any need to address in the lead the alternative and naturally obvious meaning of the speed that light happens to go in a particular medium, just like we would not have an article on the 'speed of a dog' explaining that this just means the speed that a dog is travelling at. 'The speed of light', unless qualified, has a specific meaning, even in everyday life. Martin Hogbin (talk) 08:48, 18 August 2009 (UTC)

Of course, the main difficulty in the discussion is that the "speed of light" may refer to the real, physical entity, the speed of light as found in the universe. Or, it may refer to the "numerical value of the speed of light in some system of units". What is said about the first, the physical speed of light, does not necessarily apply to the second (its value in some system of units). In fact, the numerical value in the SI system sometimes referred to as "the speed of light" actually is a misnomer, as it does not refer to either the physical speed of light, nor its numerical value, but to a conversion factor that allows the length standard to be replaced by the time standard. The true numerical value of the real speed of light is not accessible from within the SI system of units. For example, if the speed of light changed, the number 299 792 458 m/s would not change, but the metre would. Likewise, if accuracy improved in measurement, the number 299 792 458 m/s would not change, but the metre would. See NIST publication, Jespersen, Sydenham and Bortfeldt ("Since 1983 any measurement of the speed of light is neither necessary nor possible"); Brush ("Thus the speed of light ceased to be a measurable constant.") Brews ohare (talk) 06:24, 18 August 2009 (UTC)

Suggestion: Way back in July, the lead started with "The speed of light normally refers the speed of light in a vacuum, and is an important physical constant in modern physics. Light travels at different speeds through different materials..." (followed by an explanation which said why the speed in vacuum was particularly important). I'd suggest the following minor wording changes "The speed of light normally refers the speed of light in a vacuum, which is an important physical constant in modern physics. Although light travels at different speeds through different materials..." LouScheffer (talk) 13:06, 18 August 2009 (UTC)


 * I have no problem with this sort of introduction. My problem is the statement "In the International System of Units, the length of the metre is now defined such that the speed of light is exactly 299 792 458 m/s." Although sources can be found that state such a thing, the notion that the speed of light is exactly known is nonsense, and in fact, in the SI units, the speed of light is not even measurable in principle. Misinformation should be avoided. Brews ohare (talk) 13:14, 18 August 2009 (UTC)
 * Possibly a way out of this dilemma would be to state the measured value in 1972, with its error bar, and then describe the transition away from measurement of c in 1983 to the use of a conversion factor for c that removes it from the experimental domain and enables the use of an arbitrary exact value. Brews ohare (talk) 13:22, 18 August 2009 (UTC)
 * Putting this info about the switch to a conversion factor into the intro is one way to introduce many important properties of the speed of light, because it is just these properties that make c a candidate for a standard that is readily reproduced. Brews ohare (talk) 15:06, 18 August 2009 (UTC)
 * Sorry Brews but your comment, 'Although sources can be found that state such a thing, the notion that the speed of light is exactly known is nonsense' will not wash. It as very reliable secondary source which states this and it is therefore what we must say.  Your opinion is of no relevance, especially as most editors here disagree with you. Martin Hogbin (talk) 16:38, 18 August 2009 (UTC)
 * We do know that the speed of light is exactly 299 792 458 m/s, and the reason why we know this is because we defined it to be exactly 299 792 458 m/s. We cannot measure this, because it is a defined quantity. This is like saying you can't know that a square has four sides because we defined a square to have four sides.Headbomb {{{sup|ταλκ}}κοντριβς – WP Physics} 18:39, 18 August 2009 (UTC)

I'd like Martin Hogbin & Headbomb to reflect upon this matter further. The speed of light was measurable before 1983 because the metre was defined as an actual physical length (a certain number of wavelengths) determined independently of the speed of light, as was time as well. Hence, c = ℓ/t was found by measuring t and using the known, separately measured ℓ. That is standard practice in determining speed, wouldn't you say?

Post 1983, the metre was not determined separately from the speed of light, but in terms of the distance light traveled in 1/299 792 458 s, as ℓ = 299 792 458 m/s × 1/299 792 458 s = 1 m. Consequently, c = 299 792 458 m/s, no matter what. Do you two think this outline is correct?

Supposing you agree, when the BIPM definition of the metre refers to "the speed of light" as 299 792 458 m/s, I'd say they definitely expect (at a minimum) to use the relation ℓ = c × t in determining length from time-of-transit, and naturally in this context c = 299 792 458 m/s exactly. Do you think you might agree with me on this point?

However, I do not think BIPM or NIST mean that, quite beyond its use in ℓ = c × t, this numerical value for c refers to the exact physical speed of light. (By physical speed of light I refer to the speed of light with the properties explained in the theory of relativity, say.) They mean, rather, to restrict this use of c to be only the exact conversion factor when using the relation ℓ = c × t. Of course, a conversion factor c for use in ℓ = c × t can be defined and never measured (it doesn't have to be measured at all), because it is a convention decided among men to place their comparisons of length upon the same footing. It is therefore exact and does not change with advances in measurement technique. As Jespersen says, it is arbitrary and can take on any value whatsoever. As long as everybody uses the same c in finding lengths ℓ = c × t, everybody is on the same page. I believe that is the position of the sources I have cited.

Would you please discuss these points?? Brews ohare (talk) 03:39, 19 August 2009 (UTC)

pi is defined to be the ratio of the circumference to the diameter of a perfect circle
Let me see if I understand the dispute with Brews via an analogy. Suppose that for some strange reason we always express lengths of line segments in different units from curved lines. Expressed in these units the ratio between the circumference of a circle and the diameter could be some arbitrary number depending on how we would choose the units for the two type of lengths. In natural units you would choose the same units and then the ratio would be the dimensionless number pi (analogous to c = 1).

But in conventional units the ratio is expressed as 3.001(curved length)/(straight lenght) and a redefinition of the "curved length" unit makes this ratio exactly 3 (curved length)/(straight length).

Then Brews comes along and he argues that however we define things (natural units, or other units), the lengths have a physical meaning independent of our definitions. He argues that the physical ratio could change, but because we define our units so that the ratio is fixed, that would mean that the length standards change their lengths.

The only way what Brews suggests can happen would be if the metric could change from Euclidian to non-Euclidean. If he would not make explicit such a scenario and everyone would by default only consider Euclidean geometry, then Brews objections would be difficult to appreciate.

I think that in the case of speed of light, the laws of physics themselves as we understad them allow one to act according to the 1983 redefinition of the meter. I.e. c is constant and you can assign it whatever value you like, c = 1 being the most natural choice. This choice then fixes the unit for distances in the time direction to be the same as distances in the spatial direction. The "physical speed of light" can differ from c if the photon would turn out to have a small mass.

Anyway, I think it would be helpful if Brews were to specify some unambiguous theory in which making the distionctins he suggests would be relevant. Count Iblis (talk) 19:22, 18 August 2009 (UTC)


 * Count Iblis, You said above I think that in the case of speed of light, the laws of physics themselves as we understad them allow one to act according to the 1983 redefinition of the meter. I.e. c is constant .


 * But I think that Brews is trying to emphasize the fact that the constancy that you are referring to is not the same as the constancy that comes with the new set of SI units. I suspect that at least some of Brews's opponents have a vested interest in blurring this distinction. Not everybody believes in the physical constancy of the speed of light in the relativistic sense. Hence, to compound this belief with a system of units in which the speed of light has to be a numerical constant, will assist the case of those who do believe in the physical constancy. It would most likely have been a forgone conclusion at the 1983 conference that the distinction between the two types of constancy would eventually become blurred.


 * The distinction is important in its own right as part of the general understanding electromagnetism. Brews shouldn't need to point out a theory in which the distinction becomes important. Electromagnetism can be a confusing topic at the best of times for many students, and there is no better way to confuse students even more than to confuse two different concepts into one. David Tombe (talk) 20:34, 18 August 2009 (UTC)


 * The answer is that c is constant in both senses. We have no reason to believe that c changes over time, if indeed such a change were to be measurable.  It is also true that c is constant in all inertial frames.  Thus, as Count Iblis says, we can act according to the 1983 definition, because the speed of light is constant.  But I have to repeat, even if this were not the case it would not alter that fact that in metres per second the speed of light is constant by definition. Martin Hogbin (talk) 21:31, 18 August 2009 (UTC)

Martin, The speed of light differs upsteam and downstream in a column of liquid. Fizeau demonstrated this fact in 1851. The speed of light in glass changes with frequency, which is why we have dispersion. The speed of light is different in different media. So how are we supposed to do physics if the speed of light is constant by definition of the units? The situation is a nonsense. This is all new physics. It's not the physics that I was taught, or the physics that I later taught. I live in a world where a trundle wheel measures a distance. But you obviously live quite comfortably in a bizarre world where the distance calibrates the trundle wheel. It is far too presumptious to put this latest new age physics into the lead of an on-line encyclopaedia without putting a clarification alongside it in relation to traditional attitudes to the concept of speed. The public aren't ready for these silly games. You have now confirmed to me that you want to blur the distinction between real physics and new age physics. Basically, you want to delete real physics. And you have already proved to me that you want to delete it from history too. I can see the significance of 1984 coming through very clearly in all of this. The lead to the article vacuum permittivity is pure 1984 material. David Tombe (talk) 00:36, 19 August 2009 (UTC)
 * If this is not the physics you were taught, I suggest that you update yourself to take into account the last 26 years of science before coming on Wikipedia to argue you know the truth and everyone else is trying to suppress "real physics". Call the BIPM and complain if you want, in the meantime we'll keep on reporting how the world is rather than how you want it to be. Physicists have had no problem working with that definition of the speed of light, and if they had any you can bet that they would have made a ruckus about it by now. Headbomb {{{sup|ταλκ}}κοντριβς – WP Physics} 04:38, 19 August 2009 (UTC)
 * Obviously, I should have said 'in free space'. I think it is actually the last century of physics that David needs to acquaint himself with. Martin Hogbin (talk) 13:12, 19 August 2009 (UTC)

Headbomb, The point is that such a cataclysmic change in recent times, which means that we can no longer put a ruler to an object and measure a length, needs to be elaborated on for the benefit of the non-physics readership. It is far too presumptious to slip this nonsense into a mainstream encyclopaedia as if we had always lived in such an Alice in Wonderland world. An average member of the public still thinks of speed in terms of the distance that light travels in a given period of time. There has so far, to the best of my knowledge, been no public awareness campaign instructing the public that since 1984, when we measure a length, we are in fact really just calibrating our measuring device.

You have taken this nonsense on board, hook, line, and sinker. But the public haven't. I would say that 99.9999999% of the public are oblivious to this nonsense. This is all just a fad, and you have become absorbed by the fad to the extent that you want to write these articles as if it had never been any different. Brews on the other hand clearly sees that something badly needs explaining, and you and the rest of your 'consensus' are vilifying him for doing so, because you don't want it to be known that this is actually just new physics that is being slipped in through the back door unnoticed. David Tombe (talk) 12:25, 19 August 2009 (UTC)

NIST and BIPM meaning for c
I'd like Martin Hogbin & Headbomb to reflect further upon the way BIPM and NIST use the term speed of light.

The speed of light was measurable before 1983 because the metre was defined as an actual physical length (a certain number of wavelengths) determined independently of the speed of light, as was time as well. Hence, c = ℓ/t was found by measuring t and using the known, separately measured ℓ. As with all measurements, the speed of light had error bars. That is standard practice in determining speed, wouldn't you say?

Post 1983, the metre was not determined separately from the speed of light, but in terms of the distance light traveled in 1/299 792 458 s, as ℓ = 299 792 458 m/s × 1/299 792 458 s = 1 m. Consequently, c = 299 792 458 m/s, no matter what. Do you two think this outline is correct?

Supposing you agree, when the BIPM definition of the metre refers to "the speed of light" as 299 792 458 m/s, I'd say they definitely expect (at a minimum) to use the relation ℓ = c × t in determining length from time-of-transit, and naturally in this context c = 299 792 458 m/s exactly. Do you think you might agree with me on this point?

Of course, a conversion factor c for use in ℓ = c × t can be defined and never measured (it doesn't have to be measured at all), because it is a convention decided among men to place their comparisons of length upon the same footing. It is therefore exact and does not change with advances in measurement technique. As Jespersen says, it is arbitrary and can take on any value whatsoever. As long as everybody uses the same c in finding lengths ℓ = c × t, everybody is on the same page. A change in c everybody agrees upon simply changes the length of the metre that everybody uses. I believe that is the position of the sources I have cited.

However, I do not think BIPM or NIST mean that, quite beyond its use in ℓ = c × t, this numerical value for c refers to the exact physical speed of light. (By physical speed of light I refer to the speed of light with the properties explained in the theory of relativity, say.) They mean, rather, to restrict this use of c = 299 792 458 m/s to be only the exact conversion factor when using the relation ℓ = c × t.

Such a point of view makes clear why c from the BIPM viewpoint is exactly 299 792 458 m/s with no error bar and no connection to measurement. On the other hand the physical speed of light, like all real, physical properties, is known only by observation and always has error bars and never is exactly known, just as common sense suggests. Moreover, for comparing lengths, one never needs to know what the exact physical speed of light is: one needs only the confidence that, whatever it is, this same speed always is realized in "vacuum".

Would you please discuss these points?? Brews ohare (talk) 03:39, 19 August 2009 (UTC)


 * Up to the sentence starting with "However", yes, that is accurate. What follows is more or less unclear. There's no difference between "defining the speed of light" and "defining the conversion factor", these are completely equivalent actions (up to the limit that photons are indeed massless if you want to be picky about it, but that's irrelevant to the discussion at hand). The "physical" speed of light does not have error errors bars on it, it is exactly 299 792 458 m/s, by definition. If you do a measurement, and there are errors bars somewhere, yes, but the error bars are not on the speed of light. Rather they are on the other quantities. For example, if you take a pulse of light traveling in vacuum (speed = c, by definition), and check where it's at three seconds later, then you will probably have errors bars on time because your clock is probably not a single atom of caesium-133, at rest, isolated from the universe, in its ground state, at 0 K. You have an uncertainty on elapsed time, thus will have one on the measured distance. Since t = d/c, then &Delta;t = &Delta;d/c. Since &Delta;t is directly proportional to &Delta;d, then if you are off by 1 part in 10^9 on time, you will be off by 1 part in 10^9 on distance. c however, is unaffected by this, as it is defined to be a certain precise value. Headbomb {{{sup|ταλκ}}κοντριβς – WP Physics} 04:57, 19 August 2009 (UTC)

Headbomb: It seems to me that it doesn't actually matter what the "exact" speed of light is. Suppose it was 2 × 106 wavelengths of some transition per second. We still can use c=299 792 458 m/s exactly as we do today, with absolutely no effect upon anything we do, even though 2 × 106λ/s was totally different from c=299 792 458 m/s. In particular, we'd still find lengths as ℓ = 299 792 458 m/s × t, with t the transit time in vacuum. Do you agree?? Brews ohare (talk) 05:13, 19 August 2009 (UTC)
 * No, we can't use that, we'd have to measure the speed of light in that scenario in order to know it was 299 792 458 m/s, and there would be error bars on the speed of light, and the scaling between meters and seconds would be uncertain. There would also be error bars on the meter, since there is no such thing as as purely monochromatic light source whose spectrum can be described by a delta function located at a certain wavelength. And there would be error bars on the second just as before. So you're worse off doing things this way (and that is why that we are now using a better way). Headbomb {{{sup|ταλκ}}κοντριβς – WP Physics} 07:21, 19 August 2009 (UTC)

This seems to me to be the exact same situation we have with mass today. The kg is defined as the mass of some metal cylinder kept in a vault in Paris. Now what is the mass of that object? It's EXACTLY 1.000000000000 kg, with no error bars. This is a result of the definition, just as the speed of light is. Of course, just like light, you can (and do) measure the mass (speed) using other realizations of the kg (meter), so at least you have some idea of the uncertainty in your definition. This is a much bigger problem with mass rather than the meter, and they would love to replace the standard kg with a count of some kind of atom, but the technology is not quite there yet. LouScheffer (talk) 11:48, 19 August 2009 (UTC)

The next step in discussion, it seems, is to say, Hey, since we can't tell the difference anyway ... Brews ohare (talk) 05:15, 19 August 2009 (UTC)


 * From 1960 to 1983, the wavelength of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum in SI units had no error-bar on it and could not in principle be determined by measurement - it was by definition 1 / 1 650 763.73 metres. Does this fact bother you as much as the post-1983 situation with the speed of light? Why or why not? EdwardLockhart (talk) 05:16, 19 August 2009 (UTC)
 * EdwardLockhart: Off the top of my head I'd say that defining the metre in terms of some reproducible length is logically different from defining it in terms of a standard of time. The precise definition of the metre in terms of some λ does not afford a problem to me: the λ is a well-defined length in principle (Nature's metre, so to speak). When using λ = c t it is the idea that c so-used is supposedly the same thing as exactly the actual speed of light, when in fact the set-up makes measurement of this speed in SI units logically impossible, and c so-used is an arbitrary convention. Brews ohare (talk) 05:27, 19 August 2009 (UTC)
 * But it is a reproducible length! Take a pulse of light, wait until 9,192,631,770/299,792,458 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom, at rest, in its ground state, at 0 K have ellasped and the distance light will have travelled is your meter, no matter where you are. This is a well defined length, just as much as the other, only that instance of counting wavelenghts, you count ticks from a clock. Headbomb {{{sup|ταλκ}}κοντριβς – WP Physics} 07:29, 19 August 2009 (UTC)


 * Headbomb, Does that work for the circumference of a trundle wheel too, even though light doesn't travel in circles? David Tombe (talk) 11:58, 19 August 2009 (UTC)


 * Brews, along with other editors, I am happy to discuss this topic with you but will you please accept that there is a consensus of editors and reliable sources here to state the exact value.


 * Your point that we cannot control or define nature remains true. Although the speed of light in free space is defined exactly in m/s we can never exactly determine the length of (delineate) the metre any more.  Under the current definitions there is, and always must be, some uncertainty when the metre is delineated.  This will be due to experimental error and due to the fact that, as you have often pointed out, although the speed of light is a defined constant in free space, we can never actually delineate the metre in free space because we do not have any. So, when the meter is delineated, we have to make corrections, as best we can, for the actual experimental conditions.  All the experimental and theoretical issues that once applied to measuring the speed of light now apply to the delineation of the metre.  Martin Hogbin (talk) 08:07, 19 August 2009 (UTC)

Can I suggest that this discussion is continued on a page in my user space User_talk:Martin_Hogbin/Speed_of_light_set_by_definition so that the rest of us can use this page to discuss improving the article. Martin Hogbin (talk) 13:15, 19 August 2009 (UTC)


 * No you can't suggest that. Your suggestion is just a bureaucratic tactic to hide the fact that there is a matter to be discussed. This is the page to discuss ways of improving the article, so we will stick to this page. David Tombe (talk) 13:18, 19 August 2009 (UTC)
 * This parrot is no more! He has ceased to be! Martin Hogbin (talk) 14:53, 19 August 2009 (UTC)

Proposed start to the lead.
Bearing in mind what reliable sources say and the opinions of other editors, my suggestion for the start of the lead is:

''The term 'speed of light', usually denoted by 'c', generally refers to a fundamental dimensionful physical constant which is the speed that light travels in a vacuum. The length of the metre is defined such that the speed of light is exactly 299 792 458 metres per second but it is often conveniently approximated to 300,000 km/s ''

Links and references would need to be added. Let me give reasons for my choice of words:

I added 'the term' to 'speed of light' because to say  The speed of light generally refers to is not good English...  Perhaps 'speed of light' in quotes would be OK.

There is no 'now', 'currently' or 'according to current theory' as this is always expected to be the case in an encyclopedia article.

I mention that it a fundamental physical constant first then give its exact speed in SI units, then the approximation. I have added that it is a dimensionful (or dimensional) constant. This explains a lot for those interested.

I have added a minor tautology 'the length of the metre' because I think it helps understanding of what has been done. I do not think that the additional tautology of 'in SI units' is necessary.

Maybe the English could be polished a but but I think these are the right things to say, in this order.Martin Hogbin (talk) 17:04, 18 August 2009 (UTC)


 * It is a step in the right direction. I think the brief comment about the metre would confuse a general reader; and I think the approximate value is better suited to the section on value/notation (are there any sources that go into some detail about the speed of light, and introduce the approximation so prominently?); also, this wording seems to unintentionally invite the question of what the term could also refer to. Pecos Joe (talk) 17:48, 18 August 2009 (UTC)


 * I am trying to take account of the comments of other editors. The bit about the metre gives a very brief indication of how the speed can be exact.  Perhaps we could do without it?


 * I agree that the approximate speed is not essential for the lead.


 * If you mean 'generally refers to', I would be quite happy to drop the 'generally'. I think the self evident meaning of the speed that light happens to go in some medium is not particularly relevant to an encyclopedia article.


 * Basically, I am trying to cover some of the angles, so that we can end up with a well though out and intro that does not get rewritten every week. Let us see what others think. Martin Hogbin (talk) 18:07, 18 August 2009 (UTC)


 * I don't think you can cut out 'generally', unfortunately. For example, in electromagnetism, I think it is fair to say that the speed of light generally means the speed of light in some medium.  I don't know a good way to reconcile that yet.  I came back to clarify that I support replacing the first paragraph with the one you propose when the protection is gone. Pecos Joe (talk) 18:28, 18 August 2009 (UTC)

A suggestion:

''In Physics, the term 'speed of light' generally refers to the speed at which light travels in a vacuum. This is a fundamental dimensionful physical constant, usually denoted by 'c'. In SI units, c is exactly 299 792 458 metres per second.''

This adds the field of study, as is conventional. And I hope attaching the statement about exactness to "c" rather than to "the speed of light" might make it a bit clearer that it is an arbitrary choice of value for a constant rather than a happy conincidence. EdwardLockhart (talk) 09:02, 19 August 2009 (UTC)


 * Yes, let's go for it. Finally. DVdm (talk) 10:18, 19 August 2009 (UTC)


 * That reads very nicely, and I personally have no problem with it. I was trying to cover some of the points raised by other editors, but it is not easy and there is a lot to be said for a simple and straightforward approach like yours.  In the future, no doubt, someone will raise the point of how the speed can be an exact number and should we give some indication of this in the lead.  The problem is that the only way that I can think of to phrase this neatly starts with, 'The meter is ...', rather than a statement about the speed of light. I am trying to get something that is well written an bomb proof, maybe this is not possible. Martin Hogbin (talk) 11:49, 19 August 2009 (UTC)


 * Perhaps making the word "exactly" an internal link to the "speed of light set by definition" section would help. EdwardLockhart (talk) 12:27, 19 August 2009 (UTC)


 * That sounds good to me but is it not what you called a 'teaser'. Martin Hogbin (talk) 12:56, 19 August 2009 (UTC)


 * That link would not be a teaser in my opinion; just don't write "For more detail see some section." If you wanted to mention the field of study, why restrict it to physics? Chemistry and astronomy also use the term, so maybe it should read "in science'.  I don't think it is a convention to mention the field of study, as just about 1/3 of good articles do in what I saw, so either way would be acceptable.  Do you have any objection to explaining the other use of the term 'speed of light' as the next sentence of this paragraph? Pecos Joe (talk) 17:05, 19 August 2009 (UTC)
 * I'm basically OK with the above, but I'll throw in a suggestion and a comment. The suggestion is to remove the word "dimensionful". It is correct, of course, but most physical constants have dimensions: it is dimensionless constants such as the fine structure constant which really stand out for attention. The comment is that astronomy also uses the speed of light (obviously) but not with the SI definition: I think it would be fair simply to start the lead with "The term "speed of light" usually refers to…" Physchim62 (talk) 17:51, 19 August 2009 (UTC)


 * I agree. "Dimensionful" sounds pretty awkward, to say the least. DVdm (talk) 17:57, 19 August 2009 (UTC)


 * Joe, if you mean by "the other use of the term 'speed of light'" just the speed light happens to go in some particular medium, then I think this should not be in the lead. As I said above this is just the ordinary use of the English language.  We would not have an article on 'The speed of a dog'. Martin Hogbin (talk) 18:13, 19 August 2009 (UTC)


 * Here is a sentence that describes what I was thinking. It is very poor English, but it adequately describes an approach to answer the question of what else 'speed of light' would refer to.  The cleaned-up version would appear as the sentence after the one with the 299... number.  I think it would be better to cover 100% of the uses rather than only 90% (or whatever the case may be) of them, especially when the change is relatively small, as a cleaned-up version would be.
 * Sometimes, in electromagnetism, when the medium in which light propagates is clear, the term speed of light may refer to the speed of light in that medium, which is slower than the speed in vacuum by a factor called the refractive index of the medium.Pecos Joe (talk) 21:07, 19 August 2009 (UTC)


 * I think that we should mention that light travels more slowly in a medium but what we do not, in my opinion, need to say is that in some cases 'the speed of light' can be used to mean simply the speed light is that travelling at. This is just normal English.  Of course it is used in this way.  Do you see what I mean.  It is not that you are wrong, it is that we do not need an encyclopedia article to tell people what 'the speed of light' means when it is not referring to the important case of light in a vacuum. It just means exactly what those words would be expected to mean.Martin Hogbin (talk) 21:52, 19 August 2009 (UTC)


 * It seems my suggestion is the minority, so I will not continue suggesting it. Pecos Joe (talk) 17:01, 20 August 2009 (UTC)

So we now have, for the first paragraph:

''The term 'speed of light' generally refers to the speed at which light travels in a vacuum. This is a fundamental physical constant, usually denoted by 'c'. In SI units, c is exactly 299 792 458 metres per second''.

with a link to the 'Speed set by definition' for 'exactly' and other appropriate links. References are not generally liked in the lead, as I understand it, they should be where the detail is discussed in the main body. Is there anyone who does not like this wording?

Perhaps we are all wasting our time here. Finell has taken it upon himself to rewrite the lead for us, completely ignoring this ongoing discussion. Martin Hogbin (talk) 18:55, 19 August 2009 (UTC)


 * I can perfectly live with Finell's edit. And besides, indeed, we are all wasting our time here, but it's fun, isn't it? :-) - DVdm (talk) 19:42, 19 August 2009 (UTC)


 * I've changed the lead paragraph to be the one above. Let's see if it sticks! EdwardLockhart (talk) 19:53, 19 August 2009 (UTC)


 * ... and Edward's is really ok as well. So, what was al this fuss about? DVdm (talk) 19:55, 19 August 2009 (UTC)


 * Maybe I am taking it a bit too seriously, but I am trying to get this article back to being a FA. One of the first criteria for that is that it should be relatively stable.  The only way I can see to achieve that is to use wording that is acceptable to as many editors as possible. That is what I was trying to do here. Martin Hogbin (talk) 20:03, 19 August 2009 (UTC)


 * Probably you are taking it too seriously, but I appreciate your efforts. Pecos Joe (talk) 21:07, 19 August 2009 (UTC)


 * I agree that the goal should be a return to FA. —Finell (Talk) 22:53, 19 August 2009 (UTC)

Well, that is the first paragraph done, how are we going to do the rest. The basic content of the next paragraph is fine but I think it needs rewriting to improve the style and the way that it puts things. Martin Hogbin (talk) 23:05, 19 August 2009 (UTC)


 * Replied below in new section. Pecos Joe (talk) 17:01, 20 August 2009 (UTC)

This is a dead parrot
The poll and subsequent discussion clearly show a large majority of editors agreeing that we should state the exact value of the speed of light early in the lead. Many reliable sources clearly state that the speed of light has an exact value in m/s and none state that it does not.

I therefore suggest that we accept that there is a consensus, supported by reliable sources, to state the exact value of the speed of light in the lead section of this article. This need not stop discussion of the subject although I would suggest we set up a special talk page for those interested in talking about the subject further.

On this basis, I have proposed a new first paragraph for the lead, with the intention that we come up with well written section that is acceptable to everybody here. I am doing this to try to end the endless rewriting and edit waring over the lead section that we have had recently. I would therefore ask all editors to have a look at the section on the lead above and make their comments and suggestions there. Martin Hogbin (talk) 08:45, 19 August 2009 (UTC)


 * Martin, There is one point that I would like to raise. Further up the page, you supplied us with the proceedings of the conference (in your own words). You wrote,


 * Taken directly from 17th Conférence Générale des Poids et Mesures - Resolution 1, but in my words, the reasons are:


 * (1)The older definition did not allow the meter to be sufficiently precisely realised for all purposes.


 * (2)More stable sources of light (stabilized lasers) than the krypton 86 lamp used in the previous definition became available.


 * (3)Measurements (delineations of the metre) made with the new light sources agreed well with those made by the old ones


 * (4)Realizations of the metre are more reproducible using the newer light sources


 * (5)The new defined value is the numerically the same as the older (1975) recommended value


 * (6)The new definition agrees well enough with older realizations that there will me no sudden jump in the length of the meter for all practical purposes.


 * (7)After consultations and deep discussions on the subject it was agreed that this definition is equivalent to the older one, which was based on the wavelength of a defined light source.


 * I would like to draw attention to points (6) and (7). They seem to be saying that the new system will be equivalent to the old system. But I now have Headbomb telling me that I can no longer measure the distance between the plates of a capacitor under the new system. Can you clarify these two points please. David Tombe (talk) 12:14, 19 August 2009 (UTC)


 * Although they are in my words the above reasons were taken directly from Resolution 1. There is link to that resolution somewhere on this page that you can follow to get the original words.  I suggest that you follow up you discussion with Headbonb directly with him. Martin Hogbin (talk) 13:00, 19 August 2009 (UTC)

As this parrot is now dead I have set up a page in my user space User_talk:Martin_Hogbin/Speed_of_light_set_by_definition where I suggest that this discussion is continued so that the rest of us can use this page to discuss improving the article. Martin Hogbin (talk) 13:07, 19 August 2009 (UTC)


 * Well Martin, I'm going to ignore your special page because I was discussing improving the article right here where it is supposed to be discussed. The issue is that there has been too much of a cataclysmic shift in physics as a consequence of the 1983 conference, and it is too recent to allow this new physics to be slipped in without an explanation for the general non-physics readership. Brews wants to make the elaboration and you have been pulling out all stops to vilify him and hinder him because you want the article to be written up as if this new nonsense physics has always existed. David Tombe (talk) 13:14, 19 August 2009 (UTC)
 * It is your choice discuss the subject where you like but there is clearly a consensus of editors her who want to state the exact speed in the lead and there is no sign that you have convinced any of them. This parrot is no more! He has ceased to be! Martin Hogbin (talk) 13:59, 19 August 2009 (UTC)
 * Actually, it isn't David Tombe's choice to spout his pseudoscience where he likes. Physchim62 (talk) 14:14, 19 August 2009 (UTC)

Still dead?
David Tombe is still under an indefinite topic ban from this article and talk page. To those of you who who have extensive experience with Tombe's views and past edits, do the recent edits of the article by look like his work? This IP has a total of 4 contribs, all today. The first is a diatribe at AN/I against the admin who topic banned Tombe followed by the 3 article edits here. You may want to initiate an investigation of whether the IP is Tombe violating his topic ban. —Finell (Talk) 20:10, 20 August 2009 (UTC)


 * Never mind. I reported it myself at AN/I. The contribs convinced me. —Finell (Talk) 20:44, 20 August 2009 (UTC)

Second lead paragraph
''One postulate of the theory of special relativity states that the speed of light is constant in any inertial reference frame. One important consequence of relativity is that neither information nor matter may travel at a speed greater than the speed of light in vacuum.


 * It's basically the relativity paragraph, and I had thought the most important things to mention would be the limiting speed of info/matter, and the constancy in inertial frames. I dislike the "believed to be" part (if we can say it is, then say that, but otherwise leave unsettled science out of the lead if possible), and I think mentioning spacetime is confusing to the general reader and unnecessary in the lead. Part of that paragraph could read something like above. Pecos Joe (talk) 16:59, 20 August 2009 (UTC) (modified by Pecos Joe (talk) 17:13, 21 August 2009 (UTC))


 * I think we should steer clear of the constant speed in inertial frames, or at least be very careful. It is hard to explain the true situation, as is done in the relevant section, in a sentence or two.  If we do say something, I think that it should just be that light travels at the same speed in all inertial frames.


 * The other point that would be good to get across is that special relativity, general relativity and spacetime are all essentially the same topic. Martin Hogbin (talk) 21:50, 20 August 2009 (UTC)


 * I favor keeping and being careful with the inertial frames part. I'm not quite sure what you mean by equating those topics - could you be more specific?  I rearranged the first post, maybe just add to/alter the italicized paragraph up there to propose a particular wording so the section won't become too long to read through the discussion. Pecos Joe (talk) 17:13, 21 August 2009 (UTC)


 * The more modern way to treat relativity is to start with Mankowski spacetime, in which c plays the role of the constant relating the space dimensions to the time dimension. All the results that we call SR (maximum speed, composition of velocities, mass-energy equivalence etc) follow from the fact that we live in Minkowski spacetime.  In GR this same spacetime is curved by the presence of matter and energy.  In this curved spacetime c is still a constant of the spacetime but our intuitive notions of distance and time are no longer applicable.  To talk of a speed is meaningless unless you define exactly how you will determine distance and time.


 * Because of the above, I do not think that it is a good idea to talk along the lines of,'c is a constant of spacetime...according to SR this happens...according to GR that happens', as though they were three unconnected theories. I would like to say something along these lines, but in better English: c is a constant of the spacetime in which we live.  Because of this the following are true: speed of light is constant in all inertial frames; it represents the maximum speed at which matter, energy or information can travel; it is the conversion factor between mass and energy. Martin Hogbin (talk) 22:39, 21 August 2009 (UTC)

299 792 458 m/s a "fundamental constant"
Majority rule has been implemented and the lead statement now says 299 792 458 m/s is a fundamental constant of nature, despite sources (for example Wheeler; Jespersen; Sydenham) that state the contrary.

The fact is that the number 299 792 458 m/s was introduced as "the speed of light" in 1983 by defining the metre to make this so. However, far from being a fundamental constant, this number was decided by committee, and has a fixed and arbitrary value (according to Jesepersen) that (as clearly stated by NIST and CODATA) is beyond measurement, being simply a conversion factor between time and length in the SI units.

It is legitimate to state that (within the context of SI units) the speed of light in SI units is 299792458 m/s because of the definition of the metre; it is absurd to state that it is a fundamental constant, as it is a number determined by committee, as Wheeler says. Brews ohare (talk) 13:04, 21 August 2009 (UTC)


 * The lead does not say that "299 792 458 m/s is a fundamental constant of nature". It says that the speed of light is a fundamental physical constant, usually denoted by 'c', and that, in SI units, c is exactly 299792458 m/s. The constant is denoted by 'c', and c has some value in some system.
 * Good grief. DVdm (talk) 13:11, 21 August 2009 (UTC)


 * Maybe that is what you think it says: I don't agree. Here is the text:"This is a fundamental physical constant, usually denoted by 'c'. In SI units, c is exactly 299792458 metres per second."


 * To elaborate unnecessarily, the c that is a fundamental physical constant has nothing to do with the number 299 792 458 m/s that is the conversion factor between time and length in SI units, which is set by committee, independent of any recourse to measurement and entirely unrelated (logically) to the actual speed of light, as expressed let's say, in terms of wavelengths of some atomic transition traversed in a second. The closest one can come to connecting the number 299 792 458 m/s to the actual speed of light is that it is a number that was chosen by committee to be pretty close to the measured value pre-1983, when this number still was a measurable entity in the SI system.Brews ohare (talk) 13:16, 21 August 2009 (UTC)


 * It says that the speed of light is a fundamental physical constant, usually denoted by 'c', and that, in SI units, c is exactly 299792458 m/s. The constant is denoted by 'c', and c has some value in some system. Period.
 * What you think words say, seems to be a personal problem for you. DVdm (talk) 13:27, 21 August 2009 (UTC)

Well, if you say c is the fundamental physical constant, and then say c is exactly 299 792 458 m/s, it's not "my personal problem" if I take that as saying the fundamental physical constant has a value of exactly 299 792 458 m/s in SI units, a blatant untruth, contradictory to sources. Brews ohare (talk) 13:34, 21 August 2009 (UTC)


 * Well, It says the speed is "usually denoted by 'c', and that, in SI units, c is exactly 299792458 m/s".
 * You see, it is indeed your personal problem. We really can't help you with this. DVdm (talk) 13:41, 21 August 2009 (UTC)

Perhaps you could rephrase it so no confusion could result, even for those of my limited capacities? Brews ohare (talk) 13:52, 21 August 2009 (UTC)


 * You now know what the words mean - we explained. You don't have to worry about people with "limited capacities". If such people come along and express their confusion, we'll explain it to them. No worries. DVdm (talk) 13:58, 21 August 2009 (UTC)

It's a bit dubious to leave ambiguous statements in the article that have to be resolved on the talk page. Here's a possible change of phrasing:"“This is a fundamental physical constant, usually denoted by 'c'. Prior to 1983, in SI units the measured numerical value of c was determined as approximately 299 792 458 m/s. Then, in 1983, a switch was made from a measured to a defined value for c, and c was fixed arbitrarily at the value 299 792 458 m/s by forcing the metre to adapt so as to maintain this value regardless of experimental observations.”"References could be added. Brews ohare (talk) 14:03, 21 August 2009 (UTC)


 * I said: "No worries". DVdm (talk) 14:09, 21 August 2009 (UTC)

DVdm: Your reassurances are meaningless, of course, and are not a substitute for substantive comment. I'm not a dope, and I do speak English. So the approach should be to fix the language, not to make the reader the problem. Brews ohare (talk) 14:13, 21 August 2009 (UTC)


 * I also said that "we really can't help you with this". That should cover the fact that you find the reassurances meaningless. Perhaps another hobby? DVdm (talk) 14:26, 21 August 2009 (UTC)

Apparently you have adopted the responsible position of spokesman for the group "we" who cannot be helpful. I'd suggest that your approach is offensive, contrary to WP policy, and not conducive to improving either the page or the spirit of collaboration. Brews ohare (talk)


 * I'm just trying to help keeping the article and the talk page clean, and to explain the meaning of a few simple words. To no avail it seems. Sorry. DVdm (talk) 14:39, 21 August 2009 (UTC)
 * If Brews feels that his personal views carry more weight than the consensus of other editors, then there is really nothing "we" can do to help him. We can, however, attempt to minimize the damage. Physchim62 (talk) 14:54, 21 August 2009 (UTC)
 * Brews is not conveying his personal views, but reporting the contents of published sources with impeccable credentials. It is your approach to avoid confronting the cited authorities and instead to make backhanded unsupported remarks. Why not attempt to deal with the issues? Is it less fun? Brews ohare (talk) 22:23, 21 August 2009 (UTC)

If the goal is to restore the status of this page, the lead sentence must avoid ambiguity. To do that, it needs revision. I've tried to suggest the problem. The response I get is that there is no problem. My attempts to clarify the problem are not dealt with, but instead I'm told to get lost. Great way to improve things, eh? Why the animosity? I've provided sources. I've provided logic. I've proposed a possible rephrasing. C'mon, guys. Brews ohare (talk) 15:00, 21 August 2009 (UTC)


 * Brews, your proposed wording is good, but it is too detailed for the introduction. The sequence of events that led to the definition can (and should) be elaborated in the article, but doesn't need to be gone over in the first paragraph.
 * I would propose a slight addition to the third sentence, reading: "In SI units, c is defined to be exactly 299792458 metres per second." (The italics would not appear in the sentence, of course). DJ Clayworth (talk) 15:08, 21 August 2009 (UTC)

DJ Clayworth: Thanks for your comment. Your suggestion does not solve the problem entirely, however. The difficulty is this: prior to 1983 the speed of light was measurable, due to the metre being a defined number of wavelengths of a particular transition. Consequently, one could determine how long it took for light to traverse this distance, and c was just distance over time. However, post 1983 the value of c became simply a conversion factor, with length = c × time of transit. Evidently, because length is now determined this way, to determine the length of any particular object, the time of transit is measured and then multiplied by c to find the length. The numerical value of c is defined, but obviously is not the fundamental constant termed the speed of light, because it is a number fixed by fiat at the pre-1983 approximate value, and now independent of measurement altogether.

I agree that this state of affairs goes into some detail, and may not be appropriate in the introduction. However, one does not want the introduction to make a statement readily misinterpreted as suggesting the fundamental constant c has an exactly known numerical value, when this is certainly not the case. The problem with this WP article seems to be this: Within the SI system of units, the defined conversion factor is referred to by the wording "the speed of light", and this terminology is unfortunately interpreted outside this SI units context as meaning the actual speed of light. Brews ohare (talk) 16:24, 21 August 2009 (UTC)

Brews, I think your worries may be a bit off. A new reader is more likely to be impressed with the tight linkage between relativity, length-measurement and time via experiment, implied by the ability of a committee (that knew these things) to set the value and be consistent with all of these experiments, than do be thrown off by it, imho. Jok2000 (talk) 17:33, 21 August 2009 (UTC)


 * Brews, I think you are getting needlessly philosophical here. We are writing for the general reader, especially in the intro. Let's make the simple statement in the intro, and deal with any possible misunderstandings in the body. DJ Clayworth (talk) 17:46, 21 August 2009 (UTC)

Brews is not alone in doubting the wisdom of the current lead; the problem as I see it is this: Now this may seem like semantics or "needlessly philosophical" but imho the lead should be changed to avoid the implication that the SoL is known exactly when it isn't and never will be (except, by definition, in m/s). Abtract (talk) 18:38, 21 August 2009 (UTC)
 * 1) The speed of light (SoL) has not been defined as 299etc m/s but rather a metre has been defined as 1/299etc of the distance travelled by light in a second.
 * 2) It is therefore quite misleading, although completely true, to state in the lead that the SoL is exactly 299etc m/s because this is a circular definition.
 * 3) Because even readers with a passing knowledge of science will not realise this circularity, they will believe the SoL is known exactly.
 * 4) However, the SoL is not known exactly and never will be; it is measurable with a greater and greater accuracy in any length unit not defined using the SoL.


 * Perhaps it would help if you would actually read the lead. Nowhere does it say anything about defining. DVdm (talk) 19:14, 21 August 2009 (UTC)
 * Abtract, we have just had a very long discussion on this subject which you can read above, it is a pity that you chose not to take part in it. We seriously considered saying, 'The meter is defined such that the speed of light is exactly 299 792 458 meters per second', but there was a consensus against this.  As you have said, what we say in the lead is correct and it is supported by impeccable sources, thus we are entitled to say it.  The situation is complicated and, in the end, it was considered not practicable to try to explain it in the lead.  You will note that the link on 'exactly' takes you to the section where this matter is discussed in detail. Martin Hogbin (talk) 21:13, 21 August 2009 (UTC)
 * It does however say that in the wording I proposed above. I believe that the difference between defining the metre and defining the speed of light is a pretty philosophical and abstract one (again remember we're writing for the general reader and giving an overview, and also remembering that neither space nor time are absolutes). However let me propose another wording:""In SI units, c is exactly 299792458 metres per second by definition". DJ Clayworth (talk) 20:15, 21 August 2009 (UTC)
 * This suffers from the same circularity ... it is the metre that has been defined in relation to the SoL, not the other way round. Abtract (talk) 20:31, 21 August 2009 (UTC)
 * If you really must say something about definitions, then better would be: "'In SI units, c is exactly 299792458 meters per second, by virtue of the definition of the meter."
 * But just "'In SI units, c is exactly 299792458 meters per second" (as it is now) will do perfectly. That is a fact all by itself. Remarks about definitions come later in several remarks and in an entire subsection. No need to overload the lead with this. DVdm (talk) 20:35, 21 August 2009 (UTC)
 * There has been a very long discussion on whether we should say that. My original words were, 'The meter is defined such that the speed of light is exactly 299792458 meters per second'.
 * There are multiple meanings for the symbol c and for the term speed of light: there's the rub. It can be taken as 299 792 458 m/s in the SI system of units. It can be taken as a fundamental constant of nature. In the above statement by DJ Clayworth, the choice of meaning is not made clear enough.


 * Unfortunately, 299 792 458 m/s is not the numerical value of the speed of the fundamental constant of nature, not even in SI units.


 * In case that is not clear, here is the point: The number 299 792 458 m/s is defined exactly in SI units as the factor used to determine lengths of objects from the measured transit time of light: length = 299 792 458 m/s × time of transit. One consequence is that measurement of the actual, physical speed of light has to be done in a multi-step process in SI units: (i) one measures a selected wavelength in metres (ii) one measures the time of light to transit this wavelength (iii) one expresses the speed of light in wavelengths/s. Unfortunately, one cannot in principle express the speed of light in m/s because of the tautological nature of the definition of the metre.   Brews ohare (talk) 20:43, 21 August 2009 (UTC)


 * What does it take to make you understand that we can't help you with this? DVdm (talk) 20:50, 21 August 2009 (UTC)
 * This parrot is no more! He has ceased to be! 'E's expired and gone to meet 'is maker! Martin Hogbin (talk) 21:18, 21 August 2009 (UTC)

Brews, I really don't know what your objection is to this, or how it's even possible to believe that the speed of light (c) is anything else but 299 792 458 m/s (or to quote your exact words "299 792 458 m/s is not the numerical value of the [speed of light]). Meters are defined so the speed of light is exactly that number. You cannot "measure a selected wavelength in metres" independently of the speed of light. To measure a wavelength in meters, you need to compare it to the length that a pulse of light travels during 1/299 792 458 seconds. Headbomb {{{sup|ταλκ}}κοντριβς – WP Physics} 21:03, 21 August 2009 (UTC)


 * Headbomb: I understand your frustration with me over this point; it is a consequence of having one end of a stick and not realizing there is another. The point is that within the SI units system 299 792 458 m/s is used to calculate lengths as in length =  299 792 458 m/s × time of transit. I believe you understand this point. I believe you also understand that with length defined this way the speed of light becomes 299 792 458 m/s inescapably. What I think you do not understand is that this number is not related to the fundamental constant of nature referred to by (e.g.) the theory of relativity. The reason: it is not possible given the 1983 definition to express this fundamental constant in m/s: it always comes out as 299 792 458 m/s, and that is a wrong answer. Brews ohare (talk) 21:23, 21 August 2009 (UTC)


 * Well then you espouse a WP:FRINGE point of view. The number 299 792 458 m/s is the value of the speed of light / scaling factor represented by c when expressed in SI units. You will find no mainstream source siding with you here.Headbomb {{{sup|ταλκ}}κοντριβς – WP Physics} 22:47, 21 August 2009 (UTC)

Section on Speed of light by definition
Perhaps it would be helpful to rewrite this section in a context where problems of succinctness are less severe. Then, with a clear statement of the situation in hand, perhaps a more accurate short intro sentence can be constructed?

I'd suggest this section begin with the pre-1983 definition: “On October 14, 1960 the Eleventh General Conference on Weights and Measures redefined the International Standard of Length as 1,650,763.73 vacuum wavelengths of light resulting from unperturbed atomic energy level transition 2p10 5d5 of the krypton isotope having an atomic weight of 86. The wavelength is


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

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

This beginning point has the merit of being perfectly understandable: define the length standard "metre" in terms of a fundamental length of nature, a particular wavelength.

Given this metre and a time measurement, the speed of light in m/s is readily understood as a length / time of transit, and is roughly 299 792 458 m/s.

That introduction then can be followed by an explanation of the 1983 switch to a defined speed of light and a length standard based directly upon time-of-transit. That explanation would clarify why the term "speed of light" in SI units is then to be understood only in the context of length determination as in length = 299 792 458 m/s × time of transit. The number 299 792 458 m/s is explained to be a convention adopted by CODATA, and caution given that it should not be confused with a numerical value for the physical speed of light, which can no longer be expressed in m/s, but can be expressed, for example, in wavelengths of some atomic transition / second.

Of course, references to all critical statements are to be provided, such as Wheeler; Jespersen; Sydenham. What do you all think? Brews ohare (talk) 21:14, 21 August 2009 (UTC)


 * I think you already know what we all think. Can you please stop disrupting and harassing this talk page? Thank you. DVdm (talk) 21:18, 21 August 2009 (UTC)


 * The reliable sources you quote are not critical of the current state of affairs, that is just a misconception on your part. Martin Hogbin (talk) 21:20, 21 August 2009 (UTC)
 * If you want to discuss this subject further, I have set up a page for it at User_talk:Martin_Hogbin/Speed_of_light_set_by_definition Martin Hogbin (talk) 21:22, 21 August 2009 (UTC)
 * Martin: You state I am under a misconception, but do not say what that misconception is. I have provided sources, and you do not address them. I have outlined an approach and you dismiss it without comment. Humbug. Brews ohare (talk) 22:15, 21 August 2009 (UTC)
 * This clearly goes under wp:nor. DVdm (talk) 21:24, 21 August 2009 (UTC)


 * Perhaps you could elaborate by stating exactly what has been said that goes beyond the cited sources? You have never addressed the sources, the issues; go beyond bald accusation and put some meat on the bones, eh?. Brews ohare (talk) 22:13, 21 August 2009 (UTC)

Well Martin et al.: I have explained this matter carefully to you all. I have made above a simple and clear outline of a proposed change. You all simply will not engage on a detailed basis, but continue to harangue in abstractions. That is not how to advance. The article is in flat contradiction to sources despite your unsupported claims to the contrary. It is logically unsound. It is factually incorrect. It makes the absurd claim that the speed of light is known exactly, in distinction (contradiction) to all other observations of nature. All this will become clear in due course, I hope. Brews ohare (talk) 21:31, 21 August 2009 (UTC)


 * The speed of light is known exactly because it is exactly defined. You're claiming that we can't know a square has four sides because we defined squares as having four sides. Headbomb {{{sup|ταλκ}}κοντριβς – WP Physics} 01:04, 22 August 2009 (UTC)


 * The analogy with a square is a false one because we have defined "four" and "side" quite independently of "square" so when we make a statement about the square using four and side it is useful and meaningful. However, when we define the metre as 1/299.. of the distance travelled by light in a second and then state that the SoL is 299.. m/s we have a circular statement that is not a definition at all because, although we do have an independent definition of a second, we do not for the metre. To state it another way, "The SoL is 299.. m/s" says absolutely nothing scientific about the speed of light because we do not know what a metre is withought first knowing the SoL. Of course the statement is meaningful to a layman (and indeed to us all) because we all "know" what a metre is from its previous definition.Abtract (talk) 09:33, 22 August 2009 (UTC)


 * The numerical value of the speed of light in SI units is known exactly. The mechanical length of the meter has, as far as can be measured or detected, not changed.


 * Your statement:' "The SoL is 299.. m/s" says absolutely nothing scientific about the speed of light because we do not know what a metre is without first knowing the SoL' is not correct, we do not need to know the SoL, all we need to delineate the meter is some light, which we have. Martin Hogbin (talk) 09:47, 22 August 2009 (UTC)


 * Thanks. Perhaps you could enlighten (sorry couldn't resist) me as to how we delineate a metre using some light. Abtract (talk) 09:55, 22 August 2009 (UTC)


 * You, obviously, also need a means if realizing the second. In principle, all you then do is let the light travel for the appropriate fraction of a second and see how far it goes.  That is your metre.  Of course, in practice, this is done with interferometry and light of a measured frequency.  Martin Hogbin (talk) 10:50, 22 August 2009 (UTC)


 * Thanks for confirming the circularity and apologies for previouly stating the problem badly. What I should have said is:
 * "The SoL is 299.. m/s" says absolutely nothing scientific about the speed of light because we do not know what a metre is without first defining it in relation to the SoL.
 * Abtract (talk) 13:17, 22 August 2009 (UTC)

[outdent]I do not understand what you mean by 'nothing scientific'. The current article gives the speed in SI units. Obviously, if you do not know what a second and a meter are this will mean little to you. This applied just as much when the meter was defined by a metal rod. If you had never seen the rod or a meter rule, you would have no idea what the number meant. Martin Hogbin (talk) 15:12, 22 August 2009 (UTC)


 * If we had defined "four" as the number of sides a square has, and then state that a square has four sides, that would be a circular statement and not very helpful. Imho defining the metre as the distance travelled by light in 1/299.. metres and then stating that the SoL is 299.. m/s is equally circular and should be avoided without some reference to the way a metre is defined (which we have now done). That's all I'm saying. Abtract (talk) 15:29, 22 August 2009 (UTC)


 * As you say, the current lead briefly explains why the speed of light is an exact value so I suggest we leave drop the subject. Most people are getting rather fed up with. Martin Hogbin (talk) 15:51, 22 August 2009 (UTC)


 * Agreed, and thanks for your patience. Abtract (talk) 17:44, 22 August 2009 (UTC)

Proposed lead first para
In an attempt to overcome the circularity problem, I offer the following wording (second part taken more or less verbatim from the source):
 * The term speed of light generally refers to the speed at which light (or any other electromagnetic wave) travels in vacuum. This is a fundamental physical constant, usually denoted by the letter "c".


 * The metre has been defined (SI units) so that the speed of light in vacuum has the exact value:
 * c = 299 792 458 m/s ... which can be used as a standard. Abtract (talk) 22:29, 21 August 2009 (UTC)


 * This is close to what we once had before, but I am not sure why it is better that the wording we have now. There was a feeling that we wanted the fundamental constant bit first.  Also, I do not understand what you mean by the 'circularity problem'.   Also, I would prefer to say 'The metre is defined' rather than 'The metre has been defined'.  Martin Hogbin (talk) 22:46, 21 August 2009 (UTC)
 * I have explained what I mean by the 'circularity problem' in the section above (Section on Speed of light by definition) Abtract (talk) 09:36, 22 August 2009 (UTC)


 * Is or has been, makes no difference to me; I think this phrasing is superior to what's there now, though I abhor staring with "the term". My suggestion would be:


 * In physics, the speed of light is a fundamental physical constant, the speed at which light (or any other electromagnetic wave) travels in an ideal vacuum. It is usually denoted by the letter "c". In the SI system of unit, the metre has been defined such that the speed of light has the exact value c = 299 792 458 m/s.


 * Dicklyon (talk) 00:54, 22 August 2009 (UTC)


 * Abtract, I can see why you do not like saying 'the term', it is rather ugly, but I think we have to say something like that. 'The speed of light' is, even in everyday usage, is a phrase with a specific meaning, namely the speed of light in free space.  The natural English phrase 'the speed of light' can be used to mean many things, just like saying, 'the speed of a train'.


 * Until we have defined our terms, I do not think that we can say, for example 'the speed of light is 299 792 458 m/s' because this is not always the case, the speed of light in glass is less, for example. My alternative suggestion was to say something like, "the speed of light is generally taken to refer to ...".  —Preceding unsigned comment added by Martin Hogbin (talk • contribs)


 * Martin, that was me. So why did you put back "the term"?  The lead defines the meaning in the context of the article, without that.  I recommend we put back the more standard-form lead, and you tell us what it is in it that you "strongly object" to.  You can certainly say, as there is a strong consensus to say, that 'the speed of light is 299 792 458 m/s' once you've said that 'the speed of light is a fundamental physical constant, the speed at which light (or any other electromagnetic wave) travels in an ideal vacuum'.  Was I wrong in thinking that we had a consensus to go this way?  Does anyone else object? Dicklyon (talk) 22:08, 22 August 2009 (UTC)

The problem is that the words "the speed of light" in the phrase The term "speed of light" generally refers to the speed at which light (or any other electromagnetic wave) travels in vacuum does not have the same meaning as it has in the phrase:  The metre has been defined (SI units) so that the speed of light in vacuum has the exact value: c = 299 792 458 m/s .

The first occurrence refers to the physical, observable, real speed of light. The second occurrence refers to the factor used in the SI system of units to relate length to time-of-transit. This last number agrees with an approximate value for the physical speed of light as measured prior to 1983, now enshrined by definition to lie outside observation, forever immutable. Brews ohare (talk) 22:59, 21 August 2009 (UTC)


 * Brews, please stop dragging dead parrots into every discussion on this topic. Martin Hogbin (talk) 09:32, 22 August 2009 (UTC)

Rest
I think that the speed of light, and specially its talk page, desperately needs some rest ;-) - DVdm (talk) 10:55, 22 August 2009 (UTC)


 * I completely agree that we should stop discussing the speed of light being fixed by definition. This is an impeccably sourced fact that must remain in the article.  The fact that some people  seem to be having difficulty getting their heads round this fact does not change it.  I have offered a home for further discussion of the subject by those interested in it. Martin Hogbin (talk) 11:09, 22 August 2009 (UTC)

The numerical value 299 792 458 m/s is not measured in SI units
In SI units the number 299 792 458 m/s is used to relate time-of-transit to length in the formula length = 299 792 458 m/s × time.Jespersen and in Sydenham. Given this relation, the number 299 792 458 m/s is established by definition, and is not subject to confirmation by experimental observation. (See same references.) In short, the number 299 792 458 m/s in the SI system of units is not measurable.

In contrast, if a length is expressed in wavelengths of some particular atomic transition (for example, as per the definition of the metre prior to 1983), the time it takes light to traverse this length can be measured (as is done for the metre), and the speed of light then is measurable in terms of wavelengths/s. Needless to say, if the wavelength is expressed in metres, this number for the speed of light will be close to 299 792 458 m/s, but unlike 299 792 458 m/s, this measured speed of light will have an error bar associated with the errors of observation.

This measured value of the real speed of light is not used in the modern SI definition of length because (at the moment, anyway) the error bar introduced by use of the measured value of c is larger than that incurred using 299 792 458 m/s with the time-of-flight methodology. Brews ohare (talk) 23:17, 21 August 2009 (UTC)


 * Yes, so you've told us, and we get it. How does this relate to the article?  I don't see anything about "measured" that needs to be fixed.  Dicklyon (talk) 23:34, 21 August 2009 (UTC)

The number 299 792 458 m/s is not the measured speed of light in SI units, and it is not the "exact" speed of light in SI units; it's just the number in the formula length  = 299 792 458 m/s × time of flight. The measured speed of light has an error bar, and is not known exactly, but is approximately 299 792 458 m/s. The statement in the lead In SI units, c is exactly 299,792,458 metres per second. is not correct, or perhaps I should say it is not correct if c is interpreted as the actual speed of light, and not just as the proportionality factor in the equation for length. Brews ohare (talk) 00:06, 22 August 2009 (UTC)


 * Brews, the scaling constant and the speed of light are the exact same things. I suggest you drop the stick before someone files an ANI request and get topic-banned like David Tombe just got. Headbomb {{{sup|ταλκ}}κοντριβς – WP Physics} 08:07, 22 August 2009 (UTC)


 * Or you can continue the discussion somewhere else. Martin Hogbin (talk) 08:56, 22 August 2009 (UTC)

What was the measured speed of light in 1982 in m/s, before the meter was redefined to be based upon the speed of light? —Finell (Talk) 12:47, 22 August 2009 (UTC)
 * 299 792 458 m/s. Martin Hogbin (talk) 14:43, 22 August 2009 (UTC)


 * Thanks, that's what I thought. Martin, I am sure that you can explain the fallacy of the DT-BO position in terms that they are able to understand, including demonstration (not bare assertion) of how they either misinterpret their sources or that they are not WP:RSs. Whether they will agree with it is a separate matter. While I would welcome their understanding and agreement, that is not my primary concern; my primary concern is the process. The explanation belongs on this Talk page and is the the appropriate way to close this discussion topic. As tendentious as they have been, they have been trying in good faith to improve the article and their thesis is relevant to the article's topic. Therefore, the error in the thesis should be explained, here. FYI, Jimbo looked at this talk page and agrees that neither side has addressed the other's points or supported their own position by showing what the RSs actually say on the subject (he doesn't claim to understand the topic, but he got the gist of the dispute). The explanation will serve in the future to show how the issue was resolved, if it arises again. Thank you. —Finell (Talk) 22:10, 22 August 2009 (UTC)

I will make an attempt here to state my case in response to Finell with supporting references as a summary of how I see the matter. I invite the other editors on this page to support their views in a similar manner. The following is not intended to be argumentative, but a straightforward response to Finell.
 * Reply to Finell.


 * Background:

There are several different points to make. The first is that there does exist in the universe something called the speed of light that, according to relativity, relates to the structure of spacetime and the bounds on the transport of information and of matter. Second, the numerical value of this speed can be measured. One way to do this is to take a set length, for example the wavelength of an atomic transition, and determine the time it takes for light to transit this distance. The speed of light is then c = λ/t. That approach was the methodology used in the SI units prior to 1983. See this NIST timeline for details: definition: “On October 14, 1960 the Eleventh General Conference on Weights and Measures redefined the International Standard of Length as 1,650,763.73 vacuum wavelengths of light resulting from unperturbed atomic energy level transition 2p10 5d5 of the krypton isotope having an atomic weight of 86. The wavelength is


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

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

It seems to me doubtful that any of the parties disagree with this background. They probably also agree that this pre-1983 approach resulted in a value for c of approximately 299 792 458 m/s, and that this value was obtained only to within observational error (that is, this number is not an exact value).


 * SI units post 1983

In view of the status of the speed of light as a physical constant, independent of frequency, direction etc., it is possible to set up a laboratory to realize the speed of light (subject to some caveats about corrections needed to adjust for "nonideal vacuum"). That being so, lengths can be compared by comparing the times of transit of light along the lengths using a formula length = c × time of transit. Actually, one does not have to know the numerical value of c in order to make such length comparisons: one has to be sure only that the speed of light in vacuum has been realized. Thus length comparisons are simply ratios of transit times, and the actual value of c simply divides out in such ratios. For this reason, as pointed out by Jespersen of the Bureau of Standards (underscores are mine):"One fallout of this new definition was that the speed of light was no longer a measured quantity . The reason is that, by definition, a meter is the distance light travels in a designated length of time, so however we label that distance - the speed of light is automatically determined. ... turning c into a conversion factor whose value is fixed and arbitrary." As stated by Sydenham in a rather definitive article that is often cited (underscores mine):"The numerical value of the speed of light c = 299 792 458 m/s is the result of numerical standards chosen for the standards of time and of length. Thus the speed of light as a numerical value, is not a fundamental constant."


 * Summary

We see then, that while the "speed of light" was initially a measurable quantity pre-1983, post-1983 is has become a "arbitrary" value that is not "a fundamental constant". This odd situation is understood by noticing that the speed of light still is a physical quantity, but the terminology "speed of light" has two different meanings: pre-1983 it was a measured value, and post-1983 it is a "conventional value" of 299 792 458 m/s.

This change in meaning is a peculiarity of the SI units. The speed of light is used in two senses: the actual physical experimental speed of light and the SI units conventional value for the speed of light. Because of the post-1983 definition of the meter, the actual physical speed of light can no longer be expressed in m/s; all that is possible is the conventional, defined value 299 792 458 m/s. To state the actual physical speed of light in SI units, that is the physically observable behavior of light, one must resort to expressing the speed of light in terms of some physical length, for example, wavelengths of some transition/second.

The debate on this WP page stems from confusion between the conventional defined value for the speed of light of 299 792 458 m/s, (which is an exact number chosen by definition to be a value chosen for convenience to be close to the measured value prior to 1983), and the actual physical speed of light, which cannot be expressed any longer in m/s because of the convention that the metre is defined so this number always is 299 792 458 m/s.

Within the new definition, the actual physical speed of light must be expressed not in m/s (because that results in tautology) but in terms of some physical length, for example in terms of wavelengths of some atomic transition/s, as was done prior to 1983. That is, c can be expressed as some number of λ's per second. The real, physical speed of light is not calculable from some basic theory, and so cannot be stated as a definite numerical value, but must result from measurement with an observational error bar attached to it.

I hope that this exposition lays out the situation clearly. It is not intended to be argumentative, but simply to point out the confusing usage of the term "speed of light" to refer both to a fact of physical nature (that is beyond human capacity to measure without an error bar) and also to an exact conventional "conversion factor" of 299 792 458 m/s Brews ohare (talk) 16:04, 23 August 2009 (UTC)


 * ... and if everyone politely ignores this clearly laid out exposition of your confusion, will you then finally stop repeating it on this talk page? DVdm (talk) 16:47, 23 August 2009 (UTC)


 * DVdm: You have made what is simply a nasty uncooperative response to a sincere effort on my part. Your failure to be constructive and your snarky attitude does nothing to advance matters. My view is that your ideas about these matters are misinformed, and if you disagree, you should support your views, not badmouth earnest effort. Brews ohare (talk) 19:23, 23 August 2009 (UTC)


 * Perhaps I should have ignored it myself to begin with. My apologies. DVdm (talk) 21:23, 23 August 2009 (UTC)


 * True apology is accompanied by changed behavior. False apology is just a form of sarcasm. Brews ohare (talk) 02:01, 24 August 2009 (UTC)


 * This was not an apology to you but to the others, for not following my own hint to politely ignore your repetitive talk page disruptions. DVdm (talk) 10:00, 24 August 2009 (UTC)