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Wikipedian64, I have reverted your removal of italics and your change of 'exactly' to 'precisely'. The use of italics is as per WP guidelines in that they are used sparingly for emphasis.
The word 'exactly' is a better word in this context because 'precisely' could be taken to mean that there is some, very small, room for error or change, which there is not. Martin Hogbin ( talk) 16:54, 26 July 2009 (UTC)
The fact that the value of physical quantity, when stated in SI units, should have an exact value is surprising to most people. This makes the point worthy of emphasis in the lead in my opinion. Martin Hogbin ( talk) 16:22, 28 July 2009 (UTC)
In view of the conventional nature of 299,792,458 m/s, it seems misplaced to make it the central point of the introduction. Instead, the importance of the physical nature of the speed of light in setting the maximum rate of information and of matter transfer, and its connection to the speed of EM radiation should be the main focus. The numerical value in SI units is subsidiary. Brews ohare ( talk) 16:18, 28 July 2009 (UTC)
It's surprising that Sarah Palin quit midterm, but that doesn't mean it's important. I'm inclined to think the placement of an accidental value in the lead, however surprising, tends to make it look like it is more significant than it is, and sets the article up with unnecessary explaining to do. A primary position in the lead also diverts attention from the actual importance of the speed of light to physical theory. Brews ohare ( talk) 16:39, 28 July 2009 (UTC)
Martin: this is an incomplete thought at best. It does not approach the subject, which is the diversion from what is important to highlight the marginally relevant. Brews ohare ( talk) 17:01, 28 July 2009 (UTC)
Apparently there is resistance form Martin to the notion that 299,792,458 m/s is a conversion factor from time to length. However, it will be noted that in support of this notion was a verbatim quote from
Bertrand Russell (2009).
ABC of Relativity (Revision by Felix Pirani of 1925 4th ed.). Taylor & Francis. p. 12.
ISBN
0415473829. (just prior to the quote, the NIST decision to "define" the value of "c" is discussed), and two other sources: A Connes (2006). "On the foundations of noncommutative geometry". In Izrailʹ Moiseevich Gelʹfand, Pavel I. Ètingof, Vladimir Retakh, Isadore Manuel Singer (ed.).
The unity of mathematics. Gulf Professional Publishing. p. 175.
ISBN
0817640762.{{
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link)
and
Richard Wolfson (2003). Simply Einstein: Relativity Demystified. W. W. Norton & Company. p. 170. ISBN 0393325075..
Previously I provided JA Wheeler and Mendel Sachs, not exactly slouches in the realms of physics. Here's three more sources: Jespersen; Brzeziński; ND Mermin.
In the discussion with Softvision this conversion factor role was very extensively described, and Martin himself suggested the arbitrariness of 'c' was evidenced by the "natural" units where c=1.
So what is the problem here? Use of λ = c/f (or ℓ = c t) means c is a conversion factor. Stating that makes it very clear that any value of c may be used (in principle), and that 299,792,458 m/s has nothing to do whatsoever with the physical entity known as the speed of light. That realization defuses 98% of the controversy around the "speed of light by definition". Brews ohare ( talk) 02:21, 28 July 2009 (UTC)
The term "conversion factor" is not mine. It appears in the cited sources. Brews ohare ( talk) 11:17, 28 July 2009 (UTC)
I agree that there are two usages involved here, of which one is strictly the space-time notion that the metric ds2 involves c2dt2, so c converts the time component to a distance allowing the computation of ds.
However Gelʹfand very clearly is describing a second usage: “This definition was replaced in 1983 by the current definition which, using the speed of light as a conversion factor, is expressed in terms of inverse frequencies rather than wavelength …” Likewise, “By an international agreement, made in 1983, 'the metre is the length of the path travelled in vacuum by light during a time 1/299,792,458 of a second'. From the physicist's point of view, the speed of light has become a conversion factor, to be used for turning distances into times… ” ABC's of Relativity; Wheeler mentions conversion factor: “The conversion factor between seconds and meters is the speed of light, c = 299,792,458 m/s. … The speed of light is the only natural constant that has the necessary units to convert a time to a length.” John Archibald Wheeler. Jespersen is very clear about the arbitrariness of the value for c: “One fallout of this new definition was that the speed of light was no longer a measured quantity; it became a defined quantity. The reason is that, by definition, a meter is the distance that light travels in a designated length of time, so however we label that distance – one meter, five meters, whatever – the speed of light is automatically determined. And measuring length in terms of time is a prime example of how defining one unit in terms of another removes a constant of nature by turning c into a conversion factor whose value is fixed and arbitrary.″ Jesperson
I bring up all these quotes from the already cited sources (of which there are more above) to persuade that the arbitrariness of the value for c is not my idea, and has multiple published occurrences. Likewise, the term "conversion factor". Of course, the quotes should be unnecessary because the idea is a logical consequence of the definition, as NIST has pointed out, and as Jespersen states in so many words.
In sum, the realization that the choice of 299,792,458 m/s is simply a practical choice of number and it could equally well be 1 m/s defuses the controversy around this numerical value, by showing any number can be taken when using the "speed of light by definition". Naturally, it is the very arbitrariness of 299,792,458 m/s that raises people's eyebrows, because they think of c as a definite value that can be measured. Introducing the term "conversion factor" places the emphasis where it belongs: on human conventions that are human choices, and not in nature outside man's grasp. Unlike the famous quip about a senator introducing a bill to define π to be a round number, a senator could introduce a bill to make c a round number. Brews ohare ( talk) 11:17, 28 July 2009 (UTC)
It is not easy to determine your goal in the above remarks. My take is that you wish to explain how it is that the speed of light cannot be taken as a fixed value. There is some ambiguity in such a remark, which may be contained in your observations, but eludes me. The distinction to be made is that the physical speed of light may in fact change over time (as suggested by some cosmologies), or may be observed with greater precision in the future (resulting in more precise numerical values for wavelengths of selected atomic transitions). However, the numerical value of the speed of light in SI units will never change, short of an agreement by the CIPM ( Comité International des Poids et Mesures) that the definition should be changed. The numerical value in SI units simply is one of man's conventions, and is not an experimental matter, but a matter of convenience. Do we agree? Brews ohare ( talk) 15:13, 28 July 2009 (UTC)
Have we abandoned logic here? First, we say that c = 299,792,458 m/s. Then we say that 299,792,458 m/s is a mere conversion factor, not related to physical entities. This is a logical contradiction because "m/s" means "meters per second" or, in general, "movement through space in a certain time." What moves through space in a certain time? A physical entity. Lestrade ( talk) 19:51, 30 July 2009 (UTC)Lestrade
Please explain deletion of this quote:
“One fallout of this new definition was that the speed of light was no longer a measured quantity; it became a defined quantity. The reason is that, by definition, a meter is the distance that light travels in a designated length of time, so however we label that distance – one meter, five meters, whatever – the speed of light is automatically determined. And measuring length in terms of time is a prime example of how defining one unit in terms of another removes a constant of nature by turning c into a conversion factor whose value is fixed and arbitrary.″ [1]
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Brews ohare ( talk) 16:59, 28 July 2009 (UTC)
I find this quote to be (i) authoritative (ii) very clear and (iii) important to have in the article in view of the "very surprising" [Martin's words] nature of an exact value for the speed of light in SI units. Brews ohare ( talk) 16:59, 28 July 2009 (UTC)
If this point is as "surprising" as you said it is, the point bears some amplification, and this text does it very well. Also, your own resistance to this viewpoint indicates the need for a direct quote, because nothing less proves convincing. The point must be made in a way that very clearly is not a WP editor's flaky opinion, or it will be the source of unending battles on this page. Brews ohare ( talk) 17:18, 28 July 2009 (UTC)
Martin: I don't know what "quote from the article" you are talking about. First, the article doesn't presently even cite Jespersen, never mind quote him. Second, the only item in quotes is from NIST. Third, the removed quotation uses the phrase "by turning c into a conversion factor whose value is fixed and arbitrary", which I find a very eloquent statement of the situation compared to the existing text. IMO you have deleted the quote precisely because it is so clear, and you would rather preserve an element of mystery about this subject. Brews ohare ( talk) 17:41, 28 July 2009 (UTC)
This "quote" appears to be the result of your slight rewording of an entry of my own words 21:46, 24 July 2009. So, it lacks the clout of a real verbatim quote from a published source.
As I've said, the removed quotation from Jespersen uses the phrase "by turning c into a conversion factor whose value is fixed and arbitrary", which I find an eloquent statement of the situation compared to the existing text. In addition, as I'm sure you are aware, this subject of an exact speed of light comes up over and over here, the most recent exchange being Softvision, but prior to that Venus10041, Physchim62 and others. It's worth a try to see whether a clearer statement would improve matters. It is a commentary on the present wording that readers don't find the explanation satisfying. Brews ohare ( talk) 18:03, 28 July 2009 (UTC)
You have stated your preference, without arguments in support, and I disagree with it, for substantial reasons provided by me, and not challenged by you. Brews ohare ( talk) 23:54, 28 July 2009 (UTC)
Where do you get the notion that "conversion factor" is used vaguely when all the verbatim quotes you have deleted from several different sources all define what is meant in absolutely starkly clear and unambiguous terms, and include specific examples? Brews ohare ( talk) 14:44, 29 July 2009 (UTC)
I have added Jespersen as a reference (no quote), and a sentence stating that the value of c is not a property of nature, but a convention. Brews ohare ( talk) 15:19, 29 July 2009 (UTC)
The article as revised by Martin now states:
A more complete theory of light that describes the interaction of photons with matter is given by quantum electrodynamics (QED) in which c plays the role of a fundamental constant. [1]
Brews ohare ( talk) 17:14, 28 July 2009 (UTC)
The problems with this are:
Brews ohare ( talk) 17:14, 28 July 2009 (UTC)
Martin:
The photon does not directly feel the strong or weak force, but the photon feels quarks and quarks feel the strong force. The way Feynman diagrams work means that that's enough for the photons to feel the strong interaction a little bit. Here's an example: The question "What is the electron magnetic moment" appears to be a purely electromagnetic question. And it can be answered to 99.9999999999% accuracy within QED. But to get to the parts-per-trillion level, you need to include strong interactions (see Figure 2 of [3], "hadronic" means strong force) despite the fact that neither the electron nor photon feels the strong force. :-) --Steve (talk) 03:55, 24 July 2009 (UTC)
In addition:
Brews ohare ( talk) 17:33, 28 July 2009 (UTC)
Let us ask Steve what he thinks. Martin Hogbin ( talk) 17:45, 28 July 2009 (UTC) This is what was said above:
I have no objection to mentioning photons. Martin Hogbin ( talk) 17:53, 28 July 2009 (UTC)
It also is not what is said in the deleted statement. Brews ohare ( talk) 23:53, 28 July 2009 (UTC)
The source is written by an expert, There is no argument that the source has misstated things. It is clear and unambiguous. Martin apparently doubts that he understands it, but nobody else will have that problem. Possibly Martin's statement can be interpreted as he would like it to be, but it is ambiguous and casts QED in a somewhat odd light, as "more complete", as though we just ain't got there yet. Why not stick with the source, which is "more complete" yet? And why make the reader dig through an entire text on the subject when all that is wanted is a sentence that makes the entire matter clear? Is that Steve humoring Martin, or some real effort to be useful to the reader??? Brews ohare ( talk) 14:27, 29 July 2009 (UTC) I have re-written the note to agree with Martin's text. Brews ohare ( talk) 15:17, 29 July 2009 (UTC)
I have just removed the following phrase about photons from the article, "which in vacuum travel at the speed of light, c".
Photons are quantum entities and to talk of them travelling with a particular speed is not a good idea, although it is often done. Also what speed do photons travel at when they are not in a vacuum? Martin Hogbin ( talk) 22:27, 28 July 2009 (UTC)
I believe you should answer these questions, which are natural, and source your answers to a reference available on line. BTW, the article already states (and sources) c as the speed of rest-mass-zero particles. Brews ohare ( talk) 23:51, 28 July 2009 (UTC)
What statement is "that statement" that you are referring to?? Brews ohare ( talk) 15:44, 29 July 2009 (UTC)
You are arguing among yourselves here. What Martin's comments "clearly show" is a lack of civility. Brews ohare ( talk) 14:21, 29 July 2009 (UTC)
I have not made any suggestions like that. I understand that the juxtaposition of the speed of photons in vacuum next to a description of their role as force carriers was unfortunate, allowing virtual photons to be confused with photons moving freely in space. However, it easily could be corrected without a diatribe on background, and "little balls". I won't engage in argument over the speed of photons - do whatever you like. Brews ohare ( talk) 17:57, 30 July 2009 (UTC)
There was no need to change any of this. The cited reference on the independence of the speed of light on this source velocity gives examples of many experiments, including light from double stars, rotating mirrors, light from limbs of the sun and more.
The relativity postulates were modern and concise ones from a reliable source. Martin Hogbin ( talk) 22:44, 28 July 2009 (UTC)
If you look at the table of contents, you will see that the sub-sub-sub-sections I mention are sub-sub-sub-sections of the Inertial frames sub-sub-section, not of the sub-section Fundamental importance. Hence my observations about inappropriateness of the heading. Brews ohare ( talk) 14:18, 29 July 2009 (UTC)
"What does 'Speed in relativity' mean?" As a subsection title, it means "the role of the speed of light in relativity". Previously you chastised me about this, saying that the article was about the "speed of light" and it need not be repeated. Brews ohare ( talk) 15:39, 29 July 2009 (UTC)
I've changed the sub-title to a more complete description of its subject that fits with its own sub-sub-sections. Brews ohare ( talk) 17:30, 29 July 2009 (UTC)
We are hardly "repeating everything in every article" by listing Einstein's statements of his postulates and maintaining some consistency. In addition, his names for the postulates are helpful in this context because they are divided nicely and are labeled exactly as is needed. I do not find your statements more succinct, you just dumped the labeling and chose a formulation that requires some logic to attach to the text instead of being direct statements of the postulates. I hope you have enough objectivity to see that. Brews ohare ( talk) 14:18, 29 July 2009 (UTC)
I find little support for your statement that Einstein's statements of the postulates have been superseded in modern times. For example: Chow (2008) states them virtually verbatim as in Einstein's papers, and I find many other recent texts that do likewise. Moreover, the statement in terms of the "principle of relativity" and separately as "the principle of the constancy of the velocity of light" is a formulation that (i) suits the article, and (ii) is logically preferable to the conglomeration you presented, which fails to separate these two ideas, and does not state the invariance of all physical laws in inertial frames.
If a reformulation of Einstein's postulates exists, it is a stress upon the identification of inertial frames as those where the physical laws are not only invariant, but have their simplest form, a notion Einstein put forward in other writings, but not relevant here. Brews ohare ( talk) 14:18, 29 July 2009 (UTC)
I have modified the presentation of the postulates more in keeping with your text, but using a reference accessible on-line, and a more accurate enunciation. Brews ohare ( talk) 17:33, 29 July 2009 (UTC)
I would like to change the sentence:
'[The speed of light] has also been confirmed by the Michelson-Morley experiment and the Kennedy-Thorndike experiment (an improved version of the Michelson-Morley experiment) that the two-way speed of light; between a source and a mirror, and back again; is the same, independent of the common constant velocity of the observer, source and mirror (all moving together), regardless of its magnitude or direction'.
Which I consider to be long winded and containing a lot of unnecessary and irrelevant detail.
To
'The two-way speed of light, that is from a source to a mirror and back again, has also been confirmed by the Michelson-Morley and later, more accurate, experiments to be constant'.
The MM experiment is by far the best known and should be mentioned by name but there are at least 14 others that we need not mention specifically. I think that giving detail on some of the experiments is not just unnecessary but undesirable. I would add the summary page from Zhang as a reference as this the most comprehensive source on the subject. I would suggest leaving just one online reference. Martin Hogbin ( talk) 10:51, 29 July 2009 (UTC)
Once again, you response to the suggestion that we discuss changes before making them has been to make yet another series of rapid-fire edits to the article. Please stop this. Martin Hogbin ( talk) 00:20, 30 July 2009 (UTC)
It is important to separate the definition of a physical quantity from the definition of the unit in which it is measured.[96]Defining the metre as the distance light travels in a specified time simply has the effect of setting the speed of light to a definite numerical value when measured in the SI units of m/s:[97][98] Said in different words, the exact value, c = 299 792 458 m/s, is an international convention, not a property of nature.[99]
I suggest that the following using a quote from Jespersen is better:
Defining the metre as the distance light travels in a specified time has the effect of setting the speed of light to a definite numerical value when measured in the SI units of m/s.[97][98] “One fallout of this new definition was that the speed of light was no longer a measured quantity; it became a defined quantity. The reason is that, by definition, a meter is the distance that light travels in a designated length of time, so however we label that distance – one meter, five meters, whatever – the speed of light is automatically determined. And measuring length in terms of time is a prime example of how defining one unit in terms of another removes a constant of nature by turning c into a conversion factor whose value is fixed and arbitrary.″[99]
Brews ohare ( talk) 17:01, 29 July 2009 (UTC)
“Also verifying the postulates, the speed of light has been shown experimentally to be independent of the motion of the source, [1] [2] as shown by experiments on the speed of γ-rays emitted during the decay of rapidly moving pions. [3] Although the speed of propagation is independent of motion of the source, the observed frequency can change due to the Doppler effect.”
Brews ohare ( talk) 02:32, 30 July 2009 (UTC)
This statement is well sourced and also of interest in providing some details of how the experiment was done, an experiment that is neither obvious nor easy to do. I represent one class of readers interested in the speed of light who would find this information about it interesting and would appreciate some sources where details could be found. Brews ohare ( talk) 02:48, 30 July 2009 (UTC)
As you know, Zhang is not available on-line. I have read his co-authored book that is on line about this topic, and he refers to the gamma ray experiment. At a minimum here: (i) Provide an on-line available source ( I provided three, which you deleted) (ii) Mention the nature of these tests or provide links where they may be found (The present state of this matter appears to be represented in §3.2 of Will, and on p. 23 of Field, although the binary star observation may be less striking than normally stated because of gravitational (curved space-time) effects neglected in its interpretation. Baird). Brews ohare ( talk) 15:06, 30 July 2009 (UTC)
“Einstein (who was aware of this fact) postulated that the speed of light should be taken as constant in all cases, one-way and two-way. This postulate, that the speed of light is the same in all inertial systems together with the postulate that all inertial observers are equivalent forms the basis of Einstein's theory of relativity.″
As carefully pointed out earlier, inertial systems are not identified by the fact that the speed of light is c in such frames. Rather, these frames are identified by the fact that all physical laws take on the same and simplest forms in such frames. In addition, the postulate of invariant speed of light is more appropriately phrased for the discussion of the text.
As also was pointed out, the cited source is not available on line and should be supplemented by one that is. Two such sources were provided (and deleted), both of which contain the correct statement of the postulates, which are in accord with the statements by Einstein cited in Special relativity. Examples are Chow and Banerjee. Brews ohare ( talk) 02:37, 30 July 2009 (UTC)
The statement says: "This postulate ... together with the postulate that ..." .There is no way to read this as anything short of a statement of the two postulates of relativity, which they are not. In addition, the first of the true postulates does not assume an inertial frame is understood, it defines an inertial frame. Moreover, various correct statements of the two postulates have been summarily deleted and replaced with this misinformation twice, along with Martin's claims on this Talk page that his so-called "modern" statement of the postulates is more succinct and accurate than Einstein's own version. See Martin's posture here and here, and an explanation that had no impact upon Martin here.
Again, modern examples of correct statements of the postulates are Chow and Banerjee, which are very similar to Einstein's version cited in the WP article Special relativity. Brews ohare ( talk) 07:39, 30 July 2009 (UTC)
As you know, d'Inverno is not available on line. I cannot check the accuracy of your reading of this source, which reading may well have selected a few notions out of context in an ill-fated attempt to be brief. In any event the text as placed in the article is incorrect; the most recent, deleted, correct version, which followed your sentence construction, and is supported by on-line available sources, is hardly longer than your incorrect version, which cannot be supported as it stands. Brews ohare ( talk) 14:55, 30 July 2009 (UTC)
The quotation of a boxed text highlight without the context it refers to has led you astray. However, this argument can be avoided entirely as we really don't need to refer to inertial frames at all. Brews ohare ( talk) 18:00, 30 July 2009 (UTC)
The speed of light has been shown experimentally to be independent of the motion of the source.[14
First, this source is not available on-line, and several that are available were simply deleted.
Second, this lead sentence properly belongs with the last sentence of the section, which treats the subject, and which follows the necessary introduction of this idea via the postulates.
This sentence along with the last sentence of this subsection should be replaced by the deleted subsection noted above. Brews ohare ( talk) 02:41, 30 July 2009 (UTC)
Martin Hogbin ( talk) 09:02, 30 July 2009 (UTC)
Nonresponsive to misplacement of sentence when topic has not been introduced and is not subject of the paragraph it serves as topic sentence for. Subject is brought up again at end of subsection as a dangling sentence. Poor construction. Source not available on line, and supplementary sources that are available deleted. Brews ohare ( talk) 14:49, 30 July 2009 (UTC)
The heading Role of the speed of light in relativity was replaced summarily as Constant speed in inertial frames, with Martin's complimentary comment "Restored sensible version", despite the carefully worded observation that the sub-sub-sections of this sub-section fit better under the deleted heading, being in fact about relativity, not about inertial frames. Apparently relevance to the topic is not sensible. Brews ohare ( talk) 02:45, 30 July 2009 (UTC)
Constant speed in inertial frames is a statement of the experimentally observed facts (and a generally accepted postulate) concerning the subject of the article. I might add that it is similar to the wording in the article when it originally became an FA.
Now that I have explained my actions, perhaps you can explain you recent series of edits Brews. I count 23 of them in the last day.
My suggestion was that we should now discuss changes before making them. I do not think that there are any serious problems with the content of this page. If you disagree let us discuss what you think is wrong before you make substantial changes.
Martin Hogbin ( talk) 09:18, 30 July 2009 (UTC)
It is not an issue of what is acceptable to WP, but of convenience for the reader. To suggest that every reader be prepared to go to a technical library (hardly " a little work") to follow up on sources is impractical. It also is common for editors to misread sources or abridge them without retaining the sense of the source, so citing a source that is not readily checked adds to WP's greatest problem: readers' assessment of credibility. Brews ohare ( talk) 17:52, 30 July 2009 (UTC)
I have not disputed the text of the guidelines. I have made an observation. Please re-read my remarks.
My own experience with technical libraries, both in industry and in universities, is that the book you want is often not present (either lost, on loan, or never acquired (a situation more and more prevalent)) and must be ordered in from a remote source with a delivery time of several weeks. That applies even to common textbooks, never mind specialized sources. Working on line is the most common method of acquiring information both in universities and in industry.
Of course, the nonacademic, or the person not employed by the corporate host, is denied access to the technical library.
I don't know about sources that are "not standard". I'd guess that sources from reputable publishers are frequently a cut above WP, and citing them adds to WP's credibility far beyond the unsupported assertions from WP editors, including their assertions about the contents of these sources. Brews ohare ( talk) 18:30, 30 July 2009 (UTC)
The guideline may be a bit Utopian if it entails that a reader or an editor purchase a source to find out whether it has been accurately represented as supporting a statement in the WP article. (Typically these sources are about $100 apiece.) My view is that if the book is published by, say Cambridge University Press, and it is available in its relevant portions on-line at, say Google books, that is a useful source that could be cited as an aid to the reader, especially where some alternative sources are not available in this manner. Not every Cambridge University Press book is definitive, but I'd probably be more inclined to accept the editor's stated views if a Cambridge University Press title could be seen to support their viewpoint. That is even more the case if I happen to disagree with an editor who resorts only to inaccessible sources in a continuing debate. Brews ohare ( talk) 21:26, 30 July 2009 (UTC)
I am adding this ony as supplement to logicaly enclose my discussion. - - - The calibration of physical SI metre unit implements the number 299792458 as a space/time factor in relation with the speed of light. When performing measurements with SI calibrated devices, all measurements will, according to the precission of the measurement, converge to the implemented value 299792458. The question is, why to perform speed of light measurements using the space/time units independent of the speed of light physical phenomenon ? If the results of independent measurement are expressed in SI units, the speed of light quantity will allways converge to the implemented value 299792458 in SI units, according to the precission of the measurements. Because constant 299792458 is exact numerical value, the uncertainity of this value has changed to the physical uncertainity, that is, uncertainity what is the physical meaning of this value. We can obtain the physical meaning of this value only by calibration of the physical metre/second units. The more precisely we calibrate the units, the more precisely we now the physical meaning of the number 299792458. The measurement of the speed of light is therefore in the context of SI definitions made nonstandardly by calibration of the physical metre/second units, instead of measurement of the numerical quantity of property. But the number itself is not only relation of SI units - saying : "our space/time units are this way exactly related". If c is maximal speed of interaction - propagation of cause - the number itself is quantitative expression of this phenomenon, expressed in SI units. That means, the number itself has substantial significance when implementing apropriate physical devices. If the number itself has substantial significance, the valid calibration of the metre/second units in apropriate physical devices has substantial significance, because without that the number may generate errors, because the meaning of the number 299792458 (exact constant) is fully bound to the physical metre/second units realizations. In this sense the meaning of the number must be physicaly realised, and therefore measured by units calibration. In this sense the speed of light is allways measured when calibratinng the metre/second units. In this sense the meaning of the constant 299792458 is physicaly uncertain. Physical uncertainity is not related to the number itself, but it is related to the precission of the physical realizations of the metre/second units. Physical uncertainity of the constant 299792458 can be quantified in context of individual metre/second units realizations. Therefore the lowest possible uncertainity can be evaluated. Anybody can start to do this considering the ruler and the wristwatch. Softvision ( talk) 13:49, 30 July 2009 (UTC)
There is no need to state both postulates of relativity, so I changed this section to avoid that issue. That requires a title change for the subsection, which no longer mentions inertial frames. Some sources are added and the paragraphs rearranged for better flow. Brews ohare ( talk) 16:53, 30 July 2009 (UTC)
Hi Martin:
Please respond to the points raised instead of reiterating your viewpoint without addressing the objections to it. Brews ohare ( talk) 20:26, 30 July 2009 (UTC)
Joe: Your edit far exceeded your stated goal. Regarding the stated goal, the references provided (and many more general discussions of this work) refer only to the two experiments cited as being decisive. Of course there were earlier experiments that established weaker bounds, or that were later found to be misinterpreted. The source Zhang & Hsu has a long discussion. Brews ohare ( talk) 21:12, 30 July 2009 (UTC).
Is your objection met if it is restated that there are "two experiments primarily cited in this connection"? See, for example, Zhang & Hsu; Baierlein; Will (§2.2); Field, p. 29; etc. etc. Brews ohare ( talk) 21:37, 30 July 2009 (UTC)
Incidentally, I note that you reverted to "This postulate, that the speed of light is the same in all inertial systems, together with the postulate that all inertial observers are equivalent, forms the basis of Einstein's theory of special relativity." which is an incorrect statement of the postulates. I had hoped to diffuse this debate by stating only Einstein's postulate about the behavior of light, which is all that is necessary. For example Banerjee states this postulate as: "The velocity of light in empty space is a constant, independent not only of the direction of propagation but also of the relative velocity between the source of light and the observer". What are you aiming at here?? Brews ohare ( talk) 21:43, 30 July 2009 (UTC)
The only way forward that I can see for this page now and the only way it will ever get back to being an FA is for Brews ohare to stop editing it to make points and to state his own opinions and private research on the subject. I have tried to get him to discuss things before making changes but his response is always the same, a bunch of rapid-fire and ill conceived edits to the article.
I suggest that we need to raise an RfC on his conduct to save this article from becoming Brews' personal soapbox. Martin Hogbin ( talk) 20:28, 30 July 2009 (UTC)
I think a good way to determine which points should be in the article would be to follow the lead of published sources that treat the subject speed of light in detail. Are there any books or other lengthy publications which we could compare to determine whether a particular subject should be covered here, and to what extent? Also, would this method be acceptable to the editors of this article?
Pecos Joe (
talk) 21:17, 30 July 2009 (UTC) edited to clarify at
Pecos Joe (
talk)
21:26, 30 July 2009 (UTC)
I hoped this would have conversation unrelated to the dispute about Zhang, in hopes to address what I think is a more persistent problem - "Should we include stuff about this particular topic, and if so, in how much detail?" I hoped that by following the lead of other publications, we could arrive at well-supported answers for these questions. Maybe I am trying to solve a problem that doesn't really exist? Pecos Joe ( talk) 00:09, 31 July 2009 (UTC)
"This postulate, that the speed of light is the same in all inertial systems, together with the postulate that all inertial observers are equivalent, forms the basis of Einstein's theory of special relativity."
The above statements are an incorrect statement of the postulates, unlike a correct statement that provides a careful definition of "inertial frames". This situation cannot be left like this.
I had hoped to diffuse this debate by stating only Einstein's postulate about the behavior of light, which is all that is necessary. For example Banerjee states this postulate as: "The velocity of light in empty space is a constant, independent not only of the direction of propagation but also of the relative velocity between the source of light and the observer".
The statement of the postulates due to Einstein are:
The first postulate defines inertial frames.
It appears that this will have to go to RfC. Brews ohare ( talk) 22:19, 30 July 2009 (UTC)
Apparently Martin believes the statements of Einstein's postulates by Einstein himself at A Einstein and at §2 On the relativity of lengths and times are a “personal reinterpretation”. An indication of Martin's state of mind, perhaps. Brews ohare ( talk) 14:10, 31 July 2009 (UTC)
It is an interesting form of cooperation to refer to Pecos Joe's attempted compromise as a "mangled version" and take that as an excuse to re-insert exactly the form causing the difficulties. These difficulties have been pointed out repeatedly, with documentation from published sources, including Einstein. Brews ohare ( talk) 15:08, 31 July 2009 (UTC)
An alternative lead is proposed in the section located here. Brews ohare ( talk) 15:18, 31 July 2009 (UTC)
The speed of light is postulated to be constant in all directions, regardless of motion. According to this postulate, the SI speed of light constant is postulated to be exact numerical value. According to this, noone can talk about speed of light as comparable numerical quantity, in context of speed of light measurement. Therefore the concept "measurement of the speed of light quantity" is obsolete. According to current postulates, comparable quantities are physical metre and second units. If you are talking about comparsion of the speed of light quantity in different directions, you are talking about physical comparsion of the physical metre/second units calibrated in different directions. If space is physicaly contracted in the direction of motion, and time is proportionaly dilated, calibration of the physical metre/second units in different directions, and their physical comparsion, must lead to conformity of physical metre/second units, confirming the uniformity of the speed of light in apropriate directions. It is hard to imagine, how to turn the time device carrying the physical second unit. Time units calibrations in different directions must be compared simultaneously. But comparing the time units simultaneously, we must compare the space units simultaneously, to perform valid comparsion. If difference in physical space units is proportional to the difference in physical time units, the speed of light is uniform in appropriate directions, as measured in SI units. (...) This is not my observation, or my own research. This is consequence of current physical postulates. I am submiting this to help the editors to solve the article formulation. Softvision ( talk) 14:23, 31 July 2009 (UTC)
Proposed lead-in:
It is not possible to measure the one-way speed of light (for example from a source to a distant detector) without some convention as to how clocks at the source and detector should be synchronized. [1] Einstein postulated both that the speed of light should be taken as constant in all cases, one-way and two-way, and that the speed of light was independent of the motion of its source. [2]
- ^ Zhang, Yuan Zhong (1997). "Special Relativity and Its Experimental Foundations". Advanced series on theoretical physical science. 4. World Scientific: 31, 171, 173. ISBN 9810227493.
- ^ See the article Special relativity and, for example, Sriranjan Banerji, Banerji & Banerjee (2004). "Postulates of the special theory of relativity and their consequences". The Special Theory of Relativity. PHI Learning. p. 20. ISBN 812031963X.
Brews ohare ( talk) 14:34, 31 July 2009 (UTC)
Rationale:
The goal is first to introduce the two topics of two-way light speed, and of motion of the source, and in follow-on discussion describe the experimental supporting data. This order is better than the reverse order in providing orientation as to the point of the entire subsection.
The above statement of Einstein's postulates concerning these matters is accurate, and does not go into undue detail. The footnotes provide opportunity for the reader to pursue matters further. The Banerjee source is on-line at Google books, and states the second postulate in a form very suitable for the purpose of this subsection. Brews ohare ( talk) 14:34, 31 July 2009 (UTC)
Experimental evidence concerting the two-way speed of light would appear in the following paragraph. I find the present lead two sentences fit each other.
The objections to the current order are (i) no basis is laid for introducing the selected experimental data from the (possibly) myriads of things that could be discussed. The revised order introduces the subject of Einstein's postulates about light and the experiment then fits as substantiation. (ii) the present order tends to force one into an unduly elaborate statement of the postulates, which so far has proved a stumbling block. The revised order allows introduction only of what Einstein said about the speed of light, which is all that is germane. Brews ohare ( talk) 16:05, 31 July 2009 (UTC)
Martin: (i)The experimental facts: There is no argument here about the experimental facts. The point is that stating the Einstein assumptions about light speed automatically raises the question in the reader as to whether they are true. Therefore this sentence order placing the Einstein assumptions first is natural, while the reverse order is less natural. Brews ohare ( talk) 04:37, 1 August 2009 (UTC)
Martin: (ii) "I do not understand what you mean by elaborate statements of the postulates." I can see very well that you have no understanding of this point, and after a week of unending effort on my part, I am convinced that you never will. That does not mean that you are right, it means just that you can understand only your own view. My most recent attempt is this subsection. An earlier attempt is this subsection. Brews ohare ( talk) 04:37, 1 August 2009 (UTC)
Martin: Of course, the answer to this is this subsection. To summarize: the present text in the WP article appears to claim to state the two postulates of the special theory. However, (i) that is not the case, and (ii) it is not necessary to undertake some (inadequate) summary of the two postulates; a simple statement of the assumptions concerning speed of light is all that is needed.
The proposed lead above provides a succinct statement of these assumptions appropriate for this section, expressed in a similar fashion by, for example, Banerjee, as cited in the lead. See the proposed lead here. Brews ohare ( talk) 09:46, 1 August 2009 (UTC)
The first postulate of relativity, as explained carefully above, and in greater detail in inertial frames establishes what an inertial frame is. The statement you've attributed to d'Inverno assumes this definition is understood from elsewhere (possibly the main text, instead of the boxed highlight you chose to quote). Clearly, omission of this definition is an emasculation of the postulates that do indeed include this definition. It is, therefore, incorrect to parade the emasculated version as the postulates of relativity. However, of greater interest, we do not need any of this for this discussion, as is evident form the proposed lead. Brews ohare ( talk) 11:18, 1 August 2009 (UTC)
An inertial frame does enter the postulates of relativity, and could be included by making a full statement of the postulates as originally proposed a long, long time ago and reverted by you. That is what Banerjee does. However, for the purposes of introducing the experimental work, that does not seem necessary, especially as the experimental work does not take place in an inertial frame, and makes little or no reference to it. Brews ohare ( talk) 11:18, 1 August 2009 (UTC)
Martin: You are not engaging here. The postulates themselves define what an inertial frame is. Your attribution to d'Inverno does not. Therefore, your attribution to d'Inverno is not equivalent to the postulates. OK? Brews ohare ( talk) 11:51, 1 August 2009 (UTC)
No Martin. There is nothing idiosyncratic about a literal quotation from multiple sources including Einstein. What is a bit odd is that the simple logic that A includes B and C does not include B; therefore A and C are not the same, eludes you. Brews ohare ( talk) 12:13, 1 August 2009 (UTC)
Let me elaborate: A=Einstein's Postulates; B=Definition of inertial frame; C =Your attribution to d'Inverno. A includes B, C does not include B so A is not C and C is not A. Brews ohare ( talk) 15:35, 1 August 2009 (UTC)
Which still does not address the point that introduction of "inertial frames" is just distracting in terms of introducing the experiments about two-way speed of light and independence from motion of the source. The experimental papers barely mention if they do at all, the role of "inertial frames", which is not a major point to be made in this WP subsection. Avoidance also allows deletion of the present incorrect WP text that suggests the idea of "inertial frames" is a matter outside the original postulates. It is not. Brews ohare ( talk) 17:23, 1 August 2009 (UTC)
Martin: It is not the issue of 'inertial frames' being inside or outside the postulates. The issues are (i) the WP phrasing of the postulates is incorrect and (ii) for purposes of framing the role of the experiments, we don't need to bring up inertial frames.
The purpose here is to introduce the experimental work, and sufficient understanding of the role of the experiments can be gained without inertial frames. If you insist that inertial frames are needed, then use the real statement of the two postulates instead of a bastardization that misrepresents them. Brews ohare ( talk) 03:50, 2 August 2009 (UTC)
If you insist that inertial frames are needed, then use the real statement of the two postulates instead of a bastardization. Brews ohare ( talk) 03:50, 2 August 2009 (UTC)
The simplest way to handle this is to leave inertial frames out of this paragraph altogether. The more precise approach is to put in both postulates (correctly, not in some elliptic version), for example as done by Banerjee. I'd call this approach pedantic for the purpose of this section, and suggest the role of inertial frames is better discussed under other sections. 22:45, 2 August 2009 (UTC)
I am OK with the present version. Brews ohare ( talk) 00:47, 3 August 2009 (UTC)
I happened to notice this from the comments section above, I moved it here so it would eventually be archived with the rest of this talk page: "Francis Bacon argued that the speed of light was not necessarily infinite..." No. That was Roger Bacon. However I hesitate to edit a page that is the subject of so much heated discussion about the scientific method, philosophy of science, and definitions of simple words, merely to correct a fact. —Preceding unsigned comment added by 67.9.148.203 ( talk) 13:36, 29 July 2009 (UTC)
Possibly this passage can be construed as saying the speed of light is not infinite? "The difference between light, sound and odour can be expressed in another way. Light travels far more quickly in air than do the other two; thus when somebody far away strikes a blow with a mallet or stick, we see the blow before we hear the sound it generates...Therefore when Aristotle claims that there is a difference between light and the other sensibles, this should be understood as a difference not between an instant and time, but between less time and more time..." Lindberg Brews ohare ( talk) 10:33, 1 August 2009 (UTC) I've reinstated a remark here about Roger Bacon with this source. Brews ohare ( talk) 11:00, 1 August 2009 (UTC)
Dicklyon: The verbatim text indicates that Bacon thought the speed of light was finite in air, and that he based that view on analogy between the propagation of light and sound. One of the two examples he used is exactly the one stated in the text you reverted. Yes, it is illogical. No, it is not different from his statements. Yes your explanation for your reverts is annoying and unjustifiable, as well as non-factual. Brews ohare ( talk) 14:45, 2 August 2009 (UTC)
I notice that, without discussion, Brews has decided to rewrite and reorganise the lead section. There is a change of order and emphasis to make the fundamental spacetime aspect come first. This is an major change to the article for which it would have been a good idea to seek a consensus first.
As it happens, I do not disagree with this change of emphasis, but if we are going to do it it needs careful thought and needs to be written very carefully. This is not the case at present. Brews has also taken the opportunity to insert the chatty line, 'Said in different words, the exact value, c = 299 792 458 m/s, is an international convention concerning the metre, not a property of light', that he could not get in elsewhere, into the lead. This point is fully discussed in the relevant section. Martin Hogbin ( talk) 10:30, 1 August 2009 (UTC)
There is nothing "verbatim" about this, it's just an English sentence, and it fits here perfectly; as it says, it is a restatement of the preceding sentence in words that emphasize the meaning of the preceding sentence, which requires more emphasis. You seemingly prefer the meaning of the previous sentence to go unnoticed. Brews ohare ( talk)
I prefer to make the point that is being made where it is introduced, and where the reader will find it most appropriate. That takes two sentences, it seems... Brews ohare ( talk) 11:32, 1 August 2009 (UTC)
"We" are not taking that view. It is preposterous to say a one line restatement for emphasis is an exhaustive discussion. Brews ohare ( talk) 12:00, 1 August 2009 (UTC)
The sentences in question are "In SI units, the magnitude of the speed of light in vacuum is exactly 299,792,458 metres per second (m/s)[6] because of the way the metre is defined. Said in different words, the exact value, c = 299 792 458 m/s, is an international convention concerning the metre, not a property of light.[7]". I don't like having two sentences, it's redundant and unhelpful. The way it's written makes it come across a deep and important and complicated insight, when it's really a very simple point. I suggest instead:
In SI units, the magnitude of the speed of light in vacuum is exactly 299,792,458 metres per second (m/s), [1] because of the way the metre is defined. [2]
- ^ "Fundamental Physical Constants: Speed of light in vacuum; c, c0". physics.nist.gov.
- ^ The metre is defined as the distance that light travels in 1/299792458 seconds. The second is defined independently, via an atomic transition frequency. See below for more information.
OK for me to make that change? -- Steve ( talk) 13:17, 1 August 2009 (UTC)
If we are talking about the quantity c = 299 792 458 m/s, we are talking about the property of reality expressed in SI units. Because we know what is second and what is metre, according to the approximate human perception space/time uncertainities, the property c is well explained by this quantity. Both, property c and quantity 299 792 458 m/s, can be considered as real, because our senses are real and we have real perception of the metre and second units, limited by the approximate human space/time perception uncertainities. Theese uncertainities of the speed of light quantity perception can be minimized by expressing the speed of light in following context : Around Earth's equator 0.13 seconds. This is very well done at the begining of the atricle. In the era of intercontinental flights, this is the best expression of the speed of light quantity, in relation to the human perception. The numbers 299792458 and 9192631700 itself, are the international agreement, that interconnects the Caesium 133 etalon with historical metre/second units, not to invalidate them, and to extend their precission.
Quantity 299792458 m/s is the speed of light physical phenomenon in vacuum expressed in SI units. Nothing less, nothing more. We know what is speed, we know what is light, we know what is metre, we know what is second. The perception of these realities is very good and intuitive, even without exact technical SI definitions. It could be good to stress the "Around Earth's equator 0.13 seconds" - click time scale - context. Afterwards, the fact, that the current speed of light constant is synthetic constant (exact) should be mentioned, and the subject of the interconnection of the SI metre unit with the speed of light physical phenomenon should be explained, including the explanation of the meaning of the word "exact". Therefore the motive for this interconnection should be explained, probably before the metre interconnection subject details. In context of this, the consequences of this interconnection should be explained, including positive and negative consequences. I have mentioned some of the negative consequences in my previous submissions. I think this is the way how to put everything together. Softvision ( talk) 14:34, 1 August 2009 (UTC)
The speed of light in vacuum in SI units has an exact value: 299,792,458 metres per second (m/s). In fact, the metre is officially defined as the distance light travels in vacuum in exactly 1/299,792,458 seconds.
Steve: You say I hope we can get the point across without saying anything too extreme, for example that the number 299,792,458 is purely 100% arbitrary convention and nothing else. I am more concerned that the point that comes across is that the number 299,792,458 is (from a strictly scientific stance) something more than 100% arbitrary convention. Of course, all conventions have their social context, and do not exist in limbo created from thin air. But the hazard here is not that; the hazard is that people will read that the speed of light is exactly 299,792,458 m/s and say "What? No experimental error? What, no need for future measurement? Are these guys nuts? " Maybe more significantly, they may say: "Look at that; one more example of how whacked out WP is - they even allow stuff like this to go unchallenged."
The way to cope with this reaction is to make clear at the outset that the reason an exact value is possible, and that no measurement is necessary is precisely because this number is not the physical speed of light, it's just a conversion factor. As Jespersen says: its value is a matter of convention, its value is entirely arbitrary. Once the reader understands that we are not dealing here with a physical property of light, the amazement over how it can be exact and independent of measurement goes away. Then the question arises: "Why this particular value?" and that discussion is the one you have presented above. Your discussion cannot be absorbed until the "conversion factor" idea has taken hold. Brews ohare ( talk) 03:59, 2 August 2009 (UTC)
In this connection, a discussion by David Mermin about the "foot" and the "phoot" (the distance light travels in 1 ns) might be fun to read and lead to some good ideas on this subject. Brews ohare ( talk) 04:24, 2 August 2009 (UTC)
Softvision, this is just an example of the very kind of thing that I was talking about. Nobody ever analyses the speed of sound in such philosophical depth, in connection with the system of units. This is all a product of the fact that by removing the medium of propagation of light from the textbooks, they have removed all rational physical basis for the speed of light. The end result is the endless philosophical arguments that we can see above, while everybody tries in vain to make sense out of nonsense. David Tombe ( talk) 15:08, 1 August 2009 (UTC)
Two items are not addressed here by you all:
(i) I've explained a reader's astonished reaction to reading the speed is exact, no measurement necessary, no error bars, because the reader takes the numerical value to refer to the physical speed of light, which seems intuitively to be a measurable quantity. A number of editors here share this view that it is measurable, or at least have not fully abandoned it. If you read the sentence in the lead, it is technically correct, but will not avoid this reaction or dispel the misconception.
(ii) The treatment of c as a conversion factor is beyond dispute, and is the only viewpoint from which an exact value seems reasonable: its exact because we could define it to be any value whatsoever were it not for the widespread use of the foot rather than the phoot (to use David Mermin's example). Steve has said above that defining the second in terms of a particular atomic transition (under a variety of conditions like 0 K, zero gravity, zero linewidth etc. that cannot be realized) is exact, but it is not a conversion factor. Of course, that is not the case. Instead of 1/9192631770 s, this transition could equally be chosen to be 100 ps or 100 s. The physical entity atomic transition must be kept separate from the unit second. The fraction 1/9192631770 s is a conversion factor from the physical standard to units of seconds, just like converting the speed of light to 299,792,458 m/s could equally be 1 ft/ns.
The resistance to these facts, and the desire to hide behind a form of words that is technically correct, but not terribly obvious to the non-legal mind, indicates an unwillingness to accept the real status of 299,792,458 m/s and a continuing belief that 299,792,458 m/s is somehow a real, physical, constant of nature. It is a failure to separate 299,792,458 m/s from the physical entity speed of light. The resulting lack of clarity in the WP article will cause a reader reaction that the lead is paradoxical and the article is not authoritative. Brews ohare ( talk) 13:44, 2 August 2009 (UTC)
Martin: Your comment doesn't address one key point: The WP single-sentence treatment will cause a reader reaction that the lead is paradoxical and the article is not authoritative. That is the natural reaction to reading that the speed of light in vacuum is exactly 299,792,458 metres per second, which is exactly the reason you italicized "exactly" - your stated purpose in doing that is to produce that reaction. Brews ohare ( talk) 16:02, 2 August 2009 (UTC)
If this sentence is so clear, why the insistence on italics with: 16:48, 26 July 2009 Martin Hogbin (The italics are in accordance with WP guidelines to add emphasis.); and earlier 18:29, 21 July 2009 Martin Hogbin (Restore emphasis); and the first insertion of these italics according to: 16:32, 28 July 2009 Martin Hogbin (Surprising fact (to many people) needs emphasis.)?
It appears very clear that you, Martin, wish to startle the reader with this statement, rather than explain why this statement should be only a ho hum minor observation as Steve suggests it to be. Brews ohare ( talk) 20:19, 2 August 2009 (UTC)
The revised version is OK with me, so I guess I can graciously ignore assault and hair-splitting. Brews ohare ( talk) 00:24, 3 August 2009 (UTC)
I'm still trying to focus on what is the real underlying cause for this ongoing argument. I had a look at the article entitled speed of sound for ideas as to how an introduction might look. The first sentence in the speed of sound article reveals the entire problem as regards the 'speed of light' article. The speed of sound article begins by explaining the rational physical basis behind sound waves. In modern texbooks, no such rational physical basis exists to explain light waves. This is of course a perfect recipe for endless argument. The ongoing arguments here must surely be about the differing points of view and opinions that exist amongst the different editors as regards trying to make sense out of something that does not make sense.
That's why we are seeing the sword fight straying far away from the original starting point. It has now entered into the terrain of the definition of the metre which is hardly in anyway connected to the actual speed of light. I think that in order to bring this dispute to a swift conclusion, all parties need to clearly state in simple terms what they would like to add to the introduction and why, and what they would like removed from the introduction and why. There also needs to be a more explicit statement of the individual points of view in order to avoid all the tip-toeing around the core issues. David Tombe ( talk) 14:28, 1 August 2009 (UTC)
Softvision, this is just an example of the very kind of thing that I was talking about. Nobody ever analyses the speed of sound in such philosophical depth, in connection with the system of units. This is all a product of the fact that by removing the medium of propagation of light from the textbooks, they have removed all rational physical basis for the speed of light. The end result is the endless philosophical arguments that we can see above, while everybody tries in vain to make sense out of nonsense. David Tombe ( talk) 15:08, 1 August 2009 (UTC)
OK, I'll move my reply up to join it. David Tombe ( talk) 10:48, 2 August 2009 (UTC)
Martin, I'm merely trying to ascertain the underlying cause of the argument between yourself and Brews. Brews has suggested that it is to do with the fact that you see the speed of light as being something in the realms of a platonic constant like π. Would you agree with that suggestion? David Tombe ( talk) 10:46, 2 August 2009 (UTC)
Martin, I've made a suggestion on Brew's talk page that he moves matters relating to the first line of the introduction (regarding 'c' being a physical constant) to further down. The idea of this is to maintain a historical chronology in the introduction and to separate the issues of dispute. David Tombe ( talk) 18:29, 2 August 2009 (UTC)
Much is being made in the article (in the lead the word exactly is stated thus) and in this talk page of the exactness of the speed of light (SoL). It seems to me that this has been overdone and indeed may even be incorrect. The SoL is only exact because we have defined it in reference to the metre and the second. The second has been defined separately but the metre has not, it owes its definition to the relationship ("conversion factor") with the SoL and the second. This was not always so; the metre used to be defined, by the French Academy of Sciences, as the length between two marks on a platinum-iridium bar, which was designed to represent 1⁄10,000,000 of the distance from the equator to the north pole through Paris. It was only defined in relation to the SoL in 1983. I find it difficult to accept that this redefinition made the speed of light accurate when it clearly wasn't before 1983. Where did I go wrong? Abtract ( talk) 17:09, 2 August 2009 (UTC)
Martin, you removed some edits of mine in the historical section on the grounds that Maxwell's molecular vortex theory has no role in modern physics.
It was of course the history section. Secondly, the way that you have left it is now totally inaccurate. You have got the contents of Maxwell's 1861 paper totally mixed up with the contents of his 1865 paper.
I assume that you overlooked the fact that these edits were in the history section and so I will restore them. David Tombe ( talk) 22:37, 2 August 2009 (UTC)
The current lead has too many problems to be left as it is. Let me start with the first sentence:The term speed of light generally refers to a fundamental physical constant of spacetime that limits the rate of transfer of matter or information.
The first problem is that it would appear from this construction that the only effect of this fundamental constant is to limit the speed at which matter or information can travel. (I know that it is possible to derive SR from this kind of premise but I do not think that is particularly relevant here). The implications of c as a fundamental constant of spacetime are much wider that indicated by this statement. It is better to show this speed limit as a result of the spacetime in which we live
More importantly limits the rate of transfer of matter or information is ambiguous. The rate of transfer of information could refer to bytes per second for example. Better to say something along the lines of maximum speed at which the can travel. Martin Hogbin ( talk) 15:58, 2 August 2009 (UTC)
The basic point of c is the spacetime connection, given prominence in the lead, which is fundamentally associated with information transfer, as that sets up the clocks. Perhaps the information transfer needs a separate sentence to get it more correctly stated. The EM connection is the next line, which hardly is burying this aspect. However, it is more or less accidental that EM radiation travels at c - many formulations of relativity simply posit a maximum speed of transfer, as Martin points out. (In fact, c does not actually refer to an extant physical speed of light in any realizable medium; it is fundamentally a theoretical construct.) This is a key point, and not at all one that is "not particularly relevant". Brews ohare ( talk) 16:20, 2 August 2009 (UTC)
I'm not sure what "this way" refers to. Every source agrees there is a limiting speed. Most take it as the "speed of light" but do not examine what that phrase actually means: light is medium for the transfer of information the maximum speed of interaction ... is equal to the speed of light By and large the view is that the Lorentz invariance of the Maxwell equations in classical vacuum carries over to all of physics, and implies a limiting speed of information transfer. The inverse procedure is more fundamental in that Maxwell's equations are just one "law of nature" and so are subsumed under "the laws of nature" in the postulates of relativity. In short, historically the Maxwell symmetry "discovers" the Lorentz transformation, but then the Lorentz invariance is posited, and so subsumes the Maxwell relations as a special case. Brews ohare ( talk) 16:43, 2 August 2009 (UTC)
Looks OK to me. Brews ohare ( talk) 21:45, 2 August 2009 (UTC)
Sorry, I was trying to consolidate it with the same thing said in the second paragraph. You did it better. Brews ohare ( talk) 04:55, 3 August 2009 (UTC)
We have a mini edit war going on the appropriateness of talking about Maxwell's "molecular vortices." I think this needs to be resolved in terms of what's in secondary sources. Providing an interpretation of, and assessment of the important of, of Maxwell's original primary writings, in the history section, is inappropriate per WP:NOR. If we find such interpretion and assessment in secondary sources that connect to the topic of the speed of light, then maybe it's OK. So, David, please cite a source if you want this material in. Dicklyon ( talk) 23:12, 2 August 2009 (UTC)
I don't think it is our place to determine whether molecular vortices played a significant part in the history. Obviously it was important at the time of Maxwell, and may have played an important role in how thought evolved. History of science is not just about what survives in today's textbooks of physics. It also is about the dynamics of the evolution of thought and the context in which it arose. For example, it seems the molecular vortices were the underlying intuition behind the curl operation. See Siegel and for vortices in fluid mechanics see Durand-Vidal et al. . Brews ohare ( talk) 13:47, 3 August 2009 (UTC)
This has all been a bit of a storm in a tea cup over what was just a correction edit in the history section. But the subject matter of that correction has clearly triggered off sensitivities in relation to the ongoing dispute regarding the modern day quagmire. Within the history section, there is nothing at all controversial about mentioning the point in history when Maxwell first discovered the connection between the speed of light and the electric and magnetic constants. And there is nothing wrong with giving a brief description as to how he did it. That brief description has been made, fully in line with primary, secondary, and tertiary sources. We have plenty of sources, so what exactly is the problem? Is anybody challenging the accuracy of the description of how Maxwell arrived at his conclusion?
My guess is that the controversy over this historical edit is based on the contrast between the logical approach that Maxwell used in the 19th century as compared to the total nonsense situation that we have reached in the 21st century. I opted to keep out of the dispute in the modern sections of the article because I have nothing useful to contribute to those sections. But while I was watching the dispute, I decided to correct some fundamental errors that I noticed in the historical section, and that seems to have ruffled some feathers. Martin has revealed his total disdain for Maxwell's approach, and so it seems that he would prefer to have Maxwell's approach reported inaccurately, rather than accurately.
If Maxwell's approach was the joke that Martin is suggesting that it was, then it should hardly be of any concern to Martin that it is described down in the history section. But I suspect that the real fear surrounding any mention of Maxwell's approach is that his old fashioned classical values put the modern nonsense to shame, and as such it must either be distorted or brushed out of view. David Tombe ( talk) 17:18, 3 August 2009 (UTC)
Pecos, I examined the situation very carefully before making the correction. The original paragraph was totally confused and it needed to be re-worded. The first thing that I saw wrong was the use of the term 'electromagnetic field' in conjunction with Maxwell's 1861 paper. That is clearly wrong terminology in the context. So what term should we use in its place? We cannot use the term 'aether' because Maxwell clearly states that he sees his sea of molecular vortices to be partly aether and partly ordinary matter. So why not use the exact term that Maxwell used. Maxwell's concept was a sea of molecular vortices, so why not call a spade a spade? We cannot have a situation in which we have to play down a historical term in a historical context in a historical section simply because one editor holds such a prejudice against Maxwell's work. Martin deleted my corrections twice, and both times the grounds stated were specious. The first time, he declared that molecular vortices have no place in modern physics. Nobody was claiming that they do. The second time, he declared that he was removing crackpot physics. It is not for him to decide that Maxwell's work is crackpot physics and then to proceed to eliminate an accurate description of it from the history section and replace it with a distorted version.
I'm glad that you agree that my correction was valid, and I can see that you are looking for a compromise. I also tried to consider a compromise, knowing that the very words 'molecular vortices' seem to severely aggravate Martin. But I simply can't think of what other term to use for Maxwell's sea of molecular vortices, and on reflection, I don't see why we should be bothering to pander to these kind of prejudices. David Tombe ( talk) 21:30, 3 August 2009 (UTC)
There is a line in the introduction that reads,
The speed of light can be viewed as simply a conversion factor between space and time in spacetime
Can anybody elaborate on the meaning of this sentence? Having already stated that the speed of light is a speed, and that it is the speed at which EM radiation moves, then how can the speed of light suddenly become a conversion factor? It is not sufficient to refer to the 1983 definition of the metre in terms of the speed of light. The statement in question is much more profound than anything to do with systems of units. It needs some kind of elaboration. By the same token, could we say that the speed of sound is a conversion factor between air and time?
As regards the 1983 definition of the metre in terms of the speed of light, it is somewhat tautologous. It neither tells us what a metre is nor what the speed of light is. I would blame this entire edit war on the person that introduced that definition of the metre in 1983. They were clearly too fascinated with their belief that the speed of light is a universal constant to be able to think rationally about the underlying purpose of what they were doing. The end result has been to sew additional confusion into an already badly confused scientific community.
On reading the entries above, perhaps this is what editor Abtract was also driving at. David Tombe ( talk) 09:25, 3 August 2009 (UTC)
David, you complain "It neither tells us what a metre is nor what the speed of light is." But that's the point, sort of. The meter and the speed of light are not independent of each other; the speed of light is a physical constant that anyone can measure, and the metre is an arbitrarily defined distance defined in terms of it; this definition ended the redundancy of having separate standards for time and distance, when the standard for the second and the physical constant c are enough. The definition recoginzes that there are just two degrees of freedom: you choice of time unit and you choice of conversion factor. What other way would tell you more about what a meter is, or what c is? For practical purposes, to make you comfortable, take the meter as the length of bar in a vault, and then you'll know pretty well what the measurements of c come out to. Dicklyon ( talk) 14:52, 3 August 2009 (UTC)
Dick, The 1983 definition is a total abomination. The man who concocted this definition chose a time fraction based on the most up to date speed of light as measured using classical values in line with the normally understood concept of speed. He then turned the whole thing upside down and defined the metre as the distance that light travels in this fraction of time. This had the effect of fixing the value of something whose value could yet be subjected to more refined meausurements. It is a fraud.
And why did he do it? In 1983, we were already languishing in the doldrums of a relativistic era in which the speed of light was supposed to be a universal constant. This man obviously wanted to impress the high priests of relativity. The relativistic priests probably indicated that they would like to see the speed of light being elevated to the sainthood, and this was his opportunity to oblige. The 1983 definition is tantamount to a standing proclamation of 'Amen!' to relativity. It firmly locked the door to any future variations in the speed of light using an elaborate conjuring trick.
I can detect that Brews knows that something needs explaining here. Although he claims to be comfortable enough with the 1983 definition, he can still see that the actual speed of light is something different than what this definition is implying. He can see that this definition is about something that has to be constant by definition, yet light will still nevertheless move in conjunction with a traditional style speed that might be subject to ongoing revisions and perfections of its value.
Maybe I've picked it up wrongly, but is Brews pointing to the fact that the 1983 definition allows a divergence between the classical speed of light and " Saint Speed of Light" as per this definition? And are his opponents trying to argue that no such dichotomy exists? Have I got the basis of the dispute correct? David Tombe ( talk) 18:35, 3 August 2009 (UTC)
I have been involved in this page for only a few days but I am sorry to say I must now leave it ... there is a major problem - Brews ohare. I have looked back over the edit history on this page and the article and it is clear that he is exactly the type of editor that has caused me much anguish in the past. I suggest that those of you who know the position more intimately than I do refer him for comment with a view to a topic ban; that's the only way progress will be made imho. Good luck. Abtract ( talk) 10:58, 3 August 2009 (UTC)
Some of the physics was settled years ago. However, as quantum gravity, doubly special relativity and cosmology show, there is ongoing activity, even about the basics. Brews ohare ( talk) 13:39, 3 August 2009 (UTC) And the role of c as a defined standard of comparison for speed contrasted with its role in the metric of spacetime may be understood by some, but is hard to explain nonetheless. Brews ohare ( talk) 13:42, 3 August 2009 (UTC)
The issue is really communication. Dicklyon and Martin share the trait they they are always right about (i) their views, and (ii) about what is appropriate in a WP article vis-à-vis level of presentation and topics. These two editors simply know no response to resistance to their "authority" other than reversion and reiteration of their personal standards in the face of sources beyond dispute. Brews ohare ( talk) 15:29, 3 August 2009 (UTC)
The accepted equation for the dispersion of light traveling though a plasma medium is ω2 = ωp2 + c2k2, which leads to a group velocity less than c and a phase velocity greater than c. I thought someone might be interested in knowing this, because it not even alluded to in the section on superluminal light. —Preceding unsigned comment added by Odellus ( talk • contribs) 22:55, 5 August 2009 (UTC)
Should the notion that one can measure a defined property be deleted from speed of light? Presently stated in the article speed of light:
"According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. This has been verified to a high degree of accuracy by experiment."
Comment: It is a defined property of free space that its electromagnetic properties εo and μo are frequency-independent. To claim that this defined lack of frequency dependence is experimentally verifiable is a logical error, as one cannot (in principle) measure a defined property: its value is, after all, what the definition says it is. All that experiment can do is confirm whether some realizable medium, like outer space say, has this property. Such confirmation serves to support the notion that "free space" is a useful model, but it cannot change the model. It can only support its utility. These statements in the article should be replaced with something like:
"According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. This behavior has been verified by experiment to a high degree of accuracy for media such as outer space or ultra-high vacuum, showing that in this respect these media are good approximations to free space."
The present statement should be replaced. Brews ohare ( talk) 20:41, 15 July 2009 (UTC)
Comments
My original statement was: 'According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. This has been verified to a high degree of accuracy by experiment'.
Two of the quoted sources are entitled: 'Severe limits on variations of the speed of light with frequency' and 'Probing the Speed of Light with Radio Waves at Extremely Low Frequencies'.
I am not sure how it could be any simpler. Theory claims that the speed of light is independent of frequency and experiment verifies this. Is there a physicist in the house???? Martin Hogbin ( talk) 23:34, 15 July 2009 (UTC)
I hesitate to enter into "define". However, NIST does post c0 as a unique, exact number for "vacuum". I take "vacuum" as free space. Thus "vacuum" exhibits no dispersion. Moreover c0 is exact, so there is no doubt that it is not a measured value. Do you agree?
In addition, NIST posts this remark: "The effect of this definition [of the meter] is to fix the speed of light in vacuum at exactly 299 792 458 m/s." The notion of a definition being able to fix the speed of light appears to rule out measurement, and a value independent of frequency rules out dispersion. Brews ohare ( talk) 14:30, 16 July 2009 (UTC)
Brews, I don't know how I could make it any more clear, so I'll just repeat myself: "If NIST defines a unit U as a quantity Q, then one cannot automatically infer that the NIST believes that Q is an absolute constant." Do you agree with the previous sentence? Yes or no? I say yes, with the kilogram as proof. What about you? -- Steve ( talk) 17:01, 16 July 2009 (UTC)
Steve: There is no analogy here. The weight of the object may change. BIPM is aware of that. Work is ongoing to replace the specimen with a better standard. There is no specification of light frequency in the BIPM c0; do you think that is simply an oversight? It is an exact number for all frequencies. It took a resolution to adopt this number: why do you think it might change without need for another such resolution? Brews ohare ( talk) 21:01, 16 July 2009 (UTC)
OK, where are we? BIPM "fixes" an equality between the kg and the mass of the IPK. BIPM "fixes" an equality between the meter and the distance light travels in vacuum in 1/299792458 s. BIPM is not implying that the mass of the IPK is fixed. BIPM, according to you, is implying with 100% certainty that the distance light travels in vacuum in 1/299792458 s is fixed. How do you know? First you said something obvious and irrelevant: The number 299792458 will not change. Yes, this is true and irrelevant. Next, you said that BIPM explicitly says that they're aware that the mass of the IPK is not fixed, and didn't give a similar warning about the meter. OK, fair enough. So to summarize, you believe that NIST/BIPM makes it 100% clear and unambiguous that the distance light travels in vacuum in 1/299792458 s is an absolute constant independent of frequency, date, direction, etc., and you believe this because of the absence of an explicit warning to the contrary. Is that really enough to base your entire argument on? Is that really the basis for 100% certainty? Or can we agree that NIST/BIPM don't quite make it 100% clear whether vacuum dispersion is possible in principle? --
Steve (
talk)
23:13, 16 July 2009 (UTC)
Although a clear definition of free space would aid in this discussion, I propose that the aforementioned change be made anyway. Even if free space is not in fact defined as possessing the property that all electromagnetic radiation travels at the same speed, the proposed change,
"According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. This behavior has been verified by experiment to a high degree of accuracy for media such as outer space or ultra-high vacuum, showing that in this respect these media are good approximations to free space."
is still a more accurate, and less ambiguous statement. SemitoneSonata ( talk) 15:10, 22 July 2009 (UTC)
Perhaps the following statement would be sufficiently accurate, without depending on too many other definitions. It should also be more accessible and easy to understand.
"According to the classical theory of electromagnetism, the speed of electromagnetic radiation in the absence of matter is the same for all frequencies. This has been experimentally confirmed to a high degree of accuracy in outer space and ultra-high vacuum."
This dispenses with the task of defining free space, and implies only (rather obviously) that outer space and ultra-high vacuum are good approximations to the absence of matter. Fizyxnrd ( talk) 13:56, 5 August 2009 (UTC)
"According to the classical theory of electromagnetism, the speed of electromagnetic radiation in the absence of matter is the same for all frequencies. This has been confirmed to a high degree of accuracy by experiments in outer space and ultra-high vacuum."
"According to the classical theory of electromagnetism, the speed of electromagnetic radiation in the absence of matter is the same for all frequencies. This has been experimentally confirmed to a high degree of accuracy in outer space and ultra-high vacuum."
Martin, it seems that you are not prepared to discuss this issue. You have now reverted my correction three times, leaving a short bogus excuse on each occasion, and without coming to the talk page to explain yourself. You have taken it upon yourself to deem Maxwell's work in 1861 to be 'crackpot physics'. We have a history section which touches on the subject of the aether ideas of the 19th century. That of course necessarily touches upon the very important work that Maxwell did in 1861. He linked the speed of light to the electric and magnetic constants. He did this by modelling Faraday's lines of force in terms of a sea of molecular vortices.
As the section now stands subsequent to your reversions, it contains gross inaccuracies. How do you propose that we fix those inaccuracies? Is there any particular aspect of my corrections in particular which you find to be inaccurate? Or is it the use of the term 'sea of molecular vortices' that you object to? What name would you propose to use instead?
You cannot simply impose distortions into the history section just because it is an aspect of history which you clearly don't like to be reminded about. Your actions are totally contrary to wikipedia's rules. You are censoring history in order to favour your own prejudices about the nature of the speed of light.
I've noticed that you have just removed the link to the 1861 paper claiming it to be irrelevant in the context of the 1861 paper, yet you have left the link to the 1865 paper which is not being discussed. You are clearly messing this whole paragraph up deliberately. It is clear that you hold Maxwell's 1861 paper in total contempt, and you are simply trashing the paragraph that discusses this paper. I think that we are going to have to get administrator intervention here, because your actions are starting to appear like vandalism. David Tombe ( talk) 21:53, 3 August 2009 (UTC)
It was in his 1864 paper that Maxwell came up with his famous wave equation that predicted EM radiation and allowed its speed to be calculated. He used the results of Weber and Kohlrausch in that paper (see link) to calculate the speed.
The sentence about the speed of sound is just a general comment about the way in which values for the properties of the transmitting medium can be put into a wave equation to calculate the wave speed speed. It was not intended to relate directly to Maxwell's work. (unsigned comment by Martin Hogbin)
In 1864, Maxwell derived the electromagnetic wave equation . [1] This allowed the speed of light to be calculated in much the same way as the speed of sound can be calculated in normal matter. Maxwell use this theory to calculate the speed of light from ratio of electrostatic to electromagnetic units by Weber and Kohlrausch. This was in good agreement with the measured value of the speed of light and supported the view that light was a form of electromagnetic radiation. [1]
Hi David: I believe you are learning how Martin does things. It is a mistake to think it has something to do with the subject, with your edits, or with WP. It is all about Martin. Brews ohare ( talk) 23:11, 3 August 2009 (UTC)
Martin, Well at least that has cleared up what the problem was all about. You do of course know that this is the history section and that nobody has been saying that molecular vortices form any part of modern electromagnetism. Nevertheless, the fact that Maxwell arrived at the link between electromagnetism and light using that method is very interesting, and it is certainly not crackpot physics. It's true that Maxwell didn't use the 1861 method in his 1864 paper but he did nevertheless retain a less explicit luminiferous medium with all the characteristics of his sea of molecular vortices. The characteristics of that luminiferous medium are clearly described in the 1864 paper. Maxwell still talks about an aethereal medium filling all of space (section 4 part I) with a rotatory nature such that the axes of rotation are along the magnetic lines of force (section 8 part I), and an elastic nature (section 15 of part I). He may not have mentioned a sea of molecular vortices explicitly but the concept is still implicit in the 1864 paper. Maxwell never gave up the vortex sea idea. He merely played it down because he was uncertain about the details. Maxwell's luminiferous medium remained a major part of physics right up until the arguments that followed in the wake of the Michelson-Morley experiment in 1887, after Maxwell's death.
So it is somewhat misleading to state so boldly, as you have done, that Maxwell did not use the sea of molecular vortices concept in his 1864 paper. He had already done the calculation in his 1861 paper. In 1864, he merely transferred the result into a wave equation using the well known 'Maxwell's equations' of which the most important ones can also be found throughout his 1861 paper.
I’m going to re-word it again along the lines that in his 1864 paper, Maxwell used a less explicit luminiferous medium and that he never mentioned the specific idea of molecular vortices. David Tombe ( talk) 17:54, 5 August 2009 (UTC)
Brews, I'll get back to you on this in more detail later. But just off the top of the head, yes, Maxwell wasn't the only one to be playing about with vortices at that time. The young John Bernoulli seems to have been the one who first came up with the idea of a sea of tiny vortices pressing against each other with centrifugal force. I do re-call reading a long time ago that it was Maxwell who actually coined the terms curl, div, and grad. And yes, curl and vorticity are intricately linked. If a curl is non-zero, then we have vorticity in the sytem. Magnetism has got plenty of non-zero curl. David Tombe ( talk) 07:52, 6 August 2009 (UTC)
This sentence is entirely misleading in this context, as it can be construed as meaning the exact number 299,792,458 m/s is considered by some to be a fundamental constant of nature, which absolutely no-one believes.
Duff himself doesn't think this way: Duff says "Asking whether c has varied over cosmic history (a question unfortunately appearing on the front page of the New York Times [7], in Physics World [8]4, in New Scientist [10, 11, 12], in Nature [3] and on CNN [13]) is like asking whether the number of liters to the gallon has varied." Unfortunately, however, in saying this, Duff confuses the units 299,792,458 m/s with the physical entity the speed of light. Naturally, 299,792,458 m/s is arbitrary (as all agree; for example Jespersen), but that has nothing to do with whether the physical entity speed of light has fundamental meaning. Of course, relativity shows that it does. So this is a bit of a straw man.
Duff also suggests, however, that actually determining whether the physical entity speed of light has, for example, changed over time, has to be based on dimensionless ratios involving c. In that regard he has a point, as suggested by Smolin, who points out the role of ratios. Brews ohare ( talk) 20:12, 3 August 2009 (UTC)
Pecos, I'm not sure, but I think that Brews is trying to say that the pre-1983 speed of light was a physical thing whereas the post 1983 speed of light is just a number by human definition. Maybe I've picked it up wrongly. David Tombe ( talk) 22:08, 3 August 2009 (UTC)
To say c dt is on the same level as dx is perfectly fine, and completely consistent with saying c is a conversion factor to make dt into an element of length. That is said in several of the cited sources. However, to say the revolution caused by this revelation is nothing to talk about would be silly. Brews ohare ( talk) 05:27, 4 August 2009 (UTC)
Quantity 299,792,458 m/s is not arbitrary. Arbitrary are the units of space and time. If the space and time units are correctly derived from the stable effects of reality, than the speed of light quantity is the real quantity expressed in these units, with uncertainity determined by the measurement precision and the units definition.
Current metre/second units are not arbitrary in the sense, that they are based on continuity of the historical units. Historical units are not arbitrary in the sense, that they are based on human perception of the space and time and on human intelligence. Human perception and human intelligence in this context certainly does not mean arbitrarity. Thanks to this nonarbitrarity this internet discussion about arbitrarity of the speed of light constant is possible. I am sure, that the result of this discussion will be nonarbitrary.
The meaning of the speed of light is essentially dependent on the concept of the time. According to current SI units definitions, the time unit is fundamental independent unit. It could be very dangerous, to undermine the concept of time. (already partially undermined)
"In mathematics and its applications, a coordinate system is a system for assigning an n-tuple of numbers or scalars to each point in an n-dimensional space." If the n-dimensional space is the real space-time, then you can assign as "numbers" in a coordinate system only real dimensions. Universe has three same dimensions and one specific dimension. That is the reason, why we have two types of units. There is no way to constitute the real coordinate system on one type of unit. Softvision ( talk) 00:51, 5 August 2009 (UTC)
Brews, yes I see your point about the ratios. But getting back to the main point, the number 299,792,458 was arbitrarily chosen because it related to the most accurate measured value of the speed of light in 1983. The effect of that choice was to keep the metre close in size to the already existing metre. They could however have chosen any number. They could have decided that a metre is the distance that light travels in 1/ 400,000,000 of a second. That would have yielded a different number for the speed of light.
So, just as you said, the speed of light, post 1983, is a consequence of the definition of the metre and it has got no physical significance in its own right. It is just a tautology. It is just a number, and that number doesn't even have any of the mysterious qualities that are possessed by e or π. The physical significance of the traditional speed of light, post 1983, has been delegated to the definition of the metre.
Is that the point that you have been trying to make in the introduction? And have others been trying to say that the speed of light is about the speed of light? It would seem to me that since 1983, the speed of light is no longer about the speed of light. I can't even quite see it as a conversion factor, although I can see why others might. I see it as a tautology, plain and simple. We are in a new era in which Saint 'Speed of Light' is not only a constant by belief in relativity, but also as an unalterable fact embalmed in our international system of units. David Tombe ( talk) 18:18, 5 August 2009 (UTC)
Dicklyon is mistaken in his remark above. It is true that the modern value is the value that it is because of the existence of the metre in its earlier forms. Convenience dictates that the new measure be close to the old one to avoid conflict where possible, and for purposes where great accuracy is not an issue, the old and new metres are the same. However, from a logical standpoint there is a big difference. Today c no longer means what it did. It is now, from a logical standpoint, an entirely arbitrary number, and is divorced completely from measurement. That means that the "meaning" of the speed of light is no longer what it used to be. Dicklyon has not absorbed the WP article nor the references in it. In particular, Dicklyon should read the article cited in the WP article by Jespersen. Brews ohare ( talk) 03:12, 8 August 2009 (UTC)
This discussion seems to ignore two properties of the speed of light which definitely are not simply human constructs. Firstly, the speed of light is not infinite: it it were infinite, it would be infinite in any set of units. Secondly, the mainstream view is that the speed of light in free space is constant (although there is also a respectable minority hypothesis that it has changed over cosmological time, see Variable speed of light). This is equivalent to saying that dc⁄dt = 0. Again, either c is constant (the time-derivative is exactly zero) or it isn't, regardless of the units you use to measure it. Physchim62 (talk) 10:18, 8 August 2009 (UTC)
Since 1983, the speed of light has meant that light travels 299 792 458 times the distance that light travels in 1/299 792 458 of a second, in one second. It is a total tautology based on the definition of the metre. The number, 299 792 458, that is connected with the new definition of the metre was specially chosen so as to create a smooth and unnoticeable transition between the old real physics and the new nonsense physics. The new nonsense physics was inspired by a desire on the part of some, to elevate the speed of light to the sainthood because of its role in Einstein's theories of relativity. David Tombe ( talk) 09:18, 9 August 2009 (UTC)
Dicklyon: In response to the turd you left on my user page, you can't read, and are too lazy to look at the source. The source explains how the history of Roemer's work has been mangled, not that the WP article has a mangled version of history. Wake up, and get that chip off your shoulder. Brews ohare ( talk) 05:32, 4 August 2009 (UTC)
In response to your second reversion, this source, Roemer: a cautionary tale, if read, explains how Roemer's work was misreported in a series of historical discussions. It often was said that Roemer calculated the speed of light, when in fact he did not, and it was Huygens who used Roemer's work to estimate a speed (and using his data incorrectly, at that). Inasmuch as this widespread misreporting is out there, a reader might well wonder if the WP treatment is indeed accurate, and this reference provides the necessary discussion to put everything straight.
All this would be completely understood by anyone who spent a minute reading this source, which is available at google books at the provided link. To remove this source once, without proper examination of the source and without taking the time to understand its purpose is pretty sloppy. To do it twice, when reminded about its purpose is really sloppy. And to accompany the first reversion with a turd on my user page about it??? What is that???.
Sloppy and impolite and intemperate editing. Brews ohare ( talk) 06:14, 4 August 2009 (UTC)
Well Martin, your research into this mater is flawed. The WP article has a carefully documented version of this history that agrees with the version in this source. This source also explains very carefully with additional sources that your notion of the "generally accepted views" (which you apparently feel are being challenged) is incorrect, and just how that view came to be promulgated. As I said, this account of misinformation is helpful in supporting the WP account. All this entry amounts to is a note supporting what has been in this article for ages; suddenly this minor addition becomes a cause célèbre. As I am sure you are perfectly capable of understanding my summary of the source (and checking its accuracy), of reading the WP article, and perfectly capable of assisting with this entry instead of being obstructive, I wonder what drives your actions? Could it be because a person on your hit list made the addition? Or, could it be that your action is based upon sycophantic acceptance of Dicklyon's sloppy assessment? Or, is it just Martin's usual Not Invented Here attitude? These traits combined make a mighty force! Brews ohare ( talk) 13:10, 4 August 2009 (UTC)
David Wilson has looked into this matter carefully and assembled several sources. The article French cites a number of reputable sources and, being a mea culpa, does not appear to be grinding any axes. I have read other accounts that support the view that Huygen's did the calculation (not Roemer) and got the wrong answer. Just why that happened, I don't know. I confess to not having carefully read the WP account, and no, I cannot read Danish. So I'd suggest that David put together what appears to him to be a suitable version. Brews ohare ( talk) 13:18, 7 August 2009 (UTC) BTW I am not responsible for posting the WP account: I simply sought to add a footnote with a source. Brews ohare ( talk) 13:18, 7 August 2009 (UTC)
Yeah, if the current version is wrong you should probably blame me, not Brews. And having read a bit more on the topic since I last edited this article I have got a bit of doubt on the "22min/80 orbits of Io" version myself. It's something I've been meaning to get back to.
Regarding Teuber's article his wording doesn't actually leave that much room for doubt (in my own translation: "The only explanation of the numbers that make even the slightest bit of sense is..."), but in my view the article has a significant weakness in that it only seems to use the 1676 article and the correspondence between Rømer and Huygens. If the 1676 article is so cryptic why not use the other key article on the topic, Meyer, Kirstine (1915) Om Rømers opdagelse af Lysets Tøven (in Danish), The Royal Danish Academy of Sciences. Kirstine Meyer discovered Rømer's observation tables at the University of Copenhagen Library in 1913, documented that they were the basis of the value of 22 minutes and published them with extensive commentary in 1915. I haven't yet gotten hold of a copy of her article, which is why I haven't gotten back to this yet, but I've read summaries of it in Pedersen, Kurt Møller (1976) Ole Rømers opdagelse af lysets tøven (in Danish), Astronomisk Tidsskrift, vol. 9, pp. 160-66 and in Friedrichsen, Per; Tortzen, Christian Gorm (2001) Ole Rømer - Korrespondance og afhandlinger samt et udvalg af dokumenter, C. A. Reitzel, and neither of the two seem to question the "22min/diameter of the Earth orbit" explanation.
So in conclusion I think I have to agree with David that Teuber's explanation appears not to be a widely held view and the "22min/diameter of the Earth orbit"-version is more appropriate here.
Hemmingsen
07:42, 9 August 2009 (UTC)
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Wikipedian64, I have reverted your removal of italics and your change of 'exactly' to 'precisely'. The use of italics is as per WP guidelines in that they are used sparingly for emphasis.
The word 'exactly' is a better word in this context because 'precisely' could be taken to mean that there is some, very small, room for error or change, which there is not. Martin Hogbin ( talk) 16:54, 26 July 2009 (UTC)
The fact that the value of physical quantity, when stated in SI units, should have an exact value is surprising to most people. This makes the point worthy of emphasis in the lead in my opinion. Martin Hogbin ( talk) 16:22, 28 July 2009 (UTC)
In view of the conventional nature of 299,792,458 m/s, it seems misplaced to make it the central point of the introduction. Instead, the importance of the physical nature of the speed of light in setting the maximum rate of information and of matter transfer, and its connection to the speed of EM radiation should be the main focus. The numerical value in SI units is subsidiary. Brews ohare ( talk) 16:18, 28 July 2009 (UTC)
It's surprising that Sarah Palin quit midterm, but that doesn't mean it's important. I'm inclined to think the placement of an accidental value in the lead, however surprising, tends to make it look like it is more significant than it is, and sets the article up with unnecessary explaining to do. A primary position in the lead also diverts attention from the actual importance of the speed of light to physical theory. Brews ohare ( talk) 16:39, 28 July 2009 (UTC)
Martin: this is an incomplete thought at best. It does not approach the subject, which is the diversion from what is important to highlight the marginally relevant. Brews ohare ( talk) 17:01, 28 July 2009 (UTC)
Apparently there is resistance form Martin to the notion that 299,792,458 m/s is a conversion factor from time to length. However, it will be noted that in support of this notion was a verbatim quote from
Bertrand Russell (2009).
ABC of Relativity (Revision by Felix Pirani of 1925 4th ed.). Taylor & Francis. p. 12.
ISBN
0415473829. (just prior to the quote, the NIST decision to "define" the value of "c" is discussed), and two other sources: A Connes (2006). "On the foundations of noncommutative geometry". In Izrailʹ Moiseevich Gelʹfand, Pavel I. Ètingof, Vladimir Retakh, Isadore Manuel Singer (ed.).
The unity of mathematics. Gulf Professional Publishing. p. 175.
ISBN
0817640762.{{
cite book}}
: CS1 maint: multiple names: editors list (
link)
and
Richard Wolfson (2003). Simply Einstein: Relativity Demystified. W. W. Norton & Company. p. 170. ISBN 0393325075..
Previously I provided JA Wheeler and Mendel Sachs, not exactly slouches in the realms of physics. Here's three more sources: Jespersen; Brzeziński; ND Mermin.
In the discussion with Softvision this conversion factor role was very extensively described, and Martin himself suggested the arbitrariness of 'c' was evidenced by the "natural" units where c=1.
So what is the problem here? Use of λ = c/f (or ℓ = c t) means c is a conversion factor. Stating that makes it very clear that any value of c may be used (in principle), and that 299,792,458 m/s has nothing to do whatsoever with the physical entity known as the speed of light. That realization defuses 98% of the controversy around the "speed of light by definition". Brews ohare ( talk) 02:21, 28 July 2009 (UTC)
The term "conversion factor" is not mine. It appears in the cited sources. Brews ohare ( talk) 11:17, 28 July 2009 (UTC)
I agree that there are two usages involved here, of which one is strictly the space-time notion that the metric ds2 involves c2dt2, so c converts the time component to a distance allowing the computation of ds.
However Gelʹfand very clearly is describing a second usage: “This definition was replaced in 1983 by the current definition which, using the speed of light as a conversion factor, is expressed in terms of inverse frequencies rather than wavelength …” Likewise, “By an international agreement, made in 1983, 'the metre is the length of the path travelled in vacuum by light during a time 1/299,792,458 of a second'. From the physicist's point of view, the speed of light has become a conversion factor, to be used for turning distances into times… ” ABC's of Relativity; Wheeler mentions conversion factor: “The conversion factor between seconds and meters is the speed of light, c = 299,792,458 m/s. … The speed of light is the only natural constant that has the necessary units to convert a time to a length.” John Archibald Wheeler. Jespersen is very clear about the arbitrariness of the value for c: “One fallout of this new definition was that the speed of light was no longer a measured quantity; it became a defined quantity. The reason is that, by definition, a meter is the distance that light travels in a designated length of time, so however we label that distance – one meter, five meters, whatever – the speed of light is automatically determined. And measuring length in terms of time is a prime example of how defining one unit in terms of another removes a constant of nature by turning c into a conversion factor whose value is fixed and arbitrary.″ Jesperson
I bring up all these quotes from the already cited sources (of which there are more above) to persuade that the arbitrariness of the value for c is not my idea, and has multiple published occurrences. Likewise, the term "conversion factor". Of course, the quotes should be unnecessary because the idea is a logical consequence of the definition, as NIST has pointed out, and as Jespersen states in so many words.
In sum, the realization that the choice of 299,792,458 m/s is simply a practical choice of number and it could equally well be 1 m/s defuses the controversy around this numerical value, by showing any number can be taken when using the "speed of light by definition". Naturally, it is the very arbitrariness of 299,792,458 m/s that raises people's eyebrows, because they think of c as a definite value that can be measured. Introducing the term "conversion factor" places the emphasis where it belongs: on human conventions that are human choices, and not in nature outside man's grasp. Unlike the famous quip about a senator introducing a bill to define π to be a round number, a senator could introduce a bill to make c a round number. Brews ohare ( talk) 11:17, 28 July 2009 (UTC)
It is not easy to determine your goal in the above remarks. My take is that you wish to explain how it is that the speed of light cannot be taken as a fixed value. There is some ambiguity in such a remark, which may be contained in your observations, but eludes me. The distinction to be made is that the physical speed of light may in fact change over time (as suggested by some cosmologies), or may be observed with greater precision in the future (resulting in more precise numerical values for wavelengths of selected atomic transitions). However, the numerical value of the speed of light in SI units will never change, short of an agreement by the CIPM ( Comité International des Poids et Mesures) that the definition should be changed. The numerical value in SI units simply is one of man's conventions, and is not an experimental matter, but a matter of convenience. Do we agree? Brews ohare ( talk) 15:13, 28 July 2009 (UTC)
Have we abandoned logic here? First, we say that c = 299,792,458 m/s. Then we say that 299,792,458 m/s is a mere conversion factor, not related to physical entities. This is a logical contradiction because "m/s" means "meters per second" or, in general, "movement through space in a certain time." What moves through space in a certain time? A physical entity. Lestrade ( talk) 19:51, 30 July 2009 (UTC)Lestrade
Please explain deletion of this quote:
“One fallout of this new definition was that the speed of light was no longer a measured quantity; it became a defined quantity. The reason is that, by definition, a meter is the distance that light travels in a designated length of time, so however we label that distance – one meter, five meters, whatever – the speed of light is automatically determined. And measuring length in terms of time is a prime example of how defining one unit in terms of another removes a constant of nature by turning c into a conversion factor whose value is fixed and arbitrary.″ [1]
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Brews ohare ( talk) 16:59, 28 July 2009 (UTC)
I find this quote to be (i) authoritative (ii) very clear and (iii) important to have in the article in view of the "very surprising" [Martin's words] nature of an exact value for the speed of light in SI units. Brews ohare ( talk) 16:59, 28 July 2009 (UTC)
If this point is as "surprising" as you said it is, the point bears some amplification, and this text does it very well. Also, your own resistance to this viewpoint indicates the need for a direct quote, because nothing less proves convincing. The point must be made in a way that very clearly is not a WP editor's flaky opinion, or it will be the source of unending battles on this page. Brews ohare ( talk) 17:18, 28 July 2009 (UTC)
Martin: I don't know what "quote from the article" you are talking about. First, the article doesn't presently even cite Jespersen, never mind quote him. Second, the only item in quotes is from NIST. Third, the removed quotation uses the phrase "by turning c into a conversion factor whose value is fixed and arbitrary", which I find a very eloquent statement of the situation compared to the existing text. IMO you have deleted the quote precisely because it is so clear, and you would rather preserve an element of mystery about this subject. Brews ohare ( talk) 17:41, 28 July 2009 (UTC)
This "quote" appears to be the result of your slight rewording of an entry of my own words 21:46, 24 July 2009. So, it lacks the clout of a real verbatim quote from a published source.
As I've said, the removed quotation from Jespersen uses the phrase "by turning c into a conversion factor whose value is fixed and arbitrary", which I find an eloquent statement of the situation compared to the existing text. In addition, as I'm sure you are aware, this subject of an exact speed of light comes up over and over here, the most recent exchange being Softvision, but prior to that Venus10041, Physchim62 and others. It's worth a try to see whether a clearer statement would improve matters. It is a commentary on the present wording that readers don't find the explanation satisfying. Brews ohare ( talk) 18:03, 28 July 2009 (UTC)
You have stated your preference, without arguments in support, and I disagree with it, for substantial reasons provided by me, and not challenged by you. Brews ohare ( talk) 23:54, 28 July 2009 (UTC)
Where do you get the notion that "conversion factor" is used vaguely when all the verbatim quotes you have deleted from several different sources all define what is meant in absolutely starkly clear and unambiguous terms, and include specific examples? Brews ohare ( talk) 14:44, 29 July 2009 (UTC)
I have added Jespersen as a reference (no quote), and a sentence stating that the value of c is not a property of nature, but a convention. Brews ohare ( talk) 15:19, 29 July 2009 (UTC)
The article as revised by Martin now states:
A more complete theory of light that describes the interaction of photons with matter is given by quantum electrodynamics (QED) in which c plays the role of a fundamental constant. [1]
Brews ohare ( talk) 17:14, 28 July 2009 (UTC)
The problems with this are:
Brews ohare ( talk) 17:14, 28 July 2009 (UTC)
Martin:
The photon does not directly feel the strong or weak force, but the photon feels quarks and quarks feel the strong force. The way Feynman diagrams work means that that's enough for the photons to feel the strong interaction a little bit. Here's an example: The question "What is the electron magnetic moment" appears to be a purely electromagnetic question. And it can be answered to 99.9999999999% accuracy within QED. But to get to the parts-per-trillion level, you need to include strong interactions (see Figure 2 of [3], "hadronic" means strong force) despite the fact that neither the electron nor photon feels the strong force. :-) --Steve (talk) 03:55, 24 July 2009 (UTC)
In addition:
Brews ohare ( talk) 17:33, 28 July 2009 (UTC)
Let us ask Steve what he thinks. Martin Hogbin ( talk) 17:45, 28 July 2009 (UTC) This is what was said above:
I have no objection to mentioning photons. Martin Hogbin ( talk) 17:53, 28 July 2009 (UTC)
It also is not what is said in the deleted statement. Brews ohare ( talk) 23:53, 28 July 2009 (UTC)
The source is written by an expert, There is no argument that the source has misstated things. It is clear and unambiguous. Martin apparently doubts that he understands it, but nobody else will have that problem. Possibly Martin's statement can be interpreted as he would like it to be, but it is ambiguous and casts QED in a somewhat odd light, as "more complete", as though we just ain't got there yet. Why not stick with the source, which is "more complete" yet? And why make the reader dig through an entire text on the subject when all that is wanted is a sentence that makes the entire matter clear? Is that Steve humoring Martin, or some real effort to be useful to the reader??? Brews ohare ( talk) 14:27, 29 July 2009 (UTC) I have re-written the note to agree with Martin's text. Brews ohare ( talk) 15:17, 29 July 2009 (UTC)
I have just removed the following phrase about photons from the article, "which in vacuum travel at the speed of light, c".
Photons are quantum entities and to talk of them travelling with a particular speed is not a good idea, although it is often done. Also what speed do photons travel at when they are not in a vacuum? Martin Hogbin ( talk) 22:27, 28 July 2009 (UTC)
I believe you should answer these questions, which are natural, and source your answers to a reference available on line. BTW, the article already states (and sources) c as the speed of rest-mass-zero particles. Brews ohare ( talk) 23:51, 28 July 2009 (UTC)
What statement is "that statement" that you are referring to?? Brews ohare ( talk) 15:44, 29 July 2009 (UTC)
You are arguing among yourselves here. What Martin's comments "clearly show" is a lack of civility. Brews ohare ( talk) 14:21, 29 July 2009 (UTC)
I have not made any suggestions like that. I understand that the juxtaposition of the speed of photons in vacuum next to a description of their role as force carriers was unfortunate, allowing virtual photons to be confused with photons moving freely in space. However, it easily could be corrected without a diatribe on background, and "little balls". I won't engage in argument over the speed of photons - do whatever you like. Brews ohare ( talk) 17:57, 30 July 2009 (UTC)
There was no need to change any of this. The cited reference on the independence of the speed of light on this source velocity gives examples of many experiments, including light from double stars, rotating mirrors, light from limbs of the sun and more.
The relativity postulates were modern and concise ones from a reliable source. Martin Hogbin ( talk) 22:44, 28 July 2009 (UTC)
If you look at the table of contents, you will see that the sub-sub-sub-sections I mention are sub-sub-sub-sections of the Inertial frames sub-sub-section, not of the sub-section Fundamental importance. Hence my observations about inappropriateness of the heading. Brews ohare ( talk) 14:18, 29 July 2009 (UTC)
"What does 'Speed in relativity' mean?" As a subsection title, it means "the role of the speed of light in relativity". Previously you chastised me about this, saying that the article was about the "speed of light" and it need not be repeated. Brews ohare ( talk) 15:39, 29 July 2009 (UTC)
I've changed the sub-title to a more complete description of its subject that fits with its own sub-sub-sections. Brews ohare ( talk) 17:30, 29 July 2009 (UTC)
We are hardly "repeating everything in every article" by listing Einstein's statements of his postulates and maintaining some consistency. In addition, his names for the postulates are helpful in this context because they are divided nicely and are labeled exactly as is needed. I do not find your statements more succinct, you just dumped the labeling and chose a formulation that requires some logic to attach to the text instead of being direct statements of the postulates. I hope you have enough objectivity to see that. Brews ohare ( talk) 14:18, 29 July 2009 (UTC)
I find little support for your statement that Einstein's statements of the postulates have been superseded in modern times. For example: Chow (2008) states them virtually verbatim as in Einstein's papers, and I find many other recent texts that do likewise. Moreover, the statement in terms of the "principle of relativity" and separately as "the principle of the constancy of the velocity of light" is a formulation that (i) suits the article, and (ii) is logically preferable to the conglomeration you presented, which fails to separate these two ideas, and does not state the invariance of all physical laws in inertial frames.
If a reformulation of Einstein's postulates exists, it is a stress upon the identification of inertial frames as those where the physical laws are not only invariant, but have their simplest form, a notion Einstein put forward in other writings, but not relevant here. Brews ohare ( talk) 14:18, 29 July 2009 (UTC)
I have modified the presentation of the postulates more in keeping with your text, but using a reference accessible on-line, and a more accurate enunciation. Brews ohare ( talk) 17:33, 29 July 2009 (UTC)
I would like to change the sentence:
'[The speed of light] has also been confirmed by the Michelson-Morley experiment and the Kennedy-Thorndike experiment (an improved version of the Michelson-Morley experiment) that the two-way speed of light; between a source and a mirror, and back again; is the same, independent of the common constant velocity of the observer, source and mirror (all moving together), regardless of its magnitude or direction'.
Which I consider to be long winded and containing a lot of unnecessary and irrelevant detail.
To
'The two-way speed of light, that is from a source to a mirror and back again, has also been confirmed by the Michelson-Morley and later, more accurate, experiments to be constant'.
The MM experiment is by far the best known and should be mentioned by name but there are at least 14 others that we need not mention specifically. I think that giving detail on some of the experiments is not just unnecessary but undesirable. I would add the summary page from Zhang as a reference as this the most comprehensive source on the subject. I would suggest leaving just one online reference. Martin Hogbin ( talk) 10:51, 29 July 2009 (UTC)
Once again, you response to the suggestion that we discuss changes before making them has been to make yet another series of rapid-fire edits to the article. Please stop this. Martin Hogbin ( talk) 00:20, 30 July 2009 (UTC)
It is important to separate the definition of a physical quantity from the definition of the unit in which it is measured.[96]Defining the metre as the distance light travels in a specified time simply has the effect of setting the speed of light to a definite numerical value when measured in the SI units of m/s:[97][98] Said in different words, the exact value, c = 299 792 458 m/s, is an international convention, not a property of nature.[99]
I suggest that the following using a quote from Jespersen is better:
Defining the metre as the distance light travels in a specified time has the effect of setting the speed of light to a definite numerical value when measured in the SI units of m/s.[97][98] “One fallout of this new definition was that the speed of light was no longer a measured quantity; it became a defined quantity. The reason is that, by definition, a meter is the distance that light travels in a designated length of time, so however we label that distance – one meter, five meters, whatever – the speed of light is automatically determined. And measuring length in terms of time is a prime example of how defining one unit in terms of another removes a constant of nature by turning c into a conversion factor whose value is fixed and arbitrary.″[99]
Brews ohare ( talk) 17:01, 29 July 2009 (UTC)
“Also verifying the postulates, the speed of light has been shown experimentally to be independent of the motion of the source, [1] [2] as shown by experiments on the speed of γ-rays emitted during the decay of rapidly moving pions. [3] Although the speed of propagation is independent of motion of the source, the observed frequency can change due to the Doppler effect.”
Brews ohare ( talk) 02:32, 30 July 2009 (UTC)
This statement is well sourced and also of interest in providing some details of how the experiment was done, an experiment that is neither obvious nor easy to do. I represent one class of readers interested in the speed of light who would find this information about it interesting and would appreciate some sources where details could be found. Brews ohare ( talk) 02:48, 30 July 2009 (UTC)
As you know, Zhang is not available on-line. I have read his co-authored book that is on line about this topic, and he refers to the gamma ray experiment. At a minimum here: (i) Provide an on-line available source ( I provided three, which you deleted) (ii) Mention the nature of these tests or provide links where they may be found (The present state of this matter appears to be represented in §3.2 of Will, and on p. 23 of Field, although the binary star observation may be less striking than normally stated because of gravitational (curved space-time) effects neglected in its interpretation. Baird). Brews ohare ( talk) 15:06, 30 July 2009 (UTC)
“Einstein (who was aware of this fact) postulated that the speed of light should be taken as constant in all cases, one-way and two-way. This postulate, that the speed of light is the same in all inertial systems together with the postulate that all inertial observers are equivalent forms the basis of Einstein's theory of relativity.″
As carefully pointed out earlier, inertial systems are not identified by the fact that the speed of light is c in such frames. Rather, these frames are identified by the fact that all physical laws take on the same and simplest forms in such frames. In addition, the postulate of invariant speed of light is more appropriately phrased for the discussion of the text.
As also was pointed out, the cited source is not available on line and should be supplemented by one that is. Two such sources were provided (and deleted), both of which contain the correct statement of the postulates, which are in accord with the statements by Einstein cited in Special relativity. Examples are Chow and Banerjee. Brews ohare ( talk) 02:37, 30 July 2009 (UTC)
The statement says: "This postulate ... together with the postulate that ..." .There is no way to read this as anything short of a statement of the two postulates of relativity, which they are not. In addition, the first of the true postulates does not assume an inertial frame is understood, it defines an inertial frame. Moreover, various correct statements of the two postulates have been summarily deleted and replaced with this misinformation twice, along with Martin's claims on this Talk page that his so-called "modern" statement of the postulates is more succinct and accurate than Einstein's own version. See Martin's posture here and here, and an explanation that had no impact upon Martin here.
Again, modern examples of correct statements of the postulates are Chow and Banerjee, which are very similar to Einstein's version cited in the WP article Special relativity. Brews ohare ( talk) 07:39, 30 July 2009 (UTC)
As you know, d'Inverno is not available on line. I cannot check the accuracy of your reading of this source, which reading may well have selected a few notions out of context in an ill-fated attempt to be brief. In any event the text as placed in the article is incorrect; the most recent, deleted, correct version, which followed your sentence construction, and is supported by on-line available sources, is hardly longer than your incorrect version, which cannot be supported as it stands. Brews ohare ( talk) 14:55, 30 July 2009 (UTC)
The quotation of a boxed text highlight without the context it refers to has led you astray. However, this argument can be avoided entirely as we really don't need to refer to inertial frames at all. Brews ohare ( talk) 18:00, 30 July 2009 (UTC)
The speed of light has been shown experimentally to be independent of the motion of the source.[14
First, this source is not available on-line, and several that are available were simply deleted.
Second, this lead sentence properly belongs with the last sentence of the section, which treats the subject, and which follows the necessary introduction of this idea via the postulates.
This sentence along with the last sentence of this subsection should be replaced by the deleted subsection noted above. Brews ohare ( talk) 02:41, 30 July 2009 (UTC)
Martin Hogbin ( talk) 09:02, 30 July 2009 (UTC)
Nonresponsive to misplacement of sentence when topic has not been introduced and is not subject of the paragraph it serves as topic sentence for. Subject is brought up again at end of subsection as a dangling sentence. Poor construction. Source not available on line, and supplementary sources that are available deleted. Brews ohare ( talk) 14:49, 30 July 2009 (UTC)
The heading Role of the speed of light in relativity was replaced summarily as Constant speed in inertial frames, with Martin's complimentary comment "Restored sensible version", despite the carefully worded observation that the sub-sub-sections of this sub-section fit better under the deleted heading, being in fact about relativity, not about inertial frames. Apparently relevance to the topic is not sensible. Brews ohare ( talk) 02:45, 30 July 2009 (UTC)
Constant speed in inertial frames is a statement of the experimentally observed facts (and a generally accepted postulate) concerning the subject of the article. I might add that it is similar to the wording in the article when it originally became an FA.
Now that I have explained my actions, perhaps you can explain you recent series of edits Brews. I count 23 of them in the last day.
My suggestion was that we should now discuss changes before making them. I do not think that there are any serious problems with the content of this page. If you disagree let us discuss what you think is wrong before you make substantial changes.
Martin Hogbin ( talk) 09:18, 30 July 2009 (UTC)
It is not an issue of what is acceptable to WP, but of convenience for the reader. To suggest that every reader be prepared to go to a technical library (hardly " a little work") to follow up on sources is impractical. It also is common for editors to misread sources or abridge them without retaining the sense of the source, so citing a source that is not readily checked adds to WP's greatest problem: readers' assessment of credibility. Brews ohare ( talk) 17:52, 30 July 2009 (UTC)
I have not disputed the text of the guidelines. I have made an observation. Please re-read my remarks.
My own experience with technical libraries, both in industry and in universities, is that the book you want is often not present (either lost, on loan, or never acquired (a situation more and more prevalent)) and must be ordered in from a remote source with a delivery time of several weeks. That applies even to common textbooks, never mind specialized sources. Working on line is the most common method of acquiring information both in universities and in industry.
Of course, the nonacademic, or the person not employed by the corporate host, is denied access to the technical library.
I don't know about sources that are "not standard". I'd guess that sources from reputable publishers are frequently a cut above WP, and citing them adds to WP's credibility far beyond the unsupported assertions from WP editors, including their assertions about the contents of these sources. Brews ohare ( talk) 18:30, 30 July 2009 (UTC)
The guideline may be a bit Utopian if it entails that a reader or an editor purchase a source to find out whether it has been accurately represented as supporting a statement in the WP article. (Typically these sources are about $100 apiece.) My view is that if the book is published by, say Cambridge University Press, and it is available in its relevant portions on-line at, say Google books, that is a useful source that could be cited as an aid to the reader, especially where some alternative sources are not available in this manner. Not every Cambridge University Press book is definitive, but I'd probably be more inclined to accept the editor's stated views if a Cambridge University Press title could be seen to support their viewpoint. That is even more the case if I happen to disagree with an editor who resorts only to inaccessible sources in a continuing debate. Brews ohare ( talk) 21:26, 30 July 2009 (UTC)
I am adding this ony as supplement to logicaly enclose my discussion. - - - The calibration of physical SI metre unit implements the number 299792458 as a space/time factor in relation with the speed of light. When performing measurements with SI calibrated devices, all measurements will, according to the precission of the measurement, converge to the implemented value 299792458. The question is, why to perform speed of light measurements using the space/time units independent of the speed of light physical phenomenon ? If the results of independent measurement are expressed in SI units, the speed of light quantity will allways converge to the implemented value 299792458 in SI units, according to the precission of the measurements. Because constant 299792458 is exact numerical value, the uncertainity of this value has changed to the physical uncertainity, that is, uncertainity what is the physical meaning of this value. We can obtain the physical meaning of this value only by calibration of the physical metre/second units. The more precisely we calibrate the units, the more precisely we now the physical meaning of the number 299792458. The measurement of the speed of light is therefore in the context of SI definitions made nonstandardly by calibration of the physical metre/second units, instead of measurement of the numerical quantity of property. But the number itself is not only relation of SI units - saying : "our space/time units are this way exactly related". If c is maximal speed of interaction - propagation of cause - the number itself is quantitative expression of this phenomenon, expressed in SI units. That means, the number itself has substantial significance when implementing apropriate physical devices. If the number itself has substantial significance, the valid calibration of the metre/second units in apropriate physical devices has substantial significance, because without that the number may generate errors, because the meaning of the number 299792458 (exact constant) is fully bound to the physical metre/second units realizations. In this sense the meaning of the number must be physicaly realised, and therefore measured by units calibration. In this sense the speed of light is allways measured when calibratinng the metre/second units. In this sense the meaning of the constant 299792458 is physicaly uncertain. Physical uncertainity is not related to the number itself, but it is related to the precission of the physical realizations of the metre/second units. Physical uncertainity of the constant 299792458 can be quantified in context of individual metre/second units realizations. Therefore the lowest possible uncertainity can be evaluated. Anybody can start to do this considering the ruler and the wristwatch. Softvision ( talk) 13:49, 30 July 2009 (UTC)
There is no need to state both postulates of relativity, so I changed this section to avoid that issue. That requires a title change for the subsection, which no longer mentions inertial frames. Some sources are added and the paragraphs rearranged for better flow. Brews ohare ( talk) 16:53, 30 July 2009 (UTC)
Hi Martin:
Please respond to the points raised instead of reiterating your viewpoint without addressing the objections to it. Brews ohare ( talk) 20:26, 30 July 2009 (UTC)
Joe: Your edit far exceeded your stated goal. Regarding the stated goal, the references provided (and many more general discussions of this work) refer only to the two experiments cited as being decisive. Of course there were earlier experiments that established weaker bounds, or that were later found to be misinterpreted. The source Zhang & Hsu has a long discussion. Brews ohare ( talk) 21:12, 30 July 2009 (UTC).
Is your objection met if it is restated that there are "two experiments primarily cited in this connection"? See, for example, Zhang & Hsu; Baierlein; Will (§2.2); Field, p. 29; etc. etc. Brews ohare ( talk) 21:37, 30 July 2009 (UTC)
Incidentally, I note that you reverted to "This postulate, that the speed of light is the same in all inertial systems, together with the postulate that all inertial observers are equivalent, forms the basis of Einstein's theory of special relativity." which is an incorrect statement of the postulates. I had hoped to diffuse this debate by stating only Einstein's postulate about the behavior of light, which is all that is necessary. For example Banerjee states this postulate as: "The velocity of light in empty space is a constant, independent not only of the direction of propagation but also of the relative velocity between the source of light and the observer". What are you aiming at here?? Brews ohare ( talk) 21:43, 30 July 2009 (UTC)
The only way forward that I can see for this page now and the only way it will ever get back to being an FA is for Brews ohare to stop editing it to make points and to state his own opinions and private research on the subject. I have tried to get him to discuss things before making changes but his response is always the same, a bunch of rapid-fire and ill conceived edits to the article.
I suggest that we need to raise an RfC on his conduct to save this article from becoming Brews' personal soapbox. Martin Hogbin ( talk) 20:28, 30 July 2009 (UTC)
I think a good way to determine which points should be in the article would be to follow the lead of published sources that treat the subject speed of light in detail. Are there any books or other lengthy publications which we could compare to determine whether a particular subject should be covered here, and to what extent? Also, would this method be acceptable to the editors of this article?
Pecos Joe (
talk) 21:17, 30 July 2009 (UTC) edited to clarify at
Pecos Joe (
talk)
21:26, 30 July 2009 (UTC)
I hoped this would have conversation unrelated to the dispute about Zhang, in hopes to address what I think is a more persistent problem - "Should we include stuff about this particular topic, and if so, in how much detail?" I hoped that by following the lead of other publications, we could arrive at well-supported answers for these questions. Maybe I am trying to solve a problem that doesn't really exist? Pecos Joe ( talk) 00:09, 31 July 2009 (UTC)
"This postulate, that the speed of light is the same in all inertial systems, together with the postulate that all inertial observers are equivalent, forms the basis of Einstein's theory of special relativity."
The above statements are an incorrect statement of the postulates, unlike a correct statement that provides a careful definition of "inertial frames". This situation cannot be left like this.
I had hoped to diffuse this debate by stating only Einstein's postulate about the behavior of light, which is all that is necessary. For example Banerjee states this postulate as: "The velocity of light in empty space is a constant, independent not only of the direction of propagation but also of the relative velocity between the source of light and the observer".
The statement of the postulates due to Einstein are:
The first postulate defines inertial frames.
It appears that this will have to go to RfC. Brews ohare ( talk) 22:19, 30 July 2009 (UTC)
Apparently Martin believes the statements of Einstein's postulates by Einstein himself at A Einstein and at §2 On the relativity of lengths and times are a “personal reinterpretation”. An indication of Martin's state of mind, perhaps. Brews ohare ( talk) 14:10, 31 July 2009 (UTC)
It is an interesting form of cooperation to refer to Pecos Joe's attempted compromise as a "mangled version" and take that as an excuse to re-insert exactly the form causing the difficulties. These difficulties have been pointed out repeatedly, with documentation from published sources, including Einstein. Brews ohare ( talk) 15:08, 31 July 2009 (UTC)
An alternative lead is proposed in the section located here. Brews ohare ( talk) 15:18, 31 July 2009 (UTC)
The speed of light is postulated to be constant in all directions, regardless of motion. According to this postulate, the SI speed of light constant is postulated to be exact numerical value. According to this, noone can talk about speed of light as comparable numerical quantity, in context of speed of light measurement. Therefore the concept "measurement of the speed of light quantity" is obsolete. According to current postulates, comparable quantities are physical metre and second units. If you are talking about comparsion of the speed of light quantity in different directions, you are talking about physical comparsion of the physical metre/second units calibrated in different directions. If space is physicaly contracted in the direction of motion, and time is proportionaly dilated, calibration of the physical metre/second units in different directions, and their physical comparsion, must lead to conformity of physical metre/second units, confirming the uniformity of the speed of light in apropriate directions. It is hard to imagine, how to turn the time device carrying the physical second unit. Time units calibrations in different directions must be compared simultaneously. But comparing the time units simultaneously, we must compare the space units simultaneously, to perform valid comparsion. If difference in physical space units is proportional to the difference in physical time units, the speed of light is uniform in appropriate directions, as measured in SI units. (...) This is not my observation, or my own research. This is consequence of current physical postulates. I am submiting this to help the editors to solve the article formulation. Softvision ( talk) 14:23, 31 July 2009 (UTC)
Proposed lead-in:
It is not possible to measure the one-way speed of light (for example from a source to a distant detector) without some convention as to how clocks at the source and detector should be synchronized. [1] Einstein postulated both that the speed of light should be taken as constant in all cases, one-way and two-way, and that the speed of light was independent of the motion of its source. [2]
- ^ Zhang, Yuan Zhong (1997). "Special Relativity and Its Experimental Foundations". Advanced series on theoretical physical science. 4. World Scientific: 31, 171, 173. ISBN 9810227493.
- ^ See the article Special relativity and, for example, Sriranjan Banerji, Banerji & Banerjee (2004). "Postulates of the special theory of relativity and their consequences". The Special Theory of Relativity. PHI Learning. p. 20. ISBN 812031963X.
Brews ohare ( talk) 14:34, 31 July 2009 (UTC)
Rationale:
The goal is first to introduce the two topics of two-way light speed, and of motion of the source, and in follow-on discussion describe the experimental supporting data. This order is better than the reverse order in providing orientation as to the point of the entire subsection.
The above statement of Einstein's postulates concerning these matters is accurate, and does not go into undue detail. The footnotes provide opportunity for the reader to pursue matters further. The Banerjee source is on-line at Google books, and states the second postulate in a form very suitable for the purpose of this subsection. Brews ohare ( talk) 14:34, 31 July 2009 (UTC)
Experimental evidence concerting the two-way speed of light would appear in the following paragraph. I find the present lead two sentences fit each other.
The objections to the current order are (i) no basis is laid for introducing the selected experimental data from the (possibly) myriads of things that could be discussed. The revised order introduces the subject of Einstein's postulates about light and the experiment then fits as substantiation. (ii) the present order tends to force one into an unduly elaborate statement of the postulates, which so far has proved a stumbling block. The revised order allows introduction only of what Einstein said about the speed of light, which is all that is germane. Brews ohare ( talk) 16:05, 31 July 2009 (UTC)
Martin: (i)The experimental facts: There is no argument here about the experimental facts. The point is that stating the Einstein assumptions about light speed automatically raises the question in the reader as to whether they are true. Therefore this sentence order placing the Einstein assumptions first is natural, while the reverse order is less natural. Brews ohare ( talk) 04:37, 1 August 2009 (UTC)
Martin: (ii) "I do not understand what you mean by elaborate statements of the postulates." I can see very well that you have no understanding of this point, and after a week of unending effort on my part, I am convinced that you never will. That does not mean that you are right, it means just that you can understand only your own view. My most recent attempt is this subsection. An earlier attempt is this subsection. Brews ohare ( talk) 04:37, 1 August 2009 (UTC)
Martin: Of course, the answer to this is this subsection. To summarize: the present text in the WP article appears to claim to state the two postulates of the special theory. However, (i) that is not the case, and (ii) it is not necessary to undertake some (inadequate) summary of the two postulates; a simple statement of the assumptions concerning speed of light is all that is needed.
The proposed lead above provides a succinct statement of these assumptions appropriate for this section, expressed in a similar fashion by, for example, Banerjee, as cited in the lead. See the proposed lead here. Brews ohare ( talk) 09:46, 1 August 2009 (UTC)
The first postulate of relativity, as explained carefully above, and in greater detail in inertial frames establishes what an inertial frame is. The statement you've attributed to d'Inverno assumes this definition is understood from elsewhere (possibly the main text, instead of the boxed highlight you chose to quote). Clearly, omission of this definition is an emasculation of the postulates that do indeed include this definition. It is, therefore, incorrect to parade the emasculated version as the postulates of relativity. However, of greater interest, we do not need any of this for this discussion, as is evident form the proposed lead. Brews ohare ( talk) 11:18, 1 August 2009 (UTC)
An inertial frame does enter the postulates of relativity, and could be included by making a full statement of the postulates as originally proposed a long, long time ago and reverted by you. That is what Banerjee does. However, for the purposes of introducing the experimental work, that does not seem necessary, especially as the experimental work does not take place in an inertial frame, and makes little or no reference to it. Brews ohare ( talk) 11:18, 1 August 2009 (UTC)
Martin: You are not engaging here. The postulates themselves define what an inertial frame is. Your attribution to d'Inverno does not. Therefore, your attribution to d'Inverno is not equivalent to the postulates. OK? Brews ohare ( talk) 11:51, 1 August 2009 (UTC)
No Martin. There is nothing idiosyncratic about a literal quotation from multiple sources including Einstein. What is a bit odd is that the simple logic that A includes B and C does not include B; therefore A and C are not the same, eludes you. Brews ohare ( talk) 12:13, 1 August 2009 (UTC)
Let me elaborate: A=Einstein's Postulates; B=Definition of inertial frame; C =Your attribution to d'Inverno. A includes B, C does not include B so A is not C and C is not A. Brews ohare ( talk) 15:35, 1 August 2009 (UTC)
Which still does not address the point that introduction of "inertial frames" is just distracting in terms of introducing the experiments about two-way speed of light and independence from motion of the source. The experimental papers barely mention if they do at all, the role of "inertial frames", which is not a major point to be made in this WP subsection. Avoidance also allows deletion of the present incorrect WP text that suggests the idea of "inertial frames" is a matter outside the original postulates. It is not. Brews ohare ( talk) 17:23, 1 August 2009 (UTC)
Martin: It is not the issue of 'inertial frames' being inside or outside the postulates. The issues are (i) the WP phrasing of the postulates is incorrect and (ii) for purposes of framing the role of the experiments, we don't need to bring up inertial frames.
The purpose here is to introduce the experimental work, and sufficient understanding of the role of the experiments can be gained without inertial frames. If you insist that inertial frames are needed, then use the real statement of the two postulates instead of a bastardization that misrepresents them. Brews ohare ( talk) 03:50, 2 August 2009 (UTC)
If you insist that inertial frames are needed, then use the real statement of the two postulates instead of a bastardization. Brews ohare ( talk) 03:50, 2 August 2009 (UTC)
The simplest way to handle this is to leave inertial frames out of this paragraph altogether. The more precise approach is to put in both postulates (correctly, not in some elliptic version), for example as done by Banerjee. I'd call this approach pedantic for the purpose of this section, and suggest the role of inertial frames is better discussed under other sections. 22:45, 2 August 2009 (UTC)
I am OK with the present version. Brews ohare ( talk) 00:47, 3 August 2009 (UTC)
I happened to notice this from the comments section above, I moved it here so it would eventually be archived with the rest of this talk page: "Francis Bacon argued that the speed of light was not necessarily infinite..." No. That was Roger Bacon. However I hesitate to edit a page that is the subject of so much heated discussion about the scientific method, philosophy of science, and definitions of simple words, merely to correct a fact. —Preceding unsigned comment added by 67.9.148.203 ( talk) 13:36, 29 July 2009 (UTC)
Possibly this passage can be construed as saying the speed of light is not infinite? "The difference between light, sound and odour can be expressed in another way. Light travels far more quickly in air than do the other two; thus when somebody far away strikes a blow with a mallet or stick, we see the blow before we hear the sound it generates...Therefore when Aristotle claims that there is a difference between light and the other sensibles, this should be understood as a difference not between an instant and time, but between less time and more time..." Lindberg Brews ohare ( talk) 10:33, 1 August 2009 (UTC) I've reinstated a remark here about Roger Bacon with this source. Brews ohare ( talk) 11:00, 1 August 2009 (UTC)
Dicklyon: The verbatim text indicates that Bacon thought the speed of light was finite in air, and that he based that view on analogy between the propagation of light and sound. One of the two examples he used is exactly the one stated in the text you reverted. Yes, it is illogical. No, it is not different from his statements. Yes your explanation for your reverts is annoying and unjustifiable, as well as non-factual. Brews ohare ( talk) 14:45, 2 August 2009 (UTC)
I notice that, without discussion, Brews has decided to rewrite and reorganise the lead section. There is a change of order and emphasis to make the fundamental spacetime aspect come first. This is an major change to the article for which it would have been a good idea to seek a consensus first.
As it happens, I do not disagree with this change of emphasis, but if we are going to do it it needs careful thought and needs to be written very carefully. This is not the case at present. Brews has also taken the opportunity to insert the chatty line, 'Said in different words, the exact value, c = 299 792 458 m/s, is an international convention concerning the metre, not a property of light', that he could not get in elsewhere, into the lead. This point is fully discussed in the relevant section. Martin Hogbin ( talk) 10:30, 1 August 2009 (UTC)
There is nothing "verbatim" about this, it's just an English sentence, and it fits here perfectly; as it says, it is a restatement of the preceding sentence in words that emphasize the meaning of the preceding sentence, which requires more emphasis. You seemingly prefer the meaning of the previous sentence to go unnoticed. Brews ohare ( talk)
I prefer to make the point that is being made where it is introduced, and where the reader will find it most appropriate. That takes two sentences, it seems... Brews ohare ( talk) 11:32, 1 August 2009 (UTC)
"We" are not taking that view. It is preposterous to say a one line restatement for emphasis is an exhaustive discussion. Brews ohare ( talk) 12:00, 1 August 2009 (UTC)
The sentences in question are "In SI units, the magnitude of the speed of light in vacuum is exactly 299,792,458 metres per second (m/s)[6] because of the way the metre is defined. Said in different words, the exact value, c = 299 792 458 m/s, is an international convention concerning the metre, not a property of light.[7]". I don't like having two sentences, it's redundant and unhelpful. The way it's written makes it come across a deep and important and complicated insight, when it's really a very simple point. I suggest instead:
In SI units, the magnitude of the speed of light in vacuum is exactly 299,792,458 metres per second (m/s), [1] because of the way the metre is defined. [2]
- ^ "Fundamental Physical Constants: Speed of light in vacuum; c, c0". physics.nist.gov.
- ^ The metre is defined as the distance that light travels in 1/299792458 seconds. The second is defined independently, via an atomic transition frequency. See below for more information.
OK for me to make that change? -- Steve ( talk) 13:17, 1 August 2009 (UTC)
If we are talking about the quantity c = 299 792 458 m/s, we are talking about the property of reality expressed in SI units. Because we know what is second and what is metre, according to the approximate human perception space/time uncertainities, the property c is well explained by this quantity. Both, property c and quantity 299 792 458 m/s, can be considered as real, because our senses are real and we have real perception of the metre and second units, limited by the approximate human space/time perception uncertainities. Theese uncertainities of the speed of light quantity perception can be minimized by expressing the speed of light in following context : Around Earth's equator 0.13 seconds. This is very well done at the begining of the atricle. In the era of intercontinental flights, this is the best expression of the speed of light quantity, in relation to the human perception. The numbers 299792458 and 9192631700 itself, are the international agreement, that interconnects the Caesium 133 etalon with historical metre/second units, not to invalidate them, and to extend their precission.
Quantity 299792458 m/s is the speed of light physical phenomenon in vacuum expressed in SI units. Nothing less, nothing more. We know what is speed, we know what is light, we know what is metre, we know what is second. The perception of these realities is very good and intuitive, even without exact technical SI definitions. It could be good to stress the "Around Earth's equator 0.13 seconds" - click time scale - context. Afterwards, the fact, that the current speed of light constant is synthetic constant (exact) should be mentioned, and the subject of the interconnection of the SI metre unit with the speed of light physical phenomenon should be explained, including the explanation of the meaning of the word "exact". Therefore the motive for this interconnection should be explained, probably before the metre interconnection subject details. In context of this, the consequences of this interconnection should be explained, including positive and negative consequences. I have mentioned some of the negative consequences in my previous submissions. I think this is the way how to put everything together. Softvision ( talk) 14:34, 1 August 2009 (UTC)
The speed of light in vacuum in SI units has an exact value: 299,792,458 metres per second (m/s). In fact, the metre is officially defined as the distance light travels in vacuum in exactly 1/299,792,458 seconds.
Steve: You say I hope we can get the point across without saying anything too extreme, for example that the number 299,792,458 is purely 100% arbitrary convention and nothing else. I am more concerned that the point that comes across is that the number 299,792,458 is (from a strictly scientific stance) something more than 100% arbitrary convention. Of course, all conventions have their social context, and do not exist in limbo created from thin air. But the hazard here is not that; the hazard is that people will read that the speed of light is exactly 299,792,458 m/s and say "What? No experimental error? What, no need for future measurement? Are these guys nuts? " Maybe more significantly, they may say: "Look at that; one more example of how whacked out WP is - they even allow stuff like this to go unchallenged."
The way to cope with this reaction is to make clear at the outset that the reason an exact value is possible, and that no measurement is necessary is precisely because this number is not the physical speed of light, it's just a conversion factor. As Jespersen says: its value is a matter of convention, its value is entirely arbitrary. Once the reader understands that we are not dealing here with a physical property of light, the amazement over how it can be exact and independent of measurement goes away. Then the question arises: "Why this particular value?" and that discussion is the one you have presented above. Your discussion cannot be absorbed until the "conversion factor" idea has taken hold. Brews ohare ( talk) 03:59, 2 August 2009 (UTC)
In this connection, a discussion by David Mermin about the "foot" and the "phoot" (the distance light travels in 1 ns) might be fun to read and lead to some good ideas on this subject. Brews ohare ( talk) 04:24, 2 August 2009 (UTC)
Softvision, this is just an example of the very kind of thing that I was talking about. Nobody ever analyses the speed of sound in such philosophical depth, in connection with the system of units. This is all a product of the fact that by removing the medium of propagation of light from the textbooks, they have removed all rational physical basis for the speed of light. The end result is the endless philosophical arguments that we can see above, while everybody tries in vain to make sense out of nonsense. David Tombe ( talk) 15:08, 1 August 2009 (UTC)
Two items are not addressed here by you all:
(i) I've explained a reader's astonished reaction to reading the speed is exact, no measurement necessary, no error bars, because the reader takes the numerical value to refer to the physical speed of light, which seems intuitively to be a measurable quantity. A number of editors here share this view that it is measurable, or at least have not fully abandoned it. If you read the sentence in the lead, it is technically correct, but will not avoid this reaction or dispel the misconception.
(ii) The treatment of c as a conversion factor is beyond dispute, and is the only viewpoint from which an exact value seems reasonable: its exact because we could define it to be any value whatsoever were it not for the widespread use of the foot rather than the phoot (to use David Mermin's example). Steve has said above that defining the second in terms of a particular atomic transition (under a variety of conditions like 0 K, zero gravity, zero linewidth etc. that cannot be realized) is exact, but it is not a conversion factor. Of course, that is not the case. Instead of 1/9192631770 s, this transition could equally be chosen to be 100 ps or 100 s. The physical entity atomic transition must be kept separate from the unit second. The fraction 1/9192631770 s is a conversion factor from the physical standard to units of seconds, just like converting the speed of light to 299,792,458 m/s could equally be 1 ft/ns.
The resistance to these facts, and the desire to hide behind a form of words that is technically correct, but not terribly obvious to the non-legal mind, indicates an unwillingness to accept the real status of 299,792,458 m/s and a continuing belief that 299,792,458 m/s is somehow a real, physical, constant of nature. It is a failure to separate 299,792,458 m/s from the physical entity speed of light. The resulting lack of clarity in the WP article will cause a reader reaction that the lead is paradoxical and the article is not authoritative. Brews ohare ( talk) 13:44, 2 August 2009 (UTC)
Martin: Your comment doesn't address one key point: The WP single-sentence treatment will cause a reader reaction that the lead is paradoxical and the article is not authoritative. That is the natural reaction to reading that the speed of light in vacuum is exactly 299,792,458 metres per second, which is exactly the reason you italicized "exactly" - your stated purpose in doing that is to produce that reaction. Brews ohare ( talk) 16:02, 2 August 2009 (UTC)
If this sentence is so clear, why the insistence on italics with: 16:48, 26 July 2009 Martin Hogbin (The italics are in accordance with WP guidelines to add emphasis.); and earlier 18:29, 21 July 2009 Martin Hogbin (Restore emphasis); and the first insertion of these italics according to: 16:32, 28 July 2009 Martin Hogbin (Surprising fact (to many people) needs emphasis.)?
It appears very clear that you, Martin, wish to startle the reader with this statement, rather than explain why this statement should be only a ho hum minor observation as Steve suggests it to be. Brews ohare ( talk) 20:19, 2 August 2009 (UTC)
The revised version is OK with me, so I guess I can graciously ignore assault and hair-splitting. Brews ohare ( talk) 00:24, 3 August 2009 (UTC)
I'm still trying to focus on what is the real underlying cause for this ongoing argument. I had a look at the article entitled speed of sound for ideas as to how an introduction might look. The first sentence in the speed of sound article reveals the entire problem as regards the 'speed of light' article. The speed of sound article begins by explaining the rational physical basis behind sound waves. In modern texbooks, no such rational physical basis exists to explain light waves. This is of course a perfect recipe for endless argument. The ongoing arguments here must surely be about the differing points of view and opinions that exist amongst the different editors as regards trying to make sense out of something that does not make sense.
That's why we are seeing the sword fight straying far away from the original starting point. It has now entered into the terrain of the definition of the metre which is hardly in anyway connected to the actual speed of light. I think that in order to bring this dispute to a swift conclusion, all parties need to clearly state in simple terms what they would like to add to the introduction and why, and what they would like removed from the introduction and why. There also needs to be a more explicit statement of the individual points of view in order to avoid all the tip-toeing around the core issues. David Tombe ( talk) 14:28, 1 August 2009 (UTC)
Softvision, this is just an example of the very kind of thing that I was talking about. Nobody ever analyses the speed of sound in such philosophical depth, in connection with the system of units. This is all a product of the fact that by removing the medium of propagation of light from the textbooks, they have removed all rational physical basis for the speed of light. The end result is the endless philosophical arguments that we can see above, while everybody tries in vain to make sense out of nonsense. David Tombe ( talk) 15:08, 1 August 2009 (UTC)
OK, I'll move my reply up to join it. David Tombe ( talk) 10:48, 2 August 2009 (UTC)
Martin, I'm merely trying to ascertain the underlying cause of the argument between yourself and Brews. Brews has suggested that it is to do with the fact that you see the speed of light as being something in the realms of a platonic constant like π. Would you agree with that suggestion? David Tombe ( talk) 10:46, 2 August 2009 (UTC)
Martin, I've made a suggestion on Brew's talk page that he moves matters relating to the first line of the introduction (regarding 'c' being a physical constant) to further down. The idea of this is to maintain a historical chronology in the introduction and to separate the issues of dispute. David Tombe ( talk) 18:29, 2 August 2009 (UTC)
Much is being made in the article (in the lead the word exactly is stated thus) and in this talk page of the exactness of the speed of light (SoL). It seems to me that this has been overdone and indeed may even be incorrect. The SoL is only exact because we have defined it in reference to the metre and the second. The second has been defined separately but the metre has not, it owes its definition to the relationship ("conversion factor") with the SoL and the second. This was not always so; the metre used to be defined, by the French Academy of Sciences, as the length between two marks on a platinum-iridium bar, which was designed to represent 1⁄10,000,000 of the distance from the equator to the north pole through Paris. It was only defined in relation to the SoL in 1983. I find it difficult to accept that this redefinition made the speed of light accurate when it clearly wasn't before 1983. Where did I go wrong? Abtract ( talk) 17:09, 2 August 2009 (UTC)
Martin, you removed some edits of mine in the historical section on the grounds that Maxwell's molecular vortex theory has no role in modern physics.
It was of course the history section. Secondly, the way that you have left it is now totally inaccurate. You have got the contents of Maxwell's 1861 paper totally mixed up with the contents of his 1865 paper.
I assume that you overlooked the fact that these edits were in the history section and so I will restore them. David Tombe ( talk) 22:37, 2 August 2009 (UTC)
The current lead has too many problems to be left as it is. Let me start with the first sentence:The term speed of light generally refers to a fundamental physical constant of spacetime that limits the rate of transfer of matter or information.
The first problem is that it would appear from this construction that the only effect of this fundamental constant is to limit the speed at which matter or information can travel. (I know that it is possible to derive SR from this kind of premise but I do not think that is particularly relevant here). The implications of c as a fundamental constant of spacetime are much wider that indicated by this statement. It is better to show this speed limit as a result of the spacetime in which we live
More importantly limits the rate of transfer of matter or information is ambiguous. The rate of transfer of information could refer to bytes per second for example. Better to say something along the lines of maximum speed at which the can travel. Martin Hogbin ( talk) 15:58, 2 August 2009 (UTC)
The basic point of c is the spacetime connection, given prominence in the lead, which is fundamentally associated with information transfer, as that sets up the clocks. Perhaps the information transfer needs a separate sentence to get it more correctly stated. The EM connection is the next line, which hardly is burying this aspect. However, it is more or less accidental that EM radiation travels at c - many formulations of relativity simply posit a maximum speed of transfer, as Martin points out. (In fact, c does not actually refer to an extant physical speed of light in any realizable medium; it is fundamentally a theoretical construct.) This is a key point, and not at all one that is "not particularly relevant". Brews ohare ( talk) 16:20, 2 August 2009 (UTC)
I'm not sure what "this way" refers to. Every source agrees there is a limiting speed. Most take it as the "speed of light" but do not examine what that phrase actually means: light is medium for the transfer of information the maximum speed of interaction ... is equal to the speed of light By and large the view is that the Lorentz invariance of the Maxwell equations in classical vacuum carries over to all of physics, and implies a limiting speed of information transfer. The inverse procedure is more fundamental in that Maxwell's equations are just one "law of nature" and so are subsumed under "the laws of nature" in the postulates of relativity. In short, historically the Maxwell symmetry "discovers" the Lorentz transformation, but then the Lorentz invariance is posited, and so subsumes the Maxwell relations as a special case. Brews ohare ( talk) 16:43, 2 August 2009 (UTC)
Looks OK to me. Brews ohare ( talk) 21:45, 2 August 2009 (UTC)
Sorry, I was trying to consolidate it with the same thing said in the second paragraph. You did it better. Brews ohare ( talk) 04:55, 3 August 2009 (UTC)
We have a mini edit war going on the appropriateness of talking about Maxwell's "molecular vortices." I think this needs to be resolved in terms of what's in secondary sources. Providing an interpretation of, and assessment of the important of, of Maxwell's original primary writings, in the history section, is inappropriate per WP:NOR. If we find such interpretion and assessment in secondary sources that connect to the topic of the speed of light, then maybe it's OK. So, David, please cite a source if you want this material in. Dicklyon ( talk) 23:12, 2 August 2009 (UTC)
I don't think it is our place to determine whether molecular vortices played a significant part in the history. Obviously it was important at the time of Maxwell, and may have played an important role in how thought evolved. History of science is not just about what survives in today's textbooks of physics. It also is about the dynamics of the evolution of thought and the context in which it arose. For example, it seems the molecular vortices were the underlying intuition behind the curl operation. See Siegel and for vortices in fluid mechanics see Durand-Vidal et al. . Brews ohare ( talk) 13:47, 3 August 2009 (UTC)
This has all been a bit of a storm in a tea cup over what was just a correction edit in the history section. But the subject matter of that correction has clearly triggered off sensitivities in relation to the ongoing dispute regarding the modern day quagmire. Within the history section, there is nothing at all controversial about mentioning the point in history when Maxwell first discovered the connection between the speed of light and the electric and magnetic constants. And there is nothing wrong with giving a brief description as to how he did it. That brief description has been made, fully in line with primary, secondary, and tertiary sources. We have plenty of sources, so what exactly is the problem? Is anybody challenging the accuracy of the description of how Maxwell arrived at his conclusion?
My guess is that the controversy over this historical edit is based on the contrast between the logical approach that Maxwell used in the 19th century as compared to the total nonsense situation that we have reached in the 21st century. I opted to keep out of the dispute in the modern sections of the article because I have nothing useful to contribute to those sections. But while I was watching the dispute, I decided to correct some fundamental errors that I noticed in the historical section, and that seems to have ruffled some feathers. Martin has revealed his total disdain for Maxwell's approach, and so it seems that he would prefer to have Maxwell's approach reported inaccurately, rather than accurately.
If Maxwell's approach was the joke that Martin is suggesting that it was, then it should hardly be of any concern to Martin that it is described down in the history section. But I suspect that the real fear surrounding any mention of Maxwell's approach is that his old fashioned classical values put the modern nonsense to shame, and as such it must either be distorted or brushed out of view. David Tombe ( talk) 17:18, 3 August 2009 (UTC)
Pecos, I examined the situation very carefully before making the correction. The original paragraph was totally confused and it needed to be re-worded. The first thing that I saw wrong was the use of the term 'electromagnetic field' in conjunction with Maxwell's 1861 paper. That is clearly wrong terminology in the context. So what term should we use in its place? We cannot use the term 'aether' because Maxwell clearly states that he sees his sea of molecular vortices to be partly aether and partly ordinary matter. So why not use the exact term that Maxwell used. Maxwell's concept was a sea of molecular vortices, so why not call a spade a spade? We cannot have a situation in which we have to play down a historical term in a historical context in a historical section simply because one editor holds such a prejudice against Maxwell's work. Martin deleted my corrections twice, and both times the grounds stated were specious. The first time, he declared that molecular vortices have no place in modern physics. Nobody was claiming that they do. The second time, he declared that he was removing crackpot physics. It is not for him to decide that Maxwell's work is crackpot physics and then to proceed to eliminate an accurate description of it from the history section and replace it with a distorted version.
I'm glad that you agree that my correction was valid, and I can see that you are looking for a compromise. I also tried to consider a compromise, knowing that the very words 'molecular vortices' seem to severely aggravate Martin. But I simply can't think of what other term to use for Maxwell's sea of molecular vortices, and on reflection, I don't see why we should be bothering to pander to these kind of prejudices. David Tombe ( talk) 21:30, 3 August 2009 (UTC)
There is a line in the introduction that reads,
The speed of light can be viewed as simply a conversion factor between space and time in spacetime
Can anybody elaborate on the meaning of this sentence? Having already stated that the speed of light is a speed, and that it is the speed at which EM radiation moves, then how can the speed of light suddenly become a conversion factor? It is not sufficient to refer to the 1983 definition of the metre in terms of the speed of light. The statement in question is much more profound than anything to do with systems of units. It needs some kind of elaboration. By the same token, could we say that the speed of sound is a conversion factor between air and time?
As regards the 1983 definition of the metre in terms of the speed of light, it is somewhat tautologous. It neither tells us what a metre is nor what the speed of light is. I would blame this entire edit war on the person that introduced that definition of the metre in 1983. They were clearly too fascinated with their belief that the speed of light is a universal constant to be able to think rationally about the underlying purpose of what they were doing. The end result has been to sew additional confusion into an already badly confused scientific community.
On reading the entries above, perhaps this is what editor Abtract was also driving at. David Tombe ( talk) 09:25, 3 August 2009 (UTC)
David, you complain "It neither tells us what a metre is nor what the speed of light is." But that's the point, sort of. The meter and the speed of light are not independent of each other; the speed of light is a physical constant that anyone can measure, and the metre is an arbitrarily defined distance defined in terms of it; this definition ended the redundancy of having separate standards for time and distance, when the standard for the second and the physical constant c are enough. The definition recoginzes that there are just two degrees of freedom: you choice of time unit and you choice of conversion factor. What other way would tell you more about what a meter is, or what c is? For practical purposes, to make you comfortable, take the meter as the length of bar in a vault, and then you'll know pretty well what the measurements of c come out to. Dicklyon ( talk) 14:52, 3 August 2009 (UTC)
Dick, The 1983 definition is a total abomination. The man who concocted this definition chose a time fraction based on the most up to date speed of light as measured using classical values in line with the normally understood concept of speed. He then turned the whole thing upside down and defined the metre as the distance that light travels in this fraction of time. This had the effect of fixing the value of something whose value could yet be subjected to more refined meausurements. It is a fraud.
And why did he do it? In 1983, we were already languishing in the doldrums of a relativistic era in which the speed of light was supposed to be a universal constant. This man obviously wanted to impress the high priests of relativity. The relativistic priests probably indicated that they would like to see the speed of light being elevated to the sainthood, and this was his opportunity to oblige. The 1983 definition is tantamount to a standing proclamation of 'Amen!' to relativity. It firmly locked the door to any future variations in the speed of light using an elaborate conjuring trick.
I can detect that Brews knows that something needs explaining here. Although he claims to be comfortable enough with the 1983 definition, he can still see that the actual speed of light is something different than what this definition is implying. He can see that this definition is about something that has to be constant by definition, yet light will still nevertheless move in conjunction with a traditional style speed that might be subject to ongoing revisions and perfections of its value.
Maybe I've picked it up wrongly, but is Brews pointing to the fact that the 1983 definition allows a divergence between the classical speed of light and " Saint Speed of Light" as per this definition? And are his opponents trying to argue that no such dichotomy exists? Have I got the basis of the dispute correct? David Tombe ( talk) 18:35, 3 August 2009 (UTC)
I have been involved in this page for only a few days but I am sorry to say I must now leave it ... there is a major problem - Brews ohare. I have looked back over the edit history on this page and the article and it is clear that he is exactly the type of editor that has caused me much anguish in the past. I suggest that those of you who know the position more intimately than I do refer him for comment with a view to a topic ban; that's the only way progress will be made imho. Good luck. Abtract ( talk) 10:58, 3 August 2009 (UTC)
Some of the physics was settled years ago. However, as quantum gravity, doubly special relativity and cosmology show, there is ongoing activity, even about the basics. Brews ohare ( talk) 13:39, 3 August 2009 (UTC) And the role of c as a defined standard of comparison for speed contrasted with its role in the metric of spacetime may be understood by some, but is hard to explain nonetheless. Brews ohare ( talk) 13:42, 3 August 2009 (UTC)
The issue is really communication. Dicklyon and Martin share the trait they they are always right about (i) their views, and (ii) about what is appropriate in a WP article vis-à-vis level of presentation and topics. These two editors simply know no response to resistance to their "authority" other than reversion and reiteration of their personal standards in the face of sources beyond dispute. Brews ohare ( talk) 15:29, 3 August 2009 (UTC)
The accepted equation for the dispersion of light traveling though a plasma medium is ω2 = ωp2 + c2k2, which leads to a group velocity less than c and a phase velocity greater than c. I thought someone might be interested in knowing this, because it not even alluded to in the section on superluminal light. —Preceding unsigned comment added by Odellus ( talk • contribs) 22:55, 5 August 2009 (UTC)
Should the notion that one can measure a defined property be deleted from speed of light? Presently stated in the article speed of light:
"According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. This has been verified to a high degree of accuracy by experiment."
Comment: It is a defined property of free space that its electromagnetic properties εo and μo are frequency-independent. To claim that this defined lack of frequency dependence is experimentally verifiable is a logical error, as one cannot (in principle) measure a defined property: its value is, after all, what the definition says it is. All that experiment can do is confirm whether some realizable medium, like outer space say, has this property. Such confirmation serves to support the notion that "free space" is a useful model, but it cannot change the model. It can only support its utility. These statements in the article should be replaced with something like:
"According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. This behavior has been verified by experiment to a high degree of accuracy for media such as outer space or ultra-high vacuum, showing that in this respect these media are good approximations to free space."
The present statement should be replaced. Brews ohare ( talk) 20:41, 15 July 2009 (UTC)
Comments
My original statement was: 'According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. This has been verified to a high degree of accuracy by experiment'.
Two of the quoted sources are entitled: 'Severe limits on variations of the speed of light with frequency' and 'Probing the Speed of Light with Radio Waves at Extremely Low Frequencies'.
I am not sure how it could be any simpler. Theory claims that the speed of light is independent of frequency and experiment verifies this. Is there a physicist in the house???? Martin Hogbin ( talk) 23:34, 15 July 2009 (UTC)
I hesitate to enter into "define". However, NIST does post c0 as a unique, exact number for "vacuum". I take "vacuum" as free space. Thus "vacuum" exhibits no dispersion. Moreover c0 is exact, so there is no doubt that it is not a measured value. Do you agree?
In addition, NIST posts this remark: "The effect of this definition [of the meter] is to fix the speed of light in vacuum at exactly 299 792 458 m/s." The notion of a definition being able to fix the speed of light appears to rule out measurement, and a value independent of frequency rules out dispersion. Brews ohare ( talk) 14:30, 16 July 2009 (UTC)
Brews, I don't know how I could make it any more clear, so I'll just repeat myself: "If NIST defines a unit U as a quantity Q, then one cannot automatically infer that the NIST believes that Q is an absolute constant." Do you agree with the previous sentence? Yes or no? I say yes, with the kilogram as proof. What about you? -- Steve ( talk) 17:01, 16 July 2009 (UTC)
Steve: There is no analogy here. The weight of the object may change. BIPM is aware of that. Work is ongoing to replace the specimen with a better standard. There is no specification of light frequency in the BIPM c0; do you think that is simply an oversight? It is an exact number for all frequencies. It took a resolution to adopt this number: why do you think it might change without need for another such resolution? Brews ohare ( talk) 21:01, 16 July 2009 (UTC)
OK, where are we? BIPM "fixes" an equality between the kg and the mass of the IPK. BIPM "fixes" an equality between the meter and the distance light travels in vacuum in 1/299792458 s. BIPM is not implying that the mass of the IPK is fixed. BIPM, according to you, is implying with 100% certainty that the distance light travels in vacuum in 1/299792458 s is fixed. How do you know? First you said something obvious and irrelevant: The number 299792458 will not change. Yes, this is true and irrelevant. Next, you said that BIPM explicitly says that they're aware that the mass of the IPK is not fixed, and didn't give a similar warning about the meter. OK, fair enough. So to summarize, you believe that NIST/BIPM makes it 100% clear and unambiguous that the distance light travels in vacuum in 1/299792458 s is an absolute constant independent of frequency, date, direction, etc., and you believe this because of the absence of an explicit warning to the contrary. Is that really enough to base your entire argument on? Is that really the basis for 100% certainty? Or can we agree that NIST/BIPM don't quite make it 100% clear whether vacuum dispersion is possible in principle? --
Steve (
talk)
23:13, 16 July 2009 (UTC)
Although a clear definition of free space would aid in this discussion, I propose that the aforementioned change be made anyway. Even if free space is not in fact defined as possessing the property that all electromagnetic radiation travels at the same speed, the proposed change,
"According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. This behavior has been verified by experiment to a high degree of accuracy for media such as outer space or ultra-high vacuum, showing that in this respect these media are good approximations to free space."
is still a more accurate, and less ambiguous statement. SemitoneSonata ( talk) 15:10, 22 July 2009 (UTC)
Perhaps the following statement would be sufficiently accurate, without depending on too many other definitions. It should also be more accessible and easy to understand.
"According to the classical theory of electromagnetism, the speed of electromagnetic radiation in the absence of matter is the same for all frequencies. This has been experimentally confirmed to a high degree of accuracy in outer space and ultra-high vacuum."
This dispenses with the task of defining free space, and implies only (rather obviously) that outer space and ultra-high vacuum are good approximations to the absence of matter. Fizyxnrd ( talk) 13:56, 5 August 2009 (UTC)
"According to the classical theory of electromagnetism, the speed of electromagnetic radiation in the absence of matter is the same for all frequencies. This has been confirmed to a high degree of accuracy by experiments in outer space and ultra-high vacuum."
"According to the classical theory of electromagnetism, the speed of electromagnetic radiation in the absence of matter is the same for all frequencies. This has been experimentally confirmed to a high degree of accuracy in outer space and ultra-high vacuum."
Martin, it seems that you are not prepared to discuss this issue. You have now reverted my correction three times, leaving a short bogus excuse on each occasion, and without coming to the talk page to explain yourself. You have taken it upon yourself to deem Maxwell's work in 1861 to be 'crackpot physics'. We have a history section which touches on the subject of the aether ideas of the 19th century. That of course necessarily touches upon the very important work that Maxwell did in 1861. He linked the speed of light to the electric and magnetic constants. He did this by modelling Faraday's lines of force in terms of a sea of molecular vortices.
As the section now stands subsequent to your reversions, it contains gross inaccuracies. How do you propose that we fix those inaccuracies? Is there any particular aspect of my corrections in particular which you find to be inaccurate? Or is it the use of the term 'sea of molecular vortices' that you object to? What name would you propose to use instead?
You cannot simply impose distortions into the history section just because it is an aspect of history which you clearly don't like to be reminded about. Your actions are totally contrary to wikipedia's rules. You are censoring history in order to favour your own prejudices about the nature of the speed of light.
I've noticed that you have just removed the link to the 1861 paper claiming it to be irrelevant in the context of the 1861 paper, yet you have left the link to the 1865 paper which is not being discussed. You are clearly messing this whole paragraph up deliberately. It is clear that you hold Maxwell's 1861 paper in total contempt, and you are simply trashing the paragraph that discusses this paper. I think that we are going to have to get administrator intervention here, because your actions are starting to appear like vandalism. David Tombe ( talk) 21:53, 3 August 2009 (UTC)
It was in his 1864 paper that Maxwell came up with his famous wave equation that predicted EM radiation and allowed its speed to be calculated. He used the results of Weber and Kohlrausch in that paper (see link) to calculate the speed.
The sentence about the speed of sound is just a general comment about the way in which values for the properties of the transmitting medium can be put into a wave equation to calculate the wave speed speed. It was not intended to relate directly to Maxwell's work. (unsigned comment by Martin Hogbin)
In 1864, Maxwell derived the electromagnetic wave equation . [1] This allowed the speed of light to be calculated in much the same way as the speed of sound can be calculated in normal matter. Maxwell use this theory to calculate the speed of light from ratio of electrostatic to electromagnetic units by Weber and Kohlrausch. This was in good agreement with the measured value of the speed of light and supported the view that light was a form of electromagnetic radiation. [1]
Hi David: I believe you are learning how Martin does things. It is a mistake to think it has something to do with the subject, with your edits, or with WP. It is all about Martin. Brews ohare ( talk) 23:11, 3 August 2009 (UTC)
Martin, Well at least that has cleared up what the problem was all about. You do of course know that this is the history section and that nobody has been saying that molecular vortices form any part of modern electromagnetism. Nevertheless, the fact that Maxwell arrived at the link between electromagnetism and light using that method is very interesting, and it is certainly not crackpot physics. It's true that Maxwell didn't use the 1861 method in his 1864 paper but he did nevertheless retain a less explicit luminiferous medium with all the characteristics of his sea of molecular vortices. The characteristics of that luminiferous medium are clearly described in the 1864 paper. Maxwell still talks about an aethereal medium filling all of space (section 4 part I) with a rotatory nature such that the axes of rotation are along the magnetic lines of force (section 8 part I), and an elastic nature (section 15 of part I). He may not have mentioned a sea of molecular vortices explicitly but the concept is still implicit in the 1864 paper. Maxwell never gave up the vortex sea idea. He merely played it down because he was uncertain about the details. Maxwell's luminiferous medium remained a major part of physics right up until the arguments that followed in the wake of the Michelson-Morley experiment in 1887, after Maxwell's death.
So it is somewhat misleading to state so boldly, as you have done, that Maxwell did not use the sea of molecular vortices concept in his 1864 paper. He had already done the calculation in his 1861 paper. In 1864, he merely transferred the result into a wave equation using the well known 'Maxwell's equations' of which the most important ones can also be found throughout his 1861 paper.
I’m going to re-word it again along the lines that in his 1864 paper, Maxwell used a less explicit luminiferous medium and that he never mentioned the specific idea of molecular vortices. David Tombe ( talk) 17:54, 5 August 2009 (UTC)
Brews, I'll get back to you on this in more detail later. But just off the top of the head, yes, Maxwell wasn't the only one to be playing about with vortices at that time. The young John Bernoulli seems to have been the one who first came up with the idea of a sea of tiny vortices pressing against each other with centrifugal force. I do re-call reading a long time ago that it was Maxwell who actually coined the terms curl, div, and grad. And yes, curl and vorticity are intricately linked. If a curl is non-zero, then we have vorticity in the sytem. Magnetism has got plenty of non-zero curl. David Tombe ( talk) 07:52, 6 August 2009 (UTC)
This sentence is entirely misleading in this context, as it can be construed as meaning the exact number 299,792,458 m/s is considered by some to be a fundamental constant of nature, which absolutely no-one believes.
Duff himself doesn't think this way: Duff says "Asking whether c has varied over cosmic history (a question unfortunately appearing on the front page of the New York Times [7], in Physics World [8]4, in New Scientist [10, 11, 12], in Nature [3] and on CNN [13]) is like asking whether the number of liters to the gallon has varied." Unfortunately, however, in saying this, Duff confuses the units 299,792,458 m/s with the physical entity the speed of light. Naturally, 299,792,458 m/s is arbitrary (as all agree; for example Jespersen), but that has nothing to do with whether the physical entity speed of light has fundamental meaning. Of course, relativity shows that it does. So this is a bit of a straw man.
Duff also suggests, however, that actually determining whether the physical entity speed of light has, for example, changed over time, has to be based on dimensionless ratios involving c. In that regard he has a point, as suggested by Smolin, who points out the role of ratios. Brews ohare ( talk) 20:12, 3 August 2009 (UTC)
Pecos, I'm not sure, but I think that Brews is trying to say that the pre-1983 speed of light was a physical thing whereas the post 1983 speed of light is just a number by human definition. Maybe I've picked it up wrongly. David Tombe ( talk) 22:08, 3 August 2009 (UTC)
To say c dt is on the same level as dx is perfectly fine, and completely consistent with saying c is a conversion factor to make dt into an element of length. That is said in several of the cited sources. However, to say the revolution caused by this revelation is nothing to talk about would be silly. Brews ohare ( talk) 05:27, 4 August 2009 (UTC)
Quantity 299,792,458 m/s is not arbitrary. Arbitrary are the units of space and time. If the space and time units are correctly derived from the stable effects of reality, than the speed of light quantity is the real quantity expressed in these units, with uncertainity determined by the measurement precision and the units definition.
Current metre/second units are not arbitrary in the sense, that they are based on continuity of the historical units. Historical units are not arbitrary in the sense, that they are based on human perception of the space and time and on human intelligence. Human perception and human intelligence in this context certainly does not mean arbitrarity. Thanks to this nonarbitrarity this internet discussion about arbitrarity of the speed of light constant is possible. I am sure, that the result of this discussion will be nonarbitrary.
The meaning of the speed of light is essentially dependent on the concept of the time. According to current SI units definitions, the time unit is fundamental independent unit. It could be very dangerous, to undermine the concept of time. (already partially undermined)
"In mathematics and its applications, a coordinate system is a system for assigning an n-tuple of numbers or scalars to each point in an n-dimensional space." If the n-dimensional space is the real space-time, then you can assign as "numbers" in a coordinate system only real dimensions. Universe has three same dimensions and one specific dimension. That is the reason, why we have two types of units. There is no way to constitute the real coordinate system on one type of unit. Softvision ( talk) 00:51, 5 August 2009 (UTC)
Brews, yes I see your point about the ratios. But getting back to the main point, the number 299,792,458 was arbitrarily chosen because it related to the most accurate measured value of the speed of light in 1983. The effect of that choice was to keep the metre close in size to the already existing metre. They could however have chosen any number. They could have decided that a metre is the distance that light travels in 1/ 400,000,000 of a second. That would have yielded a different number for the speed of light.
So, just as you said, the speed of light, post 1983, is a consequence of the definition of the metre and it has got no physical significance in its own right. It is just a tautology. It is just a number, and that number doesn't even have any of the mysterious qualities that are possessed by e or π. The physical significance of the traditional speed of light, post 1983, has been delegated to the definition of the metre.
Is that the point that you have been trying to make in the introduction? And have others been trying to say that the speed of light is about the speed of light? It would seem to me that since 1983, the speed of light is no longer about the speed of light. I can't even quite see it as a conversion factor, although I can see why others might. I see it as a tautology, plain and simple. We are in a new era in which Saint 'Speed of Light' is not only a constant by belief in relativity, but also as an unalterable fact embalmed in our international system of units. David Tombe ( talk) 18:18, 5 August 2009 (UTC)
Dicklyon is mistaken in his remark above. It is true that the modern value is the value that it is because of the existence of the metre in its earlier forms. Convenience dictates that the new measure be close to the old one to avoid conflict where possible, and for purposes where great accuracy is not an issue, the old and new metres are the same. However, from a logical standpoint there is a big difference. Today c no longer means what it did. It is now, from a logical standpoint, an entirely arbitrary number, and is divorced completely from measurement. That means that the "meaning" of the speed of light is no longer what it used to be. Dicklyon has not absorbed the WP article nor the references in it. In particular, Dicklyon should read the article cited in the WP article by Jespersen. Brews ohare ( talk) 03:12, 8 August 2009 (UTC)
This discussion seems to ignore two properties of the speed of light which definitely are not simply human constructs. Firstly, the speed of light is not infinite: it it were infinite, it would be infinite in any set of units. Secondly, the mainstream view is that the speed of light in free space is constant (although there is also a respectable minority hypothesis that it has changed over cosmological time, see Variable speed of light). This is equivalent to saying that dc⁄dt = 0. Again, either c is constant (the time-derivative is exactly zero) or it isn't, regardless of the units you use to measure it. Physchim62 (talk) 10:18, 8 August 2009 (UTC)
Since 1983, the speed of light has meant that light travels 299 792 458 times the distance that light travels in 1/299 792 458 of a second, in one second. It is a total tautology based on the definition of the metre. The number, 299 792 458, that is connected with the new definition of the metre was specially chosen so as to create a smooth and unnoticeable transition between the old real physics and the new nonsense physics. The new nonsense physics was inspired by a desire on the part of some, to elevate the speed of light to the sainthood because of its role in Einstein's theories of relativity. David Tombe ( talk) 09:18, 9 August 2009 (UTC)
Dicklyon: In response to the turd you left on my user page, you can't read, and are too lazy to look at the source. The source explains how the history of Roemer's work has been mangled, not that the WP article has a mangled version of history. Wake up, and get that chip off your shoulder. Brews ohare ( talk) 05:32, 4 August 2009 (UTC)
In response to your second reversion, this source, Roemer: a cautionary tale, if read, explains how Roemer's work was misreported in a series of historical discussions. It often was said that Roemer calculated the speed of light, when in fact he did not, and it was Huygens who used Roemer's work to estimate a speed (and using his data incorrectly, at that). Inasmuch as this widespread misreporting is out there, a reader might well wonder if the WP treatment is indeed accurate, and this reference provides the necessary discussion to put everything straight.
All this would be completely understood by anyone who spent a minute reading this source, which is available at google books at the provided link. To remove this source once, without proper examination of the source and without taking the time to understand its purpose is pretty sloppy. To do it twice, when reminded about its purpose is really sloppy. And to accompany the first reversion with a turd on my user page about it??? What is that???.
Sloppy and impolite and intemperate editing. Brews ohare ( talk) 06:14, 4 August 2009 (UTC)
Well Martin, your research into this mater is flawed. The WP article has a carefully documented version of this history that agrees with the version in this source. This source also explains very carefully with additional sources that your notion of the "generally accepted views" (which you apparently feel are being challenged) is incorrect, and just how that view came to be promulgated. As I said, this account of misinformation is helpful in supporting the WP account. All this entry amounts to is a note supporting what has been in this article for ages; suddenly this minor addition becomes a cause célèbre. As I am sure you are perfectly capable of understanding my summary of the source (and checking its accuracy), of reading the WP article, and perfectly capable of assisting with this entry instead of being obstructive, I wonder what drives your actions? Could it be because a person on your hit list made the addition? Or, could it be that your action is based upon sycophantic acceptance of Dicklyon's sloppy assessment? Or, is it just Martin's usual Not Invented Here attitude? These traits combined make a mighty force! Brews ohare ( talk) 13:10, 4 August 2009 (UTC)
David Wilson has looked into this matter carefully and assembled several sources. The article French cites a number of reputable sources and, being a mea culpa, does not appear to be grinding any axes. I have read other accounts that support the view that Huygen's did the calculation (not Roemer) and got the wrong answer. Just why that happened, I don't know. I confess to not having carefully read the WP account, and no, I cannot read Danish. So I'd suggest that David put together what appears to him to be a suitable version. Brews ohare ( talk) 13:18, 7 August 2009 (UTC) BTW I am not responsible for posting the WP account: I simply sought to add a footnote with a source. Brews ohare ( talk) 13:18, 7 August 2009 (UTC)
Yeah, if the current version is wrong you should probably blame me, not Brews. And having read a bit more on the topic since I last edited this article I have got a bit of doubt on the "22min/80 orbits of Io" version myself. It's something I've been meaning to get back to.
Regarding Teuber's article his wording doesn't actually leave that much room for doubt (in my own translation: "The only explanation of the numbers that make even the slightest bit of sense is..."), but in my view the article has a significant weakness in that it only seems to use the 1676 article and the correspondence between Rømer and Huygens. If the 1676 article is so cryptic why not use the other key article on the topic, Meyer, Kirstine (1915) Om Rømers opdagelse af Lysets Tøven (in Danish), The Royal Danish Academy of Sciences. Kirstine Meyer discovered Rømer's observation tables at the University of Copenhagen Library in 1913, documented that they were the basis of the value of 22 minutes and published them with extensive commentary in 1915. I haven't yet gotten hold of a copy of her article, which is why I haven't gotten back to this yet, but I've read summaries of it in Pedersen, Kurt Møller (1976) Ole Rømers opdagelse af lysets tøven (in Danish), Astronomisk Tidsskrift, vol. 9, pp. 160-66 and in Friedrichsen, Per; Tortzen, Christian Gorm (2001) Ole Rømer - Korrespondance og afhandlinger samt et udvalg af dokumenter, C. A. Reitzel, and neither of the two seem to question the "22min/diameter of the Earth orbit" explanation.
So in conclusion I think I have to agree with David that Teuber's explanation appears not to be a widely held view and the "22min/diameter of the Earth orbit"-version is more appropriate here.
Hemmingsen
07:42, 9 August 2009 (UTC)