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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 ε_o and μ_o 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

1. The issue is one of misuse of a definition by suggesting that a defined value is the same as a measured value. That incorrect view should be amended, as was attempted by LouScheffer and summarily reverted by Martin Hogbin. Brews ohare (talk) 20:44, 15 July 2009 (UTC)

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)

Theory regarding properties of real media can be confirmed by measurement. Measurement cannot confirm the defined properties of a hypothetical entity, free space. Brews ohare (talk) 00:09, 16 July 2009 (UTC)

• It may not be clear from Brews's description: The real dispute is over how the term "free space" is defined. If you accept that Brews's definition of free space is correct, then obviously what he's saying here is right. (Pure logic!) But if you reject Brews's definition, as I do, it's not so obvious. --Steve (talk) 00:21, 16 July 2009 (UTC)

Brew has repeatedly failed to find any accepted definition which states that in free space the speed of EM radiation is independent of frequency. It is, of course, true that classical EM theory predicts this and as far as I know, so do all accepted theories on the subject. But, as you know, all theories are subject to experimental falsification which is what several experimenters have tried to do, but their results have been consistent with accepted theories on the subject and thus the experiments verify the theories. This is all I want to say. Martin Hogbin (talk) 00:35, 16 July 2009 (UTC)

The properties of vacuum are specified by NIST (reporting for CODATA) as μ₀ ε₀, and c₀ all of which have exact numerical values independent of frequency. Accordingly, "vacuum" has no dispersion. Inasmuch as the values are exact, they cannot be measured values. No theory exists that predicts these numbers (e.g. field theory or QCD) - they are postulated independent of any theory. As the article Free space points out, "vacuum" refers to no known real medium. As a defined, hypothetical medium, it may serve as a model for some real media (to be established within some error bounds), but it is not itself amenable to observation and measurement. It is defined to have the defined properties.

It's fine for Steve to reject this definition, but that is a personal matter, independent of WP or physics until some basis for this odd view is provided. As for Martin??? Brews ohare (talk) 00:42, 16 July 2009 (UTC)

This does not definethe speed of EM radiation to be the same at all frequencies in free space. Martin Hogbin (talk) 08:24, 16 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?

Your claim is that, 'It is a defined property of free space that its electromagnetic properties ε_o and μ_o are frequency-independent'. Show me the definition that says this. Martin Hogbin (talk) 17:57, 16 July 2009 (UTC)

I've pointed out the links μ₀ ε₀, and c₀. I believe you have agreed that the number c₀ is not subject to experiment. As I understand you, you think somehow the meter is the point where experiment intrudes. I have pointed out that the meter can be measured only in real media. You do not grasp that it is not possible to realize a physical example of free space in which to make such a measurement. I have invited you to engage this statement, but you have not done so. Brews ohare (talk) 18:46, 16 July 2009 (UTC)

It is not a definition it is a theory which says this and, as such, it is subject to experimental verification. Martin Hogbin (talk) 17:57, 16 July 2009 (UTC)

What theory is this? It is not QED or QCD. Brews ohare (talk) 18:46, 16 July 2009 (UTC)

Our currently accepted theories on the subject say that EM radiation of all frequencies travels at the same speed, but you claim that it does so by definition. Please show me that definition. Martin Hogbin (talk) 20:39, 16 July 2009 (UTC)

In addition, NIST posts this remark: "The effect of this definition 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)

You're reading way to much into that comment. For example, NIST defines the kilogram as the mass of the IPK. The effect of that definition is to fix the mass of the IPK. But the IPK has a real, slightly-varying mass. Do you see what I'm saying? Let me be absolutely explicit: "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? --Steve (talk) 16:04, 16 July 2009 (UTC)

I hesitate to broaden the discussion to other standards. My uninformed view of what you have said is simply that the standard kilogram is related to a material object, and therefore may change with the history of that object. I see no relevance to the specification of the speed of light, except possibly that in this case no such accidental history is involved. Brews ohare (talk) 16:22, 16 July 2009 (UTC)

As a practical matter, if vacuum is shown to have a measurable dispersion, I suspect NIST and other agencies would simply specify the wavelenght to be used. I don't think they are saying no dispersion exists, or that the meter depends on the measuring wavelength - it's just that the dispersion, if any, is too small to matter to the current accuracy. LouScheffer (talk) 16:14, 16 July 2009 (UTC)

I'd disagree with you Lou. The hypothetical here should be stated differently. Here is an attempt: if it proves that using a free space definition that exhibits no dispersion is cumbersome in practical use, the BIPM might conclude that a different standard would be advisable, and adopt a different definition that employs some "standard dispersion". The effect of this change simply would reflect the accumulation of experience that showed all material media were subject to a universal dispersion correction, and rather than be burdened with subtraction of this universal dispersion in all calculations, it would be simpler to include it in the baseline.

Notice that such a change in the definition of free space does not alter its role as a defined, unrealizable medium, not a real medium. Consequently, the possibility in principle of such shifts in definition does not impact the present argument in any way. Brews ohare (talk) 16:22, 16 July 2009 (UTC)

And, for Martin's benefit, notice that the impetus for such changes in definition stem from measurements of real media, not from "observations" of free space. Brews ohare (talk) 16:34, 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)

You can make it more clear by saying what you are driving at instead of trying the cat and mouse game. As I have said (clearly): the kg case is not comparable. So to answer your question: the answer is "no" for the speed of light, and "yes" for the kg. Have to be careful about the double negatives here.

Let me explain why the kg case is not comparable: BIPM "fixes" an equality between the kg and an object; in the case of light BIPM fixes c to a numerical value. The object can change by historical accident, like accretion of some rust. The number cannot change except by formal international agreement. For example, see above dialog with LouScheffer on adoption of a new standard.

Relevance? Brews ohare (talk) 17:13, 16 July 2009 (UTC)

Brews, I'll restate it to make the analogy even more clear. It seems that you believe the following:

"NIST/BIPM state that the IPK is exactly 1kg, but they obviously don't believe that the IPK has a constant mass. Meanwhile, NIST/BIPM state that "the distance that light travels in vacuum in 1/299792458 s" is exactly 1 meter, and this statement is 100% absolutely clear-cut evidence that NIST/BIPM believe that "the distance light travels in vacuum in 1/299792458 s" is a universal constant independent of light-frequency, direction, date, or anything else."

This is your point of view, correct? You really don't see anything wrong with this? --Steve (talk) 17:49, 16 July 2009 (UTC)

That is consistent with my view. You have not responded to my reason for separating these two cases. Brews ohare (talk) 18:10, 16 July 2009 (UTC)

I thought my restatement would help. BIPM "fixes" an equality between the kg and the IPK. BIPM "fixes" an equality between the meter and the distance light travels in vacuum in 1/299792458 s. These are analogous. The object might change by the accumulation of rust. The distance light travels in vacuum in 1/299792458 s might change by using a different light frequency. These are analogous. There is no new formal international agreement involved in either case. --Steve (talk) 20:33, 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)

Let me repeat my statement: "The distance light travels in vacuum in 1/299792458 s might change by using a different light frequency." You'll notice that I did not say that the number 299792458 will ever change. You seem to have a perpetual confusion: "The speed of light in vacuum as measured in meters per second is a constant" is an obviously-true statement (it's 299792458). "The speed of light in vacuum is a constant" is an entirely different statement. This is the statement under dispute, not the other one. This statement has nothing to do with the number 299792458.Steve (talk) 21:40, 16 July 2009 (UTC)

Well, yes, it's a different statement. So we might measure the speed using different system of units; is that the point? And which units might you choose? Units in which c=1?? Because "free space" is defined in the SI units, I don't think a switch of units is going to help because everything has to be translated anyway. Brews ohare (talk) 22:00, 16 July 2009 (UTC)

You also asked why isn't the frequency specified in the definition of the meter. This is explained in Jackson's Classical Electrodynamics: We're using a definition of the meter which is informed by the experimental fact that, as accurately as we can measure today, the speed of light is independent of frequency. If that experimental fact changed, the definition would presumably be made more specific. --Steve (talk) 21:40, 16 July 2009 (UTC)

Independent of frequency in what medium?? No, the corrections of "good practice" would change, to bring the measurements in that medium back to free space, that is all. Brews ohare (talk) 22:00, 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)

The whole issue boils down to "vacuum". I think "vacuum" is an unachievable medium with μ₀ ε₀, and c₀. Real media may approximate "vacuum", but that can be established only within error bars (notably absent in the specification of "vacuum"). Brews ohare (talk) 00:04, 17 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)

I would regard that statement as a bit weak. It is generally accepted that the experiments in real vacua verify the theory that the the speed of light in free space is independent of frequency. One editor here does not agree with that view. A slightly stronger statement might be

"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."

What do you think of that? Martin Hogbin (talk) 14:25, 5 August 2009 (UTC)

Hi Fizyxnrd: Your statement is accurate. Of course, "absence of matter" is one version of "classical vacuum". I'd be happy with your statement if it used the link absence of matter to free space. You will recognize that the version by SemitoneSonata is more explicit, and is not subject to any misinterpretaion that "absence of matter" is the same as "outer space" or "ultrahigh vacuum". Brews ohare (talk) 14:45, 5 August 2009 (UTC)

Martin: Your thought process is the same as before: you state It is generally accepted that the experiments in real vacua verify the theory that the the speed of light in free space is independent of frequency. Not only is this not generally accepted, it is patent nonsense: no experiment in a real medium can "confirm' the behavior of a model medium like free space. Experiment can only confirm how well the model fits the real medium. So an accurate version of your statement is It is generally accepted that the experiments in real vacua verify that the the speed of light in in these media is independent of frequency to within experimental error. Brews ohare (talk) 14:45, 5 August 2009 (UTC)

I am unclear about where this sentence is supposed to show up. The article has changed over time. Maybe somewhere in the section Speed_of_light#Light_as_electromagnetic_radiation? Brews ohare (talk) 16:39, 5 August 2009 (UTC)

@Martin Hogbin: I would (personally) prefer the phrase "experimentally confirmed" to "confirmed... by experiment", although the difference is mostly semantic. The first informs the reader that experiment implies this conclusion. The second states that we know a truth, and that we arrived at it by experiment. I feel that the first statement is a little more modest and consistent with the care with which scientific statements should be made.

@Brews ohare: I think the statement

"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."

is satisfactory.

I agree that the sentence suggested by SemitoneSonata is technically quite accurate. However, it is a somewhat complicated statement to make a relatively simple point. I think that the editing standard on NPOV:Undue may have something to contribute here. Since the important concept here is that all light travels through the vacuum at the same speed (which is a fact well known to science, despite the possible exceptions that may exist) this is the statement that should be made. Highly technical definitions obscure this point unnecessarily for all but a relatively small group of researchers and scientists who are interested in the more fundamental nature of light that may arise as a consequence of non-zero vacuum energies (or the non-existence of vacuum). I suggest that this angle, while not precisely non-neutral, represents an argument of very narrow appeal, and to obscure the basic point with exacting precision is counter-productive. If a further discussion of the (possibly) dispersive nature of the vacuum is desired, it should be included in another location. Fizyxnrd (talk) 21:18, 6 August 2009 (UTC)

You are, of course, quite right Fizyxnrd. Unfortunately, one editor has not grasped this simple fact yet. Martin Hogbin (talk) 09:10, 7 August 2009 (UTC)

should c be considered nothing more than purely a human construct

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 , in Physics World 4, in New Scientist , in Nature and on CNN ) 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)

Which sentence are you talking about? Your argument seems confused about this, and you can't claim a source is both right and wrong. I'm also unclear on the apparent distinction between c and the speed of light. At any rate, the speed of light is a fundamental constant in the SI system, and should be introduced as such. Pecos Joe (talk) 21:22, 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)

The sentence occurs in Speed_of_light#Light_as_electromagnetic_radiation and is as follows: Whether the speed of light can be considered to be a fundamental constant of Nature, or whether only dimensionless combinations of constants such as α can be considered fundamental, while dimensionfull constants such as c should be considered nothing more than purely a human constructs fixing our unit system, is disputed among theorists.

It's a confusing subject. Here's how I see it; see if you agree:

In Nature there is light, and light travels. It travels at a constant rate, at least in free space. This rate can be called c, but we don't know what it's actual numerical value is (there is no theory that calculates it from other facts). However, we can compare this speed with other speeds, e.g. the speed of sound in some specified medium, and say it is x times faster than that speed of sound. We can take a yardstick and measure how long it takes light to traverse it, and measure it again and again. If the time of traversal does not change, we can say the speed of light hasn't changed enough to notice during our series of tests (or the yardstick and the speed of light conspired to keep the transit time the same.) Likewise, we can check on the frequency of an atomic transition on earth and compare it with the transition emitted by an atom in the outer universe to see if the frequency changed. Any change in c could affect that frequency. The comparison now is more complicated because some things could have happened to apparently change the frequency even if c did not change, but still we can say: Hey, as far as we can tell, c hasn't changed over the life of the universe. So there are lots of things we can say about c that have nothing to do with its actual value, and they all have to do with comparisons.

So the point is, there is a physical something we can call the speed of light, without involving ourselves in its numerical value. See Okum's contribution to Trialogue.

Then there is the question of its numerical value. That is arbitrary, because it involves comparison with a "standard" which is itself arbitrary. We could pick the length of some king's arm and ask how many of those does light cover in a second. That gives the speed as (king's arms)/second. Or, we can say the speed of light is 299,792,458 (king's arms/s) and then the king's arm is not his arm any more but the distance light travels in 1/299,792,458 s. This last method is preferable if the king doesn't want to travel a lot, to bring his arm around for people to compare to.

Duff confuses these two things: the physical speed of light and the numerical value of the speed of light. However, though this confusion is unfortunate, his point that real physical quantities have properties discovered by comparisons, by ratios, not by looking at the numerical values that depend upon the choice of units, makes sense. He carries the question a bit further by saying unless you can discover a property of c by looking at the behavior of a catalog of various dimensionless ratios, you actually cannot say anything experimentally verifiable about that property of c. He may go too far, however, because I don't think he admits a dimensionless ratio like c(f) / c(f₁) tells you anything about the dispersion of c.

So ratios matter, units don't. Ratios determine physical properties, units facilitate commerce.

I don't think the WP sentences above convey the point at all well. Brews ohare (talk) 22:56, 3 August 2009 (UTC)

Yes, I agree that there can be some confusion about this. I think one should just expand that paragraph a bit and explain things better. You could say that as soon as one chooses units for space and time, the speed of light is also fixed, assuming the validity of Special Relativity and that electromagnetism is described by Maxwell's equations (so, the photon is massless). This choice of units is arbitrary, albeit that the natural choice would be to choose the same units for space and time.

In his other article on this subject, where the authors Duff, Okun and Veneziano disagree about this issue, Duff makes the point on page 23 that you could just as well define three different units for the three spatial directions in some coordinate system and define a unit "xylophone" for distances in the x-directions, a unit "yacht" for distances in the y-direction and a unit "zebra" for distances in the z-direction. Then we have a line element:

ds^2 = -c^2 dt^2 + c_x^2 dx^2 + c_y^2 dy^2 + c_z^2 dz^2

Of course, c is the speed of light and t is time, but x, y, and z are not lengths, they are xylophones, yachts, and zebras respectively and these are physically not the same as length :). c_x, c_y and c_z have dimensions of length/xylophone, length/yacht and length/zebra, respectively. Then, as silly as all this sounds (given the fact that we have rotational symmetry so there is no preferred x, y and z direction), it is no more silly than defining time to have a different dimension from space given Lorentz invariance. So, Duff argues that c is no more fundamental than the superfluous c_x, c_y or c_z that you could introduce. Count Iblis (talk) 23:47, 3 August 2009 (UTC)

I don't think there's much dispute that c is a "fundamental physical constant" in the colloquial sense. The dispute is whether or not c is so fundamental that shame on us for even bringing it up, rather than incorporating it implicitly into our thought process.

If we did measure xylophones and yachts, the conversion from xylophone to yacht would sure as heck be a fundamental physical constant (insofar as it's "fundamental" in importance, it's related to "physical" measurements, and it's "constant"). :-)

Maybe we can say "From the standpoint of special relativity, space and time are so closely intertwined (with c as the conversion between them) that some physicists say c isn't really such a "fundamental constant", but rather c is just an artifact of our naive inclination to measure space and time in different units. Indeed, in "natural units", space and time are measured in the same units, and light travels at a speed of 1, so here the speed of light is not taken as a new and meaningful physical constant."...or something like that... :-) --Steve (talk)

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)

Brews, on thinking more about it, I can see now that what has happened is that the physical significance that was implicit in the speed of light prior to 1983 was transferred into the definition of the metre in 1983. The speed of light subsequent to 1983 is therefore merely a tautology. All the phyiscal significance of it lies in the definition of the metre. The 1983 definition of the metre was mischievous because it was bound to cause the kind of confusion that you are currently trying to unravel. David Tombe (talk) 11:30, 4 August 2009 (UTC)

I recall that prior to 1983 the meter was defined in terms of the wavelength of a certain atomic transition. That so, the numerical value of c was based upon how many wavelengths were traveled in a second: c = λf. The argument was made that f was known very precisely, but λ wasn't. That made c imprecise. So they figured λ = c/f fixed the problem, because (whatever its value) c was, so far as theory and experiment were concerned, a constant of nature, and could be counted upon to always have the same value (whatever that might be). At that point, one could pick a value for c with impunity. The error in λ still is not the same as the error in f, however, because there are some experimental difficulties in assessing corrections to the measurement to bring it into accord with "vacuum", which is where c has its fundamental value. Brews ohare (talk) 14:21, 4 August 2009 (UTC)

If we use the symbols, they are allways "impune". Impunity of SI speed of light constant is in fact, that we don't need to perform very precise measurement of c, published and then refined, that will be used in devices. According to definitions, the number is constant. All we need to do now, is same as before - correctly calibrate our devices. Now we don't see uncertainity of our speed of light constant, because it is individual to every device and every single device calibration. Now, every device calibrated directly by caesium 133 etalon measurement, is performing the speed of light measurement, because it says - this is the second and this is the metre. Valid motivation for this model of units definitions is technical simplification and independence of technical realizations, based on the speed of light physics. That means, that the valid motivation is related to units, not to equations. Softvision (talk) 00:51, 5 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)

I think I can see what Brews is getting at. The value 299,792,458 m/s is totally arbitrary because it is a consequence of the choice of the definition of the metre in 1983. It doesn't tell us anything about the speed of light as such. It is merely a knock-on tautology from the definition of the metre. Any physical significance that is associated with the speed of light is now firmly devolved to the definition of the metre. The fraud is that prior to 1983 the constancy of the speed of light was a belief held by relativists, whereas since 1983, the constancy of the speed of light is now locked into arbitrary definition so that even if they did perchance perform experiments that showed up a variation in the speed of light (in the traditional sense), this would instantly be masked out by the modern definition of the metre. David Tombe (talk) 08:56, 5 August 2009 (UTC)

It is the same, like when you are speaking. The meaning of what you say is nonarbitrary. The language you use is arbitrary. In the sense, that you must use the language that is intelligible, the language you use is not arbitrary.

The SI units are the language, that we have not arbitrarily choosed, that the Nature will be speeking to us. The Nature can speek many languages, but the meaning is allways the same. This is the fundamental assumption of scientific research. When the Nature speeks about the speed of light in our SI units, the meaning is the same as in any valid space/time units. The better language we choose for the Nature to speek to us, the better is the meaning of what the Nature has spoken.

The language of current SI speed of light-metre definitions is a little bit confuzing. Nevertheless, the current SI definitions are valid, even though problematic. If we assume, that the caesium 133 etalon is valid and stable on the level of precision expected, the physical meaning of the the metre, time and speed of light units are according to the measurement precision converging to the real meaning of the SI definitions, given by the caesium 133 etalon. An absolute precision is not possible, and therefore the exactness of the SI speed of light quantity is virtual, not the meaning of the quantity in SI units. Current technological level is sufficient, to consider the question of precision as nonproblematic. There are other, more serious disadvandages of this model of units, especially when used in relation with the speed of light, under scientific level, that did not solved important questions regarding the speed of light physical phenomenon and causality. Validity of devices calibration should be verified. Softvision (talk) 10:51, 5 August 2009 (UTC)

David: Much of what you say I agree with. However, experiments to measure the variation of light with time and with frequency are ongoing to test theories in cosmology and in quantum gravity where such variations are predicted. Likewise measurements are ongoing to detect field-amplitude dependence to test QCD. They are not masked out by the definition of the metre, because these experiments are measurements of ratios, for example, c (t_now) / c(t_then) or c(f₁) / c (f₂) in which the functional variation of c can be detected because the ratio is independent of the definition of the metre in terms of the speed of light: they are basically time-of-flight ratios. Brews ohare (talk) 14:27, 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)

Yes - "The physical significance of the traditional speed of light, post 1983, has been delegated to the definition of the metre." When you ask the Nature "What is the quantity of the speed of light in vacuum in SI units ?", the answer will be allways 299,792,458 m/s. More exactly - you don't need to ask the Nature, you just ask your definitions. However, when you want to implement the speed of light based device, you must calibrate it using the real speed of light effect. Than you are asking the Nature what is the real speed of light quantity.

Generally, the physical concepts are the units of the language, that we have nonarbitrarily choosed, that the Nature will be speeking to us. What is constant in one "language" can be variable in other "language". What is flat in one "language", can be round in other "language". Nature is not simple, nature is complex. If the words of what the Nature is saying to us are contradictious, it is the matter of invalid language we have choosed, not the contradiction of the Nature. The fundamental scientific requirement is to understand, what is constant and what is variable, what is flat and what is round - curved. Therefore selection of the "language" cannot be arbitrary. Softvision (talk) 12:53, 6 August 2009 (UTC)

Attempts are made to define c as a mere conversion factor or simply a human construct or merely the unit "1" because it is meaningless to square meters per second with the result being meters/second.Lestrade (talk) 13:07, 6 August 2009 (UTC)Lestrade

Speed of light, or better maximal causal speed, is the physical phenomenon, that according to its nature, or better - essence, can be considered as one of the fundamental causes. That means, the speed of light phenomenon should be considered as fundamental cause, event propagation, not just as an effect or just as quantity.

Energetical balance of valid experimental data confirms rapid increase of causal energy needed to create kinetic effects near the speed of light. This is most important evidence of existence and character of the causal speed limit. That means, the causal speed limit is not only the separate limit of the "light" propagation, but it has deep fundamental physical context. The constantness of the speed of light phenomenon should be considered in this context.

The speed of light phenomenon is not human construct, it is essential fundamental physical cause that should be considered completly, in causal context of event propagation, not just like an effect, speed or quantity. The quantity 299,792,458 m/s is expression of the speed of this physical phenomenon in vacuum in SI units. SI units are not arbitrary in the sense, that they are the language that we have nonarbitrarily choosed, that the Nature will be speeking to us. Selection of the "language" of units cannot be arbitrary. The "language" of units can affect the interpretation of measurements. Softvision (talk) 16:47, 6 August 2009 (UTC)

Softvision, It's all pretty simple. Prior to 1983, the speed of light referred to the speed of light. It was a quantity that had physical significance. Relativists believed it to be a universal constant.

Since 1983, the speed of light is simply a human construct. It is a universal constant by definition. It has got nothing to do with the pre-1983 understanding of the speed of light. It is a tautology. David Tombe (talk) 23:39, 7 August 2009 (UTC)

And that statement is an example of why this talk page and article are such a mess. It's absurd to say that "It has got nothing to do with the pre-1983 understanding of the speed of light" just because we recognized it as such a good constant that we take c, instead of the length of a bar in a vault, as the basis for length standards. The speed of light still means what it always did, but its expression in meters per second has subtly shifted meaning. Dicklyon (talk) 23:48, 7 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)

I've read the article and Jespersen; they seem OK. But I don't see where they back you up. I'm talking about "the speed of light"; perhaps you and David are talking about the numerical value that represents it, or the logic of international standards. I don't think the changes in the latter make such a fundamental difference in the meaning of the former. Jesperson says they "removed a constant of nature by turning c into a conversion factor whose value is fixed and arbitrary." OK, we all agree there. But c is still how fast light travels; we just have different standards for what to define in terms of what; you can still measure how light slows down in different media, for example; and if we ever see light of different wavelengths moving at different speeds through whatever you want to call empty space, then we'll probably just revise the standards definitions again, not argue that it's impossible. I know you like to belabor this point, but to say that "It has got nothing to do with the pre-1983 understanding of the speed of light" is a bit extreme (shall I say idiosyncratic); are there sources that agree with this view? Dicklyon (talk) 05:33, 8 August 2009 (UTC)

The phrase "the speed of light" may refer to the "numerical value" or to the physical concept and it makes statements unclear when no distinction is drawn. Thus, "the speed of light still means what it always did" is ambiguous. The "it" in "It has got nothing to do with the pre-1983 understanding of the speed of light" refers to the numerical value and is a correct statement, as pre-1983 the numerical value was still found by experiment, while post-1983 it was a defined (arbitrary) value. Maybe a little more precision in language would fix all this debate?? Brews ohare (talk) 19:35, 8 August 2009 (UTC)

Can anyone tell me what this line means, 'The speed of light can be viewed as simply a conversion factor between space and time in spacetime', and why we need it in the lead. For some reason, Brews now wants to include the term 'conversion factor' throughout the article. In the next line of the lead we have, '...the numerical value of this conversion factor is fixed...'. We already have, '...In Einstein's theory of special relativity space and time are viewed as a four dimensional unification of space and time, known as spacetime, with c playing the fundamental role of a conversion factor between the units of space and time'.

The sentence Martin cannot understand: The speed of light can be viewed as simply a conversion factor between space and time in spacetime. is made in several sources that have appeared and disappeared from this article, (e.g. ABC's of Relativity) and has only the simple meaning that the metric in spacetime is ds = dx − (c dt) so c "converts" time to units of length in the metric. Not too wild a statement, I guess?? It should be in the intro somewhere because it is a fundamentally important property of the physical entity "speed of light", not a minor point. Brews ohare (talk) 14:55, 8 August 2009 (UTC)

What we do not need is an individual editor's personal philosophy scattered throughout the article. The arbitrary nature of the numerical value of c is already summed up in this statement, 'As the speed of light is a dimensionful constant its numerical value is dependent on the system of units used. In the SI system, the metre is defined as the distance light travels in vacuum in 1⁄299,792,458 of a second. The effect of this definition is to fix the speed of light in vacuum at exactly 299 792 458 m/s.' Anyone who wants to delve into the philospohy of the subject is better to follow the dimensionful constant link where there is a fuller and better though out discussion of the subject. Martin Hogbin (talk) 09:01, 8 August 2009 (UTC)

This reference to "an individual editor" might refer to me, but I am not the creator of the third paragraph in the intro. Brews ohare (talk) 15:08, 8 August 2009 (UTC)

I believe I wrote that bit, as an attempt to simplify a rather more drawn out explanation by Brews into something that nobody would object to. Sounds like I didn't succeed. I'm not strongly attached to "simply"; really it's the explanation that needs to be simple. There are tons of sources about the conversion factor concept, here. I'm not sure what to think of the dimensionful constant link, as there's nothing sourced there; is it just someone's opinion or OR? Dicklyon (talk) 22:20, 8 August 2009 (UTC)

OK, blast from the past. The dimensionful constant section was created by the crank User:Rbj back in 2005-2006; he was my first arch-nemesis on WP, for his wedged position on the Nyquist–Shannon sampling theorem back in 2006; he's been blocked over 20 times, and is now blocked indefinitely, so I think it would be OK to clean up that section, deleting unsourced stuff and finding sourced stuff to replace it with, if anyone cares, but to point to it as some kind of authoritative section is a bit absurd. Dicklyon (talk) 22:33, 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)

All this philosophical musing is fine here. I only object when it begins to find its way into the article, with lines like, "The speed of light can be viewed as simply a conversion factor between space and time in spacetime" .

A discussion on which science and maths is, and which is not, simply a human construct, is way beyond the scope of this article. Let us stick to stating the established facts on the subject, as given in reliable sources. Martin Hogbin (talk) 12:38, 8 August 2009 (UTC)

The distinction between the properties of the physical entity speed of light and the numerical value attached to this speed in the SI system of units, is a major emphasis of the article. Physchim62 appears to have missed this distinction, so perhaps some change in wording or organization is warranted? I don't see Martin's comment as germane to this issue, unless he feels that the distinction is a "philosophical issue", which it may be, but it also is a scientific issue and a major point that cannot be ignored. 19:22, 8 August 2009 (UTC)

A time depend c has no operational meaning, as pointed out here. Dimensionfull constants are simply conversion factors that relate physical quantities that were assigned different dimensions by humans. Count Iblis (talk) 20:03, 8 August 2009 (UTC)

Any speed can be used as a conversion factor between space and time. The physical meaning of this speed-space-time conversion is - the space distance overcomed per time, or time needed to overcome the space distance. The space needed to overcome the time is nonsense.

To overcome the distance you need time and motion. To overcome the time you don't need motion or space distance, you need only time. In this sense, to overcome the space you need causal energy, but to overcome the time, you don't need causal energy, just stability. Stability, equilibrium, is the cause of time overcoming. In this sense, the speed (motion) is not time overcoming, but space overcoming.

Space and time dimensions are not reciprocal. Space and time dimensions are fundamentaly not the same type.

Space and time units can be "converted" only in appropriate physical context, typically in causal context of field propagation. In the sense, that speed of light is the real causal limit, the speed of light is very specific and constant speed, that can be under specific conditions used as conversion factor, considering all consequences, and the fact, that this is contextual operation, that can be (does not need to be) misleading. However, the physical context of the speed of light is much deeper. Speed of light is not speed of time. Softvision (talk) 22:12, 8 August 2009 (UTC)

I'm unsure what the claim made by Count Iblis is. Is it a simple statement about time dependence of c, or is it a statement about any property of c? As regards the source, it argues actually that physically observable variations are variations in dimensionless constants, as these are independent of any choice of units. The source illustrates this point using the expression for entropy, which is shown to be expressible in terms of such dimensionless constants.

I'd have two issues with drawing a wide conclusion about the observability of properties of c: first, c may not appear in the same manner in every dimensionless constant, so observing a catalog of such constants might make possible observation of some properties of c. Perhaps this is not possible, but an illustration based on a limited example requires more support. A second issue is that there is no theory expressing the properties of c, such as its time or frequency dependence. Thus, we do not have a formula expressing the properties of c in terms of other fundamental constants. In particular, it seems entirely possible to measure the wavelength in SI units of an atomic transition repeatedly, and to determine whether this wavelength changes. Already ongoing are experiments to track whether the standard second changes. It would seem that putting these two types of observation together one could determine whether c varies over time. Moreover, these observations do not involve the choice of units, as the measurements all are ratios independent of the units chosen. So in a way these observation do not differ with Duff on the use of dimensionless quantities, but extends the catalog of dimensionless constants that could be used. Brews ohare (talk) 22:21, 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)

Once there was a good definition of the second, reprodicuble in most labs, and a bad definition of the meter, reproducible in no lab. There also was a good and highly successfull theory which used the osbserved fact that light speed was invariant, fact that could be experimentally verified in most labs, and without using the bad definition of the meter. Thus, we had a good second, a good light speed and a bad meter. So what did we do? We did the right thing and redefined the meter in terms of second and light speed, allowing most labs in the world to reliably reproduce it. Now, people who, due to some kind of allergy against "Einstein's theories of relativity", think this was the wrong thing to do, should know that there are plenty of forums available to vent their frustration, but that an article's talk page in Misplaced Pages is not the place to do it. Please take your frustrations elsewhere? Thanks. DVdm (talk) 11:33, 9 August 2009 (UTC)

The only people are trying to "elevate the speed of light to sainthood" are those commentators in this discussion who seem to believe that you can change the speed of light in free space by a humble choice of units. Nobody makes all this fuss about the frequency of the hyperfine transition of caesium-133 (definition of the second) or the wavelength of light from a certain transition in the krypton-86 atom (definition of the metre, 1960-1983), and yet these physical constants are (or were) also "fixed" in SI units by the very definition of the units themselves. It's metrologically useful to measure distances in terms of the speed of light: just get over it. Physchim62 (talk) 12:12, 9 August 2009 (UTC)

The Edit War at The History Section

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)

Martin, I'm watching your ongoing edits. You're simply making the matter worse. You are now attempting to change the subject to Maxwell's 1865 paper instead. But if you do that you will have two questions to answer.

The first will be why you are trying to skip the chronology? Maxwell first used the results of Weber and Kohlrausch in 1861 when writing the 1861 paper. He looked those results up when he was down in London on a break from his home in Galloway. So why are you insisting on changing all this to his 1865 paper?

Secondly, the method that paralled the speed of sound is in his 1861 paper. It is not in his 1865 paper.

You have clearly realized that your reversions were wrong, and supported by nobody, and you are now trying to wriggle out of it without losing face. David Tombe (talk) 22:15, 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)

Martin, Maxwell's 1865 paper came after his 1861 paper. Maxwell first made the link between the speed of light and the electric and magnetic constants in his 1861 paper. In 1861 he returned from Scotland to London to look up the 1856 results of Weber and Kohlrausch regarding the ratio of the two kinds of units of charge. He applied those results to an equation in his 1861 paper that then linked this ratio to the magnetic permeability and the dielectric constant. He also equated these constants with the density and transverse elasticity of his sea of molecular vortices and then applied the result to Newton's equation for the speed of sound. It was then, in 1862, that he first saw the link between electricity, magnetism and the speed of light.

You have, for whatever reason decided that nobody is allowed to know about the 1861 paper. You have decided to jump ahead to the 1865 paper where some things have been carried forward from the 1861 paper, such as the 1856 Weber/Kohlrausch ratio. In doing so, you have got the contents of the two papers mixed up. And in doing so, you have distorted historical truth by trying to imply that it was in the 1865 paper that Maxwell first discovered the link between the speed of light and electromagnetism.

So why are you so keen to suppress all references to the 1861 paper? David Tombe (talk) 22:46, 3 August 2009 (UTC)

Here is Martin's re-written section, complete with the bad grammar,

In 1864, Maxwell derived the electromagnetic wave equation . 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.

He deliberately skips the fact that the link between light and EM had already been made in Maxwell's 1861 paper. He then makes a false statement to the extent that the electromagnetic wave equation allowed the speed of light to be calculated in much the same way as the speed of sound is calculated in normal matter. He is getting totally confused with equation (132) in Maxwell's 1861 paper.

And all this seems to be because he doesn't want to have any mention of the sea of molecular vortices which was the entire basis of the important calculations in the 1861 paper. So he jumps ahead to the 1865 paper and attempts to re-write history. David Tombe (talk) 23:03, 3 August 2009 (UTC)

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, I've elaborated the paragraph in question in order to clearly segregate matters which belong in the 1861 paper from matters that belong in the 1864 paper. As a compromise, I've explicitly mentioned about Maxwell's own reservations about the molecular vortex model. I believe that it is the very mention of this model that is the source of your determined opposition. Nevertheless, it is part of history and it led Maxwell to a landmark result in relation to the speed of light. We cannot brush that aspect of history under the carpet just because you have decided that Maxwell's 1861 paper was crackpot physics. David Tombe (talk) 11:26, 4 August 2009 (UTC)

David, It is better how it is now but I would still ask the question of what molecular vortices have to do with the speed of light. It is quite true that they formed part of Maxwell's initial thinking on the subject but he later dropped the idea and it most certainly forms no part of current physics.

In the road leading to our current understanding of the subject there were may dead ends and the molecular vortex model is just one of them and, in my opinion, there is reason why we should mention it, however, I am not going to start edit warring over the subject but I would like to make just one small change so that it is clear that molecular vortices for no longer relevant to the speed of light. Martin Hogbin (talk) 10:11, 5 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)

David: I may be all wet on this, but is it possible that the curl operation is an outgrowth of vortices? For example, Siegel, Barrow, The second coming of vortices and some other history texts might help with this. I know that vortices still show up in fluid mechanics, see Durand-Vidal and I would not be surprised to see a lot of mathematical similarities. (For example, mutual attraction of vortices in fluids, etc.) The point is this: the derivation of the wave equation involving the speed of light requires the curl equation, and so a case can be made that the molecular vortices, whatever their residual intuitive power today, served their purpose in getting the math right in Maxwell's time. Another point to be made here is that Maxwell was only one of many physicists of his time to talk about vortices: it wasn't the brain child of only one genius. And finally, I think Maxwell used the vortices because his colleagues were all about mechanical models, not trusting purely math manipulations (who can blame them). Today we are less inclined to mechanical intuition, but at that time to sell the notion of waves such a model was needed. The modern reaction to molecular vortices is that it is really weird, but if some social context is provided it may seem less so.

Oh, and one more thing, vortex theory still has its interest: see The fifth coming of vortices and Baker & Steinke. Brews ohare (talk) 22:16, 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)

Brews, Maxwell's molecular vortices played a crucial role in the 1861 paper in his derivation of the two curl equations. The two curl equations are of course Ampère's circuital law at equation (9) and Faraday's law at equation (54). When it came to the wave mechanics and Newton's equation for the speed of sound in part III, the emphasis was more on linear elasticity, ratios, and analogies.

The 1865 paper produced a wave equation using the equations that had been derived in the 1861 paper using the molecular vortices, and of course the same numerical ratios led to the same conclusion as in the 1861 paper. What is interesting is the insistence by some that the molecular vortices are not necessary because they are not mentioned in the 1865 paper.

The natural thing to do would therefore be to examine the 1865 paper in order to see how Maxwell derived the two curl equations if molecular vortices were not being considered. You'll probably notice that Faraday's law does not appear in the 1865 paper. There will of course be the equation (D) which corresponds to the modern Lorentz force equation, and hence to Faraday's law. Maxwell used equation (D) to derive the EM wave equation, where modern textbooks use the clipped (partial time derivative) Faraday's law.

At any rate, I think that you all ought to carefully study how Maxwell derived Ampère's circuital law and the Lorentz force equation in his 1865 paper without using molecular vortices. That might be quite an eye opener. David Tombe (talk) 23:33, 7 August 2009 (UTC)

Sloppy editing

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)

I already followed up on this on your talk page. What was mangled was your brain; I mean your edit. Dicklyon (talk) 06:02, 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)

Brews if you are going to challenge the generally accepted views on Roemer's measurement of the speed of light you will need very good quality and convincing evidence. One page in a newly published book is not enough. Does the book cite its sources and what are the credentials of its author? Martin Hogbin (talk) 07:52, 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)

Brews, I have no problem with that version of the history, and sm wondering why Martin jumped in to object; maybe he's the one that didn't read the source. My problem was with your mangled edits; the latest one is OK, though not really necessary. Dicklyon (talk) 15:18, 4 August 2009 (UTC)

In my opinion, the account of Roemer's discovery currently given in the article is not satisfactory. Among several problems with it, the following statements constitute the most serious:

• "On the basis of those observations, Rømer concluded that it took light 22 minutes to cross the distance the Earth traversed in 80 orbits of Io."
• " Christiaan Huygens ... misinterpreted Rømer's value of 22 minutes to mean the time it would take light to cross the diameter of the orbit of the Earth."

According to several reliable source—including I. Bernard Cohen's definitive account, Carl Boyer's widely cited paper, as well as the account given in French's Roemer: a cautionary tale (p.121), already cited by Brews ohare—Roemer gave the value of 22 minutes as the time it took light to cross the diameter of the Earth's orbit, and not the time it took it "to cross the distance the Earth traversed in 80 orbits of Io." Moreover, on my reading of the original French report of Roemer's findings (available on-line here), as well as the 1677 English translation from the Philosophical Transactions of the Royal Society (also available on-line here), Cohen, Boyer and French's interpretations are clearly correct while the article's is clearly not.

The source cited for the article's account appears to be in Danish (which I do not understand). In view of the inconsistencies between the article's account and those of the sources I have cited above, I am sceptical that the article's interpretation of this Danish source is accurate. Is ((Brews ohare)) the editor responsible for the article's account sufficiently confident of his fluency in Danish to assure us that his interpretation of the source is accurate? A google search brings up a couple of web-sites whose interpretation of the source appears to be the same as the article's, or at least very similar to it. Could ((Brews ohare)) the responsible editor please let us know whether he relied on information from any such web-sites to arrive at his interpretation, or whether he arrived at it independently of any them?.

The article's account of Roemer's work was put in its current form with this edit, by an editor who appears to be fluent in Danish. I have therefore struck out my above questions as inappropriate.

—David Wilson (talk · cont) 16:10, 6 August 2009 (UTC)

This post appears to support the point that I made above, which is not a criticism of any particular editor or view, it just a reminder that we should stick to standard WP practice.

Many reliable sources, such as physics text books, indicate that Roemer measured the speed of light. I have no knowledge or strong opinion on this matter but, if we are going to state a different view, we need to cite good quality and reliable sources. It is not acceptable to base the article on an individual editor's assessment of one or two online sources. We need to find out what current, established and expert opinion on the subject is. Martin Hogbin (talk) 10:59, 7 August 2009 (UTC)

While I generally agree with these sentiments, there is one point I would take issue with. Physics textbooks are reliable sources for physics. They should not be regarded as reliable sources for the history of science, or even for the history of physics. Their accounts of Roemer's discovery of the finite speed of light are generally quite poor, and this is well-documented in Carl Boyer's paper, already cited above.

On the issue of the article's current treatment of Roemer, I have now stumbled across further evidence that the article has not interpreted the cited Danish source (Ole Rømer og den bevægede Jord – en dansk førsteplads? (2004) by Jan Teuber) accurately. This web-cite claims to be quoting comments directly from the source. On the interpretation of a crucial part of the 1667 English report of Roemer's work, Teuber is quoted as saying:

"This cryptic part is difficult to understand but an interpretation could be that the proportion between the velocity of light and the velocity of the Earth is like (40 revolutions of Io seen from one side of the Earth orbit + 40 seen from the other)*42½*60 to 22, this gives 9300 very near today's value 10000."

It is clear from this wording that Teuber is not at all certain of his interpretation, and is only proposing it as one possibility. The article, however, presents it as if there were no doubts at all about the interpretation.

But in fact it seems to me that Teuber has exaggerated the difficulty of understanding the report of Roemer's work. I. Bernard Cohen's interpretation (in the article I have already cited above) seems to me to make perfectly good sense, while Teuber's own doesn't. It's not clear whether Teuber has explicitly rejected Cohen's interpretation, or whether he was simply unaware of it. But in any case, Cohen's article has been widely cited in the scholarly literature, and no-one else seems to have raised any problems with his interpretation. I therefore propose to amend the article accordingly.

—David Wilson (talk · cont) 13:08, 7 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)

"BTW I am not responsible for posting the WP account: .... "

Grovelling apologies for jumping to the unwarranted conclusion that you were. I should have read the preceding discussion more carefully.

—David Wilson (talk · cont) 13:28, 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)

The use of a definition for c replaces the length standard with the time standard

Martin removed this explanation from the article without discussion on this Talk page:

The first effective measurements of the speed of light were made in the 17th century, and these were progressively refined. Modern time standards are more reproducible than length standards, so in 1983 the standard metre was defined in terms of the speed of light in a vacuum, replacing the length standard with the time standard.

and replaced it with

The first effective measurements of the speed of light were made in the 17th century, and these were progressively refined, until in 1983 the metre was defined in terms of the speed of light in a vacuum.

Unfortunately the replacement sentence contains a non sequitor. Why should improvements in measurement of the numerical value of c lead to use of c as a defined value, thereby making no use of this improved measurement skill at all, but making it moot? The earlier text has the merit of making sense out of this change as a means of improving the accuracy with which the metre can be used.

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

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

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

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

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

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

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

Physchim62, What I am beginning to witness here is a denial on the part of most editors, that something fundamentally changed in relation to the concept of the speed of light with the 1983 definition of the metre. It changed from a speed to a tautology. Few seem to want to admit that fact. David Tombe (talk) 16:42, 9 August 2009 (UTC)

Not at all, you are getting mixed up between the speed of light, a physical quantity which most people believe is unchanging, and its value in a given set of units of measurement. The distance between Paris and Marseilles (for example) hasn't changed significantly dispite the various redefinitions of the metre! There is only a tautology if you believe that we can somehow choose the speed of light to be whatever we want it to be. We can't, of course, but we can choose the metre (the human construct) to be whatever we want it to be, which is exactly what happened in 1983 (and several times previously, for that matter). Physchim62 (talk) 17:39, 9 August 2009 (UTC)

Physchim62, I know this is not the place to discuss this, but the second of course is most certainly not, as you say, defined "in terms of a frequency standard". It is defined in terms of a count, and that is what makes the difference.

David Tombe, what you need, is a soapbox. Try for instance Usenet. You'll feel much better at ease there, and the place was created for people like you in the first place. Good luck. DVdm (talk) 17:48, 9 August 2009 (UTC)

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

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

The statement by D Tombe

"What I am beginning to witness here is a denial on the part of most editors, that something fundamentally changed in relation to the concept of the speed of light with the 1983 definition of the metre. It changed from a speed to a tautology."

is not "soap boxing", but an observation upon editor conduct (or misconduct). It is not an abuse of Talk pages. Brews ohare (talk) 00:20, 10 August 2009 (UTC)

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

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

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

So let me "call a spade a spade" three times:

• Spade 1: The second is defined as a count.
• Spade 2: David Tombe is soap boxing his anti-relativity POV.
• Spade 3: Either David and Bruce don't understand what a count is, or they don't understand what a tautology is.

There you go, spades galore. DVdm (talk) 07:06, 10 August 2009 (UTC)

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

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

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

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

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

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

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

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

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

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

New lead paragraphs

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

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

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

Third paragraph, measurement history.

Fourth paragraph, practical consequences.

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

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

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

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

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

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

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

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

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

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

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

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

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

Chez37's input got reverted because he tried to express light speed exactly in miles per hour. You can't do that with decimal numbers. Check the definition(s) of mile and try it. Go ahead.

What about reducing the whole article to "Light goes pretty fast and relativity is all wrong". Would that make you happy?

DVdm (talk) 21:30, 10 August 2009 (UTC)

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

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

What would an encyclopedia writer do?

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

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

this speed is measured to find a more exact definition of the metre

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

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

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

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

For most purposes, the speed of light has been defined... as exactly 299,792,458

Exactly what are the "other" purposes suggested here, and should they not be mentioned explicitly? My feeling is that the words "for most purposes" should be deleted, inasmuch as the metre always is defined so the numerical value of the speed of light is always 299,792,458 m/s. These words are just nonsensical filler. Brews ohare (talk) 03:32, 11 August 2009 (UTC)

Astronomical measurements (within the inner solar system) don't use the SI definition: in the astronomical system of units, the speed of light is still an empirical constant with an associated measurement uncertainty. Physchim62 (talk) 03:39, 11 August 2009 (UTC)

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

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

Pre 1983 vs. post 1983

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

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

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

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

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

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

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

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

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

The Light-Year

The 1983 definition of the metre is a similar concept to that of the light-year. Therefore, this same problem would have existed before 1983 should we have wished to state the speed of light in terms of light-years per year. The speed of light has always been one light-year per year. But that hardly tells us very much about the speed of light. The post-1983 situation in SI units is just as useless. David Tombe (talk) 11:07, 11 August 2009 (UTC)

The definition of the light-year tells us all we need to know about the light-year. The definition of the metre tells us all we need to know about the metre. Your comments tells us all we need to know about about you knowledge of metrology and you usefulness as an editor on this article. Please desist from soapboxing your nonsense here. Physchim62 (talk) 12:33, 11 August 2009 (UTC)

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

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

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

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

Lou: it is a tautology in the sense that the speed of light must come out to be the magic number because λ = ct means that c=λ/t for any length λ and any time-of-flight t. The measure of the length "adjusts" to insure the result. If you prefer, c becomes a conversion factor. If you can't measure the thing, its value is what it is defined to be, it's tautological.

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

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

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

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

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

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

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

==Colloquial use as metaphor==

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

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

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

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

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

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

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

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

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

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

It makes the same stupid mistake that you do

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

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

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

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

Making points in the article

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

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

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

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

c^2 = 1/(εμ)

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

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

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

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

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

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

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

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

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

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

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

avoid confusion between 299,792,458 m/s and physical constancy of the speed of light

It does not fix confusion engendered by one sentence to suggest that the reader "should not be confused" by that sentence. This paragraph:

Currently the speed of light is used to define the metre and hence the speed of light in a vacuum in SI units is now fixed at 299,792,458 metres per second. This fact should not be confused with the physical constancy of the speed of light that was used as a postulate in the original formulation of the special theory of relativity, and which is independent of the choice of units.

is a disaster and should be rewritten.

Here are some things wrong with it:

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

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

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

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

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

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

Here are some problems:

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

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

Constancy of the speed of light

Sorry Lou but there are far too many problems in this section - see below

The speed of light is constant in many ways.

Unhelpful and chatty start

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

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

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

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

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

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

Finally, and somewhat confusingly,

Is it really that hard to understand?

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

'Numerically stable' it is fixed by definition.

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

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

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

What are the problems with the lead?

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

As the speed of light is a dimensionful constant

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

its numerical value

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

is dependent on the system of units used.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Speed of light in non-inertial frames and GR

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

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

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

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

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

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

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

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

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

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

Martin walks right into the trap

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

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

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

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

As I have said before, David seems to be reading a different book from the rest of us. Martin Hogbin (talk) 19:35, 13 August 2009 (UTC)

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

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

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

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

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

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

Martin: Pardon me: I am asking you whether you think "free space" is a medium with measurable properties, and if so, how come its electric constant and magnetic constant have exact values with no error bars (see links to NIST provided here), postulated values that are not measured?

It appears that instead, free space is a model medium with postulated exact properties, to which measurements upon real media may be compared. Do you agree with that?? Brews ohare (talk) 00:02, 14 August 2009 (UTC)

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

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

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

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

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

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

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

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

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

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

Reasons the speed of light was set by definition.

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

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

1. The new definition allows the meter to be more precisely realised.
2. More stable sources of light (stabilized lasers) than the krypton 86 lamp used in the previous definition became available.
3. Measurements (delineations of the metre) made with the new light sources agreed well with those made by the old ones
4. Realizations of the metre are more reproducible using the newer light sources
5. The new defined value is the numerically the same as the older (1975) recommended value
6. The new definition agrees well enough with older realizations that there will me no sudden jump in the length of the meter for all practical purposes.
7. After consultations and deep discussions on the subject it was agreed that this definition is equivalent to the older one, which was based on the wavelength of a defined light source.
8. The new definition still allowed the older krypton 86 source to be used for realization of the metre but also newer and better sources.

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

1. ^ See Maxwell p. 499 in A Dynamical Theory of the Electromagnetic Field (1864).

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