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I have just changed all the UK 'travelling' to US 'traveling' as the article seems to mainly use US English, however, I notice that the earliest versions of the article appear to have been written in UK English. We need to chose one and stick to it. Martin Hogbin ( talk) 09:32, 13 January 2009 (UTC)
Isn't it a bit bold to say that the speed of light "is the speed of not just visible light, but of all electromagnetic radiation, as well as gravitational waves"? After all, no one has ever detected a gravitational wave. Lestrade ( talk) 00:01, 18 January 2009 (UTC)Lestrade
Direct detection is so passé. The luminiferous æther was real for a while, why can't gravitational waves and black holes be real for us, too? Lestrade ( talk) 01:07, 18 January 2009 (UTC)Lestrade
Joseph H. Taylor and Joel M. Weisberg have observed the binary star system PSR B1913+16 for several decades. They claim that their observations indirectly prove the reality of gravitational waves and are within 0.2 % agreement with the prediction of general relativity. This claim is based on the logical conclusion that the orbits of the two stars should become smaller due to the emission of gravitational waves. Any other possible cause of the orbital decay is not considered. On the basis of this logical conclusion, they claim indirect proof of gravitational wave radiation. The orbits shrunk so there are gravitational waves. The scientific community has seized on this indirect proof and has been content to agree that gravitational waves are real. Due to the effect of the acceleration of the galaxy, Taylor and Weisberg do not think that the results of the test will improve. So, this is as close as we can get to proof of the existence of gravitational waves. Unlike electromagnetic waves, gravitational waves will have to remain a strong belief rather than an observed fact. Many sources of information, however, simply claim the reality of these waves and do not communicate their actual basis to the public. The Wikipedia article follows this trend. Lestrade ( talk) 16:13, 19 January 2009 (UTC)Lestrade
The sentence "gravitational waves travel at the speed of light" is a categorical, unconditional, unqualified, explicit, declarative assertion. Can such a direct statement be made about something that has, and possible never will be, directly experienced? Lestrade ( talk) 12:59, 20 January 2009 (UTC)Lestrade
Thanks for your conciliatory tone. A useful set of guidelines is Wikipedia:Lead section. In particular, it suggests the lead should provide an overview of the article, not just a few points that might interest 80% of the readers. That objective might be a handful here. Brews ohare ( talk) 20:31, 20 January 2009 (UTC)
If c is the fastest speed in the universe, then c2 must be p r e t t y, p r e t t y fast. Lestrade ( talk) 00:06, 18 January 2009 (UTC)Lestrade
Speed is a quantity that has the dimension length/time. Isn't it true that when dimensions of quantity are multiplied by each other, the product is of the same dimension of quantity, squared? For example, three feet times two feet are six square feet, but it is still feet. So, speed times speed is speed squared, but it is still speed. What else could it be that makes sense mathematically and physically? Lestrade ( talk) 14:17, 18 January 2009 (UTC)Lestrade
Height or distance is a one dimensional quantity, measured as a line. A square is a two dimensional quantity, measured as a surface or area. Some people say that c2 is to be considered as a proportionality or conversion factor. As such, only the absolute number of approximately 34,704,709,264 (if the English miles/second dimensions are used) or 89,875,543,056,250,000 (if the Metric meters/second dimensions are used) would be considered as the factor. In either case, it seems almost silly to say that we know that this number indicates precisely the ratio of energy to a unit of mass. Rather, it probably merely means "a really big quantity." Lestrade ( talk) 20:51, 18 January 2009 (UTC)Lestrade
Using those units, and assuming the literal truth of the equation e = mc2, one part of mass would convert to 0.0000000001492 parts of energy when a uranium nucleus is split. That's not much of a fission bomb. Lestrade ( talk) 14:05, 19 January 2009 (UTC)Lestrade
The section entitled 'overview' is not really an overview, and anyway this is the function of the lead section. I therefore propose that we merge the 'overview' section with the physics section and make it the first section of the article after the lead.
I also suggest that we remove references from the lead, which should be a brief overview of the article as a whole, and add them to the relevant parts of the body. Martin Hogbin ( talk) 10:45, 21 January 2009 (UTC)
I have just done a proposed rearrangement of the first half of the article, which you can find here. Have a look and see what you think Martin Hogbin ( talk) 13:26, 21 January 2009 (UTC)
If no one objects I will past the new bit in place. We can then work on improving it. Martin Hogbin ( talk) 22:46, 21 January 2009 (UTC)
I have added a section on light as EM radiation. At the end, it says that the speed of EM radiation is independent of frequency. I know this has been confirmed by observations on pulsars but I cannot find a good reference. Any offers? Martin Hogbin ( talk) 10:35, 24 January 2009 (UTC)
Brews, you have raised some points about this subject, but I am not quite sure what you are saying. Are you suggesting that further explanation of this topic is in required in the article? Martin Hogbin ( talk) 10:37, 24 January 2009 (UTC)
According to general relativity, gravitational waves travel at c, this is well known, as indicated by the reference from Hartle that I gave. There may be other, unconfirmed and not generally accepted theories, in which this is not the case.
The paper by Carlip that you quote refers to a recent attempt to measure the 'speed of gravity', and what is says is that the experiment did not actually measure the speed of gravity but the speed of light - there was some controversy over this at the time. It does not suggest in any way that they are different, just that the particular experiment failed in its objective to actually measure the speed of gravity.
It is true that the speed of gravity has not been confirmed experimentally but the only accepted, and well well-tested, theory of the subject (GR) predicts that it will travel at c. I believe that it is therefore justified to make the stronger statement that I made. Martin Hogbin ( talk) 16:27, 24 January 2009 (UTC)
No one has ever detected gravitational waves, but waves of gravitation travel at the speed of light. Lestrade ( talk) 20:38, 12 February 2009 (UTC)Lestrade
Brews, are these quotes from the FA review or your comments? Martin Hogbin ( talk) 17:00, 24 January 2009 (UTC)
The article frequently stumbles over whether statements like "all EM radiation travels at speed c" is (i) a definition, or (ii) a physical observation or (iii) a posit of some physical theories (e.g. relativity theory, quantum gravity theory etc.).
The article also frequently fails to point out what medium is under discussion: for example: is it the ideal free space which is the unobtainable reference state where c = c0, or is it outer space where the speed of light may not be c0 but possibly any measured value whatsoever?
I've changed some of these occurrences, but more are out there. Brews ohare ( talk) 16:41, 24 January 2009 (UTC)
Hi Martin: Your comment: "Now if it transpires that there is some small variation in the speed of EM radiation in free space with frequency (and no definition could prevent this)" is, in my opinion, based upon a misconception that measurement can impinge on the defined properties of "vacuum". In practice, any measurement must be in a medium. Hence, according to BIPM, the measurement must be corrected for its non-ideal behavior. Changes in accuracy of the measurement will affect the corrections to be made in this medium in order to refer it to the defined state of "vacuum", but do not change the defined "vacuum". It appears to me that you see free space as something real, while it is only an unobtainable reference state, like absolute zero. Brews ohare
Hi Martin: Our editing session overlapped. Where the "vacuum" of BIPM or "Free space" is concerned, experiment is not an arbiter; rather it is simply a matter of convention and in turn a matter of convenience. Just like any other standard.
In particular, if the meter established in air at two different light frequencies were to differ, the difference would be attributed to the medium "air" and not to the vacuum of free space. If some "universal dispersion correction" showed up in all media, and if the international community decided that making this correction all the time was a nuisance the definition of "vacuum" could be changed by international agreement to incorporate "the dispersion of the vacuum". The properties of the "vacuum" are not a reflection of nature, but of convention. The properties of nature are expressed in permeabilities and permittivities relative to the "vacuum". Instead, a 'standard medium" could be adopted, but that would require tests to see of the standard were met. Apparently, corrections to refer to a "standard medium" is viewed as a more cumbersome approach. Brews ohare ( talk) 18:27, 24 January 2009 (UTC)
Hi Martin: Yes. By definition of "free space", rather than by definition of the speed of light. Brews ohare ( talk) 14:09, 25 January 2009 (UTC)
When I look at the article you link to above I get this (my emphasis),'In the reference state of free space, according to Maxwell's equations, electromagnetic waves, such as radio waves and visible light (among other electromagnetic spectrum frequencies) propagate at the defined speed of light'. Is that what you are referring to? Martin Hogbin ( talk) 14:55, 25 January 2009 (UTC)
Firstly, can you confirm that the quote above is what you believe defines the speed of all EM radiation to be c?
I am unsure what quote is referred to here. However, the speed of all EM radiation in free space is c by definition, because, by definition, free space has no dispersion, no dichroism, no nonlinearity, and no anisotropy. The only issue in any actual measurement is whether it actually was done in a medium that approximates "free space", and just what corrections have to be made. Present theory on the QCD vacuum and the quantum vacuum suggest that realization of one of these vacuums is not tantamount to realization of free space, as these vaccua in theory are not isotropic, do exhibit dichroism and are nonlinear at large field strengths. Brews ohare ( talk) 16:26, 25 January 2009 (UTC)
The speed of all EM radiation in free space is c by definition, because, by definition, free space has no dispersion, no dichroism, no nonlinearity, and no anisotropy. The proof of these statements is the defined values of electric constant, magnetic constant, impedance of free space and speed of light in free space. All these properties of free space by definition are dispersionless, field independent, and scalars (not tensors). If they are plugged into Maxwell's equations, they result in dispersionless, isotropic, field-independent propagation of EM waves in free space.
Of course, every realizable medium, be it outer space, partial terrestrial vacuum, QCD vacuum, or whatever, has a permittivity and permeability that can be measured to some level of accuracy and will exhibit (to a degree determined theoretically and/or experimentally) anisotropy, dichroism, nonlinearity and dispersion. Brews ohare ( talk) 16:49, 25 January 2009 (UTC)
Granted; not by definition; also, "speed of all frequencies of EM radiation" is meaningless without stating whether "free space" is meant, or some real medium. If "free space" is meant, then experimental observation is impossible and irrelevant. Brews ohare ( talk) 00:46, 26 January 2009 (UTC)
"Free space" can be viewed a limiting case, I agree. It can be approached by some real media, but it cannot be reached. Theoretically, there is no "vacuum" with its properties. Experimentally, we can establish a medium resembles free space within experimental error, but of course there is always the possibility that further refinements in technique will reveal differences.
The EM properties of free space are defined by ε0 and μ0 (or by some pair of these two and Z0 and c0). These values constitute a reference case. It might be possible that a real medium comes close to the reference. Unfortunately, because of experimental error, it never can be established that any real medium is free space, only that it is close to free space.
That is why the logical impossibility of "measuring" free space has to be recognized. All we can do is measure a real medium and point out that it is as close to "free space" as we can tell within our experimental limitations.
Therefore, no statement in the article should suggest that we can measure free space. Nor, can we say measurement puts "limits" on the free space parameters: they are defined; they are not measured. They could be totally different, they could be tensors, etc etc. There is no underlying philosophical "meaning" to free space having the parameters it does. The values of ε0, μ0, Z0 and c0 are simply historical accidents.
We could choose instead a real standard medium like air that could be realized. That would not prove to be an advantage, because we could never be sure that our reference sample of air was exactly the same as the standard. We'd have to maintain a standard air sample for comparison and reassure ourselves that it was not changing with time because of improper storage, etc.
So choosing an arbitrary ideal reference is easier, especially if we can maintain a vigilant list of "best practices" to relate any measurement in real media to the standard. Hopefully, the standard has the merit that these "best practices" are not too difficult to apply in practice. (You tell me if preparation of an atomic clock is simple?) The BIPM and its associates decide by international agreement whether the reference is appropriate, and will change it if there appears to be a simpler or a more accurate standard that would make for "best practices" that were easier to use, or accessible to greater accuracy experimentally.
A statement like: we would expect any variation of speed with frequency to be less in the medium of ideal free space that it is in the medium of outer space. Thus it is justifiable to say that experimental evidence sets limits on the variation of speed with frequency in free space. is misleading: given Maxwell's equations we know that "free space" has absolutely zero dispersion. And we know that the validity of Maxwell's equations has nothing to do with the values of the parameters in free space. Maxwell's equations are established by measurements in the real world, on real media. Whatever real media behave like has no bearing upon the parameters of free space, neither upon the actual values nor their scalar dispersionless nature.
We could imagine a hypothetical Universe where every real medium we measured exhibited dispersion. These physical facts need not impact the parameters of free space. It might be decided that reference to free space was still the simplest methodology. Or, the BIPM might decide that free space would be modified. That is a decision based upon practicality and politics.
Again: because of its defined properties, no statement in the article should suggest that we can measure free space or that experiment "constrains" the parameters of free space to have particular values, or to be scalars, or to be dispersion free, or … . Brews ohare ( talk) 13:00, 26 January 2009 (UTC)
Martin: I have no trouble with free space. I have trouble with statements in the article that suggest measurements have something to do with free space. Measurements are corrected to refer to free space, but do not impinge upon the defined properties of free space. Brews ohare ( talk) 18:29, 26 January 2009 (UTC)
Thank you, Martin. Brews ohare ( talk) 19:30, 26 January 2009 (UTC)
Hi Steve: An interesting formulation of the discussion. Under (2), Maxwell's equation can be written for any medium: you just need the constitutive equations. The relevance of free space under (2) is that the constitutive equations are simply specification of ε0 and μ0. Evidently this specification does not need to apply to any real material (although it might). Free space is just this hypothetical material. As regards (1) there may be a sequence of steps (e.g. evacuation of a flask) that cause ε → ε0 and μ → μ0). Such a sequence might suggest free space is most nearly to be realized by pursuing this sequence. But so what? You can measure c in any medium and make corrections so your results are referred to free space, whether or not media approximating free space actually exist. There is zero information in free space and its constants.
Is there literature support for saying that [some limiting sequence] is not actually the true definition of free space? I'd say the NIST web site where ε0 and μ0 a stated as defined values is the true definition, not the possibility of some limiting sequence. Maybe there is some historical commentary on this point, but it has nothing to do with our final definition. And, just to be nasty, is there any literature supporting the limiting sequence idea?
In the context of this part of the article, I believe it makes more sense to discuss (1) than (2), since (1) is the basis for statements about the universe. Free space is not about the universe. Measurements of real constitutive relations for real media, like outer space or terrestrial vacuum, is about the real universe, quite independent of what we adopt for free space.
Historically, the yard may be related to the arm-length of monarchs and the metre to the dimensions of the planet Earth, but neither has any relation to the Universe as observed in science. Brews ohare ( talk) 20:40, 26 January 2009 (UTC)
To put things differently: there is a continuing effort experimentally to determine if the speed of light in space is isotropic, and to see if it varies with frequency. Evidently, Maxwell's equations using ε0 and μ0 predict no success. Would a success prove that space is not a good realization of "free space", but a medium with its own constitutive equations, or would it prove Maxwell's equations are wrong, or something else (maybe some weird aspects of general relativity: varying speed of light theories)? None of these three categories of conclusion would change ε0 and μ0. Brews ohare ( talk) 20:57, 26 January 2009 (UTC)
Hi Steve: Changing terms to "vacuum" we still have cO as the speed of light in a hypothetical medium. Only now "vacuum" is vague and subject to misinterpretation as a "real" medium.
if the meter is defined in relation to the speed of light in free space, and no real-universe measurement could ever give information regarding the speed of light in free space, then there isn't any way to confirm that a meter isn't the size of an atom
The first condition is met: the meter is defined in relation to the speed of light in free space
The second condition is met: no real-universe measurement could ever give information regarding the speed of light in free space
The conclusion is false: there isn't any way to confirm that a meter isn't the size of an atom
Here is how it could be done. Measure the meter in air, assuming the definition applies in air, not free space. Measure the refractive index of the air. Adjust the size of your meter according to the index. Brews ohare ( talk) 22:46, 26 January 2009 (UTC)
Hi Steve: The issue here is the usual one that occurs in setting up a standard, namely the "accumulated wisdom" effect. For example, how do we determine that one clock is better than another clock? It is partly accumulated experience on how well supposedly identical clocks agree, and partly a theoretical assessment as to what influences might disturb the clocks and how big these disturbances are expected to be. When experience shows the clocks don't all agree, we look around and find that it is gravitational dilation, an effect within theoretical estimation, one we should have seen coming, but one that simply was overlooked. Or, maybe its something else.
As technique improves, discrepancies in indices will show up. Sample vacuums that are supposedly identical turn out to be different. In accounting for the difference, maybe some of the observed discrepancy is due to quantum fluctuations of the vacuum, or whatever. In conjunction with theory we decide what these contributions are, learn how to avoid them, or account for their effects in the sample vacuums, and subtract them from measurements to get the "free space" value.
In dealing with air, present practice seems to be based upon some formulas that people feel have been well tested against different air samples. You just measure the partial pressures of water, CO2 etc. in your sample and calculate the index for your case. Brews ohare ( talk) 03:30, 27 January 2009 (UTC)
Rather than continue the long discussion on the philosophical nature of free space, I would like to suggest the reversion of the wording of the section above from Brews' version:
'According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. Observations in outer space and in good terrestrial vacuums confirm that in such mediums any variation of the this speed with frequency is extremely small, suggesting that, in this regard, they are good approximations to free space'.
to my original version:
'According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. Observations confirm that any variation of the this speed with frequency is extremely small'.
I believe that my version is clearer, fully justified, and says all that needs to be said. I am not against adding more detail but I think that Brews' attempt to make some kind of philosophical point in the article is wrong and unhelpful to the reader. Martin Hogbin ( talk) 09:37, 27 January 2009 (UTC)
I suggest:
'According to classical electromagnetism, the speed of electromagnetic radiation in a perfect vacuum is the same for all frequencies. Observations confirm that any variation of this speed with frequency is extremely small'.
I think everyone will understand what a perfect vacuum is; it's the extrapolation of a better and better physical vacuum; while OTOH a lot of readers will not have heard of free space. The phrase "speed of light in vacuum" is an order-of-magnitude more common on Google than "speed of light in free space". NIST and CODATA both use "vacuum", not "free space". And moreover, to the extent that that "free space" means something different from "perfect vacuum" (and I'm still not sure it does in common usage), "perfect vacuum" is what we want to talk about, since "free space" (as Brews uses the term) has no necessary relationship to anything in our universe, while this is a physics article about our universe and its laws. -- Steve ( talk) 17:18, 27 January 2009 (UTC)
'Measurements based on the arrival of electromagnetic radiation from distant astrophysical events puts severe limits on the possible variation in the speed of light with frequency'. This is much closer to what is actually stated in our reliable source and it shows the context of radiation travelling through outer space. Martin Hogbin ( talk) 19:37, 27 January 2009 (UTC)
I prefer Martin's version. It's overly modest to say that they merely set limits on dispersion in outer space; with a bit of simple analysis, it actually sets quantitative limits on the dispersion in the "extrapolated better and better vacuum" (call it what you will). -- Steve ( talk) 22:00, 27 January 2009 (UTC)
The reason I dislike this, as I said above, is that it makes it sound like this is a measurement of a certain property of the interstellar medium. It's more than that, it's an experimental constraint on the basic laws that govern our universe. It's as if you were describing the Millikan oil-drop experiment as a "demonstration that small droplets of oil suspended in air have a quantized charge". It's accurate but gives readers the wrong idea. :-) -- Steve ( talk) 04:51, 28 January 2009 (UTC)
Brews, all you have written above and in the section below is your opinion, which you are entitled to, as I am entitled to mine, but articles in WP should be based on reliable sources, not opinion. Now I am not fan of quoting sources verbatim, I am happy with a degree of interpretation for the benefit of non-technical readers, but only when there is general agreement about the facts. In this case there is a difference of opinion, which I am happy to discuss, but until there is a consensus to do otherwise we should say what what our source says, in the words that it says it in. Martin Hogbin ( talk) 13:57, 28 January 2009 (UTC)
Steve and Martin: You both seem to believe that the measurement of of the properties of the interstellar medium has ramifications that go far beyond the properties of this particular medium. However, all the measurement actually does is establish the properties. Whatever you two want to read into it requires that this fact be placed within a theory where the theory provides a significance beyond the simple results of the observation. So bite the bullet and dig up the sources and write the summary paragraph that shows that the existence of some media with ε and μ as close to ε0 and μ0 as present accuracy can determine is a much more meaningful thing than the simple fact.
As far as Maxwell's equations go, they are totally indifferent to whether outer space approximates ε0 and μ0 or has some wild material properties. EM waves can propagate in any medium, and at any speed up to c0. The measurement showing some media exhibit ε ≈ ε0 and μ ≈ μ0 doesn't do more than establish the existence of media with these properties.
It's like the M - M measurement of the speed of light as a function of the movement of the observer. : you need relativity to interpret the facts before the implications become clear. And it seems all special relativity demands is that an upper limit on speed of light exists, regardless of whether c0 applies to outer space.
Maybe an approach to your discussion could be to show that if outer space exhibited dispersion it would have amazing repercussions according to some theory? Brews ohare ( talk) 13:17, 28 January 2009 (UTC)
Brew, I still cannot work out what your point is. Perhaps it would help if we see what are the things that we agree on. Do we agree the following?
Free space is an idealization that is physically unobtainable.
The speed of light is defined to be a certain value in free space.
Actual measurements of the speed of light are made in media other than free space.
According to Maxwell's equations, all frequencies of EM radiation travel at the same speed in free space.
Martin Hogbin ( talk) 18:11, 28 January 2009 (UTC)
We don't have to infer or speculate about the properties of free space: they are defined, and therefore certain. We can speculate whether outer space behaves like free space, and attempt to support these ideas by measuring outer space. Brews ohare ( talk) 05:49, 29 January 2009 (UTC)
Where are the properties of free space defined? Martin Hogbin ( talk) 21:27, 29 January 2009 (UTC)
Maxwell used Newton's equation for the speed of sound in a long solid rod in order to obtain the speed of light on the basis of an elastic medium with a density related to the magnetic permeability, and a transverse elasticity related to the dielectric constant.
This suggests to me that light is a coherent dispersionless wave in an elastic solid.
Maxwell obtained the concept of displacement current from his own postulated elastic medium, and the concept was used in his derivation of the EM wave equation in 1864, three years after he did the analyis above with Newton's equation.
The modern derivation of displacement current bears no relationship to Maxwell's derivation of displacement current, and the modern derivation doesn't have the correct divergence properties to allow it to connect to the electromagnetic wave equation.
This is all telling us something about the nature of the so-called vacuum. David Tombe ( talk) 16:07, 28 January 2009 (UTC)
Martin, All I did was corrected the facts in the already existing section about the aether. You are claiming that the Michelson-Morley experiment discredited the aether. That is not what the historical record tells us. The Michelson-Morley experiment could have confirmed the Stokes aether drag model. But Lorentz believed that the Stokes aether drag model contradicted stellar aberration. Lorentz himself believed that the aether blew right through the Earth and he devised his transformation equations on the basis that the aether wind contracted the Michelson interferometer. The aether was dropped from physics when Einstein gave a new interpretation to the Lorentz transformations. But the Michelson-Gale-Pearson experiment of 1925 detected an aether wind due to the Earth's diurnal motion.
Is there anything that I have said above that is not true? You have undone my revisions and told me not to add sections on the aether. I did not add any section on the aether. There was already such a section there and it is inaccurate. All I was doing was correcting the historical record.You are obviously intent on upholding the inaccuracy. You are pushing your own point of view here. David Tombe ( talk) 06:48, 29 January 2009 (UTC)
Martin. Yes, diurnal refers to the rotation of the Earth. The Michelson-Gale-Pearson experiment in 1925 got interference fringes in that regard. David Tombe ( talk) 04:33, 30 January 2009 (UTC)
Steve, there have been so many contradictory experiments on this issue, that I don't know who to believe. We must also remember that we need to scrutinize the theory behind the interpretation of an experiment to ensure that we are not dealing with any tautologies. I had a look at Mossbauer spectroscopy in this regard and I could see that it was using the Lorentz transformation equations in the interpretation. One paper that I dragged up on google then left the interpretation open as between Einstein and Lorentz aether theory.
I don't mind whether or not you include Michelson-Gale in the main article. I was merely objecting to the outright assertion that Michelson-Morley in 1887 had discredited the idea of a luminiferous aether altogether. I was pointing out that the historical sequence of events since 1887 never led to grounds for any such absolute assertion.
Do you have any links to the 1970 experiment which you have referred to? I would like to read it. If not, does it involve the use of Einstein's special theory of relativity in the interpretation? David Tombe ( talk) 08:24, 30 January 2009 (UTC)
Steve, I don't have the book immediately to hand. Can you please tell me if the experiments in question use Einstein's special theory of relativity in the interpretation.
Also, in the other experiments which you mention in your recent edits to the main article, in which the speed of light has been measured at 'c' relative to sources that are moving at 99% c, is the measured value of 'c' in that case both relative to the source and to the frame of reference in which the source is moving at 99% c? David Tombe ( talk) 17:10, 30 January 2009 (UTC)
Newton didn't write equations. He wrote quasi–geometrical proportions of ratios. Lestrade ( talk) 20:26, 30 January 2009 (UTC)Lestrade
My point is merely that there can be no "Newton's equation" because Newton never published an equation. All of his published mathematics were in the form of proportions. You may consider this to be insignificant. Someone else may consider it to be significant. However, the world is so full of fictitious fabrications that, I guess, there is always room for one more. I had thought that it might be better if we only spoke about what we know to be true. Lestrade ( talk) 14:19, 31 January 2009 (UTC)Lestrade
Lestrade, OK I take your point. I admit that it is an interesting historical piece of information in its own right which I had not been previously aware of. I do intend to study Newton in detail eventually. David Tombe ( talk) 05:04, 1 February 2009 (UTC)
I think that there may be some confusion concerning experiments to detect the aether. The MMX was intended to detect the Earth's motion through the aether due to its orbit round the sun. It gave a null result as did all following experiments designed to detect the same thing; thus the simple rigid aether is ruled out.
The Earth's rotation (spin on its axis), however, can be detected by more sensitive interferometric apparatus, this effect is the basis of the laser gyro, and it is consistent with both SR and a simple aether theory. However the simple aether cannot be considered a valid explanation for these observations as it had already been ruled out as a possibility. Martin Hogbin ( talk) 20:51, 30 January 2009 (UTC)
I'll try to clarify a few points here. I am suspicious about the experiments referred to in Jackson, and as to what they exactly mean. But I haven't fully investigated them yet, so I can't properly comment.
In my view, the Michelson-Gale experiment showed up the aether drift in relation to the Earth's diurnal motion, just as Steve has described above.
Martin has claimed that the 1887 Michelson-Morley experiment has ruled out a rigid aether. How exactly has it ruled out a rigid aether if the aether were to be entrained in the Earth's orbital motion right up to a cut off line where the Earth's gravitational field gives way to the gravitational field of a neighbour? Such entrainment would fully account for Michelson-Morley. Also, the Earth could be rotating within that entrained aether, accounting for Michelson-Gale.
I am not insisting on mentioning Michelson-Gale in the main article. I am not insisting on pushing the aether entrainment model either. I was merely neutralizing the wording to match the historical facts that related to why the aether was abandoned in modern physics. It is a matter of point of view to categorically state that Michelson-Morley actually discredited aether theory. It didn't do that. It began a series of postulates and investigations which ultimately led to the aether being abandoned.
Meanwhile, I would like to know more about those Jackson experiments. I am very suspicious of claims of highly accurate experiments involving measuring the speed of light coming from tiny particles that are travelling at 99.9% of the speed of light.
I would also like to know why they were doing such experiments in relation to the Earth's diurnal motion. Did they have some doubts about a certain matter? And do they use the special theory of relativity in the interpretation, because if they do, then it is a tautology to use a theory which depends on the constancy of the speed of light to prove the constancy of the speed of light.
David Tombe (
talk) 06:15, 31 January 2009 (UTC)
At present the article mentions aether and the MMX twice, once under 'Luminfiferous aether' and again under 'Michelson-Morley experiment'. I suggest that we combine these two sections into one, giving a clear historical perspective to the aether and the MMX and showing current mainstream scientific though on the subject. I have created a page and copied the current two sections there. We can combine them into one to produce a new section and then put the completed section back here. Martin Hogbin ( talk) 10:31, 31 January 2009 (UTC)
Steve, I removed one of your references because nobody is disputing the point that it makes and it is not related to the issue in question. I left the other reference because it would appear to be relevant. I'd certainly like to check that reference out. David Tombe ( talk) 11:08, 31 January 2009 (UTC)
I had done a draft of my proposed section which I think gives a fair view of the history and current status of the aether. Martin Hogbin ( talk) 13:53, 31 January 2009 (UTC)
The sentence stating that for many practical purposes the speed of light can be considered infinite has been changed many times, practically going full circle.
The concept that we are trying to convey is that for many real purposes the speed of light is so high that we can assume that it takes no time at all to get from A to B without causing any significant errors or problems. Examples would be, counting seconds to get the range of lightning, or starting signals for Olympic athletes.
The problem is, how to convey this message clearly, succinctly, and in good English. Martin Hogbin ( talk) 20:56, 30 January 2009 (UTC)
Steve, I am going to take it that your Jackson reference refers to a 1960 Mossbauer effect experiment by Ruderfer. It is a null result experiment with a highly far fetched interpretation. It has been widely criticized, including by Ronald Hatch who was heavily involved in the Global Positioning system.
Here is a web link listing some of that criticism, [3]
I don't think that your Jackson/Ruderfer reference is a balanced reference. You certainly have no basis whatsoever to claim its superiority over the Michelson-Gale-Pearson experiment. David Tombe ( talk) 15:22, 31 January 2009 (UTC)
Steve, was the experiment carried out indoors or outdoors? And ultimately, does the experiment disprove the existence of the aether or not? David Tombe ( talk) 17:11, 31 January 2009 (UTC)
Martin, In 1845,Stokes advocated an aether theory in which the aether was entrained with the Earth. It drew criticism on the grounds that the material of the aether would have to be such that it would behave like a solid at low pressure, such as to allow the transmission of transverse electromagnetic waves, yet it would have to behave like a liquid when under high pressure at the shear lines where it tapered off into distant space. Stokes didn't know the solution to this problem, but he was convinced that such a solution might eventually be found.
That was Stokes' opinion. Lorentz opposed the Stokes model because he believed that it contradicted the observed measurements of stellar aberration. That was Lorentz's opinion, based on no greater a knowledge than Stokes had about the nature of the material of the luminiferous aether.
Lorentz devised his own aether wind and aether contraction model to account for the Michelson-Morley experiment. The Lorentz theory was modified by Einstein and the aether was dropped from the physics books.
It is important to record this sequence of events correctly. There is no basis to categorically state that Michelson-Morley discredited the aether.
The aether has been creeping in to modern physics again through the back door. First there was the electron-positron Dirac sea, and more recently the polarized vacuum in quantum mechanics. David Tombe ( talk) 04:59, 1 February 2009 (UTC)
You still have not defined what you mean by aether, thus it is impossible to agree or disagree with you. If you mean by aether 'everything that is not yet understood in physics' then I guess it is creeping back. Martin Hogbin ( talk) 10:56, 1 February 2009 (UTC)
I have provided a reference that places limits upon the dispersion of light using astronomical observations. This reference refers to these limits as a limit upon propagation in "space" or "vacuum". Martin insists that by use of these terms, the reference means to refer to "free space" where ε = ε0 and μ = μ0 by definition. That is, Martin suggests that in fact definitions are constrained by experimental measurements.
It is obvious that these astronomical measurements refer to transmission through outer space (that is the medium pervading the nearly empty regions of the universe). It is furthermore obvious that although outer space may be well approximated by ε = ε0 and μ = μ0, the accuracy of such approximation can be established only by measurement of the medium of outer space itself (which is, of course, exactly the purpose of the cited paper). It is further obvious that there is absolutely zero dispersion if the medium has an ε = ε0 and μ = μ0. Thus, the reference has succeeded in placing limits upon how far outer space strays from such a medium with ε = ε0 and μ = μ0. I simply cannot understand Martin's objections to my statement of this fact, which he now no longer bases upon reason, but upon exactly parroting the words of the reference, and moreover, parroting these words in a misleading context, in a fashion that makes the ridiculous and unsourced implication that measurement of ε and μ in the medium of outer space somehow limits the definitions of the defined values for ε0 and μ0 Brews ohare ( talk) 01:45, 1 February 2009 (UTC)
So, propose a definition and support it with a reference. Personally I think an adequate definition of free space is a medium in which ε = ε0 and μ = μ0 exactly. Brews ohare ( talk) 02:43, 1 February 2009 (UTC)
In real terms, permittivity and permeability are EM properties of real measurable media. ε0 and μ0 are defined permittivity and permeability of a hypothetical medium free space. Some real media have ε≈ε0 and μ≈μ0. Of course, measurement error means we can never say of any real medium that its ε=ε0 and μ=μ0, only that equality exists to within experimental error. More than that, the existence of media with ε≈ε0 and μ≈μ0 is not a prerequisite for choosing to define a hypothetical medium with these defined values, although it may add to the convenience of said definition if there are in fact some real media that do exhibit ε≈ε0 and μ≈μ0. Brews ohare ( talk) 05:37, 1 February 2009 (UTC)
In my view, and not being trained in interpreting historical documents, not a sophisticated view, Maxwell viewed the aether as just another medium, as real as quartz, say, but with its own permittivity and permeability explained by him in what is to me a very complex fashion involving vortices etc. etc. Thus, in my unversed interpretation, one could ask the question today, does the aether have ε≈ε0 and μ≈μ0? Apart from various practical difficulties in arranging to assuredly have an example of aether to measure, the matter would be settled by appeal to experiment. Whatever the outcome of this exercise, it would have absolutely no bearing upon whether free space, a very technical term with a very specific meaning, has ε=ε0 and μ=μ0. In fact, because free space is hypothetical and its properties based upon definition alone, these properties are untouched by any experimental observation. The only question one can reasonably ask about free space is whether it is at all useful, for example, as a reference state. In support of its utility one might advance that outer space or extremely good terrestrial vacuum approaches the behavior of free space, so it serves as some kind of idealization of some real media. Or, one could argue that use of a an idealized free space is more practical than maintaining a "standard medium" with "standard ε, μ" in some lab in Paris to which one would have to refer all measurement of ε, μ for any other sample of a real medium. Brews ohare ( talk) 06:21, 1 February 2009 (UTC)
Martin: you have not got the point here. I think it boils down to a belief on your part that "free space" is real. Is that your belief? If you do believe that, my next question is: how can the ε and μ of some real medium be established by definition (for free space, ε=ε0 and μ=μ0 by definition as you can find on the NIST website)? It seems to me that ε and μ of some real medium must be established by measurement (or by calculation based on related measured properties such as the density of constituent polarizable components with established EM properties of their own). Such values never could be set by definition but always would be accompanied by a ± error bar due to experimental uncertainty. In contrast, free space has ε=ε0 and μ=μ0 exactly. Can you fill me in on the origin of your ideas?
The purpose and objective in setting up free space is to supply a standard reference state to which measurements in all real media may be referred. It is unnecessary to establish these free space values by experiment. What is necessary is a set of "best practices" to correct measurements so as to refer to the reference state.
Steve seems to suggest that a sequence of measurements on vacuums made more and more "perfect" (that is, by pumping down further and further to eliminate polarizable constituents) can be extrapolated to the value of "free space". That approach probably could be adopted as a best practice for present technology. A different "best practice" would be needed to correct the pulsar observations of the cited article on dispersion, inasmuch as we cannot tinker with outer space to make it a more perfect vacuum. (I'd guess that observations would be screened to eliminate those whose light went too close to distorting celestial objects. In any event, the observations lead to "limits" that is ± error bars, according to the article, not to exact values for the dispersion.) We have yet to develop best practices that anticipate the advance in techniques that will allow observation of the nonlinear and dichroic behavior of quantum vacuum, which will lead to departures of quantum vacuum from "free space" despite following the extrapolation procedure Steve suggests. That discrepancy might lead to changes in the definition of "free space". However, it equally might not, and instead just lead to adding some more corrections to the list of "best practices". Which course of action is taken will be a judgment based upon the entire metrology, and international consensus on what is the simplest and most accurate way to define the reference state. It is a stretch to call this kind of deliberation over best practices a "measurement" of the properties of free space. Brews ohare ( talk) 16:31, 1 February 2009 (UTC)
A departure in the form of Maxwell's equations due to observation of various media, of which quantum vacuum and outer space are examples (as already explained earlier in this discussion) might lead to a revision of the definition of free space, but that revision of the standard of "free space" is not based upon "agreement" of ε0 and μ0 with experimental data, but is based upon the practicality and accuracy of a revised standard versus the present standard. (In this context, "accuracy" refers not to how closely ε0 and μ0 agree with experimental data, but rather refers to how revised measurement procedures compare in terms of the ultimate ± errors of the entire procedure that corrects observations to refer to "free space".) Those considerations of metrology are widely based (involving possibly the standards for time, length, and other issues of accuracy, convenience and what not etc.) and may simply lead to a new set of "best practices" without any change in the definition of free space. How the cookie will crumble cannot be assessed until the nature of the departures is understood. I believe this kind of discussion about how metrology selects a standard is somewhat outside the present scope, which I would take to be a discussion of how free space as used today fits in with the presently accepted form of Maxwell's equations. Brews ohare ( talk) 21:44, 1 February 2009 (UTC)
The sentence in the Wiki article that I dispute is
There are two problems with this sentence.
Next, let us ask whether a modified form of Maxwell's equations could impact the above points were it to evolve that free space exhibited dispersion when used with these updated Maxwell equations.
I would suggest not. The observations suggest that outer space does not exhibit dispersion (to within the accuracy of the observations). Were the updated Maxwell's equations to be adopted, we would then have to explain why outer space does not exhibit dispersion.
We would not have to explain why free space does exhibit dispersion, because that would be a logical deduction using ε=ε0 and μ=μ0 and the updated Maxwell equations, and not an observation.
We then might open an inquiry into whether a new definition of free space were desirable, for example, a new definition that would lead to zero dispersion in free space despite the propensity of the new Maxwell equations to predict dispersion. The considerations entering this deliberation could be the subject of discussion in this article, but do not bear upon the numbered objections above. Brews ohare ( talk) 00:13, 2 February 2009 (UTC)
Martin: You are simply ducking the issue. What medium other than "outer space" could possibly be meant???? Please make a suggestion for an alternative medium to pervade the Universe. Brews ohare ( talk) 00:30, 2 February 2009 (UTC)
Go ahead and state your interpretation of observations in outer space that are independent of the material properties of the interstellar medium. Brews ohare ( talk) 00:39, 2 February 2009 (UTC)
Martin: You have made no attempt to convince: you do not respond to the arguments presented, but wander about in your own world. What medium other than "outer space" could possibly be meant???? Brews ohare ( talk) 00:49, 2 February 2009 (UTC)
BTW, the source actually never mentions "space" or "vacuum" or anything at all except the "speed of light" without any reference as to the medium referred to, rather a large gaff, I'd say. Brews ohare ( talk) 01:03, 2 February 2009 (UTC)
As the error bars of EM measurements are reduced, for example, we might be able to measure a polarization dependence of the speed of light in quantum vacuum. Free space as presently defined with Maxwell's equations as presently understood does not exhibit such a dependence. So what do we do? Do we revise the definition of free space so it also exhibits dichroism using the accepted Maxwell equations, or not? Will we want to modify Maxwell's equations so free space as presently defined also exhibits dichroism? Are these questions ones we want to explore in this Wiki article? Brews ohare ( talk) 00:30, 2 February 2009 (UTC)
Martin: You are indulging in deliberate distortion. I have said nothing of the kind. I have explicitly considered the possibility that dispersion in free space could occur, and demonstrated the irrelevance of this possibility. I have merely suggested that the statement in the article is inadequately phrased and positioned. See my numbered points 1 and 2 above:There are two problems with this sentence.
Martin: Below is my earlier presentation of the role of dispersion, which you have ignored. Brews ohare ( talk) 15:16, 2 February 2009 (UTC)
Martin, it's got everything to do with the topic in question. Aren't you talking about the physical meaning of permittivity and permeability? Maxwell gave good evidence that these terms refer to elasticity and density in a sea of molecular vortices. How can you discuss a subject like this and block the key points from the discussion by upholding wikipedia's rules and regulations on speculation. Could we discuss the second world war without mentioning the Germans? Maxwell's papers were alot more than speculation. They introduced important concepts and equations which we still use today.
The point that I was making here is that Maxwell's luminiferous medium was never discredited. A series of events led to it being abandoned, and that's how you need to write it up in the article. You cannot make rash and inaccurate statements to the extent that Michelson-Morley discredited the aether.
And now you ask me what is the aether. Well Maxwell's vortex sea was not the aether as such, but it involved the aether. His luminiferous medium was an elastic solid composed of electric particles that existed around the edge of aethereal vortices. It was an incomplete theory. There were no sinks or sources in his vortices.
A polarized vacuum is creeping back into physics again through the back door, on the back of Dirac's electron-positron sea. Make the electrons into aether sinks and the positrons into aether sources and consider an electron and a positron in mutual orbit. That dipole would be pretty close to a Maxwellian vortex, and a sea of such dipoles would be pretty close to the Dirac sea. And what is the aether itself? It is space. But not the rigid static space that you have in mind. It is a dynamic compressible and stretchable space with field momentum A, equivalent to the magnetic vector potential. That's how Maxwell saw the A vector, and Dirac is also on record as having said that A must be a velocity. So space as you know it is a sea of tiny whirlpools with their axes aligned along the magnetic field lines. The fine-grained angular momentum density is B = curl A.
But as far as the main article is concerned, I think that all you can say is what Brews is saying, and that is that the permittivity and the permeability, as in free space, are constants of free space. The textbooks probably wouldn't permit any deeper knowledge on the matter. There is an equation which links these two constants to the speed of light. That equation is a skeleton version of Newton's equation for the speed of sound. Perhaps you could put the two equations side by side and make the inference that permeability is a density and that permittity is the inverse of transverse elasticity. David Tombe ( talk) 13:21, 1 February 2009 (UTC)
Steve, As you say, the spinning Mossbauer effect experiment is published in a peer reviewed journal. But it is also disputed by quite a few physics professionals in other peer reviewed journals. I have therefore left your reference in place but re-worded it slightly in order to give a more accurate balance. The experiment did take place and it did get a null result. But there is dispute over the interpretation of that null result. David Tombe ( talk) 05:24, 1 February 2009 (UTC)
Martin, I know of a few professors who are disputing Mossbauer and I could probably get books and publications if I tried. But we don't really need to go down that road. What I want to know is, were the above experiments all carried on outdoors? Aether entrainment by a wall would shield indoor experiments from any aether wind. Also, aether entrainment in the sensory material would also negate any aether wind effects. It's not all as cut and dried as some people like to think. I don't want to be involved any more in this article, as regards the aether wind issue because it would only end up in endless counter citations. Just bear in mind that it's not all cut and dried, and so try and word it so as not to give the impression that the aether is definitely a discredited concept.
I might however reword some of the things that you say about Maxwell, because I have read his 1861 paper many times. David Tombe ( talk) 08:11, 2 February 2009 (UTC)
Martin, I was specifically thinking in terms of the words permittivity and permeability. Maxwell, didn't use those words. So if you are doing a historical section and talking about how Maxwell related the aether to the speed of light in terms of permittivity and permeability, I might be inclined to change the wordings to 'a sea of tiny aethereal vortices', dielectric constant/transverse elasticity, and density. One of the features of Maxwell's 1861 paper was a lengthy analysis in which he linked dielectric constant to transverse elasticity. That of course involved his famous link up of mechanical stress with electric displacement current. Maxwell never claimed that light propagated in the pure aether itself. It is important to make that clear. David Tombe ( talk) 10:52, 2 February 2009 (UTC)
As stated above, in order to consolidate the two original sections and address some of the criticisms recently made, I have done a draft of a proposed new aether section. This, I now maintain, gives a fair and balanced view of the historical and current status of the aether. It could do with a few more references.
Please take a look as, if no one objects, I intend to paste it into the article to replace the current two sections. Martin Hogbin ( talk) 11:31, 1 February 2009 (UTC)
The title of this section expresses a misconception. It suggests that a definition is subject to experimental observation. Perhaps one might change a definition were it to prove too much of a Procrustean bed, encumbering discussion and analysis. However, that means only that the definition proves impractical, not that it fails an experimental test.
What makes a definition impractical is hard to define outside of a particular setting.
For example, let us suppose that an operational approach to realization of free space is to make an extrapolation of measurements on a sequence of samples prepared using higher and higher pumping down, so the sequence of samples contain fewer and fewer atoms or molecules. Within some assumptions as to how such an extrapolation should be conducted, the extrapolation extends the measurements to the case of zero atoms or molecules, a case not actually realizable by any technique known at the moment. Then the speed of light resulting from this extrapolation is taken to be the speed of light in free space, a hypothetical medium where ε = ε0, μ = μ0, both defined constant values.
This extrapolation works fine until measurement technique advances to a point where the fluctuations of quantum vacuum become measurable. Theory predicts, for example, that quantum vacuum may exhibit a speed of light that depends upon the polarization of the light. Supposing this prediction to be accurate, application of the extrapolation technique will lead to different speeds of light depending upon the polarization used in the measurements.
On the other hand, application of Maxwell's equations as we know them to free space (a hypothetical medium where ε = ε0, μ = μ0) predicts no polarization dependence.
Does this disagreement mean that the definition of "free space" must be changed? That is a possible choice, but not a necessary one. One could refine the operational approach using extrapolation to free space. Or, instead, one could simply implement a "best practices" correction to the extrapolation that corrects for the polarization effect, so measurements referred to free space agree regardless of the polarizations used.
Whether a change in operational approach to realization of free space, or a correction using the "best practices" approach, or a redefinition of free space is the better route will be decided by various standards organizations based upon a wide ranging view of metrology, and the best solution from the standpoint of accuracy and convenience will be selected. These considerations extend far beyond whether a particular extrapolation technique leads to ε ≈ ε0, μ ≈ μ0.
The conclusion is that the title of this subsection of discussion is mistaken. Brews ohare ( talk) 16:07, 2 February 2009 (UTC)
Brews, I'm not sure what the purpose of this discussion is within the confines of wikipedia's rules. The textbooks say that free space is nothing, and that there is no aether. What else could you expect to find out from this discussion? Those two constants, permittivity and permeability are constants of space related to magnetism and electricity, which are also related to the speed of light waves through the vacuum.
I noticed your discussion taking place last week, and so I decided to draw your attention to Maxwell's use of Newton's equation of the speed for sound, for the purpose of pointing out that the two constants in question might be a density and a Young's modulus. But I was soon ruled out of order under the rules and regulations. Maxwell is old fashioned and out of date and doesn't apply anymore. So under the rules and regulations, what kind of an answer would you be expecting to get? What modern up to date kind of answer would satisfy you regarding the physical nature of those two constants?
I fail to understand what all this could be leading up to. David Tombe ( talk) 12:18, 4 February 2009 (UTC)
This statement is restricted to the medium to which it refers. We know that the speed of light is dependent upon the ε(ω) and μ(ω) of the medium through which it passes. Because the statement refers to "distant astrophysical events", which are observable only through the medium of outer space, it would appear that a more correct statement would be:
This modification of the sentence is almost trite. It assumes more importance in the context of the article, however, because this sentence appears directly following the sentence "According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies."
This juxtaposition, when done without inclusion of the limiting phrase "in outer space", has the seeming implication that the astronomical observation somehow has relevance to the properties of free space, which is clearly untrue, as these are defined, not measured. See the discussion above. Brews ohare ( talk) 17:10, 2 February 2009 (UTC)
Martin Hogbin ( talk) 18:02, 2 February 2009 (UTC)
Martin: My statements in this subsection stand alone, and do not hark back to anything previous. If some statement made in this subsection requires amplification, please explain further. Brews ohare ( talk) 20:57, 2 February 2009 (UTC)
Martin: You can pick any few words you like out of the article and say they are the gist. How about "speed of light" or "dispersion"? What is the bearing of your remarks upon including "in outer space" at the end of the sentence? Brews ohare ( talk) 06:43, 4 February 2009 (UTC)
This topic is about the article but it has gone on for a long time, monopolizing this page. Should we take it to a dedicated page?
This article is about the speed of light and how it is understood by physicists today. It has a brief mention of the aether because it was of historical significance. The simple rigid fixed aether was the mainstream theory at the time of the Michelson Morley and the only theory that need be mentioned in this article. Many excellent physicist indulged in musings about the aether and proposed various theories but none of these ever came to anything and they have no place in this article. Martin Hogbin ( talk) 19:54, 3 February 2009 (UTC)
Martin, do you mean by fixed aether, that the aether is fixed in space relative to the universe and that the Earth moves through it, causing an aether wind?
For the record, I don't actually support Lorentz's theory, but my understanding of the facts are that Lorentz's theory followed from Michelson-Morley and that the aether wasn't actually abandoned until Einstein's theories superseded Lorentz's theories. It was on that basis that I removed your reference to fact that Michelson-Morley directly disproved the fixed aether theory.
By all means keep your references to the aether short, but I was only trying to make your references to Maxwell's aether more accurate. Why give a reference to Larmor's aether in relation to a paragraph on Maxwell's aether?
On second thoughts, you're the one that wrote the article on the aether. It wasn't me. I corrected some details based on my knowledge of Maxwell's original work. You undid those corrections and then went on to complain that there was too much in the article about the aether. Well so there still is, but it's worse now because it is inaccurate. Would it not just be better to delete the whole section altogether?
David Tombe (
talk) 05:57, 4 February 2009 (UTC)
Martin, On third thoughts, there was already a section about the aether in this article when I came here last week. I made some corrections to it. You immediately told me that I should not be putting articles about the aether into this article, even though the article was there already. You undid my edits, but nevertheless let the article remain in place, when by your statement, it would have been more logical if you had simply deleted the entire aether article.
You then proceeded to replace it with a larger article on the aether which contains factual inaccuracies. I corrected those inaccuracies. I put quite a bit of work into it, getting quotes, and links, and dates. You undid the corrrections and went to somebody else's talk page requesting them for some input. And they came and made a statement to the extent there was too much about aether in this article.
It seems to me that it not so much a question of there being too much aether in the article, as it's a question of what point of view is being emphasized. You have just happily supplied a large section on aether yourself, which contains a strong point of view that the aether has been disproven. Tell that to quantum mechanics experts about their polarized vacuum. It seems to me that the issue is that you will not be happy about an aether article if it is not critical enough about the concept. I think that has been what's it's been about all along.
You now have some badly written information about Maxwell's aether, with copious references to Larmor's aether to back it all up. The one aether theory which you mention above in relation to Michelson-Morley was Maxwell's aether theory. I fixed up the details of Maxwell's theory for you. So what do you want? It seems that you want to draw attention to aether in a negative light, make it vague with inappropriate terminologies such as permittivity and permeability which Maxwell didn't use, and give references to Larmor's aether theory, and complain that correct information about Maxwell's theory is not needed because there is only one aether theory that is relevant? It doesn't make sense. David Tombe ( talk) 12:30, 4 February 2009 (UTC)
I'd suggest that a brief mention without prejudice be made here and a link provided to Aether theories, which should be the definitive article on this matter. Brews ohare ( talk) 18:41, 4 February 2009 (UTC)
Martin, my entire focus in this debate was on density and transverse elasticity. That's what brought me into all this. Brews and some others, including yourself were debating the vacuum and what its physical characteristics might be. You were all focused on permeability and permittivity. I drew attention to the fact that Maxwell had shed more light on this subject than anybody else. He had firmly connected the equation that links permeability and permittivity to the speed of light, with Newton's equation for the speed of sound.
That's all I was saying. Then I noticed that Maxwell's aether had been written up carelessly in terms of permeability and permittivity. Those words don't give it its true significance. They are the very words that you guys were debating. So we came full circle.
I decided to make the bit on Maxwell's aether factually accurate in as little space as possible, and also in relation to Lorentz and how the aether was eventually abandoned. It was not abandoned instantly after Michelson-Morley as you have implied.
But what I detected was a reluctance to acknowledge the essence of what Maxwell had said. That's why I asked Brews yesterday what he expected to be the final outcome of his enquiry into free space. If we are going to restrict the terms and conditions of the discussion to what is written in modern textbooks, then there is nothing to discuss. Space is nothing, and it was a conversation about nothing.
I was finally suggesting that we therefore remove all references to the aether, or else, if we want a short section on the aether in relation to Michelson-Morley, then we can at least describe Maxwell's aether correctly and not back it up with lots of references to Larmor's aether. Ultimatley Maxwell's luminiferous medium was not the aether. He never called it the aether. It was a sea of molecular vortices. If you want to mention it in connection with Michelson-Morley, then why not use the correct name. If you don't like the name, then why bother writing about the subject at all? David Tombe ( talk) 08:42, 5 February 2009 (UTC)
Martin, the Michelson-Morley experiment was done specifically in connection with Maxwell's model. Maxwell's model was that very fixed aether model that you keep referring to. But anyhow, what about putting those details back again and shifting the whole section to some other article which you may feel is more appropriate for the content? David Tombe ( talk) 18:57, 5 February 2009 (UTC)
It certainly didn't disprove the Stokes aether entrainment model. It was only Lorentz's claim that the Stokes model disagreed with stellar aberration that caused Lorentz to look at an aether wind contraction phenomenon. But in the absence of any details of the material of the Stokes model, Lorentz had no basis upon which to object to the Stokes model on the grounds of stellar aberration. David Tombe ( talk) 13:53, 6 February 2009 (UTC)
Here is another try at this paragraph:
I believe the merits of this paragraph are:
I do not think there is any debatable point in this paragraph other than its literary merits. Brews ohare ( talk) 20:01, 4 February 2009 (UTC)
Martin: Please explain what is wrong with this paragraph. Your statement: "severe limits have been put on the variation in the speed of light with frequency " is elliptic because it does not explain: (i) the implications of classical EM upon such variation, or (ii) how it was done, and most importantly (iii) to what medium the limitation applies. Brews ohare ( talk) 23:11, 4 February 2009 (UTC)
Question: Is its "perfectly clear meaning" the same as that of my paragraph (so far as you can understand my paragraph in view of its "confusion")? If not, why not? Brews ohare ( talk) 07:20, 5 February 2009 (UTC)
You two are arguing about nothing. You are literally arguing about nothing, as in pure empty vacuum. As per the textbooks, the only thing that you can know about this 'nothing' is that c^2 = 1/με. There is nothing more to be said about it. There is nothing that can be added to this discussion that would be permitted by the modern textbooks. The summary is that we have a 'nothing' in which c^2 = 1/με. David Tombe ( talk) 19:01, 5 February 2009 (UTC)
Well, Martin I see you have simply restored your version. I am sorry you cannot come to grips with this topic. I do not understand how you arrive at the view that somehow this arrival time data confirms Maxwell's equations in some degree. First of all, if dispersion were observed, as you have admitted earlier, we would not discredit Maxwell's equations, but attribute the dispersion to the medium of outer space, which is not "vacuum". Hence, there is no "test". Second, the observations do not establish zero dispersion, but only set limits upon it. Thus they cannot establish that outer space is free space, but only that it approximates free space to some degree. Your statement of the "content" of the article is nothing but words out of context that can be misconstrued. Brews ohare ( talk) 22:37, 5 February 2009 (UTC)
A different, formal RfC is found Here.
The dispute concerns the wording at the end of the 'Light as electromagnetic radiation section. Martin Hogbin ( talk) 18:12, 6 February 2009 (UTC)
The current text is:
'According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. Measurements based on the arrival of electromagnetic radiation from distant astrophysical events put severe limits on the possible variation in the speed of light with frequency. {{cite journal |journal=Phys. Rev. Lett. |volume=82 |pages=4964-4966 |year=1999 |url= http://arxiv.org/abs/astro-ph/9810479v1 '
One editor would like to keep this text. Another would like to replace it with:
'According to Maxwell's equations, the speed of electromagnetic radiation in any medium with frequency independent permittivity ε and permeability μ (in particular, free space) is the same for all frequencies. Measurements have been made of differences in arrival time on Earth of electromagnetic radiations of various frequencies, radiations that originate simultaneously in distant astrophysical events.Bradley E Shaefer (1999). "Severe limits on variations of the speed of light with frequency". Phys. Rev. Lett. 82: 4964–4966. The upper bounds placed upon the observed differences in delay set severe limits on any possible variation in the speed of light with frequency, suggesting that in reaching Earth this radiation traversed a medium with a very nearly frequency-independent permittivity ε and permeability μ'.
A third alternative is :
‘According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. Measurements based on the arrival of electromagnetic radiation from distant astrophysical events put severe limits on the possible variation with frequency of the speed of light transmitted through outer space.’Bradley E Shaefer (1999). "Severe limits on variations of the speed of light with frequency". Phys. Rev. Lett. 82: 4964–4966.
There has been considerable discussion (see above on this page) but no consensus has been reached. Please comment below:
My thoughts - observed limits on speed variation with frequency in outer space imply (given a model) limits on speed variation with frequency in free space (or indeed in any other medium). So the first option seems to me to be clear, correct, and close to the source. The second option seems to be importing too much theoretical and philosophical baggage into what should be a straightforward observation. I dislike the (unsignposted) segue from free space to outer space in the third option, and the implicit assumption that experiment can have nothing to say about the properties of free space (or presumably therefore any other idealisation). EdwardLockhart ( talk) 19:44, 6 February 2009 (UTC)
I have added this for discussion of the comments to keep things tidy. Martin Hogbin ( talk) 20:03, 6 February 2009 (UTC)
The speed of light in the vacuum of free space c0 is not measured. It has an exact fixed value when given in standard units. Since 1983 the metre has been defined by international agreement as the length of the path traveled by light in vacuum during a time interval of 1/299,792,458 of a second. This makes the speed of light exactly 299,792.458 km/s. This exact speed of light is found at c0. The exact permeability is found at μ0. The electric and magnetic fields in EM waves are related by the value of the characteristic impedance of vacuum, found at Z0 =√(μ0/ε0). The permittivity is at ε0=1/(μ0c02). The "standard uncertainties" of all four variables is labeled by NIST as exact (i.e. zero) because they are not measured; zero measurement error means, ipso facto, that free space is not a real, measurable medium.
Martin: the BIPM definition is: The meter is the length of the path traveled by light in vacuum during a time interval of 1/299 792 458 of a second. One therefore needs a second. The standard for the second is in the microwave region using a Cesium fountain. I suppose one could go into a lot of detail here. You are fixated upon light being optical. What about the Doppler shift: would you suggest that relativity is also part of the discussion? It suggests that a moving source is just red shifted, but still propagates light at the speed of light. What about radio astronomy? Brews ohare ( talk) 19:08, 8 February 2009 (UTC)
Consequently, Shaefer's paper does not refer to a delay measured in free space, but to a delay in transmission through the medium of outer space, a real measurable medium. If measurement of zero dispersion in outer space has any implication for free space, the logic behind this implication must be explained. The present wording of the article seemingly implies some connection without explanation. Brews ohare ( talk) 23:33, 7 February 2009 (UTC)
Hello again. It seems to me that this is a commonplace situation in the interpretation of experiment. There are some electrical signals in a telescope of some sort. Via a whole bunch of physical theory (much of it embodied in hardware and software), we interpret these as light of multiple frequencies from a single explosive astronomical event. With the employment of some more physical theory, we interpret this to mean that there are limits on the variation of the speed of light with frequency in outer space. And after a bit more theory, we deduce limits on the variation of the speed of light in free space in a Maxwell-like theory.
There are large amounts of theory involved in going from the actual physical observations made to deducing the implied limits on physical theories, of which the fact that outer space is a close approximation to free space is just one (and perhaps one of the simplest bits).
EdwardLockhart ( talk) 09:34, 8 February 2009 (UTC)
I disagree with your statement "any dispersion found would be attributed to the medium of outer space". We would have a choice between ascribing significant dispersion to the physical properties of outer space, or to the theoretical properties of free space. Determining which is the best explanation would require further experimentation and theorising. But Shaefer's negative result shows that (at least for now) that we do not need to revise either one. EdwardLockhart ( talk) 17:25, 8 February 2009 (UTC)
I have added a Cosmology section. It appears Shaefer's paper is of interest in this context. That suggests a possibility for compromise, and I have added a sentence to the disputed paragraph. Brews ohare ( talk) 22:16, 8 February 2009 (UTC)
I have just looked at this section. In my opinion it makes a number of spurious claims that are not supported by the quoted references. Martin Hogbin ( talk) 23:19, 8 February 2009 (UTC)
My understanding is at the level of the verbal introductions and conclusions of the various papers, and does not extend to doing the math. That approach can be risky if there is controversy, because each author wants to put their slant on things. However, no statements in the Wiki subsection favor one or another model. The predictions of dispersion etc. seem to be accepted as theoretical predictions worth testing, and some tests have been made. The accuracy that can be claimed by these tests is debated, but most are null results. No attempt has been made in the Wiki article to sort this out in any detail. I don't think there are any huge gaffs here, but I agree that an expert could write something more cogent. It is (by Wiki standards) very well documented, so the more sophisticated reader can pursue matters themselves. Possibly, having an attempt here will encourage expert action: I'm often told it is easier to edit a proposal than to start from scratch. Put up a template requesting assistance if you like. Brews ohare ( talk) 03:00, 10 February 2009 (UTC)
An improvement in clarity of this article would result if a clear distinction were made throughout between the speed of light c in various media (such as that of outer space, or quantum vacuum, or the vacuum of quantum gravity) and the defined speed of light in free space c0 = 299 792 458 m s–1. The speed of light c is a matter for theory and experiment. The defined c0 = 299 792 458 m s–1 is useful for metrology, but doesn't play a role in various theories and measurements of c (in particular, that of outer space, or quantum vacuum, or the vacuum of quantum gravity), which mainly are interested in departures of c from c0 = 299 792 458 m s–1, departures not only in numerical value, but also functional differences such as anisotropy and dispersion and variation with the age of the Universe. Brews ohare ( talk) 16:43, 9 February 2009 (UTC)
I have modified the Intro as follows:
In my mind this description better distinguishes between measurement and corrected measurement, and more carefully explains the role of c0. Brews ohare ( talk) 18:23, 10 February 2009 (UTC)
What I wanted to say is that c is extracted from a measurement by application of CIPM corrections that refer the measurements to free space, while c0 is a number attached to the idealization of free space (and is defined, so no measurement is implied). Do you agree with this objective in principle? Brews ohare ( talk) 20:46, 10 February 2009 (UTC)
I agree that c is the common symbol, and that "vacuum" is the common description of where c can be found. However, it is clear that the speed of light has multiple meanings, all of which should enter the intro:
These important roles transcend setting the speed of light at any particular value, such as 299,792,458 m/s, and these roles would be unaffected if this were not the case.
Therefore, I would support a different intro to this article, which right now puts way too much emphasis on 299,792,458 m/s. Brews ohare ( talk) 21:47, 10 February 2009 (UTC)
In addition, any observation of the speed of light takes place in a realizable vacuum (such as terrestrial vacuum at very low pressure, or outer space) and then is corrected to refer to free space. These real vacuums are not perfect, of course, and were technique up to it, would reveal nonlinearities , dichroism and anisotropy. In contrast, the reference state of free space has none of these behaviors. In addition it has a defined speed of light 299,792,458 m/s, and the use of a defined value (zero error bars) sets it apart from real media that are known only to within an error bar.
None of these fundamental facts are expressed in the present article, which remains very deficient in relating standards, measurements, and ongoing research into the properties of realizable vacuums.
No recognition is given to the fact that free space is a construct, and no vacuum extant in our Universe can be proven to be exactly like free space. Nor can corrections intended to extrapolate measurement in these media to free space values be shown to be exact.
In addition, the above numbered properties of the speed of light are under scrutiny, and may have to be generalized. No indication of the tentative nature of these properties and how they are being re-examined enters the article, with the exception of the Cosmology section I have just added. Brews ohare ( talk) 21:47, 10 February 2009 (UTC)
The point is that theoretical physics is always a long way ahead of metrology and engineering. Metrologists are only interested in very well established physics that has been tried an tested for years. The constancy of the speed of light in free space is just that, it was first proposed over a century ago and to date no evidence against it has been found. For that reason (with others) when it comes to the delineation of the metre, the a definition based on the speed of light (under defined conditions and of preferred wavelength) has been chosen by the standards authorities as the most stable and practical way of doing things.
Theoretical physicists, on the other hand, have speculated that there might be variations in the speed of light in free space but these variations are many orders of magnitude below those that would affect metrology. No experimental evidence of any of these theories has been found to date. If some large and unexpected effect were to be found then metrology might need to reconsider some of its definitions but that has not happened yet so there is no need to change anything.
As far as anyone can tell so far, the different meanings of the speed of light that you give above are all identical. Martin Hogbin ( talk) 22:46, 10 February 2009 (UTC)
Martin: Maybe you are mistaken in your thoughts about free space: check this out:
In free space the principle of linear superposition of potentials and fields holds: for example, the electric potential generated by two charges is the simple addition of the potentials generated by each charge in isolation. Superposition is a defined property of free space even though the electric field near a point charge can become extremely large. See, for example:
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help)Therefore, the ideal vacuum of free space is not the same as any physically obtainable vacuum that exhibits nonlinearity. Quantum vacuum is predicted to do so. See, for example:
So, Martin, maybe this work will not come up with departures from free space, but it is possible in principle, eh?. Whether differences are measured or not, the bottom line is that free space is an idealization (with an exact, linear, ε0 μ0) that may or may not apply to any realizable vacuum. That is not your belief, as I understand you.
Similar references can be advanced that predict anisotropy and dispersion of quantum vacuum, which also are not properties of a medium with the exact EM properties ε0 μ0. Brews ohare ( talk) 01:41, 12 February 2009 (UTC)
Hi Dick: That is not exactly the point. To follow along with your sentence "well-sourced speculation that an ideal vacuum might turn out to be nonlinear and affect the speed of light", there is no need to speculate about ideal vacuum in the sense of free space because it has an exact speed of light set by definition and is exactly linear. The issue is that what often is called "vacuum" is outer space or ultra-high vacuum, which is not necessarily ideal, and may indeed exhibit nonlinearity. There is tendency to confuse the ideal, model vacuum with an actually realizable vacuum, and that tendency is exacerbated by using "vacuum" in a variety of meanings, and referring to "the speed of light" as though it existed in some Platonic universe, instead of stating "speed of light in ideal vacuum", or "speed of light in outer space". Remarkably, in describing Shaefer's observations of light from distant astronomical objects, Martin has repeatedly removed my addition of the limiting words "in outer space" and demands to state only "the speed of light". It seems trite to say that light that travels to Earth through the interstellar medium characterizes that medium, and does not characterize the defined c of free space, but apparently not. Personally I just don't grasp this resistance, which seems to me irrational and has been supported only by saying we cannot depart from a paraphrase of the source (not even represented as a direct quote) to make this distinction. We can't even add "it seems probable that these observations based on light traveling through outer space refer to the speed of light in that medium." Brews ohare ( talk) 15:25, 12 February 2009 (UTC)
Dick: This dispute arose originally in the last paragraph of the subsection Light as electromagnetic radiation, not in the Introduction, so this version of Martin's reasoning doesn't apply. I now have added the following well-documented lines in a later subsection to present this viewpoint, rather than continue the battle to add the three simple words "in outer space" to the earlier sentence:
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link)This added paragraph is not a complete antidote to the logical failure to discriminate between definitions and reality, which permeates the speed of light presentation. Brews ohare ( talk) 17:32, 12 February 2009 (UTC)
Hi Martin: I find your actions unsupported by any argument or discussion; they are high-handed, arbitrary, and based on your unsupported opinion. Brews ohare ( talk) 20:52, 12 February 2009 (UTC)
We agreed to an informal RfC and we got one person to take an interest. They agreed with me. If you want to we can have a full RfC but I am not sure how many it will take to convince you that you are pursuing a nonsensical point. Martin Hogbin ( talk) 22:07, 12 February 2009 (UTC)
We need some more editors here to stop this nonsense. Is there anybody there? Martin Hogbin ( talk) 23:23, 12 February 2009 (UTC)
Comment is invited on the accuracy of the following proposed insertion in the subsection of speed of light discussing transparent media. A subsection to collect your observations is Here.
Here is the suggested paragraph followed at bottom with the supporting references:
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link)This section is to collect your observations:
The quotation given does not support the proposed insertion. The authors are discussing new theoretical models for an idealised 'free space', in part via experiments on physically realisable vacua. As they explicitly say, "constancy of the speed" is a "pre-quantum approximation" - in their models, free space would behave (slightly) differently from classical free space. The work they are doing is to improve our model of free space, not to better understand the difference between physically realisable vacua and an idealised vacuum, which is uncontroversially considered to be small but non-zero. EdwardLockhart ( talk) 08:57, 13 February 2009 (UTC)
Hi Folks: The problem here as I see it is the use of "vacuum" and "free space" in multiple meanings. The cited literature refers variously to "electromagnetic vacuum", quantum vacuum" and just plain "vacuum". All these terms are meant to refer to a vacuum (or vacuums) where ε≠ε0 and μ≠μ0.
In the suggested Wiki paragraph the term "free space" is introduced to represent a idealized medium, in which ε=ε0 and μ=μ0, and therefore definitely not the medium meant by the authors. None of the authors uses the term "free space", but all refer to a baseline medium from which their vacuums depart. The baseline medium is one with ε=ε0 and μ=μ0. For example, the verbatim quote from Delphenich says he wishes to describe the departure of (in his case) "electromagnetic vacuum" from a medium where ε=ε0 and μ=μ0.
That is, I think the only reason to suggest that the sources are misrepresented is because you two bring your own meaning of "free space" into the discussion, and whatever you two have in mind, it is different from the meaning stated in the proposed paragraph. So for Edward, he reinterprets the term "free space" as some more general concept for which ε≠ε0 and μ≠μ0 are possible. That contradicts the stated usage in the proposed paragraph.
I could coin a new phrase, like "baseline vacuum" to represent ε=ε0 and μ=μ0 medium. Would that make you all happy? Then I could discuss how "baseline vacuum" oddly has exactly the same ε=ε0 and μ=μ0 as the "vacuum" used by BIPM and NIST, and oddly has the same properties of linearity and zero dispersion attributed to "free space" in text books. Would this circumlocution serve the purpose of separating unwanted connotations and then putting only the desired connotations back in? Brews ohare ( talk) 17:55, 13 February 2009 (UTC)
To me, the important idea for this article is that the very idea of *the* speed of light is only true in the "classical free space" with constant u and h. But a chunk of space with nothing in it (also confusingly called free space) does not have these properties, due to at least QM, GR, and perhaps other explanations. In empty space as it really exists, EM fields do not behave linearly. In particular, perhaps we do not have just c, but maybe c(lamda) or c(lambda, energy) or something even more complex. This article is not the right point to go into the details of what the correct model for a physical vacuum is, but I think it is reasonable to point out that the idea of "the speed of light" is an approximation (though a very good one in normal circumstances), and that physicists agree that if you look closely enough, even empty space needs to be considered as a transparent medium with non-trivial optical properties. LouScheffer ( talk) 18:30, 13 February 2009 (UTC)
Drawing upon all three comments above, how does this look:
Martin: The above proposed paragraph is not the same as its predecessors: it has evolved. However, there is no fresh input from you as to what direction would bring it closer to your desires. I see nothing wrong with it; we'll have to see what the response of others is. It has been amended to account for their comments. I have not made any posts about this topic on the article page since this RfC has been posted. Brews ohare ( talk) 22:28, 13 February 2009 (UTC)
There is no reason one cannot use a term to avoid continuous repetition of a cumbersome phrase. Maybe you would like to see "a medium with ε = ε0 and μ = μ0" repeated over and over? Brews ohare ( talk) 23:07, 13 February 2009 (UTC)
Martin, that very simply is nonsense. There is a simply overwhelming amount of literature on this subject. Take the quotes below and show me how you can possibly stretch them to contradict the proposed paragraph of Revision 2 below.
Brews ohare ( talk) 00:30, 14 February 2009 (UTC)
In the following, all mention of "free space" in any form is paraphrased to eliminate any direct reference:
I hope against hope that this version is so smooth that Martin can swallow it. Brews ohare ( talk) 00:03, 14 February 2009 (UTC)
Following suggestions of Dick Lyon:
Yes, that's right unless the anomalies were so large that they exceeded these contributions, which, in that case, would become a mere "background noise". Can you put your finger on what needs change? Brews ohare ( talk) 12:24, 14 February 2009 (UTC)
This version adds a footnote to flesh out the sort of implications that might arise from observations of light:
Proposed footnote
Brews ohare ( talk) 14:29, 14 February 2009 (UTC)
This list is now an assembly of articles of which perhaps three or four would be selected.
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help)Brews ohare ( talk) 20:05, 13 February 2009 (UTC)
Brews, I salute your indefatigability. How about the following?
In the classical model of electromagnetism, the speed of light is the same for all wavelengths in free space. Modern models of small-scale physics predict some variation - the constancy of the speed of light with wavelength is considered to be a "pre-quantum approximation" [1]. Experimental results, such as analysis of arrival times of light of different wavelengths from distant astronomical events [2] [3], show that the divergence from classical behaviour must be very small.
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EdwardLockhart ( talk) 09:15, 14 February 2009 (UTC)
Hi Edward: I also made "yet another proposal" (number 4 above); can they be merged? I am particularly interested in retaining the first paragraph, which has relevance to the overall article. Brews ohare ( talk) 14:33, 14 February 2009 (UTC)
I propose it to go in the section on " transparent media" as it is about transparent media and that section introduces the notion that the speed of light is affected by refractive index. Whether these few lines represent the cutting edge of research in four or five prolific areas may be not so important, because the gist is simply to point out that questions in these areas are under investigation and to present some representative links that can lead the reader further, if they want to go into the literature. In any event, a deeper discussion would have to go into nuance that is a bit more than this article needs. Brews ohare ( talk) 17:36, 14 February 2009 (UTC)
Although it is a laudable goal to make speed of light a superhumanly impeccable article that expresses the universe in a grain of sand, it might be noted that the standards being applied to this short section are more stringent than those applied anywhere else in this article, and way beyond most Wiki articles, which often are lucky to get the definition of the topic accurate, and may be full of unreferenced opinons. (I'm thinking of my experience with matter acceleration frame of reference centrifugal force and others.) Brews ohare ( talk) 17:48, 14 February 2009 (UTC)
An attempt to combine the two proposals; it partially answers Martin's questions about "which models" and "are they accepted". I believe the reference to Delphenich covers the "pre-quantum approximation" issue. Here's the proposal :
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Brews ohare ( talk) 16:13, 14 February 2009 (UTC)
Sorry Brews but I still cannot see what point you are trying to make. All I can say is that I would object less if it were put in your new section, which I would suggest calling something like 'current theories' or 'new theories'. Martin Hogbin ( talk) 18:42, 14 February 2009 (UTC)
To be a little more constructive I have no objection to pointing out in the 'Light as EM radiation' section that classical electromagnetism (or Maxwell's equations if you prefer) does not predict any anisotropy, dispersion, nonlinearity, or dichroism in free space.
Beyond that all, we are really saying is that some theories under current consideration may predict some of the above effects in free space but no experimental evidence of any of them has been found to date. I am not convinced that any of the current editors have the necessary knowledge and understanding to give sensible summaries of current research papers, but if you want to give it a go, who am I to stop you. Again I strongly suggest that this goes in a separate section. Martin Hogbin ( talk) 19:02, 14 February 2009 (UTC)
Hi Dick: A possibility is Delphenich, who says in part:
I'd say all the references in the reference list are aiming to find departures from the customarily assumed values for ε0 and μ0. I'd say none of them use the term "free space", so they won't settle the meaning of that terminology. You will find many textbooks do refer to free space, and they all cite the usual values for ε0 and μ0. However, one is left wondering if they have even thought about a vacuum that wasn't one with ε0 and μ0. Thus, there is an issue here as to what one wants to do with the term "free space" as time marches on: I prefer to keep it back at square one with ε0 and μ0; Martin prefers to let the term evolve with the times. The advantage of the first course is (i) there is continuity with the past if "free space" is taken to mean ε0 and μ0, and (ii) one has a handy term to use in place of an awkward circumlocution, and (iii) one has consistency with NIST & BIPM ε0 and μ0. The drawback is that there are those who follow Martin's path, and also suggest the symbols ε0 and μ0 evolve with the times, in which case the NIST BIPM defined values for ε0 and μ0 must be viewed as anachronisms in the making. Another hang-up of the view that ε0 and μ0 evolve with the times is that the theory suggests that a scalar constant is insufficient: real vacuums are described by εij(E, B, ω) and μij(E, B, ω). Brews ohare ( talk) 21:45, 14 February 2009 (UTC)
Hi Dick: I don't see that there is any "mixing up" going on here; the paragraph is "remarkably lucid and straightforward" as it stands. However, nothing is beyond improvement: see the next version below. Brews ohare ( talk) 23:23, 15 February 2009 (UTC)
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All vestiges of imperfect vacuum are removed; there is no mention of free space. Are we ready for prime time? Brews ohare ( talk) 00:00, 16 February 2009 (UTC)
How about a sub-subsection of "transparent media"?? Brews ohare ( talk) 00:11, 16 February 2009 (UTC)
I have tried blending it into the transparent media section. If you don't like it, try moving it to Cosmology, but I think some reworking will be needed. Brews ohare ( talk) 00:34, 16 February 2009 (UTC)
Brews I am still trying to understand the point that you are tying to make about free space. Do you take the words 'free space' to automatically imply the free space of classical electromagnetism - what you earlier called 'classical free space'? Martin Hogbin ( talk) 10:46, 15 February 2009 (UTC)
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Well, different groups will use terms differently; I find physicists among themselves are not consistent either (see Jackson, for instance). In any event, I have avoided this term in the proposed paragraph, which therefore is not subject to any reservations about usage. Brews ohare ( talk) 23:20, 15 February 2009 (UTC)
Well Martin, stick with whatever version of free space you like: it does not appear in the proposed paragraph (Merger II) and so it is a non-issue at the moment. Brews ohare ( talk) 00:07, 16 February 2009 (UTC)
This is an archive of past discussions. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page. |
Archive 1 | ← | Archive 4 | Archive 5 | Archive 6 | Archive 7 | Archive 8 | → | Archive 10 |
I have just changed all the UK 'travelling' to US 'traveling' as the article seems to mainly use US English, however, I notice that the earliest versions of the article appear to have been written in UK English. We need to chose one and stick to it. Martin Hogbin ( talk) 09:32, 13 January 2009 (UTC)
Isn't it a bit bold to say that the speed of light "is the speed of not just visible light, but of all electromagnetic radiation, as well as gravitational waves"? After all, no one has ever detected a gravitational wave. Lestrade ( talk) 00:01, 18 January 2009 (UTC)Lestrade
Direct detection is so passé. The luminiferous æther was real for a while, why can't gravitational waves and black holes be real for us, too? Lestrade ( talk) 01:07, 18 January 2009 (UTC)Lestrade
Joseph H. Taylor and Joel M. Weisberg have observed the binary star system PSR B1913+16 for several decades. They claim that their observations indirectly prove the reality of gravitational waves and are within 0.2 % agreement with the prediction of general relativity. This claim is based on the logical conclusion that the orbits of the two stars should become smaller due to the emission of gravitational waves. Any other possible cause of the orbital decay is not considered. On the basis of this logical conclusion, they claim indirect proof of gravitational wave radiation. The orbits shrunk so there are gravitational waves. The scientific community has seized on this indirect proof and has been content to agree that gravitational waves are real. Due to the effect of the acceleration of the galaxy, Taylor and Weisberg do not think that the results of the test will improve. So, this is as close as we can get to proof of the existence of gravitational waves. Unlike electromagnetic waves, gravitational waves will have to remain a strong belief rather than an observed fact. Many sources of information, however, simply claim the reality of these waves and do not communicate their actual basis to the public. The Wikipedia article follows this trend. Lestrade ( talk) 16:13, 19 January 2009 (UTC)Lestrade
The sentence "gravitational waves travel at the speed of light" is a categorical, unconditional, unqualified, explicit, declarative assertion. Can such a direct statement be made about something that has, and possible never will be, directly experienced? Lestrade ( talk) 12:59, 20 January 2009 (UTC)Lestrade
Thanks for your conciliatory tone. A useful set of guidelines is Wikipedia:Lead section. In particular, it suggests the lead should provide an overview of the article, not just a few points that might interest 80% of the readers. That objective might be a handful here. Brews ohare ( talk) 20:31, 20 January 2009 (UTC)
If c is the fastest speed in the universe, then c2 must be p r e t t y, p r e t t y fast. Lestrade ( talk) 00:06, 18 January 2009 (UTC)Lestrade
Speed is a quantity that has the dimension length/time. Isn't it true that when dimensions of quantity are multiplied by each other, the product is of the same dimension of quantity, squared? For example, three feet times two feet are six square feet, but it is still feet. So, speed times speed is speed squared, but it is still speed. What else could it be that makes sense mathematically and physically? Lestrade ( talk) 14:17, 18 January 2009 (UTC)Lestrade
Height or distance is a one dimensional quantity, measured as a line. A square is a two dimensional quantity, measured as a surface or area. Some people say that c2 is to be considered as a proportionality or conversion factor. As such, only the absolute number of approximately 34,704,709,264 (if the English miles/second dimensions are used) or 89,875,543,056,250,000 (if the Metric meters/second dimensions are used) would be considered as the factor. In either case, it seems almost silly to say that we know that this number indicates precisely the ratio of energy to a unit of mass. Rather, it probably merely means "a really big quantity." Lestrade ( talk) 20:51, 18 January 2009 (UTC)Lestrade
Using those units, and assuming the literal truth of the equation e = mc2, one part of mass would convert to 0.0000000001492 parts of energy when a uranium nucleus is split. That's not much of a fission bomb. Lestrade ( talk) 14:05, 19 January 2009 (UTC)Lestrade
The section entitled 'overview' is not really an overview, and anyway this is the function of the lead section. I therefore propose that we merge the 'overview' section with the physics section and make it the first section of the article after the lead.
I also suggest that we remove references from the lead, which should be a brief overview of the article as a whole, and add them to the relevant parts of the body. Martin Hogbin ( talk) 10:45, 21 January 2009 (UTC)
I have just done a proposed rearrangement of the first half of the article, which you can find here. Have a look and see what you think Martin Hogbin ( talk) 13:26, 21 January 2009 (UTC)
If no one objects I will past the new bit in place. We can then work on improving it. Martin Hogbin ( talk) 22:46, 21 January 2009 (UTC)
I have added a section on light as EM radiation. At the end, it says that the speed of EM radiation is independent of frequency. I know this has been confirmed by observations on pulsars but I cannot find a good reference. Any offers? Martin Hogbin ( talk) 10:35, 24 January 2009 (UTC)
Brews, you have raised some points about this subject, but I am not quite sure what you are saying. Are you suggesting that further explanation of this topic is in required in the article? Martin Hogbin ( talk) 10:37, 24 January 2009 (UTC)
According to general relativity, gravitational waves travel at c, this is well known, as indicated by the reference from Hartle that I gave. There may be other, unconfirmed and not generally accepted theories, in which this is not the case.
The paper by Carlip that you quote refers to a recent attempt to measure the 'speed of gravity', and what is says is that the experiment did not actually measure the speed of gravity but the speed of light - there was some controversy over this at the time. It does not suggest in any way that they are different, just that the particular experiment failed in its objective to actually measure the speed of gravity.
It is true that the speed of gravity has not been confirmed experimentally but the only accepted, and well well-tested, theory of the subject (GR) predicts that it will travel at c. I believe that it is therefore justified to make the stronger statement that I made. Martin Hogbin ( talk) 16:27, 24 January 2009 (UTC)
No one has ever detected gravitational waves, but waves of gravitation travel at the speed of light. Lestrade ( talk) 20:38, 12 February 2009 (UTC)Lestrade
Brews, are these quotes from the FA review or your comments? Martin Hogbin ( talk) 17:00, 24 January 2009 (UTC)
The article frequently stumbles over whether statements like "all EM radiation travels at speed c" is (i) a definition, or (ii) a physical observation or (iii) a posit of some physical theories (e.g. relativity theory, quantum gravity theory etc.).
The article also frequently fails to point out what medium is under discussion: for example: is it the ideal free space which is the unobtainable reference state where c = c0, or is it outer space where the speed of light may not be c0 but possibly any measured value whatsoever?
I've changed some of these occurrences, but more are out there. Brews ohare ( talk) 16:41, 24 January 2009 (UTC)
Hi Martin: Your comment: "Now if it transpires that there is some small variation in the speed of EM radiation in free space with frequency (and no definition could prevent this)" is, in my opinion, based upon a misconception that measurement can impinge on the defined properties of "vacuum". In practice, any measurement must be in a medium. Hence, according to BIPM, the measurement must be corrected for its non-ideal behavior. Changes in accuracy of the measurement will affect the corrections to be made in this medium in order to refer it to the defined state of "vacuum", but do not change the defined "vacuum". It appears to me that you see free space as something real, while it is only an unobtainable reference state, like absolute zero. Brews ohare
Hi Martin: Our editing session overlapped. Where the "vacuum" of BIPM or "Free space" is concerned, experiment is not an arbiter; rather it is simply a matter of convention and in turn a matter of convenience. Just like any other standard.
In particular, if the meter established in air at two different light frequencies were to differ, the difference would be attributed to the medium "air" and not to the vacuum of free space. If some "universal dispersion correction" showed up in all media, and if the international community decided that making this correction all the time was a nuisance the definition of "vacuum" could be changed by international agreement to incorporate "the dispersion of the vacuum". The properties of the "vacuum" are not a reflection of nature, but of convention. The properties of nature are expressed in permeabilities and permittivities relative to the "vacuum". Instead, a 'standard medium" could be adopted, but that would require tests to see of the standard were met. Apparently, corrections to refer to a "standard medium" is viewed as a more cumbersome approach. Brews ohare ( talk) 18:27, 24 January 2009 (UTC)
Hi Martin: Yes. By definition of "free space", rather than by definition of the speed of light. Brews ohare ( talk) 14:09, 25 January 2009 (UTC)
When I look at the article you link to above I get this (my emphasis),'In the reference state of free space, according to Maxwell's equations, electromagnetic waves, such as radio waves and visible light (among other electromagnetic spectrum frequencies) propagate at the defined speed of light'. Is that what you are referring to? Martin Hogbin ( talk) 14:55, 25 January 2009 (UTC)
Firstly, can you confirm that the quote above is what you believe defines the speed of all EM radiation to be c?
I am unsure what quote is referred to here. However, the speed of all EM radiation in free space is c by definition, because, by definition, free space has no dispersion, no dichroism, no nonlinearity, and no anisotropy. The only issue in any actual measurement is whether it actually was done in a medium that approximates "free space", and just what corrections have to be made. Present theory on the QCD vacuum and the quantum vacuum suggest that realization of one of these vacuums is not tantamount to realization of free space, as these vaccua in theory are not isotropic, do exhibit dichroism and are nonlinear at large field strengths. Brews ohare ( talk) 16:26, 25 January 2009 (UTC)
The speed of all EM radiation in free space is c by definition, because, by definition, free space has no dispersion, no dichroism, no nonlinearity, and no anisotropy. The proof of these statements is the defined values of electric constant, magnetic constant, impedance of free space and speed of light in free space. All these properties of free space by definition are dispersionless, field independent, and scalars (not tensors). If they are plugged into Maxwell's equations, they result in dispersionless, isotropic, field-independent propagation of EM waves in free space.
Of course, every realizable medium, be it outer space, partial terrestrial vacuum, QCD vacuum, or whatever, has a permittivity and permeability that can be measured to some level of accuracy and will exhibit (to a degree determined theoretically and/or experimentally) anisotropy, dichroism, nonlinearity and dispersion. Brews ohare ( talk) 16:49, 25 January 2009 (UTC)
Granted; not by definition; also, "speed of all frequencies of EM radiation" is meaningless without stating whether "free space" is meant, or some real medium. If "free space" is meant, then experimental observation is impossible and irrelevant. Brews ohare ( talk) 00:46, 26 January 2009 (UTC)
"Free space" can be viewed a limiting case, I agree. It can be approached by some real media, but it cannot be reached. Theoretically, there is no "vacuum" with its properties. Experimentally, we can establish a medium resembles free space within experimental error, but of course there is always the possibility that further refinements in technique will reveal differences.
The EM properties of free space are defined by ε0 and μ0 (or by some pair of these two and Z0 and c0). These values constitute a reference case. It might be possible that a real medium comes close to the reference. Unfortunately, because of experimental error, it never can be established that any real medium is free space, only that it is close to free space.
That is why the logical impossibility of "measuring" free space has to be recognized. All we can do is measure a real medium and point out that it is as close to "free space" as we can tell within our experimental limitations.
Therefore, no statement in the article should suggest that we can measure free space. Nor, can we say measurement puts "limits" on the free space parameters: they are defined; they are not measured. They could be totally different, they could be tensors, etc etc. There is no underlying philosophical "meaning" to free space having the parameters it does. The values of ε0, μ0, Z0 and c0 are simply historical accidents.
We could choose instead a real standard medium like air that could be realized. That would not prove to be an advantage, because we could never be sure that our reference sample of air was exactly the same as the standard. We'd have to maintain a standard air sample for comparison and reassure ourselves that it was not changing with time because of improper storage, etc.
So choosing an arbitrary ideal reference is easier, especially if we can maintain a vigilant list of "best practices" to relate any measurement in real media to the standard. Hopefully, the standard has the merit that these "best practices" are not too difficult to apply in practice. (You tell me if preparation of an atomic clock is simple?) The BIPM and its associates decide by international agreement whether the reference is appropriate, and will change it if there appears to be a simpler or a more accurate standard that would make for "best practices" that were easier to use, or accessible to greater accuracy experimentally.
A statement like: we would expect any variation of speed with frequency to be less in the medium of ideal free space that it is in the medium of outer space. Thus it is justifiable to say that experimental evidence sets limits on the variation of speed with frequency in free space. is misleading: given Maxwell's equations we know that "free space" has absolutely zero dispersion. And we know that the validity of Maxwell's equations has nothing to do with the values of the parameters in free space. Maxwell's equations are established by measurements in the real world, on real media. Whatever real media behave like has no bearing upon the parameters of free space, neither upon the actual values nor their scalar dispersionless nature.
We could imagine a hypothetical Universe where every real medium we measured exhibited dispersion. These physical facts need not impact the parameters of free space. It might be decided that reference to free space was still the simplest methodology. Or, the BIPM might decide that free space would be modified. That is a decision based upon practicality and politics.
Again: because of its defined properties, no statement in the article should suggest that we can measure free space or that experiment "constrains" the parameters of free space to have particular values, or to be scalars, or to be dispersion free, or … . Brews ohare ( talk) 13:00, 26 January 2009 (UTC)
Martin: I have no trouble with free space. I have trouble with statements in the article that suggest measurements have something to do with free space. Measurements are corrected to refer to free space, but do not impinge upon the defined properties of free space. Brews ohare ( talk) 18:29, 26 January 2009 (UTC)
Thank you, Martin. Brews ohare ( talk) 19:30, 26 January 2009 (UTC)
Hi Steve: An interesting formulation of the discussion. Under (2), Maxwell's equation can be written for any medium: you just need the constitutive equations. The relevance of free space under (2) is that the constitutive equations are simply specification of ε0 and μ0. Evidently this specification does not need to apply to any real material (although it might). Free space is just this hypothetical material. As regards (1) there may be a sequence of steps (e.g. evacuation of a flask) that cause ε → ε0 and μ → μ0). Such a sequence might suggest free space is most nearly to be realized by pursuing this sequence. But so what? You can measure c in any medium and make corrections so your results are referred to free space, whether or not media approximating free space actually exist. There is zero information in free space and its constants.
Is there literature support for saying that [some limiting sequence] is not actually the true definition of free space? I'd say the NIST web site where ε0 and μ0 a stated as defined values is the true definition, not the possibility of some limiting sequence. Maybe there is some historical commentary on this point, but it has nothing to do with our final definition. And, just to be nasty, is there any literature supporting the limiting sequence idea?
In the context of this part of the article, I believe it makes more sense to discuss (1) than (2), since (1) is the basis for statements about the universe. Free space is not about the universe. Measurements of real constitutive relations for real media, like outer space or terrestrial vacuum, is about the real universe, quite independent of what we adopt for free space.
Historically, the yard may be related to the arm-length of monarchs and the metre to the dimensions of the planet Earth, but neither has any relation to the Universe as observed in science. Brews ohare ( talk) 20:40, 26 January 2009 (UTC)
To put things differently: there is a continuing effort experimentally to determine if the speed of light in space is isotropic, and to see if it varies with frequency. Evidently, Maxwell's equations using ε0 and μ0 predict no success. Would a success prove that space is not a good realization of "free space", but a medium with its own constitutive equations, or would it prove Maxwell's equations are wrong, or something else (maybe some weird aspects of general relativity: varying speed of light theories)? None of these three categories of conclusion would change ε0 and μ0. Brews ohare ( talk) 20:57, 26 January 2009 (UTC)
Hi Steve: Changing terms to "vacuum" we still have cO as the speed of light in a hypothetical medium. Only now "vacuum" is vague and subject to misinterpretation as a "real" medium.
if the meter is defined in relation to the speed of light in free space, and no real-universe measurement could ever give information regarding the speed of light in free space, then there isn't any way to confirm that a meter isn't the size of an atom
The first condition is met: the meter is defined in relation to the speed of light in free space
The second condition is met: no real-universe measurement could ever give information regarding the speed of light in free space
The conclusion is false: there isn't any way to confirm that a meter isn't the size of an atom
Here is how it could be done. Measure the meter in air, assuming the definition applies in air, not free space. Measure the refractive index of the air. Adjust the size of your meter according to the index. Brews ohare ( talk) 22:46, 26 January 2009 (UTC)
Hi Steve: The issue here is the usual one that occurs in setting up a standard, namely the "accumulated wisdom" effect. For example, how do we determine that one clock is better than another clock? It is partly accumulated experience on how well supposedly identical clocks agree, and partly a theoretical assessment as to what influences might disturb the clocks and how big these disturbances are expected to be. When experience shows the clocks don't all agree, we look around and find that it is gravitational dilation, an effect within theoretical estimation, one we should have seen coming, but one that simply was overlooked. Or, maybe its something else.
As technique improves, discrepancies in indices will show up. Sample vacuums that are supposedly identical turn out to be different. In accounting for the difference, maybe some of the observed discrepancy is due to quantum fluctuations of the vacuum, or whatever. In conjunction with theory we decide what these contributions are, learn how to avoid them, or account for their effects in the sample vacuums, and subtract them from measurements to get the "free space" value.
In dealing with air, present practice seems to be based upon some formulas that people feel have been well tested against different air samples. You just measure the partial pressures of water, CO2 etc. in your sample and calculate the index for your case. Brews ohare ( talk) 03:30, 27 January 2009 (UTC)
Rather than continue the long discussion on the philosophical nature of free space, I would like to suggest the reversion of the wording of the section above from Brews' version:
'According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. Observations in outer space and in good terrestrial vacuums confirm that in such mediums any variation of the this speed with frequency is extremely small, suggesting that, in this regard, they are good approximations to free space'.
to my original version:
'According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. Observations confirm that any variation of the this speed with frequency is extremely small'.
I believe that my version is clearer, fully justified, and says all that needs to be said. I am not against adding more detail but I think that Brews' attempt to make some kind of philosophical point in the article is wrong and unhelpful to the reader. Martin Hogbin ( talk) 09:37, 27 January 2009 (UTC)
I suggest:
'According to classical electromagnetism, the speed of electromagnetic radiation in a perfect vacuum is the same for all frequencies. Observations confirm that any variation of this speed with frequency is extremely small'.
I think everyone will understand what a perfect vacuum is; it's the extrapolation of a better and better physical vacuum; while OTOH a lot of readers will not have heard of free space. The phrase "speed of light in vacuum" is an order-of-magnitude more common on Google than "speed of light in free space". NIST and CODATA both use "vacuum", not "free space". And moreover, to the extent that that "free space" means something different from "perfect vacuum" (and I'm still not sure it does in common usage), "perfect vacuum" is what we want to talk about, since "free space" (as Brews uses the term) has no necessary relationship to anything in our universe, while this is a physics article about our universe and its laws. -- Steve ( talk) 17:18, 27 January 2009 (UTC)
'Measurements based on the arrival of electromagnetic radiation from distant astrophysical events puts severe limits on the possible variation in the speed of light with frequency'. This is much closer to what is actually stated in our reliable source and it shows the context of radiation travelling through outer space. Martin Hogbin ( talk) 19:37, 27 January 2009 (UTC)
I prefer Martin's version. It's overly modest to say that they merely set limits on dispersion in outer space; with a bit of simple analysis, it actually sets quantitative limits on the dispersion in the "extrapolated better and better vacuum" (call it what you will). -- Steve ( talk) 22:00, 27 January 2009 (UTC)
The reason I dislike this, as I said above, is that it makes it sound like this is a measurement of a certain property of the interstellar medium. It's more than that, it's an experimental constraint on the basic laws that govern our universe. It's as if you were describing the Millikan oil-drop experiment as a "demonstration that small droplets of oil suspended in air have a quantized charge". It's accurate but gives readers the wrong idea. :-) -- Steve ( talk) 04:51, 28 January 2009 (UTC)
Brews, all you have written above and in the section below is your opinion, which you are entitled to, as I am entitled to mine, but articles in WP should be based on reliable sources, not opinion. Now I am not fan of quoting sources verbatim, I am happy with a degree of interpretation for the benefit of non-technical readers, but only when there is general agreement about the facts. In this case there is a difference of opinion, which I am happy to discuss, but until there is a consensus to do otherwise we should say what what our source says, in the words that it says it in. Martin Hogbin ( talk) 13:57, 28 January 2009 (UTC)
Steve and Martin: You both seem to believe that the measurement of of the properties of the interstellar medium has ramifications that go far beyond the properties of this particular medium. However, all the measurement actually does is establish the properties. Whatever you two want to read into it requires that this fact be placed within a theory where the theory provides a significance beyond the simple results of the observation. So bite the bullet and dig up the sources and write the summary paragraph that shows that the existence of some media with ε and μ as close to ε0 and μ0 as present accuracy can determine is a much more meaningful thing than the simple fact.
As far as Maxwell's equations go, they are totally indifferent to whether outer space approximates ε0 and μ0 or has some wild material properties. EM waves can propagate in any medium, and at any speed up to c0. The measurement showing some media exhibit ε ≈ ε0 and μ ≈ μ0 doesn't do more than establish the existence of media with these properties.
It's like the M - M measurement of the speed of light as a function of the movement of the observer. : you need relativity to interpret the facts before the implications become clear. And it seems all special relativity demands is that an upper limit on speed of light exists, regardless of whether c0 applies to outer space.
Maybe an approach to your discussion could be to show that if outer space exhibited dispersion it would have amazing repercussions according to some theory? Brews ohare ( talk) 13:17, 28 January 2009 (UTC)
Brew, I still cannot work out what your point is. Perhaps it would help if we see what are the things that we agree on. Do we agree the following?
Free space is an idealization that is physically unobtainable.
The speed of light is defined to be a certain value in free space.
Actual measurements of the speed of light are made in media other than free space.
According to Maxwell's equations, all frequencies of EM radiation travel at the same speed in free space.
Martin Hogbin ( talk) 18:11, 28 January 2009 (UTC)
We don't have to infer or speculate about the properties of free space: they are defined, and therefore certain. We can speculate whether outer space behaves like free space, and attempt to support these ideas by measuring outer space. Brews ohare ( talk) 05:49, 29 January 2009 (UTC)
Where are the properties of free space defined? Martin Hogbin ( talk) 21:27, 29 January 2009 (UTC)
Maxwell used Newton's equation for the speed of sound in a long solid rod in order to obtain the speed of light on the basis of an elastic medium with a density related to the magnetic permeability, and a transverse elasticity related to the dielectric constant.
This suggests to me that light is a coherent dispersionless wave in an elastic solid.
Maxwell obtained the concept of displacement current from his own postulated elastic medium, and the concept was used in his derivation of the EM wave equation in 1864, three years after he did the analyis above with Newton's equation.
The modern derivation of displacement current bears no relationship to Maxwell's derivation of displacement current, and the modern derivation doesn't have the correct divergence properties to allow it to connect to the electromagnetic wave equation.
This is all telling us something about the nature of the so-called vacuum. David Tombe ( talk) 16:07, 28 January 2009 (UTC)
Martin, All I did was corrected the facts in the already existing section about the aether. You are claiming that the Michelson-Morley experiment discredited the aether. That is not what the historical record tells us. The Michelson-Morley experiment could have confirmed the Stokes aether drag model. But Lorentz believed that the Stokes aether drag model contradicted stellar aberration. Lorentz himself believed that the aether blew right through the Earth and he devised his transformation equations on the basis that the aether wind contracted the Michelson interferometer. The aether was dropped from physics when Einstein gave a new interpretation to the Lorentz transformations. But the Michelson-Gale-Pearson experiment of 1925 detected an aether wind due to the Earth's diurnal motion.
Is there anything that I have said above that is not true? You have undone my revisions and told me not to add sections on the aether. I did not add any section on the aether. There was already such a section there and it is inaccurate. All I was doing was correcting the historical record.You are obviously intent on upholding the inaccuracy. You are pushing your own point of view here. David Tombe ( talk) 06:48, 29 January 2009 (UTC)
Martin. Yes, diurnal refers to the rotation of the Earth. The Michelson-Gale-Pearson experiment in 1925 got interference fringes in that regard. David Tombe ( talk) 04:33, 30 January 2009 (UTC)
Steve, there have been so many contradictory experiments on this issue, that I don't know who to believe. We must also remember that we need to scrutinize the theory behind the interpretation of an experiment to ensure that we are not dealing with any tautologies. I had a look at Mossbauer spectroscopy in this regard and I could see that it was using the Lorentz transformation equations in the interpretation. One paper that I dragged up on google then left the interpretation open as between Einstein and Lorentz aether theory.
I don't mind whether or not you include Michelson-Gale in the main article. I was merely objecting to the outright assertion that Michelson-Morley in 1887 had discredited the idea of a luminiferous aether altogether. I was pointing out that the historical sequence of events since 1887 never led to grounds for any such absolute assertion.
Do you have any links to the 1970 experiment which you have referred to? I would like to read it. If not, does it involve the use of Einstein's special theory of relativity in the interpretation? David Tombe ( talk) 08:24, 30 January 2009 (UTC)
Steve, I don't have the book immediately to hand. Can you please tell me if the experiments in question use Einstein's special theory of relativity in the interpretation.
Also, in the other experiments which you mention in your recent edits to the main article, in which the speed of light has been measured at 'c' relative to sources that are moving at 99% c, is the measured value of 'c' in that case both relative to the source and to the frame of reference in which the source is moving at 99% c? David Tombe ( talk) 17:10, 30 January 2009 (UTC)
Newton didn't write equations. He wrote quasi–geometrical proportions of ratios. Lestrade ( talk) 20:26, 30 January 2009 (UTC)Lestrade
My point is merely that there can be no "Newton's equation" because Newton never published an equation. All of his published mathematics were in the form of proportions. You may consider this to be insignificant. Someone else may consider it to be significant. However, the world is so full of fictitious fabrications that, I guess, there is always room for one more. I had thought that it might be better if we only spoke about what we know to be true. Lestrade ( talk) 14:19, 31 January 2009 (UTC)Lestrade
Lestrade, OK I take your point. I admit that it is an interesting historical piece of information in its own right which I had not been previously aware of. I do intend to study Newton in detail eventually. David Tombe ( talk) 05:04, 1 February 2009 (UTC)
I think that there may be some confusion concerning experiments to detect the aether. The MMX was intended to detect the Earth's motion through the aether due to its orbit round the sun. It gave a null result as did all following experiments designed to detect the same thing; thus the simple rigid aether is ruled out.
The Earth's rotation (spin on its axis), however, can be detected by more sensitive interferometric apparatus, this effect is the basis of the laser gyro, and it is consistent with both SR and a simple aether theory. However the simple aether cannot be considered a valid explanation for these observations as it had already been ruled out as a possibility. Martin Hogbin ( talk) 20:51, 30 January 2009 (UTC)
I'll try to clarify a few points here. I am suspicious about the experiments referred to in Jackson, and as to what they exactly mean. But I haven't fully investigated them yet, so I can't properly comment.
In my view, the Michelson-Gale experiment showed up the aether drift in relation to the Earth's diurnal motion, just as Steve has described above.
Martin has claimed that the 1887 Michelson-Morley experiment has ruled out a rigid aether. How exactly has it ruled out a rigid aether if the aether were to be entrained in the Earth's orbital motion right up to a cut off line where the Earth's gravitational field gives way to the gravitational field of a neighbour? Such entrainment would fully account for Michelson-Morley. Also, the Earth could be rotating within that entrained aether, accounting for Michelson-Gale.
I am not insisting on mentioning Michelson-Gale in the main article. I am not insisting on pushing the aether entrainment model either. I was merely neutralizing the wording to match the historical facts that related to why the aether was abandoned in modern physics. It is a matter of point of view to categorically state that Michelson-Morley actually discredited aether theory. It didn't do that. It began a series of postulates and investigations which ultimately led to the aether being abandoned.
Meanwhile, I would like to know more about those Jackson experiments. I am very suspicious of claims of highly accurate experiments involving measuring the speed of light coming from tiny particles that are travelling at 99.9% of the speed of light.
I would also like to know why they were doing such experiments in relation to the Earth's diurnal motion. Did they have some doubts about a certain matter? And do they use the special theory of relativity in the interpretation, because if they do, then it is a tautology to use a theory which depends on the constancy of the speed of light to prove the constancy of the speed of light.
David Tombe (
talk) 06:15, 31 January 2009 (UTC)
At present the article mentions aether and the MMX twice, once under 'Luminfiferous aether' and again under 'Michelson-Morley experiment'. I suggest that we combine these two sections into one, giving a clear historical perspective to the aether and the MMX and showing current mainstream scientific though on the subject. I have created a page and copied the current two sections there. We can combine them into one to produce a new section and then put the completed section back here. Martin Hogbin ( talk) 10:31, 31 January 2009 (UTC)
Steve, I removed one of your references because nobody is disputing the point that it makes and it is not related to the issue in question. I left the other reference because it would appear to be relevant. I'd certainly like to check that reference out. David Tombe ( talk) 11:08, 31 January 2009 (UTC)
I had done a draft of my proposed section which I think gives a fair view of the history and current status of the aether. Martin Hogbin ( talk) 13:53, 31 January 2009 (UTC)
The sentence stating that for many practical purposes the speed of light can be considered infinite has been changed many times, practically going full circle.
The concept that we are trying to convey is that for many real purposes the speed of light is so high that we can assume that it takes no time at all to get from A to B without causing any significant errors or problems. Examples would be, counting seconds to get the range of lightning, or starting signals for Olympic athletes.
The problem is, how to convey this message clearly, succinctly, and in good English. Martin Hogbin ( talk) 20:56, 30 January 2009 (UTC)
Steve, I am going to take it that your Jackson reference refers to a 1960 Mossbauer effect experiment by Ruderfer. It is a null result experiment with a highly far fetched interpretation. It has been widely criticized, including by Ronald Hatch who was heavily involved in the Global Positioning system.
Here is a web link listing some of that criticism, [3]
I don't think that your Jackson/Ruderfer reference is a balanced reference. You certainly have no basis whatsoever to claim its superiority over the Michelson-Gale-Pearson experiment. David Tombe ( talk) 15:22, 31 January 2009 (UTC)
Steve, was the experiment carried out indoors or outdoors? And ultimately, does the experiment disprove the existence of the aether or not? David Tombe ( talk) 17:11, 31 January 2009 (UTC)
Martin, In 1845,Stokes advocated an aether theory in which the aether was entrained with the Earth. It drew criticism on the grounds that the material of the aether would have to be such that it would behave like a solid at low pressure, such as to allow the transmission of transverse electromagnetic waves, yet it would have to behave like a liquid when under high pressure at the shear lines where it tapered off into distant space. Stokes didn't know the solution to this problem, but he was convinced that such a solution might eventually be found.
That was Stokes' opinion. Lorentz opposed the Stokes model because he believed that it contradicted the observed measurements of stellar aberration. That was Lorentz's opinion, based on no greater a knowledge than Stokes had about the nature of the material of the luminiferous aether.
Lorentz devised his own aether wind and aether contraction model to account for the Michelson-Morley experiment. The Lorentz theory was modified by Einstein and the aether was dropped from the physics books.
It is important to record this sequence of events correctly. There is no basis to categorically state that Michelson-Morley discredited the aether.
The aether has been creeping in to modern physics again through the back door. First there was the electron-positron Dirac sea, and more recently the polarized vacuum in quantum mechanics. David Tombe ( talk) 04:59, 1 February 2009 (UTC)
You still have not defined what you mean by aether, thus it is impossible to agree or disagree with you. If you mean by aether 'everything that is not yet understood in physics' then I guess it is creeping back. Martin Hogbin ( talk) 10:56, 1 February 2009 (UTC)
I have provided a reference that places limits upon the dispersion of light using astronomical observations. This reference refers to these limits as a limit upon propagation in "space" or "vacuum". Martin insists that by use of these terms, the reference means to refer to "free space" where ε = ε0 and μ = μ0 by definition. That is, Martin suggests that in fact definitions are constrained by experimental measurements.
It is obvious that these astronomical measurements refer to transmission through outer space (that is the medium pervading the nearly empty regions of the universe). It is furthermore obvious that although outer space may be well approximated by ε = ε0 and μ = μ0, the accuracy of such approximation can be established only by measurement of the medium of outer space itself (which is, of course, exactly the purpose of the cited paper). It is further obvious that there is absolutely zero dispersion if the medium has an ε = ε0 and μ = μ0. Thus, the reference has succeeded in placing limits upon how far outer space strays from such a medium with ε = ε0 and μ = μ0. I simply cannot understand Martin's objections to my statement of this fact, which he now no longer bases upon reason, but upon exactly parroting the words of the reference, and moreover, parroting these words in a misleading context, in a fashion that makes the ridiculous and unsourced implication that measurement of ε and μ in the medium of outer space somehow limits the definitions of the defined values for ε0 and μ0 Brews ohare ( talk) 01:45, 1 February 2009 (UTC)
So, propose a definition and support it with a reference. Personally I think an adequate definition of free space is a medium in which ε = ε0 and μ = μ0 exactly. Brews ohare ( talk) 02:43, 1 February 2009 (UTC)
In real terms, permittivity and permeability are EM properties of real measurable media. ε0 and μ0 are defined permittivity and permeability of a hypothetical medium free space. Some real media have ε≈ε0 and μ≈μ0. Of course, measurement error means we can never say of any real medium that its ε=ε0 and μ=μ0, only that equality exists to within experimental error. More than that, the existence of media with ε≈ε0 and μ≈μ0 is not a prerequisite for choosing to define a hypothetical medium with these defined values, although it may add to the convenience of said definition if there are in fact some real media that do exhibit ε≈ε0 and μ≈μ0. Brews ohare ( talk) 05:37, 1 February 2009 (UTC)
In my view, and not being trained in interpreting historical documents, not a sophisticated view, Maxwell viewed the aether as just another medium, as real as quartz, say, but with its own permittivity and permeability explained by him in what is to me a very complex fashion involving vortices etc. etc. Thus, in my unversed interpretation, one could ask the question today, does the aether have ε≈ε0 and μ≈μ0? Apart from various practical difficulties in arranging to assuredly have an example of aether to measure, the matter would be settled by appeal to experiment. Whatever the outcome of this exercise, it would have absolutely no bearing upon whether free space, a very technical term with a very specific meaning, has ε=ε0 and μ=μ0. In fact, because free space is hypothetical and its properties based upon definition alone, these properties are untouched by any experimental observation. The only question one can reasonably ask about free space is whether it is at all useful, for example, as a reference state. In support of its utility one might advance that outer space or extremely good terrestrial vacuum approaches the behavior of free space, so it serves as some kind of idealization of some real media. Or, one could argue that use of a an idealized free space is more practical than maintaining a "standard medium" with "standard ε, μ" in some lab in Paris to which one would have to refer all measurement of ε, μ for any other sample of a real medium. Brews ohare ( talk) 06:21, 1 February 2009 (UTC)
Martin: you have not got the point here. I think it boils down to a belief on your part that "free space" is real. Is that your belief? If you do believe that, my next question is: how can the ε and μ of some real medium be established by definition (for free space, ε=ε0 and μ=μ0 by definition as you can find on the NIST website)? It seems to me that ε and μ of some real medium must be established by measurement (or by calculation based on related measured properties such as the density of constituent polarizable components with established EM properties of their own). Such values never could be set by definition but always would be accompanied by a ± error bar due to experimental uncertainty. In contrast, free space has ε=ε0 and μ=μ0 exactly. Can you fill me in on the origin of your ideas?
The purpose and objective in setting up free space is to supply a standard reference state to which measurements in all real media may be referred. It is unnecessary to establish these free space values by experiment. What is necessary is a set of "best practices" to correct measurements so as to refer to the reference state.
Steve seems to suggest that a sequence of measurements on vacuums made more and more "perfect" (that is, by pumping down further and further to eliminate polarizable constituents) can be extrapolated to the value of "free space". That approach probably could be adopted as a best practice for present technology. A different "best practice" would be needed to correct the pulsar observations of the cited article on dispersion, inasmuch as we cannot tinker with outer space to make it a more perfect vacuum. (I'd guess that observations would be screened to eliminate those whose light went too close to distorting celestial objects. In any event, the observations lead to "limits" that is ± error bars, according to the article, not to exact values for the dispersion.) We have yet to develop best practices that anticipate the advance in techniques that will allow observation of the nonlinear and dichroic behavior of quantum vacuum, which will lead to departures of quantum vacuum from "free space" despite following the extrapolation procedure Steve suggests. That discrepancy might lead to changes in the definition of "free space". However, it equally might not, and instead just lead to adding some more corrections to the list of "best practices". Which course of action is taken will be a judgment based upon the entire metrology, and international consensus on what is the simplest and most accurate way to define the reference state. It is a stretch to call this kind of deliberation over best practices a "measurement" of the properties of free space. Brews ohare ( talk) 16:31, 1 February 2009 (UTC)
A departure in the form of Maxwell's equations due to observation of various media, of which quantum vacuum and outer space are examples (as already explained earlier in this discussion) might lead to a revision of the definition of free space, but that revision of the standard of "free space" is not based upon "agreement" of ε0 and μ0 with experimental data, but is based upon the practicality and accuracy of a revised standard versus the present standard. (In this context, "accuracy" refers not to how closely ε0 and μ0 agree with experimental data, but rather refers to how revised measurement procedures compare in terms of the ultimate ± errors of the entire procedure that corrects observations to refer to "free space".) Those considerations of metrology are widely based (involving possibly the standards for time, length, and other issues of accuracy, convenience and what not etc.) and may simply lead to a new set of "best practices" without any change in the definition of free space. How the cookie will crumble cannot be assessed until the nature of the departures is understood. I believe this kind of discussion about how metrology selects a standard is somewhat outside the present scope, which I would take to be a discussion of how free space as used today fits in with the presently accepted form of Maxwell's equations. Brews ohare ( talk) 21:44, 1 February 2009 (UTC)
The sentence in the Wiki article that I dispute is
There are two problems with this sentence.
Next, let us ask whether a modified form of Maxwell's equations could impact the above points were it to evolve that free space exhibited dispersion when used with these updated Maxwell equations.
I would suggest not. The observations suggest that outer space does not exhibit dispersion (to within the accuracy of the observations). Were the updated Maxwell's equations to be adopted, we would then have to explain why outer space does not exhibit dispersion.
We would not have to explain why free space does exhibit dispersion, because that would be a logical deduction using ε=ε0 and μ=μ0 and the updated Maxwell equations, and not an observation.
We then might open an inquiry into whether a new definition of free space were desirable, for example, a new definition that would lead to zero dispersion in free space despite the propensity of the new Maxwell equations to predict dispersion. The considerations entering this deliberation could be the subject of discussion in this article, but do not bear upon the numbered objections above. Brews ohare ( talk) 00:13, 2 February 2009 (UTC)
Martin: You are simply ducking the issue. What medium other than "outer space" could possibly be meant???? Please make a suggestion for an alternative medium to pervade the Universe. Brews ohare ( talk) 00:30, 2 February 2009 (UTC)
Go ahead and state your interpretation of observations in outer space that are independent of the material properties of the interstellar medium. Brews ohare ( talk) 00:39, 2 February 2009 (UTC)
Martin: You have made no attempt to convince: you do not respond to the arguments presented, but wander about in your own world. What medium other than "outer space" could possibly be meant???? Brews ohare ( talk) 00:49, 2 February 2009 (UTC)
BTW, the source actually never mentions "space" or "vacuum" or anything at all except the "speed of light" without any reference as to the medium referred to, rather a large gaff, I'd say. Brews ohare ( talk) 01:03, 2 February 2009 (UTC)
As the error bars of EM measurements are reduced, for example, we might be able to measure a polarization dependence of the speed of light in quantum vacuum. Free space as presently defined with Maxwell's equations as presently understood does not exhibit such a dependence. So what do we do? Do we revise the definition of free space so it also exhibits dichroism using the accepted Maxwell equations, or not? Will we want to modify Maxwell's equations so free space as presently defined also exhibits dichroism? Are these questions ones we want to explore in this Wiki article? Brews ohare ( talk) 00:30, 2 February 2009 (UTC)
Martin: You are indulging in deliberate distortion. I have said nothing of the kind. I have explicitly considered the possibility that dispersion in free space could occur, and demonstrated the irrelevance of this possibility. I have merely suggested that the statement in the article is inadequately phrased and positioned. See my numbered points 1 and 2 above:There are two problems with this sentence.
Martin: Below is my earlier presentation of the role of dispersion, which you have ignored. Brews ohare ( talk) 15:16, 2 February 2009 (UTC)
Martin, it's got everything to do with the topic in question. Aren't you talking about the physical meaning of permittivity and permeability? Maxwell gave good evidence that these terms refer to elasticity and density in a sea of molecular vortices. How can you discuss a subject like this and block the key points from the discussion by upholding wikipedia's rules and regulations on speculation. Could we discuss the second world war without mentioning the Germans? Maxwell's papers were alot more than speculation. They introduced important concepts and equations which we still use today.
The point that I was making here is that Maxwell's luminiferous medium was never discredited. A series of events led to it being abandoned, and that's how you need to write it up in the article. You cannot make rash and inaccurate statements to the extent that Michelson-Morley discredited the aether.
And now you ask me what is the aether. Well Maxwell's vortex sea was not the aether as such, but it involved the aether. His luminiferous medium was an elastic solid composed of electric particles that existed around the edge of aethereal vortices. It was an incomplete theory. There were no sinks or sources in his vortices.
A polarized vacuum is creeping back into physics again through the back door, on the back of Dirac's electron-positron sea. Make the electrons into aether sinks and the positrons into aether sources and consider an electron and a positron in mutual orbit. That dipole would be pretty close to a Maxwellian vortex, and a sea of such dipoles would be pretty close to the Dirac sea. And what is the aether itself? It is space. But not the rigid static space that you have in mind. It is a dynamic compressible and stretchable space with field momentum A, equivalent to the magnetic vector potential. That's how Maxwell saw the A vector, and Dirac is also on record as having said that A must be a velocity. So space as you know it is a sea of tiny whirlpools with their axes aligned along the magnetic field lines. The fine-grained angular momentum density is B = curl A.
But as far as the main article is concerned, I think that all you can say is what Brews is saying, and that is that the permittivity and the permeability, as in free space, are constants of free space. The textbooks probably wouldn't permit any deeper knowledge on the matter. There is an equation which links these two constants to the speed of light. That equation is a skeleton version of Newton's equation for the speed of sound. Perhaps you could put the two equations side by side and make the inference that permeability is a density and that permittity is the inverse of transverse elasticity. David Tombe ( talk) 13:21, 1 February 2009 (UTC)
Steve, As you say, the spinning Mossbauer effect experiment is published in a peer reviewed journal. But it is also disputed by quite a few physics professionals in other peer reviewed journals. I have therefore left your reference in place but re-worded it slightly in order to give a more accurate balance. The experiment did take place and it did get a null result. But there is dispute over the interpretation of that null result. David Tombe ( talk) 05:24, 1 February 2009 (UTC)
Martin, I know of a few professors who are disputing Mossbauer and I could probably get books and publications if I tried. But we don't really need to go down that road. What I want to know is, were the above experiments all carried on outdoors? Aether entrainment by a wall would shield indoor experiments from any aether wind. Also, aether entrainment in the sensory material would also negate any aether wind effects. It's not all as cut and dried as some people like to think. I don't want to be involved any more in this article, as regards the aether wind issue because it would only end up in endless counter citations. Just bear in mind that it's not all cut and dried, and so try and word it so as not to give the impression that the aether is definitely a discredited concept.
I might however reword some of the things that you say about Maxwell, because I have read his 1861 paper many times. David Tombe ( talk) 08:11, 2 February 2009 (UTC)
Martin, I was specifically thinking in terms of the words permittivity and permeability. Maxwell, didn't use those words. So if you are doing a historical section and talking about how Maxwell related the aether to the speed of light in terms of permittivity and permeability, I might be inclined to change the wordings to 'a sea of tiny aethereal vortices', dielectric constant/transverse elasticity, and density. One of the features of Maxwell's 1861 paper was a lengthy analysis in which he linked dielectric constant to transverse elasticity. That of course involved his famous link up of mechanical stress with electric displacement current. Maxwell never claimed that light propagated in the pure aether itself. It is important to make that clear. David Tombe ( talk) 10:52, 2 February 2009 (UTC)
As stated above, in order to consolidate the two original sections and address some of the criticisms recently made, I have done a draft of a proposed new aether section. This, I now maintain, gives a fair and balanced view of the historical and current status of the aether. It could do with a few more references.
Please take a look as, if no one objects, I intend to paste it into the article to replace the current two sections. Martin Hogbin ( talk) 11:31, 1 February 2009 (UTC)
The title of this section expresses a misconception. It suggests that a definition is subject to experimental observation. Perhaps one might change a definition were it to prove too much of a Procrustean bed, encumbering discussion and analysis. However, that means only that the definition proves impractical, not that it fails an experimental test.
What makes a definition impractical is hard to define outside of a particular setting.
For example, let us suppose that an operational approach to realization of free space is to make an extrapolation of measurements on a sequence of samples prepared using higher and higher pumping down, so the sequence of samples contain fewer and fewer atoms or molecules. Within some assumptions as to how such an extrapolation should be conducted, the extrapolation extends the measurements to the case of zero atoms or molecules, a case not actually realizable by any technique known at the moment. Then the speed of light resulting from this extrapolation is taken to be the speed of light in free space, a hypothetical medium where ε = ε0, μ = μ0, both defined constant values.
This extrapolation works fine until measurement technique advances to a point where the fluctuations of quantum vacuum become measurable. Theory predicts, for example, that quantum vacuum may exhibit a speed of light that depends upon the polarization of the light. Supposing this prediction to be accurate, application of the extrapolation technique will lead to different speeds of light depending upon the polarization used in the measurements.
On the other hand, application of Maxwell's equations as we know them to free space (a hypothetical medium where ε = ε0, μ = μ0) predicts no polarization dependence.
Does this disagreement mean that the definition of "free space" must be changed? That is a possible choice, but not a necessary one. One could refine the operational approach using extrapolation to free space. Or, instead, one could simply implement a "best practices" correction to the extrapolation that corrects for the polarization effect, so measurements referred to free space agree regardless of the polarizations used.
Whether a change in operational approach to realization of free space, or a correction using the "best practices" approach, or a redefinition of free space is the better route will be decided by various standards organizations based upon a wide ranging view of metrology, and the best solution from the standpoint of accuracy and convenience will be selected. These considerations extend far beyond whether a particular extrapolation technique leads to ε ≈ ε0, μ ≈ μ0.
The conclusion is that the title of this subsection of discussion is mistaken. Brews ohare ( talk) 16:07, 2 February 2009 (UTC)
Brews, I'm not sure what the purpose of this discussion is within the confines of wikipedia's rules. The textbooks say that free space is nothing, and that there is no aether. What else could you expect to find out from this discussion? Those two constants, permittivity and permeability are constants of space related to magnetism and electricity, which are also related to the speed of light waves through the vacuum.
I noticed your discussion taking place last week, and so I decided to draw your attention to Maxwell's use of Newton's equation of the speed for sound, for the purpose of pointing out that the two constants in question might be a density and a Young's modulus. But I was soon ruled out of order under the rules and regulations. Maxwell is old fashioned and out of date and doesn't apply anymore. So under the rules and regulations, what kind of an answer would you be expecting to get? What modern up to date kind of answer would satisfy you regarding the physical nature of those two constants?
I fail to understand what all this could be leading up to. David Tombe ( talk) 12:18, 4 February 2009 (UTC)
This statement is restricted to the medium to which it refers. We know that the speed of light is dependent upon the ε(ω) and μ(ω) of the medium through which it passes. Because the statement refers to "distant astrophysical events", which are observable only through the medium of outer space, it would appear that a more correct statement would be:
This modification of the sentence is almost trite. It assumes more importance in the context of the article, however, because this sentence appears directly following the sentence "According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies."
This juxtaposition, when done without inclusion of the limiting phrase "in outer space", has the seeming implication that the astronomical observation somehow has relevance to the properties of free space, which is clearly untrue, as these are defined, not measured. See the discussion above. Brews ohare ( talk) 17:10, 2 February 2009 (UTC)
Martin Hogbin ( talk) 18:02, 2 February 2009 (UTC)
Martin: My statements in this subsection stand alone, and do not hark back to anything previous. If some statement made in this subsection requires amplification, please explain further. Brews ohare ( talk) 20:57, 2 February 2009 (UTC)
Martin: You can pick any few words you like out of the article and say they are the gist. How about "speed of light" or "dispersion"? What is the bearing of your remarks upon including "in outer space" at the end of the sentence? Brews ohare ( talk) 06:43, 4 February 2009 (UTC)
This topic is about the article but it has gone on for a long time, monopolizing this page. Should we take it to a dedicated page?
This article is about the speed of light and how it is understood by physicists today. It has a brief mention of the aether because it was of historical significance. The simple rigid fixed aether was the mainstream theory at the time of the Michelson Morley and the only theory that need be mentioned in this article. Many excellent physicist indulged in musings about the aether and proposed various theories but none of these ever came to anything and they have no place in this article. Martin Hogbin ( talk) 19:54, 3 February 2009 (UTC)
Martin, do you mean by fixed aether, that the aether is fixed in space relative to the universe and that the Earth moves through it, causing an aether wind?
For the record, I don't actually support Lorentz's theory, but my understanding of the facts are that Lorentz's theory followed from Michelson-Morley and that the aether wasn't actually abandoned until Einstein's theories superseded Lorentz's theories. It was on that basis that I removed your reference to fact that Michelson-Morley directly disproved the fixed aether theory.
By all means keep your references to the aether short, but I was only trying to make your references to Maxwell's aether more accurate. Why give a reference to Larmor's aether in relation to a paragraph on Maxwell's aether?
On second thoughts, you're the one that wrote the article on the aether. It wasn't me. I corrected some details based on my knowledge of Maxwell's original work. You undid those corrections and then went on to complain that there was too much in the article about the aether. Well so there still is, but it's worse now because it is inaccurate. Would it not just be better to delete the whole section altogether?
David Tombe (
talk) 05:57, 4 February 2009 (UTC)
Martin, On third thoughts, there was already a section about the aether in this article when I came here last week. I made some corrections to it. You immediately told me that I should not be putting articles about the aether into this article, even though the article was there already. You undid my edits, but nevertheless let the article remain in place, when by your statement, it would have been more logical if you had simply deleted the entire aether article.
You then proceeded to replace it with a larger article on the aether which contains factual inaccuracies. I corrected those inaccuracies. I put quite a bit of work into it, getting quotes, and links, and dates. You undid the corrrections and went to somebody else's talk page requesting them for some input. And they came and made a statement to the extent there was too much about aether in this article.
It seems to me that it not so much a question of there being too much aether in the article, as it's a question of what point of view is being emphasized. You have just happily supplied a large section on aether yourself, which contains a strong point of view that the aether has been disproven. Tell that to quantum mechanics experts about their polarized vacuum. It seems to me that the issue is that you will not be happy about an aether article if it is not critical enough about the concept. I think that has been what's it's been about all along.
You now have some badly written information about Maxwell's aether, with copious references to Larmor's aether to back it all up. The one aether theory which you mention above in relation to Michelson-Morley was Maxwell's aether theory. I fixed up the details of Maxwell's theory for you. So what do you want? It seems that you want to draw attention to aether in a negative light, make it vague with inappropriate terminologies such as permittivity and permeability which Maxwell didn't use, and give references to Larmor's aether theory, and complain that correct information about Maxwell's theory is not needed because there is only one aether theory that is relevant? It doesn't make sense. David Tombe ( talk) 12:30, 4 February 2009 (UTC)
I'd suggest that a brief mention without prejudice be made here and a link provided to Aether theories, which should be the definitive article on this matter. Brews ohare ( talk) 18:41, 4 February 2009 (UTC)
Martin, my entire focus in this debate was on density and transverse elasticity. That's what brought me into all this. Brews and some others, including yourself were debating the vacuum and what its physical characteristics might be. You were all focused on permeability and permittivity. I drew attention to the fact that Maxwell had shed more light on this subject than anybody else. He had firmly connected the equation that links permeability and permittivity to the speed of light, with Newton's equation for the speed of sound.
That's all I was saying. Then I noticed that Maxwell's aether had been written up carelessly in terms of permeability and permittivity. Those words don't give it its true significance. They are the very words that you guys were debating. So we came full circle.
I decided to make the bit on Maxwell's aether factually accurate in as little space as possible, and also in relation to Lorentz and how the aether was eventually abandoned. It was not abandoned instantly after Michelson-Morley as you have implied.
But what I detected was a reluctance to acknowledge the essence of what Maxwell had said. That's why I asked Brews yesterday what he expected to be the final outcome of his enquiry into free space. If we are going to restrict the terms and conditions of the discussion to what is written in modern textbooks, then there is nothing to discuss. Space is nothing, and it was a conversation about nothing.
I was finally suggesting that we therefore remove all references to the aether, or else, if we want a short section on the aether in relation to Michelson-Morley, then we can at least describe Maxwell's aether correctly and not back it up with lots of references to Larmor's aether. Ultimatley Maxwell's luminiferous medium was not the aether. He never called it the aether. It was a sea of molecular vortices. If you want to mention it in connection with Michelson-Morley, then why not use the correct name. If you don't like the name, then why bother writing about the subject at all? David Tombe ( talk) 08:42, 5 February 2009 (UTC)
Martin, the Michelson-Morley experiment was done specifically in connection with Maxwell's model. Maxwell's model was that very fixed aether model that you keep referring to. But anyhow, what about putting those details back again and shifting the whole section to some other article which you may feel is more appropriate for the content? David Tombe ( talk) 18:57, 5 February 2009 (UTC)
It certainly didn't disprove the Stokes aether entrainment model. It was only Lorentz's claim that the Stokes model disagreed with stellar aberration that caused Lorentz to look at an aether wind contraction phenomenon. But in the absence of any details of the material of the Stokes model, Lorentz had no basis upon which to object to the Stokes model on the grounds of stellar aberration. David Tombe ( talk) 13:53, 6 February 2009 (UTC)
Here is another try at this paragraph:
I believe the merits of this paragraph are:
I do not think there is any debatable point in this paragraph other than its literary merits. Brews ohare ( talk) 20:01, 4 February 2009 (UTC)
Martin: Please explain what is wrong with this paragraph. Your statement: "severe limits have been put on the variation in the speed of light with frequency " is elliptic because it does not explain: (i) the implications of classical EM upon such variation, or (ii) how it was done, and most importantly (iii) to what medium the limitation applies. Brews ohare ( talk) 23:11, 4 February 2009 (UTC)
Question: Is its "perfectly clear meaning" the same as that of my paragraph (so far as you can understand my paragraph in view of its "confusion")? If not, why not? Brews ohare ( talk) 07:20, 5 February 2009 (UTC)
You two are arguing about nothing. You are literally arguing about nothing, as in pure empty vacuum. As per the textbooks, the only thing that you can know about this 'nothing' is that c^2 = 1/με. There is nothing more to be said about it. There is nothing that can be added to this discussion that would be permitted by the modern textbooks. The summary is that we have a 'nothing' in which c^2 = 1/με. David Tombe ( talk) 19:01, 5 February 2009 (UTC)
Well, Martin I see you have simply restored your version. I am sorry you cannot come to grips with this topic. I do not understand how you arrive at the view that somehow this arrival time data confirms Maxwell's equations in some degree. First of all, if dispersion were observed, as you have admitted earlier, we would not discredit Maxwell's equations, but attribute the dispersion to the medium of outer space, which is not "vacuum". Hence, there is no "test". Second, the observations do not establish zero dispersion, but only set limits upon it. Thus they cannot establish that outer space is free space, but only that it approximates free space to some degree. Your statement of the "content" of the article is nothing but words out of context that can be misconstrued. Brews ohare ( talk) 22:37, 5 February 2009 (UTC)
A different, formal RfC is found Here.
The dispute concerns the wording at the end of the 'Light as electromagnetic radiation section. Martin Hogbin ( talk) 18:12, 6 February 2009 (UTC)
The current text is:
'According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. Measurements based on the arrival of electromagnetic radiation from distant astrophysical events put severe limits on the possible variation in the speed of light with frequency. {{cite journal |journal=Phys. Rev. Lett. |volume=82 |pages=4964-4966 |year=1999 |url= http://arxiv.org/abs/astro-ph/9810479v1 '
One editor would like to keep this text. Another would like to replace it with:
'According to Maxwell's equations, the speed of electromagnetic radiation in any medium with frequency independent permittivity ε and permeability μ (in particular, free space) is the same for all frequencies. Measurements have been made of differences in arrival time on Earth of electromagnetic radiations of various frequencies, radiations that originate simultaneously in distant astrophysical events.Bradley E Shaefer (1999). "Severe limits on variations of the speed of light with frequency". Phys. Rev. Lett. 82: 4964–4966. The upper bounds placed upon the observed differences in delay set severe limits on any possible variation in the speed of light with frequency, suggesting that in reaching Earth this radiation traversed a medium with a very nearly frequency-independent permittivity ε and permeability μ'.
A third alternative is :
‘According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. Measurements based on the arrival of electromagnetic radiation from distant astrophysical events put severe limits on the possible variation with frequency of the speed of light transmitted through outer space.’Bradley E Shaefer (1999). "Severe limits on variations of the speed of light with frequency". Phys. Rev. Lett. 82: 4964–4966.
There has been considerable discussion (see above on this page) but no consensus has been reached. Please comment below:
My thoughts - observed limits on speed variation with frequency in outer space imply (given a model) limits on speed variation with frequency in free space (or indeed in any other medium). So the first option seems to me to be clear, correct, and close to the source. The second option seems to be importing too much theoretical and philosophical baggage into what should be a straightforward observation. I dislike the (unsignposted) segue from free space to outer space in the third option, and the implicit assumption that experiment can have nothing to say about the properties of free space (or presumably therefore any other idealisation). EdwardLockhart ( talk) 19:44, 6 February 2009 (UTC)
I have added this for discussion of the comments to keep things tidy. Martin Hogbin ( talk) 20:03, 6 February 2009 (UTC)
The speed of light in the vacuum of free space c0 is not measured. It has an exact fixed value when given in standard units. Since 1983 the metre has been defined by international agreement as the length of the path traveled by light in vacuum during a time interval of 1/299,792,458 of a second. This makes the speed of light exactly 299,792.458 km/s. This exact speed of light is found at c0. The exact permeability is found at μ0. The electric and magnetic fields in EM waves are related by the value of the characteristic impedance of vacuum, found at Z0 =√(μ0/ε0). The permittivity is at ε0=1/(μ0c02). The "standard uncertainties" of all four variables is labeled by NIST as exact (i.e. zero) because they are not measured; zero measurement error means, ipso facto, that free space is not a real, measurable medium.
Martin: the BIPM definition is: The meter is the length of the path traveled by light in vacuum during a time interval of 1/299 792 458 of a second. One therefore needs a second. The standard for the second is in the microwave region using a Cesium fountain. I suppose one could go into a lot of detail here. You are fixated upon light being optical. What about the Doppler shift: would you suggest that relativity is also part of the discussion? It suggests that a moving source is just red shifted, but still propagates light at the speed of light. What about radio astronomy? Brews ohare ( talk) 19:08, 8 February 2009 (UTC)
Consequently, Shaefer's paper does not refer to a delay measured in free space, but to a delay in transmission through the medium of outer space, a real measurable medium. If measurement of zero dispersion in outer space has any implication for free space, the logic behind this implication must be explained. The present wording of the article seemingly implies some connection without explanation. Brews ohare ( talk) 23:33, 7 February 2009 (UTC)
Hello again. It seems to me that this is a commonplace situation in the interpretation of experiment. There are some electrical signals in a telescope of some sort. Via a whole bunch of physical theory (much of it embodied in hardware and software), we interpret these as light of multiple frequencies from a single explosive astronomical event. With the employment of some more physical theory, we interpret this to mean that there are limits on the variation of the speed of light with frequency in outer space. And after a bit more theory, we deduce limits on the variation of the speed of light in free space in a Maxwell-like theory.
There are large amounts of theory involved in going from the actual physical observations made to deducing the implied limits on physical theories, of which the fact that outer space is a close approximation to free space is just one (and perhaps one of the simplest bits).
EdwardLockhart ( talk) 09:34, 8 February 2009 (UTC)
I disagree with your statement "any dispersion found would be attributed to the medium of outer space". We would have a choice between ascribing significant dispersion to the physical properties of outer space, or to the theoretical properties of free space. Determining which is the best explanation would require further experimentation and theorising. But Shaefer's negative result shows that (at least for now) that we do not need to revise either one. EdwardLockhart ( talk) 17:25, 8 February 2009 (UTC)
I have added a Cosmology section. It appears Shaefer's paper is of interest in this context. That suggests a possibility for compromise, and I have added a sentence to the disputed paragraph. Brews ohare ( talk) 22:16, 8 February 2009 (UTC)
I have just looked at this section. In my opinion it makes a number of spurious claims that are not supported by the quoted references. Martin Hogbin ( talk) 23:19, 8 February 2009 (UTC)
My understanding is at the level of the verbal introductions and conclusions of the various papers, and does not extend to doing the math. That approach can be risky if there is controversy, because each author wants to put their slant on things. However, no statements in the Wiki subsection favor one or another model. The predictions of dispersion etc. seem to be accepted as theoretical predictions worth testing, and some tests have been made. The accuracy that can be claimed by these tests is debated, but most are null results. No attempt has been made in the Wiki article to sort this out in any detail. I don't think there are any huge gaffs here, but I agree that an expert could write something more cogent. It is (by Wiki standards) very well documented, so the more sophisticated reader can pursue matters themselves. Possibly, having an attempt here will encourage expert action: I'm often told it is easier to edit a proposal than to start from scratch. Put up a template requesting assistance if you like. Brews ohare ( talk) 03:00, 10 February 2009 (UTC)
An improvement in clarity of this article would result if a clear distinction were made throughout between the speed of light c in various media (such as that of outer space, or quantum vacuum, or the vacuum of quantum gravity) and the defined speed of light in free space c0 = 299 792 458 m s–1. The speed of light c is a matter for theory and experiment. The defined c0 = 299 792 458 m s–1 is useful for metrology, but doesn't play a role in various theories and measurements of c (in particular, that of outer space, or quantum vacuum, or the vacuum of quantum gravity), which mainly are interested in departures of c from c0 = 299 792 458 m s–1, departures not only in numerical value, but also functional differences such as anisotropy and dispersion and variation with the age of the Universe. Brews ohare ( talk) 16:43, 9 February 2009 (UTC)
I have modified the Intro as follows:
In my mind this description better distinguishes between measurement and corrected measurement, and more carefully explains the role of c0. Brews ohare ( talk) 18:23, 10 February 2009 (UTC)
What I wanted to say is that c is extracted from a measurement by application of CIPM corrections that refer the measurements to free space, while c0 is a number attached to the idealization of free space (and is defined, so no measurement is implied). Do you agree with this objective in principle? Brews ohare ( talk) 20:46, 10 February 2009 (UTC)
I agree that c is the common symbol, and that "vacuum" is the common description of where c can be found. However, it is clear that the speed of light has multiple meanings, all of which should enter the intro:
These important roles transcend setting the speed of light at any particular value, such as 299,792,458 m/s, and these roles would be unaffected if this were not the case.
Therefore, I would support a different intro to this article, which right now puts way too much emphasis on 299,792,458 m/s. Brews ohare ( talk) 21:47, 10 February 2009 (UTC)
In addition, any observation of the speed of light takes place in a realizable vacuum (such as terrestrial vacuum at very low pressure, or outer space) and then is corrected to refer to free space. These real vacuums are not perfect, of course, and were technique up to it, would reveal nonlinearities , dichroism and anisotropy. In contrast, the reference state of free space has none of these behaviors. In addition it has a defined speed of light 299,792,458 m/s, and the use of a defined value (zero error bars) sets it apart from real media that are known only to within an error bar.
None of these fundamental facts are expressed in the present article, which remains very deficient in relating standards, measurements, and ongoing research into the properties of realizable vacuums.
No recognition is given to the fact that free space is a construct, and no vacuum extant in our Universe can be proven to be exactly like free space. Nor can corrections intended to extrapolate measurement in these media to free space values be shown to be exact.
In addition, the above numbered properties of the speed of light are under scrutiny, and may have to be generalized. No indication of the tentative nature of these properties and how they are being re-examined enters the article, with the exception of the Cosmology section I have just added. Brews ohare ( talk) 21:47, 10 February 2009 (UTC)
The point is that theoretical physics is always a long way ahead of metrology and engineering. Metrologists are only interested in very well established physics that has been tried an tested for years. The constancy of the speed of light in free space is just that, it was first proposed over a century ago and to date no evidence against it has been found. For that reason (with others) when it comes to the delineation of the metre, the a definition based on the speed of light (under defined conditions and of preferred wavelength) has been chosen by the standards authorities as the most stable and practical way of doing things.
Theoretical physicists, on the other hand, have speculated that there might be variations in the speed of light in free space but these variations are many orders of magnitude below those that would affect metrology. No experimental evidence of any of these theories has been found to date. If some large and unexpected effect were to be found then metrology might need to reconsider some of its definitions but that has not happened yet so there is no need to change anything.
As far as anyone can tell so far, the different meanings of the speed of light that you give above are all identical. Martin Hogbin ( talk) 22:46, 10 February 2009 (UTC)
Martin: Maybe you are mistaken in your thoughts about free space: check this out:
In free space the principle of linear superposition of potentials and fields holds: for example, the electric potential generated by two charges is the simple addition of the potentials generated by each charge in isolation. Superposition is a defined property of free space even though the electric field near a point charge can become extremely large. See, for example:
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help)Therefore, the ideal vacuum of free space is not the same as any physically obtainable vacuum that exhibits nonlinearity. Quantum vacuum is predicted to do so. See, for example:
So, Martin, maybe this work will not come up with departures from free space, but it is possible in principle, eh?. Whether differences are measured or not, the bottom line is that free space is an idealization (with an exact, linear, ε0 μ0) that may or may not apply to any realizable vacuum. That is not your belief, as I understand you.
Similar references can be advanced that predict anisotropy and dispersion of quantum vacuum, which also are not properties of a medium with the exact EM properties ε0 μ0. Brews ohare ( talk) 01:41, 12 February 2009 (UTC)
Hi Dick: That is not exactly the point. To follow along with your sentence "well-sourced speculation that an ideal vacuum might turn out to be nonlinear and affect the speed of light", there is no need to speculate about ideal vacuum in the sense of free space because it has an exact speed of light set by definition and is exactly linear. The issue is that what often is called "vacuum" is outer space or ultra-high vacuum, which is not necessarily ideal, and may indeed exhibit nonlinearity. There is tendency to confuse the ideal, model vacuum with an actually realizable vacuum, and that tendency is exacerbated by using "vacuum" in a variety of meanings, and referring to "the speed of light" as though it existed in some Platonic universe, instead of stating "speed of light in ideal vacuum", or "speed of light in outer space". Remarkably, in describing Shaefer's observations of light from distant astronomical objects, Martin has repeatedly removed my addition of the limiting words "in outer space" and demands to state only "the speed of light". It seems trite to say that light that travels to Earth through the interstellar medium characterizes that medium, and does not characterize the defined c of free space, but apparently not. Personally I just don't grasp this resistance, which seems to me irrational and has been supported only by saying we cannot depart from a paraphrase of the source (not even represented as a direct quote) to make this distinction. We can't even add "it seems probable that these observations based on light traveling through outer space refer to the speed of light in that medium." Brews ohare ( talk) 15:25, 12 February 2009 (UTC)
Dick: This dispute arose originally in the last paragraph of the subsection Light as electromagnetic radiation, not in the Introduction, so this version of Martin's reasoning doesn't apply. I now have added the following well-documented lines in a later subsection to present this viewpoint, rather than continue the battle to add the three simple words "in outer space" to the earlier sentence:
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link)This added paragraph is not a complete antidote to the logical failure to discriminate between definitions and reality, which permeates the speed of light presentation. Brews ohare ( talk) 17:32, 12 February 2009 (UTC)
Hi Martin: I find your actions unsupported by any argument or discussion; they are high-handed, arbitrary, and based on your unsupported opinion. Brews ohare ( talk) 20:52, 12 February 2009 (UTC)
We agreed to an informal RfC and we got one person to take an interest. They agreed with me. If you want to we can have a full RfC but I am not sure how many it will take to convince you that you are pursuing a nonsensical point. Martin Hogbin ( talk) 22:07, 12 February 2009 (UTC)
We need some more editors here to stop this nonsense. Is there anybody there? Martin Hogbin ( talk) 23:23, 12 February 2009 (UTC)
Comment is invited on the accuracy of the following proposed insertion in the subsection of speed of light discussing transparent media. A subsection to collect your observations is Here.
Here is the suggested paragraph followed at bottom with the supporting references:
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link)This section is to collect your observations:
The quotation given does not support the proposed insertion. The authors are discussing new theoretical models for an idealised 'free space', in part via experiments on physically realisable vacua. As they explicitly say, "constancy of the speed" is a "pre-quantum approximation" - in their models, free space would behave (slightly) differently from classical free space. The work they are doing is to improve our model of free space, not to better understand the difference between physically realisable vacua and an idealised vacuum, which is uncontroversially considered to be small but non-zero. EdwardLockhart ( talk) 08:57, 13 February 2009 (UTC)
Hi Folks: The problem here as I see it is the use of "vacuum" and "free space" in multiple meanings. The cited literature refers variously to "electromagnetic vacuum", quantum vacuum" and just plain "vacuum". All these terms are meant to refer to a vacuum (or vacuums) where ε≠ε0 and μ≠μ0.
In the suggested Wiki paragraph the term "free space" is introduced to represent a idealized medium, in which ε=ε0 and μ=μ0, and therefore definitely not the medium meant by the authors. None of the authors uses the term "free space", but all refer to a baseline medium from which their vacuums depart. The baseline medium is one with ε=ε0 and μ=μ0. For example, the verbatim quote from Delphenich says he wishes to describe the departure of (in his case) "electromagnetic vacuum" from a medium where ε=ε0 and μ=μ0.
That is, I think the only reason to suggest that the sources are misrepresented is because you two bring your own meaning of "free space" into the discussion, and whatever you two have in mind, it is different from the meaning stated in the proposed paragraph. So for Edward, he reinterprets the term "free space" as some more general concept for which ε≠ε0 and μ≠μ0 are possible. That contradicts the stated usage in the proposed paragraph.
I could coin a new phrase, like "baseline vacuum" to represent ε=ε0 and μ=μ0 medium. Would that make you all happy? Then I could discuss how "baseline vacuum" oddly has exactly the same ε=ε0 and μ=μ0 as the "vacuum" used by BIPM and NIST, and oddly has the same properties of linearity and zero dispersion attributed to "free space" in text books. Would this circumlocution serve the purpose of separating unwanted connotations and then putting only the desired connotations back in? Brews ohare ( talk) 17:55, 13 February 2009 (UTC)
To me, the important idea for this article is that the very idea of *the* speed of light is only true in the "classical free space" with constant u and h. But a chunk of space with nothing in it (also confusingly called free space) does not have these properties, due to at least QM, GR, and perhaps other explanations. In empty space as it really exists, EM fields do not behave linearly. In particular, perhaps we do not have just c, but maybe c(lamda) or c(lambda, energy) or something even more complex. This article is not the right point to go into the details of what the correct model for a physical vacuum is, but I think it is reasonable to point out that the idea of "the speed of light" is an approximation (though a very good one in normal circumstances), and that physicists agree that if you look closely enough, even empty space needs to be considered as a transparent medium with non-trivial optical properties. LouScheffer ( talk) 18:30, 13 February 2009 (UTC)
Drawing upon all three comments above, how does this look:
Martin: The above proposed paragraph is not the same as its predecessors: it has evolved. However, there is no fresh input from you as to what direction would bring it closer to your desires. I see nothing wrong with it; we'll have to see what the response of others is. It has been amended to account for their comments. I have not made any posts about this topic on the article page since this RfC has been posted. Brews ohare ( talk) 22:28, 13 February 2009 (UTC)
There is no reason one cannot use a term to avoid continuous repetition of a cumbersome phrase. Maybe you would like to see "a medium with ε = ε0 and μ = μ0" repeated over and over? Brews ohare ( talk) 23:07, 13 February 2009 (UTC)
Martin, that very simply is nonsense. There is a simply overwhelming amount of literature on this subject. Take the quotes below and show me how you can possibly stretch them to contradict the proposed paragraph of Revision 2 below.
Brews ohare ( talk) 00:30, 14 February 2009 (UTC)
In the following, all mention of "free space" in any form is paraphrased to eliminate any direct reference:
I hope against hope that this version is so smooth that Martin can swallow it. Brews ohare ( talk) 00:03, 14 February 2009 (UTC)
Following suggestions of Dick Lyon:
Yes, that's right unless the anomalies were so large that they exceeded these contributions, which, in that case, would become a mere "background noise". Can you put your finger on what needs change? Brews ohare ( talk) 12:24, 14 February 2009 (UTC)
This version adds a footnote to flesh out the sort of implications that might arise from observations of light:
Proposed footnote
Brews ohare ( talk) 14:29, 14 February 2009 (UTC)
This list is now an assembly of articles of which perhaps three or four would be selected.
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help)Brews ohare ( talk) 20:05, 13 February 2009 (UTC)
Brews, I salute your indefatigability. How about the following?
In the classical model of electromagnetism, the speed of light is the same for all wavelengths in free space. Modern models of small-scale physics predict some variation - the constancy of the speed of light with wavelength is considered to be a "pre-quantum approximation" [1]. Experimental results, such as analysis of arrival times of light of different wavelengths from distant astronomical events [2] [3], show that the divergence from classical behaviour must be very small.
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EdwardLockhart ( talk) 09:15, 14 February 2009 (UTC)
Hi Edward: I also made "yet another proposal" (number 4 above); can they be merged? I am particularly interested in retaining the first paragraph, which has relevance to the overall article. Brews ohare ( talk) 14:33, 14 February 2009 (UTC)
I propose it to go in the section on " transparent media" as it is about transparent media and that section introduces the notion that the speed of light is affected by refractive index. Whether these few lines represent the cutting edge of research in four or five prolific areas may be not so important, because the gist is simply to point out that questions in these areas are under investigation and to present some representative links that can lead the reader further, if they want to go into the literature. In any event, a deeper discussion would have to go into nuance that is a bit more than this article needs. Brews ohare ( talk) 17:36, 14 February 2009 (UTC)
Although it is a laudable goal to make speed of light a superhumanly impeccable article that expresses the universe in a grain of sand, it might be noted that the standards being applied to this short section are more stringent than those applied anywhere else in this article, and way beyond most Wiki articles, which often are lucky to get the definition of the topic accurate, and may be full of unreferenced opinons. (I'm thinking of my experience with matter acceleration frame of reference centrifugal force and others.) Brews ohare ( talk) 17:48, 14 February 2009 (UTC)
An attempt to combine the two proposals; it partially answers Martin's questions about "which models" and "are they accepted". I believe the reference to Delphenich covers the "pre-quantum approximation" issue. Here's the proposal :
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Brews ohare ( talk) 16:13, 14 February 2009 (UTC)
Sorry Brews but I still cannot see what point you are trying to make. All I can say is that I would object less if it were put in your new section, which I would suggest calling something like 'current theories' or 'new theories'. Martin Hogbin ( talk) 18:42, 14 February 2009 (UTC)
To be a little more constructive I have no objection to pointing out in the 'Light as EM radiation' section that classical electromagnetism (or Maxwell's equations if you prefer) does not predict any anisotropy, dispersion, nonlinearity, or dichroism in free space.
Beyond that all, we are really saying is that some theories under current consideration may predict some of the above effects in free space but no experimental evidence of any of them has been found to date. I am not convinced that any of the current editors have the necessary knowledge and understanding to give sensible summaries of current research papers, but if you want to give it a go, who am I to stop you. Again I strongly suggest that this goes in a separate section. Martin Hogbin ( talk) 19:02, 14 February 2009 (UTC)
Hi Dick: A possibility is Delphenich, who says in part:
I'd say all the references in the reference list are aiming to find departures from the customarily assumed values for ε0 and μ0. I'd say none of them use the term "free space", so they won't settle the meaning of that terminology. You will find many textbooks do refer to free space, and they all cite the usual values for ε0 and μ0. However, one is left wondering if they have even thought about a vacuum that wasn't one with ε0 and μ0. Thus, there is an issue here as to what one wants to do with the term "free space" as time marches on: I prefer to keep it back at square one with ε0 and μ0; Martin prefers to let the term evolve with the times. The advantage of the first course is (i) there is continuity with the past if "free space" is taken to mean ε0 and μ0, and (ii) one has a handy term to use in place of an awkward circumlocution, and (iii) one has consistency with NIST & BIPM ε0 and μ0. The drawback is that there are those who follow Martin's path, and also suggest the symbols ε0 and μ0 evolve with the times, in which case the NIST BIPM defined values for ε0 and μ0 must be viewed as anachronisms in the making. Another hang-up of the view that ε0 and μ0 evolve with the times is that the theory suggests that a scalar constant is insufficient: real vacuums are described by εij(E, B, ω) and μij(E, B, ω). Brews ohare ( talk) 21:45, 14 February 2009 (UTC)
Hi Dick: I don't see that there is any "mixing up" going on here; the paragraph is "remarkably lucid and straightforward" as it stands. However, nothing is beyond improvement: see the next version below. Brews ohare ( talk) 23:23, 15 February 2009 (UTC)
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All vestiges of imperfect vacuum are removed; there is no mention of free space. Are we ready for prime time? Brews ohare ( talk) 00:00, 16 February 2009 (UTC)
How about a sub-subsection of "transparent media"?? Brews ohare ( talk) 00:11, 16 February 2009 (UTC)
I have tried blending it into the transparent media section. If you don't like it, try moving it to Cosmology, but I think some reworking will be needed. Brews ohare ( talk) 00:34, 16 February 2009 (UTC)
Brews I am still trying to understand the point that you are tying to make about free space. Do you take the words 'free space' to automatically imply the free space of classical electromagnetism - what you earlier called 'classical free space'? Martin Hogbin ( talk) 10:46, 15 February 2009 (UTC)
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help)Brews ohare ( talk) 15:05, 15 February 2009 (UTC)
Well, different groups will use terms differently; I find physicists among themselves are not consistent either (see Jackson, for instance). In any event, I have avoided this term in the proposed paragraph, which therefore is not subject to any reservations about usage. Brews ohare ( talk) 23:20, 15 February 2009 (UTC)
Well Martin, stick with whatever version of free space you like: it does not appear in the proposed paragraph (Merger II) and so it is a non-issue at the moment. Brews ohare ( talk) 00:07, 16 February 2009 (UTC)