# Talk:Vacuum permittivity/Archive 1

## old comment

Maybe it would be useful to add the value without Pi^-1, just 8.8541878176 × 10−12 F/m, as it appears in the Permittivity article... IMHO

— Preceding unsigned comment added by 67.165.85.24 (talk) 20:06, 10 September 2006 (UTC)

## Why use a truncated real number approximation to describe a value that has an exact representation with transcendental numbers?

The following links should make this clear:

http://physics.nist.gov/cgi-bin/cuu/Value?eqep0

http://physics.nist.gov/cgi-bin/cuu/Value?eqmu0

http://physics.nist.gov/cgi-bin/cuu/Value?c

Therefore

${\displaystyle \varepsilon _{0}={\frac {1}{\mu _{0}c^{2}}}={\frac {1}{(4\pi N10^{-7}A^{-2})(299792458m/s)^{2}}}={\frac {1}{\pi }}{\frac {625000}{22468879468420441}}{\frac {F}{m}}\approx 8.85418781762039\cdot 10^{-12}Fm^{-1}}$

Certainly we should not be saying:

"Its exact value in SI units is

${\displaystyle \varepsilon _{0}=8.8542\cdot 10^{-12}}$ F m-1."

ChrisChiasson 07:54, 1 October 2007 (UTC)

## Electric constant is the recommended term (also: "vacuum permittivity" is official)

Electric constant is the recommended term for ε0. Also "vacuum permittivity" is used:

I think we need a seperate small article about the concept, without all the complications of materials properties. All redirect pages about the concept should point to electric constant, the recommended term. /Pieter Kuiper 17:09, 8 August 2007 (UTC)

Just because one NIST page uses the term "electric constant" doesn't mean it is the "recommended term". If you Google site:nist.gov, you can find plenty of other NIST pages that use the terms "vacuum permittivity" and "permittivity of vacuum". —Steven G. Johnson 18:05, 8 August 2007 (UTC)
In any case, whether we call the page "electric constant" or "vacuum permittivity", we should only have one page, and the other should redirect to it. This is the standard Wikipedia practice: if two terms are synonyms (as they are here), then we only have one page that lists both terms, naming the page after the most common usage. —Steven G. Johnson 18:09, 8 August 2007 (UTC)
I think wikipedia should conform to standard nomenclature. Everybody is in different subfields with different preferences and homegrown conventions. I myself tended to use "dielectric constant", but I will change (especially in teaching, but also in papers) to "electric constant". The advantage for wikipedia of adhering to a convention is that it makes discussions on dominant usage unnecessary.
Also, there are reasons for the recommended usage. The committees are experts on this kind of thing, and do a good service to the field of physics. In this case, the idea behind the recommended nomenclature may be that "permittivity" is not a materials property of vacuum, or something like that. /Pieter Kuiper 18:20, 8 August 2007 (UTC)
You haven't given any evidence that "electric constant" is an "official" standard nomenclature. You've only given instances where the term "electric constant" is used by standards bodies, and I've pointed out that the same standards bodies also use "vacuum permittivity" in other places. Both terms are clearly accepted nomenclature for ${\displaystyle \varepsilon _{0}}$. —Steven G. Johnson 18:49, 8 August 2007 (UTC)
Also, you're totally confused if you think "electric constant" is a synonym for "dielectric constant". The former refers to ${\displaystyle \varepsilon _{0}}$, a dimensionful quantity and a fundamental constant. The latter refers to the relative permittivity ${\displaystyle \varepsilon /\varepsilon _{0}}$, possibly only at zero frequency depending on the context, and is a dimensionless quantity and a property of a given material rather than a physical constant. —Steven G. Johnson 18:51, 8 August 2007 (UTC)
I may have been unclear or imprecise, but I was not confused. I am a solid state physicist, I know what ε0, ε, and εr are. Please do not patronize. /Pieter Kuiper 19:12, 8 August 2007 (UTC)
You wrote that "electric constant" could/should be used in place of "dielectric constant". This is wrong. If you tell me that this is not what you meant to write, I believe you, but correcting an unambiguous mistake is not patronizing. (I'm a physicist with a background in solid-state as well, and work on the theory of microstructured optical media; however, on Wikipedia we have no way of knowing other peoples' credentials in general.) —Steven G. Johnson 20:41, 8 August 2007 (UTC)

## Please do not delete "Electric constant" for the duration of discussions

For the second time now, User:Stevenj has deleted the article electric constant, which I tried to put in because it is usage recommended by international bodies of physical societies. Although vacuum permittivity also is inluded in official nomenclature, the term "electric constants" is preferred.

I agree that in the long run wikipedia should not have two articles about the same concept. However, in this case, little harm is caused by these two short articles. It would be good to have a discussion about this, and for that we need both articles present.

Now, "electric constant" or "vacuum permittivity" is not a big issue for me. The real problem are the two big articles dielectric constant and relative permittivity. They need to be merged, but to what. The only way we cn try to resolve this on Wikipedia is be adhering to standard nomenclature. IUPAP and IUPAC both say that "relative (static) permittivity" is recommended usage and that "relative dielectric constant" is an obsolete term. The merger of these two articles about the same materials property is a bit of a longtime goal. I will not go in and just delete the article with the obsolete title.

But I will revert the deletion of electric constant. Please refrain from deleting it. Discuss. Consider the arguments. User:Stevenj ought to give others the time to have their say. Or go ahead and delete the obsolete term dielectric constant as well. /Pieter Kuiper 20:45, 8 August 2007 (UTC)

Sorry, I'm going to change it back to a redirect. There is no excuse for having two articles for exact synonyms of the same concept (except for old articles that are pending a merge, but that doesn't apply to new articles that you are writing today!). This just encourages wasted effort that will have to be merged manually later. If we decide later that the article should have a different name, it can always be moved.
And you still haven't given a single source to back up your assertion that one usage is endorsed over another by standards bodies. As far as I can tell, standards bodies use both terms on their own web pages, and you haven't contradicted this—pointing to a particular page that uses one term is therefore not evidence of a strong preference. (It may be that there is no "official standard" term. In general, English terminology is defined by usage rather than by official bodies, and synonyms are common in science as they are everywhere else.)
I agree that relative permittivity vs. dielectric constant is a mess. Even the article on dielectric constant is a mess because it only refers to the static relative permittivity, contradicting widespread present usage.
—Steven G. Johnson 20:54, 8 August 2007 (UTC)
There is no reason at all why any of these articles on an exact constant should expand. They will always be short. There is little harm in having this pair for a while, compared to the problem of the materials property. /Pieter Kuiper 21:02, 8 August 2007 (UTC)
This is not how Wikipedia works. We don't create multiple pages on the same thing just because different people prefer different terminology. See Wikipedia:Redirect. Even for a short article, you are already wasting other editors' time. And repeatedly creating a synonym page rather than trying to civilly discuss what the best name for one page should be is destructive edit warring. —Steven G. Johnson 21:07, 8 August 2007 (UTC)
Well, who is edit warring here? Calm down, let things be for a few days. And I am discussing. Also, this is not a personal preference of mine. It differs from the terminology that I used myself. This is the recommendation of IUPAP and IUPAC, international bodies of professional societies. /Pieter Kuiper 21:19, 8 August 2007 (UTC)
You are the one who is intentionally creating redundant pages, in clear violation of Wikipedia policy and practice. (It's this I'm objecting to, not your discussion, as you well know.) Therefore the onus is on you, I'm afraid. And you still haven't provided a clear reference to support your claim of a clear preference in terminology by standards bodies. See also below. —Steven G. Johnson 21:24, 8 August 2007 (UTC)

## NIST terminology

If you search on site:nist.gov with Google (for the terms in quotes, i.e. the exact phrase), you find:

• 38 hits for "permittivity of free space"
• 33 hits for "permittivity of vacuum"
• 7 hits for "vacuum permittivity"
• 47 hits for "electric constant" (however, several of these are actually hits for "dielectric constant", which gets hyphenated "di- electric constant" around a line break in some documents and confuses Google)

Thus, NIST as a whole seems to express no clear endorsement for one term over another (even if authors of individual documents/pages obviously make a particular choice). Frankly, this is not all that surprising: most scientific terminology is defined by usage (like most natural languages), not by standards bodies. And synonyms are hardly uncommon in science.

In the absence of a clear agreement among standards bodies that one term is to be used over the other in the future (as opposed to a particular author/page that picks a particular term), the name for this article should be determined by which term is the most common and the least ambiguous, just like for every other Wikipedia page. For example, we could survey a few popular English textbooks on electromagnetism (Jackson, Griffiths, Purcell, etc.) and see what term they use.

—Steven G. Johnson 21:22, 8 August 2007 (UTC)

Well, CODATA is not just one particular author. It is the result of a collaborative undertaking, where these things are talked through in meticulous detail. Authors at NIST are of course perfectly free to write "vacuum permittivity". Nobody on wikipedia will be forced to write "electric constant". A redirect will take care of that. But if were are going to have a policy, it cannot be based on selecting a couple of a books, that ware published a long time ago. /Pieter Kuiper 21:37, 8 August 2007 (UTC)
Even if in one document NIST chose a particular synonym, that's not the same thing as an official standardization on one term over another. Can you find any document that says, "Body XXXX recommends 'electric constant' over "vacuum permittivity" because ..." ? I'm open to this possibility, but am still a bit skeptical that anyone would attempt to deprecate such a common term for an unambiguous quantity. —Steven G. Johnson 21:48, 8 August 2007 (UTC)
(Classics like Jackson, Griffiths, and Purcell are not just a "couple of books". This would be like saying that classics like Kittel and Ashcroft & Mermin are just a "couple of books" on solid-state physics that have no bearing on Wikipedia's terminology for, say, ferromagnetism...not a contrived example, because a controversy over the usage of "ferromagnetism" vs. "ferrimagnetism" was resoloved in exactly this way. —Steven G. Johnson 21:59, 8 August 2007 (UTC))

(By the way, I can think of at least one rational reason why "electric constant" would be preferred over "vacuum permittivity" or any variation thereof. Because of QED, there are small nonlinearities of vacuum, so in some sense "permittivity of vacuum" is ambiguous unless one specifies the limit of weak fields. However, language and usage are not always rational. In practice, if you say "vacuum permittivity" everyone knows what you mean. —Steven G. Johnson 21:52, 8 August 2007 (UTC))

## A google test

In the absence of a clear endorsement of one term over another by standards bodies, another possibility suggested in Wikipedia:Naming conflict is a Google test.

The following searches are for pages that use one term exclusively (using "-" to exclude the other terms):

• "electric constant": 93,000 hits (however, a large fraction of these seem to be for other meanings/usages, as in "di-electric constant", "electric constant heat sealers", and so on)
• "vacuum permittivity": 45,000 hits
• "permittivity of vacuum": 54,000 hits
• "permittivity of free space": 153,000 hits

Food for thought. —Steven G. Johnson 21:57, 8 August 2007 (UTC)

## IUPAC preferences

In the 2006 IUPAC glossary you linked to above, it writes:

0 ≈ 8.854 × 10–12 C2 J–1 m–1 is the electric constant (vacuum permittivity)"

That is, it uses the two terms interchangeably; it doesn't seem to be recommending one over the other. (The 2005 recommendations document uses "vacuum permittivity" exclusively.)

Moreover, Googling site:iupac.org finds:

• "vacuum permittivity": 5 hits
• "permittivity of vacuum": 44 hits
• "permittvity of free space": 9 hits
• "electric constant": 13 hits (several of which are really hits for "di-electric constant" as mentioned above)

—Steven G. Johnson 22:08, 8 August 2007 (UTC)

To me it seems that "electric constant (vacuum permittivity)" indicates a preference for "electric constant". The term "dielectric constant" is called "obsolete" in the 2006 glossary, and that is likely to include "dielectric constant of vacuum". However, it is surprisingly difficult to find articles about this nomenclature issue. The IUPAP 1987 red book said "permittivity of vacuum", but I cannot find anything more recent by IUPAP. /Pieter Kuiper 22:26, 8 August 2007 (UTC)
I found this paper by an engineer who studied the issue: Montgomery 1984. He uses "electric constant (vacuum permittivity)" in a table of constants and just plain "electric constant" in a note on the first page. /Pieter Kuiper 22:40, 8 August 2007 (UTC)
"Dielectric constant" is a completely different issue: the problem with it is that it is ambiguous (historically, it referred to only the static relative permittivity, but nowadays it is commonly used for the relative permittivity in general). Please don't conflate independent issues.
(Probably the reason it is "difficult to find articles about this nomenclature issue" is because, like most terminology, there is no official standard. Why is this so hard for you to believe?)
In any case, it is a huge leap for you to go from the fact that one document parenthesizes "vacuum permittivity" to say that the term is deprecated in general. In any particular document, the authors obviously have to choose a particular term. The fact that there is no consistent choice even in the same organization makes it especially hard to agree with your blanket assertion that the organization "recommends" a particular term. —Steven G. Johnson 23:58, 8 August 2007 (UTC)

## BIPM preferences

BIPM, the Internation Bureau of Weights and Measures in Paris, likes to use "electric constant". A websearch vote would be misleading, because it also counts papers from the 1960's. But in this 1998 set of physical constants it just says "electric constant". Does not that settle the issue? /Pieter Kuiper 23:05, 8 August 2007 (UTC)

Again, you're making a huge leap: the fact that one document from a particular organization uses a particular term does not imply that the organization in general has officially anointed or "recommended" one term over another. It just means that they made a choice of synonyms in a particular document.
And in any case, you are overstating the facts: it is easy to find recent documents from the BIPM site that use both terms. BIPM's 2006 official description of the SI units calls it "the electric constant ε0 (the permittivity of vacuum)". That is, they use both terms, and to go from the fact that they put one in parentheses to a blanket statement that they "recommend" the other term is not justified, in my opinion.
—Steven G. Johnson 00:16, 9 August 2007 (UTC)

## Conclusion: the article title should be decided by common usage

The sources Kuiper has pointed out argue against his point, as soon as they are examined. The standards bodies clearly have not chosen one term over the other: the undisputable facts seem to be that

• standards bodies commonly include and use either or both of "electric constant" and "vacuum permittivity" (or similar) in recent official documents
• they never make a clear statement that one term is generally to be preferred over the other.
• Even when, in a particular document, they choose the "electric constant" term, they commonly give "vacuum permittivity" (or similar) as a synonym with no statement that the latter is deprecated.

Therefore, according to Wikipedia:Naming conflict, the issue of what the Wikipedia article should be titled must be decided by the most common usage (although of course the article text must list all common names). Exactly what is the most common usage has yet to be decided.

—Steven G. Johnson 00:15, 9 August 2007 (UTC)

Prevailing usage is also decided by recourse to international organizations, according to Wikipedia:Naming conflict#Identification of common names using external references. The BIPM is the most authoritative source I can think of in this area, and one should look at their official documents, the ones authored by committees. /Pieter Kuiper 10:16, 9 August 2007 (UTC)
Yes, international organizations are one possible source to resolve conflicts as described in Wikipedia:Naming conflict, especially for proper names. But as indicated above, the official documents of international sources like BIPM indicate no strong preference (and in any case the Wikipedia guidelines indicate using the websites of international organizations when judging "prevalency", not just official documents). BIPM's 2006 official publication on SI units uses both terms, and from the fact that it puts one in parentheses we can hardly infer a clear "recommendation" or that one term is "obsolete". Similarly for IUPAC, and in fact as recently as 2 years ago IUPAC's official glossary used "vacuum permittivity" exclusively. In the absence of a strong preference, the deciding factor should be the fact that literature searches, e.g. on Google Scholar, seem to show that an overwhelming majority of practicing scientists use "vacuum permittivity" or a similar term incorporating "permittivity". (Which makes sense: both ${\displaystyle \epsilon }$ and ${\displaystyle \epsilon _{0}}$ play the same role in Maxwell's equations and are denoted by the same letter; it is rather odd not to call them both a "permittivity".) —Steven G. Johnson 15:00, 9 August 2007 (UTC)

## Extraordinary claims require extraordinary evidence

Peter had added a "Terminology" section that wrote:

Historically, the physical constant ε0 has had different names. One of these is the dielectric constant of vacuum. Although still in use, "dielectric constant" is now deemed obsolete.

This doesn't make any sense. None of the sources quoted have used "dielectric constant of vacuum" to refer to ε0. In fact, such terminology would be nonsense because the dielectric constant always refers to a dimensionless relative permittivity, while ε0 is dimensionful.

The same section also claimed:

Currently the nomenclature is electric constant or alternatively electric constant (vacuum permittivity).

Again, this is false. Currently, "electric constant" and "vacuum permittivity" are both widely accepted, both in the literature as a whole (as can be seen by any literature search, the latter term and variants thereof in fact appear to be far more prevalent) and in standards bodies (which use both terms in their official documents). Electric constant (vacuum permittivity) is not a "nomenclature", it is just standard English grammar for indicating two synonymous terms.

I've removed this section. Extraordinary claims—that one of the most common nomenclatures (if not the most common nomenclature) in science for this constant is no longer an "officially accepted" term—require extraordinary evidence before they can go into Wikipedia. Pointing to an official document or two that happens to use one of several synonyms for a given term, without explicitly saying that the other synonyms is deprecated for future use, ain't sufficient.

(Note that this is a separate issue from what the article should be titled. For the title of the article, we have to pick among several choices that are all widely accepted, and much less is required to tip the balance between them.)

—Steven G. Johnson 15:52, 9 August 2007 (UTC)

www.google.se/search?q=%22dielectric+constant+of+vacuum%22 gives 15 000 hits. /Pieter Kuiper 15:56, 9 August 2007 (UTC)
Vacuum has a dielectric constant (=1), because it has a relative permittivity just like everything else. This doesn't mean that "dielectric constant of vacuum" is ever used for the dimensionful absolute permittivity of vacuum, ε0. Again, you are confusing the usage of the term "dielectric constant"—last time, you insisted that you weren't confused and had just expressed yourself badly, but this is the second time you've made the same mistake. Perhaps you are running into difficulty here because English is not your first language? Perhaps this should tell you to be more cautious in your judgements of English terminology? —Steven G. Johnson 16:00, 9 August 2007 (UTC)
Most likely a difference between the theorists who put ε of vacuum to unity and almost everybody else who puts ε of vacuum (the dielectric constant of vacuum) to something with a value and a unit. It would not make much sense to talk about the "relative permittivy of vacuum". /Pieter Kuiper 16:10, 9 August 2007 (UTC)
PS: See a random patent for the use of obsolete language. /PK
I'm one of the theorists who routinely set ε0=1, and it's still not called a "dielectric constant". And yes, plenty of people talk about the relative permittivity of vacuum: in many circumstances you are describing everything in an inhomogeneous medium by its relative permittivity, and in that case vacuum is no exception (it just happens to be trivial). Most of the sites using "dielectric constant of vacuum" seem to be in this vein.
I don't consider non-refereed patents to be authoritative references. That being said, I did some digging and I found at least one old textbook (King, Fundamental Electromagnetic Theory) that refers to ε as an "absolute dielectric constant" and the relative permittivity as a "relative dielectric constant". So, I apologize—you are right, "dielectric constant" was sometimes used historically for the absolute permittivity. All recent textbooks (e.g. Jackson) and papers I'm aware of use "dielectric constant" exclusively for relative permittivity, so the "absolute" term is clearly obsolete.
However, we cannot cite the standards document you quoted to demonstrate this—it was talking about "dielectric constant" being "obsolete" for the static relative permittivity, so citing it as a statement regarding the absolute permittivity is incorrect. —Steven G. Johnson 17:20, 9 August 2007 (UTC)
Apology accepted. The wording is the way it is, because I have not yet found anything authoritative saying anything about "dielectric constant of vacuum". It is so obsolete that you believed it was not even used - you are probably younger than me. It is always difficult to find references about stuff from the pre-internet era. /Pieter Kuiper 17:31, 9 August 2007 (UTC)

## Please leave longer explanation of terminology

We have no lack of space, so being "wordy" is not a problem per se. This situation is complicated, and unclear because there is no unambiguous statement by any authoritative source. The situation with "dielectric constant" is especially confusing because it is so obsolete, and requires a bit of explanation.

As long as we can both agree that the current description is factually accurate, then it seems better to err on the side of completeness.

—Steven G. Johnson 18:00, 9 August 2007 (UTC)

Readers do not need all this hedging and hawing, and the situation is not unclear. Please do some work on the preceeding paragraph. Get rid of the wikilink to dielectric constant. /Pieter Kuiper 18:05, 9 August 2007 (UTC)
Also, you reduced bibliographic information in the reference to the IUPAC glossary. Why on earth are you doing that?? /Pieter Kuiper 18:10, 9 August 2007 (UTC)
Um, if you think the situation is crystal clear, please re-read our discussion above. There has been no official statement saying one term is preferred to the other, and both terms are still used in standards documents, but there is some evidence that "electric constant" is being used more consistently in a few recent standards documents. Things should be simplified as far as possible, but no further.
Regarding the reference, I had typed it in and then subsequently noticed you had typed in the same reference, but I didn't notice that you had typed in a bit more information. I've copied back the information; don't get excited.
—Steven G. Johnson 18:23, 9 August 2007 (UTC)
My three-line paragraph was perfectly clear, carefully worded, factual, and almost frase by frase supported by references. Yours includes loads of waffling words, talk about what is not said, and guesses about "probable" reasons for the change. I also think that your guesses on the motives are probably wrong. Of course it would be good to give reasons for the change in nomenclare. I think it is mostly because the international bodies want to avoid assigning materials properties to the vacuum does (a remnant from ether theories I think). These are my impressions from what I have seen. I have not yet found anything sufficiently authoritative for that. So I did not write about my guesses and impressions. Neither should you write about your guesses in an encyclopedic article. /Pieter Kuiper 18:39, 9 August 2007 (UTC)

For reference, the "perfectly clear, carefully worded, factual" three-line paragraph you proposed is:

Historically, the physical constant ε0 has had different names. One of these is the dielectric constant of vacuum. Although still in use,[2] "dielectric constant" is now deemed obsolete.[3] In the 1987 IUPAP Red book this constant was called permittivity of vacuum.[4] Currently the scientific nomenclature is electric constant,[1][5] alternatively electric constant (vacuum permittivity).[6]

Let's pull this apart, shall we?

The first sentence is fine, but omits the information that several names were not only used historically, but are still in widespread use.

The second and third sentences are confusing, because the term "dielectric constant" that is still in widespread use refers almost exclusively to a relative permittivity (and this is what the standards document deems obsolete, not the term "dielectric constant of vacuum" which is not even mentioned). For a modern reader, it needs to first be explained that "dielectric constant" was at one point also used for the absolute permittivity, and a reference needs to be given for this. As for my "guesses", the ambiguity of "dielectric constant" is self-evident, and ambiguity is practically the only reason a standards organization ever tries to deprecate a term in common use. (I didn't venture any guesses in the article as to why "electric constant" might be preferred to "vacuum permittivity", precisely because there is no ambiguity as these terms are presently used.)

The fourth sentence implies that only "electric constant" or "electric constant (vacuum permittivity)" are currently accepted scientific nomenclature. This is false. First, any literature search will show you that "vacuum permittivity" and its variants are widely accepted by themselves, arguably moreso than "electric constant". Second, there is no evidence that standards organizations have deprecated these terms, or even made an explicit statement of recommendation; their implicit preferences, expressed by there choice of synonyms in certain documents, is very weak and should be stated as such by Wikipedia in the absence of a clearer statement. Third, it is absurd to suggest that "electric constant (vacuum permittivity)" is a "nomenclature". As I mentioned earlier, standard English suggests that this is a statement that the two terms are synonyms, not that they have to be used together with parentheses like that.

To pick a different example, suppose I write "I drive a motorcar (an automobile)." Would a native English speaker understand that to mean:

(a) My vehicle should be referred to as a motorcar (an automobile), parentheses and all, every time.
(b) The proper name for my vehicle is a "motorcar", while "automobile" is an obsolete term that should be avoided by professional drivers.
(c) I happen to be calling it a "motorcar", but I realize that other readers may call it an "automobile" and I want to make sure they know the two words are synonyms.

It's hard to argue that (c) is not the most reasonable reading. And yet you are arguing that we read it as (a), or perhaps (b).

Do you have any factual objections to the present wording?

—Steven G. Johnson 18:56, 9 August 2007 (UTC)

Standards organization do not often deprecate. They try to lead by example. That is why they leave it at "electrical constant -- ε0 -- value -- unit".
Some journals have an editorial policy or editorial recommendations. I gave a 1984 example of an editorial policy of an acoustical engineering journal by Montgomery. One might be able to find more similar such documents.
In the meantime, please remove your speculations about the reasons for the change. As I said, they are probably wrong. /Pieter Kuiper 19:12, 9 August 2007 (UTC)
You're right that they don't often deprecate terms. It's very hard to change language by fiat. But in the case of "dielectric constant" they did. Explicitly. Despite the fact that the term is in widespread modern professional use. In fact, I just found an explicit reference explaining why "relative permittivity" was preferred to "dielectric constant", and I was exactly right: in an introduction to a 1946 glossary of terms used in electrical engineering by the British Standards Institution, it describes the situation where "terms are used with somewhat different meanings in different branches of electrical engineering", in which case it lists all of the meanings and gives the terms in order of preference, and "dielectric constant" is listed as one such term. I'll add it as soon as possible...the reference I found online was a brief excerpt, and I want to try to track down the original to get the full reference. —Steven G. Johnson 19:20, 9 August 2007 (UTC)
On a second reading, the snippet of the reference I found was not as clear as it seemed on first reading. Anyway, regardless of whether an explicit reference can be found for this, I've removed the statement about the ambiguity of the term, since a detailed discussion probably would be better off in dielectric constant. Happy now? —Steven G. Johnson 19:37, 9 August 2007 (UTC)
The should not be an article dielectric constant, I think that that is something we can agree on. It should be a redirect to "relative (static) permittivity". But that will take time and effort to accomplish.
No, I am not happy with your wording. But I will let it be for a while. Maybe some other physicist will come in when term starts. Hopefully someone who agrees with me ;-) In the meantime, I will see if I can find some policy papers about usage in reputable journals. /Pieter Kuiper 19:54, 9 August 2007 (UTC)
PS: That did not sound good. I am not unhappy either. I am in fact quite satisfied with what we accomplished in our dialogue. /PK 20:02, 9 August 2007 (UTC)

## Boldness

I only now noticed the bold move by User:Stevenj of a longer article from obsolete dielectric constant to relative static permittivity. I am quite surprised that nobody protested. I will now be bold here, and change some thing that I feel should be changed. /Pieter Kuiper 13:33, 11 August 2007 (UTC)

Notice that I had posted a complaint on the talk page for some time without response before making the move. This is usually a good idea before doing anything dramatic. (Note also that dielectric constant has an even bigger strike against it, from the perspective of Wikipedia article titles, than the fact that it has been explicitly termed "obsolete" despite being in widespread use. It is ambiguous: given a choice of two article titles, where one is ambiguous and the other is not, and both are widely accepted terms, Wikipedia practice favors the latter.) Your use of this completely unrelated issue to justify reverting here, after we had gone through all of that discussion and arrived (I thought) at a compromise version that we both agreed was factually accurate and complete (even if you thought it too "wordy"), is frankly astonishing. —Steven G. Johnson 16:11, 11 August 2007 (UTC)

## Consensus?

While User:Stevenj and I agree on many things, we do not have complete consensus. I advocate concise wording. Stevenj reverted maybe in an instinctive reflex. It is unlikely that he disagrees with all the changes that he made. BTW, I am no longer advocating a name change. I changed many redirects to point to this article. Something akin to "vacuum permittivity" is the most commonly used term. Maybe "electric constant" will be prevalent 50 years from now, but I doubt it. /Pieter Kuiper 16:20, 11 August 2007 (UTC)

Pieter, we had argued and argued and came up with a wording that we both (apparently) agreed (you: "I'm quite satisfied with what we accomplished") was factually accurate and complete. I'm all in favor of conciseness, but not at the expense of accuracy or misleading/confusing readers about the full situation. You reverted, without explanation, to your old "three-line" version that I had specifically explained my objections to. The fact that you mixed this in with other edits to the article does not justify keeping it. I'm frankly, shocked that after all of that discussion you would just go back to your old three-line version and expect me not to object. WTF? —Steven G. Johnson 16:25, 11 August 2007 (UTC)
You may have missed that it was not the same three-line version. It said:
Historically, the physical constant ε0 has had different names. One of these was dielectric constant of vacuum.[2] Although still in use,[3] "dielectric constant" is now deemed obsolete.[4][5] In the 1987 IUPAP Red book this constant was called permittivity of vacuum.[6] Currently the nomenclature is electric constant.[1][7] The vacuum permittivity ε = εr ε0 is equal to the electric constant ε0.
I had also incorporated you reference to King. I thought that this was in all factual matters in agreement with what you wrote. Just without your personal reflections etcetera. /Pieter Kuiper 16:47, 11 August 2007 (UTC)
It still says that the only current nomenclature for ε0 is "electric constant" which is false. Most physicists would probably still call ε0 the vacuum permittivity (as opposed to saying it "equals" the vacuum permittivity, which is subtly different). You also removed the discussion of the fact that "dielectric constant" used to be used for absolute permittivity, which is necessary for any modern reader to understand how "dielectric constant of vacuum" referred to ε0. You also removed...let me make it easier: what specifically in the current (longer) description do you find factually incorrect or misleading? —Steven G. Johnson 20:35, 11 August 2007 (UTC)

## Opening statement: "The vacuum permittivity e_0 is the permittivity of vacuum "

OK, I see that I will have to argue my improvements one by one. In Stevenj's version the article opens with the statement of which the principle clause is:

""The vacuum permittivity ε0 is the permittivity of vacuum".

Although this would seem to be an inane tautology in the eyes of most readers, I think it reflects and/or promotes a misunderstanding, namely that the vacuum (the ether) has a materials property permittivity. So I will revert to the opening line that I had:

"The vacuum permittivity is equal to the electric constant ε0 (also called the permittivity of free space, or dielectric constant of vacuum), which is a fundamental physical constant."

Anybody here is of course free to improve on this. Just do not revert it to what is at best a tautology. /Pieter Kuiper 17:37, 11 August 2007 (UTC)

I agree that it was somewhat tautological before, although I would think that most physicists would say that vacuum has a permittivity (this is not equivalent to saying that vacuum is a "material", just that Maxwell's equations in vacuum are characterized by a given ε quantity). I changed your phrasing to "vacuum permittivity or electric constant, ε0" giving prominence to "electric constant" but not implying (as your previous phrasing did) that the proper name for ε0 is "electric constant" rather than "vacuum permittivity". —Steven G. Johnson 20:10, 11 August 2007 (UTC)
'Proper name' has meanings like "given name", but that is not what you meant, I assume. 'Proper' also means "exactly as I am describing it" or "right according to the accepted standards of a society or group" (Collins). This is indeed what I meant to imply, and it is supported by the references. However, this was so subtly frased that only experts like yourself would notice it. My wording was not saying that "vacuum permittivity" is wrong or passé or less correct. It just subtly implies the rather subtle distinction that "vacuum permittivity" is not properly the designation of ε0 but of ε in free space. Your wording is scientifically less precise. /Pieter Kuiper 21:12, 11 August 2007 (UTC)
Sigh. Are we going to have this argument all over again? Yes, it's possible that some standards bodies feel that ε0 is not properly called "vacuum permittivity", although this requires a subtle inference from their word choices...after all, I quoted documents from BIPM etc. that called ε0 the "electric constant (vacuum permittivity)" which grammatically indicates synonyms. Regardless of whether this inference is justified, large numbers of practicing scientists would not use ε0 in this way...it is not hard to find innumerable papers that say things like "... where ε0 is the permittivity of vacuum." And Wikipedia policy is to be descriptive, not prescriptive...your implication was prescriptive, not descriptive of current usage.
Obviously, the article needs to make clear that ε0 is no longer tied to any experimental measurement of the permittivity of a vacuum (which can indeed vary from ε0 due to quantum nonlinear effects, in theory). I added a paragraph to try to emphasize this. But that doesn't allow us to be prescriptive about usage, regardless of what we think rationally of current usage. —Steven G. Johnson 21:57, 11 August 2007 (UTC)
My wording was not prescriptive of usage, it just very subtly made a distinction, which your wording is blurring. You now added yourself some references to theory papers about birefringence of vacuum. I had not thaught of that, but similarly there are searches for circular birefringence in strong magnetic fields to search for axions. Yes! The article should include such stuff, but I would suggest moving it down, under a separate heading.
"Some" standard bodies? I gave references to everything that is authoritative:CIPM, IUPAP, IUPAC, NIST, and NPL. You added IEEE. The PTB in Germany says "die elektrische Feldkonstante". The nomenclature is very solid. Now "electric constant" is indeed a relatively unfamiliar term for most people. While every physicist understands "the electric constant ε0", others will be helped by the clarification "vacuum permittivity". It does not indicate synonymity or juxtaposition. These committees never write "the vacuum permittivity ε0" anymore. They never write "vacuum permittivity (electric constant)". Their usage is very clear and such committees do pick their words with the utmost care. /Pieter Kuiper 22:25, 11 August 2007 (UTC)
The "subtle distinction" that you want to make, in your own words from above, is that "vacuum permittivity" is not properly the designation of ε0. How is this not prescriptive? How is this not in conflict with the multitudes of practising scientists who clearly (by their usage) think that "vacuum permittivity" is in fact a proper designation of ε0? I "blurred" this distinction precisely for this reason: it is not Wikipedia's place to say or imply, explicitly or subtly, that the common usage of a given term is not "proper". If a standards organization says this explicitly, we can report their words, but reporting our subtle inference from their choice of words is a matter for more caution. —Steven G. Johnson 22:52, 11 August 2007 (UTC)
The distinction is in the documents produced by the committees, and the wording of wikipedia articles should be accurate. Now, the naming of the articles is a different thing. Wikipedia takes usage into account. I am not advocating a name change anymore, and "electric constant" should point to this article.
An analogy: an article about some animal species flying dogs can say that they are not really dogs, without being prescriptive about usage. Accuracy matters. Readers benefit from it. /Pieter Kuiper 23:27, 11 August 2007 (UTC)

(Restarting the indenting). Your analogy with flying dogs is a bad one. First, that is a question of fact, independent of culture or language (given a precise definition of "dog"). The question of whether "vacuum permittivity" is a "proper designation" ε0 is not a question of unambiguous fact, it is a question of usage. This is not a "scientific" question in the same sense, it is a question of terminology and convention. (There is an experimental test for whether a species is related to dogs. There is no experimental test for whether a particular word "should" be used for a particular quantity, unless you count surveys of scientists to indicate usage... Terminology is never "accurate" or "inaccurate" in a "scientific" sense. It can be unconventional, it can be confusing, it can be inconsistent, but not inaccurate as long as its meaning is precisely defined in context. We could call ε0 "the balloon of bnarfbag" and it wouldn't be "scientifically inaccurate", it would merely be unconventional.) And "vacuum permittivity" as it is used in the article and as it is used in the professional world is precisely and unambiguously defined as ε0, nor is it unconventional in the slightest.

Second, in the "flying dog" example, Wikipedia itself would not conclude that the animal is not a dog, it would cite a reputable source stating that the animal is not a dog; we don't have that here. No, the distinction is not in the documents produced by the committees. The committees chose one synonym among many to use in several documents, and in other cases they used two synonyms and put one in parentheses. Those are the unambiguous facts. Everything else is inference: you are inferring that they find "vacuum permittivity" an improper designation for ε0, but this is never explicitly stated. And, as I said, when you read in e.g. the BIPM SI document that ε0 is "the electric constant (vacuum permittivity)", the most straightforward English reading is that the authors consider the two terms synonymous.

So no, we aren't justified in inserting a prescriptive implication that we merely infer from the recent standards documents. As we've been over again and again.

—Steven G. Johnson 02:51, 12 August 2007 (UTC)

I should be the one sighing... I really cannot understand why you insist om imprecise language. A term in parentheses is added for clarification, as in for example "Paprika (2006 film)". For the committees the terms are not synonyms that they use indiscriminately. /Pieter Kuiper 05:02, 12 August 2007 (UTC)
PS: Actually it was you who introduced in the opening clause that "dielectric constant of vacuum" is obsolete. I agree that this is correct, but it is probably better not to mention this until the terminology section. After all, lots of people use this term (including myself until last week, certainly when writing in Swedish or in Dutch). /Pieter Kuiper 07:55, 12 August 2007 (UTC)

## Second sentence:" It connects mechanical quantities to the units for electrical charge, for example in Coulomb's law."

Something like this should be the second sentence. The electric constant ε0 has the function of connecting the unit for charge to the units for length, time, and mass. /Pieter Kuiper 08:35, 12 August 2007 (UTC)

## Free space vs. Empty space in permittivity article

Both these terms were adequately introduced in the "Terminology" section in the earlier version on a par with each other. I see absolutely no basis for putting "free space" in the intro and putting "empty space" in a later section, and a long shopping list of aliases is not necessary in the intro. The "terminology" section explains a special status for the "vacuum" terminology and that with the "electric constant" is all that is needed in the intro.

What ever is done, "empty space" and "free space" should be treated alike. The Sears and Zemansky reference, for example, has been a standard reference and textbook for decades and uses "empty space". Jackson, a more modern standard, uses "free space". So I'd say: different treatment of these two terms cannot be supported.Brews ohare (talk) 03:23, 13 February 2008 (UTC)

"Permittivity of free space" is far more common than "permittivity of empty space". e.g. google turns up more than 60 times as many references for the former term. Google Scholar turns up more than 100 times as many references for the former phrase. The INSPEC and Factiva literature databases (which only index abstracts and titles for the most part) turn up no references at all for the latter phrase, and a number of references for the former. There's no comparison in frequency of usage. That being said, I have no problem with mentioning both in the article lede, but there is much more justification for the former being prominently mentioned.
In any case, standard Wikipedia practice is to list common synonyms in the lede. The "Terminology" section of this particular article is more to discuss the evolution and relationship of the terms and especially the evolving usage by standards bodies and other authoritative sources. (I think it's unnecessary to list both "vacuum permittivity" and "permittivity in/of vacuum" in the lede, however, as the two are just equivalent grammatical rearrangements of the same terminology.) —Steven G. Johnson (talk) 04:43, 13 February 2008 (UTC)
I'm OK with leaving everything in this version, although it is a bit of a laundry list. I do hope you are not serious that a popularity contest is the way to go in this kind of discussion. A better basis is (i) usefulness to the reader, especially an uneducated reader, and (ii) best practices - for example, what would experts in the field advise? I'm of the opinion that "free space" is just a reflex use of terminology, and that most scientists would agree on second thought to a different choice. We just regurgitate what we learned even though better ideas are out there. How long will it take before all the scientists and engineers and all the books use "electric constant", which is the best choice? Should Wiki be a force in this direction?
Brews ohare (talk) 18:04, 13 February 2008 (UTC)
I'm totally serious about the "popularity contest". First, language is a matter of shared convention determined by usage, not by what we think would make the most sense (otherwise, English would be a lot different!). Second going by usage first is Wikipedia policy. Third, listing the names that readers are likely to see in other work, which are also the names that users are likely to be looking up the concept under, is the most helpful thing for the reader — it's confusing for a reader to search for "permittivity of free space" and come up with "vacuum permittivity," and have to read 2/3 through the article to find out that this is the same thing.
The basic philosophy here is that, as an encyclopedia, Wikipedia is descriptive rather than prescriptive — we describe the way things are rather than prescribing the way they should be (although we can report prescriptive recommendations by prominent authorities, e.g. by standards bodies). Wikipedia is not intended to be a "force" in any direction other than informing the public. —Steven G. Johnson (talk) 20:54, 13 February 2008 (UTC)
Two of your points I agree with: (i) usefulness to the reader, especially an uneducated reader.
(ii) we can report prescriptive recommendations by prominent authorities, e.g. by standards bodies
One of your points I'm unsure about: you say "Wikipedia is descriptive rather than prescriptive" - that looks a bit treacherous, as describing what a preponderance of people think is not always telling it the way it is. An encyclopedia is not like a dictionary in this respect. It is supposed to afford knowledge, not the popular view. If all we get is "entertainment tonite" then that's all we get. Eventually this type of thing crowds out any serious effort and discourages any honest effort in favor of being "cool" and making a splash.

Brews ohare (talk) 21:43, 13 February 2008 (UTC)

This is a non-issue. The standards bodies call this the electric constant. It would be best to move the article to the new name. /Pieter Kuiper (talk) 22:00, 13 February 2008 (UTC)

I agree with this suggestion Brews ohare (talk) 22:14, 13 February 2008 (UTC)
Saying that we are not prescriptive does not mean that we don't rely on reputable sources (WP:RS). It means that we try not to inject our own opinions—unlike, say, a pedagogical textbook such as Purcell, where the author may decide to promote a particular convention (e.g. cgs units) and convince people of its advantages. This is especially pertinent for questions of convention and terminology, where there are no absolute "right" and "wrong" answers, only standard/common and nonstandard/uncommon. —Steven G. Johnson (talk) 23:01, 13 February 2008 (UTC)

## Proposal to move Vacuum Permittivity

Although vacuum permittivity redirects from electric constant, electric constant is modern terminology. So it ought to work the other way around. As a similar case, magnetic constant is used in Wiki for vacuum permeability, which last is redirected. However, movement of vacuum permittivity to electric constant requires administrator. Brews ohare (talk) 20:15, 13 February 2008 (UTC)

This has already been discussed. Longstanding Wikipedia practice/policy is to use the most common unambiguous name, even if the "official" name is different; see, for example, Burma (which is, unfortunately, the subject of unending flamewars from people who disagree with the Wikipedia practice of using the common name...is that what we want here? ...and this case is not nearly as extreme as the Burma case, because we're not talking about a proper noun for a person/country that, arguably, has some right to decide on its own name). In present usage "vacuum permittivity" (and its permutations, such as "permittivity of vacuum") is vastly more common than "electric constant" by every available objective measure (e.g. literature searches of journal articles). —Steven G. Johnson (talk) 23:04, 13 February 2008 (UTC)
I did a google check on arxiv.org. Counting all years that are there, "electric constant" gets about the same number of hits as "permittivity of free space" (which seems the most common of the permittivity-cluster there). I think the time is ripe for a move. /Pieter Kuiper (talk) 23:21, 13 February 2008 (UTC)
That particular google search is deceptive, because if you look at the results, many of the hits are actually for "dielectric constant": when "dielectric" is hyphenated across a line break, "electric constant" appears on the second line by itself. I explained this several months ago when we discussed the issue then, and gave results of a number of searches — it's not like the usage will have changed much in half a year. (Also, it's more reasonable to count different rearrangements of the same phrase, like "vacuum permittivity" vs. "permittivity of vacuum" together rather than as "different" terms.) —Steven G. Johnson (talk) 00:49, 14 February 2008 (UTC)

By the way, the same argument applies to "magnetic constant", since "vacuum permeability" is still far more common. I see that Peter Kuiper moved that page a while back, despite the discussions we had here. I'm going to have to go there and clean it up. Really guys, I shouldn't need to watch these pages like a hawk to keep you guys from moving them away from the most common name, despite what Peter knows is the Wikipedia policy. Please try not to sneak counter-policy changes in while no one is looking. —Steven G. Johnson (talk) 01:01, 14 February 2008 (UTC)

Steven: I think you are way off base here. The standards terminology should be used and the rest handled by redirection. If you are unwilling to do this, I suggest this be taken "to a higher level". Care to point out how this is done?? Brews ohare (talk) 01:36, 14 February 2008 (UTC)
I'm afraid you are mistaken about Wikipedia policy and practice, which is explicitly documented in multiple places. See e.g. Wikipedia:Naming conventions (common names). If you want to change this policy, you can try to debate it on the Talk page for WP:NAME or on Wikipedia:Village pump (policy), but this is likely to be a waste of time (the naming policy is longstanding and widely accepted by other editors). —Steven G. Johnson (talk) 01:41, 14 February 2008 (UTC)
Steven: I'd rather change your policy. Can you explain why redirection is not an adequate way to deal with the "ancien regime"? In my mind, if the user types into google "vacuum permittivity" and is redirected automatically, completely without intervention or pause, to the article "electric constant" where vacuum permittivity appears in bold font in the lead, then this reader will be perfectly happy.
At the same time, the reader notices that the title is Electric constant and so is very clearly informed that the more acceptable term is this. That seems a beautiful combination of ease of use and education. What do you see differently from this?
Brews ohare (talk) 06:09, 14 February 2008 (UTC)
It's not my policy. It's Wikipedia's. We always have redirects from alternate names, but the question is what the main article should be called, and the consistent policy has been to use the most common name (while mentioning other common names in the lede). Like I said, if you want to debate general Wikipedia policy, this is not the page for it. —Steven G. Johnson (talk) 16:03, 14 February 2008 (UTC)
And by the way, "informing the reader what the more acceptable term is" is contrary to WP:NPOV. Acceptable to whom? To the standards bodies? Or to the many practicing scientists and engineers who continue to prefer the older term? One of the reasons for th policy of using the most common unambiguous name is that it obviates the need for Wikipedia to choose between competing authorities on issues like this. —Steven G. Johnson (talk) 16:08, 14 February 2008 (UTC)
A guideline in resolving naming conflicts is to check scientific nomenclature as used by professional bodies: Wikipedia:Naming conflict#Identification of common names using external references. /Pieter Kuiper (talk) 20:29, 14 February 2008 (UTC)
Read the whole page: that page lists ways to test which names are the most common, but the criterion is still which name is the most common—the guidelines are just giving examples of places to look. In this case, there is a clear disconnect in that the name that is most common with standards organizations is not even close to being the most common in professional usage by practicing scientists. (As for other languages, we're talking about terminology in English here, so it's not clear why that's relevant.) —Steven G. Johnson (talk) 23:02, 14 February 2008 (UTC)
Stevenj omits relevant passages on what the guidelines says. Wikipedia:Naming conflict#Proper nouns: "If the common name conflicts with the official name, use the common name except for conflicting scientific names". That should be clear, I think. /Pieter Kuiper (talk) 23:10, 14 February 2008 (UTC)
(Interesting, none of the other naming pages lists that exception IIRC.) In any case, that passage is not applicable because we are not dealing with an "official" name vs. an "unofficial" name. (This is not like "brontosaurus" vs. "apatosaurus" where one name has been explicitly deprecated.) As we've extensively discussed, the standards bodies haven't said that the other names are "wrong", or "incorrect", or "deprecated" (unlike, e.g. "dielectric constant"), or that there is only one "official" name for this constant. They have merely quietly, with no explicit statement of explanation or preference, chosen a particular synonym to use in their own documents (and many of the documents continue to list other synonyms as well, as we've discussed). —Steven G. Johnson (talk) 23:41, 14 February 2008 (UTC)

On this one, you raise the idea that the use of the old terms is a matter of preference to some practitioners. That is unlikely to be the case: the simple fact is that most scientists and engineers haven't even thought about the term they use - it's just habit from their early days as undergrads. If actually appraised of the situation, they all would switch to the standards committee choice. After all, that is what the standards committee does - it canvasses the community and listens to all arguments and then makes a decision. Very few actually will debate the conclusion. So using this terminology is not overriding the adherents of an alternative, its just being current, and those who are not current will come along as they learn what is happening. Brews ohare (talk) 06:37, 15 February 2008 (UTC)

Speculations about the reasons why the older terms continue to be overwhelmingly more popular in professional usage are irrelevant here, because the fact remains that they are more popular. Yes, inertia probably plays a big role, but you could say the same thing about any other aspect of the English language (which frequently makes less logical sense than one would like, but if you want to use an artificially designed rational language then you should switch to the Esperanto Wikipedia). Using terminology that is not yet popular is not "being current", it is trying to be ahead of the curve, it is trying to use Wikipedia to change the usage to something you feel is more rational, and this is not our role. —Steven G. Johnson (talk) 15:06, 15 February 2008 (UTC)
I agree that my view of the situation is not documented. I brought this up because I felt you entertained a misconception about the situation.

Brews ohare (talk) 15:25, 15 February 2008 (UTC)

Steven: No standards body is going to "deprecate" prior terms beyond just not using or advocating their use. That point has already come up on this page.
To illustrate one problem involved in sticking with the disused nomenclature, the standards bodies in the UK, France, the former British Empire and undoubtedly others all link back to NIST for the value of the electric constant. On that web page at http://physics.nist.gov/cuu/Constants/index.html typing in "vacuum permittivity" links to http://physics.nist.gov/cgi-bin/cuu/Results?search_for=vacuum+permitttivity with no value given - just a link to "electric constant". Typing in "permittivity of free space" or of "empty space" returns no result. So I'd say it is very clear that international opinion of the relevant standards organizations has taken the plunge to electric constant, and failure to use that name fails to link to the posted web-site value for the term. That is at least a head scratcher for the user of Wikipedia. I think the user running into this situation is going to say to themselves that Wikipedia is not in touch. That does not reflect well upon Wikipedia.
Isn't it a bit silly for Wikipedia policy to use Google hits as the basis for terminology for technically defined terms, which are not terms in common English but have specific application, regardless of international expert opinion?.
Brews ohare (talk) 15:32, 15 February 2008 (UTC)
Actually, standards bodies do deprecate terms; see "brontosaurus." Or more close to home, see "dielectric constant". And the point is, you are agreeing with me—the standards bodies are not inclined to deprecate a term like "vacuum permittivity" that is still widely used; if they have a preference, they prefer to lead by example. So, the situation remains as I stated: there is no single "official" name for this quantity such that other names are considered "unofficial" or "nonstandard", and the older terminology remains far more widespread. Google Scholar hits, and Google hits in arXiv, and searches of databases like INSPEC, are searches of usage by expert practitioners in the field; we're not talking about usage by random laypeople. —Steven G. Johnson (talk) 20:21, 15 February 2008 (UTC)
I'd like to add to the above Wikipedia quotations:
Scientific nomenclature. Check usage by international bodies like CIPM, IUPAP, IUPAC, and other scientific bodies concerned with nomenclature; consider also the national standards agencies NIST and NPL. Consult style guides of scientific journals.
Notice NIST in there??

Brews ohare (talk) 19:01, 15 February 2008 (UTC)

I did notice. I also notice that (a) Peter Kuiper added that whole section himself (read the history) and (b) his edit doesn't change the overall policy that the most common name should be used, it just gave examples of places to look. —Steven G. Johnson (talk) 20:21, 15 February 2008 (UTC)
The exception for scientific nomenclature was there since the beginning of the naming conflicts page in July 2005 - read the history. Also, nobody has objected the the lines that I added. /Pieter Kuiper (talk) 20:48, 15 February 2008 (UTC)
I was referring to that particular paragraph quoted above. No one objected because that paragraph did not change the policy, it only gave examples of places to look to find common terminology. As for the "exception for scientific nomenclature," as I explained above that is inapplicable here because no standards body has said that the older nomenclature is "unofficial" or "deprecated" or anything like that. (And as you and Brews have both admitted, they are unlikely to do so.) —Steven G. Johnson (talk) 01:31, 16 February 2008 (UTC)
Hi Steve: Well, I see clearly that you have a fixed view of the situation that no argument will change. I'm sorry about that. It injures my view of the Wikipedia project, and I am left feeling that a single individual is able to block all reasonable debate about what is for me a very powerful case.

Brews ohare (talk) 20:28, 15 February 2008 (UTC)

No, if you could demonstrate in 10 years that "electric constant" has become the most prevalent name, that would be a persuasive argument. The reason you haven't been able to convince me is that (a) Wikipedia's consistent policy is to use the most common unambiguous name, (b) the older terms are undisputably far more common in professional use by practicing scientists and (c) none of the official bodies have demonstrated any inclination to explicitly pronounce by fiat that the older terminology is "unofficial" or "wrong" or "nonstandard". Basically, all your arguments come down to, "Wikipedia's policy is wrong, we should follow the unstated preferences of standards bodies no matter how rare the terminology is in practice." The problem with such an argument is that you are are simply not arguing in the correct forum if you want a change in policy; I wish you luck if you try to do so elsewhere.
And frankly, you need to take a step back and calm down if you find yourself getting so excited about this. We're talking about terminology here, not whether π is 22/7. There are no absolute "right" or "wrong" answers in questions of naming, only conventions. —Steven G. Johnson (talk) 20:41, 15 February 2008 (UTC)
Hi Steve: Sorry about the bold font; I hit the colon key without the cap shift.
My position is almost as you summarize: a bit of rephrasing would be as follows: "Wikipedia's policy is wrong, we should follow the stated preferences of standards bodies as it relates to technical terms used in technical context no matter how rare the terminology is in practice." I'd note that the usage in this case actually is not rare, it's just less common than the older terminology. I'd also add that the change in terminology is not a caprice of a few dunderheads, but a considered change that took years in the making.

Brews ohare (talk) 21:14, 15 February 2008 (UTC)

## Relative permittivity of vacuum

The article now states:

These observations are at variance with some textbooks, where the relative permittivity of vacuum is claimed to be 1 by definition (see Jackson [15] or Nyce[16], or de Podesta [17]). Of course, that position would be accurate if ε0 were actually the permittivity of the vacuum as the old terminology implies but, as the new terminology indicates, the electric constant ε0 has nothing to do with the vacuum, so the real-life relative permittivity of the vacuum is up to experiment, not definition.

One has to be careful about blithely implying that these authorities are simply wrong, especially without a reference (we shouldn't be inserting our own conclusions); Jackson for one is certainly well aware that ε0 and μ0 are defined rather than measured quantities. Arguably, they are talking about the linear relative permittivity (which is what almost all unqualified references to "relative permittivity" are referring to; nonlinear effects are usually separated into nonlinear susceptibilities). And, at least in the current understanding of physical laws, the linear relative permittivity of vacuum seems indeed to be 1 by definition (because Maxwell's equations must give waves propagating at a speed c in vacuum in the linear limit). (e.g. in the Mourou Rev. Mod. Phys. paper that is cited for the quantum effects on vacuum optics, the linear relative permittivity of vacuum is still given as unity, and they describe the QED effect as a change in the vacuum nonlinear susceptibilities.)

—Steven G. Johnson (talk) 02:26, 16 February 2008 (UTC)

Two problems with your argument, Steve: (1) the value of ε0 is a defined definite numerical value and has nothing to do with the vacuum, linear or otherwise. So whatever εVacuum is, it is, and only experiment can decide that. (2) The speed of light in vacuum also is a defined definite numerical value, and it doesn't matter what the value of εVacuum is, or any other experiment, the speed of light is fixed. What does depend on measurement is the actual length of the meter, which is the result of experimental measurement. I believe the wiki articles on speed of light and meter and SI units cover this issue. Brews ohare (talk) 05:37, 16 February 2008 (UTC)
I understand that they are defined values, but that doesn't mean you can measure something different for the permittivity of a vacuum. As you say, the length of the meter is defined in terms of the speed of light in vacuum to make c a fixed number. Maxwell's equations in vacuum must give a wave propagating at the speed of light, because it is light. Hence, if any measurement of vacuum gave a (linear) relative permittivity not equal to one, the length of the meter would automatically be adjusted to make the relative permittivity equal to one. (If the linear relative permittivity of vacuum differed from unity, then an electromagnetic wave in vacuum would travel at a speed different from c. But since the meter is defined to make the speed of an electromagnetic wave in vacuum c, ... do you understand?)
Can you give any reference for your claim that (in the current understanding of physical laws), the linear permittivity of vacuum can differ from ε0? It seems like your claim is equivalent to saying that a physical measurement could obtain a vacuum speed of light different from c, which I think we can both agree is nonsense because of the way the meter is defined. —Steven G. Johnson (talk) 05:49, 16 February 2008 (UTC)
Hi Steve: I think I'm being misunderstood here: the idea is that the εVacuum can be anything experiment turns up, including having a value equal to ε0. Whatever this experimental value is, it just doesn't change the value of either ε0, or c or μ0.
Your last sentence is almost true. It should read like this: Hence, if any measurement of any quantity led to values for ε0, or c or μ0 different from the defined values, the length of the meter would automatically be adjusted to make these experiments agree with the defined values.
This way of doing things is not the most intuitive. My understanding is that the primary aim is get everything measured with the greatest possible accuracy, and with portability (you can measure stuff anywhere on the planet, you don't have to go to Paris). So the definitions are shuffled around to make the the most accurately measured quantities fundamental and the rest derivative. It's not for clarity, its for metrology.
Brews ohare (talk) 06:09, 16 February 2008 (UTC)
Um, Brews, please read what I wrote again. I'm not suggesting that the value of ε0 could change. I'm saying that, by the same token, the linear relative permittivity of vacuum is unity by definition. If you disagree, then explain how an (accurate) measurement could possibly measure a (linear) εVacuum different from ε0 without leading inexorably to the conclusion that the speed of an electromagnetic wave in vacuum is different from c, which is impossible by definition of the meter.
And again, can you give any reference to back up your understanding that it is possible for any experiment to measure a (linear) εVacuum different from ε0, or is this a conclusion you've come to on your own? (And don't respond with references for the fact that ε0 is a "defined" quantity; that is not in dispute. What I'm disputing is your interpretation of the consequences of this definition.) —Steven G. Johnson (talk) 06:25, 16 February 2008 (UTC)
The linear permittivity of the vacuum is εVacuum / ε0, so I guess the question is equally can εVacuum differ from ε0?
I'd say (as stated in the article) that there is nothing in the choice of definitions to make that impossible, it is an experimental issue. I'd take it from the article that a departure of εVacuum from ε0 is a theoretical prediction (references are provided) but no such departure has been measured yet.
A related question that I'd love to have answered is just what the ramifications of a change in εVacuum might be. Obviously, they do not change the definitions, in particular c doesn't change. But some other things would - a possibility is a change in the measured fine structure constant. (I get that idea from the NRC website). The next question I'd like to see answered is how these changes affect the basic SI units?
Whatever the answers to these questions, there is no doubt that whatever the value of εVacuum, it is an experimental issue. There is no logical requirement that εVacuum = ε0; it is in no way a result of definition. It is true that the present definitions have values that make εVacuum very nearly equal ε0; but greater precision in measurements could upset that equality. It isn't set in stone. Brews ohare (talk) 07:02, 16 February 2008 (UTC)

I do not see the problem. "Free space" means the absence of disturbing factors. If experiment would show that the vacuum inside a superconducting magnet becomes birefringent (an axion search experiment), then that is just not free space. /Pieter Kuiper (talk) 08:49, 16 February 2008 (UTC)

That would not be a change in the linear permittivity, Peter, it would be a nonlinear susceptibility of vacuum. The linear permittivity is the permittivity in the absence of disturbing factors, and my point is that the textbooks are correct in saying that this is ε0 by definition (in the current understanding of physical laws). —Steven G. Johnson (talk) 16:30, 16 February 2008 (UTC)
My take on this is that you say εVacuum = ε0 always because if you measure anything different it just isn't vacuum. What if εVacuum changes with the volume of the universe: do we discover we just can't make free space any more? Take a look at http://www.npl.co.uk/server.php?show=ConWebDoc.1144 Brews ohare (talk) 16:06, 16 February 2008 (UTC)
For me, vacuum is the absence of ordinary matter (molecules). The electromagnetic concept of free space is a bit different. A wave guide may be empty, but signals propagate att speeds lower than c. Similarly, a strong magnetic field in vacuum might show deviations from free space. It is just another good reason to avoid "of the vacuum" etcetera. /Pieter Kuiper (talk) 16:20, 16 February 2008 (UTC)
Brews, the page you link supports my position, not yours. If εVacuum were changing with time, then the speed of light in vacuum and hence our definition of a "meter" would be changing in time, rather than the numerical value of εVacuum, which would continue to be ε0 by definition. That's why the page you link to says that experiments to measure changes in physical laws over time must look at dimensionless quantities like α.
In any case, if we had a dramatic upheaval of our understanding of physical laws (e.g. if we found that the speed of light in vacuum varied with frequency), then there would have to be a corresponding upheaval in our definitions of physical constants (e.g. we would have to define the meter based on the speed of light at a particular frequency). That's why I have said several times that the textbooks are correct in the current understanding of physical laws; if you change physical laws then everything (including the standard itself) goes out the window.
Peter, read my response above; you are supporting my point that the linear permittivity is indeed ε0 by definition, and that the textbooks are correct if we understand them in this sense. And waveguide dispersion is irrelevant here; no one would interpret a measurement of a wave propagation speed in an inhomogeneous medium to (directly) measure the permittivity of only one of the constituent materials (except in the limit that the inhomogeneity is negligible). —Steven G. Johnson (talk) 16:30, 16 February 2008 (UTC)
Hi Steve: I'm happy for you that you can draw support from How can we know?. Personally, I find the question very well posed, to paraphrase, if the speed of light is defined to be c = 299 792 458 m s-1 , then how can it change? I get the question. but I don't see any answer given except some gobbledygook about how that drives use to look at mp/me, which may be true, but certainly doesn't connect the dots.
One could get involved in a whole metrology discussion here. For example the meter is
The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
and the second is:
The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. This definition refers to a caesium atom at rest at a temperature of 0 K.
So if the time unit stays fixed and only the physical speed of light changes, the meter will change. But relativity seems to suggest that space and time are not really so independent, so can we be sure that time didn't change too? Don't we have to check everything at once - I suppose that is why we end up with dimensionless constants as the best thing to measure, but again the dots haven't been connected.
My take on all this is that it is a very subtle issue just what to measure to see what is going on. However, it is clear that we have defined c, μ0 and ε0 to be certain numerical values that don't change under any circumstances. It is the rest of the measurement system that has to adapt around these numbers. No number is assigned to εVacuum, which has a role no different than εKCl. No special place is reserved for the vacuum, although, as Peter has pointed out, some definition seems to be needed.
If we want to define the vacuum as "that physical something that has a measured εVacuum = ε0", that could be done. Then we have to figure out how whether our particular candidate for vacuum does satisfy this condition, and we are right back with experimental measurement, not definition.
To support the view that εVacuum = ε0 by definition requires a definition of vacuum that makes this so. And then one has to prove that a particular candidate (like the space between my ears) satisfies the requirement.
Brews ohare (talk) 23:47, 16 February 2008 (UTC)
Brews, the current definition is such that the linear εVacuum = ε0 by definition. The fact of the matter, though, is that I don't really have to convince you. By the references already cited in the article, there are multiple textbooks that support my point (when talking about classical electrodynamics where there are no nonlinear effects of vacuum), and the Rev. Mod. Phys. article on the quantum optics in vacuum also confirms that the linear permittivity is always exactly ε0 by definition. You haven't been able to cite a single reference to support what appears to be your personal conclusion about the consequences (or lack thereof) of the ε0 definition for the value of of εVacuum. In cases like this, Wikipedia policy is to report only what is published in reputable sources (here, that the linear relative permittivity of a vacuum is unity by definition), not the interpretations of individual editors.
However, it will be easier to edit the article if I can convince you to cooperate on this point, which is what I've been trying. (This is a question of fact, about the mathematical consequences of definitions and physical laws, not of terminology or convention, so it's really something that we should be able to agree about.) Let me try one more time, because you keep failing to address my central argument.
(As I said, the current choices for definitions of the various units and fundamental constants are predicated upon the current understanding of physical laws. It is interesting to speculate how we would define the meter, for example, if the vacuum speed of light were not a constant, and indeed if physical laws were that different from what we currently understand then we might need to change our definition of seconds and other units as well. However, such speculation is not really relevant here.)
You write No number is assigned to εVacuum, which has a role no different than εKCl. This is fundamentally wrong, because the linear εVacuum in Maxwell's equations determines the speed of light in vacuum, which is defined to be c by the definition of the meter, and so the linear εVacuum must be ε0. In contrast, the speed of light in KCl is not defined to be anything a priori, so the unit definitions do not define the value of εKCl. (I'm assuming you understand how the relative permittivity affects the propagating-wave solutions of the Maxwell equations; please let me know if that's not the case, because that would explain why we're having trouble communicating.)
—Steven G. Johnson (talk) 02:40, 17 February 2008 (UTC)
Hi Steve:
I'd be most interested in working through this issue. I've two suggestions for helping me out here. First, I am not impressed by finding citations in many texts that the permittivity of vacuum is one by definition because it is my belief at the moment that that idea comes from thinking that the electric constant is really the permittivity of vacuum. These books, so far as I know of them. do not use the idea that ε0 is defined as a number in SI units. Therefore, I'm of the view that they are written from an older standpoint where ε0 was defined as the permittivity of vacuum, not as 8.854817 × 10-12.
If we can put the above aside for the moment, maybe you can explain to me somewhat further your statement
the linear εVacuum in Maxwell's equations determines the speed of light in vacuum, which is defined to be c by the definition of the meter, and so the linear εVacuum must be ε0.
To begin, I'd agree that the meter is defined so that c = 299,792,458 m/s in vacuum. I'd like to distinguish between the physical speed of light and the defined number c = 299,792,458 m/s. Can we start there? With that in mind please amplify the above remark. In particular, does εVacuum appear in Maxwell's equations, or does 8.854817 × 10-12 appear in Maxwell's equations when the meter is scaled according to its new definition? See Nondimensionalization.
Brews ohare (talk) 06:38, 17 February 2008 (UTC)
I don't know about the other books, but Jackson most certainly understands how the constants/units are defined (and has a whole section on it if I recall correctly), as does the Rev. Mod. Phys. article. I find it staggering that you can blithely claim that you understand electromagnetism and electromagnetic units better than Jackson (one of the world's most respected experts on classical electromagnetism, and the author of probably the most important modern graduate-level textbook), and that we should take your word on this. (See also Wikipedia:No original research.)
The more you write, I'm afraid the more confused you sound. I'd like to distinguish between the physical speed of light and the defined number c = 299,792,458 m/s. Such a distinction is not meaningful, because the physical speed of light defines the meter, and hence is impossible to "measure" per se; one can, however, measure dimensionless ratios like the speed of light compared at two different frequencies. When you measure a quantity, it must always be in relation to some other quantity (a "unit"); you can't measure something that defines its own unit. This is the point the NPL page you linked to above was trying to make, which you apparently missed. Second, when describing electromagnetic waves in vacuum, εVacuum does indeed appear in Maxwell's equations (if it didn't, it wouldn't be a "permittivity" of vacuum), which is why (in the linear regime) it has to equal ε0 in order for the speed of light to be c as a consequence of the unit definitions.
—Steven G. Johnson (talk) 18:12, 17 February 2008 (UTC)
Hi Steve: In response to my "staggering and blithe claim", my defense is that this is a matter of logic, not authority, and the new definitions have led to a different logical basis for interpreting the symbols ε0, μ0, and c, which no longer are experimental objects referring to anything "real", vacuum or otherwise. You don't sound like having a discussion is what you want. Maybe we should give this a rest for a while? Brews ohare (talk) 02:12, 18 February 2008 (UTC)
I just wanted to point out the staggering arrogance of you claiming to understand electromagnetism better than Jackson, and to claim that he doesn't understand the unit definitions(!), in the hope that this would inspire a little introspection on your part. In general, in science, if you ever find yourself claiming that most scientists, especially respected experts, are wrong on a basic issue, you had better to have some darn strong evidence; more commonly you want to re-check your position for errors. (Regardless of how strong you think your arguments are, and frankly they exhibit some fundamental errors on your part as I pointed out above, Wikipedia's policy is a question of authority, not logic, because Wikipedia is not in a position to evaluate truth per se, only verifiability; see WP:NOR.)
At some point, I'll have to give up trying to convince you or discuss with you if you continue not to address my explanations reasonably, of course. I don't have unlimited time (far from it). Regardless of whether I convince you, the article can only reflect reputably-sourced information. —Steven G. Johnson (talk) 19:37, 18 February 2008 (UTC)
Hi Steve: Would it be an acceptable resolution of this issue to say that there is a "conceptual" vacuum with properties c, ε0, μ0 and a question about whether any particular measurement actually took place in such a "vacuum"? Then we could say the static relative permittivity of "conceptual vacuum" is defined as 1 because ε0 is defined as the permittivity of "conceptual vacuum".
However, an experiment then is faced with the experimental question of whether or not it took place in a physical situation that mimics "conceptual vacuum", and to answer that question a number of experiments are needed (e.g. measuring the speed of light). Whatever the result of these experiments, it never can be established that vacuum was attained, only that the speed of light (or whatever) is within δ% of c, εvac is within δ% of ε0, μvac is within δ% of μ0 etc.
In particular the vacuum state may not be a "conceptual vacuum" at all.
That sounds like a pretty reasonable approach to me, and probably is what Peter said before. Brews ohare (talk) 22:10, 18 February 2008 (UTC)
See below; you are still saying things that are fundamentally wrong, like that it is possible to "measure" the "physical" speed of light in vacuum. —Steven G. Johnson (talk) 00:03, 19 February 2008 (UTC)

## Rewrite of section "What is the vacuum"

I've rewritten this section and I think it makes more sense. Brews ohare (talk) 22:50, 18 February 2008 (UTC)

It's still fundamentally wrong, I'm afraid. (Besides which it conflicts with the published references, which is more pertinent under Wikipedia's WP:NOR policy. Note that you haven't been able to cite a single published reference that supports your interpretation that the linear permittivity in a vacuum is measurable or can differ from ε0; doesn't that tell you something?)
Brews, you still have the fundamental misunderstanding of thinking that it is possible to "measure" the "physical" speed of light in vacuum. One cannot measure it because it defines its own unit. Mathematically equivalently, it is no longer possible to "measure" the linear permittivity of vacuum (= a measurement of the speed of light) now that the units are defined the way they are. It must be ε0, by definition. That's why I cannot agree with your suggested resolution; it implies the possibility of an impossible, indeed ill-defined measurement.
For an example of the fallacies that this leads you into, the way you define "vacuum" in the article is circular. (What is "vacuum"? It is the medium in which the speed of light is c. What is c? The speed of light in vacuum, which defines the meter. How do you measure speed? You look at the meters per time. How do you measure a meter? It is the distance light travels in vacuum in a certain time interval. What is "vacuum"? ...)
What one can try to measure, and what we can mention, are possible nonlinear effects in vacuum (although no one has succeeded yet), because there you have a point of reference: you can compare the speed (or whatever) with a strong external field switched on or off. Or people have tried to measure frequency-dependence of ε0 (equivalent to a frequency-dependent speed of light), because there again you can compare behavior at one frequency to behavior at another—this is also interesting to mention, but if any such dependence were measured (it hasn't, yet) it would represent a fundamental upheaval in our understanding of physical laws and all the units and constants would have to be rethought. —Steven G. Johnson (talk) 23:51, 18 February 2008 (UTC)
Hi Steven:
You say: It's still fundamentally wrong, I'm afraid. (Besides which it conflicts with the published references, which is more pertinent under Wikipedia's WP:NOR policy.
Can you cite any published references that are in conflict?
You say: Brews, you still have the fundamental misunderstanding of thinking that it is possible to "measure" the "physical" speed of light in vacuum.
One can measure the speed of light in any real medium. It is not possible in the hypothetical vacuum because it's hypothetical. So I think I agree with you.
You say: For an example of the fallacies that this leads you into, the way you define "vacuum" in the article is circular.
I don't think its circular. For example, suppose you thought your lab measurement was done in vacuum, but it wasn't. It was accidentally done in methane vapor. You could measure permittivity, epsilon and speed of light, fine structure constant or whatever is the established process and you'd discover contradictions with the notion that this was vacuum. Same story with any other "candidate" for vacuum, in particular the vacuum state of quantum space.
Comments, please, Brews ohare (talk) 00:42, 19 February 2008 (UTC)
Jackson (who doesn't consider quantum nonlinear effects) and Mourou (who does), for example, both say that the relative permittivity of vacuum is one by definition.
Again, you're totally confused when you say that you could measure the speed of light in a vacuum, or the best available approximation thereof. How do you measure distance? Remember that you can only measure a quantity in relation to another known quantity; what is your reference quantity?
Name one source that defines the vacuum in your circular fashion, or says that you can measure a linear permittivity of vacuum that differs from the electric constant. If you can't, then just stop, because your point (whether you think it is correct or not) is irrelevant for Wikipedia.
—Steven G. Johnson (talk) 01:00, 19 February 2008 (UTC)
I'll mull over your other two points. However, so far as first point goes, if Jackson refers to "hypothetical" vacuum, then relative permittivity is one by definition. Whether Jackson really refers to hypothetical vacuum or some other version of vacuum is hard to determine because "vacuum" is used in a vague sense, as though it was intuitively obvious what it is. Brews ohare (talk) 01:17, 19 February 2008 (UTC)
Do you have any reputable source that says there is more than one kind of vacuum? (And no one would consider a partial vacuum to be a "version" of vacuum, by the way; at best it would be an approximation for a true vacuum.)
Thanks for taking some time to think, but I don't have time to discuss this with you further if you don't provide reputable sources for (a) your definition of vacuum, (b) the idea that there is a "hypothetical" vacuum that is different from a "vacuum," and/or (most importantly) (c) the idea that the linear permittivity of vacuum can differ from ε0 (under the current understanding of the laws of physics) or that a difference is even measurable (under the current definitions of the units). Unless you can provide a source, WP:NOR applies and inclusion of those claims in the article is a non-starter, and I'm not going to debate with you in the absence of such references. —Steven G. Johnson (talk) 02:28, 19 February 2008 (UTC)
Hi Steve: To take the opposite tack, I doubt that you can find a source that says the vacuum state has all the properties of the hypothetical vacuum; sources just don't discuss what is the vacuum in any kind of depth. I see that you are impatient, and that this difference of opinion cannot be settled. Brews ohare (talk) 05:27, 19 February 2008 (UTC)

On this one, I agree with Stevenj. Also, "What is the vacuum?" would probably be more appropriate for the article free space. /Pieter Kuiper (talk) 08:32, 19 February 2008 (UTC)

Thanks Pieter. Brews, you went around and added the following unsourced original research, which I've reverted:
Contrariwise, a departure of a medium's measured permittivity from ε0 is a demonstration that the measured medium is not "vacuum".
This is deceptive, because you cannot measure a putative vacuum's permittivity; you need to have something to compare it against. (If you have two approximate vacuums, you can conceivably measure the permittivity of one compared to the other. If they differ, then at least one of them is not a good approximation for a perfect vacuum, although you can't say which is a better vacuum without more information. But you cannot measure deviations from ε0 in an absolute sense without a reference vacuum.) Again, unless you have any source to cite that says you can measure the permittivity of vacuum, neither that claim nor anything that implies that claim can go into Wikipedia. —Steven G. Johnson (talk) 14:08, 19 February 2008 (UTC)
Hi Steve: This is nonsense, being equivalent to saying you cannot measure ANY permittivity. All that is said is that if you do measure a permittivity in a medium (it could be KCl) and its permittivity is not eps_0 , then it isn't vacuum. Of course, you have to have a "vacuum" to compare against - nothing said to contradict that point. You may be "reading between the lines". Brews ohare (talk) 14:52, 19 February 2008 (UTC)
Nope, you can measure the permittivity of one medium relative to another. The "reference" medium for all other measurements is currently vacuum (or the best experimental approximation thereof). One of the problems with what you inserted into the article is that it is phrased ambiguously enough that it could imply your (incorrect, original research claims) of the possibility of measuring the permittivity of a putative vacuum in addition to the (correct) statement that (given an approximate experimental vacuum as a reference) one can measure other permittivities compared to that, at least at a particular frequency or range of frequencies (since media other than vacuum have frequency-dependent permittivity). Moreover, one has to be careful in claiming that, if a material has a permittivity equal to ε0, then it is vacuum—it may contain particles that don't interact with the electric field (e.g. it's not obvious what the effect of neutrinos are on the permittivity, and without a reference....), so unless you can find a reference to state that this is a test of whether a medium is vacuum, then it can't go in the article.
Please understand that, if you cannot give a reference for your claims, you cannot insert them in Wikipedia. —Steven G. Johnson (talk) 15:12, 19 February 2008 (UTC)