Talk:Vacuum permittivity/Archive 2

Archive 1

Archive 2

Speed of light in vacuum

The article now reads: " An experimental measurement of this linear permittivity of vacuum is equivalent to measuring the speed of light in vacuum, which is no longer possible because the meter is defined in terms of this speed to make c (and thus ε₀) a fixed number. Hence, the linear relative permittivity of vacuum is 1 by definition. "

It appears to suggest that there is no possible way to disqualify a proposed vacuum. In the talk section above, you raise the possibility of comparing two candidates, and suggest that no choice between them is possible without "more information". A disingenuous question: where would one turn for "more information" to aid this choice? Presumably, c ε, and μ are tautological. Brews ohare (talk) 15:10, 19 February 2008 (UTC)

The way to disqualify a vacuum is to show that it contains (non-virtual) particles. e.g. if I have a brick, I can show that it is not a vacuum because it has mass, etcetera; certainly, I can show that it is less of a vacuum than the surrounding air. And one can show that air, in turn, is less of a vacuum than a vacuum jar by showing that the former contains more particles. You cannot do it by measuring the speed of light in a putative vacuum, unless you have a reference vacuum to compare to, because of the definition of the meter. The cited sources support this fact.

Since you don't have a source for your original contention that it is possible to measure the speed of light (or the permittivity) in a putative vacuum (without a reference vacuum to compare against), your argument is a non-starter. Your "logic" doesn't matter here—all that matters is whether you can provide an authoritative reference.

—Steven G. Johnson (talk) 15:16, 19 February 2008 (UTC)

Hmm. It does appear that, for example, the question of whether the speed of light is changing with time, can be answered. Ostensibly, then, with one's standard vacuum, one can do measurements that would reveal that the speed of light was changing. The UK standards site suggest that it has to do with measuring the fine structure constant or the electron/proton mass ratio. Would you not say that if they really can determine that the speed of light is changing in their certified vacuum, that this means they CAN measure the speed of light? BTW, they provide six references.

Brews ohare (talk) 16:23, 19 February 2008 (UTC)

Nowhere on that page does it say that they can measure whether the speed of light itself is changing in time, in fact it says the opposite: "Because of this the most meaningful experiments must involve dimensionless constants such as the fine structure constant." —Steven G. Johnson (talk) 17:26, 19 February 2008 (UTC)

By no stretch of imagination does it say the opposite. It just says its tough to do, and forces the experimenter's attention toward the measurement of dimensionless constants. Brews ohare (talk) 17:56, 19 February 2008 (UTC)

Where specifically, does it say that it is possible to measure? —Steven G. Johnson (talk) 19:02, 19 February 2008 (UTC)

By the way, I just had a chance to look through Jackson. He has a long discussion of the electromagnetic units, including a discussion of the nonlinear quantum effects of vacuum, corrections to the Coulomb potential, etcetera etcetera, and various limitations of measurements and in particular how the various physical laws are measured. And he gives the vacuum speed of light and (linear) vacuum polarization as being the corresponding constants by definition of the units, as I said, with no possibility for "measuring" them and no possibility of a "physical" vacuum vs. a "hypothetical" one. —Steven G. Johnson (talk) 19:02, 19 February 2008 (UTC)

Where specifically, does it say that it is not possible to measure?

"And he gives the vacuum speed of light and (linear) vacuum polarization as being the corresponding constants by definition of the units, as I said, with no possibility for "measuring" them and no possibility of a "physical" vacuum vs. a "hypothetical" one."

That is interesting. I do not have a copy of Jackson with me. Of course, the matters of definition are beyond dispute. Does he really say in bald terms that there is no possibility of a "physical" vacuum vs. a "hypothetical one"? What is the exact language?? Brews ohare (talk) 19:10, 19 February 2008 (UTC)

The terminology needed here is from free space, namely, partial vacuum for a realizable vacuum and vacuum for the idealized unattainable reference also known as free space, which is a zero pressure state with no particles. 71.80.210.217 (talk) 21:54, 26 February 2008 (UTC)

Summary of the role of "vacuum"

I've arrived at this view of things:

Free space is a reference state, unattainable even in a simple picture because any lab vacuum is only a partial vacuum. However, corrections for poor vacuum can be, should be, and in practice are, estimated and applied. I got hung up on the idea that "free space" was something real; it isn't and was never intended to be thought of that way.

In principle, there are very, very small additional corrections that should be made for vacuum fluctuations, but they haven't even been measured yet, so the practical approach is to forget about them. Their possible existence should be mentioned only for logical completeness.

The various articles should be written, as they are right now, to point out that free space is a reference state and that c₀, μ₀, etc. refer to free space.

The various articles also should point out, as they do now, that where they are known, corrections to lab standards for partial vacuum should be applied.

The various articles probably belabor some issues because of a lack of cool on my part. They could be slimmed down, but I hope the points above are adopted as guidelines in doing this.

Various notational conventions have turned up as per ISO 31 that should be adhered to. Among them, the use of c₀ for the speed of light in vacuum and the use of ${\overset {\underset {\mathrm {def} }{}}{=}}$ for definitions.

Brews ohare (talk) 23:11, 29 February 2008 (UTC)

The article has lost persepctive. Vacuum permittivity is air permittivity to within a fraction of a percent. Perhaps we could start with something intuitive, and leave small corrections until later. Leave unmeasurable corrections to brief mentions, or even footnotes. --192.75.48.150 (talk) 16:50, 3 July 2008 (UTC)

Clumsy intro

The intro of this article is way too cluttered with links and technicalities. I've split off half of it into the a new section; but for clarity I suggest it would be better if some of the NIST technical stuff could be moved deeper into the article (a section of its own) so users of the page can find out:
1) What the constant is
2) The value of the constant
at a glance, which is what 90% of users come to the page for. Article intros should be concise and to the point without going into technicalities and discussions.
--Zebas (talk) 16:08, 2 November 2008 (UTC)

What is this article meant to be about ? (and "name" again)

Hi ! I have some expertise in this area, and have been doing some tidy-ups on this article. In doing so, I have removed some material that appears to be irrelevant to the topic under discussion, and also I have removed a couple of statements that in my view are not technically well formulated. [For example, in the same way that you cannot "put π=1", you cannot "put ε₀ = 1".] You can certainly manipulate the form of Coulomb's law, but (post 1970) this is not a technically correct way of describing how to do it. I have also added a section on why ε₀ has the value it does, which does cover the issue of other unit systems.

I have done the tidy-ups because (quite apart from the name issue) there appears to be some confusion about what this article ought to be about. In my view, it should be about the parameter ε₀ - which basically is a mathematical parameter associated with the design of the current international system of measurement, and about this alone. This article should probably not include material about the properties of the vacuum.

On the name issue, the problem with the name "vacuum permittivity" is that the Standards Organizations apparently regard names of this type as confusing or potentially confusing, because they contain the hidden message that ε₀ represents some physical property of free space and/or the vacuum. This is my view too. It may be the most popular name - but what is the point - in an encyclopedia - of using a name that the technical experts regard as confusing ? Does this expedite or slow down the progress of science ?

I appreciate that opinions are strongly held on both sides, so can I suggest a compromise - namely that this article is called: "The scientific parameter ε₀" (or something like that), and that re-direct links are put in from all the accepted alternative names. If no-one has objection to this, then I will do this in due course, and also deal with the question of disambiguation from the mathematical quantity ε₀. (RGForbes (talk) 01:02, 27 March 2009 (UTC))

I think that "The scientific parameter ε₀ is not a very good article name; and it's unclear whether subscripts can work in a name. The present name is still what this parameter is most widely known as, so I'd leave it. Dicklyon (talk) 02:13, 27 March 2009 (UTC)

I am not inclined to attempt to press this point if it is not thought helpful. "Vacuum permittivity" is ok as a title as long as the article itself is clear about the nature of ε₀ and does not contain confusing material. .(RGForbes (talk) 15:05, 27 March 2009 (UTC)).

I'd agree with Dick that naming the article "The scientific parameter ε₀" is not going to help. First, it will make the article impossible to find directly unless a host of redirects are invented. That solution simply puts the plan back to square one: it's no better than leaving things as they are. At least one can google "electric constant" or "permittivity of free space" or "vacuum permittivity" and find actual printed works where this terminology is used.

As pointed out at various places above, the article must make clear to the new reader some confusion about several closely related but different things:

1. The role of ε₀ as a property of free space
2. The role of ε₀ as a property of an ideal model of vacuum
3. The relation between "free space" and a model of "ideal vacuum"
4. The ambiguity in usage of "vacuum" to refer variously to the reference state free space and also to realizable vacuum like outer space; ultra-high vacuum; quantum vacuum; QCD vacuum; etc., leaving it to the reader to infer from context what is meant

My take is that one can, of course, define a model of vacuum (call it ideal vacuum) where ε =ε₀ and μ = μ₀. Having done that one can inquire whether (for example) outer space ultra-high vacuum etc are well approximated by this model. This is an experimental issue. One can also inquire whether QCD vacuum or quantum vacuum agree with this ideal model. That is ultimately an experimental issue also, but lacking the technique, it is more a theoretical issue right now.

One can also ask how this ideal model relates to the reference state referred to as "vacuum" by BIPM and NIST. This is a metrology issue, not an issue of accuracy of the model, but an issue of utility to the community of a particular point of reference. (Like the length of the king's arm for a yard, the choice of reference is arbitrary, but some choices are more convenient than others.)

Rewriting the article is fine, but these issues should be dealt with to avoid continually re-rewriting the article because of confusions that arise in every new reader of the article. Brews ohare (talk) 02:42, 27 March 2009 (UTC)

As I see it, the only way that "physics" rather than measurement system decisions might enter into the allocation of a numerical value to ε₀ is via the parameter c₀, commonly called the "velocity of light in free space" or the "velocity of light in vacuum". My take on this is that, if c₀ were to be considered as a measured quantity (which it was some years ago, but now is not), then it would be better to call it (or think of it as) "the velocity of light in real vacuum" (which may have all the features and problems that you describe of it). I leave aside the issue of the relative permittivity and permeability of air, which have to be corrected for. If real polarisable vacuum is different from free space and different from old-style non-interactive vacuum, then these latter two ideas are presumably hypothetical ideas that have no role (or only a limited role) to play in physics. The measurements of the velocity of light that took place some years ago (when c₀ was not a defined constant) took place in real polarisable vacuum (possibly with some gas in it). Therefore, the existing defined value of c₀ takes into account that light actually travels in a real vacuum (whatever we think that is). Therefore, discussion of hypothetical ideas of "free space" and "ideal vacuum" are simply not relevant in an article that discusses ε₀. The discussion of ε₀ is already set in the context of the real world in which light travels in a polarisable vacuum. [Alternatively, you might want to take the view that, nowadays, the term "free space" is synonymous with "real polarisable vacuum".]

This is my current view. If, however, you were able to show that, in real polarisable vacuum, Maxwell's equations are not correct, and/or that it is not true that ε₀μ₀=1/(c₀)², then the Standards Organizations would presumably take this into account (and you might get a Nobel prize !) At present they have not considered it necessary to do so, as far as I know.

I have no problem with the basic idea that the speed of a photon might be affected by its interaction with virtual electron-positron pairs. So I do not dispute the reasonableness of your views about considering the nature of a vacuum: I dispute their relevance to this article, which is about ε₀ (whatever we individually prefer to call it). I do not see how the matters you raise could affect the value allocated to ε₀. Why not add your views to an existing article on free space or vacuum or the speed of light, or put them in a separate article on the electromagnetic properties of vacuum ? There is also the question of whether, in the context of an article on ε₀, your views constitute "original research" of a kind that should not be included.

Aside from all this, I do think that you have established a case for making the point briefly in the article that (c₀), called the "velocity of light in free space", really means the "velocity of light in a real vacuum".(RGForbes (talk) 15:05, 27 March 2009 (UTC))

Hi RG: I disagree with your summary that "velocity of light in free space", really means the "velocity of light in a real vacuum". The point is that free space is not a real (realizable) vacuum, but is a reference state, unobtainable in practice. However, the defined values of speed of light, ε₀, etc. do apply to free space. Maybe they apply to real vacuum with some error bars attached, but certainly not with defined values. Brews ohare (talk) 15:47, 27 March 2009 (UTC)

Hi ! First, I think that my use of the term "in free space" may be confusing discussion, so I'll stick to "vacuum". Main point is: No - I think that logically it has to be the other way round - I agree with you that we have to make a distinction between "real vacuum" and what I'd like to call "ideal vacuum" - but the original experiments on measuring the velocity of light took place in "real vacuum" (possibly with some gas molecules in, of course, but let's assume that this can be corrected for), so the defined velocity that we now have must be the velocity that applies to real vacuum. I think that the issue is what do the words "in vacuum" mean in the definition of c₀ as the velocity of light "in vacuum". I think that - in this context - they have to be interpreted to mean "in real vacuum". Maybe in other contexts they should be interpreted to mean "in ideal (unrealizable) vacuum". Where your arguments possibly go is to suggest that Standards Organizations need to be more precise in defining what is meant by "vacuum". However, I think that, if you raise this matter, then you might get the reply that it is thought to be obvious that "vacuum" means "real vacuum", because it is is impossible to perform experimental measurements in hypothetical unrealizable vacuum (so why would Standards Organizations be interested in specifying quantities applying to this). (RGForbes (talk) 23:32, 27 March 2009 (UTC)) (Richard)

Hi Richard: I hope you will have some patience with the following summary of events. It might be helpful if we have some common background for the present discussion:

No doubt the speed of light was measured originally using a "standard meter". That led to the number for c_0 eventually settled upon. These measurements came from various sources, but I gather the definitive values came from microwave cavity resonators, which used the relation c = λf, with λ based on measured cavity dimensions using the standard meter.

Later came the decision to define c instead of the meter, and use λ = c/f = cT to determine the meter instead of measuring c using the standard meter. That decision is based upon where the errors come from, and the decision that for now the errors come from establishing frequency. To avoid massive retooling expense they stuck with a number for c close to the measured value, instead of rounding it off at some nice value.

The definition of the meter refers to "vacuum", but description of what "vacuum" means is oblique, shall we say. The closest we come is some remarks about "corrections to account for imperfection of the vacuum", and about using lengths short enough that spacetime curvature is not a factor and so forth. When push comes to shove, they will sell you a list of standard corrections they have accumulated so far.

The problem is recognized: Mohr et al., after pages of math about vacuum polarization etc. say things like "The improvements that led to the reduction in uncertainty include a more stable external reference cavity for locking the 486nm cw dye laser, thereby reducing its linewidth; an upgraded vacuum system that lowered the background gas pressure in the interaction region, thereby reducing the background gas pressure shift and its associated uncertainty;… and: "The most notable change in the experiment is that in the new apparatus the entire balance mechanism and moving coil are in vacuum, which eliminates the uncertainties of the corrections in the previous experiment for the index of refraction of air in the laser position measurements… ". However, I have yet to find any detailed discussion of how the errors contributed by imperfect vacuum compare to those in determining frequency.

So what are we doing when we correct for imperfections?

I'd suggest that what is going on is that there are theoretical expressions for the parameters of any particular medium, for example, air, in terms of measurable "imperfections" such as partial pressures of contaminants. So one measures the partial pressures, or estimates them indirectly from other considerations, and then cheerfully subtracts these "corrections" to refer the results to "vacuum".

I'd say that the brief way to say all this is that extrapolation is being used to take real measurements back to a baseline that is some "ideal vacuum" that never can be realized, a nirvana where c=c_0 μ = μ_0 by definition. For example, interference fringes are counted as a chamber is pumped down, and a theoretical fit is extrapolated to zero pressure. Should it evolve that this procedure is pure fantasy, and that no such "ideal vacuum" exists even in principle, it really doesn't matter as long as the convention is to follow the procedure with the established corrections so that everything is measured from the same reference point, be that a fiction or not. I'd say that in fact, it is abundantly clear from theory so far that, in fact, there absolutely is no such medium as one with c=c_0 μ = μ_0; it certainly will be impossible to prove existence by experiment, because experiment always has error bars. It will, however, eventually become possible to show experimentally that there exists absolutely no known medium in which the speed of light is unreservedly field-independent, polarization-independent, isotropic and dispersionless.

Is this your view of the situation? Brews ohare (talk) 01:13, 28 March 2009 (UTC)

Hi Brews! I concur that we should attempt to reach consensus on the physics, before discussing what to do about it in the context of Wikipedia. I agree with the first part of what you say, but want to analyze the second part more carefully. Let me use some temporary terminology.

Ideal classical vacuum is defined to be hypothetical classical "definitely empty" space. In particular, ideal classical vacuum has in it no gas atoms or molecules, or anything else that could be classically polarized by an electric field.

On the other hand, "ideal quantum-mechanical vacuum" is defined to be space that is free of "classical polarizable objects" (such as atoms and molecules), but may contain phenomena or entities associated with electromagnetic zero-point energy and/or virtual electron-positron pairs, and/or anything else of this general kind that theoreticians tell us is always there (I am not an expert on what's currently considered to be there).

Now, I assume that 100 years ago, when Maxwell's equations were formulated, an assumption was around that "real ideal vacuum" was what I have just called "ideal classical vacuum". However, nowadays we assume that "real ideal vacuum" is "ideal quantum-mechanical vacuum".

The 1700s and 1800s experiments that underlie Maxwell's equations were conducted in air, which we now understand to be "ideal quantum-mechanical vacuum with polarizable atoms and molecules in it".

Therefore, in my view, when corrections for the presence of polarizable atoms and molecules were made in metrology before c₀ became a defined quantity, what was done was to correct the value of the speed of light back to a reference state that is the "ideal quantum-mechanical vacuum". Obviously, at some time in the past, metrologists may have thought that they were correcting the value of light back to a reference state that was "ideal classical vacuum", but it seems to me that the "classical" method of making corrections takes us back to ideal quantum-mechanical vacuum, not to ideal classical vacuum.

Certainly, in the Mohr article, when considering energy levels, they are working in the context of "quantum-mechanical vacuum".

There might be a question of whether additional corrections should be made to take the velocity of light back to the hypothetical reference state of "ideal classical vacuum", but in my view there is no reason why this should be done (certainly not as part of normal metrological activity). This is because there is no way that experiments can be done in ideal classical vacuum (so why should a Standards Organization bother to make a correction to this hypothetical state).

Obviously, in principle, there are a number of questions that could be asked both about ideal classical vacuum and about ideal quantum-mechanical vacuum. These include: Do Maxwell's equations take exactly the same form in both cases ? Is the speed of light isotropic in both cases ? Is the speed of light the same in both cases ? What form should the constitutive relationships take ? Whether it is really sensible or useful to ask such questions, I am not quite sure, because I am not an expert on these things. But they do not appear to me to be obviously stupid questions – even if accepted answers already exist to some possible questions.

I assume that, if the velocity of light were not isotropic and uniform in "real ideal vacuum" (i.e., in "ideal quantum-mechanical vacuum"), then: either (1) we would know about it (for example, as a consequence of experiments done to test the special and general theories of relativity); or (2) any anisotropy or speed non-uniformity is so small that it is masked by other (more significant) errors and/or uncertainties. If (2), then at this point of time we do not need to concern ourselves about these things in the context of an article on ε₀.

This is my view of the physics. If there is a difference between our views, I think it would relate to the issues of: (1) what reference state actually is used by Standards Organizations when defining the velocity of light; and, possibly (2) what reference state should be used by Standards Organizations. However, I think that it is more likely that we would agree that the reference state is what I have called "ideal quantum-mechanical vacuum".

Where I think these arguments might take you is to the conclusion that the constitutive relation (relating the D-field to the E-field) needs amendment. Perhaps there could be a need to write this in the form:

D=ε₀E + P_class = ε₀E + P_total – P_quantum

where P_class is the classical polarization of a medium, P_quantum represents effects due to "quantum mechnical polarization of the vacuum", and P_total is the "total polarization of the medium". I abstain on whether this is sensible physics or not.

There are then issues relating to Wikipedia. The first is – if these are original research discussions then maybe these issues should not be included in Wikipdeia at all – I abstain on this, as I have not looked for references to discussions of this kind in the literature. The second is – should these issues be discussed in the context of an article on ε₀? My view is that they are primarily issues that relate to the speed of light and/or to the nature of the concept of vacuum, or the concept of free space. Therefore, in my view, the primary discussion should be in one or more of these places, with a link or links to it from this article on ε₀.

In particular, I would personally prefer to see the section on "Realizable vacuum and free space" moved to a more relevant article, with links to it from this article. This is because (in my view) you do not have to go into these complicated issues when trying to tell an undergraduate physicist or electronic/electrical engineer what ε₀ is. (RGForbes (talk) 19:08, 29 March 2009 (UTC)) (Richard)

Last paragraph of intro

Presently reads:

A common mistake is to think that ε₀ is a physical constant that describes some physical property of the vacuum or of free space. This is not true: ε₀ is a measurement-system constant introduced and defined as a result of international agreement. In physics there may be issues relating to the physical properties of the vacuum or of free space, but they are separate from issues relating to the meaning and value of ε₀.

This paragraph tries to do too much in too few words. Here is a proposed replacement:

A common mistake is to think that ε₀ is a physical constant that describes some physical property of a realizable "vacuum". This thought is not true: ε₀ is a measurement-system constant introduced and defined as a result of international agreement. It refers to a property of what sometimes is called free space, which is not a realizable vacuum, but is a reference state or benchmark simply used as a baseline for comparison of measurements made in all types of real media. In physics there may be issues relating to the physical properties of realizable vacuums such as outer space, ultra-high vacuum, QCD vacuum or quantum vacuum, but they are separate from issues relating to the meaning and value of ε₀, all of which are metrology issues. Brews ohare (talk) 03:00, 27 March 2009 (UTC)

I've inserted a modified version of the above proposal intended to separate the notion of free space from realizable vacuum. Brews ohare (talk) 17:24, 27 March 2009 (UTC)

Not totally happy with the revised statement, but I'll think about it. (RGForbes (talk) 23:36, 27 March 2009 (UTC)) (Richard)

Have concluded that the only further change needed is to make it clearer that the issue is the velocity of light in the reference state/situation, and have implemented this change (RGForbes (talk) 23:53, 29 March 2009 (UTC)) (Richard)