Talk:Free space

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[edit] difference between Patent office and technical definition

I thought it would be obvious but " the atmosphere, the ocean, or the earth" are not free of matter. Whether or not something is a good approximation to free space depends on the wavelength, for example, the earth is far from free space for visible light. Salsb 15:01, 7 December 2005 (UTC)

Thanks ... I would guess also that the "vacuum" of space (which contains a small proportion of elements) could be a approximation to "free space". J. D. Redding

True, but a incredibly much better approximation for all wavelengths. In terms of density, comparing outer space to air is like comparing a neutron star to a medium vacuum! Salsb 16:42, 7 December 2005 (UTC)
I don't think I did make such a comparision. Anyways ... nothing is totally "free of matter" and, more importantly, this is in the article. Sincerely, J. D. Redding 17:03, 7 December 2005 (UTC)

[edit] Sources or substantial revision needed

The statement in the second paragraph of the definition article says that "The notion of free space does not correspond to the present-day understanding of what is called the vacuum state or the quantum vacuum". So there's the "partial vacuum" that people make in their laboratories, and the "ideal physical vacuum" (or whatever the proper term is) which is the limit of a partial vacuum as temperature and pressure go to zero. (So far this is widely accepted and uncontroversial.) But then defining "free space" to be the ideal physical vacuum without quantum fluctuations seems to me to be quite unusual and unconventional definition. Moreover, making the claim that the speed of light and vacuum permittivity are defined with respect to "free space" (and not the ideal physical vacuum) is a statement of fact which is unsourced, and which appears to contradict many reliable sources. For example, it's implicit in the SI definition of the meter that by measuring the speed of light in a better and better vacuum, you should get a better and better measurement of the meter standard. Therefore, they presumably intend the speed of light to be defined in the ideal physical vacuum, not free space. (If someone did a theoretical calculation that says that turning off quantum fluctuations would change the value of the speed of light by 20%, that would not mean that everyone in the world would have to order new meter-sticks and redo all their measurements!)

  • First, someone needs to find a source that uses the same definition as the one in this article, to show that this isn't the only place on earth which is using "free space" as synonymous with a vacuum with quantum fluctuations turned off. In fact, as best as I can tell, it seems like the term bare vacuum is much more common (at least in QFT literature), and I'd suggest that the article be renamed accordingly, unless sources say otherwise.
  • Second, the claim that the vacuum permittivity and speed of light are defined with to free space, and not the ideal physical vacuum, needs to be sourced or deleted. I'm skeptical, for the reason mentioned above.
  • Third, outer space and laboratories are not approximations to "free space" as defined here. They are approximations to the ideal physical vacuum. There's a big difference: For example, in "free space" the charge of the electron is not renormalized by virtual photons, and turns out to be infinitely large. Likewise, the masses of particles in "free space" are not renormalized by virtual processes, so they're the "bare masses" instead of physical masses, and these are also infinity, I think. Therefore, these sections should be moved to the vacuum article, or deleted, unless someone finds a source for these claims.

Alternatively, and perhaps better, the definition could be changed to define "free space" as the ideal physical vacuum. This would, however, entail some redundancy with the vacuum article, which could be minimized by appropriate editing and cross-referencing. What do people think is the better direction to go with the article? --Steve (talk) 23:55, 29 February 2008 (UTC)

Well, there is way too much here to tackle all at once. And the discussion is likely to go astray because of words like "vacuum" that have multiple definitions. I'll pick the third item as a starting point:
Third, outer space and laboratories are not approximations to "free space" as defined here. They are approximations to the ideal physical vacuum.
Let me start by suggesting the term "quantum vacuum" for the physically observable vacuum that actually occurs in nature. That definition seems to be in accord with the articles. So this convention on definitions replaces the term "ideal physical vacuum". Then outer space and laboratories are approximations to the quantum vacuum, only approximations because they cannot pump down to low enough pressures and cannot remove entirely things like background fields, neutrinos etc. I'd guess that agreement exists on that point?
With the quantum vacuum in mind, I agree with the first part of the statement, namely, Third, outer space and laboratories are not approximations to "free space" . So I'd guess that there is actually no debate here at all – we agree on point three. Brews ohare (talk) 17:13, 1 March 2008 (UTC)
Let's call the above Response to Point 3. Now let's go to Point 1:
using "free space" as synonymous with a vacuum with quantum fluctuations turned off. In fact, as best as I can tell, it seems like the term bare vacuum is much more common
Most authors simply refer to "free space" as a synonym for "empty space", and references 1-3 in the article take pains to distinguish that view from the quantum vacuum as defined in Response to Point 3. I like your term bare vacuum for this case. My view was that the common interpretation of free space was bare vacuum. I suspect that most EM books and even NIST itself is unclear whether or not free space is quantum vacuum or bare vacuum. In fact, I doubt that this distinction even occurs to them, never mind reaches discussion. I'd like a bit more on what you think should be done about this. A good choice might be to define free space as the same thing as quantum vacuum. What would be the ramifications? Brews ohare (talk) 18:57, 1 March 2008 (UTC)
And here is a whole book about it. [1] Brews ohare (talk) 22:20, 1 March 2008 (UTC)
  1. ^ Henning Genz (2002). Nothingness: the science of empty space. Reading MA: Oxford: Perseus. ISBN 0738206105. 
I revised this page; see how you like it. Besides free space, electric constant and magnetic constant have been slightly altered. Basically most EM articles refer to free space, so this is the key article to get straight. Brews ohare (talk) 23:01, 1 March 2008 (UTC)

Hi again! Thanks for responding. I haven't read your changes yet, but before I do I wanted to share the following bit of research that I just did.

Evidence that NIST means quantum vacuum, not bare vacuum, when defining speed of light and vacuum permittivity: If you look at [1] p46, NIST says that in defining the meter via the speed of light in vacuum, "that in all cases any necessary corrections be applied to take account of actual conditions such as diffraction, gravitation or imperfection in the vacuum". It doesn't mention quantum fluctuations (and certainly doesn't advocate theoretical studies of speed-of-light renormalization), but nor does it explicitly say otherwise. But it's clear from the rest of the pamphlet what they mean by "vacuum". For example, if you flip to p51, it lists the "vacuum wavelengths" for certain atomic transitions. Without virtual particles, the values of atomic transitions are very different (since there's no virtual photons, the electron has infinite charge, etc.), so here they must be talking about quantum vacuum. On p62, it talks about weighing against the kilogram mass standard "in vacuum". Without virtual particles, the mass of each proton and electron is infinity, making such a comparison impossible. So I'd say there's substantial evidence that NIST means the quantum vacuum. [Just to be clear, as a matter of physics, I don't believe that quantum fluctuations affect the speed of light...but if some future theoretical advance indicated that it did, I believe that this wouldn't have any bearing on the NIST standard.]

I hope I get a chance later to comment more.--Steve (talk) 23:41, 1 March 2008 (UTC)

You make good points; the revisions just made in free space, magnetic constant and electric constant bring these articles to coincide with your views. Brews ohare (talk) 23:45, 1 March 2008 (UTC)
By the way, the point that the nature of the vacuum affects predictions of atomic transition seems very significant to me - that should come up in the discussion of frequency standards somewhere -interested in looking at that? It seems worthwhile. Brews ohare (talk) 00:53, 2 March 2008 (UTC)

I have no objections to the article as currently written. Thanks for putting in the time to improve it! :-) --Steve (talk) 02:45, 3 March 2008 (UTC)

Brews ohare, you did a great job on this article. J. D. Redding 08:17, 17 April 2008 (UTC)