Talk:Free space

Removed section on Quantum vacuum
The article didn't mention quantum vacuum in the lead or elsewhere, and the only thing this section said was that it's not the same concept as free space. Given the lack of connection, it seemed best to just leave it out. Dicklyon (talk) 03:29, 2 November 2010 (UTC)


 * I consider the premise that the term free space is necessarily restricted to a classical entity incorrect and unsupported. Moreover, the citation in the lead contradicts its preceding text which asserts this premise.
 * "Free space corresponds to the ideal classical vacuum.[1]".
 * The reference on page 28 states, "The vacuum (or free space) itself is like Janus, a deity from the Roman mythology, with quantum and classical faces."
 * Tim Shuba (talk) 14:55, 2 November 2010 (UTC)
 * I agree that there is absolutely no support for the idea that free space is limited to a classical description of vacuum. It appears to have been used as a counterpoint to the imperfect practical vacuums that were available before the space age, but I've got no reference for that. Physchim62 (talk) 15:17, 2 November 2010 (UTC)


 * I have no particular opinion on how free space should be defined, but the ref that was there before was to p.34, which didn't mention free space. If you read the rest of the paragraph quoted above, and the next, it sort of says that free space or classical vacuum is the reference medium we're talking about.  It never really defines free space.  If we could agree on a set of sources to use to define what the topic of the article is, and whether it's different from vacuum, it would be easier to proceed.  I was trying to clean up the article to better support and agree with the topic defined in the lead; if that's not it, someone should fix it.  Or if there's really no definition for free space as a topic, we don't need the article at all.  I'm not convinced that physicists use the term free space for something different than ideal vacuum.  All the complexities of quantum vacuum just get inherited by free space if that's so, in which case an article on it is redundant.  Brews has his own POV on this, which he doesn't seem to have been able to communicate while bloating the article, but which he's now lobbying me about in off-wiki email.   Dicklyon (talk) 05:15, 3 November 2010 (UTC)


 * As used in modern literature, the term mostly appears in descriptions of quantum phenomena. There is no formal, universal definition for free space. It generally refers either to a vacuum or any background which can be effectively treated as vacuum for some particular phenomena or experiment. I included a reference with a decent definition of the former, though if anything it's the latter use of the term that may be the best reason to keep this article separate from vacuum. Tim Shuba (talk) 00:26, 4 November 2010 (UTC)

Rewrote a bit
I rewrote a bit...not much content changed I think...let me know what you think. One thing is, I used phrases like "properties of vacuum in classical electromagnetism" rather than "classical vacuum" -- it's not the vacuum that is classical or quantum, it's the laws of physics describing it. Also, it's not in principle impossible to remove every last particle of matter from some volume, it just is in practice today, so I tried to make that clearer. --Steve (talk) 18:33, 6 November 2010 (UTC)
 * I can't agree that free space is defined in terms of its permeability and its permittivity. It is our systems of units which are defined by the numerical values of these constants. Maybe we should have a separate section on these, but I've always found it hard to do without falling into WP:OR. Physchim62 (talk) 02:18, 8 November 2010 (UTC)
 * I don't think it says that... It says that in classical electromagnetism, the permeability of free space is mu0 and the permittivity is epsilon0. That's not debated by anyone. It does not say that that's the definition of free space, does it? Are you misled by the phrase "has the following properties"? That's not meant to be "is defined by the following properties", just "has". Maybe there's a clearer way to say it. --Steve (talk) 04:37, 8 November 2010 (UTC)

Merge it?
Please see the merge discussion at Talk:Vacuum, and add comments there if you want. It appears that there was only one objection, without followup, to the merge proposal. I still think it's the right way to go, and maybe it's now time. Comment there. Dicklyon (talk) 04:23, 8 November 2010 (UTC)

Restore facts and information
The article as it stands now contains egregious omissions ... --J. D. Redding 00:49, 25 November 2010 (UTC)

Begin
In classical physics, free space is a concept of electromagnetic theory, corresponding to a theoretically perfect vacuum and sometimes referred to as the vacuum of free space, or as classical vacuum, and is appropriately viewed as a reference medium.

Properties
Free space is a reference medium through which electromagnetic radiation can travel without obstructions. Many physicists refer to free space as a reference medium that has a refractive index of exactly one for every wavelength; in this usage, free space corresponds to the classical vacuum. Other scientists often refer to free space as a medium that has a refractive index that may be approximated as 1.

When discussing electromagnetic potentials and fields propagating in free space, it is assumed that the principle of linear superposition holds. For example, the electric potential generated by two charges is the simple addition of the potentials generated by each charge in isolation. The value of the electric field at any point around these two charges is found by calculating the vector sum of the two electric fields from each of the charges acting alone. Furthermore, any two or more electromagnetic waves passing through the same volume of free space do not interact with each other and they are not affected by each other in any way.

The permittivity of free space and permeability of free space define the electromagnetic properties of free space. The speed of light in free space is the same as the speed of light in vacuum.

The classical vacuum is a practically unattainable reference medium devoid of any matter. Any large enough volume of space will contain at least one particle of matter, like an atom or molecule. By analogy, free space has this unattainable nature similar to the absolute zero of temperature in thermodynamics and the ideal gas in physics and chemistry.

What is the vacuum?
Physicists use the term "vacuum" in several ways. One use is to discuss ideal test results that would occur in a perfect vacuum, which physicists simply call classical vacuum or free space in this context. The term partial vacuum is used to refer to the imperfect vacuum that is obtainable in practice.

The physicist's term "partial vacuum" does suggest one major source of departure of a realizable vacuum from free space, namely non-zero pressure. Today, however, the classical concept of vacuum as a simple void is replaced by the quantum vacuum, separating "free space" still further from the real vacuum – quantum vacuum or the vacuum state is not empty. An approximate meaning is as follows: The quantum vacuum is "by no means a simple empty space," and again: "it is a mistake to think of any physical vacuum as some absolutely empty void." According to quantum mechanics, empty space (the "vacuum") is not truly empty but instead contains fleeting electromagnetic waves and particles that pop into and out of existence. One measurable result of these ephemeral occurrences is the Casimir effect. Other examples are spontaneous emission  and the Lamb shift. Related to these differences, quantum vacuum differs from free space in exhibiting nonlinearity in the presence of strong electric or magnetic fields (violation of linear superposition). Even in classical physics it was realized that the vacuum must have a field-dependent permittivity in the strong fields found near point charges. These field-dependent properties of the quantum vacuum continue to be an active area of research. The determined reader can explore various nuances of the quantum vacuum in Saunders. A more recent treatment is Genz.

At present, even the meaning of the quantum vacuum state is not settled. To quote GE Brown: : In eighteen-century Newtonian mechanics, the three-body problem was insoluble. With the birth of general relativity around 1910 and quantum electrodynamics in 1930, the two- and one-body problems became insoluble. And within modern quantum field theory, the problem of zero bodies (vacuum) is insoluble. &ensp; … GE Brown quoted by RD Mattuck For example, what constitutes a "particle" depends on the gravitational state of the observer. See the discussion of vacuum in Unruh effect. Speculation abounds on the role of quantum vacuum in the expanding universe. See vacuum in cosmology. In addition, the quantum vacuum may exhibit spontaneous symmetry breaking. See Woit and the articles: Higgs mechanism and QCD vacuum.

The discrepancies between free space and the quantum vacuum are predicted to be very small, and to date there is no suggestion that these uncertainties affect the use of SI units, whose implementation is predicated upon the undisputed predictions of quantum electrodynamics.

In short, realization of the ideal of "free space" is not just a matter of achieving low pressure, as the term partial vacuum suggests. In fact, "free space" is an abstraction from nature, a baseline or reference state, that is unattainable in practice.

Realization of free space
While only a partial vacuum, outer space contains such sparse matter that the pressure of interstellar space is on the order of 10 pPa (1×10−11 Pa). For comparison, the pressure at sea level (as defined in the unit of atmospheric pressure) is about 101 kPa (1×105 Pa). The gases in outer space are not uniformly distributed, of course. The density of hydrogen in our galaxy is estimated at one hydrogen atom per cubic centimeter. The critical density separating a Universe that continuously expands from one that ultimately crunches is estimated as about three hydrogen atoms per thousand liters of space. In the partial vacuum of outer space, there are small quantities of matter (mostly hydrogen), cosmic dust and cosmic noise. See intergalactic space. In addition, there is a cosmic microwave background with a temperature of 2.725 K, which implies a photon density of about 400 photons per cubic centimeter.

The density of the interplanetary medium and interstellar medium, though, is extremely low; for many applications negligible error is introduced by treating the interplanetary and interstellar regions as "free space".

US Patent Office interpretation
Scientists working in optical communications tend to use free space to refer to a medium with an unobstructed line of sight (often air, sometimes space). See Free-space optical communication and the What is Free Space Optical Communications?.

The United States Patent Office defines free space in a number of ways. For radio and radar applications the definition is "space where the movement of energy in any direction is substantially unimpeded, such as the atmosphere, the ocean, or the earth" (Glossary in US Patent Class 342, Class Notes).

Another US Patent Office interpretation is Subclass 310: Communication over free space, where the definition is "a medium which is not a wire or a waveguide".

EOS
Please expand and improve the existing article. J. D. Redding 00:49, 25 November 2010 (UTC)


 * Most of the stuff you're showing (above) was removed as being either not relevant to free space, or redundant with respect to vacuum.  If some of the many sources provide info about free space that is useful in distinguishing it from vacuum, or otherwise relevant to the idea of having a separate article on free space, please do point those out or add them back.  There seems to have been pretty good consensus for taking out most of this stuff that you and Brews ohear added,  but I'm sure that if you explained, we could include any bits that are important. Dicklyon (talk) 21:36, 27 November 2010 (UTC)

See Manual of Style (summary style) ... particularly, Levels of desired details [eg., Summary style is based on the premise that information about a topic should not all be contained in a single article since different readers have different needs]. It is not bad to be "redundant" with respect to vacuum.

Could you state what is 'not relevant' to free space?

Sincerely, --J. D. Redding 02:42, 28 November 2010 (UTC)

sociology
Free space is also a term used in sociology and left-wing/anarchist culture for a community center or other space which is free of state control and discrimination. See Article on autonomy or Polletta on free space or Chris Atton 2003: Infoshops in the Shadow of the state, in: Contesting Media Power: alternative media in a networked world, 2003, p.57-69.--85.183.154.44 (talk) 19:31, 26 July 2014 (UTC)