Free space
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Brews ohare
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/* What is the vacuum? */ ambigutiy
{{Electromagnetism|cTopic=[[Classical electromagnetism|Electrodynamics]]}}
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'''. The definitions of the [[ampere]] and [[meter]] [[SI units]] are based upon measurements corrected to refer to free space.
==Properties of ''free space''==
The concept of ''free space'' is an abstraction from nature, a baseline or reference state, that is unattainable in practice, like the [[absolute zero]] of temperature. It is characterized by the ''defined'' value of the parameter μ<sub>0</sub> known as the ''[[permeability of free space]]'' or the [http://physics.nist.gov/cgi-bin/cuu/Value?mu0|''magnetic constant''], and the ''defined'' value of the parameter ε<sub>0</sub> called the ''[[permittivity of free space]]'' or the [http://physics.nist.gov/cgi-bin/cuu/Value?ep0|''electric constant'']. These parameters appear in [[Maxwell's equations]] for the electromagnetic fields. Parameter ε<sub>0</sub> also enters the expression for the [[fine-structure constant]] usually denoted by [http://physics.nist.gov/cgi-bin/cuu/Value?alph| α], which characterizes the strength of the [[electromagnetic interaction]].
In the reference state of free space, according to Maxwell's equations, [[electromagnetic wave]]s, such as [[radio wave]]s and [[visible light]] (among other [[electromagnetic spectrum]] frequencies) propagate at the ''defined'' [[Speed_of_light#Speed_of_light_set_by_definition|speed of light]], [http://physics.nist.gov/cgi-bin/cuu/Value?c| ''c''<sub>0</sub>], and according to the [[Special relativity|theory of relativity]], this speed is independent of the speed of the observer or of the source of the waves. The electric and magnetic fields in these waves are related by the ''defined'' value of the [[characteristic impedance of vacuum]] [http://physics.nist.gov/cgi-bin/cuu/Value?z0| ''Z''<sub>0</sub>]. In addition, in this reference state the principle of [[linear superposition]] of potentials and fields holds: for example, the electric potential generated by two charges is the simple addition of the potentials generated by each charge in isolation.<ref name=Jackson1>
{{cite book
|author=John David Jackson
|title=Classical electrodynamics
|edition=Third Edition
|page=pages 10, 13
|publisher= Wiley
|location=NY
|year=1999
|isbn= 0-471-30932-X
|url=http://worldcat.org/search?q=047130932X&qt=owc_search}}
</ref>
The ideal ''vacuum of free space'' is not the same as a physically obtainable ''vacuum''.
==What is the ''vacuum''?==
{{seealso|Vacuum energy|Vacuum state}}
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 '''vacuum''' or '''free space''' in this context. The term '''partial vacuum''' is used to refer to the imperfect vacuo realizable 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.<ref name=Dittrich>
{{cite book
|author=Walter Dittrich & Gies H
|title=Probing the quantum vacuum: perturbative effective action approach
|publisher= Springer
|location=Berlin
|year=2000
|isbn=3540674284
|url=http://books.google.com/books?id=DyhyFSL7bNUC&pg=PP1&dq=intitle:Probing+intitle:the+intitle:Quantum+intitle:Vacuum&lr=&as_brr=0&sig=VSfMMLJnmYyWplC2L5i9oVSjurg#PPA1,M1}}
</ref> An approximate meaning is as follows:<ref name=Kane>
{{cite book
|author=Gordon Kane
|title=Supersymmetry: squarks, photinos, and the unveiling of the ultimate laws
|page=Appendix A; pp. 149 ff.
|publisher= Perseus Publishers
|location=Cambridge, MA
|year=2000
|isbn= 0738204897
|url=http://books.google.com/books?id=gxr-75Pse8YC&pg=PA149&dq=%22lowest+energy+state%22&lr=&as_brr=0&sig=KzjKOWzdlRbv6MjSzVks3drGU1E}}
</ref>
{{Quotation|Quantum vacuum describes a region devoid of real particles in its lowest energy state.}}
The quantum vacuum is "by no means a simple empty space".<ref name=Lambrecht>
{{cite book
|author=Astrid Lambrecht (Hartmut Figger, Dieter Meschede, Claus Zimmermann Eds.)
|title=Observing mechanical dissipation in the quantum vacuum: an experimental challenge; in''' Laser physics at the limits'''
|page=p. 197
|publisher= Springer
|location=Berlin/New York
|year=2002
|isbn=3540424180
|url=http://books.google.com/books?id=0DUjDAPwcqoC&pg=PA197&dq=%22vacuum+state%22&lr=&as_brr=0&sig=-gfWcR7RdymYL3W-M2VxVQPFm10#PPA197,M1}}
</ref>, and again: "it is a mistake to think of any physical vacuum as some absolutely empty void."<ref name=Ray>
{{cite book
|author=Christopher Ray
|title=Time, space and philosophy
|page=Chapter 10, p. 205
|publisher= Routledge
|location=London/New York
|year=1991
|isbn=0415032210
|url=http://books.google.com/books?id=1F7xWULz0P0C&pg=RA1-PA205&dq=%22vacuum+state%22&lr=&as_brr=0&sig=3jx_vuLEBKXfGNEY4CfUIDTPVSI}}
</ref> 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.<ref>[http://www.aip.org/pnu/1996/split/pnu300-3.htm AIP Physics News Update,1996]</ref> One measurable result of these ephemeral occurrences is the [[Casimir effect]].<ref>[http://focus.aps.org/story/v2/st28 Physical Review Focus Dec. 1998]</ref><ref>[https://www.editorial.seas.harvard.edu/capasso/publications/Capasso_STJQE_13_400_2007.pdf F Capasso, JN Munday, D. Iannuzzi & HB Chen ''Casimir forces and quantum electrodynamical torques: physics and nanomechanics'' 2007]</ref> Other examples are [[spontaneous emission]]<ref name=Yokoyama,>
{{cite book
|author= Hiroyuki Yokoyama & Ujihara K
|title=Spontaneous emission and laser oscillation in microcavities
|publisher= CRC Press
|location=Boca Raton
|page=6
|year=1995
|isbn=0849337860
|url=http://books.google.com/books?id=J_0ZAwf6AQ0C&printsec=frontcover&dq=%22spontaneous+emission%22&lr=&as_brr=0&sig=QbNGpi5P0uTHxwhHDECXhG2VA9M#PPA6,M1}}
</ref><ref name=Fain>
{{cite book
|author= Benjamin Fain
|title=Irreversibilities in quantum mechanics: Fundamental theories of physics v. 113
|publisher= Springer/Kluwer Academic
|location=New York:London
|page=§4.4 pp. 113ff
|year=2000
|isbn=079236581X
|url=http://books.google.com/books?id=9FjsaogX3TgC&pg=PA113&dq=%22spontaneous+emission%22&lr=&as_brr=0&sig=gpGI0dO9iNHwHGjkWyApaz4B_JY}}
</ref><ref name=Scully1>
{{cite book
|author=Marian O Scully & Zubairy MS
|title=Quantum optics
|publisher= Cambridge University Press
|location=Cambridge UK
|page=§1.5.2 pp. 22-23
|year=1997
|isbn=0521435951
|url=http://books.google.com/books?id=20ISsQCKKmQC&pg=PA430&dq=atom+transition+photon&lr=&as_brr=0&sig=d5TzC9UTl7CGU3PIJiCV0c0M6HU#PPA22,M1}}
</ref> and the [[Lamb shift]].<ref name=Scully2>
{{cite book
|author=Marian O Scully & Zubairy MS
|title=pp. 13-16
|isbn=0521435951
|url=http://books.google.com/books?id=20ISsQCKKmQC&pg=PA430&dq=atom+transition+photon&lr=&as_brr=0&sig=d5TzC9UTl7CGU3PIJiCV0c0M6HU#PPA13,M1}}
</ref> 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 <ref>For example, by M. Born and L. Infeld ''Proc. Royal Soc. London'' '''A144''' 425 (1934) </ref><ref name=Jackson>
{{cite book
|author=John David Jackson
|title=Classical electrodynamics
|edition=Third Edtion
|page=pp. 10-12
|publisher= Wiley
|location=NY
|year=1999
|isbn= 0-471-30932-X
|url=http://worldcat.org/search?q=047130932X&qt=owc_search}}
</ref> 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.<ref>See, for example,[http://arxiv.org/abs/hep-ph/0602039v2 Di Piazza ''et al.'']: ''Light diffraction by a strong standing electromagnetic wave'' Phys.Rev.Lett. 97 (2006) 083603, [http://arxiv.org/abs/hep-ph/0607118v1 Gies, H ''et al.'']: ''Polarized light propagating in a magnetic field as a probe for millicharged fermions'' Phys. Rev. Letts. '''97''' (2006) 140402</ref> The determined reader can explore various nuances of the quantum vacuum in Saunders.<ref name=Saunders>
{{cite book
|author=S Saunders & HR Brown Eds.)
|title=The philosophy of vacuum
|publisher= Oxford University Press
|location=Oxford UK
|year=1991
|isbn=0198244495
|url=http://books.google.com/books?id=ZU1LL4IbDKcC&pg=PA43&sig=cEOiLN537ku-k24d0dFLJD_D5FA&vq=%22The+principle+of+the+constancy+of+the+velocity+of+light+Light+is+always+propagated+in+empty+space+with+a+definite%22&source=gbs_quotes_s&cad=2#PPA172,M1}}
</ref> A more recent treatment is Genz. <ref name=Genz>
{{cite book
|author=Henning Genz
|title=Nothingness: the science of empty space
|publisher= Oxford: Perseus
|location=Reading MA
|year=2002
|isbn=0738206105
|url=http://books.google.com/books?id=Cn_Q9wbDOM0C&printsec=frontcover&dq=%22empty+space%22&lr=&as_brr=0&sig=udf6V66Xial28_JKFJZHgm92M1M#PPA290,M1}}
</ref>
At present, even the meaning of the quantum vacuum state is not settled. For example, what constitutes a "particle" depends on the gravitational state of the observer. See the discussion of vacuum in [[Unruh effect#Vacuum interpretation|Unruh effect]].<ref name=Fulling>{{cite book |title=Aspects of Quantum Field Theory in Curved Spacetime |page=p. 259 |author=Stephen A. Fulling |url=http://books.google.com/books?id=h6gUbmd973AC&pg=PA259&dq=real+particles++%22unruh+effect%22&lr=&as_brr=0&sig=e0zVAipDAZS_EztlOvNqX_h-2yQ
|publisher=Cambridge University Press |year=1989 |location=Cambridge UK}}</ref> <ref name=Cao> {{cite book |title=Conceptual foundations of quantum field theory |author= Tian Yu Cao|page=p. 179 |url=http://books.google.com/books?id=d0wS0EJHZ3MC&pg=PA179&dq=real+particles++%22unruh+effect%22&lr=&as_brr=0&sig=jA91P9oknu0JoEFANZi58xXhbyg#PPA179,M1
|isbn=0521602726 |publisher=Cambridge University Press |year=1999 |location=Cambridge UK}}</ref> Speculation abounds on the role of quantum vacuum in the expanding universe. See [[Cosmological constant#Cosmological constant problem| vacuum in cosmology]]. In addition, the quantum vacuum may exhibit spontaneous [[symmetry breaking]]. See Woit<ref name=Woit>
{{cite book
|author=Peter Woit
|title=Not even wrong: the failure of string theory and the search for unity in physical law
|publisher= Basic Books
|location=New York
|year=2006
|isbn=0465092756
|url=http://books.google.com/books?id=pcJA3i0xKAUC&pg=PA93&dq=%22Higgs+field%22&lr=&as_brr=0&sig=I168cJKoVyLqOd7z9pdeR0mC_A0#PPA71,M1}}
</ref> and the articles: [[Higgs mechanism]] and [[QCD vacuum]]. To date, there is no suggestion that these uncertainties affect the use of [[SI units]], whose implementation is predicated upon the undisputed predictions of [[Precision tests of QED|quantum electrodynamics]].<ref name=Genz2>
{{cite book
|author=Henning Genz
|title=p. 247
|isbn=0738206105
|url=http://books.google.com/books?id=Cn_Q9wbDOM0C&printsec=frontcover&dq=%22empty+space%22&lr=&as_brr=0&sig=udf6V66Xial28_JKFJZHgm92M1M#PPA247,M1}}
</ref>
In short, realization of the ideal of "free space" is not entirely a matter of achieving low pressure, as the term ''partial vacuum'' suggests.
==Realization of free space in a laboratory==
By "realization" is meant the [[reduction to practice]], or experimental embodiment, of the term "free space", for example, a ''partial vacuum''. What is the [[operational definition]] of free space? Although in principle ''free space'' is unattainable, like the [[absolute zero]] of temperature, the [[SI units]] are referred to ''free space'', and so an estimate of the necessary correction to a real measurement is needed. An example might be a correction for non-zero pressure of a partial vacuum. Regarding measurements taken in a real environment (for example, partial vacuum) that are to be related to "free space", the [[CIPM]] cautions that:<ref>[http://physics.nist.gov/Pubs/SP330/sp330.pdf CIPM adopted Recommendation 1 (CI-1983)]</ref>
:"in all cases any necessary corrections be applied to take account of actual conditions such as diffraction, gravitation or imperfection in the vacuum."
In practice, a partial vacuum can be produced in the laboratory that is a very good realization of free space. Some of the issues involved in obtaining a high vacuum are described in the article on [[ultra high vacuum]]. The lowest measurable pressure today is about 10<sup>−11</sup> Pa.<ref name=Rozanov>{{cite book
|author=LM Rozanov & Hablanian, MH
|title=Vacuum technique
|url=http://books.google.com/books?id=8yEGJCtS2XgC&pg=PA79&lpg=PA79&dq=%22measurement+of+vacuum%22&source=web&ots=RmEYhDyVOY&sig=Muaslx8-GZW6zWBCa403si5cuRM#PPA80,M1 |publisher=Taylor & Francis
|location=London; New York
|year=2002
|page=Figure 3.1, p. 80
|isbn=041527351X}}</ref> (The abbreviation Pa stands for the unit [[Pascal (unit)|pascal]], 1 pascal = 1 N/m<sup>2</sup>.)
==Realization of free space in outer space==
While only a partial vacuum, [[outer space]] contains such sparse matter that the pressure of interstellar space is on the order of 10 [[Pascal (unit)|pPa]] (1×10<sup>−11</sup> Pa)<ref>{{cite web|url=http://hypertextbook.com/facts/2002/MimiZheng.shtml|last=Zheng|first=MiMi|title=Pressure in Outer Space|work=The Physics Factbook|year=2002}}</ref>. For comparison, the pressure at sea level (as defined in the unit of [[Atmosphere (unit)|atmospheric pressure]]) is about 101 kPa (1×10<sup>5</sup> Pa). The gases in outer space are not uniformly distributed, of course. The density of hydrogen in our galaxy is estimated at 1 hydrogen atom/cm<sup>3</sup>.<ref name=Wynn-Williams>{{cite book
|author=Gareth Wynn-Williams
|title=The fullness of space
|page=p. 38
|url=http://books.google.com/books?id=wjxrloC2gyMC&pg=PA155&dq=astronomy++pressure+interplanetary&lr=&as_brr=0&sig=ecolKMxvbGQbLbNOjy3VvWVSzaI#PPA38,M1 |publisher=Cambridge University Press
|location=Cambridge UK
|year=1992
|isbn=0521426383}}</ref>
In the partial vacuum of [[outer space]], there are [[density|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 /cm<sup>3</sup>.<ref>{{citation |author=[[Martin J. Rees]]| title=Origin of pregalactic microwave background |year=1978 |url=http://www.nature.com/nature/journal/v275/n5675/abs/275035a0.html |journal=[[Nature]] |volume=275 |pages=35-37.}}</ref> <ref>This background temperature depends upon the gravitational state of the observer. See [[Unruh effect#Calculations|Unruh effect]].</ref>
The density of the [[interplanetary medium]] and [[interstellar medium]], though, is extremely low; and, for many applications, the interplanetary and interstellar regions are "free space".
== US Patent Office interpretation of free space==
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).<ref> U.S. Patent Classification System - [http://www.uspto.gov/web/offices/ac/ido/oeip/taf/def/342.htm Classification Definitions] as of June 30, 2000</ref> 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''".<ref>[http://www.uspto.gov/web/offices/ac/ido/oeip/taf/def/370.htm#310 Subclass 310: Communication over free space]</ref> This definition bears little if any relation to other technical definitions of free space outlined above.
==References and notes==
<references/>
==External links==
*[http://physics.nist.gov/cuu/Constants/introduction.html| NIST Introduction to the fundamental physical constants]
*[http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf BIPM brochure on SI units]
==See also==
<div style="-moz-column-count:3; column-count:3;">
* [[permittivity]] and [[Permeability (electromagnetism)|permeability]] of free space
* [[homogeneous media]]
* [[Vacuum energy]]
* [[Vacuum state]]
* [[Virtual particle]]
* [[Casimir effect]]
* [[Unruh effect]]
* [[Goldstone boson]]
* [[Intergalactic space]]
* [[Interplanetary space]]
* [[Interstellar medium]]
* [[Outer space]]
* [[Medium (optics)]]
* [[Electric constant]]
* [[Magnetic constant]]
* [[Speed of light]]
* [[SI units]]
* [[Dirac sea]]
* [[Characteristic impedance of vacuum]]
* [[Jaynes-Cummings model]]
* [[Maxwell's equations]]
* [[Electromagnetic wave equation]]
* [[Sinusoidal plane-wave solutions of the electromagnetic wave equation]]
* [[Mathematical descriptions of the electromagnetic field]]
</div>
[[Category:Electromagnetism]]
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