Free space

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.Fact|date=September 2008

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 μ₀ 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 ε₀ 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 ε₀ 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 waves, such as radio waves and visible light (among other electromagnetic spectrum frequencies) propagate at the "defined" speed of light, [http://physics.nist.gov/cgi-bin/cuu/Value?c| "c"₀] , and according to the 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"₀] . 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.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]

The ideal "vacuum of free space" is not the same as a physically obtainable "vacuum".

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 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.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] An approximate meaning is as follows: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] The quantum vacuum is "by no means a simple empty space".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] , and again: "it is a mistake to think of any physical vacuum as some absolutely empty void."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] 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. [ [http://www.aip.org/pnu/1996/split/pnu300-3.htm AIP Physics News Update,1996] ] One measurable result of these ephemeral occurrences is the Casimir effect. [ [http://focus.aps.org/story/v2/st28 Physical Review Focus Dec. 1998] ] [ [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] ] Other examples are spontaneous emissioncite 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] 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] 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] and the Lamb shift.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] 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 [For example, by M. Born and L. Infeld "Proc. Royal Soc. London" A144 425 (1934) ] 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] 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. [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] The determined reader can explore various nuances of the quantum vacuum in 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] A more recent treatment is 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]

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.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] 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] 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 Woitcite 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] 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 quantum electrodynamics.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]

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: [ [http://physics.nist.gov/Pubs/SP330/sp330.pdf CIPM adopted Recommendation 1 (CI-1983)] ]

:"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 10⁻¹¹ Pa.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] (The abbreviation Pa stands for the unit pascal, 1 pascal = 1 N/m².)

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 pPa (1×10⁻¹¹ Pa) [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] . 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 1 hydrogen atom/cm³.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] 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 /cm³. [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.] [This background temperature depends upon the gravitational state of the observer. See Unruh effect.]

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). [ 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] This definition matches other technical definitions of free space outlined above.

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". [ [http://www.uspto.gov/web/offices/ac/ido/oeip/taf/def/370.htm#310 Subclass 310: Communication over free space] ] This definition bears little if any relation to other technical definitions of free space outlined above.

References and notes

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]

ee also

* permittivity and 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