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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.
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Today, quantum theory has changed the vacuum from the classical concept: quantum vacuum or the vacuum state is no longer empty. Walter Dittrich & Gies H (2000). Probing the quantum vacuum: perturbative effective action approach. Berlin: Springer. ISBN 3540674284. An approximate meaning is as follows:
Gordon Kane (2000). Supersymmetry: squarks, photinos, and the unveiling of the ultimate laws. Cambridge, MA: Perseus Publishers, Appendix A; pp. 149 ff.. ISBN 0738204897.
A true, physical, quantum vacuum is an abstraction from nature that has its own properties and a vacuum energy. The quantum vacuum is "by no means a simple empty space". Astrid Lambrecht (Hartmut Figger, Dieter Meschede, Claus Zimmermann Eds.) (2002). Observing mechanical dissipation in the quantum vacuum: an experimental challenge; in Laser physics at the limits. Berlin/New York: Springer, p. 197. ISBN 3540424180. , and again: "it is a mistake to think of any physical vacuum as some absolutely empty void." Christopher Ray (1991). Time, space and philosophy. London/New York: Routledge, Chapter 10, p. 205. ISBN 0415032210. 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.AIP Physics News Update,1996 One measurable result of these ephemeral occurrences is the Casimir effect.Physical Review Focus Dec. 1998F Capasso, JN Munday, D. Iannuzzi & HB Chen Casimir forces and quantum electrodynamical torques: physics and nanomechanics 2007 Other examples are spontaneous emission Hiroyuki Yokoyama & Ujihara K (1995). Spontaneous emission and laser oscillation in microcavities. Boca Raton: CRC Press, 6. ISBN 0849337860. Benjamin Fain (2000). Irreversibilities in quantum mechanics: Fundamental theories of physics v. 113. New York:London: Springer/Kluwer Academic, §4.4 pp. 113ff. ISBN 079236581X. Marian O Scully & Zubairy MS (1997). Quantum optics. Cambridge UK: Cambridge University Press, §1.5.2 pp. 22-23. ISBN 0521435951. and the Lamb shift. Marian O Scully & Zubairy MS. pp. 13-16. ISBN 0521435951. The determined reader can explore various nuances of the vacuum in Saunders. S Saunders & HR Brown Eds.) (1991). The philosophy of vacuum. Oxford UK: Oxford University Press. ISBN 0198244495. A more recent treatment is Genz. Henning Genz (2002). Nothingness: the science of empty space. Reading MA: Oxford: Perseus. ISBN 0738206105.
Physicists often use the term "vacuum" to discuss ideal test results that would occur in a perfect vacuum, which they simply call vacuum or free space in this context, and use the term partial vacuum to refer to the imperfect vacuo realized in practice.The term partial vacuum suggests one major source of departure of a an approximate vacuum from true vacuum, namely non-zero pressure. However, there are additional possible sources of nonideality, for example, strong electric or magnetic fields. See, for example,Di Piazza et al.: Light diffraction by a strong standing electromagnetic wave Phys.Rev.Lett. 97 (2006) 083603, Gies, H et al.: Polarized light propagating in a magnetic field as a probe for millicharged fermions Phys. Rev. Letts. 97 (2006) 140402 Thus, free space is considered a baseline or reference state, unattainable like the absolute zero of temperature. In this reference state, for example, radiant energy propagates through free space in the form of electromagnetic waves, such as radio waves and visible light (among other electromagnetic spectrum frequencies) at the defined speed of light, c0 with electric and magnetic fields related by the defined value of the characteristic impedance of vacuum Z0. According to relativity, radiant energy in free space propagates at the speed of light, independent of the speed of the observer or of the source of the waves.
However, the theoretical meaning of the vacuum state is not settled. Peter Woit (2006). Not even wrong: the failure of string theory and the search for unity in physical law. New York: Basic Books. ISBN 0465092756. See, for example, Higgs mechanism and QCD vacuum. To date, there is no suggestion that these uncertainties impinge on the role of the vacuum state in the definitions of the SI units, which are more closely related to the undisputed predictions of quantum electrodynamics. Henning Genz. p. 247. ISBN 0738206105.
By "realization" is meant the reduction to practice, or experimental embodiment, of the term "free space", for example, a partial vacuum. 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:CIPM adopted Recommendation 1 (CI-1983)
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.LM Rozanov & Hablanian, MH (2002). Vacuum technique. London; New York: Taylor & Francis, Figure 3.1, p. 80. ISBN 041527351X. (The abbreviation Pa stands for the unit pascal, 1 pascal = 1 N/m2.)
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)Zheng, MiMi (2002). Pressure in Outer Space. The Physics Factbook.. 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.Gareth Wynn-Williams (1992). The fullness of space. Cambridge UK: Cambridge University Press, p. 38. 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.Martin J. Rees (1978), "Origin of pregalactic microwave background", Nature 275: 35-37., <http://www.nature.com/nature/journal/v275/n5675/abs/275035a0.html>
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".
The United States Patent Office defines "free space" in a number of ways, at least some of which bear little if any relation to the technical definition of free space outlined above. For example, 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 - Classification Definitions as of June 30, 2000 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".
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