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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. Electromagnetism is the Physics of the Electromagnetic field: a field which exerts a Force on particles that possess the property of In Physics, magnetism is one of the Phenomena by which Materials exert attractive or repulsive Forces on other Materials. Classical electromagnetism (or classical electrodynamics) is a theory of Electromagnetism that was developed over the course of the 19th century most prominently In Physics, the Lorentz force is the Force on a Point charge due to Electromagnetic fields It is given by the following equation Electromotive force ( emf, \mathcal{E} is a term used to characterize electrical devices such as Voltaic cells thermoelectric devices electrical Faraday's law of induction describes an important basic law of electromagnetism which is involved in the working of Transformers Inductors and many forms of Faraday's law of induction describes an important basic law of electromagnetism which is involved in the working of Transformers Inductors and many forms of Displacement current is a quantity that arises in a changing electric field In Classical electromagnetism, Maxwell's equations are a set of four Partial differential equations that describe the properties of the electric The electromagnetic field is a physical field produced by electrically charged objects. Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. The Liénard-Wiechert potential describes the electromagnetic effect of a moving Electric charge. The Maxwell Stress Tensor (also known as Maxwell's Stress Tensor is used to calculate the stresses on objects in magnetic or electrical fields An eddy current (also known as Foucault current) is an electrical phenomenon discovered by French physicist Léon Foucault in In Classical electromagnetism, Maxwell's equations are a set of four Partial differential equations that describe the properties of the electric This vacuum means "absence of matter" or "an empty area or space" for the cleaning appliance see Vacuum cleaner. The definitions of the ampere and meter SI units are based upon measurements corrected to refer to free space. The ampere, in practice often shortened to amp, (symbol A is a unit of Electric current, or amount of Electric charge per second The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International

## 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. Absolute zero is the point at which molecules do not move (relative to the rest of the body more than they are required to by a quantum mechanical effect called Zero-point It is characterized by the defined value of the parameter μ0 known as the permeability of free space or the magnetic constant, and the defined value of the parameter ε0 called the permittivity of free space or the electric constant. The vacuum permeability, referred to by international standards organizations as the magnetic constant, and denoted by the symbol μ 0 (also Vacuum permittivity, referred to by international standards organizations as the electric constant, and denoted by the symbol ε0 is a fundamental Physical These parameters appear in Maxwell's equations for the electromagnetic fields. In Classical electromagnetism, Maxwell's equations are a set of four Partial differential equations that describe the properties of the electric Parameter ε0 also enters the expression for the fine-structure constant usually denoted by α, which characterizes the strength of the electromagnetic interaction. The fine-structure constant or Sommerfeld fine-structure constant, usually denoted \alpha \ is the Fundamental physical constant characterizing In Physics, the electromagnetic force is the force that the Electromagnetic field exerts on electrically charged particles

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, c0, and according to the theory of relativity, this speed is independent of the speed of the observer or of the source of the waves. Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. Radio waves are electromagnetic waves occurring on the Radio frequency portion of the Electromagnetic spectrum. The electromagnetic (EM spectrum is the range of all possible Electromagnetic radiation frequencies Special relativity (SR (also known as the special theory of relativity or STR) is the Physical theory of Measurement in Inertial The electric and magnetic fields in these waves are related by the defined value of the characteristic impedance of vacuum Z0. Impedance of Free Space (also known as Z_0 is a Canadian IDM and Electronica group 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. In Physics and Systems theory, the superposition principle, also known as superposition property, states that for all Linear systems [1]

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

## What is the vacuum?

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. Vacuum energy is an underlying background Energy that exists in Space even when devoid of Matter (known as Free space) In Quantum field theory, the vacuum state (also called the vacuum) is the Quantum state with the lowest possible Energy. 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. In Quantum field theory, the vacuum state (also called the vacuum) is the Quantum state with the lowest possible Energy. [2] An approximate meaning is as follows:[3]

Vacuum describes a region devoid of real particles in its lowest energy state.

The quantum vacuum is "by no means a simple empty space". [4], and again: "it is a mistake to think of any physical vacuum as some absolutely empty void. "[5] 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. [6] One measurable result of these ephemeral occurrences is the Casimir effect. In Physics, the Casimir effect and the Casimir-Polder force are physical forces arising from a quantized field. [7][8] Other examples are spontaneous emission[9][10][11] and the Lamb shift. Spontaneous emission is the process by which a light source such as an Atom, Molecule, Nanocrystal or nucleus in an Excited state In Physics, the Lamb shift, named after Willis Lamb (1913-2008 is a small difference in Energy between two Energy levels ^2S_{1/2} [12] Related to these differences, 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 [13][14] that the vacuum must have a field-dependent permittivity in the strong fields found near point charges. These field-dependent properties of the vacuum continue to be an active area of research. [15] The determined reader can explore various nuances of the vacuum in Saunders. [16] A more recent treatment is Genz. [17]

At present, even the meaning of the 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. The Unruh effect, discovered in 1976 by Bill Unruh of the University of British Columbia, is the prediction that an accelerating observer will observe [18] [19] Speculation abounds on the role of vacuum in the expanding universe. See vacuum in cosmology. In Physical cosmology, the cosmological constant (usually denoted by the Greek capital letter Lambda: Λ was proposed by Albert Einstein as a modification In addition, the vacuum may exhibit spontaneous symmetry breaking. See Woit[20] and the articles: Higgs mechanism and QCD vacuum. The Higgs mechanism is Spontaneous symmetry breaking in a Gauge theory. The QCD vacuum is the Vacuum state of Quantum chromodynamics (QCD 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. Quantum electrodynamics ( QED) a relativistic quantum field theory of electrodynamics is among the most stringently tested theories in Physics. [21]

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. In United States patent law, the reduction to practice is a concept meaning the embodiment of the concept of an Invention. 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 operational definition is a demonstration of a process &mdash such as a Variable, term, or object &mdash relative in terms of the specific Process Absolute zero is the point at which molecules do not move (relative to the rest of the body more than they are required to by a quantum mechanical effect called Zero-point 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:[22]

"in all cases any necessary corrections be applied to take account of actual conditions such as diffraction, gravitation or imperfection in the vacuum. The International Committee for Weights and Measures is the English name of the Comité international des poids et mesures ( CIPM, sometimes written in English "

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. Ultra high vacuum (UHV is the Vacuum regime characterised by Pressures lower than about 10−7 pascal or 100 nanopascals (~10−9 The lowest measurable pressure today is about 10−11 Pa. [23] (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)[24]. Outer space, often simply called space, comprises the relatively empty regions of the Universe outside the escape velocities of Celestial bodies. For comparison, the pressure at sea level (as defined in the unit of atmospheric pressure) is about 101 kPa (1×105 Pa). The Standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of Pressure (symbol atm 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. [25] In the partial vacuum of outer space, there are small quantities of matter (mostly hydrogen), cosmic dust and cosmic noise. Outer space, often simply called space, comprises the relatively empty regions of the Universe outside the escape velocities of Celestial bodies. The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different Matter is commonly defined as being anything that has mass and that takes up space. Cosmic noise and galactic radio noise are Random noise that originates outside the Earth's atmosphere. See intergalactic space. Intergalactic space is the physical space between galaxies. Generally free of dust and debris intergalactic space is very close to a total Vacuum. In addition, there is a cosmic microwave background with a temperature of 2. 725 K, which implies a photon density of about 400 /cm3. [26] [27]

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 interplanetary medium is the material which fills the solar system and through which all the larger solar system bodies such as Planets Asteroids

## US Patent Office interpretation of free space

The United States Patent Office defines "free space" in a number of ways. The United States Patent and Trademark Office ( PTO or USPTO) is an agency in the United States Department of Commerce that issues Patents to 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). [28] 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". [29] This definition bears little if any relation to other technical definitions of free space outlined above.

## References and notes

1. ^ John David Jackson (1999). Classical electrodynamics, Third Edition, NY: Wiley, pages 10, 13. ISBN 0-471-30932-X.
2. ^ Walter Dittrich & Gies H (2000). Probing the quantum vacuum: perturbative effective action approach. Berlin: Springer. ISBN 3540674284.
3. ^ Gordon Kane (2000). Supersymmetry: squarks, photinos, and the unveiling of the ultimate laws. Cambridge, MA: Perseus Publishers, Appendix A; pp. 149 ff. . ISBN 0738204897.
4. ^ 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.
5. ^ Christopher Ray (1991). Time, space and philosophy. London/New York: Routledge, Chapter 10, p. 205. ISBN 0415032210.
6. ^ AIP Physics News Update,1996
7. ^ Physical Review Focus Dec. 1998
8. ^ F Capasso, JN Munday, D. Iannuzzi & HB Chen Casimir forces and quantum electrodynamical torques: physics and nanomechanics 2007
9. ^ Hiroyuki Yokoyama & Ujihara K (1995). Spontaneous emission and laser oscillation in microcavities. Boca Raton: CRC Press, 6. ISBN 0849337860.
10. ^ 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.
11. ^ Marian O Scully & Zubairy MS (1997). Quantum optics. Cambridge UK: Cambridge University Press, §1. 5. 2 pp. 22-23. ISBN 0521435951.
12. ^ Marian O Scully & Zubairy MS. pp. 13-16. ISBN 0521435951.
13. ^ For example, by M. Born and L. Infeld Proc. Royal Soc. London A144 425 (1934)
14. ^ John David Jackson (1999). Classical electrodynamics, Third Edtion, NY: Wiley, pp. 10-12. ISBN 0-471-30932-X.
15. ^ 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
16. ^ S Saunders & HR Brown Eds. ) (1991). The philosophy of vacuum. Oxford UK: Oxford University Press. ISBN 0198244495.
17. ^ Henning Genz (2002). Nothingness: the science of empty space. Reading MA: Oxford: Perseus. ISBN 0738206105.
18. ^ Stephen A. Fulling (1989). Aspects of Quantum Field Theory in Curved Spacetime. Cambridge UK: Cambridge University Press, p. 259.
19. ^ Tian Yu Cao (1999). Conceptual foundations of quantum field theory. Cambridge UK: Cambridge University Press, p. 179. ISBN 0521602726.
20. ^ 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.
21. ^ Henning Genz. p. 247. ISBN 0738206105.
22. ^ CIPM adopted Recommendation 1 (CI-1983)
23. ^ LM Rozanov & Hablanian, MH (2002). Vacuum technique. London; New York: Taylor & Francis, Figure 3. 1, p. 80. ISBN 041527351X.
24. ^ Zheng, MiMi (2002). Pressure in Outer Space. The Physics Factbook.
25. ^ Gareth Wynn-Williams (1992). The fullness of space. Cambridge UK: Cambridge University Press, p. 38. ISBN 0521426383.
26. ^ Martin J. Rees (1978), “Origin of pregalactic microwave background”, Nature 275: 35-37. Martin John Rees Baron Rees of Ludlow, OM, PRS (born 23 June 1942 in York) is an English cosmologist Nature, in the broadest sense is equivalent to the natural world, physical universe, material world or material universe.
27. ^ This background temperature depends upon the gravitational state of the observer. See Unruh effect. The Unruh effect, discovered in 1976 by Bill Unruh of the University of British Columbia, is the prediction that an accelerating observer will observe
28. ^ U. S. Patent Classification System - Classification Definitions as of June 30, 2000
29. ^ Subclass 310: Communication over free space