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vacuum,theoretically, space without matter in it. A perfect vacuum has never been obtained; the best human-generated vacuums contain less than 100,000 gas moleculesmolecule
[New Lat.,=little mass], smallest particle of a compound that has all the chemical properties of that compound. A single atom is usually not referred to as a molecule, and ionic compounds such as common salt are not made up of molecules.
..... Click the link for more information. per cc, compared to about 30 billion billion (30×1018) molecules for air at sea level. The most nearly perfect vacuum exists in intergalactic space, where it is estimated that on the average there is less than one molecule per cubic meter. In ancient times the belief that "nature abhors a vacuum" was held widely and persisted without serious question until the late 16th and early 17th cent., when the experimental observations of Galileo and the Italian physicist Evangelista Torricelli demonstrated its essential fallacy. Torricelli obtained a nearly perfect vacuum (Torricellian vacuum) in his mercury barometerbarometer
, instrument for measuring atmospheric pressure. It was invented in 1643 by the Italian scientist Evangelista Torricelli, who used a column of water in a tube 34 ft (10.4 m) long.
..... Click the link for more information. . A common but incorrect belief is that a vacuum causes "suction." Actually the apparent suction caused by a vacuum is the pressure of the atmosphere tending to rush in and fill the unoccupied space. There are various methods for producing a vacuum, and several different kinds of vacuum pumps have been devised for removing the molecules of gas or vapor from a confined space. In the rotary oil-sealed pump a rotor turning in a cylinder allows gas to enter through an inlet valve from a space to be evacuated and then pushes it through an outlet valve into the atmosphere. In the oil or mercury diffusion pump, gas enters the pump through an inlet and is then swept toward an outlet by heavy, fast-moving oil or mercury vapor molecules. The outlet is connected to a rotary pump that expels the gas into the atmosphere. A cryogenic pump removes gas from a container by condensing the gas molecules on an extremely cold surface in the container. An ion pump consists of a chamber containing a source of electrons that are used to bombard gas molecules from a container to be evacuated. Collisions between the electrons and gas molecules ionize the molecules, causing them to be drawn to, and held by, a collector in the pump. The first vacuum pump was invented by the German physicist Otto von Guerricke in 1650. There are many practical applications of vacuums in industry and scientific research, e.g., in vacuum distillation, vacuum processing of food, in devices such as the vacuum tube, vacuum bottle, and barometer, and in research machines.
the state of a gas at a pressure considerably lower than that of the atmosphere. The concept of a vacuum is usually applied to a gas that fills a limited volume, but it is often also applied to gas in open space—for example, in the cosmos. The behavior of gas in vacuum devices is determined by the relationship between the free-path length λ of the molecules (or atoms) and the dimension d characteristic for the given instrument or process. These dimensions may be, for example, the distance between the walls of a vacuum space, the diameter of a vacuum conduit, or the distance between electrodes in an electrical-vacuum instrument. Vacuums are classified as follows, according to the relationship of λ to d: low vacuum (λ≪d), medium vacuum (λ~d), and high vacuum (λ≫d).
In vacuum installations and instruments of dimension d ~ 10 cm, the pressure range greater than 102 newtons per sq m (N/m2) or 1 mm Hg corresponds to low vacuum; 102 to 10-1 N/m2 (1 to 10-3 mm Hg), to medium vacuum; and less than 0.1 N/m2, to high vacuum. The pressure range less than 10-6 N/m2 (10-8 mm Hg) is called ultrahigh vacuum. However, in pores or canals with diameter d ~ 1 micron, the behavior of gas corresponds to that of a high vacuum beginning at 103 N/m2 (a few dozen mm Hg), but in chambers for the simulation of outer space with dimensions reaching dozens of meters, the boundary between medium and high vacuum is considered to be a pressure of 10-3 N/m2(10-5 mm Hg).
The highest degree of vacuum attainable by existing methods corresponds to pressures of 10-13 to 10-14 N/m2(10-15 to 10-16 mm Hg). Under these circumstances a volume of 1 cu cm contains only a few dozen molecules. The degree of rarefaction attained is determined by the equilibrium between the rate of evacuation of the gas and the rate at which it enters the evacuated space. Entry may occur because of penetration of gas into the vacuum chamber from the outside through microscopic openings (leaks), and also as a result of the emission of gas adsorbed by the walls or dissolved in them.
The properties of a gas under conditions of low vacuum are determined by the frequent collisions of gas molecules with each other, which are accompanied by energy exchange among them. Such a gas has internal friction (viscosity); its flow is subject to the laws of aerodynamics. Phenomena of transfer (electrical conductivity, heat conduction, internal friction, or diffusion) under conditions of low vacuum are characterized by smooth change or constancy of the gradient of the transferred quantity. For example, the temperature of a gas in the space between the “hot” and “cold” walls in a low vacuum changes gradually. In addition, the quantity of heat (heat conduction) or matter (diffusion) transferred does not depend on pressure. If the gas is in two communicating vessels with different temperatures, the pressures in these vessels are equal at equilibrium. When there is passage of a current in a low vacuum, the ionization of gas molecules plays a determining role.
In a high vacuum the properties of a gas are determined by the collisions of its molecules with the walls. Collisions between gas molecules occur rarely and play a secondary role. The movement of molecules between the walls is rectilinear (molecular mode of gas flow). Transfer phenomena are characterized by the appearance on the walls of a jump in the gradient of the transferred quantity—for example, in the total space between the hot and cold walls, approximately half of the molecules have a velocity corresponding to the temperature of the cold wall, and the other half have a velocity corresponding to that of the hot wall (that is, the average temperature of the gas in the total space is the same and is different from that of either the hot or the cold wall). The quantity of transferred heat, matter, and so on is directly proportional to the gas pressure. The gas pressures p1 and p2 in communicating vessels of different absolute temperatures T1 and T2 are determined by the relationship .
The passage of a current through a high vacuum is possible only as a result of the emission of electrons and ions by the electrodes. The ionization of gas molecules here plays a secondary role. It is substantial in cases where the free-path length of charged particles is artificially increased and becomes significantly greater than the distance between electrodes or when there is fluctuating movement of the particles around any electrode.
The properties of a gas in a medium vacuum are intermediate between its properties in high and low vacuums.
The characteristics of ultrahigh vacuums are due not to mutual collisions of particles but to other processes on the surfaces of solid bodies in the vacuum. The surface of any body is always covered with a thin layer of gas, which can be removed by heating (outgassing). The surface properties of bodies change sharply after outgassing: the friction coefficient is greatly increased, in a number of cases the welding of certain materials becomes possible even at room temperature, and so on. The layer of gas that has been removed is gradually restored as a result of the adsorption of the gas molecules bombarding the surface; this is accompanied by a change in its surface properties. The formation of a monomolecular layer of gas is sufficient to change these properties. The time t necessary to form such a layer in a vacuum is inversely proportional to the pressure. When pressure p = 10-4 N/m2 (10-6 mm Hg), t = 1 sec; at other temperatures the time t (in seconds) may be estimated according to the formula t = 10-6p, where p is the pressure in mm Hg, or according to the formula t = 10-4p, where p is the pressure in N/m2. These formulas are correct if every molecule of gas that hits the surface remains on it (if the capture coefficient is equal to 1). In a number of cases the capture coefficient is less than 1, and the time of formation of the monomolecular layer increases accordingly. When p < 10-6 N/m2 (10-8 mm Hg), the formation of a monomolecular layer of gas occurs over a period of more than several minutes. An ultrahigh vacuum is defined as one in which, during the period of observation, there is no essential change in the properties of the surface (which was initially free of gas) as a result of its interaction with gas molecules.
A. M. RODIN
in physics, a medium in which there are no particles of matter and no field. In technology, a medium in which there are “very few” particles is called a vacuum. The fewer particles per unit volume of such an atmosphere, the higher the vacuum. However, a complete vacuum—a medium in which there are no particles at all—is not “nothingness” devoid of all properties. The absence of particles in a physical system does not mean that it is absolutely empty and nothing occurs within it.
The contemporary concept of the vacuum took shape within the framework of the quantum field theory. In the microcosm described in quantum theory, wave-particle duality occurs: all particles (molecules, atoms, elementary particles) have some wave properties and all waves have some particle properties (corpuscles). In quantum field theory, all particles—including the corpuscles of light waves, photons —perform on identical bases as quanta of their corresponding physical fields. The photon is a quantum of the electromagnetic field; the electron and proton are quanta of the electron-positron field; mesons are quanta of the meson, or nuclear, field, and so forth. The physical magnitudes characteristic of the particles are associated with each quantum: mass, energy, momentum, electrical charge, spin, and others. The state of the system and of its physical characteristics is completely determined by the number of particles in it—the quanta—and their individual states. Specifically, in any quantum system there is a vacuum state in which the system contains absolutely no particles (quanta). In this state, the energy of the system assumes the smallest possible value, and the charge, spin, and other quantum numbers that characterize the system equal zero. These facts are intuitively clear: inasmuch as there are no material carriers of physical properties in the vacuum state, it would seem that in such a state the values of all physical magnitudes should equal zero. But in quantum theory the uncertainty principle operates, according to which only a portion of the physical magnitudes of the system may simultaneously have precise values; the remaining magnitudes prove to be indeterminate. (Thus, precise assignment of the pulse of a particle entails complete indeterminacy of its coordinates.) Therefore, in any quantum system all the physical magnitudes cannot simultaneously be exactly equal to zero.
For example, among the magnitudes that cannot be assigned exactly and simultaneously are the number of photons and the intensity of the electrical (or magnetic) field. Strict recording of the number of photons leads to scattering (fluctuations) in the magnitude of the intensity of the electrical field in relation to some average value (and vice versa). If the number of photons in the system is precisely equal to zero (in the vacuum state of the electromagnetic field), then the intensity of the electrical field has no definite value: the field will undergo fluctuations all the time, although the average (observed) value of intensity will be equal to zero. All the other physical fields—the electron-positron field, the meson field, and so forth—are also subject to such fluctuations.
In quantum field theory fluctuations are interpreted as the production and destruction of virtual particles (that is, particles that are continually produced and immediately destroyed), or virtual quanta of a given field. The presence of fluctuations does not affect the values of the total electrical charge, the spin, and other characteristics of the system, which, as already stated, are equal to zero in the vacuum state. However, virtual particles participate in interactions exactly as real particles do. For example, a virtual photon is capable of producing a virtual electron-positron pair; this is analogous to the production of a real electron-positron pair by a real photon. Because of fluctuations, a vacuum acquires special characteristics that are manifested in observed effects; consequently, the vacuum state follows all the laws that apply to “real” physical states.
Let us examine a system that consists only of one real electron. There are no real photons in such a system, but fluctuations of the photon vacuum (this term means the absence of real photons) lead to the production of “clouds” of virtual photons near the electron and electron-positron pairs following the photons. Such pairs appear in the same way as bound charges in a dielectric: under the action of the coulomb field of the real electron they are polarized, and they screen (that is, effectively decrease) the charge of the electron. By analogy with the dielectric, the effect of screening the electrical charge of virtual particles is called vacuum polarization.
As a result of vacuum polarization, the electrical field of a charged particle is slightly different from its coulomb field at short distances from the particle. Thus, for example, the energy levels of electrons closest to the nucleus of an atom are displaced. Vacuum polarization also influences the behavior of charged particles in a magnetic field. The magnetic momentum of the particle, which characterizes this behavior, varies in total from its ‘’normal” value, which is determined by the mass and spin of the particle. Corrections of the energy levels as well as of the magnetic momentum amount to fractions of a percent, and theoretically calculated values agree with very great precision with experimental measurements.
V. P. PAVLOV