Vacuum

Vacuum (from Latin vacuus for “vacant” or “void”) is a region of space devoid of matter. As a first approximation, a vacuum is defined as a region of space in which the pressure is much less than atmospheric pressure. This condition, which is usually obtained in the laboratory, is defined by physicists as partial vacuum, which is opposed to the ideal perfect vacuum, simply called vacuum.

The perfect vacuum condition is not obtainable in the laboratory and has never been observed in nature. Much of intergalactic space is believed to consist of a near-perfect vacuum, with a small number of molecules per cubic meter. Furthermore, even assuming that in a certain region of physical space there were no molecules, the presence of fields (gravitational, electromagnetic, etc.) would still imply the absence of a complete vacuum.

The term “vacuum” often refers to different physical conditions depending on the field considered (e.g. thermodynamics, quantum mechanics, engineering). For example, when we talk about vacuum degree (in the sense of porosity) and vacuum pump the concept of vacuum is different: in the first case it is a part of the space pertaining to the solid itself, but not necessarily devoid of matter (for example it can be filled by a fluid, such as air or water), while in the second case the vacuum consists in a thermodynamic limit which is approached as pressure decreases. Also, when we talk about “vacuum” we are referring to a pressure less than the atmosphere, but not necessarily zero.

Vacuum technology studies the methods and experimental means of achieving very low pressures in vessels of varying sizes by means of vacuum pumps; it is of crucial importance in many technical-scientific fields (mass spectrography, electronic optics, preparation of lamps, electronic tubes, chemical manipulation, etc.). The vacuum level of an environment measures the rarefaction of the air that has been obtained in the environment itself and is given by the value of the pressure in the environment; there is a low and medium vacuum up to pressures of 10-3 torr, a high vacuum for pressures between 10-3 and 10-7 torr, and an ultra-vacuum for pressures lower than 10-7 torr. To measure the degree of vacuum, instruments called vacuometers are used. The barometric or Torricellian vacuum is the empty space at the upper end of the mercury barometer chamber; it is greatly pressurized and is occupied by the vapors that are released from the mercury. An optical vacuum is the state of a solid, liquid, or gaseous mass in which no inhomogeneity can be detected by optical means; an optically empty medium has a refractive index of 1,000,000.

The quantum vacuum

Quantum field theory reveals to us that even an ideal vacuum, with zero measured pressure, is not truly empty. In fact, quantum-mechanical fluctuations are present in the vacuum, making it a bubbling of virtual particle pairs that are constantly being born and annihilated. Such quantum phenomenon could be responsible for the observed value of the cosmological constant.

According to Heisenberg’s uncertainty principle, energy and time, like other quantities such as position and velocity, cannot be measured with infinite accuracy. If empty space had no form of energy, a particle could have zero velocity and energy, with an error equal to zero that would violate Heisenberg principle: this leads to conclude for the existence of quantum fluctuations in empty space, which generate a minimum amount of uncertainty. The vacuum is then thought of as a dynamic equilibrium of matter and antimatter particles in continuous creation and annihilation.

The virtual particles of the quantum vacuum, characterized by the usual wave-particle binomial of quantum mechanics, in an infinitely extended space have any wavelength. On the contrary, in a limited space, for example between two walls, there will exist only particles with wavelengths that are integer submultiples of the distance between the walls, with an energy lower than the one outside. Therefore it will be possible to measure a force-pressure that tends to approach the walls (Casimir effect).

Particles are called virtual because they do not normally produce physical effects; in a limited space, however, there are measurable quantities.

Another reason for the vacuum energy is that the walls of the vacuum chamber emit light in the form of black body radiation: visible light, if they are at a temperature of thousands of degrees, infrared light, if they are colder. This “soup” of photons will be in thermodynamic equilibrium with the walls, and it can be said accordingly that the vacuum has a particular temperature.

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