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 classical vacuum[9][10] or free space in this context. The term partial vacuum is used to refer to the imperfect vacua 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[11] is replaced by the quantum vacuum, separating "free space" still further from the real vacuum – quantum vacuum or the vacuum state is not empty.[12] An approximate meaning is as follows:[13]
Quantum vacuum describes a region devoid of real particles in its lowest energy state.
The quantum vacuum is "by no means a simple empty space,"[14] and again: "it is a mistake to think of any physical vacuum as some absolutely empty void."[15] 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.[16] One measurable result of these ephemeral occurrences is the Casimir effect.[17][18] Other examples are spontaneous emission[19][20][21] and the Lamb shift.[22] 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 [23][24] 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.[25] The determined reader can explore various nuances of the quantum vacuum in Saunders.[26] A more recent treatment is Genz. [27]
At present, even the meaning of the quantum vacuum state is not settled. To quote GE Brown:[28]:
| “ | In eighteen-century Newtonian mechanics, the three-body problem was insoluble. With the birth of general relativity around 1910 and quantum electrodynamics in 1930, the two- and one-body problems became insoluble. And within modern quantum field theory, the problem of zero bodies (vacuum) is insoluble. … GE Brown quoted by RD Mattuck | ” |
For example, what constitutes a "particle" depends on the gravitational state of the observer. See the discussion of vacuum in Unruh effect.[29][30] 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 Woit[31] and the articles: Higgs mechanism and QCD vacuum.
| Why doesn't the zero-point energy of vacuum cause a large cosmological constant? What cancels it out? |  |
The discrepancies between free space and the quantum vacuum are predicted to be very small, and 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.[32]
In short, realization of the ideal of "free space" is not just a matter of achieving low pressure, as the term partial vacuum suggests. In fact, "free space" is an abstraction from nature, a baseline or reference state, that is unattainable in practice
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