In modern day usage vacuum is considered to exist in an enclosed space or chamber, when the pressure of gaseous environment is lower than atmospheric pressure (760 Torr or 101 kPa), or has been reduced as much as necessary to prevent the influence of some gas on a process being carried out in that space.
Thus a vacuum of 26 inHg is equivalent to a pressure of (29.92 - 26) or 3.92 inHg.
Vacuum fluctuations may also be related to the so-called cosmological constant in the theory of gravitation, if indeed this entity were to be observed in nature on a macroscopic scale.
Due to the nonvanishing average photon population of the squeezed vacuumstate, finite corrections to the scattering matrix are obtained.
The squeezed vacuum is a fascinating nonclassical state of the quantized electromagnetic field [1].
The dependence of the scattering matrix on the vacuumstate of the theory and on exterior parameters has been studied previously for the thermal equilibrium [2], in cavity-quantum electrodynamics [3], on fractal space-time support [4] and, to some extent, in the presence of strong electromagnetic fields [5,6].
, is the element of the 
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