10_At the end .. what is vacuum?

Vacuum is defined on encyclopaedia as “absence of matter”, “a space empty of matter”, “a space in which the pressure is significantly lower than atmospheric pressure”, “a state of emptiness”, “a void”, “a state of being insulated and sealed off from external or environmental influences”.
A less philosophical and more mathematical definition however refers Vacuum as “whatever volume where the pressure is lower than the atmospheric one”.

In a volume, particles are in constant motion. They hit the walls of the container and exert a force on its surface area, which is called "pressure". The force per unit area of the vessel is measurable and it is called pressure. Therefore, a pressure measurement will just give the number and intensity of particle impacts on a unit of surface area.

The highest pressures ever obtained in a laboratory is the respectable number of 1030 bar, a pressure larger than the one obtained before a supernova explosion. It was produced in a storage ring at CERN (Centre Européenne pour la Recherche Nucléaire) at Geneva, Switzerland during head-on collisions of two fast lead nuclei. Compared with this value, the maximum static pressure is of the order of 106 bar, reached in diamond Anvil cells.

On the other hand, the lowest pressure or, in other words the highest vacuum generated in laboratory, is of the order of 10-17 bar, which corresponds to just a few hundred particles in one cm3 and which was also obtained at CERN. One should compare this with a number of ~ 1019 particles per cm3 at atmospheric pressure and room temperature.

Therefore, between the highest and lowest pressure there are 47 orders of magnitude in between. But even the best vacuum obtained on earth is a high-pressure area compared with the almost total emptiness between stars in space. Accordingly, besides pressure vacuum is characterised by the density of particles. The interstellar particle density in the Milky Way, for instance, consisting of gas, plasma and dust, is only ~ 5 x 104 particles per m3. Between Galaxies one has only one particle or at most a few of them per m3. If one would distribute homogeneously the total matter of the universe in space, one would still have an extremely low particle density of only 3 particles per m3.

The enormous range of about 60 orders of magnitude between the maximum density calculated for a black hole and almost totally empty regions in space.

Nevertheless, even the almost perfect vacuum of interstellar space is not empty at all. There is electromagnetic radiation everywhere or superstrong fields, which are even able to generate new particles. Even if we would succeed to construct an empty volume, totally shielded against outside radiation, cooled to absolute zero temperature to reduce radiation from its walls, there would still be zero-point energy radiation emitted from the wall particles which are never totally at rest due to Heisenberg´s uncertainty principle.
Absolute vacuum is unstable: at the end “Vacuum is more full than the plenum”.