Vacuum Technology Book, Volume II

1.2.3 Molecular number density

As can be seen from Formula 1-7 and Formula 1-8 pressure is proportional to particle number density. Due to the high number of particles per unit of volume at standard conditions, it follows that at a pressure of 10-12 hPa, for example, 26,500 molecules per cm³ will still be present. This is why it is not possible to speak of a void, or nothingness, even under ultra-high vacuum.

In space it is increasingly ineffective at extremely low pressures to express pressure in the unit Pascal, e. g. less than 10-18 hPa. These pressure ranges can be better expressed by the particle number density, e. g. < 104 molecules per cubic meter in interplanetary space.

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