Vacuum

Vacuum (from Latin vacuum – void) is an important concept in classical physics. There are three meanings of the term in physics, which differ in engineering, space exploration, and physics.
In engineering, vacuum is the state of a gas at a pressure below the standard atmospheric pressure of 101,325 pascals. Outer space also contains vacuum, where at an altitude of 50 thousand kilometers above the Earth’s surface, the pressure is very low – about 1*10-19 pascals, which corresponds to a concentration of about four molecules in 1 cm3. This state is already close to the space vacuum. In near-Earth interplanetary space, the concentration of atoms is a few pieces in 1 cm3 and they are mostly ionized, as they are components of the solar wind. In interstellar space, outside gas clouds, the concentration of atoms is an order of magnitude smaller. Inside gas clouds, the concentration is about the same as in interplanetary space. Given the existence of cosmic dust, the cosmic vacuum is not a void at all.
The physical vacuum is a space in which there are no particles of matter and the energy state has reached its lower limit, at which the average number of quanta of physical fields is zero. However, virtual elementary particles that appear and disappear have been found in the vacuum, and they influence physical processes. For example, polarization of electromagnetic radiation on these particles has been recorded.