In vacuum science, vacuum means the state of gas below one atmosphere of pressure in a given space. People usually refer to this rarefied gas state as a vacuum state. Compared with the atmospheric state in which human beings live, this specific vacuum state mainly has the following basic characteristics;
The earth is surrounded by the atmosphere. The troposphere is within 12km of the earth's surface in the mid-latitudes. As the height increases, it is the stratosphere, the mesosphere, the thermosphere (ionosphere), the outer atmosphere, and the interstellar space. The standard atmosphere is a mixture of various gases whose main components are 78.084% nitrogen, 20.948%, 0.934% argon, and so on.
Vacuum is relative to the atmosphere. If the pressure in a container is lower than the surrounding atmospheric pressure, then the gas state in the container is called a vacuum state. If the atmospheric pressure of a certain place is 105pa, and the pressure in the closed container is 1pa, the container is in a vacuum state, and there are 2.7*1014 gas molecules per cubic centimeter. It can be seen that the vacuum space is not empty, but full of gas molecules. With various modern pumping methods, a vacuum degree of 10-11 ~ 10-13pa can be obtained. Even in such a thin gas state, there are still Dozens to hundreds of gas molecules.