A vacuum is a space devoid of matter. The word “vacuum” comes from the Latin vacuus for “empty” or “void”. An approximate definition in classical physics is that a vacuum is a region with a gaseous pressure much less than atmospheric pressure.
In modern physics, a perfect vacuum is one without any particles, even ones as small as atoms and photons. It has been demonstrated that there are no particles in such a vacuum by means of the Casimir effect and other experiments. In everyday usage, however, the term “vacuum” usually refers to an approximation to this ideal which is good enough for most practical purposes, such as in electrical engineering and in outer space applications.
Vacuum may also refer to:
* Absolute zero, the lowest possible temperature
* Outer space
* Perfect gas
* Torricelli’s law
Ideally, an evacuated volume would contain no particles at all. However, in practice it is impossible to remove them all due to their random thermal motion even at absolute zero temperature; thus there will always be some (residual) air molecules left in an evacuation by knowledge of statistical mechanics. Other effects can also lead to a residual population of particles such as surface tension effects (the Leidenfrost effect) or streaming emission from hot bodies (e.g., the glow of incandescence).