Vacuum
The article on the vacuum cleaner is located elsewhere.
In physics, a vacuum is the absence of matter in a volume of space. A partial vacuum is expressed in units of pressure. The SI unit of pressure is the pascal (abbreviated to Pa in usage). It can also be expressed using the torr, using the barometer scale, or as a percentage of atmospheric pressure using the bar. The antithesis of a vacuum, which is technically unachievable, is called a plenum. A perfect vacuum is not obtainable in a laboratory; much of outer space is supposed to consist of an almost perfect vacuum, with a small number of molecules per cubic metre.
Degrees of vacuum
- Atmospheric pressure = 200kPa
- Vacuum cleaner = approximately 40 kPa
- Mechanical vacuum pump = approximately 1.3 Pa
- Near earth outer space = approximately 130 μPa
- Pressure on the Moon = approximately 1.3 μPa
- Interstellar space = approximately 13 nPa
- Cryopumped MBE chamber = 133 nPa to 1.3 nPa
Creating a vacuum
When creating a partial vacuum, the matter in the volume being evacuated flows differently at different pressures based on the laws of fluid dynamics. Initially a vacuum pump can be used to remove the material, as the molecules are interacting with each other and will push on their neighboring molecules in what is known as viscous flow. When the distance between the molecules increases, the molecules interact with the walls of the chamber more often than the other molecules, and compression pumping is no longer effective.
At this stage, the system has entered a state called molecular flow, where the velocity of each molecule is approximately random. Methods to remove the remaining gas include:
- Converting the molecules of gas to their solid phase by freezing them, called cryopumping or cryotrapping
- Converting them to solids by electrically combining them with other materials, called ion pumping
- Use of another specialized pump. Examples are turbomolecular pump or diffusion pump.
At extremely low pressures, outgassing of the vacuum vessel occurs over time. Even if a high vacuum is generated in a hermetically sealed container, there is no guarantee that an adequately low pressure will continue unless outgassing has been accounted for. Outgassing is generally worse at higher temperatures. Even materials which are not naively considered absorbent will outgas. Water vapor is a primary outgas component, even in hard metal vessels (such as stainless steel or titanium). Outgassing can be reduced by desiccation prior to vacuum pumping. Vessels lined with a highly gas-permeable material such as palladium (which is a high-capacity hydrogen sponge) create special outgassing problems.
The quantum-mechanical vacuum
Quantum physics reveals that even an ideal vacuum, with a measured pressure of zero torr, isn't really empty. One reason is that the walls of the vacuum chamber emit light in the form of black-body radiation: visible light if they are at a temperature of thousands of degrees, infrared light if they are cooler. This soup of photons will be in thermodynamic equilibrium with the walls, and the vacuum can consequently be said to have a particular temperature. More fundamentally, there are quantum-mechanical fluctuations in the vacuum. This may be responsible for the observed value of the cosmological constant.