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A vacuum is the absence of matter (molecules, atoms...) in a volume of space. Normally we think of vacuum cleaners, which are actually pumps that create a partial vacuum by moving air.

The partial vacuum can be measured in pascal (Pa) or torr, or as a percentage of atmospheric pressure using the bar or barometer scale.

Degrees of vacuum

Creating a vacuum Creating a partial vacuum is called sucking. Infants are born with a sucking reflex, by which they instinctively know to suck for food. More generally drinking is partly done by sucking, and especially when drinking through a straw.

Breathing in involves the expansion of the lungs, with the result that air is sucked in.

The term "sucking" is also used for fellatio. "It sucks" means it is not pleasurable or functional at all.

When creating a partial vacuum, the matter in the volume being evacuated flows differently at different pressures based on the laws of fluid mechanics. Initally 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 know 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, we have entered a state called molecular flow, where the directionality of each molecule is basically random. Three basic ways to remove the remaining gas are by 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 (ion pumping) or using a specialized machine called a turbomolecular pump or diffusion pump.

At lower pressures, outgassing of the vacuum vessel occurs over time, so that the generation of an instantaneous high vacuum in a hermetically sealed container does not ensure that an adequately low pressure will continue unless outgassing has been accounted for. Heating the vacuum vessel will accelerate outgassing. 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 a vacuum isn't really empty.

One reason is that the walls of the vacuum chamber will inevitably emit light: visible light if they are at a temperature or thousands of degrees, or perhaps infrared light if they are cooler. This soup of photons will be in thermodynamic equilibrium with the walls, and we may therefore speak of a vacuum that is at a particular temperature.

More fundamentally, there are quantum-mechanical fluctuations on space-time (refered to commonly as the quantum flux) in the vacuum medium during a Spacetime interval. This flux may be responsible for the observed value of the cosmological constant. This flux creates virtual particles.

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