A vacuum, in the simplest sense, is empty space. However, since a vacuum, either natural
or artificial, is never completely empty, the term needs a modifier. Thus
scientists speak of a hard vacuum (one that approximates the vacuum of outer space), a quantum
vacuum, and so forth. The best human-made vacuums contain less than 100,000 molecules per cubic centimeter.
Evangelista Torricelli is credited with developing the first artificial vacuum in the mercury barometer. In 1650 the German physicist Otto von Guericke invented the first vacuum pump.
| "No point is more central than this, that empty space is not empty.
It is the seat of the most violent physics."
– John A. Wheeler
'Empty space' when seen at the subatomic level is far from empty but instead seethes with energy, the rapid appearance and disappearance of all sorts of elementary particles, and fluctuations in the fabric of spacetime itself. It is the source of zero point energy which is seen as the potential basis for radically new forms of propulsion and "free energy" in the far future.
Vacuum energy density
Vacuum energy density is the amount of energy per unit volume associated with empty space itself. Although the idea of empty space having a nonzero energy associated with it seems a strange one, this idea is at the root of the cosmological constant and inflationary cosmologies.