In the ordinary world, we use the term "vacuum" to describe spaces completely devoid of air. But it would be more precise to think of a vacuum not as a space with no air, but rather as a space lacking any atoms or molecules at all, a completely empty space. This is no easy state to achieve, but near perfect vacuums do exist in outer space at points where interstellar gas is very thinly spread.

In the quantum world, however, "nothing" has a different meaning. For example, if you bombard a supposed true vacuum with powerful gamma rays, electron-positron pairs appear (pair production), and, even after the bombardment stops, there is "something" where there was supposed to be nothing. The pairs immediately collide with each other and disappear (pair annihilation), releasing gamma rays when they do so.

Positrons show almost exactly the same properties as electrons, except for the fact that they carry a positive charge. This kind of particle is known as antimatter, positrons being antimatter electrons.

Seen from the quantum world perspective, a vacuum is not a totally empty space, but one in a state of constant fluctuation, with electrons and positrons appearing and disappearing all the time. Because electrons and positrons are neutral when combined, the space they fill appears to be empty, but a quantum vacuum in fact hums with the drama of constant pair production and annihilation.