What is a vacuum?

A vacuum is a space that has less gaseous pressure than the standard atmospheric pressure at sea level on Earth. A partial vacuum can be easily created by simply pumping air out of a container. If the container is not sealed, though, the air will be replaced fairly quickly. In everyday life, vacuums are used in light bulbs, cathode ray tubes, cleaning appliances, and to package, protect and preserve a range of foodstuff s. Creating a vacuum drove the piston mechanism in the Newcomen steam engine and was also used in the braking systems of trains. 

Household vacuum cleaners work by sucking in air, which creates a lower air pressure than that outside the device. To restore the partial vacuum the outside pressure forces air, and with it dirt/dust etc, into the appliance. The purest vacuums can be found in outer space. Between galaxies, the vacuum density drops to ~0.001 atoms per cubic centimetre, while in the void between stars in the Milky Way, the vacuum is ~0.1-1 atoms per cubic centimetre. This is in contrast to a vacuum cleaner that produces a vacuum of around 1019 molecules per cubic centimetre, though highly sophisticated extreme-high vacuum (also known as XHV) lab chambers have managed to achieve a vacuum of fewer than 1,000 molecules per cubic centimetre.

 Whether man-made or natural, there is no such thing as a perfect vacuum. Even in a virtually complete vacuum, physicists have discovered the presence of quantum fl uctuations and vacuum energy. See opposite for more on fi re and sound work inside a vacuum.