What is a vacuum?

Avacuum 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.