# What is a Vacuum

Is empty space really empty?

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 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 and protect food. 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 “10 on 19” molecules per cubic centimetre, though sophisticated extreme-high vacuum (XHV) lab chambers have achieved a vacuum of fewer than 1,000 molecules per cubic centimetre.

Whether manmade or natural, there is no such thing as a perfect vacuum. Even in a virtually complete vacuum, physicists have discovered the presence of quantum fluctuations and vacuum energy.

## Characteristics of a vacuum

The amount of air pressure on our planet reduces the higher you travel in the atmosphere. At sea level, we experience one kilogram per square centimetre (14.7 pounds per square inch) of pressure produced by air molecules.

At 15 kilometres (ten miles) air pressure reduces to 0.12 kilograms per square centimetre (1.6 pounds per square inch), and in outer space, there’s no air pressure. This means it’s easier to obtain a vacuum the further you travel from Earth’s surface.

Another characteristic of a vacuum is that it does not transfer heat by convection nor conduction; this property inspired vacuum flasks, which use a vacuum sandwiched between an outer and an inner wall to keep liquids hot/cool for extended periods.