Expert: Steve Nelson - 1/4/2008

Question
What is the extent of the vacuum in space, to what degree has it generally been measured? I understand that there are many particles in space (such as earth) and that without particles there isn't an opposite to a vacuum (or indeed any 'pressure') but in a non-populated area what is the level of the vacuum? Is this a trick question as all there is to compare a non-vacuumed environment is the vacuumed environment in space, so therefore non-particled 'space' becomes the 'absolute zero'?

Answer
The vacuum of space varies greatly.  It goes down to about 4 particles per cubic meter in intergalactic space.  The best vacuum ever achieved in the laboratory, in contrast, is 350 molecules per cubic *centimeter*.  At what's considered the "edge of space" (about 100 km above the surface) on Earth, the vacuum pressure is approximately 10^-4 torr, or about 3.2 trillion molecules per cubic centimeter.  Optical constraints have been placed on the amount of matter between stars by measurement of light's extinction.

Even "empty space" however, is not truly empty.  There are particle-antiparticle pairs appearing and disappearing in empty space all the time.  See references on the Casimir force for more details.