A very good question!
Yes, you can suck the air out of a container, leaving a vacuum
inside. Molecules are really tiny and notoriously hard to grab a hold
of, so it is very difficult to get rid of all of them (I won't say it's
impossible, though).
It becomes a question of technology, what fraction of the
molecules you can get out. Molecules sometimes like to stick to the
sides of the container, and they become unstuck very slowly, so even if
you succeed in pulling out the air in a container, if you wait a while,
molecules can often be found floating around again. The container
should not be made of molecules that can break loose. Also, the
container cannot leak. Helium atoms can diffuse through a great variety
of materials (glass included) and so it is hard to get a good vacuum
with helium around. Mechanical piston pumps usually leave a little oil
around. Turbine pumps are better, but still cannot get all the
molecules out.
One of the best ways to get a really really good vacuum is to get
a mostly good vacuum with a turbine pump and then freeze the container
with liquid nitrogen (outside) or something even colder. Then the stray
molecules will just stick to the sides (these are sometimes called
"cryopumps").
Because air gets in the way of exciting new physics -- like the
study of antimatter, for example, advances in vacuum technology are
often needed to make advances in physics research. Particle
accelerators require very good vacuums in their beampipes or the beams
will collide with air molecules and make sprays of unwanted garbage in
the detectors. Similarly, if you are storing antimatter, you want as
few matter molecules lying around as possible so you can store the
antimatter longer.
Tom
(published on 10/22/2007)
