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Q & A: Making a vacuum

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Most recent answer: 10/22/2007
Q:
I was thought in school that vacuum cannot be created. Why not? how about sucking all the airs out frm a closed container? If vacuum canot be created then what about the magnetic bottles used to store antimatter?
- Anonymous
Singapore
A:
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)

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