As defined Vacuum is a space entirely devoid of matterI guess that is a reasonable theoretical definition.
In practice, it is more useful to think of a vacuum in terms of the pressure of residual gas inside the chamber, it's temperature, and it's chemical/ionic composition.
- Whether you consider this a true vacuum depends on the application
- To the human body, the effects of exposure to the atmosphere of Mars (about 1% of the air pressure at Earth's surface) would be almost indistinguishable from the effects of exposure to interstellar space.
- If you are running the Large Hadron Collider, the air pressure of Mars would immediately block the proton beam.
- There are a variety of applications that require a vacuum between the atmosphere of Mars and the vacuum in the LHC
- There have been experiments put into space that delivered a better vacuum than that in the LHC. The simplest of these was a simple metal plate, orbiting the Earth at around 28,000km/h, blocking any gas molecules that might reach the experimental bay. (Of course, you need to make the metal plate out of substances that won't boil off atoms or molecules into the space behind the speeding plate...)
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