There is really no such thing as a perfect vacuum – just varying degrees of very low pressure. Even in deep interstellar space, the closest there is to a perfect vacuum, there are still a few hydrogen molecules to be found per cubic meter.
The mass and movement of these hydrogen molecules creates pressure.
Back here on Earth we have decided to call the atmospheric pressure we feel when we are at sea level as a “zero”. But it is not zero. On our pressure gauges we note atmospheric pressure as 0 psig (the “g” signifying gauge pressure). We have chosen to call pressures less than the pressure we feel at sea level, as “vacuum.” Thus, if you live in the Rocky Mountains in Colorado or in the Swiss Alps, you are living in a partial vacuum. Let me explain.
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