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Recently I read some article about vacuum and negative pressure, and the reasoning was like this:

Assume you have a vacuum in a cylinder, and then you move the top of the cylinder (like a piston) up to create negative pressure.

That made me wonder: Does a real vacuum exist actually? I mean: I you pump 1 liter of air from a cylinder containing 1 liter of air, do you have the perfect vacuum? Or does it mean that after having extracted half a liter of air, then there is only half a liter air air in the one liter container? If so, extracting another half a liter of "air" you actually extracted a quarter liter of air (at the average)? So there would be increasingly less air in the cylinder, but you will never reach the perfect vacuum while extracting only a finite amount of "air".

And if you don't have a perfect vacuum, the pressure won't be zero, so if moving up the piston, it would just reduce the remaining pressure, but never create a negative pressure. Likewise even in the depth of space there would not be a perfect vacuum, as a few gas molecules or atoms are surely around somewhere.

Is that true?

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    $\begingroup$ Does this answer your question? Why is it impossible to have a perfect vacuum? $\endgroup$
    – John Rennie
    Commented May 12, 2023 at 6:59
  • $\begingroup$ There can never be a perfect vacuum in an arbitrarily large volume, but for sufficiently low pressure there is always some finite volume in which there are no atoms or molecules. That, of course, is only a human construct, anyway, as the fields that cause the existence of atoms and molecules permeate all of spacetime, i.e. the physical vacuum is never "empty". At most it is not excited above its ground state. $\endgroup$
    – FlatterMann
    Commented May 12, 2023 at 8:10
  • $\begingroup$ @JohnRennie The vacuum is only one part of the question; the other part is how increasing the volume/capacity can make a small, but positive pressure negative. Can there be less than nothing (anti-matter excepted)? $\endgroup$
    – U. Windl
    Commented May 12, 2023 at 8:57
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    $\begingroup$ @U.Windl Moving the piston up cannot turn a small positive pressure into a negative pressure. We get a negative pressure only when the "stuff" in the piston has an exotic equation of state. In the case of spacetime this happens because the amount of dark energy in a volume of spacetime increases when you expand that volume and decreases when you compress it. Ordinary matter like a gas decreases in energy (gets colder) when you expand it and increases in energy (gets hotter) when you compress it. If you are interested in understanding the physics behind this you might ask another question. $\endgroup$
    – John Rennie
    Commented May 12, 2023 at 10:13
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    $\begingroup$ See Have negative pressures any physical meaning? for more on this. $\endgroup$
    – John Rennie
    Commented May 12, 2023 at 10:15

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