system
Given the following post, respond as if you're commenting on Reddit:
human
Is vacuum something that is conserved or that moves from place to place?

Wife and I had a long, weird argument last night about how siphons work. She didn't understand at all, and I only vaguely do (imagine what that argument was like). But at the end of the debate, I was left with a new question.

If I fill a cup with water in a tub, turn it upside down, and raise it out of the water, keeping the rim submerged, the water doesn't fall out of the cup. My understanding is, the water is being pulled down by gravity, but can't fall because there's nothing to take its place *[edit: wrong]*, and it takes a lot of energy to create a vacuum, so the water is simply being held up by the cup *[edit: wrong]*, and is exerting some kind of negative pressure on the inside of the cup (the cup itself is being pulled down by the water, but it's sturdy and doesn't move, so neither does the water). When I make a hole in the cup, air can be pulled in to take its place in the cup, so the water can fall *[edit: wrong]*.

If I did this experiment in a vacuum, I figure something very similar would happen *[edit: this paragraph is 100% wrong, the main thing I learned in the responses below]*. The water would be held in the cup until I made a hole, then it would fall into the tub. If anything, the water will fall a little faster, since it doesn't need to do any work to pull air into the cup through the hole. But then it seems that the vacuum is coming in to fill the space, which sounds wrong since the vacuum isn't a thing that moves.

I'm missing something in all of this, or thinking about it all the wrong way. Vacuum isn't like air, it doesn't rush in through the hole in the cup to take the place of the water, allowing the water to fall. But then why does making a hole in the cup allow the water to fall?

*edit:*

*thanks all, I have really learned some things today.. but now my intuitions regarding how a siphon works have been destroyed.. need to do some studying...*

*edit 2:*

*really, though, how does a siphon work then? why doesn't the water on both sides of the bend fall down, creating a vacuum in-between?*
gpt
So in the first example the air pressure outside the cup, pushing down on the water is greater than the pressure in side the cup. If you have raised the cup up 3" the pressure in the cup is -3" of water from atmospheric. 

If you are working in a perfect vacuum there is no air pressure pushing down on the water outside the cup. So you wouldn't be able to 'lift' the water with the cup because there wouldn't be a pressure difference between inside and outside. 

It's the action of the air pressure pushing down on the water outside the cup that allows you to create the vacuum inside the cup and lift the water.