Isn’t the universe mostly vacuum?
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Way back when that phrase came about with the Greeks we had just started thinking the earth was round, but also still believed that the stars and planets were mounted up there on spheres of water air and fire.
So we didn’t know about atmospheric pressure really. The force that resulted in a created vacuum disappearing was just thought of as elements rushing to fill the vacuum because they wanted to. We didn’t even come up with bouyancy until about a hundred years after that.
The idea that space was a vacuum didn’t really even come about till like 1800 ish years later when someone got a proper barometer and realized that air pressure tended to decrease with altitude.
Well, yes, which is why this ancient saying isn’t 100% completely true. There are conditions in which vacuums can exist. But, lacking these conditions, and in the presence of some sort of fluid material, contents will equalize between areas where something exists and a vacuum.
Imagine a deep hole in the ground. Things are naturally going to fall into the hole. A vacuum is similar to that hole. Nature and the laws of physics make it so that stuff naturally finds its way into the hole over time.
Even space isn’t a pure vacuum. Like the hole, the laws if physics make it so that there are minute amounts of matter in space.
Space isn’t a pure vacuum, there’s stuff in it, just not a lot of stuff.
But nature still “abhors” it. That’s why, if you went up in space in a spaceship, and you pressure that spaceship with air so that you can breathe in it, and then you opened a window on that spaceship, all the air would rush out into space in a millisecond.
Basically, nature hates difference in air pressure, and always wants to try to equalize them. The bigger the pressure difference, the more it wants to equalize.
I want to point out that the idea of what vacuum is had changed over history, and possibly will continue to do so until physics is finally solved. Vacuum – as an imprecise idea of “nothingness” – is a fine concept in normal discussion, but when you need to get philosophical about it, such as in the above statement, you need to quantify it much more precisely, and that’s when the definition matter.
So while the universe is full of vacuum right now, what is called “vacuum” is very different to people in different era, or even the same people now with different understanding of physics. For example, the widely-believed Unruh effect will tell you that even the idea of vacuum itself is relative: an accelerating observer will see a bunch of particles in supposedly empty space.
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