In this question:
Swimming in a ball of water in space
They seem to have reached the conclusion that the pressure inside a giant ball of water in space is zero.
Quoting the second answer:
As a conclusion, you would be swimming in a bubbling sphere of water, feeling no pressure at all, having a bit less difficulty in moving your arms and legs since the water would be full of bubbles, however I am assuming it would be harder to move around for the very same reason.
Quoting the first answer:
So, the bottom line is swimming in a big ball of water pretty much feels like swimming very slowly in space - until the ball of water gets big enough (2.68 km). Then it just feels like swimming in a giant pool on a distant planet. For practicality, the ball of water doesn't work, but the lunar swimming pool is awesome.
However, I'm still confused about one thing. So far as I know, although the cause of pressure here at the surface of the earth may be the gravitational weight of all the air above us (and the cause of more pressure deeper in the ocean is the weight of all the water above it) at the local level, pressure is really a cause of the random motion of the particles, smashing into the particles around them, and delivering their momentum.
After all, that's why rising temperatures cause the pressure to rise.
Assuming this giant ball of water can hold itself together due to cohesion, wouldn't you still feel the pressure from...well, simply the water molecules themselves, moving randomly in all directions?
Edit:
To help readers, and as a response to the criticism from the person who answered (criticism taken kindly, you're right, I should clarify. Although you could've said it in a nicer way...), this is why I'm asking this question:
I'm trying to understand pressure.
I'm trying to understand whether it originates fundamentally from having to "sustain" a fluid in place due to gravitational forces, or from something intrinsic to the fluid itself (its random motion.)
Although I know that at a local level, the random motion of particles is the cause of pressure, what I'm trying to understand is whether the random motion is caused by something else, or is intrinsic to the fluid itself.
In short, I want to know if there would be pressure without gravity, and this is the best thought experiment I could come up with to explain what I mean by that.
Thanks.
Answer
The misconception that is probably causing your confusion is that
at a local level, the random motion of particles is the cause of pressure,
Random motion of particles is measured by temperature; the higher the temperature, the more intense the random motion.
If we are to talk about causes, the cause of pressure on some wall is first and foremost mutual interaction of the particles and the wall. The fact that the particles move randomly is secondary. True, in gases increase of pressure often goes with increase in this random motion, because the increase of gas pressure can be done only by putting in substantial energy. But in liquids, it is possible to increase the pressure substantially with negligible amount of work and so with negligible change in intensity of this random motion.
Pressure of such liquid is due to force interaction of the particles with walls and each other, not necessarily due to their random motion. It suffices that particles push or pull each other. They do not have to move rapidly. You can have high pressure in very cold water or in ice cold at 1 K.
When pressure of a liquid water is increased, say, by moving a piston in a blocked syringe filled with water, water temperature increase is very small and is usually neglected.
Now to your question - gravity isn't necessary for pressure either. What is necessary to increase pressure is some other body that will squeeze the gas or liquid into smaller volume. On Earth, this body is the Earth with its gravity, but the same pressure is achieved in a closed vessel, such as the International Space Station, simply by making it robust enough to withstand the pressure and pushing in enough amount of gas. There is no effective gravity there, but there is pressure close to 100kPa, due to walls not allowing the gas to escape.
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