# Is the “pressure” as we know a kind of “instantaneous action at a distance”?

This may be a very simple question, but I still dont know how that work in the essence (quantic/atomic level)

Consider the image bellow:

How the "pressure (air atoms) " known that in the other side of the barrier there is nothing and pull the bairrer to the "low pressure". There is some sort of instantanious action at a distance to the particles knowing where are holes to go ? Where does the pressure comes from ?

In a practical way, how the water in the ocean know that there is air inside the iron ship to pull it upwards out of water ?

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The air molecules collide with the barrier atoms. While colliding, they have to change direction, because they obviously can't go further, thus the barrier atoms exert a force to the air molecules, and vice versa. On the vacuum side there are no collisions so there is only a force towards the vacuum. So the pressure comes from those collisions. For your second question, the water doesn't know that there is air in the ship, but the water also doesn't "pull" the iron ship out of the water. It is just the air that is pulling the ship less inwards the water, because air weights less than water. – Rayman Aug 23 '13 at 17:55
Simply put, that makes perfect sense at all. Thanks. – Roger Aug 23 '13 at 18:08
Your query follows naturally from the way air pressure is taught in school. They teach you that if you have lots of air here, and little here, then it will want to even itself out(often using the analogy of many people walking through a door spread out as they leave). While this is correct, it doesn't explain why a bottle with no air in it gets crushed. The air can't possibly know how much air is in the bottle! – Cruncher Dec 9 '13 at 19:58

It has nothing to do with action at a distance the way you talk about it.

There is some sort of instantanious action at a distance to the particles knowing where are holes to go ?

Not quite.

Consider that case of the barrier with vacuum inside. From one side, the air atoms are constantly colliding with the surface, and exerting a force on the barrier inward. From the other side, there are no atoms(or very few atoms, in case of low pressure) colliding with the barrier. So no force is exerted on the barrier outward.

This leads to a net inward force on the barrier due to air pressure, which in some cases leads to the barrier collapsing inward(unless it is strong enough to exert a force of itself to counter this inward force).

A similar thing happens if you have a hole in this barrier. An air atom moving towards the hole goes right in, because there is nothing on the other side of the hole stopping it. If there was air inside the barrier, the air atom moving towards the hole, would then collide with some air from the other side, and reflect back from the hole.
Although some atoms will go through, and even come out of the hole, the net effect will look like there is no exchange of air through the hole.

In a practical way, how the water in the ocean know that there is air inside the iron ship to pull it upwards out of water ?

The buoyancy of the ship has nothing to do with the pressure difference between the air and water, it is in fact due to difference in apparent density of the ship.

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Thanks for the response, @Rayman also explained very well my second buoyancy question. Thanks ! :) – Roger Aug 23 '13 at 18:10
something even cooler happens with the hole. Since the air from the outside is free to fall right into the hole, the air just outside the hole will be of lower pressure than the rest of the air(which is bouncing off eachother, rather than just disappearing into a ball). Causing all of the air around it to funnel to the hole, creating the "sucking" that you would expect. – Cruncher Dec 9 '13 at 19:51