Flat-earthers oftentimes have a hard time understanding why Earth's atmosphere is "next to a vacuum" and doesn't get "sucked" by the vacuum. I explain the reason here.
Oftentimes these same flat-earthers will use vacuum chambers as examples of how vacuum "sucks" air into it: The walls of the vacuum chamber are under enormous pressure, and if it's opened, it "sucks" air in very rapidly.
This is a complete misunderstanding of what's happening.
The actual reason why vacuum chambers experience an enormous amount of pressure by the surrounding air, and why the air rushes in at enormous speeds if it's opened, is very closely related to what I explained in that other article.
To help understand, consider the atmosphere as if it were water. This is, in fact, not far-fetched because both liquids and gases behave in a fluid manner, quite similarly to each other. Their densities may be quite different, and this density behaves a bit differently when in gaseous form (gases are compressible while liquids essentially are not), but in essence both behave as fluids.
So consider this: Suppose you put a watertight container that's full of air underwater: It, too, will experience an enormous amount of inwards pressure from the water, and if it's opened, water will rush in at great speed.
With air, it's the same thing: The atmosphere is essentially a huge layer of fluid (in this case in gaseous form) laying on top of Earth's surface, and if there's a vacuum chamber, the weight of this gaseous fluid will press against the walls (because there's nothing inside pressing back in the opposite direction). And if the vacuum chamber is opened, the air will rush in, exactly as if it were a liquid.
And, indeed, the amount of pressure that air exerts on the surface of a vacuum chamber depends, quite literally, on air pressure (and that's exactly where that concept comes from!) If you take the vacuum chamber very high in the atmosphere, air pressure high up there is lower and lower the higher you are, and thus the amount of pressure against the surface of the vacuum chamber gets lower and lower. Go high above enough, when air pressure drops to almost zero, and there will be very little pressure against the surface of the chamber. Opening the chamber in this situation will do almost nothing.
In the "sealed container full of air under water" case the air inside does not "suck" water: Water rushes in if the container is inside the water. In the same way a vacuum chamber does not "suck" air into it: Instead, air rushes in if it's opened, just like a liquid would.
"But", might some more attentive flat-earther object, "if you put a watertight container full of air underwater, it will experience an enormous amount of buoyancy trying to push it to the surface. Vacuum chambers don't experience that. It's not the same situation at all."
Actually that's incorrect: Vacuum chambers do experience a buoyant force upwards. The thing is that the difference in air pressure between the inside and outside is so small that the weight of the vacuum chamber is more than enough to overcome it, and keep it on the ground. It's no different than if the watertight container full of air was made of extremely thick and heavy metal: If it's heavy enough, it will sink in the water even if it contains air inside.
That just goes to show how much stronger water pressure is compared to air pressure. And, it indeed is enormous. (That's because water weighs enormously more by unit of volume than the atmosphere does. That's why submersibles need incredibly strong walls to withstand the water pressure. In fact, much stronger than vacuum chambers do. Perhaps a bit surprisingly, small vacuum chambers don't require a huge amount of material strength.)
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