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This isn't a duplicate, but I couldn't respond here:

Why is space a vacuum? Also, why doesn't air from the Earth escape into space?

Someone said "space isn't that kind of vacuum" and "it doesn't suck".

If that is so, why not? How can this essentially pure vacuum possibly not be strong enough to overcome gravity and suck off the atmosphere?

Community
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siosophy
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    Space is not a giant vacuum cleaner. – auden Nov 25 '16 at 01:09
  • I do agree that your post is not a duplicate, but you could possibly stress a bit more strongly your point that you are seeking a clarification of an earlier, closely related question. Also, are you sure there is no answer similar, or exactly the same as there is no other answer to your point, buried somewhere in the other questions? –  Nov 25 '16 at 01:10
  • The answer is gravity. – gerrit Nov 25 '16 at 08:54
  • http://en.wikipedia.org/wiki/Atmospheric_escape – Stéphane Rollandin Nov 25 '16 at 17:42

2 Answers2

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Imagine you have two identical rooms with a door between them. One is a vacuum and the other is filled with air. The air molecules are moving in straight lines until they bounce into each other and get deflected.

Now, you open the door between the two rooms. Air molecules near the door that just happen to be traveling towards the door have nothing to bump against. So a lot of them will do so. So over a short period of time, there will be few air molecules near the door. So molecules going into that direction will also be able to keep going in that direction without bumping into anything. This chain reaction keeps happening until there is now about the same number of molecules in the two rooms.

That's what vacuums "sucking" is: molecules that are randomly traveling towards the vacuum having an unimpeded path which frees space for more molecules traveling by random in that direction.

So now look at earth. It's almost the same thing, but you need to add gravity. The molecules aren't going in random directions. Gravity pulls them down and away from the vacuum of space. So instead of many molecules being on their path towards space in the first place, few are. And so our atmosphere doesn't just escape into space.

azani
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  • Nice expanded answer, but no offence meant, gravity does not tend to pull things down, it absolutely pulls them down : ) –  Nov 25 '16 at 01:29
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    Oops. Thanks for catching that. Gravity pulls air molecules down, which means that their trajectories are biased downwards compared to the no-gravity case. They are absolutely pulled down which means they tend to go down... :) – azani Nov 25 '16 at 01:36
  • +1 A good answer in my opinion. As all this stemmed from a badly chosen phrase on a previous question, that's why I didn't want another wild goose chase on gravity :) –  Nov 25 '16 at 01:44
  • Actually, sorry about this, but someone will shout at you over it. You might want to reconsider your wording a bit more at the start. If you have two rooms, one filled with air, one a vacuum, it sorta, kinda negates your argument about slow diffusion. It will be more like a student bar just before closing time. You would have to say at what pressure. To the OP, filled with air might indicate STP conditions. It's your answer tho, sorry for butting in. –  Nov 25 '16 at 02:02
  • I would have though the pressure differential would create equalisation, rather than it just being about 'direction of movement'? It sounds as though you are saying that, if the molecules were still, they wouldn't escape, even without gravity. This makes no sense. Sorry if I misunderstood. – siosophy Nov 25 '16 at 02:12
  • I can't figure out a way to make that adjustment without making the answer overly difficult to understand. If you have edit privileges and can think of something, feel free to go ahead and make a change. :) – azani Nov 25 '16 at 03:49
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    Sisophy, there are different levels of abstractions you can use to think about this. One is pressure and temperature. Another is the movement of the individual molecules. The latter is IMO more intuitive to answer your question but they yield identical answers. But to answer your question about no movement, if there is no movement, there is by definition 0 kinetic energy. Kinetic energy is also known as temperature in this context. So the gas would be at absolute 0. At that point it would freeze into a solid and would not escape. – azani Nov 25 '16 at 04:23
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"Things" go into a vacuum, because of a difference in forces pushing them in, a vacuum does not suck anything into it. But the force from Earth's gravity is pulling things down, so they stay here. Without gravity, the atmosphere would drift away, not be pulled violently into the void.

Let me take the example of the two rooms from the other answer.

By definition, a vacuum contains nothing, absolutely completely nothing and especially nothing that can do something like pulling in molecules. There is nothing there, so no mechanism of any kind by which the vacuum can do anything.

But the other room, that is full of gas, at whatever pressure you like, is obviously different and because the air molecules are present, there is a mechanism by which it can do something.

The air molecules can travel into the empty room because, on average some of them will be heading in that direction anyway. The higher the pressure, means the more of them, just by chance, will head in to the vacuum room.

This process will keep going until both rooms contain the same number of molecules. But the vacuum has done nothing to affect this process, whereas the room with air molecules has done something. The air molecules orginally in it, especially if they are at moving fast ( under pressure) have bounced off each other like mini billiard balls and, just by chance, sent some of them into the vacuum room.

  • Surely pressure equalisation alone would mean that, without gravity, the atmosphere would immediately spread into the void (violently)? Two containers or rooms, one with a vacuum and one with atmosphere - if I joined them, the atmosphere would violently rush into the vacuum no?? – siosophy Nov 25 '16 at 02:10
  • @siosophy If you could switch gravity off, yes, the loss of atmosphere would be very fast. But CountTo10's point is that the atmosphere's molecules are nonetheless simply coasting into space unhindered at a nonincreasing velocity set by their initial momentum. Nothing is "pulling" them off. – Selene Routley Nov 25 '16 at 02:50
  • I have added a bit to my answer which might help you see what I am getting at when I say the vacuum does nothing and can't do anything to pull things into it. A vacuum cleaner does do something, and pulls thing in but it needs an electric motor to do this. –  Nov 25 '16 at 17:20