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everybody knows that if you have an

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alien in your spaceship the best course

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of action is to blow a hole in a window

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and the alien will get sucked out

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problem solved so we all know if you've

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got some air under normal pressure and

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there's a vacuum next to it and the air

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will rush over to fill the vacuum but

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the Earth's atmosphere is in the normal

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pressure and there's nothing between the

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atmosphere in the vacuum of space so why

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doesn't all the air on Earth get sucked

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into space the answer of course is

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gravity gravity is holding the

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atmosphere down but how does that work

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exactly how does gravity keep the air in

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my room with a constant pressure and not

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rushing off into space well to

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understand how that it works you could

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have think about what the atmosphere

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actually is the atmosphere of the air is

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a gas a mixture most of nitrogen and

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oxygen like all gases it's just a bunch

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of molecules flying around now we're

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used to thinking of the atmosphere in

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terms of air pressure but to understand

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that you have to consider the motion of

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individual molecules let's do some

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thought experiments in a simplified

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universe let's say the universe contain

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nothing at all except for some gas

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molecules

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tiny little objects they behave like

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other objects were in space the gas

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molecules would just move in straight

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lines not changing direction unless they

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happen to collide with each other the

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universe would just continue like this

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gas molecules moving around eventually

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we're closer together on formal nebulae

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and stars that's another story

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so suppose we had apply it into the mix

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how will that change things BAM suddenly

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our universe is a strong gravity well

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all the nearby gas molecules are

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attracted down to the planet so they

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tend to move in that direction now

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adding a planet like that has created a

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whole bunch of potential energy so

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things speed up and slosh around for a

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bit

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but eventually they settle there let's

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what do we find the planet has acquired

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an atmosphere not only that it's one

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where the density of the atmosphere

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decreases as you get higher most of the

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molecules near the planet's surface are

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bouncing around colliding with each

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other and colliding with the objects

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around them

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it's those collisions that make air

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pressure but the ones off in space such

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as following ballistic trajectories and

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because there's so few molecules there's

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far fewer collisions and so very little

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pressure of any so we can see there's no

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reason for the atmosphere to fly off

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into space all of the atmosphere has

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been pulled down towards the planet the

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only reason why air rushes to fill up

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vacuum down earth is because the

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molecules are bouncing around so much

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they just fill up any empty space so

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look at this in more detail it's the

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bottom of the screen is the surface of

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the planet we've dropped a bunch of gas

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in here again and it's settling down

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let's speed up again until it reaches

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some kind of equilibrium just lets

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again we see a pressure gradient with a

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vacuum at the top and sea level at the

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bottom you can also see there's no

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compelling reason for the molecules at

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the top of the atmosphere to whiz off

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into space some of them do but gravity

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keeps most of them returning to Earth

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another thing you can see is that there

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really isn't an edge to the atmosphere

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it just gets thinner and thinner and the

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molecules get further and further apart

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he never really ends even in the space

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between galaxies the hardest vacuum