Space doesn’t suck - the earth blows.
[QUOTE=Ludovic]
I’ve been meaning to ask something like this. What would happen if we built this, and then pumped out the air from the bottom? Would it refill from the scant atmosphere in semi-outer space to the same pressure as before the pumping, or would it only refill to the pressure in the outer atmosphere?
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The straw would collapse from the external pressure, like trying too hard to suck a thickshake up a weak straw.
Si
If a 100-mile cylinder was strong enough to remain standing, there’s a good chance that it’s strong enough to withstand 1 atm of air pressure.
Interestingly, one of the possibilities for building a supertall structure (tens of kilometers) is to make it inflatable. Sort of the opposite approach to what’s proposed here.
This assumes that the top of the straw is below the top of the atmosphere. If the top of the straw is well above the “top” of the atmosphere, then once it’s been evacuated, it should remain so indefinitely. I realize that the “top” of the atmosphere is a fuzzy thing, but if for example the straw is a couple thousand miles tall, I would expect very few atoms of earth’s atmosphere to be lofted in over the top edge of the straw.
The question did refer to there being some atmosphere at the top so I went with that assumption.
The thing is, there really isn’t a top of the atmosphere. It just sort of gradually diminishes.
What if you build a giant straw on an infinite plane on an infinite treadmill?
Sorry if I wasn’t clear. When I said “some atmosphere at the top” I meant some atmosphere at the top of the tube.
Right, that’s related to what I was saying. The pipe can’t extend to the “top of the atmosphere”, since there is no well-defined top, so there will still be some traces to leak in at the top of the pipe.
That’s true, but nevertheless the pressure at 100 km would be called a hard vacuum by any scientist. Even intergalactic space has some amount of free gas, but no one calls that part of an atmosphere. So although we can’t draw a hard boundary, we can name an altitude which is effectively outside the atmosphere. Traditionally, 100 km marks the boundary between the atmosphere and space.
So suppose the tube is 100 km tall, 1 m in diameter, and initially a hard vacuum. Can anyone here estimate how quickly the tube will come up to atmospheric pressure? Say within 0.1%?
That would require an integral which I don’t have time to do now, but I can set an approximate lower bound.
Surface pressure is about 101.325 kPa, or 101325 kg/(ms^2). Multiply by the area of the tube (0.785375 m^2) to get 79578 kgm/(s^2), and then divide by 9.8 m/s^2 to get 8120 kg of air in the tube.
Flow through an orifice is equal to
f = CAsqrt(2p(P1-P2))
C is a flow coefficient, or about 0.8 for this setup
A is the area of the opening, or again 0.785375 m^2
p is the air density, or about 0.000000019131 kg/m^3 at 100 km
P1 the pressure inside the tube, or 0 Pa
P2 is the pressure outside, or 5 Pa
Do the math and you get 2.748e-4 kg/s. Divide into the 8120 kg and you get 29548762 seconds, or nearly a year.
Of course things will go more slowly as time goes on. I’ll take a guess and say about 10 years to get to 99.9% density. I’m sure I’m within a factor of 2, at any rate…
ETA citations:
http://physics.bu.edu/~py502/lectures3/l07.pdf
Thanks Dr. Strangelove. I really had no feel for if it would fill up in a few years like you showed, or if it would take thousands or more.
Just to be clear, Dr Strangelove’s answer is for the case where the tube is not open to atmosphere at the bottom, only at the top, where there is hardly any atmosphere available. Is that what you meant? It would fill much more quickly from the bottom.
Now the question is: How can I use this to win a bar bet?
Yes, I meant open only at the top.
You wouldn’t say that if you’d tried living in one.:mad: