Would a bag collapse if I sucked the air out of it in outer space?
September 8, 2006 4:09 PM   Subscribe

Would a bag collapse if I sucked the air out of it in outer space?

Unless I completely failed physics (which I did), a bag collapses not because we suck its contents out but rather because we create less internal pressure than the pressure of the atmosphere pushing against the bag. Thus: squish!

So if you don't have any atmospheric pressure, is there any reason a bag would change its shape in any way if I sucked every last molecule of gas out of it?

(For the sake of argument, assume the bag is strong enough to not tear apart from its own internal pressure)
posted by TimeTravelSpeed to Science & Nature (32 answers total) 2 users marked this as a favorite
 
The "vacuum" in low earth orbit isn't actually a true vacuum. In fact, a true vacuum (complete absence of gas) is like absolute zero: an asymptote that cannot be reached. Even in intergalactic space there is some gas -- just not very damned much of it.

LEO is a very low pressure area, but the pressure is not zero. Therefore, if you do manage to suck enough gas out of the bag, the internal pressure will be lower than the external pressure and it will collapse. But you have to suck a hell of a lot of gas out, because the external pressure isn't very high.
posted by Steven C. Den Beste at 4:14 PM on September 8, 2006


Without taking a ride on my space shuttle to try this out, I imagine it would be fully inflated like a balloon up until you suck out all the gas and the internal pressure goes to 0. Then it might crinkle a bit where your fingers are touching it.
posted by Khalad at 4:15 PM on September 8, 2006



The "vacuum" in low earth orbit isn't actually a true vacuum.


Right. I understand that. I think a Physics professor once told me there was something like one hydrogen molecule per square meter. But mine isn't so much a practical question as a theoretical one, ie: what would happen in 0-pressure.

Then it might crinkle a bit where your fingers are touching it.

This is sort of what I thought would happen (although with a balloon, I think the stretched rubber would actually provide the force necessary to collapse it). I imagine with a bag it wouldn't change shape until you poked at it. And it would give very easily.
posted by TimeTravelSpeed at 4:25 PM on September 8, 2006


If you were using your lungs to suck the air out, I doubt that you could produce enough of a relative vacuum. Your poor alveoli would rupture and blood would fill the bag. Eww.
posted by owhydididoit at 4:27 PM on September 8, 2006 [2 favorites]


If you want to do a thought experiment, picture the bag in air, only it has holes in it so there's no pressure differential between inside and outside. Can it form whatever shape you like? Pretty much.*

* nit-pickery: when you're in vacuum and microgravity, all sorts of effects that you wouldn't notice on Earth because they were swamped by gravitational and fluid dynamic forces will be the only forces left acting on the bag and so will be the principal influences on its shape.
posted by cardboard at 4:37 PM on September 8, 2006


If you were using your lungs to suck the air out, I doubt that you could produce enough of a relative vacuum. Your poor alveoli would rupture and blood would fill the bag. Eww.

I'm not sure how to explain why, but I think your lungs would easily have the capability to suck the air out.

The way I'd attempt to explain this is by noting that since you're not "sucking" anything out of the bag (the gas molecules are forcing themselves into a container of lesser pressure), all you'd need to do is fully deflate your lungs, pucker up against that bag, open your windpipe, and the air would do the rest. It wouldn't deflate the bag, but it kinda shows how easy it might be to get at least most of the molecules out.

But you could just as easily be correct. Either way, it's against the spirit of the question, I think :)
posted by TimeTravelSpeed at 4:43 PM on September 8, 2006


It's my understanding that the bag collapses because your sucking the air out actually causes *negative* pressure inside the bag, so I would have to say that yes, it would still collapse, while the sucking is going on.
posted by clevershark at 4:47 PM on September 8, 2006


"Negative pressure" is just a shorthand way of saying "less internal pressure than the pressure of the atmosphere". If the atmospheric pressure is 0 then you can't have negative pressure.
posted by Khalad at 4:50 PM on September 8, 2006


There is no such thing as absolute negative pressure. You can go no lower than zero pressure (hard vacuum). "Negative pressure" can only be used in the sense Khalad uses.
posted by cardboard at 4:54 PM on September 8, 2006


It's my understanding that the bag collapses because your sucking the air out actually causes *negative* pressure inside the bag, so I would have to say that yes, it would still collapse, while the sucking is going on.

Is it possible to have negative pressure in relation to an external pressure of 0? I thought the "negative" was relative to the external pressure, but you can't go lower than 0.
posted by qwip at 5:18 PM on September 8, 2006


I should have left off the eww. But the rest of my response is serious. How is it against the spirit of the question?
posted by owhydididoit at 5:49 PM on September 8, 2006


There's no such thing as negative pressure. Nature doesn't suck, it pushes. Start the jokes whenever you please but it's true. Pressure can be less in one region than another, which we perceive as suction, but there is no suction. Only relative pressure. There's no negative pressure.

it might crinkle a bit where your fingers are touching it.

Yes, and where the elastic tension of the paper itself was pulling back against being stretched by inflation. In pure theory, nothing should happen to the bag, but in any kind of practical experiment, there would be all kinds of forces at work. For one thing, a paper bag might not be able to contain the pressure at all, in the first place. It might explode upon entering said theoretical vaccum, but would almost surely be torn in the midst of any effort to allow the air to escape.
posted by scarabic at 5:58 PM on September 8, 2006


It would have to be a perfect sphere (since there must be an opening, it can't be a perfect sphere), otherwise self-gravity would eventually collapse it.
posted by Chuckles at 6:17 PM on September 8, 2006


I think a Physics professor once told me there was something like one hydrogen molecule per square meter.

It's not even remotely that low.

I used to work for a company that made vacuum chambers for IC processing. One of the big problems was leak-checking them, to make sure they could achieve vacuums which were sufficiently good. (Note: atmospheric pressure is about 750 torr, depending on your altitude and the weather.)

Our chambers used a diaphragm pump to get down to about 4 torr, and then we used either a turbopump or a cryopump to try to achieve the operating pressure, usually about 3*10^-7 torr or less if possible.

Cryos are really astounding. Once the slot valve over the cryo was opened the pressure in the chamber would drop about 5 orders of magnitude in about 20 seconds. But reaching operating pressure took 15 hours.

Those pressures were lower than the gas pressure in LEO, and at those pressures gas particles in the chamber tended to follow paths a lot like billiard balls on a pool table, but it was still many, many orders of magnitude more gas particles than "1 per cubic meter".

To get pressures like that outside a lab you have to go out into intergalactic space. You won't find pressures that low occurring naturally anywhere inside or near a galaxy.
posted by Steven C. Den Beste at 6:30 PM on September 8, 2006


all you'd need to do is fully deflate your lungs, pucker up against that bag, open your windpipe, and the air would do the rest.

There's still quite a bit of air in your lungs even when you've exhaled as fully as you can. To get a bag anywhere near vacuum, you'd need something a hell of a lot more efficient than your lungs.
posted by chrisamiller at 6:35 PM on September 8, 2006


Maybe it depends on whether or not any hydrogen atoms happen to be in the bag to begin with.
posted by StickyCarpet at 6:45 PM on September 8, 2006


OK, let's take a wild leap into my best guess as to calculating the number of particles in our chambers when they were pumped down. There's got to be better information about this somewhere online but I can't find it.

Avogadro's number is the count of gas particles in one cubic centimeter at "normal" atmospheric pressure and temperature (nominally, sea level and 25 degrees C, I think)

That's 6 * 10^23.

There's a million cubic centimeters in a cubic meter. So it's 6 * 10^29 in a cubic meter.

"Normal" atmospheric pressure is 760 torr. Sometimes we managed to achieve 7.6 * 10^-8 torr, which is to say we were 11 orders of magnitude below normal atmospheric pressure. In other words, 6 * 10^18 gas particles per cubic meter.

To reach one molecule per cubic meter would have required dropping more than another 18 orders of magnitude.

The gas in LEO is a continuation of Earth's atmosphere; it isn't hydrogen, it's oxygen and nitrogen. In fact, the Moon has a bit of an atmosphere too, captured from the Earth. Earth's atmosphere stops at the point where the solar wind overpowers it. And in turn, the solar wind stops at a point where the "interstellar wind" overpowers it.

Aha! And I just found some real numbers at last: the interstellar medium inside our galaxy in this region has a density of a thousand to a million molecules per cubic centimeter. Multiply that by a million to get molecules per cubic meter: between a billion and a trillion.

To get down to 1 per cubic meter you've still got a lot of pumping to do...
posted by Steven C. Den Beste at 7:39 PM on September 8, 2006


Rats. I got the definition of Avogadro's number totally wrong, so the above calculation is trash. Alas.
posted by Steven C. Den Beste at 7:42 PM on September 8, 2006


You want the ideal gas law: PV = nRT where P is pressure in atmospheres, V is volume in cubic metres, n is the number of moles of gas, T is temperature Kelvin, and R is the gas constant 8.2057459 x 10-5 m3 · atm · K-1 · mol-1.

There are Avogadro's Number 6.023 x 1023 molecules in a mole.

So the number of molecules in a cubic metre of an ideal gas at standard temperature (25°C = 298K) and pressure (1 atm) is 6.023 x 1023 molecules mol-1 x 1 atm * 1 m3 / 8.2057459 x 10-5 m3 · atm · K-1 · mol-1 / 298 K = 2.46 x 1025 molecules.

Eleven orders of magnitude below that is something of the order of 1014 molecules, which is still a metric assload of molecules.
posted by flabdablet at 9:08 PM on September 8, 2006


I think the shape of the bag would be dominated by the stresses in the bag material more than anything else. Yes, there's a tiny amount of gas in low earth orbit, but not enough to crumple the bag. (Plausibility check: If there were enough gas in LEO to crumple the bag, the "breeze" of that gas whipping past you at 8 km/sec would rip the bag fro your hands. But in fact there isn't that much drag, although there is enough drag to cause satellites in low orbit to very gradually slow down and eventually fall out of orbit.)

Whatever the bag is made of (plastic, paper, rubber) probably has some preferred, natural shape, and in the absence of air pressure keeping the bag inflated, it'll probably try to assume that shape.

Some quick googling gives me an atmospheric pressure at 500km altitude — which is a bit higher than LEO — of 10-8 Pascals (the back of my envelope and a glance through a CRC handbook converts that to 10-10 torr, 10-15 grams/cm3, maybe 108 molecules/cm3, and a molecular mean free path of many kilometers). Things like the wake shield facility can be used to create regions of higher vacuum, though.
posted by hattifattener at 9:11 PM on September 8, 2006


You wouldn't need to orbit any vaccum pumps to see what would happen. Just hold open a bag on earth. A (near) zero pressure bag in (near) zero pressure space wouldn't behave any differently than an atmospheric pressure bag in an atmospheric pressure, er, atmosphere.
posted by Popular Ethics at 10:04 PM on September 8, 2006


... or what cardboard said.
(Sorry, missed that answer amongst the how-many-hydrogen-molecules-are-there-in-orbit sidetrack)
posted by Popular Ethics at 10:06 PM on September 8, 2006


I should have left off the eww. But the rest of my response is serious. How is it against the spirit of the question?

Because I'm really only interested in what would happen to a somewhat flexible container that has the gas inside it sucked out in a theoretical complete vacuum. Not whether or not its physically conceivable that a human being could do such a thing in the first place.
posted by TimeTravelSpeed at 10:26 PM on September 8, 2006


Those pressures were lower than the gas pressure in LEO, and at those pressures gas particles in the chamber tended to follow paths a lot like billiard balls on a pool table, but it was still many, many orders of magnitude more gas particles than "1 per cubic meter".

Although I didn't mean to imply this was in LEO, I still didn't know that nowhere in this galaxy is there as low a density as 1 per cubic meter. Thanks for the info!
posted by TimeTravelSpeed at 10:30 PM on September 8, 2006


Ultimately, cardboard's answer made the most sense to me. A bag with the same internal pressure as the outside will behave the same in any atmosphere (including a theoretical zero).

And it's the theoretical solution I was most interested in. While I appreciate all the info about Earth-specific atmospheres and lung power, I was really just imagining some sufficiently strong mechanism sucking (er... creating a lower pressure differential) the air out of a non-explodable bag in a perfect vacuum. A lot of good info and research though! Thanks!
posted by TimeTravelSpeed at 11:17 PM on September 8, 2006


No.
posted by delmoi at 6:32 AM on September 9, 2006


The "vacuum" in low earth orbit isn't actually a true vacuum.

Well, the guy asked about outer space, not low earth orbit. Wtf? You didn't answer his question at all.
posted by delmoi at 6:34 AM on September 9, 2006


Is it possible to have negative pressure in relation to an external pressure of 0? I thought the "negative" was relative to the external pressure, but you can't go lower than 0.
--
There's no such thing as negative pressure. Nature doesn't suck, it pushes.


No, there is such a thing as absolute negative pressure, which can be caused by things like surface tension. Absolute negative pressure has been observed in a lab, if I'm remembering my high school physics class correct. There's also this wikipedia article on the subject. (Scroll down to the last subsection)

It would have to be a perfect sphere (since there must be an opening, it can't be a perfect sphere), otherwise self-gravity would eventually collapse it.

That's only true if the gravitational forces were stronger then the tensile strength of the bag, which I seriously doubt.

Also Steven stop fucking talking about LEO. LEO is not "outer" space, and the questioner was obviously talking about very near vacuum conditions.
posted by delmoi at 6:44 AM on September 9, 2006


Forget "sucking". It isn't necessary. A vacuum pump (your lungs) simply produces a lower pressure, causing the external pressure to push the air out of whatever you're vacuuming. I the case of a container with solid walls, it simply equalizes the low pressure between the container and the pump. So in space, with no external pressure, a vacuum pump is useless, or simply not necessary. Just open the bag. If you imagine the pump being inside a spaceship with a hose going to the bag outside the spaceship the lowest pressure it could create would not be zero, so the effect would, again, not be any different from just opening the bag. If the bag were inflated from internal pressure (air), then it would simply revert to its "natural" shape, or the shape it would be if you simply opened the bag. A balloon would revert to a flat shape due to the elasticity of the rubber. The natural shape of the bag would have much more independence in space. It would be like opening the valve on an over-inflated tire on earth.
posted by weapons-grade pandemonium at 7:44 AM on September 9, 2006


You'd never get 1 per cubic meter near your bag no matter where you were, because of outgassing from the bag itself.

All matter in a vacuum will outgas at least to some extent, even things like glass. And it will continue doing so for literally months at a gradually reducing rate.
posted by Steven C. Den Beste at 8:50 AM on September 9, 2006


Consider that in this thought experiment you're not simply making all of the atoms in the paper bag disappear, together, instantly. What you're doing is trapping the atoms at the front of the bag in your lungs, then trapping the ones behind them, and so on, and meanwhile you've got matter moving around in different ways. You probably wouldn't get the same shape you'd get in an atmosphere, but the bag would certainly deform somewhat due to the fact that the process takes place over time and a lot would be happening inside the bag.
posted by Hildago at 9:19 AM on September 9, 2006


But that wouldn't matter much in space, Hildago. If nothing is pushing from the outside, the pressure inside would instantly equalize, pushing out evenly in all directions, even with almost no air in the bag. Once the pressure was low enough inside to relieve stretching, the bag would simply stay put--unless, of course, you're moving it or poking it. The effect would be exactly like taking a plastic shopping bag, inflating it, then opening it up. Try it. It just stays there. Pressure inside equals pressure outside. Doesn't matter if the pressure is atmospheric or zero.
posted by weapons-grade pandemonium at 9:49 AM on September 9, 2006


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