would a sealed vacuum be buouyant?

If you created (and maintained) a vacuum in a container that was large and/or light enough would it float in air? Picture a rigid airtight balloon, except instead of helium, it’s filled with empty space (not even air).

going by a volume to mass ratio alone, you’d think so, but my brain keeps saying the lack of outward pressure could change that.

Yes, it would. The outward pressure doesn’t figure into it; if its effective density is less than that of air, it will rise.

The question of whether it’s possible to enclose a vacuum in a chamber that is both large enough and light enough to be less dense than air is another matter entirely…

The container would have to be lighter than the air it displaces. Like a helium balloon.

The idea would work but the building of the container will be really difficult.

It might float at 5 feet in death valley in the cold dry winter but it would have to be much bigger to get higher or in warmer or moister air. Air density, ergo weight decreases as you leave the earths surface, unlike say, water, which does not thin out so much.

Still, the container is probably mythical or only imaginary at this time unless you have access to a super material balloon.


Yeah, building the container is the hard part. There are several videos on youtube of rail cars getting crushed by drawing a vacuum on them, and rail cars are designed not to crush easily.

There’s a patentfor a vaccuum balloon which doesn’t require Unobtanium.

I expect that actually manufacturing it might be prohibitively expensive.

Yeah, I totally agree that it’d require some sort of unobtanium. Still, it’d theoretically be more buoyant than hydrogen, making it a useful concept for airship fiction.

I’ve heard that, in theory, a perfectly spherical aluminum container could achieve this without being crushed. The fact that nobody has built one suggests that theory has a flaw.

Others have correctly answered in the affirmative, but haven’t said why.

The reason things float is because there’s more force on the bottom face than the top face. This upward buoyant force counteracts the downward force from the weight of the object.

Why does the bottom face have more force on it? Because the air is just a little more dense than on the top face, and has a higher pressure. The bottom face might have 14.7 psi of air pressure pushing it up; the top face, (say) 100 feet up, has 14.65 psi pushing it down. It mostly cancels out, but that tiny difference over a large surface accounts for all the lift.

It doesn’t matter at all what’s inside the balloon. This only contributes to the weight; the internal arrangement doesn’t matter. Although for the record, given the choice between a hydrogen balloon and a prototype vacuum balloon, I’ll go with the hydrogen.

It might help to imagine the balloon (or whatever object it is) being dipped into the atmosphere from above, from outer space. For the object to go into the atmosphere, an equivalent volume of atmosphere must be displaced up and out, just as when you climb into the bathtub a lot of water has to be displaced up (but hopefully not out) to accommodate your volume.

Well, naturally that air weighs something, and if it weighs more than the object that displaced it – which is exactly what happens when the average density of the object is less than the density of air – then the force of gravity on the displaced air will exceed that on the object, and the net force will pull the air down and pop the object back up and out again. Bouyancy, as discovered by Archimedes.

Concrete is strong under compression. Concrete vacuum airships would be really nice for fiction. Probably not for reality.

I think the OP’s question was just asking if you need the helium to get the lift,
or does the vacuum provide lift…

Well the insides do not cause the lift, the lift comes from the outside… the pressure of the air decreases as you go up, its this pressure which can be seen to be pushing a net force up on the container, and causing density to decrease as you go up too … and so the container (Balloon, or whatever) goes up its average density is the same as the air around it.

I guess you could build a zeppelin frame from something that is light and strong under compression, and then cover it with a material that is thin, airtight and doesn’t stretch.

However, air weighs 1.2 grams per liter, helium 0.18 grams per liter and hydrogen 0.09 grams per liter. So a vacuum balloon only needs 85% of the volume for the same weight compared to a helium balloon. But there’s no way the structure to keep a vacuum balloon inflated is going to come in at under 15% extra weight compared to a regular balloon.

Too bad the mythbusters are stopping. This would be a great one for them to try out.

A great high-school/refresher physics question.