# Tiny spheres of vacuum **URL:** \ **Category:** Factual Questions\ **Created:** [June 27, 2025, 4:38am UTC](https://boards.straightdope.com/t/tiny-spheres-of-vacuum/1020185 "2025-06-27T04:38:37Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Pleonast](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pleonast/32/1183_2.png) [@Pleonast](https://boards.straightdope.com/u/Pleonast)\ **Post date:** [June 27, 2025, 4:38am UTC](https://boards.straightdope.com/t/tiny-spheres-of-vacuum/1020185/1 "2025-06-27T04:38:37Z")
Continuing the discussion from [­xkcd thread](https://boards.straightdope.com/t/xkcd-thread/854574/4521): > [@­xkcd thread](https://boards.straightdope.com/t/xkcd-thread/854574/4521): > > It’s been suggested that microscopic glass hollow spheres filled with dry nitrogen could behave as high-altitude aerosols with persistence times in the decades. Is it possible to make a tiny glass sphere strong enough to hold a vacuum? Would it have enough buoyancy to float in water? In air? What about other materials, like titanium, carbon, or some composite? What if it’s not a true vacuum? How much internal pressure would be needed to keep this tiny sphere from collapsing, and would it still float? ---
**Author:** ![moes\_lotion](https://avatars.discourse-cdn.com/v4/letter/m/7ba0ec/32.png) [@moes\_lotion](https://boards.straightdope.com/u/moes_lotion)\ **Post date:** [June 27, 2025, 6:38am UTC](https://boards.straightdope.com/t/tiny-spheres-of-vacuum/1020185/2 "2025-06-27T06:38:29Z")
I can’t answer the first question - how to manufacture a tiny hollow glass sphere filled with a vacuum, but glass is extremely strong in compression so yes, it would be strong enough to resist atmospheric pressure. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [June 27, 2025, 6:59am UTC](https://boards.straightdope.com/t/tiny-spheres-of-vacuum/1020185/3 "2025-06-27T06:59:11Z")
In order to be buoyant in air, an evacuated vessel would need to have a mass slightly less than the mass of the air it displaces - sorry if that’s obvious, but that is how buoyancy works and it’s not a lot to work with. Calculations at a really small scale yield results that are difficult to envisage, so just to begin, consider a sphere that has a volume of 1 cubic metre; the mass of that amount of air at sea level etc is about 1.25kg - so you’d have to make a sphere of some impermeable material with excellent compressive strength, out of less than 1.25kg of that material. There will be scaling effects that bring this closer to being possible at smaller scales, but I believe it never quite gets to the break-even point where vacuum could be used practically to create net positive buoyancy in air. Here are a few previous threads on the topic: > [@Vacuum balloon](https://boards.straightdope.com/t/vacuum-balloon/825556): > > Is it possible to create a balloon rigid enough to contain a vacuum, but whose structure is not too heavy to not negate the bouyancy? How does feasibility change with scale? > [@Why no vacuum or hydrogen based airships?](https://boards.straightdope.com/t/why-no-vacuum-or-hydrogen-based-airships/763811): > > I know that the Hindenburg clearly put a dent in people’s confidence in hydrogen’s usefulness in lighter than air travel, but that was a very long time ago, and there are still questions as to whether the hydrogen or the paint involved was responsible for that disaster. One would think that after th better part of a century, with new materials and technology that someone could engineer a way to use hydrogen safely in lighter than air travel. It weighs half of what helium does and is nowhere n… > [@would a sealed vacuum be buouyant?](https://boards.straightdope.com/t/would-a-sealed-vacuum-be-buouyant/746610): > > 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. ---
**Author:** ![Schnitte](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/schnitte/32/9033_2.png) [@Schnitte](https://boards.straightdope.com/u/Schnitte)\ **Post date:** [June 27, 2025, 7:18am UTC](https://boards.straightdope.com/t/tiny-spheres-of-vacuum/1020185/4 "2025-06-27T07:18:11Z")
Perhaps I’m on the entirely wrong track here, but I’d guess that the pressure on the glass sphere, and hence the strength it needs to have, increase with the square of the diameter of the sphere, but its volume (and hence buoyancy) increases with the cube. So as you make your sphere progressively smaller, you’re actually making it harder to reach the point where it floats in air compared to a larger sphere. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [June 27, 2025, 7:52am UTC](https://boards.straightdope.com/t/tiny-spheres-of-vacuum/1020185/5 "2025-06-27T07:52:29Z")
> [@Pleonast](#): > > Would it have enough buoyancy to float in water? In air? I suspect buoyancy is not how it is proposed to work. Very small heavier than air objects can remain in the atmosphere a long time due to drag, diffusion, and updrafts. This is why dust and tiny spiders and so on can remain in the atmosphere for extended periods despite being heavier than air. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [June 27, 2025, 8:12am UTC](https://boards.straightdope.com/t/tiny-spheres-of-vacuum/1020185/6 "2025-06-27T08:12:27Z")
Even if it was possible, the process to create a “bubble” of vacuum is hazy at best. ---
**Author:** ![Si\_Amigo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/si_amigo/32/2877_2.png) [@Si\_Amigo](https://boards.straightdope.com/u/Si_Amigo)\ **Post date:** [June 27, 2025, 8:57am UTC](https://boards.straightdope.com/t/tiny-spheres-of-vacuum/1020185/7 "2025-06-27T08:57:00Z")
Aren’t light bulbs glass spheres containing a vacuum? A vacuum is a very small force of a maximum of 1 atmosphere at the most. Even a high vacuum is limited to this because the definition of a vacuum is little or no molecules. Getting little amount of molecules is easy, getting none is not possible. Unless I am missing something here I believe the answer is yes. I suspect I am missing something, I need coffee. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [June 27, 2025, 9:00am UTC](https://boards.straightdope.com/t/tiny-spheres-of-vacuum/1020185/8 "2025-06-27T09:00:26Z")
I think light bulbs typically contain inert gas because (for a tungsten bulb) this slows down the loss of atoms boiling off the filament (or something like that). But the question isn’t whether you can make a rigid container that can be evacuated (you can - we make vacuum chambers out of glass) - the question is whether you can do it _using an amount of material that weighs less than the evacuated air._ ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [June 27, 2025, 11:18am UTC](https://boards.straightdope.com/t/tiny-spheres-of-vacuum/1020185/9 "2025-06-27T11:18:01Z")
@Mangetout utterly nailed it in the first post and more succinctly here in the second. Those prior threads he cites go into great detail. Worth reading. There are others I think. Vacuum is only slightly lighter than hydrogen. But hydrogen at ambient pressure of your outside atmosphere at whatever altitude you want to cruise is incredibly powerful at supporting your sphere from the inside. Conversely vacuum provides zero support. You can make your hydrogen spheres out of the thinnest lightest imaginable plastic sheet, _if_ they’re filled with hydrogen (or any gas really) to the same pressure as outside. But if it’s vacuum inside and atmosphere outside, you need very stiff, relatively thick walls to keep the sphere from being crushed. The extra lightness of the vacuum doesn’t come close to repaying the extra heaviness of the container. It’s a losing game all the way around. More cites: > [@Have we been able to construct vacuum buoyant structures yet?](https://boards.straightdope.com/t/have-we-been-able-to-construct-vacuum-buoyant-structures-yet/787114): > > If you build a structure that is simultaneously light enough and rigid enough to stand up to atmospheric pressure, you could suck out all the air and make it float. Have we developed the engineering prowess to make something like this yet? If not, how close are we and what are major engineering constraints that still limit us? > [@Vacuum balloon ?](https://boards.straightdope.com/t/vacuum-balloon/241462): > > I have a feeling this is an obviously dumb question, but here goes anyway. A hypothetical experiment is set forth below. First obtain a very lightweight yet very strong carbon fiber sphere, such that it resembles a large tough plastic balloon. Said sphere, is about, one meter in diameter and hermetically sealed except for one small valve. Said shut-off valve is used for the following two experiments: A: The sphere is filled up with helium gas; the valve is then shut, whereupon the sphere acts… ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [June 27, 2025, 11:41am UTC](https://boards.straightdope.com/t/tiny-spheres-of-vacuum/1020185/10 "2025-06-27T11:41:34Z")
It’s not hard to calculate the necessary dimensions. For the spherical shell of glass with outer radius r\_1 and inner radius r\_2 to have the same overall mass as a spherical volume of air of radius r\_1, we must have \rho\_g (r\_1^3 - r\_2^3) = \rho\_a r\_1^3 where \rho\_g is the density of glass and \rho\_a is that of air. Solving for this, we get r\_2 = r\_1 \sqrt[3]{1 - \frac{\rho\_a}{\rho\_g}} \approx r\_1 \left( 1 - \frac{\rho\_a}{3 \rho\_g} \right). Or to put it another way, the wall thickness must be \rho\_a / (3 \rho\_g) times the radius of the sphere to make it neutrally buoyant. This number is very very small — about 180 parts per million, assuming air at room temperature and soda-lime glass. A glass sphere with a radius of 1 mm would have to have walls about 180 nanometers thick — less than a wavelength of light. I don’t want to say this is impossible to manufacture, but it would be quite tricky. And even if you could construct it, it probably couldn’t withstand an external pressure of 1 atm. If [the formula on this page](https://engineering.stackexchange.com/questions/15717/how-to-calculate-external-pressure-resistance-in-spheres-and-cylinders) is to be believed, such a sphere would buckle at an external pressure of 0.046 atm. And that’s if it was perfectly constructed with no microscopic flaws; otherwise, it couldn’t even withstand that. Note that this same logic would apply to a glass sphere of any size, microscopic or not, since the ratio t/r is fixed by the mass requirement. (Floating in water is much easier, because the density of water is much closer to the density of glass. In that case it works out that the wall thickness needs to be about 15% of the radius, and it can withstand about 32,000 atm.) ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [June 27, 2025, 12:22pm UTC](https://boards.straightdope.com/t/tiny-spheres-of-vacuum/1020185/11 "2025-06-27T12:22:02Z")
Beautiful. Thank you. The money quote is right here: > [@MikeS](#): > > such a sphere would buckle at an external pressure of 0.046 atm. So the glass is ~1/20th of the needed strength enough to survive. Assuming _perfect_ glass. IRL, we’d need to design in a safety factor or nearly every sphere would fail as soon as it was made. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 27, 2025, 12:41pm UTC](https://boards.straightdope.com/t/tiny-spheres-of-vacuum/1020185/12 "2025-06-27T12:41:52Z")
And while we’re at it, folks sometimes suggest things like rigid hoops with a membrane stretched across them, but those are even less efficient. A continuous sphere is the best you can do. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [June 27, 2025, 12:51pm UTC](https://boards.straightdope.com/t/tiny-spheres-of-vacuum/1020185/13 "2025-06-27T12:51:01Z")
I assume the same would be true of anything where the membrane (in tension) is supported with internal radial spokes in axial compression (like a circus tent, but in the form of a sphere). ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [June 27, 2025, 2:14pm UTC](https://boards.straightdope.com/t/tiny-spheres-of-vacuum/1020185/14 "2025-06-27T14:14:42Z")
Yup. This was covered in excruciating detail in some of those earlier threads. No matter what you do, no matter how nifty your structure is, you can’t beat a sphere. Every part of a sphere is in compression. This will always beat any other structure. ---
**Author:** ![Pleonast](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pleonast/32/1183_2.png) [@Pleonast](https://boards.straightdope.com/u/Pleonast)\ **Post date:** [June 27, 2025, 3:01pm UTC](https://boards.straightdope.com/t/tiny-spheres-of-vacuum/1020185/15 "2025-06-27T15:01:31Z")
Oh, thank you for the formulas! Very helpful. So, we have the thickness of the material as t = \frac{1}3r\rho\_a/\rho given the radius of sphere, the density of air, and the material’s density, respectively. And the maximum pressure, from your cite, is P = \frac{2}{\sqrt{3(1-\nu^2)}} (\frac{t}r)^2 E given the material’s [Poisson ratio](https://en.wikipedia.org/wiki/Poisson%27s_ratio), material’s thickness, sphere’s radius, and the material’s [Young’s modulus](https://en.wikipedia.org/wiki/Young%27s_modulus), respectively. (This is a best case scenario with a perfectly manufactured ~~cow~~ sphere.) Combining, this gives the maximum pressure a neutrally buoyant sphere can withstand, P = \frac{2}{9\sqrt{3(1-\nu^2)}} (\frac{\rho\_a}\rho)^2 E Note that this does not depend on the radius or thickness of the sphere–it’s is purely a function air density and properties of the material. I’d like to throw in values for various materials, but out of time at the moment. ---
**Author:** ![abcdefghij](https://avatars.discourse-cdn.com/v4/letter/a/43a26b/32.png) [@abcdefghij](https://boards.straightdope.com/u/abcdefghij)\ **Post date:** [June 27, 2025, 3:33pm UTC](https://boards.straightdope.com/t/tiny-spheres-of-vacuum/1020185/16 "2025-06-27T15:33:43Z")
E is of order 100 GPa or 10^6 atm. \rho\_{air} is order 1 \mathrm{kg/m^3} For structural materials \rho ranges from maybe 300 \mathrm{kg/m^3} for some woods to 20000 \mathrm{kg/m^3} for tungsten. The rest of the numbers don’t matter. It looks like an oak balloon might support almost an atmosphere. Edit: plugging in the numbers that “don’t matter” knocks it down another order of magnitude ---
**Author:** ![DesertDog](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/desertdog/32/11503_2.png) [@DesertDog](https://boards.straightdope.com/u/DesertDog)\ **Post date:** [June 27, 2025, 3:49pm UTC](https://boards.straightdope.com/t/tiny-spheres-of-vacuum/1020185/17 "2025-06-27T15:49:56Z")
> [@beowulff](#): > > Even if it was possible, the process to create a “bubble” of vacuum is hazy at best. (Hand wave) The engineers will take care of it. I’m just the idea guy. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [June 27, 2025, 4:12pm UTC](https://boards.straightdope.com/t/tiny-spheres-of-vacuum/1020185/18 "2025-06-27T16:12:25Z")
Yeah, but… Hear me out… A carbon fibre cylinder with titanium domes on the ends. If there’s any risk of it buckling we could listen out for that and just back it off. ---
**Author:** ![mixdenny](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mixdenny/32/2962_2.png) [@mixdenny](https://boards.straightdope.com/u/mixdenny)\ **Post date:** [June 27, 2025, 4:50pm UTC](https://boards.straightdope.com/t/tiny-spheres-of-vacuum/1020185/19 "2025-06-27T16:50:46Z")
Mythbusters got a lead balloon to float. And it wasn’t even a sphere. It was a bloated cube. A _blube._ I forget the size of the blube, around 10 feet or so. And the thickness of the lead was 0.002". That gives a blube diameter to shell thickness ratio of 60,000 to 1. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [June 27, 2025, 5:33pm UTC](https://boards.straightdope.com/t/tiny-spheres-of-vacuum/1020185/20 "2025-06-27T17:33:10Z")
> [@MikeS](#): > > A glass sphere with a radius of 1 mm What about a glass sphere with a radius of one micron, or even 100 nanometers? The math you cited is beyond my ability to follow but I’d be surprised to hear that there is _no_ advantage to making the sphere smaller. Can’t the shell thickness be proportionally less at smaller radii? ETA: and of course the original cite was for a sphere filled with dry nitrogen so the shell only has to be strong enough to remained superpressurized at constant altitude. [Next page](https://boards.straightdope.com/t/tiny-spheres-of-vacuum/1020185.md?page=2)