And not mentioned (unless I skimmed it) was to float in the upper atmosphere, the buoyancy would have to be much greater, since density (hence weight of a given volume of atmosphere at altitude) is less than sea level.
IIRC, the density of the atmosphere (and hence, Magetout’s 1.25kg/m^3) halves every 18,000 feet. High altitude vaccum bubbles would have to correspondingly lighter - the buoyancy goes down, but not the weight of the container. An alternative would be stretchy balloons ffilled with hydrogen or helium, because unless the pressure inside the vessel goes down too, at a certain point denser H or He becomes a lot less buoyant. If instead of stretching, the contents leaked to balance pressure, then they risk being too heavy (or crushed) if turbulence brings them to lower levels.
As always with engineering, the devil is in the details.
Well, there’s no scaling with size at normal sizes. Sometimes things get a little weird down at nano-scale… but you can’t just wave your hand and say the magic word “Nano!” and expect it to fix everything.
The original proposal was just an ordinary balloon. No reason that can’t work: Keep the pressure the same on both sides, and you can make the envelope arbitrarily thin. Nitrogen is pretty weak as lifting gases go, and I’m not sure why it was proposed… Just because it’s abundant? But if you insisted, you could make it work.
The shell thickness is proportionally less at smaller radii. Specifically, it’s always about \rho_a/(3 \rho_g) \approx 0.00048 times the radius. The easiest way to see this is that for the sphere to have the same overall density as air, the fraction of the sphere taken up by the glass has to be a particular value (\rho_a/\rho_g) no matter what the radius is. That means that if you double the radius of the sphere, you double the wall thickness as well.
The problem is then that the exterior pressure that a glass sphere can withstand (assuming perfect materials) is also determined by the ratio of the thickness to the radius. You could, of course, get around this by filling the sphere with a gas — but then your sphere would not longer be neutrally buoyant because of the mass of the air inside, so you’d have to make the wall even thinner in response.
Right, but the net effect is that for any given material, the minimum average density of a vacuum balloon is constant at any (non-nano) size (and maybe at nano size, too). And for any known real-world material, that minimum average density is orders of magnitude more than the density of air, so it wouldn’t float.
One other thing to consider is that for some materials, the kinds of defects that are present and that are really hard to engineer out, have their own typical scale - or in other words, when you try to scale down your glass bubble factory, you don’t necessarily get to scale down the defects along with everything else - that means they are more likely to represent critical flaws at smaller scales.
The original proposal quoted by the OP states that the proposal was that the tiny spheres be aerosols. Aerosols don’t stay suspended because they are lighter than air.
Consequently pretty much this entire thread is irrelevant to the original proposal.
That’s true, but the flip side of that is that, for very small scales, it’s easier to have some specimens with no defects at all. So if you’ve got good quality control to separate out the critically-flawed from the unflawed, you might get better material properties at very small scales.