# eli5 – Lighter than Air

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If one were able to create a “balloon” out of carbon nanotubes and evacuate the air inside, would it act like a helium balloon and be lighter-than-air? It is my understanding that the “lift” of a helium balloon is due to the gas inside creating more buoyancy than the weight of the balloon itself. So, if we could create a balloon out of a super-lightweight material that is structurally sound, but void of any gasses, it should create lift in our environment, correct?

In: Physics

Basically, yes, but that void inside it is going to be under some pretty intense pressure from the air outside pressing down on it, and nothing inside pressing out to balance that out. If it’s strong enough to withstand that, and big enough to displace more air than its own weight, then it will float.

I don’t know what carbon nanotubes are strong enough to withstand, but large surface areas basically have literal tonnes of weight on them on account of their size. It’s all about surface area – the more physical area exposed to air, the more place air can push down. If you can withstand that, sure you can float anyway.

Yes, if you create a balloon that occupies 5 liters of space which has a total mass, inside and out, of less than 5 liters of air, then it will rise in air the same way a helium balloon does.

Answer: the point is that the “balloon” would need to weigh less than the weight of the volume of air it displaced for it to float. That is what buoyancy means and why things float in a fluid (like air or water).

So it is POSSIBLE, but you need to displace some volume of air AND the contraption needs to weigh less than that air.

It might be easier to think about water. A cubic foot of water weighs 62 lbs. A cubic foot of polyethylene (milk bottle plastic) weighs about 53 lbs, so it floats in water.