[marksbrown]

Vacuum is the ideal. Taking the molar mass of air at 28.8g/mol at STP we can see a density of air at about 1.225 kg per metre cubed. Combined with the volume of the bag we have the uplift. We quickly find that empty flexible bags collapse with nothing to provide an atmosphere of pressure back out thus the compromise of low mass gasses. These still add weight to our vessel diminishing our carrying capacity.

Therefore the best solution is hot hydrogen.

[i5heu]

There is no light and strong enough Material that can keep a vacuum in the size needed for a Zeppelin on earth so it could fly.

On mars this could work.

The brain confusion that a room of vacuum is lighter then air and will therefore have a lift is amusing.

[jfim]

I think you misinterpreted what the parent is saying.

He's saying that a bag that contains only vacuum would be the lightest, but as it contains nothing, it would just collapse into having no volume due to the atmospheric pressure. That would obviously provide no lift. As such, you need something in the bag that will fill the space.