Children ask a lot of fun questions, often quite persistently and while you are trying to cook food. One such question comes from the 9-year-old child of a Reddit user, who asked: "If helium is lighter than air, would a balloon with a vacuum in it, also float?"
An excellent and enjoyable question. First, sorry 9-year-old, let's look at why helium balloons float. Buoyancy is an upwards force in a fluid (any flowing substance, including air) exerted on all bodies within it. The force comes from the pressure within the fluid being greater the further down the fluid you go. The pressure on the bottom of an object within the fluid is higher than at its top, causing the upwards force.
If the buoyant force of a fluid is greater than the weight of an object placed within it, the object will float.
Helium, being lighter than the other elements in our atmosphere, rises. It's the same when air is heated inside a hot air balloon, making it less dense per volume inside the balloon than it is outside, causing it to rise.
A vacuum is a volume that is empty of matter, so it's less dense (and lighter) than the air in our atmosphere. So, if you could fill a balloon with a vacuum (or really, draw out the membrane of a balloon so that it contains a vacuum and is stable), then it wouldn't just rise – it would float better than helium balloons.
Helium is so light it floats to the edge of our atmosphere to sit until it is blown away by solar winds, so presumably any perfect vacuum balloon would sit on top of any helium and hydrogen it encountered, being lighter than anything inside our atmosphere.
The idea of making transport using vacuums goes back a surprisingly long way. In 1690, following the recent invention of the vacuum pump, priest and mathematician Francesco Lana-Terzi proposed the idea of a vacuum airship, a conventional wooden ship held aloft by several vacuum balloons.
It would have been impossible back then to make a vacuum large, empty, and rigid enough to achieve liftoff, and the story is the same today. The problem in building it is the same thing that would help lift it – the pressure outside of a vacuum is higher than the inside of a vacuum. When the pressure becomes too much for the surrounding material to withstand, the result is a violent implosion, equalizing the pressure within and outside the object.
As yet, we haven't made ships that can float through the air carried by vacuums. Here's hoping the material scientists pull through.