Every child learns this lesson at a birthday party. Blow up a balloon with your own breath, let go, and it drifts to the floor. But a balloon filled with helium tugs at its string and, if you let go, sails away into the sky. Same balloon, very different results.
The secret is buoyancy, the same force that keeps boats afloat. Air is a fluid, just like water. A balloon floats when the gas inside it is lighter than the air it pushes aside. Helium is much lighter than air, so the surrounding air pushes the balloon upward with more force than gravity pulls it down.
Archimedes and the Upward Push
The basic rule was worked out by the ancient Greek thinker Archimedes, and it's still called Archimedes' principle. Any object surrounded by a fluid gets pushed upward by a force equal to the weight of the fluid it displaces.
Here's why that happens. The pressure in a fluid increases the deeper you go, because of the weight of the fluid above. So the bottom of an object feels slightly more pressure than the top. That difference creates a net upward push. If that push is bigger than the object's weight, up it goes.
The air inside a balloon you've blown up yourself is no lighter than the air around it, and the rubber adds weight, so it sinks. A balloon full of helium weighs less than the air it displaces, so it rises.
Hot Air Does the Trick Too
Hot air balloons use the same principle without any special gas. Heating the air inside the balloon makes it less dense than the cooler air outside. The bigger the temperature difference, the more lift the balloon gets.
This was actually the first successful technology to carry people into the air. On November 21, 1783, two men made the world's first untethered human flight in a hot air balloon over Paris, in a balloon created by the Montgolfier brothers.
Why Not Hydrogen?
Hydrogen is even lighter than helium and can lift things too. The problem is that hydrogen is extremely flammable. The Hindenburg disaster is often pointed to as an example of the risks. Helium doesn't burn, which makes it the go-to choice for party balloons and airships.
Because helium is so expensive compared with hydrogen, some researchers have taken a fresh look at hydrogen's safety, especially for uncrewed craft flown away from populated areas. With good engineering and careful handling, they say, the risks can be greatly reduced. Hydrogen is still allowed for recreational ballooning in some countries, including the United States.
Helium has an interesting American connection. In 1903, a gas well in Dexter, Kansas, produced gas that wouldn't burn. Scientists at the University of Kansas analyzed it and found helium, showing that large amounts of it were trapped under the Great Plains.
Balloon Bits and Pieces
The rubber balloon was invented by the scientist Michael Faraday in 1824, during experiments with different gases. He made them for use in his lab. Earlier balloons were sometimes made from dried animal bladders.
Helium is a limited, non-renewable resource, so some party decorators have started making more of their creations with plain air instead. And remember, balloons that float away eventually come back down. Animals can mistake them for food or nesting material, so it's kinder to keep them tied.
So the next time you see a balloon bobbing on its string, you're watching a little piece of ancient Greek science in action. It's floating on an invisible sea of air.
