If you t hink back to your eighth-grade science class, you may remember that a fluid is anything that can’t hold a shape. Your car may be fast, but it’s still a solid, not a fluid. But liquids like the petrol in your tank and the coffee in your cup take the shape of whatever they’re poured into. They are fluids. Gases like the air you’re breathing and the CO2 pouring out of your exhaust pipe also take the shape of their containers; they are fluids, too.
The most important characteristic of a fluid is its ability to flow . The atoms in a solid — like an iron beam — are locked rigidly into place like Lego blocks in a Lego castle you (or your kids) built. But the atoms in a fluid can roll and tumble and cascade around each other (like those same Legos in the plastic bin when your kids have torn the castle apart).
It’s that flowing freedom that gives fluid motion its hypnotic quality. Allow yourself to become mesmerized by the flow of a fast-moving river around a bridge trestle and you’ll know what I mean. There is poetry in the massive cascade of waves as the river water pushes past an obstacle. And there is dance in the roiling turbulence that emerges downstream.
But, most importantly, the choreography of matter and motion you’re watching right before your eyes doesn’t care about place and time. What you see before your eyes today is being repeated all across the cosmos.
If you don’t believe me, go flush your toilet.
As soon as you push that handle down on that most personal of household appliances, the universal laws of fluid dynamics kick in. The swirling pattern of water down the toilet drain is an example of what physicists call vortical motion . A vortex occurs whenever a fluid is forced to flow in circles. The polar vortex that made everyone’s life a wintry hell last month originates with the atmosphere (a fluid) driven into circular motion by the Earth’s rotation. But look at the graceful arcs of a spiral galaxy across 10 million light-years of space and you will see the same principles at work on scales so vast it will make your imagination cry uncle! (It takes the water in your toilet about a half-second to complete one rotation; the gas in a spiral galaxy makes the same trip in 200 million years).
From the first moments after the Big Bang, to clouds rolling across the sky, to you staring down into your toilet — it’s all the same. So if you really want to find your inner child and know the secrets of the cosmos at the same time, it’s simple.
All you have to do is flush.