(Every day until December 25, I’m posting a science-related image or video and description.)
Candles are strongly associated with many holiday seasons, including Advent, Hanukkah, and Diwali. They are also enjoyable as everyday objects, providing a soft form of light that many of us (maybe most of us) find pleasing. And when you extinguish a candle flame, it produces a beautiful example of the physics of the complex in normal life.
The smoke from a candle consists of a variety of molecules, wafted up on a current of hot air. Directly above the wick, the flow is laminar, meaning it follows smooth regular streams. “Laminar” refers to layers: you can treat the path of a stream of particles as independent of a neighboring stream. (The word has the same root as “laminated”.)
However, higher above the wick, the flow has cooled enough that it is buffeted by air molecules, which move randomly. That produces the whirling motion known as turbulent flow, which is a fascinating physical phenomenon. Turbulence is self-similar: if you zoom in on a whorl in the smoke, you’ll see it’s made up of smaller whorls that are equally complex. Self-similarity is the basis of fractal geometry, of which the most famous example is probably the Mandelbrot set.
Turbulence is often destructive from an engineering standpoint, but it’s often strikingly beautiful. Consider the atmosphere of Jupiter: we see alternating dark- and light-colored band, but within those, the clouds are marvelously chaotic. The forces involved are different from those producing turbulence in candle smoke, but the result is self-similarity — and beauty.