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Pendulum Wave

Eighteen glowing pendulums start in one perfect line. Within seconds they twist into a snake, split into two lines, then three, dissolve into what looks like chaos and finally snap back into a single line on the very last frame of the video.

No pendulum ever interacts with another. The trick is in the timing: in one 30-second cycle the top pendulum swings back and forth exactly 15 times, the next one 16 times, and so on down to 32 times for the bottom one. Because each completes a whole number of swings, all of them are back at the start when the cycle ends. Halfway through, the odd ones have done half a swing more than the even ones, which is why you see two lines. At a third of the cycle you get three groups.

Fun fact: a pendulum's period depends almost only on its length. The top one here needs 2 seconds there and back, one second each way, like the classic "seconds pendulum" of grandfather clocks, which is just under one meter long. The fastest one would be only about 22 centimeters.