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When I looked at the photo in this article, I didn’t see a “pentagonal shape” in the swirling water. I saw a sine wave, wrapped around a circle. I was immediately reminded of the Bohr–de Broglie model of electron orbits forming standing waves. Rather than swirling water and glycol forming “unexplained” polygons, isn’t this simply a standing-wave phenomenon?

Ellery Frahm
Minneapolis, Minn.

Combinations of circles and standing sine waves might explain the observed shapes, agrees Tomas Bohr of the Technical University of Denmark. But his team has been unable to use the circle–sine wave model to explain how shapes vary as experimental parameters change .—P. Weiss