When you knock on a melon to see if it's ripe, you are using sound waves to probe the structure of the material inside. Physicists at the University of Chicago were using the same concept to explore how sound waves travel through patterned structures when they noticed an oddity: completely different structures sounded the same.

This was a surprising thing—sort of like knocking on a melon and a pineapple, and discovering they both made the same sound.

"What got us excited was that we could not explain our findings using existing concepts, such as spatial symmetries," said Vincenzo Vitelli, a professor of physics in the James Franck Institute.

What Vitelli and his group had discovered was a duality, a "hidden" symmetry linking apparently unrelated systems. Published in Nature, their study could one day help to design metamaterials or even that process information encoded in .

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