If you’ve ever watched a basketball game, you’re familiar with the white noise of staccato squeaks from the players’ shoes as they maneuver across the court. Now, new research published in Nature is shedding light on this phenomenon.
“This project started with a simple question: Why do basketball shoes squeak?” Harvard University applied physicist Adel Djelloul said in a statement.
To answer that question, Djelloul and his colleagues used high-speed imaging to record the movement of sneakers and blocks of silicone rubbed against glass. “We combined total internal reflection imaging with cameras capturing up to 1 million frames per second to visualize the evolving contact between rubber and glass,” Djelloul explained. “To drive sliding, we adapted a configuration conceptually similar to Leonardo da Vinci’s friction experiments from the 15th century.”
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The high-speed cameras revealed that the sneaker tread intermittently slipped and stuck to the surface at supersonic speeds, resulting in a sound wave. The ridges on sneaker tread, they discovered, were key to producing the squeaks. While smooth silicone produced broadband noise like the kind you might hear from a malfunctioning white noise machine, ridged rubber acted as a waveguide, producing a clear tone.
They also found that the pitch of the tone produced was a function of the thickness of the rubber blocks they used, with thinner blocks producing higher pitches. In fact, this relationship was so precise the team was able to custom-make blocks to play “The Imperial March” from Star Wars: The Empire Strikes Back.
Interestingly, this new research could help explain some of the dynamics at play during earthquakes, when two tectonic plates quickly slip against one another.
“These results bridge two fields that are traditionally disconnected: the tribology of soft materials and the dynamics of earthquakes,” study co-author Shmuel Rubinstein of Hebrew University said. “Soft friction is usually considered slow, yet we show that the squeak of a sneaker can propagate as fast as, or even faster than, the rupture of a geological fault, and that their physics is strikingly similar.”
Some scientific discoveries arise from a pressing need; others, like this one, come from the simple desire to scratch the itch of curiosity. 
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