I found out why my shoes squeak, and it has something to do with miniature lightning bolts.

The ubiquitous sneaker squeak on basketball courts may be caused by more than just friction, new research suggests.

Researchers found that the sharp sound of rubber on hard floors occurs when a small slippery area between the sole and the floor moves at supersonic speeds. And in some experiments, the process produced miniature lightning-like sparks. Additionally, this discovery could lead to improved understanding of earthquakes and help in the design of slippery surfaces.

The new study, published February 25 in the journal Nature, shows that soft rubber isn’t as slippery as many people imagine. Rather than the entire sole sticking together and then slipping off at once, the movement is concentrated at the front in a fast, wrinkle-like motion known as an “opening slip pulse,” which causes the rubber to peel and reattach across the contact zone. These repeated pulses create vibrations, which our ears hear as a screeching sound.

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Scientists have long explained squealing sounds from shoes, bicycle brakes, and tires using stick-slip friction, a stop-and-go cycle in which surfaces repeatedly catch and release. This model is suitable for many hard-on-hard systems, such as door hinges.

However, soft materials like rubber behave differently when sliding over hard surfaces.

To understand the physics of this process, researchers from Harvard’s John A. Paulson School of Engineering and Applied Sciences (SEAS) collaborated with experts from the University of Nottingham in the UK and the French National Center for Scientific Research. Using high-speed optical imaging and synchronized audio, they observed soft rubber moving quickly along smooth glass.

But what they saw wasn’t a smooth slide. Instead, the movement focused on opening slip pulses across the rubber at starts and stops.

“Essentially, these findings call into question the long-held assumption that friction in soft materials can be fully captured by a simplified one-dimensional ‘stick-slip’ model,” study first author Adele Geroli, a postdoctoral fellow at Harvard University, told Live Science in an email.

Small lightning bolts occur here and there.

This discovery reveals much more about the physics of friction. In classic stick-slip friction, the entire contact surface repeatedly sticks and slips. However, in this study, the movement was more localized, as only small areas opened or slipped and moved on, while other areas remained in full contact.

In some experiments, the team also observed small flashes caused by friction, which they described as miniature “lightning” sparks. In some tests, these sparks or discharges appeared to cause slip pulses. Although the spark was not the main cause of the squeak, it was indicative of how electrical energy builds up within the system when the rubber moves.

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The researchers also found that the shape of the rubber, rather than its movement, was the main factor determining the pitch of the squeak.

As the flat rubber block slid across the glass, the slip pulses were irregular, producing a wide “swoosh” sound rather than a clean squeak. But when the researchers added thin ridges to the rubber, the ridges trapped the pulses and caused them to repeat at regular intervals.

In fact, the ridges acted like guides, guiding the pulses through repeated cycles. This fixed the sound to a specific frequency or tone. The research team found that this squeaking frequency primarily depends on the height of the rubber ridges.

In fact, the pattern was so reliable that the team designed blocks of varying heights and used them to manually play the Imperial March theme from “Star Wars.”

“When it came time to actually perform the Star Wars theme song, we had to rehearse for three solid days to get the video right,” Gerouri said. “None of us are trained to make music using squeaky rubber blocks, so it took a lot of practice to master the timing and technique. I think the funniest part was the feeling of relief in the lab when we finally finished recording after three days of high-pitched squeaking. Our colleagues were so happy to finally have some quiet time again!”

What sneakers and earthquakes have in common

This discovery has implications beyond shoe design. The slip pulses in the experiment share important characteristics with the rupture front of an earthquake, where a section of a fault suddenly breaks and slides down at very high speeds.

“Although soft friction is typically thought of as slow, we have shown that sneaker squeaks can propagate as quickly or even faster than geological fault ruptures, and that their physical properties are surprisingly similar,” study co-author Shmuel Rubinstein, professor of physics at the Hebrew University of Jerusalem and SEAS visiting professor, said in a statement.

In addition to understanding the physics of earthquakes, this research could help engineers design surfaces that switch between slippery and slippery states on demand.

“Tuning friction behavior on the fly has been a long-standing engineering dream,” Katia Bertholdi, a professor of applied mechanics at Harvard University, said in a statement. “This new insight into how surface topography controls slip pulses paves the way for tunable friction metamaterials that can transition from low-friction to high-grip states on demand.”