科学美国人60秒:Math Cracks a Knuckle-Cracking Mystery

<<knuckle-cracking sound>>

Knuckles cracking. You may not mind the sound. You may despise it. Or you could study it.

Couple years back, Vinny Suja was taking a biomechanics class at the French Polytechnic School, outside Paris. And he was on the hunt for the perfect class project.

"Even though they suggested many projects, I couldn't find one which was both practical and that I could complete within the framework of this class. So in frustration, I was cracking my knuckles one day and that's when I realized—'Huh, that's interesting.'"

And so a project was born: the physics of knuckle cracking. It's actually a subject of intense scientific investigation. Back in 1971, scientists figured they knew how it worked: the cracking sound was caused by bubbles popping within the fluid surrounding the knuckles.

<<cracking sound>>

Or so they thought—because in 2015 shots were fired, in the form of MRI visualization of the knuckles post-cracking.

<<cracking sound>>

In fact, the bubbles were still there. The whole process happens too fast for imaging technology to visualize in real time—you’d need to shoot at 1,200 frames per second, 10 times faster than the best x-ray and MRI machines on the market. "And that's when we realized that a model could help people better understand the origin of this sound."

So, using mathematical models, Suja and his colleague Abdul Barakat found that just a partial collapse of the bubbles could cause cracking sounds of the same degree, which might explain why the 2015 researchers still saw bubbles after the crack. The details are in the journal Scientific Reports [V. Chandran Suja & A. I. Barakat, A Mathematical Model for the Sounds Produced by Knuckle Cracking].

Further modeling of bubble behavior, both pre- and post-pop, will be needed, they say, before they’re confident that they've truly cracked the case.

<<more cracking sounds>>

—Christopher Intagliata