Science Friday - The Surprising Science Of Why Sneakers Squeak

Episode Date: March 9, 2026

March Madness is almost upon us, which means basketball arenas across the country will be filled with the thunderous roar of fans and the surprisingly loud squeaks of basketball shoes. At his first NBA game, physicist Adel Djellouli was surprised by the constant noise from the court and wondered, why do basketball shoes squeak? Turns out, the physics of a squeak involves lightning bolts and earthquakes. Host Flora Lichtman talks with Djellouli about his research and the joy of investigating seemingly simple questions. Guest: Dr. Adel Djellouli is an experimental physicist at Harvard University. Transcripts for each episode are available within 1-3 days at sciencefriday.com. Subscribe to this podcast. Plus, to stay updated on all things science, sign up for Science Friday's newsletters.

Hey there, it's Flora Licksman, and you're listening to Science Friday. March Madness is around the corner, and of course that means a lot of this. What is the science behind that signature sound? Look, if you're tempted to tag out, don't, because it turns out the physics is sticky and surprising and involves lightning bolts and earthquakes. I'm not kidding. So, lace up, get your mind off the bench, because joining me now. to talk about this is the lead author on a new paper in nature that investigates the squeak. Dr. Adele Joluli is an experimental physicist at Harvard. Adel, why this question?

Thank you for having me, Flora. Well, the reason why I ask myself this is because most of my projects are curiosity-driven. I learned through my training during my PhD that even the most simple questions can be deceptive. So when I first arrive to the US, one of the first things that you do as an immigrant is to get accustomed with the local people is that you go to their sports events.

And let me tell you that Boston people and Bostonians are very much fans of their different sports. We know. We know. We know, we know. And so I went to a Celtics game for the NBA. And, you know,

there are two things that strike you. The first thing is how enthusiastic the crowd is. And the second one is this omnipresent sound that never leaves your ears when you're in the stadium, which is the squeaking of basketball shoes when players are sliding, right? And so for me, it was why?

Simple question. Why do basketball shoes squeak? Okay, so run me through your setup. How did you figure this out? Well, one of my interns was a fan of basketball, and he had these beat down basketball shoes. So I asked him to borrow just one shoe, not the two. I love that you needed a used shoe.

I feel like there's, you know, you couldn't buy a. shoe. I love that. You know, it's it's not that easy because if you go to your purchasing department and you tell them, I want new basketball shoes, they start asking questions, you know, even though the money that you got is from a grant that you wrote, it's your sweat and, but they still ask your questions, you know, checks and balances. I got it. I got it. Okay. So you have this used basketball shoe, and then what? And then, you know, we're a friction lab. So we're used to visualizing frictional interfaces.

And it's, and it is quite simple to do. You take an acrylic plate transparent. You put LED around it and you put black tape all around that LED strip. You connect it to light and then you have this very simple yet powerful optical setup called Totally Internal Reflection. It's a great tool, a great setup to visualize just the contact. So when you have no contact or loss of contact, it's dark. When you have contact, it's bright.

Okay? Because it's reflecting it back, like a mirror. Reflecting back. And then you put in a camera, you put in a microphone, you synchronize it to, you take a beat down basketball issue, and you do a test. And we were recording with this very high-speed camera that can go up to a million images per second. And we saw something that we did not foresee. Tell us.

All right. So what you see when you rub or when you slide a basketball shoe on the smooth, dry surface is these ripples. Think about them as wrinkles. so the sole of the shoe wrinkles, and that wrinkle travels at supersonic speed. And the frequency of repetition of these fast-traveling wrinkles sets the frequency of the sound. Hmm.

Yes. So your shoe is not sliding uniformly. No. It's like sticking and slipping. Is that right? It's what we thought shoes did as they all. all stick in a block and they all move in a block. Instead, what we see are what we call slip pulses.

So what does it mean slip pulses is that the whole interface, frictional interface between the shoe and the smooth surface, rigid surface, is stuck. Nothing moves except the regions where this wrinkle is traveling. And so think about it as almost like when you take a rug and then you give it a shake, and you see that fold travels through it. That's how I would try to convey what's going on at the frictional interface. Your physicist, was that cool and surprising?

Like, put that finding in context. Okay, so usually what you think about rubbers are very boring. No disrespect to the rubber tribology community, but it's usually boring. It's usually slow. And when you see these supersonic slip pulses, it is quite exciting for us because usually these kind of events, this kind of phenomena, you see mostly in geophysical settings. So, for example. Like plane tectonics? Yes.

Yes. When you have an earthquake or when you have a rupture, the dynamics of this rupture versus the shoe squeaking or the shoe. shoe moving, I share a lot of similarities that we did not foresee. Okay, so a shoe squeaking across the surface is acting like two continental plates banging against each other? Sliding against each other, yes. Yes. So it's like an earthquake on the basketball court.

I would say maybe like a shoe quake, you know? So it's a quake at a different scale. instead of being hundreds of miles long, it's a few inches. A shoe quake. Yeah. Amazing. Okay. What else did you see?

Because I know you were watching this in high speeds. You saw these waves. You find the shoe quake. What else do you find? You know, this is why I love experiments is because it's the ultimate simulation and it's very surprising. It was 10.30 p.m. on the winter night. And Gabriele, my co-first author and I, we were in the lab doing experiments.

And, you know, we watched the movie after we do the experiment. And it's several millions of images that we record. So we go one by one to try to look at what's going on. And what we saw there was something very surprising that I assumed at first to be a glitch or the camera misbehaving. But then when you look closely and when you repeat the experiments, you see systematically lightning as a trigger for these opening slip pulses. And, you know, I don't blush.

I don't, I don't get, I don't get this rush usually. But at that moment, I was like, this is so cool. This is really one of the coolest moment in my life to share that kind of happiness and that kind of, I don't know, excitement to see lightning. Lightning under a shoe or lightning under rubber, right? Shoe quakes and now shoe lightning. Exactly. That's happening every time. So it's just a charge, an electrostatic charge that happens every time that you hear a squeak?

Yeah, so think about it. You know, when you have this kind of wool sweater and that you remove, you probably, or your audience probably has experienced this before where you get a lot of tiny jolts or zips, electricity zips. Basically what you do when you rub two objects against each other, you create an imbalance in electric charges. And when this imbalance becomes big enough when it has sufficient potential, it discharges, it discharges. to equilibrate or to balance back these charges. Okay? When you slide a rubber on a smooth surface, you create this imbalance by rubbing,

and then that same rubbing creates this discharge, that think about it as a mini explosion that drastically increases temperature locally, which increases the pressure, and triggers these open impulses. I mean, is the squeak the thunder to the shoe lightning? In a way, yes, but you need a certain condition to be met. So it's not a general means for triggering these slipples, but it's one of them that we have observed, yes.

For me, this actually really makes me appreciate the squeak, you know, as someone who doesn't really think of the squeaking as a value add to my basketball. experience. Now I do because every time I hear it, I'm going to be thinking, a little lightning bolt was created and this is like the physics that's happening in an earthquake. That's amazing. Yeah. And it's like several thousands of shoe quakes happening per second. So in a sense, it's a super cool set of phenomena under a deceptively simple question. I feel bad for the microbiome on those shoes. It's just, it must be really turbulent for them. Yeah, and let me tell you, Flora, so we were very curious to see how universal this is. Does it just happen for rubbers or can it happen to your hand, for example? And if you slide your hand on a smooth surface and you slide

it quite fast, let's say a mirror, you will hear squeaking. You will hear your hand hissing at quite a high pitch. And if you image your hand sliding on an acrylic plate with these LEDs and this high-speed camera, you would see these hand quakes traveling at hundreds of meters per second and repeat in tens of thousands of times per second. Okay. Adele, it seems obvious that you had fun doing this study. Did you have a favorite element?

Yes, I would say it felt to me like a scientific R.Cu Poirot-O story, right, with a lot of unexpected twists. We had to challenge our assumptions all the time, confront our biases, and more than once go back to, you know, to the drawing board and say, hey, what did we miss? right? And it's a story about stubbornness, obsession, perseverance, and I would say creativity. You know, interestingly, that's like the story of many great sports achievements, too. I agree, yeah. You need to be stubborn. And creative and persistent. Yes, yes. We can't end this interview without hearing some art that you made to go along with your study,

which is quite unusual. Let's play it and then you can tell us why. How long did it take to do Darth Vader's Imperial March with rubber blocks? It took us three days, basically. Three days to rehearse this improbable squeaky band. With three different people, each one of us had two or three notes that we needed to slide. And you need to determine what is the length of sliding to get the proper tempo. So there's a lot of coordination. to produce that, I don't know, 15 seconds of music.

But it was a lot of fun, at least for us. Adele, I can't wait for your next study. Well, I'm probably leaving academia to produce something more, let's say, impactful. Really? Okay, well, that's a story for another time. Dr. Adel Joluli is an experimental physicist at Harvard University. Thank you so much for joining me today. Thank you very much, Flora. This episode was produced by Rasha Auretti.

We will catch you tomorrow. I'm Flora Lichtman.