Unexplainable or Not: Bikes, planes, ice skates
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It's time for another episode of Unexplainable or Not, the game show where we finally get some answers.

This week, our guest is Avery Truffleman. She's the host of the excellent Articles of Interest podcast. You might also know her from her time at 99% Invisible or Nice Try. Welcome, Avery. Thanks for having me. I'm terrified. You nervous about these scientific mysteries? Yeah, yeah. This is like so not my wheelhouse, but, you know, thanks for taking me in. I'm going to try to make you proud. I'm going to do my best. Oh, wait, no, we're adversaries. I'm going to beat you.

You're not really friendly. That is the attitude we want. Yeah, I'm not here to make friends. Yeah, I'm here to guess mysteries. Yeah, that's right. Okay, so here we go. Unexplainable or Not is a game show where you have to guess what we know and what we don't. You're going to hear three stories about scientific mysteries and the scientists searching for answers. But one of the mysteries has recently been solved. After you hear all three mysteries, you'll get a chance to guess which one you think scientists have actually figured out.

If you get it right, we're going to tell all our listeners exactly why Articles of Interest is the best show around. If you get it wrong, you're going to have to tell everyone you know why Unexplainable is the greatest podcast you've ever heard. I totally do that anyway, but I'm going to be like, yeah, yeah, you drive a hard bargain. Yeah, yeah, just do it with a little more verve this time. All right, Hasenfeld. So this week, we're doing three mysteries all about movement.

First up, we got a mystery about planes from our supervising producer, Meredith Hodna. Hey, Avery. Hi. How are you feeling about planes? You like a nervous flyer? You a... Uh...

I'm not the biggest fan of planes, but I have to say, I feel like the people I know who are the most flight-phobic are the ones who are the most plane nerdy. So it's like this weird horseshoe theory where the people who are most into it actually, like, hate it the most, I find. Well, I have a plane nerd for you. My wife loves air crash investigations. Oh, she's one of those. She loves the TV shows. Yeah.

She loves learning all about horrific plane crashes and why they happen. Oh, my God. And, of course, she's always reminded of, like, what she's learned and wants to share right when we're on a flight and, like, hit a bad patch of turbulence. Not ideal timing. But we actually don't understand how turbulence works. So I'm sorry if any listeners are going to have to board a plane. Oh, no.

My apologies. Wait, what do you mean we don't understand how turbulence works? So we'll have to start with the idea that air is a fluid. Right. So the best way to imagine this is actually a throwback to an old episode of ours where we talked about clouds. And I talked to Gavin Peter Penny. He's the founder of the Cloud Appreciation Society.

And he told me that we don't live beneath the sky. We actually live in it. And the sky is an ocean that we inhabit. We just happen to live on the bed of that ocean. Bro! Right? So when you're in the plane, imagine your plane is actually a submarine in this ocean of air. And what we feel is turbulence is our submarine getting pushed and pulled by these air currents.

And scientists are desperately trying to like tease apart any patterns to be able to predict

how these air currents will flow. Turbulence isn't like a huge problem for like planes because in a wide open sky, pilots can fly above or below these currents of turbulent air. But understanding turbulence is critically important for like lower flying machines like helicopters or drones navigating a downtown city area with skyscrapers or like if we ever want jetpacks, you know, we should probably figure this stuff out.

So the closest tool we have to even beginning to describe how these currents works is a set of equations called the Navier-Stokes equations, and they

They were developed like over 150 years ago. And they work. Well, there was a million dollar prize for solving these equations because understanding how fluids flow in different ways goes way beyond just avoiding a bumpy flight. Like it's the key to understanding how like blood flows through the body or how the ocean waters flow around the planet or even like how gases flow around the universe. Whoa.

So Avery, what do you think about this whole we don't understand how turbulence works thing? I just, I feel like this is the kind of like beautiful thing that we should never figure out. It does not bother me that we do not understand why there are like ripples in the beautiful ocean of air that we are all swimming in and that they're unpredictable. You know what I mean? Even when you're in a plane? Yeah, even when I'm in a plane. You're just like, you know, turbulence happens. I...

I find there's comfort in the fact that the captain in his Chuck Yeager voice is like experiencing a little bit of turbulence. And like, I feel like if we could map out the turbulence, I'd be more pissed. I'd be like, why didn't you know this turbulence was happening? You didn't check the Google Maps for the sky ocean. It's so big that it's cool to realize that we don't know it. Like, I didn't even realize that this was a thing we didn't know. But I'm also OK with

Me, personally, an idiot, am like totally fine with not knowing this. Unless we already know it. Unless it's already solved. So that's how turbulence impacts planes. Okay, okay, okay, okay. Next up, we got science editor Brian Resnick, and he's got a mystery all about ice skating. Ooh, I love ice skating. I'm terrible at it, though I appreciate it. Yeah.

I'm one of those, like, my knees buckle inward. I'm like, you know, kind of like searching for the railing. But yeah, so this mystery is a lot more quite literally down to earth than what Meredith was describing. It's about ice. And a deceptively simple question here is, why is it slippery? What?

Specifically, we're talking about the scenario of ice skating, but this applies to a lot of scenarios. You put a blade to ice and you can immediately glide on it.

And of course, this isn't weird, you know, for us because we've seen this, we've done it. But it's really weird in the sense that, like, if it's cold enough, ice is just kind of like a rock, you know, it's frozen water. And it's not like we can just skate on, you know, quartz or your granite countertops or concrete or any sufficiently smooth surface. Right.

There's something about ice that makes us able to skate on it. And this has been a question that's been up in debate over centuries. And what I really like about it is, on one hand, it's kind of trivia. It's like, oh, how does ice skating work? But really, it's about the weirdness of water.

So if we understand why ice is slippery, we can understand a little bit better, like, how glaciers move, which have, you know, formed, you know, a lot of the surface features of the Earth. Okay, but isn't it just that, like, it's not like a piece of quartz. It's made of water, so it's, like, sweating all the time, you know? And so obviously it's, like, a little bit wet. Yeah, yeah. An igneous rock isn't about to transform back.

at any time and ice is on the precipice of transforming. That's a good idea. You're very nice. Ice is a solid at below 32 degrees Fahrenheit. Yeah. But there are some answers that have been proposed. So one, you can demonstrate with a very simple experiment. So imagine you have a big cube of ice.

And you lay a wire across the top of it. And on each side of the wire, you hook up a weight. And the wire is putting pressure down the ice. What you'll see over time is that wire will cut clean through the ice.

So this was worked out in the 19th century by a scientist called James Thompson. I'm told he's actually more famous for being the brother of another famous scientist called Lord Kelvin, who has the temperature thing named after him. It's funny comparing scientific siblings because Lord Kelvin's Wikipedia page is much longer than James Thompson's. But Kelvin's got real cred. Kelvin's like the Linux of science.

Temperatures, you know what I mean? Absolute zero. This is like the black sheep brother who discovered something really cool, or at least worked out the math for something really cool, in that this is a really weird property of ice. It's not like you can squeeze any rock and it becomes more liquid, at least immediately. A weird property of ice is that it's less dense than water, so your ice cubes float on the top of the surface. And Thompson worked out, well...

If ice is less dense than water, if you put pressure in it, if you squeeze ice, it might want to become more like water again because water is more dense than ice. But turns out, like, this can't be the answer because you have to be extremely heavy to make the pressure melt the ice enough to skate on it. Like, even tiny children can skate on ice. Right.

So this is not the answer. Wait, isn't it that like, okay, with ice skating, you can't make the room too freezing because people are in it. Like the fact is that it still has to be warm enough for people to have a good time. And so the ice is not going to be absolutely frozen and it's going to be sweating a bit, right? Yeah.

Yeah, but you can ice skate on, you know... On a pond. Outdoor ranks. Yeah, it can happen. That's true. And there have been other ideas, too. So, like, maybe gliding on the ice, the blade creates some friction that, like, you know, when you rub your hands together, it warms up, and maybe that melts the water. That also doesn't work out because...

Friction doesn't work instantaneously, and ice is immediately slippery. It's an instantaneous thing. So these are so intuitive, like either pressure melting ice or friction. But when you put the math to it, they turn out they don't. And what I like about here is that ice being this super mundane substance is telling us it has some deeper truth to it. It's something else happening. And do we know what that something else is?

Oh my God, both of these are so...

Existentially gorgeous, like our ocean of air and our mysterious water. My God. Okay, so we got turbulence, we got ice skating, and now I've got a third potential mystery for you. And it's all about bikes. I love all these activities. This is so fun. So up until a couple years ago, embarrassing fact about me is that I did not know how to ride a bike. That is so cute. Oh, thank you. That's such a nice...

Kind reaction to that fact. I was always embarrassed about it. I was always like afraid to try to learn in public. Totally. I eventually learned it's great. Love biking. But I became less embarrassed about the whole delayed learning situation when I learned that we don't know how bikes work.

Get out. Come on. That's a thing. We made those. Those are man-made. Yeah, we made bikes over 200 years ago, but we don't really know how they stay up so well. So to be specific, what we're talking about here is when you take a bike and you push it with no rider and it just kind of rolls away, you'll see that it actually stays up for a while. It doesn't just immediately topple over as long as it's moving forward. Huh. Yeah.

And this is called self-stability. Like the design of a bike itself makes it very, very good at balancing. It's why bikes are so easy to learn to ride and hard to forget. Or easy if you're not me. But basically, there have been a couple theories floated, two main theories.

The first one has to do with a gyroscope. So a gyroscope, you know, like a spinning top, and it tends to balance itself. So if you kind of nudge it to one side, it'll sort of right itself because it's spinning so fast. Right. And then if you think of the gyroscope, you kind of flip it on its side. The gyroscope is turning forward. Oh.

that's sort of what the bike wheels are doing. So if you kind of nudge the bike wheel to one side, it's going to turn, balance itself, and then continue moving forward. Wait, so why is that not the answer? That sounds like the answer. It really sounds like the answer, right? Yeah, that sounds right to me. That was accepted as the answer for 60, 70 years. It came around in the 1900s, was accepted till the 70s. Then in the 70s, this guy's like,

These equations are wrong. There are some like basic math problems in these equations. And what he did is he decided to build a bike that had counter-rotating wheels to balance out the effect of the gyroscope.

And he realized that the bike still has this magic effect of self-stability. So it doesn't come down to the gyroscopic effect. So he made a fixie. He was like, behold. Exactly. He made this weird bike that still technically counts as a quote bike, but doesn't have the effect that we thought kept a bike up. Yeah.

So he thought it came down to the caster effect, which is basically that on a bike, the wheel where it touches the ground is kind of behind the steering axis, like the line that holds the wheel. So it's kind of set back a bit. And you can imagine this by thinking of the wheels on like a shopping cart, which are kind of set back so they can turn left and right. Yeah, yeah, yeah. And the idea is that because the wheel touches the ground behind the steering axis, it's

When the cart or the bike is pushed, the wheel just kind of trails the steering axis. So if you nudge it,

it left, it'll just trail the steering axis, bounce itself out. And then you've got this sort of self-stable bike. But then someone just came along and built another weird bike that canceled out the Caster effect. I hope they like did a derby with all of these bikes. I hope they took all of them and raced them. These bikes are the weirdest looking things. They have these weird kind of like big diagonal lines sloping in all these different directions. And

But basically, this one also still balanced if it moved forward. So ultimately, we have these two ideas that we thought were really important. We know they're part of the explanation. We just don't know. As the scientist I spoke to said, he said, they're not the keys to the castle here. They're not the core things that are keeping the bike balancing. And if we could figure this out, we might be able to

you know, make other things balance better. Like, you know, e-scooters are kind of hard to balance on. And if we figured out the like core thing that makes a bike able to balance, we might be able to make those e-scooters better. Or we might have already figured that out and made those e-scooters enough to balance on because we actually do understand how bikes work. And I've been playing you. Ah, yeah.

My God. Okay, the fundamental nature of air, the fundamental nature of ice, or the fundamental nature of balance. Those are your three potential mysteries. And you'll get a chance to guess which one has actually been solved after the break. Support for Unexplainable comes from Greenlight. People with kids tell me time moves a lot faster. Before you know it, your kid is all grown up. They've got their own credit card.

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We're back. It's unexplainable or not. Avery, welcome back. Thank you. We've got three mysteries here. Mystery one, how does turbulence work? Mystery two, why is ice slippery? And mystery three, how do bikes balance so well? Which one do you think we've solved? Which ones do you think are still unexplainable?

I... Okay. Gut impulse is turbulence feels too completely gigantic and holy to, like, even touch. Okay. Ice is almost the opposite. Ice is like...

It's so obvious. Like, why don't we know this? This is infuriating. And I don't know if that means it's more likely to be solved. But that's when I'm like furious because I want to know the answer to. I mean, the thing about the bikes is like there's so many like bike people and bike scientists. I'm leaning towards I'm leaning towards bikes right now. OK, are you ready to lock it in? This is your chance. Uh.

- We'll give you like a sound effect. We'll give you like the who wants to be a millionaire, like . - Yeah, exactly. I feel like I'm gonna regret this. I'm gonna say bikes. I'm gonna say we figured it out. I'm gonna say we figured out how bikes work. - Bikes. Final answer. - Sure, yes. Yes! - Okay. Here is the answer. - Ice is wonderful because its properties are difficult to understand, even though it is so omnipresent in our world.

We've figured out why ice is slippery. So what's the answer? Why is ice slippery? What is it? For the answer, Brian's going to tell you all about it. So is me. I'm the problem. It's me. That tape you just heard, I've been talking to David Limmer. He's a chemistry professor at University of California, Berkeley. And he told me all about this. You know, ice keeps its shape, but that the surface is so slippery, it's

is like the surface is acting more like a liquid instead of the solid that's retaining its shape. It was actually confirmed in the 80s, but the answer goes back to another weirdo property of ice that has less to do with why it's slippery and more to do with why, in certain circumstances, it's sticky. So imagine you go into your freezer and you take two ice cubes and you stack them on top of each other in the freezer and you leave them there.

You come back some time later and you find that those ice cubes are stuck together. You know, I don't have to worry about putting my phone on the table and it sticking. Most solids don't behave this way. So back in the 1800s, a scientist called Michael Faraday thought, well, there's something coating ice that allows it to stick together.

I mean, it's an incredible amount of insight from him. He realized that there was somehow an intrinsic layer of liquid water that always sits atop the surface of ice. Like it has a skin or something. Has a skin. So you were saying before, like, oh, maybe ice is just sweating all the time. You couldn't throw it.

Come on. I was, I was preserving the mystery as an agent of the show. I had to execute the mission. My, my greater devotion is to the truth, which I'm telling you now. You like Michael Faraday thought it right. That maybe there's like a thin coating of water on ice at all time that lends it some lubrication. But, you know, back in the 1800s, Faraday,

Faraday had no way to prove this. Like, ice just looks like a solid. And I think largely that's why his explanation went ignored. You know, the fact that there was no direct evidence for this let people dismiss it. Until the 1980s when scientists in Japan used X-ray imaging and saw it. They call it the quasi-liquid layer. It's like this little infinitesimally small skin of water on the surface of ice. The largest it gets is about six nanometers.

So a nanometer is about a thousandth the size of a micrometer, which is about the size of a piece of bacteria. So very, very small. And the reason why this happens, like on a molecular level, if you're interested in this, I think it's cool. So like imagine you're in a line of people and you're all holding hands.

Everyone in the middle of the line is holding two hands, and so they're stable in the line. But the person at the end has one arm flailing. That's kind of what's happening on the surface of water, that the molecules at the surface of water, they aren't as stable because they don't have as many other molecules to hold on to. And so what can they do? Well, maybe if they can't hold on to each other, maybe they can wander around.

And then with ice skating, it's like you already have this film of less friction on top of the ice so you can get started. And it just rests on the surface, just always. Yeah, most of the time. It actually can get too cold to ice skate. So as you get colder and colder, that surface of water gets thinner and thinner. So there are actually these reports of...

of explorers in the Arctic and Antarctic, when it gets to around negative 40 degrees Fahrenheit, it actually becomes really hard to ski. They say it's like skiing in gravel. Oh. Ice will become just as rough as any other solid because that intrinsic liquid-like layer will go away. What I really love about this is, like, apparent on the surface of ice, like,

It looks like a rock. You know, we usually think of matter in three phases, you know, solid, liquids, gases, but

But it's not like something, like, once it becomes a solid is a perfect solid. There's always that interface, always that, like, weird zone. And, you know, that's the zone we explore a lot on this show of, like, the is it one thing or is it another or is it somewhere in between? That's what I really like about ice. Like, every time you look at it, you can imagine, like, this weird boundary, this infinitesimally small area where ice is both ice and water at the same time. Man!

God, I feel like I could have guessed that. But yes, me and Michael Faraday are on the same page. You did. You kind of did. Yeah. So close. Yes. So you definitely get some credit. And I would have gotten away with it, too, if it wasn't for you rotten science podcasters. But what's interesting, too, is this kind of also intuitive idea that came up in the 1800s. We just did not have the technology to prove this for a very long time. So even simple ideas...

To fill in that blank space of knowledge, we sometimes need to invent whole new technologies. You need to wait for the right thing to come along.

That's beautiful. Do you think you'll be more excited about ice skating now, Brian? I'm still terrible at it. Actually, I want to know the secrets of balancing on bikes because I'm also not very balanced as a person. So I'm all these things. So I don't balance well on ice skates. If I fall, I create turbulence. You know, like I need all these horses aligned in me. All of these things that we understand or don't understand are things that lead to crashes. So Avery...

Because we did best you. You could, I doff, I doff my hat to you. Oh, thank you so much. You are now required to tell everyone you know how great we are. Oh, easy. I will also say that Articles of Interest is one of my favorite podcasts. And I really hope our listeners check it out if they haven't already. But I have a parting gift for you as well. Oh, man, is it what I think it is? It's song time. Yeah, it's a song. And yeah, here we go. I love this.

This is a banger.

Turns out pretty much all the time there's a tiny quasi-liquid layer on the outside So the ice is solid but it's kinda also liquid and it's Y-E-I-E, ice is slippery When it comes to water, the solid-liquid border is a bit more slippery than people thought it ought to be And that's Y-E-I-E, ice is slippery Oh, the drop! So the quasi-liquid layer on the outside Ice, ice

Yay! How much time do you have? This is your job. That's the real unexplainable thing. That's it. Avery, thanks so much for coming on the show. What'd you think of the game? This is like the greatest luxury to have the best...

the best correspondents for the best podcast. Thank you. Regale me with story. Like, thank you all so much. This was so cool. This was so fun. Thank you for coming on the show, Avery. Thank you to our storytellers, Meredith Hodna. And Brian Resnick. See ya. And thank you to the audience for joining us. If you want to come on the show and present one of these mysteries, or if you want to be a contestant and play our game,

Let us know. Write us at unexplainable at vox.com. And if you got a mystery for us, let us know too. We'll see you next time on Unexplainable or Not. This episode was reported, produced, and edited by Brian Resnick, Meredith Hodnot, and me, Noam Hassenfeld. Christian Ayala did the mixing and sound design. I wrote the music. And Zoe Mullick checked the facts. Neil Dinesha forgot what Deja Vu is.

Manding Nguyen is copying plants. And Bird Pinkerton walked toward the noise at the end of the hall. She looked down and found the source of the loud rumble. It was a small, spinning penny. And Neil Dineshia forgot what déjà vu is.

Thank you so much to Avery Truffleman for playing our game this week. Go check out her show, Articles of Interest. It's all about the complicated history and politics of fashion. And season two is about how preppy clothing is way more interesting and global than you might expect. You're going to love it. Special thanks this week to Andy Ruina and Holger Babinski. And since I know you're still listening through to the end of the credits, happy 70th, Abba.

If you have thoughts about this episode or ideas for the show, please email us or unexplainable at Vox.com. We'd also love it if you wrote us a review or a rating. Unexplainable is part of the Vox Media Podcast Network, and we'll be back next week.

Ice, ice, slipping on the ice on the quasi-liquid layer on the outside.