Radiolab - What a Slinky Knows

Episode Date: September 10, 2012

"Hey kids," said physicist Tadashi Tokieda, "Wanna see a magic trick?" He pulled out a Slinky and did something that amazed the kids, & their dad Steve Strogatz. Steve, along with Neil deGrasse Tyson, explains what the gravity-defying Slinky trick reveals about the nature of all things great and small (including us).

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From. W. N. Y. Se. C. Yes. And NPR. Hey, I'm Chad Abramrod. I'm Robert Crowicz. This is Radio Lab.

The podcast. Okay, so a little while ago, we were talking to our friend. Hi, am I on? And regular guest, Steve Stroggatz. Try that. He's a math professor at Cornell. Talk out of the side of my mouth, seen. He does have some mic positioning issues, but he came to our studio because he wanted to tell us about something he'd seen.

They kind of made him go. Whoa, man. And I just had all these funny, deep thoughts. And it had to do, oddly enough, with a slinky. Yeah. No, there's, I mean, there's actually a little more of a story. I don't know if you want to make it a real story. Yeah, story it up.

Okay, the real story is I was in England, you know, the past few months, and there's this character, like England is good with characters. They have these eccentric kind of guy. So they have this eccentric physicist named Tadashi. He was a philologist. He speaks like 12 languages, and he decided to be a physicist. And not only is Tadashi a multilingual pseudo-phalologist turned physicist. He also likes magic. Yeah, he's very good, like a magician, at entertaining little kids. And so one day, Steve and his kids are having... lunch with Tadashi. And at some point during that lunch, Tadashi turns to Steve's kids and he's like,

hey, you guys want to see a trick? A magic trick? And he shows my kids, you know, here, you know what a slinky is. Yes, the kids know. And then he holds out this slinky, which is at first it's all compressed. You know, it's a tight coil. Then he lets it the bottom stretch as much as it wants to. But he's still holding onto the top. You know, so it's all stretched out. And then he says, now watch carefully. I'm going to let go of the top. But watch the bottom. Watch closely. So he's holding it out. The kid's eyes are open wide.

And then, you let's go. It's like he falls to the ground. But the weird thing, the unbelievable thing, is that the bottom doesn't move at first. The bottom doesn't go down? The bottom does not go down.

He's held the top. He released the top. And the top starts to fall, but the bottom doesn't. When you see the bottom doesn't go down, what does that mean? What is the bottom doing? You're just hovering?

The bottom is hovering in mid-air. No. Like a flying carpet. It's just sitting. there, but not for long. For a fraction of a second, the bottom of the slinky seemed to defy the laws of physics. It looks impossible. Wait, wait, wait. Now, why is it, so what would we normally think the slinky should do? That it would fall. It would all fall together. It's a coil, so couldn't you say the top is pulling on the bottom, holding it up?

Well, let's remember how we prepared it. First, we let the slinky come to equilibrium. We hold the top, We let the bottom stretch out as far as it wants to until everybody's happy. It goes, woo, woo, woo, and then it damps itself out and sits there. Yeah. Okay, so that means the bottom is... At rest. It's at rest because it's being pulled up just as hard as it's being pulled down.

It's being pulled up by the slinky. By the tension in that slinky coil, pulled down by gravity, they're exactly matched. So it says, I'm cool, I'm here, I'm not going anywhere. Now, at the top, the tension. in the spring is pulling the top down, as well as gravity is pulling it down. The only thing holding it up is the fingers. The next thing that happens is

the sinister magician, let's go of the top. Now, there are actually videos of this online where you can see people dropping gigantic slinkies, like 100 feet long slinkies, off of balconies. And in the video, they also slow the frames way down so you can see everything that happens. And here's what you see. You see the top

of the slinky, the first coil, turn, collapses onto the turn just below it, and then just below that one, and just below that one, and you can watch the thing collapse, hitting towards the bottom. Meanwhile, the bottom of the slinky is just hovering. Even though the top has been let go, the bottom doesn't know anything has happened. Wait, what? The bottom doesn't know that the top is free. The bottom doesn't know. The verb knowing is throwing me up. So here's the way the physicist would talk about it is the bottom doesn't feel anything except that little bit of metal right above it. And as long as the little bit of metal right above hasn't changed in any way, then the very bottom doesn't know that it's supposed to fall.

That is exactly right. There's these two ideas there. There's a kind of physics-y idea about what the top is doing relative to the bottom. And then there's this concept of knowledge, which is why I thought this would be fun to talk about. Because we don't usually think of things like Slinkies as having information and knowledge. There's a much deeper meaning to the idea of information in the world around us than we're used to thinking. And Steve says this goes way beyond slinkies. When I drop a pen, in fact, the bottom hovers for a few billions of a second while the top is falling and compressing the pen. You're kidding? Yeah, it really has to be like that.

When anything falls, it falls like a slinky, just the difference being does it so fast that you can ignore it. Oh, by the way. Go ahead. Oh, you should know this. That's Neil. I'm Neil deGrasse Tyson. An excitable physicist with the American Museum of Natural History. We actually brought him this argument so we could find out what he thought.

I've played with the Slinky or two in my day. I mean, it's my generation's toy, right? You never knew, however, that there was this levitating thing. I never knew about the levitating properties of a slinky. But after he thought about it for a bit, he told me something surprising, that there's an awesome consequence to the fact that one part of an object has no idea of it. what's going on in an adjacent part. For example, in anti-tank warfare,

you want to get through armor, or an armored tank. You can fire a bullet into the side of the tank, and the bullet flattens and puts a little dink, and nothing else happens. So what the military... According to Neil, what they do... They shoot rods at the tank. Not bullets, rods.

Why do they do that? Why do they do that? If you shoot a rod, the first part of the rod hits the tank, It gets pulverized. But the second part of the rod does not know that yet. It's waiting around for the wave to reach it. So then the next piece hits and the next piece, every molecule of that rod is hitting the tank

without ever knowing that there was a molecule in front of it that got vaporized. There's a bow and arrow. Same thing with a bow and arrow, he says. Can you take an arrow and just use your arm and shove it into a tree? No. But with a bow you can. With a bow, a powerful enough bow, you can fire an arrow fast enough so that every segment of the arrow doesn't know that the front of the arrow just hits something and it keeps pushing it. So in other words, you think once the front of the arrow hits something, it slows down the whole arrow.

Yes, that's what I think. It doesn't. The last bit of it hits it at the same speed that the front of it did. The arrow is saying, coming in, ooh. Go on. Next part. Coming in, ooh, goes away. Coming in, sequentially, straight in.

But now the next bit of arrow comes in and it gets further, Just again. Exactly. So it just keeps getting a bigger and bigger hole. Right. So if you have a rod that is the length of 10 bullets, that single rod is effectively 10 consecutive bullets hitting exactly the same spot. So the military has exploited this fact, this simple slinky fact, that one part of the

slinky has no clue what's going on with the rest of it. That's right. This is the nature of reality. The slinky shows you how it really is. So you're saying the slinky reveals something about the nature of everything? There you go. That's why I think it's so cool. What it's revealing is what physicists call locality. All you can feel in this world and in this life is what's right on you. What's right on your fingertips, what's right on your senses. And you don't know, I don't want to say what I want to say. You don't know anything until the right wave hits you. The example I like to think of is the sun. The sun is there. We feel it's gravity. It's warmth. It's light. If some cosmic force, pull. the sun from the center of the solar system instantaneously,

we would have no clue. We would continue to orbit this empty spot in space. We would continue to feel sunlight and warm. Would we see a glow? You would see the sun for eight minutes. Even though it's not there. You don't know it's not there. Okay.

The edge of the wave hasn't reached you yet. The earth doesn't, nobody knows it. For eight minutes and 20 seconds. Because eight minutes and 20 seconds after it's plucked from the center of the solar system, your world goes dark, it goes cold, and Earth flies away at a tangent to be lost forever in interstellar space. Have a nice day. Now, this feels like an extraordinarily rich notion in some Greek tragic way or something. Like, you are, when something changes in your world, you are fated to some kind of experience,

but you don't know it, and yet it's inevitable. It's coming at you. Yes. Coming at you is the perfect way to think about it. But what does this do to the thought that you are in charge of your own experiences? If... Well, it's just a time delay. I mean, you want to get philosophical, and I respect that,

and you're a philosophical man about town, but it's just a time delay. I mean, suppose it happened instantaneously. Then it would happen now instead of a half a second from now. What difference does it make to you? Well, because I have a period of... being in the dark. That is, you know, later on, I'll have knowledge that I was fated to a certain consequence. And all that time, the rest of the slinking was coming at me, coming at me, coming at me, coming at me, coming at me, coming at me, coming at me, coming at me, coming at me, coming at me,

and I had no idea. I was just innocent in my end of slinkiness. By the way, that happens all the time, for example, the dinosaurs, there was an asteroid headed their way, and not a last one of them knew about it until it was too late. This happens all the time. But that makes it seem sort of matter of fact. I, being the man about town, as you just described, the philosophical man about that. I feel that there's something somewhat frightening about this notion as opposed to just simply

there. It's not my fault if your brain operates differently from the inevitability of something, from not knowing that it was going to happen at all. You are so, you are so calm about this. I'm a practical guy. I'm not going to, you know, I'm not going to scratch my head over this one. No, it's when you know about it, you know about it. But you're angst about something about to happen to you, but you don't know.

So you're going to worry about the thing that might have happened to you that you don't know. I can't, life is too short. For me to worry about something that I have no control over that I don't even know what will happen. Someone said if Earth is going to be swallowed by a black hole or if there's some disturbance in the space time continuum, should we worry about it? No, because you won't know it until it crosses your place in space time. So the life lesson here is like you live in your own tempo, you know, and you get your beats and that's just it. Your beats come to you when nature decides it's the right time. Yeah.

Be it the speed of sound, the speed of light, the speed of electrical impulses. We will forever be victims of the time delay between information around us and our capacity to receive it. Yeah, that's it. That's the picture. You're always the bottom of the slinky. Big thanks to Steve Strogatz, who is a professor of mathematics at Cornell University. His new book is called The Big X.

No, it's the joy of X. Or the joy of X, which is like the joy of sex, but it's missing the S. And the E. And the E. And the sex. And the sex. But you get the last letter,

which is, of course, the only thing that really matters. As many copulating couples have always said at the end, You know, just before the cigarette, I really like the X part. And I think we all agree with that. It takes to Neil deGrasse Tyson. His latest book is called Space Chronicles Facing the Ultimate Frontier. And thanks also to Matthew Kielte for production help. Hi, this is Brian Ross from Brooklyn, New York, and I am a radio lab listener.

Radio Lab is supported in part by the National Science Foundation and by the Alfred P. Sloan Foundation, enhancing public understanding of science and technology in the modern world. More information about Sloan could be found at www.sloan.org. End of message.