[RADIOLAB INTRO]

JAD ABUMRAD: Hey, I'm Jad Abumrad.

ROBERT KRULWICH: I'm Robert Krulwich.

JAD: This is Radiolab.

ROBERT: The podcast.

JAD: Okay. So a little while ago, we were talking to our friend ...

STEVE STROGATZ: Hi. Am I on, or do I ...?

JAD: ... and regular guest Steve Strogatz.

JAD: Yeah, try that.

JAD: He's a math professor at Cornell.

STEVE STROGATZ: Talk out of the side of my mouth, see?

ROBERT: He does have some mic positioning issues, but he came to our studio because he wanted to tell us about something he'd seen ...

JAD: That kind of made him go ...

STEVE STROGATZ: "Whoa, man!" [laughs] And I just had all these funny, deep thoughts.

ROBERT: And it had to do, oddly enough, with a Slinky.

STEVE STROGATZ: 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.

JAD: Yeah, story it up.

STEVE STROGATZ: 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 guys. So they have this eccentric physicist named Tadashi. He was a philologist. He speaks like 12 languages. And he decided to be a physicist.

JAD: And not only is Tadashi a multilingual pseudophilogist turned physicist, he also likes magic.

STEVE STROGATZ: Yeah, he's very good, like a magician at entertaining little kids.

JAD: 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?"

STEVE STROGATZ: 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 the bottom stretch as much as it wants to.

JAD: But he's still holding onto the top.

STEVE STROGATZ: You know, so it's all stretched out. And then he says, "Now watch carefully. I'm gonna let go of the top, but watch the bottom."

JAD: Watch closely.

STEVE STROGATZ: So he's holding it out. The kid's eyes are open wide.

JAD: And then ...

STEVE STROGATZ: He lets go.

JAD: It's like he falls to the ground. But ...

STEVE STROGATZ: The weird thing, the unbelievable thing, is that the bottom doesn't move at first.

ROBERT: The bottom doesn't go down?

STEVE STROGATZ: 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.

ROBERT: When you say the bottom doesn't go down, what does that mean? What is the bottom doing?

STEVE STROGATZ: The bottom is hovering in midair.

ROBERT: No.

STEVE STROGATZ: Like a flying carpet. It's just sitting there. But not for long.

ROBERT: For a fraction of a second, the bottom of the Slinky seemed to defy the laws of physics.

STEVE STROGATZ: It looks impossible.

JAD: Wait, wait, wait. Now, why is it—so what would we normally think the Slinky should do?

ROBERT: That it would fall.

JAD: It would all fall together. But it's a coil, so couldn't you say the top is pulling on the bottom, holding it up?

STEVE STROGATZ: 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 'til everybody's happy.

JAD: It goes waa-waa-waa.

STEVE STROGATZ: And then it damps itself out and sits there.

JAD: Yeah.

STEVE STROGATZ: Okay? so that means the bottom is ...

ROBERT: At rest.

STEVE STROGATZ: 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.

ROBERT: By the tension in that Slinky coil ...

STEVE STROGATZ: Pulled down by gravity. They're exactly matched. So it says, I'm cool, I'm here. I'm not going anywhere. [laughs]

ROBERT: Now at the top ...

STEVE STROGATZ: 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 lets go of the top.

JAD: Now there are actually videos of this online where you can see people dropping gigantic Slinkies, like hundred-feet-long Slinkies off of balconies.

ROBERT: 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, first 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, heading towards the bottom. Meanwhile, the bottom of the Slinky is just hovering.

STEVE STROGATZ: Even though the top has been let go, the bottom doesn't know anything has happened.

JAD: Wait, what?

STEVE STROGATZ: The bottom doesn't know that the top is free.

JAD: Wait, the bottom doesn't know? The word "knowing" is throwing me off.

STEVE STROGATZ: So here's the way that a physicist would talk about it is the bottom doesn't feel anything except that little bit of metal right above it.

ROBERT: 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.

STEVE STROGATZ: 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.

JAD: Hmm.

STEVE STROGATZ: There's a much deeper meaning to the idea of information in the world around us than we're used to thinking.

JAD: And Steve says this goes way beyond Slinkies.

STEVE STROGATZ: When I drop a pen, in fact, the bottom hovers for a few billionths of a second while the top is falling and compressing the pen. But it's all—yeah, it really has to be like that.

ROBERT: Huh.

STEVE STROGATZ: When anything falls, it falls like a Slinky. Just the difference being, it does it so fast that you can ignore it.

NEIL DEGRASSE TYSON: Oh, oh, oh, oh. By the way ...

JAD: Go ahead.

NEIL DEGRASSE TYSON: Oh, you should know this.

ROBERT: That's Neil.

NEIL DEGRASSE TYSON: I'm Neil DeGrasse Tyson.

ROBERT: An excitable physicist.

NEIL DEGRASSE TYSON: With the American Museum of Natural History.

ROBERT: We actually brought him this argument so we could find out what he thought.

NEIL DEGRASSE TYSON: I've played with a Slinky or two in my day. I mean, it's my generation's toy, right?

ROBERT: And you never knew, however, that there was this levitating thing.

NEIL DEGRASSE TYSON: I never knew about the levitating properties of a Slinky. [laughs]

ROBERT: But after he thought about it for a bit, he told me something surprising. That there's an ...

NEIL DEGRASSE TYSON: Awesome consequence to the fact ...

ROBERT: That one part of an object has no idea what's going on in an adjacent part.

NEIL DEGRASSE TYSON: 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 ...

ROBERT: According to Neil, what they do ...

NEIL DEGRASSE TYSON: They shoot rods at the tank.

ROBERT: Not bullets, rods.

NEIL DEGRASSE TYSON: And why do they do that?

ROBERT: Why do they do that?

NEIL DEGRASSE TYSON: 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.

ROBERT: Same thing with a bow and arrow, he says.

NEIL DEGRASSE TYSON: 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 hit 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?

ROBERT: Yes, that's what I think.

NEIL DEGRASSE TYSON: It doesn't. The last bit of it hits it at the same speed that the front of it did.

ROBERT: The arrow's saying, "Coming in! Ooh!" Gone. Next part, "Coming in! Ooh!" Goes away. "Coming in! Ooh!"

NEIL DEGRASSE TYSON: Sequentially straight in.

ROBERT: But now the next bit of arrow comes in and it gets further. So it just keeps getting a bigger and bigger hole.

NEIL DEGRASSE TYSON: 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.

STEVE STROGATZ: That's right. This is the nature of reality. The Slinky shows you how it really is.

JAD: So you're saying the Slinky reveals something about the nature of everything?

STEVE STROGATZ: There you go! That's why I think it's so cool. [laughs] 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.

NEIL DEGRASSE TYSON: The example I like to think of is the sun. The sun is there. We feel its gravity, its warmth, its light. If some cosmic force plucked 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 warmth.

ROBERT: Would we see a glow?

NEIL DEGRASSE TYSON: You would see the sun for eight minutes.

ROBERT: Even though it's not there.

NEIL DEGRASSE TYSON: You don't know it's not there.

ROBERT: Okay.

NEIL DEGRASSE TYSON: 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.

ROBERT: [laughs] 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.

NEIL DEGRASSE TYSON: Yes.

ROBERT: It's coming at you.

NEIL DEGRASSE TYSON: Yes, coming at you is the perfect way to think about it.

ROBERT: But what does this do to the thought that you are in charge of your own experiences, if ...

NEIL DEGRASSE TYSON: 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?

ROBERT: 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 Slinky was 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.

NEIL DEGRASSE TYSON: 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.

ROBERT: But that makes it seem sort of matter of fact. I being the man about town as you just described.

NEIL DEGRASSE TYSON: The philosophical man.

ROBERT: The philosophical man. Yes. I feel that there's something somewhat frightening about this notion as opposed to just simply there.

NEIL DEGRASSE TYSON: It's not my fault if your brain operates differently from the inevitability of something, from not knowing that it was gonna happen at all.

ROBERT: You are so—you are so calm.

NEIL DEGRASSE TYSON: I'm a practical guy! I'm not gonna—you know, I'm not gonna 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 gonna 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 ...

ROBERT: [laughs] Maybe this is like a Jewish thing.

NEIL DEGRASSE TYSON: ... that I have no control over that I don't even know will happen. Someone said if Earth is gonna 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.

ROBERT: So the life lesson here is like, you live in your own tempo and you get your beats, and that's just it.

NEIL DEGRASSE TYSON: Your beats come to you when nature decides it's the right time.

ROBERT: Yeah.

NEIL DEGRASSE TYSON: [laughs] 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.

STEVE STROGATZ: Yeah, that's it. That's the picture. You're always the bottom of the Slinky.

ROBERT: Big thanks to Steve Strogatz, who is a professor of mathematics at Cornell University. His new book is called The Big X.

JAD: No, it's The Joy of X.

ROBERT: Oh, The Joy of X, which is like the joy of sex, but it's missing the S.

JAD: And the E.

ROBERT: And the E.

JAD: And the sex.

ROBERT: 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 liked the X part." And I think we all agree with that.

JAD: [laughs] And thanks to Neil DeGrasse Tyson. His latest book is called Space Chronicles: Facing the Ultimate Frontier. And thanks also to Matthew Kielty for production help.

[LISTENER: Hi, this is Brian Ross from Brooklyn, New York, and I am a Radiolab listener. Radiolab 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 can be found at www.sloan.org.]

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