Radiolab - The Nothing Behind Everything

Episode Date: July 25, 2025

This week, two conversations from the archives about parts of the world that are imperceptible to us, verging on almost unthinkable. We start with a moment of uncertainty in physics. Inspired by an essay written by physicist and novelist Alan Lightman, called The Accidental Universe (https://zpr.io/4965dUdNqtpQ), taken from a book of the same name. Former Radiolab co-host Robert Krulwich pays a visit to Brian Greene to ask if the latest developments in theoretical physics spell a crisis for science. He finds that we've reached the limit of what we can see and test, and we’re left with mathematical equations that can't be verified by experiments or observation.Then, come along as we kick rocks. And end up tumbling down a philosophical rabbit hole where the solid things around us might not be solid at all. We talk to Jim Holt, author of Why Does the World Exist? (https://zpr.io/UqHpLnDx2QNx) who points out that when you start slicing and sleuthing in subatomic particle land, trying to get to the bottom of what makes matter, you mostly find empty space. Your hand, your chair, the floor, it's all made up of mostly nothing. Robert and Jim go toe-to-toe over whether the universe is made up of solid bits and pieces of stuff, or a cloudy foundation that more closely resembles thoughts and ideas.Signup for our newsletter!! It includes short essays, recommendations, and details about other ways to interact with the show. Sign up (https://radiolab.org/newsletter)!Radiolab is supported by listeners like you. Support Radiolab by becoming a member of The Lab (https://members.radiolab.org/) today.Follow our show on Instagram, Twitter and Facebook @radiolab, and share your thoughts with us by emailing [e-mail] support for Radiolab’s science programming is provided by the Simons Foundation and the John Templeton Foundation. Foundational support for Radiolab was provided by the Alfred P. Sloan Foundation.

Hey, this is Radio Lab, I'm Lethifan Ossiter, and today I have got for you two Radio Lab segments that came out before I ever worked here. Both of them do a thing that I find myself craving more and more these days, which is they pop you out. They pop you out of the news cycle, they pop you out of whatever interpersonal drama you're stuck thinking about, they pop you out of your own body. These are pieces about dimensions and even universes that are imperceptible

verging on almost unthinkable. I mean, I think I get it. I think I understand. Maybe I don't, but I still found it all really fun. So I hope you enjoy this pre-scheduled break from your perceptual reality. It begins with our emeritus host, Robert Kralwicz, talking to our other emeritus host, Jad Abherrad,

about a conversation he had with a legendary physicist. Okay, so this is about what you do for a living. You know that I have this neighbor and friend, Brian Green. Brian Green, professor of physics and mathematics, Columbia University. Yes living. You know that I have this neighbor and friend, Brian Green. Brian Green, professor of physics and mathematics, Columbia University. Yes, I do know that. And the thing about Brian is he is a theoretical physicist. Now, theoretical physicists say that it's theoretically possible to know everything there is to know in the universe. So one day

they'll be able to explain not only how you could send a rocket to the moon but the laws that govern space and energy and time and gravity, everything, the whole universe, one day they think might be totally understandable using logic and mathematical equations. Now you can't take that too far. None of us really imagined that if you ask the equations what are we gonna have for dinner tomorrow night, the equations will spit out fried tofu and you know, spring rolls or something like that. But at the level of the fundamental ingredients,

the particles that make up the universe, their properties, the hope and the goal is that the theories that we work out will apply everywhere and tell us about everything. You just said everything. Yes. As in everything. Yes, that's the big, big goal. This is like playing poker.

You're helping me, I don't know what you're gonna do. All right, we'll take it to the next step, okay. Wait, so what are you up to here, Krollwitz? All right, so. Well, you know we argue, that's the fun thing we do, but, but unlike him, my position has always been that it's going to be very hard to answer all the puzzles in the universe. And frankly, it's not a bad thing if some mysteries remain mysterious. Yeah, that's my view. But because Brian's so smart, when I tell him, how do you know this, whatever, he always

wins the arguments. But a few months ago, this is the thing that got this whole thing started, I was reading Harper's Magazine and I found an article written by another physicist and a novelist, Alan Lightman. And I thought, oh boy, this is going to drive Brian bats. Because Alan says, there is a group of physicists, and Brian happens to be one of them, who've embraced a very exciting idea with an unfortunate effect.

If this idea turns out to be true, Alan writes, it will then be impossible for physicists to know everything. Which I thought, ah, excellent. What is the idea? It has to do with more than one universe. You know, there is a very, you know this, we've talked about it before, that there is a vogue now for the idea that instead of one universe encompassing everything, there might be more than one. Right, so there actually are a number of ways that

physics comes upon this idea of other universes. Maybe the most intuitive is to think about the Big Bang. That sent space rushing outward, then matter could cool and yield to stars and galaxies. That wonderful picture that we've had with us since the 1920s. We have in the interim decades come to the possibility that the Big Bang may not be a one-time event. That is, there may have been many Big Bangs,

there may continue to be Big Bang-like events, each spawning its own universe. If that were the case, then our universe would then be viewed as one of many in this grand collection emerging from all of these Big Bang-like events. Now, in this view of things, there could be not just one universe or three or 19, there could be 10,000, there could be a hundred, there could be trillions, there could be an infinite number.

And here's the crucial thing. Each and every one of these universes can be different from its neighbor, vastly different. That's right. So when we study the equations for the production of these universes, we see in the mathematics that the other universes could have different features, different particle compositions, different masses of the particles, different forces.

Some of them might have atoms, some of them might not have atoms. You could have a universe with lots of stars, some with no stars, some could be made of Muenster cheese. I don't know, the fundamental properties of each universe could be very different. That's exactly right. And that's the key to Alan Lightman's argument. Well then, going back to the beginning of our conversation,

if a physicist's job is to explore everything, that is the universe, now the universe has just been demoted to a sub-universe, then when you get your diploma from a great university, the president of the university says, my friends, we are gathered here to meet the people who have earned the credentials to describe the sub-universe. A little bit of what we could know. It's like you've been demoted. You thought that you were going to get to learn about everything, your words, and now

it turns out that everything is very sub. Oh, I wouldn't describe it like that at all, as you might imagine. Rather than view this as an incredible loss of understanding, the right way of viewing it, I think, is to recognize that certain questions that we were asking when we thought there was just one universe were the wrong questions. Meaning, meaning what? Well, he says here's the way to think about it. This is how it always goes. We've seen this before in the history of science. Take Kepler. Johannes Kepler was an astronomer and a kind of mapper of the solar system. He was trying to figure out where the planets were and the nature of their orbits and stuff.

And Kepler spent a long time trying to find an explanation for why the Earth is 93 million miles away from the Sun. 93 million. Kepler thought that has to be a really important number, a key to a deeper mystery. But we now know that he was barking up the wrong tree. Why? There isn't just one planet. There are many planets. In fact, many planets around many stars,

and the distances of those planets from their host star varies over a wide range of possibilities. Mars for example is a hundred forty one million miles from the Sun. Jupiter 483 million and when you start comparing the different distances of planets from the Sun you realize that the fact of the earth is 93 million miles away it doesn't seem like a deep law of the universe anymore it It just feels kind of arbitrary. And then that forces you to change the question. Not, why 93 million?

No, why are all these different planets at different distances from the sun and yet they all stick around the sun? They're all trapped in the neighborhood. That question puts you on the road to a deeper thought. The theory of gravity. The point is, says Brian, if you're focused on one thing, you're going to think that one thing is the key to everything. When your one turns to many, then you think, ah, well, the one thing really wasn't so special.

But the way Brian sees it... That is progress. That is understanding. And then it frees you up to ask other kinds of questions such as what's the law of gravity what is the equation that allows us to understand how the Sun forms so those are real questions and when you can toss out the ones that are red herrings that you thought were deep but they're actually just asking the wrong question that frees you up to make progress and Brian says you can make the exact same kind of progress if you compare universes.

So instead of asking why is our one universe the way it is, now you can ask well what do all of these universes, so different one from the other, still have in common? That would be pretty heavy and exciting to describe the underlying laws that govern all universes regardless of their detailed features and what it would be like in that universe or that universe or that universe way over there. But there are an infinite number of them so if I told you that you could write anything down and it might be a universe, black universes, white universes, green universes, soft universes, hard universes, muscular universes, teeny universes, huge universes, then the only one you know intimately is your own. It seems to me that what do you know about those other universes other

than that they might be very different? We don't know very much observationally, sure, we can't see them, we don't know very much experimentally, so they're definitely on a very different footing from that perspective. But Brian believes that one day we might be able to experimentally detect these other universes and somehow, you know, kind of pick up their distant vibrations, kind of like the way you hear your neighbor's music, it's just emanating through the walls. We might be able to listen in, he says, and take a couple of measurements. Which would be quite wonderful.

And in that case, at least there's a chance that we'd get observational evidence of the existence of these other realms. And at that point, I would begin to say, hmm, maybe there's something really to this. So the physics you're doing says, I can't go there. I can't observe it, at least for the moment, all I have is my brain and my math. And I say from my brain, I'm going to just assume certain things are always true, there's always going to be gravity to say, there's always going to be some particle or wave that creates matter, there's always going to be, I don't know, what else, are there things that are always going to be? What are they? They're always going to be, I don't know what else, are there things that they're always going to be?

What are they? That are always going to be? The things that you were describing need not always be the case. Really? Yes. What would be the case is that the fundamental governing equations, the mathematical laws, would be the underlying architecture that governs

what happens in those places. But environmental details can change things Gravities and environmental details? Yes, well that's actually something you know at some level right now, right? On the moon you could jump a lot higher than you can here. So if you didn't realize... But I do think that two bodies do attract each other. That's right, so there is a fundamental law of gravity that manifests itself in different ways based on the environment.

All right. So let me say that again. Ask it again. Are there fundamental laws that you think operate in all universes? Yes, absolutely. And why do you think that? That is the starting point. When we come upon this possibility of other universes, It's not a crazy idea that we

dream up late at night when there's nothing else to think about. These are ideas that emerge from the fundamental equations that we use to describe the things that we do see in the world around us and we follow the equations and the equations suggest to us there might be these other universes. So we have equations, we analyze them and we interpret what they're telling us about reality. But those are the very equations that come to this possibility of other universes, then those are the equations that govern

those other universes. The starting point is let's assume that these are the fundamental gifts. Doesn't it sound an awful lot like, why is God three in one? Or why was the world made in seven days? Aren't we getting close to some sort of, Why was the world made in seven days? Aren't we getting close to some sort of, you're believing in certain things to be always true

the way religious people believe certain things are always true. Not because you've seen it, or it's just because you can't, you have a faith in it. I couldn't disagree with you more. I thought not. It has absolutely nothing to do with faith. The reason why we trust the equations is because we've got centuries worth of

observational experimental evidence that the equations take us in the right direction. Here. Here. And it's those very same equations that work here that we are following to their logical conclusion to see where the mathematics takes us right so if you remember the train of reasoning you may have just projected here into there that's faith talking no you can't go there all you can do is say well what works with my deep understanding of here has to be there I don't know why it has to be, but that's what you just said. No, it's actually the reasoning goes in somewhat reverse order from that. We build mathematical equations to describe here. We then follow those equations and say, oh my goodness, those equations that we develop to describe here are telling us that there is something over there. And then we're

like, wow, the equations do a great job of describing things here and the equations have this feature that they tell us there's another place over there. Maybe that's possible. The key thing also – This is logic in your mind. Logic and reason, not belief. Just logic. This is just logic. Aren't you worried though that there's another Brian Gre Green in universe number 3790,208,645 who is sitting there talking to another radio reporter in another university and he's saying well we know all about the other universes because we're assuming that the math here

is the same as the math there in that other place. But as it turns out, their math and our math aren't the same. So there will not be this. You may just be wrong. Oh, that's always the possibility. In fact, it's likely the possibility. In fact, 99.99% of everything we do is wrong. Not from the point of view, we make a mistake, but which math. But the wrongness is a deep wrongness that you somehow are feeling that the math is a

clue that everything follows your math. If at some point the maths collide and then the universes collide, then that would be very unsettling to both of you, I would assume. In terms of whether the math is somehow contradictory and coheres in some way. Well, it can be your tools of learning are not working. Yes, that would suggest that we were both wrong and that there's a deeper overarching framework. I mean I I hate to use the word faith, but the one point where I'll give you faith is this. I do have a deep faith

that the universe is coherent and by universe call it multiverse whatever word you want to use the whole thing. I do believe that it's coherent. Now, whether that means it follows mathematical laws, I don't know. It could be the case that, you know, when we talk to those aliens that we encounter one day and they say, okay, show us what you got, we bring out our equations and they kind of laugh at us

and say, oh, you guys are still stuck on math, you know? And they said, yeah, you know, 10,000 years ago we were doing math too, but here's the real way of describing it. Now, what they'd be showing us with the real way of describing it, I have no idea. I can't even imagine what it would be that would be non-mathematical.

So I do have a deep faith that it's coherent, and the only tool that I know how to encapsulate that coherence are mathematical equations. So if Zantar Brian and Brian here come up with equations that collide with one another and don't work, to me it just means that both were wrong and there's some bigger overarching coherence that we've yet to find. That's it.

That's it. I don't even, I can't even begin to figure out if you, did you just win? Did you lose? I can't tell. Wait, so this all came from Alan Leitman's article. Right, so. Do you think he beat the objections in the article? Did he beat the article? Well, I thought it would be fair to ask the author of the article. So I called Alan, who happened as it turns out to be in Phnom Penh, Cambodia. I make all of my international calls on Skype. And I sent him the interview with Brian. He listened.

And I asked, well, what do you think about Brian's argument? Well, I don't think that he's wrong, but I think that the problem is philosophically more disturbing than what he is confessing. He said, well, I think it's going to be much harder than Brian thinks to actually sense or encounter or measure these other universes if they exist at all. We don't even know whether the other universes exist in the same space and time that we do. And there are other physicists who feel that these universes are, even in principle, never, never observable by us, that we will never be able to have any physical evidence of their

existence. And that possibility is what I find disturbing. It may be that this is the way nature is. What does that mean? Well, I mean, it may be that we've done as much explaining as is possible. And that we'll never ever really understand everything? Yes, in other words we may have pushed the human mind as far as it can possibly go. Huge thanks to Brian Greene, Professor of Physics and Math at Columbia University, as well as Alan Lightman up at MIT whose essay Robert read, The Accidental Universe.

It appears in a book of the same name. When we come back, we have another story that will break your brain in a whole different way. This time, it's not distant, unobservable universes, but maybe every single thing around you right now. Stick around. Lotthiff, Radiolab, we are back. Today's episode is about the nothing behind everything.

And I'll pass it back over to our emeritus hosts, Chad and Robert. Let's just start it up. I'm Chad Iboomrod. I'm Robert Krulwich. This is Radiolab. The podcast. We're going to continue the conversation we were just having. We've been having all week about, uh, well, perfection, you know, like striving

for things which seem perfect versus living in the real world. Right. And, uh, you know, like striving for things which seem perfect versus living in the real world. And recently... I got into a bit of a kerfuffle with a guy who yearns like you do for an ideal. His name is Jim Holt. And he wrote this really good book called Why Does the World Exist? And just to get us started, in that book, he quotes a poem. Yeah. Remember the line?

Yeah. Kick at the rock, Sam Johnson, break your bones, but cloudy, cloudy is the stuff of stones. Cloudy, cloudy is the stuff of stones. Yeah. Meaning what? It's something, well, Samuel Johnson, who lived in the 18th century, it

was a contemporary of Bishop Barkley. And Bishop Barkley was an idealist. He believed that the world was essentially pure appearance. It was like a thought, not like a solid reality. It was a thought in the mind of God. Like the rock really had no substance. And Samuel Johnson, when he heard this, he thought it was ridiculous, and he went and kicked a stone

and said, I refute Barkley thus. Anyhow, that's the story. Wait, one guy thought it was a thought, the other guy thought the rock was a, what are they arguing about exactly? Well, they're arguing about reality. Just what is this world? What is its essential nature?

When you hold a rock in your hand, like what's it made of? What's it made of? Yeah. Minerals? No, I'm really asking is what is the most essential nature of the rock? So if you look deep deep deep down into the rock do you find something concrete? Do you find a little bit of thing? Yeah. Or do you find something more ethereal? Something you can't touch? Something you can't pin down, something like, oh, a thought. This is Jim's notion.

And this sounds like a, it sounds like I've been eating lotus leaves. I mean, it's a pipe dream. But this is what science has increasingly led us to. That rocks are thoughts? Well, to follow Jim's logic, he goes all the way back to the Greeks, to the first real attempt to get to what's really at the bottom of a rock. You know, even in ancient times, the atomists, democratists and leucippists thought that if you keep cutting up the stuff of reality that we see around us, tables and chairs and rocks and so forth, eventually you cut them up into such itty bitty pieces that you can't

cut any further. And then you've got atoms. So there you've got, you've clearly got a fundamental stuff, the atoms. Yeah, that sounds very pleasing. But even going back to Newton, there were reasons to suspect that there was something a little funny about reality. It wasn't quite as substantial as we believed. Newton, of course, came up with the theory of gravity,

and the theory of gravity says if you've got the sun and a planet, the sun exerts a gravitational force on the planet. And Newton's contemporaries wanted to know, well how does it do that? What is the mechanism by which gravity is mediated? How does the sun, as it were, reach out to the earth and force it to move around this orbit? reach out to the Earth and force it to move around this orbit. So if I were an atomist, if I were looking for stuff, then I'd need some kind of thing that carried gravity. Yeah, yeah. But the problem is, it looks like there's nothing between the Earth and the Sun

except a void. All that Newton had to fill that void was a mathematical equation that told him how the Sun and the Earth interact. And the thing is it worked. You could plug in the numbers and you could know how one was influencing the other. But Newton had no idea at all why the equation worked. He couldn't point to any like a little particle thing like a graviton and say, there's your reason. It almost seemed like gravity was created from the equation itself.

And this disturbed a lot of people. Because at that time, everybody thought that nature has to be made out of hard, durable stuff. You know, gears, sprockets, pushing and pulling. That's the essence of reality. Then in the 20th century, of course, it got much, much worse. You know, the atom, which was thought to be very, very tiny, and you couldn't cut it any further.

It was the limit to this splitting process. And as we know all too well from the 20th century, you can split an atom. Yeah, it has pretty interesting consequences. But we also discovered the atom is almost entirely empty space. If you took a baseball and put it in the middle of Madison Square Garden, that would be like the nucleus, and the first level of electrons are as far away as the exterior of the garden.

So you can think of this baseball, this nucleus, as a tiny dot all along. So it's basically, the atom is a big empty space. Well, it doesn't feel that way. Like, watch this. I'm going to do this. Right. Right.

Yeah. If my hands are all atoms, and as you say, atoms are mostly empty space, then why don't my hands just go right to each other like two clouds? But you'll notice they don't. Yeah, yeah. Why don't I fall through the floor here? Because the floor is mostly empty space and I'm mostly empty space. That too, if you look at it in the micro level,

this apparent solidity is the product of a purely mathematical relation. Well, that can't, isn't it more like my electrons don't like similar electrons? So the electrons in my hands just hate the electrons on the other hand? No, it basically comes down to a pair of mathematical relations, the Pauli exclusion principle and

the Heisenberg uncertainty principle. I mean, all of this gets very abstract. I understand it perfectly, of course, but I don't want to bore you with the details of his argument. You have no idea what he's saying, do you? I'll say this. According to Jim, it's not that the electrons in my left hand are repelling the electrons in my right hand. It has to do with a law of nature that says two particles, identical particles, cannot be in the same place at the same time.

So when you hear that sound, you can hear it as the sound of a law saying, no, not allowed, not in nature. Exactly. And here's a slightly different way of putting that. But wait, isn't this law that we are announcing, isn't this law about particles? Like we're talking about atoms and electrons, those are things. So we're still talking about things. Well, if you study quantum field theory, which is what all physics graduate students begin with in graduate school, you discover that even particles are unreal.

They're just temporary properties of what are called fields, and fields are just distributions of mathematical quantities through space-time. It's all, they're not, they don't seem to be grounded in anything. According to Jim, a field is kind of like a stream of numbers. Pure information. Numbers that tell you where a particle like an electron might be. So maybe the electron's over here, oh no, no, maybe it's over there, or maybe it's with this group, or maybe it's with that group. The problem is you can't ever see the thing itself.

You can only see the effect it has on other things. So you can't observe it. And if something is in principle unobservable, you may as well say it doesn't exist. Wait a second. No, no, no. I mean, I'm on his side, but you could say that it's just not observable down there

at the microscale. Up here, it's pretty observable. I mean, this table exists, this mixer. I mean, something is happening to give the world substance. Well, according to Jim, what we think happens, and this, admittedly, is a gross oversimplification, but in these fields, you're going to get these little fluctuations, these little vents, sudden hiccups of energy, little bursts, and that's where stuffiness flickers into existence. But it's a very flickering existence.

Stuff isn't permanent. So what is a rock? I mean, a rock looks like a good solid persisting object, but it's really, our perception of it is energy transitions, changes in the distribution of energy from one state to another. When that happens, the energy is irradiated. It goes through my retina, it goes through my pupil rather, and strikes my retina,

and I perceive the rock. I don't know if Jim would call a rock like Bishop Barclay did a thought in the mind of God, but he might say that deep down, what a rock is, is an expression of rules or math. It's just here like a shadow of an idea. Yeah.

Yeah. I've heard one physicist say that the cosmos is ultimately a concept. Are you increasingly convinced that the reason you can clap, the reason you don't fall through the floor, the reason that gravity works is all because of certain ideas that govern, ideas rule the world. Yeah, yeah. Maybe, you know, in 100 years from now when string theory is finally worked out, we might have a very different conception of it, but what it looks is it's going to be mathematics

and structure all the way down. You're okay with this. Well, I'm a sort of mathematical romantic. I love the idea that the essence of reality is not stuff. You know, who, what, stuff is kind of ugly. I mean, you want to get rid of stuff. There's too much stuff in your apartment. It's flutter. It's gross, niscus, absurd.

I don't know what to do if I don't have stuff. Well, you know, this is a temperamental difference between us. I like the idea that reality consists. It's a, it's a flux of pure information with no further substance. I don't know why this makes you so happy. I mean, here I, I would love if I'm clapping or if I'm hitting someone in the face, I would love to think the billiard ball of me is hitting the billiard ball of them and that explains what's going on. Now you've offered…

But we're living in almost in a spiritual realm. You want to live in this gross material realm where there's a lot of stuff. But your spiritual realm, it's literally empty. It feels so intuitively wrong. But if you go back to the old 19th century view that were made up of these little hard particle atoms that are all bumping around, is it any more plausible that you and I are just a bunch of dumb hard particles in a certain configuration? And if that's true, how are certain configurations of these particles tantamount to the horrible feeling of pain?

You could say, pain, oh, that's just a lot of elementary particles in a certain configuration. But we all know that explanation isn't enough. So when you look down to the bottom of everything, whether it's a mathematical object or whether it's little billiard balls knocking around, it's still miraculous and improbable that it should produce subjective experience, that it should produce pleasure and pain. And that mystery, how you go from the most basic things, or actually the most basic nothings,

to everything we see around us. I find that to be exhilarating to worry about the metaphysics of physics and the nature of reality even though it doesn't lead you to any sort of comfortable intellectual closure. It makes for it's a good way of idling away an otherwise boring afternoon as we've just proved. It also explains why when I head-butted him with my very strong forehead, he seemed to think of it as a fascinating thought. Special thanks to Jim Holt, who actually we're both too shy to ever head-butt each other

and too weak ever to try it. But anyway, his book is called Why Does the World Exist? An Existential Detective Story. Okay, well I guess that's it for this podcast. I'm Chad Abumrad. I'm Robert Kralwicz. Thanks for listening and existing. Temporarily. Hi, I'm Amy Beth and I'm from Longmont, Colorado and here are the staff credits.

Radio Lab was created by Jad Abumrad and is edited by Sorin Weelan. Lulu Miller and Latif Nasser are our co-hosts. Dylan Keefe is our director of sound design. Our staff includes Simon Adler, Jeremy Bloom, Becca Bressler, W. Harry Fortuna, David Gable, Rebecca Latt, Maria Paz Gutierrez, Sindhu Nyan Nansambandan, Matt Kielke, Annie McEwan, Alex Neeson, Our fact checkers are Diane Kelly, Emily Krieger, Anna Pujol-Mazzini, and Natalie Middleton. Leadership support for Radiolab Science Programming is provided by the Simons Foundation and the John Templeton Foundation. Foundational support for Radiolab is provided by the Simons Foundation and the John Templeton Foundation. Foundational support for Radiolab was provided by the Alfred P. Sloan Foundation.