[RADIOLAB INTRO]

LATIF NASSER: Hey, this is Radiolab. I'm Latif Nasser. And today I have got for you two Radiolab 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—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 Krulwich talking to our other emeritus host Jad Abumrad about a conversation he had with a legendary physicist.

ROBERT KRULWICH: Okay, so this is about what you do for a living.

ROBERT: You know that I have this neighbor and friend, Brian Greene.

BRIAN GREENE: Brian Greene, professor of physics and mathematics, Columbia University.

JAD ABUMRAD: Yes, I do know that.

ROBERT: 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.

BRIAN GREENE: Now you can't take that too far. None of us really imagined that if you asked 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.

ROBERT: [laughs]

BRIAN GREENE: But at the level, 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.

ROBERT: You just said "everything."

BRIAN GREENE: Yes.

ROBERT: As in everything.

BRIAN GREENE: Yes. That's the big, big goal.

ROBERT: This is like playing poker. You're helping me. I don't know what you're gonna do. All right, well take it to the next step. Okay. [laughs]

JAD: Wait, so what are—what are you up to here, Krulwich?

ROBERT: All right, so ...

ROBERT: Well, you know we argue. That's the fun thing we do.

JAD: Sure.

ROBERT: But unlike him, my position has always been that it's gonna be very hard to answer all the puzzles in the universe, and frankly it's not a bad thing if some mysteries remain mysterious.

BRIAN GREENE: Yeah. That's my view.

ROBERT: But because Brian's so smart, when I tell him, "How do you know this?" or whatever, he always wins the arguments. But a few months ago—this is—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 gonna 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 "Oh, excellent!"

JAD: What is the—what is the idea?

ROBERT: It has to do with more than one universe. You know, there is a—you know this, we've talked about it before.

JAD: Yeah.

ROBERT: That there is a vogue now for the idea that instead of one universe encompassing everything, there might be more than one.

ALAN LIGHTMAN: 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 sense of space rushing outwards, and 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 and 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 these Big Bang-like events.

ROBERT: Now in this view of things, there could be not just one universe or three or nineteen, there could be 10,000, there could be a hund—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.

ALAN LIGHTMAN: 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 composition, different masses of the particles, different forces.

ROBERT: Some of them might have atoms, some of them might not have atoms. You could have universes 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.

JAD: That's exactly right.

ROBERT: And that's the key to Alan Lightman's argument.

ALAN LIGHTMAN: Well then ...

ROBERT: 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."

JAD: [laughs]

ROBERT: It's like you've been demoted. You thought that you were gonna get to learn about everything—your words—and now your everything is very—it's sub.

BRIAN GREENE: Oh, I wouldn't describe it like that at all, as you might imagine.

ROBERT: [laughs]

BRIAN GREENE: 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.

JAD: Meaning—meaning what?

ROBERT: Well, he says here's the way to think about it: this is how it always goes.

BRIAN GREENE: We've seen this before in the history of science.

ROBERT: 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.

BRIAN GREENE: And Kepler spent a long time trying to find an explanation for why the Earth is 93 million miles away from the Sun.

ROBERT: 93 million. Kepler thought that has to be a really important number, a key to a deeper mystery.

BRIAN GREENE: 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 stars varies over a wide range of possibilities.

ROBERT: Mars, for example, is 141 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 that the Earth is 93 million miles away, it doesn't seem like a deep law of the universe anymore. 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.

ROBERT: The point is, says Brian, if you're focused on one thing, you're gonna think that one thing is the key to everything. When your one turns to many, then you think, "Ah, well the one thing wasn't really so special." But the way Brian sees it ...

BRIAN GREENE: 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 formed? 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.

ROBERT: 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?"

BRIAN GREENE: 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.

ROBERT: 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. In other words ...

BRIAN GREENE: We don't know very much observationally, sure.

ROBERT: Well, obviously.

BRIAN GREENE: I mean, you can't see them. We don't know very much experimentally, so they're definitely on a very different footing from that perspective.

ROBERT: But Brian believes that one day we might be able to experimentally detect these other universes, and somehow they'll kind of pick up their distant vibrations kind of like the way you hear your neighbor's music just emanating through the walls?

JAD: Hmm.

ROBERT: We might be able to listen in, he says, and take a couple of measurements.

BRIAN GREENE: Which would be quite wonderful. And in that case, at least there's a chance that we'd gain 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."

ROBERT: 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 gonna just assume certain things are always true. There's always gonna be gravity, say. There's always gonna be some particle or wave that creates matter. There's always gonna be—I don't know what else. Are there things that are always gonna be? What are they that are always gonna be?

BRIAN GREENE: The things that you were describing need not always be the case.

ROBERT: Really?

BRIAN GREENE: 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 dramatically.

ROBERT: Gravity is an environmental detail?

BRIAN GREENE: Yes. 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 ...

ROBERT: Realizing that two—two bodies do attract each other.

BRIAN GREENE: That's right. So there is a fundamental law of gravity that manifests itself in different ways based on the environment.

ROBERT: All right, so let me say that again—or ask it again. Are there fundamental laws ...

BRIAN GREENE: Yes.

ROBERT: ... that you think operate in all universes?

BRIAN GREENE: Yes. Yes, absolutely.

ROBERT: And why do you think that?

BRIAN GREENE: That is a 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 that it 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, and those are the equations that govern those other universes. The starting point is let's assume that these are the fundamental ...

ROBERT: But doesn't this sound an awful lot like, "Why is God three in one?" Or, "Why—why was the world made in seven days?" It—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.

BRIAN GREENE: I couldn't disagree with you more.

ROBERT: I thought not. [laughs]

BRIAN GREENE: 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.

ROBERT: Here.

BRIAN GREENE: 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 here ...

ROBERT: 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—" but my deep understanding of "here" has to be "there." I don't know why that has to be, but that's what you just said.

BRIAN GREENE: No, that'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 developed 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 ...

ROBERT: This is all logic in your mind.

BRIAN GREENE: Oh, this is—this is ...

ROBERT: Not a—not belief. This is logic.

BRIAN GREENE: This is just logic.

ROBERT: Aren't you worried, though, that there's another Brian Greene in Universe number 3,000,790,208 600,485 who's 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 ours aren't the same, so they will not—you may just be wrong.

BRIAN GREENE: Oh, that's always the possibility. In fact, it's likely the possibility. In fact, 99.99 percent of everything we do is wrong, not from the point of view of we make a mistake, but ...

ROBERT: But the wrongness is a deep wrongness that you—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.

BRIAN GREENE: In terms of whether the math is somehow contradictory or incoherent in some way?

ROBERT: Well, it can be your tools of—yes, your tools of learning are—are not working.

BRIAN GREENE: 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?

ROBERT: [laughs]

BRIAN GREENE: And they say "Yeah, you know, a thousand, ten thousand 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.

ROBERT: That's it.

JAD: I don't even—I can't even begin to figure out if you—did you just win? Did you just lose? I can't tell. Wait, so this all came from Alan Lightman's article?

ROBERT: Right. So ...

JAD: Do you think he beat the objections in the article?

ROBERT: Did he beat the article? Well, I thought it would be fair to ask the author of the article, so I—I called Alan ...

ALAN LIGHTMAN: Yeah.

ROBERT: ... who happened, as it turns out, to be in Phnom Penh, Cambodia.

ALAN LIGHTMAN: I make all of my international calls on Skype.

ROBERT: And I sent him the interview with Brian. He listened.

ALAN LIGHTMAN: The thing that I listened to ...

ROBERT: And I asked, "Well, what do you think about Brian's argument?"

ALAN LIGHTMAN: Well, I don't think that he's wrong, but I think that the problem is philosophically more disturbing than what he is confessing.

ROBERT: He said, "Well, I think it's gonna be much harder than Brian thinks to actually sense or encounter or measure these other universes if they exist at all."

ALAN LIGHTMAN: 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 the 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.

ROBERT: What does that mean?

ALAN LIGHTMAN: Well, I mean, it may be that—that we've done as much explaining as is possible.

ROBERT: Oh. And that we'll never ever really understand everything?

ALAN LIGHTMAN: Yes. In other words, we may be—we may have pushed the human mind as far as it can possibly go.

LATIF: 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.

LATIF: Latif. Radiolab. We are back. Today's episode is about the nothing behind everything. And I'll pass it back over to our emeritus hosts Jad and Robert.

JAD: Let's just start it up. I'm Jad Abumrad.

ROBERT: I'm Robert Krulwich.

JAD: This is Radiolab.

ROBERT: The podcast.

JAD: We're gonna continue the conversation we were just having—been having all week—about well, perfection. You know, like, striving for things which seem perfect versus living in the real world.

ROBERT: Right.

JAD: And recently ...

ROBERT: I got into a bit of a kerfuffle with a guy who—who yearns like you do for an ideal. His name is Jim Holt.

JIM HOLT: Okay.

ROBERT: 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.

JIM HOLT: Yeah.

ROBERT: Remember the line?

JIM HOLT: Yeah. "Kick at the rock, Sam Johnson, break your bones. But cloudy cloudy is the stuff of stones."

ROBERT: Cloudy cloudy is the stuff of stones.

JIM HOLT: Yeah.

ROBERT: Meaning what?

JIM HOLT: It's something—well, Samuel Johnson, who lived in the 18th century, was a contemporary of Bishop Berkeley. And Bishop Berkeley 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.

ROBERT: Like the rock really had no substance.

JIM HOLT: And Samuel Johnson, when he heard this, he thought it was ridiculous, and he went and kicked a stone and said, "I refute Berkeley thus."

ROBERT: [laughs]

JIM HOLT: Anyway, that's the story.

JAD: Wait, one guy thought it was a thought, the other guy thought the rock was a—what are they arguing about, exactly?

ROBERT: Well, they're arguing about reality.

JIM HOLT: Just what is this world? What is its, you know, essential nature?

ROBERT: When you hold a rock in your hand, like, what's it made of?

JAD: What's it made of?

ROBERT: Yeah.

JAD: Minerals?

ROBERT: No. What 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?

JAD: Yeah?

ROBERT: 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.

JIM HOLT: And this sounds like a—it sounds like I've been eating lotus leaves. It's a pipe dream. But this is what science has increasingly led us to.

JAD: That rocks are thoughts?

ROBERT: 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.

JIM HOLT: You know, even in ancient times, the atomists Democritus and Leucippus 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 atom.

ROBERT: Yeah, that sounds very pleasing.

JIM HOLT: Right. 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. You know, Newton, of course, came up with a 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.

ROBERT: Right.

JIM HOLT: And Newton's contemporaries wanted to know ...

ROBERT: Well, how does it do that?

JIM HOLT: 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 in this orbit?

ROBERT: So if I were an atomist, if I were looking for stuff, then I'd need some kind of thing that carried gravity.

JIM HOLT: Yeah, yeah. But the problem is that it looks like there's nothing between the Earth and Sun except a void.

ROBERT: 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 ...

JIM HOLT: That nature has to be made out of hard, durable stuff.

ROBERT: You know, gears and sprockets.

JIM HOLT: Pushing and pulling. That's the essence of reality. And then in the 20th century, of course, it had gotten 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, you know, splitting process.

ROBERT: Right.

JIM HOLT: And as we know all too well from the 20th century, you can split an atom. Yeah, and it has pretty interesting consequences. But we also discover the atom is almost entirely empty space.

ROBERT: Huh.

JIM HOLT: 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.

ROBERT: So you can think of this baseball, this nucleus, as a tiny tot all alone.

JIM HOLT: So it's basically—the atom is a big empty space.

ROBERT: Well, it doesn't feel that way. Like, watch this. I'm gonna do this. [claps hands]. Right? [claps hands] If my hands are all atoms, and as you say, atoms are mostly empty space, then why don't my hands just go right through each other like two clouds? But you'll notice ... [claps hands]

JIM HOLT: 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.

ROBERT: Well, that can't—isn't it more like my electrons don't like similar electrons? So the electrons on my hands just hate the electrons on the other hand?

JIM HOLT: 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 ...

ROBERT: I understand it perfectly, of course, but I don't want to bore you with the details of his argument.

JAD: [laughs] You have no idea what he's saying, do you?

ROBERT: [laughs] Well, 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, [claps hands] you can hear it as the sound of a law saying, "No! Not allowed! Not in nature!"

JIM HOLT: Exactly. And here's a slightly different way of putting that.

ROBERT: 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.

JIM HOLT: 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.

ROBERT: [laughs] Oh, God!

JIM HOLT: They're just temporary properties of what are called "fields." And fields are just distributions of mathematical quantities through space-time. So they're not—they don't seem to be grounded in anything.

ROBERT: According to Jim, a field is kind of like a stream of numbers.

JIM HOLT: Pure information.

ROBERT: 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.

JIM HOLT: And if something is in principle unobservable, you may as well say it doesn't exist.

JAD: Wait a second. No. No, no.

ROBERT: What?

JAD: I mean, I'm on his side, but you could say that it's just not observable down there at the micro-scale. Up here, it's pretty observable. I mean, [bangs table] this table exists. [bangs mixer] This mixer. I mean, something is happening to give the world substance.

ROBERT: Well, according to Jim, what we think happens—and this is admittedly is a gross oversimplification. But in these fields, you're gonna get these little fluctuations, these little ...

JIM HOLT: Events.

ROBERT: ... 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.

JIM HOLT: 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.

ROBERT: I don't know if Jim would call a rock, like Bishop Berkeley 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.

JIM HOLT: Yeah. Yeah. I've heard one physicist say that the cosmos is ultimately a concept.

ROBERT: 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?

JIM HOLT: Yeah. Yeah. Maybe, you know, a hundred years from now when string theory is finally worked out, we might have a very different conception of it. But it looks as though it's going to be mathematics and structure all the way down.

ROBERT: You're okay with this?

JIM HOLT: Well, I'm a sort of mathematical romantic. I love the idea that the essence of reality is not stuff, you know, stuff is kind of ugly. I mean, you want to get rid of stuff, there's too much stuff in your apartment.

ROBERT: I like stuff!

JIM HOLT: It's clutter. It's gross, viscous, absurd.

ROBERT: I don't know what to do if I don't have stuff.

JIM HOLT: Well, you know, this is the temperamental difference between us. I like the idea that reality consists—it's a flux of pure information with no further substance.

ROBERT: I don't know why this makes you so happy. I mean, here—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 ...

JIM HOLT: But we're living in almost a spiritual realm. You want to live in this—in this gross material realm, where there's a lot of stuff.

ROBERT: Yeah, but your spiritual realm, it's literally empty. It feels so intuitively wrong.

JIM HOLT: But if you go back to the old 19th-century view, that we're 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, you know, how are certain configurations of these particles tantamount to the horrible feeling of pain?

ROBERT: You could say pain, oh, that's ...

JIM HOLT: ... just a lot of elementary particles in a certain configuration.

ROBERT: But we all know that explanation isn't enough. So when you look down to the bottom of everything ...

JIM HOLT: Whether it's a mathematical object, or whether it's little billiard balls knocking around, it's still miraculous and probable that it should produce subjective experience, that it should produce, you know, pleasure and pain.

ROBERT: And that mystery, how you go from the most basic things—or actually, the most basic nothings to everything we see around us ...

JIM HOLT: I find that to be exhilarating. To worry about, you know, 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.

ROBERT: [laughs]

ROBERT: It also explains why when I head butted him with my—with my very strong forehead, he seemed to think of it as a fascinating thought.

JAD: [laughs]

ROBERT: Special thanks to Jim Holt, who—actually, we're both too shy to ever head butt each other. But anyway, he has a book, it's called Why Does the World Exist?: An Existential Detective Story.

JAD: Okay, well I guess that's it for this podcast. I'm Jad Abumrad.

ROBERT: I'm Robert Krulwich.

JAD: Thanks for listening. And existing.

ROBERT: Temporarily.

[LISTENER: Hi. I'm Amy Beth, and I'm from Longmont, Colorado. And here are the staff credits. Radiolab was created by Jad Abumrad, and is edited by Soren Wheeler. 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 Gebel, Rebecca Laks, Maria Paz Gutiérrez, Sindhu Gnanasambandan, Matt Kielty, Annie McEwen, Alex Neason, Sarah Qari, Sarah Sandbach, Anisa Vietze, Arianne Wack, Pat Walters, Molly Webster and Jessica Yung. With help from Rebecca Rand. Our fact-checkers are Diane Kelly, Emily Krieger, Anna Pujol-Mazini and Natalie Middleton.]

[LISTENER: Leadership 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.]

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