Justin E.H. Smith  [00:07]
Hello, and welcome to “What Is X?” I'm your regular host of this podcast for The Point magazine, Justin E.H. Smith. Regular listeners will know the rules. Each episode I have a guest on and we talk about given X. And we try to come to a shared, accepted definition of what the X in question is. Today's episode is going to be focused on matter. And my guest is Sean Carroll, a research professor of physics at Cal Tech, and also fractal faculty at the Santa Fe Institute in New Mexico, and most importantly, the host of a podcast called “Mindscape,” which I'm sure many of you already know, and love. So welcome, Sean.

Sean Carroll  [01:10]
Thanks very much for having me on.

Justin E.H. Smith  [01:12]
Now, matter is a difficult thing for the two of us to discuss, for a lot of reasons. One of the reasons is that I generally say that my own competence for explaining how the world works kind of gets a bit laggy around, say, 1800. I understand everything up until then. But everything after that moment is really a blur for me. Whereas I think you're somewhat the opposite of that. Now, I thought it might be fun, just as a kind of warm-up exercise, to do the following little game with you. I'm going to summarize some old-school theories of matter. And you're going to give me a grade on these. I'm not gonna say who held them, I'm just going to shoot them out at you. And then we're going to see which old-school philosopher you're closest to. Does that sound…?

Sean Carroll  [02:18]
…Okay. Well, but first, let me ask, what is the basis for the grading? Is it how clever and useful the idea was? Or is it how close it is to our most modern conception?

Justin E.H. Smith  [02:30]
It's how close it is to what you think matter is.

Sean Carroll  [02:36]
Got it.

Justin E.H. Smith  [02:37]
Okay. So, let's go: “An idea in the mind of God.”

Sean Carroll  [02:42]
That is an… F.

Justin E.H. Smith  [02:47]
All right. “Their extension.”

Sean Carroll  [02:51]
That's a D-minus.

Justin E.H. Smith  [02:54]
This is great. “A vehicle through which form, which is the only thing that truly exists, realizes itself.”

Sean Carroll  [03:04]
I'll give that a D-plus.

Justin E.H. Smith  [03:06]
[Laughs] Amazing. “Some kind of force that derives from some kind of activity.”

Sean Carroll  [03:19]
I like that better. I'll give that a C-minus.

Justin E.H. Smith  [03:21]
Amazing. That's so much what I was anticipating you would come up with. And let me just briefly say, that was respectively George Barkley, Rene Descartes, Aristotle, and Leibniz.

Sean Carroll  [03:37]
Right.

Justin E.H. Smith  [03:38]
And this is interesting to me. I mean, he only got C-minus. But every semester that I teach Leibniz's Metaphysics, my students—or invariably one of my students—will raise their hands and say, “That sounds so contemporary.” And I always say to them, “Maybe, but let's try not to talk about contemporary physics. That's someone else's domain. That's for people like Sean Carroll.” But still, this exercise was extremely revealing to me, and I think it's going to be a good way of working our way into what interests you. Why is…

Sean Carroll  [04:24]
Let me mention very quickly, you know, a sense in which one can explain why all of these grades are not very good.

Justin E.H. Smith  [04:31]
Yeah.

Sean Carroll  [04:31]
Even though, I think if you ask the person on the street here “What is matter?” they would get a better grade than any of those folks. And I think that's an important thing, because all of these that you mentioned are sort of attempts to discern some fundamental essence to this idea. And they went further away from the idea, I would argue, then the sort of folk understanding of what's going on.

Justin E.H. Smith  [04:58]
Right, right, right. And that would have been the case for the peasant in the field had you asked them during the during the time of these people.

Sean Carroll  [05:07]
I think the peasant in the field would have given me a better answer than any of those.

Justin E.H. Smith  [05:10]
[Laughs] Amazing, amazing. Now can you briefly sum up the order of your grading—that is, why the broadly speaking Leibnizian account of matter as "a derivative force coming from activity" sounds most appealing to you?

Sean Carroll  [05:35]
Well, I think it's a very subtle set of ideas that come in when we talk about the modern point of view here. And we don't need to go into all the details. But there is a question being begged by the question "What is matter?" Namely, is there matter?

Justin E.H. Smith  [05:49]
Right. Yeah, yeah.

Sean Carroll  [05:51]
And that's, you know, that has also its own levels, I'm not worried about being a true skeptic, worried about the existence of the external world or anything like that. But in our everyday lives, we run against tables and chairs, and the sidewalk, and so forth. And we take these for granted as part of the architecture of the world. And then modern physics kind of undermines that a little bit with quantum mechanics and relativity and so forth. So there is some role that the idea of matter plays in our contemporary understanding—and we can get to what that is—but it might not be a fundamental part of the architecture of reality. It might be a higher-level emergent concept. And so what I liked the most—or what I disliked the least—about Leibniz's is that it's a more functional description. It's a set of words about the role that matter plays in the world. And if you think that matter is some higher-level emergent thing, that's the right kind of attitude to take on the question.

Justin E.H. Smith  [06:49]
Right, right, right, right, right. Now, is there a possibility that our use of the term in the 21st century is just a vestigial hang-around from earlier period when it seemed like it made sense to call something out there “matter”? Might it be best to just discard it? Or is there a continued real sense in which it does work for us?

Sean Carroll  [07:22]
I absolutely think there's a real sense in which it does work for us. But again, this is a potentially contentious perspective, because I am someone who believes that higher-level emergent things count as real. You know, I believe that consciousness is real and free will is real.

Justin E.H. Smith  [07:37]
Yeah.

Sean Carroll  [07:37]
But I don't think that any of these ideas are there in the fundamental laws of physics.

Justin E.H. Smith  [07:41]
Right.

Sean Carroll  [07:41]
Matter is an idea that you might very well have guessed is there already in the most fundamental laws that we have to describe the world. And there I disagree. I think that's unlikely to be true, but we don't know for sure. Okay, so we can we can talk about the reasons to be pro or con there. But I think that, you know, I'm someone who thinks that it's very hard to get through the day talking like a non-crazy person if you refuse to use ideas like matter and free will and consciousness for that matter.

Justin E.H. Smith  [08:15]
Mmhmm. So it's a useful fiction for you.

Sean Carroll  [08:18]
It's not a fiction, it's real. It's useful as a higher-level non undamental description. That's the crucial distinction.

Justin E.H. Smith  [08:26]
Sure, sure, sure, sure. Consciousness is another interesting case may be a parallel case that's worth discussing for at least a moment. This is something I think many people are more comfortable recognizing as an emergent phenomenon, out of the activity of a hundred billion neurons, something arises that is so central to human lives that it would be disastrous to try to eliminate it from the way we account for our own existence, at least. Right?

Sean Carroll  [09:04]
Yeah.

Justin E.H. Smith  [09:05]
Matter is usually not seen like that by non-physicists, in part because getting rid of talk of matter is even more an unreasonable demand. Would that be why if we look at this in comparison with consciousness…?

Sean Carroll  [09:27]
Yeah, you know, I think that there are absolutely people out there in the world—respectable people who might even end up being right, who knows?—who will take this hardcore perspective that higher-level emergent things do not count as real. Right? People who are illusionist about consciousness or even, you know, gases and solids and things like that. And so they would say, like, only the most fundamental level should be given this privileged status of being real. I think, you know, along with the words you just used that that makes it almost impossible to talk about the world in any way that real people talk about the world. Talking about the world is an important part of what we try to do to apprehend it and get through it. And so, I'm much more comfortable treating things like, you know—the paradigmatic example of emergence is the air I'm breathing right now I describe as a "gas." It has a temperature, it has a pressure and a velocity and so forth. I know that there's a deeper level in which it's molecules and atoms, okay. But that doesn't stop me from calling it a gas, or attributing realness to that description as a gas. And I think that matter is going to be in exactly the same classification.

Justin E.H. Smith  [10:47]
Do you think physicists are especially well positioned to make calls on the question of whether emergent objects count as real or not?

Sean Carroll  [11:00]
No [laughs]. I think that when whenever you get to subtle philosophical questions, physicists are generally never in the right position to think about it. But I think you need someone who is more philosophically inclined, but also aware of and respectful of the most modern ideas in physics. So, let me just give a little preview here, because this is—I told you when you invited me that I'm going to give nonstandard answers.

Justin E.H. Smith  [11:25]
Yeah, that's…

Sean Carroll  [11:26]
So there's two steps to just get on the table for future discussion. One is that a big shift happened in the early twentieth century with quantum mechanics—okay? Before quantum mechanics, you know, up to 1920, the idea that we had about matter in modern physics wasn't that different than the idea that Newton, or maybe even Aristotle, really had about it. No matter what they said, they didn't, you know—they treated it as a thing, some stuff, right, that exists in space, and so forth. So number one, quantum mechanics comes along and calls that into question. Number two—and that's just an obvious thing to discuss and fret about. Now, number two, even among people who understand and agree and appreciate quantum mechanics, we don't agree on what it says, right? We know that, okay, quantum mechanics is right, and we get it, and we can do calculations with it. But the fundamental ontology of quantum mechanics is contentious. And that is a question in which I personally have an extremist position, where I think that the best way to talk about the fundamental ontology of quantum mechanics is as a pure vector in some gigantic abstract vector space. In other words, not referring to things like space and time and matter as specific, separately real elements. Almost everyone else in the world of quantum mechanics—so I'm trying to recognize which things are contentious here—many people who are quantum mechanics experts, and really will appreciate what it has to say, will nevertheless also say that there is a sensible category called “matter” that we discuss quantum mechanically rather than classically. So both the shift from classical mechanics to quantum mechanics and the question of the actual fundamental ontology of quantum mechanics are steps that are very tricky when it comes to asking what is matter.

Justin E.H. Smith  [13:31]
Now, your extremism, then, if you could maybe articulate that in a slightly different way. For those of us…

Sean Carroll  [13:41]
Sure. The best way to articulate it is to give the alternative.

Justin E.H. Smith  [13:45]
Yeah, yeah.

Sean Carroll  [13:46]
Before quantum mechanics came along, we had classical mechanics, okay? And classical mechanics handed down by Newton and followed up on all the way through people like Maxwell, and Einstein, and so forth. They revolutionized physics in their own ways, but they were still doing classical mechanics broadly construed. And in fact, the way I like to say it is if you were physicists, circa 1900, you would have been forgiven for thinking that we were almost done. Some people did think we were almost done. We had this view, or there were particles, like electrons, and so forth, that made up matter, right. And then we had fields that that gave us the forces between those matter particles—the electric field, the magnetic field, the gravitational field, and what have you. And we describe both of these particles and fields using the rules of classical mechanics. And so, you say where the particle is, how fast it's moving, you can observe all those things. If you have two particles, then you have two separate features of those. There's one particle that has its position and its velocity, and another particle as its position, its velocity. So there's a set of rules that is very, very clear. You can combine particles to make tables and chairs. It's a pretty straightforward operation. So quantum mechanics comes along in the 1920s. And it says, “No [laughs], the world is not made up of things with positions and velocities.” This is the absolutely revolutionary move that we still haven't come to grips with. It says that things like position and velocity are measurements that you can make, and you can get a result, right. But when you're not looking at it, the system is not described in terms of positions and velocities. Right? So how is it described? Well, this abstract weird thing that we call the “wave function,” and the wave function is, basically—for right now let's think of it as it's a tool we can use to make predictions, where what we say about the possible outcomes of measuring the position or the velocity or whatever. So quantum mechanics comes along, and for the first time in the history of physics, separates out what is truly going on from the results of what you see when you measure or observe what is going on. And again, that's something we still haven't come to terms with. So someone like me, is a wave-function realist. Okay? So I will say that, “Well, the reason why every version of quantum mechanics needs to invoke this thing called the wave function is because that's what the world is!” It’s the simplest version of saying that, right? It's not just a tool, it's not just a black box that lets us make predictions. It's just the world, okay? But there are plenty of other people who say, “No, no, no, the world is something different.” And the wave function represents, somehow, either our knowledge of the world, or some mediator between us and the world when we interact with it, or something like that. I think it all those other attempts kind of are shaky, because none of them tell me what the world is. But whereas I can say what the world is, in my view… Okay, but this is the fundamental cleavage—the first fundamental cleavage between people trying to understand quantum mechanics: Is the wave function reality, or is it just a tool for accessing reality?

Justin E.H. Smith  [17:08]
What is the nature of your commitment? What grounds your commitment here? Is it—it's surely not a question of you having access to a different set of experimental data than other people, I don't think.

Sean Carroll  [17:25]
No, it's not.

Justin E.H. Smith  [17:26]
It's some kind of interpretation of the data that understands it differently. But what I want to know beyond that is: Is there any sense when you stake this claim, that you are consciously engaging in philosophical speculation? Or is it rather simply your—what you take to be the sound and correct and only possible interpretation of the shared data that everyone has?

Sean Carroll  [18:01]
I think it's somewhere in between those two things. I mean, philosophy absolutely is helpful in discerning the differences between these different positions and adjudicating them. But really, we're just doing science. There's no certainty here, there's no absolutism, there's a very fallibilistic scientific theorizing. My view is a theory. Other people's views are other theories, and we attach credences to all these theories, and we judge them by our personal lights, given our feeling for the simplicity, the fruitfulness, the fit to the data—you know, the possible compatibility with other things we know about the world, etc. And we hope that through further theoretical investigation and experimental data collection, we will all come to converge on one of these theories or another one.

Justin E.H. Smith  [18:50]
How much—I don't want to ask for a percentage, but how much are you prepared to concede that there's a certain amount of—I don't want to say aesthetic, but just kind of a sense of a personal sense of what the world is like that defies any possible scientific grounding, that causes you to commit yourself to the view that the world is—just is a wave function?

Sean Carroll  [19:24]
So I think that I'm 100 percent of the opinion that there is something personal and aesthetic going on here, and 0 percent of the opinion that it defies scientific investigation. I think personal and aesthetic considerations are part of the scientific process. I mean, again, ultimately, long term, we would like to be able to just say, given what the data are telling us, certain points of view are untenable. You got to get rid of them, right? I mean, there's no phlogiston out there. There are no celestial crystal spheres on which the planets are moving. Those are those are ideas that have been bypassed by the data. But when you're in the position where you're not sure yet, which is always going to happen along the way to making scientific progress, people are going to assign credences to different possibilities, in different ways. And some of them will turn out to be right, some of them will turn out to be wrong. None of us has invented an algorithm for getting it right every time. Otherwise, science would be a lot easier than it is. So I think that the the disputes between different versions of quantum mechanics and their implications for what matter is are, on the one hand, very much grounded in personal choices about what is likely to be true; on the other hand, are 100 percent amenable to scientific investigation and will eventually be resolve.

Justin E.H. Smith  [20:45]
Sure, sure. Sure, sure. Okay, so tell me a bit about—forget about the background motivation behind the credence you assign. And tell me more about this credence: the world, reality, just is the wave function, whereas other people are trying to explain the wave function in terms of something that is more loyal to what we took the world to be before it came along…?

Sean Carroll  [21:15]
That's exactly right. That's a perfect way to put it. When we teach ourselves or our students quantum mechanics—okay?—which again, I I'm gonna say this over and over again, we don't agree on what quantum mechanics and says, and that makes all of our statements about it a little wishy-washy sounding because I'm trying to be fair to people who disagree with me. But what we tell them is, you know, let's start our investigation with a classical picture—a model of the world—whether it's, you know, a ball rolling down a hill, or an electron and an atom or whatever. But let's, let's pretend it were classical. It's not, let's pretend it was. And then there's a procedure we teach them to promote that classical theory to a quantum mechanical theory. It's called quantization. Okay, so we say, here's the theory of the electron, let's quantize it. Here's the theory of the electromagnetic field, let’s quantized that. Here's the standard model of particle physics, let's quantize that, okay. And this procedure has been extraordinarily successful overall, again, and again and again. So we don't teach it to students because we're afraid of reality, it's because it's empirically it's worked. And so therefore, the very common-sense point of view is: Sure, the world is quantum mechanical, as described by wave functions. But the correct wave function that describes the real world corresponds to some precursor, classical understanding, where there's space, and there's time and there's stuff, that stuff is matter-like. And then we quantize it to get the quantum version of that, and that's the real world. Whereas I'm coming along and saying, you know, that's not how nature thinks about itself, if I can anthropomorphize nature a little bit. [Laughs] Nature doesn't start with some classical precursor point of view, and quantize it. Nature was just always quantum mechanical, and the arrow should go the other way. We should start with some intrinsically quantum mechanical description and figure out why the world looks approximately classical to us—which it does really, really well. You don't need to know quantum mechanics to fly a rocket to the moon, or something like that. Right? To know that when you put your coffee cup on the table, it won't just fall down. These are things that work pretty well, even though they are intrinsically classical statements. So my program, as it were, is to say, let's be extremist about the ontology of the universe. It's some abstract wave function. And we can go into—if your listeners are interested, we can describe exactly what I mean by the abstract mathematical notion of a wave function.

Justin E.H. Smith  [23:58]
I think we're going to get to that, yeah, yeah.

Sean Carroll  [24:00]
Okay. But—but the point is, to me, the project of reconstructing physics from the ground up is starting there, and locating the classical world within it, and explaining why there are things like tables and chairs and planets and puppies and people that come out as emergent higher-level phenomena from the wave function. Whereas a more traditional thing would be to say the the theory that we're working with is a quantum mechanical theory of electrons and photons and things like that. So that's our starting point. And then we show how to quantize it and how they interact.

Justin E.H. Smith  [24:37]
This is the first time I've really grasped this particular point, I think, quite so clearly—so thank you for that. But what it seems to me now is—what you're saying, of course, when you initiate students and classical mechanics first and then you quantize, this is really just a matter of didactics. Right? This is a question of pedagogical approaches. You've got to do the one thing first, because it's easier to get your head around, and then you can, you can step it up a level. But what's so interesting to me here is if you compare this with the previous revolution in the history of physics, when people who were promoting mechanical physics in the seventeenth and eighteenth centuries were talking to their disciples, they didn't say, analogously, “Let's do Aristotle first, and then step it up a level.” Right? They just said, ”Aristotle's wrong. Let's do it right.” Right?

Sean Carroll  [25:49]
Well, that's a very, very good point. But I don't think it is purely a matter of convenience. It largely is. But there's a sense in which, you know, maybe they did do Aristotle first, in the sense that, you know, when you, when you—the example I always like to use is: you push a coffee cup that is sitting on the table. Like, as I'm talking to you here, the audience doesn't know, but I have a coffee cup in front of me, I can push it on the table in front of me. And if you take overly seriously Newton's laws, or Galileo's kinematics, the acceleration of the coffee cup depends on the force you put on it. And the natural motion of the coffee cup is just to move in a straight line at a constant velocity. Whereas Aristotle says, “If you stop pushing the coffee cup, it will stop moving,” right? So guess which one of those is right? Aristotle is 100 percent, right, for the real coffee cup on the real table. Right?

Justin E.H. Smith  [26:52]
Right. Right.

Sean Carroll  [26:53]
Galilei or Newton would tell you this story about, “Oh, yes, but there's friction, and you're not looking at the correct idealization and so forth.” And so, there is a relationship between those two theories, it's not a complete break. But, but nevertheless, you put your finger on something real because there's a sense in which—that Galileo, Newton idealization of like, ignore friction, put all that stuff away, and start by imagining a frictionless world and then put the friction in later—that's a more fruitful way of thinking about the world than putting the friction in from the start. And so, you can imagine just being Newtonian from the start, and then learning quantum mechanics afterward. And that's a very, completely fruitful way of understanding the world. Whereas you don't need to hear that Aristotelian thing first, because you can sort of derive it from the Newtonian thing. Whereas—the world in which… Sorry, I'll say one more thing about this, because it's because it is important. You know, when we—I think that when we philosophize about science and theory choice, we underrate the role of compatibility with the manifest image, with our folk picture of the world, right? And Newtonian—so, Aristotelian physics fits in very well with our folk image of the world. But Newtonian physics fits in well enough, especially now that we live in the modern world. And we know that there are spaceships, and things like that—we can get there pretty quickly. Whereas quantum mechanics is just an entirely different beast, that is the best image of the world. So the idea that someday we will be so advanced that we're teaching third graders quantum mechanics, and then just taking the classical limit in sixth grade, I don't think that's ever gonna happen.

Justin E.H. Smith  [28:41]
Right. But there's certainly been already a century of lag, right? And we don't seem any more spontaneously inclined to a quantum mechanical view of the world than people were in the 1920s. Right?

Sean Carroll  [28:55]
That's right.

Justin E.H. Smith  [28:56]
We've made no progress. And that's actually something that really interests me a great deal. I've always been inclined to think that once you all get things worked out and come to kind of unanimity on what's actually going on, there's going to be a kind of cultural spread of this shared understanding, and one day it will be not intuitive, but it'll be something we can take for granted. Because in any case, the manifest image is so systematically distorting, right?

Sean Carroll  [29:33]
Right.

Justin E.H. Smith  [29:33]
…on all levels, that that we're able to, you know, from early in our schooling compensate for the way it’s—the world comes to us through a distorting filter. Right?

Sean Carroll  [29:48]
But you're right in that the one thing that is really holding us back from making the deep understanding of quantum mechanics more widespread is that we don't agree on that deep understanding should be. You know, if physicists and philosophers can't get their act together, then why in the world should we be surprised that people in the street don't understand quantum mechanics?

Justin E.H. Smith  [30:11]
Right. Well, that's why I'm saying once you guys get it all worked out and you come to a shared understanding…

Sean Carroll  [30:16]
Yeah, I agree…

Justin E.H. Smith  [30:16]
…then the public will start to think quantum mechanically, that's kind of my optimistic kind of prediction of how this is going to go. We might want to learn a little bit more from you about, about the wave function. And then maybe we can turn specifically to the question of what this where this leaves matter. In particular, you promised us a somewhat more sophisticated account than we've heard so far.

Sean Carroll  [30:50]
So think about, again, to get quantum mechanics into your brain, you really have to at least provisionally be willing to accept the idea that the act of measurement and observation plays a fundamental role in how we describe nature at its deepest levels. Eventually, someone like me who believes in the Everett interpretation or or most other most other modern versions, will explain what is meant by observation in some mechanical way, rather than as a fundamental category, but but it's a good place to start. So the point is, you know, when you describe an electron, you're going to measure it. So an electron is just a stand in here for some little point particle, okay, you don't need to know anything specific about electrons, it's the, it's the smallest particle that is easily manipulated. So it's a good choice for us. And you want to find out where it is, you know, like you shoot it at a at a plate in a camera, and you're going to get a dot where the electron arrives, okay? So what that.is telling you is where the electron hit the plate, you measured its position, right? Okay, so you are very naturally inclined to think I measured its position, therefore, that was its position. That's not too crazy, that's a very natural intellectual move. What quantum mechanics says is, before you measured it, you could have measured it anywhere, right, there was a probability for measuring it at different places on the plate. And depending on the specific state of the electron, that probability may have been highly concentrated near where you saw it. But there is some other probability of measuring it elsewhere. So what the wavefunction is, at the at the abstract level, is an assignment of a number to every possible measurement outcome. And that number will tell us the probability of measuring it there. And just to be super careful, the by telling us what I mean is you take that number that's in the wavefunction, and you square it, and it gives you the probability and that's just a fancy version of Pythagoras theorem, the, you know, sides of a triangle squared, add them up, if it's a right triangle, they give you the hypothesis, that's where the square is coming from here. So you can think of the electron as sorry, the wavefunction of an electron as a number at every location in space, namely, what is the number you square to get its position. And so it looks very much like a wave. So you know that that naturally kind of gives the name of a wavefunction. And you think of it, you draw pictures of it, you go online, Google wavefunction, and you get these sort of bell curves, or waves or whatever. And it kind of looks like a function here is where it gets tricky. When you have two electrons. Because in classical physics, you know, you we've had waves before, there's no problem with that. But the waves are just waving up and down in space, like at every point in space, there's some value for the wave if the wave is like the height of water on the ocean, or something like that. But this abstract quantum mechanical wave function is an assignment of a number to every possible measurement outcome, space of possible measurement outcomes is enormously larger than the space of points in space, right? space is three dimensional. But if I have two particles, and I measure the locations of both of them, guess what, that's a six dimensional space, because I get to different points, right. And if I have Avogadro's number of particles, 10 to the 26, or whatever, then I have three to the power of 10, to the 26. That no three times 10 to the 26 dimensional space that we're living in. So and that's the space in which the wavefunction lives, enormously high dimensional space, the space of all possible measurement out guys. It's a wave in that space. And so our feeble human imaginations kind of boggle when it comes to even visualizing such a thing. Happily, within the history of the human race, we have produced brilliant mathematicians like David Hilbert and john von Neumann and so forth. And they have told us how to understand these high dimensional functions and waves and whatever in very precise mathematical terms. The lingo is Hilbert space is the space of all possible wave functions. And the wave function can be thought of as a vector in Hilbert space. Okay? And but this is just fancy lingo for saying that, for every possible simultaneous position of everything in the world, there's a value of the wave function. And so again, this is so far from our experience, that people are rightfully a little bit reluctant to just say, that's the real world doesn't look like the real world at all. Okay, and so, to someone like me, that's the real world, and we have the hard work ahead of us in connecting it to our observations. Do other people, they're like, no, our observations are pretty solid. I'm gonna stick close to them. I'm gonna use this fancy mathematical formalism to make predictions about my observations without reifying it too abruptly?

Justin E.H. Smith  [36:02]
Uh huh. That's fantastic. Yeah, that's a very good explanation. Again, I come back—and maybe I'm just repeating the same question in a different way, but it's really crucial to me—these other people who are attached to the real world, or are attached to what we used to call the real world… Is this some kind of—what's the word I'm looking for—nostalgia, perhaps, but also just some kind of loyalty—maybe loyalty is the better word—to something that we've got stuck with for historical reasons, as a result of a particular historical legacy? Because if that’s so, then you could say, one of the grounds for your own credence in your own extremism, is less loyalty for whatever reason that comes from, you know, the particularities of your personality or whatever. Right?

Sean Carroll  [37:06]
Yeah. So I do think that that does play a role. And I think that you've diagnosed me accurately, and that I am very willing to leap to something that I think is the simplest possible underlying explanation, no matter how far away it is from the manifest image and trust that we will do the important work to build our way back. But if I'm a little bit more fair to the people who disagree with me—you know, if I channeled what they would say here, it's like: “Look, we have a very hard project ahead of us, connecting quantum mechanics to the real world—let's not make our lives much, much, much harder than it already is. After all, we see tables and chairs, we see electrons and photons; starting with them isn't the craziest idea in the world, and furthermore, if you don't even allow yourself—if you don't help yourself to that piece of information, it's going to be almost hopeless, to go from a purely abstract image of this vector in some giant, humungous mathematical space, to all the nuanced richness of reality.” Okay? So they would say, “We're just using the data that nature is giving us to help us build a good scientific theory.” And, in fact, I would agree with that. I mean, I would be on their side if it weren't for the existence of gravity.

Justin E.H. Smith  [38:30]
Right. Okay.

Sean Carroll  [38:31]
And some of your listeners might be experts at this, and some might not be, but we have had over the last hundred years, not only a struggle to understand quantum mechanics, but we've had extraordinary success at using quantum mechanics to describe the world. Okay? Even though we don't understand the details, we can make predictions with it to extraordinary accuracy. Many, many Nobel Prizes have been given out for constructing what we call the Standard Model of particle physics, the theory of the different interactions and the different particles of matter and so forth. And, you know, maybe we can even describe those in more detail just to get some meat on the bones of what matter is in modern physical understanding, if you want to, but the one Ugly Duckling in this success story is gravity. As you know, Einstein explained to us over 100 years ago, the gravity can be thought of as the curvature of space-time. It's a classical theory—general relativity is the name of the theory. It works extraordinarily well in predicting the outcome of astrophysical observations. But we can't quantize it, or when we try to do this standard move of taking that classical theory and turning it into a quantum theory, it fails.

Justin E.H. Smith  [39:44]
That's so interesting. You know, usually when I hear gravitation described as a “fundamental force,” I take this to mean that it's fundamental in the sense of primitive—it’s just, you know, you just have to take it. And you can't go back any further behind it. I've always wondered whether that's arbitrary, to state it that way. Now, your account is that the problem with gravity is that it's the one lingering vestige of classical mechanics that can't be quantized. Once it's quantized will it remain a fundamental force?

Sean Carroll  [40:26]
Well, so here's where I radicalize. Because my feeling is that the difference—the reason why gravity is hard to quantize isn't just some epiphenomenon. It's not just an accident. It's because gravity as given to us by Einstein is a theory of the dynamics of space-time itself. So the other forces of nature that we have been able to successfully quantize take space-time for granted and live inside it. And in those cases, we can successfully quantize it. Gravity suggests that space-time itself has a wave function. In other words, it's not in a unique configuration, we might measure it to be—if we did that—but the correct description of it is something much more abstract and mathematical. And therefore, I think, starting with some classical description of it and trying to quantize it is never going to get us the right answer. That's the only reason why I become radical. It's because I think that the more conservative route is going to fail us when it comes to gravity, and we have no choice but to be radical.

Justin E.H. Smith  [41:35]
So it will never be quantized. Do you have colleagues who are working away at this futility?

Sean Carroll  [41:42]
[Laughs] Well, I have many, many colleagues, highly trained and super brilliant people who are working on a quantum theory of gravity, okay. And almost all of them do start from some classical precursor theory and try to quantize it. But they're not—you know, they're smart enough to know that that might lead us somewhere, which is not the starting point. You know, that's perfectly okay. And again, there's nothing wrong with starting with the phenomena and trying to build on them. You know, this ambition that I have to start with the idea and work back to the phenomenon can be very, very difficult to do. So I don't think it's futile and they will necessarily fail. But I think that they're going to be limited in what they can uncover by saddling themselves with this classical starting point.

Justin E.H. Smith  [42:33]
Right, right, right, right. That's fascinating. Now to try to work back to the question of matter, more narrowly, I feel kind of bad for the guy who got an F in your matter class, George Berkeley, because I think that you two would actually get along in—at least uncertain points. It's true he said that “matter is an idea in the mind of God,” and that's not much of a promising starting point for theoretical physics today. But part of his shifting matter in that direction was kind of just trying to get rid of it, to make it kind of something that we can hold on to for higher purposes. But that—and here's where I hear echoes of Berkeley in you—but that nonetheless, have never had any real purchase for the common people. If you go out and ask an Irish peasant, what it is that makes the stone hurt your foot when you kick it, the Irish peasant is not going to say, “Oh, well, it's the material substratum behind the phenomenal stone.” Right? He's going to say it's the stone. And so the fact that the philosophers including the natural philosophers, the physicists, up until now, Berkeley says—up until the early eighteenth century—have theorized something behind the phenomenal experience of the stone that makes it hurt your toe is something that really is just an added layer of unnecessary theorizing on top of the manifest image if that makes any sense. And so I'm not arguing, I'm not defending that view of matter myself. But it does seem like it might share some affinity with your own view.

Sean Carroll  [44:43]
There's some affinity there, but I would argue that it's actually pretty intangible and pretty, pretty tenuous. And the idea that I always have in mind—not always, because it's actually a fairly recent conversion to what is known as “structural realism” in the philosophy of science game, right? And the idea of structural realism—the way that I think about it, which I'm not sure is the right way to its proponents think about it—but when we change physical theories, when we go from something like Aristotle to Newton, Newton to Einstein, Einstein to quantum mechanics, very often, the fundamental ontology that we're talking about changes utterly. This is Thomas Kuhn’s point in The Structure of Scientific Revolutions, right? Like a wave function is just a completely different kind of thing than a bunch of particles moving in space. But the phenomena that it predicts in our telescopes and microscopes match to a very high precision up to some, you know, once you push things too far, so there's something preserved also. So Kuhn was emphasizing the radical ontological shift. The structural realists are saying, “Yes, but there's something that is preserved. And maybe it's those structures, even though you're using different ontological underlying language, but the structures are sort of progressing. We're getting better and better at it.” Now, I think they can go too far. Because they become sort of purely epistemic in how they describe the world and say, “Forget about fundamental ontology, all we have are the structures.” I am a thoroughgoing realist about physics, I think that there is the real world out there. And I'm not even sure how to get along without imagining there is something called the real world out there. So that's where I differ from both the the hardest core of structural realists and from Berkeley. Because I think that precisely the move the Berkeley wants to make is—you know, he's annoyed with the baggage that comes along with some particular view of the ontology of the world, and therefore becomes an idealist and focuses in on, you know, this “idea” nature. So most of his bad grade from me came from using the word “idea” not from using the word “God,” but both both of them hurt his grade. I gotta say, I don't think of it as an idea. I think the world is, is the world and it's real. And it's fundamental, and it's important, its ultimate physical description might appear very, very different than the manifest image. But I don't think we can get rid of it. I think it's absolutely central. And then on this account, is matter—whatever it is—that which, on the final reckoning, is making the world real? On this account—my account?

Justin E.H. Smith  [47:24]
Yeah, on your account, yeah.

Sean Carroll  [47:25]
So let me let me say some buzzwords very quickly, just because they're in the back of my mind. And, you know, I should let the audience in on what I'm thinking. So, you know, like I said, I think that there is some deep underlying picture of the world, which is very abstract, and very far away from what we observe every day. And it seems like just a wave function in some giant space that is evolving according to the laws of physics. But the project is to connect that to what we observe. And the first stepping stone—long before we get to tables and chairs—is, you know, particle physics and atoms and things like that. Okay? And that's where things get very, very rich and intricate and successful and surprising in modern physics. And so if you talk to most other modern physicists, they would have spent the entire discussion talking about bosons and fermions. Okay. This is a way of saying that the particles that we know about in the Standard Model of particle physics, which is extraordinarily successful, come in these two buckets. Bosons are particles that can pile on top of each other, and give us a big classical field, like the electromagnetic field, or the gravitational field. Fermions are these particles that take up space, so they can't pile on top of each other. And so—like electrons and quarks and so forth, right. And so these two kinds of particles in modern physics, the bosons and fermions, map on very nicely to that pre-quantum idea that the world is made of matter and forces. Right? Now we say it's made of fermions and bosons, then they would have said it's made of matter and forces. So this particular quantum-mechanical feature of certain kinds of particles—that they take up space, you can't take two electrons in exactly the same state and put them in exactly the same place—That's what makes tables solid. Okay? That's what gives us the phenomenological features of matter that we were so familiar with. So in my mind, there's this long chain, several of the steps of which are still very tentatively constructed. Starting from a pure abstract quantum theory, to an emergent version of space-time with fields in it, then to the specific version of space-time and fields in it that we call the Standard Model of particle physics that has all these bosons and fermions in it, and thence to a picture of atoms and molecules and chemistry and then we get to the matter that we know and love.

Justin E.H. Smith  [50:01]
So the matter only comes down at the final stage of that?

Sean Carroll  [50:05]
You can attach the word, whatever you want. I mean, it's not in the first stage. But you could, this is—the word “matter”… This is why I said at the beginning, “Is there matter?” Right? Because we invent the word “matter” thousands of years ago in some language, or whatever, and they didn't know that there were quantum fields and bosons and fermions. They knew there were tables and chairs. So I am all in favor of attaching the word “matter” to tables and chairs. Does it also belong in the world as a good label for quantum fields that are electrons and bosons? You know, whatever maybe? Yeah, I'm happy to do that. If you don't want to do that. I'm happy to do that, too. Like it was ever meant for that purpose. So let's not, let's not pretend that we start with a preexisting idea called “matter” and ask what it applies to. We learn about the universe, and then ask how best to deploy our language.

Justin E.H. Smith  [50:58]
Yeah. And nothing of any theoretical significance hinges on finding the precise boundary. Right?

Sean Carroll  [51:08]
No, not at all.

Justin E.H. Smith  [51:08]
Like, you could draw it at fermions, or atoms.

Sean Carroll  [51:11]
Or tables, yeah.

Justin E.H. Smith  [51:12]
Yeah, yeah.

Sean Carroll  [51:13]
Yeah, that's right.

Justin E.H. Smith  [51:13]
It sounds like you don't really—I mean, okay, now if we go back to consciousness, again, you might, by analogy, similarly say that we don't have any firm grasp of what degree of neural activity yields up consciousness, when we look at the broader animal kingdom, we're sure it starts somewhere. And we're not have any wide agreement as to just how many features should be there, whether self-awareness, or the mirror test, or whatever, need to be passed in order to be speaking of consciousness. And we also suppose that believing in consciousness—believing there's such a thing as consciousness doesn't require us to find the precise line where it leaves off and mere neural activity is going on. So is this what you would say of matter as well? That it definitely exists? We definitely know some paradigm cases of it. But we're not going to be able to solve definitively certain boundary questions like whether a fermion is a bit of matter or not?

Sean Carroll  [52:40]
Well, the way that I would say it is almost that. I'm very happy saying that, yes, matter definitely exists in the form of tables and chairs. There are two things to do. One is to put together the entire comprehensive picture of all the different levels of reality and how they connect with each other. Right? And, and also understand why the world appears to us as a series of levels. Why is it—why does there exist these useful higher level immersion descriptions? That's the hard part and the interesting part and the fun part. And then there is the sort of part, “Okay, what words are we going to attach to these different levels?” And yeah, there, I wouldn't say we'll never answer the question. I'll just say your question’s not that interesting. It's completely subjective. If you want to use the language in some particular way that makes you happy, go nuts. As long as we agree on the underlying thing going on, I don't care what words you want to use.

Justin E.H. Smith  [53:39]
Could physics do away with the term “matter” altogether and still do everything it wants to do?

Sean Carroll  [53:46]
I mean, it could, but why would you want to do that? The word “matter” is really, really useful for describing things. You know, in cosmology, for example, we talk about radiation-dominated universes and matter-dominated universes. And those words signal very real physical things going on. So you'd make your life much harder if you eliminated that word from your vocabulary.

Justin E.H. Smith  [54:12]
Right, right. Right. You know, we're coming close to the point in the discussion where I start to try to move towards a judgment on whether we agree or not, and I think this is the clearest case I've ever had in a conversation of [gushing wind] aporia. And this is through no fault of the interlocutor, but just because I'm still kind of as perplexed as when we began as to what matter is, but I think you… That said, I think there's a shared sensibility here coming at the question from very different directions, in that we seem equally at ease with convention dictating, where we're going to apply certain fundamental philosophical terms. And matter is a philosophical term as much as as much as a term of physical theory. And also a comfort with treating the emergent as real. Certainly, I have absolutely no problem taking as real—let's again, go back to the analogy of consciousness, even though I think it's quite likely that it emerges from neural activity. Um, I could be wrong there. But, and I think we're all kind of working in a context again, in which being wrong about the sources is something that we're prepared to be. Right? So then the only the only reason for the aporia then is because to be perfectly honest, I still don't really understand quantum mechanics.

Sean Carroll  [56:11]
Yeah, that's a problem. You're not alone.

Justin E.H. Smith  [56:15]
And this brings me maybe to a kind of wrapping question that, that I somewhat hesitate to ask, but that I think you might have some insights on and years of experience dealing with: Why does quantum mechanics attracts so many crackpots? So many people who think they understand it and use it for such wild, unhinged things? This clearly has something to do with the lack of scientific consensus, right?

Sean Carroll  [56:46]
Yep. Yeah.

Justin E.H. Smith  [56:46]
And if there were—if scientists were more univocal about it, you'd probably hear less disinformation from the broader public. But what is this phenomenon? And what—and is there anything perhaps of value to be extracted from it at a cultural level?

Sean Carroll  [57:07]
Well, I think it's not just the lack of scientists being univocal. It’s the lack of scientists themselves understanding what is happening. Right? I mean, it's not that they don't just speak with a universal tone, but they don't even agree with each other. So in that kind of circumstance, where scientists themselves disagree on how to talk about what is going on—and some scientists kind of just get out there and say some crazy things, honestly—then, I'm a little bit reluctant to place most of the blame on non-scientists for these kinds of misunderstandings. But at the same time, the other thing that you can't completely gloss over—that's sort of a procedural question—there's also a substantive issue that, like I said, quantum mechanics in the way that we teach it seems to involve the act of measurement or observation, in our statement of fundamental laws of physics. Many of us want to get rid of that, but still it’s there. When we teach quantum mechanics, when you open the textbook, it's there. So that is a substantive claim that does open the door for all sorts of crackpottery, right? Putting human beings back in the center of the operation of the universe. We thought that, you know, we'd gotten past that since Copernicus or whatever but, you know, quantum mechanics is threatening to have that happen again. So I think it's both our lack of perfect understanding and the fact that quantum mechanics sort of raises a flag that certain people will naturally run toward.

Justin E.H. Smith  [58:41]
Right. Do you think scientists in their work as public intellectuals—and that would certainly include you—have some kind of duty or perhaps calling to help guide potential crackpots towards something more sober and cautious?

Sean Carroll  [59:01]
Well, two things with that. One is that I generally locate the responsibilities or the duties not in individual scientists, but in the field. Some scientists, as I'm very happy to say, I would just as soon not talk to them. Like, [laughs] they're really good at doing science. But, you know, explaining things to the wider world is not their expertise. But as a field we absolutely have a responsibility. So I think that we should really value and encourage the people who are good at it to do it, which is not what we do, in academia anyway. But the other point is that it's not the crackpots we want to save. There will always be extremists in other directions, other than me, who just wants to be wrong about things, but there will also always be good faith, curious people who just don't know a lot. And it's not that we want to convince the crackpots that they're wrong. It’s that we want to save the people on the street from being captured by the crackpots,. That's an important thing to try to do.

Justin E.H. Smith  [1:00:07]
Yeah, yeah. Well, you're doing amazing work. And I really appreciate your efforts to help us understand a bit more clearly what some of the deep theoretical issues at stake and the question of what matter is, are, and thanks again for coming on. This was really a lot of fun.

Sean Carroll  [1:00:28]
My pleasure.

Justin E.H. Smith  [1:00:29]
And once again, this is “What Is X?” I've been talking to Sean Carroll, who has his own podcast called “MindScape” that you should listen to. And we've been talking about what matter is, and I hope you'll join us again on the next episode. Thanks again. We'll see you here soon. Bye bye.