What is so beautiful about string theory?

Daniel and Kelly talk to Thomas Van Riet about how string theory unravels the puzzle of quantum gravity and whether math can be beautiful. See omnystudio.com/listener https://omnystudio.com/listener for privacy information.

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Daniel Whiteson?

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• 1:23Daniel Whiteson?

  People might disagree about what kind of art they like. In fact, pretty much everybody does, but we all know what it means when we say that something is beautiful. It means that we appreciate it, that it moves us, that it strikes us, that we see an elegance in it. But what does it mean if you say a theory of physics or a little bit of math is beautiful? How can math be gorgeous? How can physics be elegant? What does that really mean? Well, string theory is the theory of physics that's most often described as a bit of 21st century gorgeous physics that fell into our laps. What does that really mean? What is so beautiful about string theory?

• 2:07Daniel Whiteson?

  And just because something is beautiful, does that tell us whether it's more likely to describe our universe to actually be right? That's the question we're going to be asking today on the podcast. What's so beautiful about string theory? Welcome to Daniel and Kelly's extraordinary universe.

• 2:41Kelly Weinersmith?

  Hello, I'm Kelly Weiner Smith, and I know nothing about string theory. In fact, sometimes my eyes cross and I just blankly stare at the wall when the conversation comes up. But today, I'm gonna understand it.

• 2:52Daniel Whiteson?

  Hi, I'm Daniel Whitehead. I'm a particle physicist, which might make it sound like I should know string theory, but actually it means I just smash particles together without understanding the nature of the universe.

• 3:02Kelly Weinersmith?

  Oh, all right. Well, so I've got a question for you. I listened to your beautiful opening about what does it mean to have a gorgeous equation? So my question for you is, what is the most beautiful equation?

• 3:15Daniel Whiteson?

  Most beautiful equation? Oh, my gosh. To me, the most beautiful equation is actually not in physics, it's in math. Euler's identity, it says e to the i pi plus 1 equals 0. And I just think it's incredible because it combines like a bunch of different stuff. You have e, pi, 0, 1, and i all together, and it's so compact and there's just so much encoded into it. It's like so dense with useful information. It tells you about how you can think about signs and cosines in terms of complex numbers. To me, it's just fascinating to have so much information packed so tightly and so beautifully into a single equation.

• 3:58Kelly Weinersmith?

  Awesome. A fine choice.

• 4:00Daniel Whiteson?

  But I also have to say that in grad school, the moment I discovered I was not going to be a theoretical physicist was when I was sitting next to my office mate and I realized that he did his homework just like I did, but he did it two or three times in different fonts because he got really excited about like writing these equations. Equations. He's like, oh, I'm writing in italics, or I can write these symbols another way. And I realized like, wow, this kid really jams out about like, writing down the equations. It's something about being a theoretical physicist that I just didn't have. I was like, happy to be done with it once.

• 4:33Kelly Weinersmith?

  Yeah, I gotta be honest, that doesn't strike me as super efficient. I'm gonna do the same thing three times, but I'm glad that he's super into it.

• 4:41Daniel Whiteson?

  Yeah, but there's something about the equations and the formalisms and the expressions, and even the fonts, the way you're writing these mathematical symbols, that you gotta be excited about if you're gonna work in the nitty gritty of figuring these things out. Because being a theoretical physicist is a lot about writing equations on paper. So if you don't like that, then probably you shouldn't be one.

• 4:58Kelly Weinersmith?

  Yeah, fair enough.

• 5:01Kelly Weinersmith?

  Today, we're talking about a theory that is regularly described as beautiful. And I'll tell you, by the end of the episode, I'm moderately convinced that string theory is beautiful. Maybe even more than moderately convinced. But I think we should see what our audience thinks about what's so beautiful about string theory. Is this something that people know already?

• 5:20Daniel Whiteson?

  That's right. I reached out to our listeners to ask them, what do they think is beautiful about string theory? If you'd like to contribute your voice for future episodes, please write to us to [email protected]. We will sign you up. Also, send us questions about anything. I got a recent question about somebody's dating life, which I totally couldn't answer, but I enjoyed reading anyway. So feel free to write to us.

• 5:41Kelly Weinersmith?

  You should have sent that to me. I was on Dan Savage's podcast, and I feel like that makes me a relationship expert. So you can just send those to me. I've got it covered.

• 5:51Daniel Whiteson?

  There you go, folks. We are self-proclaimed experts in anything. So think about it for a minute. What do you think is beautiful about string theory? Here's what some listeners had to say.

• 6:02Speaker 04

  I don't think the universe would be so inelegant to be a tangled, knotty, strand-filled mess.

• 6:13Speaker 05

  Unifies general relativity and quantum mechanics. Physicists love elegance and symmetry. It may also provide a new basis.

• 6:23Speaker 06

  Baseline of what's the tiniest thing? And then we get to ask the question, is that it? Or is there something beyond that?

• 6:30Speaker 04

  The amount of money that Brian Greene was able to make by taking advantage of popularizing it. We are able to mathematically explain why gravity is so weak compared to the other forces. Son asks his dad, Dad, what is string theory? The dad says, why you ask such difficult questions?

• 6:51Speaker 05

  Ask me something easier. So the son says, okay, why does mom get so angry? Ah, well, string theory is a theoretical framework. It's a kind of symmetrical beauty. The beauty to me is that we keep searching for the boundaries. That would have to be the G string. Nothing's beautiful about string theory, except that our confusion is beautiful.

• 7:14Speaker 04

  People want to try to bring everything all together into one unified theory that explains everything. I think that's what makes it beautiful.

• 7:24Daniel Whiteson?

  And they look like worms. String theory and all the theories that try to bridge this gap really show the spirit of scientists and researchers and physicists everywhere to keep on trucking. The totality of its failure. In the same way that a lot of other unfalsifiable and self sealing things are in life that we love.

• 7:47Kelly Weinersmith?

  That the maths of it all is quite elegant. What I like about the idea of it is that rather than trying to think of the universe in discrete particles,

• 7:59Kelly Weinersmith?

  kind of just thinking about it in like pulses of energy. I call it confusing. Well, Daniel, it looks like we're teaching the controversy today because our our answers range from, I don't think there's anything beautiful about it. To, you know, it's pleasing aesthetically and there was a good range of answers. What do you think? Is it beautiful?

• 8:19Daniel Whiteson?

  I think it's fascinating to apply this subjective standard to something which is supposed to be objective, right? We're talking about like the answer to the question of how the universe runs itself, the machinery of the cosmos. Why do we care about whether it's beautiful? Why should beauty be a guide? Like if we have two theories, should we pick the one that's more beautiful and follow that because we think it's more likely? I think it's this sort of like bias we have that we think nature should be beautiful because we mostly look around and we're like, oh yeah, the world is pretty. I wonder if aliens evolving on an ugly planet, one that they find like, eh, kind of yucky, would tend to be biased towards yucky theories of physics because their life is pretty yucky.

• 9:03Daniel Whiteson?

  Or maybe everybody evolves to think that their planet is beautiful and everybody tends towards beauty. I don't know. To me, it's a deep sort of philosophical question of what is beauty anyway? And why do we appreciate it in our world? And why do we look for it in our physics?

• 9:16Kelly Weinersmith?

  All right. So first, an observation. In my experience, it seems to me that when people say, oh, this equation is beautiful, what it usually means is it makes their life easier. It explains a lot of things and maybe this is, uh, human laziness is the wrong answer because most of the people working on these equations are anything but lazy, but like, oh, it's nice. It explains a lot of things. I don't, I don't have to worry about that stuff. So you, you did seem earlier to think that Euler's equation was beautiful, but now you seem to be a little bit more critical of people saying, talking about equations that describe the universe as beautiful. Do you feel like there's some difference there?

• 9:52Daniel Whiteson?

  No, I can see beauty and I can appreciate it. It's like when you see a piece of machinery and there's only a few moving parts, but it can do something really complex. Or you look at a piece of code and you're like, wow, that is so simple and yet powerful. You can appreciate the beauty of that. I just don't know why the universe has to work that way. Like the universe could be a total mess. We could discover the way it works, be like, actually I have some notes. This could have been done better, you know, like more documentation, please. So I can definitely appreciate beauty when I see it. And I think I can even capture what is beautiful about something. I just don't know why we expect the universe to be beautiful. I mean, I hope that it is, but we'll see.

• 10:31Kelly Weinersmith?

  I mean, I think the universe is beautiful, like the sunsets are beautiful. The biodiversity is beautiful, but it certainly seems to me that anytime we try to explain what's happening, there's nothing beautiful in there. But maybe that's just the biologist working on ecological models where we're like, this is a mess and cells are a mess and everything's a mess. But we're all just muddling forward.

• 10:51Daniel Whiteson?

  And let's not even get into chemistry because that's a disaster.

• 10:53Kelly Weinersmith?

  No, we weren't going to get into chemistry, Daniel. That's not what we do.

• 10:57Daniel Whiteson?

  That's right. Exactly. But neither of us are also string theorists. And so I reached out to somebody I know online, Thomas van Riet, who is a string theorist and writes about this stuff. And I've seen him on social media venting gently about how string theory is not well understood and misexplained and misunderstood by the general public. So I invited him to come on the podcast and tell us all about it.

• 11:19Kelly Weinersmith?

  Let's jump right in.

• 11:24Daniel Whiteson?

  So then it's my great pleasure to welcome to the podcast Professor Thomas van Riet. He's a theoretical physicist at the Institute for Theoretical Physics at the KU Leuven. Some of his recent work include papers called "The Stability of Axion-Saxion Wormholes" and "The Quantum Theory of Gravitation: Effective Field Theories and Strings," yesterday and today. So I thought he'd be a good person to talk to about string theory and its alleged elegance. Thomas, thank you very much for joining us. It's my pleasure to be here. So my first question for you is, what is this big problem that everybody's trying to solve? We hear a lot in popular science about how we have general relativity and we have quantum mechanics and these two theories don't work well together and we need some

• 12:05Daniel Whiteson?

  theory of quantum gravity. Why do we need a theory of quantum gravity? What is this big issue? Why can't we just have GR and quantum mechanics and be happy with those? So in everyday life, gravity is a classical force and there's no problem in understanding gravity. Sometimes it's a bit complicated, especially, you know, when you're looking at, say,

• 12:26Speaker 05

  Black hole mergers, you need full blown relativity, but it's still a classical theory. You can put it on a computer. You can do advanced calculations and you can understand what's going on.

• 12:38Daniel Whiteson?

  And what do you mean when you say a classical theory? What does classical mean? It sounds like a technical word you're using.

• 12:43Speaker 05

  Classical can mean two things in physics. It's very confusing. So classical can mean that you do Newton mechanics and you don't do relativity. Relativity is a correction to Newton mechanics. And the correction takes into account that the speed of light is finite. So in physics, theories are always corrected by so numbers. And you can think of the difference between relativity and Newton mechanics to be that one theory is corrected by the other. By numbers which go like one divided by the speed of light, which is a very tiny number. So that's the first sense of classical. I actually meant the second sense of classical. And that's where you say, I take.

• 13:26Speaker 05

  Say, it doesn't matter whether it's a Newtonian or a relativistic theory, but I had quantum mechanics. Okay, then in quantum mechanics, we also have a small number called Planck's constant. Right? So very informally speaking, you could say that quantum mechanics correct classical mechanics. By terms in equations that are powers of this small number.

• 13:51Daniel Whiteson?

  So classical is a fuzzy word that basically means old fashioned. The same way like you might call classical music Mozart, but the kind of music I like to listen to is called classic rock on the radio, even though it's not that old. And so you're talking about two different senses in which physics has evolved from Newton to Einstein and then from Einstein to like Schrodinger and Heisenberg and stuff. And so in this sense, when you say a classical theory of physics, you mean without quantum mechanics.

• 14:15Kelly Weinersmith?

  Daniel, some of that music is pretty old by now, man. We are getting a little bit old, I'm sorry.

• 14:20Daniel Whiteson?

  And for the record, Mozart totally rocks, okay?

• 14:22Kelly Weinersmith?

  Okay, no, I'm not disagreeing there. So the biologist who's trying to keep up with the physicists here, alright, so it sounded to me like you were saying quantum mechanics and general relativity can be reconciled if you just divide by the right terms. I was under the impression that they describe completely different phenomena and kind of don't work together at all.

• 14:43Speaker 05

  It's too quick to say that they can easily be combined, but it's also too quick to say that they cannot be combined. So first of all, indeed, are there regimes of interest where the two theories should be combined? Because usually, gravity, we think of very large things. Gravity is so weak that in order to see it, you need to have large objects, massive objects. You know, the Earth is pretty big, and I can still lift my glass of water from my table, meaning that, you know, the electromagnetic forces in my body are stronger than the gravity of the full Earth. So gravity is weak, and things need to be big to be able to see it. There's another option. If things are dense enough, you know, imagine taking the Earth

• 15:26Speaker 05

  and compressing it into the size of my water cup, okay? And then even compressing it more. So then, of course, I will get into a regime where you say, well, you know, it becomes very small, and then the theory of quantum mechanics becomes important. Yet also, gravity becomes strong, and you can ask, do we know of such regimes? And we do. And I would say it's the most important regime for all of physics. It's the early universe. So if we go back in time, and we look with our telescopes, so looking with a telescope means that you look back into the, you look into the past, you see that the universe was denser. And if we

• 16:06Speaker 05

  just follow our classical equations, it actually tells us that the density will go to infinity, which is of course not true, but it is an indication that in the very early universe, you know, everything was very tiny, so quantum mechanics was absolutely important, and gravity was huge. So we need The theory of quantum gravity. One other example that we have already measured are black holes. So black holes have always been a sort of a theoretical invention. But they're not anymore. We have seen them, they're out there, okay? And what you sometimes wrongly hear when people, you know, talk about science in a for the for the bigger public.

• 16:50Speaker 05

  They would tell you that black holes for sure are objects where quantum gravity is important because gravity is strong near a black hole. That's actually not entirely correct. If you look at a big black hole, for instance, a black hole in the in the middle of our galaxy, the gravitational force at the horizon of that black hole is big, but it's not ridiculously big. Okay, and the bigger a black hole, the weaker gravity is

• 17:14Speaker 05

  at the horizon of a black hole. It's kind of counterintuitive a bit.

• 17:17Daniel Whiteson?

  Is that because you're further from the center?

• 17:20Speaker 05

  Exactly, and it's also because the density of the black hole goes down as the black hole grows. Like the dense, the black hole, I think, if I'm not mistaken, you can always fact check this. But I think the black hole in the center of our galaxy has a density compared to water. So it's wrong to say that for sure quantum mechanics will be important near the horizon of a black hole. But what we are pretty convinced of is that if you would jump in a black hole, we don't know what's there. But it cannot be classical physics anymore. Because at some point, the classical equations tell you rubbish. They tell you things which are impossible. So we know the classical theory has to break down. And so the assumption is that, just as in the early universe, in the very center of a black hole,

• 18:03Speaker 05

  there's also quantum mechanics and gravity at play at the same time.

• 18:08Daniel Whiteson?

  So I want to get back to what you said about things breaking down. But in a minute, first, I want to focus on this question of quantum mechanics and gravity at play at the same time. So you told us earlier that general relativity, or we can call it gravity, describes usually big things and quantum mechanics usually describes small things. And now you're saying that at the beginning of the universe and inside black holes, we think both of those are relevant. And that's why we need a unified theory, because we need some way to describe that and the two disagree, the two conflict. Why is there a conflict in their predictions? Couldn't it just be that they make the same prediction for what happens in that scenario? Couldn't there just be beautiful, fortunate harmony among the theories?

• 18:48Speaker 05

  How, what's the quickest way to explain? So let me give an example that I hope more people know.

• 18:57Speaker 05

  High school or first year of university say, okay.

• 19:01Speaker 05

  Think of an electric field. So you have a charged particle. And a charged particle is surrounded by an electric field that it sources itself. So when you look at it classically. And when you think of a particle classically, it means that the particle is a point. And maybe if people remember this still, there was this formula that said that the strength of this electric field. Went like a negative power of the distance from the particle.

• 19:28Speaker 05

  Say, you know, take it's one over R squared. That's actually the force. Then you find that this force becomes infinitely big as you approach the particle.

• 19:41Speaker 05

  Okay, it's the same for the energy. The total energy carried by that particle would be infinite. And we know that that cannot be, that cannot be correct. And then we have learned later on. Hundred years ago, when people understood the quantum theory of charged particles. There was nothing going infinite. Things were just super well behaved. Numbers were finite. Nothing weird was happening. And gravity, you see, that sounds completely analogous, right? Even the the formula for the force of gravity is almost the same as a formula for the Coulomb force. Both can give you infinities. And for the Coulomb force, we learned. That that infinity is gone when you treat it quantum mechanically.

• 20:23Kelly Weinersmith?

  Both general relativity and quantum mechanics at some point start giving you infinities that make no sense when you push them to their extremes.

• 20:31Speaker 05

  So quantum mechanics doesn't give you infinities, but the classical theory does. So relativity does. Yeah. So then the question is, if you treat relativity in a quantum mechanical way, would you get sensible numbers?

• 20:43Kelly Weinersmith?

  And do you?

• 20:46Speaker 05

  It's a, well, that's a good question. So.

• 20:48Daniel Whiteson?

  And do you? Yeah.

• 20:49Speaker 05

  To be able to answer it, I should, I should tell you, this is the theory that of quantum gravity. And to say that something is the theory of quantum gravity. I mean, you can write down a theory which is extremely hard, but imagine you succeed and people succeed it.

• 21:04Speaker 05

  You don't know whether it's the only option, right? So you need to test. Normally you test the theory. And the problem is that to go out and test it requires you to look for these, you know, places where gravity is so strong. And I guess none of us wants to jump in a black hole. You could, but you know, you could test it. But unfortunately, you would not be able to tell anymore to anybody else because you cannot escape from the black hole. Right? So nature is playing a very mean trick on us. It seems that humanity, in order to to test the theory of quantum gravity, it is forced to do something that kills you.

• 21:41Speaker 05

  You know, this is actually goes under the name of censorship, cosmic censorship. It's actually something quite serious in the physics community. Gravity works such that you could actually just see quantum gravity. Unfortunately, there's always what we call a cosmic horizon preventing you to see it. Either it's the horizon of a black hole or you have to go back in time. But if you take your telescope, it's actually impossible to to directly look at the Big Bang. So that's kind of mean. Otherwise, you could just observe it.

• 22:07Kelly Weinersmith?

  It's frustrating, no doubt.

• 22:09Speaker 05

  If I was in religion, I would say God is playing, you know, is is an evil person, so to speak.

• 22:15Daniel Whiteson?

  Yeah, I also believe in cosmic free speech. I think the universe should be free to tell us how it works. And I'm bummed about all this censorship. Okay, I have a lot more questions, but first let's take a quick break and let our brains rest a moment.

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• 25:00Daniel Whiteson?

  Okay, we're back and we're talking to Thomas van Reed, a self-proclaimed string theorist, about how string theory works and why it's so pretty. So let's go back to this question of infinities. You said just a moment ago that it's hard. And we hear this a lot, quantum gravity is hard, it's the hardest problem. And you're telling us that we have general relativity, which works beautifully outside of event horizons and after some critical Vertical density in the universe. We have quantum mechanics, which works beautifully and very effectively for very small things and very high energies. Why is it hard to bring these two things together? What is the challenge? I mean, we've made a quantum version of electromagnetism. We've made a quantum version of the nuclear forces. Like, why is it so hard to take gravity and make it quantum?

• 25:43Speaker 05

  It is hard for two reasons. Maybe there are more, but there are two that are very sort of prominent. So one of them technically goes under the name that the theory of gravity is non-renormalizable. We can come back to that. It has to do with infinities of sorts. But actually, we have some experience with non-renormalizable theories in the past, and we solved them. But still, it usually means bad. It's tough. That's already what it tells you. The second thing is that gravity, at the same time, is a theory, a classical theory of space and time, a theory of the background. Okay? So in quantum mechanics, the way the laws of quantum mechanics are formulated is that you assume that this background is fixed.

• 26:28Speaker 05

  What does it mean in practice? Is that you have to, for instance, in, let me try to give a good example. That doesn't sound too abstract. In quantum mechanics, you use the notion of two points in spacetime, whether they are what we call causally connected. Whether they can talk to each other. Whether, you know, you can send a light ray from one point to the other.

• 26:56Daniel Whiteson?

  You're talking about lycons?

• 26:57Speaker 05

  Exactly. But some more, maybe less technical. Imagine that right now, there are people, I don't know, 10 kilometers away from us. If we want to communicate with them, we cannot do it right now. We can do it in a, you know, a little bit of time, the little time it takes to send a light ray. But so the two points that are us here right now and then people a little bit away, we are what you call disconnected. But imagine now that you have a theory of space and time where space and time are wobbly. That's what relativity tells you. Then maybe that that whole notion changes, right? So quantum mechanics is telling you that things fluctuate. Things are very wobbly at a small scale.

• 27:38Speaker 05

  They're so wobbly that maybe, you know, in your equations, what you thought are disconnected points, maybe they're not disconnected anymore. Because you're wobbling the background that is telling you that it's connected or not. And that's what makes it kind of annoying.

• 27:53Kelly Weinersmith?

  Daniel, is this going back to our map analogy that we keep bringing up on the show? Or is this something different? This is the same?

• 27:59Daniel Whiteson?

  Yeah, that's exactly right. In GR, we have the concept of distances which are not fixed, right? Which can change. Whereas you're saying in quantum mechanics, this is essentially the background. It's like quantum mechanics is assuming that there's a stage on which everything is happening. And GR is like the theory of that stage and it's changing underneath it. But what I don't understand is why that makes it hard. Like, can't we do quantum mechanics on curved space? You know, you can think about your fields and my quantum mechanical view of spacetime is like, yeah, you have this backdrop and then you put fields on top of it. And then you do the physics of the fields and stuff is propagating. Is it hard to do that quantum mechanical field theory in a curved spacetime? Haven't people been able to do that? What's so hard about that?

• 28:42Speaker 05

  Very good. So indeed, relativity can curve spacetime. And then you need to formulate quantum mechanics on curved spacetime. That is not easy, but that people have done for sure. One example why that is not so easy is that quantum mechanics tells you there has to be a global time direction. Things move forward or backward in time. On a curved spacetime, what you thought was time can actually at some point become space or vice versa. And a famous example is a black hole. Imagine you approach a black hole. You jump through the horizon. What do you know? Is that you have to move forward to the singularity. But you see, that means that that spatial direction became time. Because what is time for us?

• 29:27Speaker 05

  Time is the only dimension in which we cannot stop moving forward. I can decide now to sit on my chair, but I cannot decide to move backward in time or be still in time. I always have to move forward. So the fact that once you pass the black hole horizon, you are moving forward in time, forward towards that singularity means that that forward direction became time. And time became actually a spatial direction. So for quantum mechanics, for Schrodinger equation, that's pretty annoying. But we learned how to deal with it. And dealing with it gives you this amazing phenomenon like talking evaporation of black holes. But what I said, quantum gravity is still something different.

• 30:10Speaker 05

  Because in quantum gravity, it's that curved background that is itself fluctuating under the quantum fluctuations. Right? So there's no problem doing quantum mechanics on a curved background. It's just a bit more complicated because of the problem I told you. But now the background itself should become dynamical in the quantum theory. So your standard Schrodinger equation is not well formulated to deal with the fact that the background itself is the thing that is part of the theory.

• 30:40Daniel Whiteson?

  I see. So you can do quantum mechanics on flat space. That's easy. You can do quantum mechanics when space gets curved. That's a little bit more technical, but people with big brains have figured that out. But having the space itself respond to the quantum mechanics to have it all be dynamical and linked together and be harmonious. To have this back and forth where energy is telling space how to bend and space is telling matter how to move. That is too technical for people to have figured out, where that's the challenge.

• 31:07Speaker 05

  That's a challenge. Yeah, absolutely.

• 31:10Daniel Whiteson?

  And this, I think, also connects to the other comment you made about renormalizable theories, which I think is worth digging into for a minute. Because it connects to the example you talked about a moment ago about an electron having apparently infinite charge or apparently infinite energy, right? If you take an electron's charge and you look at it from a distance, it appears to have charge of negative one, right? But as you say, an electron is surrounded by its field. And that field you can think of as a cloud of potential particles. And so if you actually think about what the charge of the electron is that we measure, it's the charge of the electron surrounded by the cloud, right? And as you penetrate deeper into the cloud, you measure a more and more negative charge. And then that charge, if you get all the way through the cloud to the electron, the charge apparently becomes negative infinity,

• 31:54Daniel Whiteson?

  which is crazy and bonkers and unphysical. And so you were talking earlier about renormalizable theories and how we've managed to patch this up with quantum mechanics. Can you say a few words about what it means to renormalize a theory? How do you get rid of an infinity in a theory? How do you solve that kind of problem where you're like, hold on a second, electrons can't have negative infinity charge? How do you solve that? What is renormalizability?

• 32:15Kelly Weinersmith?

  I just clarify real quick, so the biologist in me wants to confirm. So when you guys say you're getting infinities, that's just a fancy way of saying we're wrong. Like, it's just, this is not working, right? Okay, got it. All right.

• 32:27Speaker 05

  Yes, absolutely, absolutely. To be honest, I think to explain the normalizability, the story of the infinities is what usually is told, is, I think, leading people astray. It's not the right way of explaining it. May I try to explain it differently? But please go ahead if it's... Okay, yeah. Maybe it's good to go back to, you know, what Newton got famous for, you know, his theory. And what is his theory saying? Maybe people remember that, you know, there's an equation that he got famous for, which is called F equal m times A, which is essentially telling you that a force on a particle equals the mass of the particle times the acceleration that the particle is undergoing because of the force.

• 33:10Speaker 05

  So you can ask yourself, is this really a law? A law in physics means that you suddenly, there are three things you know, and you actually found a connection between them. I would say, isn't this a definition? Did Newton just define the word force by saying it's m times A? It looks like that. But there's another famous example that, you know, in high school, you learn Ohm's law. And usually you call it, you say that resistance is voltage divided by current. See, that's not a law, that's the definition of resistance. There's no information in that equation, but a definition. Ohm was successful because he said this R is a constant. That's the definition, that's the law.

• 33:54Speaker 05

  R is a constant, and then it becomes something with predictive power. Namely, I measure the voltage over a resistance, and then I know the current that goes through it. But it's only because my real equation is R equal to a constant. So what's the equation of Newton? Newton's idea was only successful because he essentially wanted to tell us this F is universal. Imagine the way an apple falls under this F, whether, how it falls in Cambridge or in China. It will be the same formula. So that means that once you have the formula for the F and you say it's true all over the universe. It becomes very strong as a prediction. So Newton and his program was having a lot of predictive power by, you know, moving around in the universe.

• 34:39Speaker 05

  He found something true all over the universe. Okay, so this is same with normalizability. So instead of moving around from the left to the right, up, down, whatever. Future, past, renormalizability has to do with zooming in and zooming out. If I have an equation and I wanted to have predictive power. It has to tell me what also happens when I zoom in or I zoom out. Okay, and zoom, let, let us think of zooming in, that's where really the problem lies. It means that if I go to very small length scales. I want a theory which gives me a single equation with all the constants known and measured. So I can tell you what happens at small distance scales when I zoom in. A non-renormalizable theory doesn't give you infinities.

• 35:21Speaker 05

  People should stop saying that they give you completely finite numbers after some mathematical trickery. But what it does is that the more you zoom in, each time the equation gains another constant. That we don't know the value of and we have to go out in nature, nature and measure it. Right, so imagine you want to, somebody's asking me, okay, Thomas, what happens in a gravitational field? At, you know, a micrometer. I'll say, oh my God, I have to, you know, already maybe correct Newton's law for quantum gravity. And I say, yeah, there's an extra constant in the equation. It's not, you know, the force is not one over R squared, there's maybe one over R cube. But it's not one, it will be some number multiplying that equation. Okay, I go out in nature and measure it. Good, I have the number.

• 36:03Speaker 05

  And if somebody wants to go 10 times smaller or 20 times smaller, suddenly a term which goes like 1 R to the power 4 becomes important. And you need to know the coefficient of that term. I again have to do a measure. So you see, I don't have predictive power. That's the definition of a non-renormalizable theory. It means that, you know, the smaller you get, the more constant the theory has to be more precise. But it cannot predict what the constants are. Whereas a renormalizable theory, it says, hey, I don't need any new constant, guys. I mean, I can tell you with a computation of what, you know, how the theory behaves at the smallest length scales. And so unfortunately, the way historically this came about, and that's where the word renormalizable comes from, is that, you know,

• 36:47Speaker 05

  we were getting infinities and then we found, we always say a mathematical trick, but in fact, it's a physics trick to get rid of them. But you see, that's, that's not the essential part. The essential part is whether the theory is predictive, whether there's only a few constants that you can get out, go out and measure, or whether you need an infinite amount of constants if I want to get infinitely small. And so if we take Einstein's theory, classical theory of gravity, we apply our usual techniques of quantum theory, we find that it's non-renormalizable, not meaning that it gives you infinities. This is actually, I think, a bad explanation. It gives you too many constants that we don't know what they are, and we will have to measure.

• 37:30Daniel Whiteson?

  I see. So a renormalizable theory, you can say, I don't really know what's going on inside the electron. Maybe there's other particles, maybe not. But I can measure the charge and I can move on. And I could say it's all wrapped up in this number, the charge of the electron. And as long as I can make a finite number of measurements, like I don't have to measure an infinite number of properties of the electron, then I have a theory I can use because I can make a finite number of measurements in a finite amount of time. So a non-renormalizable theory you're saying is one where you can't ever capture all those details in, in a single number or two numbers or even a finite number of numbers. You'd need to like measure an infinite number of parameters to have a theory that you can actually use to make calculations. But you said a minute ago that we have other non-renormalizable theories.

• 38:13Daniel Whiteson?

  I think, for example, quantum chromodynamics is non-renormalizable. And we've made that work. I mean, I know it's a headache, but we've made it work. What is it about gravity that's so special that we can't use our non-renormalizable fancy clever tricks to get quantum gravity to work?

• 38:28Speaker 05

  Right, so you're saying humanity has dealt with non-renormalizable theories, made them to work, and I can tell you what the problem is. So I guess, don't forget that before, we already said that gravity had two problems for it to be hard to quantize. Non-renormalizability was one of them. So there's still the other one. So that's part of my answer. But it's still, I believe it's very different. Like, the other part of my answer will be the following. Usually what happens in physics, actually every instance we have seen so far, where a theory was non-renormalizable, we actually cured it by realizing that we didn't have all what we usually call degrees of freedom. Okay. So what does that mean?

• 39:12Speaker 05

  So I zoom in, I go to small distances. And I always assume that, you know, if I have the theory of the electron, there's only the electron, say. Well, maybe there are very massive particles out there, which require a lot of energy to be created. In physics, having a lot of energy is the same as going to very small distances. So all non-renormalizable theories we have encountered

• 39:40Speaker 05

  were always made renormalizable by realizing that we didn't take into account fluctuation fields, particles that were just very massive, so that we didn't measure them yet.

• 39:49Kelly Weinersmith?

  Just real quick, saying you needed more degrees of freedom means there was something else that wasn't included in the equation that needed to be there.

• 39:54Speaker 05

  Exactly, absolutely, absolutely. And then you see, oh, this is nice, my theory becomes mathematically renormalizable. But then, actually, we went out in nature. We found technologies to increase our energy in our experiment, and then we saw those particles that we predicted mathematically because we wanted the theory to be renormalizable. Okay, I think that's extremely beautiful. Like you, you do something on mathematical grounds. It predicts new particles for it to work out. And there you measure them, okay? And here is a funny thing with gravity. What string theorists will typically tell you, but I think more and more people are leaning towards it, is that if you want to make gravity normalizable, it looks like you need an infinite amount of particles with ever-increasing energy. And that sounds super bad when you say it at first, because you're like, oh, my God,

• 40:39Speaker 05

  infinite amount of particles to solve your problem is like measuring an infinite number of constants. You're not better off, okay? So of course, now I'm gonna sell string theory here. No. What is so beautiful is that this infinite tower of particles groups together in the motion of a string. It just meant that what we thought were particles, no, it was just a single object. There's no tower. It's just a string that can vibrate in different ways. So there's a lot of structure in that infinite amount of particles that you need to invoke to get a normalizable theory. Yeah, otherwise it looks very bad, like every time you're taking a new particle, you find that renormalizability still requires a new one, and you think, oh, my God, you guys are just, you know, in a never-ending street of problems. No. We see that every single particle we have to add has exactly properties that we could

• 41:24Speaker 05

  have predicted from the previous one. So there's a beautiful structure. And what looks like an infinite tower of particles just becomes a single string, a single object with almost no constants associated to it.

• 41:36Daniel Whiteson?

  So there you just said the word beautiful. What is beautiful about that? Is it because, wow, this is a hard problem, and now I have a solution? Is it like my headache is gone? Or is there something objectively beautiful about this particular solution?

• 41:49Kelly Weinersmith?

  Can I go back to a real quick question? And then can we move to beauty? Because I don't want to miss my chance to understand this, because I'm actually really following everything. I'm excited. Okay, so instead of needing to measure an infinite number of constants, can we measure that string? Do we know how to measure the string? Does that make our situation any better?

• 42:07Speaker 05

  I'm gonna be honest, in practice, no. But this, we could have predicted you in advance, okay? So this is, has nothing to do with string theory. I just told you that the regime where gravity... And quantum mechanics are relevant. It's either we have to jump through a black hole, which is not nice as an experience, or we somehow have to be able to move back in time to the Big Bang, or people are able to build, you know, galaxy-sized accelerator. That is a true statement, independent of what the theory of quantum gravity is. It just, you know, you predict what is the energy density needed to see those effects. Of course, one can be lucky, and some effects of the highest energy densities

• 42:51Speaker 05

  or the smallest length scales can trickle, you know, how do you say trickle down? Is it correct English? I don't know. But can leave an imprint on larger distances and smaller energies. Is it something that we're looking into? We are hoping, you know, praying, but we don't know for sure. So I hope that that explains a bit, yeah. And after the beauty, it's, I like the question, so but I'm trying to find an analogy, okay? So imagine, I don't know whether this reminds you of the word beauty, but imagine you have a super complicated puzzle in front of you. I don't know, one billion pieces, and you just don't know how to put them together. And suddenly you find two connecting. And because you see two pieces that connect, you suddenly see the third piece lying there. And the more you put them together,

• 43:35Speaker 05

  suddenly it just won't structure that is like extremely simple, okay? It's like imagine you have a blackboard full of equations, and you cannot solve them. And suddenly you realize that your equation was too complicated, that terms are dropping against each other. And you keep on canceling terms, and suddenly you have an equation left which is just one centimeter in size. You're like, oh, my God, this is, you know, this is amazing. That's the kind of beauty we're talking about, that we think that renormalizing gravity is a nightmare. It gives you ugly theories. And then the first thing we try, which, you know, just on mathematical grounds, and we get something that is, in terms of the length of equations, is even smaller than any equation that we have had in the past.

• 44:22Speaker 05

  And that is what I think why so many people like it. And then the confusing part is that it's not because the the size of the equation is small that it's easy to solve. It just means that it's very elegant. Okay. In the sense that, for instance, maybe elegance is better than beauty. The elegant thing of string theory is that there are no constants in the theory.

• 44:44Daniel Whiteson?

  No numbers at all.

• 44:46Speaker 05

  No numbers at all. Exactly. Any other theory you know, in physics, has to have a lot of numbers that you go out and measure. String theory doesn't have a number. Actually, it only has one. It's the size of the string.

• 45:01Kelly Weinersmith?

  As variables, but no constants? Is that I'm having trouble imagining an equation with no numbers?

• 45:06Speaker 05

  No, that's exactly correct.

• 45:08Daniel Whiteson?

  So it's like x equals y, not x equals 2.74 times y.

• 45:12Speaker 05

  Right. Where I didn't know the 2.7, I had to go out and measure it.

• 45:15Kelly Weinersmith?

  All right. So I'm excited because this is the most that I've understood string theory in my life so far, but I could still use a break. So let's go ahead, get some more coffee, a little bit more brain fuel, and we will be right back to talk more about string theory.

• 45:36Kelly Weinersmith?

  Hey, it's Niall Rogers, host of Before the Moment, a Lincoln original series.

• 45:42Speaker 05

  I'm driving across the country, meeting some of today's most fascinating artists. We're exploring the stories before the moment that changed everything. This week, I'm joined by Miguel. Man, I'm a black and Mexican kid from San Pedro, and I grew up in Inglewood as well. I never liked to be like boxed in. Both of my parents colored outside of the line. I mean, even them being together at the time in L.A. was just not typical. And so I think that has always been a part of my DNA. There's a defiant something in me that feels like what we get to do with with what we're given that's greater than us is to maybe connect people to ideas that they wouldn't naturally.

• 46:26Speaker 04

  You know, connect to. That's really me. I'm just kind of like defiant with love, though.

• 46:30Daniel Whiteson?

  Experience before the moment. A Lincoln original series on the iHeart Radio app, YouTube, or wherever you get your podcasts.

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• 47:11Speaker 02

  What's that? My fresh detector. Uh, your what? There it is. What is it detecting?

• 47:17Speaker 01

  Fresh produce, fresh meat. OK, what happens if it detects something really fresh?

• 47:25Speaker 01

  Apparently, we celebrate. At Food Lion, you can count on fresh food at fresher prices when your fresh detector is going off. Head to your local Food Lion or shop at foodlion.com.

• 47:49Kelly Weinersmith?

  All right, we are back with Thomas van Riet. Let's jump back into string theory. So, string theory, we've discussed that it can help when you're in those really tiny little situations where you'd usually have to get a lot more or calculate a lot more constants. Does it also work if you zoom out? Or is it just a theory for when you're super zoomed in?

• 48:08Speaker 05

  This is an excellent question. So that's where it gets hard, surprisingly. OK, so when you zoom out in physics? It means, okay, large distance also means low energy. And what is the hardest part of working with high-energy theories like string theory? Is to understand if I take the theory and I run it to low energies, like the energy densities that we like have, you know, in your office, okay? Then it's not unique. And so it becomes very difficult to understand. So how would the world look like on this low density or large distances? I mean, string theory predicts a completely unique world at small distances. You see little vibrating strings behaving in a certain way. But then if you zoom out, it's not obvious what's gonna happen.

• 48:54Speaker 05

  Okay, that's why we always say we have trouble. We are not sure whether we can reproduce the large, the universes we typically know.

• 49:02Speaker 05

  But this is not a problem of string theory. This is effect of all high-energy theories. And maybe I can give an analogy. Okay, so imagine that you have a rocky landscape. Hills, mountains, whatever, very, very complicated, many valleys. And you have a football, but the football has a lot of energy. You know, so then it's like up there, up the tops of the mountains, right? Because it just has lots of velocity. It's moving through those valleys and it's just all the way up. But then, you know, there's friction and the velocity is going down. Well, if I have many valleys, I don't know where the ball is gonna roll down and where the valley is that it's gonna end. That's a problem we have. But I don't think it's a problem with theory.

• 49:47Speaker 05

  It's just, it's typical. Actually, even the standard model has this property that this difficulty.

• 49:53Daniel Whiteson?

  That's a great explanation. Thank you. Can you circle back and help us understand more specifically how string theory solves these problems of quantum gravity? You talked about how having these strings replaces the infinite number of parameters you might have to measure. How does it solve the problem of quantum mechanics and general relativity working together on this dynamic spacetime?

• 50:14Speaker 05

  So first, I think it's very important to have a disclaimer. We don't have the full theory, right? So whether it solves all problems that we know of quantum gravity, we do not know. I have to be honest on this. I would say that from a mathematical point of view, the way it solves this is by not quantizing gravity. That's very strange to say, but it is. That's what string theory did. So quantizing, when we use the word quantizing, it means that we take our classical theory.

• 50:41Speaker 05

  It has a certain amount of variables, okay, like electromagnetism has the electric field and has the electron field. And we said, let's quantize it. Let's make it quantum mechanical. So you could say, well, relativity has what we call the metric field. That's a field that describes how space and time curve, okay? And that's what people usually do. They say, oh, we learned in history, you have to quantize series. We take that classical, what we call field and we turn it into a quantum mechanical field.

• 51:12Daniel Whiteson?

  But how do you do that? How do you quantize a theory? You don't just like tap your magic wand on it and say, and now you're quantum mechanical.

• 51:18Speaker 05

  Oh, my God, this is stuff. So mathematically, you would say you turn a field into an operator. But that probably means a little too people listening. Here is an attempt. I'm not sure it's even good. Quantum mechanics tells you that things, they're only probabilities. Right? What you thought was a particle, we sometimes say it's a wave. It's not a very good word. What we have instead is a probability distribution of where the particle should be. So objects are turned into probability distributions.

• 51:50Speaker 05

  That's quantizing a theory. And then there's a certain equation for the probability distribution.

• 51:57Speaker 05

  But more mathematically, it means that you take a field and you make it into an operator.

• 52:01Daniel Whiteson?

  No, that's a great way to think about it. A classical theory says that everything is specified and there is infinite information, even if you don't have it. Whereas a quantum theory leaves some uncertainty and says, well, this isn't determined. Maybe the electron is here, maybe the electron is there, maybe the field has this value, maybe it has that value. And for those of you playing along at home, an operator here is making a measurement. It's like applying something to it and getting a result out. And so that's crucial element of quantum mechanics. Okay, so now we're going to try to quantize spacetime. And you said, we can think of the metric as a field. The metric is like how much curvature there is at every point in space. So if we think of like the curvature in space as a field, why is it hard to quantize that? Why can't we think of that as like, well, maybe the curvature is this value, maybe

• 52:46Daniel Whiteson?

  the curvature is that value? Why can't we just think of that probabilistically?

• 52:49Speaker 05

  That was a problem of the problems we talked about before. Then you run into the problem of non-renormalizability if you do it that way. Or you run into the problem that, you know, it's a background itself that has to become a probability, so. The formalism of quantum mechanics gets very confusing at that point. And as I said, we already knew that a theory that is non-renormalizable means that you're not having the right degrees of freedom. You're missing information. So the way string theory went about is people that discovered it were not trying to quantize gravity. Let's be clear on this, okay? They wanted to solve another puzzle.

• 53:27Speaker 05

  And for some reason, which is a long story by itself, they were interested into string-like objects and how they move and how strings move quantum mechanically. So they do their computation. And they suddenly see that the string can fluctuate in what we mathematically call a spin two field. If you don't know what a spin two field is, it's just a fancy way of saying it describes what I call a metric field. They just say, but that's strange. There's no space and time. And yet they found a structure which is what they knew from relativity. And then they started to look into it deeper. And they wanted to understand the equations that that metric field obeyed. And they were completely surprised that, you know, they didn't ask for it.

• 54:11Speaker 05

  They found Einstein's equations. So this is also what I call absolute beauty. Okay, other approaches to quantum gravity, they say, let me take Einstein equations for true.

• 54:23Speaker 05

  Just have them. Einstein also just wrote them down. Einstein didn't know why. He said, these are my equations, okay? He didn't derive them. And so the other approaches to quantum gravity say, let me take those equations and, you know, quantize them. String theorists did something else. They were looking at strings vibrating for a completely different reason. They not only recover Einstein's equations, the classical ones, they predict, they literally predict them, but they immediately have them quantum mechanically. And it meant that they needed to have all these possible vibrations of the string. It's just one vibration that gives you this metric field. But a string can vibrate in so many other ways. And then suddenly it gave them other things they knew. For instance, all the forces in nature, they come in two kinds. Okay, there's gravity. This is a separate guy. It is described by this metric field.

• 55:08Speaker 05

  And the other forces are, with a mathematical term, are gauge forces, Yang-Mills forces. Electromagnetism is an example of it, okay? And the two other are the nuclear forces. And they're all described by one equation, which is called the Yang-Mills equation. And a special kind of Yang-Mills equation that maybe people listening who had a little bit of a scientific, you know, education remember are what we call the Maxwell equations, which are the equations of electric and magnetic fields. But this is part of a general mathematical equation, which is called the Yang-Mills equation, which, which also mathematicians study for completely different reasons. But guess what? They were looking at the other modes of vibration of the string and without asking for it, the Yang-Mills equations appeared. And at that point, people were like, my God, this is insane.

• 55:51Speaker 05

  Okay, so this is where all the hype came from, all right? From string theory, like the excitement of people all came from this. Not only do we, you know, we get, we get the classical theories. We didn't ask for that, we got them. So you get all at once, so to speak.

• 56:05Daniel Whiteson?

  So I'm getting a sense from you that the elegance of string theory comes from the sort of discovery that it answers questions simply and that sometimes it answers questions we weren't trying to answer. And in that sense, it feels more like you're accidentally revealing a big chunk of truth rather than you're like laboriously putting together an overcomplicated answer that's just an invention in your mind. Is, is that the feeling here that we've like uncovered a vein of reality?

• 56:34Speaker 05

  Absolutely. And I think that's for all of science. Like I can imagine in biology, when you understood like gene structure and you suddenly realize how things work, it becomes so simple. Like I think the same evolution happened in biology. You have all these phenomena. At some point, we learn about the cell and the smaller organisms. And all of these phenomena suddenly can be explained in a more microscopic way. So the theory becomes simpler. It became, maybe I'm simplifying. I mean, I'm not the expert here, but I would say biology at some point became a theory of the cell, which is so much smaller and so much in a way simpler.

• 57:12Daniel Whiteson?

  So is it then about the theory itself? Or is it about the insights the theory gives you about? How the universe works or just the place where we are where we're like, wow, we're frustrated by these problems for decades. And now finally, the headache has gone away. I mean, can you look at the theory itself and say, this theory is beautiful? Is there a chance that we could have revealed a theory which feels truthful? But then you're like, actually, I don't like it. It's kind of ugly.

• 57:35Speaker 05

  Right. I guess different versions of beauty were felt at different stages, right? So the people first making these discoveries of seeing Einstein's equations and so on. When you read their biographies, they really like, they talk about it extremely emotional. Like they could be crying because they saw that part of beauty, but my generation came later. So we kind of, how do you say, you got used to it. You don't feel that beauty anymore. The thing that strikes me is that when you first time learn about Newton's second law, I don't think none of us feels what Newton felt at the time. And I think we are underestimating the emotions. Because one of the thing that I didn't tell you about Newton's second law is it is it tells you about what we call

• 58:20Speaker 05

  the deterministic view on nature. Newton realized that this equation was telling you that, you know, I'm sitting on my chair and later on I will walk away, but according to Newton's equation, I don't have the choice. I have no freedom. There's no freedom of, you know, there's no free will.

• 58:35Kelly Weinersmith?

  Do the philosophers know you all have solved that problem?

• 58:38Speaker 05

  No, that's a classical theory, right? So what physics is deterministic. So there, according to physics, I think we should emphasize is there is no free will in physics. Anybody that tells you that there is is wrong. Physics has, I don't say we have solved it, but there is no free will in physics, absolutely not. Free will is completely in contradiction with physics. Not the illusion of free will, but free will by itself, no way. There's no room in physics for free will. Not in the usual notion of the word free will. Anyhow, I wanted to say that the beauty that we feel or now the students, you know, which are younger than me. There are different versions of beauty. It's more where you start applying the theory. I can give you one example of a thing that I found beautiful is like you have equations with singularities, like these infinities we talked about.

• 59:21Speaker 05

  And in string theory, you can do calculations and you see there's no infinity. And then you learn how string theory tells you that there's no infinity. And it does it in a very creative and funny way. There's often a picture, like you can even see it literally. It's not an equation, it's not formulas, you can just see it. And that's kind of beautiful.

• 59:38Kelly Weinersmith?

  Too bad the listeners can't see your face because you have the biggest grin on that you've had the whole interview, explaining how beautiful it is. Like, you're clearly getting a total kick out of this. It's awesome.

• 59:47Daniel Whiteson?

  I think it is really hard to put yourself in the minds of earlier generations to appreciate how big some of those steps forward were. Like, it seems pretty basic what Aristotle accomplished. You're like, uh, things fall down, duh, I could have said that. But, you know, to systematize the world at all was a big step forward. I think you're right that it's underappreciated. So what is it that we're underappreciating now in terms of string theory? I mean, there's a lot of popular writing about string theory, a lot of popular conceptions about it. But from somebody on the inside, what do you feel like is most often misunderstood or misrepresented about the nature of string theory?

• 1:00:21Speaker 05

  I have to be careful not to become too sort of drawn into the sociological discussion. But I feel like I cannot not say it. So string theory, say, thirty, twenty years ago, when it's, people discussed it in science outreach, it was only the one, okay? And now it's the opposite. And I think that the opposite went so far that it's completely misrepresenting the field.

• 1:00:48Daniel Whiteson?

  When you say the opposite, you mean like people being critical of string theory because it hasn't yet predicted some experiment and been proven right? Is that the backlash you're talking about?

• 1:00:57Speaker 05

  Exactly, this is an example, exactly. So they will tell you this. And then, of course, you have to tell them, because they don't tell you this, that this is so for any theory of quantum gravity. So, and I told, we knew this in advance, we knew this before we started working on string theory. With absolute certainty that if you have access to the smallest length scales, you can falsify a theory, one from the other, okay? String theory from the other examples. What is completely non-obvious is that some of these high-energy, small-distance effects have an imprint at larger distances. At the moment, we don't know. And we're actually looking into it. And this program has a name. I think it's super exciting. It's called the Swamp Land Program.

• 1:01:36Speaker 05

  And it's where we try to look into that question, but at the moment, we do not know. Whereas any other supposed alternative to string theory is not even there at that stage where they can even ask this question. Does my theory predict something at a bigger length scale? Because normally you don't expect that to be the case, right? So can I get a way of not sending a student into a black hole to learn about quantum gravity? We don't know.

• 1:02:02Kelly Weinersmith?

  I mean, master's students are expendable. You could send like four or five of them.

• 1:02:06Speaker 05

  Unfortunately, they could not explain what they're what they're seeing, right? So they couldn't explain it. That's that's a otherwise I would, you know, be interested in maybe jumping into a black hole just to see. Because if you jump into a big black hole, actually, it doesn't need to be a painful experience. You can pass the horizon without, you know, feeling it too much. And then you could actually see the singularity, you know, Interstellar is a little bit about this, right? You jump into a black hole, there's a movie about it. But yeah, so this, I think, is my frustration that there is a back reaction to the original hype. But the back reaction, especially, you know, now on social media, but also in, I'd say, conventional science outreach.

• 1:02:48Speaker 05

  To give an example, I saw my children, they're at an age where they get interested in science and they start Googling things. So I see their first hit on Google when they ask a question, which is about fundamental physics. And the first hit that they have is criticism on string theory. It became so mainstream that this is the first thing you see, and that's not healthy anymore. Okay.

• 1:03:10Daniel Whiteson?

  So just to make sure I understand, you're saying it's fair to criticize string theory and say you haven't made a prediction which has been verified. But all theories of quantum gravity also have that issue that we can't go inside a black hole. And many theories of quantum gravity haven't even come together and coalesced in enough detail to make any predictions, not to mention ones that can be tested. So then let me wrap up by asking you a last question, which is about the truth of string theory. I mean, you're excited about string theory because you think it's simple and it feels like a compelling potential answer to the question of like, what's really happening in the universe? So do you think, for example, in some hypothetical scenario where aliens arrive on Earth and they're very advanced scientifically and we could figure out how to communicate

• 1:03:54Daniel Whiteson?

  with them, et cetera, et cetera, what do you think are the chances that alien physicists are doing string theory, that they have also stumbled upon this explanation?

• 1:04:03Kelly Weinersmith?

  Daniel always has to get aliens into the show at least once. And so, so here we go.

• 1:04:08Speaker 05

  We all love aliens, actually. Okay. I don't know whether my answer is of any meaning, but I would say they will discover string theory. I actually don't even doubt it. I'm 100% convinced. And as to the question before, people tell you that a theory without predictions is not, is not science. And I think we have to really step away from this. So in science, there are two things, there are observables and there are computables, especially in theoretical sciences. And what a theory has to get right are the computables. For instance, if I have a theory that can explain phenomena at large distances, but

• 1:04:48Speaker 05

  I look at small distances and the theory tells me that I can go back in time and kill my mother before I was born, I know that theory is nonsense. But I cannot make an experimental verification. But the theory is just nonsense. It's not logical. And the thing that people, that the audience and, you know, the greater public needs to understand, quantum gravity is so extremely constraining in terms of just logical consistency, that you almost uniquely arrive at an answer. And that's where this is true science. Okay. Despite not having access at the moment to an experiment to test it, you almost uniquely are pushed into

• 1:05:31Speaker 05

  a direction to solve this problem of non-retrovalizability. Now, I'm selling it too much, but I hope you're understanding what I'm trying to say.

• 1:05:39Kelly Weinersmith?

  I've read a couple of books on string theory and never understood them, but I've totally understood our conversation today. So I'm, this has been awesome.

• 1:05:46Speaker 05

  Happy to hear that. Awesome.

• 1:05:49Daniel Whiteson?

  And if aliens arrive and they don't do string theory, maybe they can listen to this episode to get a primer on how string theory works.

• 1:05:55Speaker 05

  Exactly. Exactly.

• 1:05:58Daniel Whiteson?

  Wonderful. Well, thank you very much for coming on the show and talking to us about the hard problem of quantum gravity and how string theory might be the solution. Thanks very much.

• 1:06:06Speaker 05

  It was a lot of fun. Thank you so much.

• 1:06:16Kelly Weinersmith?

  Daniel and Kelly's Extraordinary Universe is produced by iHeartRadio. We would love to hear from you.

• 1:06:21Daniel Whiteson?

  We really would. We want to know what questions you have about this extraordinary universe.

• 1:06:27Kelly Weinersmith?

  We want to know your thoughts on recent shows, suggestions for future shows. If you contact us, we will get back to you.

• 1:06:33Daniel Whiteson?

  We really mean it. We answer every message. Email us at [email protected].

• 1:06:40Kelly Weinersmith?

  Or you can find us on social media. We have accounts on X, Instagram, Blue Sky, and on all of those platforms you can find us at D and K Universe.

• 1:06:49Daniel Whiteson?

  Don't be shy. Write to us.

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• 1:09:13Daniel Whiteson?

  Hey Barb.

• 1:09:13Kelly Weinersmith?

  Morning. Quick question, just one second.

• 1:09:15Daniel Whiteson?

  Sorry. Carmen's worked at this office for 20 years. She knows everything, which is why Carmen has to help with, well, everything.

• 1:09:22Kelly Weinersmith?

  Hey Carmen, what did Faye pitch in 2023? Who owns the Nordic account?

• 1:09:26Speaker 02

  Oh, uh, where's the printer?

• 1:09:27Daniel Whiteson?

  But Carmen's revenue report is due in five minutes, so she shows them work IQ. Hey Copilot, who owns the Nordic account? And what did we pitch them in 2023? Get the context you need with Microsoft Copilot. Work lives on.

• 1:09:42Kelly Weinersmith?

  This is an iHeart Podcast. Guaranteed human.