transcript

Speaker 1:
[00:04] String Theory remains a leading candidate for the long sought theory of everything, one that unites classical and quantum physics in a tidy, elegant mathematical package. But nearly six decades after it was first proposed, it's still about as controversial as theoretical physics gets, with vocal proponents and critics and a whole bunch of physicists just trying to navigate the swampland in between. As a theory, though, it hasn't been stagnant, and in fact, a few recent papers continue this tradition it has of having surprising, almost miraculous mathematical powers to make things make sense. But does it make any sense, that is? Welcome to The Quanta Podcast, where we explore the frontiers of fundamental science and math. I'm Samir Patel, Editor-in-Chief of Quanta Magazine. Our regular columnist, Natalie Wolchover, has been covering physics, including string theory, for us for more than a decade. So it's natural that she'd be our guide for navigating this quagmire. She's here with us today to talk about her latest essay, Our Strings Still Our Best Hope for a Theory of Everything. Welcome back to the show, Natalie.

Speaker 2:
[01:23] Nice to be here, Samir.

Speaker 1:
[01:25] Okay. So Natalie, what's the big idea?

Speaker 2:
[01:29] The big idea is really this disconnect that I think exists between the way string theory is talked about for the general public and the voices that are loudest. The message people get is that this is a dead theory, and maybe even that people who are still studying it have this false hope, but that actually it's this dead end. So a disconnect between that and what I hear from theorists I talk to, which is that basically people think, yeah, it could be right. Not only that, but there is a huge community that's still pursuing it, and thinks it really is still the most promising theory of everything that we have as a candidate. So I wanted to try to understand why that is the view you come to when you know more about this, and then in the course of reporting on this, I also am aware of this new approach to the subject that's developed over the past few years, that's actually adding more life and more excitement to this possibility, string theory.

Speaker 1:
[02:34] So in order for us to get into this, like why there's a disconnect between the critics and general public perception of it, and the way that it's seen within the physics world, I think it'll help to have a little string theory 101, and I wonder if it's not useful to start at the beginning with where this theory came from, because I think a lot of people don't even understand that part. So can you start us at where string theory was first initiated, then we could talk a little bit about how it's evolved over the last few decades.

Speaker 2:
[03:03] So it didn't originate as an idea for a theory of everything, which is to say a theory of where all four of the fundamental forces come from. It actually initially started as an explanation of what these particles called hadrons are that were popping up in experiments in the 50s and 60s. So people would bash together particles, and then they would form momentarily this huge variety of different composite particles called hadrons. Physicists were trying to figure out what was going on. It was a force called the strong force, and it kind of glues these particles together into composite hadrons, and to what were the rules there, and what was really going on. Gabriele Veneziano was a theorist from Italy who was working in Israel, and he had this idea, basically this equation, that seemed to match a lot of the data that was coming out about hadron. So Veneziano came up with this equation, and he actually had like an epiphany and thought of this equation. So it did describe this hadron data. So then for a couple of years, everyone was really excited about this equation. It was called the Veneziano Amplitude. And very quickly, some other researchers realized that what it was describing is two strings ricocheting off each other and moving apart. And sometimes two strings colliding, forming an intermediate state, and then those move apart. So this Veneziano Amplitude was actually describing the hadrons as being like little strings.

Speaker 1:
[04:43] When we say they're like strings, I think this actually might be part of where the public misunderstanding of string theory comes from. What do we mean by the word string in this context?

Speaker 2:
[04:57] Basically, it's a band of energy, and it's a one-dimensional kind of line that's oscillating. So the original idea is this is open-ended line oscillating, basically vibrating in these different patterns. Then you can also close the ends of the string, and they have a loop that's vibrating. Those are the string can just do that, attach to itself or be freely moving as an open string.

Speaker 1:
[05:24] This is in contrast to the way that we usually think of particles as a point, like an infinitesimally small point, is the way we tend to treat particles mathematically. And so, Veneziano had this epiphany that if you didn't treat it as a point, but as a vibrating one-dimensional string, something interesting happens with the math.

Speaker 2:
[05:46] Yeah, and I'll clarify that he actually did not himself think of these as strings. He came up with the equation. Other people realized, oh, the things being described by that equation are little strings. And there's always a question of, well, what's really going on with the point? What is a point really? How can a point have properties? So that mystery goes back decades before Veneziano came along, and it's still an ongoing question. So strings actually get around a lot of problems that come up through the infinitesimal nature of points.

Speaker 1:
[06:24] And this is, for context, this is in the mathematical descriptions we're using to describe ultimately the way stuff behaves.

Speaker 2:
[06:33] Exactly.

Speaker 1:
[06:34] Okay.

Speaker 2:
[06:35] But Veneziano's amplitude, it really didn't describe hadrons. What happened was within a few years, maybe five years, an alternative theory of hadrons was discovered, invented, and that treated hadrons as composites of quarks and gluons, which are more fundamental particles that are fluctuations in this quantum field. And so that picture was correct, and that became established. But string theory, this idea that there might be these little wiggling lines of energy, that stuck around. And it became a theory about those fundamental quarks and gluons themselves, as well as all the other fundamental particles. Yet basically people realized actually, okay, so hadrons are actually composites of smaller things, and that's actually why they behave like strings, because you can imagine two things stuck together and wiggling around each other as the two ends of strings. But those smaller things themselves could be strings. In fact, when you have an open string wiggling around, that can give rise to all three of the forces that we now know to be quantum forces, we have equations for, and then when the string connects its ends and wiggles around, that closed string can underlie the properties of gravity. So in the early 70s, some people who would continue to study string theory, even though it was fell out of fashion, realized actually this is potentially a theory of everything.

Speaker 1:
[08:10] Prior to this and in other contexts, we don't have a mathematical framework where you can, in this relatively elegant way, connect the other three forces with gravity. Gravity is always kind of the problem here in developing a theory of everything. But people look at it and say, okay, this actually, if we think about this as a vibrating string instead of a point, and we are able to do things that connect the ends of it, mathematically, it does describe everything that we wanted to describe. So what's the problem?

Speaker 2:
[08:47] Well, there's a couple of problems. One is the strings themselves would be so small that we just have no hope of ever seeing them. We're talking basically if an atom was made of atoms that were equivalently small as atoms are to us and those atoms had atoms, that would be the size of strings.

Speaker 1:
[09:07] Okay.

Speaker 2:
[09:07] We're never going to see them.

Speaker 1:
[09:09] That's a big problem.

Speaker 2:
[09:10] It's a big problem.

Speaker 1:
[09:10] That stops us from making predictions and then testing them on the basis of this theory.

Speaker 2:
[09:15] Yeah.

Speaker 1:
[09:16] Okay.

Speaker 2:
[09:17] Another problem is very early on again, it was realized that in order for the math of string theory to really make sense, there have to be initially it was 26 dimensions of space-time, and then later it changed to 10 because of an extra symmetry, wasn't recognized originally. So there have to be 10 space-time dimensions, which means that we have to swallow the idea that there are six hidden dimensions of space. That was a bitter pill. It's still probably one of the main criticisms of string theories. That's just a lot of people would say, well, it's a 10-dimensional theory, therefore, it's not the theory of our world. String theory has four dimensions.

Speaker 1:
[09:59] That we can perceive. We don't know that there aren't six other dimensions, but it is a complicated thing to say that in order for this to work, there has to be six other dimensions that are too small or in some direction that we can't ever possibly see.

Speaker 2:
[10:14] Yeah. So, it's a really big stumbling block for people. Then another problem was realized based on how those dimensions are organized, what configuration they're in. The idea is they're all bundled up together on small scales. So, if you could zoom in really small, you would suddenly see that in every position in our big four dimensions, you can actually move a short way in some additional dimensions, six additional ones. But the way those six dimensions are all curled up, affects the pattern of how the strings vibrate in the dimension. So, it's almost like a flute changing size will change the sound of the sound waves inside the flute. So, the states you get in the universe, the fields, the set of particles depends on the configuration of those small dimensions. No one has found the configuration of those small dimensions that could underlie the exact set of quantum fields and particles in our world. And the hope of finding the exact configuration is kind of hopeless because there's so many possibilities. So, it's like looking through the biggest taste stack you can ever imagine.

Speaker 1:
[11:30] So, we see here some of the criticisms and why they would resonate with people, right? Which is that mathematically, this theory does a lot. But, it asks a lot of us, which is to say, it's very complicated, it needs six other dimensions, it is hard to visualize, and then, on top of that, it's impossible to test. And the actual specifics of how it would literally describe our universe are lost in an infinite, seemingly infinite number of haystacks.

Speaker 2:
[12:04] Right. Right.

Speaker 1:
[12:06] I guess so then the next question is, what's gone right with string theory that has it persisting as a potential component of a theory of everything?

Speaker 2:
[12:15] The thing that goes right always with string theory is the math. It's just very elegant and self-consistent. So the more it's studied, the more things work out. And there were two big revolutions in string theory when everyone was super excited about it. The first one was in the mid-80s. These two guys, they were studying string theory when no one cared, no one thought it was interesting because of these issues we talked about. And it was just kind of on the fringes. So they realized it has this beautiful, almost self-healing property to it where these anomalies that could be a problem in the theory kind of all cancel each other out and then it's consistent. And that's exactly what happens also with the standard model of particle physics, our current kind of best quantum field theory of the world. So people pay attention to this. And when that happened, it was like, okay, this is a unified theory of the four forces. It's self-consistent, amazing. That's probably the theory of everything.

Speaker 1:
[13:22] When they're in these theories, when we're in these mathematical frameworks and you use a word like elegance, what we're talking about is, it's almost like the smoke to the fire, right? If the math is working out this beautifully, if things are canceling each other out in ways that solve problems, if surprising connections come out of the math, it's an indication that something about the theory that you're working with holds water. Because otherwise, why would these things just start happening? It's an indication that there's something right about it, right?

Speaker 2:
[13:55] Yeah, that's right. And the other revolution was of this nature. It was in the mid-90s when Edward Whitten, famous important physicist, realized that, so at that point, there were five different versions of string theory, like these anomaly cancellations I was talking about. And these five versions, that was kind of annoying. It's like, well, why are there five? It would be nicer if we just only had one that everything led to. And people were realizing there's dualities connecting all of these five. One is actually the other one in disguise where you're changing, you're making one of the variables really big and it turns into one of the other string theories with that variable small, their equivalent. So these equivalences between the five all suggested they're really one thing. And that's an 11-dimensional theory. And the variables you can change about the five different string theories are all like the most different positions in this 11th dimension that unites them all. So once that happened, once again, it was like, okay, there's one theory here and that's got to be it. And, you know, it's really the draw is these mathematical kind of qualities it has.

Speaker 1:
[15:11] To keep up, though, with the theme of the PR problem is that these kinds of advances that to theoretical physicists and mathematical physicists make a lot of sense and indicate something deeper and more profound and potentially more universal are not easy to translate into something tangible that people can hold on to. And now, and I think this is part of the impetus for you writing this essay is that we're sort of at another place recently where string theory is getting another moment or getting some more smoke, as I put it. So tell me a little bit about what's been happening last few years here.

Speaker 2:
[15:55] There's this ascendant technique in physics theory called the bootstrap that's being applied to the question of string theory true, that has really kind of revitalized that question. So this way of doing it is basically you assume things that you think might be true about the world or are true based on what you know. And then you use those assumptions, kind of put them in mathematical terms and pose equations that you want. And then you try to bootstrap what the only possible outcome could be. And what these amplitude ologists, people who focus on scattering amplitudes, have started applying the bootstrap to try to understand when particles collide and they get to be more and more energetic. And what these bootstrappers find is that based on various assumptions about scattering, about particles, about the world, you can arrive at that Veneziano amplitude as the only possibility for particle scattering at high energies. Another paper is more striking to a lot of people and a lot of experts. And that's a paper that assumes the maximal level of supersymmetry, which basically means there are these families of particles that have different amounts of spin. It's an intrinsic property, but they all share the same set of interactions, like they all slot into the same equations. And so this supersymmetry really simplifies calculations, and so that's why it's so useful as an assumption. And by assuming this, you can again show that the Veneziano amplitude is the only high-energy form of the scattering equation you can get.

Speaker 1:
[17:51] But our universe is not maximally supersymmetric.

Speaker 2:
[17:54] Yeah, our universe is not. The other way to read this is if maximally supersymmetric quantum field theory is actually string theory in disguise, if actually those particles of that theory, when you zoom in, they're really strings, then some physicists would assume that other quantum field theories, like the ones we know, describe the particles of our world, also are string theory in disguise. That conclusion is a matter of opinion, so we're never gonna resolve, again, this like controversy around string theory because of this, but it adds more fuel to the debate.

Speaker 1:
[18:36] It's such an interesting negative space in science in as much as, as you mentioned, it has this beautiful mathematical structure, yet it's untestable. We're not running against a brick wall, like we can make more and more progress in understanding that mathematical structure, and yet we still can't find out what the particular aspects of the six hidden dimensions are. So it's not stuck, but it's also never going to be what we want it to be. I don't know if that's even a fair way to put it.

Speaker 2:
[19:10] Yeah, yeah. And I think that's where so much of the sociology comes in.

Speaker 1:
[19:14] Yeah, yeah. It's psychological, it's sociological.

Speaker 2:
[19:17] Yeah. And it's partly because of this being just different than what happened in the past. Like now physicists are asking these questions about realms of the universe that they can't possibly probe. And so the tools we have are just the kind of the logic and the math and seeing how they work. And some people just don't think that that's real physics and that it's a waste of time. It's something that kind of riles people up a lot. Yeah. So I think the critics have taken on this role of reminding everyone that this is not a proven idea, that other ideas should be welcomed into the fold. And I think that, of course, is important to do, but it is, I think, they have set themselves up as the adversaries of string theory. When really, they're not saying it's wrong, they're just saying it's stagnated or can never be proven right, or we should be going back to the drawing boards more than we are, or.

Speaker 1:
[20:23] Yeah, and everyone is entitled to their opinion on what, like, the arc of big B, big science should be, right? That's fine. But, I mean, like, I noticed in your essay that one of the authors of one of the bootstrap papers is like, okay, here's the situation. We don't have to be emotional about it. I think it's a direct quote. It's like, we don't have to be emotional about it. And it is a very interesting situation where, like, you can't avoid needing to state that. Like, we are just, like, we're calculating, we're theorizing, we're trying to understand things from the mathematical framework perspective where this all started. And I think that's kind of the bootstrapping ethos in a way. But we do have to say to each other and to people, like, people in the physics world have to say to each other, like, let's talk about the thing. We don't have to get really emotional about it. And I think there may have been reason over the years to be emotional about it, but we might be in, like, a different place now. And it's probably not fair to say that string theory is stagnating exactly, even if some of the criticisms about it being untestable and unprovable remain valid.

Speaker 2:
[21:29] Yeah. And, I mean, he says we don't have to get emotional, but that advice did not work.

Speaker 1:
[21:35] It does not. No, it doesn't. I don't think that advice has ever worked in any context in human history. Let's not get emotional about this. Like, that doesn't work.

Speaker 2:
[21:45] But yeah, even in the impression I've gotten of the reception of this article, it's like so many people involved have something to be mad about. You know, people who don't like that these critics were even quoted because why do these critics even have a right to have a say versus people who think that they can't believe we even covered String Theory. It's so dead that why are we even talking about it? You know, everyone is still feeling very emotional.

Speaker 1:
[22:14] It's raw. It's interesting. And we want to be fair and understanding and think about the actual science, but it is, especially in this case, just impossible to divorce from its own history, from our history, from what it can and can't do at the end of the day.

Speaker 2:
[22:33] Yeah.

Speaker 1:
[22:35] I loved reading this essay. I feel like I understand, if not actual String Theory more, at least I understand the scientific slash cultural context around it in a way that I think is really useful. And it gives this really great sense of perspective on scientific debate, on what we know and don't know, but also what we can and can't know. I think everyone should check it out. Thank you for coming on the show, Natalie.

Speaker 2:
[23:04] Thanks so much, Samir. Glad you liked the essay.

Speaker 1:
[23:08] Before we let you go, we always like to ask our guests for a recommendation. So what's exciting your imagination this week?

Speaker 2:
[23:15] I got this review copy of this amazing book that just came out. It's called Geology and Illustrated History by David Bainbridge. It's just got the most incredible images of infographics that give you a sense of Earth's history, illustrations from people trying to understand how canyons or mountains form throughout history. So a visual guide to the understanding of our planet. It's just an awesome book to look through that, I feel like it would be the perfect present for a lot of people.

Speaker 1:
[23:55] Thank you, that sounds awesome. Also on Quanta This Week, you can read about the parts of our immune systems that we have inherited from bacteria and a piece about quantum jamming or techniques to interfere with quantum systems or communication. We're gonna leave you today with a different kind of strings. This is a clip of Alchemical String Theory, a collective of avant-garde musicians performing a live improvisational piece at the Red Light Cafe in Atlanta in 2024. It's more than just a name, members of the collective say they're inspired by quantum mechanics and the vibrations of string theory. If you've been enjoying The Quanta Podcast, please take a moment to rate the show and leave a review. We'd love to hear from you. The Quanta Podcast is a podcast from Quanta Magazine, an editorially independent publication supported by the Simons Foundation. I'm Quanta's editor-in-chief, Samir Patel. Funding decisions by the Simons Foundation have no influence on the selection of topics, guests or other editorial decisions in this podcast or in Quanta Magazine. The Quanta Podcast is produced in partnership with PRX Productions. The production team is Ali Budner, Deborah J. Balthazar, Genevieve Sponsler, and Tommy Bazarian. The executive producer of PRX Productions is Jocelyn Gonzalez. From Quanta Magazine, Simon France and myself provide editorial guidance, with support from Samuel Velasco, Simone Barr, and Michael Kenyongolo. Our theme music is from APM Music. If you have any questions or comments for us, please email us at quanta at simonsfoundation.org. Thanks for listening. From PRX.