Welcome to "On A Tangent", where we tell the stories behind the mathematics. In each episode, we meet a different early career mathematician from Mathematics Münster, and learn about their research, their path towards mathematics, and their hopes for the future. We explore the many different shapes that mathematical research can take, the early memories that led people towards the subject, and try to understand a little bit better the voices that make the mathematical community of today.

The podcast episodes are available on this webpage, on Podigee and on popular podcast platforms such as Spotify, Apple Podcast or Deezer.

A new episode is published every last Monday of the month.

About the host: Simone is a doctoral researcher in model theory and occasional science communicator. He likes all things related to stories and fiction.

Episode 7: From headbutting gnomes to infinity categories, with Edith Hübner

In this episode of On A Tangent, Simone is joined by Edith Hübner, a doctoral researcher in topology. We learn about her journey from physics to mathematics, what headbutting gnomes can teach young children, and the mysteries of infinity categories.

Link to Edith's website

Episode 6: Finding structure in groups, with Marjory Mwanza

In this episode of On A Tangent, Simone is joined by Marjory Mwanza, a Young African Mathematicians Fellow working in group theory. We learn about how one goes from statistics to group theory, and where to find deep structure in seemingly simple objects.

Link to Young African Mathematicians (YAM) Fellowship Programme

• Transcript

  Simone: Hello, and welcome to "On a Tangent" the podcast where the main characters are the stories behind the mathematics. My name is Simone, and in each episode, I meet a different early career researcher from Münster to learn about their paths towards mathematics and their hopes for the future. In this episode, I interview Marjory, a young African fellow at the Centre of Excellence, to learn about her journey from statistics to group theory and how to find deep structure in groups. I hope you enjoy this episode.

  Okay, so. Hi. Welcome. Thank you for joining us.

  Marjory: I'm happy to be here. Thank you.

  Simone: On this podcast, we try to understand the stories of the people in the cluster, the mathematicians in the cluster. So I’d like to ask you, what is your earliest memory of mathematics? When did you first encounter mathematics in your life?

  Marjory: Well, I would say my memory of mathematics goes back to primary school, actually. I’ve always been interested in mathematics as a subject since then, and growing up, I had very good mathematics teachers who motivated my interest in the subject. In high school, we had career days where they’d ask what you want to do in the future. I had no idea that someone could pursue mathematics as a career.

  Simone: Yes, you don't really see many mathematicians in everyday life, right?

  Marjory: Yes, exactly. So I wanted to find out which careers involved mathematics. I was told engineering involved maths and sciences, so I used to say I wanted to be an engineer — not necessarily because I wanted to be one, but because I wanted to study maths.

  Simone: I understand that completely.

  Marjory: When I finished high school, I applied for a bachelor’s programme at a university in Zambia. The universities there have a general first year where you study natural sciences if you’re aiming for sciences or engineering. I applied for natural sciences, intending to go into engineering. However, while I was there, I researched the programmes offered and discovered that mathematics was one of the options. That was intriguing to me.

  Simone: So you realised you could actually pursue what you wanted to do.

  Marjory: Yes, I found out that mathematics could be a career. So I started looking into it — what it would involve, what options there would be after completing a mathematics programme. I wanted to study maths passionately, but I also wanted it to lead somewhere practical.

  Simone: Yes, that’s what my parents used to say. They’d ask, "What are the jobs afterwards?" because, while passion is good, you still need to make a living.

  Marjory: Precisely. And as the firstborn in my family, I felt I needed to set an example for my siblings by doing something worthwhile. I did some background research on the mathematics programme, asking about people who had graduated from it. I spoke to someone who told me about a fantastic opportunity for top students — a sponsored master’s programme in various centres across Africa, called the African Institute for Mathematical Sciences. There are centres in South Africa, Cameroon, Rwanda, and Senegal. The person I spoke with had studied there and was doing well, which motivated me to pursue this path.

  So, I entered my second year, specialising in mathematics, with the goal of performing well to qualify for this programme. It was a full scholarship covering everything, including transport. As I was progressing through my bachelor’s degree, my understanding of mathematics deepened. In high school, it was just maths, but during my bachelor’s, I started seeing it as something real. We had both applied and pure courses.

  Simone: So a comprehensive overview.

  Marjory: Yes. In research, you can then specialise. I was particularly drawn to statistics as an applied course and algebra. I liked the applied aspect of statistics, but choosing between the two was tough.

  Simone: That’s challenging. In some programmes, you have to choose a theoretical or applied curriculum from the start, which is tricky since you barely know what mathematics is about in the beginning. I initially wanted to do physics, but my physics professor suggested I was more suited to mathematics.

  Marjory: Yes, I understand. My journey wasn’t straightforward either. I liked group theory. We studied linear algebra, then group theory and other topics, and I was drawn to the challenging nature of group theory. However, I didn’t get to choose my final-year project; they assigned us topics for ease since our class was large. I was given a project in statistics.

  Simone: So someone else made the decision for you?

  Marjory: Yes, unfortunately. I liked statistics and numerical analysis, but deep down, I knew I wanted to delve into group theory. I completed my bachelor’s degree and applied for the master’s scholarship in South Africa. While waiting for acceptance, I worked as a tutor because I love teaching. Mathematics is something I enjoy sharing with others.

  Simone: So, did you get accepted?

  Marjory: Yes, I was thrilled when I received the acceptance email. I was accepted to pursue my master’s in Cape Town, South Africa.

  Simone: And what was it like when you arrived there?

  Marjory: It was fantastic. At the African Institute for Mathematical Sciences, you’re exposed to various topics in mathematics before choosing your research focus. I even took a course in group theory.

  Simone: So you could explore different areas before specialising.

  Marjory: Yes. We were introduced to applied courses, machine learning, coding, and pure courses. I mostly chose pure courses because I knew I wanted to do a pure mathematics project. I took modules in quantum theory, logic, and group theory. The best part was the diversity—students came from across Africa, and we had lecturers from around the world, not just South Africa. We had lecturers from Germany, the UK, and other places.

  Simone: You were exposed to different approaches to mathematics from various countries.

  Marjory: Exactly. At the end, we got to choose our own research topic and supervisor. I had a very enthusiastic group theory lecturer, which helped solidify my decision to pursue group theory.

  Simone: It's fascinating how having a motivating teacher can really make a difference. It can turn the subject into something exciting and alive.

  Marjory: Yes, it really gave me a boost. My lecturer during my bachelor’s wasn’t so enthusiastic about group theory; I developed the interest largely on my own. But what truly made me certain this was what I wanted to do was my lecturer in my master’s. He was so passionate—one of those people who talk with such love for what they’re doing.

  Simone: Yes, the passion really comes through, doesn’t it?

  Marjory: Absolutely. I was motivated, and I spoke to him after class. At first, he had his doubts because my bachelor’s project wasn’t in a pure field; I’d done statistics. But I convinced him, and he agreed. So I began my research with him, and it was incredible. I learnt so much during my first pure project, although it wasn’t easy.

  Simone: Right, especially since your background was more, well, applied. I mean, your previous project was in statistics, wasn’t it? So, what was the project about?

  Marjory: I studied the structure of groups that satisfy the Glauberman-Thompson theorem. This theorem is one of the most important in finite group theory. However, not all groups satisfy it, but some do, and my focus was on the structure of those groups. That was the main subject of my master’s project.

  Simone: And now that you’re here as a fellow, what has your project been about?

  Marjory: Here in Münster, I actually shifted fields slightly. During my master’s, I developed a love for graph theory through a course I took on it. It’s fascinating because it allows you to represent mathematics visually, which I really enjoy. When I came here, my supervisor specialised in geometry, topology, and group theory, specifically in geometric group theory. So I thought, why not switch to geometric group theory?

  Simone: That seems like a perfect blend — you’ve got the groups, but also the visual element through graphs.

  Marjory: Exactly. You get the groups, and you can also have all the visual aspects you enjoy in one place. It’s great. At first, I was a bit anxious because I hadn’t done much geometry before, but I thought, there’s always room to learn.

  Simone: I think that’s something people fear quite a lot. I remember when I started my PhD, there was this constant feeling of not knowing enough, but eventually, you realise that there’s always room to learn.

  Marjory: Yes, there’s always room to learn. So, I took a course in geometric group theory to understand how it connects with the group theory I already knew. It was fascinating—like finding a missing piece. I really appreciated diving into geometric group theory. When it came to choosing a project for my Young African Mathematician programme, I wanted something that combined groups and graphs, and my supervisor and I found a perfect topic. It’s on constructing graph products of groups from finite groups. My master’s focused on finite groups, and now I’m working on constructing graph products of groups from these finite groups.

  Simone: The puzzle is really coming together, isn’t it?

  Marjory: Yes, it feels like the puzzle is complete.

  Simone: It’s wonderful. How would you explain the topic of your project to someone without a mathematics background, perhaps at a party? Although people don’t often ask about our work at parties!

  Marjory: True! Well, if I were to explain it to someone unfamiliar with mathematics, I’d say, since I’m working on graph products of groups, I’d start with the concept of a graph. Imagine connections between people — say, your family and friends. You’re connected to certain people, and those people are connected to others. In a graph, people are represented as vertices, and their relationships as edges. So, when I work on graph products of groups, I’m talking about how these connections can be represented in a mathematical structure. Cayley graphs, for example, are another interesting area — they allow us to represent groups in terms of graphs. If I were explaining it at a party, I’d talk about relationships and connections, which is how I link the concept of graph products of groups.

  Simone: Graphs are everywhere! They’re so versatile, and people can easily picture them. It’s a great subject because it’s so visual.

  Marjory: Yes, exactly. And with Cayley graphs, you don’t have to represent every element of the group — just a generating set. A group can be generated by different sets, so you can have various graphs representing the same group depending on the generating set you choose. It’s really quite interesting.

  Simone: Yes, I think it’s fascinating. Personally, groups are a bit daunting for me because they have less structure compared to fields, which have more operations. I find groups intimidating because they only have one operation; it feels limiting.

  Marjory: That’s true! Groups on their own don’t have much structure. My master’s project didn’t have a lot of structure, and I almost forced one into it! Groups themselves might not seem structured, but when you start relating them to geometric objects, that’s when you start to see structure.

  Simone: Yes, that’s reassuring! So, we’ve talked a bit about your past and present. Now, let’s look to the future. I always ask our guests: what’s one big question or problem you’d like to see answered, maybe by yourself or someone else?

  Marjory: That’s a great question. I’ve recently come across something that has been unsolved for a long time, and I’d love to see it solved. It’s connected to my current project on graph products of groups. There’s a specific type of group called the right-angled Artin group, which is a type of graph product of groups. Right-angled Artin groups are simpler in structure due to commutativity, but in general, Artin groups are quite diverse and complex. One major open question is determining the quasi-isometry of these groups. Yeah. So, this is one issue I would like to see resolved because it's challenging. When you want to determine the quasi-isometry, which is like a map between certain groups, you need to understand the large-scale geometric structure. This understanding enables you to compare spaces effectively. These groups act on spaces and complexes, making it difficult for people to determine the quasi-isometry without a deep understanding of the large-scale geometric structure of these groups. I would really like to see this issue resolved in the coming years.

  Simone: At some point, yes.

  Marjory: Yes, at some point. I think solving this could actually have a significant impact in the field of geometric group theory. Since groups combine both algebraic and geometric properties, it seems that one way to approach a solution might involve collaboration between mathematicians from different fields. Bringing together combinatorial group theory, geometric group theory, and topology, for example, could lead to a breakthrough. I look forward to seeing this issue resolved.

  Simone: Well, thank you very much for the insightful thoughts. See you around in the corridors.

  Marjory: All right. Thank you.

Episode 5: How an inventor becomes a mathematician, with Rin Ray

In this episode of On A Tangent, Simone is joined by Rin Ray, a postdoc in Topology. We learn about why an inventor becomes a mathematician, and how to build bridges between deep questions in different areas.

Link to Rin's website

• Transcript

  Simone: Moin! Welcome to On a Tangent, the podcast where the main characters are the voices behind the mathematics. My name is Simone and in each episode I will interview a different early career mathematician from Münster to discover their paths towards mathematics and their hopes for the future. In this episode, I meet Rin, a postdoc from topology, and learn about how an inventor turns into a mathematician. I hope you enjoy the episode. (...) Thank you for joining us. Welcome.

  Rin: Hello.

  Simone: So, let's start from the past. Let's start from your earliest memory of maths. What is the earliest time maths entered your life?

  Rin: I think the first time… I have kind of three candidates for this. The first one was that my mom had a lot of jewellery, and one of the things she found that kept me occupied was undoing the knots in the jewellery. So probably that's my earliest. Just like I'm doing knots in, like, shoestrings and in jewellery and such. Also, my dad is a professional antique repairman and has a full woodworking studio, and often people would come with just totally broken and splintered pieces of furniture that we would carefully glue back together. And I also remember that. But probably my first like maths memory, where I was like, ooh, there's something here, and I knew that that was called maths was when I took calculus. I remember like learning about Riemann sums. This sounds really silly, but I remember when I was learning about Riemann sums, I felt like this thing that I had done when I was younger and bored in class is that I would sometimes kind of draw lines in the air and then mentally, like stack things from around the room under the lines. And I was like, oh my God. They packaged this thing that I do in my head in like one tiny package. And however else anyone else thinks about it, they can unpack it in that way. And I was like, that's really cool that you can really like package it because it's a huge amount of like tools and information in this, like really small symbolic framework. And that's like the earliest time I was like, ooh.

  Simone: Is this something I want to see more of in my life?

  Rin: Yeah, exactly. I enjoyed this ability of talking about one thing many different ways in like a unifying language, I guess.

  Simone: And do you feel like these three experiences you talked about somehow were already precognition of what later you would do in maths or not really?

  Rin: No. I mean, I do think that I'm a pretty visually thinking person, but I don't think they necessarily pointed to the area of maths that I ended up working in.

  Simone: Which, now that this comes up, what is it that you do in maths?

  Rin: Yeah. So, um, I work on kind of a combination of a lot of different fields, but mostly I do kind of arithmetic geometry, homotopy theory, representation theory, number theory, I guess.

  Simone: And how do you explain this to people at parties? I mean, if they ask.

  Rin: Of course. So I mean, I guess, well, the first thing that I say, if someone asks me what I do, I say I like thinking about shapes and numbers. Um, and if they ask more than that, then I say, well, I study the way that maps between higher dimensional spheres are related to counting points on curves.

  Simone: So spheres we can imagine, curves we can more or less imagine.

  Rin: Yeah. And counting. People know what that is roughly.

  Simone: Yes.

  Rin: Right. And that's like, yeah I struggle sometimes, but so do we all.

  Simone: It's always a humbling experience, right? When you're the mathematician in the room and they expect you to sort of… can you please compute the bill at the restaurant and. Well, no, I can't. I mean, I could write I mean, with some time, but not this. Definitely not what I consider within my skill set.

  Rin: Ah, yes. It's very different from, I think, the kind of everyday type of maths that we do for sure. Although I have had a few of these moments over the years where sometimes I'll be doing math and I'm like doing math, you know, computing a five by five matrix or like counting, uh, you know, comparing a bunch of numbers to each other and trying to find a common factor. And I'm like, this is what people think mathematicians do. And right now I'm actually doing this.

  Simone: Exactly. And this is what mathematicians often struggle at doing. I mean, the high dimensional spheres are fine. It's the matrix of integers which we sort of not necessarily grasp.

  Rin: Right, I mean, this is somehow more of a trained skill in like the ICM crowd or sorry, ICM is the wrong word. What's the word for the competition?

  Simone: You mean the IMO? Were you a competitor?

  Rin: I was not an IMO competitor, but I do think that IMO competitors tend to be better at these sorts of kind of quick calculation skills than the average mathematician is.

  Simone: Yeah, there's often, I think the belief that if you're a good IMO competitor, then then this means you're going to enjoy a lot of of mathematical research, which I don't think there is necessarily a big correlation there.

  Rin: No, it burns a lot of mathematicians out. But I do think that the IMO is pretty good at effectively teaching proof methods. In fact, when I eventually kind of decided to professionally enter mathematics, the thing I struggled with is that I had never written a proof before, and I used a book for IMO students to learn how to do basics of proof writing.

  Simone: So how did you get to the maths? And how did you get to what you do nowadays?

  Rin: I have a somewhat convoluted path towards the way I got into math. So I was working in robotics and engineering… Actually, it kind of started even earlier. When I was at university, I really, kind of didn't really quite know what I wanted to do. And then eventually I took this calculus class. I got really excited about that. And I wanted to do pure math. And at the time, the person who is mentoring me was a burnt out postdoc and he was like, Rin, you don't want to do mathematics, you can actually help people. And I was like, okay, so then I did like robotics and plasma physics for a while, and then I got a grant to be an inventor, and I was doing that for a while, and I was doing maths…

  Simone: I need to backtrack on this a bit. An inventor, as in, because somehow, maybe to me, the image of an inventor is like Archimedes in the Mickey Mouse comics, like somebody who is in the lab the whole day and actually build things. Is that actually…?

  Rin: That was what I was doing. I mean, the grant that I got is called the Teal Fellowship, and it's given to 20 people under 20 years old who kind of have a project in mind that they want to do. And it's for $100,000 and it's given to the person, not the project. So I mean, what my project was, was doing medical technology, improving wheelchairs in various ways and neuroprosthetics and stuff like that and improving pre-clinical trials. So being able to detect stuff that we weren't previously able to detect in pre-clinical trials until they reached human stage earlier using technology. And um, so I was doing that and I was doing math as a hobby. And then I just like really reflected on what aspects of my job I liked and I didn't like. And I realized the part I liked the most was the math part. And I decided that I would, you know, try that, just wrap up the projects I had at the moment, give them to other people and try just doing math for a month and see how I felt. And the month never ended.

  Simone: Yeah. So how did you go from somehow medical technology to arithmetic geometry? I mean, it's quite a jump.

  Rin: Yeah. Um, so I guess the way that I kind of went through it was, I was at this like party where the person who is behind the Teal Fellowship, Peter Teal, has meetings with the fellows. And we had this kind of questionnaire that was open and kind of we could all ask him questions. And at the time, I had a friend who was a pure scientist who was struggling in various ways, and someone who was then a hedge fund manager, but had been a theoretical physicist before, Eric Weinstein, he came up to me and he was like, oh, you're a pure science Teal fellow, why haven't I met you before? And I was like, well, because I'm a mechanical engineering fellow. And he was like, well, tell me about like some of the things that you're interested in. And this was the day before I was going to start studying math for a month.

  Simone: “A month.”

  Rin: Yeah, a month… eight years.

  Simone: Uh, and counting.

  Rin: And counting. Yeah, so I kind of asked him various things, and he was like, you know, have you heard of the Atiyah-Singer Index Theorem? And I was like, no, what a good place to start. Let's look at it. And I was like, damn, this is really cool, you know? And that was one of my entering points into maths. And one of the ways that I still learn and the way that I learned then, was I would print out a paper that I found interesting, I would outline all the words I didn't know, which were most of them, and then I would look up John Baez and that word, and I just read every post he wrote on the topic.

  Simone: And on most words you would find something.

  Rin: So I actually was able to learn enough from doing this, just locking myself in a room for like three months and doing this, enough to start attending classes at Berkeley, which was nearby because I was living in San Francisco at the time. And so I started sitting in the back of lectures, and at some point, I kind of reached a point in Eric Weinstein's mentorship where he realized I needed to actually be talking to a real mathematician. And Edward Frenkel was also affiliated to the Teal Fellowship, who's a professor at Berkeley. And he started meeting with me weekly. And at some point, I reached a point in his mentorship of me where he was like, the questions you're asking me are a little too topological for me to understand. And he started to introduce me to people in the mathematical world. And also I met them through just showing up in classes and asking questions.

  Simone: So somehow by this sort of repeated process of reaching the limits of the expertise of somebody and going to the next one, is how you eventually ended up in your field.

  Rin: Yeah. And I think that's still how I do math. I mean, something I have really enjoyed about my Uni Münster postdoc is that I think the way that I best learn is by doing this. And so if I want to learn a topic, I’m able to fly to wherever that is, take a train to wherever it is, talk to that person for a week, a month, however long, then go to the next place, or write up what I learned and then go to the next place.

  Simone: And somehow do you think there was any connection between your exam or your past as an inventor, and your daily life as a mathematician? I don't want to get into the topic of is math created or discovered…

  Rin: No, I don't care about that.

  Simone: And it would take, you know, infinitely many hours. So let's not get into that. But somehow this idea of letting ideas flow and coming up with something new. Do you think somehow this was always in, in at least hidden away somewhere?

  Rin: Yeah. So I think I've always known that I wanted to be a researcher of some kind, or maybe a creator, and preferably both. And so I think that has continued to influence my development as a scientist and artist, as a person, like in all areas of my life.

  Simone: So what would you like to see happen in this somehow, remainder of this “one month in maths” that you've been doing for eight years and a bit more? Uh, what is some what is an answer to some question that you would like to see?

  Rin: I've always been really fascinated about the interplay between manifolds and topology and number theory and arithmetic geometry. And this has been evolving quickly and like beautifully in the past couple of years. I mean, I feel like a paper comes out almost every month now that just puts together another piece of this puzzle. And it's kind of a broad picture that I'm devoting myself to, but let me try to express it. So there's somehow a connection between L-functions, topological genuses, and like Euler characteristics and boundary conditions on topological quantum field theories or kind of Riemann-Roch type statements, the Atiyah-Singer index theorem, etc.. And I mean, it's really just amazing how these things are starting to feed into each other. I mean, for example, this kind of started with me looking and seeing that the Bernoulli numbers were showing up all over homotopy theory and in homotopy groups of spheres. And my goal is to kind of sit down and be like, okay, so we know that really numbers show up, but what other arithmetic invariants show up? What other L functions, what other generalizations of the Bernoulli numbers show up? And as I sat down to do that, I realized that we in fact don't understand why the Bernoulli numbers show up.

  Simone: They just happen to show up.

  Rin: Yes. And right now it's just kind of a set of numerical coincidences. And together with my collaborators Noah Riggenbach and Andreas Mejia, we put together this picture that kind of categorizes all of the different instances that the Bernoulli numbers show up in the homotopy groups of spheres, and seems to imply that they come from kind of this greater structure. And so that's nice and in some sense a key part of like the perspective that we developed is that the Bernoulli numbers are capturing the lattice of Z sitting inside of the real numbers. And you can ask… If you generalize to other lattices, you know SL2(Z), or like the symplectic group on Z or any other group you'd like, that's discrete, living inside of a topological space. You can also ask for a generalization of the zeta function there. You can ask what sorts of Gauss theorems do you get? Do you get a generalization of this beautiful statement that haunts me to this day, that the zeta function of one minus 2g is equal to the Euler characteristic of the moduli stack of genius g curves with one marked point. So you have this like topological invariant of a moduli space being equal to the zeta function of the dimension of the moduli space. And you can ask, well, what if you put something other than the mapping class group there? What if you put the symplectic group. What if you put SLn. Do you get on the other side the generalization of the zeta function to that lattice? And this kind of fits into a different picture. Do you get that the orbifold Euler characteristic of like SLn for example, or Spn? And to the symplectic group is equal to a product of zeta functions. But there's no known connection between the mapping class group case, which gives you one zeta function like a zeta function of one minus two n, and the symplectic group case, which gives you a product of the zeta function minus one all the way to zeta function of one minus two n. And like those, there's a map from one of these groups to the other, and there should be a connection, but we don't know what it is. And a lot of this amazing recent work. So for example, this recent work of Amina Abdurrahman and Akshay Venkatesh, where they were really able to use Reidemeister torsion and the topology of knots to prove things about L functions of symplectic representations. And I feel like in recent years we've really been able to have these two fields fully talk to each other. These analogies are becoming actual proof techniques that are emerging into like a bridge in a language.

  Simone: No more heuristics, but but actual frameworks to understand things.

  Rin: But for now, the frameworks still need more work. I mean, somehow they're just they're hinting at this very simple, beautiful global structure that specializes to these two worlds. It specialized to the topological case and specialized to the zeta function case. But right now, getting in between those two worlds is still unbelievably difficult. I mean, their paper is really, really hard work. And again, it comes down to a sort of numerical coincidence which would be really interesting to try and see if we can specialize. And it's also kind of cool because you can see things like, the Euler characteristic could be written as a type of duality statement, like the Euler characteristic of a manifold is equal to the Chern character of the manifold times the Chern character of the dual of the manifold evaluated on fundamental class, and in fact, in a lot of path integrals that show up in field theories, you get these bosonic fermionic cancellations that make your path integral converge. And it seems like there's a connection between some of the path integrals that show up in string theory, where you're not just mapping, say, S1 into your manifold, but you're mapping some other world sheet, you know, genus g curve into your manifold. It seems that those path integrals in the statements, for example, for genus one curves, you get that the Riemann zeta function of minus one shows up as a correcting factor, which is the Euler characteristic of the moduli of genus one curves of one marked point. And right now there's no framework that really explains why that happens. I'll just like reiterate this, somehow this triangle of like L functions and zeta functions, they're generalizations to other lattices, topological genuses or Euler characteristics. And these like Riemann-Roch duality statements which show up in boundary conditions of invertible topological quantum field theories and invertible is important here, because somehow the way that it's connected is showing up as something and its dualising object. So a lot of the arguments that show up there end up being some sort of Fourier transform that picks up a term, a measure on a space that allows it to converge. And this re normalizing factor seems to be the Euler characteristic I see in like elliptic curves. So I'm really, really curious to see, first of all if we can really make this picture that I've been developing work that really shows that the Bernoulli number instances that we know specialize. That this categorification really does give you an explanation of where all the Bernoulli numbers show up in the homotopy groups of spheres, and also in general, this kind of a categorical framework which allows us to talk in the same breath about symplectic groups and the mapping class group, um, and their measures and their Euler characteristic just living in the same world.

  Simone: That’s really fascinating. Thank you very much for this.

Episode 4: Weddings and limit structures, with Rob Sullivan

In this episode of On A Tangent, Simone is joined by Rob Sullivan, a postdoc in Combinatorics and Model Theory. We learn about what children do to entertain themselves at boring weddings, and why ultrahomogeneous structures are hard to understand.

Link to Rob's website

• Transcript

  Simone: Welcome to On a Tangent, the podcast where the main characters are the stories behind the mathematics. My name is Simon and in each episode I am joined by a different early career mathematician from Muenster to learn about their paths towards mathematics and their hopes for the future. In this episode, I am joined by Rob, a postdoc in combinatorics and model theory, to learn about what children do when they're very bored at weddings and why ultra homogeneous structures are quite hard to understand. I hope you enjoy the episode. (...) All right. Hi, Rob. Welcome. Thank you for joining us. So let's start a bit with who you are and what kind of mathematics you're interested in. How do you explain this to your family, to friends, to people at parties?
  
  Rob: Okay. So, yeah. So the general I think generally I've always been interested in combinatorics, I'd say it is the general theme. And then how do I explain this to people? I generally lie, first of all, and sort of give them a sort of mild misrepresentation of what I'm doing. And generally I think it's better... I often find it's better to give someone a problem to play with because, you know, I can tell them all kinds of terminology and whatever. But I think the best thing to do is to give people like, you know, a case. So what I often do is someone says to me, what do you do? And I say to them, okay, let's prove that the Ramsey number three is equal to six, right. And you know, obviously I don't say it like that. I start off with, you know, particularly if you're speaking to someone who's got, you know, very little mathematical background, I'll say, so you've got six people at a party and then, you know, I claim and it's supposed to be this sort of, you know, confounding fact that, you know, so you've got six people at a party and you can choose any six people you like, and there's always going to be three of them who know each other, or three of them who don't know each other. Right. And so then people sort of say to me, so some people say, why do you care? Which is a very interesting question.
  
  Simone: That is not an easy one.
  
  Rob: Yeah, yeah. But I don't necessarily have a good answer to. And but generally what people do is particularly if they've not got a mathematical background, is they'll start off and they'll try and define the problem. They might say, oh, but hang on. So, you know, Annabel is Dan's mate, but then Bella is Dan's wife. So does Bella know Dan more than Annabel does?
  
  Simone: Right. They sort of focus on the irrelevant part of the problem.
  
  Rob: Yeah, exactly. And so it's really interesting because you see, you see people trying to go through this process of abstraction and... Yeah, so we sort of essentially play a little mini tutorial. And so I say, okay, so points and lines and then, you know, colourings and I don't know, you know, blue means know and red means don't know and so on. And then we start talking about the pigeonhole principle. And we do small little cases. And so this is, you know, and I think that's more, that's much more instructive because I think that much more closely represents what I actually do on a daily basis rather than, you know, theorem statement or this is my field or whatever, you know.
  
  Simone: It gives them a taste of what actually, I mean, what your work actually sort of feels like or what the concepts are behind.
  
  Rob: Yeah. And more than that, it tells them why I might care as well, because the goal is to essentially, for me, is to convince people that these questions that we're asking are natural, because a lot of people have this preconception that maths somehow exists in this separate platonic, existential realm. And, you know, I just want to convince people that, you know, everyone's got a certain set of problems that they're trying to solve, and we're not doing kind of any magic. It's just often the problems have a certain flavour.
  
  Simone: And people prefer certain problems over others because somehow they feel like that line of reasoning is more natural to them.
  
  Rob: Yeah, or because they've been educated on that, exactly.
  
  Simone: And so, so how did you come to combinatorics? What brought you to the topic? I mean, somehow you've sort of already implied that the taste of doing it, this idea of concrete problems that you can do, maybe you can draw the things and this on is something you like about your subject.
  
  Rob: How. Yeah. And I think it's something that you can you can teach children or teenagers in a very attractive way, because you don't have to lead them through a whole series of prerequisites. You can literally and, you know, people who enjoy problem solving, you can just sort of get them going, right. So for me, …, so I did maths Olympiads, and so I was sort of involved in this stuff in the UK. So I went to a school that as a matter of course, enrolled everyone in. So there was something I think it's called the kangaroo or something. There's some sort of very Junior maths Olympiad. And then there was the British. I'm probably getting all the names wrong, but there was the British Junior Maths Olympiad as well. So if you've got a good score in that then you've got a magical Hogwarts letter in the post, and you got invited to go to a summer camp. And so this is sort of, where I had my first sort of taste of combinatorics, I suppose, because I don't think I was really at that time capable of differentiating different styles of mathematical thinking, and so I just naturally gravitated toward whatever seemed the most interesting, you know, without any necessarily having any preconceptions. And I remember at this summer camp, there was... they had some, you know, professional mathematicians there as well. And one of them was Imre Leader, who's at Cambridge, and I think it was after dinner, maybe he gave a little proof of Van der Waerden's theorem. Shall I state Van der Waerden's theorem?
  
  Simone: Why not?
  
  Rob: Why not. Okay, so it's to do with colourings of the natural numbers. So let's say that we colour the natural numbers in two colours, red and blue. Then Van der Waerden's theorem states that one of the colours contains arbitrarily long arithmetic progressions. So if I want an arithmetic progression of length five where you know all the points are of the same colour, right?
  
  Simone: Okay.
  
  Rob: Um, and so. It's a great theorem to sort of present at a summer camp, because what do you do? It's basically a clever induction. And you start off with small cases and so you say, okay, so you start talking about the pigeonhole principle and you say, okay, well, obviously if we colour three integers into colours, then we're always going to get two integers of the same colour, right. Yes. You know, and obviously it forms an arithmetic progression because it's trivial. And so you start off with that as the base case, and then you sort of build it up and you say, okay, so what if we want an arithmetic progression of length three where everything's sort of the same colour and so on. And so he literally did this. So he sort of had a series of slides and he had a blackboard. And he got us to prove it collectively as a group. And I don't think we proved the full theorem, but, you know, I think we did enough of the sort of low value cases to sort of work out how it worked in general. And this sense of like play. I think something that I found very intimidating as a child was this sort of 19th century romantic idea of a mathematician, which is often what you see in films where, you know, I'm sitting on top of a mountain and, you know, a lightning bolt strikes and suddenly I prove the Riemann hypothesis.
  
  Simone: Possibly in some awfully little office and, you know, sort of closed there for eight years, and then you come up with the...
  
  Rob: Exactly. You know. And this gets glamorised. You know, if you look at Hollywood films, I don't know. People always love the story of Andrew Wiles, where no one knew what he was doing for seven years and so on. And I was always very frightened by that because there was always this question of, well, I'm probably not good enough to do that. Right. And so I think that's a very frightening stereotype.
  
  Simone: I think we went through this already in a previous episode. It's somehow in a way, it's not a very standard way for people to live, and even you think, okay. I mean, I'm not going to do that. Like that's not natural for me.
  
  Rob: Yeah.
  
  Simone: And then you think, okay, well, does this mean I cannot be a mathematician?
  
  Rob: Yeah, and it intersects with lots of really troubling notions, I think, of personal worth and things like authorship, where, you know, you have to be able to put your flag down and say, I did this, right. And so what am I trying to say? You know, having this theorem being presented to me and, you know, it's a theorem with a capital T, and it's got a guy's name, Van der Waerden and, you know, 1929. And I say, oh, well, he must have been some genius guy. And then, you know, it's presented to us as like, you play around with some small cases, you spot some patterns and you generalise. And I thought, well, this is great. You know, this is what I wanted to do. I mean, it's a little paradoxical because often combinatorial proofs involve having, you know, one of these lightning bolt style ideas, maybe, or at least they're often presented as that. Right?
  
  Simone: Yeah. Somehow, I've always, or well, I've often had that feeling when some combinatorics entered the picture. So doing model theory myself, there is always sort of a moment where you have to do a little bit of combinatorics, even if you do very algebraic model theory. And then people sort of drop from the sky as if, you know, here is here's what you should do, here's the set you should consider, here's the, you know, the bound that actually happens. And you're like, okay, how did you figure this out? I mean, it's not clear, but I mean, I guess if you do a lot of small cases, right, then heuristically you can sort of start guessing what the general picture might be. And that's one of the good things you can do in combinatorics that you maybe cannot do elsewhere, because maybe concrete examples don't really exist in other, when other questions are considered.
  
  Rob: Yeah. And no, I mean, there is there is a paradox there in the sense that I like combinatorics because it's accessible and you can play around with it. But at the same time, I think a lot of the culture around it is this kind of...
  
  Simone: Miraculous.
  
  Rob: Yeah, miraculous. I had some amazing idea. And then I publish a paper and it's one page long and it solves this, this, you know, this long standing problem.
  
  Simone: And it's sort of saw in a dream that the bound was supposed to be log, log, log log ten times and then but not 11. Yeah, ten is enough.
  
  Rob: And so I think when you're much younger, this combination of the fact that you can play with it, but also, you know, because people are very contradictory. Right. And, you know, combinatorics has got these big personalities like Erdos with these very colourful biographies. And so I think, it's naturally something that, you know, if you're exposed to this, you can feel very attracted to. And the environment I grew up in said that combinatorics was important because it was hard. And, I think there was this idea that it really tested you out because you couldn't come along having read a textbook, and then do any better on the problem than anyone else necessarily. I'm not sure whether that's actually true.
  
  Simone: Yeah, somehow it feels a bit like a stereotype, but it does seem true, right? That in a certain stereotypical way that what's been tested is here. And again, this is a problematic notion. Right. But sort of your cleverness because there's not much theory that goes into that. So it's not like you can actually know better than others.
  
  Rob: I mean, it's not true, right?
  
  Simone: It's not true, of course.
  
  Rob: But if you've been thinking about this style of problem for a long time, you build up a little compendium of problem solving, and it's clear that experience goes into it and knowledge goes into it.
  
  Simone: Yeah, but somehow I think it's true that stereotypically, combinatorics is seen as the field where the actual clever people can maybe just go in, dip in, fix a problem, go out. Yeah, because you don't actually need anything else rather than some clever intuition. And of course this is false, right? I mean, but there is a bit of a stereotype there, and I can understand its attractiveness because it really tests. It's a bit like street smart versus book smart. In a sense.
  
  Rob: Yeah. I mean, and this is that's actually a very good way of putting it. I've always liked this idea of, um, you know, I'm sitting there having a coffee and another mathematician comes and says, what are you working on? And I say, oh, well, you know, and I just sort of sketch it out for them because I don't need to say to them, ah, so do you know what an abelian variety is? Yeah. Right. Yes. I do think there's a fundamental paradox there in terms of my attraction to the subject and in terms of how the subject is generally perceived as well. And so then, you know, after the Olympiad background, I went on to Cambridge and this was a place where, again, there were lots of. I don't know. It's difficult because no one ever actually sits down and says to you, oh, so this subject is important here, and this subject is more important elsewhere. And no one, because, you know, it can be incredibly rude to the people working in these fields to do that. But yeah, there was a sense of... combinatorics is exciting and fun.
  
  Simone: And if anything, because probably there were a lot of combinatorialists, or still are a lot of combinatorialists in Cambridge.
  
  Rob: I mean, a lot of them have moved to Oxford now, but certainly, at the time I was there, they were big names in arithmetic combinatorics who were doing lots of exciting things, you know, so this is sort of late 2000, early 20 tens. And so they're teaching courses on graph theory, they're teaching courses on functional analysis. And then at the master's level combining things, and often these people tend to be very good lecturers and presenters as well.
  
  Simone: Yeah. Of course this is sort of these naturally builds an exciting fields. Not just cool in the sense of, you know, popular to do this subject, but also you're naturally drawn into it. Because if, of course, if the lecturer is are very good in it and there's a lot of exciting courses and seminars, then I mean, you're going to be exposed to more, more results and more, more enthusiasm even.
  
  Rob: Yeah, exactly.
  
  Simone: Okay. So let's go a bit further back and further back than your Olympics. So what is sort of your early...
  
  Rob: Olympiacs? Olympiads.
  
  Simone: What did I say? The Olympics?
  
  Rob: Mathlete.
  
  Simone: Yeah. Mathletes. People call themselves mathletes, I think. So let's go back to maybe even further back. So what is your sort of earliest memory of doing something mathematical, which, of course, mathematical is defined as whatever you felt was mathematical at the time.
  
  Rob: So we should probably distinguish between my earliest memory and my mum's earliest perception of my mathematics.
  
  Simone: I don't know about you, but I have like zero memories from when I was born to like age ten or something. So I might as well have done infinitely many things back then.
  
  Rob: But I mean, so my mom always tells me that when I was in the the pram...
  
  Simone: The pram?
  
  Rob: Oh yeah, the pushchair, I guess people call it in the US. So I'm originally from Australia and obviously because a lot of the towns in Australia were built in this very sparse way. You have these sort of American style grid systems a lot of the time. And so it's very normal for your house number to be like 1152. Right. And so my mum always tells me, you know, when I was in the pram, I used to point at a number and say, mum, what's that number. And you know, then she'd say 217 and I'd say wow, you know.
  
  Simone: Yeah, I mean kids point at things and ask, but you had a particular fascination for numbers.
  
  Rob: I actually don't think my original interest in maths was... I think I was more into computers, really, and codes and things like that. So I remember I was always trying to code messages, you know, like these Caesar shift ciphers, you know, where instead of writing A, you write B and instead of B, you write C.
  
  Simone: So these are the ones that are sort of alleged to be the ones that Caesar actually used to move information.
  
  Rob: To send messages during times of war. Exactly. I was trying to invent my own alphabet and, you know, God knows why. There was something that my dad always did that I always found very, very impressive. But I don't think he did it deliberately. It was that he was very good at not knowing what was age appropriate.
  
  Simone: Let's see where this goes.
  
  Rob: Yeah. You know, and so we had a computer and, you know, a computer at home. And in the early 90s, that's like a non-obvious statement, obviously. My dad was an engineer, still is, I suppose. We had a computer at home and one day I can't remember where we were. I think we were at some kind of second-hand boot sale. You know, where people sell things out of the back of their cars and things they want to get rid of. And he bought a book, and he said, oh, doesn't this sound interesting, Robert, wouldn't you like to do this? And it was called QBasic for dummies, right?
  
  Simone: Okay. I mean, I sort of expected this to go into like, Marquis de Sade situation.
  
  Rob: You can edit that out. So, he bought this book for me, you know, QBasic for dummies. And what does it do? I have to program for a for loop. And, you know, the computer screen prints out all the numbers from 1 to 100. And then this was back in the day where not everything was done on windows. So this was mS-DOS. And so you could do things like you could change the terminal background to be pink, of course.
  
  Simone: Yeah.
  
  Rob: And this is amazing.
  
  Simone: Yeah. Of course, of course. I mean when all you have is just the terminal.
  
  Rob: Yeah. And so I loved it. So and the other thing was when I was a child, I used to, completely differently to how I am as an adult, I used to wake up very, very early, like 5 a.m. early and go and jump on my parents bed.
  
  Simone: Um, which I'm sure they loved.
  
  Rob: Yeah. And so we eventually reached a compromise, which was that I could go on the computer, if I got up early and, you know, unsupervised.
  
  Simone: Yeah. What age were you?
  
  Rob: Oh, like 4 or 5.
  
  Simone: I see. Okay. You could barely read.
  
  Rob: I don't know. I mean, I definitely feel like, I learned to read via the computer or via programming rather than... I mean, I read lots of books as well. And so I used to sit there, and so the family cat used to come up and curl up next to me next to the computer, and I used to sit there when I was five, and I used to try out these programming exercises. And, you know, I used to play computer games as well, but this is how I initially got into it. Oh, I've got a much better answer. I've got a much better answer. I've just suddenly remembered. (...) So I was at a wedding when I was four. I'm at this wedding, and I'm absolutely bored out of my skull. Right? Because I'm a four year old. I'm at a wedding. You know, what's a wedding?
  
  Simone: You're not allowed to run around.
  
  Rob: Yeah, exactly.
  
  Simone: Sit quiet.
  
  Rob: There's no children at a wedding, usually, because people don't bring children to a wedding because they make too much noise, right? So my dad was an engineer, and his idea to entertain me because we were both very bored was to teach me how to count in binary.
  
  Simone: Right. Okay.
  
  Rob: And so, you know, I already knew how to count. I think I could already count, I don't know, over ten at least. And he said, oh, well, there's another way of counting. And then, you know, we were sort of putting each finger up and down. And then can you convert between them? And I remember it just seemed like impossibly difficult and I didn't understand anything, but like, uh, it seemed very interesting.
  
  Simone: Yeah. So enough about the past. What about the future? So what is a big problem in your area you would like to see solved? Of course, not necessarily by yourself, although one can, of course hope, but unlikely. But no, what is a theorem you would like to see proven, or a question you would like to see answered, or really, maybe even more. I mean, a concept that you think should be explored more and so on.
  
  Rob: Yeah, I think I just tend to think more in terms of... I often feel like we're just sort of swimming in a sea of complete ignorance.
  
  Simone: Sort of in the deep sea. We can't see anything.
  
  Rob: Yeah. I just think I have this feeling that we're only really scratching the surface. And so what do I do? A lot of the stuff that I do is to do with ultrahomogeneous structures. So, there is this sort of generic limit objects, these sort of generic countable objects that result as limits, of course, of finite structures. And the point is they kind of, from the finite perspective, they look the same everywhere. And so if you want to do arguments, you know, about, I don't know, finite graphs, you can look at this limit object called the random graph or the Rado graph. And the kind of the interplay between them is very interesting. I think... we don't know very much about ultrahomogeneous structures in general, ultrahomogeneous structures where all the relations are binary. We know some stuff, but I think in general we just don't know that much. And a lot of the intuition goes out the window when you start looking at ternary things or, you know, more complicated things in general, really.
  
  Simone: Also, it feels like in mathematics there's a lot of emphasis on what is the next big problem, what is the big problem. But of course, most of the work, which is maybe sort of... submerged. Yeah, I mean, the sort of submerged part of the iceberg is just understanding the things we don't know. I mean, which is of course, much less popular, much less fancy to say, oh, I want to expand knowledge on this class of objects. It's less fancy than saying, I want to prove this big conjecture.
  
  Rob: But I think this big conjecture style thinking feels a bit again. It goes back to what I was saying before. It feels a bit Hollywood. It feels a bit. If you look at if, you look at Grigory Perelman, who, you know, who declined, the Fields Medal and you know, declined the Millennium Prize as well. I say this because, Simone and I have just paused the interview to check exactly what prizes he declined because I think he declined about five of them. And, you speak to people about this, and I think there's this sort of general idea that, you know, this is a bit of an odd guy, right?
  
  Simone: In a sense it is an extreme view of the problem.
  
  Rob: Yeah. I mean, I don't know how many people would decline a Fields medal, but you know, if you look at what he says, he says, well, you know, a lot of this stuff was building on previous people's stuff and some of it was done collaboratively. And he doesn't like this idea of the money and fame and the, you know, the sort of the focus just being put on him and saying he solved it, you know.
  
  Simone: Yeah. It is this submerged iceberg idea that, of course, in the end, most of us will not prove a big theorem or a big conjecture in their lifetime. And the idea to me is more like every one of us gives a bit of a push, and then there's a sort of, a critical mass is reached and the big thing can be proved. Yeah, but all of this is just the sum of all the little pushes of people.
  
  Rob: Yeah. And you do need the genius people, of course. But if you start to think to yourself, oh, well, why don't we just take the genius people and...
  
  Simone: Lock them in a room?
  
  Rob: Yeah. And just we don't need any of the rest of them. They're all kind of not really doing anything particularly substantial. I think that's a completely wrong way of seeing it.
  
  Simone: It's actually probably factually false. I mean, somehow someone will have to do a maybe have some big intuition. But yeah, even that big intuition will be nothing without all the work of the previous people behind them. I mean, even Newton, who was not the most agreeable fellow on earth, used to say this about being on the shoulders of giants, right? I mean, yeah, it was Newton, right? Maybe we have to pause again.
  
  Rob: It was Newton. And this impetus towards ownership, sometimes it's forced on people externally and it's not something that they would necessarily want. I mean, we don't want to and you have to sort of ask yourself, how much is it got to do with the scarcity of academic positions?
  
  Simone: The job market being what it is. A stronger focus on, you know, who proved what and what is the exact contribution.
  
  Rob: What's the more important thing? The what or the person who proofed it? I think it's obviously the theorem that's the more important thing.
  
  Simone: And of course you want to give recognition to the person. But this doesn't mean that it's a scarce substance that you cannot give to more people. I mean, I always feel like you're not going to run out of acknowledgements.
  
  Rob: No, certainly.
  
  Simone: And somehow it's always... it's how I feel about giving a single name to a theorem, because of course, this person proved the theorem, and it's a useful mnemonic to know who proved a theorem.
  
  Rob: Yeah. And often it's just a little placeholder that you put in. Right. Just because you need a name for it, there's no better way and so on.
  
  Rob: But yeah, but this whole concept of ownership I find very... I mean, look, you know, when you're selecting people for a position, you've got to have a rough idea of what they've proved.
  
  Simone: But I think that this is really just because the market has become so competitive. To the point where you actually have to sort of surgically compare the contributions to choose which one is better according to some metrics.
  
  Rob: Right. I mean, you can't give everyone a job.
  
  Simone: I mean, I agree, but I'm not sure that pushing so hard on, you know, how many papers you have and what percentage of that paper is your contribution is actually a healthy thing.
  
  Rob: Yeah. And it's often very murky.
  
  Simone: I mean, I wouldn't know what percentage of a certain paper I have.
  
  Rob: But certainly during my PhD. The first time I gave a talk on the results from my PhD, my supervisor had to come up to me afterwards, and he said to me, okay, so two comments. He said, number one, you didn't have any theorems in that talk. You just labelled everything as a proposition.
  
  Simone: Yeah, I've done this. I've done the same in my first talk actually. It felt too much to just call them theorems.
  
  Rob: Yeah. Yeah, exactly. And then secondly, he said to me, actually, you forgot to attribute some of these theorems to yourself. And you wrote my name. And I said to him, you know, because I don't feel like I proved them. And he said, well, you did, and I really struggled with it because, it's still not clear to me now, you know.
  
  Simone: But I think this is common, right? I mean, it's definitely not that someone comes out of nowhere with a single author paper, and they've come up with every single idea in the paper. It's just not how it works.
  
  Rob: Yeah. I mean, sometimes it does.
  
  Simone: Sometimes. Occasionally. Yeah. But it's not how the standard thing goes.
  
  Rob: And you can wrap yourself, you can tie yourself up in knots with this stuff.
  
  Simone: Definitely.
  
  Rob: The solution is okay. Did the theorem get out there? You know? Yes. Am I going to be able to get a job? Yes. And then if those two preconditions are met, then nothing else really matters, you know?
  
  Simone: All right. So thank you very much for the lovely conversation.

Episode 3: Paradoxical sets and ice cream, with Azul Fatalini

In this episode of On A Tangent, Simone is joined by Azul Fatalini, a doctoral researcher in Set Theory. We learn about how the Axiom of Choice transforms the universes it holds in, how logic is the mathematics of mathematics, and what’s the quickest path to ice cream.

Link to Azul's website

• Transcript

  Simone: Welcome to On a Tangent, the podcast, where the main characters are the stories behind the mathematics. My name is Simon and in each episode I am joined by a different early career mathematician from Munster to learn about their paths towards mathematics and the hopes for the future. This episode, coming out during Pride Month, I am joined by Azul, a doctoral researcher in set theory. We learn about how the axiom of choice transforms the universes it holds in, how set theory is the mathematics of mathematics, and what's the quickest path to ice cream. I hope you enjoy this episode. (...) So, Azul… Welcome to the podcast.

  Azul: Hi. It's nice to be here.

  Simone: Yeah. Thank you for joining us. So as you know, in the podcast, we sort of explore the stories of our guests. And so I would like to start from your past. So what is your earliest memory of mathematics?

  Azul: Actually I love that question. So when you told me you were going to do this podcast, I ask you what type of questions you would ask. And you told me this example, and I was so glad because I have this story and nobody ever asked me this. And I didn't even know this until you ask me them. So I'm super happy to tell this. So the thing is, when I was a kid, in my hometown, the map of the city is a grid. And there is this main square in the city centre that is a square. So it's 100m times 100m, but it also has a cross in the middle. And one of the things we did with my mom a lot, was walking from my house to the city centre and going to have an ice cream in the corner of the square. And to do that, the best way was to cross the square through the diagonal, right? And most of these, most of the blocks in my city don't have this, it's just that part that you can cross in diagonally. And when I was a kid, I was always thinking, so first, if it was really better, I think, that one I could see, like, I could just look around and see that it was shorter than doing two blocks of 200m. But I always wanted to know precisely…

  Simone: How much better.

  Azul: How much better it was, because it seemed to me that it was more than one block, but less than two.

  Simone: So better than two.

  Azul: But yeah. But not like so much better. So… how much. And I was obsessed with this and I was trying to estimate. But it was super hard.

  Simone: Getting faster to the ice cream.

  Azul: Of course. How much faster are we getting to the ice cream?

  Simone: Exactly.

  Azul: But there was always trying to do this, and in my mind estimation was like 1.5. I didn't even know decimals I think, at this point. But like half.

  Simone: Half. Yeah.

  Azul: But I never could answer if the diagonal was less or more than 1.5. And this was just something I had in my mind for a long time.

  Simone: Until many years later in school, you suddenly found out?

  Azul: Yeah. Yeah, definitely. It was much longer, and I was not even living there anymore. It came the answer ten years later.

  Simone: And do you somehow think that this way… Well, this is in a way, a very geometric problem then sort of connects to what you do nowadays in your research. Have you always been a very visual thinker or a geometric thinker?

  Azul: I would say so. When I was in a Math Olympiad in high school, my favourite branch of problems was geometry. So Euclidean geometry. So in that regard, yes. But I would say I'm terrible at visual thinking in the sense of… It's very hard for me to imagine figures. Okay. In that sense I would say no, but because I cannot imagine so much, then I would draw things. I mean, I like to compensate this lack of imagination with like doing diagrams and drawings to help myself.

  Simone: And so… I just sort of already went into this topic: what is it, that you do nowadays in research?

  Azul: So I'm studying set theory. That's the branch. It's inside logic, as you know.

  Simone: Of course. We finally have a logician on the podcast.

  Azul: Yes. I mean, it's easier to tell you, right?

  Simone: Of course. But for our audience…

  Azul: Yes. So I study problems that are related with the axiom of choice and the subsets of the real numbers. So somehow the objects are very much like normal math, let's say. And there are some geometrical objects that I study. But from the perspective of the theory, because the questions are related to how much axiom of choice do you need to construct these objects or not, which is essentially a set theoretic question.

  Simone: Yes. And is this how you explain it to non maths friends? If they ask, of course.

  Azul: Of course not. I mean they don't know what axiom of choice is.

  Simone: And why set theory is important for people I mean.

  Azul: I mean, I don't even know if a mathematician… I'm not sure mathematicians know why set theory is important in general, I think. Yeah.

  Simone: Except for the fact that, I mean, yes, this is the foundation.

  Azul: That's it. Not mathematicians. I usually say I do something in logic. Logic is a word that people know, right? And I mean, of course they don't know what logic, mathematical logic means, but there is a concept of logic, an idea. And then if they ask me more, I say, okay, there is a branch of logic that is set theory that studies more or less the foundations of math and the structure of how math works. So there are these meta mathematical questions that set theory takes care of.

  Simone: I think I usually tell people we do, like what linguists do to the real world, like, people study the language in which people communicate and logicians study the linguistics of mathematics.

  Azul: Yeah, it could be.

  Simone: In terms of funding, probably it's a similar parallel.

  Azul: In terms of funding?

  Simone: We get as much funding as the linguistics people.

  Azul: Yes. I'm not sure I don't have any idea of this.

  Simone: Yeah. But I imagined it’s not much, as all humanities people.

  Azul: I mean, I agree on what you're saying, but in particular set theory I like more to simplify as the math of math. I think because you're not using linguistics to study, you know, the language of real life, it's like a different thing. Right? Linguistics is different from real world or, or the language that is spoken. But the theory is itself a part of math. And I think that's one of the most beautiful things.

  Simone: And the most confusing.

  Azul: And it's super confusing. But this thing… I mean, math can study things that are things of math. And when you say this to people, even if they are not mathematicians, it is like, what? How do you do that?

  Simone: Yeah. And I do remember my first set theory class, there is all the sort of paradoxes which seem to stem from the fact that you actually talk about set theory when you do the set theory of math. I mean, when you do the maths of maths, you're in particular studying the maths. I mean, you know…

  Azul: Yeah, there's this self-reference. Self-reference at the beginning.

  Simone: Yes. I mean, it is confusing. I mean, it's also something to explore.

  Azul: Well it's definitely both for me. The fact that is so weird in a sense, in comparison to other branches that don't have this feature makes it more interesting.

  Simone: And so your research deals with sort of, let's say, normal mathematical objects, which if another mathematician said this to me, that it would be offensive, but we can say it. Yeah. I mean, you know, because if somebody told me, ah, yeah, we do the normal mathematics, you do the logic, I would be a bit offended. But I can say, you know, the natural mathematical objects that you find in nature and in nature, well, you know, in nature, like in, in mathematics, I mean, mathematical nature. In mathematical nature. Exactly. And you often hear about how the axiom of choice builds these sort of paradoxical objects. Is this the kind of things you're interested in?

  Azul: Yeah. So my PhD thesis is called paradoxical sets and the axiom of choice. That's definitely the object. And yeah, I agree this about the normal math and not normal math.

  Simone: But would paradoxical sets be considered a normal mathematical object.

  Azul: Yeah, I think so. I mean, for example, you have this example that I think is one of the most well known. I think that is the Vitali set. So the standard example of a non measurable set in the reals. And this is I mean this is part of the course of real analysis. This is just something most of people would have seen at some point or Banach-Tarski that is so famous. And, well some people don't like actually Banach-Tarski, like it's weird. Right. But for example, the Vitali set is really a part of understanding of measure theory.

  Simone: No, we don't work in the, … what is it? The Solovay paradise where all sets are measurable?

  Azul: Yeah, exactly. So, I mean, but of course, when you start asking a theoretical question, then the object kind of doesn't satisfy the same role as if it is because you're not studying the object itself, but rather also or in my case, which are the axioms you need to construct, or how consistent is this existence of this object with some other set of axioms or some other objects? So, even if the name is right the same, like Vitali set, then what you're doing with it is very… different, you know.

  Simone: You’re not studying the properties of it, but rather what, what its existence says about the universe you find it in.

  Azul: Yeah, somehow. Where does it lie? In the map of things.

  Simone: And I know you're not meant to play favourites, but what is your favourite paradoxical set?

  Azul: What is my favourite paradoxical set? Let's see. Well, I think if I have to pick one, it would be the one I thought more about. So there is this theorem in ZFC. So using the axiom of choice. That R3 can be partitioned. So the space can be partitioned in circles, so only with the border. And these circles can be taken to have radius one. So they are all the same radius. But still, you can cover R3.

  Simone: Which is, I think you the first time you hear it, like of course I can do this. And then you mentally start placing circles one into the other and then very quickly get to the point where you don't know where to place the next one. Right?

  Azul: Exactly. I mean, you can put more and more circles, right? Because each one is kind of small in inside the space. But how would you end up completing everything over there?

  Simone: Yes.

  Azul: Yeah. Like, imagine you put a lot of circles and like it's dense in R3. But there is some space that you have to fill. If you have like some isolated points, for example, then of course you cannot do it. So even if you have like a full circle, but a few points are not already taken, then you cannot put a circle again.

  Simone: So this means you did it wrongly from the start?

  Azul: Yes. I mean at some point there was something that you did that didn't allow to continue the procedure. Right. But there is a way to do this process, wisely so that this doesn't happen. And this is a theorem of ZFC. So you really need the axiom of choice. Well, less than that, but some form of choice. Some part of choice. Yeah. I think this is my favourite. It’s nice that you can tell people you are studying something like this.

  Simone: Because they can visualise it.

  Azul: They can visualise it is just Euclidean geometry. So I can even tell my family about this if they are patient enough to hear this.

  Simone: Yeah. Or to look at the pictures at least.

  Azul: Yes.

  Simone: And do you feel like because this feels a lot like one of those problems in number theory where the statement is very easy. But then the math behind it is very complicated. So I presume in a similar way the problem can be stated very easily. But then proving it is not Euclidean geometry is actually.

  Azul: Yes. So the proof of this theorem is not well, of course it has some Euclidean geometry because the object is. But the main tool is doing transfinite induction on the cardinality of the reals, whatever that is. But the point is you do this induction, but it's longer than the natural numbers, but even more. You keep going after you did countable many steps, and then you keep going. Yes. And then you keep going on and you keep going until you reach the cardinality of the reals, which is the same cardinality of all the points in the space that you have to cover. So this tool is not from Euclidean geometry. It's set theoretical. So the existence of this paradoxical set already has a proof that is set theoretical.

  Simone: So you said you need less than choice. So maybe something like a, well, ordering of the reals or dependent choice, whatever this means for the non logicians. But then maybe in the vein of what we said before, from the existence of this set, just so you look sort of look at some universe where the set exists. Can you then say anything about what axioms are true in this universe? If, for example, is some amount of choice still true?

  Azul: Right. So that's exactly the questions we were trying to think of while my PhD was happening and so basically the answer is no. There is I mean, you could have this set and basically no choice. I mean. This, you can formalise this. For example, there is this concept of countable choice. Doing the choice countable many times. And we got the result that there is a model of the theory in which you have this set, so this partition of the space in unit circles, but you still you don't even have countable choice.

  Simone: And I guess for the non logicians or maybe even for the non set theorists, the striking idea is building a model of set theory because it's kind of feels like building a universe of but we are in one. So it's a bit confusing.

  Azul: Of course.

  Simone: If you start thinking about this philosophically you can go on for hours, I guess. But yes. How does one build a like a universe of set theory or model of set theory?

  Azul: I mean, I think that's the full thing of what a theory does, building models of set theory that satisfy the things you want. So how do you build the model? Well, first you assume there is one.

  Simone: Which is already… Yeah.

  Azul: And then from there, I mean, there are many techniques. So one way to build a model is build a smaller model, inside the one you have. So from all the sets you have in your model, let's take only the definable sets which have some definition and then you'll get something in principle, smaller. It could be strictly smaller. It could be all of it, depending on what you're doing. But there is also a way to construct bigger models. Okay. So there is this technique that is called forcing that allows you from a model and some other elements, construct a bigger model than the one you started with. So those are the two ways people do it.

  Simone: I think I remember in my set theory class, it was described to me as… So in the model, there are people and they believe in some sort of like entity. They know sort of how this entity looks like. They know some things, you know. Like so in the first thing you have some properties encoded, but then you sort of look up at the sky and this entity doesn't exist. But then like in the level above, the entity is there and looks down on the people and sort of has an effect on them. So it's a bit like religion, which I don't know. I mean, you do have a lot of cardinals in set theory.

  Azul: I do think it's a bit like that. So I don't know if you know this. Is it a book or…, I think it's a book.

  Simone: The higher set theory?

  Azul: No, no. Nothing to do with this. Okay. Flat something.

  Simone: Flatlandia.

  Azul: Flatlandia. Yeah. Is it English?

  Simone: Flatland.

  Azul: Maybe. Flatland.

  Simone: Yeah.

  Azul: Yes, flatland. I didn't know it in English. Sorry. So in flatland, there is this people that cannot. Well, actually, did I read this book?

  Simone: I'm sure I read it in school, like, when I was very young.

  Azul: Yeah, but. So the point is, if you are in flatland, you cannot see three dimensional things, right? You're a circle. Yeah, or a square or something. But then a sphere in flatland is like a circle that moves and has different radius. Right. Because depending on the section of the sphere, the circle changes. So somehow from flatland you can see this three dimensional things as another thing which is not three dimensional, right. Because you cannot but you can guess what it is somehow and can imagine what three dimensionality is by all the sections, for example. And this is more or less what forcing is. Yeah, it's a simplification of course. But what forcing is about. So from the original model you can kind of imagine what the exterior model would be like.

  Simone: And so from your research you're doing nowadays, let's maybe look into the future a bit. What is something you would like to see prove in the next ten years by somebody? Not necessarily by you, but… some questions you would like to see answered?

  Azul: I don't know. I think I would like… so I have this maybe vision about what I want to do in. I don't know the future very abstractly in general at some point with math, and it is trying to connect a set theory with, yeah, like subset of the reals or things that are like normal math now.

  Simone: That you find in nature.

  Azul: Yes, I am not comfortable with it.

  Simone: I think I often say, you know, model theory does logic on objects found in nature. This is what I usually say.

  Azul: Well, yeah, I don't feel it is in nature. Right. So, I feel uncomfortable with that concept. In theory the real numbers, for example, are not the real line. If you tell any mathematician, imagine the real numbers, they probably imagine the real line or maybe they just say pi or something. But the set of reals, which is the set of reals? They will draw a line. And for the set theorists usually it is not. For example the power set of the natural numbers or all the functions from the natural numbers to the natural numbers or things like this. Like there are several sets that we consider them all as the real numbers. They are all bijection with each other. So for most of the questions, this is the same if you care, for example, only about the cardinality of that set, then they are all the same. But for the objects I'm looking at, for example circles in the space. You are thinking about the real line. I mean, the space is three times the real line. So you have to look at the reals as the real line to just approach these questions. So what I would like to be solved is the things I'm curious about, of course. Which is this abstract idea of some theoretical questions that are related to the reals as the reals of the mathematicians, which is the real line.

  Simone: Mhm. To do a more geometric version of the reals.

  Azul: So yeah, but not only, I mean, even if you consider it for example the reals as a field, it's also something that is not… I mean, the power set of the natural numbers is not a field.

  Simone: It’s just a set.

  Azul: Yes. So even if you ask more algebraic questions, this also has to do with this view that I have, that we have a gap there.

  Simone: Okay. Well thank you very much for the insights!

  Azul: Thank you so much.

  Simone: See you in the corridors.

Episode 2: Springs and memory alloys, with Mert Bastug

In this episode of On A Tangent, Simone is joined by Mert Bastug, a doctoral researcher in PDEs and Calculus of Variations. We discuss Mert’s first meeting with mathematics, how we can understand materials with the help of PDEs, and where they might take us next.

Link to Mert's website

• Transcript

  Simone: Welcome to On A Tangent, the podcast where the main characters are the stories behind the mathematics. My name is Simone, and in each episode I am joined by a different early career mathematician from Muenster to learn about their paths towards mathematics, and their hopes for the future. In this episode, I am joined by Mert, a PhD student in PDEs and Calculus of Variations, to find out about what mathematics can say about memory alloys, why they're not the same as memory mattresses, and how mathematicians think about their work. I hope you enjoy the episode!

  Simone: Hi. Welcome. And thank you for joining us.

  Mert: Hello. Yeah, thank you for having me.

  Simone: So let's start a bit further in the past. And so let's start from your very first memory of maths. So what's the first time you've seen maths enter your life?

  Mert: Well, the first time. I mean, I guess I could surely mention all those times I would draw on numbers or learn how to count. But I think my first moment where I consciously thought about a math problem was when my dad asked me a simple counting problem. But to me, at the time, it didn't look simple at all because it involved very large numbers. Actually, I can talk about the problem. It was about some number of horses all needing some horse shoes and each horse shoe requiring some number of nails. And then one would need to tell the total number of nails one would need. And the total came up to something in the millions which, as a child in kindergarten or maybe elementary school, it was ... I didn't really grasp this.

  Simone: A lot of zeros!

  Mert:  Right. I mean, somehow… Yeah. Yeah, definitely. And I don't even think I attempted to solve the problem. It was just to me that defined what a difficult math problem was. So that's as far as I can remember, right.

  Simone: So from somehow seeing what a difficult math problem was when you were a child, now you come to dealing with difficult math problems now, in your life.

  Mert: Yes, totally.

  Simone: So what is it that you do? How do you explain it to your friends? How do you how would you have explained it to yourself as a child?

  Mert: Oh, myself as a child, that would certainly be difficult, but, I mean, I guess I would use some tricks. Now it is okay to lie to others when you say you're doing something in math when you're not actually doing it. For me personally, the kind of math that I'm interested in ultimately goes back to problems in physics. So if I want to talk about math, well, not want to, but maybe if people ask me to explain to them what I do, I will start with some physical motivation. In my case, it would be something related to understanding materials. More specifically, what happens to materials when you try to bend them or when you just in general try to deform them? What sort of shapes are possible or why do they bend the way they do? Or, if they retain their form after you've let go of them, why that happens? I mean, okay, these are certainly not all questions that I try to deal with, but yes, if I'm trying to give people some sense of what I do, then I will freely talk about other things as well.

  Simone: I mean, somehow it's always helpful, right, when you can appeal to some physical motivation or so. There are any particular materials, I mean, that you actually work with or…

  Mert: Me personally, no. But I know. Well, yeah. So different people, of course, maybe they try to understand. I think they're called memory shape alloys.

  Simone: Are these a bit like memory mattresses, like you know, the ones where you sort of you lay down and over time, they sort of take your shape?

  Mert: I guess not really. Yeah, I would say these are a little more different than memory mattresses. They actually retain their form again. Maybe if you heat them or you expose them to some other external condition.

  Simone: So it's like those videos on TikTok of these materials that you bend over in strange shapes, and then the moment you sort of heat them up, they go back into, I don't know, a line or…

  Mert: Right, right.

  Simone: Maybe this goes back to earlier than TikTok, I mean… on YouTube in the early 2010s or something. This is magic of these alloys. Right. So this is the physical motivation. Right. So this is what you tell people. But what is the maths that you actually do for this. Or I mean I'm assuming you don't do experiments.

  Mert: No. Well I don’t. I’m sure some people out there do. The part of mathematics that deals with these questions is related to calculus of variations and also partial differential equations. In calculus of variations, one looks at minimisation problems. Now the quantities that one tries to minimise typically come from geometrical considerations, but as in my case, also from physical problems.

  Simone: So these materials you were referring to at the beginning.

  Mert: Exactly. So if you look at a material, then the shape that material wants to assume, will actually be the shape that minimises some sort of quantity, and this can be some sort of elastic energy or some other type of energy that one might want to consider.

  Simone: So, the natural state of this material is the one where this energy is minimal. And I think if everybody has ever played with a spring, you know. You extend the spring somehow. This is, you feel that there's some tension and you let it go. There is none.

  Mert: Yes. Right. And also, when you apply force to these materials, they will still try to minimise the energy in the best way they can. But now there will be some external condition, which is the force you're applying to it. Right. And in partial differential equations one looks at again, maybe physical or geometrical models of something. And one tries to understand how that model changes over time or over space and these are then described in terms of some equations that involve derivatives, which is where the name differential equation comes from.

  Simone: So somehow these in this case you're again thinking about models of materials and how they change over time when you give them some input.

  Mert: Right. And actually the two areas are related because it turns out that minimising something, amounts to solving a particular equation. So in other words, to every minimisation problem you can associate well, in most cases at least a partial differential equation. Yes, but I mean, if I wanted to talk about math to my friends or my past self say, I don't think I would necessarily mention the actual work that I do, but I would just try to give them a feeling of what doing math is like. That's why I often also mention, um, number theory, because, well, naturally, numbers are the objects that everyone can relate to mathematically. And I might also mention famous open problems from number theory, because in the end, no matter what it is in math, you're doing, um, you're trying to solve a problem that hasn't been solved before. And it's just the nature of the problem differs from area to area. But the motivating force, which is trying to understand something that hasn't been done so far, is the same in each area, I would say.

  Simone: And somehow you write model theory. Sorry, number theory professional deformation, there's all these open problems which are somehow easy to state, but then require years and years of difficult or difficult maths to be solved. Right? I mean, like Fermat's Last Theorem, where you would say, okay, I want to solve this, this equation with integers, and then, okay, everyone can more or less think, okay, how do you solve this? And then it takes, I don't know, eight years of Andrew Wiles to do this, which is of course the striking fact.

  Mert: Um that's right.

  Simone: So so you use often number theory as an example. Do you think somehow number theory and what you do then actually feel the same when doing them. Do they, do they sort of. Let me let me try to explain. Do you when you do maths on your on a daily basis, what is your guiding intuition for example? Last time we had Alex, who had a very geometric intuition about things. Uh, in this case, do you also somehow imagine your materials actually somehow evolving in space visually? Because, for example, some mathematician I can give the example of myself, I'm not a visual mathematician. It also doesn't help that my field is not visual at all. Um, but in other areas. In GR we had the discussion last time. It's a very visual area. You can imagine the geometric configuration. Does this also help in your problems? And do you think it's somehow, um, do you also think in this way or is it or is there some alternative way of people think about problems in PDEs or in, in calculus of variations?

  Mert: Well, um, I would say that I am also quite a visual thinker. Um, now. For me, it's difficult to apply my visual thinking into problems directly, but it's always helpful to draw a sketch of what I'm doing. Um, just so I have something that I can work on. Uh, even though the methods that maybe I require in solving a problem don't necessarily appear in the picture that I'm drawing. Um, I mean. So to answer your question, whether geometric thinking helps in, um, say, solving a differential equation or maybe more broadly, and in the two areas that I mentioned. Um. I would. Well, I would first have to say that I don't think I'm qualified enough to answer it, but my impression is that most of the time in analysis the proofs come down to some smart estimates or some smart, um, identities that one should recognise or, somehow rather the than the intuition about geometry often is useful the intuition about the quantities that you actually computing with uh or some standard tricks.

  Simone: I mean, because I always have this, this clash when I talk to people who maybe do differential geometry or this or analysis where they there's a lot of estimations and computations going on. There is no computation going on in what I do on a daily basis. And somehow I always struggle a bit to imagine how that feels like, because there is hardly any computation you can do to solve a theorem or to prove a theorem in model theory, you usually have to maybe give some structural argument or appeal to some standard tricks. Um, so somehow what you're, you're saying is, is there is a more quantitative, maybe intuition that people have rather than geometric.

  Mert: Right. Um, I mean, I would imagine, I would imagine that the situation changed actually, as one, gains more experience, um, because as I've said, the, the problems that one deals with, um, well, at least in my case come from physics and there maybe not the geometrical intuition, but the physical intuition plays a greater role. Um, now, certainly if one has a mathematical proof of something, that proof doesn't have to relate to the physical world, but. It is usually the case that the proof is based upon some physical intuition that the author that the mathematician had. And um, for me so far, it has been more or less possible to work without the physical intuition. But I think for everyone wanting to work in an applied field that is, um, quite an important tool that one should have in their arsenal.

  Simone: And so I guess then the natural question is how did you come to an applied field? I mean, what brought you to working in analysis, but not just analysis, but applied to some actual concrete problems like materials.

  Mert: Right. Um, well, actually, analysis was the first math subject that I wanted to learn. Um, and this was back in high school when I was trying to think about what to study in university. And, um, somehow math, um, became, for me, the most reasonable choice. And then I wanted to, before even starting university, see whether I would like it. Um, and then I looked online. I searched what people recommended, and I found this one book that was, um, on analysis. And as I studied more and more from this book, I came to notice that analysis was, for me, full of, um, nice ideas and geometrical intuition, at least at the level I was learning. Um, and then when I got to university, I didn't want to immediately cling onto this one subject, but rather keep my options, um, open. However. Yeah. Due to having a sort of head start in analysis, I felt more comfortable working with it. And ultimately, in my case, it turned out that professors and analysis were more pleasant. Uh uh. Well, they gave more pleasant lectures to attend. Um, and around the same time, I was also getting, um, interested in the idea of seeing how math could be applied to real life, because at a certain point, the math curriculum at the university can get quite abstract. And I noticed that there was a sort of a certain rewarding sense I felt whenever I saw math being applied to a real world problem. Or at least I could find. I found it that for me, it was more interesting to feel attracted to the problem, knowing that it was coming from something physical.

  Simone: There is a natural satisfaction that comes from the fact that you are actually having an impact, which I think is something that maybe people who do very theoretical maths struggle a bit with. Certainly I can say, I mean, or, um, that your math does not have a direct impact. And it's very common to say, well, yes, but I don't know. Alan Turing developing the computer. His theoretical maths also did not have any impact on the world. And see now what happens. Well, okay. Yes. But I don't think we can use exceptions as our motivating examples. And I understand somehow it's, something that Alex was saying is, you can motivate yourself a lot through the fact that you're dealing with a concrete problem that actually has an impact.

  Mert: But I would actually like to argue in another way. For me, of course, it's helpful to know that what I do might be impactful in real life for the development of science or whatever you can call it. But I would say the, the selling point for me was that. I have a more immediate way of seeing why this problem is interesting. Now you have problems in, say, algebraic geometry that require a lot of background information just to be able to understand. And I'm sure if I could understand those problems, I would find most of them interesting as well. It was just the amount of sophistication necessary to appreciate problems from analysis was lower. And it is not to say that the problems have lower quality.

  Simone: Of course not. As we've said before, right? Is this number theory problems that you can state to a child, then they require all the work in the world to be solved. Right. And somehow, in the same way, just because you can appeal to the physical intuition, it doesn't mean that the problem is less interesting or less worthy or less hard, right? I mean, yes, sure. Of course we don't want to discriminate in any way. So what are your hopes for the future of your field now then somehow, what is something that you would really like to be to see solved or to see addressed?

  Mert: Well, I have to admit that I am still quite fresh in my field. So I don't feel qualified, to pick a problem that would be interesting. Now I can say, for example, that if one thinks of materials in a broader sense as, things that are made of continua, such as maybe fluids as well, then the biggest open problem at the moment would probably be the problem relating to Navier Stokes equation. But again, even though. This problem is quite popular. I don't feel quite invested in the problem yet. So it is difficult for me to pick a particular subject. But I've always had certain thoughts about the future of mathematics. You hear often that there isn't a unified theory for partial differential equations, and that each equation is by itself, very special and requires different methods to be dealt with. I mean, this is natural when you think that for every physical system that you can describe, there is a differential equation that would maybe associated with that system. So we cannot hope to maybe find a unified theory for the whole field of partial differential equations. But then I'm reminded of the history of math and how calculus came to develop at a time when there were lots of problems, all requiring a different methods to be solved and how calculus built the ground for, well, not necessarily a unified treatment for each of them, but at least a language to describe all those problems then. I always find it exciting to think what could happen in the future in the field of partial differential equations or more generally, in analysis. In general I would be curious to see something like a new revolution happening, a new kind of calculus appearing and forcing us to think about the things we already know in a different light, but in a way that opens up new avenues of research. Of course, I don't know what that would look like, but it's always, for me at least, interesting to ponder about the possibilities, because we tend to think that what we know is the edge of knowledge and things will remain the way they are. But often we're surprised by advancements in our knowledge.

  Simone: Well, thank you for the lovely insight. And we'll see you in the corridors.

  Mert: Thank you. See you.

Episode 1: Oranges and eclipses, with Alex Tullini

In this episode of On A Tangent, Simone is joined by Alex Tullini, a doctoral researcher in General Relativity. We discuss Alex’s journey towards mathematics, going through oranges, eclipses, and the odd similarities between a career in mathematics and surgery.

Link to Alex's website

• Transcription

  Simone: Welcome to On A Tangent, the podcast where the main characters are the stories behind the mathematics. My name is Simone, and in each episode I will meet a different early career mathematician from Muenster to learn about their stories, their paths towards mathematics, and their hopes for the future. In this episode, I am joined by Alex, a PhD student in General Relativity, 'GR' for short, to learn about eclipses, cosmic censorship, and surgery. I hope you enjoy the episode!

  Simone: Hello, Alex and welcome. Thank you for joining us.

  Alex: Hello, Simone. It's really nice to be here.

  Simone: So you're a PhD student in general relativity. So when you go home and your friends ask you, what do you do? What do you tell them?

  Alex: So first, I take a very deep breath. Actually, I think that as a mathematician, being in general relativity makes this job of sharing what I do easier, because it's one thing to say, oh, I'm a model theorist, and I study this and that.

  Simone: This is a personal attack to myself.

  Alex: It is. I mean, it's another thing to mention, say, a black hole, which is much more part of pop culture. And that helps me in some sense. It also saddens me because I realise that I have to be imprecise in order for them to receive the message. But okay, what I typically tell them is that I try to study through math the stability of black hole solutions to Einsteins equations, and this is kind of easy to get through because then people imagine, oh, black holes, there's these things that are described using math and okay, these things are hard, but I can do something with this math. But it creates an idea in their mind typically. Then the next thing they ask me is, so do you make experiments, which I think is something unique to people in my field, I don't think people ask you about experiments.

  Simone: No, definitely not.

  Alex: Okay, then I have to explain to them that what I do cannot really be experimented, but it's more connected to questioning the consistency of a theory. And I do think that what I do, the message really arrives to say my family, my friends. So I think I feel like I'm in a privileged position.

  Simone: I was once reading a message, I think maybe on Twitter by a professor who was saying, you know, everybody learns what a black hole is in high school or what the DNA is, right. Nobody learns what a manifold is in high school, which might be trickier, but somehow it is a bit easier if you can explain it with the pretty pictures.

  Alex: Yeah, and the physical phenomenon maybe is maybe easier to relate to. I think the weird thing is that they don't realise that what I do is still very abstract, and they think of it as being, um, enjoyable in a way that is not really what I go through. Um, there's much more technicality involved, but there's no hope of passing it through, of really creating a picture of this type of technicality. So I just accept that I'm going to mention interstellar and they'll be happy and very excited.

  Simone: Having a mainstream reference for it is a very good attack point.

  Alex: Yeah. I'm almost as lucky as a physicist, let's say, even if I don't really do what they do. I mean, I don't have the abilities to do what they do.

  Simone: So this is you now. Let's take a step back. What is the earliest moment you can remember in your life where maths entered the picture?

  Alex: Um, so I'm afraid I might be unrelatable to the general public when I actually answer this question. But I have to be honest, math has always been with me, and that for whatever reason that I couldn't really understand, among all the things that I like, it was my greatest interest and the first memory of it I that I have, which might seem also unrelated, but to me it was a moment of mathematical interest and understanding was when at some point we were waiting for an eclipse to happen. I was home with my mom and dad, and they just told me, the eclipse is going to happen. And I had some trouble understanding what it what it meant, um, because I couldn't picture in my head the kind of the geometric configuration that would allow for the eclipse to happen. And so I asked my dad to explain to me, and we went, you know, over his big bed, and we took some balls, I think an orange and, like, maybe a lemon or something. And my dad, like, put them on the on the bed and said, okay, this is the solar system, this is the moon, this is the Earth. And what we're going to witness today happens because of this geometric configuration. Right? And for me, that was my first approach with math. And I think it also in some sense it is coherent with the sort of interests I've had later, which were more connected to geometry.

  Simone: I was going to ask, do you think somehow this is what brought you to a field where maybe geometric intuition or imagining visually what's going on is important?

  Alex: I don't know if it brought me. I mean, I don't see it as a cause, rather I see it as a maybe as a symptom of something already there. Maybe I don't have that story of a little kid thinking about numbers, but I was always intrigued by geometry, by a perception of distance, by just configurations of objects in space. And this definitely is something that I also witnessed in the choices I made later during university and uh, and everything that led me to where I am right now.

  Simone: So how did you arrive where you are? How did you choose what to do?

  Alex: Um, so as I said, I was always drawn to it. It almost felt like a choice that was made by someone else. And that for whatever reason, I always felt like, oh, this is actually what I like the most. Then I also liked other things very much. I liked philosophy very much. I really wanted to go into that. I liked ancient languages very much. Part of me wanted to study Latin and Greek.

  Simone: Did you study them in school?

  Alex: I only studied Latin in school, unfortunately. But I made this choice because I thought, okay, if I want to study math, and it is certainly what I want to know the most before anything else. wI also believe that there is a right time to do it, because I felt like my brain would be better off studying math at an earlier age, and then maybe other things at a later age, then the opposite. And so this made me confident that what I felt was right for me, which was to study math, was also objectively, in some sense, the most optimal choice\'85 in terms of long term plans, even if it meant sacrificing other interests that I maybe had. And sometimes I wondered, okay, could I try doing something else? And something which may seem unrelated, but that I was very interested in: I've always wanted to be a surgeon, just because how cool is it? Okay, this is going to sound weird, but to actually see the inside of a body.

  Simone: It's certainly something, right, you don't see every day. It's a bit of a mysterious thing.

  Alex: And also, okay, the idea of having that role where some problems that are very hard to solve, they maybe can be solved through surgery. And somebody that has, you know, studied for years has a special set of skills. They can do that for you. And, um, I try to imagine myself into that career. But then I realised I would have suffered too much the loss of the mathematical knowledge, whereas I could have, you know, accepted the loss of, say, the surgical knowledge, or the knowledge that I've had to sacrifice because of course, there is only one thing you you can reasonably master. I mean, unless you're specially gifted or something.

  Simone: Or you have a lot of free time, and somebody at home that takes care of everything. So this is how you came to math. But how did you come to general relativity then? I mean, you told me you've always been geometrically inclined.

  Alex: Yeah, but I would say I came to GR because at some point I really took into my own hands my journey. I studied in Italy, where, as you know, we tend to have less freedom than in other countries. Which okay it can be both good and bad. It certainly allowed me to see a lot of different math, which was good. But then I reached a point where I realised that even if I was very much interested in geometric problems and certain type of problems specifically, there was something about GR and black holes because of course, I live in the same world as anybody. And as we said before, GR is much more part of pop culture. And of course this has an effect on me too. And at some point I realised, okay, maybe I like it. This is kind of the base, you know, the bottom line, it's that I like it, but it's also something that it is probably easier to maintain interest in because of course, studying math is hard, right? Sometimes you're like, it's the equivalent of walking into a dark room. Um, and at least for me, I thought it was a good move to pursue an interest in something that, um, I can much more easily renew my interest for. Because in some sense, it's at least for me much easier to renew my excitement for black holes than it is to renew my excitement for the homology groups of positively curved manifolds.

  Simone: And also, how cool is it that you can go and tell people "I work on black holes", right?

  Alex: Yes, yes. I mean, let's not hide that. There is, at least for me, an influence in my choices given by the fact that I like to be the person that does this, other than I like to just do it. And of course it is. It is cool. I mean, I like to think of myself as a person that has this interest but at a certain point either you're lucky and the professors and the people around you in the university where you study actually share those interests, or you have to create a path for yourself. And this is what I did. I did it a specific time. I don't think you can do it at all times, like when you're much younger, of course, it's much harder. It certainly would have been too hard for me. But it also was somehow natural because I reached a point where I had a I felt like I had a decent understanding of all the tools needed to pursue this, this interest. And I realised, who are the people that could help me get into this field? And even if they were not in my home university, I looked for ways to get in touch with these people. And I don't know, for whatever reasons, the planets aligned again.

  Simone: Like in the eclipse.

  Alex: Like an eclipse.

  Simone: The orange, the lemon...

  Alex: And somehow I've ended up doing something that that I like very much, which was not the only option here, I want to say again, but it's the one on which I bet.

  Simone: So you moved to Switzerland first and then here.

  Alex: And then I came here, yes. Because there was some previous student of a professor that I had that was somehow between what I was doing before, which was a geometric analysis, and GR, which is what I'm doing right now. And this person was a bridge. But most importantly, moving away from my hometown, which was actually not my hometown, but my home university. It allowed me to meet other people that also made a bridge. And, you know, most of the times, unless you meet the people, you don't even realise that certain areas of research exist. This was my case. I didn't realise that it was possible to study black holes using this set of skills that a mathematician has, which is the set of skills that I have at my disposal. Until I went to Zurich and I met a person that was doing this. So I think overall, other than believing in yourself and kind of for once not doing exactly what you're supposed to do, more than that, you also need to be a bit lucky right, to meet the right person that tells you yes, you can do this. And then you realise, oh, I could do this, and then you do this. Um, so it's a series of factors and. Okay, luck is always a factor.

  Simone: So this is the now. This is why you do it. So what is the future ahead of you that you would like to see? What is a theorem that you would like to see proven well by yourself maybe, or others of course, in the next ten, 20, 30 years.

  Alex: Among all the things that I initially looked at when I started exploring this mathematical general relativity field, there is something specifically that really caught my attention, and that's something called strong cosmic censorship conjecture.

  Simone: Which is a very metal name.

  Alex: I mean, it's very cool. It's very easy to sell. And I mean, they sold it to me effectively. Yes. I mean, reasonably, this is something that is probably gonna take on a hundred years, maybe they won't even be enough because also my field is relatively young compared to other fields, but in essence, what I like about it, and I think this is like further proof of how much more easily I can communicate what I do if compared to other people. Basically, this conjecture says that the theory of general relativity is deterministic, and which you would say, oh, why? Why shouldn't it be deterministic? Well, it turns out that this is not something to take for granted because, okay, in very layman terms, there are some black hole solutions to Einstein equations that present something that's called Cauchy horizon, which I won't go into the the reason why it's called like that, but in essence, it's like a geometric locus where the theory at times appears not to be capable of predicting uniquely what happens to an observer or a person or an object passing this horizon. And this conjecture that I mentioned, the strong cosmic censorship conjecture, in essence, it says that although you can write down solutions of the Einstein's equations which display this behaviour, they are not generic in the sense that if you consider the initial data that give rise to the solution and you perturb them, then once you apply this perturbation, you no longer end up with a spacetime or a solution that that has this feature.

  Simone: So somehow the solutions that don't work like we expect, we expect physical theories to work deterministically, right? Yes, we put in the initial data and we know and we predict what's going to happen. Yes. So the solutions that don't behave like this are just an exception. So they just happen. And then but if you move a bit away from that then it works just fine.

  Alex: Yeah. Like trying to balance a picture upside down. Right. There is an equilibrium point. Ideally you could balance it and put it up on the wall. But you know the moment you perturb it a little bit, it's just going to go down and eventually it's going to stabilise along the other reasonable equilibrium point that you can use to put a picture up on the wall. And this is the same idea. And also I feel like this is the best representation of what it means to do general relativity. As a mathematician with the skills of a mathematician, I don't have the ability to do what the physicists do. But you know, with our set of skills, we can investigate questions such as this one, which they are really about the consistency of the theory. They are really giving meaning to the words determinism of general relativity. And they're doing it in a very reliable way, because I'm really, you know, taking the equations and rigorously trying to prove that the theory is deterministic.

  Simone: And actually predicts our reality as we expect it.

  Alex: Yes, yes. Um, so this would be very nice. Um, yeah. I think whoever puts the last brick on this collective effort is certainly going to be a famous man. Uh, but I don't know how long it will take. I would be happy if I gave my contribution. But not necessarily for the glory of it, which I confess for sure. And for the first years of my journey in math it was part of the motivation. But then I think, certainly once you entered the PhD, what I really want for myself is to have given some contribution and at the same time have a certain set of skills and of knowledge and of understanding that allows me to sit with my peers and give something to them in conversation, right? What I envy is not, say, the list of awards that professors have or may have. I mean, in part, we all envy that, right? But what I envy on a daily basis is the fact that they, they can navigate their department and talk to other people, and they have conversations where there's an actual like exchange between them. They don't just have to sit there and take as much as they can, which is what you just have to do at the beginning, right. Somehow it's also a necessary step, I mean, to have these conversations, right? Because nobody will prove the strong cosmic censorship conjecture or any other big conjecture on their own.

  Simone: Math is necessarily a collective effort that we all have to do together.

  Alex: And then we need to be able to interact and communicate. Yes. I feel like this is in the end, what once you're when you're inside, when you're doing your PhD or doing some research, or maybe if you're just a master's student, you realise that really what gives you the feeling of having owned your spot inside the community? It's really just this, you know, sitting down, having a coffee after lunch and being able to contribute to a conversation. It's not, I don't know, the list of pieces of paper that you've earned. And really it comes from the human interaction, which is something that I only discovered and touched with firsthand once I started doing my PhD. And maybe, maybe this is something that professors and researchers and everybody in charge should try to pass on to students while they're studying, maybe to boost motivation, because I think people think of math as a very lonely career. Unless maybe you're at the top of the top.

  Simone: Yes. Because we have these images, right? We see movies or in or in on also the stories we hear. I mean, of Wiles proving Fermat's Last Theorem alone for eight years.

  Alex: Yeah, and okay, this can happen. But actually the majority of the work is not like this, right? I mean, it's more about really sitting down, having a coffee, discussing what's going on. Slowly. Brick by brick is anybody would do in their generic work like office work. You have your colleagues, you work together on something and you collaborate. If anything, we're luckier because we are actually working for ourselves. And so yeah, I think that is the best part. That is what I also what reasonably I think keeps us motivated because one does not just blindly pursue this career in hope of one day maybe receiving an incredible piece of recognition. It really is a daily life to daily department life that fuels you and it allows you to move forward.
  
  Simone: Yeah. Also because right, you cannot force maths to work.
  
  Alex: No, sometimes it doesn't work.
  
  Simone: Right. Uh, and so sometimes you, you go and go and go at the same problem for, for infinite time and then you still don't get results because that's how it goes. So yeah, that's true.
  
  Alex: I mean, motivation can hardly come from the success because you cannot control it. You cannot predict it. You have to find a way to make the process, if not enjoyable, because maybe this is aiming too high, but sustainable. So a good balance of positive emotions and negative emotions otherwise I mean you're not going to go anywhere because I feel like it's just against the human nature. And it's weird that the narrative that that people receive from the outside is of people somehow going against the human nature, which is probably the reason why those that are depicted, maybe movies are this very, um, um, this genius people or this very, um, uncommon people in some sense. And I feel like this is what makes them uncommon in the eye of the, of whoever is watching. But really, this is far from the reality of the department, or at least of the departments that I've inhabited and navigated.
  
  Simone: Well, thank you very much for this lovely chat.
  
  Alex: See you soon around the corridor.