Coffee Meeting Log

2026-02-13

Failure of Scale Separation in Physics

Wei-Hsiang Shao (Special Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

The success of modern physics relies heavily on the principle of scale separation, which allows complex systems to be described by simplified effective theories that are largely insensitive to microscopic details. In this talk, I will review this Wilsonian framework and illustrate its usefulness across a wide range of physical contexts. I will then discuss situations in which this separation of scales breaks down, focusing on examples motivated by quantum gravity as well as by certain physical settings in our universe. These examples show how high-energy physics can have unexpectedly strong effects at large scales.

2026-02-06

On how to detect selection in a set of genomes

Aina Colomer i Vilaplana (Postdoctoral Researcher, Mathematical Genomics RIKEN ECL Research Unit, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

How do organisms adapt and evolve? Population genetics provides us with the theoretical framework to answer this question. A chance mutation can give rise to an advantageous mutation in one individual, but how does this mutation then spread in a population? How quickly does a mutation increase in frequency, what role does chance play in this process, and how does it impact the diversity of our genomes? This talk will introduce how natural selection appears in our genomes and how we can detect it in genetic data. I will introduce the main concepts behind some of the classical selection statistics developed early in the field. I will then show how Ancestral Recombination Graphs have revolutionized our ability to infer selection in recent years. One of the ongoing debates around selection is the extent to which genomes have been (or are currently being) shaped by this force, and what this means for how organisms adapt.

2026-01-30

Chaos bound and thermalization

Osamu Fukushima (Special Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

Chaos refers to complex dynamical behavior, where the long-time behavior becomes effectively unpredictable even though the underlying equations of motion are deterministic. On the other hand, thermalization concerns the relaxation of observables toward equilibrium, which is a pivotal dynamical phenomenon in many-body systems. Chaos and thermalization share some common aspects, and their relation has been extensively discussed. In this talk, I will introduce how the eigenstate thermalization hypothesis, which is a central criterion for thermalization, is related to a universal bound on chaos in quantum systems.

2026-01-23

Evolution of Evolution?: Modifier Theory as a First Step

Kenji Okubo (Special Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

Evolution successfully explains how populations change across generations, but it usually treats key parameters such as mutation and recombination rates as fixed. In this talk, I introduce modifier theory as a first step toward a theory of the evolution of evolution, where these parameters themselves can evolve. Using simple finite-population toy models, I show that mutation-rate modifiers can be favored not by higher fitness, but by producing beneficial mutations earlier. This leads to history-dependent evolutionary outcomes driven by timing rather than averages. I conclude by discussing implications for mutator evolution in bacteria, cancer dynamics, and the possibility of inferring past environments from present-day evolutionary parameters.

2026-01-16

Recent progress in understanding quantum black holes

Masamichi Miyaji (Senior Research Scientist, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

Black holes are ubiquitous in our universe, yet their microscopic structure is still far from being understood. In particular, they pose serious theoretical puzzles, such as the black hole information paradox and the firewall problem. Recent progress based on holography has shed new light on these issues, as well as on the low-temperature quantum dynamics of black holes, connecting black holes with strongly chaotic quantum systems. In this coffee meeting, I will explain the key ingredients needed to understand these recent and fascinating theoretical developments.

2026-01-09

Many-faceted KP hierarchy

Zhe Wang (Research Scientist, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

The Kadomtsev-Petviashivili (KP) hierarchy is an integrable hierarchy of partial differential equations generalizing the well-known KdV hierarchy describing the dynamics of shallow water. The KP theory is extremely rich, and it relates different fields of mathematics. In this talk, we will focus on Sato’s theory for KP hierarchy and interpret it using analytic/geometric/algebraic languages.

2025-12-19

Brunn-Minkowski inequality and the Borell-Brascamp-Lieb inequality

Takashi Satomi (Special Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

The Brunn-Minkowski inequality (or Brunn-Minkowski theorem) is one of the fundamental geometric inequalities in Euclidean space. This inequality states that the volume of the sum of two sets (called the Minkowski sum) can be bounded from below in terms of the volumes of the two original sets. There are several proofs of this inequality, and one approach is to establish a functional extension known as the Borell–Brascamp–Lieb inequality. In this talk, we will review these inequalities and briefly indicate how the Borell–Brascamp–Lieb inequality may be proved by mathematical induction.

2025-12-12

Aspects of Quantum Resonance Revisited

Okuto Morikawa (Special Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

Quantum resonances appear in many areas of physics, yet their definitions—through scattering peaks, S-matrix poles, or non-Hermitian eigenvalues—often seem unrelated. In this talk, I revisit these aspects from a unified viewpoint: resonances as pseudo-bound states defined by analytic continuation into the complex plane. This perspective clarifies how methods such as complex scaling, Gamow states, and semiclassical tunneling fit together. I will briefly show how exact WKB naturally captures these structures, without assuming any background in resurgence theory.

2025-12-05

Graphs, Linear Algebra, and the "Champagne Glass Problem"

Junnosuke Koizumi (Special Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

When people clink glasses in a toast, it is surprisingly hard for four or more glasses to all touch each other at the same time. What is the largest number of people whose glasses can be arranged so that every pair of glasses is in contact simultaneously? If we model the glasses as infinite cylinders, this turns into an open problem in mathematics. In this talk I will present an approach to this problem that uses tools from graph theory and linear algebra.

2025-11-28

Distances and Durations: Cosmology 101

Amaury Micheli (Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

Cosmology aims to describe the evolution of the Universe and its contents. The light we receive from distant objects serves as our primary observable for understanding this evolution. I will outline how this light enables us to infer the distance of its source and the time of its emission. These considerations swiftly lead to a discussion of the expansion of space, which is the main driver of the Universe's evolution and lies at the heart of many active questions in cosmology, such as the Hubble tension and the nature of dark energy.

2025-11-21

Lower bound on ground state energy in quantum chemistry

Seishiro Ono (Visiting Scientist, Division of Global Collaborations and Research Talent Development, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS) / Assistant Professor, Division of Condensed Matter Theory, The Institute for Solid State Physics (ISSP), The University of Tokyo)

Thanks to the variational principle, we can obtain a meaningful upper bound on the ground-state energy if we have a good variational state. Here, I will review how to obtain a lower bound on the ground-state energy, in contrast to the variational approach. In fact, this problem can be formulated as a well-known optimization problem.

2025-11-14

Symmetries of gravity

Puttarak Jai-akson (Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

Symmetry lies at the foundation of modern physics, and gravity offers a unique setting where symmetry and geometry deeply intertwine. This talk will trace how gravitational symmetries evolve: from exact isometries of spacetime to asymptotic symmetries at infinity, and ultimately to the broad class of corner symmetries defined on finite surfaces.

2025-11-07

Hilbert space fragmentation: breakdown of quantum thermalization

Yuta Sekino (Postdoctoral Researcher, Quantum Mathematical Science Team, Division of Applied Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS) / Postdoctoral Researcher, Nonequilibrium Quantum Statistical Mechanics RIKEN Hakubi Research Team, RIKEN Cluster for Pioneering Research (CPR))

Thermalization is a ubiquitous phenomenon in nature, ranging from the early universe in cosmology to electron systems in condensed matter physics. However, its understanding based on microscopic dynamics by quantum mechanics is still not so clear. In this talk, I will introduce the microscopic mechanism called Hilbert space fragmentation, which leads to dynamics violating thermalization.

2025-10-31

Graphs and quantum groups

Kan Kitamura (Special Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

I will introduce a seemingly classical problem in graph theory that I encountered through a motivation from (relatively modern) quantum group theory. The problem is to find examples of certain strongly regular graphs: collections of nodes and edges having exceptional combinatorial properties, which will be illustrated in the talk.

2025-10-24

Social hierarchy formation in animal groups

Isaac Planas Sitja (Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

Dominance hierarchies are common in group-living animals, emerging from repeated social interactions that establish social ranks and often involve escalated aggressive contests that can lead to fatal outcomes. Yet, the mechanisms underlying this process remain challenging to model, as decisions to fight or retreat depend on complex interactions among social information, environmental context, and resource pay-offs. I will discuss key biological and theoretical challenges in understanding the formation of social dominance.

2025-10-17

Birational classification of algebraic varieties

Chi-Kang Chang (Special Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

One of the goal of algebraic geometry is to classify algebraic varieties (that is, zero locus of polynomial function(s)) up to certain equivalence relations. For example, two algebraic varieties are called birationally equivalent if they have open dense subsets that share the same structure as algebraic subvarieties (intutiatively, if they have the same structure after removing "small" closed subsets). In this talk, we will give an introduction about the birational classification theory of algebraic varieties.

2025-10-10

Gravitational redshift and wave packets in QFT

Sotaro Sugishita (Research Scientist, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS) / Assistant Professor, Department of Mathematics, Kyoto University)

Light emitted near a black hole appears redshifted to a distant observer; this is the gravitational redshift. I will explain how to describe it using wave packets in quantum field theory.

2025-10-03

Gelfand duality

Vladimir Sosnilo (Research Scientist, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

In a broad sense, geometry studies those mathematical objects that require geometric intuition, while algebra studies abstract structures that are common in all fields of mathematics. In various instances, it turns out that there is a way to describe geometric objects entirely using algebraic objects. As one such example, we discuss Gelfand duality between compact hausdorf topological spaces and commutative C*-algebras.

2025-09-26

Representation learning for astronomy

Keiya Hirashima (Special Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

Stellar mass is a fundamental driver of stellar evolution, yet estimating it in star-forming regions is difficult due to heavy obscuration and strong inhomogeneity, which undermines simple dynamical models. Supervised ML is promising but limited by the cost of generating large, high-quality labeled datasets from high-resolution MHD simulations. In this talk, I will present a data-efficient alternative: pretraining a Vision Transformer with DINOv2 on one million synthetic fractal images, then transferring the frozen encoder to limited MHD maps. Synthetic pretraining improves frozen-feature stellar-mass predictions, slightly outperforming a supervised model trained on the same limited simulations. Principal component analysis of the embeddings reveals semantically meaningful structure (e.g., dense cores and inflows), enabling unsupervised segmentation without labels or lightweight fine-tuning.

2025-09-19

Scale transformations in physics and beyond

Álvaro Pastor Gutiérrez (Special Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

The evolution of systems across different scales is a common theme in many areas of research, from fundamental physics to biology and machine learning. Understanding how macroscopic behaviour arises from microscopic dynamics is essential both for explaining emergent phenomena and for reconstructing a fundamental picture of nature. In this talk, I will introduce the renormalisation group in a non-technical way, highlighting its interpretation as a framework for performing scale transformations applicable to a broad range of contexts beyond physics. Beginning with the block-spinning idea, I will show how scale transformations lead to phase transitions and universal phenomena, and then sketch the modern Wilsonian perspective. Finally, I will show how this framework underlies our most fundamental picture of nature and offers a toolkit for addressing the open problems and puzzles in the Standard Model of particle physics.