Coffee Meeting Log

2025-03-07

Steady-State Size Distribution for a Collision Cascade

Misako Tatsuuma (Research Scientist, iTHEMS)

A collision cascade describes a self-similar process where large bodies fragment into smaller ones through successive collisions, maintaining a constant mass flux over time. This phenomenon is observed in various contexts, from space debris evolution to asteroid collisions. In this talk, I will derive the steady-state mass distribution for a collision cascade and introduce its application to the observed grain size distributions in planet-forming disks.

2025-02-28

Space-time as syntax of Particle Physics

Yuto Moriwaki (Special Postdoctoral Researcher, iTHEMS)

In natural language, syntax is the vocabulary and grammar, and semantics is the meaning (model) of a sentence constructed according to syntax. Lawever duality in mathematics states that from all semantics (possible models), the category of syntax (grammar) can be recovered. In particle physics, we interpret syntax as the conditions that a physical theory must satisfy (grammar of the theory), and semantics as specific models (gauge theory, Standard Model, etc.). Then, from all the models of quantum field theories (semantics of QFT), we argue that the category of spacetime is expected to be recovered. In other words, spacetime seems to be the grammar of quantum field theory.

2025-02-21

Renormalization: Awareness of Ignorance

Masazumi Honda (Senior Research Scientist, iTHEMS)

Renormalization is one of the most complicated concepts in learning quantum field theory despite realistic field theories cannot be predictable without renormalization. In this talk, I am going to introduce the concept of renormalization into broad audience by using a simple example in quantum mechanics which is technically an eigenvalue problem of ordinary differential equation. While I will mainly explain a "physicist" way of doing renormalization, I will also briefly mention more mathematical understanding from perspective of self adjoint extension of Hamiltonian

2025-02-14

Newton polytopes and topology of hypersurfaces

Yuto Yamamoto (Special Postdoctoral Researcher, iTHEMS)

The set of points where a polynomial equals zero, called a hypersurface, is a fundamental object in algebraic geometry. Important information about its shape is encoded in the Newton polytope, which is a convex polytope associated with the polynomial in a specific way. In this talk, I will explain this connection in an intuitive way using ideas from tropical geometry.

2025-02-07

On the Buchdahl’s limit and its relation to black hole mimickers

Che-Yu Chen (Special Postdoctoral Researcher, iTHEMS)

Are black holes really black holes? According to Einstein's General Relativity with a few assumptions, static fluid spheres in hydrostatic equilibrium can only reach a certain value of compactness, dubbed Buchdahl limit. In this talk, I'll discuss how this bound is obtained, and how one can evade it by relaxing some of its underlying assumptions, constructing horizonless compact objects as black hole mimickers.

2025-01-31

A canonical size of sets in mathematics, biology, and statistics -- in physics too?

Ryosuke Iritani (Senior Research Scientist, iTHEMS)

I will talk about "magnitude" (the effective number of a collection of objects) and how we interpret it in terms of mathematical, biological, and statistical viewpoints by drawing a few examples. As the magnitude has strongly to do with entropy maximization, there could be something interesting with physics.

2025-01-24

The Jeans Instability: why it happens, why it doesn't happen, and why it happens (?)

Derek Beattie Inman (Research Scientist, iTHEMS)

2025-01-17

Learning how to count: from infectious virions to tangled brain proteins

Catherine Beauchemin (Deputy Program Director, iTHEMS / Professor, Department of Physics, Toronto Metropolitan University, Canada)

Being able to count the number of disease-causing agents over the course of an infection is essential for developing mathematical models to correctly describe, predict, and control diseases. In a virus infection, the agents are infectious virions. In neurodegenerative diseases like Alzheimer's, Parkinson's, and prion diseases, the suspected (or confirmed) agents are misfolded proteins that form aggregates, disrupting normal processes, and causing cell death. I will explain why it is difficult to count these agents and show some nice mathematical solutions to some of the problems. I will also describe ongoing challenges that perhaps you would like to help me tackle.

2025-01-10

What is Chaos?

Gabriele Di Ubaldo (Postdoctoral Researcher, iTHEMS)

Chaos is an intuitive concept yet notoriously hard to define and study by nature. I will introduce different notions of chaos that are applicable to classical and quantum systems and discuss their relations, giving examples of very different chaotic systems.

2024-12-20

Re-summing quasi-normal modes - where divergence meets our universe -

Ryo Namba (Senior Research Scientist, iTHEMS)

Series expansions provide a powerful control over problems in physics. Yet such expansions are often divergent, and it is a non-trivial task to interpret their consequences. This is, however, not something that is depressing, but rather something we should be excited about. Even though an expansion is a perturbative one, its infinite series inherits information about fully non-perturbative effects. In this short coffee meeting talk, I present one such example: quasi-normal modes of peturbations around black hole background spacetime. This is essentially to look for eigenvalues of Regge-Wheeler-Zerilli-type equations by re-summing a conventional WKB series solution, which is divergent in general, and applying proper boundary conditions.

2024-12-06

Recipe for soup of quarks and gluons

Yuuka Kanakubo (Postdoctoral Researcher, iTHEMS)

Can you imagine what the hottest soup on the earth is like? Is it viscous as honey or runny as water? How can we make such a hot soup? The hottest soup of quarks and gluons, known as quark-gluon plasma, has been studied through relativistic heavy-ion collision experiments at the Relativistic Heavy Ion Collider (RHIC) at BNL and the Large Hadron Collider (LHC) at CERN. In this talk, I will explain what is known and unknown about quark-gluon plasma, and how we study the properties with numerical simulations of relativistic heavy-ion collisions.

2024-11-29

What is “equilibrium”?

Kyosuke Adachi (Research Scientist, iTHEMS / Research Scientist, Nonequilibrium Physics of Living Matter RIKEN Hakubi Research Team, RIKEN Center for Biosystems Dynamics Research (BDR))

In interdisciplinary environments like iTHEMS, effective communication can be challenging due to differences in terminologies. In this 15-minute online talk, I will explore the concept of “equilibrium,” a term whose meaning varies across disciplines, ranging from mathematics to thermodynamics. By examining how “equilibrium” is understood in different research fields, I aim to bridge gaps in communication and foster a deeper interdisciplinary dialogue.

2024-11-22

Quark deconfinement in superdense matter: baryons or quarks? 

Yuki Fujimoto (Postdoctoral Researcher, iTHEMS)

I will explain the qualitative picture regarding the quark deconfinement in superdense matter. Naively, we expect that superdense matter, found at neutron star core densities and beyond, consists of liberated quarks rather than of confined baryons because of the asymptotic freedom. However, QCD at large Nc implies a different interesting possibility: quark confinement persists, so the system is composed of baryons, while the thermodynamics is still governed by quarks.

2024-11-15

Kinetic plasma turbulence and geometry

Motoki Nakata (Visiting Scientist, iTHEMS / Associate Professor, Faculty of Arts and Sciences, Komazawa University)

Plasmas in the laboratory and in space are often collisionless, but still exhibit collective fluid-like dynamics. Most plasmas are also turbulent, and microscopic fluctuations, vortices, and even large-scale flows develop due to nonlinear interactions. Kinetic description is an effective approach for such collisionless or weakly collisional turbulent plasmas, and the basic ideas will be introduced in this talk. In particular, I will present on the interplay between turbulent dynamics and magnetic geometry such as torus that confine the plasma. This is also a question of how turbulence and flows can be activated or deactivated by geometry.

2024-11-08

A subjective view on what is a quantum effect

Amaury Micheli (Postdoctoral Researcher, iTHEMS)

In this talk I will first sketch the difference in formalism and predictions between a classical and a quantum model of the same system.The differences will be illustrated by taking a spin-1/2 system as an example. Based on these I will give my point of view on what is a genuine quantum effect: an effect that cannot be predicted in whole class of classical theories. The landmark example of such effect is a Bell inequalities violation. Finally, I will shortly discuss the implications of the existence of such genuine quantum effect at a practical and interpretational level.

2024-11-01

Bell's inequality

Yantao Wu (Research Scientist, iTHEMS)

I will explain what Bell's inequality is without assuming you know quantum mechanics.

2024-10-25

Arrangement of cells: how are they organized in your body (or not)?

Akihisa Yamamoto (Research Scientist, iTHEMS)

Living organisms are composed of cells that adhere to each other and extracellular matrices (ECM), a complex network of proteins and biologically active molecules. Cells dynamically renew and replace each other through division and death while preserving the structure of tissues. This stability, known as homeostasis, is vital for proper tissue functions. However, diseases such as cancer can disrupt the typical arrangement of cells in tissues. Although there is some empirical understanding of the relationship between tissue structure and disease state, where skilled doctors can diagnose by examining tissue images, it is still largely unknown why tissue structure gets altered according to disease progression and what aspects of the physiological and mechanical changes of cells and tissues are responsible for it. In my talk, I will present examples of how disease affects tissue structure and demonstrate how we can characterize these changes.

2024-10-18

Mathematical aspects of the entropic uncertainty relation

Takashi Satomi (Special Postdoctoral Researcher, iTHEMS)

The uncertainty relation is originally a fundamental concept in quantum mechanics. This relation limits the precision with which two physical quantities (such as position and momentum) can be simultaneously known. In mathematical aspects, the uncertainty relation implies that either the original function or its Fourier transform has a spread graph for any function. In this talk, we see a variant of the uncertainty relations by using the entropy, which is called the entropic uncertainty relation. In addition, we observe that this is stronger than the variance version (and hence the standard deviation version) of the uncertainty relation.

2024-10-11

On mixed-state topological phases

Osamu Fukushima (Special Postdoctoral Researcher, iTHEMS)

Topological phases are quantum phases of matter characterized by the robustness against local perturbations. While the understanding of topological phases in pure states has reached a mature stage over the past few decades, their mixed-state counterparts remain less explored. In this talk, I will review the concept of symmetries for mixed states, which is essential for characterizing mixed-state topological phases, and introduce an intrinsically mixed-state topological phase that does not belong to the same equivalence class as any pure states.

2024-09-27

Quadratic residues and domino tilings

Junnosuke Koizumi (Special Postdoctoral Researcher, iTHEMS)

Quadratic residues are a classical concept in number theory, with a history tracing back to the times of Fermat and Euler. On the other hand, the number of ways to tile a rectangular grid with dominoes was given an explicit formula by Kasteleyn and Temperley-Fisher in the 1960s, in the context of research on the dimer model. In this talk, I will introduce recent research by myself, Y. Kamio, and T. Nakazawa, which connects these two concepts.