Black holes exhibit thermodynamic properties and provide an important window into the quantum aspects of gravity. In this context, nonlinear electrodynamics (NLED) offers a useful framework for constructing and analyzing charged black-hole solutions beyond Maxwell theory. Requiring causality - namely, excluding superluminal signal propagation - imposes nontrivial constraints on the allowed form of the NLED Lagrangian.

In this talk, we focus on two quantities: the charge-to-mass ratio and the entropy density (entropy-to-mass squared ratio). The charge-to-mass ratio is expected to obey a monotonic behavior consistent with the Weak Gravity Conjecture, while the entropy density is also anticipated to be monotonic, reflecting the expectation that higher-energy effective theories contain more degrees of freedom. We show that these monotonic behaviors follow directly from the causality constraints on the NLED sector.

The two fundamental questions—“What is quantum?” and “What is spacetime?”—are deeply intertwined. On one hand, the formulation and interpretation of quantum theory depend both implicitly and explicitly on our conceptions of time and space. On the other hand, we believe that fully taking into account the quantum character of nature will force us to revise our understanding of spacetime. These two conceptual problems lie at the heart of the unsolved challenge of how to quantize classical spacetime, and conversely, how (semi-) classical descriptions of spacetime emerge from quantum theory. Furthermore, if the entire matter-spacetime system is a kind of quantum many-body system, thermodynamics—which governs its statistical behaviors—should play a key role in elucidating these problems.

This workshop will discuss the question “How can quantum theory and spacetime be understood in a consistent manner?” from a fundamental and broad perspective. To tackle this challenge, we gather researchers in foundations of quantum theory, quantum gravity, and related fields from around the world, providing a "space and time" to share various ideas with open minds and engage in lively discussions. By exploring new concepts and principles, we hope to uncover directions to guide quantum theory over the next 100 years.

This workshop covers…

Foundations of quantum theory
Quantum gravity and emergence of spacetime
Formulation of semi-classical gravity
Experimental aspects of fundamental properties in nature and quantum gravity
Foundations of quantum many-body systems and thermodynamics
Other related topics are welcome.

We welcome short talk presentations and poster presentations.

This event is a workshop jointly organized by KEK Theory Center and RIKEN iTHEMS.

Living systems operate far from equilibrium under continuous time-varying forcing across multiple temporal and spatial scales. From neural and cardiovascular rhythms to microcirculatory dynamics and circadian cycles, physiological processes are inherently nonautonomous. Classical stability concepts based on autonomous attractors and stationary limit cycles are therefore insufficient to explain how such systems remain robust yet adaptable.
In this talk, I will introduce chronotaxicity as a framework for nonautonomous oscillatory systems possessing time-dependent point attractors and contraction regions. Chronotaxic systems maintain stability under continuous forcing, providing a rigorous theoretical description of dynamic robustness.
To illustrate the generality of this concept, I will show how chronotaxicity can be observed in a controlled physical experiment. I will then present a new order parameter based on angular velocity for quantifying phase dynamics in numerical simulations of coupled nonautonomous oscillators, along with the methods collected in the Multiscale Oscillatory Dynamics Analysis (MODA) toolbox for analysing time-dependent oscillatory behaviour.
This approach provides a unified perspective on dynamic stability in complex systems, highlighting how living systems remain robust yet adaptable and suggesting quantitative signatures of dysfunction in health and disease. While the focus is on physiological and numerical models, it is broadly applicable to complex nonautonomous systems, underscoring its generality as a dynamical principle.

Quantum invariants are invariants of knots and 3-manifolds which relate deeply to mathematical physics and representation theory.
In recent years, it has become increasingly clear that it is also deeply related to number theory, that is, quantum modularity for quantum invariants.
This topic is interesting from a topological viewpoint since this is a refinement of establishing asymptotic expansions of quantum invariants, which is an important problem in quantum topology,
and is interesting from a number-theores[tic viewpoint since this gives examples of quantum modular forms, which are mysterious objects in number theory.

I obtained two linked results on topology and number theory:
Establishing explicit asymptotic expansions of quantum invariants for negative definite plumbed 3-manifolds and establishing quantum modularity of false theta functions in full generality.

In this talk, I will outline previous progress on quantum modularity for quantum invariants and my results.

We investigate the critical behavior of a family of Z2-symmetric scalar field theories on the Bethe lattice (the tree limit of regular hyperbolic tessellations) using both the non-perturbative Functional Renormalization Group and perturbation theory. Due to the hyperbolic nature of Bethe lattices, the Laplacian lacks a zero mode and exhibits a spectral gap. We demonstrate that closing the spectral gap via a modified Laplacian leads to novel critical behavior governed by interacting fixed points. This stands in contrast to the nearest-neighbor Ising model, which exhibits a phase transition with mean-field critical exponents. We further comment on the possible reasons for such a deviation.

PROGRAM:

9h45 - 10h15 Registration & Coffee

10h15 - 10h20 Opening Remarks - Satoshi Iso (RIKEN), Director of iTHEMS

10h20 - 11h20 SESSION 1 - Chair: Tetsuo Hatsuda (RIKEN)

Yoshihiko Susuki (Kyoto University): Koopman resolvents in dynamical systems and control

11h20 -11h40 Free Discussions

11h40 - 13h00 Lunch Break & Discussions

13h00-14h00 SESSION 2 - Chair: Narumi Fujii (Institute of Science Tokyo)

Alexandre Mauroy (University of Namur, Belgium): Analytic EDMD method for spectral analysis of fixed point dynamics

14h00 - 14h30 Coffee Break & Discussions

14h30 - 15h30 SESSION 3 - Chair: Tetsuo Hatsuda (RIKEN)

Hiroya Nakao (Institute of Science Tokyo): Koopman operator analysis of coupled oscillator systems

15h30 - 16h00 Coffee Break & Discussions

16h00 - 17h00 SESSION 4 - Chair: Riccardo Muolo (RIKEN)

Yuzuru Kato (Future University Hakodate): Analysis of quantum nonlinear oscillators on the basis of Koopman operator theory

17h00 - 17h05 Closing Remarks - Tetsuo Hatsuda, Chair of the Workshop

17h05 - 18h00 Free Discussions

Understanding how complex organs reliably form during development remains a key question in biology. In this talk, I discuss how gene regulatory networks may generate skeletal patterns in the vertebrate limb, using Sox9 expression as a proxy, as it marks the earliest stages of cartilage formation. To address this, I developed new computational tools for reconstructing spatiotemporal gene expression and built models ranging from machine learning approaches to mechanistic frameworks. These analyses reveal that limb patterning cannot be explained by a single universal mechanism. Instead, different regions of the limb appear to use distinct regulatory strategies, uncovering an unexpected qualitative modularity in skeletal development. Together, these findings lead to a new hypothesis in which other systems, such as the vasculature may actively shape skeletal spacing in specific limb regions.

Correlation functions of local operators in Quantum Field Theory (QFT) on hyperbolic space can be fully characterized by the set of QFT data. These are the scaling dimensions of boundary operators, the boundary Operator Product Expansion (OPE) coefficients and the Boundary Operator Expansion (BOE) coefficients that characterize how each bulk operator can be expanded in terms of boundary operators. For simplicity, we focus on two dimensional QFTs and derive a universal set of first order Ordinary Differential Equations (ODEs) that encode the variation of the QFT data under an infinitesimal change of a bulk relevant coupling. In principle, our ODEs can be used to follow a renormalization group flow starting from a solvable QFT into a strongly coupled phase and to the flat space limit.

Gravitational-wave detections by the LIGO-Virgo-KAGRA network have turned compact-object mergers into precision probes of strong gravity. The post-merger ringdown is particularly incisive: it is governed by quasinormal modes (QNMs), the damped oscillations that encode the remnant's structure and provide a fingerprint of the final object. While current detectors constrain the dominant mode, next-generation observatories will resolve multiple modes with high precision, placing stringent demands on the accuracy of theoretical predictions. Computing QNMs for rotating black holes is, however, a non-trivial task, as it requires solving highly coupled, complex-valued perturbation equations where standard methods struggle. In this talk, I present SpectralPINN, a hybrid solver combining Pseudo-spectral methods with Physics-Informed Neural Networks, validated at 10⁻⁵ relative accuracy. I will present results for Kerr and Kerr-Newman black holes, demonstrating the method's robustness and accuracy across parameter space, and discuss its potential for extension to more exotic compact objects relevant to next-generation detector science.

Recent advances in X-ray spectroscopic observation have enabled researchers to reveal distinct clumpy structures in the super-Eddington outflows from the supermassive black hole in PDS 456 (XRISM Collaboration 2025), initiating detailed investigation of fine-scale structures in accretion-driven outflows. In this talk, I will introduce our high-resolution, two-dimensional radiation-hydrodynamics simulations with time-varying and anisotropic initial and boundary conditions that reproduce clumpy outflows from super-Eddington accretion flows. The resulting clumpy outflows extend across a wide range of radial distances and polar angles, exhibiting typical properties such as a size of ~10 rg (where rg is the gravitational radius), a velocity of ~0.05–0.2 c (where c is the speed of light), and about five clumps along the line of sight. Although the velocities are slightly smaller, these characteristics reasonably resemble those obtained from the XRISM observation. The gas density of the clumps is on the order of 10^-13–10^-12 g cm^-3, and their optical depth for electron scattering is approximately 1–10. The clumpy winds accelerated by radiation force are considered to originate from the region within <300 rg.