Japan-UK Workshop on Quantum Gravity

22 Sept 2025, 09:00 → 26 Sept 2025, 17:00 Asia/Tokyo

8F Lecture theatre (RIKEN Kobe Integrated Innovation Building (理化学研究所の神戸キャンパスの融合連携イノベーション推進棟))

Daniele Dorigoni, Tobias Hansen, Masazumi Honda, Pratik Nandy (RIKEN iTHEMS)

The universe at extremely early times is expected to be described by some theory of quantum gravity, although we still do not know precisely what quantum gravity actually is.

In modern approaches to quantum gravity, the path integral point of view provides a fundamental framework towards answering this pressing question. However, an evaluation or even just a precise definition of the path-integral for a full-fledged quantum gravity is one of the most important open problems in modern theoretical physics. 

With the "Japan-UK Workshop on Quantum Gravity" we want to bring together experts working on different aspects of the gravitational path-integral, such as

• Gravitational Scattering Amplitudes;
• Complex Geometries and Exact WKB;
• Quantum Cosmology;
• Exact Methods and Resurgence Analysis;
  
  

with the long-term goal of providing innovative ways of tackling modern problems in quantum gravity.

This workshop serves as the kick-off meeting for the Royal Society International Collaboration Award Grant "Re-PaInt: A Resurgence Path-Integral approach to quantum gravity" shared between Masazumi Honda at Riken iTHEMS and Daniele Dorigoni at Durham University, and aimed at fostering and developing an international partnership between the two institutes, as well as the greater Japan and UK theoretical physics communities working on quantum gravity, broadly intended.

Poster Image Credit: Alessia Dorigoni

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Confirmed Speakers:

• Nick Dorey (Cambridge U.)
• Alessandro Georgoudis (Queen Mary U.)
• Tobias Hansen (Durham U.)
• Simeon Hellerman (IPMU)
• Paul Heslop (Durham U.)
• Simon Ekhammar (King's College)
• Hiroki Matsui (Nihon U.)
• Okuto Morikawa (Riken)
• Rishi Mouland (Imperial College)
• Yu Nakayama (Rykkio)

• Ryo Namba (iTHEMS)

• Kazumi Okuyama (Shinshu U.)

• Simon Ross (Durham U.)
• David Skinner (Cambridge U.)

• Yuya Tanizaki (YITP)

• Masahito Yamazaki (IPMU)

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Registration:

Registration is open to anyone (not just participants from Japan or UK).

There will be a poster session and we encourage students and young postdocs to present a poster.

The registration deadline is August 15, 2025.

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Organisers:

• Daniele Dorigoni (Durham University);
• Tobias Hansen (Durham University);
• Masazumi Honda (Riken, iTHEMS);
• Pratik Nandy (Riken, iTHEMS).

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Venue:

RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences, Kobe Campus

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Acknowledgements:

We would be grateful if you could acknowledge this workshop in a paper which is initiated or conducted during this workshop. Here are the samples of acknowledgments:

• The authors thank the Royal Society under the International Collaboration Award Grant \R2\242058 and the RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program where this work was initiated [completed] during the "Japan-UK Workshop on Quantum Gravity".
• The authors thank the Royal Society under the International Collaboration Award Grant \R2\242058 and the RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program. Discussions during the "Japan-UK Workshop on Quantum Gravity" were useful in completing this work.

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Supporting Organisations:

This workshop is supported by the Royal Society International Collaboration Award Grant \R2\242058 "Re-PaInt: A Resurgence Path-Integral approach to quantum gravity".

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Monday 22 September

09:00 → 09:25 Welcome

09:25 → 09:30 Opening Remarks

Speaker: Masazumi Honda (RIKEN iTHEMS / Saitama University)

Kobe_opening.pdf

09:30 → 10:30 Recent developments in double-scaled SYK

SYK model is an interesting toy model of holographic duality. In particular, the so-called double-scaled SYK (DSSYK), which is defined by taking a large N limit with the number p of the p-body interaction scaled as p=N^(1/2), has attracted a lot of attention recently since DSSYK is exactly solvable thanks to the underlying quantum group symmetry. In this talk, I will briefly review the recent developments in DSSYK.

Speaker: Kazumi Okuyama

okuyama.pdf

10:30 → 11:00 Coffee Break

11:00 → 12:00 T Tbar and Sqrt(T Tbar) deformations in 4d Chern-Simons Theory

Speaker: Masahito Yamazaki

12:00 → 14:30 Lunch Break

14:30 → 15:30 Higher point N=4 SYM correlators

Speaker: Paul Heslop

heslop.pdf

15:30 → 16:00 Coffee Break

16:00 → 17:00 Giant Gravitons, Volume Minimisation, M2/M5 Duality, and Vertex Algebras

Speaker: Nick Dorey

Tuesday 23 September

09:30 → 10:30 Resonances, Spectral Theory, and the Resurgent Structure

An attempt is made to unify the understanding of resonance phenomena, analytic spectra, and resurgence by developing and applying a mature exact-WKB framework. We begin by formulating non-perturbative resonances in quantum mechanics via the exact WKB method, demonstrated concretely in the inverted Rosen–Morse potential, where barrier resonances and quasi-stationary states are shown to match exact and WKB predictions. Next, we extend this framework to incorporate Zel’dovich regularization, complex scaling, and a modified rigged Hilbert space, thereby clarifying the equivalence and complementarity of these regularization methods and the functional analytic structure they define. Finally, we apply exact WKB and complex scaling to scattering problems, compute the continuum spectrum and the S-matrix, and reinterpret classical theorems such as Aguilar-Balslev-Combes and the Siegert boundary condition in this unified setting. The result is a coherent picture in which resonant poles, continuum spectra, and resurgent structures are seamlessly interconnected, giving new insight into the spectral theory of unstable quantum states.

Speaker: Okuto Morikawa

morikawa.pdf

10:30 → 11:00 Coffee Break

11:00 → 12:00 AdS string amplitudes from single-valuedness

It has long been known that string theory amplitudes have intriguing single-valuedness properties. When considering string theories on curved backgrounds, which are still lacking a complete worldsheet description, these properties become even richer. For AdS/CFT, single-valuedness can be combined with the structure of the OPE in the dual CFT to fix AdS string amplitudes in a small curvature expansion. In this way we found curvature corrections to the AdS Virasoro-Shapiro amplitude for graviton scattering in type IIB on AdS5xS5, type IIA on AdS4xCP3 and the AdS Veneziano amplitude for gluon scattering in orientifolds of type IIB on AdS5xS5. The results have the form of worldsheet integrals involving single-valued multiple polylogarithms. Our answers determine the CFT data for unprotected operators in planar N=4 SYM theory, ABJM theory and certain N=2 SCFTs at strong coupling, making contact with integrability, localization and conformal bootstrap. Furthermore, the high energy limit of the amplitudes agrees with classical scattering computations in AdS.

Speaker: Tobias Hansen (Durham University)

hansen.pdf

12:00 → 14:30 Lunch Break

14:30 → 17:00 Free Discussion / Excursion

Wednesday 24 September

09:30 → 10:30 Extremal Couplings and Gluons in AdS/CFT

In AdS/CFT, extremal couplings induce a mixing between single- and double-trace operators. I will diagnose this mixing and compute it coefficient using the conformal block expansion of 4-point Witten diagrams involving extremal couplings. I will then study the simplest dual pair in which such couplings arise: 4d N=2 theories dual to AdS5 x S5 with some 7-branes. Summing a tower of extremal Witten diagrams provides the final piece of the gluon 4-point function to order 1/N^2.

Speaker: Rishi Mouland

10:30 → 11:00 Coffee Break

11:00 → 12:00 Yang-Mills/QCD on R2xT2, adiabatic continuity, and eta' potential

We show that 4d Yang-Mills/QCD can be semi-classically solved on R2xT2 with nontrivial 't Hooft flux. In this setup, 4d instanton splits into N center-vortex fractional instantons, and its dilute gas describes qualitative features of 4d confinement vacua via the adiabatic continuity. When we apply this technique to QCD with fundamental quarks, we can learn about the global structure of the eta' potential.

Speaker: Yuya Tanizaki

Tanizaki-UK-Japan-QGKobeWorkshop2025.pdf

12:00 → 14:30 Lunch Break

14:30 → 15:30 Towards Celestial Holography for Self-Dual Einstein Gravity

Celestial Holography posits the existence of a holographic description of gravitational theories in asymptotically flat space-times. To date, top-down constructions of such dualities involve a combination of twisted holography and twistor theory. The gravitational theory is the closed string B model living in a suitable twistor space, while the dual is a chiral 2d gauge theory living on a stack of D1 branes wrapping a twistor line. I’ll talk about a variant of these models that yields a theory of self-dual Einstein gravity in four dimensions. This is based on work in progress with Roland Bittleston, Kevin Costello and Atul Sharma.

Speaker: David Skinner

15:30 → 16:00 Coffee Break

16:00 → 17:00 Quantum Cosmology from Perturbative and Non-Perturbative Perspective

Speaker: Hiroki Matsui

matsui.pdf

17:00 → 18:30 Poster Session

Presenters:
Yongchao Lu (Korea institute of advanced study)
Taiga Miyachi (Osaka Metropolitan University)
Kanta Nakano (Saitama University)
Pratik Nandy (RIKEN iTHEMS)
Jaeha Park (Imperial College London)
Cihan Pazarbasi (Okinawa Institute of Science and Technology)
Ryosuke Suda (Saitama University)
Takahiro Yokokura (Tohoku University)

18:30 → 20:30 Banquet

Thursday 25 September

09:30 → 10:30 Integrable Regge Trajectories of N=4 SYM

Maximally supersymmetric four-dimensional Yang-Mills appear to be solvable in the planar limit. In particular, the non-perturbative spectrum of local operators can be calculated using the integrability-based Quantum Spectral Curve (QSC) formalism. However, the usefulness QSC extends beyond local operators. I will demonstrate how it can be used to understand what happens upon analytic continuation to negative spin, into the regime of horizontal Regge trajectories. Taking a weak coupling limit I will formulate a novel ”Asymptotic Baxter-Bethe Ansatz”: a set of equations, reminiscent of the Beisert-Eden-Staudacher equations, that controls the trajectories.

Speaker: Simon Ekhammar

ekhammar.pdf

10:30 → 11:00 Coffee Break

11:00 → 12:00 Comments on Deep Learning as a String Theory

Speaker: Simeon Hellerman

12:00 → 12:05 Photo

12:05 → 14:30 Lunch Break

14:30 → 15:30 Gravitational Waveform from Amplitudes

In this talk I will present some recent developments in the study of classical observables for gravitational waves physics. This topic has received a lot of attention in the recent years due to the observation of gravitational waves emitted by black hole mergers. Our approach will be based on quantum scattering amplitudes methods, which have been developed to study scattering of fundamental particles. We will describe how the same tools can be used to also study binary black hole processes in the largely separated and weakly interacting post-Minkowskian (PM) regime.
I will focus in particular on the 2 to 3 amplitude, which can be connected to the emitted gravitational waveform itself, and after introducing the amplitude methods used for the one-loop computation I will show how the choice of the asymptotic BMS frame is crucial for comparing with more traditional methods (i.e. the multipolar post-Newtonian (PN) expansion). I will conclude by presenting some recent results at two-loop for the non-linear memory.

Speaker: Alessandro Georgoudis

15:30 → 16:00 Coffee Break

16:00 → 17:00 Resumming perturbations in curved spacetime - Black hole QNMs via exact WKB

Divergences in perturbative expansions often signal the
presence of rich physics. Perturbations around curved spacetime can be
systematically handled using the (J)WKB series. However, while being
asymptotic expansions, these series are typically divergent. Recent
developments have demonstrated that exact WKB analysis, combined with
Borel resummation, provides a powerful framework to go beyond the
limitations of perturbation theory and to access the global structure of
exact solutions. This approach unveils nonperturbative effects that
cannot be captured by standard expansions and thus offers deeper
insights into the dynamics in curved spacetime. In this talk, I will
focus on black hole perturbations as a particularly illuminating
application and discuss how exact WKB techniques clarify the role of
singularities, providing a novel analytical technique to compute
quasi-normal modes (QNMs).

Speaker: Ryo Namba

Friday 26 September

09:30 → 10:30 To boost or not to boost, that’s the question: dS/CFT vs dS/SFT

Speaker: Yu Nakayama

10:30 → 11:00 Coffee Break

11:00 → 12:00 Multiparty entanglement and holography

Speaker: Simon Ross

12:00 → 12:15 Concluding Remarks

12:15 → 17:00 Free discussion