Given the current travel and gathering lockdown situation due to the coronavirus, a few of us from Osaka University, KIAS at Seoul, GIST at Gwangju and National Taiwan University at Taipei are starting a biweekly webinar series.

The webinar's address is announced in our mailing list, to which you can register via this Google form.

To add the seminars to the calendar of your choice (Google, Apple, Microsoft...), please go to the bottom of this page on ResearchSeminars.org.

Organizers: Heng-Yu Chen, Koji Hashimoto, Norihiro Iizuka, Keun-Young Kim, Sungjay Lee; Webmaster: Yuji Tachikawa

Juan Pedraza (University Coll. London / Brandeis U.),

"Bit threads, Einstein's equations and bulk locality" [slides]

June 18, **4pm** Taiwan time, **5pm** Japan/Korea time **(Note the unusual time!)**

Entanglement plays a crucial role in questions pertaining to the emergence and dynamics of bulk spacetime. The proposal of Ryu and Takayanagi, which relates the entanglement entropy of a boundary region to the area of a codimension-two bulk surface, has been crucial to this effort. However, holographic entanglement can also be formulated using `bit threads', i.e., divergenceless, norm-bounded vector fields. In this talk, I will review recent work that studies how Einstein’s equations are encoded in bit threads and how to understand bulk metric reconstruction in this language. Time permitting, I'll also comment on the Lorentzian analog of this problem, which is formulated in the context of the "complexity=volume" proposal. This talk will be based on 2007.07907, 2105.08063, 2105.12735 and work in progress.

Aninda Sinha (Indian Institute of Science),

"QFT, EFT and GFT" [slides]

June 4, **4pm** Taiwan time, **5pm** Japan/Korea time **(Note the unusual time!)**

I will talk about an intriguing correspondence between 2-2 scattering in quantum field theory (QFT) and an area of mathematics called geometric function theory (GFT). The area of GFT that we will focus on is the theory of typically real, univalent functions which are the relevant objects to understand scattering amplitudes. This understanding relies heavily on a crossing symmetric dispersion relation for 2-2 scattering that was recently resurrected and furthered in the context of effective field theories (EFT). Time permitting, I will explain how to get the crossing symmetric EFT-hedron in effective field theories.

Hitoshi Murayama (UC Berkeley/ Kavli IPMU),

"Some Exact Results in QCD-like and Chiral Gauge Theories" [slides]

May 21, **11:30am** Taiwan time, **12:30pm** Japan/Korea time **(Note the slightly unusual time!)**

I present some exact results in QCD-like chiral gauge theories. They are exact when supersymmetric gauge theories are perturbed by anomaly-mediated supersymmetry breaking (AMSB). Thanks to the UV-insensitivity of AMSB, SUSY results can be perturbed with no ambiguities even when applied to composite fields. I find two phases for QCD-like theories, one with chiral symmetry breaking and another conformal. Our results for chiral gauge theories do not agree with what had been suggested by tumbling. We suggest alternative schemes of tumbling-like interpretations. We see no evidence that large SUSY breaking leads to phase transitions, perhaps protected by holomorphy.

Pavel Kovtun (U. Victoria),

"Hydrodynamics beyond hydrodynamics" [slides]

May 7, 11am Taiwan time, 12pm Japan/Korea time

Hydrodynamics is a well-established field with a venerable history. In this talk, I will focus on the foundational aspect of hydrodynamics which came to light in recent years. Do the equations of hydrodynamics even make sense? To what degree can the crudeness of hydrodynamics be improved? How do we describe long-distance phenomena that hydrodynamics is supposed to describe but fails to? And how universal is it really?

Ashoke Sen (Harish-Chandra),

"Action principle for self-dual forms" [slides]

April 23, **4pm** Taiwan time, **5pm** Japan/Korea time **(Note the unusual time!)**

In this talk I'll describe recent progress towards finding an action for describing chiral 2n-form fields in 4n+2 dimensions.

Roberto Emparan (ICREA and U. de Barcelona),

"Digging traversable wormholes" [slides]

April 9, **3pm** Taiwan time, **4pm** Japan/Korea time **(Note the unusual time!)**

In recent years there have appeared several constructions of traversable wormholes, in four and other dimensions, which only involve physically acceptable, controllable ingredients. They connect in deep ways many aspects of gravity, quantum field theory, and quantum information. I will discuss several features of these constructions, with a focus on traversability, connectivity between multiple mouths, and the (im)possibility of time travel.

Dalimil Mazáč (IAS),

"Sharp Boundaries for the Swampland" [slides]

Mar 26, 11am Taiwan time, 12pm Japan/Korea time

I will discuss the problem of bounding higher derivative couplings in consistent weakly coupled gravitational theories. The starting point will be general assumptions about analyticity and Regge growth of the S-matrix. Higher derivative couplings are expected to be of order one in the units of the UV cutoff. Our approach justifies this expectation and allows us to prove precise bounds on the order one coefficients. I will explain that the notorious difficulties presented by the graviton pole can be overcome by measuring couplings at small impact parameter, rather than in the forward limit. I will illustrate the method in theories containing a massless scalar coupled to gravity, and in theories with maximal supersymmetry. My talk will be based on arXiv:2102.08951.

Geoff Penington (Berkeley),

"Entanglement wedge reconstruction and the Petz map"

Feb 12, **12pm** Taiwan time, **1pm** Japan/Korea time **(Note the unusual time!)**

I will introduce the Petz map and describe how it can be used in quantum gravity to implement entanglement wedge reconstruction. In particular, I will focus on the example of reconstruction the interior of a black hole from its Hawking radiation.

Yuya Kusuki (Yukawa),

"Wedge holography as generalization of AdS/CFT" [slides]

Jan 29, 11am Taiwan time, 12pm Japan/Korea time

I will describe a recent work about a new duality between D+1 dimensional AdS space and D-1 dimensional CFT, so-called the “codimension two holography”. I will show how we can realize this new holography and show a good deal of evidence of this conjecture by performing explicit computations of physical quantities from both sides.

Po-Shen Hsin (Caltech),

"Applications of Berry phase in quantum field theory" [slides]

Dec 18, 11am Taiwan time, 12pm Japan/Korea time

We will discuss Berry phase in family of quantum field theories and its implication to the low energy physics. The family is labelled by parameters which we promote to be spacetime-dependent background fields, and the Berry phase is equivalent to topological term of the parameters in the effective action. We use Berry phase to study diabolic points in the phase diagram and discuss applications including deconfined quantum criticality in 2+1d and new tests for boson/fermion dualities.

Justin Kaidi (SCGP),

"Adventures in Non-Supersymmetric String Theory" [slides]

Dec 4, 11am Taiwan time, 12pm Japan/Korea time

It has long been known that there exist strings with supersymmetry on the world sheet, but not in spacetime. These include the well-known Type 0 strings, as well as a series of seven heterotic strings, all of which are obtained by imposing unconventional GSO projections. Besides these classic examples though, relatively little is known about the full space of non-SUSY theories. One of the reasons why non-SUSY strings have remained understudied is the fact that nearly all of them have closed string tachyons, and hence do not admit ten-dimensional flat space as a stable vacuum. The goal of this talk is two-fold. First, using recent advances in condensed matter theory, we will reinterpret GSO projections in terms of topological phases of matter, thereby providing a framework for the classification of non-SUSY strings. Having done so, we will show that for all non-SUSY theories in which a tachyon exists, it can be condensed to give a stable lower-dimensional vacuum. In many cases, these stable vacua will be two-dimensional string theories already known in the literature.

Yang-Hui He (Oxford),

"Universes as Bigdata: Superstrings, Calabi-Yau Manifolds and Machine-Learning" [slides]

Sep 18, **4pm** Taiwan time, **5pm** Japan/Korea time **(Note the unusual time!)**

We review how historically the problem of string phenomenology lead theoretical physics first to algebraic/diffenretial geometry, and then to computational geometry, and now to data science and AI. With the concrete playground of the Calabi-Yau landscape, accumulated by the collaboration of physicists, mathematicians and computer scientists over the last 4 decades, we show how the latest techniques in machine-learning can help explore problems of physical and mathematical interest.

Yang Zhou (Fudan Univ),

"Reflected entropy for an evaporating black hole" [slides]

Sep 4, 11am Taiwan time, 12pm Japan/Korea time

Black hole information paradox is a problem over 40 years. Recent progress shows hint towards a new understanding of the black hole interior as part of the Hawking radiation, which they called island. In particular the island formula for the Von Neumann entropy of Hawking radiation is consistent with unitarity. In this talk I will discuss how to extract more information of the island by computing the correlation in Hawking radiation. In particular we study reflected entropy (as a correlation measure of mixed states) and propose a quantum extremal surface for reflected entropy. Based on that, we find a formula for reflected entropy with island cross section as its area term in gravitational system.

Richard A. Davison (Herriot-Watt Univ),

"Relations between transport and chaos in holographic theories" [slides]

Aug 21, **4pm** Taiwan time, **5pm** Japan/Korea time **(Note the unusual time!)**

I will describe recent work illustrating general relations between the transport properties and chaotic properties of quantum field theories with holographic duals. I will firstly show how a simple analysis of near-horizon dynamics yields exact constraints on the spectrum of collective excitations. I will then describe how this can be exploited to identify a universal feature in the spectrum, and its implications for the collective transport properties of strongly interacting field theories with gravity duals.

Julian Sonner (Univ of Geneva),

"Causal symmetry breaking: late time physics of holographic quantum chaos" [slides]

Aug 7, **4pm** Taiwan time, **5pm** Japan/Korea time **(Note the unusual time!)**

Quantum chaotic systems are often defined via the assertion that their spectral statistics coincides with, or is well approximated by, random matrix theory. In this talk I will explain how the universal content of random matrix theory emerges as the consequence of a simple symmetry-breaking principle and its associated Goldstone modes. Within AdS/CFT this gives a universal framework to express correlations between "different universes", even for individual theories rather than ensembles. I will illustrate the bulk picture of this EFT using the example minimal string theory. I will end by discussing the construction of the EFT of quantum chaos also in higher dimensional field theories, as applicable for example for higher-dimensional AdS/CFT dual pairs.

Xiangyu Cao (UC Berkeley),

"Scrambling vs chaos" [slides]

July 24, 11am Taiwan time, 12pm Japan/Korea time

How to characterize information scrambling in dynamical systems is a question of interest to physicists across energy scales. Inspired by classical chaos, which addresses this question quite successfully in classical physics, out of order time correlators (OTOC) have been considered a promising approach for quantum systems. In this talk, I will first discuss the relation between scrambling (as defined by exponential growth of OTOC) and chaos in the semiclassical regime, and show that they can differ qualitatively (joint work with Tianrui Xu andThomas Scaffidi). In a second part, I will discuss scrambling in a family of generalized Sachdev-Ye-Kitaev models, which are arguably more realistic, and which allow us to clarify the behavior of the Lyapunov exponent in non-maxically chaotic Fermi and non-Fermi liquids (joint work with Jaewon Kim and Ehud Altman).

David Tong (University of Cambridge),

"Boundary Conditions for Chiral Fermions" [slides]

July 10, **4pm** Taiwan time, **5pm** Japan/Korea time **(Note the unusual time!)**

I will describe boundary conditions for fermions in d=1+1 dimensions, using the framework of boundary conformal field theory. I will explain how boundary states are classified by a mod 2 index, related to their SPT phase, describe how one can follow RG flows from one boundary state to another, and give a novel perspective on the Z_8 classification of d=2+1 SPT phases.

João Penedones (École Polytechnique Fédérale de Lausanne),

"Nonperturbative Mellin Amplitudes" [slides]

June 26, **4pm** Taiwan time, **5pm** Japan/Korea time **(Note the unusual time!)**

I will argue that nonperturbative CFT correlation functions admit a Mellin amplitude representation. I will discuss the main properties of Mellin amplitudes: subtractions, analyticity, unitarity, Polyakov conditions and polynomial boundedness at infinity. These can be used to write dispersion relations for Mellin amplitudes. Combining crossing, dispersion relations and Polyakov conditions I will write down a set of (extremal) functionals that act on the OPE data.

Miranda Cheng (Univ. of Amsterdam/National Taiwan University),

"Quantum Modularity from 3-Manifolds" [slides]

June 12, 11am Taiwan time, 12pm Japan/Korea time

Quantum modular forms are functions on rational numbers that have rather mysterious weak modular properties. Mock modular forms and false theta functions are examples of holomorphic functions on the upper-half plane which lead to quantum modular forms. Inspired by the 3d-3d correspondence in string theory, a new topological invariants named homological blocks for (in particular plumbed) three-manifolds have been proposed a few years ago. My talk aims to explain the recent observations on the quantum modular properties of the homological blocks, as well as the relation to logarithmic vertex algebras. The talk will be based on a series of work in collaboration with Sungbong Chun, Boris Feigin, Francesca Ferrari, Sergei Gukov, Sarah Harrison, and Gabriele Sgroi.

Chang-Tse Hsieh (Kavli IPMU & ISSP, Univ. of Tokyo),

"Anomaly of the Electromagnetic Duality of Maxwell Theory" [slides]

May 29, 11am Taiwan time, 12pm Japan/Korea time

Every physicist knows that the classical electromagnetism is described by Maxwell's equations and that it is invariant under the electromagnetic duality S: (E, B) → (B, −E). However, the properties of the electromagnetic duality in the quantum theory might not be as well known to physicists and are in fact not very well understood in the literature. Recently, we uncovered a feature of Maxwell theory in the situation where going around nontrivial paths in the spacetime involves the action of the duality transformation, namely that it has a quantum anomaly. We found that the anomaly of this system in a particular formulation is 56 times that of a Weyl fermion. Our result reproduces, as a special case, the known anomaly of the all-fermion electrodynamics — a version of the Maxwell theory where particles of odd (electric or magnetic) charge are fermions — discovered in the last few years.

Kantaro Ohmori (Simons Center for Geometry and Physics),

"1+1d adjoint QCD and non-invertible topological lines" [slides]

May 15, 11am Taiwan time, 12pm Japan/Korea time

In 1990s, it is claimed that the QCD with massless adjoint quark in 1+1-dimensions is confined, although naively the fermions cannot screen the Wilson line in the fundamental representation. In this talk, we will see that the confinement can be explained from the topological line operators live in the theory. In particular, non-invertible topological lines, which do not correspond to a symmetry, play a crucial role. Based on a ongoing work with Zohar Komargodski, Konstantinos Roumpedakis and Sahand Seifnashri.

Tadashi Takayanagi (Yukawa Institute, Kyoto Univ.),

"Holographic Pseudo Entropy" [slides]

May 1, 11am Taiwan time, 12pm Japan/Korea time

In this talk, we will study a generalization of holographic entanglement entropy. This quantity is defined by a mininal area surface in an asymptotically AdS space with Euclidean time-dependence. We will give its CFT counterpart, which we call pseudo entropy. We will also provide quantum information theoretic interpretations of this quantity in several specific examples of qubit systems, which help us to understand our results of pseudo entropy in CFTs and gravity duals.

Daniel Baumann (Univ. of Amsterdam),

"The Cosmological Bootstrap" [slides]

Apr 17, 11am Taiwan time, 12pm Japan/Korea time

The past decade has seen an explosion of progress in our understanding
of scattering amplitudes in gauge theory and gravity. New bootstrap
methods have revealed hidden symmetries and new mathematical
structures that are completely invisible in the standard approach of
Lagrangians and Feynman diagrams. Inspired by these developments, the
bootstrap philosophy has recently been applied to cosmology.
In this talk, I will describe our work on the bootstrapping of
cosmological correlations.

The talk will have two parts: In the first part, I will describe the
conceptual foundations of the "cosmological bootstrap” as developed
together with Arkani-Hamed, Lee and Pimentel in [arXiv:1811.00024].
In the second part, I will describe the extension of these ideas to
massless particles with spin, where locality provides important new
constraints. This is work to appear with Duaso Pueyo, Joyce, Lee and
Pimentel.