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This page catalogues past speakers, along with the titles of their talks and their slides. Links to the videos will be shared on Discord
#25 LETHEP SEMINAR: "The holographic nature of quantum information storage in asymptotically flat spacetimes"
by Siddharth Prabhu (TIFR)
Abstract: Knowledge of all observables on a Cauchy surface enables us to determine these observables everywhere else in the spacetime. Thus any Cauchy surface can be deemed to be containing all information. Here, we argue that asymptotically flat spacetimes possess a holographic nature, so that all this information is stored just on the boundary of a Cauchy surface. Allowing for a few reasonable assumptions about the low energy structure of any complete theory of quantum gravity enables us to find that an asymptotic observer who resides at the past boundary of future null infinity has access to all the information about massless excitations. We also show that information available in any cut of future null infinity is also available in any later cut, but the converse doesn't hold. Similar results hold for past null infinity. We will comment on several interesting and exciting questions that this line of investigation sheds light on, including work in progress.
#24 LETHEP SEMINAR : "Riding a dark bubble"
by Souvik Banerjee
Abstract : In this talk I will discuss the possibility to induce dark energy on a generalized braneworld geometry realized on the wall of an expanding bubble. I will show that the construction would eventually convert a problem of instability into a virtue that would help us to set up a reliable cosmological model of the universe.
#23 LETHEP SEMINAR : Quantum information scrambling
by Lata Joshi
Abstract : In this talk, I will discuss the quantum informations scrambling from the point of view of quantum simulations. I will begin with operator spreading and the OTOC, and then discuss proposal for measurement of the thermal OTOC and comment on the previous experimental works in this direction. At the end, I will review recent works on wormhole teleportation as information processing through coupled chain of qubits.
See here for slides.
# 22 Reparametrization modes in CFT and chaos
by Rohan Poojary (ITP, TU Wien)
Abstract : Recently introduced reparametrization mode operators in CFTs have been shown to govern stress tensor interactions via the shadow operator formalism and seem to govern the effective dynamics of chaotic system. We initiate a study of Ward identities of reparametrization mode operators i.e. how two dimensional CFT Ward identities govern the behaviour of insertions of reparametrization modes ϵ in correlation functions: ⟨ϵϵϕϕ⟩. We find that in the semi-classical limit of large c they dictate the leading O(c^{−1}) behaviour. While for the 4pt function this reproduces the same computation as done by Heahl, Reeves & Rozali in [1], in the case of 6pt function of pair-wise equal operators this provides an alternative way of computing the Virasoro block. We compute a maximally out of time ordered correlation function in a thermal background and find the expected behaviour of an exponential growth governed by Lyapunov index λL=2π/β lasting for twice the scrambling time of the system t∗=(β/2π)logc. From a bulk perspective for the out of time ordered 4pt function we find that the Casimir equation for the stress tensor block reproduces the linearised back reaction in the bulk.
# 21 M-theory scattering amplitudes from 6d (2,0) and 3d N=8 ABJM
by Himanshu Raj (Weizmann Institute of Science)
Abstract: In this talk, I will show how to reproduce the one-loop four-point graviton scattering in 11d supergravity (the low energy approximation of M-theory) on AdS_7 * S^4 and AdS_4* S^7 and their orbifolds using analytic superconformal bootstrap of the respective holographic duals: six dimensional (2,0) SCFTs and the three dimensional N=8 ABJM theories. We will also present results for the effect of higher derivative vertices on the graviton scattering amplitude.
See here for talk slides.
# 20 APS η-invariant, path integrals, and mock modularity
by Diksha Jain
Abstract: I will show that the Atiyah-Patodi-Singer η-invariant can be related to the temperature-dependent Witten index of a noncompact theory and will re-derive the APS theorem using scattering theory. I will then motivate how the η-invariant can be computed using path integral and use this method to compute η-invariant for Cigar. The relation between η-invariant and Mock modular forms will also be hinted.
See here for talk slides.
# 19: Uniqueness of Galilean conformal electrodynamics and its dynamical structure
by Akhila Mohan (BITS-Pilani Goa) on Mar 5, 2021
Abstract: The Galilean field theories are developed to understand the low energy physics, where the velocity is very small compared to that of light. In this talk, I will look into the existence of action both for the magnetic and electric sectors of Galilean Electrodynamics using Helmholtz conditions. We prove the existence of unique action in magnetic limit with the addition of a scalar field in the system. The check also implies the non existence of action in the electric sector of Galilean electrodynamics. Further I will move on to the Dirac constraint analysis of the theory.
See here for talk slides.
#18: Analytic Functionals: Results and Applications to O[N] Global Symmetries
by Apratim Kaviraj (DESY, Hamburg) on Feb 26, 2021
Abstract: The analytic functionals provide a nice approach to both numerical and analytic analysis of the Conformal Bootstrap. In 1d it is understood quite well and also proves the Mellin/Polyakov Bootstrap approach. It is however challenging when more than one Bootstrap equations are involved. A simple example is a CFT with global symmetry. With a short general review, I will discuss how to set up analytic functionals for global symmetries. Finally I will present some new non-perturbative results on the OPE data of O[N] models, and 3d Ising twist defect.
See here for talk slides.
#17: Classical Features of (Super-)Gravity in 3D with Positive Cosmological Constant
by Arindam Bhattacharjee (Indian Institute of Science Education and Research, Pune) on February 19, 2021.
Abstract: In this talk I'll briefly discuss the construction of supergravity theory with positive cosmological constant in (2+1) dimensional spacetime. Using Chern-Simons/Gravity duality we will write one such theory and discuss its asymptotic phase space. Then we will construct the classical holographic dual of this supergravity theory in an appropriate 'screen'.
See here for talk slides.
#16: Asymptotic Symmetries at Spatial Infinity
by Sucheta Majumdar (Université Libre de Bruxelles and International Solvay Institutes) on February 12, 2021.
Abstract: The study of asymptotic symmetries is quite sensitive to the gauge choices and boundary conditions imposed on the fields at infinity. A key example is the familiar BMS group, which arises as the symmetry group of Einstein's theory at null infinity. At spatial infinity, however, the standard Regge-Teitelboim boundary conditions only allow for the Poincar\'e group and not the infinite-dimensional BMS group. The purpose of this talk will be to discuss how these infinite-dimensional symmetries can be recovered at spatial infinity in case of electromagnetism and general relativity, thereby, resolving a longstanding disparity between the results at null and spatial infinity. The invariance under Lorentz boosts plays a central role in distinguishing the "proper'' gauge transformations from the "improper'' ones, that lead to non-trivial asymptotic symmetry algebra.
See here for talk slides.
#15: Overcounting of Interior Excitations: A Resolution to the Bags of Gold Paradox in AdS
by Joydeep Chakravarty (International Center for Theoretical Sciences, Bengaluru) on January 8, 2021.
Abstract: In this talk, we will look at how single-sided and eternal black holes in AdS can host an enormous number of semiclassical excitations in their interior, which is seemingly not reflected in the Bekenstein Hawking entropy. In addition to the paradox in the entropy, we argue that the treatment of such excitations using effective field theory also violates black holes’ expected spectral properties. We propose that these mysteries are resolved because apparently orthogonal semiclassical bulk excitations have small inner products between them; and consequently, a vast number of semiclassical excitations can be constructed using the Hilbert space which describes black hole’s interior.
#14: New Recursion Relations for Scattering Amplitudes with Massive Particles
by Arkajyoti Manna and Sourav Ballav (Institute of Mathematical Sciences, Chennai) on January 6, 2021.
Abstract: We use the recently developed massive spinor helicity formalism by Arkani-Hamed et al. to propose a new class of recursion relations for tree-level amplitudes in gauge theories. These relations are based on a combined complex deformation of massless as well as massive external momenta. We use these relations to study tree-level amplitudes in scalar QCD as well as amplitudes involving massive vector bosons in Higgsed phase of Yang-Mills theory. We prove the validity of our proposal by showing that in the limit of infinite momenta of two of the external particles, the amplitude once again is controlled by an enhanced Spin-Lorentz symmetry paralleling the proof of BCFW shift for massless gauge theories. Simple examples illustrate that the proposed shift may lead to efficient computation of tree-level amplitudes.
See here for talk slides.
#13: New Asymptotic Conservation Laws for Electromagnetism
by Sayali Bhatkar (Indian Institute of Science Education and Research - Pune) on December 18, 2020.
Abstract: Soft theorems are universal statements about low energy limit of amplitudes. Strominger and his collaborators proposed an equivalence between soft theorems and asymptotic conservation laws. This equivalence has been explored in depth for tree level soft theorems. At 1-loop order, Sahoo-Sen derived a new $\log\omega$ soft theorem in four dimensions that is closely related to long range forces. An interesting question to explore is whether this new soft theorem is related to an asymptotic conservation law. In this talk, we will discuss the effect of long range forces on scattering particles and show that we get a new class of $\mathcal{O}(e^{2m+1})$ asymptotic conservation laws for classical electromagnetism. The first of these laws has been discussed by Campiglia-Laddha and is related to the $\log\omega$ soft theorem.
#12: On the Canonical Energy of Weak Gravitational Fields with a Cosmological Constant Lambda in R
by Jahanur Hoque (Institute of Theoretical Physics, Charles University) on December 11, 2020.
Abstract: We discuss the canonical energy of vacuum linearised gravitational fields on light cones on a de Sitter, Minkowski, and Anti-de Sitter backgrounds in Bondi gauge. We derive the associated asymptotic symmetries. When Lambda > 0 the energy diverges, but a renormalised formula with well-defined flux is obtained.
See here for talk slides.
#11: From 2d Droplets to 2d Yang-Mills
by Neetu (Indian Institute of Science Education and Research - Bhopal) on December 4, 2020.
Abstract: Classical phases of (0+1) dimensional unitary matrix models can be characterised by free Fermi surfaces in two dimensions. We establish a connection between time evolution of these surfaces and partition functions of (q-deformed) Yang-Mills theories on Riemann surfaces through quantization of fluctuations about a minimum energy configuration. Modes of fluctuations satisfy an abelian Kac-Moody algebra. We construct a Hilbert space of the algebra and define a one-to-one mapping between states in the Hilbert space and different shapes of the free Fermi surfaces (droplets). The partition function of Yang-Mills theory on a genus g Riemann surface can then be obtained as transition amplitudes of coherent states in the Hilbert space. Higher point correlators in the Yang-Mills theory can be related to higher point correlators in the Hilbert space. The advantage of the analysis lies in the fact that the same Hilbert space encodes the information of different topologies.
#9: Analogue Quantum Black Holes
by Tanay Kibe (Indian Institute of Technology - Madras) on November 27, 2020.
Abstract: Quantum black holes past their Page time are interesting information processors. They demonstrate fast scrambling and information mirroring. Infalling qubits are rapidly encoded and revealed in the outgoing Hawking radiation. In order to avoid paradoxes the information needs to be encoded in the Hawking radiation in extremely complex ways. Motivated to explore these aspects, we construct phenomenological models of black holes inspired by the fragmentation of the near horizon geometry of near extremal black holes. Our model is a lattice of two dimensional geometries interacting via mobile gravitational charges. In this talk I will show that our model captures the semi-classical features of a black hole and I will also demonstrate an explicit realization of the classical Hayden-Preskill protocol for information mirroring. I will also discuss how we can reproduce the entropy in our model.
#9: Accurate Mapping of Spherically Symmetric Black Holes in a Parameterised Framework
by Prashant Kocherlakota (Institute of Theoretical Physics, Frankfurt) on August 19, 2020.
Abstract: Quantum black holes past their Page time are interesting information processors. They demonstrate fast scrambling and information mirroring. Infalling qubits are rapidly encoded and revealed in the outgoing Hawking radiation. In order to avoid paradoxes the information needs to be encoded in the Hawking radiation in extremely complex ways. Motivated to explore these aspects, we construct phenomenological models of black holes inspired by the fragmentation of the near horizon geometry of near extremal black holes. Our model is a lattice of two dimensional geometries interacting via mobile gravitational charges. In this talk I will show that our model captures the semi-classical features of a black hole and I will also demonstrate an explicit realization of the classical Hayden-Preskill protocol for information mirroring. I will also discuss how we can reproduce the entropy in our model.
#8: New Higher Derivative Invariant for Tensor Multiplet in N=2 Conformal Supergravity in Four Dimensions
by Subramanya Hegde (Harish-Chandra Research Institute) on August 14, 2020.
Abstract: Matching of leading order and higher derivative corrections of black hole entropy between the microscopic and the macroscopic computations is one of the stringent consistency checks of string theory as a theory of quantum gravity. To compute the higher derivative corrections, one needs Poincare supergravity invariants which are often difficult to construct by brute force. Conformal supergravity by its gauge equivalence to Poincare supergravity provides a manageable approach to construct such invariants. This is done with the help of superconformal tensor calculus where one constructs various actions by use of density formulae and embedding of different multiplets inside one another. In this talk, we will discuss the construction of a new density formula in N=2 conformal supergravity in four dimensions. We will discuss how to embed the real scalar multiplet/tensor multiplet inside this density formula thereby obtaining a new higher derivative invariant for tensor multiplet in conformal supergravity. We will discuss some future directions and ongoing work towards the end.
See here for talk slides.
#7: Classification of Four-Point Local Gluon S-Matrices
by Subham Dutta Chowdhury (Tata Institute for Fundemental Research) on August 7, 2020.
Abstract: In this paper, we classify four-point local gluon S-matrices in arbitrary dimensions. This is along the same lines as a recent work by Minwalla et al. where four-point local photon S-matrices and graviton S-matrices were classified. We do the classification explicitly for gauge groups SO(N) and SU(N) for all N but our method is easily generalizable to other Lie groups. The construction involves combining not-necessarily-permutation-symmetric four-point S-matrices of photons and those of adjoint scalars into permutation symmetric four-point gluon S-matrix. We explicitly list both the components of the construction, i.e permutation symmetric as well as non-symmetric four point S-matrices, for both the photons as well as the adjoint scalars for arbitrary dimensions and for gauge groups SO(N) and SU(N) for all N. In this talk, we will explicitly list the local Lagrangians that generate the local gluon S-matrices for D ≥ 9 and present the relevant counting for lower dimensions. Local Lagrangians for gluon S-matrices in lower dimensions can be written down following the same method. We also express the Yang-Mills four gluon S-matrix with gluon exchange in terms of our basis structures.
#6: Seeing by CCing - Semi-classically Accessing the Entanglement Wedge with Connes Cocycle Flow
by Ronak M Soni (Stanford University) on July 31, 2020.
Abstract: One of the many mysteries of the AdS/CFT duality is entanglement wedge reconstruction. For a boundary subregion, the entanglement wedge (EW) is a region of the bulk that is naturally associated to a boundary subregion that generically contains a causally inaccessible region. Despite this, every bulk operator in the EW is encoded in the boundary subregion. This seeming stark departure from causality is not a contradiction, because of entanglement, but the exact nature of this resolution remains somewhat mysterious. In cases where the inaccessible region is perturbatively small, we demonstrate the existence of a causal operation that brings all of the inaccessible region into causal contact with the boundary subregion, via gravitational backreaction. The causal operation is the Connes cocycle (CC) flow within the causally accessible region; CC flow is a version of one-sided modular flow without any UV divergences. Further, we show that the causal CC flow along with its backreaction has the same effect as CC flow in the boundary subregion, which is an ingredient in a known entanglement wedge reconstruction protocol.
See here for talk slides.
#5: Area and Volume of Multiboundary Wormhole Models
by Aranya Bhattacharya (Saha Institute of Nuclear Physics) on July 24, 2020.
Abstract: We will discuss about various multiboundary wormhole models that help us to understand the island progress from a purely classical (coarse grained) scenario. We will also discuss about realising the island areas from the perspective of entanglement of purification and the volume dual to the minimal surface at different times as the Subregion Complexity analogue of Page Curve.
#4: Ergodicity and Thermalisation in Low Dimensional Theories
by Pranjal Nayak (University of Geneva) on June 26, 2020.
Abstract: In my talk, I will be interested in studying chaotic behavior of the SYK model using two different definitions of quantum chaos. In the first part of my talk, I study the Eigenstate thermalisation hypothesis (ETH) in this model and demonstrate that ETH implies that individual pure states are chaotic in the low energy limit of the SYK model which is given by the Schwarzian theory. Another putative definition of chaos in quantum theories is in terms of their Wigner-Dyson spectral statistics. Ergodic limit of a theory is defined as one in which this universal behavior becomes relevant. In the later part of the talk, I will discuss our ongoing work on the analysis of operator correlation functions in the SYK model in this ergodic limit, without having to take the IR limit.
#3: Periods, Deformations, and Resurgence
by Madhusudhan Raman (Tata Institute of Fundamental Research, Mumbai) on June 19, 2020.
Abstract: We study the geometry and mechanics (both classical and quantum) of potential wells described by squares of Chebyshev polynomials. We show that in a small neighbourhood of the locus cut out by them in the space of hyperelliptic curves, these systems exhibit low-orders/low-orders resurgence, where perturbative fluctuations about the vacuum determine perturbative fluctuations about non-perturbative saddles.
See here for talk slides.
#2: Bulk Reconstruction in Black Holes
by Nirmalya Kajuri (Chennai Mathematical Institute) on June 12, 2020.
Abstract: The AdS/CFT correspondence tells us that in principle one should be able to answer all questions about physics in AdS from the boundary theory. But how do we translate questions about bulk physics to questions about boundary theory? That is the aim of the bulk reconstruction program: to translate bulk physics in terms of the conformal field theory on the boundary. I will review the bulk reconstruction program and present some new results on bulk reconstruction in spinning BTZ black holes.
See here for talk slides.
#1: Constraining Conformal Field Theories
by Parijat Dey (Uppsala University) on June 3, 2020.
Abstract: We discuss the basic properties of conformal field theories (CFTs) in d > 2 dimensions. Our main focus will be on the constraints coming from conformal invariance that can be used to develop a framework to compute the critical exponents. In particular we show how the critical exponents can be obtained using the ideas of conformal bootstrap. We propose a dispersion relation for the CFT four point function that computes the correlator from its discontinuity. This is a Lagrangian free approach and can be thought of as an alternative way of computing the CFT correlators bypassing the Feynman diagrams.
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