Triangle Seminars
Tuesday, 30 Sep 2025
Negative Energy
📍 London
Stefan Hollands
(Leipzig University)
Abstract:
Ordinarily, the actual energy is not physically significant but only energy differences are. But in general relativity, the absolute energy (density) appears on the right side of the Einstein equations as a component of the stress tensor. In this colloquium I explore how negative energy densities are thereby related to exotic phenomena such as warp drive spacetimes or wormholes. Quantum fluctuations enable negative energies and can be tiny, e.g. in the halos of black holes, or astronomical, e.g. inside black holes. In many interesting cases, the laws of physics limit the amount of possible negative energy, and such laws can be seen as a fundamental bridge between gravity and quantum information..
Ordinarily, the actual energy is not physically significant but only energy differences are. But in general relativity, the absolute energy (density) appears on the right side of the Einstein equations as a component of the stress tensor. In this colloquium I explore how negative energy densities are thereby related to exotic phenomena such as warp drive spacetimes or wormholes. Quantum fluctuations enable negative energies and can be tiny, e.g. in the halos of black holes, or astronomical, e.g. inside black holes. In many interesting cases, the laws of physics limit the amount of possible negative energy, and such laws can be seen as a fundamental bridge between gravity and quantum information..
Posted by: Sebastian Cespedes
Wednesday, 1 Oct 2025
Integrability without Elastic Scattering
📍 London
Sibylle Driezen
(ETH)
Abstract:
Integrability provides a rare tool for accessing strongly coupled quantum field theories. To extend our understanding, it is important to study cases where the integrable structure departs significantly from known examples. I will discuss recent work on Drinfel’d twists of the Jordanian type in the spin chain formulation of the AdS_5 x S^5 superstring, which can be interpreted as spin chain avatars of worldsheet non-abelian T-duality. On the string side these deformations are Lax integrable, yet they lead to particle production, raising sharp questions about the scope and fate of integrability studies based on factorised and elastic S-matrices. From the spin-chain perspective, however, a different picture emerges: the natural eigenstates are organised not by particle number but by a residual non-Cartan symmetry. In this basis the spectrum follows from a simple Baxter equation, which we show to agree with (semi-)classical string energies computed from the algebraic curve. This provides not only the first evidence for a non-abelian deformed AdS/CFT duality (which is relevant for non-AdS holography), but also highlights a broader lesson: particle production need not signal the end of integrability, but may instead point to new forms in which it survives.
Integrability provides a rare tool for accessing strongly coupled quantum field theories. To extend our understanding, it is important to study cases where the integrable structure departs significantly from known examples. I will discuss recent work on Drinfel’d twists of the Jordanian type in the spin chain formulation of the AdS_5 x S^5 superstring, which can be interpreted as spin chain avatars of worldsheet non-abelian T-duality. On the string side these deformations are Lax integrable, yet they lead to particle production, raising sharp questions about the scope and fate of integrability studies based on factorised and elastic S-matrices. From the spin-chain perspective, however, a different picture emerges: the natural eigenstates are organised not by particle number but by a residual non-Cartan symmetry. In this basis the spectrum follows from a simple Baxter equation, which we show to agree with (semi-)classical string energies computed from the algebraic curve. This provides not only the first evidence for a non-abelian deformed AdS/CFT duality (which is relevant for non-AdS holography), but also highlights a broader lesson: particle production need not signal the end of integrability, but may instead point to new forms in which it survives.
Posted by: Jesse van Muiden
Quantum Gravity in Near-Extremal Black Holes
📍 London
Roberto Emparan
(University of Barcelona)
Abstract:
Recent developments have revealed that black holes near extremality exhibit large quantum fluctuations in their geometry, marking a controllable breakdown of semiclassical quantum field theory in curved spacetime. In this talk, I will discuss how these fluctuations can be revealed through scattering waves off the black hole. In particular, we find that extremely cold black holes become transparent to low-frequency light or gravitational radiation. These effects provide concrete signatures of quantum gravity at play in near-extremal regimes.
Recent developments have revealed that black holes near extremality exhibit large quantum fluctuations in their geometry, marking a controllable breakdown of semiclassical quantum field theory in curved spacetime. In this talk, I will discuss how these fluctuations can be revealed through scattering waves off the black hole. In particular, we find that extremely cold black holes become transparent to low-frequency light or gravitational radiation. These effects provide concrete signatures of quantum gravity at play in near-extremal regimes.
Posted by: Andrew Svesko
Friday, 3 Oct 2025
Wall-crossing and scattering diagrams in geometry and beyond.
📍 London
Mark Gross
(Cambridge)
Abstract:
I will discuss the notion of scattering diagrams, which encode often very complex data of wall-crossing for various kinds of geometric problems. These originally originated in work of Kontsevich-Soibelman, but have been found to occur naturally in many different contexts, from mirror symmetry, cluster algebras, wall-crossing for moduli spaces of sheaves, and theoretical physics.
I will discuss the notion of scattering diagrams, which encode often very complex data of wall-crossing for various kinds of geometric problems. These originally originated in work of Kontsevich-Soibelman, but have been found to occur naturally in many different contexts, from mirror symmetry, cluster algebras, wall-crossing for moduli spaces of sheaves, and theoretical physics.
Posted by: Yang-Hui He
Integrable models, spin and statistics
📍 London
Alessandro Torrielli
(University of Surrey)
Abstract:
Integrable models in 1+1 dimensions allows for exact solutions of their scattering problems, and often a complete derivation of the spectrum by means of algebraic methods. However, we rarely stop and wonder what the particles which we are scattering really are. In such low dimensionality, bosons, fermions, spin and statistics are actually far stranger than in our 3+1 dimensional world, and interesting phenomena are possible. We will discuss some generalities about spin and statistics in 1+1 dimensions, and then delve into one manifestation of this strange behaviour in AdS_3 string theory.
Integrable models in 1+1 dimensions allows for exact solutions of their scattering problems, and often a complete derivation of the spectrum by means of algebraic methods. However, we rarely stop and wonder what the particles which we are scattering really are. In such low dimensionality, bosons, fermions, spin and statistics are actually far stranger than in our 3+1 dimensional world, and interesting phenomena are possible. We will discuss some generalities about spin and statistics in 1+1 dimensions, and then delve into one manifestation of this strange behaviour in AdS_3 string theory.
Posted by: Evgeny Sobko