Format results
Computational trade-offs in graph models
Quentin Berthet (University of Cambridge)Cosmological collider physics beyond the Hubble scale
Arushi Ravindra Bodas University of Maryland, College Park
Wormholes in the partially disorder-averaged SYK model
Kanato Goto University of Tokyo
Gravitational waves from inflation
Ema Dimastrogiovanni University of Groningen
Making sense of semiclassical gravity
André Großardt Friedrich Schiller University Jena
Twisted eleven-dimensional supergravity and exceptional Lie algebras
Ingmar Saberi Ludwig-Maximilians-Universität München (LMU)
Secrets of the Universe
Manuel Calderón de la Barca Sánchez University of California, Davis
FRB science results from CHIME
Kendrick Smith Perimeter Institute for Theoretical Physics
Worst-Case Robustness in Machine Learning
Aditi Raghunathan (Stanford)Current machine learning (ML) systems are remarkably brittle, raising serious concerns about their deployment in safety-critical applications like self-driving cars and predictive healthcare. In such applications, models could encounter test distributions that differ wildly from the training distributions. Trustworthy ML thus requires strong robustness guarantees from learning, including robustness to worst-case distribution shifts. Robustness to worst-case distribution shifts raises several computational and statistical challenges over ‘standard’ machine learning. In this talk, I will present two formal settings of worst-case distribution shifts motivated by adversarial attacks on test inputs and presence of spurious correlations like image backgrounds. Empirical observations demonstrate (i) an arms race between attacks and existing heuristic defenses necessitating provable guarantees much like cryptography (ii) increased sample complexity of robust learning (iii) resurgence of the need for regularization in robust learning. We capture each of these observations in simple theoretical models that nevertheless yield principled and scalable approaches to overcome the hurdles in robust learning, particularly via the use of unlabeled data.Dynamics of thermalization in isolated quantum many-body systems: A simple solvable example
Katja Klobas University of Oxford
When a generic isolated quantum many-body system is driven out of equilibrium, its local properties are eventually described by the thermal ensemble. This picture can be intuitively explained by saying that, in the thermodynamic limit, the system acts as a bath for its own local subsystems. Despite the undeniable success of this paradigm, for interacting systems most of the evidence in support of it comes from numerical computations in relatively small systems, and there are very few exact results. The situation changed recently, with the discovery of certain solvable classes of local quantum circuits, in which finite-time dynamics is accessible and the subsystem-thermalization picture can be verified. After introducing the general picture I will present a recent example (arXiv:2012.12256) of a simple interacting integrable circuit, for which the finite-time dynamics can be exactly described, and the model can be shown to exhibit generic thermalization properties.
Cosmological collider physics beyond the Hubble scale
Arushi Ravindra Bodas University of Maryland, College Park
Non-gaussianity of primordial density perturbations can be sensitive to very heavy particles at the inflationary Hubble scale (H < 10^(13) GeV). However, the window of observability is often constrained to masses close to H. In this talk, I will discuss a mechanism (dubbed “chemical potential”) for heavy complex scalar fields that can extend this window to masses as large as 60H. The mechanism utilizes the large kinetic energy of the inflaton to enhance particle production, and can impart observable non-gaussianity, f_NL~ O(0.01-10). In the second part of the talk, I will discuss another mechanism where the distinct signature of a heavy field can be imprinted at the level of the power spectrum by violating scale-invariance. This can be achieved through the onset of classical oscillations of the heavy field during inflation, instead of quantum production. We consider the possibility of observing such a signal in the stochastic gravitational wave (GW) background originating from a first-order phase transition in a hidden sector. The signal can be observably large in the GW map while being completely hidden in the standard curvature perturbations such as those of the CMB.
Wormholes in the partially disorder-averaged SYK model
Kanato Goto University of Tokyo
Recent studies revealed that wormhole geometries play a central role in understanding quantum gravity. After disorder-averaging over random couplings, Sachdev-Ye-Kitaev (SYK) model has a collective field description of wormhole saddles. A recent paper by Saad, Shenker, Stanford, and Yao studied the SYK model with fixed couplings and found that the wormhole saddles persist, but that new saddles called “half-wormholes” also appear in the path-integral.
In this talk, we introduce a “partially disorder-averaged SYK model” and study how these half-wormholes emerge as we gradually fix the coupling constants. This model has a real parameter that smoothly interpolates between the ordinary totally disorder-averaged SYK model and the totally fixed-coupling model. For the large N effective description, in addition to the usual bi-local collective fields, we also introduce a new additional set of local collective fields. These local fields can be understood as the “half” of the bi-local collective fields, and they represent the half-wormholes in the totally fixed-coupling limit. We found that the large N saddles of these local fields vanish in the total-disorder-averaged limit, while they develop non-trivial profiles as we gradually fix the coupling constants. This illuminates how the half-wormhole saddles emerge in the SYK model with fixed couplings.Gravitational waves from inflation
Ema Dimastrogiovanni University of Groningen
Primordial gravitational waves have the potential to shed new light on the very early universe. In this talk I will discuss gravitational wave production in a variety of models beyond the simplest, single-field, scenarios and highlight some of their implications for testing inflation with interferometers.
Planckian Metals
Subir Sachdev Harvard University
Many modern materials feature a “Planckian metal”: a phase of electronic quantum matter without quasiparticle excitations, and relaxation in a time of order Planck's constant divided by the absolute temperature. I will review recent progress in understanding such metals using insights from the Sachdev-Ye-Kitaev model of many-particle quantum dynamics. I will also note connections to progress in understanding the quantum nature of black holes.
Making sense of semiclassical gravity
André Großardt Friedrich Schiller University Jena
In absence of both experimental evidence for and a fully understood theory of quantum gravity, the possibility that gravity might be fundamentally classical presents an option to be considered. Such a semiclassical theory also bears the potential to be part of an objective explanation for the emergence of classical measurement outcomes. Nonetheless, the possibility is mostly disregarded based on the grounds of arguments of consistency. I will discuss these arguments, attempting to present the broader picture of the constraints that need to be dealt with in order to formulate consistent semiclassical models of gravity, and the implications this has with regard to concrete proposals for theoretical models and
experimental tests of semiclassical versus quantized gravity.
Zoom Link: https://pitp.zoom.us/j/99590707415?pwd=MHFMZlhSMUdMbFFoMEFmQTIxSUhBQT09Twisted eleven-dimensional supergravity and exceptional Lie algebras
Ingmar Saberi Ludwig-Maximilians-Universität München (LMU)
In recent years, there has been a great deal of progress on ideas related to twisted supergravity, building on the definition given by Costello and Li. Much of what is explicitly known about these theories comes from the topological B-model, whose string field theory conjecturally produces the holomorphic twist of type IIB supergravity. Progress on eleven-dimensional supergravity has been hindered, in part, by the lack of such a worldsheet approach. I will discuss a rigorous computation of the twist of the free eleven-dimensional supergravity multiplet, as well as an interacting BV theory with this field content that passes a large number of consistency checks. Surprisingly, the resulting holomorphic theory on flat space is closely related to the infinite-dimensional exceptional simple Lie superalgebra E(5,10). This is joint work with Surya Raghavendran and Brian Williams.
Zoom Link: https://pitp.zoom.us/j/99622967785?pwd=YmlQWW1sNW1qS1FhQkV4NXFlY0Nsdz09
Twistor action for GR - Atul Sharma, University of Oxford
It has been a long-standing dream of twistor theorists to understand gravity without ever talking about gravitons in space-time. To this end, I will describe the recent discovery of a twistor action formulation of perturbative general relativity. This takes the form of a theory governing complex structure deformations on twistor space. It reduces to Plebanski's formulation of GR on performing the Penrose transform to space-time. Some promising applications include finding on-shell recursion relations like MHV rules for graviton scattering amplitudes, studying the quantum integrability of self-dual GR, etc.
Zoom Link: https://pitp.zoom.us/j/99235001602?pwd=QVN6b2ZPbTM2SkFWNkxYTEhzd0tsdz09
Secrets of the Universe
Manuel Calderón de la Barca Sánchez University of California, Davis
In the 3D giant-screen documentary Secrets of the Universe, physicist Manuel Calderón de la Barca Sánchez travels the globe to epicentres of cutting-edge science – from CERN in Switzerland to Perimeter Institute.
On Wednesday, November 3, he returns to Perimeter (virtually, at least) for a special webcast in which he’ll share and discuss clips from Secrets of the Universe, which is now screening at science centres and planetariums around the world.
The giant-format film, which was co-produced by Perimeter, is an immersive journey into some of the grandest scientific ideas and experiments of our time, and brings to life complex scientific ideas in vivid detail. It follows Calderón de la Barca Sánchez, a physics professor at the University of California, Davis, as he puts his own theories about quark-gluon plasma to the test with particle collisions at the Large Hadron Collider at CERN.
During the webcast, Calderón de la Barca Sánchez will show exclusive film excerpts and chat with Perimeter Institute’s Greg Dick about his own research, and the importance communicating the power of fundamental science.FRB science results from CHIME
Kendrick Smith Perimeter Institute for Theoretical Physics
Fast radio bursts (FRB's) are a recently discovered, poorly understood class of transient event, and understanding their origin has become a central problem in astrophysics. I will present FRB science results from CHIME, a new interferometric telescope at radio frequencies 400-800 MHz. In the 3 years since first light, CHIME has found ~20 times more FRB's than all other telescopes combined, including ~60 new repeating FRB's, the first repeating FRB with periodic activity, a giant pulse from a Galactic magnetar which may be an FRB in our own galaxy, and millisecond periodicity in FRB sub-pulses. These results were made possible by new algorithms which can be used to build radio telescopes orders of magnitude more powerful than CHIME. I will briefly describe two upcoming projects: outrigger telescopes for CHIME (starting 2022) and CHORD, a new telescope with ~10 times the CHIME mapping speed (starting 2024).
Zoom Link: https://pitp.zoom.us/j/93798160318?pwd=Z3ZlNTRNRXV5MkQ5cUJhU09sVFpOdz09