Talks

There has been much theorizing on the question of how the procedures of quantum theory might modify general relativity, perhaps leading to a resolution of the problem of the space-time singularities of gravitational collapse. However, I shall argue that these procedures cannot, alone, resolve the space-time singularity issue. Instead, I shall maintain that considerations of relativity, both special and general, could well provide pointers to how the major conundrum of quantum mechanics—the collapse of the wave-function—may eventually be resolved, via curious retro-active aspects of quantum and classical reality.

In this work, we introduce and study "hybrid" fracton orders, especially though a family of exactly solvable models. The hybrid fracton orders exhibit both the phenomenology of a conventional 3d topological ordered phase and a fracton phase. There are simple yet non-trivial fusion and braiding between the excitations between the two kinds. One example is the hybrid order of the Z2 topological order with the Z2 Xcube order, in which the fracton excitations fuse into the toric code charge, and in turn, the flux loop of the toric code can fuse into various lineon excitations. In the same way there is a hybrid ordered phase of Haah's code and the 3d toric code. Proliferating certain gapped excitations in these hybrid orders can drive a phase transition into either a fracton order or a conventional 3d topological phase. 

Reference. ArXiv 2102.09555

If we live in a supersymmetric world, SUSY has to be broken at some (high) scale. I will show how the presence of a SUSY-breaking hidden sector can lead to gravitational wave (GW) signals at future interferometers. I will focus on first order phase transitions that can occur along the pseudomodulus universally related to SUSY breaking. Current bounds on the superpartners are compatible with GW signals at future interferometers, while the observation of a GW signal from a SUSY-breaking hidden sector would imply superpartners within the reach of future colliders.  Along the way, I will analyze the new field theoretical features of the phase transition along the pseudomodulus direction. These allow us to pinpoint what are the necessary requirements for a SUSY-breaking hidden sector to lead to strong GW signals.

Within the next several years pulsar timing arrays (PTAs) are positioned to detect the stochastic gravitational-wave background (GWB) likely produced by the collection of inspiralling supermassive black holes binaries, and potentially constrain some exotic physics. Searches for a GWB in real PTA data rely on Markov-Chain Monte Carlo (MCMC) analyses, which are computationally demanding and not easily accessible to non-experts. In order to develop a more intuitive understanding of what PTAs may (or may not) be able to detect, we built a simple yet realistic Fisher formalism for GWB searches with PTAs. Our formalism is able to accommodate realistic noise properties of PTAs, and allows to forecast their sensitivity not only to an isotropic GWB, but also, looking ahead, to GWB anisotropies. It moreover provides a useful tool to guide and optimize real data analysis. In this talk, I will describe the basic physics behind PTAs, then the Fisher formalism, and illustrate some applications to a real-life PTA. This talk is based on arXiv:2006.14570 and 2010.13958.

Fracton models are characterized by an exponentially increasing ground state degeneracy and point excitations with constrained motion. In this talk, I will focus on a prototypical 3D fracton model -- the X-cube model -- and discuss how its ground state degeneracy can be understood from a foliation structure in the model. In particular, we show that there are hidden 2D topological layers in the 3D bulk. To calculate the ground state degeneracy, we can remove the layers until a minimal structure is reached. The ground state degeneracy comes from the combination of the degeneracy of the foliation layers and that associated with the minimal structure. We discuss explicitly how this works for X-cube model with periodic boundary condition, open boundary condition, and even in the presence of screw dislocation defects.

There are three distinct regions where quantum gravity becomes non-negligible in a black hole spacetime.  There is a precise sense in which these three regions are causally disconnected, and therefore arguably independent.   I illustrate a number of indications we have about what happens in each of them, coming both from the classical Einstein equations and from loop quantum gravity.  These point all to an interesting scenario: long living remnants stabilized by quantum gravity, formed by a large and slowly decreasing interior enclosed into a small anti-trapping horizon. Contrary to what too often stated, the scenario offers also a proof of principle that there is no tension between unitarity and the equivalence principle: the tension comes from postulating a version of holography which is too strong: a fad, for which there is no solid physical evidence.

Light rays can orbit the photon shell of a black hole many times before escaping to infinity.  This means that a distant observer can see a successive series of "echo"  images, each separated in time by the photon shell orbital period, and each dimmer than the previous.  I will present a study of light echos using coherent autocorrelation functions sourced by fluctuating matter in accretion flows.  I will demonstrate that coherent autocorrelation functions are peaked at integer multiples of the photon shell orbital period.  Furthermore, I will argue that the power in echos from supermassive black holes is too small to be observed on Earth.

Zoom Link: https://pitp.zoom.us/j/92751681169?pwd=V0hQeUMwaWtTQjQ3UzdxekJiT0lmQT09

With ongoing efforts to observe quantum effects in larger and more complex systems, both for the purposes of quantum computing and fundamental tests of quantum gravity, it becomes important to study the consequences of extending quantum theory to the macroscopic domain. Frauchiger and Renner have shown that quantum theory, when applied to model the memories of reasoning agents, can lead to a conflict with certain principles of logical deduction. Is this incompatibility a peculiar feature of quantum theory, or can modelling reasoning agents using other physical theories also lead to such contradictions? What features of physical theories are responsible for such paradoxes? 

Multi-agent paradoxes have been previously analysed only in quantum theory. To address the above questions, a framework for analysing multi-agent paradoxes in general physical theories is required. Here, we develop such a framework that can in particular be applied to generalized probabilistic theories (GPTs). We apply the framework to model how observers’ memories may evolve in box world, a post-quantum GPT and using this, derive a stronger paradox that does not rely on post-selection. Our results reveal that reversible, unitary evolution of agents’ memories is not necessary for deriving multi-agent logical paradoxes, and suggest that certain forms of contextuality might be. 

https://iopscience.iop.org/article/10.1088/1367-2630/ab4fc4