Talks

Superdeterminism has received a recent surge of attention in the foundations community. A particular superdeterministic proposal, named Invariant-set theory, appears to bring ideas from several diverse fields (eg. number theory, chaos theory etc.) to quantum foundations and provides a novel justification for the choice of initial conditions in terms of state-space geometry. However, the lack of a concrete hidden-variable model makes it difficult to evaluate the proposal from a foundational perspective. 

In this talk, I will critically analyse this superdeterministic proposal in three steps. First, I will show how to build a hidden-variable model based on the proposal's ideas. Second, I will analyse the properties of the model and show that several arguments that appear to work in the proposal (on counter-factual measurements, non-commutativity etc.) fail when considered in the model. Further, the model is not only superdeterministic but also nonlocal, $\psi$-ontic and contains redundant information in its bit-string. Third, I will discuss the accuracy of the model in representing the proposal. I will consider the arguments put forward to claim inaccuracy and show that they are incorrect. My results lend further support to the view that superdeterminism is unlikely to solve the puzzle posed by the Bell correlations.

Based on the papers: 

1. I. Sen. "Analysis of the superdeterministic Invariant-set theory in a hidden-variable setting." Proc. R. Soc. A 478.2259 (2022): 20210667.

2. I. Sen. "Reply to superdeterminists on the hidden-variable formulation of Invariant-set theory." arXiv:2109.11109 (2021).

Zoom link:  https://pitp.zoom.us/j/99415427245?pwd=T3NOWUxKTENnMThRVEd3ZTRzU3ZKZz09

In the first part of my talk, I present the effective-field theory (EFT)-based cosmological full-shape analysis of the anisotropic power spectrum of eBOSS quasars. We perform extensive tests of our pipeline on simulations, paying particular attention to the modeling of observational systematics. Assuming the minimal ΛCDM model, we find the Hubble constant H0 = (66.7 ± 3.2) km/s/Mpc, the matter density fraction Ωm = 0.32 ± 0.03, and the late-time mass fluctuation amplitude σ8 = 0.95 ± 0.08. These measurements are fully consistent with the Planck cosmic microwave background results. Our work paves the way for systematic full-shape analyses of quasar samples from future surveys like DESI. I also present the cosmological constraints from the full-shape BOSS+eBOSS data in various extensions of the ΛCDM model, such as massive neutrinos, dynamical dark energy and spatial curvature.

In the second part, I study the one-point probability distribution function (PDF) for matter density averaged over spherical cells. The leading part to the PDF is defined by the dynamics of the spherical collapse whereas the next-to-leading part comes from the integration over fluctuations around the saddle-point solution. The latter calculation receives sizable contributions from unphysical short modes and must be renormalized. We propose a new approach to renormalization by modeling the effective stress-energy tensor for short perturbations. The model contains three free parameters which can be related to the counterterms in the one-loop matter power spectrum and bispectrum. We demonstrate that this relation can be used to impose priors in fitting the model to the PDF data. We confront the model with the results of high-resolution N-body simulations and find excellent agreement for cell radii r≥10 Mpc/h at all redshifts up to z=0.

Zoom link:  https://pitp.zoom.us/j/92219627192?pwd=eGg4MDUrbGlrR2JqY0xyWHdwQ2lZZz09

Factorization algebras are local-to-global objects, much like sheaves, and it is natural to ask what kind of topology, geometry, and physics they are sensitive to. We will examine this question with a focus on less-perturbative phenomena, touching on topics like moduli of vacua for 4-dimensional gauge theories and Dijkgraaf-Witten-type TFTs. Apologies hereby issued in advance to the (hopefully) friendly audience (and to my collaborators!) for speaking before achieving complete clarity.

Zoom link:  https://pitp.zoom.us/j/94417858154?pwd=ak54UFpPb3hFbnBwcUlnMnhCdG1odz09

Phase transitions in everyday systems are often catalysed by the presence of impurities, but in cosmology we typically assume the initial state is a homogeneous vacuum. In this talk I will discuss how topological defects can seed first order phase transitions in the early universe, causing them to proceed much more rapidly than in the usual case. The field profiles describing the decay do not have the typically assumed O(3)/O(4) symmetry, requiring an extension of the usual decay rate calculation. To numerically determine the saddle point solutions which describe the decay we use a new algorithm based on the mountain pass theorem. I will present results showing the significance of this effect for catalysis by magnetic monopoles in a simplified model, then discuss the same effect for domain walls catalysing the electroweak phase transition. The presence of domain walls can significantly modify the predictions for gravitational wave signal which may be observed with LISA.

Zoom link:  https://pitp.zoom.us/j/96182819088?pwd=cXhnVjFlT0tkc1VsRld0Yk43bFROUT09

Inspired by the emergence of bulk diffeomorphism invariance in holography, I will give an explicit description how entanglement between quantum subsystems can lead to emergent gauge symmetry in a classical limit. Along the way, I will provide a precise characterisation of when it is consistent to treat a quantum subsystem classically in such a limit, and show that this gives strong constraints on the entanglement structure of classical states. I will explain how this generically leads to emergent fundamentally non-local classical degrees of freedom, which may nevertheless be accounted for in a kinematically local way if one employs an appropriately redundant description. The mechanism I describe is general and elementary, but for concreteness I will exhibit a toy example involving three entangled spins at high angular momentum, and I will also describe a significant generalisation of this toy example based on coadjoint orbits. If there is time, I will discuss evidence for the role this phenomenon plays in gravity. This talk is based on arXiv:2209.03979.

Zoom link:  https://pitp.zoom.us/j/92066956880?pwd=OTRySTlOVGgvM3RCRmkzWHFVSUF3Zz09

Gravitational waves emitted by compact binaries enable unprecedented tests of gravity at highly non-linear regimes, as well as the underlying cosmological model. Going beyond the current null tests of gravity requires accurate theoretical modelling of the waveforms in viable extensions of General Relativity. In the first part of this talk, I will present the recent results and physical insights from analytical modelling of the gravitational waves in the so-called Einstein-scalar-Gauss-Bonnet gravity. Being a sub-class of both Horndeski and quadratic gravity, this theory introduces non-linear curvature corrections to strong-field regime of gravity, allows for hairy-black hole solutions, and scalar-induced tidal deformations. I will present the gravitational-wave signatures of theory’s curvature corrections and the prospects of testing the features of this theory through gravitational wave observations. In the second part of the talk, I will discuss the prospects of using compact mergers for cosmological tests by solely relying on their gravitational wave signals. Using recent constraints on the equation-of-state of neutron stars from multi-messenger observations of NICER and LIGO/Virgo, I show possible bounds on the Hubble constant (H0) found from (single and multiple) neutron star-black hole standard sirens in the next-generation gravitational wave detector era. I show that such systems could enable unbiased 13% - 4% precision measurement of H0 (68% credible interval) within an observation time-frame of hours to a day.

Zoom link:  https://pitp.zoom.us/j/93964588227?pwd=cGsxcEZHRlNjd3R5eHg5dzdtT2lndz09

Central to many of the paradoxes arising in quantum theory is that the act of measurement cannot be understood as merely revealing the pre-existing values of some hidden variables, a phenomenon known as contextuality. In the past few years quantum contextuality has been formalized in a variety of ways; operation-theoretic, sheaf-theoretic, (hyper)graph-theoretic, and cohomological. In this seminar we will discuss the simplicial approach to contextuality introduced in arXiv:2204.06648, which builds off the earlier sheaf-theoretic approach of Abramsky-Brandenberger (arXiv:1102.0264) and the cohomological approach of Okay, et al. (arXiv:1701.01888). In the simplicial approach measurement scenarios and their statistics can be modeled topologically as simplicies using the theory of simplicial sets. The connection to topology provides an additional analytical handle, allowing for a rigorous study of both state-dependent and state-independent contextuality. Using this formalism we present a novel topological proof of Fine's theorem for characterizing noncontextuality in Bell scenarios.

Zoom link:  https://pitp.zoom.us/j/93748699892?pwd=SVhVaTdoRmlwaGdCZVdIWVlKTktjQT09

In this talk I will describe a topological response theory that predicts the physical manifestation of a class of topological invariants in systems with crystalline symmetry. I focus on two such invariants, the 'discrete shift' and a quantized charge polarization. Guided by theory, I discuss how these invariants can be extracted from lattice models by measuring the fractional charge at lattice disclinations and dislocations, as well as from the angular and linear momentum of magnetic flux. These methods are illustrated using the Hofstadter model of spinless fermions in a background magnetic field; they give new topological invariants in this model for the first time since the quantized Hall conductance was computed by TKNN in 1982.

Zoom link:  https://pitp.zoom.us/j/93633131128?pwd=d2h4U1l0ZVU5aE1ORURkdFNSanB4dz09

A quantum computer is a new type of computer based on quantum physics. When it comes to certain computational objectives, the computational ability of quantum computers is much stronger than that of the familiar digital computers, and their construction will enable us to factor large integers and to break most of the current cryptosystems.

The question of whether quantum computation is possible is one of the fascinating clear-cut open scientific questions of our time. In my lecture I will explain theoretical discoveries from the 1990s that suggested that quantum computation is possible and present my theory as to why quantum computation is nevertheless impossible.

At the crux of the matter is the study of noisy intermediate scale quantum (NISQ) computers. Based on the mathematical notions of "noise sensitivity vs noise stability" (Benjamini, Kalai, and Schramm 1999, Kalai and Kindler 2014), we identify the inherent noise sensitivity of probability distributions arising from NISQ computers. This leads to a very low complexity class of probability distributions that can be robustly described by such quantum computers; consequently, NISQ computers will not allow good-quality quantum error-correction which are the necessary building blocks for larger quantum computers.

The lecture will be self-contained and will start with a gentle explanation of some basic notions about computation, and quantum computers.

Zoom link:  https://pitp.zoom.us/j/93847236670?pwd=T2Z2emZ5ZExaZTVYcTNCdU1FNkxOdz09

We consider four-point correlators in an arbitrary excited state of a quantum field theory.  We show that when the theory and state are holographic, such correlators can produce high-quality movies of point-like bulk particles, revealing the geometry in which they move. In some situations, Einstein’s equations amount to a local differential equation on the correlator data. In theories or states that are not holographic, images are too blurry to extract a bulk geometry. Calculations are performed by adapting formulas from conformal Regge theory, to excited states and out-of-time-order correlators.

Zoom link: https://pitp.zoom.us/j/94153545930?pwd=YUFsUW44S1Z5Ri83a2xQdEN0Vk9XZz09