Physics

The classical spacetime manifold of general relativity disappears in quantum gravity, with different research programs suggesting a variety of alternatives in its place. As an illustration of how philosophers might contribute to an interdisciplinary project in quantum gravity, I will give an overview of recent philosophical debates regarding how classical spacetime "emerges."  I will criticize some philosophers as granting too much weight to the intuition that a coherent physical theory must describe objects as located in space and time.  I will further argue, based in part on historical episodes, that an account of emergence needs to recover the structural features of classical GR responsible for its empirical success.  This is more demanding than it might at first appear, although the details of recovery will differ significantly among different approaches to quantum gravity.

Zoom link:  https://pitp.zoom.us/j/98331676824?pwd=VTNOakMxWWUzT2ZFZFYwYzBRdWxBUT09

Uma nova cura do câncer, celulares que podem explodir postos de gasolina e um filtro quântico que cura doenças. Diariamente inúmeras notícias atingem a nossa sociedade, mas como separar o joio do trigo? Quais pessoas teriam a obrigação de se dirigir à sociedade para evitar a proliferação de notícias que fazem mau uso das ideias científicas? Falaremos nesse colóquio sobre como a comunidade científica pode, e deve, ajudar no letramento científico da sociedade e também influenciar os políticos na adoção de políticas públicas baseadas em fatos.

Polyaromatic hydrocarbons (PAHs) such as acenes have long been studied due to its interesting optical properties and low singlet triplet gaps. Earlier studies have already noticed that use of complete valence active space is imperative to the understanding of its qualitative and quantitative properties. Since complete active space based methods cannot be applied to such large active spaces, we have used density matrix renormalization group (DMRG) based approaches. Further small modification to the PAH topology shows interesting new phases of behaviour in its optical gaps. We have understood the effect of these effects based on spin frustration due to the presence of odd membered rings. In this talk, I will discuss these observations from molecular and model Hamiltonian perspectives.Further developments based on artificial neural network based configuration interaction for strongly correlated systems will also be discussed.5  The similarities between the ANNs and the MPS wavefunctions will be leveraged for 2D systems.

Zoom link:  https://pitp.zoom.us/j/92159136836?pwd=ZFJBcXZ3R3czSUcxcThOci9ueStBZz09

Abstract: The universe presents itself as a grand laboratory to explore and investigate a range of physical phenomena. High-energy cosmic messengers serve as unique tools to unravel some of the most energetic processes beyond the capabilities of current  human-made particle accelerators. These messengers are not only instrumental in providing cosmological insights by probing the medium they traverse, but also hold the potential to detect Planck-scale effects that accumulate over long distances.  In this talk, I will focus on the theory of cosmological particle propagation. I will then describe how they can be used to search for the elusive dark matter, in particular axion-like particles (ALPs). Finally, I will discuss some of the ways in which cosmic particles have been used to investigate quantum gravity phenomena at high and ultra-high energies.

The origins of the most energetic particles in the Universe have been a long-standing puzzle. In the quest to identify their sources, it is crucial to understand how these particles are accelerated, how they can escape their production sites, and which roads they can take on their journey to Earth. The multimessenger framework has proven to be a powerful tool for exploring the Universe at these extreme energies.  In this talk I will focus on the triad of (ultra-)high-energy messengers: cosmic rays, neutrinos, and gamma rays. I will begin with a summary of the state of affairs in this field, reviewing some key theoretical developments and recent experimental results. I will then delve into the essential components required for building theoretical models that explain these measurements. Particular emphasis will be placed on modelling the propagation of these messengers, including their interactions with matter and radiation fields, as well as with the poorly understood cosmic magnetic fields. Finally, I will discuss the prospects of building a unified and self-consistent model of the Universe at ultra-high energies and the implications such a model would have for astrophysics, cosmology, and fundamental physics.

The third observing run of advanced LIGO, Virgo and KAGRA brought unprecedented sensitivity towards a variety of quasi-monochromatic, persistent gravitational-wave signals. These signals, called continuous waves, allow us to probe not just the canonical asymmetrically rotating neutron stars, but also different forms of dark matter, thus showing the wide-ranging astrophysical implications of using a relatively simple signal model. In this colloquium, I will first describe the framework within which we try to detect continuous gravitational waves from asymmetrically rotating neutron stars, as well as how we can use these signals to constrain the millisecond pulsar hypothesis for the galactic-center GeV excess. I will then generalize continuous-wave methodologies to include the different forms of dark matter that we could detect using gravitational-wave detectors, specifically: (1) the direct interaction of dark matter with the gravitational-wave detectors themselves; (2) gravitational waves from planetary- or asteroid-mass primordial black hole inspirals and so-called "mini" extreme mass ratio inspirals, and (3) gravitational waves from annihilating boson clouds around spinning black holes.

Hawking evaporation offers a unique mechanism for particle production, unlike any other process involving fundamental interactions. Moreover, a black hole will emit all existing degrees of freedom in nature, including those which might not interact with the Standard Model sector. Thus, even a small primordial black hole population could have altered different processes in the Early Universe, such as the generation of dark matter, dark radiation, baryon asymmetry or a stochastic gravitational wave background. We will analyse the production of these cosmological observables assuming monochromatic and extended black hole mass and spin distributions, and give prospects on how future observables could test the existence of primordial black holes.

In this talk, I will give a brief introduction to the basis of mathematical modeling in biology, mainly in ecology and epidemiology.  I will then focus on recent works and challenges in the modeling of the dynamics of epidemics, discussing the difference between modeling past epidemics and ongoing outbreaks.  In the end, I will discuss the place of theoretical developments in this area.

While it is relatively widely accepted that cosmic inflation took place, the actual physics behind it are still elusive. One of the main open questions concerns the number of degrees of freedom active during inflation, which can be answered through searches for primordial non-Gaussianity and isocurvature perturbations. Analyses of cosmic microwave background (CMB) data could not provide definite answers, but future large-scale structure (LSS) surveys are expected to do so by piercing into the theoretically interesting parameter space regions. In this lecture I will describe how galaxy clustering data can be used to very efficiently probe primordial non-Gaussianity and isocurvature. I will focus in particular on the special role played by the phenomenon of "galaxy bias", which has been overlooked in the past, but which must be understood significantly better to let us both (i) robustly constrain inflation models and (ii) optimize our galaxy observations for tests of inflation.

I will describe a construction that allows to understand spinors in an arbitrary number of dimensions, with arbitrary signature. I will describe what pure spinors are, and how in low dimensions all spinors are pure. The first impure spinors arise in 8 dimensions, and "purest" impure spinors are octonions. I will describe how a spinor in an arbitrary dimension defines a set of geometric structures. The easiest example of this is how a pure spinor defines a complex structure. As one increases the dimension, the types of geometric structures that are described by spinors become more and more exotic. If time permits, I will describe some examples in 14 and 16 dimensions. Almost nothing is known about spinors in dimension beyond 16.

Zoom link:  https://pitp.zoom.us/j/94776499052?pwd=RGVURlRnaEx6REJwVE10VXhqa1Q5Zz09

Nils Siemonsen, Perimeter Institute & University of Waterloo

Dark Photon Superradiance

Gravitational and electromagnetic signatures of black hole superradiance are a unique probe of ultralight particles that are weakly-coupled to ordinary matter. Considering the lowest-order interactions one can write down for spin-1 dark photons, the kinetic mixing, a dark photon superradiance cloud sources a rotating visible electromagnetic field. A pair production cascade ensues in the superradiance cloud, resulting a turbulent plasma with strong electromagnetic emissions. The emission is expected to have a significant X-ray component and to potentially be periodic, with period set by the dark photon mass. The luminosity is comparable to the brightest X-ray sources in the Universe, allowing for searches at distances of up to hundreds of Mpc with existing telescopes. Therefore, multi-messenger search campaigns are sensitive to large parts of unexplored beyond the Standard Model parameter space.

Bruno Torres, Perimeter Institute & University of Waterloo

Optimal coupling for local entanglement extraction from a quantum field

The entanglement structure of quantum fields is of central importance in various aspects of the connection between spacetime geometry and quantum field theory. However, it is challenging to quantify entanglement between complementary regions of a quantum field theory due to the formally infinite amount of entanglement present at short distances. We present an operationally-motivated way of analyzing entanglement in a QFT by considering the entanglement which can be transferred to a set of local probes coupled to the field. In particular, using a lattice approximation to the field theory, we show how to optimize the coupling of the local probes with the field in a given region to most accurately capture the original entanglement present between that region and its complement. This coupling prescription establishes a bound on the entanglement between complementary regions that can be extracted to probes with finitely many degrees of freedom.