Talks

We revisit the scattering of massless waves of helicity $|h|=0,\frac{1}{2},1,2$ in Schwarzschild and Kerr backgrounds, in the long-wavelength regime. The Newman-Penrose scattering amplitudes arising from the Black Hole Perturbation Theory (BHPT) framework are found in agreement with the classical limit of QFT amplitudes at finite values of the scattering angle and arbitrary spin orientation. The latter amplitudes are obtained from on-shell methods and describe the $2\to 2$ scattering of a massless particle of helicity $|h|$ with a massive particle of arbitrary spin $S$, where $S=0$ corresponds to the Schwarzschild case. The effect of the black hole spin in the polarization of the waves is found in agreement with previous analysis. Finally, unitarity constraints based on partial amplitudes and positive time delay are also discussed.

Recent years have seen a surge of progress in post-Minkowskian (PM, weak-field but arbitrary-speed) approximation methods for the gravitational two-body problem, complementing and reorganizing the still much further developed post-Newtonian (PN, weak-field and slow-motion) approximation. This has been driven by simplifying insights, powerful computational tools, and new results coming from the study of on-shell scattering amplitudes in quantum field theories and their classical limits. We will review some of these developments, focusing on the particularly impactful observation (ultimately also understandable from a purely classical perspective but born of the dialog with quantum amplitudes) that certain PM and PN results for arbitrary mass ratios can be determined from surprisingly low orders in the extreme-mass-ratio/self-force expansion.

We study the problem of learning a Hamiltonian given copies of its Gibbs state at a known inverse temperature. Anshu et al. recently studied the sample complexity (number of copies of the Gibbs state needed) of this problem for geometrically local Hamiltonians. In the high-temperature regime, their algorithm has sample complexity polynomial in the system size, temperature, and accuracy. Their algorithm can also be implemented with polynomial, but suboptimal, time complexity. Here, we study the same question for a more general class of Hamiltonians and present an algorithm that solves this problem with improved sample complexity and time complexity linear in the sample size. Furthermore, we prove a matching lower bound showing that our algorithm's sample complexity is optimal, and hence our time complexity is also optimal. Joint work with Robin Kothari and Ewin Tang.

 

Academic science reflects the context in which science is conducted. Thus, the academic scientific enterprise struggles with privilege, racism, sexism, ableism, and homophobia, in Canada, as around the world. In this talk, I will address the uncomfortable and sometimes difficult conversations we must have in order to get to the real work of identifying and removing barriers that limit access and engagement in the scientific enterprise for the full diversity of humanity. We live within a myth of meritocracy in academia and are failing to achieve our full potential as a sector. Moving on from self-awareness as individuals or organizations, we can start to identify the tools and skill sets that individuals and collectives need in order to create cultures of care that achieve inclusive excellence and the very best scientific outputs.

Academic science reflects the context in which science is conducted.  Thus, the academic scientific enterprise struggles with privilege, racism, sexism, ableism, and homophobia, in Canada, as around the world.  In this talk, I will address the uncomfortable and sometimes difficult conversations we must have in order to get to the real work of identifying and removing barriers that limit access and engagement in the scientific enterprise for the full diversity of humanity.  We live within a myth of meritocracy in academia and are failing to achieve our full potential as a sector.  Moving on from self-awareness as individuals or organizations, we can start to identify the tools and skill sets that individuals and collectives need in order to create cultures of care that achieve inclusive excellence and the very best scientific outputs.  

Zoom Link: https://pitp.zoom.us/j/91715376839?pwd=TnBIM1NNek4zOFNyVWFnMHMySE02Zz09

The lowered accuracy requirements at second order in the mass ratio greatly increases the utility of high-order post-Newtonian calculations at post-adiabatic order. In this talk I will present a methodology for solving the first- and second-order field equations using matched asymptotic expansions. As an application and test of the method, we solve the scalar wave equation sourced by a first order solution to the Regge-Wheeler equation with a particle moving in a circular orbit, and thus provide a construction of an analytic expansions of the Lorenz gauge metric perturbation.

Reconstructing a metric or vector potential that corresponds to a given solution to the Teukolsky equation is an important problem for self-force calculations. Traditional reconstruction algorithms do not work in the presence of sources, and they give rise to solutions in a radiation gauge. In the electromagnetic case, however, Dolan (2019) and Wardell and Kavanagh (2020) very recently showed how to reconstruct a vector potential in Lorenz gauge, which is more convenient for self-force. Their algorithm is based on a new Hertz-potential 2-form. In this talk, I will first show that the electromagnetic Teukolsky formalism takes a simplified form when expressed in terms of differential forms and the exterior calculus. This formalism makes the new Lorenz-gauge construction much more transparent, and it enables an extension to nonzero sources. In particular, I will derive a corrector term, related to the charge current, which when added to the vector potential gives a solution to the Maxwell equations with nonzero source. I will conclude by discussing prospects for extending to the gravitational case.

We identify a set of Hertz potentials for solutions to the vector wave equation on black hole spacetimes. The Hertz potentials yield Lorenz gauge electromagnetic vector potentials that represent physical solutions to the Maxwell equations, satisfy the Teukolsky equation, and are related to the Maxwell scalars by straightforward and separable inversion relations. Our construction, based on the GHP formalism, avoids the need for a mode ansatz and leads to potentials that represent both static and non-static solutions. As an explicit example, we specialise the procedure to mode-decomposed perturbations of Kerr spacetime and in the process make connections with previous results.

Understanding extended sources is an important aspect of the second-order problem in Kerr. Metric reconstruction procedures based on the Teukolsky equation bring challenges at second order due to singularities in the radiation gauge. One approach would be to soften these singularities using an effective source derived from a puncture. I will describe how the corrector-tensor reconstruction algorithm may be applied in a puncture scheme with a spatially compact effective source. To illustrate the method I will use an example in flat spacetime which provides some insights into the structure of the problem in Kerr.