# Current seminars

In addition to the Vienna relativity seminars, the calendars above sometimes contain other events of interest to members of the relativity group. The seminars of the Vienna relativity group are listed below.

Currently (unless indicated otherwise) all seminars are held via ZOOM.

The Mathematical Physics Seminars take place on Tuesdays at 13.45.

The Particle Physics Seminars take place on Tuesdays at 16.15.

- Thursday,
**October 21th, 15:00**via ZOOM

Rita Texeira de Costa (Princeton): Homogenization of the Einstein equations under symmetry

Abstract: Due to their nonlinear nature, the Einstein equations are not closed under weak convergence: failure of compactness, due to oscillations and concentrations, produces an excess energy momentum tensor. In 1989, Burnett conjectured that, for vacuum sequences with high-frequency oscillations, the matter produced in this limit is captured by the Einstein-massless Vlasov model.

In this talk, we give a proof of Burnett's conjecture under some gauge and symmetry assumptions, improving previous work by Huneau—Luk from 2019. Our methods are more general, and apply to oscillating sequences of solutions to the wave maps equation in (1+2)-dimensions.

This is joint work with André Guerra (Institute for Advanced Study).

- Friday,
**October 22th, 12:30 Lunch Seminar**via ZOOM

Marius Oancea (Vienna): The gravitational spin Hall effect of light

Abstract: The propagation of electromagnetic waves in vacuum is often described within the geometrical optics approximation, which predicts that wave rays follow null geodesics. However, this model is valid only in the limit of infinitely high frequencies. At large but finite frequencies, diffraction can still be negligible, but the ray dynamics becomes affected by the evolution of the wave polarization. Hence, rays can deviate from null geodesics, which is known as the gravitational spin Hall effect of light. By considering the WKB approximation for Maxwell's equations, I will briefly present the main steps of a covariant derivation of the polarization-dependent ray equations describing the gravitational spin Hall effect of light. I will also discuss the relation of these equations with the well-known MPD equations, as well as the observer dependence of the position of massless spinning particles.

- Thursday,
**October 28th, 14:00**via ZOOM

Ettore Minguzzi (Firenze): A proof of the constancy of surface gravity for non-degenerate compact horizons

Abstract: Smooth compact totally geodesic null hypersurfaces (horizons) arise naturally as Cauchy horizons for partial Cauchy hypersurfaces.

Here I outline a recent proof that if they admit an incomplete generator (non-degenerate) then the surface gravity can be normalized to a non-zero constant.The proof is, in its most technical part, independent of the approach of Isenberg-Moncrief and Bustamante-Reiris. Also the result holds just under the dominant energy condition, i.e. no vacuum assumption is required. If time permits, I shall also outline the proof that they are actually Cauchy horizons bounded on one-side by a region of chronology violation. (This is joint work with Sebastian Gurriaran, ENS Paris-Saclay)

- Friday,
**November 5th, 13:00 Lunch Seminar**Seminarraum A, Währinger Straße 17, 2. Stk., 1090 Wien AND via ZOOM

Mario Hudelist (Vienna): Relativistic Theory of Elastic Bodies in the Presence of Gravitational Waves (Results of a Bachelor thesis)

Abstract: In this talk an overview of the theory of general relativistic elasticity in a gravitational wave background will be presented. The main topic of interest is the response of elastic materials or bodies to the incidence of a gravitational wave in the transverse-traceless gauge. Starting from the concept of an action principle, the primary object in question is the energy-momentum or stress-energy tensor $T{\mu\nu}$ coming from a Lagrangian field theory. The vanishing divergence of $T{\mu\nu}$ then leads to a wave equation, where the gravitational wave enters through the boundary conditions (as an effective force). A one-dimensional example for an elastic material in a gravitational wave background is provided, where an exact solution is obtained.

- Friday,
**November 12th, 12:30 Lunch Seminar**Seminarraum A, Währinger Straße 17, 2. Stk., 1090 Wien AND via ZOOM

Caslav Brukner (Vienna): Relativistic spin and quantum reference frames

Abstract: A still much debated question in the field of relativistic quantum information is whether entanglement and the degree of violation of Bell's inequalities are (Lorentz) frame independent for massive relativistic particles. Central to this question is the effect that spin gets entangled with the momentum at relativistic velocities with respect to the laboratory frame. In my talk, I will present a quantum reference frame transformation that allows us to transform a particle to the rest state even if it is in a superposition of relativistic momenta. In this way, we can first go to the rest state of the particle, perform mentally the spin measurements there (using the Stern-Gerlach experiment), and then return to the laboratory frame. In this way, we find the operational "relativistic spin" and show that entanglement and Bell's inequality are invariant for a pair of particles when changing the quantum reference frame.

- Friday,
**November 19th, 13:00 Lunch Seminar**Seminarraum A, Währinger Straße 17, 2. Stk., 1090 Wien

Argam Ohanyan (Vienna): The Hawking-Penrose singularity theorem for C^1-Lorentzian metrics

Abstract: : In this talk, our recent proof of the Hawking-Penrose singularity theorem, as well as its generalization due to Galloway and Senovilla, to C^1-Lorentzian metrics will be discussed. We will start by presenting tensor distributions of finite order as the proper setting for defining curvature conditions for C^1-metrics, and then go on to address geodesic branching, a phenomenon that can occur in this regularity since the geodesic equation is no longer uniquely solvable.

Finally, we give an outline of the proof of the main result by means of regularized metrics, using refined versions of the Riccati comparison techniques that were used in the proof of the C^{1,1} Hawking-Penrose theorem.

- Thursday,
**December 2nd, 14:00**via ZOOM

Wolfgang Martin Wieland (Vienna): How the Immirzi Parameter deforms the SL(2,R) Boundary Charges on the Light Cone

Abstract: This talk describes how the Barbero--Immirzi parameter deforms the SL(2,R) symmetries on a null surface boundary. Our starting point is the definition of the action and its boundary terms. We introduce the covariant phase space and explain how the Holst term alters the symmetries on a null surface. This alteration only affects the algebra of the edge modes on a cross-section of the null surface boundar, whereas the algebra of the radiative modes is unchanged by the addition of the Barbero--Immirzi parameter. To compute the Poisson brackets explicitly, we work on an auxiliary phase space, where the SL(2,R) symmetries of the boundary fields are manifest. The physical phase space is obtained by imposing both first-class and second-class constraints. All gauge generators are at most quadratic in terms of the fundamental SL(2,R) variables. Finally, we discuss various strategies to quantise the system.

The talk is based on the paper: https://arxiv.org/abs/2104.05803