Vienna Theory Lunch Seminar

by Christopher Lepenik (UV), Maximillian Löschner (UV), Alexander Soloviev (VUT)
and David Toneian (VUT)

Tuesdays 12:15-13:30

held alternately at:

Vienna University of Technology (VUT): Wiedner Hauptstr. 8-10, yellow area, 10th floor, seminar room E136

University of Vienna (UV): Boltzmanngasse 5, 5th floor, Schrödinger Lecture Hall

We thank our kind sponsors:

Dean of physics, TU

Faculty of Physics, UV

Daniel Grumiller, TU



Wie auf vielen Universitäten praktiziert wollen wir ein Lunch-Seminar etablieren, das aktuelle Themen der Theoretischen Physik, die von DiplomandInnen, DoktorandInnen und PostDocs behandelt werden, aufgreift.

Das Niveau soll so gewählt werden, dass jeder Student und jede Studentin am Beginn des Masterstudiums dem Vortrag folgen kann. BachelorstudentInnen können besonders von dem Seminar profitieren, da es ihnen ermöglicht einen Eindruck in die Forschungsarbeit beider Universitäten zu erhalten. Die Vortragenden werden dabei auch ermutigt darüber zu sprechen, warum sie ein gewisses Forschungsgebiet gewählt haben. Dabei dürfen durchaus offene Fragen und Probleme behandelt werden und es ist nicht notwendig einen Vortrag über eine "perfekte", abgeschlossene Arbeit zu halten.

Damit es zu keinem "Zeitverlust" kommt, wird Mittagessen (Pizza) gratis zur Verfügung gestellt.

We want to establish a lunch seminar as practiced at other universities. The focus is on recent theoretical research done by Master students, PhDs and PostDocs.

The seminar is designed for graduate students but should also be comprehensible to advanced undergraduate students. Undergraduate students are particularly encouraged to attend so that they receive an overview of research activities conducted at both universities. Speakers are also encouraged to focus on their motivation for choosing their particular topic and to present open questions.

In order to avoid any "loss of time" we provide a free lunch (pizza).

Wie kann ich teilnehmen?

How can I join?

Einfach erscheinen! Um per Email informiert zu werden, bitte in die Mailingliste eintragen.

Just attend! To receive informations via email register for the Mailinglist.

Oct 11 2016


Arjun Bagchi

Tensionless strings and related fun things

I will talk about the formulation of the tensionless limit of closed bosonic and superstring theory from the point of view of worldsheet symmetries. The talk would be based principally on the papers 1507.04361 and 1606.09628.

Oct 18 2016


Prof. Dr. Jörg Schmiedmayer
Vienna Center for Quantum Science and Technology (VCQ), Atominstitut, TU-Wien

Scrambling, Relaxation and the emergence of thermalization in an isolated many body quantum system

The evolution of an isolated quantum system is unitary. This is simple to probe for small systems consisting of few non-interacting particles. But what happens if the system becomes large and its constituents interact? In general, one will not be able to follow the evolution of the complex many body eigenstates. Ultra cold quantum gases are an ideal system to probe these aspects of many body quantum physics and the related quantum fields. Our pet systems are one-dimensional Bose-gases. Interfering two systems allows studying coherence between the two quantum fields and the full distribution functions and correlation functions give detailed insight into the many body states and their non-equilibrium evolution. In our experiments we study how the coherence created between the two isolated one-dimensional quantum gases by coherent splitting slowly degrades by coupling to the many internal degrees of freedom available. We find that a one-dimensional quantum system relaxes to a pre-thermalisatized quasi steady state which emerges through a light cone like spreading of ’de-coherence’. The pre-thermalized state is described by a generalized Gibbs ensemble. Finally, we investigate the further evolution away from the pre-thermalized state. On one hand we show that by engineering the Quasiparticles we can create many body quantum revivals. On the other hand, we point to two distinct ways for further relaxation towards a final state that appears indistinguishable from a thermally relaxed state. The system looks like two classically separated objects. This illustrates how classical physics can emerge from unitary evolution of a complex enough quantum system. We conjecture that our experiments points to a universal way through which relaxation in isolated many body quantum systems proceeds if the low energy dynamics is dominated by scrambling of the eigenmodes of long lived excitations (quasi particles).

Oct 25 2016


Benjamin Ramberger
Computational Materials Physics Group, UV

RPA forces for solids

In electronic structure simulations, one is usually interested in the groudstate energy of a many electron system which can often only be calculated approximately. A particular useful method to approximate the energy of a quantum many body system is the random phase approximation (RPA). For many systems it is superior to the widely used density functional theory (DFT), especially when van-der-Waals interactions are important. However, the computational demand for RPA simulations of solids was far out of reach for many years. Fortunately there has been a lot of progress in the development of computer simulations employing the RPA recently, so that RPA-correlation energies became accessible for large systems. For solids however, there is not yet an efficient way to calculate forces in the RPA. Since the calculation of forces is crucial for the simulation of elastic and vibrational properties as well as for structure relaxations, this is a promising area for further investigation. The problem of force calculations within the RPA is adressed by a subproject of the Special Research Program (SFB) Vienna Computational Materials Laboratory (ViCoM) supported by the Austrian Science Fund (FWF). After a general introduction to the topic, the challanges of RPA forces calculations will be discussed and recent results of the project will be presented.

Nov 1 2016

No Lunch Seminar

Allerheiligen - All Saints' Day

Nov 8 2016


Esteban Castro Ruiz

Entanglement of quantum clocks through gravity

In general relativity, the picture of spacetime for a given observer assigns an ideal clock to each spacetime point. Being ideal, gravitational effects due to these clocks are ignored and the flow of time according to one clock is not affected by the presence of surrounding clocks. However, if time is defined operationally, as a pointer position of a physical clock that obeys the laws of quantum mechanics and general relativity, such a picture is at most a convenient fiction. We show that the mass-energy equivalence implies gravitational interaction between the clocks, while the superposition of energy eigenstates leads to a non-fixed metric background. Based only on the assumption that both quantum mechanics and general relativity are valid in this situation, we show that the clocks necessarily get entangled through time dilation effect, which eventually leads to a loss of coherence of a single clock. Hence, the time as measured by a single clock is not well-defined. However, the general relativistic notion of time is recovered in the classical limit of clocks.

Nov 15 2016

No Lunch Seminar

Tag des Landespatrons - Day of the Patron Saint of Vienna

Nov 22 2016


Peter Eigenschink

Global existence for the Einstein-Vlasov System with massive and massless particles

The Cauchy problem in general relativity is of fundamental interest. What is known depends crucially on the choice of a reasonable matter model coupled to the Einstein equation. One of the simplest matter models one can choose is Vlasov matter, which represents collisionless gas and is governed by a continuity equation, the Vlasov equation. For a spherically symmetric spacetime and appropriate initial data it is known that for either massive or massless Vlasov matter the Einstein-Vlasov system has global solutions without singularities. But the way those results are obtained are inherently different. For that reason it is of interest if global solutions without singularities also exist in the case of a combination of massive and massless Vlasov matter. We show that for certain initial data, global stability of solutions of the spherically symmetric Einstein-Vlasov system with a combination of both, massive and massless matter, still holds. As an intermediate result, which is crucial for the final conclusion, we also show that for such initial data, massive and massless particles separate after finite time in an appropriate way.

Nov 29 2016


Philipp Kleinert

Thermalisation of Wightman Two-Point Functions in AdS/CFT

In this talk, we will discuss applications of the AdS/CFT correspondence to far-from-equilibrium physics. In the first part, we address the issue of generalizing the holographic dictionary to out-of-equilibrium situations. In the second part, we consider a concrete example of a holographic model of non-equilibrium physics: thermalization following a global quench in 2d CFTs dual to AdS-Vaidya spacetime. At first sight, non-local and local probes in these holographic models of thermalization reach their equilibrium values on very different time scales. As an example of non-local probes, we study the evolution of Wightman two-point functions and show that these two-point functions, after being Fourier transformed to momentum space, decay towards their thermal values with a rate set by the quasinormal modes of the final state black hole spacetime. The quasinormal modes also set the thermalisation time scale of local probes which suggests a unified picture of thermalisation times of one- and two-point functions. Based on: 1412.2806 and 1511.08187

Dec 6 2016


Carla Schuler & Gabriel Sommer

Electroweak models with classical scale-invariance

We study versions and extensions of the standard model (SM) with classical scale-invariance, which are of great interest nowadays. In these models the Lagrange density does not contain any mass terms, which are responsible for the breakdown of scale-invariance. The generation of the masses of all particles is then described by the Coleman-Weinberg mechanism, where spontaneous symmetry breaking (SSB) occurs as the consequence of quantum corrections rather than the negative Higgs mass term in the Lagrangian. With the particle content of the SM, it is not possible to formulate the SM as a classically scale-invariant theory with the experimentally measured mass values of the Higgs boson and the t-quark. An enlarged SM Higgs-Sector with additional scalar fields and an extension of the SM gauge-group by (non)-Abelian factors provide possible solutions.

Dec 13 2017


Marcus Sperling
Mathematical Physics, UV

Hilbert Series and SUSY gauge theories

The Hilbert series counts gauge invariant chiral operators and encodes information on the moduli space of supersymmetric vacua of a theory. In this talk I will introduce the use of Hilbert series in example of 4d N=1 theories. Thereafter, I proceed to 3d N=4 theories and focus on the Coulomb branch Hilbert series.

Jan 10 2017


Bernadette Kolbinger

Measuring the Ground State Hyperfine Splitting of Antihydrogen

The ASACUSA (Atomic Spectroscopy And Collisions Using Slow Antiprotons) collaboration at the Antiproton Decelerator at CERN aims to test the CPT symmetry by measuring the ground state hyperfine structure of antihydrogen which, according to the theorem of CPT, is predicted to have the same electromagnetic spectrum as hydrogen.
The experimental setup consists of a CUSP trap for antihydrogen production and a Rabi-like spectrometer line consisting of a microwave cavity, a superconducting sextupole magnet and a detector for counting arriving antihydrogen atoms.
This talk will give an overview of the experiment and focuses on the detector. The challenging task of the detector is to discriminate between background events and antiproton annihilations originating from antihydrogen atoms which are produced only in very small amounts. Furthermore, first preliminary results of the 2016 beamtime will be shown.

Jan 17 2017


Friedrich Kupka
Department of Mathematics, UV

Numerical time integration and studies of stellar convection

Convection is one of the main physical mechanisms operating in stars to transport heat and angular momentum, to mix the plasma they consist of, to generate magnetic fields, and to drive and damp global oscillations that are used to study their interior structure. A deeper understanding of the physics of these processes requires numerical simulations and their main challenge is the huge spread of scales in space and time they operate on. After an overview on this basic challenge we report on new numerical methods for efficient and reliable time integration, originally developed for the ANTARES general purpose hydrodynamical simulation code, which are of general use for similar mathematical classes of equations. We demonstrate the versatility of ANTARES with examples from general hydrodynamics, solar physics, and the physics of white dwarf stars. Specifically, these are a demonstration of layer formation in double diffusive flows, how to catch driving and damping of stellar osciallations, and how fluid is mixed by convection into stably stratified regions of a star. Each of these examples is also motivated by explaining the general physical and specific astrophysical interests in underlying questions driving that kind of research.

Jan 24 2017


Prof. Stefan Fredenhagen
Mathematical Physics, UV

Aspects of higher spins

Interacting gauge theories of fields of higher spins are difficult to construct, and the only known examples are the Vasiliev theories. I will try to explain the approach taken in these constructions, in particular the so-called unfolded formulation of the equations of motion. Although the Vasiliev theories in principle provide consistent non-linear systems of higher-spin gauge fields, attempts to extract concrete equations of motion have run into difficulties, and I will sketch some of the resulting conceptual challenges.

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