Mathematisches Oberseminar: Quantenmechanische Vielteilchensysteme und relativistische Quantentheorie in Erinnerung an Prof. Dr. Detlef DÃ¼rr
In the summer term 2021 from
Dr. Dirk Deckert,
and
Prof. Dr. Peter Pickl
The seminar is on Wednesdays, 16:15h, starting on the 21st of April and will take place on Zoom. Please contact us for the session link if you are interested to join.
In honor and remembrance of our beloved member and teacher
Prof. Dr. Detlef DÃ¼rr
Organizer: Siddhant Das
News:
Updates may be distributed on short notice by mail to all people on an internal list. People interested in should contact
Dr. Dirk Deckert
or
Prof. Dr. Peter Pickl.
Talks
Date 
Room 
Title 
Speaker 
Type 
Wed 21.04.21, 16:15 
Zoom 
Effects of disturbance of single particles in a $N$ particle Newtonian system. Part I
We consider a system consisting of $N$ interacting particles subject to Newtonian time evolution. By a probabilistic meanfield approach we will show, that a small displacement of a particle at the beginning entails a small effect for the dynamics of the whole system, i.e. the distance between the true dynamics and the disturbed dynamic will be small for later times.

Manuela Feistl

internal talk 
Wed 28.04.21, 16:15 
Zoom 
Shapes, Measures, and the Universe

Dr. Paula Reichert

internal talk 
Wed 05.05.21, 16:15 
Zoom 
Tensor networks to solve relativistic quantum field theory
Tensor networks are a popular method to solve strongly coupled manybody quantum systems on the lattice nonperturbatively. I will present recent efforts to extend this success to relativistic quantum field theory (QFT), and in particular to superrenormalizable ones. To this end, I will introduce the basics of scalar quantum field theory, starting from a condensed matter perspective, explain the difficulties to define QFT properly and recall how they can be addressed rigorously (in the superrenormalizable case). Then I will discuss various ways to solve such QFT nonperturbatively with tensor networks, with a fine discretization, or even with no cutoff whatsoever in 1+1 dimensions.

Dr. Antoine Tilloy

external talk 
Wed 12.05.21, 16:15 
Zoom 
Effects of disturbance of single particles in a $N$ particle
Newtonian system. Part II
We consider a system consisting of $N$ interacting particles
subject to Newtonian time evolution.
Manuela Feistl was able to show that a small displacement of one particle only
leads to a small disturbance of the system at later times.
By considering a differentiable force we can strengthen the result and
give an approximation for the leading term of the distance between the
true and the disturbed system.

Kajetan SÃ¶hnen

internal talk 
Wed 19.05.21, 16:15 
Zoom 
cancelled

tba

tba 
Wed 26.05.21, 16:15 
Zoom 
Position measurement in Bohmian mechanics and Wilson cloud chamber problem
Wilson cloud chamber is the first detector of elementary particles, which was invented by C.T.R. Wilson around 1905. Although it is relatively simple set up â€” a box filled with supersaturated vapor of water or alcohol â€” itâ€™s the prototype of all modern particle detectors.
Related to the cloud chamber are two important questions.
1. Why what is going on in the chamber looks like that? Why do we see straight tracks of droplets resembling the trajectories of free classical particles? This question is known as Wilson cloud chamber problem and has been posed already in 1927.
2. Do the droplets really reveal the places, where the particle was? In other words, does the cloud chamber really measure the position of the particle?
My PhDthesis is devoted to these two questions. In my talk Iâ€™ll tell, which contribution we have made to their resolution.

Serj Aristarkhov

internal talk 
Wed 02.06.21, 16:15 
Zoom 
Typicality Measures
Typicality measures play a central role in the statistical analysis of deterministic theories. In fact, the empirical import of a microscopic theory comes almost entirely from typicality results. But what determines the right typicality measure? Does it have to be stationary? Does it have to be particularly simple or uniform? Is the measure itself empirical, an independent postulate of the theory, or somehow entailed by the dynamical laws? The talk will address such questions and some of the current debates surrounding them.

Dr. Dustin Lazarovici

external talk 
Wed 09.06.21, 16:15 
Zoom 
Improved spindependent arrivaltime distributions
A novel arrivaltime experimental setup and theoretical/numerical analysis thereof will be presented that displays a much more robust and experimentally significant suppression of particle arrivaltimes compared to our original proposal.

Siddhant Das

internal talk 
Wed 16.07.21, 16:15 
Zoom 
Time and Scale in Quantum Gravity
A quantum theory of gravity novel in its treatment of both time and scale is proposed. The fundamental entities are threedimensional shapes of a dynamically closed universe. The simplest invariant of the symmetry group that defines the shapes is called their complexity. It plays three roles simultaneously: as a scaleinvariant potential; as the intrinsic size of the universe; and, decisively, as time. A dimensionless wave equation determines relative probabilities of all shapes with a given complexity and hence time. A heuristic argument indicates that the theory has a unique solution. According to it, the history of the universe begins at the unique shape with lowest possible complexity and hence greatest possible uniformity; all the infinitely many possible subsequent shapes have an ever increasing complexity (time never ends). My talk will concentrate on the motivation for this radical proposal and some of its implications if it turns out to have a sound basis.

Dr. Julian Barbour

external talk 
Wed 23.06.21, 16:15 
Zoom 
Lorentz invariance and quantum mechanics
Bohmian mechanics and spontaneous collapse models are theories that overcome the measurement problem of quantum mechanics. While these theories are naturally formulated for nonrelativistic systems, it has proven challenging to provide a Lorentz invariant extension of these theories. The source of the difficulty seems to be the nonlocality which is unavoidable due to Bell's theorem. In a sense it is easy to make a theory Lorentz invariant, but it is hard to achieve what Bell dubbed ``serious Lorentz invariance''. The notion of ``serious Lorentz invariance'' itself is even hard to make precise. This is reminiscent of the debate on the meaning of general coordinate invariance in Einstein's theory of general relativity. The issue there is whether the requirement of general invariance is vacuous (in the sense that most theories can be made generally invariant) or whether it is a fundamental physical principle. In this talk, I want use two of the more promising avenues that emerged from that debate in order to try to get a handle on what serious Lorentz invariance could mean. First, I will consider Anderson's characterization based on the identification of absolute objects. As has been observed before, this characterization is not without problems and it will be seen that similar problems resurface in this context. Second, I will consider a relativity principle for isolated subsystems. I will evaluate a number of Lorentz invariant models of Bohmian mechanics and spontaneous collapse models in this light.

Dr. Ward Struyve

external talk 
Wed 30.06.21, 16:15 
Zoom 
New results about the dynamics of tracer particles in a dense Fermi gas

David Mitrouskas

external talk 
Wed 07.07.21, 16:15 
Zoom 
Positron Position Operators
I address the question of what kind of position operators should accompany the free standard quantized Dirac field. I propose a POVM on a configuration space of particle positions of two kinds of point particles, positrons and electrons, acting on the Hilbert space of the quantized Dirac field. It is supposed to serve in several roles of position operators: (i) characterizing what an ideal detector sees, (ii) clarifying how probabilities of macroscopic configurations are provided by the quantum state, and (iii) defining the distribution of Bohmian particles. The existence of this POVM, which I call P_natural, depends on a mathematical conjecture which at present I can neither prove nor disprove; in the talk I explore consequences of the conjecture. P_natural is different from the obvious POVM that one would guess from the wave function representation (as in Thallerâ€™s book) of Fock space vectors; several considerations make P_natural seem more physically plausible than P_obvious. I will also explain what the corresponding Bohmian trajectories look like, how P_natural avoids Malamentâ€™s nogo theorem, how it arises naturally as a limiting object from taking the Dirac sea literally, and how it is expected to extend to the cases with external fields, curved spacetime, and interaction.

Dr. Roderich Tumulka

external talk 
Wed 14.07.21, 16:15 
Zoom 
Spindependent arrival times of Dirac electrons

Siddhant Das

internal talk 