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**Time: ** 8. November 2024, 11:00h

**Place: ** Institute of Physics, room 360

**Speaker: ** Igor Prlina

**Title: ** If mixed states are secretly quickly oscillating pure states, weak measurements can detect it

**Abstract:**

The apparent nonunitary evolution in the black hole information paradox and recent work on describing wavefunction collapse via nonunitary nonlinear stochastic operators has motivated us to analyze whether mixed states can be distinguished from quickly oscillating pure states. We have demonstrated that the answer is no for all practical purposes if only strong nonpostselected measurements are performed. However, if weak measurements in postselected systems are used, mixed states and quickly oscillating states produce different results. An experimental procedure is proposed which could in principle determine the nature of mixed states stemming from blackbody radiation, decoherence, thermalization in solid state materials, Unruh radiation and Hawking radiation, among others. The analysis in this work applies to all fast oscillations, including those at Planck scale. As such, tabletop weak measurements can be used to probe (very specific) potential high energy behavior, where strong nonpostselected measurements cannot be applied. This work also demonstrates that weak measurements are not equivalent to a set of strong measurements without postselection since measurements which are impossible for all practical purposes need to be excluded.

**Time: ** 5. June 2024, 11:00h

**Place: ** Faculty of Physics, room 661

**Speaker: ** Juraj Tekel

**Title: ** Fuzzy physics and matrix models

**Abstract:**

The unification of quantum mechanics and gravity predicts a drastic change in the notion of space and time, at least at the length scales where both these theories are important. One realization of spaces with such quantum structures are noncommutative and fuzzy spaces. I will describe their construction and description of physics happening in such spaces in terms of random matrix models. I will then explain some of the techniques and results for relevant examples of matrix ensembles.

**Time: ** 24. May 2024, 11:00h

**Place: ** Faculty of Physics, room 665

**Speaker: ** Dusan Djordjevic

**Title: ** Topological quantum field theory

**Abstract:**

In this two-hour mini-course, we will present the motivation for studying topological quantum field theory (TQFT), give a formulation of TQFT, and work out a two-dimensional example. The lecture is intended for final-year undergraduate students, but anyone with some basic knowledge of quantum mechanics and mathematical physics (vector spaces, group theory) should be able to follow it. Some background in quantum field theory is welcomed, but not necessary for most of the talk (we will use relativistic quantum mechanics and/or quantum field theory only to motivate our study).

**Time: ** 17. May 2024, 11:00h

**Place: ** Institute of Physics, room 360

**Speaker: ** Pavle Stipsic

**Title: ** Symmetry breaking mechanisms of the 3BF action for the Standard Model coupled to gravity

**Abstract:**

We study the details of the explicit and spontaneous symmetry breaking of the constrained 3BF action representing the Standard Model coupled to Einstein-Cartan gravity. First, we discuss how each particular constraint breaks the original symmetry of the topological 3BF action. Then we investigate the spontaneous symmetry breaking and the Higgs mechanism for the electroweak theory in the constrained 3BF form, in order to demonstrate that they can indeed be performed in the framework of higher gauge theory. A formulation of the Proca action as a constrained 3BF theory is also studied in detail.

**Time: ** 26. April 2024, 11:00h

**Place: ** Institute of Physics, room 360

**Speaker: ** Benedek Bukor

**Title: ** What about quarkonia in the (non)commutative space?

**Abstract:**

Quarkonium bound states are especially promising candidates to test the probable quantum structure of space-time since they represent a system with a reasonably small characteristic distance. Although the theory of quarkonia is quantum chromodynamics, the interaction between the quarks can be heuristically described by the quantum mechanical Cornell potential as well. Firstly, we investigate the quarkonium system in the 3-dimensional smooth configuration space. Thereafter, we insert it into a non-commutative 3-dimensional configuration space, which is composed of concentric fuzzy spheres of increasing radius, called the fuzzy onion, in order to extract some consequences of the non-trivial structure of the space by means of the quarks' properties.

**Time: ** 12. April 2024, 11:00h

**Place: ** Institute of Physics, room 360

**Speaker: ** Guillaume Trojani

**Title: ** Perturbiner expansion: old and new

**Abstract:**

In their seminal, Berends and Giele identified an efficient algorithm for computing n-point tree level scattering amplitudes in gauge theory. Later, the pertubiner expansion was developed to generate the Berends-Giele recursion relation to study any quantum field theory at tree level. This talk aims to reintroduce these techniques in the light of the modern language of homotopy algebras, with an eye towards the recent development of loop level techniques. Indeed, the recursion relations are natural consequences of the Homotopy perturbation lemma relating an interacting field theory and its underlying free theory. Time permitting, I will comment on current work where we apply this algebraic machinery to braided field theories, which are noncommutative field theories deformed in a controlled manner.

**Time: ** 29. March 2024, 11:00h

**Place: ** Institute of Physics, room 360

**Speaker: ** Petar Petrasinovic

**Title: ** 2-groups and crossed modules: Classification and algorithm for constructing non-trivial ones

**Abstract:**

The 2-groups are category theory generalisations of the usual groups. They are equivalent to crossed modules, consisting of two groups, G and H, the homomorphism ∂ from H to G and the action ▷ from G to H. One can construct a gauge theory of gravity on a 4-manifold based on crossed modules (like the BFCG theory for the Poincaré 2-group). Besides the usual group axioms, crossed modules must satisfy additional axioms. For this reason, they are much more rigid structures and more difficult to construct. There are some typical examples, but they are very rare and in which either ∂ or ▷ is trivial. In this seminar, I will discuss the properties of their linear structure-differential crossed modules. A possible way to classify these modules will be shown. Together with the classification, a corresponding algorithm for constructing differential cross modules with non-trivial ∂ and ▷ will be presented, which constructs them either for given groups G and H or for given ∂ and ▷.

**Time: ** 15. March 2024, 11:00h

**Place: ** Institute of Physics, room 360

**Speaker: ** Petar Petrasinovic

**Title: ** Moore-Penrose inverse of matrix and its application to solving general linear systems

**Abstract:**

In linear algebra, the Moore-Penrose inverse stands as a generalization of the conventional matrix inverse, having been independently introduced by E.H. Moore in 1920 and later by Roger Penrose in 1955. The Moore-Penrose inverse is uniquely defined for all matrices whose entries are either real or complex numbers and extends its applicability to rectangular and even singular matrices. The versatility and uniqueness of the Moore-Penrose inverse are the major reasons for its significance (and discovery), which is reflected in solving general linear systems of equations, or tensor systems which can be reshaped into matrix form. In this seminar, I will discuss the properties of the Moore-Penrose inverse, display some of the calculation methods, and demonstrate its practical applications.

**Time: ** 23. February 2024, 11:00h

**Place: ** Institute of Physics, room 360

**Speaker: ** Mihailo Cubrovic

**Title: ** Chaos and factorization beyond classical horizons

**Abstract:**

Chaotic dynamics of black holes has proven to be of vital importance for the understanding of the black hole information problem. Within the replica formalism, a related issue is that of nonfactorization of partition functions of wormhole geometries. We consider these questions deep in the stringy regime, described by the BFSS and IKKT matrix models, where the notions of geometry and horizon do not apply anymore. Generic solutions factorize when averaged over the quantum fluctuations, whereas the chaotic properties are non-universal. We argue that the correct notion that characterizes the black hole scrambling is instead that of quantum magic (stabilizer Renyi entropy), which guarantees that the dynamics is complex enough to accommodate the total information on infalling matter.

**Time: ** 2. February 2024, 11:00h

**Place: ** Institute of Physics, room 360

**Speaker: ** Dusan Djordjevic

**Title: ** Massless minimally coupled scalar field in dS spacetime

**Abstract:**

In this talk, we will review the quantisation of scalar fields in de-Sitter spacetime. We will discuss the differences between the case of minimally coupled massive and minimally coupled massless scalar fields, as well as the possibility of obtaining, in the massless case, a vacuum state that is invariant under the full SO(4,1) group. The talk will be based on the following papers:

Klaus Kirsten, Jaume Garriga, arXiv:gr-qc/9305013

Andrew Tolley, Neil Turok, arXiv:hep-th/0108119

**Seminars for the year:**
2024
2023
2022
2021
2020
2019
2018
2017
2016
2015
2014
2013
2012
2011
2010
2009
2008
2007

**Follow our seminars online via:** GPF BigBlueButton server