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Relativity Seminar

usually takes place the first Thursday of each month at 13.30 IM, rear building, ground floor – please check the current calendar
Chair: Marcello Ortaggio. Deputy chair: Vojtěch Pravda
Old web page with some of the seminar history: .

Perturbative higher curvature corrections of black holes and black strings

Marcelo Oyarzo
Departamento de Física, Universidad de Concepción
Thursday, 22. September 2022 - 13:30 to 14:30
Institute of Mathematics of ASCR, Zitna  25, Praha 1
N.B. **Library**
I will talk about the corrections induced by higher curvature terms inspired in the low-energy effective action of string theory in the graviton-dilaton sector. We consider a suitable field redefinition in order to obtain a ghost free Lagrangian, which is consistent with the perturbative truncation coming from string theory. In this setup, we solve the field equations analytically for a spherically symmetric ansatz and for a slowly rotating black hole in D=4. Comments regarding the corrections to the spectrum of the Gregory-Laflamme instability due to the higher curvature terms in D=5 are also given.

High energy particle collisions near black holes and naked singularities: general explanation and classification of scenarios

Oleg B. Zaslavskii
Department of Physics and Technology, V. N. Kharkiv Karazin National University
Thursday, 16. June 2022 - 13:30 to 14:30
Institute of Mathematics of ASCR, Zitna  25, Praha 1
If two particles collide near a rotating black hole, their energy in the center of mass frame E_c.m. can become unbounded under certain conditions. This is the Banados-Silk-West (BSW) effect. We give a brief review and general physical explanation from the geometric and kinematic viewpoints. In doing so, the Killing energy E of debris at infinity remains, in general, restricted. If E is also unbounded, this is called the super-Penrose process. We elucidate when such a process is possible and give full classification of corresponding relativistic objects for rotating space-times. In particular, we show that it is possible for rotating wormholes. The key role in consideration is played by the Wald inequalities. We also discuss briefly the case of a pure electric super-Penrose process near black holes and singularities. It is valid even in the flat space-time.

Symmetry operators for the conformal wave equation in rotating black hole spacetimes

Finnian Gray
University of Waterloo and Perimeter Institute for Theoretical Physics
Thursday, 19. May 2022 - 13:30 to 14:30
Institute of Mathematics of ASCR, Zitna  25, Praha 1
rear building, ground floor
We present covariant symmetry operators for the conformal wave equation in the (off-shell) Kerr-NUT-AdS spacetimes. These operators fit within the general theory of conformally invariant operators and all mutually commute for the conformal class of related spacetimes. This guarantees the R-separability of the conformal wave equation therein. Key to constructing this physically motivated example is the principal Killing-Yano tensor and the tower of hidden symmetries that it generates.

The quest for best-case scenarios of energy extraction from black holes

Filip Hejda
Thursday, 5. May 2022 - 13:30 to 14:30
Institute of Mathematics of ASCR, Zitna  25, Praha 1
rear building, ground floor
Although Penrose process remains a textbook example of a mechanism to extract energy from black holes, it has several practical limitations. A major problem is that it requires a particle to split into two fragments with very high relative velocity. This issue can be circumvented either by considering particles influenced by electromagnetic interaction or by switching to particle collisions instead of decays. Regarding the collisional Penrose process, a lot of attention has been devoted to idealised, best-case scenarios of test particle collisions with arbitrarily high centre-of-mass collision energies in the vicinity of extremal black holes. Bañados, Silk and West (BSW) first described such a possibility for particles with fine-tuned angular momentum orbiting a maximally rotating black hole. However, even in this simplified setup, the energy that can be extracted from the black hole turned out to be subjected to unconditional upper bounds. On the other hand, no such bounds were found... more

Generalized Lense-Thirring spacetimes: higher curvature corrections and solutions with matter

David Kubizňák
Thursday, 3. March 2022 - 13:30 to 14:30
Institute of Mathematics of ASCR, Zitna  25, Praha 1
rear building, ground floor
The Lense-Thirring spacetime describes a 4-dimensional slowly rotating approximate
solution of vacuum Einstein equations valid to a linear order in rotation parameter.
It is fully characterized by a single metric function of the corresponding static
(Schwarzschild) solution. We shall discuss a generalization of the Lense-Thirring
spacetimes to the case that is not necessarily fully characterized by a single (static)
metric function. This generalization lets us study slowly rotating spacetimes in
various higher curvature gravities as well as in the presence of non-trivial matter
such as non-linear electrodynamics.  In particular, we construct slowly multiply-spinning
solutions in Lovelock gravity and notably show that in four dimensions Einstein gravity is
the only non-trivial theory amongst all up to quartic curvature gravities that
admits a Lense-Thirring solution characterized by a single metric function.
We will... more