Relativity seminar

Seminar is held every Tuesdat at 13:10 (if not explicitly said otherwise) in the main lecture hall of ITP FMP CUNI at the 10th floor of the department building in Troja, V Holešovičkách 2, Praha 8

Next seminars:

13 October 2026: Physical Black Holes: Properties and Observational Tests; Prof. Daniel Terno; Macquarie University in Sydney; (seminar in English); A physical black hole is a light-trapping region whose apparent horizon forms in finite time for a distant observer, irrespective of whether an event horizon exists. Recent work shows that, if such objects form, their near-horizon geometry is not merely a dynamical version of the Schwarzschild or Kerr picture: it has unusual structural properties and features that differ substantially from eternal black holes. The central question is whether these differences can become observationally relevant. We take first steps toward this problem by developing effective near-horizon and interpolating descriptions suitable for computing signatures such as light rings and quasinormal modes. In addition, cosmological embedding provides a sharp test: hypothetical physical black holes do not couple to the large-scale structure of the Universe. Together, these results begin to translate the finite-time formation requirement into observational signatures and broader tests of the standard black-hole paradigm.
18 October 2026: Beyond the Kerr paradigm: the dynamical fate of black-hole interiors; Prof. Stefano Liberati; SISSA; (seminar in English); Black holes are usually pictured as long-lived Kerr geometries formed by gravitational collapse, changed only gradually by slow, nearly adiabatic Hawking evaporation. But is this picture right? Classical black-hole interiors contain spacetime singularities, which quantum gravity is expected to resolve, and Cauchy horizons, which are generically unstable under classical and semiclassical perturbations. The usual assumption is that the resulting new physics stays hidden behind the event horizon until late in the evaporation. I will argue that this assumption should be reconsidered. Starting from the hypotheses of the singularity theorems, I will outline a geometric classification of regular black holes and their horizonless counterparts. I will then ask whether their interiors can form and persist in realistic semiclassical collapse. Recent semiclassical numerical results point to cascades of trapped and anti-trapped regions. I will show that complementary analytical models capture this behaviour qualitatively. On this view, black holes may be transient phases of a regular, evolving spacetime, and their interiors may actively shape their fate. I will discuss the evidence for this and the self-consistent backreaction calculations needed to establish it.
10 November 2026: Black hole thermodynamics beyond leading order in higher-curvature gravity; Dr. Cristóbal Laporte; University of Barcelona; (seminar in English); Higher-curvature terms in effective theories of gravity modify the thermodynamics of black holes. The Reall–Santos method offers a shortcut: the leading correction to the free energy follows from the uncorrected solution alone, with no need to solve the modified field equations. In this talk, I apply this approach to a range of higher-derivative extensions of the Einstein–Hilbert action, focusing on quasi-topological gravities up to quintic order in arbitrary dimensions and on generalized quasi-topological gravities. Since these theories admit exact or well-controlled static black hole solutions, they offer a clean testing ground for the perturbative method at high orders. I also discuss what changes when the action includes covariant derivatives of the curvature, and outline how the method can be extended to Kerr-type ansatz, where exact solutions are no longer available and a perturbative treatment becomes essential.

Past seminars:

6 October 2026: Classical-quantum gravity as quantum gravity in disguise; Dr. Sebastian Murk; Charles University; (seminar in English); Whether gravity must be quantized remains one of the biggest open problems in fundamental physics. Classical-quantum hybrid theories have recently attracted attention as a possible framework in which gravity is treated classically yet interacts consistently with quantum matter. In particular, schemes based on completely positive dynamics preserve a consistent probabilistic description while enabling a systematic treatment of quantum backreaction. In this talk, I will show that completely positive classical-quantum dynamics admits an embedding into a fully quantum theory on an enlarged Hilbert space that faithfully reproduces the hybrid evolution. Consequently, experimental agreement with the predictions of this framework would not by itself establish that gravity is fundamentally classical. In the quantum realization, the hybrid degrees of freedom form an open subsystem, for which symmetries of the dynamics need not imply the corresponding conservation laws. As an illustrative example, I will consider a fully closed hybrid system consisting of a qubit interacting with a classical particle and show that its completely positive dynamics violates total angular momentum conservation despite rotational symmetry of the equations of motion.

David Kubizňák <david.kubiznak@matfyz.cuni.cz>; Oldřich Semerák <oldrich.semerak@matfyz.cuni.cz>

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