QuEnG Seminar, Foundations - Conceptual problems in the characterization of dynamical black holes: an invitation to quasi-locality issues in (classical) Gravity

on the May 15, 2019

At 4:00pm
As part of the next QuEnG Seminar - Foundations, we are pleased to welcome José Luis Jaramillo from the Institut de Mathématiques de Bourgogne, Université de Bourgogne Franche-Comté.
Strongly dynamical black holes are often characterized in "quasi-local" terms, as world-tubes of so-called apparent horizons, namely geometric models of the black hole horizon tracking the local convergence of light rays. This is in contrast with "causal" approaches to black holes, that involve fully global notions of the spacetime and are not well adapted to the monitoring of the dynamical evolution. The quasi-local approach has proved to be a powerful tool in the study of the strongly dynamical regime and, in particular, it is part of the everyday practice in the study of astrophysical dynamical black holes (e.g. in binary collisions in gravitational wave physics). However, such "practically successful" quasi-local approach comes with a conceptual price to pay: on the one hand, the evolution of an apparent horizon is not fully specified in terms of its state at a given instant and, on the other hand, such evolution is at odds with the causal behaviour expected from physical objects. In a sense that can be made precise, apparent horizon worldtubes present a "gauge nature” whose physical content is not straightforwardly assessed. The first part of the talk aims at presenting such "physical versus gauge" issues in quasi-local approaches to black holes.

The second part of the talk has a more "open-discussion" character. Building on the presented black hole analysis as a study case, we will discuss the interplay in General Relativity among: i) quasi-locality of the gravitational field, ii) the definition of physical quantities (with a focus on Mass/Energy), and iii) the need to resort to external (rigid) structures to make specific physical statements. A tension between "fundamental versus effective" descriptions of physical objects emerges from this interplay, leading us to a reflection on the role of "universality" in characterising physically relevant properties.

If you don't usually have acces to the Institut Néel main site (CNRS campus, Presqu'île) send an email to Cyril BRANCIARD before 12:00pm on 14th of May so that your entry passes can be sent to you.

Published on June 16, 2020

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