# Global Structure in Semi-Classical Gravity

21.07.2022 – 23.07.2022

## Idea and Motivation

Semi-classical gravity (SCG)---the coupling of the dynamics of quantum fields to the classical spacetime geometry of general relativity byway of the semi-classical Einstein field equation (SCEFE)---is the framework within which black hole thermodynamics is formulated. As such, it is widely held to provide the most secure theoretical clues

to a search for a theory of quantum gravity. Considered in its own right, however, it already presents us with a plethora of foundational and conceptual problems that require investigation, even over and above the most famous one, Hawking's Information-Loss Paradox. In what sense is it legitimate to think of black holes as thermodynamical systems? What is the causal structure of an evaporating black hole spacetime? What is black hole entropy? How does the Generalized Second Law ramify into the host of other problems of fundamental physical and philosophical import that the ordinary Second Law does, from the arrow of time and the nature of spacetime to issues of probability, causality, predictability and determinism? What is the difference between geometry and matter? Are there singularities in the semi-classical regime, and does a form of cosmic censorship hold in it? Are there physically reasonable full solutions to the SCEFE? What role do classical energy conditions and quantum energy inequalities play, and how do they relate to entropy and information? How does holography relate to and constrain all this?

This will be the first major conference to address foundational issues associated with SCG, and to try to examine how they all bear on each other. Invited physicists and philosophers have been chosen to represent different approaches to SCG (quantum field theory on curved spacetime, emergent gravity, canonical gravity, holography) and to discuss different kinds of issues (locality, information loss, the nature of black hole entropy, singularities and cosmic censorship, epistemology, methodology, ontology, inter alia), in an attempt to foster a dialogue among the different fields. Contributed talks from both physicists and philosophers will be chosen in part so as to complement the perspectives of the invited speakers. Such a gathering will spur new, innovative approaches to the problem, as well as connecting and invigorating work on existing approaches. It will also provide young researchers with a comprehensive introduction to the state of the art of this central field of research, and established researchers with a comprehensive overview.