Crowther K. Why Do We Want a Theory of Quantum Gravity 2025

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Crowther K. Why Do We Want a Theory of Quantum Gravity 2025

Textbook in PDF format The search for a new scientific theory is typically prompted by an encounter with something in the world that cannot be explained by current theories. This is not the case for the search for a theory of quantum gravity, which has been primarily motivated by theoretical and philosophical concerns. This Element introduces some of the motivations for seeking a theory of quantum gravity, with the aim of instigating a more critical perspective on how they are used in defining and constraining the theory sought. These motivations include unification, incompatibilities between general relativity and quantum field theory, consistency, singularity resolution, and results from black hole thermodynamics. Introduction Motivations, Constraints, and Desiderata Relative Fundamentality Empirical Constraints Approaches to QG Incompatibilities between GR and QFT Background Independence Problem of Time Perturbative Non-renormalizability of Quantum GR UV Completeness The Primary Motivation: Reconciling the Two Pillars ‘Domains’ ‘Describes’ ‘Takes into Account Both QM and GR’ Three Pillars Perspective Delimitations Unification Quantization Hybrid Theories and Emergent Gravity Approaches Tabletop Experiments Singularity Resolution UV Divergences in QFT Landau Poles Responses to QFT Singularities Geodesic Incompleteness Curvature Singularities Responses to Spacetime Singularities Black Hole Thermodynamics Hawking Radiation Black Hole Information Paradox Entropy Bounds Holographic Principle References Acknowledgements