How Far Are We from the Quantum Theory of Gravity?

Abstract

I give a pedagogical explanation of what it is about quantization that makes general relativity go from being a nearly perfect classical theory to a very problematic quantum one. I also explain why some quantization of gravity is unavoidable, why quantum field theories have divergences, why the divergences of quantum general relativity are worse than those of the other forces, what physicists think this means and what they might do with a consistent theory of quantum gravity if they had one. Finally, I discuss the quantum gravitational data that have recently become available from cosmology.

arXiv comment106 page review article solicited by Reports on Progress in Physics

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• Perturbative Quantum Gravity as a Double Copy of Gauge Theory
• Quantum Spacetime Phenomenology
• Simplifying Multiloop Integrands and Ultraviolet Divergences of Gauge Theory and Gravity Amplitudes
• General Relativity and Cosmology: Unsolved Questions and Future Directions
• Basics of Generalized Unitarity
• Network Cosmology
• The new lunar ephemeris INPOP17a and its application to fundamental physics
• Designing a space-based galaxy redshift survey to probe dark energy
• Using Cosmology to Establish the Quantization of Gravity
• Field Parametrization Dependence in Asymptotically Safe Quantum Gravity
• Perturbative Quantum Gravity Comes of Age
• An Optomechanical Quantum Cavendish Experiment
• D = 5 maximally supersymmetric Yang-Mills theory diverges at six loops
• Quantum correlation of light mediated by gravity
• Gravity-induced entanglement in optomechanical systems
• Limit on graviton mass from galaxy cluster Abell 1689
• Vacuum Fluctuations of a Scalar Field during Inflation: Quantum versus Stochastic Analysis
• Quantum Gravity: A Brief History of Ideas and Some Prospects
• Scalar Contribution to the Graviton Self-Energy during Inflation
• Loop Quantum Gravity
• Running vacuum in QFT in FLRW spacetime: The dynamics of ${\rho }_{\mathrm{vac}}(H)$ from the quantized matter fields
• On powercounting in perturbative quantum gravity theories through color-kinematic duality
• $d+1$ formalism in Einstein-scalar-Gauss-Bonnet gravity
• Graviton and gluon scattering from first principles
• Model Comparison tests of modified gravity from the Eöt-Wash experiment
• Quantum Stress Tensor Fluctuations of a Conformal Field and Inflationary Cosmology
• One-loop quantum electrodynamic correction to the gravitational potentials on de Sitter spacetime
• Emergent Gravity as the Eraser of Anomalous Gauge Boson Masses, and QFT-GR Concord
• Volume average regularization for the Wheeler-DeWitt equation
• Slowly Rotating Black Hole Solutions to Hořava-Lifshitz Gravity
• One loop graviton corrections to dynamical photons in de Sitter
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• Quantum astrometric observables I: time delay in classical and quantum gravity
• Quantum Effects of Mass on Scalar Field Correlations and Fluctuations during Inflation
• Computing the Primordial Power Spectra Directly
• Pure Gravitational Back-Reaction Observables
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• The Structure of Multiloop Amplitudes in Gauge and Gravity Theories
• Spacetime and deformations of special relativistic kinematics
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• Non-Gaussianity from Features
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• Imaging of PbWO4 Crystals for G Experiment Test Masses Using a Laser Interferometer
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