The greatest challenge in fundamental physics attempts to reconcile quantum mechanics and general relativity in a theory of "quantum gravity." The project suggests a profound revision of the notions of space, time and matter. It has become a key topic of debate and collaboration between physicists and philosophers. This volume collects classic and original contributions from leading experts in both fields for a provocative discussion of the issues. It contains accessible introductions to the main and less-well-known known approaches to quantum gravity. It includes exciting topics such as the fate of spacetime in various theories, the so-called "problem of time" in canonical quantum gravity, black hole thermodynamics, and the relationship between the interpretation of quantum theory and quantum gravity. This book will be essential reading for anyone interested in the profound implications of trying to marry the two most important theories in physics.

### Contents

Preface; 1. Introduction Craig Callendar and Nick Huggett; Part I. Theories of Quantum Gravity and their Philosophical Dimensions: 2. Spacetime and the philosophical challenge of quantum gravity Jeremy Butterfield and Christopher Isham; 3. Naive quantum gravity Steven Weinstein; 4. Quantum spacetime: what do we know? Carlo Rovelli; Part II. Strings: 5. Reflections on the fate of spacetime Edward Witten; 6. A philosopher looks at string theory Robert Weingard; 7. Black holes, dumb holes, and entropy William G. Unruh; Part III. Topological Quantum Field Theory: 8. Higher-dimensional algebra and Planck scale physics John C. Baez; Part IV. Quantum Gravity and the Interpretation of General Relativity: 9. On general covariance and best matching Julian B. Barbour; 10. Pre-Socratic quantum gravity Gordon Belot and John Earman; 11. The origin of the spacetime metric: Bell's 'Lorentzian Pedagogy' and its significance in general relativity Harvey R. Brown and Oliver Pooley; Part IV. Quantum Gravity and the Interpretation of Quantum Mechanics: 12. Quantum spacetime without observers: ontological clarity and the conceptual foundations of quantum gravity Sheldon Goldstein and Stefan Teufel; 13. On gravity's role in quantum state reduction Roger Penrose; 14. Why the quantum must yield to gravity Joy Christian.