B.-L. Hu, Maryland
Emergent / Quantum Gravity: Macroscopic and Microscopic Structures of Spacetime

Quantum gravity is a theory for the microscopic structure of spacetime. We challenge a half-century old supposition and practice that quantizing general relativity will produce such a theory. In our view general relativity is an effective theory valid only at the long wavelength, low energy limits, and the metric and connection forms are collective variables. Quantizing them will only lead to the equivalent of phonon dynamics of crystal lattices, not quantum electrodynamics of electrons and photons. The challenge of this new paradigm is to find ways to unravel the underlying microscopic structures from observed macroscopic phenomena (many to one relation), not unlike deducing the molecular constituents from hydrodynamics and kinetic theory, or universalities of microscopic theories from critical phenomena. We explore this `bottom-up’ approach, focusing on the foundational issues in quantum-classical and micro-macro interfaces, using ideas of nonequilibrium statistical mechanics and techniques from quantum field theory for strongly correlated systems, identifying the relation of gravity with matter fields.