Quantum gravity

Quantum gravity is the search for a framework that describes gravity using quantum principles. It addresses regimes where quantum effects and spacetime curvature both matter, such as near black-hole singularities or at the universe’s earliest moments.

String theory: A framework in which fundamental entities are one-dimensional strings whose vibrational states include particles and gravity. It incorporates gravity by including a massless spin-two string mode.
General relativity: Einstein’s theory in which gravity is the curvature of spacetime produced by energy and momentum. Its classical description supplies the gravitational dynamics quantum theories must recover.
Quantum mechanics: The physical theory describing matter and interactions through quantum states, probabilities, and observables. Any quantum theory of gravity must preserve quantum principles such as superposition.
Hawking radiation: Thermal radiation predicted to arise from quantum fields near a black hole’s event horizon. It links quantum field theory, gravity, and black-hole thermodynamics.
Holographic principle: The proposal that information in a region of space can be encoded by degrees of freedom on its boundary. Black-hole entropy motivates a possible reduction in the degrees of freedom needed to describe spacetime.
Loop quantum gravity: A program that quantizes general relativity using networks of quantum states for geometry. Unlike string theory, it seeks to quantize spacetime geometry without adding extra dimensions.
Quantum field theory in curved spacetime: A framework for quantum fields propagating on a prescribed curved spacetime background. It captures important quantum effects in gravity’s presence without quantizing spacetime.
Spacetime: The four-dimensional structure combining three spatial dimensions with time in relativistic physics. Quantum gravity asks whether spacetime remains fundamental or emerges from deeper quantum structure.
Black hole information paradox: The conflict between black-hole evaporation and quantum-mechanical conservation of information. Resolving it may reveal how a quantum theory encodes spacetime and evaporation.
AdS/CFT correspondence: A conjectured equivalence between a gravitational theory in anti-de Sitter space and a conformal field theory on its boundary. It gives a precise example in which quantum gravity can be described by a nongravitational quantum theory.

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