Loop quantum gravity

Loop quantum gravity is a theoretical approach to quantum gravity that quantizes spacetime geometry. Its states assign discrete spectra to geometric quantities such as area and volume.

Spin network: A graph whose edges and vertices carry representation labels and encode quantum states of geometry. Spin networks provide the kinematic states from which loop quantum gravity constructs spatial geometry.
General relativity: Einstein’s theory in which gravity is the curvature of spacetime produced by matter and energy. Loop quantum gravity seeks to quantize general relativity without treating spacetime as a fixed background.
String theory: A framework in which fundamental entities are one-dimensional strings whose excitations include particle-like states. Unlike loop quantum gravity, it typically seeks quantum gravity within a broader unifying framework.
Loop quantum cosmology: A symmetry-reduced application of loop quantum gravity to homogeneous cosmological models. It uses related quantization ideas to study the universe’s early evolution and replace classical singular behavior.
Problem of time: The difficulty of recovering physical time evolution in generally covariant quantum theories. The Hamiltonian constraint leaves the role of time in loop quantum gravity nonstandard.
Spin foam: A history-like structure formed by evolving spin networks, used to describe quantum-geometric dynamics. Spin foams represent transitions between the theory’s spatial quantum states.
Quantum mechanics: The physical theory describing matter and interactions through quantum states, observables, and probabilities. The approach applies quantum principles to the geometry that general relativity treats classically.
Perturbative quantum gravity: An approach that quantizes small fluctuations of the gravitational field around a chosen background spacetime. Its fixed-background expansion contrasts with loop quantum gravity’s background-independent formulation.
Big Bounce: A cosmological scenario in which a contracting universe transitions into expansion instead of emerging from a singularity. Some loop quantum cosmology models predict a bounce where classical general relativity predicts a Big Bang singularity.
Semiclassical limit: The regime in which quantum descriptions reproduce the predictions of a classical theory. The theory must recover smooth spacetime and Einstein’s equations at scales far above the Planck length.

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