## Definition **Loop Quantum Gravity (LQG)** is a background-independent, non-perturbative approach to quantum gravity that quantises spacetime itself without introducing additional assumptions beyond General Relativity and Quantum Mechanics. Its central prediction is that space is discrete at the smallest scales: it is composed of indivisible "atoms of space" (spin-network nodes and their connecting loops) rather than being a smooth continuum. Time, at the fundamental level, also disappears as an independent variable. ## The Core Argument General Relativity teaches that space is not a fixed container but a dynamical field — one that bends, stretches, and ripples. Quantum Mechanics teaches that every dynamical field has a granular, quantised structure (as photons are the quanta of the electromagnetic field). Combining these two lessons leads directly to the conclusion: the gravitational field — which *is* spacetime — must also have a granular, quantised structure. ## Atoms of Space LQG predicts a minimum length scale, the **Planck length** $\ell_P \approx 1.6 \times 10^{-35}$ m (about $10^{20}$ times smaller than the atomic nucleus). Below this scale, the classical notion of space ceases to make sense. Space is woven from discrete quanta called *spin-network nodes*, connected by *loops* (hence the name). These loops are not embedded *in* space; they *are* space — the network of their mutual relations constitutes the spatial fabric. ## No Fundamental Time In LQG, the equations describing the evolution of quanta of space and matter do not contain a universal time variable. This does not mean nothing changes; it means that change is everywhere but is not ordered by a single global clock. At the Planck scale, each elementary process evolves relative to its neighbours, not relative to a background time. The experienced flow of time emerges at larger scales — ultimately from thermodynamics (see [[Entropy and the Arrow of Time]]). ## Experimental Status LQG has not yet been directly confirmed experimentally, as the Planck scale is far beyond the reach of current instruments. Research directions include: - Possible signatures in the polarisation of gamma-ray bursts (Lorentz invariance tests). - Predictions about the explosion of ancient black holes ([[Planck Star and the Big Bounce]]). - Constraints from cosmic microwave background data. ## Consequences: Planck Stars and the Big Bounce Inside a black hole, LQG predicts that matter cannot collapse to a true singularity — no truly infinitesimal points exist. Instead, quantum pressure halts the collapse at a Planck-density state (the Planck Star), after which the matter bounces outward. Applied to the Big Bang: the universe's birth may have been a *Big Bounce* — a rebound from a prior contracting phase — rather than an origin from nothing. See [[Planck Star and the Big Bounce]]. ## Relation to Other Approaches Loop Quantum Gravity is conservative in the sense that it introduces no new entities (no extra dimensions, no supersymmetry) beyond the two established frameworks. String Theory, the other major quantum gravity candidate, takes the opposite approach, hypothesising new fundamental objects (strings) and extra dimensions. As of Rovelli's writing, neither approach has experimental confirmation, but LQG makes concrete predictions at astrophysical scales that may be testable. ## Related - [[The Problem of Quantum Gravity]] - [[General Relativity]] - [[Quantum Mechanics]] - [[Planck Star and the Big Bounce]] - [[Black Holes and Hawking Radiation]] - [[Entropy and the Arrow of Time]] ## Sources - [[Seven Brief Lessons on Physics (Rovelli 2014)]]