Chapter 03 · The Nature of Space and Time
The Nature of Space & Time
Is spacetime fundamental, or does it emerge from something deeper?
General relativity treats spacetime as a smooth continuum, the arena where everything else happens. Quantum gravity questions this. One line of work treats geometry itself as emergent, derived from quantum entanglement, thermodynamic relations, or a discrete causal order. Another line accepts that spacetime is fundamental but quantizes it directly, building continuum geometry out of discrete spin networks. Neither approach has produced a clean test that decides between them. Both remain live.
Families (4)
- FrontierLoop Quantum GravitySpacetime is quantized. Geometry has a smallest possible area and volume, and the universe is built from a network of these atomic geometric units.2 variants
- FrontierEmergent Spacetime & GravitySpacetime may not be fundamental. It could emerge from quantum entanglement, thermodynamic relations, or a discrete causal order, different proposals, same starting commitment.4 variants · Verlinde bridges to Ch. 5, Holographic bridges to Ch. 6
- FrontierCausal Set TheorySpacetime is a discrete network of cause-and-effect relationships, not a smooth manifold. Order plus number equals geometry. The Sorkin program is one of the longest-running discrete-spacetime quantum-gravity research lines, with one striking quasi-empirical prediction (the magnitude of the cosmological constant).5 variants · Phenomenology variant bridges to Ch. 5 Dark Universe (Λ fluctuations)
- FrontierCausal Dynamical TriangulationBuild spacetime from simple geometric blocks glued along a strict arrow of cause and effect. Run the path integral on a computer. A four-dimensional universe self-organizes from the discrete data. The most computationally productive discrete-spacetime program, with concrete numerical results no other quantum-gravity approach has produced at comparable detail.4 variants · Quantum Microstructure variant cross-references Ch. 4 Asymptotic Safety on dimensional reduction. Locally Causal CDT serves as a lattice laboratory for testing the Asymptotic Safety fixed-point program from the discrete side.
Bridged from other chapters
These variants live in their own chapters but also answer this chapter's question.