Causal Quantum Spacetime Theory (CQST): A Framework for Emergent Geometry and Its Observational Test

The Causal Quantum Spacetime Theory (CQST) treats spacetime as emergent from a discrete causal substrate, the quantum spacetime regenerating (QSR) lattice, assembling its geometric, holographic, quantum-foundational and phenomenological strands. The framework identifies the QSR lattice with Fuller's Isotropic Vector Matrix and adopts holographic scaling together with the Bekenstein–Hawking one-quarter factor. The vacuum causal density, the squared ratio of the Planck length to the de Sitter radius fixed by Λ, is a constant of spacetime; its logarithmic relation to the fine-structure constant α is a one-parameter calibration predicting no drift. The reduced Planck constant is reinterpreted as the action of one causal tick. The empirical content lies in the photon sector: the discrete electromagnetic action on the face-centred cubic lattice is built over its twelve nearest-neighbour vectors and the propagator obtained within the Brillouin zone. The linear coefficient vanishes identically, helicity-odd birefringence is excluded by inversion, and the leading quadratic correction is subluminal in every direction, with isotropic part 1/20 in units of the squared nearest-neighbour separation, universal over centrosymmetric equal-bond graphs, and an octahedral modulation of 5/18. The quadratic sector lies far below current sensitivity but the foregoing three predictions are exact: any contrary observation rules the framework out.