The Photon Sector of Directional Anisotropy Gravity: Electromagnetic Field as the Antisymmetric Sector of the DAG Substrate Direction Field

The photon sector of Directional Anisotropy Gravity (DAG) derives classical electromagnetism from the antisymmetric differential sector of the DAG substrate direction field. The electromagnetic field tensor is identified as \(Fμν=∂_μ n_ν-∂_ν n_μ\), where \(n_μ\) is the substrate direction 1-form. From this identification the paper derives the homogeneous Maxwell equations, classical abelian gauge redundancy, the uniqueness of the parity-even Maxwell kinetic action, the inhomogeneous Maxwell equations, the Lorenz representative from the co-closed Hodge projection, and the massless two-polarisation photon structure. The paper also formulates the open quantum and phenomenological steps required to complete the sector: the derivation of vacuum permeability and the fine-structure constant from the substrate Hamiltonian, the compact \(U(1)\) matter sector, and the explicit electromagnetic Nakajima-Zwanzig memory kernel. It gives falsifiable observational predictions, including quadratic Lorentz-invariance-violation effects, zero photon mass, a fractional \(1/12\) correction to QED vacuum birefringence near magnetars, possible circular dichroism from DAG parity breaking, and a conditionally derived cosmic birefringence range of \(0.05^∘\)-\(0.5^∘\). This standalone paper is a companion to the complete DAG v12 paper, DOI: https://doi.org/10.5281/zenodo.20071357.