Four Null Microscopic Spacetime Theory

This paper proposes that spacetime has a microscopic causal structure beneath the smooth geometry of general relativity. The basic microscopic object is a spacetime cell generated by four future directed lightlike (null) directions. These directions form a local causal frame. Although each generator is individually null, the four generators and their six mutual pairings \(G_{ab}=q_a\cdot q_b\) reconstruct the spacetime metric exactly,\[ g^{\mu\nu}=\left(G^{-1}\right)^{ab}q_a^{\mu}q_b^{\nu}. \]The smooth metric of general relativity is the large scale description of this cell structure. At this microscopic level gravity is organized differently. The two physical gravitational wave polarizations are carried by the shear of the null geometry and propagate from cell to cell by a discrete Huygens law. The scalar and vector parts of gravity do not become additional waves; they remain constrained by the null Raychaudhuri and Damour equations. Each cell also determines its own local clock, a timelike direction fixed directly by its four null generators, so no independent preferred frame field is introduced. The radiative law contains one finite cell coefficient, fixed exactly by the specified four null two shell construction:\[ a_4=\frac{51491}{600000}, \]and reduces at long wavelengths to the massless gravitational wave equation of general relativity. The free theory has two degenerate polarizations, no birefringence, positive residue, causal propagation across the tested band, and positive discrete energy. Variational integrability tests reproduce the Einstein interaction at leading order and select, within the complete tensor basis tested, a unique \(O(L_c^2)\) completion with\[ x=8a_4; \]these nonlinear coefficients are supported by strong numerical certificates rather than a closed analytic proof. The same four null cell also supports electromagnetism. Potentials live on generator links and field strengths on the six bivector channels, whose exact Hodge structure splits them into electric and magnetic sectors. The resulting Maxwell construction satisfies the expected gauge constraints and Einstein--Maxwell curvature coupling. More generally, the cell carries a \((1,4,6,4,1)\) hierarchy of geometric objects connected by the boundary relation\[ \partial^2=0, \]linking its one through four dimensional structures within a single causal framework. The proposal therefore provides a candidate microscopic spacetime structure with an Einstein--Maxwell infrared limit and definite finite cell corrections. General three dimensional nonlinear constraint preservation, evolution through caustics, microscopic selection of the shell separation, and full quantum consistency remain open.