-NEW Version 4.10, old V3.75- Removed any equations and content with logitudinal waves and wave speeds. There are no longitudinal waves in UST, Gravity waves are shear sector only, and Gravity, itself, is Static Pressure: The gravitational field is a state of bulk stress, not a fluctuating wave field or with gravitons. This is why it is not connected to the EM sector, its longitudinal in the elastic response. Author's Note: In this framework, Gravity is defined as the static bulk stress of the substratum (P_g). It is a state of the medium, not a distinct entity moving through it. Because Gravity is a static condition rather than a dynamic transmission, the bulk sector must be radiatively incompressible. If it supported propagating waves (gravitons), Gravity would detach from the medium and become a particle field, contradicting the fundamental premise of UST. Therefore, the absence of the Graviton and the absence of longitudinal radiation are two sides of the same coin: Gravity is not a wave or particle. AbstractUnified Spacetime Theory (UST) treats spacetime as one continuous medium. Its everyday behavior explains gravity and electromagnetism (and all forces and physics of the Universe to be revealed in future work), and it does so with a single, common wave speed (the light-cone speed). Structural Constraint: Radiative IncompressibilityWhile the Substratum supports static bulk strain (which manifests as mass and the Newtonian gravitational potential), it strictly prohibits the propagation of longitudinal waves. The vacuum is dynamically incompressible (∇ ⋅ ξ ≈ 0 for radiation). The framework is deliberately simple: it uses only measurable quantities and adds no extra dimensions, no speculative symmetry stacks, and no exotic particles. UST is not a patchwork of existing theories; it aims to reproduce known physics mechanically, within today’s measurement reach and within its own framework (built from scratch). In familiar limits, the usual laws reappear. Stable, localized patterns in the medium act like sources by carrying energy and momentum in ways we normally attribute to mass, charge, and current. What this paper doesIt spells out the assumptions and scope, build a clean time-evolution system, and proves two essentials: causality and positive energy. We then recover standard weak-field gravitational tests and Maxwell’s equations in a vacuum, with causal propagation at c. Finally, we provide technical and numerical checks so others can audit and reproduce the results. Minkowski emerges: UST doesn’t start by assuming a Minkowski spacetime (the usual flat, geometric backdrop). Instead, it begins with a single continuous medium that has ordinary material properties, stiffness and inertia, described by bulk, shear, and torsion moduli (a generalized Hooke’s law via the Unified Spacetime Elastic Modulation Principle, USEMP) and by density. These properties set how fast signals move and how disturbances behave. When the medium is uniform and gently excited, its waves share a single universal speed and familiar Lorentz behavior appears on its own. In that calm limit, the coordinate description you’d use to track those waves is indistinguishable from Minkowski’s, so flat spacetime emerges as a consequence, not an assumption. When the medium’s properties vary, softening or stiffening with energy and configuration, paths bend and clocks shift; that’s gravity in this picture. In short, UST starts from measurable material parameters of a spacetime medium and derives the Minkowski picture as the simplest, low-stress limit, rather than building the theory on Minkowski from the start. Coming Next: The foundational series “The Mechanical Foundations of a Unified Spacetime” from UST is organized as a ten-paper series, all built on the same mechanical medium developed here. After the foundations (USEMP) and the Lorentz-invariant formulation, and Spin 1/2 paper, the next papers, in order, are Black Holes and Quantum Gravity: The Mechanical Origin of Hawking Radiation, Dark Mass, History of the Universe, The Strong Force, The Weak Force, Emergence of Quantum Dynamics: Deriving the Schrödinger Equation from Continuum Mechanics, and finally The Particle Zoo: Deriving the Mass Spectrum from Solitonic Resonance. Together they extend the same underlying medium to black holes and Hawking radiation, dark mass, cosmology, the strong and weak interactions, the emergence of quantum dynamics, and the full particle zoo, always with the goal of giving a minimal, reproducible and testable mechanical account of observed physics rather than stitching together separate theories by hand. Spin 1/2 paper https://doi.org/10.5281/zenodo.17795865 Black Hole and Quantum Gravity paperhttps://doi.org/10.5281/zenodo.17804079 Contact: jared@ustphysics.org