Expanded Theory of Relativity: Demonstrating Quantum Mechanics as an Integral Part of Relativity theory

We develop a framework based on maximal spatiotemporal symmetry in which time is promoted to an on-shell three-dimensional vector subspace. Beginning from a general basis-independent $p$-temporal plus $q$-spatial linear transformation, the Principle of Relativity together with vectorial time selects a Lorentzian invariant for physically admissible transformations in the full $(p+q)$-dimensional spacetime. Compatibility of temporal-frame transport with this invariant fixes the direction of causal temporal translation, deriving the arrow of time from preservation of the relativistic interval and excluding the usual causal pathologies of multi-time theories. The temporal subspace nevertheless retains a residual rotation within the stabilizer of the causal direction. Relating this transverse rotation to four-dimensional translational action recovers the Planck--de Broglie relations, while its anti-Hermitian generator produces intrinsic spin and admits Abelian and non-Abelian field realizations. The associated angular-velocity relation yields a covariant field-transport equation containing a propagation-orthogonal geometric-source absent from the standard Dirac equation. In the geometric-source-free sector, the spinor realization recovers Dirac dynamics, and the quadratic reduction yields the Klein--Gordon mass shell and Pauli spin--field coupling. Admissible geometric-source structures generate mass, scalar Yukawa, charged-pion isospin, and anomalous magnetic-moment interactions. The resulting theory accommodates $\mathrm{U}(1)$, $\mathrm{SU}(2)$, and $\mathrm{SU}(3)$ sectors while preserving a positive-definite conserved density, four-dimensional relativistic kinematics, and Lorentzian causal propagation.