A Unified Integral Equation of Quantum Action: From Duality Relations to Fundamental Equations of Quantum Mechanics, Relativity, and Gauge Theories

Starting from three fundamental duality relations (time-energy, space-momentum, angle-angular momentum), we establish an integral equation of quantum action and its dual counterpart. This integral equation directly links the evolution of the wave function to the accumulated action of spacetime and rotational degrees of freedom, forming a unified framework for quantum theory. Through semiclassical expansion, local approximation, and relativistic covariance, we rigorously derive the Schr\"odinger equation, Klein-Gordon equation, Dirac equation, Heisenberg equation, Schwinger-Dyson equation, Feynman path integral, Yang-Mills equations, Einstein field equations, and the Friedmann and Schwarzschild solutions. The framework also yields the kinematic effects of special relativity (time dilation, length contraction, mass-energy equivalence) as consequences of the duality structure. Moreover, the nonlocal kernel in the integral equation gives an exponential damping factor in momentum space, providing an intrinsic regularization mechanism for ultraviolet divergences. We extend the framework to quantum electrodynamics (QED), quantum chromodynamics (QCD), the electroweak theory, the full Standard Model, and quantum gravity, obtaining dual formulations for each. From the integral equation and its dual, we automatically derive quantization conditions for energy, momentum, angular momentum, and their duals, and propose experimental schemes to observe these discrete effects. All derivations are complete and rigorous, based on the same integral equation, providing a possible answer to Dirac's criticism that "quantum electrodynamics lacks a fundamental equation" and opening a new path toward a unified theory.