Paper I: The Experience Principle and the Propagation Model—A Constructive Reformulation of Electromagnetic Force

The transverse force reduction on a charge moving relative to a source, F_perp = F_s / gamma, is a well-established empirical fact. Special Relativity explains it through Lorentz transformations of the electromagnetic field tensor—a kinematic account that leaves the physical mechanism unexamined. This paper presents a constructive alternative grounded in the experience principle: electromagnetic forces are what charges physically undergo based on their interaction with propagating fields, not what observers compute from transformed field components. We first establish rigorously that force is invariant—all inertial observers measure the same force because proper acceleration is a Lorentz scalar—and locate the real puzzle: why does the interaction itself change with relative motion? We then introduce a physical picture of the electric field as a real entity co-moving instantaneously with its source charge yet composed of propagating structure at intrinsic speed c. Five postulates formalise this picture into the propagation model: quantization of charge, field decomposition E_s = psi c c_hat, field invariance under relative motion, effective coupling q_eff = kappa c_q, and interaction geometry relating c_q to the velocity triangle. A self-consistency lemma establishes that the triangle decomposition is a necessary consequence of the postulates, not an independent assumption. Using the law of sines we derive geometrically the effective propagation speed c_q = c sqrt(1 - beta^2 sin^2 theta) and the force law F = F_s (c_q / c) = F_s sqrt(1 - beta^2 sin^2 theta), which reproduces the empirical transverse force and yields the Lorentz factor from geometry alone. We prove that the derived magnetic field B = (v_perp / c^2) x E_s satisfies div B = 0 identically. The electromotive field E_m = psi v_perp is shown to satisfy E_m = c c_hat x B, demystifying magnetism as the transverse component of the electric interaction. Throughout, we distinguish the Experience Principle from Special Relativity at the conceptual level.