The double-slit experiment has been called "the only mystery" of quantum mechanics. For one hundred years the question "which slit does the electron pass through?" has lacked a physical answer: the Copenhagen interpretation declares the question meaningless until measurement; the many-worlds interpretation posits branching universes; the pilot-wave interpretation introduces an undetected guiding field. We present a complete physical answer within the stochastic electrodynamic (SED) framework established in Papers 1—6 of this series. The answer rests on three propositions, all following from first principles: 1. The electron follows a definite trajectory—one that passes through exactly one slit—described by Nelson’s stochastic differential equation (SDE) with diffusion coefficient D = ℏ/2m. 2. The electron’s zero-point field (ZPF), which fills all space as the electromagnetic quantum field of the Coulomb interaction, carries information about both slits through the osmotic velocity u = D∇ln |ψA + ψB|^2. This non-local ZPF correlation guides the electron toward the interference maxima. 3. "Measurement" is a physical disturbance of the ZPF. It replaces the joint probability density ρ = |ψA + ψB|^2 with the conditioned density ρ → |ψA|^2 (Bayes update), which eliminates the interference term 2 Re[ψ∗A ψB] and destroys the fringes. The result is confirmed by Monte Carlo simulation of both the analytical |ψ|^2 distribution (Approach A) and the Nelson SDE (Approach B). Both reproduce the interference pattern; both show its disappearance when which-slit information is available. "The observer creates reality" is replaced by "physical disturbance of the ZPF updates the probability density." The hundred-year paradox is resolved by a concrete physical mechanism requiring no new postulates beyond those established in Paper 1.