Send single particles at two slits and they build up stripes on the far wall — an interference pattern, as if each went through BOTH slits like a wave. But peek to see WHICH slit each took, and the stripes vanish: it acts like a particle through one slit. The same thing is wave or particle depending only on whether you ask which. Slide the which-path detector on and watch the fringes collapse.
At the two-slit wall the probability amplitude is a sum over paths; with no which-path information the amplitudes add coherently and the intensity is 2(1+cosφ) — bright and dark fringes, visibility 1. The instant the path becomes distinguishable (a detector, or any record), the terms add as probabilities not amplitudes, the cross term dies, the intensity flattens to 2 — visibility 0, no fringes. Wave and particle are the two readings of one object, selected by what is knowable. A fail-loud self-check throws unless visibility falls from 1 (no path) to 0 (which-path). ◆ real quantum mechanics, node-verified.
The exact two-path amplitude/probability model (full which-path vs none); real setups show partial which-path giving intermediate visibility (the duality relation V²+D²≤1) — the endpoints shown are the exact limits.