What you're seeing
- Detector OFF: press Fire 1 and a single electron's wave squeezes through both slits. The two wavelets overlap, hyperbolic fringes fill the space — yet only one dot lands on the screen. Fire thousands and the dots draw the fringes.
- Detector ON: the detector flashes as the electron passes — a which-path record now exists in the environment. The wave propagates from one slit only, no cross-term, and the dots pile up into a smooth single-slit distribution — the fringes are gone.
- Compare: runs both experiments back to back — 2000 electrons unobserved (kept as an amber ghost), then 2000 observed — same source, same slits, different record, different statistics.
Quantime narrative hook
Measurement is the moment a present-tense possibility becomes a past-tense record. With the detector off, "which slit?" has no answer yet — both routes coexist in the present and the fringes prove it. The instant the detector writes its record, "which slit?" acquires a past — and the interference becomes inaccessible. The screen is simply history's bookkeeping, one dot at a time.
Model notes: each slit is treated as a Huygens secondary source emitting a cylindrical wave with a single-slit sinc envelope; the field shown is I = |ψ₁|² + |ψ₂|² + 2V·Re(ψ₁ψ₂*) with travelling phase fronts. Screen statistics are Born-sampled from the same model, so the dots, the histogram and the blue curve always agree. Pedagogical, not a Schrödinger solver — but Fraunhofer-consistent.