Why now?

The Show HN launch drew 337 points and 146 comments, showing strong organic appetite for intuitive physics explainers right now; the demo is pure client-side, so a solo dev can ship and iterate without infrastructure or AI costs.

The gap

Existing relativity visualizations tend to be either abstract math or dry textbook diagrams; this project shows demand for a tactile, everyday-scale interactive that makes relativistic effects graspable, and the 146-comment thread suggests users want to explore variations, not just watch.

Main competitor

Free educational interactives like PhET simulations and YouTube physics explainer channels, which cover relativity conceptually but not as a single, tunable 'slow light' sandbox.

Execution plan

1. Recreate the core effect in a lightweight canvas/WebGL scene with a slider for c (e.g., 5 km/h to realistic), showing Doppler shift, time dilation, and length contraction on familiar objects.
2. Add shareable preset scenarios (walking past a lamppost, a bicycle, a train) and a URL-encoded state so users can post specific configurations in forums and classrooms.
3. Publish short explainer pages for each effect, targeting search terms like 'time dilation visualization' and 'relativistic Doppler shift demo', to capture recurring educational traffic.
4. Open-source the renderer and invite physics teachers to submit scenarios, building a small library of classroom-ready presets.
5. Instrument usage (which presets and sliders get used) to learn whether retention is one-off or repeat-visit before investing in a paid tier.

Monetization

No proven willingness to pay in the signals; realistic paths are a one-time 'classroom pack' (offline version plus lesson notes) for teachers, or museum/planetarium kiosk licensing. Treat these as hypotheses to test, not validated revenue.