Interactive Sim: Speed of Light at 5 km/h
Original: Show HN: What if the speed of light was 5 km/h?
Why This Matters
Accessible relativistic simulations help bridge the gap between theoretical physics education and intuitive understanding.
Developer Dmitry Brant released 'Relativity Park,' a browser-based interactive simulation that reduces the speed of light to 5 km/h, allowing users to directly observe special relativistic effects such as length contraction, time dilation, Doppler shifting, and Terrell rotation at human walking speeds.
Relativity Park is a free, browser-based 3D simulation created by Dmitry Brant in 2026 that sets the speed of light to 5 km/h — approximately a brisk walking pace — so that special relativistic phenomena become observable on everyday human scales. Users navigate a virtual park using keyboard and mouse controls, accelerating toward the speed of light and witnessing physically accurate effects including length contraction, time dilation, relativistic aberration, Doppler color shifts (blueshifting ahead, redshifting behind), and relativistic beaming. The simulation features on-screen readouts displaying the user's velocity, beta (v/c ratio), Lorentz factor (gamma), and two clocks — 'Your watch' and 'World clock' — to illustrate time dilation in real time. Scenario elements include lamp posts flashing once per second of world time, a Ferris wheel and carousel whose cars travel at 75% the speed of light, and a shuttle running back and forth also at 75% c, all demonstrating Terrell rotation and relativistic contraction. Individual effects such as light-travel delay, aberration, Doppler color, and relativistic beaming can be toggled independently. Brant notes that certain visual approximations are used — extreme blueshifts are rendered as silver (x-rays) and white (gamma rays) for illustrative purposes — and acknowledges physical impossibilities such as atmospheric vaporization and structural failure at relativistic speeds are not modeled. Source code is publicly available.