Interactive simulation sets the speed of light to 5 km/h

Key facts

• The demo fixes the speed of light at 5 kilometers per hour, about a brisk walking pace, inside a 3D park scene.
• Movement uses the arrow keys or WASD to accelerate, the mouse to look around, the space bar to stop, and R to reset; L, G, C and B toggle individual visual effects.
• Speeding up makes length contraction, time dilation and Terrell rotation directly visible, while the speed of light itself stays out of reach, approachable only asymptotically as in the real universe.
• The relativistic Doppler effect is rendered too: objects ahead appear blueshifted, objects behind appear redshifted, and objects passing at an angle show the transverse Doppler effect.
• Submitted to Hacker News as a Show HN post, the demo drew 262 points and 105 comments within about six hours.

Why it matters

Special relativity's effects, length contraction, time dilation, the relativistic Doppler shift, are hard to feel firsthand because nobody moves anywhere near the speed of light in daily life, so the numbers usually stay abstract. This demo works around that by fixing the speed of light inside its own simulated park at 5 kilometers per hour, a brisk walk, so ordinary human movement becomes fast enough, relative to light, for the effects to show up directly. Speeding up in the scene brings on length contraction, time dilation and Terrell rotation, along with the Doppler shift: blueshift for objects ahead, redshift for objects behind, and a transverse Doppler effect on anything passing at an angle. Just as in the real universe, the simulation keeps the speed of light itself out of reach, approachable only asymptotically no matter how long the user accelerates.

Who it affects

This is a browser-based physics demo, not a commercial product, so it speaks mainly to physics students, teachers, and anyone curious about special relativity or interactive visualization. It reached a wide slice of Hacker News, drawing 262 points and 105 comments within about six hours of being posted, though the page itself does not name who built it.

How to use it

The demo runs directly inside a web browser. Movement uses the arrow keys or WASD to accelerate and the mouse to look around; the space bar stops all motion, and R resets the scene. Four more keys, L, G, C and B, toggle individual visual effects on and off. Clicking into the scene activates the mouse-look controls, following the instructions given on the page itself.

How solid is it

The retelling here rests on the demo's own on-page description, checked fragment by fragment against the source. The page does not name who built it, nor does it explain how the simulation is built or rendered, or how physically accurate its relativistic effects are meant to be against the actual equations of special relativity. It does not identify the park setting as any real or specific place, and it states no browser, device or performance requirements, so how the demo behaves on unfamiliar hardware goes unaddressed. What it does state plainly is the mechanism itself: light fixed at 5 kilometers per hour, arrow-key-and-mouse movement, and toggleable effects, all of which a reader can check directly by opening the page.

Risks and caveats

The page makes no claim about how physically accurate its effects are, and without an explanation of how it models special relativity or who built it, a visitor has no independent way to judge whether the effects shown are exact, a close approximation, or simplified for visual clarity. Anyone using it to teach or learn relativity should treat it as an intuition builder rather than a verified simulation. Because the page states no browser, device or performance requirements, how smoothly the effects render is likely to vary by hardware in ways the page itself never addresses.

“Just like in the real world, the speed of light is not attainable, only approachable asymptotically.”

— the demo's on-page description