What if light moved at 5 km/h?
Drag the slider. Watch time slow down. Share the result with anyone who says relativity is impossible to picture.
No spam. One email when the interactive sandbox opens.
Why Relativity Visualization Matters Right Now
A single interactive demo asking "what if the speed of light was 5 km/h?" climbed to the top of Hacker News this week, collecting more than 560 points and over 250 comments. What made that thread remarkable was not the physics — the Lorentz factor has been understood for a century — but the reaction. Comment after comment said the same thing: "I finally get it." People who had read about time dilation for years suddenly understood it the moment they could see a clock tick slower in front of them.
Existing tools do not serve that moment well. Textbook diagrams are static and abstract. University simulations like PhET are accurate but heavy, built for structured coursework rather than a two-minute curiosity break. Video explainers are passive — you watch someone else drag the slider, but you never feel the number change under your own hand. The gap is not accuracy. The gap is immediacy: a tool that turns an abstract constant into something you can physically push around and immediately see the consequences of.
That gap is worth closing now because the appetite is measurable and rising. Thought experiments about altered physical constants keep resurfacing across platforms, and each wave brings a new audience of teachers looking for classroom material, parents trying to explain science to their kids, and science communicators hunting for a shareable visual. The window is open, the demand is documented in public comment threads, and the underlying math is simple enough to run entirely in a browser tab.
How It Works
Set the speed of light
Pick a preset constant — real light speed, 5 km/h, walking pace, or the speed of sound — or type your own value and watch the whole simulation rescale around it instantly.
Drag your speed slider
Move the slider toward your chosen light speed and the Lorentz factor updates live, showing time dilation and length contraction as plain numbers alongside two animated clocks ticking at different rates.
Capture and share the result
Generate a clean result card with your scenario and the counterintuitive outcome, then download it as a PNG to post in a classroom, a group chat, or a science thread.
What You Get
Live time dilation
Every slider movement recalculates the Lorentz factor and redraws two side-by-side clocks, so the difference between a stationary observer and a moving traveler becomes something you watch rather than something you memorize.
Shareable result cards
One click exports a tidy PNG containing your chosen light speed, your scenario, and the surprising outcome. It is designed to be dropped straight into a slide deck, a social post, or a message to a friend.
Preset teaching scenarios
Ready-made setups for cycling to work, driving across town, flying overseas, and orbiting the Earth let a teacher open a lesson with a concrete example instead of an abstract formula on a whiteboard.