Theory of Relativity
Relativity is mostly one stubborn idea repeated: the speed of light is the same for everyone, and almost everything strange follows from refusing to give that up. This series builds it from the ground — what “moving” even means, through special relativity, all the way to black holes and the relativity corrections inside your phone’s GPS.
Every drill stars my kids — Liza, Vika, and George — and has two settings: “explain it for a kid” (names, plain words, pictures) and “for a grown-up” (the full γ, Lorentz transforms, and honest numbers). The animations are physically accurate, not just illustrative.
19 of 19 drills live · the series is complete 🎉
Part 0 — Foundations
What Does “Moving” Even Mean?
No experiment you can run while drifting smoothly tells you who is “really” moving. The whole theory starts here — there is no special, stationary place to measure the world from.
explore →How Speeds Normally Add
Walk forward on a moving train and your speed over the ground is just the two speeds added together — the common-sense rule everyone already uses. It works perfectly in everyday life, and it is exactly the rule special relativity will later have to bend.
explore →Why Distant Objects Look Smaller
George walks away and looks tiny — but he never shrinks. Your eye measures the angle a thing makes, not its size. Just light and straight lines, no relativity yet: the perspective baseline the strange drills build on.
explore →Why Distant Things Look Fainter (But Not Dimmer)
The light companion to the drill above. The same 1/d that shrinks angular size drives the inverse-square law — total light falls as 1/d². But the slice of your eye it covers falls the same way, so surface brightness stays constant: a wall looks just as bright far as near, only smaller. Still pure geometry, no relativity.
explore →Why a Siren Changes Pitch
A passing siren drops in pitch as it goes by: the sound waves bunch up ahead of it and stretch out behind. The everyday Doppler effect, done with sound first, so the same idea with light — redshift — feels familiar when it arrives.
explore →Seeing the Past
Distance means delay. Starlight is old news — exactly like hearing thunder after you see the lightning. Still nothing relativistic; just the travel time of a signal — and the bridge to the one fact the next part is built on: light has a speed.
explore →Part 1 — Special Relativity
The One Speed Everyone Agrees On
Race toward a light beam or flee from it — you still measure light at the very same speed. The one stubborn fact the whole of special relativity is built to protect, and the reason everything after this gets strange.
explore →The Train and the Lightning
Lightning hits both ends of a speeding train at once for Liza on the platform — but not for Vika riding inside. There is no universal “now.” Dual-frame sandbox with a kid-friendly walkthrough and the full Lorentz Δt′.
explore →Why You Can’t Add Velocities
Fire a fast thing from an already-fast thing and the speeds don’t simply add. The rule that quietly keeps everything under the speed of light.
explore →Clocks Don’t Lie, But Light Does
Real time dilation — moving clocks genuinely tick slow — cleanly separated from the mere delay of a signal reaching your eye. A bouncing light-clock makes it visible.
explore →The Shrinking Spaceship
Same distance, same angle — only the speed changed, and the ship is shorter. Length contraction along the direction of motion, and why both observers are right.
explore →The Twin Paradox
The traveling twin really does come back younger. The asymmetry is simple once you see it: only one twin switches frames — and the switch is the whole story.
explore →Why Nothing Can Outrun Light
The cosmic speed limit isn’t a rule about speed — it’s the rule that keeps cause before effect. Break it and you could answer a question before it was asked.
explore →Part 2 — General Relativity
Mercury’s Stubborn Orbit
43 arcseconds per century that Newton’s gravity could never account for — the first crack in the old theory, and the first win for the new one.
explore →Gravity Is Not a Force, It’s a Shape
Falling is just moving in a straight line — through curved spacetime. The central idea of general relativity, made tangible.
explore →Bending Starlight
Light has no mass and still bends around the Sun. The 1919 eclipse measurement that turned a theory into headlines.
explore →Why Light Changes Color
The Doppler shift of motion and the gravitational redshift of a deep gravity well are two different effects that look alike — here’s how to tell them apart.
explore →Inside the Event Horizon
Tilt the light cones far enough and “out” stops being a direction you can point. What a black-hole boundary really is.
explore →How GPS Actually Works
Special and general relativity combine into a correction your phone leans on every second. The payoff, finally earned.
explore →