The weirdness that hides at the edges of speed.
Space bends. Time stretches. Mass and energy are the same thing. Einstein's special and general relativity rewrote geometry itself — and predicted everything from GPS corrections to black holes.
MODULE 1 · SR FOUNDATIONS
GALILEAN RELATIVITY AND THE FRAME PROBLEM
Why Newton's universe needed an observer who never moved.
MAXWELL AND THE SPEED OF LIGHT
The constant hiding in two SI numbers, and what it didn't depend on.
THE MICHELSON-MORLEY EXPERIMENT
An interferometer asks the wind which way it blows, and gets no answer.
EINSTEIN'S TWO POSTULATES
Drop one assumption, take one fact at face value, get a new universe.
THE RELATIVITY OF SIMULTANEITY
Two lightning bolts strike — and the train passenger disagrees with you about which came first.
MODULE 2 · SR KINEMATICS
TIME DILATION
Why a moving clock ticks slower — and why this is not an illusion.
LENGTH CONTRACTION
Why a moving rod is shorter — and why no one moving with it can tell.
THE LORENTZ TRANSFORMATION
The four lines of algebra that replaced Galileo.
RELATIVISTIC VELOCITY ADDITION
Why nothing made of matter ever crosses 299,792,458 m/s.
THE RELATIVISTIC DOPPLER EFFECT
What the source's color tells you about its motion through spacetime.
MODULE 3 · SPACETIME GEOMETRY
SPACETIME DIAGRAMS AND WORLDLINES
Draw a clock's life as a single line on a piece of paper.
THE INVARIANT INTERVAL
The one quantity every observer must agree on.
LIGHT CONES AND CAUSALITY
The shape spacetime gives "before" and "after" — and "neither."
FOUR-VECTORS AND PROPER TIME
The invariant heartbeat of every traveler in spacetime.
THE TWIN PARADOX
The longest worldline in spacetime is also the shortest in proper time.
MODULE 4 · RELATIVISTIC DYNAMICS
FOUR-MOMENTUM
Energy and momentum, packaged into one object that transforms as a vector.
MASS-ENERGY EQUIVALENCE
Why a hot cup of coffee weighs more than a cold one.
RELATIVISTIC COLLISIONS
Conservation of four-momentum, and what changes when speeds get high.
COMPTON SCATTERING
The 1923 experiment that finally made photons mechanical.
THRESHOLD ENERGY AND PAIR PRODUCTION
Why a 1.022 MeV gamma ray can become two electrons but a 511 keV one cannot.
MODULE 5 · SR APPLICATIONS AND PARADOXES
THE BARN-POLE PARADOX
Two observers, two answers, no contradiction.
BELL'S SPACESHIP PARADOX
A string between two synchronized rockets, and the geometry that snaps it.
GPS AS A WORKING RELATIVITY EXPERIMENT
Without two corrections, your position drifts ten kilometers a day.
PRECISION TESTS OF LORENTZ INVARIANCE
What happens when a theory survives a hundred years of being checked.
MODULE 6 · THE EQUIVALENCE PRINCIPLE
INERTIAL VS GRAVITATIONAL MASS
Why two completely different definitions give the same number to thirteen decimal places.
EINSTEIN'S ELEVATOR
A free-falling lab is a frame where gravity has been canceled.
GRAVITATIONAL REDSHIFT
Climb out of a gravity well and your photons get tired.
GRAVITY AS GEOMETRY
The equivalence principle's geometric implication: there is no force, only curvature.