From the speed of light to the fate of the universe through 42 interactive demonstrations. Bend light around black holes, watch spacetime curve under mass, and ride gravitational waves — all in your browser.
See also: Differential Geometry for curved spacetime as a pseudo-Riemannian manifold, Lie Groups for the Lorentz group of spacetime symmetries, and Electromagnetism for the prototype field theory that motivated special relativity.
Why nothing travels faster than light — and what happens when you try
The geometry of boosting between reference frames
Visualizing events, worldlines, and causality in 2D spacetime
Why gravity and acceleration are indistinguishable — Einstein's happiest thought
Mass tells spacetime how to curve — the rubber sheet made rigorous
Straight lines in curved spacetime — why planets orbit and apples fall
When spacetime curves so much that nothing escapes
Ripples in the fabric of spacetime
The shape and fate of the entire universe
G_μν = 8πT_μν — the most beautiful equation in physics