An interactive Minkowski (1+1-dimensional) spacetime diagram. Build a scene from events, inertial worldlines, light rays, lines of simultaneity, observers (piecewise-inertial paths) and hyperbolae (curves of constant interval), and view it at once in two independently Lorentz-boostable frames. Objects carry a rest colour, mass and other attributes; the app shows their frame-dependent statistics — energy, momentum, relativistic Doppler colour shifts and proper time. Units are natural — c = 1 by default (Settings › Show c re-displays the powers of c in the readouts) — so light runs at 45° and causal character survives any boost.
Quick start. For a quick taste of the app, try the subjective-experience demo on the twin-paradox scene it opens with: in the left sidebar click the triangle-shaped ▶ Subjective experience tool (at the bottom), then click the bent red-violet Traveller worldline, then press ▶ Play on the timeline panel below. You’ll ride the traveller’s proper time while the other panel becomes their co-moving frame and a live panel shows what they see in their past light cone.
An original tao-web applet (not a Java port), an aid for students of special relativity.
Coordinates are read in the focused panel. Boost the two panels differently to see that Δt, Δx are frame-dependent while s² = Δt² − Δx² is not.