The Geometry of Spacetime

Light Cones

Every event sits at the apex of a cone of past and future. Outside the cone, nothing is reachable.

Time Dilation

Move through space and you spend less of your motion on time. Both observers see the other's clock running slow — both are right.

03 / Simultaneity

The Loss of Now

Two events that happen "at the same time" for one observer happen in sequence for another. There is no shared present.

Space Expanding

Atoms hold themselves together. Galaxies don't. Every observer sees themselves as the center, and none of them are wrong.