World line

Summary

The world line of a point particle is the curve in spacetime made of all the events where the particle is present. In Minkowski space the world line of a massive particle is a curve whose tangent always points into the future light cone. The world line of a light pulse is a null line. World lines turn motion into geometry. A particle at rest has a vertical straight world line, a uniformly moving one a tilted straight line, and an accelerated one a curved line.