(a) In space-time, the history of a particle is represented by a world-line. The position of the particle in space at any particular time is found by slicing space-time at time and seeing where the slice cuts the world-line. [D89] (b) The trajectory of a body moving through spacetime. [c97] (c) In relativity, the path traced out in four-dimensional space-time by a given object or particle. [F88](d) The graph in spacetime coordinates which represents any continuous sequence of events relating to a given particle. In general relativity, all material particles have timelike world lines, photons have null world lines, and tachyons have spacelike world lines. [H76

In physics, the world line of an object is the unique path of that object as it travels through 4-dimensional spacetime. The concept of "world line" is distinguished from the concept of "orbit" or "trajectory" (such as an orbit in space or a trajectory of a truck on a road map) by the time dimension, and typically encompasses a large area of spacetime wherin perceptually straight paths are recalculated to show their (relatively) more absolute position states —to reveal the nature of special relativity or gravitational interactions. The idea of world lines originates in physics and was pioneered by Einstein.