What is World line

World line the history of a particle as represented in "space-time. The position of a particle at time t can be found by slicing space-time at time t and finding where the slice cuts the world line of the particle. The latter is straight if the particle moves uniformly and curved if it moves nonuniformly (i.e. There is acceleration). Light rays can be treated as the world lines of photons. The world lines of particles under the influence of a gravitational field are geodesics in space-time. The world line of a photon near a star, such as the sun, is slightly bent as the light is deflected by the gravitational field of the sun.