The history of a particle as represented on a space-time
(Minkowski) diagram. The world-line of a particle is straight if the particle
moves uniformly and is curved if it moves non-uniformly (i.e. if there is
an acceleration). Rays of light 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 due to the light being deflected by the gravitational
field of the Sun.
My Worldline is the title of the autobiography
of George Gamow, the Ukrainian-American physicist.