A curve in space-time joining the positions of a particle throughout its existence.
- ‘As in the classical twin paradox, an observer at rest relative to the preferred inertial frame measures the longest proper time between any two events on his or her world line; moving observers always measure less.’
- ‘It does possess conserved quantities, its world line does constitute a causal process, and it is not capable of moving faster than the speed of light.’
- ‘Figure 4 shows the spacetime cylinder slit open along Albert's world line and flattened to a rectangle.’
- ‘But then we have a problem, because a string oscillates in space and time, and as it oscillates, it sweeps out a two-dimensional surface in spacetime that we call a world sheet (compared with the world line of a particle).’
- ‘The proper time is not the same as the coordinate time, namely the time that would be measured for the same events along the world line by an ideal clock at the origin of the coordinate system.’