World line is a concept mentioned by German physicist Albert Einstein in his 1905 paper "On electrodynamics of moving objects". He called time and space four-dimensional spacetime, and the trajectory of particles in four-dimensional spacetime is the world line. All objects are made up of particles. If we can describe the position of particles at any moment, we can describe the whole "history" of objects. Imagine a four-dimensional space, which consists of three-dimensional space and one-dimensional time. Because a particle can only be in a specific position at any time, its whole "history" is a continuous curve in this four-dimensional space, which is the "world line". The world line of an object is the set of the world lines of all the particles that make up it.