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World line
A world line is the path that an object traces in 4-dimensional spacetime.
Explanation
In relativistic theory, proper time τ of a particle is the time measured in the area of this particle, that is in the area where it is immobile. The Lorentz factor can be written as
where
t Time τ Proper time
Additional information
The German mathematician Hermann Minkowski introduced the notion of a word line in 1908.