In: Physics
The relationship between the time t and the coordinate time is given by
Taking the derivative with respect to the proper time , we find the velocity component for ? = 0:
Using the chain rule, for 1, 2, 3, we have
where we have used the relationship
Thus, we find for the four-velocity :
In terms of the yardsticks (and synchronized clocks) associated with a particular slice of flat spacetime, the three spacelike components of four-velocity define a traveling object's proper velocity i.e. the rate at which distance is covered in the reference map frame per unit proper time elapsed on clocks traveling with the object.