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.