Suppose a particle has a velocity 
in
, then at time t its spacetime coordinates are
. In a frame 
moving at
a speed v relative to , its spacetime coordinates
will be:
The velocity in simply comes from dividing the space
parts by the time part i.e.
If 
and 
then
This looks like the Galelian relative velocity if 
. If u=c, 
, so we confirm the universality of the
the speed of light.