Consider the Rest Frame of an Observer in STR.His world line can be characterized by the Four-Vector (X=0,Y=0,Z=0;T =proper time of the observer).This Four-Vector is a Vector in flat spacetime.And his world line is just a geometrical curve(frozen curve) in spacetime.If the time-arrow of an object points away from that of an observer,we say that the object is moving(in three dimensional sense) relative to the observer.If THETA is the hyperbolic angle between time-arrow of the object and that of the observer, tanh(THETA)=V/C.V=the 3-velocity of the moving body &C=the velocity of light.When V=C,THETA becomes infinite and we may say that light travels with infinite RAPIDITY in three dimensional sense.There is NO motion in four dimensional sense.In spacetime geometrical objects such as world lines,world bands.....ONLY exist.These geometrical objects in Four-Dimensional Spacetime Represent corresponding physical objects in three dimensional space.