Relativistic to Classical Force
 4velocity becomes a 3vector:

Changes in spacetime becomes either
, a change in time for timelike events, or
, a change in space for spacelike events in the complex tangent plane.
The completely arbitrary location of the spacetime origin means in the
transformation from Minkowski spacetime to Newtonian space and time, the
slope of some worldline for massive particles are real, while others are
undefined in space and time unless the complex tangent space is included.