Gravitational Force
The gravitational force is caused by a mass charge moving in a gravitational field.
The effect is to push particles around.
 The cause is mass charge times the velocity contracted with the symmetric
field strength tensor.

The effect is to change momentum with respect to the interval .

If the sign of mass charge is inverted ,
is invariant so there is one distinguishable mass charge. 
Mass charges are forced toward each other due to the negative sign of the force.
Slide 38 looks more like a mother every day.