User blog comment:KillRoy231/How did you feel about O Brother Where Art Thou?/@comment-26982641-20151211212146/@comment-89.204.138.152-20151211222611

Well, they have, since time travels to the past are possible, while in our universe there are possible solutions for the Hilbert equations it is in all practical uses impossible to travel back to past. https://en.wikipedia.org/wiki/Einstein_field_equations You can solve that set of partial differential equations not generally, but in specific cases by using some assumptions about the global metric of our universe. The non relativistic formula for kinetic energy:

T = 1/2mv² is an approximation if space time is flat and the speed < 0,1c. Since we already know that general theory of relativity doesn't apply in the SPN universe, newtons approximation doesn't either.

--Lambda1