Why does FTL imply causality violation?

[Widdekind]

http://www.theculture.org/rich/sharpblue/archives/000089.html

 

Is there an easy way of visualizing the effect? "Everybody knows", that relativistic boosts "collapse" the moving observer's x,t "grid", a little like some form of folding fence. So, "everybody knows", that relativistic observers' "lines of simultaneity" (t=const) "slash diagonally across" some fiducial stationary observer's grid. And, exaggerating for simplicity, FTL signals that were infinitely FTL would "zip" along lines of simultaneity, covering immense distances, in negligible time. So, someone moving relativistically "to the right", firing an FTL beam "to the left", would look like they had "communicated backwards in time" (to the stationary observer).

 

Quantum non-local instaneity, exploiting entangled wave-functions, which had previously been propagated conventionally, could possibly avoid violating Causality, if the original luminal signal was "pregnant with the possibility" subsequently manifested, cp. "consistent histories". In the following figure, events Q&R could hypothetically manifest some seemingly supra-luminal "synchronicity", had they both been synchronized, Causally, from the vertex at the intersection of both events backwards light-cones. Perhaps exploiting quantum entanglement would be somewhat similar to such a "synchronization"?