Why Going Faster-Than-Light (FTL) Leads to Time Paradoxes? In this video: https://www.youtube.com/watch?v=an0M-wcHw5A&lc=UgxqC71gefTRIuVubGt4AaABAg.9jI6ltMIeu59jx2P8cpn_z
In the video the following events happen:

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*A supernova goes off.

*Earth sees the supernova

*They send a FTL signal to Vega about the supernova

*Vega receives the signal

*Vega sees the supernova

Prof David Kipping states that a slower than light (STL) ship traveling from Earth to Vega causes a change in cause and effect in a sequence.

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*Vega Receives the signal

*The supernova happens

*Earth sees the supernova

*Vega sees the supernova

To me, this makes no sense.  I understand his argument about reference frames. To me, STL world line is too long and it seems to be moving in the wrong direction.  It should not incorporate Vega or Earth.  Using the argument in the video, they should be able to see things in the future because their world line is basically extending forever.  Also why is the tine slice line moving from bottom to top?  Again, doesn't this mean that to the STL, everything beyond Vega has already happened?  Causality is broken everywhere...
Can you actually mix STL and Light Speed world lines together and get something that makes sense?
 A: You are not wrong in suspecting that scenarios involving FTL worldlines do not make sense. Hypothetical scenarios like the one described are somewhat of a gimmick.  In the 4D hyperbolic geometry of Minkowski Space (which Special Relativity describes) you simply cannot have a worldline that goes faster than lightspeed.  The scenario with the FTL ship is somewhat like taking a normal Euclidean plane and saying, "imagine we had a circle whose Circumference divided by its diameter is equal to 4."  You could take that assumption at face value and calculate all sorts of paradoxical things like its area being $4r^2$ (the same as the area of a square in which it can be inscribed), or a half circle of this type having the lengths of the arced side and the straight side both equal to $2r$, meaning it should have an area of zero.  But these are little more than fanciful ruminations on something that is simply not possible within the rules of geometry.
In the same way, an object traveling at $1.5c$ has imaginary kinetic energy.  And if it fires a projectile forward at $0.5c$ with respect to itself, a stationary observer will see the projectile moving at $1.14c$, slower than the original FTL object.  And certain observers will see it moving backwards in time, because it outpaces the light rays carrying its "past" locations in space.
So rather than saying FTL travel breaks causality, it is perhaps better to say FTL is simply not possible, in the context of Special Relativity.
