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« on: November 29, 2017, 07:40:02 pm »
Well, the exact optics are dependent on how you see FTL working, but the reason it's time travel is because it involves effects following causes that aren't actually in their past light cones. It would be like seeing a flowerpot on your windowsill fall today because you will push it tomorrow. What are you going to push tomorrow, exactly, now that the flowerpot is on the floor?
I'll draw the Minkowski diagram for the exploding stars as soon as I can, but the problem mostly comes up when, for example, B sees the ship appear at B before it leaves A; you can easily set the time, distance, and loiter time at B so it experiences two ships at once, for example*, and even if that's not a problem, when the ship leaves from A to travel to and explode B (from B's perspective), where is it going? Bear in mind we have to care about B's perspective even after it's exploded, because that's relativity, and it has to make sense according to causality for physics to make sense at all.
*incidentally, before someone brings up anything to do with rubidium and laser grids, that's not this thing.
Okay. Assuming the ship travels ~4x the speed of light, the stars are 10 years apart from each other in a line ABCD, and it lingers motionless for five years at each star in a trip going A->B->C->A, (and omitting the actual destruction events to simplify) each of them sees this:
Ship: Depart A, Arrive B, Depart B, Arrive C, Depart C, Arrive A. All good so far.
A: Depart A, Arrive B, Depart B, Arrive A, Arrive C, Depart C. Oh hey, two ships at once.
B: Arrive B, Depart B, Depart A, Arrive C, Depart C, Arrive A. It arrives before it leaves! It leaves before it leaves!
C/D: Arrive C, Arrive B, Depart C, Depart B, Depart A, Arrive A Good grief! The ship's in two places at once and it's arriving before it leaves besides!
See the problem? Mass-energy equivalence isn't conserved, effects precede causes... I can post the diagram if you want.