I'm trying to develop a story where a vast and sinister Alien Intelligence is bent on testing humanity's primitive knowledge of physics via the Q & A answer sites on their world wide communication system to make sure that they are not a threat to him (not entirely dissimilar to what Emperor Ming does in the Flash Gordon movie):
The Emperor Ming: Every thousand years, I test each life system in the Universe. I visit it with mysteries, earthquakes, unpredicted eclipses, strange craters in the wilderness... If these are taken as natural, I judge that system ignorant and harmless - I spare it. But if the Hand of Ming is recognized in these events, I judge that system dangerous to us.
What question should the AI ask the humans in order to test them and what matching answer would fully demonstrate that FTL travel violates causality? Something like this perhaps?
If I have a spaceship that can instantaneously and infallibly travel to any point within a few hundred light years show me how I can travel back in time and prevent myself from being conceived?
Obviously in order to adapt something like this to a story the answer needs to be without complicated maths (especially as I can't follow it).
The background that prompted this idea is that I've occasionally seen the phrase/assumption 'FTL travel violates causality' (or variations thereon) used by sci fi fans and authors such as Charless Stross but I have never seen a good explanation of why.
For seconds: faster than light travel would appear to be a necessary precondition to writing wide-screen space opera. But if you permit violations of special relativity, you're also implicitly permitting global causality violation — time travel. (Go read a physics textbook if you're not sure why.) Permitting violations in the first place suggests that there'll be more than one way of doing FTL travel (just as there's more than one way of doing heavier than air flight — compare a helicopter to a jet airliner and a bee). And then you've got to ask, what are the implications of time travel?
The best explanations I have seen usually involve two frames of reference moving in opposite directions at the speed of light where one observer sees an event (e.g. shooting a giant space weapon at a harmless planet) before the other observer has started the event and is thus assumed to be able to (near) instantaneously travel to the other frame of reference (thus effectively back in time) and prevent the event.
The trouble with those explanations is that surely, once an observer in one frame of reference tries to become an actor in the other frame of reference, the frames of reference must somehow merge or expand and therefore the initial conditions are no longer valid thus invalidating the violation of causality?