So I'm not a professional scientist, but rather an avid physics fan. This is one question that no matter how much I research I can't find a clear answer...(adsbygoogle = window.adsbygoogle || []).push({});

So for the sake of argument, let's assume that you could actually build something like the Alcubierre Drive and go FTL (I know a lot of people say it's not possible, but let's just for now assume it is). Let's assume that being in your Alcubierre Drive spaceship, since the spacetime your are in itself is moving (vs you moving through spacetime) you do not experience any acceleration (if I'm wrong in this assumption of how an Alcubierre Drive would work in principle, let me know).

Let's take two events that happen simultaneously, say a marble drop, both on Earth and the Sun. The distance is 1 AU, about 8:20 min travel time at velocity C. Now your spaceship can go 2C velocity and you depart for the Sun at the exact time of the marble drop on Earth (we'll call that departure time T=0). You arrive at the Sun at T=4:10, and then pointing a sufficiently powerful telescope back at Earth at T=8:20 (your reference frame) you will see both the marble drop and your departure.

In the above scenario, you could SEE into the past (hence seeing your own departure) but how would you violate causality? I also know that for such scenarios, there are situations where if you had a 3rd party observer looking at your trip it would look to them as if you traveled backwards in time... But is only observing the past equivalent to violating causality? If you only observe the past, then you still can't change it.

A classic paradox here is the grandfather paradox. To me, it's clear that if you could go FTL and have a good enough telescope you could get to a point where you could observe your grandfather being born... but how could you possibly put yourself into a situation where you could change causality and kill your grandfather? YOUR timeline, as the spaceship traveler, would always have to stay positive, would it not? Especially in something like an Alcubierre Drive, as *technically* in such a spaceship you would not be moving FTL within spacetime, but spacetime itself would be moving FTL on which there is no speed limit...

Thoughts?

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# B Trying to understand how FTL would violate causality...

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