I have heard that if faster than light travel were ever made possible, such as by an Alcubierre drive, the user would be taken backwards in time, violating causality.
This is odd to me, as it already seems explained by the twin paradox. The astronaut twin sees Earth as moving at relativistic speeds, and aging slower, but when the astronaut turns back around, time on Earth seems to accelerate, until he arrives in the "present" from Earth's perspective, where he is the one younger than his twin on Earth.
If an Alcubierre drive were ever made, the astronaut would see Earth going "back in time" as he leaves it, seeing light from further and further before he left. On the return, we are to believe he would still see Earth in the past. But why wouldn't time reconcile as in the twin paradox? Shouldn't the astronaut still see the Earth rapidly age until he arrives in the "present" from Earth's perspective?
Put another way, if we were to take an Alcubierre drive from Earth to Proxima Centauri, we would see Earth suddenly appear 4.246 years younger, as we would be seeing light that left it 4.246 years ago. But Proxima Centauri should appear to suddenly age 4.246 years, as we would now be seeing light that just left it. So, when turning back around, why should we arrive at the younger Earth we see from Proxima Centauri, as oppose to the Earth appearing to suddenly age 4.246 years, and thus arriving in the present?
