While watching a talk given by Neil DeGrase Tyson, he says that if something happens on earth, it would take light 2 million years to get to Andromeda. In theory, if you could generate a wormhole and get to Andromeda faster than light does, you can observe the event again. This all made perfect sense to me until he claimed this to be a form of time travel. Here, I lost him completely as I got into thinking about absurd scenarios.
So, what what deGrase Tyson said makes sense as distinct events need not simultaneous for different observers. But it also then implies that there will always be observers who have already witnessed an event. Then changing the past is out of the question isn't it? (Since someone already saw it happen, he would also see the time traveler trying to change it but only contributing to the occurrence of that event ) Hence, everything is already determined. Is it not? What is the reasoning behind other theories concerning the grandfather paradox?
But if that's the case what about quantum mechanics? How does uncertainty go along with different observers?
EDIT: Also, if I can use a wormhole to get to Andromeda, can I determine the result of a quantum experiment before it happened? Sorry for the number of questions but I am very confused.