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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.

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    $\begingroup$ There are indeed too many questions here. Also, you give no link or reference to the talk. There are many posts already on this site about wormholes and time travel; have you read some of them? $\endgroup$ – Stéphane Rollandin May 6 '18 at 20:20
  • $\begingroup$ There is no grandfather paradox. It is a logical hoax. 4D spacerime is static. The "logic" of the grandfather paradox is not static, but unfolds in some non-existent alternative time dimension. It is amazing how easily people get confused while thinking about time. This doesn't mean time travel is not possible, but only that it must be, as you said, "already determined". For example, if today the future you came to talk to you from tomorrow, then tomorrow you are for sure going to travel back to yesterday to talk to yourself and say exactly what you heard from the future self yesterday. $\endgroup$ – safesphere May 13 '18 at 6:12
  • $\begingroup$ Yeah, makes sense. But if that's the case, what happens to quantum mechanics? So if I conduct a quantum experiment and go back in time will I be able to predict the outcome before it happened then? $\endgroup$ – Chandrahas May 14 '18 at 7:50
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I think the basic issue here is a logical one. The question is whether a physical theory is allowed to deny the cause-effect logical sequence.

In SR (special relativity) you may hypothesize the existence of tachyons, superluminal particles. Formally they are not excluded by the theory, as SR just sets an invariant speed (an upper speed limit for bradyons, but a lower speed limit for tachyons). However the existence of tachyons would allow to send information back the past, thus clashing with causality.

GR (general relativity) is by far richer of possibilities, theoretically allowing CTC's (closed timelike curves), i.e. worldlines in a Lorentzian manifold of a material particle that are closed, that is returning to their starting point. That would imply the theoretical possibility of time travel backwards in time, raising the spectre of the grandfather paradox.

However the theoretical possibilities hinted above raise logical inconsistencies and that is why I think they should be regarded with a lot of cautiousness.

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