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I recently watched Brian Greene documentaries on parallel universes and am trying to wrap my brain around multiple quantum universes that form at every event and Einstein's relativity that in space-time, past present and future coexist so we can observe the future of a far away galaxy from Earth by moving. https://www.youtube.com/watch?v=MO_Q_f1WgQI - Around 7th minute - explains how an alien moving towards earth has a now slice cutting in Earth's future

My question is which quantum universe will be seen because future events in that galaxy would create multiple such universes.

My question is regarding many worlds interpretation of quantum mechanics according to which multiple alternate futures are possible https://en.m.wikipedia.org/wiki/Many-worlds_interpretation

The question has been answered by James K below.

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    $\begingroup$ We cannot observe the future of a far off galaxy by moving. Can you link to a source that tell you that you can. $\endgroup$
    – James K
    Feb 3 '18 at 9:22
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    $\begingroup$ Most of the viewers of your question didn't see that documentary, and it would be an unfair ask for them, wanting to google after it. So, explain, what did you see in the film with your own words. There is a rule here, that questions should be comprehensible and answerable without clicking or googling anything external. $\endgroup$
    – peterh
    Feb 3 '18 at 19:02
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So, the presenter explains that two observers who are not moving relative to one another will agree on the meaning of "now". However if one observer is moving, then they will no longer agree, and what one considers to be now will be in the past or future of the other.

He observes that a sufficiently distant observer, in another galaxy 100 million light years away that is moving at "bicycle" speed relative to us would have a 200 year discrepancy in their understanding of "now".

But this alien would have absolutely no way of observing our universe now. They could not see us in our "now", nor in their "now" of 200 years ago, because they are 100 million light years away. They can only observe how our galaxy was 100 million years ago. They can certainly not observe our galaxy in the future, even if they travel towards us at speeds approaching the speed of light.

So the explanation of different "now" is about agreement on clocks, not about actual observations.

I think then the program errs by treating Special Relativity as a full description of nature, rather than as a model. Relativity is a classical theory, in that in Relativity, if you know the initial state, you can predict the future state. There is no uncertainty principle in general relativity. It is correct to say that in Relativity the future is as real as the past. It is unclear if the same is true in Quantum Mechanics, it depends on how you interpret the maths. Whether the future exists or not cannot be answered by theory.

Since different interpretations of QM are based on the same maths, there is no way of testing which interpretation of quantum mechanics is "correct" or even whether calling an interpretation "correct" has any scientific meaning. In the "many worlds" interpretation, the notion of a single space-time which you can take slices of is lost completely.

I might note that I'm not a great fan of the explanations in that series, which seem to me to be confusing on a number of levels, and are more aimed at creating "wow" than being clear. For example, the mixed metaphor of "slices of bread and aliens riding bikes" seems to have confused you into thinking that there are aliens who are already seeing how our planet will be 200 years from now.

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  • $\begingroup$ However, the fundamental question remains. What if we orbit near a black hole to travel to future and come back to Earth, which of the quantum realities will be observed in the future Earth? $\endgroup$ Feb 4 '18 at 8:55
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    $\begingroup$ No, that is a completely different question. There is no mention of black holes in the question. $\endgroup$
    – James K
    Feb 4 '18 at 13:20

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