Quantum mechanics is a set of rules that allows an observer to predict, explain, and/or verify observations (and especially their mutual relationships) that he has access to.
No observer can detect inconsistencies within the causal diamonds. However, inconsistencies between “stories” as told by different observers with different causal diamonds are allowed (and mildly encouraged) in general (as long as there is no observer who could incorporate all the data needed to see an inconsistency).
— Raphael Bousso is right about firewalls
— Lubos Motl
There is no “god’s eye view” in physics.
— Me@2021-04-17 03:12:58 PM
Macroscopic time is with respect to an observer. Actually, physics is with respect to an observer.
In the real universe, any observer’s observations must be consistent. When two observers compare their observations, their results must be consistent, because the comparison itself is an observation of an observer.
Time travel in the absolute sense is logically impossible. Let’s assume that it is logically possible.
If a time travel story follows the principle of “an observer’s observations must be consistent”, each character in that story must see a consistent timeline, even if different characters’ timelines may be inconsistent. That is fine as long as such inconsistent observers never meet to compare their results.
If two of such observers choose to meet to compare their results, the action to “meet to compare” itself will render the results consistent. It is similar to the resolution of the twin paradox in special relativity.
There is no “god’s eye view” in physics. Every physical event must be described with respect to an observer. Every physical event, even if the event is “to compare observation results”, must be described with respect to an observer.
— Me@2017-05-10 07:45:36 AM
2021.04.17 Saturday (c) All rights reserved by ACHK