Comment by tsimionescu | Hacker News Reader

Comment by tsimionescu | Hacker News Reader

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> General relativity does show that what one observer would consider the future can exists as the present of another observer, and vice versa.

Not in any way that is inconsistent with an evolving universe, actually. Sure, an observer may exist at some coordinates in 4D spacetime where they are detecting light from Sagittarius A that in our frame is 1000 years old, but also light from the Sun that in our frame is 1000 years from now. But that observer cannot be meaningfully said to be either in our present nor in our past. And if we ever "meet" to discuss our observarions, it is guaranteed that it will be no less than 1000 years from now, after we also see the sun evolving. We'll essentially only disagree about how old the light from Sagittarius A was.

Most importantly, SR is not consistent with any ordering of events that breaks causality. If I drop a mug and it shatters, no observer can see the mug shatter and then see me dropping it. So, there is no reason to believe that the shattered mug already existed somewhere in space time when I dropped it, though there is also no reason to believe that it didn't.

Yes I agree, it’s true that causally connected events maintain their ordering. However, there is no sense where one can say definitively that some set of spatially separated events happened simultaneously. And as each reference frame is valid it’s impossible to say that there is a correct “present”. If two events happens at the same time in one reference frame, you can’t then declare that they happened at the same time “in reality” as that’s not valid. The events also happened at different times “in reality” in other reference frames. And in fact, the two observers could meet and discuss that the events happened at different times.

The result is that some reference frames have other frames future and past events, so to the grandparent posts point, there is a real sense in which the future exists along with the past. No reference frame gets to say what the official “future” is, so it must already exist.

https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity

My point is that, while there isn't a single universally defined future, for every observer there exist events that haven't happened yet, and no observer is ever going to communicate with some other observer and disagree about whether an event has happened yet or not. They can only disagree about the order in which events in their shared past happened.

Additionally, there are events that are in the future for any observer in any reference frame anywhere in the universe (except for those already caught beyond the event horizon of a black hole, but their future consists exclusively of the black hole so it's not that interesting to the discussion). For a simple example, present day on Earth was in the future for any observer anywhere in the universe 5 seconds after the big bang, regardless of their speed. So, there is no reason to say that present day Earth must have already existed 5 seconds after the Big Bang.

Of course, the block universe model is self-consistent, so it's possible it's correct. All I'm claiming is that an evolving universe is also consistent with relativity, though the way it evolves is more complex than a single universal arrow of time.

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