Wow, lots of good discussion!
Rather than try to respond individually, let me clarify a few points that I think will address the main questions that have been raised in the discussion.
(1) As DaleSpam noted, in the OP I am not, strictly speaking, advocating any particular positive viewpoint. I am only trying to show that a particular claim, that the observed fact of relativity of simultaneity is sufficient to *require* the block universe interpretation of SR, is not valid. (A quick note: I said "SR" in the OP, but I don't see how the logic would be changed at all if we extended the argument to cover any spacetime that was a solution of the EFE--or perhaps any spacetime with a Cauchy surface; more on that below. SR, as far as the argument in the OP goes, can really be considered as just one particular solution of the EFE, Minkowski spacetime; none of the key features of the argument rely on properties that are particular to that spacetime, like global flatness.)
That said, I do, of course, have a positive viewpoint, and I might as well give it. In our ordinary thinking about what is "real", we do draw a key distinction between "past" and "future": we view the "past" as fixed and certain, unchangeable, whereas we view the "future" as not yet certain, changeable. So we view the "past" as "real" in a way that the "future" is not. (Note that the block universe argument I describe in the OP implicitly adopts this view as well; the quote from Penrose I gave, that states the argument, draws a distinction between events which are "fixed and certain" and events which are not. More on that below.)
However, as I noted in the OP, that view is based on Newtonian, non-relativistic physics. Once we admit relativity into our worldview, we have to admit the existence of a third category, "elsewhere", consisting of events that are neither "past" nor "future" in our usual senses of those terms. The question then becomes, what is the "reality category", so to speak, of events that are in the "elsewhere" region? The argument that block universe proponents make, the claim that relativity of simultaneity requires the block universe interpretation, implicitly assumes that events in "elsewhere" must be put in the same "reality category" as events in the "past" (i.e., events in our past light cone). My argument in the OP is simply that that assumption is not logically required--its truth is consistent with relativity, but so is its falsity--and without it, the argument that relativity of simultaneity requires the block universe interpretation does not go through. But my own view is that the assumption is just plain false: if push comes to shove and we have to choose, we should put "elsewhere" events in the same "reality category" as "future" events (i.e., events in our future light cone). (My actual preference would be to just admit that there are three separate categories, not two, and leave it at that. But many people's intuitions can't seem to find room for that.)
(2) Just as a note, if you're looking for a quick summary of the point of view I was arguing in the OP, this quote gives it:
PeterDonis said:
Now, in those PF threads I referred to on this topic, a lot of electronic ink was spilled in arguing for proposition (1). However, all of that was really a waste of time, because I already *agree* with proposition (1)! (And so, I suspect, do others who posted in those threads expressing similar objections to mine.) Proposition (1), in itself, is not the problem. Nobody needs to be convinced that, given its premises, the conclusion of proposition (1) is true. The problem is the premises, specifically the second one.
In other words, I'm not saying that the *argument* I described in the OP, that block universe proponents give, is invalid, logically speaking. I'm saying that one of its *premises* is not established. And I'm focusing attention on that particular premise because *that* is where the real argument is; block universe proponents seem to want to just gloss over that premise as too obvious to need justification, but it isn't--it's the crux of the issue.
(3) I think it's important to distinguish our models of reality from reality itself. Any 4-dimensional spacetime, any solution of the EFE (including Minkowski spacetime), is a *model* of reality. In the model, of course the entire 4-d spacetime is determined; within the model, there is no reason to pick out any particular portion of the 4-d spacetime as being "more real" than any other. But the *reason* for that is that, within the model, the initial conditions are determined--or, more generally, the conditions on some Cauchy surface of the spacetime (assuming there is one--but the main spacetimes that get lots of practical use, Schwarzschild, Kerr, FRW, etc., and Minkowski of course, all have Cauchy surfaces) are determined, and of course it's a theorem that determining the conditions everywhere on one Cauchy surface is sufficient to determine the entire spacetime.
But in reality, we never know the conditions on an entire Cauchy surface. We only know the conditions in our past light cone, which does not completely cover any Cauchy surface. So we can never be sure that any given model, any given 4-d spacetime, exactly matches reality, *except* in our past light cone. There will always be multiple 4-d spacetimes (strictly speaking, an infinite number of them) that are consistent with the data in our past light cone. We can talk about some being more probable than others--a spacetime in which the Sun is exploding "right now", i.e., in a surface of simultaneity passing through Earth at this instant, is just as consistent with our past light cone data as one in which the Sun continues to shine normally, but I think most would agree that the former is much less probable. But we can never be *sure* which model is correct, for a given region of spacetime, until we have all the data from that region--until we actually *see* the Sun still shining normally, eight minutes from now, and thereby verify that it didn't explode. And at that point, the relevant events are in our past light cone; so we can never be forced to view any events outside our past light cone as "real" in the way we are forced to view events within our past light cone as "real".
Note, btw, that I have said "our" past light cone above. Surely, we are tempted to say, events outside our past light cone (like events happening in the Andromeda galaxy "right now") are in the past light cone of some other event in 4-d spacetime? But that is simply making the same error that I described above: confusing the model with reality. Yes, in our current *model* of 4-d spacetime that describes the universe, there are plenty of events which have, in their past light cone, events happening "right now" in the Andromeda galaxy. But that's the model, not reality.
To put it another way: if we're going to admit any distinction at all between different "categories of reality", the only sound basis on which to draw the distinction is our current past light cone, given our particular current location in spacetime, according to our best current model of it. If we refuse to admit any such distinction, then of course we can say that all events are "equally real", as block universe proponents want to say: but that's not an argument, it's just assuming the conclusion.
(4) I confess I am as confused as DaleSpam is by the repeated assertions about solipsism. It particularly confuses me in response to the OP in this thread, since I specifically addressed precisely that question:
PeterDonis said:
(2-1) The only alternative to the second premise is solipsism (only my present event is real).
...
Proposition (2-1) is false, because there *is* another alternative to the second premise that accounts for all of our observations:
(3) All events in the past light cone of a given event are real (i.e., fixed and certain) for an observer at that event.
In other words, the key issue is not "does anything exist besides me at the present moment?"--*nobody* is taking the solipsist viewpoint that the answer to that question is "no". The question is whether we are justified in considering any events outside our past light cone as fixed and certain, the way we consider "past" events (events in our past light cone) to be fixed and certain. My answer to that is "no", but that's a long, long way from solipsism.
Note, also, that I am carefully not using the word "exist" at all, and I'm putting the word "real" in scare-quotes whenever I can, and I'm trying to use the term "fixed and certain" as a better way of describing the distinction that's at issue. The question is not whether, say, there are "real" events happening in the Andromeda galaxy "right now"; the word "real" is simply too imprecise and too charged with different connotations to different people to be a useful one in this connection. But asking whether there are events happening in the Andromeda galaxy "right now" that we, here on Earth right now, can view as being fixed and certain, is much more precise and admits of a much more definite and useful answer: no, there aren't, because any such events would be outside our past light cone, here on Earth right now. We won't know for sure that any such events happened, much less exactly what happened at them, for another couple million years, when they move inside our past light cone.
Btw, I am also *not* saying that events in our past light cone are "present" events. Of course they're not: they're past events. The word "present" is another of those words that, while useful in a colloquial sense, breaks down when you try to push it too far. Strictly speaking, from the viewpoint of a given spacetime model, the "present" of any given observer is just a single event--more precisely, once you pick a particular event on a particular observer's worldline, you are *defining* that single event as that observer's "present" for the purpose of whatever problem you are analyzing. What we actually perceive as the "present", in our everyday lives, is really a small segment of our past light cone, since it takes time for our sensory apparatus to send signals to our brains and for our brains to process those signals and produce our conscious experience. So the "present" we perceive is, from the viewpoint of a spacetime model, a kind of illusion. But that's probably getting into areas like cognitive science that are out of scope for this forum.
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