Just because they are space separated events does not mean you cannot give assign them chronological order. Can't you just simply refer to a literal universal chronological reference point (i,e Big Bang) and then achieve successful ordering of events? For example: Civilization A achieved space flight x time units (whatever time units, such as Planck time unit) after Big Bang, and civilization B achieved space flight y time units after Big Bang?
Without getting too deep into this issue of relativity, let me point that that the Milky Way is per Wikipedia "100,000–120,000 light-years in diameter".
Rather than go down to Planck level granularity, if reaching space flight, or better, serious engineering works in space that are visible from afar is a low probability thing, with time scales say in the millions of years, then "first" will I think do for our purposes.
>For example: Civilization A achieved space flight x time units (whatever time units, such as Planck time unit) after Big Bang, and civilization B achieved space flight y time units after Big Bang?
as long as special relativity is applicable, for space-like separated events A and B, there exist frames of reference such that A is at x time units after Big Bang and B is at y in one frame of reference, and A is at y and B is at x in the other frame, ie. in reversed order.
Yes, but for any such reference frame if you know the distance between the observer and A and B, then after observing both A and B you can still deduct the true chronological order.
But I get the parent's point now, for our purpose of observation there is no point figuring out this "true chronological" order anyway since a civilization could have achieved space flight 100,000 years after Big Bang but is at a distance so far away from us that the information may NEVER reach us due to universe expanding.
