>Hmm your arguments make sense.

That's something I don't see much on these forums! Thanks!

>I think I was overvaluing mobility, because I figured once reaching a post-scarcity society exploration would become the top priority.

I know Star Trek isn't the most realistic TV show in the world, but I do think it (esp. TNG, esp. seasons 3+ because 1-2 sucked!) did a pretty fair job of showing what a post-scarcity society would look like, though to be fair energy was somewhat limited, but not that much.

In the show, most people/beings in the Federation lived on their home planets, or on colonies. There were a LOT of colonies; it seemed they were shown or mentioned every other episode. Most biological beings, assuming they evolve on a planet, and don't upload their minds into androids or something, are probably going to want to continue something like the lifestyle their ancestors had. In addition, there's huge logistical advantages to having stable facilities on planets. Where does the Federation build its starships? On planets, or in orbital construction yards (most likely, the components are largely made planetside, especially small but valuable ones like their computer equipment, and the ships are assembled in orbital shipyards). Where does the Federation mine the resources needed for ships and other stuff? Probably largely on planets and moons, though not necessarily the ones they live on, but the mines are probably very long-term and stable. And where does the Federation get the antimatter it needs to power its ships? According to the technical manual IIRC, they synthesize it from solar energy somehow, since antimatter is not naturally occurring in this universe. So all the infrastructure their society depends on for exploration (which is what many of those starships spend their time doing) is located in relatively fixed locations. Plus, the number of citizens in the Federation is surely enormous (they have thousands of members IIRC, and Earth alone probably has at least 5 or 10B by that time, depending on how many perished in WWIII and how many left the planet for colonization and of course the birthrate). The number of Federation citizens living full-time on starships is surely a tiny, tiny fraction of the total number.

>Two populations that sufficiently far (I'd say hundreds of light years, but it could be thousands or millions of light years) apart are essentially different civilizations, because their societies would drift out of sync.

That's true, but it really depends on the size of the civilization. If our civilization grew to span the nearest 50 star systems, for instance, they'd still all be within 16 light-years [1]. That's a pretty good-sized civilization IMO, and 16 years isn't that long to wait for a news about what's going on the homeworld (though it also means it's about 32ly between the farthest two colonies). A civilization spanning thousands or millions of ly would either be an incomprehensibly enormous civilization, or one which is really picky about which star systems it bothers with.

[1] https://en.wikipedia.org/wiki/List_of_nearest_stars_and_brow...

If you expand that radius to 30ly, now you're looking at probably hundreds of stars. Couple that with the immortality you mention, and assume very stable governments (since a civilization with governments like ours is not going to be spanning multiple star systems, and wouldn't even get far off the planet) and I do believe the rate of change will be much slower so that it's not that big a deal if you don't learn about some interesting news or new music or something from one of the other colonies until it's 50 years old. And don't forget, we consider ourselves (humanity) as having a singular civilization, but how in-sync are we between different nations? Not that much; we don't even speak the same languages much of the time. Most likely, in such a civilization, the different worlds would have different cultures, and there's only be so much interconnection between them, mostly for trade but also probably tourism and such. But I guess now we're getting into an argument over the exact definition of "civilization"; do we have a singular one now, or are the US and China two separate civilizations with trade and cultural exchange? What about centuries ago when communication was all by hand-carried letter or word-of-mouth?