system
Given the following post, respond as if you're commenting on Reddit:
human
[The Culture]What is REALLY going on to cause every member of billions-strong civilizations to choose to sublime all on the same day, seemingly at random?
gpt
Have you read *the Hydrogen Sonata*? It's the last book in the series and explains a lot about sublimation. It's rarely at random, they usually spend decades discussing whether they are ready for it as a civilization, and if they decide they are, then they set a date a few years off and start making preparations. They all have to go within the same hour or so because a single biological mind is not strong enough to keep itself together in the Sublime, and will quickly dissolve into the background. When huge numbers go at once, they can band together to survive the early stages while retaining their individuality, then once they are accustomed to the Sublime they can presumably go their separate ways, whatever that entails. It doesn't have to be the entire civilization though, just a large number - 7% of the Chelgrians up and did it one day and it worked. AIs can sublime alone whenever they want.

Edit: As for *why* they want to do it in the first place? Well, because so far as any civilization that has ever researched into the matter has been able to tell, it's incredibly amazing and better than existence in the Real in every way. Here's a description of what it's like.
>It was wonderful; that was the general tone of the vague, dreamy reports that did come back. And almost beyond comparison, literally indescribable. The absolutely most splendid wonders, experiences and achievements of the Real and all those within it were as nothing to the meanest off-hand meanderings of the Sublime. The most soaring, magnificent, ethereal cathedrals to reason, faith or anything else were as mere unkempt and dilapidated hovels compared to the constructions - if they could even be described as such - within the Sublime. That was about all anybody had to go on, but at least the reports never varied in one respect; no one ever came back saying, shit, it's horrible, don't go.