I'm not only thinking about political bubbles here, but linguistic or cultural ones. For example, when I joined a server here, it was federated only with a few other small Japanese servers, resulting in a fairly miserable experience. Another set had multiple languages but was just devoted to art. It's incredibly difficult (or was a year ago) to find servers that are general purpose and federated with both Japanese and English servers.
It's actually kind of interesting that me mentioning bubbles resulted in an immediately political assumption; I guess it shows how out of touch I am with mainstream Twitter.
I think you use "federated with" in a different sense than most people do in these discussions.
A newly created Mastodon instance by default federates with everyone in that allows messages and following from and to every other instance.
But it's federated timeline has no posts from other instances, because that view only shows posts that the instance actually sees. It only shows what people on the same instance actually follow, because otherwise there is no reason for the posts to be sent to it.
That would explain a lot of things and also clears up a lot of things for me, too! Thank you. I guess a better phrase for what I'm trying to describe is a lack of discoverability, then.
I only know one instance (awoo.space) that does whitelisting instead of blacklisting. Mastodon can't even do it out of the box (although pleroma can).
My instance bans the sort of instance described in the sibling comment to yours, like qoto (who advertised their Mastodon as "free speech twitter"... on twitter ads)
It's actually kind of interesting that me mentioning bubbles resulted in an immediately political assumption; I guess it shows how out of touch I am with mainstream Twitter.