https://plato.stanford.edu/entries/paradox-skolem/ contains this claim:
Further, the multiverse conception leads naturally to the kinds of conclusions traditional Skolemites tended to favor. Let $a$ be a set in some model $\mathbf M$ (where $\mathbf M$ lives somewhere in the multiverse). Then $\mathbf M$ has a forcing extension, $\mathbf M[\mathbf G]$, in which $a$ is only countable. This provides a natural gloss on the Skolemite claim that “every set is countable from some perspective.” Similarly, the Skolemite's bias in favor of countability (see section 3.2) can be explained by the fact that, if $a$ is countable in one model $\mathbf M$, then it stays countable in all extensions of that model. In contrast, uncountable sets can always be made countable by passing to an appropriate forcing extension. For more on the multiverse, see Hamkins 2011 and Hamkins 2012. For some criticisms, see Koellner 2013 (under Other Internet Resources).
The claim that every model of set theory can somehow be extended to make any particular set countable sounds very implausible and surprising to me. I haven't been able to figure out what the exact forcing method is that they use to make this claim.
Whatever method is used, I'd like to understand its implications for this simple example: Suppose $M$ is a transitive model of ZFC (using the notion of "model" that implies $M$ is a set), and $a \in M$ such that $M \models \mathrm{uncountable}(a)$. Furthermore suppose $a$ is uncountable. Is there a model $M'$ of ZFC such that "the same set" as $a$ is uncountable in $M'$?
- Is $a$ an object of $M'$ and $M' \models \mathrm{uncountable}(a)$, or is whoever is making the claim that "every set is countable from some perspective" using a different notion of "the same"?
- What is the notion of how $M'$ extends $M$? Is $M \subseteq M'$? Can $M'$ be made to be a standard model?
Where I got stuck:
The notion of forcing I'm most familiar with is the one described at the beginning of chapter 14 of Jech's Set Theory (http://iitp.ru/upload/userpage/300/jech_03.pdf). But for this one, a generic set $G$ is only guaranteed to exist if $M$ is countable (which by assumption, it isn't).
I've heard of forcing with Boolean algebras but don't understand the details. In particular, it seems like some equivalence relation is used to collapse the Boolean-valued model into a model of ZFC, and so I'd like to know if "every set is countable from some perspective" just boils down to "every set can be made countable if you quotient out its elements by a sufficiently strong equivalence relation" (if it does boil down to that, I'm still interested in the details).
Hamkins' 2011 paper (https://arxiv.org/abs/1108.4223) looks unhelpful; it doesn't contain much mathematical detail and appears to be full of sleights of hand of things being built out of classes and "let's pretend that". (I expect every claim involving class-sized "models" of ZFC, and all metamathematics, to be translatable into a claims about set-sized models such as my example $M$; if that's somehow not possible, I'd like to know why.)