[FOM] CH and forcing

Andreas Blass ablass at umich.edu
Sat Jul 9 14:21:57 EDT 2011


In an earlier message, I wrote:

>> Nevertheless, even
> with a non-generic ultrafilter, all the finitary, first-order consequences
> of "exact copy" remain correct; the reduction of V-check modulo any
> ultrafilter is an elementary extension of the ground model.

In reply to this, Bob Lubarsky asked:

> If a Boolean-valued model is reduced by a non-generic ultrafilter, how 
> does this get you anything coherent? I thought the point of genericity 
> is that if an existential statement is forced then so is a witness. If 
> you don't have that then the truth lemma doesn't go through, and you end 
> up having no control over the resulting structure. I don't doubt that 
> V-check looks reasonable, but I'd think the new universe wouldn't even 
> provably model ZF.

The solution to this difficulty is a property of the usual construction of 
Boolean-valued models of ZFC often called the maximal principle: The 
Boolean truth value of formula of the form "(exists x) A(x)", although 
defined as the Boolean supremum of the truth values of instances A(p) 
where p ranges over all Boolean-valued names, is in fact equal to the 
truth value of A(p) for *one* suitably chosen name p.  That p (or, more 
precisely, its equivalence class modulo the ultrafilter) serves as the 
required witness for (exists x) A(x).  The proof of the maximal principle 
obtains the desired p by patching together partial witnesses p'.  That is, 
one starts with all the Boolean algebra elements b for which some p' 
satisfies A(p') with truth value at least b, one chooses a maximal set of 
pairwise disjoint such elements b, and one defines p to agree with value 
(at least b) with an appropriate p' for each b in that antichain.  (The 
axiom of choice is needed here, in order to choose the antichain and the 
appropriate p'.  I believe I was careful enough, in my earlier message, to 
always talk about ZFC, not just ZF.  Without choice, Bob's objection is 
entirely correct.)

Andreas Blass






More information about the FOM mailing list