forcing

Forcing is the method used by Paul Cohen to prove the independence ofthe continuum hypothesis (CH). In fact, the method was used by Cohen toprove that CH could be violated.

Adding a set to a model of set theory via forcing is similar to adjoining a new element to a field. Suppose we have a field $k$, and we want to add to this field anelement $\alpha$ such ${\alpha }^2=-1$. We seethat we cannot simply drop a new $\alpha$ in $k$, since then we arenot guaranteed that we still have a field. Neither can we simplyassume that $k$ already has such an element. The standard way ofdoing this is to start by adjoining a generic indeterminate $X$, andimpose a constraint on $X$, saying that $X^2+1=0$. What we do is takethe quotient $k[X]/(X^2+1)$, and make a field out of it by taking thequotient field. We then obtain $k(\alpha )$, where $\alpha$ is theequivalence class of $X$ in the quotient.The general case of this is the theorem of algebra saying that everypolynomial $p$ over a field $k$ has a root in some extension field.

We can rephrase this and say that “it is consistent with standardfield theory that $-1$ have a square root”.

When the theory we consider is ZFC, we run in exactly the sameproblem : we can’t just add a “new” set and pretend it has therequired properties, because then we may violate something else, likefoundation. Let $𝔐$ be a transitive model of set theory, which wecall the ground model. We want to “add a new set” $S$ to $𝔐$ insuch a way that the extension ${𝔐}^{\prime }$ has $𝔐$ as a subclass, and theproperties of $𝔐$ are preserved, and $S\in {𝔐}^{\prime }$.

The first step is to “approximate” the new set using elements of$𝔐$. This is the analogue of finding the irreducible polynomial inthe algebraic example. The set $P$ of such “approximations” can beordered by how much information the approximations give : let $p,q\in P$, then $p\le q$ if and only if $p$ “is stronger than” $q$. Wecall this set a set of forcing conditions. Furthermore, it is required that the set $P$ itself and the order relation be elements of $𝔐$.

Since $P$ is a partial order, some of its subsets have interestingproperties. Consider $P$ as a topological space with the ordertopology. A subset $D\subseteq P$ is dense in $P$ if and onlyif for every $p\in P$, there is $d\in D$ such that $d\le p$. Afilter in $P$ is said to be $𝔐$-generic if and only if it intersectsevery one of the dense subsets of $P$ which are in $𝔐$. An $𝔐$-genericfilter in $P$ is also referred to as a generic set of conditionsin the literature. In general, even though $P$ is a set in $𝔐$, generic filters are not elements of $𝔐$.

If $P$ is a set of forcing conditions, and $G$ is a generic set ofconditions in $P$, all in the ground model $𝔐$, then we define$𝔐[G]$ to be the least model of ZFC that contains $G$. Thebig theorem is this :

Theorem.$𝔐[G]$ is a model of ZFC, and has the same ordinals as $𝔐$, and$𝔐\subseteq 𝔐[G]$.

The way to prove that we can violate CH using a generic extension isto add many new “subsets of $\omega$” in the following way : let$𝔐$ be a transitive model of ZFC, and let $(P,\le )$ be the set (in$𝔐$) of all functions $f$ whose domain is a finite subset of${\mathrm{\aleph }}_2\times {\mathrm{\aleph }}_0$, and whose range is the set $\{0,1\}$. Theordering here is $p\le q$ if and only if $p\supset q$. Let$G$ be a generic set of conditions in $P$. Then $\bigcup G$ is atotal function whose domain is ${\mathrm{\aleph }}_2\times {\mathrm{\aleph }}_0$, and range is$\{0,1\}$. We can see this $f$ as coding ${\mathrm{\aleph }}_2$ new functions$f_{\alpha }:{\mathrm{\aleph }}_0\to \{0,1\}$, ,which are subsets of omega. Thesefunctions are all distinct. $(P,\le )$ http://planetmath.org/node/3242doesn’t collapse cardinals since it satisfies the countable chain condition. Thus ${\mathrm{\aleph }}_2^{𝔐[G]}={\mathrm{\aleph }}_2^{𝔐}$ and CH is false in $𝔐[G]$.

All this relies on a proper definition of the satisfaction relation in$𝔐[G]$, and the forcing relation. Details can be found in Thomas Jech’s book Set Theory.

Title	forcing
Canonical name	Forcing
Date of creation	2013-03-22 12:44:17
Last modified on	2013-03-22 12:44:17
Owner	ratboy (4018)
Last modified by	ratboy (4018)
Numerical id	12
Author	ratboy (4018)
Entry type	Definition
Classification	msc 03E50
Classification	msc 03E35
Classification	msc 03E40
Related topic	ForcingRelation
Related topic	CompositionOfForcingNotions
Related topic	EquivalenceOfForcingNotions
Related topic	FieldAdjunction
Defines	forcing