# Why “adding” a single extender cannot give an L-like inner model for say, a strong cardinal?

The constructible universe $L$ is too thin for large cardinals greater than measurable. To build $L$-like inner models for large cardinal, it is natural to think about "adding" the evidences into the model. For example, $L[U]$ is an inner model for measurable cardinal. However, the situation becomes much more complicated beyond measurable. One reason is that $L[U]$ can contain only one measurable cardinal not two, which I knew. And I was told somewhere that

if $\kappa$ is a some strong cardinal witnessed by an/some elementary embedding(s) $j$, and $E$ is an extender generated from $j$, Then $L[E]=L[U]$ where $U=E_{\{\kappa\}}$.

Is the statement true? If true, how to prove it? If not, how to argue that it is necessary to develop much more complicated technique to build inner model for larger cardinals?

One point to make is that no one embedding witnesses that a cardinal is strong (that is unless it is witnessing that $\kappa$ is something much stronger, like a supercompact). The definition requires that for every ordinal $\alpha$ there is an embedding $j_\alpha$ with critical point $\kappa$ witnessing that $V_\alpha$ is the $V_\alpha$ of the target model.
I am trying to guess at what you may have seen: it is certainly true that if you take the "$\kappa+1$"-extender $E$ derived from any such embedding $j_\alpha$ for $\alpha > \kappa +1$, then this is essentially just a measure on ${\cal P}(\kappa)$, and then, yes $L[E] =L[U]$ by Kunen's analysis.
To get an inner model for, e.g., a strong cardinal, one will need some methodology that allows you to build a class-sized predicate: one will need to have encoded somehow, for a proper class of $\alpha$, those $j_\alpha$ on to the predicate. It is this methodology that makes the matter complicated; to get a fine structural inner model, such as $L[U]$ the construction is even more delicate.
• I believe Philip is referring to Kunen's paper "Some applications of iterated ultrapowers in set theory," Ann. Math. Logic 1 (1970) 179-227. Kunen shows there (among many other things) that the universe constructed from a measure on $\kappa$ depends only on $\kappa$, not on the particular measure. – Andreas Blass Nov 18 '11 at 19:44