Forcing Isomorphism II

Abstract

If T has only countably many complete types, yet has a type of infinite multiplicity then there is a ccc forcing notion Q such that, in any Q --generic extension of the universe, there are non-isomorphic models M_1 and M_2 of T that can be forced isomorphic by a ccc forcing. We give examples showing that the hypothesis on the number of complete types is necessary and what happens if `ccc' is replaced other cardinal-preserving adjectives. We also give an example showing that membership in a pseudo-elementary class can be altered by very simple cardinal-preserving forcings.