Existence of families of spacetimes with a Newtonian limit

Jurgen Ehlers developed frame theory to better understand the relationship between general relativity and Newtonian gravity. Frame theory contains a parameter lambda, which can be thought of as 1/c(2), where c is the speed of light. By construction, frame theory is equivalent to general relativity for lambda > 0, and reduces to Newtonian gravity for lambda = 0. Moreover, by setting epsilon = root lambda, frame theory provides a framework to study the Newtonian limit epsilon SE arrow 0 (i.e. c -> infinity). A number of ideas relating to frame theory that were introduced by Jurgen have subsequently found important applications to the rigorous study of both the Newtonian limit and post-Newtonian expansions. In this article, we review frame theory and discuss, in a non-technical fashion, some of the rigorous results on the Newtonian limit and post-Newtonian expansions that have followed from Jurgen's work.