  Wavefunction collapse is a puzzling aspect of quantum mechanics. Designing a
test to confirm the instantaneousness of collapse has turned out to be
challenging, especially for collapse in configuration space. We propose a test
using two identical, non-local, correlated photons in an interferometer in
which a post-selection measurement of one of the photons in its location
changes the statistical behavior of the other photon, which is an arbitrarily
large distance away, and thus its detectable subsequent behavior. Analysis of
the resulting correlations constitutes a test of the instantaneousness (or
non-instantaneousness) of collapse. Connections to some of the recent proposed
models of collapse are discussed
