Impurity Dynamics in a Bose Condensate

We estimate the changes in the condensate ground state energy induced by one or more "sizable" impurities. By "sizable", we mean an impurity whose size is within a few orders of magnitude of the trap size and not necessarily atomic in scale. A sizable impurity will "drill" a hole in the condensate wave function and alter the condensate energy. The question is whether this microscopic change can be detected macroscopically because of the Bose-Einstein condensation effect. We discuss the effect of impurity expulsion, condensate interactions, and the induced interaction between two impurities.