A black hole mimicker
Black holes have a very specific gravitational field, which one can characterize by its multipole moments. The multipole moments of black holes are all completely determined the mass and spin of the black hole. We asked the question: are there other objects that can mimick all the multipole moments of a rotating black hole? We constructed an explicit example of a Newtonian star with multipole moments identical to those of a rotating black hole, so the answer to this simple question is "yes".