I think we all share the same world. I think the world is all part of one great thing with a layer of realms. They (ghosts) enter the realm we are in from time to time or stay here because they believe they still belong in this realm and we sometimes visit their realm either through experiencing them in some way or during an OBE or a dream. To me it's like the layers of the earth beneath there are things that live below (or in the oceans) that we consider their world and not ours yet it's all one in the same. This world is intended for the living body but it is also intended for the soul/spirit as spirits on the otherside do come and live among us when they become our spirit guide. Or simply if they need to be here from to time to help us as part of their growth or to comfort us in time of need.