i am not sure, just heard about it watching ghost hunters the other night. has to do with two points in time at the exact same location where real folks on both ends perceive the simultaneous interactive haunting. no ghost, just real live folks experiencing one another through the 'slip', usually having to do with an object, perhaps furniture, or can be a whole venue, like the Ball Room at the Stanley Hotel filled with folks from the past as seen by a witness in present day. perhaps the folks at the ball interactively seeing this present day witness thought him/her to be a ghost as well.