I believe for ghosts to be real, they would have to exist in an alternate parallel universe that shares the same space in time as our universe, sort of like in multiverse string theory, but with overlaying dimensions. Ghosts would be people who die in our universe and cease to exist, but continue to exist in an alternate parallel universe, and under certain conditions, are able to interact with our universe and capable of manifesting as vusual projections. This somewhat follows the many worlds interpretation of quantum theory, that there exist an alternate reality for every possible outcome for every possible event. How would they able to interact with our universe from an alternate existence? I dont know. It's possible that ghosts exist in a literal state of quantum superposition where they both exist in don't exist in our universe (sort of like in the schrodinger's cat thought experiment), but I don't think thats possible since quantum superposition is relative to the subatomic level. This is only a theory though.