There are five basic theories about what a ghost could be. Over the years many honest and sensible witnesses have reported seeing and hearing inexplicable, anomalous entities. What have they encountered?
Theory one goes along with the Charles Dickens' style of ghost: someone like Marley, a disembodied human soul that comes back from the next world with a purpose, or a message of some kind.
Theory two refers to recordings. The strange entities reported are not disembodied souls or spirits but merely the impressions they have left behind. According to this theory, strong emotions possess energy. That energy cuts its way into the surrounding fabric and leaves a ‘recording' there. In the right conditions sensitive and perceptive people can experience the recordings - like watching the re-run of an old film.
Theory three concerns glitches in time. What are referred to as ghosts and phantoms are seen and heard because our current time has come close to a time in the past and we perceive something that happened long ago as if it was happening now. Phantom battles may fall into this category.
The fourth theory involves probability tracks: the so-called ‘worlds of if'. Some modern physicists wonder whether events that did not happen in our experiential universe are still out there somewhere with a strange quasi reality. When such a theoretical probability track lies close to our reality track, entities from that ‘world of if' become visible and audible on our world.
The fifth set of ideas looks into the possibility of extra-terrestrial aliens visiting us, sometimes appearing as phantoms or spectres while they are observing us and trying to remain more or less invisible while doing it!