I can put forth a theory as to how ghosts can become visable(maybe). Since light is supposed to be some kind of visable electro-magnetic disturbance, maybe the "energy" that makes up a ghost is sometimes able to create such a disturbance (which is why you often see a ghost when they are "upset" or "sad", maybe their energy is much higher in that state (frustration?) therefore producing enough power to generate a visable form, an electromagnetic fluctuation.
As to what a ghost is exactly, i dunno. I guess it depends on a few things, like for example whether a human soul is present before it fits itself into a body at birth or conception (like with reincarnation) an energy disturbance that has found a way to implant itself into a physical form, possibly even manipulating evolution in such a way to form over time a body that is preferrable (like how in the bible how it says man is infused with a soul, whereas animals are not, and angels are not and they are just basically a form of energy.)
However if you want to go by a "soul" or "spirit" forming as a human grows and get older (gaining memories etc) Maybe the bioelectrical part of the brain has a place in that. patterns of energy swirling in such a way that they may possibly become quantum entangled or some such type deal, and therefore once the body dies the energy remains linked in some way.
Of course im just pulling this stuff out of my ass just now, i've never had to think about this much or put my thoughts on possibilities into words. :P
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