The Traditional Theory
There are many different conjectures about ghosts, but the most common thought is that a ghost is the spirit of a departed person that has been somehow trapped between this plane of existence and the next. I am not sure why spirits would be trapped, but this explanation has been accepted for centuries. The accompanying photograph shows a white apparition at the right edge of the Buford House porch. In general, there are two different types of ghosts, and Tombstone has an abundance of both.
Intelligent. An "intelligent" haunting seems to know something about its state and what is happening in the environment. It often moves around objects, like furniture, rather than through them and seems to have a purpose for appearing. These ghosts frequently contact humans by using a medium and sending a message to the living. One type of intelligent haunting is a poltergeist, a ghost who can physically interact with the environment and is often somewhat mischievous. I occasionally hear stories about things moved in an unoccupied room, like guests who tell me something like shoes moved in their rooms while they are out, but poltergeist activity is rare in Tombstone.
Residual. In contrast to an intelligent haunting, a residual haunting is a ghost that returns periodically and performs the same action on each visit, something like a film being repeated repeatedly. In contrast to an intelligent haunting, a residual ghost will move directly through walls, vehicles, or whatever else is in the way, as if the ghost is not aware of its surroundings and is simply reliving something from the past. Tombstone has several well-known residual hauntings; for example, there are reports of an older woman dressed in clothes from the 1940s who is sometimes seen at dawn standing at the edge of Allen Street between Second and Third. She is only there for a few minutes and looks up and down Allen Street several times as if she is waiting for a ride to go somewhere. No one seems to know who this woman is or what she is doing here, but when she shows up, she tends to startle people who are not expecting to see a person there at that time of day.
A Modern Theory
Ground-breaking research being conducted in physics and metaphysics informs our understanding of ghosts. Tools available to science today, like the CERN nuclear research laboratory and the James Webb telescope, were not even imagined just a few decades ago. They significantly improve our understanding of the paranormal. We are entering a "Golden Age" of paranormal research that has never been possible before now.
One promising new field of inquiry involves the possibility of parallel universes. One exciting idea is that there can be an infinite number of universes that are remarkably similar to our own but have evolved slightly differently due to the Butterfly Effect. In those universes closest to our own, the difference may be minimal, such as how people dress or their form of government. Those universes would have, in turn, parallel equivalents that would be slightly different from them. If travelers could somehow hop from one universe to the next, they would find that each visited universe would become more and more bizarre when compared to their own.
If parallel universes are eventually proven true, then an exciting new idea about ghosts becomes possible. Perhaps a ghost is nothing more than the momentary interaction between our universe and the closest parallel. That interaction could be sensed as a fleeting light in an observer's peripheral vision. If the interaction is much more robust, it could be sensed as a spectral shape that becomes visible to a broad group of people. Of course, there are many other interactions possible, depending on the quality of the connection between the universes. The interacting universe would be our closest adjacent one. It would be similar to our own, with, maybe, just one weird difference, and that would be the source of confusion for people experiencing the interaction. For example, the interacting universe theory could explain why a poltergeist would move someone's shoes if, in that parallel world, shoes belong in the sink or refrigerator or somewhere "strange" to us.
Developing this concept further, perhaps death is nothing more than the movement of a spirit from this to a nearby parallel universe where it starts a new existence. If the spirit has an urgent message for those left behind, it will attempt to get that message back to this universe, which could explain things like mediums who communicate with departed loved ones. Perhaps our lives are merely an eternal shuffling between multi-verses, and some of those would be heavenly while others would be hellish. This understanding of the grand cosmos would explain many religious and secular experiences, including such wide-ranging phenomena as angelic appearances, extra-sensory perception, and unidentified flying objects.
At any rate, parallel universes may not be the best explanation for the presence of ghosts and other paranormal activity. However, it is at least an area that deserves more thought and investigation.