Have you ever seen a ghost? Or have you ever heard or felt something that felt creepy to you?

Ghosts are very popular subjects these days. About half the American people admit to believing in them, and there are ghost beliefs in nearly every culture in the world. People have told ghost stories and reported ghostly encounters for all of recorded history. Today, paranormal investigators are compiling film and audio evidence, and experimenting with changes in electromagnetic fields and temperature in allegedly haunted places to try to determine if there is objective evidence for the existence of something we might identify as a ghost. Growing evidence indicates that there is, indeed, some unexplained phenomena that can, for lack of a better word, be identified as a ghost or spirit.

But what, exactly, is a ghost? In folklore, a ghost is the manifestation of a dead person, and many people belief this is so. But what are other theories of what a ghost might be?

Here are some popular ideas:

1. A recapturing of some violent or deeply emotional event in the past. This is what is called a "residual" haunting, and appears to be the most common kind. The apparitions that are seen in these cases are always in the same place, doing the same thing, and show no awareness of their actual surroundings or of living beings who are present. These ghosts would have no personality or substance, but would be the psychic equivalent of holograms replaying one small bit of history. An example of this might be the well-known ghost of Alice, at the Hermitage near Myrtle Beach, SC.

2. A projection of our own thoughts. We use only a small part of our brains; some people theorize that certain circumstances combined with a certain frame of mind will trigger an extremely vivid vision that appears to be real in every way. Anyone who has ever experienced a hallucinogenic drug will concur that reality can often be a tricky thing. It is not explained how such a manifestation might be caught on film or tape, however. One of the reasons that paranormal research is so intriguing is that we do have to take into account that we usually are in a haunted place because we either really do or really don't want to see something unexplained, and that can bias our experience. If we can catch phenomena through technology, that validates our experience.

3 Angels or demons. People of a particular religious background who laugh at the idea of ghosts may be much more willing to believe that something unknown is either supernaturally good or supernaturally evil. In most cases of haunting, there is little evidence of either extreme.

4. Creatures from outer space- though why they would take forms from throughout history or hang about in places that are often empty for most of the time (such as abandoned jails and hospitals) is hard to understand.

5. Glimpses into another dimension- String theory is responsible for the new popularity of this theory. In string theory, one conjecture is that there are many, many dimensions, and not just the ones we are familiar with. Some people theorize that there may be other beings inhabiting parallel dimensions, going about their lives in similar fashion to our own, and once in a while some circumstance will create a break between the dimensions, and we catch sight of one another. This would explain why apparitions who do appear to have intelligence and emotion sometimes seem so surprised to see us.

None of these theories, which do not include the possibility of intelligent interaction between the spirits of the dead and the living, explain why ghosts often seem to be emotionally involved with the place where they are or with people in the place. It seems from my research that much more often than a ghost seems angry or dangerous, the entity will seem lonely or concerned about someone or some place left behind. This is especially true in EVP work (electronic voice phenomena, where voices are captured on tape that could not have come from anyone present in the room.) In EVPs, it is not unknown for unseen entitites to actually address people by name or refer to items in the room or the building.

Of course, it is all just theoretical, and no one can say for sure what causes ghostly phenomena. However, it seems difficult to explain away all evidence that in a small number of cases, there is intelligence, and at least a shadow of recognition and emotion, attached to what we choose to call ghosts.