They are invisible because they are energy. When is the last time you saw energy? It’s very unlikely you have easily seen energy. That’s why ghosts are only visible in certain conditions.

Yes, but they may have an electric charge or temperature difference than the surrounding atmosphere.—This means that things can happen to make them visible if conditions are right. Mists may form in their location, if they are cooler than the surrounding dew point, interactions with other local electric or magnetic fields might cause a glow, like St. Elmo’s Fire. Its electrostatic nature might cause fine dust to be attracted to it in a cloud, etc. in the shape of the “ghost”

They must be. I’ve never seen one. If you think about all the billions of people that have lived in the history of the world, plus all the other living things that have ever existed, if ghosts actually showed themselves then there would be piles of ghosts miles high everywhere you looked.

I’m going with either invisible or nonexistent in the first place.

http://www.youtube.com/watch?v=qWwwTu4-rxs

My understanding of the nature of ghosts and spirits is tempered by working with a psychic on numerous occasions as well as personal experience. It seems that the problem with seeing ghosts or spirits lies more with the viewer than with the spirit. Our pets and often small children seem to be far more able to see and react to spirits than most adults. This would seem to be in large part because, especially with small children, they haven’t learned not to see them. They do not appear as solid figures, but rather as filmy, insubstantial ones. In our Western culture, we place much emphasis on the “real world”, that is the realm that can be clearly seen and measured. Since ghosts and spirits do not belong to this realm, we are subtly encouraged not to perceive them, and thus they become invisible to us, not due to their nature, but rather due to ours.