It's got something to do with energy. Spirits draw energy from their surroundings, so the room goes cold, and batteries go flat. They can manifest in various stages, such as an orb (of heat), a mist, or a partial or full-body apparition. They are visible to some people (possibly as light). They can interfere with audio and video. They can be interactive, intelligent, and sometimes known to somebody.
We know so much already, so is it just a matter of time before we make irrefutable contact via technology with the spirit world?
EDIT: SPELLING CORRECTED
Edited by acute alan, 23 February 2013 - 11:04 PM.