Benjamin Radford has an article up over at LiveScience that looks at the "science" behind ghost hunting. It really is something that any would-be ghost hunter should read. I'd recommend it to 9/11 truthers as well to see how non-evidence is used as evidence. It's quite instructive.
The most famous ghost hunters are two plumbers who moonlight as paranormal investigators, seen in the popular Sci-Fi Channel reality show/soap opera series "Ghost Hunters." They go to haunted places and find "evidence" of ghosts such as cold spots, photographic anomalies called orbs, and other such spookiness.
The two featured investigators, Jason Hawes and Grant Wilson, are proudly blue-collar workers, not egghead Ph.D. scientists, which adds to their strong "regular guy" appeal.
Where are the ghosts?
While one doesn't need to be a scientist to search for ghosts, the pair (like most ghost hunters) could benefit greatly from a little critical thinking. They claim to be skeptics but are very credulous and seem to have no real understanding of scientific methods or real investigation. (Audiences don't seem to wonder why these "expert" ghost hunters always fail: Even after two seasons and over ten years of research, they still have yet to prove that ghosts exist!)
The uncomfortable reality that ghost hunters carefully avoid—the elephant in the tiny, haunted room—is of course that no one has ever shown that any of this equipment actually detects ghosts.
The supposed links between ghosts and electromagnetic fields, low temperatures, radiation, odd photographic images, and so on are based on nothing more than guesses, unproven theories, and wild conjecture. If a device could reliably determine the presence or absence of ghosts, then by definition, ghosts would be proven to exist. I own an EMF meter, but since it's useless for ghost investigations—it finds not spirits but red herrings—I use it in my lectures and seminars as an example of pseudoscience. The most important tools in this or any investigation are a questioning mind and a solid understanding of scientific principles.
The ghost hunters' anti-scientific illogic is clear: if one area of a home is colder than another, that may indicate a ghost; if an EMF meter detects a field, that too may be a ghost; if dowsing rods cross, that might be a ghost. Just about any "anomaly," anything that anyone considers odd for any reason, from an undetermined sound to a "bad feeling" to a blurry photo, can be (and has been) considered evidence of ghosts.
I was even at one investigation where a ghost supposedly caused a person's mild headache. Because the standard of evidence is so low, it's little wonder that ghost hunters often find "evidence" (but never proof) of ghosts.
"Evidence" without proof. Sounds kind of familiar, doesn't it? Read the whole thing, it really is quite entertaining.