Ghost stories have been around as long as there have been stories themselves. The idea of apparitions from the spirit world goes back to the very beginnings of written history, and probably even farther back in oral traditions. A recent CBS News poll concluded that nearly half of all Americans believe in ghosts, and 22 percent say they have seen or felt the presence of a ghost.
And yet mainstream science has long been clear and unequivocal: There is no scientific evidence of a supernatural explanation for ghost sightings. So how do we explain those incidents when rational people sincerely believe they have seen or felt a ghost? What are some of the scientific, non-paranormal explanations for the phenomenon of ghost sightings?
As it turns out, there are quite a lot of real-world explanations for ghost sightings. Researcher Loyd Auerbach, author of several books on the subject of hauntings, is a believer in ghosts and has been investigating reported sightings for 30 years. Yet even he concedes that the vast majority of alleged hauntings can be explained away by natural phenomena. Chief among them is the psychological state of the person who experienced the haunting.
"It's often because people are predisposed -- they've been watching too many TV shows, or something bad is going on in their lives," Auerbach said. "Sometimes people are psychologically disturbed, but most of the time I find it's people making mistakes because they're already in a sensitized state due to something else entirely. It's people who are suggestible, and when that nail in the wall finally pops out, they ascribe significance to a mundane event."
Sometimes a legitimate natural phenomenon, or a combination of different phenomena, can result in a "ghost sighting." For example, research dating back to the 1970s suggests that extremely low frequency (ELF) electromagnetic fields can stimulate certain parts of the brain and produce effects that are often associated with hauntings. "I had a case a couple years ago, a family had moved into a house and in a particular room, they got dizzy or got headaches," Auerbach said. "They saw shadows out of the corner of their eyes."
Auerbach investigated and found out the house was directly under high-tension wires that were emitting an electromagnetic field and a low-frequency hum. "It was in the frequency that would vibrate your eyeballs," Auerbach said. "That would cause you to see things out of the corner of your eye."
Low-frequency hums, sometimes called infrasound, can also produce feelings of fear and anxiety, Auerbach said. "Hollywood has known this since the 1950s at least, which is why you get those low frequency tones in horror movie soundtracks."
In many cases, an alleged haunting is caused by multiple factors that seem supernatural when combined. For instance, Auerbach said that infrasound and electromagnetic waves only explained some of the issues in the house under the high-tension wires. "It wasn't just headaches, the family said they would smell noxious odors that smelled like brimstone," he said. "They would also report bursts of fire that would singe the walls."
Further investigation revealed the house was adjacent to garbage dump, and methane gas was seeping up from the ground. "That was what they were smelling, and there was so much static electricity in the house that the methane would catch fire," Auerbach said.
In cases where people claim to have seen an apparition, but there are no psychological or emitted energy factors, the "ghost" can simply be an optical illusion. Most commonly, it's an incident of light bouncing off a window or other reflecting surface. There's also the psychological phenomenon of pareidolia, where the brain gives significance to random images or patterns -- seeing faces in clouds, say, or the ghost of your grandmother in the shadows of a wardrobe closet.
Dante Centuori, director of creative productions at the Great Lakes Science Center, said that he believes alleged ghost sightings are always the result of natural phenomenon being misinterpreted. Even with incidents we can't explain, it's simply a matter of overlooking something or ascribing false significance.
"We're very error-prone in terms of observation and perception," Centuori said. "We have a predisposition to fill in the blanks with a cultural context, with the images that we're all bombarded with. We see something that doesn't add up, and we jump to: 'It's a ghost.'"
Visual hallucinations are often cited as the scientific explanation behind ghost sightings, and they can be caused by both psychological conditions and -- according to some research -- electromagnetic waves effecting the brain. But there's a giant gray area in this topic, Auerbach said, because an experience that could be accurately termed a hallucination can also be a genuine ghost sighting.
"The key for us is, people might have experiences that could be termed hallucinations," he said. "But if what they're seeing is historically accurate -- if they're describing things they couldn't otherwise know about -- then there's something very worthy of research."
When investigating ghost sightings, researchers always have to keep an eye out for another entirely non-paranormal phenomenon -- the hoax. Auerbach said that most of the time, it's family members or kids trying to pull a fast one as a prank. Sometimes the hoaxer has other ambitions, and attempts to fool the investigator, too. Does Auerbach ever get requests from people trying to run a Scooby Doo routine?
"Infrequently," Auerbach said. "I get calls every now and then where I sense the situation is a little hinky. I always ask people what they want out of an investigation. If they say they want to sell the story to the movies and make a lot of money, then I don't want to get involved. I've also had a couple calls that, I found out later, were skeptic groups trying to trap me."
Even when all the known physical and psychological factors have been eliminated as explanations, it's possible that ghost sightings may be caused by forces that we just don't yet understand. Robert Schoch, associate professor of natural science at Boston University, has been doing research on the relationship between brain waves and geomagnetic waves. Schoch cited the phenomenon of "crisis apparitions" -- when family members see the "ghost" of a faraway relative at the exact moment that person died. "Some people will dismiss this as coincidence," Schoch said. "But there have been, in my assessment, very good statistical studies of such things that take it out of the realm of coincidence."
According to Schoch's hypothesis, such apparitions aren't ghosts at all, in the sense of otherworldly spirits. Instead, the phenomenon may be a kind of extrasensory perception that we can't yet measure. Schoch's research concerns whether brain waves of certain emotional states may be transferred between people over long distances on low-frequency wavelengths -- the same wavelengths that are detected in the Earth's geomagnetic field.
"One study looked at crisis apparitions and geomagnetic patterns on the surface of the Earth," Schoch said. "It turns out that the incidents of crisis apparitions correlate with geomagnetic flux."
Schoch said it can be professionally risky, in the academic community, to advance any theories that include parapsychology phenomena like ESP or ghost sightings. But he still believes there is value in exploring these topics. "When you get rid of all the bogus crap, which is 95 percent plus of it, there is a residuum left where it seems to be something real."