Natural Science
This passage is adapted from "The Creepy Scientific Explanation Behind Ghost Sightings" by Jack Mendoza, which appeared on cracked.com (October, 2010).
There is simply no evidence that dead people wander aimlessly around old houses, and no known scientific principle that would make it possible, but a lot of people have seen ghosts. There are certain spots and buildings where separate, unrelated witnesses have reported ghosts, without having talked to each other or being aware of the area being "haunted."
It appears that science has stumbled across the reason for it1. It has nothing to do with the supernatural, but the answer is almost as weird. While working in his robotics laboratory, Vladimir Gavreau noticed that one of his assistants was bleeding from the ears. Puzzled, Gavreau started researching the phenomenon, and soon realized that a vibrating pipe of the right length and girth can cause a number of unpleasant effects ranging from mild irritation to serious pain.
What he had discovered was infrasound. It's noise at a low enough frequency that you don't consciously hear it, but your ears still sense it. The process of receiving sensory input without your conscious mind understanding where it's coming from wreaks havoc with your emotions. Specifically, researchers found that sounds between 7 and 19 Hz could induce fear, dread or panic2.
As an experiment, acoustic scientists sneaked in low-frequency sounds at a live concert. Most of the concertgoers had no idea what was going on. At the end of the experiment, 22 percent of the people involved reported feelings of unexplainable dread, chills and depression when infrasound was blasted into the crowd3.
"Why would it have this effect?4 It may be evolution. It doesn't take a mad-scientist device to create infrasound: nature creates this type of low-frequency vibration all the time. Volcanos, earthquakes, strong ocean waves and even winds hitting the hillside in just the right spot can create infrasound. Even animals can create it. The frequency of a tiger's roar is around 18 Hz. All the things that create the sound are huge, powerful and dangerous. Evolution might have taught us that this sound means bad news.
"So now we have a phenomenon that occurs in nature, is invisible, is imperceptible on a conscious level, but can spontaneously make you feel irrational fear, even if you're sitting in an empty room. We've just described one of the first and primary signs of a "haunting"-unexplained feelings of fear or dread5. But what about actual sightings of ghosts?6 For that we need to go to a researcher named Vic Tandy. In the engineering building where Tandy worked, cleaning staff as well as fellow researchers complained of feeling dread, depression and a strange feeling that someone was watching them. Every so often, the staff would see dark figures out of the corners of their eyes. After one particularly strange experience when a gray shape sat next to his desk for several minutes, Tandy was determined to figure out what was going on. After eliminating gas poisoning and rogue equipment, he realized that the ghostly apparitions seemed to almost always occur in a certain section of the lab. He also realized that if he put a metal sheet in a vice, it would spontaneously vibrate uncontrollably for no apparent reason.
Poltergeist7? No, just infrasound. A silent exhaust fan was sending out low-frequency vibrations that bounced back and forth on the lab's walls until they formed a powerful wave at 18.9 Hz, right at the top of the panic range. According to a NASA study, it was powerful enough to resonate with the average human eyeball, causing "smeared" vision, a phenomenon where the eye vibrates just enough to register something static-say, the frame of your glasses or a speck of dust-as large, moving shapes8. Once the fan was removed, the strange apparitions and feelings of fear disappeared.
Convinced that he had stumbled onto something, Tandy went on to test this explanation for ghostly apparitions in the cellar of a nearby "haunted" abbey. According to the locals, as soon as someone would step into the cellar they would freeze up, see strange gray ghosts and have to leave because of nausea. Vic discovered that the shape of the cellar and the hallway leading to it, as well as nearby factories, all contributed to making the haunted cellar a perfect resonating chamber. The vibrations created were exactly 18.9 Hz and were most powerful at the threshold of the cellar, where most people became sick and terrified.
So if you're ever troubled by strange noises in the middle of the night or you experience feelings of dread in your basement or attic or even if someone in your family sees a ghost…. Well, call the repair guy, because it might be caused by a malfunctioning ventilation fan9.