Joe Nickell of the Committee for Skeptical Inquiry, wrote that there was no credible scientific evidence that any location was inhabited by spirits of the dead. Limitations of human perception and ordinary physical explanations can account for ghost sightings; for example, air pressure changes in a home causing doors to slam, or lights from a passing car reflected through a window at night. Pareidolia, an innate tendency to recognize patterns in random perceptions, is what some skeptics believe causes people to believe that they have 'seen ghosts'. Reports of ghosts "seen out of the corner of the eye" may be accounted for by the sensitivity of human peripheral vision. According to Nickell, peripheral vision can easily mislead, especially late at night when the brain is tired and more likely to misinterpret sights and sounds."
Some researchers, such as Michael Persinger of Laurentian University, Canada, have speculated that changes in geomagnetic fields (created, e.g., by tectonic stresses in the Earth's crust or solar activity) could stimulate the brain's temporal lobes and produce many of the experiences associated with hauntings. Sound is thought to be another cause of supposed sightings. Richard Lord and Richard Wiseman have concluded that infrasound can cause humans to experience bizarre feelings in a room, such as anxiety, extreme sorrow, a feeling of being watched, or even the chills. Carbon monoxide poisoning, which can cause changes in perception of the visual and auditory systems, was speculated upon as a possible explanation for haunted houses as early as 1921.
Alternative explanations
In the early days of the Society for Psychical Research many researchers investigated reportings of ghosts and apparitions and were seeking a scientific explanation for such phenomena. Some of the psychical researchers had rejected that ghosts were spirits of the deceased and instead claimed ghosts and apparitions were not real entities but were hallucinations caused by a telepathic mechanism. Frank Podmore was a main advocate of the telepathic theory of ghosts, he discussed this theory in his book Telepathic Hallucinations (1909). Other researchers disagreed with the telepathy theory and instead advocated the widely known spirit theory of ghosts, the belief that ghosts are discarnate spirits which was most popular amongst spiritualists.
The psychical researcher Thomson Jay Hudson also developed his own theory about ghosts which involved telepathy however Hudson did not claim ghosts are hallucinations. Instead he claimed ghosts and apparitions are memories or thoughtforms from the mind of the subject materialising or "projecting" themselves externally. Hudson also explained that where more than one person had visualised the same ghost then it can be explained by telepathic communication. The paranormal writer Paul Roland in his book The Complete Book of Ghosts (2007) argues that ghosts are the "manifestation of people still living, proving that out-of-body experiences are not as rare or as impossible as some people might think".
Ernest Holmes suggested two explanations for ghosts, that they are spirits of the deceased or a form of thought that can materialize itself then gradually fade away. Holmes wrote that further investigation was needed to prove which explanation is correct.
According to (Willin, 2005) Peter Underwood favours the "electronic impulse wave theory" which claims brain waves become more active in higher stress rates and that at a certain level will produce a ghost which is a "telepathic image that is capable of being picked up by someone else" and will fade away when people are not experiencing it. The parapsychologist George N. M. Tyrrell argued that apparitions and ghosts were telepathic hallucinations which emanate from the subconscious mind.
Jeff Danelek in The Case for Ghosts: An Objective Look at the Paranormal (2006) wrote that another explanation of ghosts is the Stone Tape theory which claims that ghosts are collections of energy that can become stored in physical matter which can be released resulting in a display of recorded activity. According to this hypothesis, ghosts are not spirits but simply non-interactive recordings similar to a movie.The parapsychologist William G. Roll agreed with the Stone Tape theory, he claimed that a person's mind can create an apparition from "psychic traces" left from the past.
Paul Devereux claims that many ghost sightings can be explained by "earthlights", clouds of plasma being charged by strong electromagnetic fields occurring in areas of seismic activity, he claims that that such natural phenenoma "can respond to witness movement and thought". He explores these ideas in his book Haunted Land (2003).
The spirit theory of ghosts in recent years has been termed the survival hypothesis and is advocated by some psychical researchers over the other alternative explanations.
