Signs
Many things that seem to occur often with poltergeists are:
Objects being thrown around, sometimes at the victim
Objects appearing in random places
Voices being heard
being pushed/dragged out of bed
A ghostly light being seen
Starting after something bad happening, e.g. a death, being checked for cancer
Forcing things on the victim, such as poison
Rapping noises (one means no, two mean yes)
Historically, several different hypotheses have been put forward to explain the poltergeist phenomenon.
Physical explanations
Some scientists and skeptics propose that all poltergeist activity untraceable to fraud has a physical explanation such as static electricity, electromagnetic fields, ultra-, and infrasound and/or ionized air. In some cases, such as the Rosenheim poltergeist case, the physicist F. Karger from the Max-Planck-Institut für Plasmaphysik and G. Zicha from the Technical University of Munich found none of these effects present and psi proponents claim that no evidence of fraud was ever found, even after a sustained investigation from the police force and CID. Whether this is true or not, police officers did sign statements that they had witnessed the phenomena. Other aspects of the case were hard to explain: The time service was rung hundreds of times, with a frequency impossible with the mechanical dialing phones of 1967. The municipal authority disconnected the office from the mains supply and hooked it up to a dedicated generator hoping to stabilize the current. But surges in current and voltage still occurred with no detectable cause according to Zicha and Karger. Others think poltergeist phenomena could be caused by more mundane phenomena, such as unusual air currents, air vibrations such as in acoustic levitation, or tremors caused by underground streams. (http://skepdic.com/poltergeist.html)
John Hutchinson has claimed that he has created poltergeist effects in his laboratory. Also worth noting is that scientist David Turner proposes that poltergeists and ball lightning may be linked phenomena. Some scientists go as far as calling them pseudo-psychic phenomena and claim that under some circumstances they are caused by obscure physical effects.
Parapsychologists William G. Roll and Dean Radin, physicist Hal Puthoff and head of electrical engineering at Duke University who specializes in electromagnetic field phenomena, claim that poltergeist phenomena [the movement of objects at least] could be caused by anomalies in the zero-point field, this is outlined in the above article and in Roll's book Unleashed and mention is made of it in a chapter of Dean Radin's book Entangled Minds. The basic theory is that poltergeist movements are repulsive versions of the casimir effect that can put pressures on objects. Thus, anomalies in this field could conceivably move objects. This theory has also been mentioned in the current book on paranormal phenomena Science by Marie D. Jones.
The theory is not complete, however, because it accounts for the movement of objects but not for the strange voices, seeming personality, and strange electrical effects displayed in some cases.
In 1979, Hutchison claims to have discovered a number of unusual phenomena, while trying to duplicate experiments done by Nikola Tesla. He refers to several of these phenomena jointly under the name "the Hutchison effect", including:
Levitation of heavy objects.
fusion of dissimilar materials such as metal and wood, while lacking any displacement
the anomalous heating of metals without burning adjacent material.
the spontaneous fracturing of metals.
changes in the crystalline structure and physical properties of metals.
disappearance of metal samples.
Hutchison has maintained a number of websites over the years, in which he posts videos and pictures of the purported effect, including short low-quality clips of objects flying around or rising from the ground, and metallic objects moving without being touched. He has offered mail-order VHS tapes of the effect for $100 each, though videos are now sold exclusively through Gryphon Productions.
Supporters like Mark Solis, his former webmaster maintain that none of these effects can be the result of known physical phenomena, such as electromagnetism. Hutchison and his supporters surmise that these phenomena arise from zero point energy or the Casimir Effect.
It has been said that researchers at NASA and the Max Planck Institute have attempted to reproduce some of Hutchison's experiments, but that so far none has succeeded. Indeed, NASA's Marc Millis remarks that Hutchison himself appears unable to reproduce his own experiments. Hutchison claims that this is due to the destruction of his lab by the military, or because he has been otherwise prevented legally by the government from repeating his experiments.
Skeptics think that the phenomena are hoaxes perpetrated by the agent. Indeed, some poltergeist agents have been caught by investigators in the act of throwing objects. A few of them later confessed to faking.
Skeptics maintain that parapsychologists are especially easy to fool when they think that many occurrences are real and discount the hoax hypothesis from the outset. Even after witnessing first hand an agent throwing objects, psi-believing parapsychologists rationalize the fact away by assuming that the agents are only cheating when caught cheating, and at no other time. One reason given is that the agents often fake phenomena when the investigation coincides with a period of time where there appears to be little or no 'genuine' phenomena occurring. Another stated reason is that some of the phenomena witnessed would be hard to fake, even for magicians when under the watch of many people, let alone untrained children and non-magicians.
The current consensus among most scientists is a mixture of the self-delusion and hoax hypotheses and a bit of the caused-by-scientifically-explained-forces hypothesis [tremors, abnormal air currents etc ].
