If a weak soul is separated from the body after the death of a human, the soul might have "forgotten" its own identity and do not know that it must pass on into the next dimension. It tries to cling onto the body and the things that the human have gathered during the life, it becomes a ghost. Ghosts are not dangerous and finds the "right way" gradually.
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You say that people who have died have got stuck in the fourth dimension, but it doesn´t have to be that way according to the RSPK-phenomena, e.g Poltergeists and knocking-spirits. Also Spectre and regular ghost doesn´t have to be this kind of phenomena. A spectre are as you probably know, a film that are stucked and keep repeating itself according to the environment. This doesn´t have to be a lost soul from the fourth dimension. Likewise is the light Phenomena that is frequently viewed on graveyards. These lights are gases that are mixed and form lights. Poltegeists is caused by internal aggressions that is canalized within (most cases) younger people. These ghost are not real ghost, but a kind of demon. A poltegeist have never lived and can not get stuck between the dimensionborders.
Ghost, spectres and poltergeist-phenomena are different things and should not be mixed up. This page treats merely ghost but will in time also deal whith other phenomena.