(Before It's News)
Just as UFOs have become synonymous with alien visitors, similarly it is often assumed that ghosts, if they are real, must be the spirits of the deceased. There are, however, several paranormal theories to explain ghosts that would be equally paradigm shattering to mainstream science if they could be scientifically verified and proven to be real, and not just the product of overactive imaginations.
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Source:
http://www.para-news.com/2014/05/have-you-ever-wondered-how-ghosts-work.html