There is however another theory that has been gaining a lot of momentum in the past few years.  This second theory, although still a theory, ties in with some cutting-edge physics research that is going on right now.  This is the theory of parallel dimensions.

The basic idea is not new.  The concept of parallel dimensions has been around since shortly after Einstein published his theory of relativity.  It is most often associated with science fiction but not anymore.  The concept says that our dimension/universe coexists along side many other dimensions/universe.  Since all of these dimensions are parallel they never intersect thus we never see them or interact with them.  These dimensions can range from slightly different to completely different from our own.  The farther you get away from our parallel the more pronounced the differences may become.  For example if you have blue eyes and were to move over 1 dimension from ours you might have green eyes.  The popular show from the 80s and 90s called "Sliders" centered around this.

Up until recently parallel dimensions were nothing more than fiction but in the past few years a great deal of research has been done within quantum physics that point to the possible existence of these dimensions.  I won't get into that aspect here as it gets way too technical but if you are interested here are a few articles worth reading:

The Harvard Crimson - Supersymmetry and Parallel Dimensions

The CERN Courier - Testing Times For Strings

Quantum physicists have already identified the connection between quantum physics and the string theory. 

Parallel Dimensions and the Paranormal
So assuming that there are millions of parallel dimensions occupying the same space/time continuum but they never intersect it stands to reason that we wouldn't actually be alone in this plane.  Taking that a step further - if these dimensions exist is it not possible for 2 extremely close dimensions to bleed over into each other?

Take for example a radio station.  If you tune in to its exact frequency the station comes in loud and clear however if you were to tune it slightly off it may still come in loud and clear BUT you might start to hear some bleed-over from a station with a close frequency.  This is the premise at play.

We are here in our dimension.  There are other people in another dimension.  If those 2 dimensions are close enough together on the same plane then perhaps what we are experiencing is not actually paranormal, spiritual, dead or anything else like that.  Instead it is bleed-over of real, living beings on another dimension.  Given certain conditions/environmental variables these bleeds can allow voices, visuals and even thermal/electromagnetic anomalies to occur. 

The biggest stumbling block with this theory is religion/spirituality.  There are huge ramifications if this were to be accepted - especially where the afterlife is concerned.  Many feel that there is an incredibly strong religious component to the paranormal and refuse to acknowledge anything else.  I personally find this to be no better than denying the possibility of anything altogether.  Open-mindedness is CRITICAL when dealing with the unknown.  This doesn't just go for whether or not you believe in ghosts but also what/how/why these anomalies exist. 

Taking one but not the other will create a bias and bias is detrimental.