The are two main theories of the paranormal (apart from the materialist view that it is all rubbish, though philosopher David Chalmers has shown in his studies of consciousness that consciousness is as intrinsic to the universe as space, time and energy, which strongly suggests materialism cannot be true).
The first is that paranormal phenomena are produced by our unconscious mind and the second that paranormal phenomena are the work of spirits. Here I concentrate on the spirit model, though the two are not mutually exclusive and either may hold in any particular case.
Are spirits independent beings, Do they live entirely in the depths of our subconscious? Do we create spirits?
Chaos magicians think it doesn't matter which answer you give if it works. Jungian Psychologists assume the things we call spirits live in the collective unconsciousness, while Jung believed his archetypes could affect the world directly and were more like independent beings.
Gather three researchers and you will find maybe ten theories. And perhaps eleven will be sort of right.
Physics and hidden universes
The universe is full of beings we cannot detect without special tools: bacteria and parasites for example. The problem with postulating bodiless spirits is mainly a difficulty in imaging what it would be like to be a consciousness without a body. Modern physics at the quantum level postulates far stranger things than spirits however, such as particles that arrive before they depart, travel backwards in time to escape black holes, or pop in and out of existence.
We do not have a physics of how spirits can exist or interact with our world but this may only be the result of a mainstream taboo on studying these matters. Also remember that much of our current knowledge would have been declared impossible by a physicist trained only in Newtonian Physics and Newton's ideas were dismissed as magical by some of his contemporaries: even if they did not know of his interest in Alchemy.
Physics does not exclude the possibility of spirit worlds. Many Worlds Quantum Theory, on which the frontier technology of Quantum Computing is based, allows for parallel universes and over the last few decades theorists have eliminated flaws in the original version of the theory. If multiple universes exist then they should be able to communicate. If not we are effectively in a single universe.
To sum up so far.
Physics allows the existence of multiple universes that could be considered “spirit” worlds.
Our world contains beings we cannot normally detect and some may be sentient.
The evidence we have is confused and contaminated with fraud, self deception, believing-is-seeing, wish fulfilment and fears arising from the subconscious. Untangling all this is a fascinating challenge. The final conclusion may be that there is no evidence spirits exist. This would be a disappointing but valuable conclusion.
Seances and Poltergeists
There is. however, suggestive evidence, from folklore, spiritualist seances , poltergeist phenomena, and various apparitions – hallucinations excluded- that spirits exist. Any item of this evidence may be debatable but the totality is harder to dismiss, and if you are devoutly religious you HAVE to believe spirits exist. Well one at least.
On the other hand many “spirit” phenomena can be explained by Super ESP, ESP of a level much higher than observed in a laboratory. This is equally unacceptable to the skeptic tendency who invoke fraud and self delusion as explanations.
The evidence about the nature of spirits and the spirit world is contaminated by wish fulfilment, images arising from the subconscious and distortion resulting from premature efforts to interpret the phenomena . It is not valueless, just very hard to separate wheat from chaff. Raudive voices, voices allegedly recorded from radios tuned to an empty channel for example are suspect because the human brain has evolved very strong pattern recognition algorithms and, as with reverse speech, the hearer tends to hear what they want in random noise.
When we look at transcripts of a séance it is almost impossible to rule out the possibility of telepathy and clairvoyance.
But the evidence cannot all be dismissed on such grounds
What might the spirit world be like?
From here on assume spirit world(s) exist. Think of the various planes of existence in Michael Moorcock's Multiverse. A theory, not yet fully accepted, known as Quantum Loop Gravity postulates a multiverse of space time bubbles, a sort of foam of universes like the sea foam said to have given birth to Aphrodite. Speculate: each world may be such a bubble. What is in the spaces between bubbles, if there are any, might be like the dwellers between worlds in the tales of H.P.Lovecraft.
It cannot be proven, but some spirits may have existed before humanity existed, though the nature of time in this world and the spirit world may make that statement meaningless: many taken for a night with he fairies returned to find they had vanished for years or decades. Time in the spirit world may flow along a dimension perpendicular to ours, and many would say that space and time are illusions. It may be however that time and space may be necessary for consciousness to exist. Or consciousness creates them.
Some apparent spirits seem to be produced by unconscious mind: figures appear in dreams and give information unknown and in some cases unknowable to the conscious mind. Such 'spirits' are something like mental complexes. They exist in the mind but often seem to act independently, beyond the control of the conscious mind, like the tune you hate but cannot help humming. An example might be a dream of a ghost, representing suppressed aspects of the personality, or of a house with hidden rooms that may indicate unused talents or a need to explore new parts of the personality.
Jungian Archetypes supposedly live in the so called collective unconscious. This region contains beings ( there is no other word for them) independent of any one human who might cease to exist if humanity ceased to exist. Again they are probably beyond the control of the conscious mind. This is a vast and contentious field and the relationship between say the Trickster archetype and certain gods and spirits such as Loki, Ghede or Eshu is wide open for research.
Finally we come to created spirits. It is generally accepted by those with unbiased eyes that Humanity created God in their own image (if the reverse is true I would like a word with the “creator” about low back pain, short sight and other design faults).
There is also evidence that humans can create what magicians call thought forms which can act independently of their creator. The most famous case is that of Alexandra David Neel, who while in Tibet created a thought form that later began to change and needed a great deal of effort to be “reabsorbed”. Her thought form was consciously created but others may be unconsciously created and maintained. The Toronto spiritualist group that created a fictitious character, complete with a biography and managed to contact him found the “spirit” was volunteering details not in the biography they had created, but no evidence they had accidentally picked up the life of a real character previously lost to history.
It is only a step from this to the poltergeist. Perhaps the most dangerous type of created spirit is the poltergeist created unconsciously. Poltergeists that are linked to a place and seen by successive occupants may have been created in this way, though mobile poltergeists who follow an individual, may be creations of that person's unconscious .
Similarly some anomalous animals ( and alien beings) that are seen but leave little or no trace may be a creation of the collective efforts of the unconscious minds of a local or national community. Perhaps we can put the posthumous sightings of Hitler, Michael Jackson and Elvis Presley down to this effect. We can perhaps also attribute those tricksters the Men in Black to this as well.
To sum up an “invisible world” is logically possible, and that there is evidence from a wide range of sources that such worlds exists and are populated, and some evidence suggests that the human mind can create such beings in this world. As usual much more research is needed.