Whether or not the great Welsh poet and philosophical novelist John Cowper Powys invented "the Multiverse" (somewhere around 1920), isn't particularly important, since Powys's romantic mind had a much more elevated discursive and philosophical use for the concept.

Powys had rejected the idea of God presiding over the universe as a tyrannical system, and adopted the idea of a “multiverse of free personalities" (and not only human ones!) all with their own power of creation and destruction. (Splitting & branching!). For Powys saw consciousness present throughout the natural world and the “Freedom of the Spirit” as the wholesome element of anarchy in our western world". ( see also “The end of Certainty”, “The Arrow of Time”, “ The Role of Chaos in Physics -- Ilya Prigogine)

Michael Moorcock (1*) and Hugh Everett III (2*) and later David Deutsch ( and probably a big number of others that I do not know) proposed each in their own fashion, a radical new way of dealing with some of the more perplexing aspects of quantum mechanics. It became known as the multiverse (1*) or Many-Worlds Interpretation (2*) and I will refer to it as the Quantum Reality System of Q.R.S.

Later, when Mandelbrot and Chaos mathematics came along, it was wonderful. It was like being given a set of maps for ones own brain! And getting lost! And enjoying getting lost!

Anyone who considers themselves even modestly educated nowadays should also be familiar with Gleick's Chaos, a more ‘popular’ book on the subject.

What is the essence of many worlds/multiverse/Q.R.S.?

Whenever numerous viable possibilities exist, the world splits into many worlds, one world for each different possibility. (In this context, the term "worlds" refers to what most people call "universes" and what I refer to as "quantum-realities").

In each of these multiple quantum-realities I will appear in a slightly different “person/character. In each of these quantum-realities, everything is would be identical, except for that one different choice; from that point on, the different quantum-realities develop independently, with no connection between them.

So the people living in those worlds (and splitting along with them) may have no idea that this is going on.

This uncertainty started long before Heisenberg, in 1881, when a physicist named Thomas Young, shone a light through a board, questioning whether light was made of waves or particles. Only 2 possibilities, but the basis of infinite potentiality within the Q.R.S.

Remember the “exploding” die, the “propensiton die” and my colleague winning the lotto “somewhere!”, in another quantum-reality?

So somewhere I have an infinite (minus one) number of colleagues, and one of them has won the lotto. My colleague’s new problem remains as complex as before: finding or defining the quantum-reality where the things have happened according to his expectations. Theoretically this might prove totally impossible…but for his sake, I hope it isn’t.

Because quantum-reality splits and –re-splits endlessly, infinity is created, at least an infinite number of possibilities. What is "the present" to us, lies in the pasts of a huge number of different futures (to be defined as "infinite minus 1"), because everything that can happen, does happen somewhere…except for my colleague, maybe here!

Time is like this ever & permanently branching tree growing into countless possible futures! Each decision, each observation affects the future! So there must be an infinite number of futures. But similarly there must be an infinite number of pasts. The question remains: ”If we change one future, do we retro-actively change the one past that was related to that future? (Henri Bergson)

In the linear river-of-time concept the future is immutable. If, on the way to work in the morning, we decide to take the bus instead of the car and are killed in a bus accident, then that death was predestined (karma).

‘now’

we weave this timeless magic
spiralling up, soaring on cycles of past ,
on stories ever-evolving
re-called, reconstructed within
without repetition ,
here everything is ever-new
and ever-old as well.

shaking the gold-dust from our winged eyes
we see the future opening ‘now’
and again ‘now’ , and now again .

weird is what we work
when time flows upstream
where waves wet our feet :
the undertow grows
and this is forever ‘now’ .
(Bert Lemmes : Poetry 2004)
.
But if time is splitting permanently according to different options into different quantum-realities, then there are at least two futures - one in which we die in the accident and another in which we live on, having taken the car instead of the bus.

It therefore follows consistently, that if there is an infinite set of futures - there must be an infinite set of pasts as well
Until the Q.R.S. appeared, the generally accepted interpretation of quantum mechanics was, and probably still is, the Copenhagen Interpretation that makes a distinction between the observer and the observed (the "noumenon").

When no one is watching, a system evolves deterministically according to a wave equation, but when someone is watching, the wave-function of the system "collapses" to the observed state, which is why the act of observing changes the system and ‘creates’ a separate quantum-reality.

The Copenhagen Interpretation gives the observer a very special status and a lot of responsibility, not allocated to any other object in quantum theory, but cannot explain the observer itself, while the quantum-realities model the entire “observer-observed” or subject-object-system.

The “Quantum Reality System” is an interpretation of quantum mechanics, and pertains to quantum events and these events in their related quantum-realities.

But it also has implications for macro-systems like you and me. Although you may think that there are certain alternatives you would never choose, can you really be sure of that? There are a practically infinite number of versions of you, who have all split off at some time in the past from the path you are now following. There may be versions of you that split off five or ten years ago, or perhaps five minutes after you were born. Their choices may not seem unthinkable. But in a very real sense, those people are still "you", (as Von Bek, the Eternal Champion, Corum, Count Brass, Earl Aubec are different realizations from one person in different Q.R.S. handled by Moorcock.

But it can be argued that we should not use the word "are", or even "were". We need to invent a new kind of tense, maybe a new verb.

Many people find the Q.R.S., and it’s consequences deeply disturbing. This number includes a great many physicists. But it shows only that many physicists, including many who teach physics, do not have a good understanding of the Q.R.S.

Polls taken among theorists have revealed that most of them believe that the Q.R.S. represents, in some sense, an accurate description of the way the world really is. The polls also show that many of them would rather not discuss the subject for fear of contradicting themselves.

They seem to forget that somewhere their quantum-alter-ego is taking a stance and saying the opposite.

It's not hard to see why so many people find these ideas disturbing. For if they are correct, they have profound implications for our understanding of the nature of the “soul”, because the soul (if there is such a thing) must branch along with the worlds that contain it. It would appear that the writings on which many contemporary religions are based make no mention of such an idea.

It is commonly thought that the Q.R.S. vision is an improvable hypothesis, but this may not be the case.
It may be possible to observe experimentally one of the predicted effects of Q.R.S.: quantum interference between adjacent worlds.

It has even been suggested that the Heisenberg Uncertainty Principle derives from this quantum interference; after you make a measurement (which is an event that splits quantum-reality, creates another branch on the tree), you cannot be sure about the subsequent state of the observed system, because you cannot be sure which world or quantum-reality you are in.

This brief description is not very rigorous, in a technical sense, and certainly not in a mathematical-physical sense, and it is intended for the lay reader, because written by a lay writer, in casu, yours truly. Others, far more qualified than I, have written much more technically on the subject.

As far as I remember, Wittgenstein says: 'The world is everything that is the case.' 'The world is the totality of facts, not of things.'

Now if you start from that premise, you are bound to welcome any theory representative of the 'case.'

The positivists have admitted that quantum mechanics describes atomic phenomena correctly, and so they have no cause for complaint.
The different “laws” we have had to add ( complementarity, interference of probabilities, uncertainty relations, separation of subject and object, etc.) strike them as embellishments, mere relapses into pre-scientific thought, not to be taken seriously.

If we consider this attitude logically defensible, one can no longer tell what we mean when we say “we have understood nature."
(Harry Harrison, The Entropy Exhibition).

In a very specific sense, it was Moorcock's fiction that got the ball rolling for me, created my important “split”, because all of a sudden my earlier studies of the mathematician-turned-philosopher, Henri Bergson, became “clear”, where before they were only “enticing” and “fascinating”, in his ‘poetic-mathematical’ vision.

Some of the theories that have been suggested in the realm of Quantum Mechanics, and particularly Chaos Theory are the “substrate” of the multiverse, and we “are” an infinite number of slightly different versions of the same person within a Q.R.S./ multiverse, where a multitude of alternate universes are intersecting sometimes with our own.

It should probably be mentioned explicitly that it will be necessary to do away with much Newtonian physics, with the concept of linear time, and with the idea of direct causality (if you haven't already!) if we are ever to visualize the Q.R.S. successfully and deal in non-linear terms with infinite versions of perception.

In fact, the Western classical concept of “reality” needs to be modified, if not deleted