Hugh Everett’s famous many-worlds theory is called The ‘Relative State’ Formulation of Quantum Mechanics (1957). He provides a solution to the peculiar collapse of the wave function which is deceptively simple. The wave function defines all possibilities, and there is no collapse; but there is the appearance of collapse.
In this case, if there is no collapse, then all possible versions of events get played out. And this means that all possible versions of the world come into being. This is where the Many Worlds concept originally came from in physics.
The picture, gratis Max Tegmark, illustrates this idea with Bob asking Alice out. She may say yes or no. If there is no collapse, then both possibilities happen. She says yes in one version of reality and no in another version. Effectively therefore, the world splits in two, and both versions exist.
An excellent article, that includes a description of this progression, was written by John Wheeler and Max Tegmark 100 Years of Quantum Mysteries.
The problems arise in the details. It has not been possible to explain exactly what is happening here. Everett essentially states that only in experience are there separate realities. Since experience cannot be relevant in physics it is assumed that he was referring to the experience of some physical process. But in that light it has not been possible to make sense of this idea, or to solve the great paradox of quantum theory, the measurement problem. The resolution in the light of logical type is given in Measurement Problem.
In order to see exactly what he did mean we need to go into a bit of detail about the theory. His concept centres on the technical process of making an observation. We are constantly observing all the things in reality we can see and hear. The human body uses the sensory apparatus, the eyes, ears and so on, to detect the sights and sounds of the world. And the neural system of the brain turns all this information into a visual image. Right now you are experiencing a visual image, a view of reality, specifically some words in black on a white background. That is simply the information, in the form of an image, that you are actually experiencing ‘in here’. The brain produces the visual representation ‘in here’ of the reality ‘out there’. This, of course, is simply how we formulate the world hologram, and thus get to experience the real world.
The brain records this image in memory. This is the process that Everett defines as the making of an observation: an observation is made when this sensory experience is recorded in memory.
The final, key component of his theory is the ‘state of the memory’. This is defined as the record of all of the observations that have been made. So the state of the memory is the sum total of all the sensory information that gets experienced and recorded in memory. This is the total record of one’s sensory experience. This is the world hologram described in The Perceiving Subject.
Now for the bit that gives rise to all the difficulty. This is what he actually says in his conclusion. When the observer interacts with the environment, and an observation is made, the result:
… is a superposition of states, each element of which assigns a different state to the memory of the observer. (1957, 462)
In other words, the physical reality itself does not split as shown in the picture above. Instead, the physical reality is literally the superimposed sum of both situations at once, as in this picture.
That is the superposition. Nonetheless, the state of the memory – the definition of the experience of reality – is different in each element of the superposition. Thus in the experience of reality, in each element of the superposition, there is just one specific outcome, and just one version of reality, as shown in each branch of the first picture. In other words, physical reality works like picture 2, but experience works like picture 1.
This is Everett’s solution. Although in physical reality, both of the different possible outcomes are superimposed – so there is no collapse of the wave function – in each element of the superposition, it appears that the possibilities have collapsed to a single physical actuality. The problem is this seems to make no sense. How can the experience of the physical world be different to the physical world? He is saying that it is in experience, and only in experience, that different, discrete versions of the physical world exist. In the current scientific worldview this sounds like complete nonsense. So the obvious assumption is that is not quite what he actually meant.
There is a great deal of literature devoted to trying to establish exactly what he did mean. The Quantum Mechanics of Minds and Worlds by Jeffrey Barrett (1999) gives an excellent overview of the decades of thinking.
There is only one way of taking all this that works. It gives the same result as QBism. The state of the memory is a frame of reference, defined as the record of observations. From the point of view of this frame of reference, one specific version of events is experienced, observed, recorded and becomes part of the definition of the determinacy of the world of this individual. Since as Everett points out, the physical body is in a state of superposition of all such states being instantiated in the memory, this individual cannot be the same thing as the physical observer. It is simply and solely the structure of information defining the world hologram.
This seems crazy in the current paradigm, but again, this is what is missing from the current paradigm. We think we are bodies which have minds in them, whatever they are, and memories, and it seems consciousness. What we actually experience, as described in The Perceiving Subject, is the reality of the world hologram. We take it, completely naturally, that the body is the big-deal identity here, instantiating the mind and the memory and, one tends to assume, the consciousness. And in terms of physical reality all that is true, except for the last bit. But inside the reality of the world hologram, experienced by consciousness, which is not produced by the body at all, the world is as defined by the record of observations, the world hologram experienced.
This, of course, is not a physical phenomenon. It is an epiphenomenon of the physical, produced by experience. Nonetheless, it is fundamental and primary in its own frame of reference. The four-dimensional, space-time, matter-and-energy movie is a whole other thing than the physical reality, as it happens in the static matrix of the universe. It is a second-logical-type phenomenon.
This is why Everett’s formulation has been incomprehensible. It is literally a “… formulation of quantum mechanics”. This is how physical reality actually works. In fact, however, this is one of the false, though all too natural, presuppositions which has led to all the confusion and pseudo problems Fuchs and Co. refer to. The linear dynamics of quantum mechanics is the dynamics of the physical. This is the wave function. The collapse dynamics is the dynamics of the inside view of the world hologram. Everett’s solution has been incomprehensible because it describes the appearance of collapse, which is what takes place as the frame of reference is altered by the addition of the observation to the record, in experience. On the inside view, this is the step from one frame of the holographic movie to the next. It is a second-logical-type phenomenon.
As Tegmark states, important scientific discoveries go though three phases:
… first they are completely ignored, then they are violently attacked, and finally they are brushed aside as well known. Everett’s discovery was no exception: it took more than a decade before it started getting noticed. But it was too late for Everett, who left academia disillusioned.
Everett’s no-collapse idea is not yet at stage three, but after being widely dismissed as too crazy during the 1970s and 1980s, it has gradually gained more acceptance. At an informal poll taken at a conference on the foundations of quantum theory in 1999, physicists rated the idea more highly than the alternatives, although many more physicists were still ‘undecided’. I believe the upward trend is clear.
Why the change? I think there are several reasons. Predictions of other types of parallel universes from cosmological inflation and string theory have increased tolerance for weird-sounding ideas. New experiments have demonstrated quantum weirdness in ever larger systems. Finally, the discovery of a process known as decoherence has answered crucial questions that Everett’s work had left dangling.
So the background is adapting to a form that makes Everett make more sense. But the central issue is logical type. His theory describes the inside view in terms of mathematical quantum mechanics. As shown in World Superposition, this is effected by a superposition of ordinary worlds. Thus the theory is defining the vital second-logical-type component of the system.