Taking the basic physics at face value, all possible worlds exist. This is the basis of Hugh Everett’s many worlds theory, as explained in the Many Worlds page. And this aspect of the theory is now generally accepted in one form or another. In this case, as Tegmark (2003) explains in Parallel Universes, there is a vast number of identical copies of each individual. In the universe of all possible worlds, every possible variation of a world that contains you, exactly as you are here and now, must actually exist. So there is a huge number of identical copies of you.
There is no question all these copies are the same person. As Deutsch reflects, given that there all these identical copies of oneself:
… which one am I? I am, of course, all of them. Each of them has just asked that question, ‘which one am I?’, and any true way of answering that question must give each one of them the same answer. (1997, 279)
The key point here is that they all have the identically the same record of observations, otherwise they would not be identically the same person. In other words, that which the individual has observed is identically the same in all of them. Therefore, with respect to the aspects of the world that have been observed, all these worlds are identically the same.
But the rest of the world is just the opposite. With respect to the aspects of the world that have not been observed, all these worlds are different. This is the set of every possible world that contains a copy of this individual. And this means that every possible variation of a world that contains this individual is included. All standard physics so far.
The Indeterminate World
One of the most difficult things to grasp in quantum theory is that all the many worlds are here and now. As stated by Lev Vaidman:
… in addition to the world we are aware of directly, there are many other similar worlds which exist in parallel at the same space and time. (2008)
They all exist in exist in superposition. This follows automatically in the many worlds theory as explained on the Many Worlds page. The implication for the reality of the individual person is crucial.
If you superimpose identical copies of something, you just get one of those things. When all the identical copies of you are superimposed, there is just one of you. And whereas before we were thinking about identical copies in similar worlds, now there is just you; and you are simultaneously present in all those worlds. And this means that your physical world is the superposed sum of all those versions of the world. It is this that gives rise to all the strange properties of the QBism world. In the superposition of worlds, everything you have experienced and observed is identically the same in all these worlds. So that is just how things are. All those things are determinate. But exactly the opposite is the case with respect to the things you have not observed. Every possible variation of those things, elsewhere in the world, is included in the superimposed sum. This means that all those things are indeterminate in this world. That is how quantum mechanics works.
This logic does not apply to the body of the observer, the physical entity. Each observer is a physical part of a specific physical world due to decoherence. However, the individual here is the entity on the inside view, the world hologram. And with respect to this individual, this logic applies absolutely.
So in the physical reality of the individual, only the things which have been observed are determinate. And, the record of observations is therefore the record of everything that is determinate. And everything else is indeterminate. That is exactly the reality of the QBism world. Once again, this is not a new idea. This is a ‘centred world’ as defined by Vaidman, one:
… centered on a perceptual state of a sentient being … In this world, all objects which the sentient being perceives have definite states, but objects that are not under her observation might be in a superposition of different (classical) states. (2008)
QBism solves all the problems with quantum theory. Now it also has an ontology in Everett’s many worlds theory, the Relative State’ Formulation of Quantum mechanics. It seems it must be correct. But the ordinary world view is correct as well. In the ordinary world of objective physical reality, the world is a single, determinate physical domain. Both are correct. The missing piece of the puzzle is that quantum theory defines two different types of frame of reference. The difference is further explained and illustrated in the page on Schrödinger’s Cat.
The objective-view ordinary world is real and valid, but this per se is not what we encounter. One encounters the superposition of ordinary worlds, and this is determinate only where observed. And of course, when we go to do experiments on reality, this is what is revealed. This is what has given rise to all the confusion about quantum theory. The equations tell us of a real objective physical universe, and the phenomenon of decoherence means this is laid out as the many worlds of Everett’s theory. But the experiments tell us that it is indeterminate except where observed. The new worldview dissolves this paradox. Both are completely correct in their own way. And both are wrong in the frame of reference of the other.
What has been missing is the fundamental nature of the superposition world. It is a different kind of frame of reference to an ordinary world. It is of different ‘logical type’. Logical type, discovered by Bertrand Russell, addresses the difference between a set and the members or elements of that set. They are utterly different kinds of things, and thus their properties may be utterly different. The physical world of the inside view is the superposition of a set of physical worlds of the outside view, which is why it has the extraordinary properties described in QBism. So the conventional idea of the objective reality of an ordinary world is right, but it is only one half of the story. In other words, it is one of two different, fundamental frames of reference. Both are equally real, valid and ‘ontological’, definitions of what actually exists. The truly major upheaval in physics is that objective physical reality is only one of three levels of logical type, all three of which have to be addressed in order to make sense of the new physics.
footnote: The explanation given above is simplified for clarity. The person is the entity defined by the record of observations, as described in Identity.