Some thoughts on the MWI for your comments. I am sending this to a few
mailing lists with overlapping memberships, so you may have received this
twice or more. I apologise if this is the case and also for the very
imprecise language and gross simplifications and analogies that I am using
to make my point. This is really a sketch of a sketch.
I think that, while Everett's Relative State formulation of quantum
mechanics makes a lot of sense, its popular interpretation as "Many Worlds"
(MWI) should be taken only as a pictorial device useful for a first
understanding of the theory.
I propose thinking of perceived realities as shadows of a more complex
reality. I suspect this is what some authors, perhaps including Everett
himself, are trying to say, and that others have said it explicitly, so I
would appreciate any pointer to relevant works.
I will use poor Schroedinger's cat as an example. Following Everett, the cat
is in a superposition of [dead cat] and [alive cat] states before the box
has been opened, and stays so after. Once opened the box an observer is in a
superposition of [observer who remembers having seen a dead cat] and
[observer who remembers having seen an alive cat]. The MWI says that the
universe is now split in two branches where the first has [dead cat] and
[observer who remembers having seen a dead cat], and the second has [alive
cat] and [observer who remembers having seen an alive cat].
The difficulty that I have is: on the one hand we are saying that there is
no such a thing as a dead cat or an alive cat, but on the other hand we are
describing the world(s) with dead cats and alive cats.
To clarify the first part of the statement: as we can choose any two
directions to form a basis to use in describing a particle's spin, all
choices generating equally valid descriptions, besides [dead cat] and [alive
cat] we should be free to use another basis to describe the cat. Any pair of
independent superpositions of [dead cat] and [alive cat] will do, of course
I have no idea of what one would "look like".
Since I cannot remember having ever seen one, I do not know what a
superposition of [dead cat] and [alive cat] would look like, so probably I
would not recognise one if I saw it. Perhaps this is the reason why I cannot
remember having ever seen one.
In other words, perhaps since reality is One Big World too complex for our
minds to process efficiently, we use a simplified representation as Many
(small) Worlds for our processing. This is not so surprising since our best
computer programs use data compression and segmentation techniques, throwing
most of the information away, to perform complex tasks such as face
recognition efficiently.
I believe thinking of shadows may be a better mental device than thinking of
parallel worlds. Using this model the realities that I, and my doubles in
the MWI model, perceive can be thought of as shadows of a more complex
reality. Observing a shadow permits saying certain things about is source,
but not other things like what the source is saying. The shadow does not
contain such information. Also, much of what we can say about shadows has
more to do with illumination and the surface where the shadow is cast than
with the actual source.
Thinking of multiple worlds as shadows brings us back to Plato's cave, but
there are two important differences: First, each of us observes shadows of
the outside world in a very large number of caves in parallel. Second, we
are shadows ourselves, our conscious computational processes being shadows
of other possibly much more complex computational processes.

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