Federico:
> The paradox consists of the fact that the theory of multiverses tells us
> that there must be infinite observers who experiment other physical laws.
> There is not only the possibility of being wrong, it is the model itself
> which proves to be wrong. In fact it tells us that there are infinite
> places and times in this multiverse where, if any people observe the world
> around them in the same way we are doing hic et nunc, they necessarly find
> another model to describe the universe. So the outcome of the model is
> that it must be wrong in infinite places and times, and the paradox is
> that we have proved that it is wrong, but we have been able to draw this
> conclusion because we have considered the hypothesis of applying the
> physical system itself. But if it was wrong, the conclusions would be
> wrong, too.

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Ciao Federico,
There is a debate about the consistency conditions that must
be satisfied by (density matrices which represent) the knowledge
that different people have about the ***same*** physical system.
These knowledges (and density matrices) are, in general,
different.
So we must always ask (with John Bell) "whose knowledge?". And then
we must impose (Rudolf Peirls) the condition that density matrices
must have a non-zero product (they must have at least one state in
common).
And that was the first level. Or, if you prefer, the first-order
paradox.
Now the question seems (to me) to be this one. What about the density
matrix of the people A in the ***world*** A, representing some knowledge
about the ***world*** B? What about the density matrix of the people B in
the ***world*** B, representing some knowledge about the ***world*** A?
The answer seems to be: more questions. Do these density matrices
commute? I suppose: no. Do these density matrices share at least
one state? I think the answer is, in general: no.
And that was the second-order paradox.
But the above was the case of people living in different ***worlds***
***but*** believing in the same QM.
Now you can imagine what is the problem when they do not even
believe in the same QM (or QM interpretation!).
And that, perhaps, is the third-order paradox.
But maybe thay are not paradoxes. Informations are always
subjective, more or less.