From: scerir (scerir@libero.it)
Date: Thu Feb 13 2003 - 14:41:15 MST
[s.]
> > We can also put it in different terms. MWI supposes there is no
> > communication between different worlds. Hence, imo, it supposes
> > there is no 'interference' between different components of the
> > wave function (at least after the splitting occurs). But having
> > no 'interference' means a transition from an original 'pure' state
> > to a 'mixture'. And we know, from a Von Neumann theorem, that in
> > the change from a 'pure' state to a 'mixture' the entropy increases.
> > (On the contrary the evolution described by the Schroedinger equation
> > is unitary and the entropy is invariant).
[Hal]
> Perhaps we mean different things by MWI? In my view the MWI is simply
> the position that Schroedinger evolution is the only thing that happens,
> there is no state function reduction.
Yes, maybe there are many different MWI.
I.e. there is one MWI assuming that quantum objects, detectors,
observers, etc., are simply *split* between the various branches.
This also means that there is only one particle shot through a
Young two-slit interferometer. And, extending to the macroscopic
domain, there is only one detector, and only one observer,
somehow *split* between different worlds. It is really difficult,
for me, to understand what does it mean when we say that an observer
(macroscopic) is *split*. (Yes I know there are macroscopic effects
with clockwise and - at same time - anticlockwise currents of electrons,
and this is a superposition indeed, but in the same world).
There is another MWI assuming that particles, observers, and instruments,
are *multiplied*. That is, this MWI states that each of the branches
contains a particle, an observer (whose consciousness is in a well
defined state), a detector (whose pointer is in a well defined state).
I can understand this one. But this MWI implies the entropy increase,
and irreversibility. This was pointed out by Peres and, as far as
I remember, was shown by Vaidman, in a gedanken experiment.
[There is also a mixture of the two above MWI theories!]
'On Schizophrenic Experiences of the Neutron or Why We Should
Believe in the Many-Worlds Interpretation of Quantum Theory'
- Lev Vaidman
http://arxiv.org/abs/quant-ph/9609006
This is a philosophical paper in favor of the Many-Worlds Interpretation
(MWI) of quantum theory. The concept of the ``measure of existence of a
world'' is introduced and some difficulties with the issue of probability
in the framework of the MWI are resolved. Brief comparative analyses of
the Bohm theory and the Many-Minds Interpretation are given.
"However, the two branches [...] correspond to different worlds,
between which there can be no communication, once an irreversible
process has occurred. This approach has several variations which
are called the 'relative state interpretation' and the 'many worlds
interpretation'. None is satisfactory because they merely replace
the arbitrariness of the collapse postulate by that of the
no-communication hypothesis."
- Asher Peres, 'Quantum Theory: Concepts and Methods',
Kluwer Ac. Press, 1998, p. 374.
> Tegmark argues that universes join as well as branch. We may have
> multiple pasts as well as multiple futures.
Yes, this seems similar to Feynman paths. But Tegmark here introduces
decoherence. And, again, I cannot understand how to explain decoherence
and reversibility at the same time.
Decoherence means that correlations between the quantum system and
the instrument (and/or observer) cannot be destroyed, but they are
downloaded into the environment. This also means that, in general, the
entropy of the quantum system, plus the entropy of the apparatus, plus the
entropy of the environment, remains [surprise] *constant*. But it also
means that, due to the extension of the environment, it is impossible to
reconstruct the *initial state*, because one should reconstruct the state
of the entire universe!
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