On 9/30/2019 3:53 PM, Bruce Kellett wrote:
On Tue, Oct 1, 2019 at 2:50 AM 'Brent Meeker' via Everything List
<[email protected]
<mailto:[email protected]>> wrote:
On 9/29/2019 6:53 PM, Bruce Kellett wrote:
On Thu, Sep 26, 2019 at 10:32 AM 'Brent Meeker' via Everything
List <[email protected]
<mailto:[email protected]>> wrote:
I think the alternative is something suggested by Zurek. He
shows that decoherence plus einselection will make the
reduced density matrix strictly diagonal, i.e. he solves the
preferred basis and derivation of the Born rule. Then he
suggests, but doesn't really argue, that the universe cannot
have enough information to realize all the non-zero states on
the diagonal and so only a few can be realized and that
realization is per the Born rule. This is what Carroll would
dismiss as a "disappearing world interpretation"; but it
would provide a physical principle for why worlds disappear,
i.e. branches of lowest probability are continually pruned.
I don't think this is exactly what Zurek is arguing. He mentions
Halliwell, but is concerned more with Quantum Darwism, which is
an account of the records left in the environment by the system,
than with the effects of decoherence on the system itself -- as
would be the case if the limits on environmental information set
some probabilities to zero. He says:
"Copying yields branches of records inscribed in subsystems of E.
Initial superposition yields superposition of branches, so there
is no literal collapse. However, fragments of E can reveal only
one branch (and not their superposition). Such evidence will
suggest 'quantum jump' from superposition to a single outcome...."
So it is the fact that our access is limited to only fragments of
the the entire environment that leads to the perception of
collapse -- our inability to see the superposition, or to reverse
the measurement. If you take only a portion of the complete state
you certainly reduce the pure state to a mixture. This is not a
particularly new position, being in line with the IGUS ideas of
Gell-Mann and others.
That seems to be the same as MWI. Our access is limited because we
are in a relative state...so each copy me has limited access. Yet
he refers to "the myth of multiple worlds".
Our access is limited because we are finite beings, with limited
information capacity. Sure, you can imagine that the other "relative
states" exist in the same way that we do, but I would take the view
that since we cannot in principle access these other states, and they
can, in principle, have no effect on us and our physics, then they are
essentially non-existent. Or rather, their existence is a metaphysical
matter, not a subject of physics.
There was a time when I thought that MWI might mean more than this,
because many MWI enthusiasts claim that many worlds does away with
Bell non-locality -- giving a purely local explanation for violations
of the Bell inequalities.
Have you read Carroll's book? I was surprised that he drew two diagrams
of a Bell experiment. In one he shows the world splitting
instantaneously (non-locally) and in the other the split propagating
within the forward light-cone. He writes that it doesn't matter
which?? I have always imagined the "split" as propagating, since the
spread of decoherence must be at the speed of light or less...which I
suppose is what is meant by "local". It is only in the future overlap
of lightcones that the non-orthogonal "worlds" (subspaces) cancel out by
interference.
This, to my mind, was the last gasp of MWI realists -- and that hope
has been dashed. First, because Bruno has resolutely been unable to
give any such local account, even in MWI; and second, extensive
searches of the MWI literature have shown many claims, and also the
same number of failed analyses. The final straw came recently when I
read David Wallace's 2012 book, "The Emergent Multiverse". In Sections
8.5 and following of that book he confidently proclaims that Everett
banishes non-locality, but he totally wimps out on giving any sort of
an account, even a half-plausible one. Section 8.7 is one of the most
disappointing accounts of Aspect's experiments that I have ever read.
I haven't read Wallace.
So MWI has no practical application -- it is pure metaphysics, and can
be relegated to the dustbin of history, along with celestial spheres
and phlogiston.
I'm not so dismissive of philosophy. I think it's role is in suggesting
ways extend or improve or replace theories. In the case of Everett's
relative state, it suggested decoherence and quantum Darwinism which I
think have gone a long way to answering the questions of preferred
pointer states and the Heisenberg cut.
Brent
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