On 01 Jun 2017, at 03:59, Bruce Kellett wrote:
On 1/06/2017 6:28 am, David Nyman wrote:
On 31 May 2017 at 04:55, Bruce Kellett <bhkell...@optusnet.com.au>
wrote:
On 31/05/2017 12:30 pm, Pierz wrote:
Thanks for these clarifications Bruce. I find your explanations to
be very lucid and helpful - they also confirm my own understanding.
Thank you for the kind comments, I try my best to be clear.
IIRC, you weren't a particular fan of MWI when I last conversed
with you on this list.
That is indeed the case. I have several reasons to be dubious about
MWI. Firstly, it amounts to reifying a complex valued function that
resides in configuration space -- I am not sure that this is a well-
defined procedure. Secondly, MWI doesn't really do what it claims
to do, which is to provide a resolution of the measurement problem
in QM. MWI doesn't provide any explanation of the transition from a
pure state to the mixed state that is required for experiments to
give definite results. At the crucial point, MWI simply says "then
a miracle happens!" To be more explicit, deterministic evolution of
the wave function by the Schrödinger equation gives a full account
of decoherence, and the dissemination of the coherence phases into
the environment. This reduces the off-diagonal elements of the
density matrix so that the diagonal elements become *almost*
orthogonal, but unitary evolution can't go the whole way. The only
way one can reduce the pure state to a mixture is to trace over the
environmental degrees of freedom, which is to say that the residual
phase information is simply thrown away. This trace operation is
non-unitary, and there is no warrant for it in the SE itself, so it
is, in the final analysis, just an appeal to magic.
Bruce, is it perhaps finally an appeal to observation per se? IOW
what you say is of course objectively the case, but perhaps in some
way the residual phase information isn't relevant (i.e. can be
'traced over') in the synthesis of the 'observer-observation'
relation. If so, this relation could perhaps resolve the
'eternalist' pure state (which after all continues simply to be
'there' in the MWI conception) subjectively into what would give
the 'instrumental' appearance, in effect, of the probability
density characteristic of a mixture?
It is often argued that the density matrix is diagonalized FAPP, and
FAPP is all that is required. Unfortunately, I don't think that that
really works. Even though the individual off-diagonal elements might
be arbitrarily small, there are a very large number of them
(increasing as further environmental degrees of freedom are
included). The end result is that the original superposition is
still intact and no 'split' has in fact occurred -- the situation is
still completely reversible. The problem I point to is that there is
nothing in the unitary mathematics corresponding to "ignoring
inessential degrees of freedom". That is what the partial trace
does, but that has to be imposed by hand, it is not automatic, and
suffers from all the old difficulties of the Heisenberg cut -- it is
arbitrary.
Then there wouldn't seem to me to be a necessary idea of 'collapse'
involved, except in a purely epistemological sense. The term
'subjective' here is more in the sense of some fundamental
compositional principle of observer-hood than a restriction to out-
and-out 'waking consciousness'. I'm not sure if this has any
necessary connection to Lockwood's idea of a 'consciousness
basis'. Of course quite clearly any of this would be highly
speculative, but I'm wondering if it could make any sense in
principle? I'd value your opinion.
I think that something like this was what Everett had in mind when
called this idea the "relative state model". I think he saw the
observer as complete in every branch, so that
|psi> = (|1> + \2>)|me>|environment> --> (|1>|me_1> + |2>|
me_2>)|environment>
--> |1>|me_1>|environment_1> + |2>|me_2>|environment_2>
and he then considered the two parts of the development to be
complete in themselves, so we found ourselves in one or the other
branch. Everett had no particular commitment to the existence of the
other branches --
Well Everett denied this, and his son, and daughter confirmed this, as
I have read in a short biography. Everett told that he had been asked
to withdraw any tlak on parallel world, and the term relative states
was proposed by the editor of the journal (Review of Modern Physics).
This seems also clear from his long text.
it was DeWitt who developed the idea of many worlds. The trouble
here, as is well known, is that the above is still a pure state, and
we require a reduction to a mixed state in order to be able to
consider one or other branch on its own. The 'collapse' can be
regarded as epistemological, but we will still need the mixed state.
OK with this.
Thirdly, the non-observed branches in MWI play no essential role in
the theory, so Occam would say that they are inessential entities
that should be discarded. If one is simply going to discard them,
and they play no observable role, why invoke these other branches
in the first place?
I wonder if you'd care to comment on my original argument on this
thread - which has of course now been swamped by the usual brawls.
Does not a single history + the physical insignificance of the
notion of a current moment mean that there is also only a single
possible future?
We will only experience a single future, but what that future is,
is indeterminate at the present instant.
And if the future is predetermined in this way, isn't this a
serious issue for single universe models of QM? How can the
outcome of quantum events be both inevitable and random?
I don't see that there is only a single possible future. The block
universe notion only requires indeterminate time ordering for
spacelike separated events. The future along and inside one's
future light cone is in the future for all observers, so need not
be determined by some other observer having already seen what
happens. The block universe only constrains the future only in very
limited sense -- it is only for spacelike separations that
simultaneity is ambiguous, timelike separations are not so
constrained.
Quantum non-locality is another matter, however, and there are
growing indications that quantum entanglement and the associated
non-locality might prove to be of fundamental significance for
physics -- such as the possibility that space-time itself might
emerge from quantum entanglements.
Yes, I'm reading Wallace's Emergent Multiverse at the moment on
this topic, amongst others. Here's some rank speculation in the
MWI vein from the observational perspective: Let's say that
entanglement does indeed turn out to be very fundamental in the
genesis of what we call 'worlds' in the first place. Mightn't it
then be the case that observers - who themselves by assumption
supervene on a very narrowly-constrained physics - thereby can't
help but find themselves observing the equally tightly-constrained
consequences of just such entanglement?
I am not sure I follow where you are going here. Entanglement is
ubiquitous in QM, but not all entanglement is monogamous in the
sense that the EPR singlet particles are maximally entangled. Of
course, if space-time is basically a manifestation of entanglement,
then observers supervening on physical space-time will be part of
this entanglement
IOW, trivially, it's a necessary condition of their own existence.
So consequently there could be the appearance of non-locality
without there being the requirement for actual simultaneity of
action across real physical separations (all the more so if this is
conceived as preceding the very emergence of space-time itself).
Showing that space-time is emergent from entanglements of some kind
might very well explain of the origin of non-locality -- basically
because, in the presence of universal conservation laws, this
entanglement is always non-local in character. But this is all still
very much in the speculative phase of development.
Entanglement is non local, but without any physical influence at a
distance, in the MWI picture.
Bruno
Bruce
I know this argument has an unavoidable flavour of circularity, but
again my question is about principle, not proof. I guess it's also
an argument for a species of super-determinism, but with all the
counterfactuals still permanently in play, as it were.
David
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