On 8/06/2017 9:06 pm, David Nyman wrote:
On 8 Jun 2017 11:40 a.m., "Bruce Kellett" <bhkell...@optusnet.com.au
<mailto:bhkell...@optusnet.com.au>> wrote:
On 8/06/2017 7:52 pm, David Nyman wrote:
On 8 Jun 2017 1:05 a.m., "Bruce Kellett"
<bhkell...@optusnet.com.au <mailto:bhkell...@optusnet.com.au>> wrote:
The question then, is whether many worlds can provide a fully
local account of this situation. I claim, with most present
day physicists, that MWI does not provide any such local account.
I suspect I'm being obtuse in some way here but, rereading the
quote attributed to Bell himself by Wikipedia about
superdeterminism, it strikes me that MWI seems to describe a
species of this sort of thing. IOW when Alice and Bob make their
measurements, the consequence in terms of branches is a spectrum
of all the possible outcomes. Indeed one could say that this is
what has been propagating from one to the other, rather than a
'particle'. Let's say then that the various versions of Alice and
Bob that consequently coexist in MWI terms, however far apart
they may have been, eventually meet to compare notes. Again, the
spectrum of possible outcomes implicit in the global MWI
perspective travels with them, as it were. However, of all the
possible pairings of the two, it appears to be 'superdetermined'
that each observed encounter must be consistent with the
predictions of QM. And so it would appear that the paired results
of their joint measurements are somehow inseparable, in Wallace's
language, without there having been any action at a distance. If
this depiction were to make any sense, one might then enquire
what common cause, or other explanatory device, could account for
this apparent superdetermination of observed outcomes?
I don't think that superdeterminism and MWI have very much in
common. Although Bell did acknowledge that superdeterminism
provides a possible local loophole to his theorem, Bell always
thought that superdeterminism was sufficiently implausible to be
disregarded as a serious contender as an explanation.
I tend to agree with the comment from Zeilinger on the same Wiki
page, to the effect that such absolute superdeterminism would
render the whole scientific enterprise otiose. I think that
non-locality is a better approach -- at least then science can
still make sense.
The problem with attempts to find local accounts of the
correlations between Alice and Bob is that their measurements are
taken to be independent. If they are independent, then they cannot
be correlated -- that is in the definition of independence.
Superdeterminism circumvents this, simply by denying that Alice
and Bob can freely choose their measurements, and are consequently
not independent.
As I understand the better attempts to give an account in MWI, it
is accepted that Alice and Bob are independent, so their results
are uncorrelated *when they are made*, but the necessary
correlation is built later when they meet to compare results. I
find this unconvincing, and no satisfactory account of any
mechanism whereby this could be achieved has been given. Accounts
along this line seem to depend on multiple worlds containing all
possible results that somehow, miraculously, pair up, without any
outside intervention, in such a way to give the necessary
correlations. This is rendered less plausible if one considers
timelike separations, where Bob, say, is always in Alice's forward
light cone, so any splitting of either observer is communicated to
the other by normal decoherence, long before the other measurement
is made, and before they meet up to compare lab books.
Yes, this is also the point where I stumble. I've been trying somewhat
inarticulately to characterise a possibly non-miraculous approach from
a slightly different perspective. Suppose we think about the matter
from the point of view of Hoyle's pigeonholes. Perhaps there are
pigeonholes that in some sense correspond to observations that are
'malformed' with respect to the predictions of QM. Now, we are
presumably to suppose that the entanglement which leads to well-formed
predictions embodies a very fundamental aspect of physical reality and
consequently also the possibility of meaningful observation. Hence any
such malformed 'observations' should by the same assumption be
considered of very low measure, in the sense of any possible
contribution to Hoyle's conceptualised sum of well-formed observation.
I suppose what I'm suggesting is that something fundamental and highly
constraining about the demands of observation of a consistent physical
environment itself effectively filters out what is possible but
incompatible with those demands. Is this irretrievably circular?
I don't think it is so much circular as conspiratorial. If physical
results were to come about in such a conspiratorial way, rather than
straightforwardly from the formalism as in quantum non-locality, one
might wonder what the scientific enterprise is really all about. (Rather
as Zeilinger wondered about superdeterminism.)
Bruce
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