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.
Bruce
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