From: *Bruno Marchal* <marc...@ulb.ac.be <mailto:marc...@ulb.ac.be>>
On 2 Aug 2018, at 12:54, Bruce Kellett <bhkell...@optusnet.com.au
<mailto:bhkell...@optusnet.com.au>> wrote:
From: *Bruno Marchal* <marc...@ulb.ac.be <mailto:marc...@ulb.ac.be>>
On 1 Aug 2018, at 21:12, Brent Meeker <meeke...@verizon.net
<mailto:meeke...@verizon.net>> wrote:
Indeed. But the common-cause explanation doesn't work for all
choices of measurement angle.
It does. Well, it does not if you assume only one Bob and Alice, but
the whole point is that it does if you take into account all Alices
and Bobs in the multiverse. QM explains why in all branches, Alice
and Bob will see the violation of Bell’s inequality, and this
without any physical instantaneous causality on a distance. The MW
theory is NOT an hidden variable theory in the sense of EPR or Bohm.
The MW theory is based on the first person indeterminacy, and
illustrate the first person plural aspect (contagion of
duplication). Hidden variable theory in the sense of de Broglie,
Böhm, or Einstein incompleteness are pure 3p theories, not involving
the role of the person in the picture.
In that case you have a different theory, which is not quantum
mechanics. You can believe anything you like about your own private
theories, but you cannot expect others to join in. If we are talking
about quantum mechanics, then it would be polite to stick to that theory.
I am talking about Quantum Mechanics without collapse. You are the one
seeming to interpret ud + du as a superposition of worlds with Alice
having a particle in state u (and Bob having the corresponding
particle in state d) with worlds with Alice having a particle in state
d (and Bob having the corresponding particle in state u). That would
contradict the rotational symmetry of the singlet state.
The rotationally symmetric singlet is ud - du. The state you mention,
ud+ du, is the spin zero component of the triplet, which is not
rotationally symmetric.
You ask how I interpret the singlet in MWI. That is quite simple -- it
is the same as in a collapse theory. In MWI you just retain all the
branches, branches that are discarded in the single world theory. In
both cases, the ud - du state is rotationally symmetric when prepared,
but that rotational symmetry is destroyed as soon as the spin component
of one particle is measured in a particular direction. The external
magnet is not rotationally symmetric, so as soon as it interacts with
the singlet, the overall rotational symmetry is lost. That is surely
obvious. That is why I don't understand why you go on about infinities
of Alice's and Bob's who can measure in any direction continuing after
the first measurement interaction. The symmetry is lost, so there can
only ever be four worlds: the uu, ud, du, dd, worlds that I have been
mentioning all along. These are the worlds that survive from one
measured singlet pair in MWI. Each branch of this can be considered a
single world, and since the branches are disjoint, the relevant
statistics must be separately satisfied in each such branch.
It is not actually very difficult to understand once you have broken the
initial symmetry. A series of trials on such singlets will just lead to
a branching tree of 2^N copies of matched Alices and Bobs. It is the
fact that they always interact with the components of the same singlet
state in each trial that keeps the worlds in order. But the measurements
that each make are made non-locally. And the relative probabilities of
their separate results (probabilities of the 'worlds' or 'branches')
depend on the non-locally set relative orientation of their
measurements. Bell's theorem is then just the observation that the
observed correlations cannot be reproduced by a local hidden variables,
such as would represent a 'common cause' that is carried along from the
point of creation of the singlet. Bell's theorem applies to each branch
in the many-worlds superposition and cannot be deflected by appeals to
counterfactuals or any such irrelevance.
Bruce
--
You received this message because you are subscribed to the Google Groups
"Everything List" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to everything-list+unsubscr...@googlegroups.com.
To post to this group, send email to everything-list@googlegroups.com.
Visit this group at https://groups.google.com/group/everything-list.
For more options, visit https://groups.google.com/d/optout.
