From: *Bruno Marchal* <[email protected] <mailto:[email protected]>>
On 13 Aug 2018, at 00:48, Bruce Kellett <[email protected]
<mailto:[email protected]>> wrote:
but the FTL are needed only if we associate the mind on Bob and
Alice to the same branche, which has no meaning for me once they are
space separated.
You might not accept that they can be in the same branch, but that
does not mean that it is not a proven fact.
If they are space separated, I am not sure I can make sense of being
in the same branch.
You appear to be referring to the presence of quantum fluctuations
continually splitting the classical Alice and Bob into multiple copies
-- the point that Jason has made. I think I have answered this in my
reply to Jason. Different disjoint classical worlds arise only if the
quantum events are amplified to classical significance by decoherence
forming thermodynamically irreversible records of the event in the
environment. This does not happen for the majority of quantum
fluctuations that underlie our classical states. Alice and Bob both have
(quasi-)classical identities that are unaffected by such fluctuations.
Just as the temperature of the air is not affected by random molecular
motions. As in statistical mechanics, these fluctuations are averaged
over and essentially all cancel out. They can be safely ignored (FAPP if
you must, but we are talking practicalities here, not undetectable
quantum events.)
Determinism has nothing to do with it. Aspect and Bell rule out
deterministic local theories, so non-locality is the only
possibility. Many-worlds does not change that.
Determinism is the issue. In a collapse theory, you need indeterminism
to assure the non signalling FTL of information. But you have still
some physical FTL/simultaneous action.
No, you just need randomness. That is necessary in MWI as well. Physical
FTL is not necessary.
With the MW, the situation is entirely deterministic and there is no
need of any FTL.
MWI may be deterministic. But then, the loss of symmetry when Alice
measures the singlet state is also entirely deterministic -- it is part
of what unitary evolution according to the Schrödinger equation gives
you. It is not different with many-worlds.
Both Bell's theorem and Aspect's results are true in many-worlds as
in any other interpretation of QM.
Of course.
Don't you understand that that is why most commentators from the
many-worlds perspective try to show that Bell's theorem does not
apply to many worlds?
They are wrong. Bell’s violation is necessary in all branches. But
what happens is that if one branch is selected, by collapse or by
hidden variables, then *that* transform the non-locality (Bell’s
violation) into FTL.
That is not true. What happens in a typical branch of a superposition is
true for all branches. And what is true for all branches is necessarily
true for the whole. The "one branch" is not selected by collapse or
hidden variables, it is selected as typical for the purposes of
calculation -- 'in the mind' as it were.
Without collapse, we don’t need hidden variables, nor any FTL, to
explain the non-locality and why it never disappears.
Many-worlds is non local because it is non-local in every branch. Can't
you see the logic of this?
Bruce
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