On 1/06/2017 10:19 pm, Bruno Marchal wrote:
On 01 Jun 2017, at 02:26, Bruce Kellett wrote:
On 1/06/2017 4:43 am, Bruno Marchal wrote:
On 31 May 2017, at 04:01, Bruce Kellett wrote:
On 30/05/2017 9:35 pm, Bruno Marchal wrote:
On 30 May 2017, at 11:28, Telmo Menezes wrote:
I get your point with decoherence.
Again, I would say that it all depends on theories of mind. What
does
mind supervene on? Perhaps it is true that every single coupling
with
the environment prevents the current observer state to become
compatible with other branches. But can we be sure? I feel that such
certainties come from a strong belief in emergentism (which I cannot
disprove, but find problematic).
It is impossible to recohere the past, FAPP.
But only FAPP. To make the blue T-rex interfereing with the
red-T-rex, we must erase the trace of particle interaction between
the T-rex in its whole light-cone, and this without forgetting the
particles "swallowed" by the black-holes, etc. It is just
completely impossible, but to derive from that the unicity of the
past, is, it seems to me (and you if I understood well) is invalid.
I think the recoherence of paths that have completely decohered is
more than just FAPP impossible, I think it is impossible in
principle. One major problem with recoherence in general is that
information leaks from the paths at the speed of light (as well as
less slowly for other interactions). Since this vital information
goes out along the light cone, it can never be recaptured and
returned to the original interaction.
In QM + special relativity OK. (note that to me QM + special
relativity => no collapse (and even the Many dreams, but we have
agreed to disagree on this if I remember well).
Reasoning in QM without SR is not very profitable. Besides, QM + SR
does not particularly imply MWI -- it is perfectly possible to have a
consistent collapse model of QM+SR. I know you don't agree because of
the non-locality implied by EPR, but this non-locality is not removed
in MWI, regardless of what you might say.
I don't see it, and the mast time we discuss this, we conclude on some
vocabulary problem. When there is no collapse, measurement tells to
people light separated in which branch theiy belongs, but to exploits
that information, they need to come into contact.
Last time we discussed this, no resolution was achieved. In QM, with or
without collapse, decoherence and the transition from the pure state to
a mixture gives a definite measurement result. Without collapse,
different branches get different results, but once obtained, these
results are fixed, and are not affected by whether Alice and Bob
exchange information or not.
Consequently, indispensable phase information is lost *in
principle*, so the recoherence is, in general, impossible.
OK. (I was reasoning in naive classical QM)
Of course, with carefully constructed systems, where the loss of
information along the light cone is prevented, recoherence is
possible in special circumstances, but not in general.
From this, the uniqueness of the past of any decoherent history is
assured. So deriving the unicity (if I understand this use of the
word) is by no means invalid -- it is proved.
In QM + SR. OK.
Even if one encounters one of those rare situations in which
recoherence is achieved, that still does not invalidate the
uniqueness of the past history -- recoherence, if it occurs, simply
means that no new branches are formed at that point, so the
decoherent history remains unique.
OK. That might suggest that we identify our indistinguishible past
in arithmetic, if we assume mechanism. I use the Y = II principle,
or the "quantum" linearity of the tensor product "@": we have that
a @ (b + c) = (a @ b) + (a @ c).
That makes sense.
Linearity is the heart of QM. It is linearity that allows
superpositions, and leads to all the "quantum weirdness".
I agree. Both "linearities" (the quantum evolution, and the tensor
products).
.......
But those state difference are accessible to the observers, and
indeed, only this makes the analogy with step 3 working.
In MWI, the differences are not observable by anyone. Any observer
has access to only one branch, so only one copy. They can say nothing
about the other branch.
The difference are not observable, but are very gross, like seeing a
cat dead, or alive. Linearity prevent any direct view of that
difference, but it exists, when we assume QM (and non collapse).
It is not linearity that prevents macro-superpositions -- it is
decoherence and the reduction to a mixed state. The difference between
the measurement outcomes exists whatever interpretation of QM you impose.
.......
Of course, it assures them in all branches, where indeed Aspect like
experiences can be made. It seems to me that we did agree on this:
that non-locality does not entail any physical influence in the
past. That does happen in the unique universe view though; even if
there is no possible communication of information is done.
Non-locality means that there is no physical transfer of information,
but that there is non-physical(non-local) transfer of information.
But this information transfer cannot be used for signalling.
Signalling is possible only with actual; physical transfer, a
consequence of SR and the fact that 'information' is physical.
So we agree. Yet, with unicity of outcomes assumed for measurement, we
still cannot signal, but must assume some "reduction of a wave packet
at a distance", or super-deterministic conspiracies.
This non-locality is even more evident in the more recent delayed
choice experiments that use entangled photons to manipulate photon
polarization states non-locally.
I have no problem with one-branch observable, apparent, non-locality.
I have a problem only with the action at a distance that you need in
case you assume one contextually well defined physical reality.
Non-locality is not removed in MWI as you appear to believe.
In the sense above, you are right. I was juste arguing against John
Clarks idea that the Bell's inequality violation introduce physical
action at a distance, even with the MWI.
There are two things:
1) no influence at a distance,
2) no signaling at a distance.
With QM-without collapse, there is no influence at a distance. The
no-locality above is explained without them.
With QM+ collapse, there is no signaling at a distance, but there is
an influence at a distance.
There is no non-local signalling whatever interpretation you adopt. But
there is non-locality -- non-local influence -- in all interpretations
since it is inherent in the quantum formalism. Non-locality is not
explained away in MWI.
For me the abandon of the collapse is the solution of the EPR
"paradox", and Aspect experience is somehow the confirmation of our
belonging to macrosuperposition.
The non-local (paradoxical) nature of EPR remains even without collapse.
I know some claims this, but I have never sen a proof. All proofs
relies on unicity of the outcome of some experiences.
You cannot get away by reversing the onus of proof. Bell's theorem is
independent of whether or not a collapse is assumed, so if you want to
argue that MWI removes the non-locality proved by Bell, then the onus of
proof is very much on you: you have to demonstrate how this can be possible.
You say that Bell's theorem relies on the unicity of outcomes. By this,
I presume you mean that Bell assumes counterfactual definiteness (in the
usual terminology). If by counterfactual definiteness you mean that a
measurement gives a definite (though unknown in advance) result, even if
that measurement is not performed. Then I accept that counterfactual
definiteness is assumed in quantum mechanics. Without such an
assumption, the whole notion of an expectation value would collapse. So
if you abandon counterfactual definiteness, you have a different theory
-- you have abandoned standard QM, and you then have to explain how you
can get and use expectation values.
Your claim appears to be that Bell's theorem is not valid in MWI. This
is nonsense. Bell's theorem is a theorem of quantum mechanics, and it is
therefore valid in all interpretations of that theory. If it is not
valid in MWI, then Many-Worlds is a different theory, and not just an
interpretation of standard QM.
As on the previous occasion we discussed this, you were unable to
demonstrate where the notion of 'collapse' is used in Bell's theorem
- all Bell requires is that measurements give results, and that is
what the whole of physics is based on: in MWI as well as in any other
interpretation.
We did eventually agree. May be reread those post. Bell's supposed
that when the two measurement are done, Alice and Bob get a precise
answer, which makes no sense without-collapse.
That is what making a measurement means. It is what happens in all
interpretations. It makes no sense to deny counterfactual definiteness
-- that is not QM.
Alice and Bob get *all* (always correlated) answers, but when
light-separated, it make no sense to compare them. They can only make
comparison with the person accessible in their light cone, where the
contagious superposition spread out.
I presume you mean "space-like separated". Alice and Bob do their
measurements; they get their results and write them in their lab books.
They meet years later and compare lab books. Are you trying to suggest
that they do not have definite answers in their lab books before then?
In MWI (with two-outcome experiments), there is a copy of Alice that
writes '+' in her lab book, and a copy who writes '-' (for a given
orientation theta). Similarly for Bob. There are, therefore, only four
possibilities when they meet: '++', '+-', '--', and '-+'. The
non-locality is necessary to set the probabilities for each of these
four possible combinations of results. If you want to eliminate the
non-locality, you have to give a non-magical way of establishing the
necessary probabilities. You have never been able to do this.
Remember that in a sequence of such experiments, the probabilities for
'+' and '-' are 50/50 for both Alice and Bob. The joint probabilities,
or correlations, depend on the relative orientations of their
polarizers. It is information about this relative orientation that must
be conveyed non-locally for the correlations to come out correctly when
they meet. It is not sufficient for them simply to exchange this
information later, because their results at particular orientations are
already fixed when they meet.
Bruce
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