On 15/11/2017 5:02 pm, Russell Standish wrote:
On Wed, Nov 15, 2017 at 02:46:21PM +1100, Bruce Kellett wrote:
On 15/11/2017 12:49 pm, Russell Standish wrote:
On Wed, Nov 15, 2017 at 11:05:22AM +1100, Bruce Kellett wrote:
One of the strongest arguments for MWI was that it eliminates the concept of
a conscious observer from the interpretation of quantum mechanics.
I disagree. The strongest argument is that it removes the need for a
mysterious nonunitary physical collapse process (that may or may not
be driven by a conscious observer).
I said "one of the strongest"! I know that you want to define QM from the
idea of observer moments. I don't think that this will work, and the usual
consensus is that one of the strengths of MWI is the elimination of the
conscious observer.
Where's your evidence that this is the usual concensus? Who argues for it?
Most people, indirectly if not directly. I am thinking of MWI proponents
such as Deutsch and Wallace. Wallace puts it like this "Some have argued
that he measurement problem of quantum mechanics gives us reason to
abandon the picture of science as describing an observer-independent
world....". He does not accept this view. The general view of scientific
realism consists in the belief that the objective external world is
independent of the observer. The thinking is related to Bell's assertion
that: "Measurement should never be introduced as a primitive process in
a fundamental mechanical theory like classical or quantum mechanics..."
Measurement, observation, observers are all related concepts in this
context.
A conscious observer (or rather just observer, really) is still
required to define the branches of the MWI, be that mediated by Zeh's
decoherence process, or otherwise. To eliminate observers entirely
requires solving the preferred basis problem without reference to an
observer or observation.
That is not true. The basis problem is solved by Zurek's einselection -- the
preferred basis is the one that is stable against further
decoherence.
I understand that the idea of einselection is still rather
controversial,
Not really. See Schlosshauer's paper and book.
but be that as it may, I can't see how it solves the
preferred basis problem. Consider an experiment where the experimenter
may choose between inserting a circularly polarised file, or a
linearly polarised one. The preferred basis (selected by einselection)
will depend on that choice.
That is a common misconception, but the angle selected for the
polarizer, or the S-G magnet in a spin measurement, is not a selection
of a measurement basis. The measurement is actually the observation
whether or not the photon/particle passes the filter. It is then an
inference from the observation of a point on a screen, or the firing (or
failing to fire) of a detector of some sort, that the
polarization/spin-component was such and such. You don't actually
measure anything in the selected orientation, you only ever measure
whether the particle passed the filter or not. So the actual measurement
is just a position measurement (position on a screen), and the
measurement basis is the position (pointer) basis.
In MWI, we normally assume that there are
two branches of the universe with different choices made by the
experimenter.
That is really an oversimplification. It is done because it is simpler
to work with two-state systems, and position measurements are of a
continuous variable, so are not neatly two-valued.
Unless there is some sort of superdeterminism in play,
where the experimenter does not have the freedom to choose. But
superdeterminism is certainly not a popular idea.
No, superdeterminism does not have many advocates.
Observers have nothing to do with it. In Zurek's account, it is the fact
that the results of interactions, be they measurements or not, are recorded
multiple times in the environment via decoherence, that is the mark of an
irreversible quantum event.
If you put a system in contact with a completely symmetric heat bath,
there will be no preferred basis selected by einselection.
The environment of a measurement or an interaction is not generally a
symmetric heat bath. If you measure a spin component (space
quantization) you get one of two spots on a screen downstream of the S-G
magnet. These are not symmetric wrt the rest of the environment. In one
world the irreversible record is of an upper spot. In the other world it
is of the lower spot. The distinction is not lost because of symmetry.
The basis for the measurement is the position basis, because that is
stable against further decoherence. The angle of the S-G magnet is not
the measurement basis.
The only
way for a basis to emerge is if there are system constraints of some
sort. I would argue that the only way these constraints could arise in
a Multiverse (which is symmetric by construction) is by considering
the environment from the point of view of some observer, ie the basic
symmetry breaking mechanism.
The observer is not a general symmetry breaking mechanism. The many
worlds in QM are not symmetric anyway.
Bruce
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