On 11/15/2017 3:20 PM, Bruce Kellett wrote:
On 16/11/2017 9:14 am, Russell Standish wrote:
On Wed, Nov 15, 2017 at 10:54:51PM +1100, Bruce Kellett wrote:
On 15/11/2017 5:02 pm, Russell Standish wrote:
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.
But not all measurements are measurements of the position of
something. What about measuring the voltage of a circuit using an A->D
converter?
A surrogate measurement of the position of a pointer on a voltmeter.
When I've tried to think of measurements that are not position
measurements, I come up with looking an emitter and seeing what color
the photons are...a momentum measurement.
Brent
Or the measurement of the momentum of a charged particle in
an electron multier?
Do you mean "electron multiplier"? Would than not be an energy
measurement? Momentum of a charged particle is often measured by
tracking the path of the charged particle in a magnetic field.
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.
The choice between circularly polarised filter and linear polarised
filters is binary. Obviously, there follows the choice of orientation,
which is continuous...
Whatever the choice of orientation, the outcome is binary, pass or not
pass.
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 there is no experimenter, just an environment, then we must
consider all possible environments in superposition. That will have
maximal symmetry.
"All possible environments in superposition"? That is seriously
under-defined. Besides, that would only have a pseudo-symmetry.
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.
That is because we're considering an SG experiment, with an SG
experimenter. That breaks the symmetry.
The environment breaks the symmetry. The environment may contain an
experimenter, but need not. A camera would do the job.
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.
I would argue that observation in a multiverse is a symmetry breaking
mechanism. In the multiverse, all possible outcomes of a measurement
exist as separate branches, and if all outcomes are equally likely,
there is a fundamental symmetry along that measurement axis. But the
action of observation fixes the outcome for a particular observer,
breaking that symmetry.
In the first place, it is unlikely that all possible outcomes of an
experiment are equally likely. But I think you are confusing symmetry
breaking with the observer self-locating in one of the possible
outcome worlds. That breaks the symmetry for him, perhaps. But he is
not important for that, because any future developments in the
separate worlds will break any residual symmetry. The observer is
nothing more than just one possibility for such a future development.
The many worlds
in QM are not symmetric anyway.
Not completely, but far more symmetric that the world we inhabit.
Only in that the many worlds of QM contain more possibilities.
Observers are not necessary for breaking this symmetry. The bottom
line is that observation is not necessary for the existence of the
world. Observation may tell us about this particular world, but that
is secondary.
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 [email protected]
<mailto:[email protected]>.
To post to this group, send email to [email protected]
<mailto:[email protected]>.
Visit this group at https://groups.google.com/group/everything-list.
For more options, visit https://groups.google.com/d/optout.
--
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 [email protected].
To post to this group, send email to [email protected].
Visit this group at https://groups.google.com/group/everything-list.
For more options, visit https://groups.google.com/d/optout.
