On 16/11/2017 5:12 pm, Brent Meeker wrote:
On 11/15/2017 9:09 PM, Bruce Kellett wrote:
On 16/11/2017 11:30 am, Russell Standish wrote:
On Thu, Nov 16, 2017 at 10:20:45AM +1100, Bruce Kellett wrote:
On 16/11/2017 9:14 am, Russell Standish wrote:
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.
The camera merely decoheres the system, which remains in a
superposition
of the two possible outcomes of the SG experiment. To break that
symmetry requires an observer looking at the photo plate, or
downloading the image from the camera's CCD and observing it on a
screen.
The observer looking at the plate merely becomes entangled with the
result on that plate -- splits along with the original split caused
by the measurement. This does not break any symmetry that might be
present. I am still not sure exactly what symmetry you are seeking to
break. At some point, separation into separate non-interacting worlds
requires the the superposed pure state be broken into a mixed state,
but I would not see that particularly as a symmetry breaking. Exactly
how the transition from pure to mixed comes about is somewhat unclear
at present. One could simply say that the 'worlds' are relative
states, relative to the original experimental result. Or one could
call on coarse graining, or take a partial trace. Zurek has what I
consider a better scheme, whereby the fact that the experimental
result is immediately repeatable -- the experiment leave the system
in an eigenstate -- is sufficient to cause the separate worlds to be
exactly orthogonal, so that the density matrix is exactly diagonal.
I like Zurek's point, but in most measurements the system measured is
destroyed. The measurement that acts as a preparation, leaving the
system in an eigenstate is rare.
Obviously Zurek was aware of this. I think his point is that it does not
really matter -- the postulate of standard QM is that a measurement
leaves the system in an eigenstate of the corresponding operator, so
measuring again will necessarily give the same result. This is the
situation in principle, if the state was destroyed post measurement,
that does not affect the principle.
Bruce
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