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.
Bruce
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