On 15/12/2017 10:15 am, smitra wrote:
On 14-12-2017 22:35, Bruce Kellett wrote:
I think this argument pre-dates the work by Zeh and Zurek developing
the idea of decoherence. Decoherence remove the oddities of Copenhagen
as presented above in that it is not consciousness that does the work,
but decoherence. Bohr was saying essentially the same thing (though he
didn't know the words) when he talked about the importance of the
whole experimental set up.
That aside, Deutsch's idea fails because he has not fully implemented
quantum erasure. If a record exists of the fact that a 'welcher weg'
measurement was made, entanglement of the rest of the world with the
result of that measurement is not erased by merely resetting the
memory of the mind or computer. So in the proposed experiment, the
interference pattern is absent, and it is not a proper 'delayed
choice' situation.
Deutsch has also given a rigorous variant of this that I summarized in
my paper. It is possible in theory to experimentally prove the
existence of other branches where a different measurement result was
obtained. It's only impossible FAPP.
From your paper, summarizing Deutsch:
"Suppose an observer measures the z-component of a spin that is
polarized in the x-direction. Then there exits a unitary operator that
disentangles the observer from the spin, causing the observer to forget
the result of the measurement. However, he would still remember having
measured the z-component of the spin. In the MWI, the spin will be in
its original state and therefore measuring the x-component will yield
spin up with 100% probability. In any collapse interpretation, measuring
the x-component will yield spin up or spin down with 50% probability."
This is equivalent to the argument John reported, and your reference
dates from 1985. That is before decoherence theory and entanglement with
the environment was fully understood, and predates the significant
quantum erasure experiments. For that reason, Deutsch has got it
seriously wrong, because the unitary operation he envisages does not
exist, even in principle, and he has not fully implemented quantum erasure.
His scenario can be approximated in quantum erasure experiments, and
because the erasure is not complete, the results obtained will be
identical to those predicted by a collapse model. In order to use time
reversal invariance to recover the initial state in MWI, you have to
reverse absolutely everything, so you cannot leave a record in the
environment that some measurement was made -- such a record means that
there has not been compete reversal, so the original state is not recovered.
Bruce
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