On 10/8/2019 5:03 PM, Bruce Kellett wrote:
On Wed, Oct 9, 2019 at 10:49 AM 'Brent Meeker' via Everything List
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On 10/8/2019 4:21 PM, Bruce Kellett wrote:
On Wed, Oct 9, 2019 at 9:53 AM 'Brent Meeker' via Everything List
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What additional assumptions do you mean?
What assumptions does he have to make to get a probability
interpretation? Probability is not an evident property of the SE.
Like many approaches to probability in Everett, he has to assume
decoherence to distinguishable branches to get anywhere. But that
relies on using the Born rule to justify ignoring branches with
low amplitude
The low amplitude branches aren't ignored. Do you mean
cross-terms in the density matrix?
Explain to me how these are functionally different from low amplitude
branches.
The branches are projections of the universal Hilbert ray onto
(approximately) orthogonal subspaces corresponding to the preferred
basis. Cross terms are what make them approximate. The cross-terms
supposedly go to zero when you compute the reduced density matrix, but
the diagonal terms don't go to zero...they measure the probability of
the branches, including branches with low probability.
-- the notorious "trace over environmental degrees of freedom".
Sean's "self-locating uncertainty" is not well-defined.
I tend to agree with you there. But if you assume that the human
brain is a classical information processor of limited capacity I
think you could get there.
Only at the price of re-introducing the human observer into your
exposition of QM. I thought that was the cardinal sin of the CI, and
here Sean is bringing it back into Everett!!!!
Well, if you're going to explain why we experience a classical world,
you're going to have to say something about experience. To say it's
information processing in the brain sounds pretty good to me.
In the lecture he hints that the observer is uncertain about the
fate of the cat until he opens the box -- until then he is
uncertain of which branch he is on. But given the timescale of
decoherence, he has branched within 10^{-20} sec, so the is no
longer any "self-locating uncertainty" -- he is definitely on one
branch or the other, he just doesn't know which. And that is just
classical probability due to lack of knowledge -- nothing quantum
about it. In another interview, he does suggest that the
"self-locating uncertainty" lasts only until decoherence reaches
the observer, at which time copies become entangled within each
branch. Now if you can think relevant thoughts in 10^{-20} sec,
then his argument might make some sense. But it fails to
convince.....
So you're faulting him for not calling 1e-20sec zero?
No. I am not faulting him for not calling it zero. I am faulting him
for calling something that is uncertain in any quantum sense for
10^{-20} sec significant for the interpretation of probabilities.
But it's relevant to showing MWI is consistent with the experimenter not
knowing which "world" he is in. He might not look at the data for a long
time.
Brent
Seems nit-picky. I see the problem as sloppy introduction of "the
observer" harking back to CI.
Yes, that is a point I have made.
If observers are quasi-classical (no funny stuff in microtubles)
then it should be enough to talk about uncertainty in the location
of the geiger counter or the environment, but I understand Sean is
trying to explain why, according to Everett, people don't see the
superposition.
That is achieved by decoherence and the preferred pointer basis. No
need to introduce observers and self-locating uncertainty.
Bruce
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