Re: For John Clark

```On 10/18/2013 1:45 PM, Jason Resch wrote:
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On Fri, Oct 18, 2013 at 11:37 AM, meekerdb <meeke...@verizon.net <mailto:meeke...@verizon.net>> wrote:
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On 10/18/2013 12:42 AM, Jason Resch wrote:
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But that's not compatible with Bruno's idea of eliminating the
physical - at
least not unless he can solve the basis problem.

Could you do me a favor and explain what the basis problem is in a way that
a 6th
grader could understand?  I've found all kinds of things said on it, and
they all
seem to be asking different things.
```
```
For physicists, it's part of the problem of explaining the emergence of the
classical world from the quantum world.  Decoherence can diagonalize
(approximately)
a reduced density matrix IN SOME BASIS.

```
Is this the same basis as in "momentum basis" and "position basis", or is it some other usage of the term?
```
Forgive my ignorance, but what does it mean to "diagonalize a reduced density
matrix"?
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It means to take an average over all the other variables except those of interest (i.e. the ones you measure). If you do this in a particular basis we think it makes the submatrix corresponding to those variables diagonal. Then it can be interpreted as the probabilities of the different values. Note that it is a mathematical operation that depends on choosing a basis, not a physical process. The MWI view is that this is a physical process - which it could be IF the basis was not an arbitrary choice but was somehow dictated by the physics. But so far there are only hand waving arguments that "it must be that way".
```
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```      Being diagonal in one basis means it's superposition in some other basis.
So for
physicists the problem is saying what privileges or picks out the
particular bases
we see in experiments. Why do our instruments have needles that are in
eigen states
of position, while some other things (e.g. atoms) are in eigen states of
energy or
eigen states of momentum.  For physicists there are some suggestive, but
not fully
worked out answers to these questions, e.g. you get position eigenstates
because the
interaction term of the Hamiltonian is a function of position.  But those
assume the physics.  If you want to reconstruct physics from experiences,
you can't
borrow the physical explanation to say why your experiences are classical.

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I think the assumption that experiences are classical comes from the classicality of Turing machines (which are the supposed mechanism by which experiences are manifest).
```
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I don't think there's anything either classical or quantum about Turing machines. They are just mathematical abstractions. And assuming they read and write qubits instead of bits doesn't change the range of things they can compute.
```
Brent

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