On 12/28/2013 6:41 PM, Jason Resch wrote:




On Sat, Dec 28, 2013 at 8:32 PM, meekerdb <meeke...@verizon.net <mailto:meeke...@verizon.net>> wrote:

    On 12/28/2013 4:45 PM, Jason Resch wrote:



    On Sat, Dec 28, 2013 at 7:12 PM, meekerdb <meeke...@verizon.net
    <mailto:meeke...@verizon.net>> wrote:

        On 12/27/2013 10:31 PM, Jason Resch wrote:

            To that I would add the purely epistemic "non-intepretation" of 
Peres and
            Fuchs.

        "No interpretation needed" -- I can interpret this in two ways, one way 
is to
        just take the math and equations literally (this leads to Everett), the 
other
        is "shut up and calculate", which leads no where really.



                2. Determined by which observer? The cat is always either dead 
or
                alive. It's just a matter of someone making a measurement to 
find out.


            So are you saying that before the measurement the cat is neither 
alive
            nor dead, both alive and dead, or definitely alive or definitely 
dead?
             If you, (and I think you are), saying that the cat is always 
definitely
            alive or definitely dead, then about about the radioactive atom? Is 
it
            ever in a state of being decayed and not decayed? If you say no, it
            sounds like you are denying the reality of the superposition, which 
some
            interpretations do, but then this leads to difficulties explaining 
how
            quantum computers work (which require the superposition to exist).

            Superposition is just a question of basis.  An eigenstate in one 
basis is
            a superposition in another.


        Can you provide a concrete example where some system can simultaneously 
be
        considered to be both in a superposition and not?  Is this like the
        superposition having collapsed for Wigner's friend while remaining for 
Wigner
        before he enters the room?


        ?? Every pure state can be written as a superposition of a complete set 
of
        basis states - that's just Hilbert space math.


    So then when is the system not in a superposition?

    When it's an incoherent mixture of pure states.


What makes it incoherent though?

If the density matrix is not a projection operator, i.e. rho^2 =/= rho, it's 
incoherent.

But really I just meant that in theory there is a basis in which any given pure state is just (1,0,0,...). In theory there is a 'dead&alive' basis in which Schrodinger's cat can be represented just like a spin-up state is a superposition is a spin-left basis.

An electron in a superposition, when measured, is still in a superposition according to MWI. It is just that the person doing the measurement is now also caught up in that superposition.

The only thing that can destroy this superposition is to move everything back into the same state it was originally for all the possible diverged states, which should practically never happen for a superposition that has leaked into the environment.

In Everett's interpretation a pure state can never evolve into a mixture because the evolution is via a Hermitian operator, the Hamiltonian. Decoherence makes the submatrix corresponding to the system+instrument to approximate a mixture. That's why it can be interpreted as giving classical probabilities.

Brent

Jason

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