On 12/29/2013 3:31 PM, Jason Resch wrote:

On Sun, Dec 29, 2013 at 5:29 PM, meekerdb <meeke...@verizon.net <mailto:meeke...@verizon.net>> wrote:

    On 12/29/2013 2:01 PM, Jason Resch wrote:

    On Sun, Dec 29, 2013 at 1:47 AM, meekerdb <meeke...@verizon.net
    <mailto:meeke...@verizon.net>> wrote:

        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, 
                way is to just take the math and equations literally (this 
leads to
                Everett), the other is "shut up and calculate", which leads no 

                        2. Determined by which observer? The cat is always 
                        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 
                    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 
                 Is this like the superposition having collapsed for Wigner's 
                while remaining for Wigner before he enters the room?

                ?? Every pure state can be written as a superposition of a 
                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, 

        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 
        Schrodinger's cat can be represented just like a spin-up state is a
        superposition is a spin-left basis.

    So if someone keeps alternating between measuring the spin on the y axis, 
and then
    the spin on the x axis, are they not multiplying themselves continuously 
    diverging states (under MWI)?  Even though these states only weakly 
interfere, are
    they not still superposed (that is, the particles involved in a simultaneous
    combination of possessing many different states for their properties)?

    Right, according to Everett, the world state becomes a superposition of 
states of
    the form |x0,x1,...> where each xi is either +x, -x, +y, or -y.  And per 
the Bucky
    Ball, Young's slit experiment, the spins don't have to observed by anyone.  
If the
    silver atom just goes thru the Stern-Gerlach apparatus and hits the 
laboratory wall,
    the superposition is still created.  If it just goes out the window and into
    space...it's not so clear.

        An electron in a superposition, when measured, is still in a 
        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 
        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 

    Are there pure states in Everett's interpretation? Doesn't one have to 
consider the
    wave function of the universe and consider it all the way into the past?

    I suppose the universe could have started in a mixed state, but most 
    would invoke Ockham and assume it started in a pure state - which, assuming 
    unitary evolution, means it's still in a pure state.  Of course since 
    there can be entanglements across event horizons, so FAPP that creates 
mixed states.

    In any case, returning to the original point that began this tangent, do 
agree that
    QM interpretations which are anti-realist (or deny the reality of the
    superposition) are unable to describe where the intermediate computations 
    produce the answer to a quantum computation, take place?

    They take place in a quantum computer.

And the quantum computer is a coherent, long-lived superposition with a number of real states exponential with the number of its qubits.

I'm not sure what you mean by "a number of real states"? It has only one state (which is in a complex Hilbert space), which can be written as a superposition of some set of basis states - but that's true of my refrigerator too.

If superpositions are real and long-lived, and involve an arbitrary number of particles, it seems there is no reason that people could not also be in superpositions.

    What would Fuchs say about quantum computation?

    It's a physical process whose outcome is predicted by QM.

We limit the power and effectiveness our own theories and stifle progress, when we don't put forward theories that make bold statements about reality.

And we divert progress when we adopt intuitively appealing theories with no operational content and try to reify them.

Bohr's (and seemingly Fuch's) positions are so conservative as to never be 

Nevertheless they both published more papers than Everett (whose interpretation doesn't seem testable either - if it were, it would be a theory instead of an intepretation).

but they also inhibit progress and new understandings. For example, general purpose quantum computers may not have been invented had Deutsch not been operating under Everett's paradigm.

Feynman wrote about quantum computation well before Deutsch.

"You are the only contemporary physicist, besides Laue <http://en.wikipedia.org/wiki/Max_von_Laue>, who sees that one cannot get around the assumption of reality, if only one is honest. Most of them simply do not see what sort of risky game they are playing with reality—reality as something independent of what is experimentally established." --- Einstein in a letter to Schrodinger

Everybody believes in reality. Nobody agrees on what it is. :-)


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