On 29 Dec 2013, at 23:29, meekerdb wrote:

On 12/29/2013 2:01 PM, Jason Resch wrote:On Sun, Dec 29, 2013 at 1:47 AM, meekerdb <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>wrote:On 12/28/2013 4:45 PM, Jason Resch wrote:On Sat, Dec 28, 2013 at 7:12 PM, meekerdb <meeke...@verizon.net>wrote:On 12/27/2013 10:31 PM, Jason Resch wrote:To that I would add the purely epistemic "non-intepretation" ofPeres and Fuchs."No interpretation needed" -- I can interpret this in two ways,one way is to just take the math and equations literally (thisleads to Everett), the other is "shut up and calculate", whichleads no where really.2. Determined by which observer? The cat is always either deador alive. It's just a matter of someone making a measurement tofind out.So are you saying that before the measurement the cat isneither alive nor dead, both alive and dead, or definitelyalive or definitely dead? If you, (and I think you are),saying that the cat is always definitely aliveordefinitely dead, then about about the radioactive atom? Is itever in a state of being decayed and not decayed? If you sayno, it sounds like you are denying the reality of thesuperposition, which some interpretations do, but then thisleads to difficulties explaining how quantum computers work(which require the superposition to exist).Superposition is just a question of basis. An eigenstate in onebasis is a superposition in another.Can you provide a concrete example where some system cansimultaneously be considered to be both in a superposition andnot? Is this like the superposition having collapsed forWigner's friend while remaining for Wigner before he enters theroom??? Every pure state can be written as a superposition of acomplete 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 whichany given pure state is just (1,0,0,...). In theory there is a'dead&alive' basis in which Schrodinger's cat can be representedjust like a spin-up state is a superposition is a spin-left basis.So if someone keeps alternating between measuring the spin on the yaxis, and then the spin on the x axis, are they not multiplyingthemselves continuously into diverging states (under MWI)? Eventhough these states only weakly interfere, are they not stillsuperposed (that is, the particles involved in a simultaneouscombination of possessing many different states for theirproperties)?Right, according to Everett, the world state becomes a superpositionof 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, thespins don't have to observed by anyone. If the silver atom justgoes thru the Stern-Gerlach apparatus and hits the laboratory wall,the superposition is still created. If it just goes out the windowand into space...it's not so clear.

`It is very clear. IF QM is exact, the superposition does not`

`disappear, but get contagious to the environment (at roughly the speed`

`of light).`

An electron in a superposition, when measured, is still in asuperposition according to MWI. It is just that the person doingthe measurement is now also caught up in that superposition.The only thing that can destroy this superposition is to moveeverything back into the same state it was originally for all thepossible diverged states, which should practically never happenfor a superposition that has leaked into the environment.In Everett's interpretation a pure state can never evolve into amixture because the evolution is via a Hermitian operator, theHamiltonian. Decoherence makes the submatrix corresponding to thesystem+instrument to approximate a mixture. That's why it can beinterpreted as giving classical probabilities.Are there pure states in Everett's interpretation? Doesn't one haveto consider the wave function of the universe and consider it allthe way into the past?I suppose the universe could have started in a mixed state, but mostcosmologists would invoke Ockham and assume it started in a purestate - which, assuming only unitary evolution, means it's still ina pure state. Of course since inflation there can be entanglementsacross event horizons, so FAPP that creates mixed states.

FAPP. yes.

In any case, returning to the original point that began thistangent, do agree that QM interpretations which are anti-realist(or deny the reality of the superposition) are unable to describewhere the intermediate computations that produce the answer to aquantum computation, take place?They take place in a quantum computer.

That beg the question ...

What would Fuchs say about quantum computation?It's a physical process whose outcome is predicted by QM.

That looks like instrumentalism and "don't ask for understanding". Bruno http://iridia.ulb.ac.be/~marchal/ -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to everything-list+unsubscr...@googlegroups.com. To post to this group, send email to everything-list@googlegroups.com. Visit this group at http://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/groups/opt_out.