# Re: What are wavefunctions?

`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" 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.
>
>
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 into 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)?

>
>   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.
>

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?

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 that produce the answer to a quantum computation, take place?

What would Fuchs say about quantum computation?

Jason

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