# Re: What are wavefunctions?

```On 12/28/2013 6:41 PM, Jason Resch wrote:
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On Sat, Dec 28, 2013 at 8:32 PM, meekerdb <meeke...@verizon.net <mailto:meeke...@verizon.net>> wrote:
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On 12/28/2013 4:45 PM, Jason Resch wrote:
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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:
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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
If you, (and I think you are), saying that the cat is always
definitely
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).
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```
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?
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```
When it's an incoherent mixture of pure states.

What makes it incoherent though?
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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.
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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.
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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.
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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
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```
Jason

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