# Re: What are wavefunctions?

```On 12/29/2013 2:01 PM, Jason Resch wrote:
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On Sun, Dec 29, 2013 at 1:47 AM, meekerdb <meeke...@verizon.net <mailto:meeke...@verizon.net>> wrote:
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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:

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
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
definitely dead?  If you, (and I think you are), saying that
the cat
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).
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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?

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?? 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.

```
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)?
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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.
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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.

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.

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

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

Brent

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