On 12/29/2013 3:31 PM, Jason Resch wrote:
On Sun, Dec 29, 2013 at 5:29 PM, meekerdb <meeke...@verizon.net
On 12/29/2013 2:01 PM, Jason Resch wrote:
On Sun, Dec 29, 2013 at 1:47 AM, meekerdb <meeke...@verizon.net
On 12/28/2013 6:41 PM, Jason Resch wrote:
On Sat, Dec 28, 2013 at 8:32 PM, meekerdb <meeke...@verizon.net
On 12/28/2013 4:45 PM, Jason Resch wrote:
On Sat, Dec 28, 2013 at 7:12 PM, meekerdb <meeke...@verizon.net
On 12/27/2013 10:31 PM, Jason Resch wrote:
To that I would add the purely epistemic
Peres and Fuchs.
"No interpretation needed" -- I can interpret this in two ways,
way is to just take the math and equations literally (this
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
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
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,
the spin on the x axis, are they not multiplying themselves continuously
diverging states (under MWI)? Even though these states only weakly
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
the form |x0,x1,...> where each xi is either +x, -x, +y, or -y. And per
Ball, Young's slit experiment, the spins don't have to observed by anyone.
silver atom just goes thru the Stern-Gerlach apparatus and hits the
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
caught up in that superposition.
The only thing that can destroy this superposition is to move
into the same state it was originally for all the possible diverged
which should practically never happen for a superposition that has
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
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
In any case, returning to the original point that began this tangent, do
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
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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