On 10/28/2014 8:14 AM, Bruno Marchal wrote:
On 27 Oct 2014, at 20:58, meekerdb wrote:
On 10/27/2014 3:38 AM, LizR wrote:
It would be nice if Mr Clark would EITHER stop joining in with discussions just to say
that he doesn't care about comp, OR state what he agrees or disagrees with in Bruno's
stated argument.
Just saying it's "obviously wrong" doesn't really cut it. So far the only real
(non-sarcastic, non-insult-based) objection I've heard comes down to a semantic
quibble to do with redefining our concept of an individual person. This is exactly the
same redefinition that was brought up by Everett in 1957. It isn't in itself
contentious - a physicist who believes the MWI to be correct will come to the same
conclusions about indeterminacy that someone using Bruno's matter transmitter would -
that it's a phenomenon experienced from a first person perspective because of the
person in question being split into two copies. The phenomena actually map onto each
other, because both comp and Everett allow for the possibility that from the third
person viewpoint the duplication could be observed - quantum computers rely on
precisely that fact.
Quantum computers (of the circuit type) rely on interference to pick out the right
solution. Interference implies superposition in the same world.
Only if you isolate the subsystem well enough. Imagine that I can isolate my room, where
I am, sufficiently, and in that room I succeed in isolating schroedinger cat (prepared
in the alive + dead state) in a box. Then, in my isolated room I look at the cat
(measuring in the alive/dead base) .QM description is that when I do that measurement, I
put myself in the superposition alive + dead. It follows from the linearity of evolution
and of the tensor product. You might say that I am in that superposed state in *one*
world. But if my room is not sufficiently well isolated, or more normally when I go out
of that room, announcing with some joy that the cat is alive, well soon enough, the
environment (the building with that room, then city, and you coming for a visit) get in
the superposition "history of the earth with that Shroedinger car alive + history of the
earth with that Shroedinger car dead.
Would you still say that it is a superposition in *one* world. Yes, the differentiation
of the galaxies will follows, at the speed of light, and I guess there will be two Milky
ways colliding with Andromeda, one with archive describing the fact that that
Schroedinger cat was alive, and one with the fact that that Schroedinger cat is dead.
Would you still say that there is one world? I like to define a physical world (in the
quantum theory) by a set of objects/events close for interaction. That makes the many
world the literal interpretation of QM. Without collapse, I don't see how the term of
the superposition can ever disappear.
The superposition doesn't disappear but it becomes dispersed into the environmental
degrees of freedom, so FAPP there are separate classical worlds. My point is that
superposition is not a defining attribute of different worlds, it's relative incoherence
so subspaces.
I highly recommend Scott Aaronson's blog http://www.scottaaronson.com/blog/ , for
straight talk about quantum computing (his book "Quantum Computing Since Democritus" is
also very good).
What is his position on Everett?
/(2) One of the first questions anyone asks on learning quantum mechanics is, "OK, but do
all these branches of the wavefunction really exist? or are they just mathematical
constructs used to calculate probabilities?" Roughly speaking, Many-Worlders would say
they do exist, while Copenhagenists would say they don't. Of course, part of what makes
the question slippery is that it's not even completely clear what we mean by words like
"exist"! Now, I'd say that quantum computing theory has sharpened the question in many
ways, and actually answered some of the sharpened versions --- but interestingly,
sometimes the answer goes one way and sometimes it goes the other! So for example, we have
strong evidence that quantum computers can solve certain specific problems in polynomial
time that would require exponential time to solve using a classical computer. Some
Many-Worlders, most notably David Deutsch, have seized on the apparent exponential
speedups for problems like factoring, as the ultimate proof that the various branches of
the wavefunction must literally exist: "if they don't exist," they ask, "then where was
this huge number factored? where did the exponential resources to solve the problem come
from?" The trouble is, we've also learned that a quantum computer could NOT solve
arbitrary search problems exponentially faster than a classical computer could solve them
--- something you'd probably predict a QC could do, if you thought of all the branches of
the wavefunction as just parallel processors. If you want a quantum speedup, then your
problem needs a particular structure, which (roughly speaking) lets you choreograph a
pattern of constructive and destructive interference involving ALL the branches. You can't
just "fan out" and have one branch try each possible solution --- twenty years of popular
articles notwithstanding, that's not how it works! We also know today that you can't
encode more than about n classical bits into n quantum bits (qubits), in such a way that
you can reliably retrieve any one of the bits afterward. And we have all lots of other
results that make quantum-mechanical amplitudes feel more like "just souped-up versions of
classical probabilities," and quantum superposition feel more like just a souped-up kind
of potentiality. I love how the mathematician Boris Tsirelson summarized the situation: he
said that "a quantum possibility is more real than a classical possibility, but less real
than a classical reality." It's an ontological category that our pre-mathematical,
pre-quantum intuitions just don't have a good name for./
http://intelligence.org/2013/12/13/aaronson/
Brent
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