On Thursday, April 19, 2018 at 2:42:37 PM UTC, Bruno Marchal wrote:
On 18 Apr 2018, at 15:45, Bruce Kellett <[email protected]>
wrote:
From: BRUNO MARCHAL <[email protected]>
On 17 Apr 2018, at 13:52, Bruce Kellett <[email protected]>
wrote
But note particularly that the spin measurement is made in the
basis chosen by the experimenter (by orienting his/her magnet).
OK.
The outcome of the measurement is + or -,
For Alice and Bob, OK.
not one of the possible infinite set of possible basis vector
orientations. The orientation is not measured, it is chose by the
experimenter. So that is one potential source of an infinite set of
worlds eliminated right away. The singlet is a superposition of two
states, + and -: it is not a superposition of possible basis
vectors.
? (That is far too ambiguous).
????? It is not in the least ambiguous. The singlet state is not a
superposition of basis vectors.
?
The singlet state is the superposition of Iup>IMinus> and (Minus>Iup>.
If you think about it for a little, the formalism of QM does not
allow the state to be written in any way that could suggest that.
I don't know what Everett says in his long text, but if it is any
different from the above, then it is not standard quantum mechanics.
Deutsch is a different case. He has a very strange notion about what
constitutes different worlds in QM. Standard QM and Everett's
interpretation are very clear: different worlds arise by the process
of decoherence which diagonalizes the density matrix. The net effect
is that worlds are, by definition, non interacting (contra Deutsch's
ideas).
?
This relates to your lack of comprehension above.
Patronising !!!!!!!
Deutsch has two distinct notions of "world" in his approach. He has
the standard Everettian notion of a "relative state" corresponding
to each term in the superposition of possible measurement outcomes.
These relative states are made definite by decoherence,
Relatively. Decoherence is only entanglement (with NON-collapse).
and then correspond to different, effectively orthogonal, worlds,
each of which represents the experimenter observing one particular
result. But Deutsch also has the idea that the infinity of possible
bases for an unpolarized qubit also represents an infinity of
worlds.
That is necessary, and Everett explains this well when he shows that
the choice of the base to describe the universal wave is irrelevant.
(A bit like the choice of the universal Turing formalism is irrelevant
to get the theology and the physics).
This is quite a different notion, and does not occur in Everettian
theory.
I disagree with this.
In this second notion of "world", the worlds remain in
superposition and continue to interfere -- there is no separation
into disjoint, non-interacting worlds. In fact, it is precisely this
continued interference of these supposed "worlds" that is the
explanation for the action of quantum computers -- which Deutsch
seems to think actually *prove* his notion of quantum "many-worlds".
He is out on a limb on this one, and few experts, even in the
quantum computing field, agree with Deutsch on this new notion of
"worlds". The essential continued interference between the different
basis states in fact means that the "worlds" remain inextricable
"one world". (See some of Scott Aaronson's comments on Deutsch and
many-worlds in his lecture notes on quantum computing.)
So when you continue to refer to an "infinity of worlds" for the
measurements on the entangled spin states, you are using a notion of
"world" that does not occur in Everett, and is inherently
controversial, if not entirely meaningless.
I use the “Herbrand” interpretation of quantum mechanics without
collapse. I mean: it is literal QM (in a sense that logicians have
made precise) without collapse up to a choice of any arbitrary base.
I don’t believe in any worlds, to be clear. It always means some
reality satisfying some formal constraints.
But even if you can manufacture an infinity of universes, you still
have not shown how this removes the non-locality inherent in the
quantum formalism.
You have not shown non locality.
I have demonstrated non-locality in the Everettian context many
times. The simplest demonstration was in the timelike separation of
Alice and Bob's measurements. It is in the archives if you don't
recall the details. The argument then is that any local influence that
would explain the timelike separated measurements must also work for
spacelike separated measurements, and that is not possible.
At all time there is an infinity of “worlds”. When Alice chose her
direction, that remains true, and her measurement will tell us if she
belongs to a world with “spin” down or up, she will automatically
know that whatever Bob she will meet, will have the corresponding
results, no action at a distance here.
Again, you keep referring to this non-existent infinity of worlds —
“worlds” would be better.
a notion that has nothing to do with Everett or his interpretation
of quantum theory. "... She will automatically know that whatever
Bob she will meet, will have the corresponding results...". This is
precisely the question that you have not answered -- how does this
happen?
Because in ALL “worlds” Alice and Bob have they spin described by
the no-separable singlet state. The statistics seems non-local, due to
their ignorance of which partition of the wave function they belong
to.
But that would be the same for all worlds; statistics which imply
instantaneous action at a distance. You haven't removed non-locality,
but in fact extended it to many worlds, and then you must ignore the
elephant in the room; the absurdity of postulating the many observers
with identical memories, histories, etc. I don't see that anything has
been gained. AG
