From: *Bruno Marchal* <marc...@ulb.ac.be <mailto:marc...@ulb.ac.be>>
On 17 Apr 2018, at 13:52, Bruce Kellett <bhkell...@optusnet.com.au
<mailto:bhkell...@optusnet.com.au>> 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.
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. 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, 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. This is quite a different notion, and
does not occur in Everettian theory. 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.
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 -- 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? What is
the particular magic that you put in the mix to ensure that the correct
correlations emerge? If there is an infinity of worlds, there must be an
awful lot of selection going on -- superfluous worlds surreptitiously
eliminated....
See Maudlin for a fuller account of the absurdity of this notion.
In the Everett, the locality is preserved by the fact that you need
interaction/measurement at some point, and the superstition get
“contagious” only at the speed of light, something zurek explained
well in his account of decoherence.
This is what you suggested above -- your view is that locality is
maintained by refusing to accept the possibility of non-locality.
Sorry, but that does not wash, scientifically or logically.
That reasoning is the same as creationist. Your theory of evolution
assumes that God has not made the world in six days, so, there is no
doubt that you can only see confirmation of it. Of course that is not
valid here, nor above.
That is your reasoning, not mine.
Bruce
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