On Mon, Dec 10, 2012 at 10:30 AM, meekerdb <meeke...@verizon.net> wrote:

>  On 12/10/2012 2:56 AM, Bruno Marchal wrote:
>  But why isn't "It's a probabilistic world and it obeys the Born rule." a
> good explanation.  I'm all for finding a better explanation, i.e. a
> deterministic one.  But simply postulating an ensemble of worlds to make
> the probabilities "deterministic" in arbitrary way doesn't strike me as any
> improvement.
>  It is, as it explains interference, without adding something not well
> defined (apparatus, observer) not obeying QM (like with the collapse needed
> to get one physical reality).
> That sounds like prejudice against probabilistic theories.  The
> interference is inherent in the complex Hilbert space states.  The
> interference happens in one world.  As Omnes says, you don't need
> 'collapse' you just need to accept that you have calculated probabilities.
> That's what probability means - some state is actualized and others aren't.
How does Omnes explain the EPR experiment without collapse?  It seems you
need to give up not only determinism, but also locality.

Also, what do you think Omnes would predict as the outcome for Deutsch's
proposed experiment:

In Deutsch's thought experiment, an atom, which has a determinate spin
state in one axis, 'left' for example, is passed through a Stern-Gerlach
apparatus which has the possibility of measuring it in another axis, as
either spin 'up' or spin 'down' in this case. This means that the atom is
then in a superposition of 'up' and 'down' states from the perspective of
an observer who has not yet become entangled with it. This superposition
travels to the AI's artificial 'sense organ'. Here it is provided with two
options, it may be detected as either spin 'up' or spin 'down'. The AI's
conscious mind then records the result. The collapse approach predicts that
this will cause the atom to collapse into one determinate state, with
either a determinate 'up' or 'down' (but not 'left' or 'right') spin. The
Everett approach predicts that the mind will branch into two, one mind will
record up and one down (but neither will record 'left' or 'right').

The whole process is then reversed so that the atom emerges from the
entrance to the Stern-Gerlach apparatus and the mind forgets which result
it recorded. This process does not erase any of the AI's other memories
however, including the memory that they did record the atom to be in a
definite state. If a 'left-right' detector was placed at the entrance of
the Stern-Gerlach apparatus then the collapse approach predicts that it
will be detected as being in either a 'left' or 'right' state with equal
probability. If the Everett approach is correct then the atom will be in
the same state that it was in before the measurement, it will still have a
'left' spin.

  --- http://www.thestargarden.co.uk/EvidenceOfParallelWorlds.html


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