scerir wrote:

> Joao wrote:
>
> > This not quite the case. In the Bohmian interpretation the "collapse"
> > is, in fact, determined by the non-local quantum potential pretty
> > much as the outcome of a critical phase transition which suppresses
> > all the branches of the superposition but the one that matches the
> > measured outcome. This is indeed "effective" but hardly pragramatic.
>
> The "collapse" or the "reduction" in the Bohmian theory is something
> obscure (to me) and - perhaps - also to the masters like Goldstein
> http://plato.stanford.edu/entries/qm-bohm/#cwf and Durr and Valentini
> and Cushing, etc. Perhaps it is the effect of the "holistic" nature
> of that model :-)

Collapse is an obscure notion in itself. What interpretations of QM try
to do is to avoid it in some vaguely consistent way by going beyond the
von Neumann conception of measurement. The bottom line is that this
can be achieved in many different ways but each involves the abandonment
of at least one of 3 cherished things: a) QM linearity (and the
superposition
principle) (b) special relativity, (c) the EPR concept of physical reality.

>
> > The Everett Interpretation is just as non-local
> > as QM with the peculiar distinction that it accommodates non-locality
> > in its peculiar way, where the unconnected "locales" are made relative
> > to the different branches of the wave function.
>
> Yes this is also my opinion (and D. Mermin's opinion!). But it is also
> true what is saying Bruno Marchal. That it to say, that we must define
> non-locality (non-separability, holism, etc.) first! (And I add that
> we must check whether non-locality is embedded in the QM formalism
> or not.)

I don't believe that there is ANY question that QM is non-local! This is
the outcome of 30 years of experiments with entangled multiparticle
states.  I also think that non-locality is pretty well defined in this
context (the way Bell put it) and we know what implications it has
in the laboratory and elsewhere.

BTW: Non-locality and (non-separability) is an effect quite distinct
from the problem of collapse or measurement  (which is better called
the Actuality Problem), but has, of course, an immense bearing on it.
This is just to point out that the several interpretations of QM address
measurement but not non-locality. They either incorporate it (Bohm)
or hide it (Everett). In this sense the two are complementary.

--

Joao Pedro Leao  :::  [EMAIL PROTECTED]
Harvard-Smithsonian Center for Astrophysics
1815 Massachussetts Av. , Cambridge MA 02140
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