On 10/09/2015 7:42 am, Brent Meeker wrote:
On 9/9/2015 10:27 AM, Bruno Marchal wrote:
On 09 Sep 2015, at 07:55, Quentin Anciaux wrote:
As for Bruce, could you then be honest, and simply say you don't
believe in the many world interpretation and don't want to explore
it... yes we're talking mostly metaphysics on this list, if you
dislike it, I wonder what you're doing here.
Indeed the point is notably that we can reason about this, that is
doing science, so that we never disagree except about the choice of
the assumptions.
Now, I disagree with Bruce, and I guess many philosophers and
scientists, except Deutsch (on this), but I do think that QM (without
collapse) is a theory of many worlds (in a perhaps admittedly more
abstract than usual notion of world).
If we define a physical world by a set of events close for interaction,
But can computationalism give a coherent account of this? Doesn't the
UD imply at every set of events will have many (countably many?)
causal histories and infinitely many causal futures.
then the "many-world" is a consequence of the linearity of the wave
evolution together with the linearity of the tensor product.
But they are only "many" FAPP. Decoherence suppresses cross terms in
the density matrix, but it doesn't make them zero - and I don't think
it's even provable that there is a unique basis in which it
diagonalized FAPP. And of course we have no theory of quantum
spacetime. QM assumes a continuous spacetime, so QM is not the last
word and if computationalism only reproduces QM it will fail when QM
fails.
I think it is clear that Bruno does not understand either QM or the MWI.
MWI is not a consequence of the linearity of the wave function.
Superposition is the consequence of linearity, and superposition implies
interactions between outcomes. Many-worlds requires decoherence in a
preferred basis, with actual zeroing of the off-diagonal terms in the
density matrix. Indeed, this latter step is just the standard "collapse"
postulate in a different guise. So, far from eliminating "collapse", MWI
relies on it as much as any other interpretation of QM. The real
difference between interpretations is whether this "collapse" is a
physical process or merely an epistemological one. In MWI, which reifies
the wave function and the elements of the superposition, the collapse is
definitely physical. So it is, after all, no different in this respect
from the von Neumann Copenhagen interpretation.
Bruce
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