On 08 Jul 2008, at 18:38, Brent Meeker wrote:

> Bruno Marchal wrote:
>> If I look to a particle in the state "(UP + DOWN)",  the state "I
>> cross (UP + DOWN)"
>> evolves (by SWE) to:
>> "I-seeing-UP cross UP + I-seeing-DOWN cross DOWN",
>> I don't see how to avoid this without abandoning QM.
> If we can't show that a quasi-classical world emerges from QM we will
> abandon it -

We have already shown (Everett ... Zurek) how quasi-classical worlds  
emerge in the memory of classical machines through QM.
QM justifies the *appearance* of classicality.

> at least as fundamental.  Perhaps the long sought quantum
> theory of gravity will come to the rescue.

All approaches now keep intact QM. Everett is the one who realize the  
more clearly that only the collapse axioms was incoherent with  
relativity. Even Bohr admitted that the collapse could not be physical.
Once you abandon the collapse, QM and (special) relativity fit very  
well together. That is why all current approaches in quantum gravity  
prefer to keep intact QM (and thus the MW) and correct only a bit GR,  
in some (different) ways.
By quantifying gravity, i.e. introducing many spaces-times, you put a  
(quantum) topological structure on the multidreams.

And why do you want a rescue? You betray you are wishing ONE universe,  
and certainly I wish ONE reality, but current physics implies Many  
alternate histories, and the current theory of mind (computer science,  
universal machine introspection) implies it too.

Scientists are (or should be) agnostic if there is 0, 1, 2, 3,  
aleph_zero, ... universes. But today evidences are in between:  0,  
aleph_zero, 2^aleph_0.

>> The collapse can
>> be explained
>> *phenomenologically* (first person plural)  through MW + decoherence.
>> And if I make a decision based on what state I measure on the
>> particle, the divergence will
>> propagate.
>> And Weinberg has given a convincing argument that once SWE is made
>> slightly non
>> linear, then, not only we keep the Many Worlds/Dreams, but  
>> interaction
>> is made
>> possible between them (falsifying then thermodynamics though: that is
>> why I don't
>> take that the delinearisation of the SWE idea very seriously). That's
>> speculation.
> Do you have a reference for Weinberg?  I'd like to read his paper.

I'm afraid I will not find quickly my copy which I have not read for  
many years, but I found a (physicist's, without title) reference:

S. Weinberg, Ann. Phys. (N.Y.) 194, 336 (1989)

Perhaps more helpful is this:


(at the time I thought I could easily proof that Plaga + Polchinsky  +  
Weinberg implies the existence of absolute elasticity, and thus on  
some continuous matter/energy/space-time/information, but of course it  
appears harder than I thought and I abandoned the project: that would  
NOT have contradicted Mechanism, on the contrary).

>  I  wonder how it would apply to the idea that there is a smallest  
> unit of
> probability amplitude.

Except perhaps for some interpretation of Loop Gravity, but I don't  
see why they would be a smallest unit of probability amplitude.
 From the UDA it is rather clear that this could not exist, unless you  
introduce some ad hoc non computable criteria of equivalence.
And from QM? You are thinking about loop gravity?

> Do you agree that the other forms of multiverse, besides Everett's,  
> are
> speculative?

i am not sure I understand. Which other forms you are thinking about?

Recall that I believe in no more than positive integers, and addition  
and multiplication. Then from inside (defined by machine  
introspection ...) an ocean of realities develop in many many  
directions, and fuse as often, at many many levels. Today the  
mechanist hypothesis still a priori implies more universes than those  
we "observe" indirectly by the quantum superposition.

Today the charge has changed. Those who speculates on *one* physical  
universe should give at least one reason or evidence of that unicity  
(other than habit or wish).
That idea just doesn't fit the facts, nor two independent theories (in  
not so unrelated matter admittedly).


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