Bruno Marchal wrote:
> 
> 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.

I don't think this problem is solved. Dowker and Kent's paper was written 
primarily as an investigation of the consistent histories interpretation, 
but it's conclusions also seem to apply to Everett+decoherence; namely that 
  in general there are no quasi-classical continuations of a state.

> 
> 
>> 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.

I don't care how many there are - I'm just concerned with the evidence.  I 
was referring to rescuing Everett+decoherence from the problem Dowker and 
Kent point to.  It seems to be the white rabbit problem within QM.

>>> 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:
> 
> http://prola.aps.org/abstract/PRL/v66/i4/p397_1

Thanks.
> 
> (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.

I would think that something like it would follow from the UDA.  Aren't all 
the values integers - reals only occurring as limits.

I've only seen one paper that considered the possibility.  I postulated a 
finite, closed universe so that probabilities could be propagated backward.


> 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?

The eternal-inflation universes of Guth for example or the sprouting 
universes of Carroll.  Or Tegmark's all-mathematical-structures.

> 
> Recall that I believe in no more than positive integers, and addition  
> and multiplication. 

Which would rule out almost all of Tegmark's mathematical structures.

>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.

Yes, I made the point that almost *any* theory that explains the existence 
of our universe as a natural event following some rules will as well imply 
the existence of other universes.  But I regard all the extant explanations 
as speculative.

Brent

> 
> 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).
> 
> Bruno
> http://iridia.ulb.ac.be/~marchal/
> 
> 
> 
> 
> > 
> 


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