Le 08-juil.-08, à 21:00, Brent Meeker a écrit :

> 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.

I have read it some times ago. I am ok with the idea that they are 
remaing questions 'course. And I do agree that in general there are no 
quasi-classical continuations of state. but this does not prevent 
quasi-classical continuations of some state to exist. In particular I 
don't see how Dowker and Kent's anaylis can be used to make 
disappearing on of the terms in Me-seeing-up CROSS up + Me-seeing-down 
CROSS down.

Also, remind that with assuming comp, then there is already 
Many-Interfering-histories with just (N, +, *). If someone really comes 
up with a "one-universe" interpretation of QM, I will most probably 
conclude that QM is most probably wrong, given that I tend to believe 
more in comp than in QM. Up to now, I take QM as confirming comp, 
because interfering dreams appears where comp predict it has to appear.

(By QM I mean QM without collapse of course).

>>> 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.

I have never pretend the contrary, especially about the first person 
white rabbits.
The only problem of QM is that it does not yet take comp seriously 
enough imo.

>>>> 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.

Yes, but by the unawareness of delay the physical and the analytical 
apperas only from inside at the tird person limit of UDA. This is why 
the ontology with comp can be very little (no more than (N,+,*)), yet 
beyond all known mathematics (including large cardinals) from inside.
most scientist just drop away that 1-3 distinction, by Aristotelian 
Of course your remark should be handled more technically, and is 
related to the more difficult 1-person/1-person-plural distinction, 
which is not yet handled by comp. remind that I am not sure at all comp 
is true. I am just sure that if comp is true then the notion of many 
alternate interfering realities is unavoidable.

> 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.

I cannot judge eternal-inflation universes of Guth. It is too early to 
compare with Mechanism.
I don't know about the sprouting universes of Carroll, and would 
appreciate a reference.
Tegmark takes too much at the ontic level, and not enough, at the first 
person level as I have already explained more than once. The notion of 
all mathematical structure is too vague for me. Attempts, like 
Lawvere's one" to capture such notions have failed, even if they have 
lead to crasily interesting generalization of mathematics, like the 
notion of variable-mathematics implemented through the notion of 
(categorical) toposes.

>> 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.

Ontically; yes. and epistemologically Tegmark's mathematical structures 
are far from enough. The first person plenitude is just above all 

>> 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.

I agree with that, except( I am not sure by what you mean with the word 

> But I regard all the extant explanations
> as speculative.

This is perhaps due to your refying of "natural", but I am not sure I 
understand clearly what you mean.


>> 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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