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
>>> 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
>>> 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
>> 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.
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,
> 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
> 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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