On 31/05/2017 12:30 pm, Pierz wrote:
Thanks for these clarifications Bruce. I find your explanations to be
very lucid and helpful - they also confirm my own understanding.
Thank you for the kind comments, I try my best to be clear.
IIRC, you weren't a particular fan of MWI when I last conversed with
you on this list.
That is indeed the case. I have several reasons to be dubious about MWI.
Firstly, it amounts to reifying a complex valued function that resides
in configuration space -- I am not sure that this is a well-defined
procedure. Secondly, MWI doesn't really do what it claims to do, which
is to provide a resolution of the measurement problem in QM. MWI doesn't
provide any explanation of the transition from a pure state to the mixed
state that is required for experiments to give definite results. At the
crucial point, MWI simply says "then a miracle happens!" To be more
explicit, deterministic evolution of the wave function by the
Schrödinger equation gives a full account of decoherence, and the
dissemination of the coherence phases into the environment. This reduces
the off-diagonal elements of the density matrix so that the diagonal
elements become *almost* orthogonal, but unitary evolution can't go the
whole way. The only way one can reduce the pure state to a mixture is to
trace over the environmental degrees of freedom, which is to say that
the residual phase information is simply thrown away. This trace
operation is non-unitary, and there is no warrant for it in the SE
itself, so it is, in the final analysis, just an appeal to magic.
Thirdly, the non-observed branches in MWI play no essential role in the
theory, so Occam would say that they are inessential entities that
should be discarded. If one is simply going to discard them, and they
play no observable role, why invoke these other branches in the first place?
I wonder if you'd care to comment on my original argument on this
thread - which has of course now been swamped by the usual brawls.
Does not a single history + the physical insignificance of the notion
of a current moment mean that there is also only a single possible future?
We will only experience a single future, but what that future is, is
indeterminate at the present instant.
And if the future is predetermined in this way, isn't this a serious
issue for single universe models of QM? How can the outcome of quantum
events be both inevitable and random?
I don't see that there is only a single possible future. The block
universe notion only requires indeterminate time ordering for spacelike
separated events. The future along and inside one's future light cone is
in the future for all observers, so need not be determined by some other
observer having already seen what happens. The block universe only
constrains the future only in very limited sense -- it is only for
spacelike separations that simultaneity is ambiguous, timelike
separations are not so constrained.
Quantum non-locality is another matter, however, and there are growing
indications that quantum entanglement and the associated non-locality
might prove to be of fundamental significance for physics -- such as the
possibility that space-time itself might emerge from quantum entanglements.
Bruce
On Wednesday, May 31, 2017 at 12:01:23 PM UTC+10, Bruce wrote:
On 30/05/2017 9:35 pm, Bruno Marchal wrote:
> On 30 May 2017, at 11:28, Telmo Menezes wrote:
>
>> I get your point with decoherence.
>> Again, I would say that it all depends on theories of mind.
What does
>> mind supervene on? Perhaps it is true that every single
coupling with
>> the environment prevents the current observer state to become
>> compatible with other branches. But can we be sure? I feel that
such
>> certainties come from a strong belief in emergentism (which I
cannot
>> disprove, but find problematic).
>
> It is impossible to recohere the past, FAPP.
>
> But only FAPP. To make the blue T-rex interfereing with the
red-T-rex,
> we must erase the trace of particle interaction between the
T-rex in
> its whole light-cone, and this without forgetting the particles
> "swallowed" by the black-holes, etc. It is just completely
impossible,
> but to derive from that the unicity of the past, is, it seems to me
> (and you if I understood well) is invalid.
I think the recoherence of paths that have completely decohered is
more
than just FAPP impossible, I think it is impossible in principle. One
major problem with recoherence in general is that information
leaks from
the paths at the speed of light (as well as less slowly for other
interactions). Since this vital information goes out along the light
cone, it can never be recaptured and returned to the original
interaction. Consequently, indispensable phase information is lost
*in
principle*, so the recoherence is, in general, impossible. Of course,
with carefully constructed systems, where the loss of information
along
the light cone is prevented, recoherence is possible in special
circumstances, but not in general.
From this, the uniqueness of the past of any decoherent history is
assured. So deriving the unicity (if I understand this use of the
word)
is by no means invalid -- it is proved. Even if one encounters one of
those rare situations in which recoherence is achieved, that still
does
not invalidate the uniqueness of the past history -- recoherence,
if it
occurs, simply means that no new branches are formed at that
point, so
the decoherent history remains unique.
>>> FWIW, you
>>> are expressing my own understanding of the situation: there
can be no
>>> superposition of red and green screens or dinosaurs, or dead and
>>> live cats,
>>> because there can be no quantum superposition of macroscopic
objects.
>>> Superpositions of wave functions are only possible for systems
>>> isolated from
>>> interaction with their environment, which is why quantum
computers
>>> are so
>>> fricking hard to make: keeping aggregates of particles
isolated from
>>> interactions with the surrounding environment is exponentially
more
>>> difficult as the system grows in size.
>>
>> The main question for me is this: can two branches hold different
>> observer states, if they differ only by things that are not
>> observable?
>
> I would say no, intuitively. I would even say "no" just for the
things
> not observed, even when observable.
I previously answered Telmo's question in the affirmative, viz., two
fully decohered branches will hold different observer states, even if
the differences are not observed or observable. So if some trivial
physical event happens to your body, such as the decay of a K 40
nucleus
in your foot, this would not be noticeable, or even particularly
observable even if you were looking for it. But such an event
causes at
least two branches to form every instant -- one in which the decay
has
occurred, and one in which it has not. And since this is a beta
decay, a
neutrino is lost along the light cone in every case of decay.
Perfect
recombination of the branches is, then, according to the above
argument,
not possible. You might object that this decay in my toe did not
alter
my conscious state -- that is correct, but there are now two
copies of
the Moscow man as in step 3, and these can evolve in different
directions while each remains unaware of the existence of the other.
They can never recombine and compare diaries!
> But this has to be tempered by the fact that any interaction will
> count as an observation, making super-exponentially hard to indeed
> recover a macroscopic superposition in the past, even the very
close
> past. Of course, that might change the day we succeed in building a
> fault tolerant (topological perhaps) quantum computer.
That will not help in the general case. Our future quantum computer
might be able to delay decoherence for some useful finite time,
but that
still only retaines the superposition in the said computer, it
does not
help with recombination of decohered branches in general.
> Unfortunately, the T-rex missed them. yet, if a T-rex made a solid
> topological quantum qubit, in the state 0+1, we would have a
past with
> 0, and a past with 1, as long as we don't look at it. I read,
already
> a long time ago, some experimental evidence of temporal Bell's
> inequality going in this direction, and I think we don't even
need to
> test them, as we get them with the usual Bell's inequality
violation,
> if we accept special relativity (and some amount of physical
realism
> (not the full materialism, to be sure).
The temporal version of Bell's inequality simply shows that the
ubiquitous non-locality of quantum entanglement occurs even over
time.
And despite your protestations to the contrary, it is now generally
accepted that MWI does not remove the essential non-locality
associated
with entangled states. This non-locality is even more evident in the
more recent delayed choice experiments that use entangled photons to
manipulate photon polarization states non-locally.
Bruce
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