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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