On 31 May 2017, at 04:01, Bruce Kellett 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.
In QM + special relativity OK. (note that to me QM + special
relativity => no collapse (and even the Many dreams, but we have
agreed to disagree on this if I remember well).
Consequently, indispensable phase information is lost *in
principle*, so the recoherence is, in general, impossible.
OK. (I was reasoning in naive classical QM)
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.
In QM + SR. OK.
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.
OK. That might suggest that we identify our indistinguishible past in
arithmetic, if we assume mechanism. I use the Y = II principle, or the
"quantum" linearity of the tensor product "@": we have that a @ (b +
c) = (a @ b) + (a @ c).
That makes sense.
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,
But those state difference are accessible to the observers, and
indeed, only this makes the analogy with step 3 working. With
computationalism, the two identical states at the correct susbt-level,
if never distinguishable, will not add ways to the probabilities (that
would entail that a computer with double cable would have a higher
probability). The measure remains on the first person view, which
eventually needs the self-referential definition. usually I just say
that if in step 3 you add a reconsitution box in Moscow, exactly
identical, the P(W) remains equal to 1/2, but a slight difference in
the future histories of the two guy in Moscow, will changes the
probability into 1/3 (but that is a typical open problem).
and these can evolve in different directions while each remains
unaware of the existence of the other. They can never recombine and
compare diaries!
OK.
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.
In general no. But I was in a thought experiment in classical QM, with
some T-rex having isolated a quantum qubit.
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.
Of course, it assures them in all branches, where indeed Aspect like
experiences can be made. It seems to me that we did agree on this:
that non-locality does not entail any physical influence in the past.
That does happen in the unique universe view though; even if there is
no possible communication of information is done.
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.
I have no problem with one-branch observable, apparent, non-locality.
I have a problem only with the action at a distance that you need in
case you assume one contextually well defined physical reality.
For me the abandon of the collapse is the solution of the EPR
"paradox", and Aspect experience is somehow the confirmation of our
belonging to macrosuperposition.
Bruno
Bruce
--
You received this message because you are subscribed to the Google
Groups "Everything List" group.
To unsubscribe from this group and stop receiving emails from it,
send an email to everything-list+unsubscr...@googlegroups.com.
To post to this group, send email to everything-list@googlegroups.com.
Visit this group at https://groups.google.com/group/everything-list.
For more options, visit https://groups.google.com/d/optout.
http://iridia.ulb.ac.be/~marchal/
--
You received this message because you are subscribed to the Google Groups
"Everything List" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to everything-list+unsubscr...@googlegroups.com.
To post to this group, send email to everything-list@googlegroups.com.
Visit this group at https://groups.google.com/group/everything-list.
For more options, visit https://groups.google.com/d/optout.
