On 02-05-2018 03:21, Brent Meeker wrote:
On 5/1/2018 4:43 PM, smitra wrote:
On 01-05-2018 20:47, Brent Meeker wrote:
On 5/1/2018 9:01 AM, Bruno Marchal wrote:
On 29 Apr 2018, at 19:59, Brent Meeker <meeke...@verizon.net> wrote:
On 4/29/2018 8:53 AM, Bruno Marchal wrote:
But that's my question: Why isn't it the same? And even if it's not
how would be know? The "conscious" quantum computer assures us that
it not only detected that there was a welcher weg photon but that
it's weg was known to the "consciousness" of the quantum computer,
before it was erased. But why would we believe it? We already have
these experiments in which we know the weg was available and could
have been recorded, but was erased. So what is the "consciousness"
that adds a secret-sauce to the experiment?
Good question. I doubt that you can fool quantum mechanics by
calling it "consciousness". I think in this case the interaction
with the welcher weg photon would amount to sufficient decoherence
-- basically information was extracted that was not restored. Also,
of course, if the QC "forgets" what it did, how can it report on the
fact that it did anything. How can we believe that it actually knew
which slit at some point?
Because in Deutsch experiment, not everything has been erased,
notably
the memory that he has known the result. He would say something like:
I remember doing the measurement and writing it in the enveloppe. Now
the envelop has been erased, and I can’t remember its content, but I
definitely remember having known the content.
But two questions remain. First, the empirical question of whether
this erasure is enough to restore interference.
I do not see why it would not been enough … in theory. You need only
a computer able to forget a memory, but not some meta-memory that it
has recorded a definite result. It isa bit like remembering we have
done a dream, without being able to remember any of its content.
In practice, that might be very difficult, if not impossible. I am
not
sure.
Second, why should we believe the quantum computer.
In Deutsch proposal, it is a human.
No, it's a conscious quantum computer. It it were a human or other
(quasi-) classical instrument decoherence would happen when there was
a detection of welcher weg and erasure would be impossible.
Brent
Yes, but note that you can make that quantum computing simulation of
the observer in that thought experiment as precise as you like. You
can in principle include a simulation of the entire Earth
And the outgoing EM and neutrino waves and their interaction with
interstellar atoms. I'm suspicious of these fantasy thought
experiments. But however detailed it may be doesn't answer my
question as to what it would mean to erase the welcher weg but not the
memory that the weg was detected. I noted that this is not like a
classical erasure of a memory because in this case the coherence is
maintained, so when the welcher weg is erased there is no long any
fact-of-the-matter as to which way it went. There is no
fact-of-the-matter that it was detected to go left or right. So the
"memory" if it exists, is a false memory.
with billions of other people and a lot of decoherence implemented by
qubits that simulate e.g. soft photons and other environmental degrees
of freedom (and all that decoherence will end up getting reversed by
the way the computation is set up ) The point is that if computation
generates consciousness, you can in principle let any given person do
the experimental verification of the existence of multiple branches by
uploading the brain to a quantum computer and letting it be subject to
such a computation.
How will the person verify it? Reversing the computation will reverse
the person and erase their memory.
Brent
It's a simple two step measurement process where you (as a virtual
person simulated by the QC) perform a measurement that tells you that
the spin (represented by a qubit) has been measured without giving you
the result. And then you perform the next measurement where you actually
measure the value of the spin component. It can then be shown that there
exists a unitary transform that will restore the original spin state
that will preserve the record of the first measurement.
Saibal
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