From: *Bruno Marchal* <[email protected] <mailto:[email protected]>>
On 3 May 2018, at 04:16, Bruce Kellett <[email protected]
<mailto:[email protected]>> wrote:
From: *smitra* <[email protected] <mailto:[email protected]>>
On 02-05-2018 03:21, Brent Meeker wrote:
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
You can prove anything in a simulation, because you get to choose the
physical laws that will be obeyed. How do you know that the
simulation will bear any relation to reality? A measurement is
something that leaves a permanent (un-erasable) record.
But for Deutsch experiments, we don’t need to erase the information,
only to discard it in the vanilla ways. That seems impossible because
we are used to leaking environment, but if we can do a quantum
computer, we can do that, and it means we did found ways to a sort of
absolute isolation, measurement without interactions, (à-la Eiltzur
Vaidman), etc.
With a quantum brain, a human can do that experience, and come back
remembering well opening the box, and saying to itself “I definiitly
saw the definite result”, but after the coming back he said "now
despite I remember all that, I get a blank when trying to remember
that specific result, I can’t recall it (and this despite the quantum
brain “knows the answer”, but is well blocked by some quantum Freudian
algorithm (grin).
I tend to think that the laws of physics are reversible.
Actually, this is the basis of MWI -- everything in physics is based on
unitary transformations. The Schrödinger equation can be derived by
assuming time evolution is unitary. So, in the wider context,
everything, even decoherence into the wider universe, is reversible, in
the sense that there is a unitary transformation that, when applied to
any final state, restores the initial state -- just take the unitary
operator that describes the time evolution, say U, and then take its
inverse, U^{-1}.
The problem, of course, is that this unitary operator is formed in the
multiverse, so to form its inverse we have to have access to the other
worlds of the multiverse. And this is impossible because of the
linearity of the SE. So although the mathematics of unitary
transformations is perfectly reversible, measurements are not reversible
in principle in the one world we find ourselves to inhabit.
So even Deutsch's quantum brain is likely to run into difficulties,
since it has to communicate with the real world.
Bruce
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